The Phase Evolution and Physical Properties of Binary Copper Oxide Thin Films Prepared by Reactive Magnetron Sputtering

materials

Article The Phase Evolution and Physical Properties of Binary Copper Oxide Thin Films Prepared by Reactive Magnetron Sputtering

Weifeng Zheng 1,2, Yue Chen 1,2,*, Xihong Peng 1,2, Kehua Zhong 1,2, Yingbin Lin 1,2 and Zhigao Huang 1,3,*

1 Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, College of Physics and Energy, Fujian Normal University, Fuzhou 350117, China; [email protected] (W.Z.); [email protected] (X.P.); [email protected] (K.Z.); [email protected] (Y.L.) 2 Fujian Provincial Engineering Technical Research Centre of Solar-Energy Conversion and Stored Energy, Fuzhou 350117, China 3 Fujian Provincial Collaborative Innovation Center for Optoelectronic, Semiconductors and Efﬁcient Devices, Xiamen 361005, China * Correspondence: [email protected] (Y.C.); [email protected] (Z.H.); Tel./Fax: +86-591-2286-7577 (Z.H.) Received: 29 May 2018; Accepted: 12 July 2018; Published: 20 July 2018

Abstract: P-type binary copper oxide semiconductor films for various O2 flow rates and total pressures (Pt) were prepared using the reactive magnetron sputtering method. Their morphologies and structures were detected by X-ray diffraction, Raman spectrometry, and SEM. A phase diagram with Cu2O, Cu4O3, CuO, and their mixture was established. Moreover, based on Kelvin Probe Force Microscopy (KPFM) and conductive AFM (C-AFM), by measuring the contact potential difference (VCPD) and the field emission property, the work function and the carrier concentration were obtained, which can be used to distinguish the different types of copper oxide states. The band gaps of the Cu2O, Cu4O3, and CuO thin films were observed to be (2.51 ± 0.02) eV, (1.65 ± 0.1) eV, and (1.42 ± 0.01) eV, 3 respectively. The resistivities of Cu2O, Cu4O3, and CuO thin films are (3.7 ± 0.3) × 10 Ω·cm, (1.1 ± 0.3) × 103 Ω·cm, and (1.6 ± 6) × 101 Ω·cm, respectively. All the measured results above are consistent.

Keywords: binary copper oxide; phase structure; band gap; contact potential difference

1. Introduction P-type binary copper oxide semiconductors with different morphologies and copper oxidation states have three distinct phases: cuprous oxide (Cu2O), paramelaconite (Cu4O3), and tenorite (CuO) [1,2]. They have great application potential in thin-film devices such as solar cell [3] and thin-film lithium-ion battery [2]. Many efforts have been made to further understand the thin film physical properties in theoretical calculations [1,4,5] and experiments [6–9]. The crystal symmetries of Cu2O, Cu4O3, and CuO vary from cubic to tetragonal and monoclinic, resulting in the diversity of optical and electronic properties. The band structure of Cu2O, with a direct gap range from 2.1 to 2.6 eV [7,10–12], was experimentally well established. Although Cu2O has the advantage of good transparency in the visible light range, its low carrier concentration or large resistivity leads to poor performances [3,10]. The second oxide phase, Cu4O3, discovered during the late 1870s [13], is a metastable mixed-valence intermediate compound between Cu2O and CuO [1,4,9,14,15]. To date, research about the electronic structure of Cu4O3 has been limited. The estimated band gap by optical methods varies from 1.3 to

Materials 2018, 11, 1253; doi:10.3390/ma11071253 www.mdpi.com/journal/materials Materials 2018, 11, 1253 2 of 13

2.5 eV, depending on whether a direct or indirect gap was assumed for the analysis [4,14]. Recently, Wang et al. predicted that the indirect band gap of Cu4O3 is 1.59 eV [4]. As for CuO, the type of band gap of CuO remains controversial; in some studies its band gap is suggested to be direct [16–18], but it is considered that its band gap is indirect in other studies [1,19,20], and its accurate band gap value is still a greater challenge for electronic structure calculations. Therefore, there is an urgent need to verify the calculated electronics structure of binary copper oxides through experiments. Various methods have been used to prepare binary copper oxides thin films. They include thermal oxidation [21,22], spray-coating [23], pulsed laser deposition [24,25], electrochemical deposition [26], and reactive sputtering [9,11,12,14]. Among those methods, magnetron sputtering at room temperature is desirable for the growth of thin films with good physical properties. Moreover, one can easily deposit the three types of binary copper oxides or their mixed phases by merely tuning the oxygen partial pressure during depositions [9,14,15]. The oxygen partial pressure during depositions does influence the oxygen chemical potential inside the deposition chamber. On one hand, the films deposited under lower oxygen partial pressure + tend to form the Cu2O phase which contains only Cu , and higher oxygen partial pressure will further oxidate Cu+ into Cu2+, resulting in the formation of the CuO phase. The calculated phase stability of the copper oxide system indicates that Cu4O3 is a metastable state [1,4], which means that the processing window of O2 flow to synthesize Cu4O3 is extremely narrow. Consequentially, the critical parameters for the synthesis of the Cu4O3 metastable phase need insightful exploration. On the other hand, the physical properties of thin films (such as preferred orientation, optical band gaps, mobilities, and carrier concentrations) can also be tuned by changing the oxygen partial pressure during deposition [9,14,15]. The effects during depositions of oxygen chemical potential on the films’ physical properties still need to be investigated further. In this work, binary copper oxide thin films including Cu2O, Cu4O3, and CuO were prepared by DC magnetron sputtering under different oxygen partial pressures. The crystal structures of those binary copper oxide films were studied using XRD and Raman spectra; band gaps were measured by introducing a UV–vis spectrophotometer; and the nanoscale electrical property was investigated by conductive AFM (C-AFM). Additionally, the oxide states of Cu on the film’s surface were determined by Kelvin Probe Force Microscopy (KPFM). It is hoped that these experimental results can facilitate the better understanding of the thin film growth mechanism and the tuning effect of physics properties of binary copper oxide thin films.

