Surface Passivation of Crystalline Silicon Wafer Using H2S Gas

applied sciences

Article Surface Passivation of Crystalline Silicon Wafer Using H2S Gas

Jian Lin 1, Hongsub Jee 1, Jangwon Yoo 1 , Junsin Yi 1, Chaehwan Jeong 2 and Jaehyeong Lee 1,*

1 Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Korea; [email protected] (J.L.); [email protected] (H.J.);[email protected] (J.Y.); [email protected] (J.Y.) 2 Smart Energy & Nano R&D Group, Korea Institute of Industrial Technology, Gwangju 61012, Korea; [email protected] * Correspondence: [email protected]

Abstract: We report the effects of H2S passivation on the effective minority carrier lifetime of crystalline silicon (c-Si) wafers. c-Si wafers were thermally annealed under an H2S atmosphere at various temperatures. The initial minority carrier lifetime (6.97 µs) of a c-Si wafer without any passivation treatments was also measured for comparison. The highest minority carrier lifetime gain of 2030% was observed at an annealing temperature of 600 ◦C. The X-ray photoelectron spectroscopy

analysis revealed that S atoms were bonded to Si atoms after H2S annealing treatment. This indicates that the increase in minority carrier lifetime originating from the effect of sulfur passivation on the silicon wafer surface involves dangling bonds.

Keywords: crystalline silicon; H2S; minority carrier lifetime; passivation; dangling bonds

1. Introduction In recent years, photovoltaic systems are becoming very popular because of their enormous amount of clean and unlimited solar energy, which can be harvested through Citation: Lin, J.; Jee, H.; Yoo, J.; Yi, J.; Jeong, C.; Lee, J. Surface Passivation photovoltaic solar cells [1–3]. Among the various technologies that increase solar cells’ of Crystalline Silicon Wafer Using efﬁciency, surface passivation treatment plays a key role in the silicon-based solar cell

H2S Gas. Appl. Sci. 2021, 11, 3527. fabrication process. The dangling bonds on the silicon wafer surface are known to work as https://doi.org/10.3390/app11083527 recombination centers and cause the loss of photon-generated carriers. Many advanced passivation materials have been developed and are widely used in commercial produc- Academic Editor: Sungjun Park tion, including SiNX:H, SiOX, and AlOX [4–9]. These materials have been shown to have good passivation effects on the silicon wafer surface. However, these materials are pro- Received: 25 March 2021 duced through complex fabrication systems that necessitate precise control of gaseous Accepted: 12 April 2021 reactants [10]. Published: 15 April 2021 According to Mead and Spitzer, the high density of the semiconductor/metal interface will pin the interfacial Fermi level, making the barrier height less sensitive to the metalwork Publisher’s Note: MDPI stays neutral function [11]. Experimentally, Ali found that sulfur-passivated silicon substrates show with regard to jurisdictional claims in greater sensitivity to the metalwork function in terms of Schottky barrier height [12]. This published maps and institutional affil- was attributed to the reduction in surface density through passivation of dangling silicon iations. bonds by S. Previous results show that S atoms are absorbed on the Si wafer surface in the form of Si–S–Si bridge bonds [13,14]. The significant minority carrier lifetime gains of multicrystalline Si recently achieved by sulfur treatment further prove that sulfur treatment can effectively passivate dangling silicon bonds on the surface [15]. Copyright: © 2021 by the authors. In this study, we report the effect of sulfur passivation on crystalline silicon wafers Licensee MDPI, Basel, Switzerland. using H2S annealing treatment at various temperatures. The results show that a significant This article is an open access article improvement in passivation can be obtained after H2S annealing treatment. Moreover, distributed under the terms and 600 ◦C was found to be the best temperature condition for crystalline silicon wafers. Under conditions of the Creative Commons this condition, the highest minority carrier lifetime gain of up to 2030% can be observed Attribution (CC BY) license (https:// using the Sinton WCT-120 measurement system. creativecommons.org/licenses/by/ 4.0/).

