The Optoelectronic Properties of a-Si, Ge:H(F) Alloys)
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THE OPTOELECTRONIC PROPERTIES OF a-Si,Ge:H(F) ALLOYS*)
S. WAGNER, V. CHU, D.S. SHEN, J.P. CONDE,a) S. ALJISHIb) AND Z E. SMITHc) Department of Electrical Engineering Princeton University, Princeton, NJ 08544
ABSTRACT We survey experimental evidence, obtained by optoelectronic measurements, and theoretical models for the density of gap states in a-Si,Ge:H(F) alloys. This survey shows that increasingly reproducible densities are obtained in the various segments of the energy gap. The results are becoming understood quantitatively. It is not yet clear if the reproducibility signals that the best material has been reached. The electron transport properties of low-gap (1.2 eV < Eopt < 1.4 eV) alloys are adequate for solar cells. Improvement of the hole transport properties, particularly (iir) which is set by the valence band tail width and the density of deep defects, is achieved through a reduction of the Urbach energy.
1. INTRODUCTION Applying a-Si,Ge:H(F) alloys in devices implies control over the carrier mobilities u and over the density of gap states N(E). Just like in un-alloyed a-Si:H(F), the band mobilities pso of electrons and holes in a-Si,Ge:H,(F) appear controlled by scattering on every atom, so that po is of the order of 10 cm 2V-'s- 1. The effective mobilities pp = ,pop/(p+ pt) and p. = p.n n/(n + nt), are controlled by the ratios of the free to trapped carrier densities P/Pt and n/nt. Thus the only realistic opportunity for improving Mpand p. is found in a reduction of the densities of the trapping band tail states. In essence, the density of gap states N(E) is the path to device-quality alloys. A survey of N(E) data produces a bewildering picture [1-7]. The published data present evidence for wide ranges in band tail widths, and in the positions and intensities of defect states. In our opinion, this spread in N(E) reflects a true spread in material properties, rather than differences in measurement techniques or their interpretation. Any device technology demands material that can be made reproducibly. Can aSi,Ge:H(F) be made reproducibly? We will discuss the evidence for, and against, reproducibility. In some cases, the next question to be answered is if reproducibility means that the best possible material has been achieved. We will attempt to answer this question, too.
*)Work supported by the Electric Power Research Institute.
Mat. Res. Soc. Symp. Proc. Vol. 118. ý 1988 Materials Research Society
624
-C Fig. 1: The positions of the band edges and dangling bond levels in a-Si,Ge:H(F) as functions of the optical gap.
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1.6 1.7 1.4 1.5 1.2 1.3 OPTICAL BANDGAP Eopt(eV)
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2. THE BAND EDGES AND DEFECT LEVELS AS FUNCTIONS OF THE OPTICAL GAP We begin our search for reproducible properties with an inspection of Fig. 1 which shows a selection of experimentally determined positions of the band edges and of the dangling bond levels for a-Si,Ge:H(F) as a function of the optical gap Eopr The reference energy (E 0) lies
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