GaNxAs1-x and Ga1-yInyNxAs1-x are the most prominent members of a novel class of non-amalgamation type semiconductor alloys where a fraction x of the anions of the host (e.g., GaAs or Ga1-yInyAs) is replaced by N isovalent impurity atoms. The localized N-states in GaNxAs1-x and Ga1-yInyAs1-x form a series of discrete energy levels (e.g., isolated N-state, N-pairs and higher order N-cluster states) resonant with the conduction band of the host. The effect of the alloying with nitrogen on the bandstructure of GaNxAs1-x and Ga1-yInyNxAs1-x can be well parameterized using a band-anticrossing (BAC) model, namely, assuming a level repulsion between an effective N-state and the conduction band-edge state. The dependence of several physical properties on nitrogen incorporation can be predicted qualitatively in the framework of this model, e.g., a tremendous increase of the electron effective mass in GaNxAsx with increasing x, a huge cross section for scattering of electrons by N impurities in electronic transport, etc. Most of these predictions can be tested and verified by performing hydrostatic pressure experiments which, within the picture of the BAC model, allow one to tune continuously the energy difference between the host-like conduction band edge and the effective N-level within one and the same specimen. Several examples of this kind will be discussed. Furthermore, we will demonstrate the limitations of the BAC model in the case of GaNxP1-x and also of GaNxAs1-x. In particular, we will show several examples where the description by a single effective N-state fails and that the multiplicity of the N-states needs to be taken into account. Again hydrostatic pressure experiments prove to be a useful and suitable tool for revealing the effects due to N-cluster states. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Theory/experiment comparisons of optical properties of dilute nitride heterostructures are presented. A type I - type II transition in In/sub 0.23/Ga/sub 0.77/As/GaN/sub x/As/sub 1-x/ heterostructures is identified. The model is used for a study of lasing in the 1.3-1.5 /spl mu/m range.

Optical interband transitions in a series of In0.23Ga0.77As–GaNxAs1−x quantum well samples are investigated. For changing nitrogen content, a type I-type II transition is identified by a detailed analysis of photoluminescence and photoreflectance spectra. Experimental results are compared systematically with spectra calculated by a microscopic theory. A valence band offset parameter of (1.5±0.5)eV is extracted for this heterostructure system.

We investigated the interband transitions of Ga0.7In0.3N0.005As0.995/GaNxAs1−x and Ga0.77In0.23As/GaNxAs1−x quantum wells with x of 0–3% by modulation spectroscopy. When N is incorporated into the barrier a change of the band alignment must occur due to the large red-shift of the Ga(N,As) band gap with increasing N. Therefore, a variation of x leads to a considerable change of the carrier confinement of holes and, in particular, of electrons. At x≈1% in the barrier a band alignment transition from type I to type II can be observed for the N-free wells. For N-containing wells, the band alignment changes at x≈3%.

We derive an analytical model to describe the conduction-band states of GaNAs-based quantum well structures, including the band anticrossing effect between N resonant states and the conduction-band edge. The predictions of the model are compared to those obtained using a full ten-band k.p model based on the same set of parameters. Both methods are then tested by comparison with the experimentally determined ground- and excited-state interband transition energies of GaNxAs1-x quantum wells of different well widths and N composition x obtained at 300 K and under hydrostatic pressures up to 2.0 GPa. We show that the transition energies can be described by a consistent set of material parameters in all the samples studied, and present how the conduction to valence-band offset ratio varies strongly with x in GaNxAs1-x/GaAs quantum well structures. We conclude that the model presented can be used to predict the transition energies and electron subband structure of any GaNxAs1-x/GaAs quantum well with well width between 2 and 25 nm, and N composition x between 1 and 4%, although further work is still required to confirm the optimum choice for the variation of band offset ratio with composition.

Exciton photoluminescence (PL) in a GaInNAs/GaNAs quantum well was measured in the temperature range from 15 K to 300 K. Two striking features of the PL were observed: the nonmonotoneous temperature dependence of the Stokes shift and the abrupt increase of the PL linewidth in a rather narrow temperature range. These features are known to be strong indications of the hopping relaxation of excitons via localized states distributed in space and energy. Computer simulations of the hopping relaxation of excitons were carried out. Comparison between the simulation results and the experimental data provides an important and reliable information on the energy shape of the density of states and also on the energy range, in which localized states for excitons are distributed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of hydrogenation on five GaNxAs1-x epitaxial layers (0.00043less than or equal toxless than or equal to0.019) grown by metal-organic vapor-phase epitaxy was investigated. Photomodulated reflectance (PR) and photoluminescence spectroscopy were used to study the electronic band structure, and x-ray diffraction (XRD) and Raman spectroscopy to probe, respectively, the static and dynamic properties of crystal lattice before and after hydrogenation. Hydrogen almost completely neutralizes the effect of N on the band structure of the GaAs host. The direct band gap E- and the spin-orbit split-off band E- +Delta(0) blueshift toward the corresponding energies in GaAs and the E+ band disappears after hydrogenation. The PR spectra of hydrogenated GaNxAs1-x resemble broad GaAs-like spectra. The XRD traces reveal that hydrogenation removes the tensile strain in GaNxAs1-x layers and even induces compressive strain. After hydrogenation the GaAs-like features in the Raman spectra persist whereas the local vibrational mode due to N disappears. Three H-related modes can be distinguished in the Raman spectra.

