We study the vibrational spectrum of AlN grown on Si͑111͒. The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the E 2 1 , E 2 2 , A 1 ͑TO͒, A 1 ͑LO͒, and E 1 ͑TO͒ phonon energies. For a 0.8-m-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 cm Ϫ1 , respectively. By combining the Raman and x-ray diffraction studies, the Raman stress factor of AlN is found to be Ϫ6.3Ϯ1.4 cm Ϫ1 /GPa for the E 2 2 phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress E 2 2 energy is determined to be 657.4 Ϯ0.2 cm Ϫ1. Fourier-transform infrared reflectance and absorption techniques allow us to measure the E 1 ͑TO͒ and A 1 ͑LO͒ phonon energies. The film thickness ͑from 0.06 to 1.0 m͒ results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness.

We report direct-backscattering Raman studies of GaAs1−xNx alloys, for x⩽0.03, grown on (001) GaAs. The Raman spectra exhibit a two-mode behavior. The allowed GaAs-like longitudinal-optic phonon near 292 cm−1 is found to red shift at a rate of −136±10 cm−1/x. This is well described by the combined effects of strain and alloying. The GaN-like phonon near 470 cm−1 is observed to increase in intensity in direct proportion to x, and to systematically blue shift at a rate of 197±10 cm−1/x. This blue shift is likewise attributed to strain and alloying. The GaAs-like second-order features are also seen to broaden slightly and diminish in intensity with increasing nitrogen concentration. These results are attributed to a weak breakdown in the zincblende-crystal long-range order, possibly related to the presence of ordered domains within the random alloy.

We report optical studies on AlxGa1−xN alloy layers across the full composition range. The series of alloy layers was grown on (111)-oriented silicon substrates using gas-source molecular beam epitaxy. From reflectance measurements, we determine the composition dependence of the energy gap to be Eg=3.42+1.18x+1.56x2, in good agreement with previous work. By combining Fourier transform infrared and Raman spectroscopy studies, we determine the composition dependence of phonons having A1(TO), E1(TO), E22, A1(LO), and E1(LO) symmetry. The longitudinal optic phonons exhibit one-mode alloy behavior. Two-mode alloy behavior is observed for all transverse optic and the E22 phonons (i.e., each mode has AlN-like and GaN-like branches). All phonons are seen to blueshift with increasing x. The influence of stress on the phonon energies is discussed.

We demonstrate selective growth of high-quality GaN by gas-source molecular beam epitaxy on Si(111) wafers patterned with SiO2. GaN was grown on wafers having two different buffer layers. The first buffer layer contains two AlGaN/GaN superlattices, separated by GaN spacer, grown on AlN, with a total thickness of 400 nm. The second is a thin AlN (1.5 nm) buffer layer. X-ray diffraction confirms (0001) growth orientation, smooth interfaces, and coherence lengths comparable to the layer thickness in both samples. In the case of the thin AlN buffer layer, the tensile stress measured by the E2 Raman line shift is attributed to the mismatch in the thermal expansion coefficients of GaN and Si. However, when the AlGaN/GaN superlattice buffer layer is grown first, a reduced stress is measured. High carrier concentrations (≈1018 cm−3) are seen in the GaN grown on the thin AlN buffer layer, which we attribute to the incorporation of silicon from the substrate during the growth process. The superlattice buffer layer is seen to inhibit this diffusion, resulting in a carrier concentration of <1017 cm−3 in the GaN.

We use Raman scattering to obtain a stress map of lateral epitaxy overgrown GaN. Isolated hexagonal islands are grown by selective area overgrowth without a seed layer. Stress mapping is obtained from shifts in the E 2 phonon. GaN in the aperture area has the greatest biaxial compressive stress, Ϸ0.18 GPa. The overgrowth region is under slightly smaller stress, about 0.15 GPa. We attribute marked variations in the A 1 (LO) phonon intensity to spatial variations in the free carrier concentration. This is found to be small in the aperture region and high in the lateral overgrowth. The position-dependent presence of the lower coupled plasmon-phonon band is consistent with this interpretation.

Raman spectroscopy is used to study GaAs heavily doped with carbon. Hole concentrations in these samples range from 2.3×1019 to 1×1020 cm−3. Three main Raman features are investigated: the longitudinal-optic (LO) phonon mode, the substitutional carbon-at-arsenic local-vibrational mode, and the coupled plasmon–LO phonon present due to the interaction between the LO phonon and the free carriers. Only one allowed phonon-like coupled mode is observed due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically redshift as carrier concentration increases. This behavior is described by a model which includes the effects of high hole concentrations on the dielectric function and an additional shift in the optic phonon we tentatively attribute to carbon size effect. The local vibrational mode intensity is found to be directly proportional to the carrier concentration p. Interestingly, the local mode intensity shows good correlation with that of the coupled plasmon–LO phonon mode as a function of p. The ratio of the coupled plasmon–LO phonon mode intensity to that of the LO phonon is found to be directly proportional to the carrier concentration.

Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

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