Silicon-Carbide (Sic) Nanocrystal Technology and Characterization and Its Applications in Memory Structures

nanomaterials

Article Silicon-Carbide (SiC) Nanocrystal Technology and Characterization and Its Applications in Memory Structures

Andrzej Mazurak 1 , Robert Mroczy ´nski 1,* , David Beke 2,3 and Adam Gali 2,3 1 Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland; [email protected] 2 Wigner Research Centre for Physics, POB. 49, H-1525 Budapest, Hungary; [email protected] (D.B.); [email protected] (A.G.) 3 Department of Atomic Physics, Budapest University of Technology and Economics, Budafoki út 8., H-1111 Budapest, Hungary * Correspondence: [email protected]; Tel.: +48-22-234-6065

Received: 28 October 2020; Accepted: 27 November 2020; Published: 29 November 2020

Abstract: Colloidal cubic silicon-carbide nanocrystals have been fabricated, characterized, and introduced into metal–insulator–semiconductor and metal–insulator–metal structures based on hafnium oxide layers. The fabricated structures were characterized through the stress-and-sense measurements in terms of device capacitance, ﬂat-band voltage shift, switching characteristics, and retention time. The examined electrical performance of the sample structures has demonstrated the feasibility of the application of both types of structures based on SiC nanoparticles in memory devices.

Keywords: silicon-carbide (SiC) nanocrystals; high-k dielectric; metal–insulator–semiconductor (MIS); metal–insulator–metal (MIM); electrical characterization

1. Introduction Due to the potential applications in the field of modern electronic and photonic devices, the investigations and feasibility studies of the structures with nanocrystals (NCs) embedded in dielectric ensembles are commonly noticeable [1]. The literature mostly reports the application of silicon (Si) nanocrystals in various semiconductor devices and structures, e.g., field-effect light-emitting devices (FELEDs) [2], photovoltaic cells [3], memory structures based on photoluminescence (PL) [4], or non-volatile semiconductor memory (NVSM) devices [5,6]. Recently,the other types of semiconductor NCs or all-inorganic perovskite NCs and nanoparticles (NPs) have significantly attracted scientific interest [7]. This study presents the technology and characterization of memory structures that are based on cubic silicon-carbide (SiC) NCs. The fabrication of SiC-NCs was based on the reactive bonding method followed by electroless wet chemical etching [8]. In the next step, SiC nanoparticles were successfully incorporated into the technology of the metal–insulator–semiconductor (MIS) and metal–insulator–metal (MIM) structures. The benefit of the proposed technology of MIS and MIM devices with SiC-NCs, as compared to the typical formation of NC-based devices, is much easier embedding of NCs into the gate-stack; most importantly, the technology can be characterized as extremely low temperature, which follows the lowering of the thermal budget of modern semiconductor device technology [9]. Moreover, the transfer of SiC-NCs from the liquid solution to the semiconductor substrate is compatible with complementary metal-oxide-semiconductor (CMOS) technology since it can be done similarly to the photoresist spin-coating procedure. Test structures with and without NCs embedded in the stack were fabricated. The electrical measurements were performed, and the results

Nanomaterials 2020, 10, 2387; doi:10.3390/nano10122387 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 2387 2 of 11 were used for device parameters extraction. A comparative discussion of the results is presented, which reveals the essential diﬀerence between the structures with and without NCs, due to the nanoparticle charging and discharging processes. Although the presented results are preliminary, relatively large charge retention was obtained. The discussion proves the feasibility of applying the investigated MIS and MIM structures in memory devices, particularly in NVSM devices or resistive random access memories (RRAMs).

2. Silicon-Carbide (SiC) Nanocrystal Technology and Characterization A bottom-up technique was used to synthesize cubic SiC powder from its elements [8,10]. Si (99%, 325 mesh, Sigma-Aldrich, St. Louis, MO, USA) and C (Norit A supra) with 1:1 ratio and 3 wt% PTFE (polytetrafluoroethylene) (Sigma-Aldrich, St. Louis, MO, USA) were mixed in a ball mill, then placed into a graphite crucible. Samples were annealed to about 1250 ◦C in an argon atmosphere for 8 min using an induction furnace. To introduce stacking faults in the cubic SiC matrix, 5 wt% Al powder (95%, <5 µm, Sigma-Aldrich, St. Louis, MO, USA) was added to the mixture mentioned above. SiC NPs were then fabricated from the SiC powder by the no-photon exciton generation chemistry (NPEGEC) technique [10,11]. The bulk cubic SiC was briefly etched in a mixture of hydrofluoric acid and nitric acid solution in an autoclave at 150 ◦C. The thin porous layer, which is the result of the NPEGEC stain etching, was broken into SiC NPs by ultra-sonication. The size distribution of the SiC NPs was controlled through the stacking faults (SF) in the cubic SiC. Aluminum was used as an additive to the mixture of silicon and carbon during SiC powder synthesis to vary the stacking fault content. The SiC NPs prepared from the cubic SiC with low SF concentration were smaller than 4 nm. Increasing the SF concentration increased the number of SiC NPs between 4 and 6 nm. In the case of the studied samples, 5 m/m% Al was used to achieve ca. 15% SF concentration, resulting in particles in the range of no more than 4–6 nm. The size distribution and crystallinity level of the particles were studied by high-resolution transmission electron microscopy (HR-TEM, Figure1a). The SiC NPs were deposited onto a TEM grid from an aqueous solution. According to the HR-TEM imagery, the SiC NPs were well crystalline without the detectable presence of an amorphous or oxide layer. Regardless of their sizes, the SiC-I and SiC-II NPs had an almost identical surface termination. Fourier transformed infrared spectra (FTIR, Figure1b) of the samples were dominated by the C =O stretching vibration of the –COOH at around 1 1 1720 cm− and corresponding C–O stretching vibration at around 1200 cm− . The spectra also showed 1 a broad band centered at 1100 cm− that was assigned to C–O–C and Si–O–Si vibrations, as well as CH 1 vibrations between 1400 and 1600 cm− . The optical bandgap varies with size from 3.87 eV (1–3 nm ultrasmall NPs) to 2.85 eV (4–6 nm NPs) (Figure1c) [11]. SiC-I and SiC-II nanoparticles shared the same surface termination. However, the difference in the bandgap originated from a different electron configuration that was influenced by the surface groups. In the case of 1–3 nm particles, the increase of the relative number of surface atoms contributing to the surface states created flat valance states or HOMO levels, and those were independent of the particle size. Such an effect was diminished for larger particles. As a result, SiC-I had size-independent, molecular-like properties, whereas the optical properties of SiC-II were strongly size-dependent [11]; the electron and hole transfer from SiC-II to a surrounding media was found to be significantly stronger [12] as well. Nanomaterials 2020, 10, 2387 3 of 11 Nanomaterials 2020, 10, 2387 3 of 11

