Principles for controlling the optical and electrical properties of hydrogenated amorphous silicon deposited from a silane plasma

Abstract

The optical, electrical, and structural properties of hydrogenated amorphous silicon (a-Si:H) films are systematically investigated as functions of the substrate temperature (Ts) and plasma parameters, such as the rf power, gas pressure, and electrode dimensions. The films are deposited by the plasma chemical vapor deposition method. The properties of a-Si:H can be controlled over a wide range by varying the plasma parameters at fixed Ts. Reducing the film deposition rate and raising Ts have the same effect on the properties of a-Si:H. A unified relationship is found to exist among those properties of a-Si:H in the range of deposition conditions in this study, which includes ‘‘device-quality’’ conditions. No apparent effects of gas-phase polymerization or ion bombardment are observed. The experimental results suggest that during device-quality a-Si:H film deposition under conventional plasma conditions, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at or near the film-growing surface. The limitations on the controllability of plasma-deposited a-Si:H, especially at low Ts, can be surmounted by adding hydrogen or helium to the plasma, and by treating a-Si:H with the hydrogen plasma.

Control of micro void fraction and optical band gap in intrinsic amorphous silicon thin films (VHF-PECVD) for thin film solar cell application

“…A suitable substrate temperature (T s ) allows hydrogen to passivate the surface but at high temperatures, hydrogen effusion occurs reducing the hydrogen content affecting the structural defect and optical band gap [8].…”

Review of thin-film silicon deposition techniques for high-efficiency solar cells developed at Panasonic/Sanyo

“…However, the higher density of defects, particularly dangling bonds, degrades the electrical characteristics of the material at higher deposition-rate [33]. This phenomenon can be understood intuitively in that the increased rate of accumulation of Si-based radicals shortens the time available to relieve stress in the film surface structure; as a result, the Si network solidifies with more defects left in an energetically unstable, non-equilibrated condition.…”

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

“…Indeed, in SiH 4 -H 2 plasmas, H 2 is not indispensable for the deposition of silicon films, since the good dissociation of SiH 4 provides both the film growth precursors SiH x (x=2,3) [61] and the H-atoms for tailoring film [27][28][29][30][31][32][33][34]. However, all the above cited works deposited microcrystalline silicon at T>230°C.…”

“…7). Those limitations have driven research on deposition of microcrystalline silicon using approaches eliminating [27][28][29][30][31][32][33][34] or limiting [9,10] H 2 dilution.…”

“…[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.…”