Collaborative Research -Analysis of Defects and Their Causes in Bulk Aluminum Nitride Crystals – Jharna Chaudhuri, DMR-0515858 Texas Tech University, Lubbock, TX
Objective: To investigate basic defect mechanisms in AlN single crystal growth and oxidation of AlN, to identify process conditions responsible for initiation of defects, and to reduce or control defect density by growth and processing conditions. High resolution transmission electron microscopy (HRTEM) provides key structural information to establish relationships between processing parameters and defects present as well as structure/property relations. Simulations and models based on diffraction theories are used to identify the origin of defects.
interface
Figure1. High resolution bright field TEM image from the interface region of an AlN single crystal oxidized at 1000°C indicating AlN is perfect without any defects present, and Al2O3 is crystalline with several large grains. Contrast in AlN is due to local bending.
Figure 2. Inverse fast Fourier transform (IFFT) of a HRTEM image from the interface region of AlN oxidized at 800°C indicating defects such as dislocations, stacking faults and domains. Regions A and B contain large and small angle domains, respectively; d = dislocations.
Results •Oxidation of AlN: at high temperature (1000 °C), a thick crystalline oxide layer (450 nm), with several large grains, is formed with an abrupt compositional interface between Al2O3 and AlN (Figure 1), with the underlying AlN nearly defect-free. The oxide layer is mostly the stable α phase except at the surface where a cubic, or γ phase, is formed. In contrast, at lower oxidation temperature (800 °C), an amorphous oxide is formed, and the nitride near the interface contains a high density of defects (dislocations, stacking faults and planar defects) as shown in Figure 2, along with a measurable amount of oxygen and nitrogen. •Mechanism: Oxidation of AlN produces oxygen and aluminum interstitials. At high temperature, the diffusion of these interstitials is rapid as is reaction kinetics involving these point defects allowing thermodynamic equilibrium to be reached throughout the nitride. In contrast, at low temperature, diffusion and reaction kinetics of the nitrogen and aluminum interstitials are much slower, leading to super-saturation of point defects in the nitride at the interface. This build up of excess point defects leads to precipitation of line and planar defects as observed near the interface. Significance •This research has demonstrated for the first time defects and structures produced during the oxidation of single crystal AlN. The results obtained will be useful in the fabrication of high quality dielectric Al 2O3 thin films. This basic materials research opens up new opportunities for making field effect transistors, and other electronic and optical devices.
Collaborative Research -Analysis of Defects and Their Causes in Bulk Aluminum Nitride Crystals – Jharna Chaudhuri, NSF Grant # DMR-0515858 Texas Tech University, Lubbock, TX
Figure 1. Ph. D. student Mr. Luke Nyakiti (left) and MS student Mr. Rac Lee (right) are learning high resolution transmission electron microscopy work at CMM, University of Ill, Urbana-Champaign, Ill.
Figure 2. Undergraduate student Ms. Jamie Armstrong (right) and principal investigator Dr. Jharna Chaudhuri (left) are discussing a research project.
Broader Impact •One Ph. D., one MS and one undergraduate (female) students (Figures 1 and 2) involved in this research are advancing their education by discovery through experiments they design and perform, through training on sophisticated instrumentation, and application of state-of-the-art modeling techniques. •This project has many learning opportunities for students beyond what is possible at Texas Tech University. Students are interacting closely with Professor Edgar and his students at Kansas State University. They are also getting trained in using high resolution transmission electron microscopy at the Center for Microanalysis of Materials (CMM), University of Illinois, Urbana-Champaign, IL, a DoE funded user facility. •Based on the current research finding one paper has been presented in the Fourteenth International Materials Research Congress in August, 2005 and one paper has been submitted for publication. We are writing another paper on the effect of Si doping on crystalline quality of AlN.