Appl. Sci. 2021, 11, 3527. https://doi.org/10.3390/app11083527 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 7 Appl. Sci. 2021, 11, 3527 2 of 7

2. Materials and Methods 2. Materials and Methods N-type flat c-silicon wafers (100) with 1–10 ohm·cm, 200 μm were used for the exper- N-type ﬂat c-silicon wafers (100) with 1–10 ohm·cm, 200 µm were used for the experi- iments. Before the experiments, the wafers were cleaned in a 3-step process: saw damage ments. Before the experiments, the wafers were cleaned in a 3-step process: saw damage removal,removal, organic organic cleaning, cleaning, and and ionic ionic cleaning. cleaning. For For saw saw damage damage removal, removal, the the wafers wafers were were soakedsoaked in in 45% 45% KOH KOH solution solution at 70 °C◦C for for 20 20 min. min. For For organic organic and and ionic ionic cleaning, cleaning, the the wafers wafers werewere cleaned inin standardstandard RCA-1RCA-1 and and RCA-2 RCA-2 procedures procedures in in sequence sequence [16 [16,17].,17]. After After cleaning, cleaning, the oxide layer induced by H2O2 was removed by 1% HF for 30 secs. Afterward, the the oxide layer induced by H2O2 was removed by 1% HF for 30 s. Afterward, the silicon siliconwafers wafers were blow-dried were blow-dried quickly quickly to prevent to prevent reoxidation. reoxidation. H2S annealing treatments were conducted in a 2-zone quartz tube furnace. The quartz H2S annealing treatments were conducted in a 2-zone quartz tube furnace. The tube was pumped to a pressure of 5 mTorr before being charged with H2S gas. To thor- quartz tube was pumped to a pressure of 5 mTorr before being charged with H2S gas. To oughly evacuate the air from the quartz tube, the tube was charged with H2S gas twice thoroughly evacuate the air from the quartz tube, the tube was charged with H2S gas twice beforebefore annealing annealing treatment treatment was was begun. begun. Then, Then, the the quartz quartz tube tube was was heated heated to to the desired temperature.temperature. The annealingannealing processingprocessing time time was was 20 20 min. min. When When the the processing processing time time ran out,ran out,the Hthe2S H gas2S wasgas pumpedwas pumped out of out the of tube, the andtube, the and heater the heater was turned was turned off. During off. During the cooling the coolingtime, argon time, gas argon was gas introduced was introduced into the into tube the at atube ﬂow at ratea flow of 20rate sccm. of 20 The sccm. silicon The waferssilicon waferswere unloaded were unloaded when thewhen temperature the temperature dropped dropped below below 50 ◦C. 50 For °C. reproducibility, For reproducibility, each eachcondition condition was testedwas tested three three times, times, and and the averagethe average value value was was taken, taken, as shown as shown in Figure in Fig-1 ureand 1 Table and Table1. 1.

FigureFigure 1. 1. TheThe plot plot of of minority minority carrier carrier lifetime lifetime gains gains vs. vs. annealing annealing temperature temperature for for H H22SS annealing. annealing.

Table 1. Table of minority carrierTable lifetime 1. Table gains of minority vs. annealing carrier temperature lifetime gains for vs. H2 Sannealing annealing temperature (initial minority for H carrier2S annealing lifetime: (ini- 6.97 µs). tial minority carrier lifetime: 6.97 μs).

H2S AnnealingH S Annealing Temperature 2 500500 525525 550550 575575 600600 625625 650650 675 700 Temperature(°C) (◦C) Minority carrierMinority lifetime 13.2/13.2/ 16.1/16.1/ 12.2/12.2/ 15.9/15.9/ 20.3/20.3/ 13.1/13.1/ 5.9/5.9/ 10.8/ 17.2/ carriergain lifetime (×100%) gain (×100%) 0.25170.25170.2646 0.2646 0.2 0.2 0.36010.3601 0.16330.1633 0.26460.2646 0.10.1 0.1732 0.17320.1732 /standard/standard deviation deviation

TheThe surface surface morphology morphology of of the the CuI CuI films ﬁlms was was characterized characterized using using a a scanning scanning electron electron microscopemicroscope (JSM-7610F, (JSM-7610F, JEOL, JEOL, Tokyo, Tokyo, Japan). The The surface surface reactions reactions were were monitored by transmission-modetransmission-mode Fourier-transform Fourier-transform infrared infrared (FTIR) (FTIR) spectroscopy. spectroscopy. The The composition of thethe silicon silicon wafers wafers was was analyzed analyzed using using XPS XPS (k-alpha, (k-alpha, Thermo Thermo Fisher Fisher Scientific, Scientiﬁc, Waltham, Waltham, MA,MA, USA). USA). Lifetime Lifetime measurements measurements were were done done wi withth a a WCT-120 WCT-120 lifetime tester from from Sinton Sinton instruments.instruments.

