Enhanced UV detection by non-polar epitaxial GaN films

Abstract

Nonpolar a-GaN (11-20) epilayers were grown on r-plane (1-102) sapphire substrates using plasma assisted molecular beam epitaxy. High resolution x-ray diffractometer confirmed the orientation of the grown film. Effect of the Ga/N ratio on the morphology and strain of a-GaN epilayers was compared and the best condition was obtained for the nitrogen flow of 1 sccm. Atomic force microscopy was used to analyze the surface morphology while the strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by reciprocal space mapping technique. UV photo response of a-GaN film was measured after fabricating a metal-semiconductor-metal structure over the film with gold metal. The external quantum efficiency of the photodetectors fabricated in the (0002) polar and (11-20) nonpolar growth directions were compared in terms of responsivity and nonpolar GaN showed the best sensitivity at the cost of comparatively slow response time.

Photodetection Properties of Nonpolar a‐Plane GaN Grown by Three Approaches Using Plasma‐Assisted Molecular Beam Epitaxy

“…The average amount of stress in the samples can be estimated from Equation .where σ is the biaxial stress expressed in GPa and Δ ω is the Raman shift in cm −1 . The list of E 2 (high) and estimated stress values for each sample is listed in Table .…”

“…Table consists of growth conditions of samples A, B, and C with various substrate temperatures. It can be seen from Table that for sample A conventional growth conditions have been utilized . This includes a high‐temperature thermal cleaning followed by nitridation, GaN buffer layer growth at low substrate temperature, and a high‐temperature GaN epilayer growth.…”

“…However, conventional c-plane-based devices suffer from spontaneous and piezoelectrical polarization due to the intrinsic asymmetry in the crystal along the growth direction and mechanical stress between substrate and thin film, respectively. [14,15] Considering UV PDs, it has been observed that the intrinsic polarization changes Schottky barrier height, which can hinder the photodetection capabilities of polar c-plane (0002) GaN. [10,[16][17][18][19] In contrast, spontaneous polarization is not found in nonpolar III-nitride materials as it has a crystal symmetry along the growth direction.…”

“…[10,[16][17][18][19] In contrast, spontaneous polarization is not found in nonpolar III-nitride materials as it has a crystal symmetry along the growth direction. It has been seen in recent reports that nonpolar a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) GaN thin films have better UV photodetection properties than polar c-plane (0002) GaN thin films. [14,20] Hence, nonpolar III-nitride materials have been explored recently for UV photodetection, but growing a high-quality nonpolar a-plane GaN still remains a challenge.…”

“…It has been seen in recent reports that nonpolar a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) GaN thin films have better UV photodetection properties than polar c-plane (0002) GaN thin films. [14,20] Hence, nonpolar III-nitride materials have been explored recently for UV photodetection, but growing a high-quality nonpolar a-plane GaN still remains a challenge. [14,20,21] In general, growth of III-nitride thin films by plasma-assisted molecular beam epitaxy (PAMBE) has four major steps: 1) thermal cleaning of substrate at high temperature (800 C) for removing residual impurities; 2) nitridation of the substrate at high temperature (650-700 C); 3) GaN buffer layer at low temperature (500-550 C); and 4) epitaxial growth of GaN (740À800 C).…”