Evidence of compositional inhomogeneity in InxGa1−xN alloys using ultraviolet and visible Raman spectroscopy.

Abstract

In this paper we report a study of phase separation in bulk InxGa1−xN films grown by metal organic chemical-vapor deposition using mid-UV Raman spectroscopy. Evidence of phase separation is observed by the occurrence of low frequency shoulders identified as minority phase in the A1(LO) Raman mode. A phase transition in the alloy from the metastable to unstable region was found to be occurring at an indium concentration of about 25%. Raman spectroscopic results also indicate that the compositional inhomogenity in our samples increase, as would be expected, with depth in the film. A direct correspondence is also found between the percentage of indium concentration in the film and the amount of compositional inhomogenity.

Raman scattering by the E2h and A1(LO) phonons of InxGa1−xN epilayers (0.25 < x < 0.75) grown by molecular beam epitaxy

“…Significant frequency variations of the A 1 (LO) mode of InGaN as a function of the excitation wavelength have been reported in the literature. 13,15,[17][18][19] Different effects such as strain/compositional gradients, 13,19 selective resonant excitation, 5,7,15,17 or disorder-induced breakdown of wave-vector conservation 18 have been invoked to explain the observed shifts.…”

“…15,16 A broad feature is also visible in all the spectra below the A 1 (LO) band. 12,13,15,17,19 The intensity of this band, which is usually labeled as "S band" in the literature, 12,17 is maximum in relation to the E 2h mode for intermediate In compositions, i.e., in the samples with the highest degree of alloy disorder. Thus, most authors attribute this feature to Raman scattering by disorder-activated optical modes arising from alloy disorder.…”

“…13,19 The presence of in-depth strain and composition gradients in the InGaN alloy seems to depend on the particular growth method and growth conditions. For instance, Pereira et al 29 showed that the relaxation of strain in InGaN epilayers grown by metal organic chemical vapor deposition (MOCVD) is accompanied by increased In contents.…”

“…Such deviations are usually attributed to the strain of the samples. 12,20 However, many authors have shown that the A 1 (LO) frequency strongly depends on the excitation wavelength and/or on temperature, and different effects have been invoked to explain the observations: i) strain and/or compositional gradients over depth; 13,19 ii) compositional inhomogeneities giving rise to selective resonant excitation of domains with a particular In content; 5,7,15,17 iii) relaxation of the wave-vector selection rule. 18 Thus, it remains necessary to identify which mechanisms are actually playing a key role in the behavior of the A 1 (LO) mode of InGaN.…”

“…3 for a review on III-V alloys). Numerous studies have been devoted to study the vibrational properties of hexagonal InGaN thin films [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and InGaN/GaN quantum wells. [22][23][24] Although it is well accepted that both the A 1 (LO) and the E 2h phonon modes of InGaN display a one-mode behavior, 15,16 there are still open questions regarding the behavior of the optical phonons in the InGaN alloy.…”