Study of Surface and Interface Roughness of GaN-Based Films Using Spectral Reflectance Measurements

Abstract

GaN films were grown using SiN treatment of sapphire substrate by metalorganic vapor-phase epitaxy in a home-made vertical reactor at atmospheric pressure. The growth was interrupted at different stages to investigate the impact of interface and surface roughness on the optical properties of the GaN layers. A transition from a three-dimensional (3D) to two-dimensional (2D) growth mode was revealed by real-time in situ laser reflectometry (k = 632.8 nm) as well as by atomic force microscopy images. A theoretical model is proposed to determine the refractive index evolution during GaN layer growth based on the Bruggeman effective medium approximation. Ex situ multiwavelength reflectivity signals were fit to the thin-film interference equations to derive the evolution of the effective refractive indexes for the surface and interface GaN layer, thereby determining the refractive index of the GaN layer during growth. Ex situ spectroscopic ellipsometry measurements of the GaN layer refractive indexes at different growth stages were compared with calculated results. Moreover, an empirical law was developed to fit the refractive index evolution during GaN layer growth and used for in situ reflectivity signal simulation in order to deduce the growth rate. Finally, good agreement was observed between the experimental and theoretical findings.

Efficient Light Absorption by GaN Truncated Nanocones for High Performance Water Splitting Applications

“…Consequently, it was experimentally found that the saturation of photocurrent increased around three times higher in the truncated nanocones compared to the planar GaN with a high incident photon to electron efficiency and charge transfer rate. In conclusion, the optimum truncated nanocone provides higher photocatalytic efficiency than the existing planar GaN-based water splitting by the only surface texturing without huge structural modification in the conventional system . Furthermore, our proposed geometries with light trapping properties can also be applicable to GaN based photocatalytic materials.…”

“…In conclusion, the optimum truncated nanocone provides higher photocatalytic efficiency than the existing planar GaNbased water splitting by the only surface texturing without huge structural modification in the conventional system. 33 Furthermore, our proposed geometries with light trapping properties can also be applicable to GaN based photocatalytic materials. We believe our approach will pave a new way to improve the water splitting efficiency.…”

Layered Heterostructures Based on MoS2/MoSe2 Nanosheets Deposited on GaN Substrates for Photodetector Applications

“…First, we utilized wet chemical etching to selectively create macropores on the GaN surface (e-GaN) based on the solvothermal treatment in KOH solution without any metal catalyst, followed by its heterojunction with MoS 2 , MoSe 2 , and MoSe 2 /MoS 2 using a simple spray coating technique. The introduction of macropores to the GaN surface enhances the optical absorption 100 times, significantly improving the light–matter interaction and facilitating the PD performance. , Interestingly, compared to single heterojunction devices, the double heterojunctions that maintained staircase-like (type-II) band alignment established better photodetection performance. The optimized heterostructure exhibits a very high photoresponsivity of 82 A/W, specific detectivity of 1.79 ×10 14 Jones, external quantum efficiency of 27,880%, and stability for over 6 months in a laboratory testing environment under the illumination of 365 nm, with an intensity of 150 nW/cm 2 and bias of 3 V. Further, DFT simulations support better charge separation and transport mechanisms for the e-GaN/MoSe 2 /MoS 2 -based hybrid 2D/3D heterostructures in terms of electron localization function and transport probability.…”