FigureFigure 1.1. ((aa)) High-resolutionHigh-resolution transmissiontransmission electronelectron microscopymicroscopy (HR-TEM)(HR-TEM) imagesimages ofof as-preparedas-prepared ultrasmall,ultrasmall, 1–31–3 nmnm SiCSiC (SiC-I),(SiC-I), andand larger,larger, 4–64–6 nmnm (SiC-II)(SiC-II) SiC,SiC, NPsNPs withwith thethe correspondingcorresponding sizesize distribution;distribution; ((bb)) FTIRFTIR spectraspectra ofof SiC-ISiC-I andand SiC-IISiC-II representingrepresenting thethe surfacesurface terminationtermination ofof thethe particles;particles; ((cc)) UV-VISUV-VIS absorptionabsorption andand photoluminescencephotoluminescence spectraspectra ofof SiC-ISiC-I andand SiC-II;SiC-II; fromfrom thethe UV-VISUV-VIS andand thethe photoluminescencephotoluminescence excitation (PLE) spectra spectra (not (not sh shown),own), one one can can calculate calculate the the optical optical bandgap bandgap of the of theNPs. NPs.

3. SampleSiC-I Devicesand SiC-II Fabrication nanoparticles and shared Measurement the same Protocol surface termination. However, the difference in the bandgapThe colloidal originated cubic from silicon-carbide a different electron nanocrystals configuration were introducedthat was influenced into metal–insulator– by the surface semiconductorgroups. In the (MIS)case of and 1–3 metal–insulator–metal nm particles, the increase (MIM) structuresof the relative based number on hafnium of surface oxide layers.atoms Twocontributing types of sampleto the devicessurface werestates fabricated: created flat NC-MIS valance (nanocrystal states or MIS)HOMO and levels, NC-MIM and (nanocrystal those were MIM),independent and they of werethe particle accompanied size. Such by an reference effect was devices, diminished i.e., without for larger nanocrystals particles. embeddedAs a result, in SiC-I the insulatorhad size-independent, layer. The schematic molecular-like cross-sections properties, and the whereas schematic the band optical diagram properties of the sampleof SiC-II devices were arestrongly presented size-dependent in Figure2; [11]; Figure the3 electronrespectively. and hole The transfer two types from of investigatedSiC-II to a surrounding devices may, media at first was glance,found to seem be significantly to be very similar stronger in Figures[12] as well.2 and 3; they di ffer essentially in terms of their operation principles. The sample NC-MIS devices were fabricated on silicon substrates with the resistivity of 1–103. SampleΩcm (boron-doped),Devices Fabrication whereas and theMeasurement NC-MIM devices Protocol used highly doped silicon substrates (the resistivityThe ofcolloidal 0.001–0.002 cubicΩ cm,silicon-carbide antimuonium nanocrystals impurities). Thewere information introduced was into stored metal–insulator– in the NC-MIS structuressemiconductor in the (MIS) form ofand an metal–insulator–metal electric charge. The nanocrystals (MIM) structures embedded based in theon gate-stackhafnium oxide constituted layers. aTwo quantum types of well sample that storeddevices the were charge. fabricated: The charge NC-M carriersIS (nanocrystal (electrons MIS) and and holes) NC-MIM were delivered(nanocrystal or removedMIM), and by they the tunnelwere accompanied communication by reference between thedevi wellces, andi.e., thewithout gate electrodenanocrystals (JeG ,embeddedJhG), or the in well the andinsulator the semiconductor layer. The schematic substrate cross-sections (JeS, JhS). In the and case the of schematic the investigated band diagram NC-MIM of structures, the sample the devices highly dopedare presented substrate in playedFigure the2; Figure role of 3 the respectively. bottom electrode. The two The types stored of investigated information devices was represented may, at first by theglance, resistance seem to value be very extracted similar at in the Figures outer electrodes.2 and 3; they The differ resistivity essentially of the in insulatorterms of their layer operation that was encapsulatedprinciples. The between sample the NC-MIS top and devices the bottom were electrodefabricated could on silicon be modulated substrates by with creating the resistivity and breaking of 1- conducting10 Ωcm (boron-doped), filaments. The whereas nanocrystals the NC-MIM embedded devices in the layerused mayhighly have doped played silicon the role substrates of seeds (the for inducedresistivity electrical of 0.001–0.002 current Ω paths.cm, antimuonium impurities). The information was stored in the NC-MIS structures in the form of an electric charge. The nanocrystals embedded in the gate-stack constituted a quantum well that stored the charge. The charge carriers (electrons and holes) were delivered or removed by the tunnel communication between the well and the gate electrode (JeG, JhG), or the well and the semiconductor substrate (JeS, JhS). In the case of the investigated NC-MIM structures, the