Appl. Sci. 2021, 11, 3527 3 of 7

3. Results and Discussion Passivation quality was measured with a Sinton WCT-120 instrument. It is widely known that silicon wafers show huge fluctuations in minority carrier lifetime. Due to wafer quality fluctuations, the gain in minority carrier lifetime is used to represent passivation quality. After the wafer cleaning procedure, H2S annealing experiments were carried out to investigate the relationship between temperature and passivation quality. Figure1 and Table1 show the minority carrier lifetime gain vs. temperature. In this minority carrier lifetime gain-temperature plot, 2 peaks were observed; one is 1610% at 525 ◦C, and the ◦ other one is 2030% at 600 C. Yinghuang found that H2 is the desorption product of thermal decomposition of H2S on silicon [9]. Later, Arunodoy experimentally proved that passivation quality on silicon is related to the combined effect of hydrogen and sulfur atoms [14]. Hence, the 2 peaks in the table probably stem from the same passivation mechanism as reported by Arunodoy. Compared with the highest minority carrier lifetime gain (2750%) obtained by Arunodoy, the H2S passivation quality seems inferior. However, it should be noted that minority carrier lifetime gain is strongly related to the silicon wafer properties itself. Specifically, the differences in material properties between the monocrystalline silicon wafer and the multicrystalline silicon wafer used in the previous case most likely resulted in the different upper limits of minority carrier lifetime gain. A large number of bulk defects are known to exist along the grain boundaries in multicrystalline silicon [18,19]. However, in monocrystalline silicon, the absence of grain boundaries means that these kinds of bulk defects are practically negligible. In other words, monocrystalline silicon is superior to multicrystalline silicon in terms of bulk defects. This is also proven by the low initial minority carrier lifetime of multicrystalline vs. monocrystalline silicon. Hence, a more profound gain in minority carrier lifetime can be obtained on multicrystalline silicon by the same method. Moreover, a strange phenomenon was observed: the minority carrier lifetime gain increased again when the annealing temperature was higher than 650 ◦C. To elucidate this phenomenon, XPS characterization was used to analyze silicon surface composition and further investigate the passivation mechanism. Figure2 shows the S 2p XPS spectra for the H2S-passivated Si surface. Most conditions exhibit peaks at 162.3 eV, corresponding to Si–S bonds [20]. At 500, 525, 550, and 600 ◦C, the peaks’ intensity exhibits the same tendency as the minority carrier lifetime gain when the annealing temperature increases from 500 ◦C to 600 ◦C. For example, the highest and second-highest peak perfectly corresponds to the largest and second-largest minority carrier lifetime gain. This means that sulfur plays a dominant role in passivation performance. However, it seems that the 550 ◦C condition does not follow this trend. It exhibits a lower Si–S peak intensity than that seen at 500 ◦C, but a larger minority lifetime gain. Both hydrogen and sulfur are believed to play important roles in passivation performance. Thus, it is possible that the more positive passivation effect of hydrogen compensates for the loss of sulfur and enhances the total passivation performance to a higher level than that seen under 500 ◦C conditions. However, there is no peak at 162.3 eV at 700 ◦C, meaning that no Si–S bonds form on the Si surface ◦ after 700 CH2S annealing treatment. However, a high minority carrier lifetime gain was observed in reality, which contradicted the conclusion that sulfur plays a dominant role in passivation performance. The answer to this conundrum can be found in Figure3. The peaks at 103.8 and 99.1 eV correspond to SiO2 and Si, respectively [21]. From Figure3, it is clear that the SiO2 peak is much higher than the Si bulk peak. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 7 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 7 Appl. Sci. 2021, 11, 3527 4 of 7

Figure 2. XPS spectra of S 2p for an H2S-passivated Si surface. FigureFigure 2. 2.XPSXPS spectra spectra of of S 2p forfor anan H H2S-passivated2S-passivated Si Si surface. surface.

