Michael J. Manfra and Oana Malis, Purdue University

III-nitride semiconductors have unique electronic properties that make them promising for extending the functionality of semiconductor light sources into spectral ranges currently inaccessible with other material systems. We propose to demonstrate and investigate a new class of far-infrared semiconductor lasers emitting in the 1-10 THz range (30-300 ìm wavelength range). These lasers will utilize intersubband transitions in the conduction band of lattice-matched III-nitride heterostructures and employ the general operating principles of quantum cascade lasers (QCLs). Our novel approach involves using low Al-composition, lattice-matched quaternary nitrides (AlInGaN/GaN) and high quality quasi-bulk GaN substrates to mitigate material quality issues that have hampered progress of nitride intersubband devices in the past. Our choice of lattice-matched heterostructures has the additional advantage of eliminating the effect of piezoelectric fields at hetero-interfaces, therefore facilitating conduction band engineering. To completely remove polarization discontinuities at hetero-interfaces, laser structures will also be grown on non-polar GaN substrates. The research effort will be interdisciplinary and will involve material design and growth, structural characterization, device fabrication, and device testing.

Intellectual merit: This project investigates the feasibility of a new class of semiconductor lasers to fill the underutilized THz gap that is currently inaccessible with any other semiconductor technology. If successful, this project will enable a novel compact, coherent, tunable THz light source with power output suitable for technological applications (milliwatt level). In addition to broader wavelength flexibility, the THz nitride QCLs are expected to have superior performance in terms of operating temperature and efficiency at the longer wavelengths currently available to GaAs THz QCLs. The focus of this research project will be on THz lasers using lattice-matched nitrides, but the knowledge acquired will also be relevant to nitride optoelectronic devices operating in other spectral ranges such as the near-infrared (telecom) range. Moreover, the acquired knowledge will also be valuable for other types of devices, such as transistors, and to other material systems. This project will advance the understanding of MBE growth of lattice-matched III-Nitride materials for complex and thick device structures. Underlying processes of material growth on polar and non-polar GaN substrates will be studied in detail and compared. The goal is to achieve adequate understanding and control of the microstructure in order to realize the theoretical potential of the material system. The research program proposed here will also give opportunities for major contributions to the understanding of the physics of charge transport and optical transitions in nitride materials. Basic nitride band structure parameters, such as conduction band offsets and intrinsic polarization fields, will be established as a function of composition for the quaternary alloys. The fundamental and practical limitations of resonant tunneling in nitride heterostructures will be identified.

Broader Impact: This project will enable a new class of compact, efficient terahertz power sources that will immediately impact a number of technological applications with broad benefits to society. The applications loosely fit into one of two main categories: THz spectroscopy, and THz imaging. THz spectroscopy is currently used in fields ranging from astronomy, and atmospheric science, to plasma fusion diagnostics and bio-chemical weapons detection. THz imaging has broad applications from airport security to medical imaging. This research program will provide unique interdisciplinary research opportunities to a diverse group of students at Purdue University. Special attention will be given to providing hands-on fabrication experience to under-represented undergraduate students. The excitement of scientific discovery will be conveyed through collaborations with several research groups in academia, industry (Kyma Technologies) and a federally funded laboratory (MIT Lincoln Laboratories). The PI's will be actively involved in mentoring minority students, in particular females, and students at risk. The research techniques and findings of this project will be integrated into graduate/undergraduate courses on the fundamentals of material growth by Molecular Beam Epitaxy and Optics, respectively. Outreach activities will include development of educational modules about basic optical properties of matter in the invisible ranges of the electromagnetic spectrum for public demonstrations in the Purdue's Physics on the Road Program. The PIs will also host activities for ScienceScape, a summer camp for middle-school girls. These activities will be focused on illustrating fundamental principles of light generation, propagation, and detection in a fun, project-oriented environment by asking students to create visually-compelling near- and far-infrared images.

