High responsive UV photodetector on epitaxial non-polar GaN nanostructures grown on sapphire (10-10) using laser-MBE

Mostly, the GaN has been grown on conventional substrates such as sapphire (0001), Si (111), etc. which promote the growth of polar GaN along the c-axis. However, the c-plane GaN exhibits large spontaneous and piezoelectric fields, which influence the radiative recombination rates along with the carrier transport of the heterojunction barriers and hence reduce the efficiency of the device. To address this, we have grown epitaxial nonpolar GaN nanostructures on m-plane sapphire (10-10) by changing the growth sequence in the laser molecular beam epitaxy process. A high-resolution x-ray diffraction (2 theta-omega) scan confirmed GaN orientation along non-polar directions for GaN growth on bare m-sapphire whereas mixed-polar GaN was grown on AlN buffered m-sapphire substrate. Field emission scanning electron microscopy study revealed the formation of tilted GaN nanocolumns and island film-like structures that facilitate a high surface-to-volume ratio. The electronic properties of GaN nanostructures were also studied using X-ray photoemission spectroscopy. Further, metalsemiconductor-metal type ultra-violet (UV) photodetectors were fabricated on the various GaN nanostructures. The photo-response behaviour has been studied at different bias voltages using an illumination source of wavelength 325 nm and the photo-responsivity was found to be similar to 15 A/W at 3 V for non-polar GaN. Finally, we discussed the crystalline and optical properties of various polar GaN nanostructures and their impact on the UV photodetector characteristics for future applications.