2. Experiments The binary copper oxide films were grown at room temperature by reactive magnetron sputtering. In the experiment, a Cu target of 2 inches with 99.999% purity was used. By using deionized water, acetone, and methanol, the glass substrates were rinsed ultrasonically. By blowing nitrogen gas, these substrates were dried in case of deposition. Then, the substrates were installed on a holder 10 cm away from the target. The rotation rate of 15 rpm was fixed during the deposition. The vacuum chamber was evacuated until the base pressure reached 4 × 10−4 Pa. The operating pressure varies from 0.5 to 2.0 Pa. Mixed argon–oxygen was used as the reactive gas. The oxygen flow rate was changed from 1 to 30 sccm, while the argon flow rate was kept at 50 sccm. A fixed DC power of 160 W was used for all the films. The deposition time was set to be 10 min. In order to measure the optical characterization, transparent glass substrates were used. However, in order to obtain J–E curves, a conductive substrate (ITO—indium tin oxide) was also considered. The crystalline structures of the films were measured using XRD (RigakuMiniFlex II, Rigaku, Tokyo, Japan) with Cu Kα radiation of λ = 1.5418 Å and using Raman spectroscopy (HORIBA Jobin Yvon Evolution, Jobin Yvon, Paris, France). The scanning speed of XRD characterization was set to 5◦/min in order to ensure sufficiently strong diffraction intensity. The surface morphologies were observed using SEM (Hitachi SU-8010, Tokyo, Japan). Based on Dektak XT (Bruker, Hamburg, Germany), the thickness of the films was obtained. Using a UV–vis spectrophotometer (Shimadzu Materials 2018, 11, 1253 3 of 13

UV-Vis 2450, Kyoto, Japan), we measured the optical reflectance and transmission spectra. The work functionsMaterials and 2018 I–V, 11 curves, x FOR PEER were REVIEW recorded using KPFM (Bruker Dimension Icon, Hamburg, Germany)3 of 13 and C-AFM measurements, respectively (AFM, Bruker Dimension Icon, Hamburg, Germany). In order functions and I–V curves were recorded using KPFM (Bruker Dimension Icon, Hamburg, Germany) to avoid the influence of moisture and gas absorption on the measured results, the whole AFM was and C-AFM measurements, respectively (AFM, Bruker Dimension Icon, Hamburg, Germany). In put intoorder a glove to avoid box the with influence water andof moisture oxygen and content gas ab <0.1sorption ppm. on the measured results, the whole AFM was put into a glove box with water and oxygen content <0.1 ppm. 3. Results and Discussion 3. Results and Discussion Figure1a shows the XRD patterns for pure phase Cu 2O, Cu4O3, and CuO deposited at 0.5 Pa with the flowingFigure rates1a shows of 8 sccm,the XRD 14 patterns sccm, and for 24pure sccm, phase respectively. Cu2O, Cu4O3 From, and CuO the figure, deposited one at can 0.5notice Pa that thewith peaks the flowing of the threerates of samples 8 sccm, 14 are sccm, consistent and 24 sc withcm, respectively. those characteristic From the figure, of the one cuprous can notice oxide, paramelaconite,that the peaks and tenoriteof the three phases samples (JCPDS are consistent NO. 65-2388, with 49-1830, those characteristic and 65-2309), of the respectively. cuprous oxide, All the observedparamelaconite, diffraction peaks and tenorite are summarized phases (JCPDS in Table NO.1 65-2. Raman388, 49-1830, spectra and were 65-2309), also introduced respectively. to All confirm the the filmobserved structure diffraction and detect peaks the traceare summarized impurity. Asin shownTable 1. in Raman Figure spectra1b, all thewere Raman also introduced peaks marked to confirm the film structure and detect the trace impurity. As shown in Figure 1b, all the Raman peaks using vertical bars agree well with experiments [15] and with previous calculations [27]. The XRD and marked using vertical bars agree well with experiments [15] and with previous calculations [27]. The Raman results indicate that the three types of Cu2O, Cu4O3, and CuO films can be prepared through XRD and Raman results indicate that the three types of Cu2O, Cu4O3, and CuO films can be prepared magnetron sputtering by only tuning the O flowing rate. through magnetron sputtering by only tuning2 the O2 flowing rate.

Figure 1. The XRD (a) and Raman spectra (b) of Cu2O, Cu4O3, and CuO deposited at 0.5 Pa with the Figure 1. The XRD (a) and Raman spectra (b) of Cu2O, Cu4O3, and CuO deposited at 0.5 Pa with the ﬂow ratesflow of rates 8 sccm, of 8 sccm, 14 sccm, 14 sccm, and 24and sccm, 24 sccm, respectively. respectively.

By using XRD and Raman measurements, the phase diagram of CuxOy, deposited under Bydifferent using XRD O2 flow and rates Raman and measurements, total pressures, theis shown phase on diagram Figure 2a. of CuFromxO ythe, deposited figure, we under can see different that O2 flowthe rates increase and totalof thepressures, oxygen flowing is shown rate at on 0.5 Figure Pa results2a. Fromin the theevolution figure, from we pure cansee Cu2 thatO, to thea mixture increase of the oxygenof Cu2O flowing and Cu rate4O3, to at pure 0.5 Pa Cu results4O3, to ina themixture evolution of Cu4 fromO3 and pure CuO, Cu and2O, to pure a mixture CuO. ofHowever, Cu2O and further increase of the O2 flow rate will give rise to the deterioration of the film crystallinity of CuO. Cu4O3, to pure Cu4O3, to a mixture of Cu4O3 and CuO, and to pure CuO. However, further increase This is consistent with previous results [9,14]. of the O2 flow rate will give rise to the deterioration of the film crystallinity of CuO. This is consistent As the total pressure is enhanced to 1.0 Pa, the processing windows of O2 flow rate to synthesize with previous results [9,14]. a mixture of Cu2O and Cu4O3 disappeared, and the O2 flow processing window for pure-phase Cu2O As the total pressure is enhanced to 1.0 Pa, the processing windows of O2 flow rate to synthesize and Cu4O3 became narrower. Moreover, the pure phase domains of Cu2O and Cu4O3 are moved to a mixture of Cu2O and Cu4O3 disappeared, and the O2 flow processing window for pure-phase Cu2O

