Cubic and hexagonal gallium nitride/aluminum nitride superlattice structures for near-infrared detector applications

Intersubband transitions and photoconductivity were observed for the first time in metastable GaN/AlN cubic superlattice structures. Further studies of intersubband transitions and photoconductivity in the well-studied stable hexagonal system were also presented herein. The main motivation prompting this study is linked to the polar nature of the hexagonal crystal structure system grown on c-plane sapphire substrates, which gives rise to the presence of large polarization fields (up to 10 MV/cm). Since these fields have been shown to complicate the design of intersubband transitions, a study of the nonpolar cubic phase, lacking these large built-in fields due to its symmetric nature, is presented in an effort to reduce the complexity of future device design.;Structural characterization of GaN/AlN superlattices was performed using X-ray Diffraction, Atomic Force Microscopy, and Transmission Electron Microscopy. Optical and electrical characterization of intersubband absorption and photoconductivity in cubic and hexagonal GaN/AlN superlattices were recorded respectively using Fourier-transform spectroscopy in the temperature range of 77K--300K. An Electrochemical capacitance voltage technique was used to measure and confirm the three dimensional carrier concentrations in doped GaN layers since the presence of free carriers in the conduction is required for intersubband transitions. Reactive ion etching and e-beam evaporation was employed in the fabrication of mesa device structures with ohmic contacts. Theoretical calculations of the bound state energy levels in quantum well structures were made using a transfer matrix technique and then compared with experimental results.