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. Al_xGa_1‐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 Al_xGa_1−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.