Materials 2018, 11, x FOR PEER REVIEW 4 of 13 lowerMaterials O20182 flow, 11 ,rate 1253 magnitude, which indicates that, for the larger total pressure, a lower oxygen 4flow of 13 rate can produce copper oxide with higher valence. Additionally, it is worth mentioning that the pure Cu4O3 and Cu2O phases disappear at 1.5 Pa and 2.0 Pa, respectively. It is also found that the phases and Cu4O3 became narrower. Moreover, the pure phase domains of Cu2O and Cu4O3 are moved to are represented by Cu2O, Cu4O3, CuO, Cu, and their mixtures, which means that grains of lower O2 flow rate magnitude, which indicates that, for the larger total pressure, a lower oxygen flow intermediate composition CuxOy may not present under such deposition conditions. As seen in Figure rate can produce copper oxide with higher valence. Additionally, it is worth mentioning that the pure 2b, the film thicknesses deposited with 0.5 Pa, 1.0 Pa, 1.5 Pa, and 2.0 Pa under 1 sccm O2 flow rate Cu O and Cu O phases disappear at 1.5 Pa and 2.0 Pa, respectively. It is also found that the phases are were4 obtained3 2 at about 700 nm, 620 nm, 550 nm, and 400 nm, respectively. Moreover, the film represented by Cu O, Cu O , CuO, Cu, and their mixtures, which means that grains of intermediate thickness is reduced2 with4 increasing3 oxygen flowing rate for same total pressure, reducing with composition CuxOy may not present under such deposition conditions. As seen in Figure2b, the film increasing total pressure for same O2 flowing rate. thicknesses deposited with 0.5 Pa, 1.0 Pa, 1.5 Pa, and 2.0 Pa under 1 sccm O2 flow rate were obtained at about 700 nm, 620 nm,Table 550 1.nm, The andsummary 400 nm, of diffraction respectively. peaks Moreover, in XRD patterns. the film thickness is reduced with increasing oxygen flowing rate for same total pressure, reducing with increasing total pressure Cu2O Cu4O3 CuO for same O2 flowing rate. 2θ (°) (h k l) 2θ (°) (h k l) 2θ (°) (h k l) 29.6 1Table 1 0 1. The summary 17.6 of diffraction 1peaks 0 1 in XRD patterns. 32.6 1 1 0 36.5 1 1 1 30.7/31.1 2 0 0/1 0 3 35.5/35.7 0 0 2/ 111 42.4 Cu2O 2 0 0 35.6/35.7/36.3 Cu 4O 2 30 2/0 0 4/2 2 0 38.9/39.1CuO 1 1 1/2 0 0 61.5 2θ (◦) 2 (h 2 k1 l) 63.9/65.0 2θ (◦) 4 0 (h 0/2 k l)0 6 2θ 65.6(◦) (h k l) 0 0 2 73.6 36.5 3 11 11 1 30.7/31.1 2 0 0/1 0 3 35.5/35.7 0 0 2/111 42.4 2 0 0 35.6/35.7/36.3 2 0 2/0 0 4/2 2 0 38.9/39.1 1 1 1/2 0 0 At a fixed61.5 argon flow 2 rate, 2 1 the increase 63.9/65.0 of the total 4pressure 0 0/2 0 6 means that 65.6 of the O2 0partial 0 2 pressure. 73.6 3 1 1 The O2 partial pressure influences the morphology of deposited films.

FigureFigure 2. 2. ((aa)) The The schematic schematic deposition deposition diagram diagram of of films films deposited deposited under under different different total total pressure pressure and and OO22 flowflow rate; rate; ( (bb)) film film thickness thickness prepared prepared under under different different conditions. conditions.

TheAt a evolution fixed argon of the flow film rate, morphologies the increase ofunder the totalvarious pressure total pressures means that is ofshown the O in2 partial Figure pressure.3. From theThe figure, O2 partial one notices pressure that influences the surface the roughness morphology of the of depositedbinary copper films. oxide increases with increasing oxygenThe partial evolution pressure. of the The film surface morphologies of the Cu under2O thin various film consists total pressures of a lot of is “spherical” shown in Figure grains,3. while From thethe figure,Cu4O3 oneand notices CuO thatthin the films surface consist roughness of many of the “r binaryoof-type” copper and oxide “pyramidal-shape” increases with increasing grains, respectively.oxygen partial Especially, pressure. the The Cu surface4O3 thin of the films Cu 2depositedO thin film under consists 1.5 ofPa a contain lot of “spherical” the CuO phase grains, which while

Materials 2018, 11, 1253 5 of 13

the Cu4O3 and CuO thin ﬁlms consist of many “roof-type” and “pyramidal-shape” grains, respectively. Materials 2018, 11, x FOR PEER REVIEW 5 of 13 Especially, the Cu4O3 thin ﬁlms deposited under 1.5 Pa contain the CuO phase which forms many

“pimples”forms onmany top “pimples” of the Cu4 onO3 top“roof”. of the As Cu shown4O3 “roof”. in Figure As shown3d, an in EDX Figure compositional 3d, an EDX analysiscompositional of Cu 4O3 depositedanalysis at of 0.5 Cu Pa4O and3 deposited 1.0 Pa indicatesat 0.5 Pa and that 1.0 Cu-to-O Pa indicates atomic that ratios Cu-to-O are atomic 1.26:1 andratios 1.27:1, are 1.26:1 respectively, and which1.27:1, is close respectively, to the stoichiometric which is close ratio to withthe stoich 1.33:1.iometric However, ratio thewith Cu-to-O 1.33:1. However, atomic ratio the ofCu-to-O deposited filmsatomic at 1.5 ratio Pa deviates of deposited from films 1.26:1, at 1.5 which Pa deviates indicates from that 1.26:1, CuO which phase indicates may existthat CuO in the phase Cu4 mayO3 films. In addition,exist in the Cu existence4O3 films. of In a CuOaddition, impurity the existence phase wasof a CuO also confirmedimpurity phase by thewas following also confirmed optical by band characterization.the following The optical morphology band characterization. of pure-phase The thin morphology films is closely of pure-phase related tothin their films crystal is closely structure, related to their crystal structure, which was discussed in detail in other studies [9]. From our which was discussed in detail in other studies [9]. From our measured results, it is suggested that measured results, it is suggested that binary copper oxide films with fine electrical quality should be binaryprepared copper under oxide lower films total with pressure. fine electrical quality should be prepared under lower total pressure.

FigureFigure 3. SEM 3. SEM images images for the for morphology the morphology evolution evolution of the of deposited the deposited ﬁlms films under under various various total pressurestotal pressures for (a) Cu2O; (b) Cu4O3, and (c) CuO; (d) the EDX characterization of Cu4O3 films deposited for (a) Cu2O; (b) Cu4O3, and (c) CuO; (d) the EDX characterization of Cu4O3 ﬁlms deposited under under 0.5 Pa, 1.0 Pa, and 1.5 Pa, respectively. 0.5 Pa, 1.0 Pa, and 1.5 Pa, respectively.