◦ Figure 3. XPS spectra of Si 2p for an H2S-passivated Si surface (700 C). Figure 3. XPS spectra of Si 2p for an H2S-passivated Si surface (700 °C). Figure 3. XPS spectra of Si 2p for an H2S-passivated Si surface (700 °C). This means that the silicon wafer was thoroughly covered by SiO2. It explains why the passivationThis means performance that the silicon was still wafer good was when thoroughly there was nocovered sulfur bondedby SiO with2. It explains silicon why This means that the silicon wafer was thoroughly covered by SiO2. It explains why thedangling passivation bonds. performance It also coincides was with still previous good wh researchen there showing was no that sulfur sulfur bonded atoms with will silicon the passivation performance was still good when there was no sulfur bonded with silicon danglingdesorb at bonds. high temperatures It also coincides [22]. Thermal with previo oxidationus research for silicon showing oxide growth that sulfur is widely atoms will dangling bonds. It also coincides with previous research showing that sulfur atoms will desorbused in at laboratory high temperatures or industrial [22]. production. Thermal Here, oxidation the 2-zone for furnacesilicon providedoxide growth a similar is widely desorb at high temperatures [22]. Thermal oxidation for silicon oxide growth is widely usedenvironment. in laboratory Even or though industrial the experiment production. was Here, conducted the 2-zone in a vacuum furnace environment, provided a a similar usedtiny in amount laboratory of O cannotor industrial be evacuated production. from the Here, furnace. the Because2-zone surfacefurnace dangling provided bonds a similar environment. Even2 though the experiment was conducted in a vacuum environment, a environment.were not terminated Even though by sulfur, the the experiment bare silicon surfacewas conducted is more positive in a vacuum and sensitive environment, to O2, a tiny amount of O2 cannot be evacuated from the furnace. Because surface dangling bonds tinyespecially amount under of O2 high-temperaturecannot be evacuated conditions. from the In thefurn nextace. section, Because the surface effects dangling of various bonds were not terminated by sulfur, the bare silicon surface is more positive and sensitive to wereannealing not terminated temperatures by on sulfur, the wafers’ the bare morphology silicon surface will be discussed. is more positive Figure4 shows and sensitive SEM to Oimages2, especially of silicon under wafers high-temperature after H2S annealing conditions treatment.. InInthis thestudy, next section, the wafers the (100) effects were of vari- O2, especially under high-temperature conditions. In the next section, the effects of vari- ousprocessed annealing with temperatures saw damage etching on the by wafers’ KOH solution. morphologyFigure 4willa shows be discussed. an SEM image Figure of a4 shows ous annealing temperatures on the wafers’ morphology will be discussed. Figure 4 shows SEMSi wafer images without of silicon H2S annealing wafers after treatment. H2S annealing Many square treatment. shapes were In this formed study, on the waferwafers (100) SEM images of silicon wafers after H2S annealing treatment. In this study, the wafers (100) weresurface, processed which is with caused saw by damage isotropic etching etching [by23 ,KOH24]. Previous solution. research Figure showed 4a shows that an H2 SEMS im- weretreatment processed would with result saw in damage the formation etching of aby thin KOH sulfur solution. layer on Figure Si or Ge 4a substrates shows an [ 20SEM]. image of a Si wafer without H2S annealing treatment. Many square shapes were formed on ageThe of morphologya Si wafer without shown in H Figure2S annealing4b,c is obviously treatment. different Many from square that shownshapes in wereFigure formed4a . on the wafer surface, which is caused by isotropic etching◦ [23,24]. Previous research showed theThe wafer silicon surface, surface which was covered is caused with by a thinisotropic layer afteretching 525 [23,24].CH2S annealing Previous treatment, research asshowed that H2S treatment would result in the formation of a thin sulfur layer on Si or Ge sub- thatshown H2S intreatment Figure4b. would The identity result ofin thisthe layerformation was confirmed of a thin to sulfur be sulfur layer by on the Si above- or Ge substrates [20]. The morphology shown in Figure 4b,c is obviously different from that shown stratesmentioned [20]. The XPS morphology spectra and EDX shown mapping in Figure results. 4b,c In is Figure obviously4b, the different wafer surface fromis that flat, shown inand Figure the boundaries 4a. The silicon between surface the square was covered shapes are with difficult a thin to layer distinguish, after 525 which °C H can2S annealing be in Figure 4a. The silicon surface was covered with a thin layer after 525 °C H2S annealing treatment,attributed toas theshown increased in Figure thickness 4b. The ofthe identity sulfur of film. this It layer can be was further confirmed concluded to be that sulfur by treatment, as shown in Figure 4b. The identity of this layer was confirmed to be sulfur◦ by thethe above-mentioned thin sulfur layer becomes XPS spectra thicker and under EDX the mappi higherng annealing results. temperatureIn Figure 4b, of the 600 waferC, sur- the above-mentioned XPS spectra and EDX mapping results. In Figure 4b, the wafer sur- facewhich is isflat, consistent and the with boundaries the intensities between of the the XPS square peaks. Basedshapes on are the difficult above discussion to distinguish, face is flat, and the boundaries between the square shapes are difficult to distinguish, which can be attributed to the increased thickness of the sulfur film. It can be further con- which can be attributed to the increased thickness of the sulfur film. It can be further concluded that the thin sulfur layer becomes thicker under the higher annealing temperature cluded that the thin sulfur layer becomes thicker under the higher annealing temperature of 600 °C, which is consistent with the intensities of the XPS peaks. Based on the above of 600 °C, which is consistent with the intensities of the XPS peaks. Based on the above discussion of the XPS spectra, we concluded that the passivation performance reflects the discussion of the XPS spectra, we concluded that the passivation performance reflects the Appl. Sci. 2021, 11, 3527 5 of 7 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 7

