Nanoscale Properties of Wide-Band Gap Semiconductor Surfaces

该项目旨在推进半导体表面的结构和传输特性的基础知识，前者涉及更好地理解和优化GaN和其他III族氮化物的生长，特别是后者作为对纳米级电子器件具有潜在影响的区域的探索。宽带隙半导体表面的结构和电子特性将结合原子尺度的显微镜和光谱探针进行研究。自然终止GaN的双层厚金属Ga层将在约750 ℃的温度下通过低能电子衍射（LEED）进行研究。这些层的形态和覆盖率将被观察到作为入射Ga通量的函数。交替终止层，如铟的行为也将被调查。将对AlN进行类似的实验。GaN上的终止层的电子状态也将通过扫描隧道光谱（STS）进行研究。表面电子结构将探测STS测量原位制备的GaN表面，比较干净的表面，已暴露于氧气或其他吸附物。高动态范围STS测量，使用精心准备的（金属）探针，将用于观察表面状态密度的演变作为曝光的函数。LEEM观察将用于观察吸附物的总体形态效应。STS将研究表面电子带的输运性质，包括在宽范围的隧道电流和从300 K到7 K的温度下的测量。作为电流的函数的光谱位移的观察将用于推导表面带的传输参数。光谱位移将进行比较，预期从一个3维的解决方案，泊松方程的尖端半导体几何形状。包括静电效应的理论所产生的表面积累或耗尽的电荷，由于在半导体有限的运输预计允许识别和评估的限制运输机制。该项目解决了材料科学领域的基础研究问题，具有高度的技术相关性。现在已有实验工具，可以在原子水平上观察基本的表面过程，当更好地理解这些过程时，可以促进基础科学和技术的发展。这项工作的更广泛的影响在于它在半导体薄膜生长（氮化物）中的应用，以及它在未来纳米电子学中的潜力。该计划的一个重要特点是通过在一个基本和技术上重要的领域对学生进行培训来整合研究和教育。***

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. ***