MaterialsMaterials2018 ,201811, 1253, 11, x FOR PEER REVIEW 66 of of 13 13

The optical band gaps of Cu2O, Cu4O3, and CuO were also analyzed. The transmittance and reflectanceThe optical spectra band for gaps different of Cu 2copperO, Cu 4oxidesO3, and depo CuOsited were under also various analyzed. total pressures The transmittance are present and in reﬂectanceFigure 4. spectra By using for differentthe Tauc copperrelation, oxides one can deposited estimate underthe Eg variousvalues from total the pressures transmittance are present and in Figurereflectance4. By [12,28]: using the Tauc relation, one can estimate the Eg values from the transmittance and reﬂectance [12,28]: n (αhυn ) = A(hυ − E ) (1) (αhυ) = A(hυ − Eg) g (1) wherewherehν is h theν is incidentthe incident photon photon energy, energy, and andA is A a is constant a constant related related to theto the materials. materials. The The magnitudes magnitudes of n areof considered n are considered to be 2,to 1/2,be 2, 3,1/2, and 3, and 3/2 3/2 corresponding corresponding to to allowed allowed direct, direct,allowed allowed indirect, forbidden forbidden direct,direct, and and forbidden forbidden indirect indirect transitions, transitions, respectively. respectively.

55 80 50 70 0.5Pa Cu2O 0.5 Pa 1.0Pa 45 1.0 Pa 60 1.5Pa 1.5 Pa 40 50 40 35 30 30 T/% R/% 20 25 10 20 0 15 -10 10 400 500 600 700 800 900 400 500 600 700 800 900 Wavelength(nm) Wavelength(nm) 50 40 0.5 Pa Cu O 0.5Pa 40 4 3 1.0 Pa 35 1.0Pa 1.5 Pa 1.5Pa 30 30

20 25 T/% R/% 10 20

15 0 10 400 500 600 700 800 900 400 500 600 700 800 900 Wavelength(nm) Wavelength(nm) 60 45 0.5 Pa CuO 0.5 Pa 50 1.0 Pa 40 1.0 Pa 1.5 Pa 1.5 Pa 40 35

30 30

T/% 20 R/% 25

10 20

0 15

400 500 600 700 800 900 400 500 600 700 800 900 Wavelength(nm) Wavelength(nm)

FigureFigure 4. The 4. The transmittance transmittance and and reﬂectance reflectance spectra spectra of of Cu Cu2O,2O, Cu Cu4O4O33,, andand CuOCuO thinthin ﬁlmsfilms deposited underunder various various total total pressures. pressures.

Here, for CuO, the indirect band gap is considered, so n = 1/2. Moreover, Cu2O and Cu4O3 are Here, for CuO, the indirect band gap is considered, so n = 1/2. Moreover, Cu O and Cu O are supposed to a direct transition so n = 2 is considered [1,4,19]. The absorption coefficient2 α 4can3 be n supposedobtained to based a direct on transitionfollowing relation: so = 2 is considered [1,4,19]. The absorption coefﬁcient α can be obtained based on following relation: " # 1 (1 − R)2 α = ln (2) d T

Materials 2018, 11, x FOR PEER REVIEW 7 of 13

− 2 α = 1 (1 R)  Materials 2018, 11, 1253 ln  7(2) of 13 d  T  where d is the thickness of the film, and R and T are the reflectance and transmittance. where d is the thickness of the film, and R and T are the reflectance and transmittance. n Figure 5 presents the photon energy dependence of the (αhν)n values. The calculated optical Eg Figure5 presents the photon energy dependence of the ( αhν) values. The calculated optical Eg values can be obtained as 2.51 ± 0.02 eV, 1.65 ± 0.1 eV, and 1.42 ± 0.01 eV for Cu2O, Cu4O3, and CuO, values can be obtained as 2.51 ± 0.02 eV, 1.65 ± 0.1 eV, and 1.42 ± 0.01 eV for Cu2O, Cu4O3, and CuO, respectively. These are consistent with the previous reported results [4,2,7,12,15,28]. Furthermore, the respectively. These are consistent with the previous reported results [2,4,7,12,15,28]. Furthermore, measured results of the band gap indicate that, although the morphologies of the films under various the measured results of the band gap indicate that, although the morphologies of the films under O2 partial pressures are different, the band gap value of each type of single-phase copper oxide various O2 partial pressures are different, the band gap value of each type of single-phase copper oxide remains almost constant. This informs us that the band gap of binary copper oxide films can be tuned remains almost constant. This informs us that the band gap of binary copper oxide films can be tuned by controlling the ratio of Cu2O/Cu4O3/CuO in the mixed phase. by controlling the ratio of Cu2O/Cu4O3/CuO in the mixed phase.

Figure 5. (αhν)n as a function of photon energy (hν) for pure-phase Cu2O, Cu4O3, and CuO thin films. n Figure 5. (αhν) as a function of photon energy (hν) for pure-phase Cu2O, Cu4O3, and CuO thin ﬁlms.

