Resistive Switching in Non-Stoichiometric Germanosilicate Glass Films Containing Ge Nanoclusters

electronics

Article Resistive Switching in Non-Stoichiometric Germanosilicate Glass Films Containing Ge Nanoclusters

Vladimir A. Volodin 1,2,* , Pavel Geydt 2,* , Gennadiy N. Kamaev 1,2 , Andrei A. Gismatulin 1,2, Grigory K. Krivyakin 1,2, Igor P. Prosvirin 3, Ivan A. Azarov 1,2, Zhang Fan 2 and Michel Vergnat 4 1 Rzhanov Institute of Semiconductor Physics SB RAS, Lavrentiev Ave., 13, 630090 Novosibirsk, Russia; [email protected] (G.N.K.); [email protected] (A.A.G.); [email protected] (G.K.K.); [email protected] (I.A.A.) 2 Physical Faculty, Novosibirsk State University, Pirogova Street, 2, 630090 Novosibirsk, Russia; [email protected] 3 Boreskov Institute of Catalysis SB RAS, Lavrentiev Ave., 5, 630090 Novosibirsk, Russia; [email protected] 4 CNRS, IJL, Université de Lorraine, F-54000 Nancy, France; [email protected] * Correspondence: [email protected] (V.A.V.); [email protected] (P.G.)

Received: 10 November 2020; Accepted: 7 December 2020; Published: 10 December 2020

Abstract: Metal–insulator–semiconductor (MIS) structures based on thin GeO[SiO2] and GeO[SiO] films on Si substrates were fabricated with indium-tin-oxide as a top electrode. The samples were divided it two series: one was left as deposited, while the second portion of MIS structures was annealed at 500 ◦C in argon for 20 min. The structural properties of as-deposited and annealed non-stoichiometric germanosilicate (GeSixOy) films were studied using X-ray photoelectron spectroscopy, electron microscopy, Raman and infrared absorption spectroscopy, spectral ellipsometry, and transmittance and reflectance spectroscopy. It was found that the as-deposited GeO[SiO] film contained amorphous Ge clusters. Annealing led to the formation of amorphous Ge nanoclusters in the GeO[SiO2] film and an increase of amorphous Ge volume in the GeO[SiO] film. Switching from a high resistance state (HRS OFF) to a low resistance state (LRS ON) and vice versa was detected in the as-deposited and annealed MIS structures. The endurance studies showed that slight degradation of the memory window occurred, mainly caused by the decrease of the ON state current. Notably, intermediate resistance states were observed in almost all MIS structures, in addition to the HRS and LRS states. This property can be used for the simulation of neuromorphic devices and related applications in data science.

Keywords: germanosilicate glass; memristor; Ge nanoclusters; resistance states; thin ﬁlms

1. Introduction The development of information technologies requires devices for storing and processing huge amounts of information. Therefore, the creation of a universal storage device with a high speed of writing and reading information, along with a long storage time, high information capacity, and low operation power, is highly demanded in the ﬁeld of nanoelectronics [1]. The capacity of memory matrices continues to grow while the planar integration of memory elements has almost reached its physical limits, which stimulates research of new physical principles and new materials for memory elements. In 1971, Leon Chua theoretically predicted the existence of the fourth element of electronics: a memory resistor, also called a memristor [2]. In 2008, Stanley Williams and his co-authors discovered

Electronics 2020, 9, 2103; doi:10.3390/electronics9122103 www.mdpi.com/journal/electronics Electronics 2020, 9, 2103 2 of 17 a memristor effect in a thin titanium oxide film, manifesting what “the missing memristor found” [3]. The memristor effect is based on the controlled switching of dielectrics to high and low resistance states (HRS and LRS, respectively). In 2013, Panasonic Semiconductor Solutions launched eight-bit MN101LR series microcontrollers with integrated, resistive random-access memories (ReRAMs) based on Ta2O5/TaO2 double layers using 180-nanometer technology [4]. With a giant capacity of information storage in nonvolatile ReRAM, it is assumed that its energy consumption will be low. Presumably, only spintronic-based memory and ferroelectric RAM (FeRAM) can compete with ReRAM in energy consumption. To date, the memristor effect has been observed in a wide class of different materials: perovskite films [5], organic films [6], fluorides and graphene oxides [7], high-k dielectrics such as TiO2, HfO2 [8], ZrO2, Ta2O5 [9], Ta2O5 x/TaO2 x [10], Nb2O5, Al2O3, germanium oxides like − − GeOx [11], and silicon nitrides like SiNx [12]. It is especially important to develop memristors based on materials that are fully compatible with simple silicon planar technology, such as materials like silicon oxide SiOx [13–17]. The important advantage of non-stoichiometric germanosilicate glasses (GeSixOy solid alloys) are that the technology of their deposition is simple, inexpensive, and fully compatible with modern silicon technology [18]. These films are also interesting because they contain charge traps of various types. The nanoscale fluctuations of the potential in these solid alloys are due to the presence of SiOx, GeOx, Si, or Ge nano-inclusions. The bandgap values in SiO2 (8–9 eV), GeO2 (4–5 eV), and Ge (0.7 eV) differ significantly. The nano-inclusions of germanium oxides in silicon oxide are relatively shallow traps. Nano-precipitates with an excess of germanium atoms (or oxygen deficit regions) and Ge nanoclusters are quite deep traps. Recently, there has been an increasing interest in materials with a low enthalpy of bonds between an atom of the fourth group of the periodic table of elements and an oxygen atom for the formation of memristors. This is because an easier formation of conductive filaments and their easier rupture in such materials are assumed. This softening should reduce the operation power for switching between the states, but it is also expected to reduce retention and endurance. On the one hand, softening can lead to the appearance of regimes of the intermediate resistance states (IRSs) in memristors. These states, supposedly, can be useful for the simulation of neuromorphic devices (e.g., stimulating synaptic plasticity and preliminary spike-enhanced plasticity) [19]. Multilevel resistance states in nanoscale neuromorphic devices with retention time satisfaction can be used as prospective artificial synapses [20].

2. Experiment

The samples of the non-stoichiometric germanosilicate glasses (i.e., GeSixOy solid alloys) were fabricated with a heating evaporation technique similar to the one previously described in [18]. 8 After simultaneous evaporation of the GeO2 and SiO2 (or SiO) powders in a high vacuum (10− Torr), deposition onto heavily doped n+-type (resistivity 0.005 Ohm cm) and p+-type (resistivity 0.01 Ohm cm) · · Si (001) substrates heated up to 100 ◦C occurred. The standard quartz microbalance method allowed for controlling the deposition rate at the level of ~0.1 nm/s. Two series of films were prepared by (1) evaporation of GeO2 and SiO2 powdered sources in accordance with the chemical composition, which we will refer to as GeO[SiO2], and (2) evaporation of GeO2 and SiO powdered sources in accordance with the chemical composition, marked as GeO[SiO] film. Note that, according to previous studies [21,22] on the condition of the evaporation of germanium dioxide powder, GeOx films of a composition very close to germanium monoxide (x ~ 1 1.1) were deposited on the substrate. ÷ Presumably, this was due to the fact that germanium monoxide is a very volatile compound (i.e., it evaporates when an electron beam heats the targeted germanium dioxide powder and condenses on a cold substrate). The accompanying evaporating oxygen did not interact actively with the deposited film and practically did not oxidize germanium monoxide. The silicon substrates were cleaned in deionized water and dried. Thus, the growth of films was carried out on the native silicon oxide layer with a thickness of 2–3 nm. In addition, thick (~400 nm) reference samples were deposited on n-type Electronics 2020, 9, 2103 3 of 17

