A New Material: a-Si,Ge:H,F in a-Si,Ge:H,F/a-Si:H,F Multilayer Structures
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A NEW MATERIAL: a-Si,Ge:H,F IN a-Si,Ge:H,F/a-Si:H,F MULTILAYER STRUCTURES J.P. CONDE, V. CHU, D.S. SHEN, M. ANGELL AND S. WAGNER Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA
ABSTRACT We present a detailed study of the electron transport parallel to the layers in a a-Si,Ge:H,F/aSi:H,F amorphous semiconductor multilayers. The optical gap of the well layer is shifted from the bulk alloy due to quantum confinement. This shift W can be modeled using a finite barrier height, and electron effective masses m-=0.35 for the barrier and m;=1.0 for the well. Multilayer-induced changes in Od and its activation energy Ead are completely determined by W. The photoconductivity aph, its temperature dependence (Eaph) and its dependence -y on carrier generation rate reflect a stretching of the conduction band tail near the band edge as the quantum shift W increases. INTRODUCTION 1 The observation of resonant tuneling in a a-Si/a-SiN multilayer structure culminated years of 5 intensive research on the transport properties of amorphous semiconductor superlattices.2" Since the introduction of the concept of amorphous superlattices in 1983 as a demonstration that amorphous materials can display some of the most subtle properties of their crystalline counterparts,6 the studies on transport properties have emphasized analogy to superlattice crystals. Recently, suggestions have been advanced that sandwiching ultra-thin layers of amorphous materials affects their structural 7 9 properties. - The multilayer-induced changes will superimpose on the crystal-like built-in potential barriers and account for the variety of newly observed phenomena in amorphous multilayer structures. Our purpose is to concentrate on the transport properties of a-Si,Ge:H,F/a-Si:H,F multilayers parallel to the plane of the layers. We will show that these measurements reflect essentially the characteristics of the well layer material. Parallel transport perhaps is less spectacular than the perpendicular properties, but also is less complex and thus constitutes a first step towards the thorough understanding of the physics of amorphous heterostructures. EXPERIMENTAL PROCEDURES The a-Si,Ge:H,F alloys and a-Si:H,F/a-Si,Ge:H,F multilayer structures were grown using a 10 plasma-enhanced chemical vapor deposition technique. The glow discharge was radio-frequency excited 2 (13.56 M]Hz). The RF diode power density was 250 mW/cm . In all deposition runs the gas flow rates were 28 sccm SiF 4 , 4.6 sccm H2 and 0 to 1.0 sccm GeF 4. These deposition conditions produce low 11 13 bandgap alloys of good quality. - 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 glass substrates (Corning 7059) with 1/3 of the surface area coated with 1000A of evaporated Cr. The period of the superlattices was measured with X-ray diffraction. It agreed with the period calculated from the growth rates of the individual layers and
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