Nanomaterials 2020, 10, 2387 4 of 11 Nanomaterials 2020, 10, 2387 4 of 11 highlyNanomaterials doped 2020substrate, 10, 2387 played the role of the bottom electrode. The stored information4 of 11 was representedhighly doped by thesubstrate resistance played value the extracted role of atthe th ebottom outer electrodes. electrode. TheThe resi storedstivity information of the insulator was layerrepresentedhighly that wasdoped by encapsulatedthe substrate resistance played betweenvalue the extracted rolethe topof atthe an th debottom outerthe bottom electrode.electrodes. electrode The The stored resicouldstivity information be ofmodulated the insulatorwas by creatinglayerrepresented that and was breaking byencapsulated the resistance conducting between value filaments. extracted the top atThe anth ednanocrystals outer the bottomelectrodes. embeddedelectrode The resi stivity couldin the ofbe layerthe modulated insulator may have by creating and breaking conducting filaments. The nanocrystals embedded in the layer may have playedNanomaterialslayer the that role 2020was of ,seeds 10encapsulated, 2387 for induced between electrical the top current and the paths. bottom electrode could be modulated4 of by 11 playedcreating the role and of breaking seeds for conducting induced electricalfilaments. current The nanocrystals paths. embedded in the layer may have played the role of seeds for induced electrical current paths.

(a) (b) (a()a ) (b()b ) Figure 2. Sample devices fabricated for this study: (a) NC-MIS and (b) NC-MIM types. FigureFigureFigure 2. Sample 2.2. Sample devices devices fabricated fabricated for for thisthis study:study:study: ( a()a ) NC-MIS NC-MIS and and ((b) )( NC-MIMNC-MIMb) NC-MIM types.types. types.

Figure 3. The schematic band diagram of the sample device. FigureFigure 3. The3. The schematic schematic band band diagramdiagram ofof the the sample sample device. device. Figure 3. The schematic band diagram of the sample device. TheThe processingprocessing sequencesequence ofof thethe fabricatedfabricated structuresstructures waswas asas follows:follows: Si substratessubstrates werewere cleanedcleaned utilizingThe processing a modified sequence RCA method of the (Piranhafabricated+ SC1structur+ SC2es +wasHF as dipping). follows: In Si the substrates first step, were a 20 cleaned nm utilizingThe processing a modified sequence RCA method of the (Piranhafabricated + SC1structur + SC2es +was HF asdipping). follows: In Si the substrates first step, were a 20 nmcleaned utilizingbottom a modified hafnium oxide RCA (HfO methodx) layer (Piranha was deposited. + SC1 + In SC2 the next+ HF step, dipping). the SiC NPsIn the spinning first step, off on a the20 nm utilizingbottom a modifiedhafnium oxide RCA (HfO methodx) layer (Piranha was deposited. + SC1 In+ SC2the next + HF step, dipping). the SiC NPs In the spinning first step,off on a the 20 nm x bottomtoptop hafnium ofof the hafnia oxide surface surface (HfO was was) layer performed, performed, was deposited. followed followed by byIn 20 20the nm nm next top top stHfO HfOep,x layerxthelayer SiC deposition. deposition. NPs spinning Two Two types off types onof the bottom hafnium oxide (HfOx) layer was deposited. In the next step, the SiC NPs spinning off on the top ofofcolloidal the colloidal hafnia nanoparticles nanoparticles surface was were were performed, used; used; those those followed with with the the dimensionsby dimensions 20 nm top of of1–3 HfO 1–3 (SiC-I),x (SiC-I),layer and deposition. and those those measuring measuring Two types 4– of top of the hafnia surface was performed, followed by 20 nm top HfOx layer deposition. Two types of colloidal4–66 nm nm nanoparticles (SiC-II) (SiC-II) (Figure (Figure were 4).4 ).After Afterused; the thethose formation formation with ofthe of Si SiC-NCsdimensionsC-NCs embedded embedded of 1–3 in (SiC-I), indielectric dielectric and layer layerthose ensembles, ensembles, measuring in 4– 6colloidal nminthe (SiC-II) the case nanoparticles case of (Figure ofNC-MIS NC-MIS 4). structures,were After structures, used; the the formation thethose aluminum aluminum with of the (Al)Si (Al)C-NCs dimensions contact contact embedded pads pads of were were1–3 informed(SiC-I), formed dielectric through throughand layerthose the ensembles,measuring standardstandard in4– the6 nm caseUVUV (SiC-II) (@365(@365of NC-MIS nm)(Figure photolithography structures, 4). After thethe formationprocess aluminum and of (Al) wet SiC-NCs etching,contact embedded whereaspads were in in the formed dielectric case of through NC-MIM layer ensembles,the devices, standard in titanium nitride (TiN) contacts were obtained using the “lift-off” procedure. The HfO , Al, and TiN UVthe (@365casetitanium of nm) NC-MIS nitride photolithography (TiN) structures, contacts the wereprocess aluminum obtain anded wet using(Al) etching, contact the “lift-off” whereaspads procedure.were in theformed caseThe throughHfOof NC-MIMxx, Al, theand standard devices,TiN UV (@365layerslayers werenm)were photolithography depositeddeposited inin thethe pulsed-DCpulsed-DC process reactiveandreactive wet magnetron magnetronetching, whereas sputteringsputtering in process.process.the case Post-metallizationPost-metallization of NC-MIM devices, titanium nitride (TiN) contacts were obtained using the “lift-off” procedure. The HfOx, Al, and TiN annealing (PMA) at 300 ◦C in a vacuum atmosphere was performed at the end of the processing layerstitaniumannealing were nitride deposited (PMA) (TiN) at incontacts300 the °C pulsed-DC in were a vacuum obtain reactive atmosphereed using magnetron the was “lift-off” performed sputtering procedure. at the process. end The of Post-metallizationHfOthe processingx, Al, and TiN layerssequence.sequence. were deposited The performed in the PMA pulsed-DC process not reactive only improvedimmagnetronproved the sputtering contact properties process. of Post-metallization thethe processedprocessed annealingsample (PMA) structures, at 300 but also°C in significantly a vacuum enhanced atmosphere the quality was performed and electrical at parameters the end of of the HfO processingfilms, annealingsample (PMA) structures, at 300but also°C insignificantly a vacuum enhanc atmosphereed the quality was performedand electrical at parameters the end ofof HfOthe xxprocessing films, sequence.as had The been performed previously demonstratedPMA process [13 not,14 ].only improved the contact properties of the processed sequence.as had Thebeen performed previously demonstratedPMA process [13,14]. not only improved the contact properties of the processed sample structures, but also significantly enhanced the quality and electrical parameters of HfOx films,

