Defect densities, hydrogen/silicon

The deposition of amoriDhous hydrogenated silicon (a-Si H) from a silane plasma doped witli diborane (B2 Hg) or phosphine (PH ) to produce p-type or n-type silicon is important in tlie semiconductor industry. The plasma process produces films witli a much lower defect density in comparison witli deposition by sputtering or evaporation. 

As described earlier, the covalently bonded hydrogen, by passivating dangling bond defects and removing strained weak Si—Si bonds from the network, dramatically improves the semiconducting quality of amorphous silicon. Hence without the presence of hydrogen, effective amorphous semiconductor devices such as solar cells or thin film transistors would not be possible. Unfortunately, low defect density, high electronic quality... 

Electronic defects reduce the photosensitivity, suppress doping and impair the device performance of a-Si H. Their high density in pure amorphous silicon makes this material of lesser interest and is the reason for the attention on the hydrogenated material, in which the defect density is greatly reduced. The remaining defects in a-Si H control many electronic properties and are centrally involved in the substitutional doping process. The phenomena of metastability, which are described in Chapter 6, are caused by the defect reactions. 

In an amorphous semiconductor with a low defect density such as hydrogenated amorphous silicon (a-Si H), charge transport takes place in the electronic states in the vicinity of the conduction- and valence-band edges. However, no complete theory of the electronic structure near the band edges in a-Si H or any amorphous semiconductor has yet been devised. The problem appears to be extraordinarily complex. The disorder generates localized states near the band edges that are not present in crystalline material. 

Nonetheless, owing to its temperature stability, dopabUity and widely tunable optical behavior, amorphous SiC doped with hydrogen (a-SiC H) has been investigated for its application in photovoltaic devices. One of several structural renderings of amorphous SiC is shown in Figure 11.9. In this case, the refractive index can be varied from 2.3 to 3.7, and the band gap from 2.4 to 2.0eV, simply by altering a few parameters of the plasma-enhanced CVD technique that is used to deposit an amorphous silicon carbide layer with low defect density onto, for example, a substrate of RSiC. Attention has also been focused on the surface passivation performance of layers deposited on crystalline siUcon. Indeed,... 

Robertson has summarized the three recent classes of models of a-Si H deposition [439]. In the first one, proposed by Ganguly and Matsuda [399, 440], the adsorbed SiHa radical reacts with the hydrogen-terminated silicon surface by abstraction or addition, which creates and removes dangling bonds. They further argue that these reactions determine the bulk dangling bond density, as the surface dangling bonds are buried by deposition of subsequent layers to become bulk defects. 

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