Controlled epitaxial growth of GaN nanostructures on sapphire (11–20) using laser molecular beam epitaxy for photodetector applications

Abstract

We have grown various epitaxial GaN nanostructures on sapphire (11–20) substrates by tuning the buffer layer growth conditions in laser molecular beam epitaxy (LMBE) process. The pre-nitridation and buffer layer GaN growth at low temperature (LT) on sapphire (11–20) critically affect the surface morphology and structural properties. Granular GaN thin film (~ 160 nm) was grown on pre-nitridated sapphire whereas nano-column (NC)-GaN was obtained on LT-GaN buffer layer on bare sapphire having a height of ~ 370 nm at the growth temperature of 700 °C. Nano-porous (NP)-GaN was obtained with pore sizes in the range of 70 ~ 110 nm having vertical height of ~ 560 nm under similar growth conditions on LT-GaN buffered pre-nitridated sapphire. In-situ reflection high energy electron diffraction, high-resolution x-ray diffraction and Raman spectroscopy measurements indicated the epitaxial growth of c-axis oriented, wurtzite crystalline GaN nanostructures on sapphire (11–20) substrate with nearly negligible biaxial stress (0.03–0.23 GPa). Further, metal-semiconductor-metal (MSM) ultra-violet (UV) photodetectors were fabricated on epitaxial GaN nanostructures. The photo responsivity studies revealed that the NP-GaN MSM device has a photoresponse of ~ 358 mA/W at an applied bias of 1V. The photo-responsivity of NP-GaN MSM device is higher than that of GaN film (~ 36 mA/W) and NC-GaN (~7 mA/W) which revealed the importance of shape and size of GaN nanostructures on the responsivity of UV-photodetector devices. These results demonstrate the capability of LMBE technique to grow different GaN nanostructures on sapphire (11–20) substrate by tuning buffer layer conditions for their application as UV-photodetectors.