assample had been structures, previously but also demonstrated significantly [13,14]. enhanc ed the quality and electrical parameters of HfOx films, as had been previously demonstrated [13,14]. Density of SiC Nanoparticles in Average Diameter Methanol Solvent 1–3 nm Density of 0.5 SiC mg/mL Nanoparticles in Average4–6 Diameter nm Density 0.8of SiCmg/mL Nanoparticles in Average Diameter Methanol Solvent 1–3 nm Methanol 0.5 mg/mL Solvent 4–61–3 nmnm 0.8 0.5 mg/mLmg/mL (a) 4–6 nm (b) 0.8 mg/mL

FigureFigure 4.4. (a) Colloidal silicon-carbide nanocrystals (SiC-NCs)(SiC-NCs) disperseddispersed inin methanol,methanol, andand ((b)) speciﬁcsspecifics

ofof thethe liquidliquid solution.solution.( a) (b) (a) (b) FigureThe 4. electrical(a) Colloidal measurements silicon-carbide always nanocrystals started with(SiC-NCs) measurements dispersed performedin methanol, for and a fresh(b) specifics device. Then,ofFigure the subsequentliquid 4. (a) solution.Colloidal electrical silicon-carbide pulses (i.e., nanocrystals the electrical (SiC-NCs) stress) dispersed were imposed, in methanol, followed and by (b) electrical specifics of the liquid solution.

Nanomaterials 2020, 10, 2387 5 of 11

Nanomaterials 2020, 10, 2387 5 of 11 The electrical measurements always started with measurements performed for a fresh device. Nanomaterials 2020, 10, 2387 5 of 11 Then, subsequentThe electrical electrical measurements pulses always(i.e., the started electr wiicalth measurementsstress) were imposed,performed followedfor a fresh by device. electrical measurementsThen, subsequent (i.e., the electrical sense pulsescomponent (i.e., the of electrthe procical edure),stress) were as presented imposed, infollowed Figure by5. Theelectrical charging pulsesmeasurements were performed (i.e., the with sense a voltage component or a currentof the procedure),proc source.edure), The as measured presented inparameters Figure5 5.. TheThe were chargingcharging the current (staticpulses measurements) were performed or withthe small-signal a voltage or a capacicurrent tance source. (admittance The measured measured measurements) parameters were conducted the current in the (static measurements) or the small-signal capacitance (admittance measurements) conducted in the voltage(static or measurements)time-domain, ori.e., the in small-signalthe sweeping capacitance or sampling (admittance modes, measurements)respectively. For conducted the program-erase in the voltage or time-domain, i.e., in the sweeping or sampling modes, respectively. For the program-erase voltagevoltage investigations, or time-domain, subsequent i.e., in the sweepingvoltage pulses or sampling of the modes, opposite respectively. polarity Forwere the used. program-erase The electrical voltage investigations, subsequent voltage pulses of the opposite polarity were used.used. The electrical measurements were conducted with the Keithley 4200 semiconductor characterization system and measurements were conducted with the Keithley 4200 semiconductorsemiconductor characterization system and SUSS PM-8 probe station, as presented in [15]. The procedure of electrical parameters extraction used SUSS PM-8 probe station, as presented in [[15].15]. The The procedure procedure of electrical parameters extraction used in thisin this thiswork work work was was was described described described in in detail in detail detail in in in [16].[16]. [16 The]. The resultsresults results presented presented presented in inthis in this thisstudy study study were were were obtained obtained obtained for NC- forfor NC- MISNC-MISMIS and and NC-MIM andNC-MIM NC-MIM devices, devices, devices, with with witha a top top a electrodeelectrode top electrode areaarea area of of 1.8 1.8 of × 1.8 ×10 10−4 cm−104 cm24. cm2. 2. × −

