Publications Search Results

Now showing 1 - 10 of 79
  • Publication
    Growth of thick films CdTe from the vapor phase
    ( 2008)
    Sorgenfrei, R.
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    Greiffenberg, D.
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    Bachem, K.H.
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    Kirste, L.
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    Zwerger, A.
    ;
    Fiederle, M.
    CdTe films with a thickness of 100 µm were grown by molecular beam epitaxy (MBE) on semi-insulating LEC GaAs (0 0 1) substrates. The films were characterized by X-ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), I/V characteristics and detector measurements using a 241Am source. The grown layers were highly oriented as revealed from X-ray pole figure measurements and high-resolution XRD experiments. Low temperature PL measurements showed the formation of A-center complexes including Ga and In acting as donors. Furthermore, the distribution of these impurities in the films was determined by spatially resolved PL measurements. XPS demonstrates the formation of a TeO2 overlayer due to Cd depletion at the surface of the films. The electrical measurements including I/V characteristics and detector measurements resulted in resistivity values of around 3×10(exp 8) ohm cm and a µt product of electrons of 10(exp -5) cm2/V.
  • Publication
    Group III-nitride based blue emitters
    ( 1999)
    Obloh, H.
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    Bachem, K.H.
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    Behr, D.
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    Kaufmann, U.
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    Kunzer, M.
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    Ramakrishnan, A.
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    Schlotter, P.
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    Seelmann-Eggebert, M.
    ;
    Wagner, J.
    (AlGaIn)N-based heterostructures are of considerable current interest for the realization of short-wave length light-emitting semiconductor devices, covering the UV- to-blue spectral region. Recent advances in group Ill-nitride layer growth using MBE and, in particular, MOCVD allow the preparation of high-quality n- and p-type conducting GaN layers as well as GaN/(AlGa)N/(InGa)N hetero- and quantumstructures, in spite of the lack of lattice matched substrates. This progress in growth technology enabled the realization, and commercialization, of high-efficiency blue light emitting diodes and the demonstration of a diode laser emitting in the violet spectral range. Despite this tremendous technological progress, important questions regarding basic material properties and growth related issues remain yet to be answered. In the present paper we shall discuss the effect of GaN surface termination on layer quality and surface morphology, the mechanisms limiting the maximum obtainable hole concentration in GaN:Mg, and the effect of compositional inhomogeneity in low In-content (InGa)N on the photoluminescence, reflection, and dielectric function spectrum.
  • Publication
    Light-emitting diodes and laser diodes based on a Ga(1-x)In(x)As/GaAs(1-y)Sb(y) type II superlattice on InP substrate
    ( 1999)
    Peter, M.
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    Kiefer, R.
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    Fuchs, F.
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    Herres, N.
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    Winkler, K.
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    Bachem, K.H.
    ;
    Wagner, J.
    We report on the fabrication and characterization of light-emitting diodes (LEDs) and laser diodes with a staggered type II Ga(1-x) In(x)As/GaAs(1-y)Sb(y) superlattice (SL) as the active region. SLs were grown strain compensated on the InP substrate using metalorganic chemical vapor deposition. The LEDs show room-temperature electroluminescence up to 2.14 mu m, the index-guided diode lasers displayed cw laser emission at 1.71 mu m up to 300 K. The spontaneous emission spectrum was found to show a significant blueshift with increasing injection current density, resulting in shorter laser emission wavelengths for the diode laser than for the LED.
  • Publication
    Resonant raman scattering as a selective probe for compositional inhomogeneity in low In content (InGa)N
    ( 1999)
    Behr, D.
    ;
    Wagner, J.
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    Ramakrishnan, A.
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    Obloh, H.
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    Kunzer, M.
    ;
    Maier, M.
    ;
    Bachem, K.H.
    Raman spectroscopy has been used to study compositional inhomogeneity in MOCVD grown In(x)Ga(1-x)N with x about 0.1, taking advantage of the selective enhancement of Raman scattering by the A1 (LO) phonon from regions with different alloy composition. A clear high- frequency shift of the A1(LO) mode relative to the E2 mode is observed, when the experimental conditions are varied from excitation below the fundamental energy gap of the (InGa)N to above band gap excitation. This frequency shift is taken as direct evidence for compositional inhomogeneity, which results in localised regions with different In content, and thus different gap energies and A1(LO) phonon frequencies.