(10 Ohm cm) Si (100) silicon and quartz substrates in the same deposition regimes for specific structural · and optical studies. The X-ray photoelectron spectra (XPS) were measured using the SPECS photoelectron spectrometer (Germany) with a PHOIBOS-150-MCD-9 hemispherical analyzer and a FOCUS-500 X-ray monochromator (AlKα line, hν = 1486.74 eV, 200 W). The binding energy scale was previously calibratedElectronics by the 2020 position, 9, x FOR PEER of the REVIEW peaks of the core levels of the Au4f7/2 (84.00 eV) and the Cu2p3 of 17 3/2 (932.67 eV). The sample was mounted on double-sided copper conductive tape. For calibration, the C1s line (bindingThe energy X-ray is 284.8photoelectron eV) from spectra carbon, (XPS) contained were inmeasured hydrocarbons using presentingthe SPECS onphotoelectron the surface of spectrometer (Germany) with a PHOIBOS-150-MCD-9 hemispherical analyzer and a FOCUS-500 X- the samples, was used [23]. ray monochromator (AlKα line, hν = 1486.74 eV, 200 W). The binding energy scale was previously The JEM-2200FS transmission electron microscope (JEOL, Akishima, Japan) was used to investigate calibrated by the position of the peaks of the core levels of the Au4f7/2 (84.00 eV) and the Cu2p3/2 the structural(932.67 properties eV). The sample of the was samples. mounted The on double-sid high-resolution,ed copper cross-sectional conductive tape transmission. For calibration, electron the microscopyC1s (HRTEM)line (binding mode energy was is 284.8 used, eV) while from the carbon, acceleration contained voltage in hydrocarbons was 200 presenting kV. The Leicaon the EM TXP targetsurface preparation of the samples, device was (Leica used Microsystems,[23]. Mannheim, Germany) was used to prepare the cross-sectionalThe samples JEM-2200FS by two-step transmission mechanical electron polishing, microscope followed (JEOL, byAkishima, ion grinding. Japan) was used to It isinvestigate known that the analysis structural of properties vibrational of the spectroscopy samples. The datahigh-resolution, provides informationcross-sectional about transmission chemical bonds. Theelectron presence microscopy of nonpolar (HRTEM) Ge–Ge modebonds was used, in thewhile films the acceleration was verified voltage from was analysis 200 kV. of The the Leica Raman EM TXP target preparation device (Leica Microsystems, Mannheim, Germany) was used to prepare spectroscopy data. Raman spectra were recorded at room temperature in the backscattering geometry the cross-sectional samples by two-step mechanical polishing, followed by ion grinding. + by using an ArIt is laserknown for that excitation, analysis of whose vibrational wavelength spectroscopy was 514.5 data provides nm, and information the polarization about chemical of the laser irradiationbonds. was The linear. presence A T64000 of nonpolar (Horiba Ge–Ge Jobin bonds Yvon) in Ramanthe films spectrometer was verified from was analysis used. The of the polarization Raman of the scatteredspectroscopy light data. was notRaman analyzed. spectra were A liquid recorded nitrogen-cooled, at room temperature silicon-based in the backscattering charge-coupled geometry device (CCD) matrixby using was an usedAr+ laser as thefor excitation, spectrometer’s whose photodetector,wavelength was 514.5 allowing nm, and a spectral the polarization resolution of the no laser worse irradiation1 was linear. A T64000 (Horiba Jobin Yvon) Raman spectrometer was used. The polarization than 2 cm− . The BX41 optical microscope accessory (Olympus, Tokyo, Japan) was used for the micro-scaleof the recording scatteredof light the was Raman not analyzed. spectral A data. liquid To ni avoidtrogen-cooled, local heating silicon-based of the filmscharge-coupled by the laser device beam, (CCD) matrix was used as the spectrometer’s photodetector, allowing a spectral resolution no worse the sample was placed slightly further from the focus. The laser beam power on the sample was than 2 cm−1. The BX41 optical microscope accessory (Olympus, Tokyo, Japan) was used for the micro- ~1 mW, while the spot diameter was ~20 µm. The polar Si–O and Ge–O bonds were studied using scale recording of the Raman spectral data. To avoid local heating of the films by the laser beam, the Fourier-transformsample was infrared placed slightly (FTIR) further absorption from the spectroscopy. focus. The laser An beam FT-801 power spectrometer on the sample (SIMEX was ~1 analytical mW, 1 equipment,while Novosibirsk, the spot diameter Russia) was with~20 µm. a spectral The polar resolution Si–O and ofGe–O 4 cm bonds− was were used. studied As using was mentioned Fourier- earlier, fortransform the FTIR infrared analysis, (FTIR) the absorption thicker (400 spectroscopy. nm) reference An FT-801 films spectrometer were deposited (SIMEX onto analytical an n-type (resistivityequipment, 10 Ohm Novosibirsk,cm) Si (001) Russia) substrate with in a the spectral same resolution growth condition. of 4 cm−1 was used. As was mentioned · Theearlier, optical for properties the FTIR ofanalysis, the non-stoichiometric the thicker (400 nm) germanosilicate reference films were films deposited in the visible onto an region n-type near the IR and(resistivity ultraviolet 10 Ohm (UV)·cm) regions Si (001) were substrate studied in the using same spectralgrowth condition. ellipsometry. An ELLIPS-1891-SAG The optical properties of the non-stoichiometric germanosilicate films in the visible region near (Institute of Semiconductor Physics, Novosibirsk, Russia) spectral ellipsometer was used for the the IR and ultraviolet (UV) regions were studied using spectral ellipsometry. An ELLIPS-1891-SAG ellipsometric(Institute analysis of Semiconductor of the thin Physics, films onNovosibirsk, nontransparent Russia) spectral silicon substratesellipsometer [was24]. used The for spectral the dependenciesellipsometric of the ellipsometricanalysis of the angles thin filmsΨ(λ )on and nont∆(λransparent) were measured silicon substrates in the range [24]. of The 250–1100 spectral nm. The spectraldependencies resolution of the of ellipsometric the device angles was about Ψ(λ) and 2 nm, Δ(λ) andwere themeasured angle in of the light range incidence of 250–1100 was nm. 70 ◦. The four-zoneThe spectral measurement resolution techniqueof the device was was used, about and 2 nm, averaging and the angle over of all light four incidence zones was was performed. 70°. The The SF-56four-zone spectrophotometer measurement (LOMO-Spectr, technique was used, Saint and Petersburg, averaging Russia)over all wasfour appliedzones was for performed. the study The of the transmissionSF-56 and spectrophotometer reflection of light. (LOMO-Spectr, The spectral Saint resolution Petersburg, was Russia) better was than applied 6 nm for the study measurement of the range oftransmission 190–1100 nm. and To reflection record the of light. reflection The spectral spectra, resolution a PS-9 modulewas better (LOMO-Spectr, than 6 nm for the Saint measurement Petersburg, range of 190–1100 nm. To record the reflection spectra, a PS-9 module (LOMO-Spectr, Saint Russia) was utilized. The angle of incidence was 9 with respect to the normal. The recorded spectra Petersburg, Russia) was utilized. The angle of◦ incidence was 9° with respect to the normal. The were normalizedrecorded tospectra thereference were normalized spectrum to obtainedthe referenc fore spectrum Si, with aobtained natural for oxide Si, with thickness a natural of 3 oxide nm like in previousthickness works of [ 253 nm,26 ].like in previous works [25,26]. Square indiumSquare indium tin oxide tin oxide (ITO) (ITO) contacts contacts (0.7 (0.7 mm mm 0.7× 0.7 mm) mm) withwith a thickness of of 200 200 nm nm were were × depositeddeposited using magnetron using magnetron sputtering. sputtering. The The layer layer resistance resistance was was 40 40 OhmOhm// ϒ , and , ϒ and the the contact contact area area was 0.5 mm2.was The 0.5 metal–insulator–semiconductor mm2. The metal–insulator–semiconductor (MIS) samples(MIS) samples were were divided divided into into two two series: series: the the first portion offirst structures portion of was structures left without was left annealing without annea (as-depositedling (as-deposited MIS), while MIS), the while second the second series series was furtherwas further annealed at 500 °C for 20 min. The current–voltage (I–V) characteristics of the MIS structures annealed at 500 C for 20 min. The current–voltage (I–V) characteristics of the MIS structures were were measured◦ using a B2902A parameter analyzer (Agilent, Santa Clara, USA) with a micro-probe measuredstation. using The a B2902A I–V measurements parameter and analyzer positive (Agilent, and negative Santa voltages Clara, CA,were USA) applied with to athe micro-probe top ITO station.electrodes, The I–V measurementswhile the back Si and substrate positive electrod andes negative had always voltages been grounded. were applied The equipment to the top was ITO electrodes,automated, while the which back made Si substrate it possible electrodes to measure hadthe currents always in been ON and grounded. OFF states The after equipment the ON and was OFF pulses, respectively, in order to investigate the endurance of the memristors.

Electronics 2020, 9, 2103 4 of 17 automated, which made it possible to measure the currents in ON and OFF states after the ON and OFF pulses, respectively, in order to investigate the endurance of the memristors.

3. Results and Discussion

3.1. Structural Studies

Figure1 displays the XPS of the as-deposited thin GeO[SiO 2] and GeO[SiO] films. In addition to the survey XPS (Figure1a), narrow spectral regions of the valence band (Figure1b), Ge3d (Figure1c), Si2p (Figure1d), and O1s (Figure1e) were measured. Survey spectra and individual spectral regions were obtained using the fixed analyzer pass energy of 50 eV and 20 eV, respectively. The survey spectra (Figure1a) exhibited intense photoelectron peaks characteristic of germanium, silicon, oxygen, and carbon. No additional peaks belonging to other elements were found. The relative content of elements near the surface of the samples and the ratio of atomic concentrations were determined using the integrated intensities of photoelectron lines corrected for the corresponding atomic sensitivity factors, based on the Scofield’s calculations of the cross-sections [27]: 1.42 for Ge3d, 0.817 for Si2p, 1.0 for C1s, and 2.93 for O1s. As such, for the as-deposited thin GeO[SiO2] sample, the O/Si ratio was 4.89, and the Ge/Si ratio was 1.43. For the GeO[SiO] sample, the O/Si ratio was 3.17, and the Ge/Si ratio was 1.04. The near-surface composition of the samples is presented in Table1. It should be noted that the oxygen content in the near-surface region of the samples was found to be larger than assumed, according to the growth conditions (60% for GeO[SiO2] and 50% for GeO[SiO]). This could have resulted from the oxidation of the near-surface region of the samples during their storage in the air atmosphere.

Table 1. The near-surface composition of the studied samples in atomic percents.

Sample Ge, at.% Si, at.% O, at.%

GeO[SiO2] 19.5 13.7 66.8 GeO[SiO] 20.0 19.2 60.8

Figure1c shows the spectra of the Ge3d region of the as-deposited samples. The binding energy of the most intense peak in both spectra was 32.8 0.1 eV. This value is characteristic of germanium ± bound to four oxygen atoms, for example, in GeO [23,28]. The low-intensity peak at ~31.2 0.2 eV can 2 ± be attributed to germanium in a non-stoichiometric oxide, GeOx [28]. In addition to the two indicated peaks, the shoulder with a lower binding energy at ~29.5 0.1 eV was observed in the spectrum of the ± GeO[SiO] sample, which can be attributed to the germanium clusters (Ge–Ge bonds) [29]. Figure1c also shows the result of the Ge3d spectra’s deconvolution into separate spectral components, carried out using XPS Peak 4.1 software [30], including the relative contents of Ge–O (32.8 0.1 eV), Ge–Ge 4 ± 4 (29.5 0.1 eV), and Ge–OxGe(4 x) (x = 1, 2, 3, 31.2 0.2 eV) tetrahedrons. The Ge–OxGe(4 x) (x = 1, ± − ± − 2, 3) tetrahedrons actually should have appeared as three separate peaks, but they were not resolved in the spectra. Figure1d,e shows the spectra of the Si2p and O1s regions of the GeO[SiO 2] and GeO[SiO] samples, respectively. It was suggested that they were a superposition of the SiO + SiO2 peaks in the Si2p spectra, and oxygen peaks arose from the germanium and silicon oxides in the O1s spectra. Detailed analysis of Figure1a–e revealed that there were no Si–Si bonds (at least not in detectable quantities) in both samples, while Ge–Ge bonds were detected in the GeO[SiO] sample. This conclusion was conﬁrmed by the analysis of the valence band spectra (Figure1b), where the tails in the density of states propagating almost to zero binding energy could be observed, which was due to the GeOx inclusions in the ﬁlms. The peak ~3 eV in the spectrum of GeO[SiO] was also due to Ge–Ge bonds. The cross-sectional HRTEM images of the as-deposited samples are shown in Figure2 (2a for GeO[SiO2]) and 2b for (GeO[SiO]). Darker spots correspond to the Ge reach areas. It can be seen that only a few dark spots existed in the GeO[SiO2] sample. They presumably correspond to the GeOx areas. Electronics 2020, 9, x FOR PEER REVIEW 4 of 17

