The Influence of the Bonding Structure on Disorder and Band-Tails in a-B:H

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Abstract
A thorough investigation of plasma-CVD amorphous hydrogenated boron (a-B:H) has been conducted with the main emphasis on how in this amorphous semiconductor the variation of the coordination number in comparison to a-Si:H influences the structural disorder and density of states in the band tails. a-B:H was deposited from different concentrations of B2 H6 diluted in H2 by both DC- and RF-plasma-CVD. The influence of the change of substrate temperature, pressure, flow and deposition power on the structural, optical and electronic properties of the material was examined. Raman-scattering and IR-absorption reveal the clear non-crystallinity of the deposited films. Almost all samples show some photoconductivity with a σphoto/σdark ratio from 10-1 to 3×101. Although a strong influence of some of the deposition parameters on bandgap and refractive index, hydrogen content, dark and photoconductivity was observed, the density of states in the band tails as measured by PDS was relatively high, showing always a rather flat Urbach slope of about 180 – 220 meV. An explanation for this unexpected almost uniform huge Urbach slope might be that even in the amorphous state the behaviour of boron is still dominated by its electron deficiency character which leads to a certain amount of three centre bonds (as seen in IR-absorption) and results in comparison to amorphous hydrogenated silicon in even stronger constraints of the amorphous network and obviously in more potential fluctuations.