Intersubband Transitions in Nonpolar m-Plane AlGaN/GaN Heterostructures

Nonpolar AlGaN/GaN heterostructures have the potential to supplant polar heterostructures in infrared optoelectronic devices due to their theoretical advantages stemming from the absence of built-in polarization fields along nonpolar directions of the nitride wurtzite lattice. However, development of nonpolar m-plane infrared devices in a broad spectral range has been hampered, so far, by challenges to grow homogeneous high Al-composition AlGaN on m-plane GaN. AlxGa1-xN layers with 0.6 < x < 0.8 are found to be kinetically unstable under metal-rich growth conditions by plasma-assisted molecular-beam epitaxy. After reviewing recent progress in the field, this paper focuses on the effect of the structure of m-plane AlxGa1-xN/GaN (x < 0.6) superlattices on near-infrared intersubband absorption. Even at these intermediate Al-compositions, the effective growth rate of AlGaN is drastically reduced, and the AlGaN-GaN interface roughness is unexpectedly high. Consequently, accurate determination of layer thicknesses and alloy composition necessitates structural characterization by a combination of scanning transmission electron microscopy and high-resolution X-ray diffraction. The energy and linewidth of near-infrared intersubband transitions are also significantly affected by this unusual growth behavior. The experimental results for intersubband absorption of m- and c-plane superlattices are compared to each other and with numerical calculations, and the main reasons for discrepancies are discussed.