3. Results and Discussion

3.1. Structural Studies

Figure 1 displays the XPS of the as-deposited thin GeO[SiO2] and GeO[SiO] films. In addition to the survey XPS (Figure 1a), narrow spectral regions of the valence band (Figure 1b), Ge3d (Figure 1c), Si2p (Figure 1d), and O1s (Figure 1e) were measured. Survey spectra and individual spectral regions were obtained using the fixed analyzer pass energy of 50 eV and 20 eV, respectively. The survey spectra (Figure 1a) exhibited intense photoelectron peaks characteristic of germanium, silicon, oxygen, and carbon. No additional peaks belonging to other elements were found. The relative content of elements near the surface of the samples and the ratio of atomic concentrations were determined using the integrated intensities of photoelectron lines corrected for the corresponding atomic sensitivity factors, based on the Scofield’s calculations of the cross-sections [27]: 1.42 for Ge3d,

Electronics0.817 for2020 Si2p,, 9, 21031.0 for C1s, and 2.93 for O1s. As such, for the as-deposited thin GeO[SiO2] sample5 of, the 17 O/Si ratio was 4.89, and the Ge/Si ratio was 1.43. For the GeO[SiO] sample, the O/Si ratio was 3.17, and the Ge/Si ratio was 1.04. The near-surface composition of the samples is presented in Table 1. It Remarkably,should be noted many that dark the spots oxygen can becontent seen in in the the GeO[SiO] near-surface sample. region They of presumablythe samples correspond was found to to the be amorphouslarger than assumed Ge clusters, according [31] (a-Ge), to the with growth an average conditions size of (60% about for 3 GeO[SiO nm. The2 surfaces] and 50% of for both GeO[SiO]). samples hadThis very could low have values result of surfaceed from roughness the oxidation in the of scale the ofnear 1 nm,-surf whichace region was controlled of the samples by an atomicduring forcetheir microscopystorage in the study air atmosphere. of their surface topographies (See Figures S1 and S2 in Supplementary Materials).

Electronics 2020, 9, x FOR PEER REVIEW 5 of 17

Figure 1. X-ray photoelectron spectra (XPS) data of the as-deposited thin GeO[SiO2] and GeO[SiO] Figure 1. X-ray photoelectron spectra (XPS) data of the as-deposited thin GeO[SiO2] and GeO[SiO] ﬁlms. (a) Survey spectra; (b) valence band region; (c) Ge3d region; (d) Si2p region; and (e) O1s region. films. (a) Survey spectra; (b) valence band region; (c) Ge3d region; (d) Si2p region; and (e) O1s region.

Figure3a displays the Raman spectra of both the as-deposited and annealed GeO[SiO 2] and GeO[SiO] thinTable films, 1. The together near-surface with a composition reference spectrum of the studied measured samples from in atomic a virgin percent Sis (001). substrate. 1 For the as-deposited films as wellSample as for the substrate,Ge, at.% oneSi, couldat.% identifyO, at.% a peak at ~305 cm− , which was due to two-phonon scatteringGeO[SiO on transversal2] acoustic19.5 phonons13.7 (2TA)66.8 [32 ]. This was because these thin 1 films were semi-transparent. Thus,GeO[SiO] both the one-phonon20.0 19.2 scattering 60.8 peak (520 cm− ) and two-phonon 1 scattering peculiarity (2TA, ~305 cm− ) could be observed for the Si substrate. It should be noted 1 that someFigure asymmetry 1c shows the of thespectra 520 cmof the− peak Ge3d was region due of to the Fano as- interferencedeposited samples. (interference The binding of phonon energy and oflight the hole-heavymost intense hole peak scattering) in both spectra in the wa p+-types 32.8 heavily± 0.1 eV. doped This value silicon is [characteristic33]. In the spectrum of germanium of the bound to four oxygen atoms, for example, in GeO2 [23,28]. The low-intensity peak at ~ 31.2 ± 0.2 eV can be attributed to germanium in a non-stoichiometric oxide, GeOx [28]. In addition to the two indicated peaks, the shoulder with a lower binding energy at ~29.5 ± 0.1 eV was observed in the spectrum of the GeO[SiO] sample, which can be attributed to the germanium clusters (Ge–Ge bonds) [29]. Figure 1c also shows the result of the Ge3d spectra’s deconvolution into separate spectral components, carried out using XPS Peak 4.1 software [30], including the relative contents of Ge–O4 (32.8 ± 0.1 eV), Ge–Ge4 (29.5 ± 0.1 eV), and Ge–OxGe(4−x) (x = 1, 2, 3, 31.2 ± 0.2 eV) tetrahedrons. The Ge–OxGe(4−x) (x = 1, 2, 3) tetrahedrons actually should have appeared as three separate peaks, but they were not resolved in the spectra. Figure 1d,e shows the spectra of the Si2p and O1s regions of the GeO[SiO2] and GeO[SiO] samples, respectively. It was suggested that they were a superposition of the SiO + SiO2 peaks in the Si2p spectra, and oxygen peaks arose from the germanium and silicon oxides in the O1s spectra. Detailed analysis of Figure 1a–e revealed that there were no Si–Si bonds (at least not in detectable quantities) in both samples, while Ge–Ge bonds were detected in the GeO[SiO] sample. This conclusion was confirmed by the analysis of the valence band spectra (Figure 1b), where the tails in the density of states propagating almost to zero binding energy could be observed, which was due to the GeOx inclusions in the films. The peak ~3 eV in the spectrum of GeO[SiO] was also due to Ge– Ge bonds. The cross-sectional HRTEM images of the as-deposited samples are shown in Figure 2 (2a for GeO[SiO2]) and 2b for (GeO[SiO]). Darker spots correspond to the Ge reach areas. It can be seen that only a few dark spots existed in the GeO[SiO2] sample. They presumably correspond to the GeOx areas. Remarkably, many dark spots can be seen in the GeO[SiO] sample. They presumably correspond to the amorphous Ge clusters [31] (a-Ge), with an average size of about 3 nm. The surfaces of both samples had very low values of surface roughness in the scale of 1 nm, which was controlled by an atomic force microscopy study of their surface topographies (See Figures S1 and S2 in Supplementary Materials).

Electronics 2020, 9, x FOR PEER REVIEW 6 of 17

Electronics 2020, 9, 2103 6 of 17

1 as-deposited GeO[SiO] film, one could observe a broad peak centered at about 275 cm− , which could be attributed to amorphous Ge [34]. One could see the growth of the intensity of this peak in the GeO[SiO] film and the appearance of this peak in the GeO[SiO2] film after annealing. Therefore, annealing led to an increase of the volume of amorphous Ge nanoclusters in the GeO[SiO] film and to the formation of amorphous Ge nanoclusters in the GeO[SiO ] film. Electronics 2020, 9, x FOR PEER REVIEW 2 6 of 17

Figure 2. High-resolution transmission electron microscopy (HRTEM) cross-sectional image of the as- deposited thin films, (a) GeO[SiO2] and (b) GeO[SiO].

Figure 3a displays the Raman spectra of both the as-deposited and annealed GeO[SiO2] and GeO[SiO] thin films, together with a reference spectrum measured from a virgin Si (001) substrate. For the as-deposited films as well as for the substrate, one could identify a peak at ~305 cm−1, which was due to two-phonon scattering on transversal acoustic phonons (2TA) [32]. This was because these thin films were semi-transparent. Thus, both the one-phonon scattering peak (520 cm−1) and two- phonon scattering peculiarity (2TA, ~305 cm−1) could be observed for the Si substrate. It should be noted that some asymmetry of the 520 cm−1 peak was due to Fano interference (interference of phonon and light hole-heavy hole scattering) in the p+-type heavily doped silicon [33]. In the spectrum of the as-deposited GeO[SiO] film, one could observe a broad peak centered at about 275 cm−1, which could be attributed to amorphous Ge [34]. One could see the growth of the intensity of this peak in the GeO[SiO] film and the appearance of this peak in the GeO[SiO2] film after annealing. Therefore, annealing led to an increase of the volume of amorphous Ge nanoclusters in the GeO[SiO] film and to the formation of amorphous Ge nanoclusters in the GeO[SiO2] film. Figure 3. (a) Raman spectra of the GeO[SiO2] and GeO[SiO] thin films, measured before and after Figure 3. (a) Raman spectra of the GeO[SiO2] and GeO[SiO] thin films, measured before and after annealing at 500 C for 20 min. A Raman spectrum from a virgin Si (001) substrate is also shown. annealing at 500 °C◦ for 20 min. A Raman spectrum from a virgin Si (001) substrate is also shown. (b) (b) Fourier-transform infrared spectroscopy (FTIR) absorption spectra of the thick GeO[SiO2] film Fourier-transform infrared spectroscopy (FTIR) absorption spectra of the thick GeO[SiO2] film (reference sample grown on a substrate not heavily doped with Si), measured before and after annealing. (reference sample grown on a substrate not heavily doped with Si), measured before and after (c) FTIR absorption spectra of the thick GeO[SiO] film (reference sample grown on a substrate not annealing. (c) FTIR absorption spectra of the thick GeO[SiO] film (reference sample grown on a heavily doped with Si), measured before and after annealing. substrate not heavily doped with Si), measured before and after annealing. As it was assumed earlier [18,31,35], the a-Ge clusters appeared due to the following solid-state chemical reaction:

GeO + SiO Ge + SiO , (1) → 2 After annealing, the intensity of the a-Ge related peak increased in the GeO[SiO] ﬁlm, which suggests that not all germanium monoxide had deoxidized during deposition, but part of it remained in the form of GeOx suboxides. It is known that GeOx is unstable and decomposes into germanium and germanium dioxide under annealing with temperatures higher than 300 ◦C[21,36 ].

Figure 3. (a) Raman spectra of the GeO[SiO2] and GeO[SiO] thin films, measured before and after annealing at 500 °C for 20 min. A Raman spectrum from a virgin Si (001) substrate is also shown. (b)

Fourier-transform infrared spectroscopy (FTIR) absorption spectra of the thick GeO[SiO2] film (reference sample grown on a substrate not heavily doped with Si), measured before and after annealing. (c) FTIR absorption spectra of the thick GeO[SiO] film (reference sample grown on a substrate not heavily doped with Si), measured before and after annealing.