Compared with XPS, Raman, and FTIR with spatial resolution at the micrometer scale, the KPFM measured method allows us to distinguish between the Cu oxide states with nanometer resolution, andMaterials to observe2018, 11, 1253the local morphology of thin films simultaneously [29]. There exists a contact potential8 of 13 difference (VCPD) between the scanning tip and the surface of sample; VCPD can be described as follows [30]: and to observe the local morphology of thin ﬁlms simultaneously [29]. There exists a contact potential ϕϕ− difference (VCPD) between the scanning tip and() thetip surface s of sample; VCPD can be described as V = (3) follows [30]: CPD q (ϕtip − ϕs) VCPD = (3) where φs is the work function of the sample, φtip is thatq of the tip, and q is the electronic charge. By measuring the work function of a standard sample (such as Au), the magnitude of φtip can be gained. where ϕs is the work function of the sample, ϕtip is that of the tip, and q is the electronic charge. Therefore, according to Equation (3), by measuring the value of VCPD, φs can be determined. By measuring the work function of a standard sample (such as Au), the magnitude of ϕtip can be Figure 6a,b present VCPD and the work function distribution on the respective surfaces of Cu2O, gained. Therefore, according to Equation (3), by measuring the value of VCPD, ϕs can be determined. Cu4O3, and CuO thin films. These data were obtained inside a 1 × 0.3 (μm)2 scanning region on the Figure6a,b present VCPD and the work function distribution on the respective surfaces of Cu2O, surface of the films, and the measured mean VCPD values for Cu2O, Cu4O3, 2and CuO thin films are Cu4O3, and CuO thin films. These data were obtained inside a 1 × 0.3 (µm) scanning region on the 231.0 mV, 98.5 mV, and 8.7 mV, respectively. According to Equation (3), the positive VCPD values surface of the films, and the measured mean VCPD values for Cu2O, Cu4O3, and CuO thin films are indicate that the work functions of the thin films are lower than the value of φtip. The results indicate 231.0 mV, 98.5 mV, and 8.7 mV, respectively. According to Equation (3), the positive VCPD values indicate that the thin films of CuO and Cu4O3 containing Cu2+ have lower surface potential. From Figure 6b, that the work functions of the thin films are lower than the value of ϕtip. The results indicate that the it is found that φφ<< φ, which2+ is consistent with other experimental results [29]. In thin films of CuO andCu24 O Cu4 CuO3 O3containing CuO Cu have lower surface potential. From Figure6b, it is found < < addition,that φCu2 Othe copperφCu4O3 oxideφCuO state, which can be is identifi consistented with with KPFM other by experimental a corresponding results measurement [29]. In addition, VCPD valuethe copper range oxide or work state canfunctions, be identified and KPFM with KPFM facilit byates a correspondingthe undamaged measurement characterizationVCPD value of the range Cu oxidationor work functions, state on andbinary KPFM copper facilitates oxide the thin undamaged film surfaces, characterization which should of the have Cu oxidationwide application state on prospects.binary copper oxide thin film surfaces, which should have wide application prospects. To furtherfurther studystudy the the electronic electronic properties properties of of the th binarye binary copper copper oxide oxide thin thin ﬁlms, films, we usedwe used the C-AFM the C- AFMmeasurement measurement system, system, as seen as seen in Figure in Figure7a. Here, 7a. Here, a conductive a conductive tip (tipR c(R≈c ≈20 20 nm nm and andk k= = 2.82.8 N/m)N/m) was used and a constant force (150 nN) was applied. This is similar to a tip-to-sample space mold in measuring J–E [31].[31].

Figure 6. The VCPD (a) and work function distribution (b) on the surface of Cu2O, Cu4O3, and CuO thin Figure 6. The VCPD (a) and work function distribution (b) on the surface of Cu2O, Cu4O3, and CuO films.thin ﬁlms.

Our studied Tip–CuO–base should belong to the metal–insulator–metal (MIM) system. For this Our studied Tip–CuO–base should belong to the metal–insulator–metal (MIM) system. For this MIM case, a nonresonant tunnel transport has been established [31,32]. There exists a metal–insulator MIM case, a nonresonant tunnel transport has been established [31,32]. There exists a metal–insulator contact barrier ϕ produced by the insulator in MIM. Now, a bias voltage V is applied to the MIM contact barrier ϕ produced by the insulator in MIM. Now, a bias voltage V is applied to the MIM system. system. Then, as the value of ϕ is less than qV, an injection tunnel current will be produced. Then, as the valueϕ of ϕ is less than qV, an injection tunnel current will be produced. However, as ϕ > qV, However,a direct current as will > qV arise., a direct In order current to analyze will arise. the properties In order ofto the analyze ﬁeld emission,the properties the following of the fieldF–N emission,equation isthe generally following used F–N [31 equation–34]: is generally used [31–34]:

A β2E2 −Bϕ3/2 J = exp( ) (4) φs βE

Materials 2018, 11, x FOR PEER REVIEW 9 of 13

AEβ 22 −Bϕ 3/2 Materials 2018, 11, 1253 J = exp( ) 9 of 13 φβ (4) s E

Equation (4) can be rewritten asas thethe following:following: βϕ23/2 JABJ A β2 Bϕ3/2 11 ln(( ) ) ==− ln(( ) )− ( () ) (5) ln 22 ln ϕβ (5) E ϕss β EE

ϕϕ=− ϕ −−2 where ϕ = ϕtiptip − ϕ s s; ;EE isis the the applied electric ﬁeld;field; JJ isis thethe currentcurrent densitydensity (A(A·cm·cm );); ββ isis the ﬁeldfield enhancement factor; and A andand BB areare constants.constants.

Copper oxide Conductive base

(a)

0.0008

Cu2O Cu O 0.0006 4 3 CuO )

2 0.0004

0.0002 J /(A/m

0.0000

0 5 10 15 20 E /(V/μm) (b)

Cu O -12 injection tunnel 2 Cu4O3 CuO

) -14 2

ln(J/E -16 direct tunnel

-18

0.0 0.5 1.0 1.5 2.0 2.5 3.0 1/E

(c) Figure 7. (a) A sketch of the C-AFM measurement system; (b) J–E curves of field emission; (c) ln(J/E2) Figure 7. (a) A sketch of the C-AFM measurement system; (b) J–E curves of ﬁeld emission; (c) ln(J/E2) versus 1/E plots of samples 1–4. versus 1/E plots of samples 1–4.

