Microcrystalline Silicon Thin film Growth and Simultaneous Etching of Amorphous Material
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*I.P.E., Universitat Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany. **Dept of A.P.E.M.E., Dundee University, Dundee, DD1 4HN, Scottland. ABSTRACT
An investigation of silicon plasma deposition and etching using both a VHF plasma in dilute SiH 4/H2 and the pulsed silane flow method is presented. It is possible to find preparation conditions at which simultaneous growth of /c-Si:H and etching of amorphous silicon (a-Si:H) on the same substrate is observed. The results clearly demonstrate that microcrystalline silicon growth proceeds via the preferential etching of amorphous tissue during film growth, and that observations of crystallization during hydrogen plasma treatment without etching are due to chemical transport of silicon within the reactor. INTRODUCTION
Hydrogenated microcrystalline silicon (j•c-Si:H) has been shown to combine several interesting properties of both amorphous and crystalline silicon. Thin pc-Si:H films may be produced by low temperature plasma deposition (PECVD) from the gas phase. Upon doping, they show much higher conductivities than amorphous silicon (a-Si:H)[1] and the bandgap may be increased by alloying with carbon[2]. For this reason microcrystalline silicon is, among other application of increasing interest to thin film solar cell manufacture: Doped contact layers a-Si:H solar cells are employed to enhance efficiencies, also fully microcrystalline cells have been demonstrated[3]. pc-Si:H thin films were first produced by chemical transport of silicon in a hydrogen plasma[41[5]. From these findings the explanation of tMc-Si:H growth by the partial chemical equilibrium model[6][7][8] was developed. The deposition of microcrystalline silicon is described as the result of continual breaking, or etching, of strained bonds from disordered material at the growth surface, which are energetically less stable than those associated with the crystalline phase, by the atomic hydrogen in the plasma. The net growth of the film is therefore the difference of growth and etching. As a second method the plasma deposition at 13.56 MHz from silane (SiH 4) highly diluted with hydrogen at high power densities[9] was employed. Under these conditions the formation of microcrystalline films was attributed to an enhanced surface mobility of film precursors due to the increased hydrogen concentration, such that they may migrate to stable crystalline sites. Hydrogen on or below the growth surface was also proposed to lead to bond strain relaxation and hence to crystallization[10][ 11][12]. It was also shown, that in plasma deposition at very high frequencies (VHF)[1][13] the parameter window for /c-Si:H formation is greatly enhanced compared to 13.56 MHz, as well as by switching between silane and hydrogen plasmas[10][14]. Fluorinated and chlorinated compounds have also been employed[15], though we will restrict our discussion to methods involving only silane and hydrogen. The growth of microcrystalline silicon has been reported to be selective with respect to the substrate[10][15] and using this effect
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