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Schottky barrier photovoltaic detectors have been fabricated on n-AlxGa1−xN(0⩽x⩽0.35) and p-GaN epitaxial layers grown on sapphire. Their characteristics have been analyzed and modeled, in order to determine the physical mechanisms that limit their performance. The influence of material properties on device parameters is discussed. Our analysis considers front and back illumination and distinguishes between devices fabricated on ideal high-quality material and state-of-the-art heteroepitaxial AlxGa1−xN. In the former case, low doping levels are advisable to achieve high responsivity and a sharp spectral cutoff. The epitaxial layer should be thin (<0.5 μm) to optimize the ultraviolet/visible contrast. In present devices fabricated on heteroepitaxial AlxGa1−xN, the responsivity is limited by the diffusion length. In this case, thick AlxGa1−xN layers are advisable, because the reduction in the dislocation density results in lower leakage currents, larger diffusion length, and higher responsivity. In order to improve bandwidth and responsivity, and to achieve good ohmic contacts, a moderate n-type doping level (∼1018 cm−3) is recommended.

Polarization-sensitive photodetectors for the ultraviolet spectral range based on M-plane GaN films grown on LiAlO 2 substrates have been fabricated and characterized. These detectors exploit the dichroic properties of strained, M-plane GaN films. For a 400-nm-thick film, a maximum contrast of 7.25 between the detection of light polarized perpendicular and parallel to the c-axis is reached at 363 nm. Considerations for the detector design show that thin strained M-plane GaN films will enhance the polarization-sensitive bandwidth, while the maximum contrast can be obtained for relaxed thick films under weak signal detection conditions.

The authors demonstrate a photodetection configuration where the responsivity in the ultraviolet spectral region is limited to a few nanometers, representing high-quality-factor, narrow-band detection together with polarization sensitivity. Both features are obtained by utilizing a polarization-sensitive photodectector in combination with a polarization filter made from two identical M-plane GaN films on γ-LiAlO2 (100) substrate. The optical band gap of these films depends on the direction of the in-plane polarization vector of the incident light beam with respect to the c axis. Electronic-band-structure calculations show that the naturally present anisotropic in-plane strain in these films is the crucial parameter to achieve both a high responsivity and a high polarization contrast.

GaN:Mg/AlGaN single-heterojunction light-emitting diodes were grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy. High-quality Mg-doped GaN layers with hole concentrations up to 1.2×1017 holes/cm3 and intense low-temperature photoluminescence, which increases in annealed samples, were obtained. Smooth AlGaN layers, with surface roughness below 5 nm, were used as buffer layers. Continuous-wave room-temperature ultraviolet electroluminescence was observed at 365 nm with a full width at half maximum of 8 nm. An estimated optical power output of 1.5 μW was achieved under 15 V/35 mA operation.

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The in-plane asymmetry of M-plane GaN films was used to fabricate polarization-sensitive photodetectors. The measured contrasts between the detected light polarized perpendicular and parallel to the c-axis are as high as 7.25 at 363 nm for a 0.4 µm thick film. For other film thicknesses, the polarization-sensitive bandwidth can be increased. At the same time, the position of the maximum contrast is also affected by the film thickness through a different strain state. Considerations for the detector design show that the film thickness affects both, the contrast and polarization-sensitive bandwidth, through the absorption and the effect of the strain on the valence-band splitting. Due to the absence of piezoelectric fields, heterostructures with nonpolar surfaces allow for the increase of the overlap between the electron and hole wavefunctions, improving the radiative efficiency for quantum wells. The growth along nonpolar orientations has so far mainly produced M-or A-plane GaN films so that the c-axis lies in the film plane, giving rise to an optical anisotropy due to the reduced in-plane crystal symmetry [2]. This intrinsic optical anisotropy results in polarization-sensitive properties near the fundamental band gap [3]. In addition, the in-plane polarization anisotropy can be further enhanced by anisotropic strain in the film plane, which is generated by the lattice mismatch between the GaN film and the substrate. In particular, the valence-band structure (VBS) of M-plane GaN grown on a lithium aluminate (LiAlO 2 ) substrate is modified in such a way that transitions between the two topmost VBs and the conduction band become polarized either purely x-like (polarization perpendicular to the c-axis) or z-like (polarization parallel to the c-axis). Both theoretical and experimental investigations have shown that these properties can be exploited to realize polarization-sensitive photodetectors (PSPDs) in the ultraviolet spectral range [3]. In this work, we present the fabrication, optical characterization, and design concepts of M-plane GaN PSPDs.