Electronics 2020, 9, 2103 7 of 17

Therefore, both annealed films consisted of a germanosilicate suboxide, which contained amorphous germanium (a-Ge) nanoclusters and possibly also a high concentration of oxygen vacancies. It is known that the analysis of Si–O and Ge–O polar bond vibrations can provide information about the stoichiometry of SiOx and GeOx suboxides. Figure3b,c shows the FTIR absorption spectra of the as-deposited and annealed thick GeO[SiO2] and GeO[SiO] films (these samples were grown on a substrate not heavily doped with Si). The virgin n-type Si substrate with an electrical resistivity of 10 Ohm cm was used as a reference when measuring the FTIR absorbance spectra. The spectra · 1 were dominated by a major line of approximately 1035–1075 cm− , that was attributed to the Si–O–Si stretch mode in the SiOx films [37,38]. Pai et al. [38] established that the position of this peak (in inverse centimeters) in SiOx films almost linearly depends on the stoichiometric parameter x, approximately expressed as ν(Si-O-Si stretching mode) = 925 + 75x (2)

The position of the Si–O–Si stretching mode peak in the as-deposited GeO[SiO] film (red curve, 1 Figure3c) was 1037 cm − . Therefore, according to Equation (2), the stoichiometric parameter x in the as-deposited GeO[SiO] film was equal to 1.5. The position of the Si–O–Si stretching mode peak in the 1 annealed GeO[SiO] film (blue curve, Figure3c) was 1063 cm − , so the stoichiometric parameter x in this case was equal to 1.85. Earlier it was supposed [39] that during deposition, not all germanium monoxide participates in a chemical redox such as Reaction (1), and by using the stoichiometric parameter x in the SiOx part of germanosilicate solid alloys, one can determine the share of Ge clusters:

GeO + SiO z Ge + (1 z) GeO + SiO(1 + z) (3) → · − · Since the parameter z was equal to 0.5 in the as-deposited GeO[SiO] film, then about half of the germanium monoxide had been deoxidized in the chemical redox, shown in Reaction (1), during its deposition. In the annealed GeO[SiO] film, the parameter z was equal to 0.85, so about 85% of the germanium monoxide had been deoxidized in the chemical redox, show in Reaction (3), under deposition and subsequent annealing. As for the as-deposited and annealed GeO[SiO2] films (black and green curve, Figure3b), the positions of the Si–O–Si stretching mode peak were 1070 and 1 1075 cm− , respectively. This suggests that the SiOx inclusions in the germanosilicate solid alloy were close to stoichiometric silicon dioxide. –1 Other peaks visible in the GeO [SiO2] films with a position of ~990 cm (as-deposited film, black curve, Figure3b) and ~1000 cm –1 (annealed film, green curve, Figure3b) were due to the absorption of Ge–O–Si vibrations in the germanosilicate glass [31,39,40]. In addition to the Si–O and Si–O–Ge peaks, peaks associated with the stretching vibrations of the Ge–O bonds in all films [41,42] were seen. Like non-stoichiometric silicon oxide, the position of this peak (in inverse centimeters) in the GeOx films almost linearly depended on the stoichiometric parameter x [42], as

ν(Ge-O stretching mode) = 743 + 72.4x (4)

From the analysis of the Ge–O stretching mode position according to Equation (4), we can conclude that the stoichiometric parameter x in the GeOx inclusions in the germanosilicate solid alloy was higher than 1.5. An important point is that, for both as-deposited films, one can observe a peak at about 3300 cm−1 due to the presence of O–H bonds in those films. The water could be either adsorbed on the surface of the films or contained in the voids of the as-deposited films. In the annealed films, the peaks related to water were much less intense. Thus, the annealing dried the films and presumably removed the voids by a densification process. Electronics 2020, 9, 2103 8 of 17 It is known that the density of electronic states determines the optical properties of materials; in particular, the mobility gap in dielectrics correlates with the optical gap in them [43]. Therefore, it is are presentedimportant into study Figure the4. spectral Figure 4dependencea,b shows of the the spectral optical constants dependencies of dielectrics. of the For complex this, we refractive used index,the with methods its real of partspectral n and ellipsometry the extinction and coe transmissionfficient (its and imaginary reflection part spectroscopy, k) for the theas-deposited results of thin which are presented in Figure 4. Figure 4a,b shows the spectral dependencies of the complex GeO[SiO ] and GeO[SiO] films, respectively. These data were obtained using ellipsometry angles Ψ(λ) refractive2 index, with its real part n and the extinction coefficient (its imaginary part k) for the as- and ∆(λ) followed by determining the thickness and optical constants of the films from the model, deposited thin GeO[SiO2] and GeO[SiO] films, respectively. These data were obtained using consideringellipsometry one angles film on Ψ(λ) the and substrate. Δ(λ) followed The by thickness determining of the theGeO[SiO thickness 2and] film optical was constants determined of the to be 85 nm,films and from the the thickness model, considering of the GeO[SiO] one film films on the was substrate. 64 nm. The One thickness can see of thatthe GeO[SiO almost2 no] film absorption was was registereddetermined in to the be 85 GeO[SiO nm, and2 ]the film, thickness which of was the relatedGeO[SiO] to films the factwas that64 nm. the One sensitivity can see that of thealmost method was insuno absorptionfficient to was determine registered weak in the absorption. GeO[SiO2] Infilm, the which GeO[SiO] was related film, absorptionto the fact that was the observed sensitivity in the entireof spectral the method range, was with insufficient growth to in determine the short weak wavelength absorption. edge. In the The GeO[SiO] refractive film, index absorption of the GeO[SiO] was observed in the entire spectral range, with growth in the short wavelength edge. The refractive index film was larger than that of the GeO[SiO2] film, due to the presence of a-Ge clusters in the first case. of the GeO[SiO] film was larger than that of the GeO[SiO2] film, due to the presence of a-Ge clusters For estimation of the volume ratio of the Ge clusters in the solid alloy films, an effective medium in the first case. For estimation of the volume ratio of the Ge clusters in the solid alloy films, an approximation was used in the Bruggeman approach [44,45]. According to this approach, the volume effective medium approximation was used in the Bruggeman approach [44,45]. According to this ratio of a-Ge in the as-deposited GeO[SiO ] film was less than 1%, and the volume ratio of a-Ge in approach, the volume ratio of a-Ge in the2 as-deposited GeO[SiO2] film was less than 1%, and the the as-depositedvolume ratio GeO[SiO] of a-Ge in filmthe as was-deposited about 10%. GeO[SiO] For film the annealedwas about films 10%. (ellipsometry For the annealed data films are not shown(ellipsometry here), these data ratios are werenot shown 5% and here) 17.5%, these for rat theios GeO[SiOwere 5% and2] and 17.5% GeO[SiO] for the GeO[SiO films, respectively.2] and 1 TheseGeO[SiO] findings films, correlate respectively. with the These Raman findings spectroscopy correlate with data the (intensity Raman spectroscopy of the broad data 275 (intensity cm− peak) in Figureof the3a. broad 275 cm−1 peak) in Figure 3a.

Electronics 2020, 9, x FOR PEER REVIEW 9 of 17

Figure 4. (a,b) Refractive index (its real part n) and extinction coeﬃcient (imaginary part of refractive Figure 4. (a,b) Refractive index (its real part n) and extinction coefficient (imaginary part of refractive index k) of the as-deposited thick GeO[SiO ] and GeO[SiO] ﬁlms, respectively. (c,d) The sum of the index k) of the as-deposited thick GeO[SiO22] and GeO[SiO] films, respectively. (c,d) The sum of the

transmissiontransmission and and reflection reflection spectra spectra of of the the as-deposited as-deposited and and annealed annealed thick thick GeO[SiO GeO[SiO2] and2] andGeO[SiO] GeO[SiO] filmsfilms on quartz on quartz substrate, substrate, respectively. respectively. SeparateSeparate transmission and and reflectance reflectance spectra spectra are shown are shown in in FigureFigure S3 in S3 Supplementary in Supplementary Materials. Materials.

It should be noted that it was not possible to apply a simple one-film-on-the-substrate model for the analysis of the spectral ellipsometry data of the annealed films. This was because annealing had led to the inhomogeneity of the non-stoichiometric germanosilicate films due to diffusion of the germanium atoms in the Si substrate [46]. The transmission and reflection spectroscopy data (shown in Figure 4c,d) confirmed the spectral ellipsometry results. Recall that the special reference samples on a quartz substrate in the same growth conditions were grown to measure absorbance. To avoid the features associated with interference, the sum of the transmittance and reflectance spectra are compared in Figure 4. One can see almost no absorption in the as-deposited GeO[SiO2] film in the visible and IR ranges (Figure 4c, black curve). The annealing led to the formation of a-Ge clusters in this film and, consequently, to the appearance of absorbance in the visible and IR ranges (Figure 4c, green curve). Since the as-deposited GeO[SiO] film had already contained a-Ge clusters, there was a clear absorbance in this film in the visible and IR ranges (the blue curve in Figure 4b and the red curve in Figure 4d). The annealing led to an increasing amorphous Ge volume in this film and, consequently, increased absorbance in the visible and IR ranges (Figure 4d, blue curve). Therefore, all structural and optical methods confirmed the following statements. The as- deposited GeO[SiO] film contained a-Ge nanoclusters, while no such clusters were found in the as- deposited GeO[SiO2] film. Annealing led to an increase in the volume of a-Ge nanoclusters in the GeO[SiO] film and to the formation of a-Ge nanoclusters in the GeO[SiO2] film. All films contained Si–O and Ge–O bonds.

Electronics 2020, 9, 2103 9 of 17

It should be noted that it was not possible to apply a simple one-film-on-the-substrate model for the analysis of the spectral ellipsometry data of the annealed films. This was because annealing had led to the inhomogeneity of the non-stoichiometric germanosilicate films due to diffusion of the germanium atoms in the Si substrate [46]. The transmission and reflection spectroscopy data (shown in Figure4c,d) confirmed the spectral ellipsometry results. Recall that the special reference samples on a quartz substrate in the same growth conditions were grown to measure absorbance. To avoid the features associated with interference, the sum of the transmittance and reflectance spectra are compared in Figure4. One can see almost no absorption in the as-deposited GeO[SiO2] film in the visible and IR ranges (Figure4c, black curve). The annealing led to the formation of a-Ge clusters in this film and, consequently, to the appearance of absorbance in the visible and IR ranges (Figure4c, green curve). Since the as-deposited GeO[SiO] film had already contained a-Ge clusters, there was a clear absorbance in this film in the visible and IR ranges (the blue curve in Figure4b and the red curve in Figure4d). The annealing led to an increasing amorphous Ge volume in this film and, consequently, increased absorbance in the visible and IR ranges (Figure4d, blue curve). Therefore, all structural and optical methods confirmed the following statements. The as-deposited GeO[SiO] film contained a-Ge nanoclusters, while no such clusters were found in the as-deposited GeO[SiO2] film. Annealing led to an increase in the volume of a-Ge nanoclusters in the GeO[SiO] film and to the formation of a-Ge nanoclusters in the GeO[SiO2] film. All films contained Si–O and Ge–O bonds.

