Conduction and Valence Band Offsets at the Hydrogenated Amorphous Silicon-Carbon/Crystalline Silicon Interface Via Capac

  • PDF / 359,593 Bytes
  • 6 Pages / 414.72 x 648 pts Page_size
  • 55 Downloads / 196 Views

DOWNLOAD

REPORT


CONDUCTION AND VALENCE BAND OFFSETS AT THE HYDROGENATED AMORPHOUS SILICON-CARBON/CRYSTALLINE SILICON INTERFACE VIA CAPACITANCE TECHNIQUES John M. Essick*, Richard T. Mather*, Murray S. Bennett** and James Newton** * Department of Physics, Occidental College, Los Angeles, CA 90041 ** Thin Film Division, Solarex Corporation, Newtown, PA 18940

ABSTRACT Heterostructure Schottky diode samples each composed of a sub-micron thick layer of intrinsic hydrogenated amorphous silicon-carbon (a-Si1 xC,:H) deposited on an n-type crystalline silicon (c-Si) substrate are used to measure the a-Sil- Cx:H/c-Si band offsets via junction capacitance techniques. The samples range in carbon concentration from x=0.0-0.3. First, a thermally activated capacitance step due to the response of defects at the amorphous/crystalline interface is evident in capacitance vs. temperature spectra taken on all these samples. The bias-dependence of this step's activation energy provides a direct measure of the a-Si,-xCx:H/c-Si interface potential as a function of c-Si depletion width in each sample. By application of Poisson's equation, we find that the a-Sil-xCx:H/c-Si conduction band offset AE, increases from 0.00 to 0.10 eV as x increases from 0.00 to 0.26. Second, while under reverse-bias at low temperature, we optically pulsed each sample with c-Si band-gap light to create trapped holes at the a-Si1 _xCx:H/c-Si valence band offset AE,. By noting the threshold for the subsequent optical release of these trapped holes by sub-band gap light, we found that zAE, increases from 0.67 to Ž0.83 eV as x increases from 0.00 to 0.26.

INTRODUCTION Mobility gap tunability, a material property required in the development of many multijunction optoelectronic devices, has provided the impetus for the study of a-Si:H based alloy systems. In particular, the optical gap for a-Si1 _xCx:H can be continuously varied from 1.75 eV to greater than 3.0 eV as x increases from x=0 to x= 1 [1]. As a result, a-SiC:H has been employed as the "window-side" p-layer in p-i-n photovoltaic cells as well as a top passivation layer in electrophotographic devices. Of future potential, visible light electroluminescence has been observed at room temperature in a-SiC:H p+-i-n' diodes. Band discontinuity measurements then are of great import for the application of these alloy materials. For example, accurate knowledge of the band offsets at various junctions is required for optimizing the efficiency of a solar cell. A systematic photoemission study of the a-SilxCx:H/a-Si interface [2] determined that the valence band offset AEv (1) for x < 0.5, is insensitive to x and is = 0.8 eV, and (2) for x >0.5, increases linearly with x and reaches a value near 2.0 eV. Photoemission cannot measure the conduction band offset AE, directly, but from the known optical gap, AE, is inferred to be small. This finding, however conflicts with earlier photoemission work [3], which concluded AEv = 0 for this interface. In this paper, we implement our previously developed combination of junction capacitance tech

Data Loading...