of the XPS spectra, we concluded that the passivation performance reflects the amount amount of sulfur. Here, the SEM findings show the same result, that passivation perfor- of sulfur. Here, the SEM findings show the same result, that passivation performance mance is related to the amount of surface sulfur. The sulfur thin layer becomes rough is related to the amount of surface sulfur. The sulfur thin layer becomes rough again again under 700 °C annealing treatment, as shown in Figure 4d. This finding is following under 700 ◦C annealing treatment, as shown in Figure4d. This finding is following XPS XPS results showing that there are no sulfur atoms on the silicon surface when the anneal- results showing that there are no sulfur atoms on the silicon surface when the annealing ing temperature is raised to 700 °C or higher. Silicon-based solar cell passivation perfor- temperature is raised to 700 ◦C or higher. Silicon-based solar cell passivation performance mance mainly stems from two factors: termination of surface dangling bonds and rejection mainly stems from two factors: termination of surface dangling bonds and rejection of the of the minority carrier due to the field effect. Herein, the potential second passivation the- minority carrier due to the field effect. Herein, the potential second passivation theory was ory was investigated. Electrostatic force microscopy (EFM) is a very useful technique to investigated. Electrostatic force microscopy (EFM) is a very useful technique to investigate investigatesurface charge-related surface charge-relat phenomena,ed phenomena, such as the such surface as the field surface so, EFM field was so, used EFM to was measure used tothe measure silicon waferthe silicon surface, wafer as shownsurface, in as Figure shown5. Thein Figure existence 5. The of aexistence fixed charge of a fixed in SiNx charge and inBSF SiNx (Al and contact) BSF (Al is known contact) to is bring known pretty to br gooding pretty passivation good performancepassivation performance to commercial to commercialsolar cells [25 solar–27]. cells Hence, [25–27]. we suspect Hence, that we the susp causeect that must the be relatedcause must to the be surface related charge to the if surfacethe possibility charge ofif the dangling possibility bond of termination dangling bond was denied. termination The detection was denied. and distributionThe detection of anda surface distribution charge of on a silicon surface were charge first on investigated silicon were by first Morita investigated [28]. The by response Morita of[28]. surface The responsecharge to of bias surface voltage charge is believed to bias tovoltage result is in believed differences to result in the topographyin differences or in EFM the images.topog- raphyBefore or measurements EFM images. wereBefore taken, measurements aluminum we contactre taken, was aluminum deposited contact on half was of the deposited samples’ onarea half for of comparison the samples’ purposes. area for Then, comparison forward purposes. bias voltage Then, (5 V) forward was applied bias tovoltage the samples (5 V) wasfirst, applied and reverse to the biassamples voltage first, (− and5 V) reverse was applied bias voltage second. (−5 TheV) was measurement applied second. area wasThe measurementdefined as 40 areaµm2 .was According defined toas previous40 μm2. According research, to the previous appearance research, of new the patterns appearance and ofcolor new changes patterns in and the samecolor changes area would in the strongly same indicatearea would the existencestrongly indicate of a surface the existence charge. It ofshould a surface be noted charge. that It theshould color be changes noted that of linesthe color A and changes B come of from lines the A and height B come difference. from theThey height were difference. caused by They Al contact were caused and chemical by Al contact solution and etching chemical (saw solution damage etching removal), (saw damagerespectively. removal), Hence, respectively. it can be concluded Hence, it that can nobe surfaceconcluded charge that exists no surface on the charge silicon exists wafer onsurface. the silicon wafer surface.

