Parallel Conduction in a-Si:H/a-Si 1-x C x :H Multilayers
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PARALLEL CONDUCTION IN a-Si:H/a-SilC.:H MULTILAYERS J. BERTOMEU, J. PUIGDOLLERS, J.M. ASENSI, J.C. DELGADO AND J. ANDREU Universitat de Barcelona, Departament de Ffsica Aplicada i Electrbnica. Av. Diagonal 647, 08028-Barcelona, Spain. ABSTRACT
This paper deals with the electrical properties in the parallel direction of compositionally modulated amorphous silicon/amorphous silicon-carbon multilayers. Conductivity of three series of samples with varying well and barrier thicknesses is studied. The results show that dark conductivity decreases when reducing a-Si:H layer thickness. This is interpreted as an alloy effect at interfaces. The role of the a-Sil-xCx:H layers in the photoconductivity decrease observed in series with variable mean composition and constant well thickness is discussed. INTRODUCTION
The electrical properties in the parallel direction of a-Si:H based compositionally modulated multilayers is usually dominated by the conduction through a-Si:H layers. But even maintaining the deposition conditions for these layers, the electrical properties of the multilayers show a wide variation range with respect to homogeneous material when changing their geometrical factors. Since the first studies on amorphous semiconductor multilayers [1] the behavior above mentioned was attributed mainly to quantum size effects analogous to their crystalline counterparts. Recently, several of the properties of amorphous multilayers such as the blue shift of the optical gap, the increase in activation energy, and the presence of resonant tunneling through double barrier structures have been explained without considering quantum size effects [2,3,4]. In this paper, we study the electrical properties of a-Si:H-/a-Sij_ 1 C,,:H multilayers in the parallel direction. The results show that the electrical behavior in the parallel direction is mainly controlled by the a-Si:H layers (i.e. well layers) and the interfaces. An increase in activation energy is observed when the width of a-Si:H layers is reduced, which would be consistent with the presence of quantum size effects. However this effect is interpreted as a consequence of some interface effects such as the existence of non-strictly abrupt interfaces. EXPERIMENTAL
Three series of a-Si:H/a-Sil-xCx:H multilayers were deposited in an RF capacitively coupled reactor with an automated substrate holder described elsewhere [5]. All the samples were prepared at a substrate temperature of 300°C and an RF (13.56 MHz) power of 5 W (27 mW cm 2). The gas pressure was maintained at 20 Pa, and pure silane (20 sccm) and a mixture of silane (1 sccm) and methane (19 sccm) were respectively used in a-Si:H and a-Sil.xCx:H deposition. The plasma was maintained during the gas switching process, but substrates were removed from plasma influence until conditions were stable [5]. The carbon content in a-SilxCx:H alloys was found to be 0.36 by XPS, and optical gap was 1.67 and 2.36 eV for a-Si:H and a-Sil-xCx:H layers respectively. Two series with variable mean composition (labeled A and B) and a seri
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