Materials 2018, 11, 1253 10 of 13

2 Figure7b,c present the J–E curves and their ln(J/E ) versus 1/E plots of Cu2O, Cu4O3, and CuO, respectively. Both clear direct and injection tunnel regimes can be found in the figure. As found in Equation (5), according to the slope of ln(J/E2) versus 1/E plots, the ϕ information can be acquired. From Figure7c, we can see that the slope of ln( J/E2) versus 1/E plots in the injection region increases in the order of Cu2O, Cu4O3, and CuO, which means that the value of ϕ is reduced in this order. That < < is, φCu2O φCu4O3 φCuO, which is consistent with the observed result in Figure5b. Moreover, in the direct tunnel region, it is found that, compared with Cu2O film, the current density J is evidently enhanced for Cu4O3 and CuO thin films, which indicates that the carrier concentration at room temperature increases for thin films deposited under higher O2 partial pressure. The above result is related to the observed fact of the band gap in Figure5. Finally, the resistivities of Cu2O, Cu4O3, and CuO thin films measured by the four-point probe method are (3.7 ± 0.3) × 103 Ω·cm, (1.1 ± 0.3) × 103 Ω·cm, and (1.6 ± 0.6) × 101 Ω·cm, respectively. > > Clearly, ρCu2O ρCu4O3 ρCuO. The resistivity values of CuO thin films are nearly 2 magnitudes less than those of Cu2O and Cu4O3, which should be attributed to the higher intrinsic carrier density of CuO [28,35,36]. The measured result indicates that the Cu2O film with the largest resistivity has the largest band gap and the least Cu valence state, while the CuO film with the least resistivity has the smallest band gap and the largest Cu valence state. All the measured results above are consistent. The evolution of Cu valence states and the thickness of binary copper oxide films are typically affected by total pressure, O2 flow rate, substrate temperature, and so on. At room temperature, the interplay of total pressure and O2 flow rate leads to the complex change of the phase. The phase diagram and corresponding thickness change in Figure2 should be associated with the deposition rate and energy of impinged atoms. For the same O2 flow rate, the low deposition rate for a high Pt value gives rise to the decrease in the incoming atom flow. High total pressure can reduce the contribution of the atomic bombardment, because the collisions of the sputtering atoms are enhanced. As a result, with increasing total pressure, the deposition thickness is decreased. Usually, a larger O2 flow rate can lead to higher energy of negative oxygen ions (O−)[37], which indicates that the bombardment effect on the deposition surface should be severer in binary copper oxide films with larger O2 flowing rate. Thus, for the same total pressure, with increasing O2 flow rate, the deposition thickness is decreased. − However, on the other hand, higher energy of O under larger O2 flow rate can give rise to a more + − sufficient reaction between Cu and O . As a consequence, at low O2 flow rate, Cu2O phase is mainly − formed due to insufficient O2 and lower energy of O . With increasing O2 flow rate, the reaction between Cu+ and O− is gradually enhanced, which leads to some of Cu+1 being oxidized to become +2 Cu . Thus, Cu4O3 phase (Cu2O + 2CuO) is formed. Similarly, a larger O2 flow rate can lead to all of Cu+1 being oxidized to Cu+2, which gives rise to the formation of pure CuO. Based on the measured results of the band gap and work function in Figures5 and6, an illustration of the band diagrams of Cu2O, Cu4O3, and CuO films is presented in Figure8. From the figure, it is found that the magnitudes of the band gap for Cu2O, Cu4O3, and CuO films are consistent with the other experimental results [38]. However, the experimental gap for Cu2O is in good agreement with that calculated based on hybrid functional calculations, while there are discrepancies between experiment and theory for CuO and Cu4O3 [1]. This may be associated with the defects in the prepared films, which need to be clarified by further experimental and theoretical investigations. The developments of film characterization techniques supply more tools to produce insight into the microscopic mechanism of physical properties for the films. Here, we introduced a nondestructive characterization approach, KPFM, to distinguish the surface electronic states depending on the composition. In general, the moisture, surface charge, absorption, and so on can evidently influence the measured accuracy of the work function [39–41]. Thus, in the measuring process, these adverse factors should be overcome. The direct valence measurement by X-ray photoelectron spectroscopy (XPS) can detect not only the information from the film’s surface, but also a depth of penetration. Therefore, the integration of KPFM with XPS may be a tremendously exciting endeavor. Materials 2018, 11, 1253 11 of 13 Materials 2018, 11, x FOR PEER REVIEW 11 of 13

Conduction band 5.10 eV 5.10 4.96 eV 5.19 eV

≈ Conduction band ≈

≈ Conduction band

E ≈2.51 eV CuO Cu2O g Cu4O3

f E ≈1.65 eV φ φ E Ef g f Eg≈1.42 eV

φ E

Valence band Valence band Valence band

Cu2OCuOCu4O3

Figure 8. The band diagrams of Cu2O, Cu4O3, and CuO thin films. Figure 8. The band diagrams of Cu2O, Cu4O3, and CuO thin ﬁlms.

4. ConclusionsThe developments of film characterization techniques supply more tools to produce insight into the microscopic mechanism of physical properties for the films. Here, we introduced a nondestructiveThe Cu2O, Cucharacterization4O3, and CuO filmsapproach, were preparedKPFM, to through distinguish magnetron the sputteringsurface electronic by changing states the dependingO2 flowing on rate the and composition. total pressure. In general, The phase the mo diagramsisture, surface and morphologies charge, absorption, of Cu2O, and Cu 4soO 3on, CuO, can evidentlyand their mixturesinfluence were the measured established accuracy by structural of the analysis work usingfunction XRD, [39–41]. SEM, andThus, Raman in the spectrometry. measuring process,One notices these that adverse the binary factors copper should oxide be films over withcome. fine The electrical direct qualityvalence should measurement be prepared by underX-ray photoelectronlower total pressure. spectroscopy Moreover, (XPS) the can contact detect potentialnot only the difference information and the from field the emission film’s surface, property but werealso < < ameasured depth of penetration. by KPFM and Therefore, conductive the AFM(C-AFM). integration of ItKPFM is found with that XPSφCu may2O beφ aCu tremendously4O3 φCuO. The exciting band endeavor.gaps of Cu 2O, Cu4O3, and CuO thin films were observed to be 2.51 ± 0.02 eV, 1.65 ± 0.1 eV, and 1.42 ± 0.01 eV, respectively. The resistivity values of the Cu2O, Cu4O3, and CuO thin films are (3.7 ± 0.3) 4.× Conclusions103 Ω·cm, (1.1 ± 0.3) × 103 Ω·cm, and (1.6 ± 0.6) × 101 Ω·cm, respectively. Moreover, the measured results indicate that the Cu2O film with the largest resistivity has the largest band gap and the least Cu The Cu2O, Cu4O3, and CuO films were prepared through magnetron sputtering by changing the valence state, while the CuO film with the least resistivity has the smallest band gap and the largest O2 flowing rate and total pressure. The phase diagrams and morphologies of Cu2O, Cu4O3, CuO, and Cu valence state. All the measured results above are consistent. their mixtures were established by structural analysis using XRD, SEM, and Raman spectrometry. OneAuthor notices Contributions: that the binaryConceptualization, copper oxide W.Z., films Y.C. with and fine Z.H.; electrical Methodology, quality W.Z., should Y.C., X.P.;be prepared Formal Analysis, under lowerK.Z., Y.L. total and pressure. Z.H.; Investigation, Moreover, W.Z.,the contact Y.C., Y.L. potential and Z.H.; difference Writing-Original and the Draft field Preparation,emission property W.Z., Y.C. were and Z.H.; Writing-Review & Editing, Z.H. measured by KPFM and conductive AFM(C-AFM). It is found that φφ<< φ. The band Cu24 O Cu O3 CuO Acknowledgments: This research was funded by the Natural Science Foundations of China (No. 61574037, gaps21203025, of Cu 11344008,2O, Cu4O 11204038).3, and CuO thin films were observed to be 2.51 ± 0.02 eV, 1.65 ± 0.1 eV, and 1.42 ± 0.01 eV, respectively. The resistivity values of the Cu2O, Cu4O3, and CuO thin films are (3.7 ± 0.3) × Conflicts of Interest: The authors declare no conflict of interest. 103 Ω·cm, (1.1 ± 0.3) × 103 Ω·cm, and (1.6 ± 0.6) × 101 Ω·cm, respectively. Moreover, the measured resultsReferences indicate that the Cu2O film with the largest resistivity has the largest band gap and the least Cu valence state, while the CuO film with the least resistivity has the smallest band gap and the largest1. Heinemann, Cu valence M.; state. Eifert, All B.; the Heiliger, measured C. Band results structure above and are phase consistent. stability of the copper oxides Cu2O, CuO, and Cu4O3. Phys. Rev. B 2013, 87, 115111. [CrossRef] Author2. Zoolfakar, Contributions: A.S.; Rani, Conceptualization, R.A.; Morfa, A.J.; W.Z., O’Mullane, Y.C. and A.P.;Z.H.; Kalantarzadeh,Methodology, W.Z., K. Nanostructured Y.C., X.P.; Formal copper Analysis, oxide K.Z., Y.L. and Z.H.; Investigation, W.Z., Y.C., Y.L. and Z.H.; Writing-Original Draft Preparation, W.Z., Y.C. and semiconductors: A perspective on materials, synthesis methods and applications. J. Mater. Chem. C 2014, 2, Z.H.; Writing-Review & Editing, Z.H. 5247–5270. [CrossRef] Acknowledgments:3. Musselman, K.P.; This Marin, research A.; was Schmidt-Mende, funded by the L.; Natura Macmanus-Driscoll,l Science Foundations J.L. Incompatible of China length(No. 61574037, scales in

