Intersubband Transitions in GaN/Al x Ga 1-x N Multi Quantum Wells
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Intersubband Transitions in GaN/AlxGa1-xN Multi Quantum Wells E. A. DeCuir Jr.1), Y.C. Chua1), B.S. Passmore 1), J. Liang, 1), M.O. Manasreh1) J. Xie2) H. Morkoc2), A. Asghar3), I. T. Ferguson3), and A. Payne3) 1) University of Arkansas, Department of Electrical Engineering, Fayetteville, AR 72701, USA 2) Virginia Commonwealth University, Department of Electrical Engineering and Physics Department, Richmond, VA 23284, USA 3) Georgia Institute of Technology, Department of Electrical Engineering, Atlanta, GA 30332, USA ABSTRACT Intersubband transitions (ISTs) in GaN/AlxGa1-xN multiple quantum wells (MQWs) were investigated using an optical absorption technique. Several samples were grown by either Molecular Beam Epitaxy (MBE) or Metal-Organic Chemical Vapor Deposition (MOCVD) and were investigated using both normal incident and waveguide configurations. The waveguides were fabricated by dicing each sample into 2 mm wide by 5 mm long pieces with two facets polished at 45 degrees with respect to the surface such that light propagates across the sample’s width. Preliminary results indicate that ISTs are observable in Si-doped and undoped GaN/AlxGa1-xN MQWs. The source of these charge carriers in the undoped samples are explained as being due to the spontaneous polarization effect which exists at the GaN/AlxGa1-xN interfaces where the GaN surface has Ga-polarity. Scanning Electron Microscopy indicates that a sample containing what appeared to be a large number of cracks and or hexagonal voids lacked the presence of ISTs. INTRODUCTION There has been much consideration given to III-nitride semiconductors in optical devices due to the availability of direct bandgaps ranging from 0.7 eV for InN to 6.1 eV for AlN [1]. The insertion of the lower bandgap GaN material between two layers of larger bandgap ternary AlxGa1-xN alloy gives rise to a quantum well structure whose periodicity can be utilized to create MQWs of AlxGa1-xN/GaN. These MQWs structures could be tuned by varying the aluminum content, doping, well parameters, and barrier parameters to cover a multitude of wavelengths for detector applications. The observation of ISTs in III-V semiconductor quantum wells was first reported in 1983 by Smith et al [2] benchmarked an era that would witness a flurry of research which culminated in quantum well infrared photodetectors (QWIPs). While these transitions have been well documented, mainly in AlGaAs/GaAs quantum structures in the subsequent years [3-8], there has been only recent attention given to AlxGa1-xN/GaN MQWs particularly for the communication wavelength of 1.55 µm [9-14]. Inherent properties of nitride semiconductors such as large electron effective mass [12] (m*~0.2-0.3m0), large LO phonon energy [12] (ELO~91meV), large conduction band offsets, and fast IST relaxation times [10,11] (~150-370 fs) make GaN based nitride semiconductors an ideal contender for high speed detectors. Growth related issues attributed to lattice mismatch [1] of sapphire (Aluminum Oxide) and GaN ( ≈ 13% ) have continued to plague the
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