Figure 5. The measurement measurement procedure which was implemen implementedted during the electrical characterization Figureof investigated 5. The measurement structures. procedure which was implemented during the electrical characterization of investigated structures. 4. Results and Discussion 4. Results and Discussion The results depicted in Figure 6 6 show show thethe high-frequencyhigh-frequency C–VC–V curvescurves ofof NC-MISNC-MIS structuresstructures withwith SiCThe nanocrystals results depicted as compared in Figure to the 6 show reference the high-fre device. The quency noticeable C–V curvesdifferent different of quantity NC-MIS of structures frequency with dispersion within the curves suggests that the built-up charge in the examined structures with NCs SiC dispersionnanocrystals within as compared the curves tosuggests the reference that the buildevice.t-up Thecharge noticeable in the examined different structures quantity with of frequency NCs can be observed. Such an effect can be noticed for both dimensions of SiC NPs, which is due to the dispersioncan be observed. within the Such curves an effect suggests can be that noticed the builfor botht-up dimensions charge in theof SiC examined NPs, which structures is due to with the NCs introduction of NCs in the hafnia films and the formation of the quantum well that allowed for the can introductionbe observed. of Such NCs inan the effect hafnia can films be noticed and the forformation both dimensions of the quantum of SiC well NPs, that whichallowed is for due the to the charge carriers trapping. Another Another effect effect that that can can be be also noticed is the lowering of the maximum introduction of NCs in the hafnia films and the formation of the quantum well that allowed for the capacitance value with the increase of NCs size in the accumulationaccumulation regime due to nanocrystals charge carriers trapping. Another effect that can be also noticed is the lowering of the maximum embedded in HfO ensembles. capacitanceThe memorymemoryvalue with eeffectffect the in in the increasethe examined examined of NC-MISNCs NC-MIS size devices devin icesthe was wasaccumulation investigated investigated by regimedetermining by determining due theto flat-band nanocrystalsthe flat- voltage (V ) shift revealed from the C–V characteristic after applying specific voltage stress. The V embeddedband voltage in fbHfO (V ensembles.fb) shift revealed from the C–V characteristic after applying specific voltage stress.fb value can be treated as the distinctive parameter of the MIS device, similar to the threshold voltage TheThe V memoryfb value can effect be treated in the as examined the distinctive NC-MIS parame devterices of wasthe MIS investigated device, similar by determining to the threshold the flat- (V ) value of the MOSFET. Furthermore, the examinations of V changes of the MIS device at specific bandvoltage voltageth (V th(V) valuefb) shift of therevealed MOSFET. from Furthermore, the C–V characteristic the examinationsfb after of applyingVfb changes specific of the MIS voltage device stress. stress values can be valuable for memory effects characterization [17]. Based on the V examination The atV fbspecific value stresscan be values treated can as be the valuable distinctive for memory parame effectster of thecharacterization MIS device, [17]. similar Based to onthe the threshold V from the C–V curves after stressing (i.e., after excitation at specific voltage value), the hysteresis loop of voltageexamination (Vth) value from of the the C–V MOSFET. curves afterFurthermore, stressing (i.e the., examinationsafter excitation ofat Vspecificfb changes voltage of thevalue), MIS the device the examined NC-MIS device V = f(V ) was obtained; this is presented in Figure7. Such a hysteresis at specifichysteresis stress loop valuesof the examined can be fbvaluable NC-MISp/e devicefor memory Vfb = f(V effectsp/e) was characterizationobtained; this is presented [17]. Based in Figure on the V can be considered as the memory window of the examined test device. examination7. Such a hysteresis from the canC–V be curves considered after as stressing the memory (i.e window., after excitation of the examined at specific test device. voltage value), the hysteresis loop of the examined NC-MIS device Vfb = f(Vp/e) was obtained; this is presented in Figure 7. Such a hysteresis can be considered as the memory window of the examined test device.

Figure 6. Obtained high-frequency high-frequency families families of of C–V C–V curves curves of of examined examined in in this this work work MIS MIS structures; structures; f f= =0.01,0.01, 0.02, 0.02, 0.04, 0.04, 0.1, 0.1, 0.2, 0.2, 0.4, 0.4, 1 1MHz. MHz.

Figure 6. Obtained high-frequency families of C–V curves of examined in this work MIS structures; f = 0.01, 0.02, 0.04, 0.1, 0.2, 0.4, 1 MHz.

Nanomaterials 2020, 10, 2387 6 of 11 Nanomaterials 2020, 10, 2387 6 of 11

Nanomaterials 2020, 10, 2387 6 of 11

Figure 7. Memory window (expressed as Vfb change) of NC-MIS devices with both types of SiC-NCs Figure 7. Memory window (expressed as Vfb change) of NC-MIS devices with both types of SiC-NCs Figure 7. Memory window (expressed as Vfb change) of NC-MIS devices with both types of SiC-NCs p/e examinedexamined in inthis this study; study; V Vp /=e 0–±12= 0– 12 V,V, and and stress stress time time == 11 s swere were used. used. examined in this study; Vp/e = 0–±12± V, and stress time = 1 s were used. In Inthe the case case of of the the NC-MIS NC-MIS structure structure with with SiC-I SiC-I NPs, a changechange ofof VV ofof the the order order of of 7.6 7.6 V V was was In the case of the NC-MIS structure with SiC-I NPs, a change of V of the order of 7.6 V was obtained,obtained, whereas whereas in in the the case case of of devices devices with with SiC- SiC-IIII NPs, the obtainedobtainedmemory memory window window of of the the order order obtained, whereas in the case of devices with SiC-II NPs, the obtained memory window of the order of 8.6 V was observed. However, it is worth noting that in the case of the NC-MIS structure with of 8.6of V8.6 was V was observed. observed. However, However, it itis is worth worth noting noting thatthat in thethe casecase of of the the NC-MIS NC-MIS structure structure with with SiC- SiC- SiC-I NPs, the symmetry of maximum and minimum values of V after the positive and negative V I NPs,I NPs, the thesymmetry symmetry of ofmaximum maximum an andd minimum minimum valuesvalues of VV afterafter the the positive positive and and negative negative V aroundV around around the flat-band voltage of a “fresh” device, i.e., the MIS structure without any program or erase the theflat-band flat-band voltage voltage of ofa a“fresh” “fresh” device, device, i.e.,i.e., thethe MIS structurestructure withoutwithout any any program program or orerase erase procedure is increased. Such a finding allows for a more convenient implementation of the examined procedureprocedure is increased. is increased. Such Such a findinga finding allows allows for for aa moremore convenient implementation implementation of ofthe the examined examined structures in a specific memory application. structuresstructures in ain specific a specific memory memory application. application. A further performed examination of the permanence of introduced charge into examined NC-MIS A furtherA further performed performed examination examination of of the the permanenpermanence ofof introducedintroduced charge charge into into examined examined NC- NC- structures confirmed the large charge retention. The results of retention investigations are collected in MISMIS structures structures confirmed confirmed the the large large charge charge retention.retention. The resultsresults ofof retention retention investigations investigations are are Figure8. The obtained retention of the V shift extrapolated to ten years exhibited similar persistence of collectedcollected in Figurein Figure 8. The8. The obtained obtained retention retention of of ththee VV shift extrapolatedextrapolated to to ten ten years years exhibited exhibited similar similar charges for both types of the investigated sample devices (i.e., SiC-I and SiC-II NCs). In the case of persistence of charges for both types of the investigated sample devices (i.e., SiC-I and SiC-II NCs). persistenceNC-MIS structures of charges with for smaller both types NCs (i.e.,of the 1–3 investig nm), theated charge sample loss wasdevices of the (i.e., order SiC-I of ~64%, and whereasSiC-II NCs). in In the case of NC-MIS structures with smaller NCs (i.e., 1–3 nm), the charge loss was of the order of In the casecase of of larger NC-MIS NPs structures (i.e., 4–6 nm), with the smaller charge lossNCs was (i.e., increased 1–3 nm), to the ~66%. charge Those loss values was correspondedof the order of ~64%,~64%, whereas whereas in inthe the case case of of larger larger NPs NPs (i.e., (i.e., 4–6 4–6 nm),nm), the chargecharge loss loss was was increased increased to to~66%. ~66%. Those Those tovalues the memory corresponded windows to widththe memory of the orderwindows of 2.7 widt V andh of 2.9the V order for NC-MIS of 2.7 V structures and 2.9 V with for SiC-INC-MIS and values corresponded to the memory windows width of the order of 2.7 V and 2.9 V for NC-MIS SiC-IIstructures nanocrystals with SiC-I (extrapolated and SiC-II nanocr to 10 years).ystals (extrapolated to 10 years). structures with SiC-I and SiC-II nanocrystals (extrapolated to 10 years).

