Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity

Abstract

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

From amorphous to microcrystalline silicon: Moving from one to the other by halogenated silicon plasma chemistry

“…Differently from SiH 4 -H 2 plasmas where H 2 %>90% is used to deposit microcrystalline films [15,17,[22][23][24][25][26], the a-Si to µc-Si transition in SiF 4 -based plasmas does not require a high H 2 dilution, as demonstrated by various papers. As an example, Shimizu and his group have reported [75] almost complete (400)-oriented µc-Si:H,F growth at gas flow ratios of SiF 4 /H 2 = 60/3 sccm at 300°C and 30 /14 sccm at 200°C, while at smaller SiF 4 /H 2 gas flow ratios such as 30/10 sccm, (220)-oriented µc-Si:H,F films were obtained using very high frequency (VHF: 100MHz).…”

“…However, all the above cited works deposited microcrystalline silicon at T>230°C. Nevertheless, H 2 , i.e., the hydrogen-dilution ratio R (H 2 /SiH 4 ), has been reported by other groups to be important for aiding the amorphous-to-microcrystalline silicon phase transitions [15,17,[22][23][24][25][26], defects [62,63] and electronic properties [64][65][66][67][68].…”

“…The life-motif of improving stability against photodegradation of a-Si:H led researchers to demonstrate recently that "protocrystalline Si", produced near the amorphous-to-microcrystalline silicon transition regime, exhibits a higher degree of ordering with increased stability under light illumination [14][15][16][17][18][19].…”

“…[15][16][17][22][23][24][25][26] or without H 2 dilution [27][28][29][30][31][32][33][34], in the course of years various halogenated Si gas precursors, e.g. SiF 4 [35][36][37][38][39][40], SiCl 4 [41][42][43], and SiH 2 Cl 2 [44,45] have also been investigated and reported.…”

Optoelectronic properties of hot-wire silicon layers deposited at 100°C

“…The structural, optical and electrical properties of the silicon layers around the amorphous-to-microcrystalline transition are presented. It has long been known that a-Si:H films deposited just before the onset of crystallinity show the best properties in terms of stability against light induced degradation, low defect densities and improved medium-range order [3]. In the fabrication of layers at temperatures around 100°C, the use of hydrogen dilution becomes essential for reaching a reasonable quality for photovoltaic applications.…”