GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505 eV at 12 K and 3.437 at room temperature. All the parameters obtained in the present investigation are compared with those reported in the literature.

Photoluminescence studies of excitonic transitions in GaN epitaxial layers

The optical properties of GaN nanowires grown by catalyst free plasma-assisted molecular beam epitaxy on Si (111) are investigated by photoluminescence (PL) spectroscopy. The influence of the Si- and Mg-flux as well as the III-V ratio during growth on the PL properties is discussed. The Mg concentration as determined by secondary ion mass spectroscopy ranges from 5×1018 to 1×1020 cm−3. Raman scattering reveals that the nanowires are strain-free, irrespective of Si- or Mg-doping. The near band-edge emission of undoped or slightly Si-doped material is dominated by the narrow D0X recombination at 3.4715 eV with a full width at half maximum of 1.5 meV at 4 K. For high Si-fluxes, a blueshift of the D0X peak by 1 meV is found, which is attributed to band-filling effects. For moderate Mg-fluxes the acceptor-bound exciton recombination was detected at 3.4665 eV. Point defects due to the N-rich growth conditions are discussed as the origin of the emission band at 3.45 eV. Recombination at coalescence boundaries were identified as the origin of an emission band at 3.21 eV. The luminescence properties below 3.27 eV in highly Mg-doped samples are shown to be affected by the presence of cubic inclusions in the otherwise wurtzite nanowires.

Optical properties of Si- and Mg-doped gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy

We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <1016 cm–3 in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 meV and 33.2 meV for the O donor. The PL transients of the DBEs reveal a non-exponential decay for the NP lines. The DBE NP transient lineshape is assumed to be influenced by optical dispersion and scattering in the vicinity of exciton resonances, as well as by surface effects. The DBE decay time can most properly be deduced from the PL decay of the respective TETs and LO replicas, leading to values in the range of 1.1–1.8 ns. These values differ significantly from previous theoretical predictions, where values about two orders of magnitude shorter were obtained. A tentative discussion of the main observed features of acceptor bound excitons (ABEs), which are much less studied in GaN, is given. A decay time of about 0.9 ns for the shallowest 3.466 eV ABE is estimated, i.e. shorter than that for the shallow donor BEs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recombination of free and bound excitons in GaN

The hot electron relaxation time is studied in an n-type GaN film grown by molecular beam epitaxy on sapphire. A femtosecond pump–probe technique is used in which the electrons are excited by an infrared pump and the carrier dynamics are monitored by a tunable near ultraviolet probe. Complex transients, showing bleaching or induced absorption, are observed. The data are fitted by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The LO-phonon emission time is measured to be 0.2 ps.

Hot electron relaxation time in GaN

Recent work has demonstrated that unique insight into the electronic structure and optical properties of solid state transition metal and lanthanide systems is achievable through high pressure studies. In this review, we present selected examples that illustrate the advances in understanding possible from high pressure luminescence experiments. The ability of pressure to continuously vary energy level structure and coordination environment is emphasized and related to variations in luminescence properties. Pressure is shown to influence luminescence properties through crystal field, covalency, and spin-orbit coupling effects. The effect of pressure on luminescence energy, intensity, and decay properties will be considered. Additional phenomena to be discussed include electronic crossovers, vibrational coupling, admixing of electronic states, energy transfer and site selective spectroscopy.High pressure luminescence properties of transition metal and lanthanide ions in crystalline solids, metal complexes, and glasses will be reviewed.

High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems

Femtosecond degenerate four-wave mixing of gan on sapphire : measurement of intrinsic exciton dephasing time

We have measured photoluminescence (PL) and reflectance spectra of high-quality wurtzite GaN on sapphire at pressures up to 8 GPa $(T=10 \mathrm{K}).$ All three intrinsic exciton transitions arising from the $A,$ $B,$ and $C$ interband transitions were observed in reflectance measurements. The PL spectra are dominated by the $A$ and $B$ free exciton transitions and the recombination of an exciton bound to a neutral donor. Transitions due to the excited $n=2$ state of the $A$ exciton and the first- and second-order LO phonon replicas of the $A$ free exciton were also resolved. The pressure dependence of the $A$ band gap and exciton binding energy is obtained from the intrinsic exciton energies. The experimental results clearly reveal a sample-dependent change of the biaxial strain in the GaN layers with increasing hydrostatic pressure.

Photoluminescence and reflectance studies of exciton transitions in wurtzite GaN under pressure

Application of femtosecond-excitation correlation to the study of emission dynamics in hexagonal GaN

We performed time-resolved and continuous wave photoluminescence on two samples of hexagonal GaN, one with free exciton emission and the other without. For the sample with free exciton emission, very different decay dynamics are observed between the front and backside emission. We find that the strain caused by the lattice mismatch between the sapphire substrate and the GaN film has a large influence on the population decay of the sample with free exciton emission and a minor influence on the decay properties of the sample dominated by bound exciton emission. A polariton picture is used to describe the observed behavior.

Time-resolved photoluminescence study of excitons in hexagonal gan layers grown on sapphire

The density and temperature dependence of the exciton dephasing time of two hexagonal GaN films on sapphire is measured using degenerate four-wave mixing (DFWM). The residual 4 ps dephasing time at low temperature and density is caused by exciton-impurity scattering. We present a theory of DFWM for various amount of inhomogeneous broadening. Good agreement of the temperature dependence of the dephasing time is found between theory and experiment.

S. Pau

Papers

Femtosecond degenerate four-wave mixing of gan on sapphire : measurement of intrinsic exciton dephasing time

Photoluminescence and reflectance studies of exciton transitions in wurtzite GaN under pressure

Application of femtosecond-excitation correlation to the study of emission dynamics in hexagonal GaN

Time-resolved photoluminescence study of excitons in hexagonal gan layers grown on sapphire

Measurement of phonon-exciton dephasing rate in GaN on sapphire by degenerate four-wave mixing