Figure 8. Memory window (expressed as Vfb change) of NC-MIS devices with both types of SiC-NCs; Figure 8. Memory window (expressed as Vfb change) of NC-MIS devices with both types of SiC-NCs; Vp/e = 0–±12 V, and stress time = 1s were used. Vp/e = 0– 12 V, and stress time = 1s were used. Figure 8. Memory± window (expressed as Vfb change) of NC-MIS devices with both types of SiC-NCs; Vp/e The= 0–±12 observed V, and effect stress of time the =slightly 1s were wider used. memo ry window and higher retention loss for NC-MIS structures with a larger dimension of SiC-NCs compared to structures with SiC-I NPs was further The observed effect of the slightly wider memory window and higher retention loss for NC-MIS structures with a larger dimension of SiC-NCs compared to structures with SiC-I NPs was further

Nanomaterials 2020, 10, 2387 7 of 11

NanomaterialsThe observed 2020, 10, x e FORffect PEER of the REVIEW slightly wider memory window and higher retention loss for NC-MIS 7 of 11 structures with a larger dimension of SiC-NCs compared to structures with SiC-I NPs was further investigated. The basic electricalelectrical parametersparameters estimatedestimated fromfrom the the C–V C–V curves curves were were calculated. calculated. Figure Figure9 9depicts depicts the the e ffeffectiveective charge charge density density (Q (Qeffeff/q)/q) and and the theinterface interface traptrap densitydensity inin thethe middlemiddle of the silicon bandgap (D itmb) )for for the the NC-MIS NC-MIS devices devices examined examined in in this this study. study. It It has has to to be be taken into account that the estimated electrical parameters represent thethe “e"effective"ffective” values for the whole gate-stack.

Figure 9. ExtractedExtracted parameters parameters showing showing the the effective eﬀective charge density (Qeeffﬀ/q)/q) and interface traps density in the middle of the silicon bandgap (D(Ditmbitmb)) for for NC-MIS NC-MIS structures structures and and reference reference MIS devices investigated in this work.

