The Influence of the Bonding Structure on Disorder and Band-Tails in a-B:H
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THE INFLUENCE OF THE BONDING STRUCTURE ON DISORDER AND BANDTAILS IN a-B:H N. BERNHARD, K. EBERHARDT, M. B. SCHUBERT and G. H. BAUER Institut fdr Physikalische Elektronik, Universitht Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80, Germany
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 H 6 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 Gphoto/'dark ratio from 10-1 to 3x10 1. 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.
INTRODUCTION Plasma Chemical Vapour Deposited amorphous hydrogenated boron (a-B:H) is known to be a semiconductor with a smooth decrease of the absorption coefficient at the band edges towards midgap [1]. The optical bandgap can be varied over a range of more than 1 eV [2,3]. Dopability and evidence of photoconductivity have been reported [4,5]. Film properties obtained by non-standard deposition methods have been examined recently [6,7], as was the high temperature deposition range (300 - 600 °C) which is interesting with respect to high microhardness of the films [8]. So far it has not been explicitly investigated how in comparison to a-Si:H the differing atomic coordination number influences the steepness of band tails, which is usually related to the degree of strain in the structural network, caused by an overdetermination of the network configuration due to the given bonding angles and lengths of the single atoms. As pointed out by Phillips [9] for an ideal amorphous (i. e. glassy) substance a coordination number greater than 2.45 (--V6V) means an overdetermination resulting in a constrained network. Boron with one valency less than silicon could build an amorphous network with less constraints than in a-Si:H, if one were able to avoid incorporation of too many three centre bonds (i. e. one t
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