We have studied the influence of nitrogen impurities in CH4/H2 gas mixtures on the structure and morphology of polycrystalline diamond films prepared by microwave plasma assisted chemical vapor deposition. The nitrogen concentration in the process gas was varied between 1 and 1000 ppm. Optical emission spectroscopy was applied to detect the nitrogen in the plasma via emission from CN radicals. The morphology and texture of polycrystalline films prepared with various N2 impurity levels and CH4 concentrations in the range 0.5%–2% was investigated using scanning electron microscopy and x-ray texture analysis. For the films prepared with low methane concentrations (e.g., 0.5%) only a minor influence of the nitrogen was observed. However, most interestingly, for higher methane concentrations (1%–2%) the addition of small amounts of nitrogen turned out to have a tremendously beneficial effect on the film morphology and structure. Films prepared without additional nitrogen are of nanocrystalline structure and of minor quality, whereas films prepared with nitrogen concentrations in the range 10–100 ppm exhibit a pronounced 〈100〉 texture and a considerably improved crystalline quality as judged by Raman spectroscopy.

We performed resonant Raman scattering in hexagonal GaN using discrete laser lines in the violet and UV spectral range for optical excitation. To tune the energetic position of the fundamental gap E0 of GaN relative to the exciting photon energy the sample temperature was varied between 77 and 870 K. Analyzing both Stokes and anti-Stokes Raman spectra, the resonance profiles for Fröhlich-induced one-E1(LO) and two-E1(LO) phonon scattering could be deduced, covering the energy range from 0.5 eV below the E0 gap up to the gap energy. The strength of deformation-potential scattering by the A1(TO) mode was used as an internal reference. For excitation slightly above the E0 gap energy E1(LO) multiphonon scattering up to the fourth order was observed, which reflects the stronger polarity of the Ga-N bond as compared to conventional III-V semiconductors.

No abstract

We report on the composition dependence of the band gap energy of strained hexagonal In x Ga 1-x N layers on GaN with x≤0.15, grown by metal-organic chemical vapor deposition on sapphire substrates. The composition of the (InGa)N was determined by secondary ion mass spectroscopy. High-resolution X-ray diffraction measurements confirmed that the (InGa)N layers with typical thicknesses of 30 nm are pseudomorphically strained to the in-plane lattice parameter of the underlying GaN. Room-temperature photoreflection spectroscopy and spectroscopic ellipsometry were used to determine the (InGa)N band gap energy. The composition dependence of the band gap energy of the strained (InGa)N layers was found to be given by E G (x)=3.43-3.28⋅x (eV) for x≤0.15. When correcting for the strain induced shift of the fundamental energy gap, a bowing parameter of 3.2 eV was obtained for the composition dependence of the gap energy of unstrained (InGa)N.

Resonant Raman scattering has been used to study hexagonal In(x)Ga(1-x)N films with x about 0.1, grown by metal-organic chemical vapor deposition on sapphire substrates. To vary the energy difference between the fundamental gap energy of the (InGa)N and the photon energy of the discrete laser emission lines used for recording the spectra, the sample temperature was varied between 300 and 870 K. Raman scattering by the (InGa)N A1 (L0) phonon shows a clear resonance profile when the fundamental energy gap approaches the incident photon energy, with a maximum enhancement in scattering efficiency of 10 measured relative to the scattering strength of the E2 phonon mode. The (InGa)N A 1 (L0) phonon was found to shift to higher frequencies with respect to the E2 mode when the experimental conditions were varied from excitation below the fundamental energy gap of (InGa)N to above-band-gap excitation. This frequency shift is explained by the presence of compositional inhomogeneity, which result s in localized regions with higher In content, and thus, lower gap energy and phonon frequency, and regions with lower In content, and consequently, higher gap energy and phonon frequency

Resonant Raman scattering has been used to study longitudinal optical (LO) phonon modes in 2-4-nm-wide GaN/Al(0.15)Ga(0.85)N single quantum wells (QW). Raman spectra recorded using subband gap excitation were found to be completely dominated by the phonon modes of the (AlGa)N barriers. In contrast, for excitation close to resonance with the lowest transition between confined electron and hole states in the GaN QW scattering by the A(1)(LO) phonon in the QW became dominant in spite of the narrow width of the QW. For well widths of 3 and 4 nm, the frequency of the A(1) (LO) phonon in the QW was found to be close to that in bulk GaN, whereas for a well width of 2 nm the QW phonon was broadened and shifted towards the frequency of the (AlGa)N A(1)(LO) phonon mode. This broadening and frequency shift indicate some cation intermixing for the narrowest QW with the widths of the interface regions being comparable to the well width of 2 nm, demonstrating the usefulness of resonant Raman scatter ing for quality assessment of GaN/(AlGa)N QWs and interfaces

We report on resonant Raman scattering in Al0.15Ga0.85N/GaN single quantum wells (QWs) and AlxGa1-xN/GaN/lnyGa1-yN heterostructures. By choosing appropriate excitation conditions we could probe selectively the GaN quantum well or the Al0.15Ga0.85N barrier of Al0.15Ga0.85N/GaN single quantum wells. For the InxGa1-xN material system a linear frequency shift of the E2- and A1(LO) phonon mode to lower frequencies was found with increasing In content. The shift was determined to -0.79cm-1 per % In content for the A,(LO) phonon frequency. Resonant excitation of AlxGa1-xN/GaN/InyGa1-YN heterostructures enabled us to detect phonon signals from the InxGa1-xN layer in the heterostructure and to determine its In content.

We have used Raman spectroscopy and spectral ellipsometry to investigate InAs/GaSb short-period superlattices (SLs), grown by molecular-beam epitaxy, with either InSb- or GaAs-like interfaces. Room-temperature ellisometric measurements show spectral features in the dielectric function due to the E1 and E1 plus Delta1 interband transitions of GaSb and InAs. For SLs with small InAs layer thickness (4 ML InAs/10 ML GaSb) the critical point energies are found to depend on the type of interfacial bonding, with an energy shift of up to 50 meV observed between SLs with GaAs- and InSb-like interfaces. Resonant Raman measurements show a pronounced enhancement in scattering efficiency for the superlattice phonons and, in particular, for the interface modes for incident photon energies matching the critical point energies of the S