Raman spectra of semi-polar (11-22) InGaN thick films

Abstract

We report the semi-polar (11-22) InGaN thick films (∼1 μm) grown on m-plane sapphire by a low pressure metal-organic vapor phase epitaxy (MOVPE) system. The Raman spectra of semi-polar InGaN thick films have been studied. For Raman spectra excited by the laser with wavelength of 532 nm, the E2(h) modes of GaN around 569 cm−1 and longitudinal optical (LO) phonon-plasma coupled modes (LPP) were mainly discussed. The E2(h) mode of GaN in the InGaN thick film would correspond to the lower frequency with wavenumber of 569.1 cm−1, compared to that in the InGaN thinner film (569.6 cm-1) in our case. Meanwhile, LPP modes also exhibited red shift with the increase of InGaN film thickness, which would be attributed to the stress relaxation and the decrease of free carrier concentration. The E2(h) modes of InGaN around 560 to 565 cm-1 and A1(LO) modes of InGaN around 729 to 731 cm-1 were found in Raman spectra excited by the laser with wavelength of 325 nm, but not probed in Raman spectra excited by a 532 nm laser. With the increase of In component, both modes were shifted towards low frequency. In addition, it was found that the Frölich electron-phonon interaction would significantly affect the peak position (729.2 cm-1) and full width at half maximum (FWHM, 17.2 cm-1) of A1(LO) mode of InGaN in the thicker film. Moreover, we have compared the Raman spectra of polar and semi-polar InGaN thick films so that the difference of Raman spectra between the two counterparts could be indicated due to lattice mismatch, In component incorporation efficiency, crystal quality and phase separation. The stress distribution of semi-polar InGaN materials is not as uniform as that of polar materials.