Nonpolar-Oriented GaN Films for Polarization-Sensitive and Narrow-Band Photodetectors
- PDF / 527,194 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 73 Downloads / 141 Views
conductor material as shown in Figure 1. More recently, the optical polarization properties of bulk-like GaN with semipolar and nonpolar orientations have been investigated by photoluminescence and reflectance spectroscopy.5 The control of light polarization is of great importance in a wide range of scientific and technological areas. The realization of polarized light-emitting diodes,6 tunable polarization converters,7 polarization threshold switches,8 and polarizationsensitive photodetectors9,10 has been reported. One mechanism to increase the optical anisotropy and therefore the polarization sensitivity in GaN films is the application of anisotropic strain, which reduces the symmetry of the crystal structure. Currently, there is increasing interest in the control of the polarization state for the ultraviolet (UV) spectral range, in particular, for data storage, sensing applications, and biophotonics. In the visible a
CB
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.
VB
| X +/– iY> ≅ | X +/– iY> ≅|Z>
A, B, C excitons
b
CB
VB
≅|Z>
Introduction Group III nitride films and heterostructures grown along nonpolar (corresponding to the a-plane described by the surface – normal parallel to the [1120] direction or the m-plane described by the surface nor– mal parallel to the [1100] direction) or semipolar orientations have received much attention in the last few years due to the absence or reduction, respectively, of piezo- and pyroelectric polarizations along the growth direction, which in heterostructures grown along the polar direction (corresponding to the c-plane described
Data Loading...
