• Meyers, Vincent
• Rocco, Emma
• Chan, W. K.
• Shahedipour-Sandvik, F.
• Mcewen, Benjamin
• Hill, David

Abstract

Quantum well infrared photodetectors (QWIP) based on III-Nitrides are a subject of increasing interest due to their potential for applications in telecommunications, optical gas sensors, military, and imaging. These detectors rely on intersubband transitions (ISBTs) of an electron (hole) between the ground state and the first excited state in the conduction (valance) band. Large conduction band offset (CBO) of nearly 2 eV [1], [2] and valance band offset (VBO) of nearly 0.85 eV [3] in AlN/GaN QWIPs make this material system suitable for a wide range of ISBTs depending on Al composition and specifically in the spectral regions where GaAs-based QWIPs are limited due to their smaller bandgap offsets. Also, the longitudinal-optical (LO) phonon energy of GaN (92 meV) is more than two times higher than GaAs [4], which makes this material an ideal choice for room temperature detection application. By employing GaN-based QWIPs, ISBT energies can be tuned from a few meV to 2 eV based on QWIP design, allowing for full NIR to IR band tunability within a single material system. While III-N materials can be grown in cubic zincblende structure, only hexagonal wurtzite structure is thermodynamically stable. In this structure, lack of inversion symmetry along the (growth) c-axis, generates a spontaneous polarization (Psp) along this axis [5]. Additionally, at AlGaN/GaN heterointerface, the lattice mismatch causes strain leading to piezoelectric polarization [6]. The discontinuity of polarization at heterointerfaces results in an uncompensated sheet charge at the interface. Further due to the polarization selection rule, normal incidence absorption is not allowed in n-QWIPs [7], requiring a light coupler and adding to design complexity. To circumvent this issue, p-QWIPs are being considered. In this work, we report on the growth of 5 periods Mg-doped Al0.4Ga0.6N/GaN QWIP, by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire for a target 1.55 µm absorption. Critical to the performance of the QWIP are high controllability of layer thicknesses and composition in superlattice and interface quality. Sub-nanometer control of layer thicknesses is challenging in MOCVD growth due to the high temperature and the relatively poorer control of precursor flux on the sample surface, compared to other methods such as MBE. To overcome this, a pulsed growth method was used to achieve higher control of growth rate [8]. A TCAD-based physics model was implemented in the commercial Synopsys Sentaurus to model the energy band diagram, electric field and calculate the band offsets between AlxGa1-xN/GaN heterointerfaces by considering polarization charges. Based on the CBO and VBO in the QW, layers thicknesses and compositions can be adjusted to achieve desired absorption wavelength. X-ray diffraction and transmission electron microscope (TEM) are used to assess strain, interface quality, layer thicknesses, and aluminum composition. Transmission/reflection measurement is used to determine the absorption spectrum of the QWIP. [1]D. Cociorva et al., ‘Quasiparticle calculations of band offsets at AlN–GaN interfaces’, Solid State Commun, 2002 [2]A. N. Westmeyer et al., ‘Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys’, J Appl Phys, 2006 [3]G. Martin et al., ‘Valence-band discontinuities of wurtzite GaN, AlN, and InN heterojunctions measured by x-ray photoemission spectroscopy’, Appl Phys Lett, 1996 [4]K. Ahi, ‘Review of GaN-based devices for terahertz operation’, Optical Engineering, 2017 [5]H. Ünlü et al., ‘Band offsets in III-nitride heterostructures’, J Phys D Appl Phys, 2002 [6]S. Nicolay et al., ‘Strain-induced interface instability in GaN/AlN multiple quantum wells’, Appl Phys Lett, 2007 [7]B. F. Levine, ‘Quantum-well infrared photodetectors’, J Appl Phys, 1993 [8]P. Suvarna et al., ‘Design and growth of visible-blind and solar-blind III-N APDs on sapphire substrates’, J Electron Mater, 2013

Topics

• impedance spectroscopy
• surface
• x-ray diffraction
• aluminium
• nitride
• transmission electron microscopy
• chemical vapor deposition
• luminescence