Surface Condition in the Plasma-CVD of a-Si:H,F from SiF 4 and H 2
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SURFACE CONDITION IN THE PLASMA-CVD OF a-Si:H,F FROM SiF 4 AND H2 A. Maruyama, D.S. Shen, V. Chu, J.Z. Liu, J. Jaroker, I. Campbell, P.M. Fauchet and S. Wagner Department of Electrical Engineering, Princeton University, Princeton, N.J. 08544 ABSTRACT We present a detailed study of the growth of a-Si:H,F from SiF 4 and H 2. The growth surface appears to have a high density of surface states. These surface states can be thermally relaxed by keeping the films at growth temperature after the termination of growth, suggesting that the states were created during film growth. When frozen in, the surface state density is found to depend on the conditions during film growth. The density is related to the sharpness of the valence band tail as measured by the Urbach Energy. We believe that a reaction on the growth surface resulting in fluorine elimination creates these surface states and also affects the formation of the Si-network. INTRODUCTION The deposition of amorphous hydrogenated and fluorinated silicon (a-Si:H,F) from SiF 4 and H2 was first reported by Ovshinsky and Madan [1]. Since then, fluorinated source gases have been employed in the growth of high quality a-Si,Ge:H,F alloys[2], especially with low optical gaps (down to 1.2 eV)[3], and of low-temperature epitaxial Si [4,5]. Although the growth mechanism from fluorides and hydrogen is not well understood, a reductive reaction between fluorinated gas and hydrogen must occur either in the gas phase or on the growth surface. This reaction should affect the formation of the three-dimensional network. a-Si:H,F and a-Si,Ge:H,F contain only about lat.% of fluorine [6]. This means that most of the fluorine atoms introduced with the source gas must be eliminated during film growth. No film grows when precursors (SiFn), produced in a glow discharge, are mixed with molecular H2 [4]. This suggests that the hydrogen radicals are needed to eliminate the fluorine atoms. Does this elimination reaction affect the structure of the films?
We have studied the growth of a-Si:HF from SiF 4 and H2 by varying the growth conditions to provide a wide range of film properties. The density of surface states Nss was found to be highly sensitive to the conditions of growth termination. Nss is correlated with the characteristic Urbach Energy, Eu, of the grown film. In this paper, we describe the identification of surface states, the relation between this density and the Urbach Energy, and the nature of the surface states. EXPERIMENTAL PROCEDURES Sample preparation We used a film growth system employing a DC-excited glow discharge in a triode reactor, which has been described elsewhere [7]. The area of the circular electrodes is 120 cm2 , the distance between the cathode grid and the substrate is normally fixed at 1 cm. The deposition parameters were: gas ratio SiF 4/H2 = 1:4 to 8:1, growth pressure P = 0.30 to 0.65 torr, a fixed total gas flow rate = 35 sccm, DC power = 34 W and substrate temprature Ts = 250 C. The growth conditions for the best bulk properties are: SiF 4/H2 = 2:1, P = 0.65 tor
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