Polarization-sensitive ultraviolet photodetectors based on M-plane GaN grown on LiAlO2 substrates

C. Rivera, J. L. Pau, E. Muñoz, P. Misra, O. Brandt, H. T. Grahn, K. H. Ploog
2006 Applied Physics Letters  
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
more » ... ilms will enhance the polarization-sensitive bandwidth, while the maximum contrast can be obtained for relaxed thick films under weak signal detection conditions. Light polarization control is of great importance in a wide range of scientific and technological areas. For semiconductor optoelectronic devices, polarization control is usually obtained as a consequence of the selection rules in lowdimensional structures fabricated along low-symmetry directions. The realization of polarized light-emitting diodes, 1 tunable polarization converters, 2 polarization threshold switches, 3 and polarization-sensitive photodetectors 4,5 ͑PSPDs͒ has been reported. One mechanism to increase the optical anisotropy and therefore the polarization sensitivity is the application of anisotropic strain, which reduces the symmetry of the crystal structure. Currently, there is an increasing interest in the control of the polarization state for the ultraviolet ͑UV͒ spectral range, in particular, for data storage, sensing applications, and biophotonics. UV PSPDs based on M-plane GaN were proposed a few years ago by Ghosh et al. 6 M-plane group-III nitrides exhibit highly anisotropic optical properties that allow for the polarization-sensitive detection in the visible and UV spectral ranges. The anisotropy can be further enhanced by the presence of in-plane anisotropic strain generated by the lattice mismatch between GaN and the LiAlO 2 substrate. Both static 7 and dynamic 8 polarization rotations have been recently demonstrated, achieving polarization rotation angles as high as 40°͑static͒ and 35°͑ dynamic͒. Theoretical studies and photoreflectance ͑PR͒ spectra have revealed that the anisotropic strain generated in M-plane films grown on LiAlO 2 substrates leads to modifications of the valence band structure ͑VBS͒ so that the two interband transitions in the vicinity of the energy gap become completely linearly polarized either parallel ͑E ʈ c͒ or perpendicular to the c axis ͑E Ќ c͒. 9 In this letter, we describe the fabrication and performance of PSPDs based on anisotropically strained M-plane GaN films on LiAlO 2 . In addition, we discuss some important issues for the design of such photodetectors. The M-plane GaN film was grown by rf plasma-assisted molecular-beam epitaxy ͑PAMBE͒ on a ␥-LiAlO 2 ͑100͒ substrate. 10 The M-plane orientation of the film and its single phase nature ͑i.e., lack of C-plane-oriented inclusions͒ were verified by means of triple-axis high-resolution x-ray diffraction ͑HRXRD͒. The HRXRD also showed that the M plane was under biaxial compressive strain with an out-of-plane tensile strain of yy = 0.43% ͑cf. inset of Fig. 1 for the choice of coordinates in the wurtzite unit cell of GaN͒. A film thickness of 400 nm was determined by scanning electron microscopy. The carrier concentration in the sample studied was about 3.5ϫ 10 15 cm −3 ͑n type͒ as extracted from capacitance-voltage ͑C-V͒ measurements of metal-insulatorsemiconductor ͑MIS͒ diodes, indicating a complete depletion of the M-plane GaN film. Both semitransparent Schottky barrier and MIS structures were fabricated on M-plane GaN films as PSPDs. We used a planar geometry to form the contacts. The semitransparent electrode was made of Au ͑12 nm͒, while the pad contacts were made of Ni ͑50 nm͒ /Au ͑200 nm͒ and Ti ͑50 nm͒ /Al ͑200 nm͒ for the rectifying and Ohmic electrodes, respectively. The insulator used in the MIS structures was a 120-nm-thick SiN x layer deposited by plasmaenhanced chemical vapor deposition. Photoluminescence ͑PL͒, photocurrent, and C-V measurements were performed on these PSPDs. Continuouswave ͑cw͒ PL was excited with the 244 nm line from a second harmonic generator pumped by an Ar + laser. Photocurrent spectra for front illumination were obtained in the 325-400 nm range by using a linearly polarized beam from a 150 W xenon arc lamp filtered by a monochromator. The dependence of the photocurrent on the in-plane polarization angle of the incident beam, where = 0°corresponds to E ʈ c, was carried out by rotating the sample keeping the polarization of the incident beam fixed. The relative responsivity was then determined by normalizing the photocurrent to the system's response measured with a calibrated Si photodiode. The absolute responsivity was accurately determined using a He-Cd laser ͑325 nm͒. The C-V characteristics were measured using an HP4284A impedance analyzer with 50 mV test signal at 10 kHz. The effect of the in-plane anisotropic strain in the M-plane GaN film on the optical polarization properties has a͒ Electronic
doi:10.1063/1.2206128 fatcat:ajymdqzp4bdjhci4btz52qe4aq