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  • 1
    Electronic Resource
    Electronic Resource
    Study of defects in GaN grown by the two-flow metalorganic chemical vapor deposition technique using monoenergetic positron beams (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 181-186 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and acceptor-type defects in −c GaN. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1372163
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  • 2
    Electronic Resource
    Electronic Resource
    Exciton spectra of an AlN epitaxial film on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 81 (2002), S. 652-654 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Exciton resonance energies in an AlN epilayer on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral line shape was fitted considering A (Γ7vu→Γ7c) and BC (Γ9v,Γ7vl→Γ7c) exciton transitions. The fitting gave the values of them at 0 K to be 6.211 and 6.266 eV, giving the crystal- field splitting (Δcr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature aitch-thetaE was estimated to be 580 K. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1493666
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  • 3
    Electronic Resource
    Electronic Resource
    Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 2006-2008 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and FPZ, produces a confined electron-hole pair whose wave functions are still overlapped (quantized excitons) provided that L〈aB. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.122350
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  • 4
    Electronic Resource
    Electronic Resource
    Localized quantum well excitons in InGaN single-quantum-well amber light-emitting diodes (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 5153-5157 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optical properties of the InGaN single-quantum-well amber light-emitting-diodes were investigated to verify the importance of localized quantum well (QW) excitons in their spontaneous emission mechanisms. The internal piezoelectric field (FPZ) across the QW of the order of 1.4 MV/cm was confirmed to point from the surface to the substrate. Transmittance and photovoltaic spectra exhibited a broad band tail state, and the emission lifetime increased with decreasing detection photon energy. The electroluminescence spectra did not show remarkable energy shift between 10 and 300 K. The spectra exhibited an exponential tail and the higher energy portion increased more rapidly than that of the lower energy one, reflecting a thermal distribution of the localized carriers within the tail states. Since the well thickness is only 2.5 nm and is atomically flat, the device is considered to exhibit reasonably efficient emission with the external quantum efficiency of 5% at 20 mA in spite of the presence of FPZ and large number of threading dislocations due to radiative decay of the localized QW excitons. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1314906
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  • 5
    Electronic Resource
    Electronic Resource
    Helicon-wave-excited-plasma sputtering epitaxy of ZnO on sapphire (0001) substrates (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 874-877 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A promising approach to obtain epitaxial films of oxide semiconductors was demonstrated, namely helicon-wave-excited-plasma sputtering epitaxy. Due to the surface-damage-free nature, completely a-axis-locked c(0001)-orientation ZnO epilayers were successfully grown on sapphire (0001) substrates having ultrasmooth surfaces with atomic steps. The ZnO epilayer exhibited a dominant near-band-edge photoluminescence (PL) peak at 300 K. Since the PL was considered to be due to the recombination of excitons bound to an impurity or a defect and certain tilting and twisting of the films were observed when Ar/O2 were used as sputtering gases, purification and optimization of the overall process are necessary to obtain improved epilayer qualities. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1426238
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  • 6
    Electronic Resource
    Electronic Resource
    Band gap bowing and exciton localization in strained cubic InxGa1−xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 3600-3602 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1−xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1−xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1−xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material's inhomogeneity is much enhanced compared to that of the hexagonal polytype. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1421082
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  • 7
    Electronic Resource
    Electronic Resource
    Photoreflectance spectra of a ZnO heteroepitaxial film on the nearly lattice-matched ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 80 (2002), S. 2860-2862 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy exhibited clear excitonic resonances due to three excitons associated with uppermost valence bands (A, B, and C excitons). The oscillator strengths of the B and C excitons are larger than that of the A exciton. Their broadening was interpreted to be due to the contribution by exciton–polaritons in terms of large longitudinal–transverse splitting of respective excitons. Dependence of the exciton energy on temperature was well fitted assuming the Bose–Einstein statistics giving the Einstein characteristic temperature aitch-thetaE of 380 K (33 meV). © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1471374
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  • 8
    Electronic Resource
    Electronic Resource
    Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 4319-4321 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.© 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1428404
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  • 9
    Electronic Resource
    Electronic Resource
    Erratum: "Evidence of localization effects in InGaN single-quantum-well ultraviolet light-emitting diodes" [Appl. Phys. Lett. 76, 1671 (2000)] (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 78 (2001), S. 679-679 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1343504
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  • 10
    Electronic Resource
    Electronic Resource
    Impact of growth polar direction on the optical properties of GaN grown by metalorganic vapor phase epitaxy (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 78 (2001), S. 28-30 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The growth polar direction during metalorganic vapor phase epitaxy of wurtzite GaN was shown to affect the optical properties in terms of impurity and vacancy incorporation during the growth. The GaN film grown toward the Ga (0001) face (+c polarity) exhibited clear excitonic features in its optical absorption and luminescence spectra up to room temperature. Conversely, the film with the N (0001¯) face (−c polarity) exhibited a broad emission band, which locates in the broad absorption tail. The difference between the two was explained in terms of the presence of impurity-induced band-tail states in −c GaN due to increased impurity density and incorporation of large volume vacancy-type defects, which were confirmed by secondary ion mass spectrometry [Sumiya et al., Appl. Phys. Lett. 76, 2098 (2000)] and monoenergetic slow positron annihilation technique. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1337641
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