Technical: This project aims to understand the chemistry, structure, and optical sensitivity of the Mg impurity used to produce p-type GaN so that the present limitations to achieve a high dopant density can be understood and new approaches to doping may be realized. It employs electron paramagnetic resonance (EPR) spectroscopy to probe the optical sensitivity and chemical reactivity of the Mg impurity in GaN. Specifically, changes in the thermal annealing behavior between films grown with the typical Mg concentration used in the present day devices and those grown by methods intended to achieve high hole densities are monitored. Further, the research involves a new method to probe the Mg impurity: time-dependent photo-EPR. Analysis of such data could provide information about charge exchange between Mg and other defects, which will likely change as Mg concentration is increased. Additional experiments include high frequency EPR available at the National High Magnetic Field Laboratory. The high frequency experiments may reveal the local symmetry of the impurity and the role of strain. Overall, the project is designed to investigate factors that limit effective p-type doping and may provide direction to maximize hole density and conductivity of GaN.
The project addresses basic research issues in a topical area of materials science with high technological relevance. A success of the research project could contribute to development of better light emitting diodes and electronic devices. The PI, who has mentored many undergraduates in science, mathematics and engineering, teaches a sophomore-level Science and Technology Honors seminar at the University of Alabama, which includes a section on future lighting technologies and efficient high power/high frequency electronic devices. The collaboration of the Honors students with PhD candidates in the PI's laboratory will foster a sense of interdisciplinary research essential for success complement to their formal education.
Recently, the introduction of the highly efficient white-light LED has revolutionized lighting technology. The fundamental motivation for using LEDâ€™s is that a much higher fraction of the energy extracted from the wall outlet becomes visible light than in the case of the light bulb. However, the ultimate success of the white light LED depends on the quality of a relatively new class of materials known as nitride semiconductors. Like silicon, another semiconductor which permeates our life, the nitridesâ€™ utility lies in the ability to controllably add small amount of impurities - less than one in a million normal atoms. The focus of our work is the study of one of those impurities, magnesium (Mg), which is required to achieve a particular type of conductivity. Our work shows that this essential impurity is not chemically or electrically the same in all nitrides. Specifically, the results show that in the most basic material, gallium nitride, the impurity reacts with hydrogen at different temperatures and these temperatures are related to the amount of Mg in the material. The hydrogen-Mg reaction is essential for forming an effective semiconductor; thus our studies demonstrate that optimization of the materials requires knowledge of the concentration of Mg incorporated in the nitride. We also show that the local surrounding of the Mg is not the same in all gallium nitride samples. When aluminum is incorporated into gallium nitride to form one of the important alloy variations, the situation gets even more complicated. Not only does the local surroundings of the Mg change, but the number of effective Mg atoms is reduced by as much as 50%. In such material the conductivity is reduced, making the material of limited use. Mg in indium-gallium-nitride, another alloy variant, also suffers from a perturbed environment. Here, our results suggest that the Mg is strained by the presence of the indium. Fortunately, however, there does not appear to be a significant loss of effective Mg. The work described above provided several opportunities for students of all ages, post-doctoral to undergraduates, to participate in the advancement of LED-based lighting technology. In particular, several undergraduates spent their summer learning about the role of the Mg impurity and making measurements pertinent to our present understanding. Two PhD students were granted their M.S. in Physics while working on this project, and presented their work at several local and international conferences. In summary, our work has examined the Mg impurity in a variety of nitride semiconductors, a material essential to the manufacture of white-light LEDs. We show that one of the hurdles that must be conquered for improved nitride material is variations in the surroundings of the Mg impurity itself. In particular, the loss of effective Mg needs to be addressed by devising new growth techniques which focus on issues directly related to impurity incorporation.