J.P. Bergman

The excitonic bandgap of GaN: Dependence on substrate

Abstract The large variation in the bandgap and exciton energies reported in the literature for GaN is discussed. We compare our photoluminescence data for unstrained bulk GaN with thin GaN layers grown on sapphire and SiC, respectively. It is demonstrated that the built-in strain has a strong effect on the spectral positions of the intrinsic excitons, and consequently also on the bandgap. GaN layers on sapphire have an increased bandgap, while growth on SiC leads to a lower bandgap, compared to unstrained bulk GaN. The temperature dependence of the free excitons has been studied for all three cases, and remarkably strong differences are reported.

Effects of defect scattering on the photoluminescence of exciton-polaritons in n-GaN

Temperature dependent studies of the resonant and phonon-assisted radiative recombination of free excitons (FEs) in GaN are performed and are analyzed within the polariton concept. A strong impurity scattering of exciton-polaritons is proposed to be responsible for the revealed unusual behavior of the free A exciton in GaN, i.e. an enhanced intensity of the resonant FE emission in comparison with its longitudinal optical (LO) phonon replicas at low temperatures, as well as the narrow line shape of the 1-LO assisted transitions.

Optical characterisation of GaN and related materials

Abstract Recent experimental results on optical properties of GaN and related materials are discussed. Photoluminescence data of free excitons for sufficiently pure GaN samples demonstrate the dominance of excitonic recombination well above room temperature. Transient PL data give a radiative lifetime of about 200 ps for the A-exciton at 2 K in strain-free samples. A corresponding value of about 2 ns at room temperature is extrapolated. Radiative lifetimes for bound excitons are measured as about 250 ps for shallow donors and about 1.5 ns for shallow acceptors. Photoluminescence spectra from the 2D electron gas at a GaN AlGaN heterointerface are also demonstrated.

Exciton dynamics in homoepitaxial GaN

Photoluminescence transients have been studied with picosecond resolution in homoepitaxial GaN layers grown with MOCVD. In the studied samples, with doping concentration in the 1017 cm−3 range, the free exciton (FE) decay time is short, less than 100 ps, mainly due to capture to the shallow impurities, but also to some extent due to nonradiative transitions at defects. The donor bound exciton (DBE) decay time is also short, <140 ps, indicating some excitation transfer to nonradiative defects. The acceptor bound exciton (ABE) has a well-defined decay time of 800 ps below 15 K. The lower decay times for the FE and DBE in the homoepitaxial layers, compared to previously studied heteroepitaxial samples, is tentatively ascribed to the much lower dislocation density in the homoepitaxial layers, which suppresses the gettering of nonradiative point defects.

Photoluminescence of exciton-polaritons in GaN

Abstract Temperature dependent studies of the resonant (no-phonon) and phonon-assisted radiative recombination of free excitons (FEs) in GaN are performed, and are analyzed within the polariton concept. The parameters of the exciton–phonon coupling are estimated by analyzing the thermal broadening of the no-phonon (NP) FE line. The interaction with acoustic phonons is shown to be the dominant broadening mechanism for temperatures lower than 150 K, while for higher temperatures the contribution from the interaction with optical phonons is important. Strong defect/impurity scattering of exciton-polaritons is proposed to be responsible for the revealed unusual behavior of the free A exciton in GaN, i.e. an enhanced intensity of NP FE emission in comparison with its longitudinal optical (LO) phonon replicas, as well as the narrow line shape of the 1-LO assisted transitions.

Electronic structure and temperature dependence of excitons in GaN

Abstract The influence of epitaxial strain and temperature on free exciton properties for GaN is discussed, in relation to optical spectra. The exciton-polariton coupling is also briefly discussed, from the temperature dependence of the LO phonon replicas.

Photoconductivity in n-type modulation-doped GaN/AlGaN heterostructures

Abstract Photoluminescence and photoconductivity data from GaN/AlGaN modulation doped heterostructures, in the temperature interval between 2 and 250xa0K, are presented. The results show a strong influence of potential fluctuations on both optical spectra and photo-induced transport. The electric field fluctuations in the interface region of the two-dimensional electron gas is estimated to be in the several kVxa0cm −1 range, sufficient to ionize the excitons. Persistent photoconductivity effects may be related to the same fluctuations in these structures.

Excitation and recombination processes of Yb in InP and InAsP

Abstract The excitation and decay mechanisms of the Yb 3+ 4f-4f intra-shell emission in InP and InAsP (4% and 7% of As) are analysed. Exciton binding by Yb ion is discussed. The photoluminescence transient measurements indicate that the Yb PL emission is deactivated at increased temperature by energy back-transfer to band states of InP and InAsP.

Electron gas in modulation doped GaN/AlGaN structures

Abstract We report low temperature photoluminescence (PL), time resolved PL and electrical transport measurements related to the two-dimensional electron gas (2DEG) in GaN/AlGaN heterostructures and quantum wells grown by MOVPE on sapphire. Electrical measurements of the heterostructure samples show a room temperature mobility of up to 1300 cm2 V−1 s−1 and carrier concentration of 1013 cm−2. The PL spectrum at low temperatures shows a broad emission about 50 meV below the bulk exciton emission, attributed to recombination involving electrons from the lowest subband of the 2DEG at the GaN/AlGaN heterointerface and photoexcited holes in the GaN layer. The data agrees with a self consistent calculation of the energy levels and the electron concentration at the interface. The modulation doped GaN/AlGaN quantum well had an electron concentration of 3.0 × 1012 cm−3 and a mobility of 850 cm2 V−1 s−1 at 300 K. In the low temperature PL spectra we observed three well defined peaks, at 3.53, 3.58 and 3.62 eV, which we attribute to recombination processes in the quantum well (QW).

Radiative recombination in modulation-doped GaAs/AlGaAs heterostructures in the presence of an electric field

The radiative recombination processes involving two dimensional (2D) carriers from the notch potential formed at the interface of modulation doped GaAs/AlGaAs heterostructures have been studied by means of photoluminescence (PL) and photoluminescence excitation spectroscopy in the presence of an external electric field applied perpendicular to the layers via a gate electrode. Two PL bands related to the 2D electron gas are interpreted as the radiative recombination between 2D electrons and holes from the valence band (HB1) and from residual acceptors (HB2), respectively. The band bending in the active layer, which determines the energy positions of these H-bands, can be controlled by applying an external electric field. However, also the separation between the Fermi edge, EF, and the second 2D electron subband is deliberately varied by applying an electric field. At a sufficiently small separation, an efficient scattering path near k=0 is available for electrons at the Fermi energy. This can be observed in the PL spectra as a striking enhancement of the many-body excitonic transition, usually referred to as the Fermi edge singularity (FES). The enhancement of the FES is usually explained in terms of an efficient scattering for electrons at the Fermi edge via the nearly resonant adjacent subband. The efficiency of this process is dependent on the separation between the Fermi edge, EF, and the next subband, which can be controlled via the applied field in our experiments.