A comparative investigation of the optical properties of polar and semipolar GaN epi-films grown by metalorganic chemical vapor deposition

Abstract

We report on the structural and optical properties of polar gallium nitride on c-plane sapphire substrates and semi-polar (11-22) GaN films on m-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD). Polar GaN on c-plane sapphire and semi-polar GaN on m-place sapphire both show good crystal quality, luminescence, absorption, and Raman characteristics. GaN on c-place sapphire shows a high crystal quality as compared to GaN on m-plane sapphire. Surface roughness of polar GaN is lesser than semi-polar GaN. The biaxial structural stress in GaN switches from compressive to tensile as the temperature is increased. This stress-switch temperature is higher in GaN/c-plane than GaN/m-plane. GaN in polar and semi-polar orientation shows ultraviolet emissions but yellow-emissions are only observed in GaN/c-plane sapphire. Raman spectroscopy-related oscillations show systematic variations with temperature in both GaN configurations (polar and semi-polar). This work provides a framework of characterizations for GaN with different crystal polarities. It contributes towards identifying suitable crystal growth mechanisms based on the application and requirements for doping (In, Al, etc.), crystal quality, emission, absorption, and photonic oscillations.

Optical, Structural, and Synchrotron X-ray Absorption Studies for GaN Thin Films Grown on Si by Molecular Beam Epitaxy

“…To extrapolate the linear part of the curve of ( αhν ) 2 vs. ( hν − E g ) to the x-axis, i.e., the so-called Tauc plot [ 31 ], the GaN bandgap energy ( E g ) was obtained. We tried this method on our GaN/Si samples, as our team did for GaN/sapphire [ 29 ] and obtained E g values almost exactly identical to what we obtained from the peaks of n~λ curves, as listed in Table 3 . Also, our seven GaN films with large film thickness variation between 160–1500 nm had their E g values with only ~0.1% differences, indicating the excellent GaN film quality grown on Si by MBE.…”

“…To analyze the structural features of five GaN/Si samples and compare their crystal characteristics, further quantitative calculations were performed. First, the average crystallite size of five GaN films can be evaluated using the Debye–Sheller formula [ 15 , 29 ]: where D is the crystallite size, β is the full width at half maximum of the (0002) XRD peak, k is the Scherrer constant that is 0.9, θ is the diffraction angle, and λ is the X-ray diffraction wavelength which is 1.5406 Å.…”

“…In addition, the microscopic strain ( ε ) can be calculated using the following expression [ 29 ]: with β and θ defined as above.…”

“…Usually, plots of ( αhv ) 2 versus photon energy are applied to obtain the bandgaps for GaN [ 29 , 30 ] and other materials. The relationship between the absorption coefficient (α) and photon energy (eV) is described by the equation [ 31 ]: where is the photon energy, is the band gap of the semiconductor, and C is a constant.…”

“…Figure 4 presents the RT visible (532 nm) excitation Raman spectra of five PAMBE-grown GaN films on Si. It shows clearly the E 2 and A 1 (LO) modes of w-GaN [ 14 , 19 , 29 ] from the thickest film S3 (NS38, 1522 nm) and the second thickest film S4 (NS42, 579 nm). The E 2 mode was found to be weaker for S2 (NS37, 204 nm) than for the above two samples, it was also found to be a clear shoulder for S1 (NS36, 182 nm) and appeared as a dim shoulder for S5 (NS45, 315 nm).…”