Technical: High In-composition alloys (InGaN, xIn > 0.25) are among the most unexplored materials in the III-nitride system. While the bandgap range of InGaN alloys can cover the entire spectrum from ultra-violet (Eg, GaN ~ 3.4 eV) to infrared (Eg, InN ~ 0.7 eV), technological applications until now are restricted to the higher energy range of this spectrum. The relatively low number of applications in the lower bandgap III-nitride alloys is due to the challenges associated with epitaxial growth, and poor understanding of the electrical, defect, and optical characteristics of these materials. This project will address the critical issues relating growth and structural properties to electrical, optical, and defect properties to achieve higher composition InGaN films on N-polar orientation of GaN. This project will investigate the growth kinetics for this material in metal-rich and nitrogen-rich regimes to create a comprehensive growth model for Ga- and N-polar InGaN. This study will include thick InGaN films and strained InGaN films on GaN to understand the physical origins of phase segragation, dislocation-mediated relaxation, and surface morphologies. Electrical and optical characterizations of the films will lead to an understanding of p-type and n-type doping, as well as the background defect incorporation in the films (through deep level optical spectroscopy).
The study will enable device applications that exploit the large bandgap range of III-nitrides. Extending the tremendous commercial success of III-nitride emitters to a larger range of optical and electronic applications will be transformative to the semiconductor technology industry. The PIs have a strong record of extending their research activities to involve high school students. The interdisciplinary nature of this project requires close collaboration between the different groups involved, and will be excellent training for undergraduate and graduate students involved.
Intellectual merit: The objective of this project is to investigate the growth and characterization of InGaN alloy semiconductors with high Indium composition. The growth of these alloys has been challenging due to the high volatiity of Indium during growth of these alloys. During the course of this project, the growth and properties of InGaN were investigated in detail. Nanometer scale InGaN films were then used to demonstrate unique device structures such as inverted polarity light emitting diodes and quantum tunnel junctions. The project research led to unprecedented low resistance tunnel junctions in the III-nitride system. These tunnel junctions could enable a new class of devices that were not feasible earlier. In particular, they can have a significant impact on the field of solid state lighting. The research done in this project showed that tunnel junctions could enable us to stack light emitting diodes on top of each other. This enables them to be operated at low current density while giving high power, thereby leading to a solution to the problem of efficiency droop, which has been the main challenge limiting the cost of solid state lighting. Broader Outcomes: The research done in the course of this project enabled the training of several graduate students and undergraduates in the fields of materials science, device engineering, and semiconductor physics. The work also led to intellectual property (patents) through the university. Some of this intellectual property has already been licensed and is under commercialization. Graduate students were involved in the commercialization process through writing of patents and disclosures.