Electronic structure and quantized surface electron accumulation of narrow band gap semiconductors.

摘要

Narrow band gap semiconductors play a crucial role in thin film photovoltaic cells and optoelectronics devices operating in the infrared region of visible spectrum. The interactions between the valence and conduction bands due to the narrow band gap have a big influence on the electronic structure and the device performance of these materials. The surface and bulk electronic properties of narrow band gap semiconductors were investigated using angle resolved photoelectron spectroscopy (ARPES), x-ray absorption spectroscopy and x-ray emission spectroscopy. Comparisons were made between the experimental results and density functional theory band structure calculations.;Intrinsic electron accumulation near the surface of clean InN was directly observed by ARPES. The accumulation layer is discussed in terms of the bulk Fermi level (EF) lying below the pinned surface E F, with a confining potential formed normal to surface due to the downward band bending facilitated by donor type surface states or nitrogen vacancies. Various spectroscopic techniques were used to measure this band bending. The energy of the Fermi level and the size of the Fermi surface for these quantum well states could be controlled by varying the method of surface preparation and by the adsorption of potassium on the surface. Intermixing between the heavy and light hole valence bands in the intrinsic quantum well potential associated with the surface electron accumulation layer results in an inverted band structure, with the valence band maximum lying away from the Brillouin zone center. Similarly, the electronic band structure of CdO was investigated and quantized electron subbands were observed above the valence band maximum. The origin of the accumulation layer is discussed in terms of the bulk band structure of CdO calculated using quasi particle corrected density functional theory.;High electron density at the surface of these materials provides new opportunities for potential device structures such as sensors, high frequency transmitters and field effect transistors. Therefore the study of their near surface electron accumulation and electronic structure is of importance in understanding the properties of these materials and discovering new application areas.