As shown, both Qeeffff/q/q and Ditmb valuesvalues were were higher higher for for MIS MIS st structuresructures with SiC-NCs than the reference device. Due to the presence of nanoparticles embedded in the HfOx layerlayer bulk, the results were also proved by the comparison depicted in Figure 66.. ItIt isis alsoalso evidentevident thatthat thethe eeffectiveffective charge was about one third higher for the structure with SiC-II compared to NC-MIS devices with SiC-I, which resulted in a higher density of charge that may have been trapped in the gate-stack of the examined structure,structure, explaining explaining the higherthe higher memory me windowmory (i.e.,window 2.9 V) (i.e., reported 2.9 above.V) reported Simultaneously, above. Simultaneously,the slightly higher the D slightly value that higher was obtainedD value that for thewas NC-MIS obtained structure for the withNC-MIS SiC-II structure may have with been SiC-II the maycause have of higher been the leakage cause current of higher (sequential leakage current tunneling (sequential process via tunneling the interface process and via border the interface traps) and and a borderhigher probabilitytraps) and a of higher charge probability loss from theof charge NCs film loss that from was the embedded NCs film inthat the was HfO embeddedx insulator in layer. the AsHfO thex insulator interface layer. trap density As the directly interface aff ectedtrap density the leakage directly current affected [18], itthe is aleakage reason forcurrent a slightly [18], higherit is a reasonretention for loss a slightly that has higher been retention observed loss for NC-MISthat has devicesbeen observed with SiC-II for NC-MIS NCs (compare devices the with retention SiC-II NCs loss (compareof 64% vs. the 66% retention for NC-MIS loss of SiC-I 64% and vs. NC-MIS66% for NC-MIS SiC-II, respectively). SiC-I and NC-MIS SiC-II, respectively). The literature demonstrates several examples of applications of structures with nanocrystalline islands in memorymemory devices.devices. These These exemplary exemplary devices devices are are mainly mainly established on Si nanocrystalsnanocrystals introduced in silicon dioxidedioxide (SiO(SiO2)) films. films. The The reported reported memory memory wi windowsndows of of these devices differ differ depending onon such such factors factors as theas studiedthe studied type oftype a gate of structure,a gate structure, the conditions the conditions of electrical polarization,of electrical polarization,and stressing and pulses. stressing As an pulses. example, As an the example, devices the were devices deposited were deposited using the low-pressureusing the low-pressure chemical vaporchemical deposition vapor deposition (LPCVD) method;(LPCVD) thermally method; recrystallizedthermally recrystallized Si-NCs that Si-NCs were introduced that were introduced in SiO2 can inbe SiO characterized2 can be characterized by the memory by the window memory manifested window manifested as the change as the ofV changefb under of V fbp /eunderof the V order p/e of theof 1.6 order V and of 1.6 3.5 V V, and for 3.5 a bipolar V, for a asymmetric bipolar asymmetric and unipolar and unipolar sequence sequence of program of program and erase and pulses, erase pulses,respectively respectively [19]. In the[19]. case In the of Sicase clusters of Si cluste obtainedrs obtained through through thermal thermal evaporation, evaporation, the memory the memory window windowof the order of the of 5.1 order V was of 5.1 shown V was [20 shown]. If one [20]. takes If one into takes account into the account threshold the voltagethreshold (V tvoltage) shift, the (Vt values) shift, theof the values order of of the 1.5 order V [21 ]of or 1.5 1–3 V V [21] [22] or have 1–3 been V [22] reported. have been The reported. simulation The studies simulation forecasted studies the forecastedcharge carriers the charge retention carriers with theretention memory with window the memory of the order window of 3 Vof for the up order to 10 of years 3 V [for23] up with to the10 yearsapplication [23] with of high- the kapplicationdielectric films. of high- Thus,k dielectric according films. to the Thus, reported according results, to the the findings reported demonstrated results, the findingsin this work demonstrated should encourage in this further work should investigations encourage on the further application investigations of SiC-NCs on in the NVSM application structures. of SiC-NCsIt has to be in underlinedNVSM structures. that a large It has memory to be underlined window of that the a order large of memory 2.7 V and window 2.9 V, for of the smallerorder of and 2.7 Vlarger and SiC-NCs,2.9 V, for respectively, the smaller asand well larger as being SiC-NCs, extrapolated respectively, to 10 years,as well was as obtained.being extrapolated to 10 years, was obtained. In the last part of this study, the fabricated MIM structures with embedded SiC-I and SiC-II nanoparticles were examined. The representative I–V characteristics of the sample structures are depicted in Figure 10. The results presented distinctly show that the application of SiC-NCs resulted

Nanomaterials 2020, 10, 2387 8 of 11

V andNanomaterials 2.9 V, for2020 the, 10 ,smaller 2387 and larger SiC-NCs, respectively, as well as being extrapolated8 of 11to 10 years, was obtained. In the last part of this study, the fabricated MIM structures with embedded SiC-I and SiC-II In the last part of this study, the fabricated MIM structures with embedded SiC-I and SiC-II nanoparticles were examined. The representative I–V characteristics of the sample structures are nanoparticles were examined. The representative I–V characteristics of the sample structures are depicted in Figure 10. The results presented distinctly show that the application of SiC-NCs resulted depicted in Figure 10. The results presented distinctly show that the application of SiC-NCs resulted in in a highera higher value value of of the the high-resistance high-resistance state (HRS)(HRS) to to low-resistance low-resistance state state (LRS) (LRS) ratio. ratio. It is It particularly is particularly evidentevident in the in case the case of NC-MIM of NC-MIM structures structures with with Si SiC-IIC-II NCs (i.e.,(i.e., 4–6 4–6 nm). nm). Moreover, Moreover, the the HRS HRS to LRS to LRS ratio ratioof the of structure the structure with with SiC-II SiC-II NCs NCs increased increased compared compared to to the the device device based based on SiC-I on SiC-I NCs. ItNCs. may It be may be connectedconnected to to the the fact thatthat the the application application of largeof large NPs NPs resulted resulted in a larger in a locallarger electric local ﬁeld, electric that field, in the that in thecase case of of LRS, LRS, resulted resulted in a in higher a higher leakage leakage current, current, and thus and a higherthus aHRS higher to LRS HRS ratio. to LRS However, ratio. However, such a such hypothesisa hypothesis needs needs further further study study as well as as well more as statistical more statistical data, both data, of which both goof beyondwhich go the beyond aims of the aims thisof this preliminary preliminary study. study.

FigureFigure 10. Comparison 10. Comparison of ofrepresentative representative resistiveresistive switching switching I–V I–V curves curves of NC-MIM of NC-MIM and reference and reference MIM MIMdevices devices investigated investigated in thisin this work; work; the numbersthe numbers indicate indicate the sequence the sequence of measurements. of measurements.