Project Report

The major goal of this project was to investigate a new class of far-infrared semiconductor lasers emitting in the largely inaccessible 1-10 THz range. These lasers utilize intersubband transitions in the conduction band of III-nitride heterostructures and employ the general operating principles of quantum cascade lasers. Our unique approach involved using low Al-composition, lattice-matched quaternary nitride heterostructures and high quality polar and non-polar GaN substrates to mitigate material quality issues that have hampered progress to date. Moreover, our choice of lattice-matched heterostructures has the advantage of eliminating the effect of piezoelectric fields at hetero-interfaces. Major activities Molecular Beam Epitaxy (MBE) growth of III-nitrides Intersubband absorption in c-plane and m-plane nitride heterostructures Theoretical calculations of nitride band structure Fabrication of nitride intersubband devices Characterization of nitride intersubband devices Specific objectives Advance the understanding of MBE growth processes on polar and non-polar GaN substrates Determine the optical properties of lattice-matched nitrides Demonstrate resonant tunneling in nitride heterostructures Establish a theoretical understanding of optical properties of polar and non-polar nitrides Significant scientific results Demonstration of repeatable low-temperature negative-differential resistance from Al0.18Ga0.82N/GaN resonant tunneling diodes grown by MBE on free-standing GaN substrates Investigation of the effect of Si doping profile on the energy and linewidth of near-infrared intersubband absorption in AlGaN/GaN superlattices Comparative study of near-infrared intersubband absorption in AlGaN/GaN and AlInN/GaN superlattices on c-plane GaN First demonstration of terahertz intersubband absorption in non-polar m-plane AlGaN/GaN quantum wells Scientific outcomes and dissemination to the research community The research resulted in publication of 10 journal papers, 2 book chapters, and 3 PhD theses. The students and PIs presented the results of the research in 27 invited or contributed talks at prestigious conferences and seminars throughout the country. Educational outcomes Three graduate students and 7 undergraduate students performed research and received training while participating in this project. The project participants were also involved in outreach to the local community. 1. Graduate Research Three graduate students, Liang Tang, Mohammed Imrul Hossain, and Colin Edmunds, have been supported in part by this project. Liang was responsible with design of complex heterostructures, the MBE growth, and materials characterization via AFM and HRXRD. Colin Edmunds and Imrul Hossain have been responsible with theoretical design of heterostructures, near and far-infrared absorption measurements, nitride device fabrication and characterization. 2. Graduate and undergraduate teaching Profs. Manfra and Malis have developed an integrated graduate course on the physics and material science of optoelectronic devices. This course was simultaneously offered to and taken by Purdue graduate and undergraduate students in physics, materials engineering, electrical and computer engineering, nuclear engineering and chemical engineering (PHYS570) in the Springs of 2012, 2013, and 2014. The course covered fundamental concepts of semiconductor nanostructure fabrication with a special emphasis on growth by MBE, state-of-the-art nano-material characterization, properties brought about by quantum confinement (quantum states, electronic transport, and optical properties of low-dimensional systems), and the applications of these properties into novel devices. The research aspects of this project have been incorporated into new dedicated teaching modules on intersubband devices for infrared optoelectronic devices. 3. Undergraduate Research and Training Seven undergraduate students contributed to the nitride quantum cascade laser project and made invaluable contributions to the infrared lab infrastructure in the last year. Lisa Nash, Emily Finan, and Amanda Landcastle were summer REU students supported from the Purdue Physics REU program. Matthew Smith, a minority Purdue Physics undergraduate student, Karl Johnston, Matthew Turczi, and Yifan Zhang were all supported in part with funds from this NSF grant. The undergraduates performed near- to far-infrared absorption measurements. They also provided specifications for far-infrared optics for the THz absorption set-up. 4. Outreach activities In the summer of 2012, Prof. Malis hosted in her infrared spectroscopy lab a high-school teacher and a high-school student (Fig. 21). The teacher, John Simmons from Pike High School in Indianapolis, worked on designing experiments to illustrate the properties of infrared radiation to cover the Indiana Standards (Physics I: SCI.P.6.2, Integrated Chemistry-Physics: SCI.CP.4.4) using an FTIR spectrometer available for loan to local schools from Purdue’s Science Express. Science Express is a project developed by the Chemistry and Biology Departments at Purdue that acquires, maintains, and delivers research-grade instruments to high-schools throughout central Indiana. The high-school student, Ryan Hancock, a junior at Harrison High School in West Lafayette, IN, worked on developing simple experiments using commonly available cameras and materials to take visually compelling infrared pictures. Prof. Malis gave laboratory tours to the participants to the 4th Annual Midwest Conference for Undergraduate Women in Physics, January 2011. Prof. Malis is currently the faculty advisor of the Undergraduate Women in Physics program at Purdue University. Matthew Smith and Mohammed Imrul Hossain participated in the outreach activities related to Purdue’s Nanodays at the Birck Nanotechnology Center.

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Purdue University
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