3.2. Studies of Resistivity Switching and Endurance The schematic image of the MIS structures, energy band diagram, and the scheme of the endurance measurements are shown in Figure5a–c, respectively. According to our previous studies [ 47], nanoscale fluctuations of electric potential in sub-oxides can play a considerable role in their electron transport. One of the significant advantages of germanosilicate solid alloys is that they offer the possibility to utilize these nanoscale fluctuations of the potential (i.e., parameters of the energy bands). The bandgap values in SiO2 and GeO2 differ significantly, being ~9 eV and ~5 eV, respectively. This difference allows modulation of the parameters of the traps consisting of germanium oxide inclusions in silicon oxide. Other possibilities to establish the nanoscale fluctuations of electric potential are based on the presence of suboxide areas, such as SiOx and GeOx, or on the formation of local areas with excess germanium atoms (which can possibly act as traps for both electrons and holes), as well as a-Ge nanoclusters. The valence band offset between crystalline Si and Ge is approximately 0.5 eV, − while the conduction band offset is only about 0.1 eV (as shown in Figure5b). Since the band o ffset for Ge is mainly in the valence band, a-Ge nanoclusters act as deep traps for holes rather than for electrons. It is important to note that the quantum size effect leads to a broadening of the bandgap in germanium nanoclusters (Figure5b), so the larger the a-Ge nanocluster, the deeper the trap for electrons and holes. The band offsets for Si/GeO2 heterojunctions are known [48]. Thus, yielding barriers for electrons and holes in such a system are about 1.2 eV and 3.3 eV, respectively. The corresponding barriers for Si/SiO2 heterojunctions are also shown in Figure5b. Unfortunately, the band o ffsets for Si/GeO heterojunctions are unknown. The typical I–V characteristics of MIS structures grown on a p+-type Si substrate are shown in Figure6a–d. Switching from an HRS to an LRS (set) was seen at a positive voltage, and reverse switching from an LRS to an HRS (reset) was seen at a negative voltage. The set and reset voltages varied from +2 to +3.5 V and from 2.5 to 3 V for both as-deposited films (Figure6a,c). It is worth − − noting that the current limitation was set during I–V measurements. Therefore, when the structures were switched to an LRS, the current value jumped, and a sharp decrease in the applied voltage was observed (Figure6b). Insignificant fluctuations of the set and reset voltages of the films could be seen in the studied switching cycles. These results are similar to data obtained earlier using similar as-deposited MIS structures [18]. The ratio of currents in the ON and OFF states (the so-called memory Electronics 2020, 9, x FOR PEER REVIEW 10 of 17 bands). The bandgap values in SiO2 and GeO2 differ significantly, being ~9 eV and ~5 eV, respectively. ElectronicsThis difference2020, 9, 2103 allows modulation of the parameters of the traps consisting of germanium 10 oxide of 17 inclusions in silicon oxide. Other possibilities to establish the nanoscale fluctuations of electric window)potential wasare based rather on low the (about presence two toof three suboxide orders areas of magnitude),, such as SiO butx and it should GeOx, be or noted on the that formation we carried of local areas with excess germanium atoms (which can possibly act as traps for both electrons and out the measurements in an air atmosphere. In some cases, for the SiOx-based memristor (with Si holes), as well as a-Ge nanoclusters. The valence band offset between crystalline Si and Ge is nanocrystals in SiOx film), the films did not show any switching effects when I–V measurements were approximatelycarried out in an −0.5 air eV atmosphere,, while the conduction while the high band ON offset/OFF is ratioonly about was obtained 0.1 eV (as only shown in a in vacuum Figure [5b).15]. SinceIt is remarkable the band offset that nofor need Ge is to mainly carry outin the the valence special band, forming a-Ge procedure nanoclusters existed act foras ourdeep structures. traps for Thisholes is rather a noteworthy than for benefit electrons. of our It structures, is important in contrast to note with that thethe structures quantum reported size effect previously leads to by a broadening of the bandgap in germanium nanoclusters (Figure 5b), so the larger the a-Ge other authors (e.g., the memristors based on SiOx with Si nanocrystal films [15], or in the case of SiOx nanocluster,layers deposited the deeper by magnetron the trap sputtering for electrons [49 ]).and Our holes. MIS The structures band offsets on the for p+ Si/GeO-type Si2 heterojunctions substrates were areforming-free. known [48]. As Thus, one can yielding see from barriers Figure for6b,d, electrons a considerably and holes stable in such memristor a system e ffareect about was observed 1.2 eV and in 3.3both eV, annealed respectively. films. However,The corresponding in the case barriers of the annealed for Si/SiO GeO[SiO]-based2 heterojunctions MIS are structure, also shown a preliminary in Figure 5b.forming Unfortunately, procedure the with band an appliedoffsets for voltage Si/GeO up heterojunctions to 5 V was required. are unknown.

Figure 5. (a) Schematic image of the metal–insulator–semiconductor (MIS) structures. (b) The scheme Figure 5. (a) Schematic image of the metal–insulator–semiconductor (MIS) structures. (b) The scheme of the corresponding energy band diagram. (c) The scheme of the measurements of endurance. of the corresponding energy band diagram. (c) The scheme of the measurements of endurance. The endurance data are shown in Figure7a–d for MISs grown on the p +-type Si substrate. The typical I–V characteristics of MIS structures grown on a p+-type Si substrate are shown in The endurance measurement scheme is shown in Figure5c. The various V set and Vreset pulse values wereFigure applied 6a–d. toSwitching the MIS structuresfrom an HRS (the to values an LRS are shown(set) wa ins Figure seen at7 a–d) a positive with a voltage5 ms pulse, and duration. reverse Vswitchingwas alwaysfrom an LRS1 V. Oneto an can HRS see (reset) some degradationwas seen at a of negative the memory voltage window. The s foret and all MIS reset structures, voltages read − variedmainly from caused +2 to by +3.5 a decrease V and from in the −2.5 ON to current −3 V for (Figure both 7as).-deposited Note that films in almost (Figure all 6a,c). MIS structures,It is worth notingin addition that the to thecurrent HRS limitation and LRS, was the IRSset during was observed. I–V measurements. The IRSs were Therefore, not clearly when distinguishable the structures wereonly inswitched the annealed to an LRS, ITO/ GeO[SiO]the current/p value+-Si (001) jump structureed, and a (Figure sharp 7decreased). The existencein the applied of IRSs voltage can be was of observedsigniﬁcant (Figure importance 6b). Insignificant for the simulation fluctuations of neuromorphic of the set and devices. reset voltages The study of of the the films eﬀect could of the be pulse seen induration the studied and amplitude switching on cycles. the switching These results processes are (studysimilar of to the data so-called obtained preliminary-spike-enhanced earlier using similar as- plasticitydeposited [ 19MIS]) isstructures the outlook [18]. for The future ratio experiments. of currents in the ON and OFF states (the so-called memory window) was rather low (about two to three orders of magnitude), but it should be noted that we carried out the measurements in an air atmosphere. In some cases, for the SiOx-based memristor (with Si nanocrystals in SiOx film), the films did not show any switching effects when I–V measurements

Electronics 2020, 9, x FOR PEER REVIEW 11 of 17 were carried out in an air atmosphere, while the high ON/OFF ratio was obtained only in a vacuum [15]. It is remarkable that no need to carry out the special forming procedure existed for our structures. This is a noteworthy benefit of our structures, in contrast with the structures reported previously by other authors (e.g., the memristors based on SiOx with Si nanocrystal films [15], or in the case of SiOx layers deposited by magnetron sputtering [49]). Our MIS structures on the p+-type Si substrates were forming-free. As one can see from Figure 6b,d, a considerably stable memristor effect was observed in both annealed films. However, in the case of the annealed GeO[SiO]-based MIS structure,Electronics 2020 a preliminary, 9, 2103 forming procedure with an applied voltage up to 5 V was required. 11 of 17

+ Figure 6. Current–voltage (I–V) characteristics of the MIS structures. (a) As-deposited ITO/GeO[SiO2]/p Figure 6. Current–voltage (I–V) characteristics of + the MIS structures. (a) As-deposited -Si (001) heterojunction; (b) Annealed ITO/GeO[SiO2]/p -Si (001) heterojunction; (c) As-deposited ITO/GeO[SiO2]/p+-Si (001) heterojunction; (b) Annealed ITO/GeO[SiO2]/p+-Si (001) heterojunction; (c) ITO/GeO[SiO]/p+-Si (001) heterojunction; and (d) annealed ITO/GeO[SiO]/p+-Si (001) heterojunction. As-deposited ITO/GeO[SiO]/p+-Si (001) heterojunction; and (d) annealed ITO/GeO[SiO]/p+-Si (001) heAttemptsterojunction. were made to estimate the energy needed to switch on and off the MIS structures and to normalize it to the contact area. It took a 5 ms long pulse of 4–6 V of voltage, with a maximal current The endurance data are shown in Figure 7a–d for MISs grown on the p+-type Si substrate. The up to 4 mA. The contact area was 0.5 mm2. It can be easily calculated that the energy density value was endurance measurement scheme is shown in Figure 5c. The various Vset and Vreset pulse values were about 200 J/m2. If we assume that, in the future ReRAM matrix, the area of the element is 25 25 nm, applied to the MIS structures (the values are shown in Figure 7a–d) with a 5 ms pulse duration.× Vread then the switching energy will be as small as ~100 fJ. It should be noted that these estimates were was always −1 V. One can see some degradation of the memory window for all MIS structures, mainly made without considering the influence of possible edge effects. However, it should be noted that the caused by a decrease in the ON current (Figure 7). Note that in almost all MIS structures, in addition present structure was not optimized and the thickness of the dielectric was too large, while in a real to the HRS and LRS, the IRS was observed. The IRSs were not clearly distinguishable only in the application it should be to the order of 10 nm. Moreover, the switching pulse duration should be a few annealed ITO/GeO[SiO]/p+-Si (001) structure (Figure 7d). The existence of IRSs can be of significant orders of magnitude smaller. Then, we can hope that the energy required for switching of one element importance for the simulation of neuromorphic devices. The study of the effect of the pulse duration (bit) will be less than 1 fJ, which is satisfactory for a memory element. and amplitude on the switching processes (study of the so-called preliminary-spike-enhanced The contact-limited and volume-limited models were used in order to study the charge transport plasticity [19]) is the outlook for future experiments. mechanism of the as-deposited films in an HRS and an LRS like in [18]. It should be remembered that local variations in the chemical composition of the layers of solid germanosilicate alloys led to local variations in the bandgap value (Figure5b). Moreover, the local bandgap fluctuations led to the local electron potential fluctuations. The charge transport in a non-periodic fluctuating potential can be described according to the Shklovskii–Efros (S-E) percolation model [50]. This model assumes that excited electrons—that is, with an energy higher than the percolation energy level W—are delocalized and transfer the charges. In accordance with Equation (5), the j and F characteristics are exponentially dependent:  1   W CeFaVγ 1+γ   0  j = j exp −  (5) 0  kT  −  Electronics 2020, 9, 2103 12 of 17

