Vacancy defect and defect cluster energetics in ion-implanted ZnO

We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation.Peer reviewe

Topological Dirac Semimetal Na3Bi Films in the Ultrathin Limit via Alternating Layer Molecular Beam Epitaxy

Ultrathin films of Na3Bi on insulating substrates are desired for opening a bulk band gap and generating the quantum spin Hall effect from a topological Dirac semimetal, though continuous films in the few nanometer regime have been difficult to realize. Here, we utilize alternating layer molecular beam epitaxy (MBE) to achieve uniform and continuous single crystal films of Na3Bi(0001) on insulating Al2O3(0001) substrates and demonstrate electrical transport on films with 3.8 nm thickness (4 unit cells). The high material quality is confirmed through in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). In addition, these films are employed as seed layers for subsequent growth by codeposition, leading to atomic layer-by-layer growth as indicated by RHEED intensity oscillations. These material advances facilitate the pursuit of quantum phenomena in thin films of Dirac semimetals.Comment: 11 pages, 5 figure

Surfaces and interfaces of electronic materials

An advanced level textbook covering geometric, chemical, and electronic structure of electronic materials, and their applications to devices based on semiconductor surfaces, metal-semiconductor interfaces, and semiconductor heterojunctions. Starting with the fundamentals of electrical measurements on semiconductor interfaces, it then describes the importance of controlling macroscopic electrical properties by atomic-scale techniques. Subsequent chapters present the wide range of surface and interface techniques available to characterize electronic, optical, chemical, and structural properti

Final Report for Department of Energy Grant DE-FG02-97ER45666 ?Interface Diffusion and Deep Level Formation of SiC and Other Wide Gap Materials?

This final report describes the research effort focusing on the nature of charge transfer and localized electronic structure at semiconductor interfaces, one of the most fundamental issues in the solid state. The basic charge exchange between two materials in general is directly connected with the systematic atomic bonding changes and redistribution that occurs at the nanoscale interface. Our programhas extended our understanding of the atomic-scale nature of electrostatic barrier formation, heterojunction band offsets, and the optical and electronic features of impurity confinement in a set of model materials systems, including nanometer-scale wide band gap semiconductor and insulator structures. This fundamentally new class of materials investigation utilizes a powerful and unique combination of techniques that are revealing the atomic-scale movement, chemical bonding, and resultant charge transfer across well-defined model interfaces

Glennan Microsystems Initiative

During the 2001-2002 award period, we performed research on Pt/Ti/bare 6H-SiC and bare 4H-SiC interfaces in order to identify their electronic properties as a function of surface preparation. The overall aim of this work is to optimize the electronic properties of metal contacts to SiC as well as the active SiC material itself as a function of surface preparation and subsequent processing. Initially, this work has involved identifying bare surface, subsurface, and metal induced gap states at the metal-SiC contact and correlating energies and densities of deep levels with Schottky barrier heights. We used low energy electron-excited nanoluminescence (LEEN) spectroscopy, X-ray photoemission spectroscopy (XPS), and Secondary Ion Mass Spectrometry (SIMS) in order to correlate electronic states and energy bands with chemical composition, bonding, and crystal structure. A major development has been the discovery of polytype transformations that occur in 4H-SiC under standard microelectronic process conditions used to fabricate SiC devices. Our results are consistent with the stacking fault generation, defect formation, and consequent degradation of SiC recently reported for state-of-the-art ABB commercial diodes under localized electrical stress. Our results highlight the importance of -optimizing process conditions and material properties - anneal times, temperatures and doping to control such structural changes within epitaxial SiC layers. Thus far, we have established threshold times and temperatures beyond which 4H-SiC exhibits 3C-SiC transformation bands for a subset of dopant concentrations and process conditions. On the basis of this temperature time behavior, we have been able to establish an activation energy of approximately 2.5 eV for polytype transformation and dislocation motion. Work continues to establish the fundamental mechanisms underlying the polytype changes and its dependence on material parameters

Impact of ultrathin Al[sub 2]O[sub 3] barrier layer on electrical properties of LaLuO[sub 3] metal-oxide-semiconductor devices

Temperature-dependent current-voltage measurements showed Poole–Frenkel conduction behavior through high-κ LaLuO3 films made by atomic layer deposition on Si. The energy levels that trap electrons were around 0.66 eV below the conduction band and were identified as oxygen vacancy levels. Oxygen treatments were done to decrease oxygen vacancies but an interfacial layer formed and the interface state density (Dit) increased. Therefore, ultrathin Al2O3 was used to protect the interface during oxygen treatments. Electrical properties were improved and no interfacial layer developed. Dit was below 9×1011eV−1cm−2 and leakage was 5×10−4A/cm2 at 1 V for 1 nm equivalent oxide thickness.Chemistry and Chemical Biolog