21203025,nanostructured 11344008, 11204038). Cu2O solar cells. Adv. Funct. Mater. 2012, 22, 2202–2208. [CrossRef] Conflicts4. Wang, of Y.;Interest: Lany,S.; The Ghanbaja, authors J.;declare Fagot-Revurat, no conflict Y.; of Chen, interest. Y.P.; Soldera, F.; Horwat, D.; Mucklich, F.; Pierson, J.F. Electronic structures of Cu2O, CuO, and Cu4O3: A joint experimental and theoretical study. Phys. Rev. B References2016, 94 , 245418. [CrossRef] 5. Ghijsen, J.; Tjeng, L.H.; Van, E.J.; Eskes, H.; Westerink, J.; Sawatzky, G.A.; Czyzyk, M.T. Electronic structure 1. Heinemann, M.; Eifert, B.; Heiliger, C. Band structure and phase stability of the copper oxides Cu2O, CuO, of Cu2O and CuO. Phys. Rev. B 1988, 38, 11322. [CrossRef] and Cu4O3. Phys. Rev. B 2013, 87, 115111. 2. Zoolfakar, A.S.; Rani, R.A.; Morfa, A.J.; O’Mullane, A.P.; Kalantarzadeh, K. Nanostructured copper oxide semiconductors: A perspective on materials, synthesis methods and applications. J. Mater. Chem. C 2014, 2, 5247–5270.

Materials 2018, 11, 1253 12 of 13

6. Liu, A.; Nie, S.; Liu, G.; Zhu, H.; Zhu, C.; Shin, B.; Fortunato, E.; Martins, R.; Shan, F. In situ one-step synthesis of p-type copper oxide for low-temperature, solution-processed thin-ﬁlm transistors. J. Mater. Chem. C 2017, 5, 2524–2530. [CrossRef] 7. Matsuzaki, K.; Nomura, K.; Yanagi, H.; Kamiya, T.; Hirano, M.; Hosono, H. Epitaxial growth of high

mobility Cu2O thin ﬁlms and application to p-channel thin ﬁlm transistor. Appl. Phys. Lett. 2008, 93, 202107. [CrossRef] 8. Subramaniyan, A.; Perkins, J.D.; O’Hayre, R.P.; Lany, S.; Stevanovic, V.; Ginley, D.S.; Andriy, D.Z.

Non-equilibrium deposition of phase pure Cu2O thin ﬁlms at reduced growth temperature. Appl. Mater. 2014, 2, 022105. [CrossRef] 9. Wang, Y.; Ghanbaja, J.; Soldera, F.; Migot, S.; Boulet, P.; Horwat, D.; Mücklich, F.; Pierson, J.F. Tuning the structure and preferred orientation in reactively sputtered copper oxide thin ﬁlms. Appl. Surf. Sci. 2015, 335, 85–91. [CrossRef] 10. Wang, Y.; Miska, P.; Pilloud, D.; Horwat, D.; Mucklich, F.; Pierson, J.F. Transmittance enhancement and

optical band gap widening of Cu2O thin ﬁlms after air annealing. J. Appl. Phys. 2014, 115, 073505. [CrossRef] 11. Moharam, M.M.; Elsayed, E.M.; Nino, J.C.; Abou-Shahba, R.M.; Rashad, M.M. Potentiostatic deposition

of Cu2O ﬁlms as p-type transparent conductors at room temperature. Thin Solid Films 2016, 616, 760–766. [CrossRef]

12. Dolai, S.; Das, S.; Hussain, S.; Bhar, R.; Pal, A.K. Cuprous oxide (Cu2O) thin ﬁlms prepared by reactive d.c. sputtering technique. Vacuum 2017, 141, 296–306. [CrossRef] 13. Zhao, L.; Chen, H.; Wang, Y.; Che, H.; Gunawan, P.; Zhong, Z.; Hong, L.; Su, F. Facile solvothermal synthesis

of phase-pure Cu4O3 microspheres and their lithium storage properties. Chem. Mater. 2012, 24, 1136–1142. [CrossRef] 14. Pierson, J.F.; Thobor-Keck, A.; Billard, A. Cuprite, paramelaconite and tenorite ﬁlms deposited by reactive magnetron sputtering. Appl. Surf. Sci. 2003, 210, 359–367. [CrossRef] 15. Alajlani, Y.; Placido, F.; Barlow, A.; Chu, H.O.; Song, S.; Rahman, S.U.; Bold, R.D.; Gibson, D. Characterisation