where j is the current density, F is the electric field intensity, j0 is the pre-exponential factor, W is the percolation energy, a is the spatial scale of fluctuations, V0 is the amplitude of the energy fluctuation, C 0, 25 is a numerical constant, and γ = 0.9 is a critical index. The percolation energy W and the ≈ pre-exponential factor j0 determine the homogeneous vertical displacement of j (in a logarithmic scale, as in Figure8). The spatial scale of the fluctuations and the amplitude of the fluctuation of energy V0 determined the slope of the experimental data curve seen in Figure8. As mentioned above,

Electronicsthe simulation 2020, 9, x was FOR carriedPEER REVIEW out only in the enrichment mode for an Si substrate in order to disregard12 of 17 the contribution of the space charge region in the Si substrate.

Figure 7. a + The endurance characteristics of the MIS structures. ( ) As-deposited ITO/GeO[SiO2]/p + -Si Figure 7. The endurance characteristics of the MIS structures.+ (a) As-deposited ITO/GeO[SiO2]/p -Si (001) heterojunction; (b) Annealed ITO/GeO[SiO2]/p -Si (001) heterojunction; (c) As-deposited (001) heterojunction; (b) Annealed ITO/GeO[SiO2]/p+-Si (001) heterojunction; (c) As-deposited ITO/GeO[SiO]/p+-Si (001) heterojunction; and (d) annealed ITO/GeO[SiO]/p+-Si (001) heterojunction. ITO/GeO[SiO]/p+-Si (001) heterojunction; and (d) annealed ITO/GeO[SiO]/p+-Si (001) heterojunction.

The comparison of the experimental results with the S–E percolation model (for the GeO[SiO2] film onAttempts the n+ andwere p made+-type to substrates) estimate the was energy made earlierneeded [ 18to]. switch The charge on and transport off the MIS in the structures as-deposited and toGeO[SiO] normalize film it onto the ncontact+-type area. substrate It too wask a well5 ms describedlong pulse by of the 4– S–E6 V model,of voltage as, seen with in a Figuremaximal8a, 2 currentalthough up slight to 4 mA. deviations The contact by the area experimental was 0.5 mm data. It in can the GeO[SiO]be easily calculated film on the that n+-type the energy substrate density from 2 valuethe S–E was percolation about 200 model J/m . If curve we assume can be seenthat, forin the low future electric ReRAM fields. matrix, The fitting the parametersarea of the element of the S–E is 25 × 25 nm, then the switching energy will be as 21 small as2 ~100 fJ. It should be noted that these model for GeO[SiO] were the following: j0 = 5.1 10 A/cm , W = 1.5 eV, V0 = 1.8 eV, and a = 2.8 nm. estimates were made without considering the influence× of possible edge effects. However, it should From the change in the V0 a value, it became possible to suggest that the composition of the GeO[SiO] be noted that the present structure× was not optimized and the thickness of the dielectric was too large, film differed from the GeO[SiO2] film [18]. A considerably fine approximation of the experimental whiledata forin a the real GeO[SiO] application film it should on the be p+ -typeto the Siorder substrate of 10 nm. by Moreover, the S–E percolation the switching model pulse can duration be seen shouldin Figure be8 b. a fewThe fittingorders parameters of magnitude of the smaller. S–E model Then,for we GeO[SiO] can hope in that an HRS the energy were the required following: for switching of21 one element2 (bit) will be less than 1 fJ, which is satisfactory for a memory element. j0 = 7.4 10 A/cm , WHRS = 1.64 eV, V0 = 2.8 eV, and a = 5 nm. Meanwhile, the fitting parameters for The× contact-limited and volume-limited models21 were2 used in order to study the charge transport GeO[SiO] in an LRS were as follows: j0 = 7.4 10 A/cm , WLRS = 1.53 eV, V0 = 2 eV, and a = 3.5 nm. mechanism of the as-deposited films in an HRS× and an LRS like in [18]. It should+ be remembered that The same V0 a value in an LRS for the GeO[SiO] and GeO[SiO2] films on a p -type substrate indicate local variations× in the chemical composition of the layers of solid germanosilicate alloys led to local that the filaments had similar compositions [18]. In addition, the different V0 a values in an HRS for variations in the bandgap value (Figure 5b). Moreover, the local bandgap fluctuations× led to the local GeO[SiO] and GeO[SiO2] correspond to the difference in their compositions [18]. electron potential fluctuations. The charge transport in a non-periodic fluctuating potential can be described according to the Shklovskii–Efros (S-E) percolation model [50]. This model assumes that excited electrons—that is, with an energy higher than the percolation energy level W—are delocalized and transfer the charges. In accordance with Equation (5), the j and F characteristics are exponentially dependent:

1  1 W () CeFaV0 jj0 exp (5) kT 

Electronics 2020, 9, x FOR PEER REVIEW 13 of 17

where j is the current density, F is the electric field intensity, j0 is the pre-exponential factor, W is the percolation energy, a is the spatial scale of fluctuations, V0 is the amplitude of the energy fluctuation, C ≈ 0, 25 is a numerical constant, and γ = 0.9 is a critical index. The percolation energy W and the pre- exponential factor j0 determine the homogeneous vertical displacement of j (in a logarithmic scale, as in Figure 8). The spatial scale of the fluctuations and the amplitude of the fluctuation of energy V0 determined the slope of the experimental data curve seen in Figure 8. As mentioned above, the Electronicssimulation2020 was, 9, 2103 carried out only in the enrichment mode for an Si substrate in order to disregard13 of the 17 contribution of the space charge region in the Si substrate.

FigureFigure 8.8. ExperimentalExperimental I–VI–V characteristicscharacteristics ofof thethe highhigh resistanceresistance statestate (HRS)(HRS) andand lowlow resistanceresistance statestate (LRS)(LRS) inin as-depositedas-deposited GeO[SiO]GeO[SiO] ﬁlmsfilms andand simulationssimulations (colored(colored lines)lines) withwith thethe Shklovskii–EfrosShklovskii–Efros (S–E)(S–E) percolationpercolation modelmodel ((aa)) onon the the n n++-type-type SiSi substratesubstrate and ((b)) onon thethe pp++--typetype Si Si substrate. substrate.

TheThe standardcomparison room of temperaturethe experimental was used results for with modeling the S– inE bothpercolation the HRS model and LRS (for cases. the GeO[SiO Since we2] usedfilm on aslow the n voltage+ and p+- sweeptype substrates) to measure was the made I–V earlier characteristics, [18]. The thecharge effects transport of the in local the temperatureas-deposited shiftsGeO[SiO] in the film percolation on the n channels+-type substrate were not was considered well described in the present by the study.S–E model Therefore,, as seen the in real Figure average 8a, temperaturealthough slight of the deviations entirety ofby the the structures experimental was closedata toin roomthe GeO[SiO] temperature. film on the n+-type substrate fromWe thehighlight S–E percolation that switching model curve to an can LRS be inseen our for system low electric took fields. place dueThe tofitting the parameters formation of of thethe filamentsS–E model of for oxygen GeO[SiO] vacancies were (like the infollowing: [15]) or oxygen j0 = 5.1 multi-vacancies.× 1021 A/cm2, W = The 1.5 schematicseV, V0 = 1.8 of eV, the and proposed a = 2.8 mechanismnm. From the of thechange formation in the andV0 × rupturea value, ofit thesebecame conducting possible to filaments suggest between that the electrodescomposition via of a-Ge the nanoclustersGeO[SiO] film are differdemonstrateded from in the Figure GeO[SiO9. The2] absence film [18]. of theA c needonsiderably for forming fine has approximation been demonstrated of the inexperimental the experiments data (e.g., for the by GeO[SiO] applying a film primary on the high p+- positivetype Si substrate voltage to by the the upper S–E electrode).percolation The model bottom can + electrode,be seen in made Figure of p 8b.-Si, The was fitting grounded. parameters No filaments of the (center S–E model image) for could GeO[SiO] be observed in an HRS in the were absence the offollowing: an external j0 = electric 7.4 × 10 field.21 A/cm The2, essenceWHRS = of1.64 the eV, proposed V0 = 2.8 model eV, and lies ain = the5 nm organizing. Meanwhile action, the of fitting a-Ge particlesparameters with for a GeO[SiO] size of ~3 in nm an inside LRS were a 64–85 as follows nm thin: j0 layer = 7.4 of× 10 germanosilicate21 A/cm2, WLRS = glass, 1.53 whicheV, V0 = builds 2 eV, aand conducting a = 3.5 nm. channel The same of oxygen V0 × a value vacancies in an even LRS atfor a the low GeO[SiO] voltage setting and GeO[SiO (one to2] the films order on ofa p++-2type V), leadingsubstrate to indicate the formation that the of filaments a conducting had similar filament composition (shown ins the [18]. right In addition, image). Thethe different absence ofV0 the× a formingvalues in stage an HRS reduced for GeO[SiO] the time and theGeO[SiO total energy2] correspond consumption to the difference necessary forin their reaching compositions the operating [18]. state of the memory element. At voltages of about 3 V, a reset mode was observed (shown in the left The standard room temperature was used for− modeling in both the HRS and LRS cases. Since image),we used after a slow which voltage the write–erase sweep to measure cycle of the information I–V characteristics, in the memristor the effects could of be the eff localectively temperature repeated manyshifts times. in the The percolation model is consistentchannels were with our not recent considered experimental in the present observations study. (Figures Therefore,5 and the6 ) and real withaverage earlier temperature models for of similar the entire systems.ty of Thethe noveltystructures of thewas present close to model room is temperature. based on taking into account the organizingWe highlight influence that switching of a-Ge nanoclusters. to an LRS in The our sink system for oxygen took place ions duringdue to electromigrationthe formation of canthe notfilaments only be of the oxygen bottom vacancies Si electrode, (like in but [15 also]) or the oxygen Ge nanoclusters. multi-vacancies. The schematics of the proposed mechanismHowever, of the it should formation benoted and rupture that additional of these conducting explanation filaments of the resistive between switching electrodes can via be a-Ge in thenanoclusters charging and are dischargingdemonstrated of thein Figure deep traps 9. The in theabsence GeSix O ofy films. the need Charged for forming traps, with has their been extrademonstrated potential, in change the experiments the flow conditions (e.g., by forapplying electrons a primary and holes high in the positive S–E percolation voltage to model the upper [51]. Suchelectrode). deep trapsThe bottom could be electrode a-Ge nanoclusters., made of p+- TheSi, was distance grounded. between No themfilaments exceeds (center the image) value favorable could be forobserved sub-barrier in the tunneling absence of (a fewan external nm, as oneelectric can seefield. in The Figure essence2). The of LRSthe stateproposed can bemodel achieved lies in only the if conducting filaments are formed between a-Ge clusters, as shown in Figure9. Thus, the model of filament formation, in our opinion, is the main explanation for resistive switching. As a further outlook, the elucidation of this issue should be done. Other further directions of this research will be related to optimization of the structure of the films in order to increase the ON/OFF ratio in resistance switching and to improvement of the endurance. Electronics 2020, 9, x FOR PEER REVIEW 14 of 17