FigureFigure 4. 4. SEMSEM images images of of the the silicon silicon surface: surface: (a ()a saw) saw damage damage etched etched wafer wafer without without H2 HS 2annealingS annealing ◦ treatment,treatment, ( (bb)) saw saw damaged damaged etched etched wafer wafer under under 525 525 °CCH H2S2 Sannealing annealing treatment, treatment, (c () csaw) saw damage damage ◦ ◦ etchedetched wafer wafer under under 600 600 °CCH H2S2 Sannealing annealing treatment, treatment, (d ()d saw) saw damage damage etched etched wafer wafer under under 700 700 °C C H2S annealing treatment. H2S annealing treatment. Appl.Appl. Sci.Sci. 20212021,,11 11,, 3527x FOR PEER REVIEW 66 ofof 77

◦ FigureFigure 5. ElectrostaticElectrostatic force force microscopy microscopy (EFM) (EFM) images images of H of2S H(6002S °C)-treated (600 C)-treated silicon silicon wafers. wafers. (a,b) (werea,b) weremeasured measured under under forwarding forwarding bias biasvoltage, voltage, while while (c,d) ( cwere,d) were measured measured under under a reverse a reverse bias bias voltage.voltage.

4.4. Conclusions

InIn this study, the the passivation passivation effect effect of of H H2S2 Sgas gas annealing annealing on on silicon silicon wafers wafers was was inves- in- ◦ vestigated,tigated, and and the theoptimal optimal annealing annealing temperat temperatureure was 600 was °C. 600 TheC. XPS The spectra XPS spectra study shows study showsthe passivation the passivation performance performance reflects reflects the amount the amount of sulfur, of sulfur, and the and SEM theSEM analysis analysis also alsoshows shows that thatpassivation passivation performance performance is related is related to the to amount the amount of surface of surface sulfur, sulfur, while while the thepossibility possibility of surface of surface charge charge passivation passivation was wasexcluded excluded by EFM. by EFM. This This research research shows shows that thatthe amount the amount of sulfur of sulfur plays plays a dominant a dominant role role in in the the passivation passivation performance performance and thatthat allall sulfur will thoroughly disappear from the silicon surface when the temperature is increased sulfur ◦will thoroughly disappear from the silicon surface when the temperature is in- tocreased 700 C. to These 700 °C. findings These showfindings that show sulfur that passivation sulfur passivation is an excellent is an candidate excellent for candidate a simple andfor a economical simple and passivation economical method. passivation For furthermethod. work, For further this technology work, this will technology be applied will to photovoltaic devices for making high-efficiency solar cells. be applied to photovoltaic devices for making high-efficiency solar cells.

