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, clean- ing, 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 passiva- tion 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.