  • Publication
    Band gaps and band offsets in strained GaAs(1-y)Sb(y) on InP grown by metalorganic chemical vapor deposition
    ( 1999)
    Peter, M.
    ;
    Herres, N.
    ;
    Fuchs, F.
    ;
    Winkler, K.
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    Bachem, K.H.
    ;
    Wagner, J.
    Metastable GaAs(1-y)Sb(y) with 0.22 < y < 0.70 has been grown pseudomorphically strained on (001) InP substrates using metalorganic chemical vapor deposition. The Sb concentration and layer thicknesses, ranging from 24 to 136 nm, were determined by high resolution x-ray diffraction (HRXRD) measurements. Low-temperature photoluminescence (PL) spectroscopy revealed spatially indirect band-to-band emission of electrons localized in the InP and holes in the GaAs(1-y)Sb(y). At increased excitation power densities samples with layer thicknesses above 65 nm showed, also, spatially direct PL across the band gap of the strained GaAs(1-y)Sb(y). From the PL data the band gap energy and the band offsets of GaAs(1-y)Sb(y) relative to InP were derived and compared with the predictions of the Model Solid Theory.
  • Publication
    Composition dependence of the band gap energy of In(x)Ga(1-x)N layers on GaN(x < = 0.15) grown by metal-organic chemical vapor deposition
    ( 1999)
    Wagner, J.
    ;
    Ramakrishnan, A.
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    Behr, D.
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    Maier, M.
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    Herres, N.
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    Kunzer, M.
    ;
    Obloh, H.
    ;
    Bachem, K.H.
    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 EG(x)=3.43-3.28x(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.
  • Publication
    X-ray analysis of the texture of heteroepitaxial gallium nitride films
    ( 1999)
    Herres, N.
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    Obloh, H.
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    Bachem, K.H.
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    Helmig, K.
    The preparation of gallium nitride films by deposition from the vapour phase (MOCVD, MBE) may suffer from an occurrence of multiple orientations and a mixture of the wurtzite (2H) and the sphalerite (3C) polytype if nucleation and deposition conditions are not optimized. We use X-ray texture diffractometry with dedicated beam optics and various scanning techniques to check for the presence of unwanted orientations and phases in a quick and semi-quantitative manner. A variety of GaN films showing multiple orientations were analyzed with respect to the nature of polycrystalline growth. These films were deposited by MOCVD on sapphire substrates. From X-ray polefigures and approximations of the texture by model components we found that the 2H and 3C orientations in these films are interrelated via stacking faults on (0001) and (111) planes, respectively. To obtain a single crystalline film orientation, a suppression of the formation of stacking faults is required. This has been achieved under optimized MOCVD conditions. As is evidennt by X-ray diffractometry, these films are single crystalline, phase pure 2H GaN films with small mosaicity spreads.
  • Publication
    MOCVD growth of (Ga(1-x)In(x)As-GaAs(1-y)Sb(y))superlattices on InP showing type-II emission at wavelengths beyond 2 mu m
    ( 1998)
    Peter, M.
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    Winkler, K.
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    Herres, N.
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    Fuchs, F.
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    Müller, S.
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    Bachem, K.H.
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    Wagner, J.
    We report, on (Ga(1-x)In(x)As- GaAs(1-y)Sb(y)) superlattices grown strain-compensated on ( 100) InP:Fe substrates using new organic chemical vapor deposition. Low temperature photoluminescence measurement show a spatially indirect type-II recombination of electrons in the conduction band of the Ga(1-x)In(x)As and holes in the valence band of the GaAs(1-y)Sb(y) layers at 2.20 mu m. Type-II emission was observed up to room-temperature with 300 K emission centered at 2.30 mu m. The valence and conduction band offsets between strain-compensated Ga(0.24)In(0.58)As and GaAs(1-y) Sb(y) (y about 0.27 to 0.3) layers were estimated to 0.21 eV and 0.33 - 0.39 eV, respectively.
  • Publication
    Evidence for compositional inhomogeneity in low In content (InGa)N obtained by resonant Raman scattering
    ( 1998)
    Behr, D.
    ;
    Wagner, J.
    ;
    Ramakrishnan, A.
    ;
    Obloh, H.
    ;
    Bachem, K.H.
    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.