Hydrogen Bonding in Low-Gap a-Si,Ge:H,F Superlattices
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HYDROGEN BONDING IN LOW-GAP a-Si,Ge:H,F SUPERLATTICES J.P. CONDE, G. HAGER ANT) S. WAGNER Department of Electrical Engineering, Princeton University, Princeton, NJ 08544
ABSTRACT The integrated absorption of the Si-H, Ge-H and Si-H 2 IR stretching modes were measured for three series of superlattices where: a) the well to barrier thickness ratio was kept constant while the period was varied; b) the barrier thickness was kept constant while the well thickness was varied and c) the well thickness was kept constant while the barrier thickness was varied. The dark (ad) and photo (aph) conductivities, the photoconductivity exponent -1 and the activation energy of the dark conductivity Ea were measured perpendicularly to the plane of the layers for series a) and b). Structural changes induced by sandwiching were documented by IR absorption and correlated with the optoelectronic properties of the thin films. INTRODUCTION Since Abeles and co-workers suggested, in 1983, the concept of amorphous super1 lattices structures, these materials have been intensively studied. The first studies 2 4 focussed on the structure of interfaces between amorphous materials, - taking advantage of the large number of reproducible interfaces available in a superlattice structure. Since then the optical and electronical properties of these novel semiconductor 6 9 materials, 5- 8 their application to solar cells and their properties as photoreceptors have been the major topic of study. Recent evidence 8 , 10, 11 suggests that the thin layers (10-100A ) of amorphous semiconductor in multilayers structure are structurally different from those grown in the bulk. We present here a study of the hydrogen concentration and bonding in aSi,Ge:H,F/a-Si:H,F superlattices. With this study we probe the structure of the thin layers in the superlattice, and we compare the structural information thus obtained with the electronic properties of the films. EXPERIMENTAL PROCEDURES The a-Si,Ge:H,F alloys and a-Si:H,F/a-Si,Ge:H,F multilayer structures were 12 The glow grown using a plasma-enhanced chemical vapor deposition technique. discharge was radio-frequency excited (13.56 MHz). The RF diode power density was 2 250 mWcm- . In all deposition runs the gas flow rates were 28 sccm SiF 4 , 4.6 sccm H2 and either 0 or 0.4 sccm GeF 4. The superlattice structure was obtained by periodically diverting the GeF 4 flow, using a computer controlled pneumatic valve without interrupting the discharge. The films were grown on c-Si substrates, polished to 1 wedges to reduce fringing in the infrared. The thickness of the films was measured with a Dektak surface profiler. It agreed with the product of the growth rates of the individual layers, the layers deposition time and the number of layers. The infrared transmittance spectra were measured in a
Mat. Res. Soc. Symp. Proc. Vol. 103. -1988 Materials Research Society
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1 range between 400 and 4000 cm- using a Digilab-20C Fourier transform infrared spectrophotometer. From the spectra we determined the integrated intensity of
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