Nanoscale Properties of Wide-Band Gap Semiconductor Surfaces

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

This project aims to advance fundamental knowledge of structures and transport properties of semiconductor surfaces, the former with respect to greater understanding and optimization of growth of GaN and other III-nitrides, in particular, and the latter as exploration of an area with potential impact on nanoscale electronic devices. The structural and electronic properties of wide-band-gap semiconductor surfaces will be studied with a combination of atomic-scale microscopic and spectroscopic probes. Bilayer-thick metallic Ga layers that naturally terminate GaN will be studied by low-energy electron diffraction (LEED) at temperatures of about 750 C. The morphology and coverage of these layers will be observed as a function of incident Ga flux. The behavior of alternate terminating layers such as indium will also be investigated. Similar experiments will be performed for AlN. The electronic states of terminating layers on GaN will also be investigated by scanning tunneling spectroscopy (STS). The surface electronic structure will be probed with STS measurements on in-situ prepared GaN surfaces, comparing clean surfaces to ones that have been exposed to oxygen or other adsorbates. High- dynamic-range STS measurements, using carefully prepared (metallic) probe-tips, will be used to observe evolution of surface state-density as a function of exposure. LEEM observation will be used to observe overall morphological effects of adsorbates. Transport properties of surface electronic bands will be studied by STS, including measurements over a wide range of tunnel currents and at temperatures from 300 K down to 7 K. The observation of spectral shifts, as a function of current, will be used to deduce transport parameters of surface bands. Spectral shifts will be compared with those expected from a 3-dimensional solution of Poisson's equation for the tip-semiconductor geometry. Inclusion of electrostatic effects in the theory arising from surface accumulation or depletion of charge due to limited transport in the semiconductor is expected to permit identification and evaluation of limiting transport mechanisms. %%% The project addresses basic research issues in a topical area of materials science with high technological relevance. Experimental tools are now available which allow atomic level observation of elementary surface processes which when better understood allow advances in fundamental science and technology. Broader impact of the work lies in its application to semiconductor thin film growth (for nitrides) and for its potential in future nanoscale electronics. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. ***