This project advanced the understanding of Molecular Beam Epitaxy (MBE) growth of nonpolar III-nitride materials for complex optoelectronic applications such as infrared lasers and detectors. Our innovative approach eliminated the adverse effects of built-in (piezoelectric and spontaneous) polarization fields in nitrides by engineering quantum charge-transport along the polarization-free [1-100] direction using heterostructures grown on m-plane GaN substrates. Basic mechanisms of material growth such as adatom kinetics and impurity incorporation on polar and non-polar GaN substrates have been studied in detail and compared to achieve adequate understanding and control of the growth process, resulting microstructure and infrared optical properties. The research program also gave opportunities for major contributions to the understanding of the physics of optical transitions in nitride materials. Important nitride band structure parameters have been tested for the AlGaN/GaN material systems. Realistic material modeling tools and techniques have been developed to explain and predict infrared optical properties.

The research activities included material design, growth, and characterization, as well as device modeling, fabrication, and testing. AlGaN/GaN single and multi-quantum wells (QWs) were grown by MBE on high-quality free-standing m-plane GaN substrates and characterized in detail with high-resolution x-ray diffraction and transmission electron microscopy. The optical properties of the materials and devices were examined with Fourier transform infrared spectroscopy, and visible photoluminescence. Intersubband absorption and emission measurements were employed to experimentally determine the parameters that are critical for optical device performance (energies, linewidths, lifetimes, etc.), and to test the validity of the developed models. A detailed comparison of experimental results on c- and m-plane GaN with theoretical predictions provided guidance for infrared nitride device design, in particular for operation at the telecommunication wavelength of 1.55-?m. Intersubband devices were fabricated and their performance was characterized with optical and electrical transport measurements.

Significant research results include:

1. Study of surface morphology evolution of m-plane GaN during MBE growth: impact of Ga/N ratio, miscut direction, and growth temperature
2. Growth of homogeneous low-Al AlGaN/GaN superlattices on free-standing m-plane GaN substrates by MBE
3. First demonstration of terahertz intersubband absorption in non-polar m-plane AlGaN/GaN quantum wells
4. Growth of homogeneous, high-aluminum AlGaN/GaN superlattices on free-standing m-plane GaN substrates by plasma-assisted molecular beam epitaxy
5. Effect of Al-composition and doping profile on the near-infrared optical properties of m-plane AlGaN/GaN superlattices and comparison between m-plane and c-plane heterostructures
6. Interface roughness effects on coherent vertical electron transport in AlGaN/GaN resonant tunneling diodes and sequential tunneling devices
7. Kinetic instability of AlGaN alloys during MBE growth on m-plane GaN under metal-rich conditions

Four graduate students, nine undergraduate students, and two postdoctoral associates in Physics and Engineering performed research and received training while participating in this project. The research resulted in publication of 12 peer-reviewed papers, two PhD theses, and 27 presentations at conferences or seminars.

Outreach

Dr. Malis hosted a high-school student, Arpitha Gadag, West Lafayette High-School, in the infrared spectroscopy lab in the summers of 2013 and 2014. Arpitha performed experiments to demonstrate the impact of infrared radiation into every-day life. The experiments were recorded into instructional videos to be used by high-school teachers and students. Arpitha also performed experiments related to infrared spectroscopy of gases. For this purpose, she designed and fabricated a gas cell. Arpitha also designed lesson plans to supplement the high-school curriculum.

Matthew Smith and Mohammed Imrul Hossain participated in the outreach activities related to Purdue?s Nanodays (April 25-26, 2013) at the Birck Nanotechnology Center.

As the faculty advisor for the Undergraduate Women in Physics at Purdue, Prof. Malis organized the APS Conference for Undergraduate Women in Physics (CUWiP) 2015 at Purdue (Jan 16-18, 2015). 165 undergraduate women majoring in Physics from the Midwest participated in this 3-day career development event intended to increase awareness of undergraduate women of their career options with or without a PhD. It was also designed to combat isolation, counteract the effects of gender stereotyping and to build persistence by providing role models and networking opportunities. Prof. Malis has played a leading role in all aspects of the conference planning including session organization, fund raising, advertising, etc.

Prof. Malis organized and conducted outreach activities at Imagination Station, a local children?s museum in West Lafayette, IN (March 2017). The activities introduced children age 4-12 to hands-on optics experiments involving light reflection, refraction, color formation, optical illusions and other physical phenomena. The children used visible and ?invisible? ultraviolet and infrared light. Specifically, the children were able to view the world through and take pictures with two commercial (FLIR) infrared cameras.

Prof. Malis and her graduate students developed and performed optics-related outreach activities at the ?Physics Inside Out? summer camp for middle-school students from economically disadvantages area around Lafayette, IN, organized by the Dept. of Physics and Astronomy in June 2017 and July 2018.  

Last Modified: 03/15/2019
Modified by: Oana Malis