Electron and Hole Transport Perpendicular to the Planes of a-Si:H/a-Si,Ge:H Compositional Superlattices
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ELECTRON AND HOLE TRANSPORT PERPENDICULAR TO THE PLANES OF a-Si:H/ a-Si,Ge:H COMPOSITIONAL SUPERLATTICES,) J. Kolodzey,a) S. Aljiahi, R. Schwarz, D.-S. Shen, S. Quinlan, S.A. Lyon and S. Wagner, Department of Electrical Engineering,
Princeton University, Princeton, New Jersey 08544. ABSTRACT Drift mobilities, drift mobility-lifetime products and minority carrier diffusion lengths perpendicular to the planes of a-Si:H,F/a-Si0 3. 5,Ge0.6:H,F superlattices have been measured over a range of well sublayer widths. The transport data suggest a transition from scattering dominated transport in states at the top of the barriers for short superlattice periods to recombination dominated transport at the bottom of the wells for large periods. The characteristic period for this transition is -3 nm. INTRODUCTION Compositionally periodic structures or superlattices (SL's) based on hydrogenated amorphous silicon (a-Si:H) have become the subject of intensive study since they were reported 2 1 first by Abeles and Tiedje, and others. ,3, 4,5 These structures can serve as vehicles for the 6 investigation of the electronic properties of amorphous alloys on a nanometer scale. This scale is of particular interest in a-Si:H because it is the scale of electron wave function coher8 7 ence length, electron scattering length and of the structural inhomogeneities presumed to 6 exist in this class of material. In this letter we report the first comprehensive carrier transport data perpendicular to the planes of such superlattices. Our materials system is a-Si:H,F for the large gap barrier layer, and a-Si 0.35Ge 0.&s:H,F alloys for the low gap partner. The a-Si,Ge:H alloy system is 9well suited to superlattice work because of the low density of defects expected at its interfaces. PREPARATION AND CHARACTERIZATION TECHNIQUES The structures were grown from SiF 4, GeF 4 and H2 in a radio frequency (RF)-excited (13.56 MHz) glow discharge. The RF power density was 250 mW-cm-2, the gas flow rates 28 standard cubic centimeters per minute (sccm) SiF 4 , 0 to 0.55 sccm GeF 4 and 4.6 sccm H2, the deposition pressure 14.4 Pa (0.108 Torr) and the substrate temperature 270 'C. The deposition procedure for alloys in the ultra high vacuum deposition station has been described in 0 other publications.1 , 11 Compositional modulation was achieved by taking advantage of the strongly preferential deposition of Ge over Si from our feed gas mixtures. The GeF 4 flow was turned on and off periodically with a function generator-controlled pneumatic valve. Fig. la shows the discharge glow intensity recorded with a silicon photodetector vs. deposition time, for a structure whose X-ray diffraction pattern is rendered in Fig. lb. The period (repeat distance) of this superlattice (SL), computed from the first and second order X-ray diffraction peaks, is 5.02 nm. One period was grown in 79 seconds. The average rate of growth was 0.07 nm/s which agrees well with the growth rates calculated from deposition of the individual bulk materials, i.e. 0.05 nm/s for a-Si:H,F and 0.09
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