organizing action of a-Ge particles with a size of ~3 nm inside a 64–85 nm thin layer of germanosilicate glass, which builds a conducting channel of oxygen vacancies even at a low voltage setting (one to the order of +2 V), leading to the formation of a conducting filament (shown in the right image). The absence of the forming stage reduced the time and the total energy consumption necessary for reaching the operating state of the memory element. At voltages of about −3 V, a reset mode was observed (shown in the left image), after which the write–erase cycle of information in the memristor could be effectively repeated many times. The model is consistent with our recent experimental observations (Figures 5 and 6) and with earlier models for similar systems. The novelty of the present model is based on taking into account the organizing influence of a-Ge nanoclusters. The sink for

Electronicsoxygen 2020 ions, 9, during 2103 electromigration can not only be the bottom Si electrode, but also the14of Ge 17 nanoclusters.

Figure 9.9. SchematicsSchematics ofof thetheformation formation ( right(right)) and and rupture rupture (left (left)) of of a conductinga conducting a-Ge-based a-Ge-based ﬁlament filament in

thein the GeSi GeSixOxyOstructurey structure under under the the external external electric electric field, field, taking taking into into account account thethe movementmovement ofof oxygenoxygen ionsions andand oxygenoxygen vacancies.vacancies. 4. Conclusions However, it should be noted that additional explanation of the resistive switching can be in the chargingStable and resistive discharging switching of wasthe deep observed traps in in MIS the memristor GeSixOy films. structures Charged based traps on non-stoichiometric, with their extra germanosilicatepotential, change (GeSi the flowxOy )conditions films with for and electrons without and amorphous holes in the Ge S– nanoclusters.E percolation model The prevailing [51]. Such mechanismdeep traps could of the be observed a-Ge nanoclusters. resistive switching The distance is the formationbetween them and ruptureexceeds of the conducting value favorable filaments. for Thesub- formationbarrier tunneling and rupture (a few of nm the, as filaments one can issee caused in Figure by the2). The electromigration LRS state can ofbe oxygen achieved ions only and if oxygenconducting vacancies. filament Thes are endurance formed between studies showeda-Ge clusters, that some as shown degradation in Figure of 9. the Thus, memory the model window of occurredfilament formation,for all structures, in our and opinion, it was is mainly the main caused explanation by a decrease for resistive of the ON switching. current. InAs almosta further all MISoutlook, structures, the elucidation intermediate of this resistance issue should states be (IRSs) done. were Other observed, further directions in addition of tothis the research HRS and will LRS. be Therelated studied to optimization memristors of can the be structur used fore of the the simulation films in order of prospective to increase neuromorphicthe ON/OFF ratio devices. in resistance switching and to improvement of the endurance. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-9292/9/12/2103/s1. Figure S1: The 2D AFM topography images of the GeO[SiO2] samples: (a) as-deposited; (b) annealed; Figure S2: Power4. Conclusions spectral density (PSD) of roughness measured for fast scan axis direction X shows small values of object 3 µ 1 sizes (StableX-axis, nm resistive) and amount switching of objects was (Y -axis, observedm− ) for in the MIS entire memristor spectral range; structures Figure S3: Transmittancebased on non and- reflectance curves for GeO[SiO2] and GeO[SiO] films on quartz substrates: left image—as-deposited films; x y rightstoichiometric image—annealed germanosilicate films; Figure (GeSi S4: ExemplaryO ) films experimental with and I–Vwithout characteristics amorphous of the as-depositedGe nanoclusters. GeO[SiO The2]; Tablepreva S1:iling Roughness mechanism parameters of the observed of Si surfaces. resistive switching is the formation and rupture of conducting Authorfilaments. Contributions: The formationConceptualization, and rupture V.A.V.;of the formalfilaments analysis, is ca G.N.K.,used by A.A.G., the electromigration G.K.K., I.P.P., I.A.A. of andoxygen Z.F.; investigation,ions and oxygen V.A.V., vacancies. G.N.K., A.A.G., The endurance G.K.K., I.P.P., studies I.A.A., Z.F.showed and M.V.;that projectsome degradation administration, of P.G.; the resources,memory M.V.;window visualization, occurred P.G.; for writing—originalall structures, and draft, it V.A.V.;was mainly writing—review caused by and a decrease editing, P.G. of Allthe authors ON current. have read In and agreed to the published version of the manuscript. almost all MIS structures, intermediate resistance states (IRSs) were observed, in addition to the HRS Funding: This work was supported by the Ministry of Education and Science of the Russian Federation, grantand LRS. FSUS-2020-0029, The studied while memristors XPS and can I-V be curve used analysis for the was simulation done with of financial prospective support neuromorphic of the grant 075-15-2020-797devices. (13.1902.21.0024). The Raman spectroscopy and TEM were done on the equipment of CKP “VTAN” in the ATRC department of NSU.

Acknowledgments: The authors are thankful to Alexandre Bouché for help in growing the samples. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Zidan, M.A.; Strachan, J.P.; Lu, W.D. The future of electronics based on memristive systems. Nat. Electron. 2018, 1, 22. [CrossRef] 2. Chua, L.O.; Kang, S.M. Memristive devices and systems. Proc. IEEE 1976, 64, 2. [CrossRef] Electronics 2020, 9, 2103 15 of 17

3. Strukov, D.B.; Snider, G.S.; Stewart, D.R.; Stanley Williams, R. The missing memristor found. Nature 2008, 453, 80. [CrossRef][PubMed]

4. Terai, M.; Sakotsubo, Y.; Kotsuji, S.; Hada, H. Resistance Controllability of Ta2O5/TiO2 Stack ReRAM for Low-Voltage and Multilevel Operation. IEEE Electron. Device Lett. 2010, 31, 204. [CrossRef] 5. Yan, K.; Peng, M.; Yu, X.; Cai, X.; Chen, S.; Hu, H.; Chen, B.; Gao, X.; Dong, B.; Zou, D. High-performance perovskite memristor based on methyl ammonium lead halides. J. Mater. Chem. C 2016, 4, 1375. [CrossRef] 6. Battistoni, S.; Erokhin, V.; Iannotta, S. Organic memristive devices for perceptron applications. J. Phys. D Appl. Phys. 2018, 51, 284002. [CrossRef] 7. Romero, F.J.; Toral, A.; Medina-Rull, A.; Moraila-Martinez, C.L.; Morales, D.P.; Ohata, A.; Godoy, A.; Ruiz, F.G.; Rodriguez, N. Resistive Switching in Graphene Oxide. Front. Mater. 2020, 7, 17. [CrossRef] 8. Lee, H.Y.; Chen, P.S.; Wu, T.Y.; Wang, C.C.; Tzeng, P.J.; Lin, C.H.; Chen, F.; Tsai, M.-J. Low power and high

speed bipolar switching with a thin reactive Ti buﬀer layer in robust HfO2 based RRAM. In Proceedings of the IEEE International Electron Devices Meeting, San Francisco, CA, USA, 15–17 December 2008. 9. Wei, Z.; Kanzawa, Y.; Arita, K.; Katoh, Y.; Kawai, K.; Muraoka, S.; Mitani, S.; Fujii, S.; Katayama, K.; Iijima, M.; et al. Highly reliable TaOx ReRAM and direct evidence of redox reaction mechanism. In Proceedings of the IEEE International Electron Devices Meeting, San Francisco, CA, USA, 15–17 December 2008. 10. Lee, M.J.; Lee, C.B.; Lee, D.; Lee, S.R.; Chang, M.; Hur, J.H.; Kim, Y.B.; Kim, C.J.; Seo, D.H.; Seo, S.; et al.