Author Contributions: Conceiving the idea, J.L. (Jaehyeong Lee), J.Y. (Junsin Yi), C.J., J.L. (Jian Lin); Author Contributions: Conceiving the idea, J.L. (Jaehyeong Lee), J.Y. (Junsin Yi), C.J., J.L. (Jian Lin); designing the study, J.L. (Jian Lin), H.J., J.Y. (Junsin Yi); experiments, J.L. (Jian Lin), J.Y. (Junsin Yi); designing the study, J.L. (Jian Lin), H.J., J.Y. (Junsin Yi); experiments, J.L. (Jian Lin), J.Y. (Junsin Yi); data analysis, J.L. (Jian Lin), H.J.; writing—original draft, J.L. (Jian Lin), J.L. (Jaehyeong Lee), writing— data analysis, J.L. (Jian Lin), H.J.; writing—original draft, J.L. (Jian Lin), J.L. (Jaehyeong Lee), writ- review and editing, H.J., J.L. (Jaehyeong Lee); supervision, J.L. (Jaehyeong Lee). All authors have ing—review and editing, H.J., J.L. (Jaehyeong Lee); supervision, J.L. (Jaehyeong Lee). All authors read and agreed to the published version of the manuscript. have read and agreed to the published version of the manuscript. Funding:Funding: This study was supported by the KoreaKorea ElectricElectric PowerPower CorporationCorporation (Grant(Grant number:number: R17XA05-1).R17XA05-1). DataData AvailabilityAvailability Statement:Statement: The data that support thethe findingsfindings ofof thisthis studystudy areare availableavailable fromfrom thethe correspondingcorresponding author,author, J.L.,J.L., uponupon reasonablereasonable request.request. ConflictsConflicts ofof Interest:Interest: TheThe authorsauthors declaredeclare nono conflictconflict ofof interest.interest.

