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

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of GaN/(Al,Ga)N heterostructures grown by ammonia molecular beam epitaxy (NH3 MBE). Twenty-period GaN/(Al,Ga)N multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding GaN substrates and heteroepitaxial a-plane GaN on r-plane sapphire templates (a-GaN/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 105cm-2 are compared with those grown on a-GaN templates on (101¯2) r-sapphire with BSF, PSF, PD, and TD densities of about 4 × 105 to 106cm-2, 5 × 103 to 2 × 104cm-2, about 9 × 1010 to 2 × 1011cm-2, and about 1010cm-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 μm) 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-GaN/r-sapphire templates compared with those on freestanding GaN 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.