Optical Characterization of AlxGa1?xN Alloys (x < 0.7) Grown on Sapphire or Silicon

Abstract

The optical properties of Al x Ga 1--x N samples (x < 0.7) have been studied by photoluminescence (PL) and reflectivity in the 10-300 K temperature range. Various physical properties have been studied as a function of composition, such as Stokes shift, alloy broadening, exciton localization, and Huang-Rhys factor. Up to x % 0.3, a band gap bowing factor of $0.9 eV accounts for the variation of PL and reflectivity energies. At higher compositions, luminescence energies deepen with regard to this behaviour. This is interpreted as a consequence of the G 9 -G 7 crossover of the valence band maxima. This is confirmed by the linear polarization of the luminescence studied under oblique observation.

Investigations on alloy potential fluctuations in AlxGa1−xN epilayers using optical characterizations

“…Similar behavior has been reported previously for the In x Ga 1Ϫx N 29 and Al x Ga 1Ϫx N epilayers. 11,13,24,25 In the In x Ga 1Ϫx N-based light-emitting device structures, In alloy inhomogeneity, and/or quantum-dot-like In phase separation have been proposed as the origin of localized states, and temperature-induced blueshift attributed to band-tail states in the density of states has been observed. The theoretical calculation also predicts a high tendency of phase separation in the In-Ga-N alloy system, but not in Al x Ga 1Ϫx N epilayers.…”

“…However, Cho et al 23 showed that the Stokes shift between the PL emission peak energy and PLE absorption edge monotonically increases with varying Al contents. Comparing our results with previously reported one, [11][12][13][14][15][16][23][24][25][26][27] the large Stokes shift between PL emission and OA or PC strongly implies that the potential fluctuation does exist in Al x Ga 1Ϫx N epilayers. These results can be understood in terms of increase in the depth of localized states from the band edge of Al x Ga 1Ϫx N epilayers with large Al contents.…”

“…Accordingly, no phase separation is predicted in Al x Ga 1Ϫx N alloys. 10 However, in practice, the Al x Ga 1Ϫx N alloys with high Al contents also show the S-shaped PL shift as observed in In x Ga 1Ϫx N alloys or InGaN/GaN MQWs, and the S-shape behavior and Stokes shift have been explained in terms of the effects of localized states induced by alloy potential fluctuations in Al x Ga 1Ϫx N. [11][12][13][14][15][16] The amplitude of the potential fluctuation at the band edges caused by the alloy fluctuation is strongly correlated to the energy gap difference between the two semiconductors, 17 e.g., between GaN and AlN for Al x Ga 1Ϫx N (⌬E g ϭ2.8 eV). It is well known that the potential fluctuation plays an important role in determining the optical properties of alloy semiconductors including ZnS x Se 1Ϫx (xϽ0.18), 18 GaAs 1Ϫx P x , 19 CdS x Se 1Ϫx , 20,21 and Zn 1Ϫx Cd x Te.…”

Optical properties of high-Al-content crack free AlxGa1−xN (x<0.67) films grown on Si(111) by molecular-beam epitaxy

“…at high x to transitions involving mainly the G 9 or the G 7C valence bands and only weakly the absolute G 7B valence band maximum. This explains, at least partly, the Stokes shift increase [14]. It seems that the reflectivity data are still keeping a bowing parameter of 1 eV, but the increasing uncertainty on the A exciton energy value taken from reflectivity measurements makes it difficult to conclude on the exact value of the bowing parameter for xO0.4.…”

Investigating the nature of Si doping in Al0.23Ga0.77N

“…Two longitudinal optical (LO) phonon replicas of the I 2 peak, separated by an energy of 93 meV, are also observed. The fullwidth at half-maximum (FWHM) of the PL peak of the undoped layer is 32 meV, in good agreement with literature [19]. This is attributed to alloy inhomogeneous broadening.…”