Defect Tolerance of Intersubband Transitions in Nonpolar GaN/(Al,Ga)N Heterostructures: A Path toward Low-Cost and Scalable Mid- to Far-Infrared Optoelectronics

Abstract

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_{3}$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^{5}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ to ${10}^{6}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, 5 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{4}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, about 9 \ifmmode\times\else\texttimes\fi{} ${10}^{10}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{11}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, and about ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250--300 meV (wavelength range 4.1--4.8 \textmu{}m) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%--20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.