M ‐plane GaN‐based dichroic photodetectors

Abstract

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.

(Al,In,Ga)N‐based photodetectors. Some materials issues

“…Besides, the in-plane crystal symmetry can be further reduced by the effect of in-plane anisotropic strain generated by the lattice mismatch between the GaN and the substrate. In particular, the valence band structure of M-plane GaN grown on LiAlO 2 substrates is modified in such a way that the topmost transitions become purely polarized either perpendicular (E ⊥ c) or parallel (E || c) to the c-axis [9,10,81,82]. These properties have been used to fabricate SB as well as MIS polarization-sensitive UV photodetectors [10].…”

“…UV PSPD based on M-plane GaN films grown on LiAlO 2 substrates have been recently fabricated [10,81]. These detectors exploit the dichroic properties of strained M-plane GaN films.…”

“…Design considerations under weak signal detection have shown that the optimum film thickness is obtained for partially relaxed films. Device photo-response was linear as a function of excitation power in the measured range of 2 × 10 -5 to 1 W/cm 2 [10,81].…”

M‐plane III‐nitride materials for polarization sensitive devices grown by PAMBE with real time analysis by spectroscopic ellipsometry

“…This anisotropic optical response can be used to fabricate polarization sensitive photodetecotors. Recently, Rivera et al [8] have shown that such photodetectors can be fabricated using GaN with improved photoresponse and large polarization selectivity. Here we describe the fabrication and performance of PSPDs based on strained InGaN epilayers grown on GaN/LiAlO 2 substrates.…”

Strained M-plane GaN for polarization-sensitive applications

“…The specific detectivity in these devices was higher than 5xlO9 cmHz'/2W-' in photovoltaic operation [11]. The maximum sensitivity in the noise measurement was limited by the low parallel resistance of the devices.…”