Appl. Sci. 2021, 11, 3527 7 of 7

References 1. Hongsub, J.; Jinho, S.; Daehan, M.; Jaehyeong, L.; Chaehwan, J. Improvement in power of shingled solar cells for photo-voltaic module. J. Nanosci. Nanotechnol. 2020, 20, 7096–7099. 2. Ajitha, A.; Nallapaneni, M.K.; Xiao, J.; Guduru, R.R.; Arunkumar, J.; Kadapalla, P.; Anil, K. Underwater performance of thin-film photovoltaic module immersed in shallow and deep waters along with possible applications. Results Phys. 2019, 15, 102768. [CrossRef] 3. Pramod, R.; Maria, M.; Nallapaneni, M.K.; Sastry, O.S.; Arunkumar, J. Operational performance and degradation influenced life cycle environmental-economic metrics of mc-Si, a-Si and HIT photovoltaic arrays in hot semi-arid climates. Sustainability 2020, 12, 1075. [CrossRef] 4. Anton, E.; Christian, R.; Armin, R.; Jan, B. Influence of layer thickness on passivation properties in SiOx/Al2O3 stacks. J. Appl. 2020, 127, 235303. 5. Teng, C.K.; Kean, C.F.; Matthew, S.; Keith, M.; Evan, F.; Sieu, P.P.; Wensheng, L.; Andrew, B. Excellent ONO passivation on phosphorus and boron diffusion demonstrating a 25% efficient IBC solar cell. Prog. Photovolt. 2020, 28, 1034–1044. 6. Kai-Yuan, C.; Shang-Chi, W.; Chia-Jui, Y.; Tong-Wen, W.; Jyun-Hao, L.; Meng-Chyi, W. Comparative study on performance of AlGaN/GaN MS-HEMTs with SiNx, SiOx, and SiNO surface passivation. Solid State Electron. 2020, 170, 107824. 7. Mickael, L.; Shota, N.; Hitoshi, S.; Koji, M. Passivation property of ultrathin SiOx:H/a-Si:H stack layers for solar cell applications. Sol. Energy Mater. Sol. Cells 2018, 185, 8–15. 8. Armin, G.A. Surface passivation of crystalline silicon solar cells: A review. Prog. Photovolt. 2000, 8, 473–487. 9. Mohammad, Z.R.; Shahidul, I.K. Advances in surface passivation of c-Si solar cells. Mater. Renew. Sustain. Energy 2012, 1, 1–11. 10. Zongjie, S.; Yanfei, Q.; Ivona, Z.M.; Cezhou, Z.; Steve, H.; Li, Y.; Tian, L.; Yanbo, H.; Chun, Z. Effect of annealing temperature for Ni/AlOx/Pt RRAM devices fabricated with solution-based dielectric. Micromachines 2019, 10, 446. 11. Mead, C.; Spitzer, W.G. Fermi level position at metal-semiconductor interfaces. Phys. Rev. 1964, 134, A713–A716. [CrossRef] 12. Muhammad, Y.A.; Meng, T. Effect of sulfur passivation of silicon (100) on Schottky barrier height: Surface states versus surface dipole. J. Appl. 2007, 101, 103708–1–103708–5. 13. Ying-Huang, L.; Chuin-Tih, Y.; Yi-Hsin, L.; Wei-Hsiu, H. Adsorption and thermal decomposition of H2S on Si(100). Surf. Sci. 2002, 519, 150–156. 14. Qifei, W.; Boris, V.Y.; Theodore, E.M. Adsorption and decomposition of H2S on UO2(001). Surf. Sci. 2003, 523, 1–11. 15. Arunodoy, S.; Haifeng, Z.; Wen-Cheng, S.; Meng, T. A new method for bulk passivation in multicrystalline-Si by sulfur. In Proceedings of the IEEE 42nd Photovoltaic Specialist Conference (PVSC), New Orleans, LA, USA, 14–19 June 2015. 16. John, V. Thin Film Processes, 1st ed.; Academic Press: Cambridge, MA, USA, 1978. 17. Werner, K. Handbook of Semiconductor Cleaning Technology; Noyes Publishing: Park Ridge, NJ, USA, 1993. 18. Jerry, G.F.; Fredrik, A.L. Theory of grain-boundary and intragrain recombination currents in polysilicon p-n-junction solar cells. IEEE Trans. Electron Devices 1980, 27, 692–700. 19. Amal, K.G.; Charles, F.; Tom, F. Theory of the electrical and photovoltaic properties of polycrystalline silicon. J. Appl. 2008, 51, 446–454. 20. Tae, J.S.; Young, J.C.; Hyun, S.J.; Dae, H.K.; Dae, W.K.; Sang-Moon, L.; Jong-Bong, P.; Jung-Yeon, W.; Seong, K.K.; Cheol, S.H.; et al. High quality interfacial sulfur passivation via H2S pre-deposition annealing for an atomic-layer deposited HfO2 film on a Ge substrate. J. Mater. Chem. C 2016, 4, 850–856. 21. Wenwu, W.; Toshihide, N.; Yukihiro, S. Interface structure of HfNx/SiO2 stack grown by MOCVD using TDEAHf precursor. Surf. Sci. 2005, 588, 108–116. 22. Hibino, Y.; Ishihara, S.; Oyanagi, Y.; Sawamoto, N.; Ohashi, T.; Matsuura, K.; Wakabayashi, H.; Ogura, A. Suppression of sulfur desorption of high-temperature sputtered MoS2 film by applying DC bias. ECS Trans. 2018, 85, 531–539. [CrossRef] 23. Sana, R.; Alain, C.; Virginie, L.; Christian, V.; Jean-Michel, H.; Sebastien, B.; Nicolas, P. Isotropic dry etching of Si selectively to Si0.7Ge0.3 for CMOS sub-10 nm applications. J. Vac. Sci. Technol. 2020, 38, 033002. 24. Jianhua, Z.; Aihua, W.; Martin, A.G. 19.8% efficient “honeycomb” textured multi-crystalline and 24.4% monocrystalline silicon solar cells. Appl. Phys. Lett. 1998, 73, 1991–1993. 25. Bas, W.H.L.; Bart, M.; Manuel, S.; Maciej, K.S.; Agnes, A.M.; David, L.Y.; William, N.; Paul, S.; Wilhelmus, M.M.K. On the hydrogenation of Poly-Si passivating contacts by Al2O3 and SiNx thin films. Sol. Energy Mater. Sol. Cells 2020, 215, 110592. 26. Wendell, D.E.; Richard, M.S. Calculation of surface generation and recombination velocities at the Si-SiO2 interface. J. Appl. Phys. 1985, 58, 4267–4276. 27. Jerry, G.F. Physical operation of back-surface-field silicon solar cells. IEEE Trans. Electron Devices 1977, 24, 322–325. 28. Morita, S.; Uchihashi, T.; Okamoto, K.; Abe, M.; Sugawara, Y. Microscale contact charging on a silicon oxide. In Scanning Probe Microscopy: Characterization, Nanofabrication and Device Application of Functional Materials; Vilarinho, P.M., Rosenwaks, Y., Kingon, A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2002; Volume 186, pp. 289–308.