A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta2O(5 x)/TaO(2 x) − − bilayer structures. Nat. Mater. 2011, 10, 625. [CrossRef]

11. Shaposhnikov, A.V.; Perevalov, T.V.; Gritsenko, V.A.; Cheng, C.H.; Chin, A. Mechanism of GeO2 resistive switching based on the multi-phonon assisted tunneling between traps. Appl. Phys. Lett. 2012, 100, 243506. [CrossRef] 12. Yen, T.J.; Chin, A.; Gritsenko, V. High Performance All Nonmetal SiNx Resistive Random Access Memory with Strong Process Dependence. Sci. Rep. 2020, 10, 2807. [CrossRef] 13. Zackriya, M.; Kittur, H.M.; Chin, A. A Novel Read Scheme for Large Size One-Resistor Resistive Random Access Memory Array. Sci. Rep. 2017, 7, 42375. [CrossRef][PubMed] 14. Mehonic, A.; Shluger, A.L.; Gao, D.; Valov, I.; Miranda, E.; Ielmini, D.; Bricalli, A.; Ambrosi, E.; Li, C.; Yang, J.J.; et al. Silicon Oxide (SiOx): A Promising Material for Resistance Switching? Adv. Mater. 2018, 30, 1801187. [CrossRef][PubMed] 15. Kawauchi, T.; Kano, S.; Fujii, M. Forming-free resistive switching in solution-processed silicon nanocrystal thin ﬁlm. J. Appl. Phys. 2018, 124, 085113. [CrossRef] 16. Fan, J.; Kapur, O.; Huang, R.; King, S.W.; de Groot, C.H.; Jiang, L. Back-end-of-line a-SiOxCy:H dielectrics for resistive memory. AIP Adv. 2018, 8, 095215. [CrossRef] 17. Yen, T.J.; Gismatulin, A.; Volodin, V.; Gritsenko, V.; Chin, A. All Nonmetal Resistive Random Access Memory. Sci. Rep. 2019, 9, 6144. [CrossRef] 18. Volodin, V.A.; Kamaev, G.N.; Gritsenko, V.A.; Gismatulin, A.A.; Chin, A.; Vergnat, M. Memristor eﬀect in

GeO[SiO2] and GeO[SiO] solid alloys ﬁlms. Appl. Phys. Lett. 2019, 114, 233104. [CrossRef] 19. Hwang, H.G.; Woo, J.U.; Lee, T.H.; Park, S.M.; Lee, T.G.; Lee, W.H.; Nahm, S. Synaptic plasticity and

preliminary-spike-enhanced plasticity in a CMOS-compatible Ta2O5 memristor. Mater. Des. 2020, 187, 108400. [CrossRef] 20. Wang, R.; Shi, T.; Zhang, X.; Wang, W.; Wei, J.; Lu, J.; Zhao, X.; Wu, Z.; Cao, R.; Long, S.; et al. Bipolar analog Memristors as artificial synapses for neuromorphic computing. Materials 2018, 11, 2102. [CrossRef] 21. Ardyanian, M.; Rinnert, H.; Devaux, X.; Vergnat, M. Structure and photoluminescence properties of evaporated GeOx thin films. Appl. Phys. Lett. 2006, 89, 011902. [CrossRef] 22. Marin, D.V.;Volodin,V.A.;Gorokhov,E.B.; Shcheglov,D.V.;Latyshev,A.V.;Vergnat, M.; Koch, J.; Chichkov,B.N. Modification of germanium nanoclusters in GeOx films during isochronous furnace and pulse laser annealing. Appl. Phys. Lett. 2010, 36, 439. 23. Moulder, J.F.; Stickle, W.F.; Sobol, P.E.; Bomben, K.D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, USA, 1992; p. 190. 24. Gritsenko, V.A.; Kruchinin, V.N.; Prosvirin, I.P.; Novikov, Y.N.; Chin, A.; Volodin, V.A. Atomic and Electronic Structures of a-SiNx:H. J. Exp. Theor. 2019, 129, 924. [CrossRef] 25. Zhigunov, D.M.; Kamaev, G.N.; Kashkarov, P.K.; Volodin, V.A. On Raman scattering cross section ratio of crystalline and microcrystalline to amorphous silicon. Appl. Phys. Lett. 2018, 113, 023101. [CrossRef] Electronics 2020, 9, 2103 16 of 17

26. Fan, Z.; Kochubey, S.A.; Stoffel, M.; Rinnert, H.; Vergnat, M.; Volodin, V.A. On the Formation of Amorphous Ge Nanoclusters and Ge Nanocrystals in GeSixOy Films on Quartz Substrates by Furnace and Pulsed Laser Annealing. Semiconductors 2020, 54, 322. 27. Scofield, J.H. Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV. J. Electron. Spectros. Relat. Phenom. 1976, 8, 129. [CrossRef] 28. Kalimuthu, V.; Kumar, P.; Kumar, M.; Rath, S. Growth mechanism and optical properties of Ge nanocrystals embedded in a GeOx matrix. Appl. Phys. A 2018, 124, 712. [CrossRef] 29. Tabet, N.; Faiz, M.; Hamdan, N.M.; Hussain, Z. High resolution XPS study of oxide layers grown on Ge substrates. Surf. Sci. 2003, 523, 68. [CrossRef] 30. Xpspeak Software Web Site. Available online: http://xpspeak.software.informer.com/4.1/ (accessed on 9 November 2020). 31. Gambaryan, M.P.; Krivyakin, G.K.; Cherkova, S.G.; Stoffel, M.; Rinnert, H.; Vergnat, M.; Volodin, V.A. Quantum Size Effects in Germanium Nanocrystals and Amorphous Nanoclusters in GeSixOy Films. Phys. Solid State 2020, 62, 492. [CrossRef] 32. Kolobov, A.V. Raman scattering from Ge nanostructures grown on Si substrates: Power and limitations. J. Appl. Phys. 2000, 87, 2926. [CrossRef] 33. Cerdeira, F.; Fjeldly, T.A.; Cardona, M. Raman study of the interaction between localized vibrations and electronic excitations in boron-doped silicon. Phys. Rev. B 1974, 9, 4344. [CrossRef] 34. Wihl, M.; Cardona, M.; Tauc, J. Raman scattering in amorphous Ge and III–V compounds. J. Non-Cryst. Solids 1972, 8–10, 172–178. [CrossRef] 35. Volodin, V.A.; Kamaev, G.N.; Vergnat, M. Negative and Positive Photoconductivity and Memristor Effect in Alloyed GeO[SiO] Films Containing Ge Nanoclusters. Phys. Stat. Sol. (RRL) 2020, 14, 2000165. [CrossRef] 36. Gorokhov, E.B.; Volodin, V.A.; Marin, D.V.; Orekhov, D.A.; Cherkov, A.G.; Gutakovskii, A.K.; Shwets, V.A.; Borisov, A.G.; Efremov, M.D. Effect of quantum confinement on optical properties of Ge nanocrystals in

GeO2 films. Semiconductors 2005, 39, 1168. [CrossRef] 37. Lucovsky, G.; Yang, J.; Chao, S.S.; Tyler, J.E.; Czubatyj, W. Oxygen-bonding environments in glow-discharge-deposited amorphous silicon-hydrogen alloy films. Phys. Rev. B 1983, 28, 3225. [CrossRef] 38. Pai, P.G.; Chao, S.S.; Takagi, Y.; Lucovsky, G. Infrared spectroscopic study of SiOx films produced by plasma enhanced chemical vapor deposition. J. Vac. Sci. Technol. A 1986, 4, 689. [CrossRef] 39. Cherkova, S.G.; Volodin, V.A.; Skuratov, V.A.; Stoffel, M.; Rinnert, H.; Vergnat, M. Infrared photoluminescence

from GeO[SiO2] and GeO[SiO] solid alloy layers irradiated with swift heavy Xe ions. J. Lumin. 2020, 223, 117238. [CrossRef] 40. Seck, M.; Devine, R.A.B.; Hernandez, C.; Campidelli, Y.; Dupuy, J.C. Study of Ge bonding and distribution in

plasma oxides of Si1 xGex alloys. Appl. Phys. Lett. 1998, 72, 2748. [CrossRef] − 41. Shabalov, A.L.; Feldman, M.S. Optical properties of thin GeOx ﬁlms. Phys. Status Solidi A 1984, 83, K11. [CrossRef] 42. Jishiashvili, D.A.; Kutelia, E.R. Infrared Spectroscopic Study of GeOx Films. Phys. Status Solidi B 1987, 143, K147. [CrossRef] 43. Perevalov, T.V.; Volodin, V.A.; Kamaev, G.N.; Krivyakin, G.K.; Gritsenko, V.A.; Prosvirin, I.P. Electronic structure and nanoscale potential ﬂuctuations in strongly nonstoichiometric PECVD SiOx. J. Non-Cryst. Solids 2020, 529, 119796. [CrossRef] 44. Tompkins, H.G.; Irene, E.A. Handbook of Ellipsometry; William Andrew Pub: Norwich, NY, USA, 2005.

45. Marin, D.V.; Gorokhov, E.B.; Borisov, A.G.; Volodin, V.A. Ellipsometry of GeO2 films with Ge nanoclusters: Influence of the quantum-size effect on refractive index. Opt. Spectrosc. 2009, 106, 436. [CrossRef] 46. Volodin, V.A.; Gambaryan, M.P.; Cherkov, A.G.; Stoffel, M.; Rinnert, H.; Vergnat, M. Structure and infrared

photoluminescence of GeSi nanocrystals formed by high temperature annealing of GeOx/SiO2 multilayers. Mater. Res. Express 2016, 3, 085019. [CrossRef] 47. Gritsenko, V.A.; Volodin, V.A.; Perevalov, T.V.; Kruchinin, V.N.; Gerasimova, A.K.; Aliev, V.S.; Prosvirin, I.P. Nanoscale potential ﬂuctuations in nonstoichiometrics tantalum oxide. Nanotechnology 2018, 29, 425202. [CrossRef]

48. Broqvist, P.; Binder, J.F.; Pasquarello, A. Band oﬀsets at the Ge/GeO2 interface through hybrid density functionals. Appl. Phys. Lett. 2009, 94, 141911. [CrossRef] Electronics 2020, 9, 2103 17 of 17

49. Tikhov, S.V.; Gorshkov, O.N.; Antonov, I.N.; Kasatkin, A.P.; Korolev, D.S.; Belov, A.I.; Mikhaylov, A.N.; Tetel’baum, D.I. Change of immitance during electroforming and resistive switching in the metal-insulator-metal memristive structures based on SiOx. Tech. Phys. 2016, 61, 745–749. [CrossRef] 50. Shklovskii, B.I.; Efros, A.L. Percolation theory and conductivity of strongly inhomogeneous media. Sov. Phys. Usp. 1976, 18, 846–862. 51. Efremov, M.D.; Kamaev, G.N.; Volodin, V.A.; Arzhannikova, S.A.; Kachurin, G.A.; Cherkova, S.G.; Kretinin, A.V.; Malyutina-Bronskaya, V.V.; Marin, D.V. Coulomb blockade of the conductivity of SiOx ﬁlms due to one-electron charging of a silicon quantum dot in a chain of electronic states. Semiconductors 2005, 39, 910. [CrossRef]

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