Nanoscale Properties of Wide-Band Gap Semiconductor Surfaces

该项目旨在促进半导体表面结构和输运性质的基础知识，前者尤其是关于更好地了解和优化GaN和其他III-氮化物的生长，而后者是对可能对纳米级电子设备产生影响的领域的探索。宽禁带半导体表面的结构和电子性质将用原子尺度的微观和光谱探针相结合的方法来研究。在750℃左右的温度下，用低能电子衍射(LEED)研究了自然终止GaN的双层厚的金属Ga层，观察了这些层的形貌和覆盖率随入射Ga通量的变化。还将研究诸如铟的交替终止层的行为。将对氮化铝进行类似的实验。还将用扫描隧道谱(STS)研究GaN上终端层的电子态。表面电子结构将通过在原位制备的GaN表面上的STS测量来探测，将干净的表面与暴露在氧气或其他吸附物中的表面进行比较。高动态范围的STS测量，使用精心准备的(金属)探头尖端，将被用来观察表面态密度随曝光的变化。LEEM观察将被用来观察吸附物的整体形态效应。表面电子能带的输运特性将被STS研究，包括在大范围的隧道电流和从300K到7K的温度范围内的测量。作为电流的函数的光谱移动的观测将被用来推断表面电子能带的输运参数。光谱位移将与针尖-半导体几何的泊松方程的三维解的预期值进行比较。由于半导体中有限的输运引起的表面电荷积累或耗尽引起的理论中的静电效应有望使人们能够识别和评价极限输运机制。%该项目解决了材料科学中具有高度技术相关性的主题领域的基础研究问题。现在有了实验工具，可以对基本的表面过程进行原子水平的观察，如果更好地了解这些过程，就可以推动基础科学和技术的进步。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，以及它在未来纳米级电子产品中的潜力。该计划的一个重要特点是通过在一个具有根本意义和技术意义的领域对学生进行培训，将研究和教育结合起来。***

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. ***