L. Eckey

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.

Dynamics of bound-exciton luminescences from epitaxial GaN

Free‐ and bound‐exciton luminescences of GaN epitaxial layers grown by a sublimation technique on 6H‐SiC substrates were investigated using time‐integrated and time‐resolved photoluminescence measurements at low temperatures. Lifetimes were determined for the donor‐bound exciton at 3.4722 eV and for two acceptor‐bound excitons with energies of 3.4672 eV and 3.459 eV. On the basis of our results we obtain an upper limit of the free‐exciton oscillator strength of 0.0046 for GaN. Luminescences between 3.29 eV and 3.37 eV are identified as due to excitons deeply bound to centers located near the substrate‐epilayer interface. Free excitons are captured by these centers within 20 ps.

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.

Mechanisms of optical gain in cubic gallium nitrite

We report on the mechanisms of optical gain in cubic GaN. Intensity-dependent gain spectra allow a distinction of the processes involved in providing optical amplification. For moderate excitation levels, the biexciton decay is responsible for a gain structure at 3.265 eV. With increasing excitation densities, gain is observed on the high energy side of the cubic band gap due to band filling processes. For the highest pump intensities, the electron-hole plasma is the dominant gain process. Gain values up to 210 cm−1 were obtained, indicating the high potential of cubic GaN for device applications. The observed gain mechanisms are similar to those of hexagonal GaN.

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.

Zeeman spectroscopy of the Fe3+ center in GaN

We report an optical investigation of the Zeeman behavior of the deep iron acceptor in GaN grown on 6H–SiC. The characteristic ground state splitting of the near‐infrared luminescence transition at 1.2988 eV allows for an unambiguous assignment to Fe3+previously proposed on the basis of ODMR results. The observed luminescence lifetime of 8 ms as well as the fine structure of the excited state are consistent with a 4T1(G)–6A1(S) transition. The 4T1(G) state is found to couple only weakly to e‐type phonon modes.

Dynamical study of the yellow luminescence band in GaN

Abstract A comprehensive study of the yellow photoluminescence (YL) in GaN epitaxial films grown by hydrid vapor phase epitaxy and by metal organic vapor phase epitaxy is presented including time-integrated and time-resolved photoluminescence (PL), PL excitation (PLE) and optically detected magnetic resonance (ODMR) experiments. ODMR reveals the participation of shallow and deep double donors based on the analysis of the g -values. This recombination model is supported by time-resolved investigations. PLE spectra show a close connection between the excitation processes of the YL band and of the inner transition of Fe 3+ at 1.293 eV. Two-color stimulation experiments prove energy transfer between YL and the Fe 3+ center by hole transfer, strongly confirming the YL recombination model involving a deep level 1.2 eV above the valence band.

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

Properties of the Biexciton and the Electron-Hole-Plasma in Highly Excited GaN

High-excitation processes like biexciton decay and recombination of an electron-hole-plasma are discussed as efficient mechanisms for lasing in blue laser diodes [1]. Therefore, the investigation of these processes is of fundamental importance to the understanding of the properties of GaN as a basic material for optoelectronical applications. We report on comprehensive photoluminescence and gain measurements of highly excited GaN epilayers grown by metal-organic chemical vapor deposition (MOCVD) over a wide range of excitation densities and temperatures. For low temperatures the decay of biexcitons and the electron-hole-plasma dominate the spontaneous-emission and gain spectra. A spectral analysis of the lineshape of these emissions is performed and the properties of the biexciton and the electron-hole-plasma in GaN will be disscused in comparison to other wide-gap materials. At increased temperatures up to 300 K exciton-exciton-scattering and band-to-band recombination are the most efficient processes in the gain spectra beside the electron-hole-plasma.