Nonpolar-Oriented GaN Films for Polarization-Sensitive and Narrow-Band Photodetectors

Abstract

This article reviews the optical polarization properties of unstrained and strained GaN films with a nonpolar orientation. In unstrained a -plane GaN films, the A exciton becomes completely linearly polarized perpendicular to the c-axis, whereas the B and C excitons are only partially polarized. In m -plane or a -plane GaN films under anisotropic in-plane compressive strain, all three interband transitions between the three uppermost valence bands and the conduction band can become linearly polarized for sufficiently large strain values. The complete linear polarization can be directly observed in reflection, transmission, or photoreflectance by a polarization-dependent energy gap. This complete linear polarization can be used to realize polarization-sensitive photodetectors in the ultraviolet spectral range, which do not need a polarization filter in front of the photodetector. By combining a polarization filter and photodetector or two photodetectors from the same material with their c-axes oriented perpendicular to each other, a narrowband photodetection configuration can be achieved in the ultraviolet spectral range with a band width below 8 nm. Since both realizations are also polarization sensitive, a configuration with four photodetectors is necessary to achieve narrow-band sensitivity regardless of the polarization state of the incident light. At the same time, the configuration with four photodetectors allows for the determination of the absolute angle of polarization.