In the case of NC-MIM structures with large SiC nanoparticles, the ratio between LRS to HRS In the case of NC-MIM structures with large SiC nanoparticles, the ratio between LRS to HRS was about four orders of magnitude, compared to two orders of magnitude in the case of reference was about four orders of magnitude, compared to two orders of magnitude in the case of reference structures. Furthermore, the NC-MIM structures with SiC-II NCs exhibited lower set/reset (Vset/Vreset) structures. Furthermore, the NC-MIM structures with SiC-II NCs exhibited lower set/reset (Vset/Vreset) voltage values that controlled the electroforming process. As can be drawn from Figure 10, the Vset was set voltage~ 6.2, values and ~that7.4 controlled V, whereas the Velectroformingreset was ~4.8, and process. ~6 V for As NC-MIM can be withdrawn SiC-II from NCs Figure and reference 10, the V − − was structures,~−6.2, and respectively.~−7.4 V, whereas This was the due Vreset to the was fact ~4.8, that insertedand ~6 V nanoparticles for NC-MIM may with enhance SiC-II the NCs local and referenceelectric structures, field [24]. respectively. In this case, SiC-NCs This was effectively due to supportedthe fact that the inserted formation nanoparticles of current paths may and enhance the the localleakage electric filament field formation [24]. In this under case, the SiC-NCs influence ofeffectively electrical polarization,supported the thus formation affecting theof current switching paths and theproperties leakage of filament the memory formation cell. Such under a finding the is influenc of a greate importanceof electrical for polarization, the reduction thus of the affecting device’s the switchingpower properties consumption of level.the memory cell. Such a finding is of a great importance for the reduction of the device’s5. Conclusions power consumption level.

5. ConclusionsThe technology of silicon-carbide nanoparticles has been developed and fabricated using a method is based on reactive bonding, followed by electroless wet chemical etching. The performed Thestructural technology and optical of silicon-carbide characterization nanoparticles of fabricated nanocrystalshas been developed showed that and the fabricated NPs were wellusing a methodcrystalline is based without on reactive the detectable bonding, presence followed of anyby electroless amorphous wet or oxide chemical phases. etching. SiC-NCs The were performed also structuralincorporated and optical into MIS characterization and MIM memory of fabricat structures.ed nanocrystals Two types of showed NPs with that diﬀerent the NPs dimensions were well crystallinewere investigated, without the i.e., detectable nanocrystals presence in the rangeof any of amorphous 1–3 nm (SiC-I), or oxide and 4–6 phases. nm (SiC-II). SiC-NCs During were the also incorporatedfabrication into of sample MIS and devices MIM with memory SiC-NCs structures embedded. Two in hafnium types oxideof NPs thin with ﬁlms, different SiC nanocrystals dimensions werewere investigated, transferred i.e., from nanocrystals a methanol solution.in the range The obtained of 1–3 nm sample (SiC-I), devices and were 4–6 examinednm (SiC-II). throughout During the fabrication of sample devices with SiC-NCs embedded in hafnium oxide thin films, SiC nanocrystals were transferred from a methanol solution. The obtained sample devices were examined throughout the stress-and-sense measurements in terms of device capacitance, flat-band voltage shift, switching properties, and retention time.

Nanomaterials 2020, 10, 2387 9 of 11 the stress-and-sense measurements in terms of device capacitance, flat-band voltage shift, switching properties, and retention time. The demonstrated findings in this study have shown a significant hysteresis of the V shift induced by the program and erase voltages of the fabricated NC-MIS devices. A change of V due to the V of the order of 7.6 V and 8.6 V was demonstrated in the case of structures with SiC-I and SiC-II, respectively. The obtained charge retention for the examined structures revealed relatively high stability. Taking into account the dimensions of SiC-NCs, the memory window of the order of 2.7 V and 2.9 V for the smaller (i.e., 1–3 nm) and larger (i.e., 4–6 nm) NCs were calculated (extrapolated to 10 years) respectively. A slightly higher memory window width and higher retention loss for NC-MIS structures with SiC-II were found. This finding is related to deteriorated electrical parameters, i.e., higher Qeff and Dit values that were proved by the analysis of high-frequency C–V characteristics. Moreover, the presented results of the analysis of electrical characteristics of the fabricated NC-MIM devices have shown the narrower set and reset operation range and a larger HRS to LRS ratio in the devices with SiC-NCs as compared to the reference devices. The results presented in this study are promising for potential applications of SiC nanocrystals, particularly in NVSM devices, or the possible replacement of silicon–oxide–nitride–oxide–silicon (SONOS) gate-stack or RRAM structures. The presented results are original and initial, as they deal with SiC-NCs dispersed in methanol solution that were spin-coated to form an active part of the device and make the examined devices interesting for further research and technology development.

Author Contributions: Conceptualization and idea of this study, A.M. and R.M.; fabrication of SiC-NCs and characterization, D.B. and A.G.; electrical measurements and analysis, A.M. and R.M.; writing—original draft preparation, A.M., R.M., D.B., and A.G.; writing—review and editing, A.M., R.M., D.B., and A.G.; visualization, A.M., R.M., D.B., and A.G.; supervision, R.M.; project administration, R.M., D.B., and A.G.; funding acquisition, R.M., D.B., and A.G. All authors have read and agreed to the published version of the manuscript. Funding: This work has been supported by The National Centre for Research and Development (NCBiR) under grant No. V4-Jap/3/2016 (“NaMSeN”) in the course of “V4-Japan Advanced Materials Joint Call” and under grant No. TECHMATSTRATEG1/347012/3/NCBR/2017 (HYPERMAT) in the course of “Novel technologies of advanced materials—TECHMATSTRATEG”. Acknowledgments: D.B. acknowledges the “Bolyai” Research Scholarship of HAS, the scholarships of the National Talent Program, Grant No. NTP-NFTÖ-18-B-0243, and the National Excellence Program, Grant No.ÚNKP-19-4-BME-83. Á.G. acknowledges the National Office of Research, Development, and Innovation Office of Hungary (NKFIH) Grant No. 127902 of the EU QuantERA Nanospin project, the NKFIH Grant No. 2017-1.2.1-NKP-2017-00001 within the National Quantum Technology Program, and the support of Quantum Information National Laboratory sponsored by the Ministry of Innovation and Technology of Hungary through NKFIH. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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