High Quality Microcrystalline Silicon-Carbide Films Prepared by Photo-CVD Method Using Ethylene Gas as a Carbon Source
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ABSTRACT Hydrogenated boron-doped microcrystalline silicon-carbide (p-pc-SiC:H) films were grown by a photo chemical vapor deposition (photo-CVD) method from silane (SiH 4), hydrogen (H2), diborane (B2H6), and ethylene (C2H4) gases. Since the photo-CVD is a mild process (-100W/cm), we can avoid the ion damage of the film, which is inevitable during the deposition of jtc-SiC:H employing conventional PECVD technique. A dark conductivity as high as 5 X 10" S/cm, together with an optical bandgap of 2 eV, was obtained by the C2H4 addition, which is the first approach in photo-CVD systems. From the Raman and FTIR spectra, it is clear that our p-Kc-SiC:H films are made up of crystalline silicon grains embedded in amorphous siliconcarbide tissue. We investigate the role of the hydrogen dilution and ethylene addition on the electrical, optical, and structural properties of p-pc-SiC:H films. INTRODUCTION We have reported on the amorphous silicon (a-Si:H) solar cells and the a-Si:H thin film light emitting diodes (TFLEDs), which have the superstrate configuration of glass/TCO/p-i-n, fabricated by a photochemical vapor deposition (photo-CVD) technique [1-2]. We have used the hydrogenated p-type amorphous silicon-carbide (p-a-SiC:H) films having a conductivity of 10- S/cm as a window layer material. The incorporation of the thin p-a-SiC:H into these devices as a window layer material has been one of the major advances in this technology [3]. Since the increase of the optical bandgap of p-a-SiC:H film through the introduction of carbon increases the series resistance of the film, the overall performance of such films remains severely limited [4]. In general, the most necessary conditions of good window layer material are for the material to have a wide bandgap and high electrical conductivity. Wide optical bandgap window player of a-Si:H solar cell is essential to minimize light absorption in this layer. On the other hand, high electrical conductivity of the p-layer is indispensable to form a high built-in potential and to generate a strong electrical field in the active intrinsic layer of a-Si:H solar cells. In the case of a-Si:H TFLEDs, wide bandgap p-type carrier injector into the luminescent i-layer is the key to improve the brightness. From this point of view, p-type microcrystalline silicon-carbide (p-1ic-SiC:H) thin films are expected as better window layer materials due to the higher electrical conductivity, optical transmittivity, carrier mobility, and dopability compared to p-a-SiC:H thin films. Plasma enhanced chemical vapor deposition (PECVD) [4-6] and electron cyclotron resonance chemical vapor deposition (ECR CVD) techniques [7] have been the conventional growth methods of 1tc-SiC:H thin films. However, the PECVD technique has the problem of ion-damage on the films and interfaces because of involving high power density (>100mW/ct). One the other hand, the ECR CVD technique has the problem of non-uniform deposition over large area and the requirement of very complex system. Dasgupta et al. [8] have reported p-type i
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