H. Siegle

Quantitative determination of hexagonal minority phase in cubic GaN using Raman spectroscopy

We show that Raman scattering is a very sensitive and straightforward tool for the quantitative determination of a structural minority phase in GaN. In- and on-plane excitations, as well as polarization dependent measurements on predominantly cubic and hexagonal GaN samples, were performed and forward scattering effects were found. We were able to verify as an example the phase purity of a cubic GaN sample down to the 1% level.

Vertical strain and doping gradients in thick GaN layers

We report on spatially-resolved low-temperature luminescence and Raman experiments on a 220-μm-thick GaN layer grown on sapphire by hydride vapor phase epitaxy. Our measurements reveal that the peak position of the near-band-gap luminescence strongly depends on the distance to the substrate interface. The luminescence shifts continuously to lower energies with decreasing distance but a strong blue shift occurs directly at the interface. We correlate these effects with the inhomogeneous free-carrier distribution and the strain gradient found by our Raman experiments.

Local vibrational modes in Mg-doped GaN grown by molecular beam epitaxy

Local vibrational modes in the region of the acoustic and optical phonons are reported for Mg-doped GaN grown by molecular beam epitaxy. The modes, studied by Raman spectroscopy, appear in addition to the known modes in the high-energy region around 2200 cm−1. We suggest disorder-activated scattering and scattering from Mg-related lattice vibrations to be the origin of the low-energy modes. Our assignment is supported by calculations based on a modified valence-force model of Kane. Temperature-dependent measurements between 4 and 300 K exclude an electronic Raman-scattering mechanism. We also report a new line at 2129 cm−1 and discuss the origin of all five observed high-energy modes.

Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.

Stress analysis of selective epitaxial growth of GaN

Stress distributions in selectively overgrown self-organized GaN hexagonal pyramids have been analyzed by continuum elasticity theory. This has been carried out using the values for the moduli of elasticity found in the literature and an effective lattice mismatch between the GaN and the substrate that was determined from the Raman shift of the GaN buffer layer. The results of compressive stress in the buffer layer, tensile stress on the lower half of the pyramids’ facet surface, and full relaxation for approximately the upper 2/3 of the pyramids are in satisfactory agreement with the experimental observations that were deduced from cathodoluminescence microscopy and micro-Raman spectroscopy.

Spatially resolved photoluminescence and Raman scattering experiments on the GaN/substrate interface

We present results from spatially resolved photoluminescence and Raman experiments on the substrate interface region of wurtzite GaN layers. We show that the broad photoluminescence band with an intensity maximum at 2.4 eV is not an intrinsic property of GaN. We found that this photoluminescence band is strong only near the interface. Our investigations reveal that both the substrate interface and a region of structural reorientation of the layer near the interface act as a source of the photoluminescence.

Pressure and temperature effects on optical transitions in cubic GaN

Pressure and temperature effects on optical transitions in cubic GaN grown on a GaAs substrate have been studied by photoluminescence (PL) spectroscopy at hydrostatic pressures up to 9 GPa (10 K) and as a function of temperature (10–300 K) at ambient pressure. The dominant emissions at 10 K and ambient pressure are assigned to the bound-exciton transition (zero-phonon line), the donor-acceptor-pair (DAP) emission, and, tentatively, to the first three LO-phonon replicas of the bound exciton. These PL features shift to higher energy with increasing pressure. The pressure coefficients indicate that the observed recombination processes involve states which are closely related to the band edges. Temperature-induced evolutions from bound to free-exciton (FE) transition and DAP emission to free-to-bound transition are resolved. The binding energies of the FE and donor and acceptor levels in cubic GaN have been determined from the temperature and power-density dependence of the PL emission energies.

Comment on “Shallow donors in GaN studied by electronic Raman scattering in resonance with yellow luminescence transitions” [Appl. Phys. Lett. 69, 1276 (1996)]

In a recent letter, Ramsteiner et al. reported the observation of electronic Raman transitions in GaN grown on GaAs. The excitation curves of these Raman spectra were in resonance with the maximum of the broad luminescence of their samples at 2.2 eV, from which the authors deduced the observation of electronic states of shallow donors involved in the yellow luminescence. We present here evidence which makes the proposed direct link to the yellow luminescence doubtful. We performed Raman-scattering experiments on GaN layers on different substrates. One series of samples was grown on GaAs by molecular beam epitaxy ~MBE! and two other series on sapphire by metal-organic chemical-vapor deposition ~MOCVD! and by hydride vapor phase epitaxy ~HVPE!. While the samples grown on sapphire exhibit a strong yellow luminescence with an intensity maximum at 2.2 eV, most of the layers grown on GaAs show a rather broad luminescence ranging from the donor-acceptorluminescence region ~;3.1 eV! to less than 1.8 eV ~Fig. 1!. The most striking observation we made was that the additional peaks reported by Ramsteiner et al. were present only in spectra taken from samples grown on GaAs. Samples grown on sapphire did not exhibit these features, although they show a much stronger yellow luminescence ~see Fig. 1!. Therefore, a direct link to the yellow luminescence as proposed by Ramsteiner et al. appears doubtful. We found the additional Raman modes also in purely cubic layers ~as evidenced by the weakness of the E2 mode! contradicting the

Impact of the ZnO buffer on the optical properties of GaN : time resolved micro-photoluminescence

Spatially-resolved and time-resolved micro-photoluminescence (μ-PL) investigations were performed on thick GaN layers. Our measurements reveal that the peak position of the excitonic transition lines strongly depends on the distance to the substrate interface. The luminescence is shifted continuously to lower energies with decreasing distance, however, a strong blue shift occurs directly at the interface. The sample exhibits morphologically separated channels emitting at different energies and having different temporal behaviors. These different channels could be explained by the incorporation of O and Zn impurities, since ZnO buffer layers were used and disappeared after growing GaN on the substrate/buffer layer. Time-resolved luminescence investigations demonstrate that there is a strong energy transfer between the Zn and O channels.

Influence of Doping on the Lattice Dynamics of Gallium Nitride

We present results of Raman-scattering experiments on GaN doped with Si, C, and Mg, respectively, grown by molecular beam epitaxy (MBE). The influence of the different dopants on strain and freecarrier concentration was investigated. Furthermore, we report on several local vibrational modes (LVM) around 2200 cm in Raman spectra of highly Mg-doped GaN. A possible explanation of these high-energy modes in terms of hydrogen-related vibrations is given. We also found a variety of new structures in the range of the GaN host lattice phonons. Secondary ion mass spectroscopy (SIMS) was applied to determine the concentration of magnesium and unintentionally incorporated hydrogen.