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GROWTH PROCESSES OF HYDROGENATED AMORPHOUS SILICON John Robertson Engineering Dept, Cambridge University, Cambridge CB2 1PZ, UK. [email protected] ABSTRACT

INTRODUCTION

Urbach energy (meV) Hydrogen content (%)

The surface and subsurface processes occurring during the growth of a-Si:H are analysed to understand why dangling bond defects and weak Si-Si bonds form. We argue that hydrogen elimination to form the Si-Si network is the rate limiting process at low temperature, and this leads to the creation of weak Si-Si bonds. Dangling bonds form subsequently from weak bonds by a defect pool type process. Plasma processes, such as ion bombardment, which dehydrogenate the surface layers, can reduce the weak bond density. 25 20

Roca

growth rate

0.1W 5W 10W 15W Beyer

Spin density (cm-3)

There has been great effort to optimise 15 the deposition conditions of hydrogenated amorphous silicon (a-Si:H), in order to 10 produce stable devices such as solar cells and 5 thin film transistors. This has lead to various 0 criteria for deposition of high quality a-Si:H Roca 0.1W in terms of deposition temperature, pressure, 90 5W growth 10W RF power and hydrogen dilution [1-4]. The 80 rate 15W growth mechanism of a-Si:H is understood at Yamasaki 70 a basic level [5-7]. It is known that the growth 60 species of electronic grade a-Si:H is the silyl 50 radical SiH3. This monoradical has a low sticking coefficient and reaction probability 10 18 on the growing surface, so that this gives rise growth to chemical (rather than physical) vapour rate 17 10 deposition surface coverage [8]. The processes which occur in the plasma are also reasonably well understood, because they can 10 16 be measured. The processes occurring in the solid, such as creation and removal of defects, 10 15 are also understood within models such as the 0 100 200 300 400 500 600 Deposition temperature (C) defect pool [9,10]. The parts which have not been so far addressed are the exact processes Fig. 1. Variation of hydrogen content, Urbach by which species incident from the plasma energy and defect density in a-Si:H with lead to the formation of the defects and deposition temperature [3,11-13]. disorder in a-Si:H, and in particular how to describe these processes in terms of realistic elementary reactions, which could ultimately be used in a computer model of the growth process.

A1.4.1

The principle defect in a-Si:H is the Si dangling bond (DB). The other main type of disorder are the Si-Si weak bonds (WBs). These form an exponential density of tail states above the valence band edge with an energy width similar to that of the Urbach tail. There is also the medium-range disorder related to microvoids and poly-hydride bonding. Experimentally, the defect density and Urbach energy depend mainly on deposition temperature, each having a minimum for a deposition temperature of about 250°C, with a weaker dependence on the plasma conditions [3,11-13](Fig. 1). This paper focuses on a-Si:H growth by plasma enhanced chemical vapour deposition (PECVD). The growth process consi