Resonant Raman Scattering in GaN/Al_0.15Ga_0.85N and In_yGa_1-yN/GaN/Al_xGa_1-xN Heterostructures

Materials Research Society Symposium Proceedings 468, 213 (1997).

Below is the abstract submitted to the meeting, not the abstract of the published paper:
Resonant Raman scattering is well known for a variety of semiconductors with band gap energies in the visible or near infrared spectral region. Recently this technique was successfully applied to GaN, a material with the fundamental gap energy in the near UV. We now report on resonant Raman scattering in GaN/Al_0.15Ga_0.85N single quantum wells. The investigated quantum wells (QWs) consisted of a thin GaN layer embedded in between Al_0.15Ga_0.85N barriers. The width of the GaN QWs varied from 2 nm to 16 nm, whereas the widths of the barriers below and above were fixed at 1 μm and 0.1 μm, respectively. For reference purpose, we also studied pure Al_xGa_1-xN samples with an Al content varying between x = 0 and x = 0.15. All samples were grown by metalorganic chemical vapor deposition on c-plane sapphire substrates. Using sub-bandgap optical excitation, the series of AI_xGa_1-xN samples revealed an almost linear high-frequency shift of the A1(LO) phonon with increasing Al content. For this off-resonance excitation the Raman spectra of the QW, samples were indistinguishable from that of the Al_0.15Ga_0.85N reference sample. Changing the excitation energy to 3.54 eV, which is close to resonance with the interband transition between the lowest quantized electron and hole levels, the Raman spectra changed drastically. In spite of the small GaN scattering volume compared to the Al_0.15Ga_0.85N barriers, the Raman spectra were dominated by Froehlich induced scattering from the GaN A₁(LO) phonon. Under these resonance conditions a certain frequency shift and broadening of the A₁(LO) phonon signal were observed for the 2 nm QW, indicating some cation intermixing at the interface. For optical excitation at 3.68 eV, which is close to the fundamental gap energy of Al_0.15Ga_0.85N, Froehlich induced scattering by the Al_0.15Ga_0.85N A₁(LO) phonon becomes resonant and its intensity exceeds that of the scattering by the GaN A₁(LO) phonon.

This paper is part of Gallium Nitride and Related Materials II

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