of Cu2O, Cu4O3, and CuO mixed phase thin films produced by microwave-activated reactive sputtering. Vacuum 2017, 144, 217–228. [CrossRef] 16. Ekuma, C.E.; Anisimov, V.I.; Moreno, J.; Jarrell, M. Electronic structure and spectra of CuO. Eur. Phys. J. B 2014, 87, 23. [CrossRef] 17. Tombak, A.; Benhaliliba, M.; Ocak, Y.S.; Kiliçoglu, T. The novel transparent sputtered p-type CuO thin films and Ag/p-CuO/n-Si schottky diode applications. Results Phys. 2015, 5, 314–321. [CrossRef] 18. Akaltun, Y. Effect of thickness on the structural and optical properties of CuO thin films grown by successive ionic layer adsorption and reaction. Thin Solid Films 2015, 594, 30–34. [CrossRef] 19. Rödl, C.; Sottile, F.; Reining, L. Quasiparticle excitations in the photo emission spectrum of CuO from first principles: A GW study. Phys. Rev. B 2015, 91, 045102. [CrossRef] 20. Tripathi, T.S.; Terasaki, I.; Karppinen, M. Anomalous thickness-dependent optical energy gap of ALD-grown ultra-thin CuO films. J. Phys. Condens. Matter 2016, 28, 475801. [CrossRef][PubMed] 21. Liang, J.B.; Kishi, N.; Soga, T.; Jimbo, T.; Ahmed, M. Thin cuprous oxide films prepared by thermal oxidation of copper foils with water vapor. Thin Solid Films 2012, 520, 2679–2682. [CrossRef] 22. Li, A.; Song, H.; Wan, W.; Zhou, J.; Chen, X. Copper oxide nanowire arrays synthesized by in-situ thermal oxidation as an anode material for lithium-ion batteries. Electrochim. Acta 2014, 132, 42–48. [CrossRef] 23. Pattanasattayavong, P.; Thomas, S.; Adamopoulos, G.; Mclachlan, M.A.; Anthopoulos, T.D. P-channel

thin-film transistors based on spray-coated Cu2O films. Appl. Phys. Lett. 2013, 102, 163505. [CrossRef] 24. Kawwam, M.; Alharbi, F.; Aldwayyan, A.; Lebbou, K. Morphological study of PLD grown CuO films on

SrTiO3, sapphire, quartz and MgO substrates. Appl. Surf. Sci. 2012, 258, 9949–9953. [CrossRef] 25. Kawwam, M.; Alharbi, F.H.; Kayed, T.; Aldwayyan, A.; Alyamani, A.; Tabet, N.; Lebbou, K. Characterization of CuO(1 1 1)/MgO(1 0 0) ﬁlms grown under two different PLD backgrounds. Appl. Surf. Sci. 2013, 276, 7–12. [CrossRef]

26. Liau, C.K.; Jhan, J.L. Investigation of rapid thermal annealing on Cu2O properties and n-p Cu2O homojunction performance by electrochemical deposition processing. J. Electrochem. Soc. 2016, 163, D787–D793. [CrossRef] Materials 2018, 11, 1253 13 of 13

27. Debbichi, M.C.; Lucas, M.; Pierson, J.F.; Kruger, P. Vibrational Properties of CuO and Cu4O3 from First-Principles Calculations, and Raman and Infrared Spectroscopy. J. Phys. Chem. C 2012, 116, 10232–10237. [CrossRef] 28. Gan, J.; Venkatachalapathy, V.; Svensson, B.G.; Monakhov, E.V. Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering. Thin Solid Films 2015, 594, 250–255. [CrossRef] 29. Berthold, T.; Benstetter, G.; Frammelsberger, W.; Rodríguez, R.; Nafría, M. Nanoscale characterization of copper oxide films by Kelvin probe force microscopy. Thin Solid Films 2015, 584, 310–315. [CrossRef] 30. Melitz, W.; Shen, J.; Kummel, A.C.; Lee, S. Kelvin probe force microscopy and its application. Surf. Sci. Rep. 2012, 66, 1–27. [CrossRef] 31. Casuso, I.; Fumagalli, L.; Samitier, J.; Padrós, E.; Reggiani, L.; Akimov, V.; Gomila, G. Electron transport through supported biomembranes at the nanoscale by conductive atomic force microscopy. Nanotechnology 2007, 18, 465503. [CrossRef][PubMed] 32. Simmons, J.G. Generalized formula for the electric tunnel effect between similar electrodes separated by a thin insulating film. J. Appl. Phys. 1963, 34, 1793–1803. [CrossRef] 33. Tang, C.M.; Wang, Y.B.; Yao, R.H.; Ning, H.L.; Qiu, W.Q.; Liu, Z.W. Enhanced adhesion and field emission of CuO nanowires synthesized by simply modified thermal oxidation technique. Nanotechnology 2016, 27, 395605. [CrossRef][PubMed] 34. Chen, Y.; Zhang, L.; Zhang, H.T.; Zhong, K.H.; Zhao, G.Y.; Chen, G.L.; Lin, Y.B.; Chen, S.Y.; Huang, Z.G. Band gap manipulation and physical properties of preferred orientation CuO thin films with nano wheatear array. Ceram. Int. 2018, 44, 1134–1141. [CrossRef] 35. Prabu, R.D.; Valanarasu, S.; Kulandaisamy, I.; Ganesh, V.; Shkir, M.; Kathalingam, A. Studies on copper oxide thin films prepared by simple nebulizer spray technique. J. Mater. Sci. Mater. Electron. 2017, 28, 6754–6762. [CrossRef]

36. Murali, D.S.; Kumar, S.; Choudhary, R.J.; Wadikar, A.D.; Jain, M.K.; Subrahmanyam, A. Synthesis of Cu2O from CuO thin films: Optical and electrical properties. AIP Adv. 2015, 5, 047143. [CrossRef] 37. Mahieu, S.; Depla, D. Correlation between electron and negative O−O− ion emission during reactive sputtering of oxides. Appl. Phys. Lett. 2007, 90, 121117. [CrossRef] 38. Meyer, B.K.; Polity, A.; Reppin, D.; Becker, M.; Hering, P.P.; Klar, J.; Sander, T.; Reindl, C.; Benz, J.; Eickhoff, M.; et al. Binary copper oxide semiconductors: From materials towards devices. Phys. Status Solidi B 2012, 249, 1487–1509. [CrossRef] 39. Guo, L.Q.; Zhao, X.M.; Bai, Y.; Qiao, L.J. Water adsorption behavior on metal surfaces and its influence on surface potential studied by in situ SPM. Appl. Surf. Sci. 2012, 258, 9087–9091. [CrossRef] 40. Rodriguez, B.J.; Yang, W.C.; Nemanich, R.J.; Gruverman, A. Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures. Appl. Phys. Lett. 2005, 86, 3522–3526. [CrossRef] 41. Gaman, V.I. Influence of oxygen adsorption on the surface potential of a metal oxide semiconductor. Russ. Phys. J. 2012, 54, 1137–1144. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).