J. Koch

From N isoelectronic impurities to N-induced bands in the GaNxAs1−x alloy

GaNxAs1−x samples with x<3% grown by metalorganic vapor phase epitaxy were studied by low-temperature photoluminescence under hydrostatic pressure and photomodulated reflectance spectroscopy. The transformation from N acting as an isoelectronic impurity to N-induced band formation takes place at x≈0.2%. The N level does not shift with respect to the valence band edge of GaNxAs1−x. Concentration as well as hydrostatic-pressure dependence of the GaNxAs1−x bands can be described by a three band kp description of the conduction band state E− and E+ and the valence band at k=0. The model parameters for T<20 and T=300 K were determined by fitting the model to the experimental data. Modeling the linewidth of the E− transition by combining the kp model and ion statistics leads to the conclusion that the electron-hole pairs are strongly localized.

Direct-band-gap Ga(NAsP)-material system pseudomorphically grown on GaP substrate

Compressively strained Ga(NAsP) multi-quantum-well heterostructures with As concentration above 85% have been grown pseudomorphically on GaP substrates by metal organic vapor phase epitaxy. Detailed structural analysis applying high-resolution x-ray diffraction proves the high crystalline perfection of the samples. Optical spectroscopy appyling photoluminescence and excitation spectroscopy verify the direct-band-gap characteristic of this novel material system. The comparison of the experimental data with elemental calculations via the band anticrossing model demonstrates that the formation of direct band structure can be understood by the strong bowing of the band gap energy typical for diluted III-V nitrides.

Optical Spectroscopic Studies of N‐Related Bands in Ga(N, As)

We have investigated the unusual band formation at the Γ-point and in the vicinity of the L-point in the alloy system Ga(N, As) by various spectroscopic methods. A series of GaNxAs1—x epitaxial layers with x varying from 0.05 to 2.8% was grown on (100) GaAs by metal-organic vapour phase epitaxy. The samples were studied by photoluminescence (PL) as well as photoluminescence excitation (PLE) spectroscopy, photomodulated reflectance (PR), and conventional reflectance (R) spectroscopy at room temperature and liquid helium temperature. The low-temperature PL and PLE spectra in the spectral region of the E0 band gap show clear evidence for in-gap nitrogen-pair and cluster states at low concentrations (x < 0.1%), and for higher nitrogen concentrations the formation of a new band. The dependence of the E0 band gap on N-content for x < 1% at 8 K is considerably stronger than at 300 K. Furthermore, R spectra of the E1 and E1 + Δ1 transitions show an uncommonly strong disorder-induced broadening with increasing N-content.

(GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser with ultrabroad temperature operation range

The temperature dependence of the emission of a (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is investigated. We find laser emission over an extremely broad temperature range from 30 K up to 388 K. The laser threshold varies from 5 kW/cm2 at 373 K down to a minimum of 1 kW/cm2 at 180 K and increases again to 4 kW/cm2 at 30 K. Picosecond emission dynamics after femtosecond optical excitation is obtained with peak delays below 33 ps and pulse widths below 20 ps over the entire operation range.

Emission dynamics and optical gain of 1.3-/spl mu/m (GaIn)(NAs)/GaAs lasers

The ultrafast emission dynamics of a 1.3-/spl mu/m (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is studied by femtosecond luminescence upconversion. We obtain a minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps. Laser operation with picosecond emission dynamics is demonstrated over a temperature range from 30 to 388 K. The bandgap shift with temperature of (GaIn)(NAs)/GaAs is determined to be about -2.9/spl middot/10/sup -4/ eV/K, which is smaller than for GaAs. Our measurements of the optical gain provide gain spectra similar to those of commercial (GaIn)(PAs)/InP-structures at moderate densities but broaden considerably for elevated carrier densities due to the stronger carrier confinement. We compare our experimental results with gain spectra calculated from a microscopic model and confirm the predictive capability of the model. The theoretical gain spectra are used as the input for a calculation of the temperature dependence of the (GaIn)(NAs)/GaAs surface-emitter emission which results in very good agreement with experiment.

Interband transitions of quantum wells and device structures containing Ga(N, As) and (Ga, In)(N, As)

The unusual N-induced band formation and band structure of Ga(N, As) and (Ga, In)(N, As) alloys are also reflected in the electronic structure of quantum wells (QWS) and device structures containing these non-amalgamation-type alloys. This review is divided into three parts. The first part deals with band structure aspects of bulk Ga(N, As) and motivates the possibility of a k · p-like parameterization of the band structure in terms of the level repulsion model between the conduction band edge of the host and a localized N-level. The second part presents experimental studies of interband transitions in Ga(N, As)/GaAs and (Ga, In)(N, As)/GaAs QW structures addressing band offsets, electron effective mass changes and an intrinsic mechanism contributing to the blueshift of the (Ga, In)(N, As) band gap on annealing. The observed interband transitions can be well described using a ten-band k · p model based on the level repulsion scheme. The third part deals with (Ga, In)(N, As)-based laser devices. The electronic structure of the active region of vertical-cavity surface-emitting laser and edge-emitter laser structures is studied by modulation spectroscopy. The gain of such structures is measured by optical methods and analysed in terms of a model combining the ten-band k · p description of the band structure and generalized Bloch equations.

Pressure and Temperature Dependent Studies of GaNxAs1–x/GaAs Quantum Well Structures

The pressure and temperature dependence of quantum well transitions in GaNxAs1—x/GaAs quantum well structures with xN = 1.8% and various well widths grown by metal organic vapour phase epitaxy were studied by photomodulated reflectance (PR) spectroscopy. The quantum well transition energies and their pressure dependence can be well described by a ten-band k · p Hamiltonian. Comparing experiment and theory demonstrates that the band alignment of the quantum well structures is type I with a chemical valence band offset of about 30% ± 5% for xN = 1.8%. The temperature coefficients of the quantum well states are to a good approximation independent of well width and considerably smaller than that of GaAs.

TEM investigations of (GaIn)(NAs)/GaAs multi-quantum wells grown by MOVPE

Quantum wells of the quaternary (GaIn)(NAs) alloy are grown compressively strained on GaAs by metal-organic vapor phase epitaxy (MOVPE) at low temperatures under non-equilibrium conditions. Growth experiments of particular heteroepitaxial multilayer systems are reported and the influence of varying conditions, namely of the arsenic source partial pressure and of the growth rate on the structural quality of the quantum wells is studied. Up to a critical amount of incorporated nitrogen, high perfection layers can be obtained which show a roughness of the interfaces between the wells and the barriers in the range of only a few monolayers. Any phase separation effects have been excluded by exact control of the particular growth conditions. For the structural characterization of the layer systems, conventional and high resolution transmission electron microscopy have been applied.

Vibrational properties of Ga As 0.915 N 0.085 under hydrostatic pressures up to 20 GPa

We investigated a

N-Composition and Pressure Dependence of the Inter Band Transitions of Ga(N,As)/GaAs Quantum Wells

\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}