An overview is presented of experimental and theoretical work oil band structure aspects of (Ga,In)(N,As) and Ga(N,As) quantum well Structures and epitaxial layers grown by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE). The evolution Of unusual band structure and phonon features in GaNxAs1-x with increasing x caused by the impurity character of nitrogen in GaAs is discussed. Hydrogenation of Ga(N,As) allows one to virtually switch off the N-induced changes of the band structure and the vibrational modes. A strong blue shift up to about 100 meV of the bandgap of quaternary samples is observed on thermal annealing. The magnitude of the blue shift depends strongly on In and N concentrations as well as on the growth and annealing conditions. Raman spectra of MOVPE-grown (Ga,hi)(N,As) epitaxial layers reveal local In-N and Ga-N modes. On annealing, the intensity ratios of the local modes change, indicating a rearrangement of the nitrogen nearest-neighbour environments from Ga-rich to In-rich environments. Tight binding calculations Suggest that this might contribute strongly to the observed blue shift. Other possible contributions to the blue shift are also discussed.

We have investigated the optical properties of edge-emitting laser structures containing three Ga0.7In0.3N0.005As0.995 quantum wells embedded in GaNxAs1-x barriers grown by metal-organic vapour-phase epitaxy. In a series of three samples the nitrogen content x of the barrier was varied from 0% to 3%. We studied the optical transitions using pressure-dependent photomodulated reflectance (PR) up to 20 kbar at room temperature. Additionally we measured the pressure dependence of the lasing energy and the threshold current of the corresponding laser structures as function of hydrostatic pressure. Due to the large redshift of the Ga(N,As) band gap with increasing N of about 150 meV per percent N, the variation of x leads to a considerable change of the carrier confinement particularly of electrons. The strong increase of the threshold current of the laser device with pressure suggests a swiftly increasing threshold carrier density due to the increasing effective mass and non-parabolicity. Comparing the pressure dependence of the lasing energy and the conduction band edge supports this conclusion.

Dramatic changes of the electronic band structure occur when incorporating even a small fraction of N into GaAs. One important consequence of the N-incorporation is a strong non-parabolicity of the conduction band of GaNxAs1−x yielding already for x less than 1% a considerable increase in the electron effective mass and a strong variation of the electron effective mass with increasing k-vector. We demonstrate how this N-induced non-parabolic dispersion of the conduction band in Ga(N,As) can be determined by a careful analysis of the interband transitions of Ga(N,As)-based quantum wells as a function of hydrostatic pressure. A series of GaN0.018As0.982/GaAs wells of various widths was studied by photomodulated reflectance (PR) at 300 K and hydrostatic pressures up to 20 kbar. The PR spectra were fitted using derivative-like line shapes to extract the energy positions of the interband transitions. The transition energies were compared with theoretical values calculated using a 10-band k.p-model including the effects of nitrogen. The good agreement between experiment and theory allows one to extract the valence band offset as well as the conduction band dispersion and hence the change of the effective mass with pressure and energy.

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.

A series of GaNxAs1-x/GaAs quantum well structures with well widths of about 20 nm and x varying between 1% and 3.5% has been grown by metal-organic vapour phase epitaxy. We have studied the evolution of the quantum well states under hydrostatic pressure up to 20 kbar at 300 K by photomodulated reflectance (PR) spectroscopy. The energy positions of the quantum well transitions have been obtained by fitting the PR spectra. The pressure dependence of the allowed heavy-hole transitions enhhn decreases with increasing n. This directly reflects the strong non-parabolic dispersion of the conduction band originating from the interaction of the N-impurity level with the bands of the GaAs host. The fitted energy positions and their pressure dependence can be well described by a 10 band k.p model. The observed splitting between the lowest light-hole and heavy-hole transitions are in agreement with a type I band alignment.

An intrinsic property of quaternary alloys A(1-y)B(y)C(1-x)D(x) (x approximate to 1 - 3 %) with D being an isovalent trap is reported: a set of discrete band gaps occurs due to substitution of the isovalent trap D on sites with different nearest-neighbor environments. Exemplary, this phenomenon is demonstrated for (Ga, In)(N,As) by experiment and explained by tight-binding supercell calculations. The band gap of this nitrogen-poor alloy is blueshifted by simply moving the nitrogen isovalent traps from Ga-ligand rich sites to In-ligand rich sites, without changing the alloy composition.

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.

The unique properties of the band structure of Ga(N,As) and (Ga,In)(N,As) grown by metal-organic vapour-phase epitaxy were studied experimentally and explained theoretically. The transition from nitrogen acting as an isovalent center to a nitrogen-induced band is found at a concentration x(N) approximate to 0.2 %. The tremendous red shift of the band gap is caused by a repulsion of the N-level and the conduction band edge. The band alignment of GaAs/GaNAs has is type I. In case of (Ga,In)(N,As) a fine structure of the band gap due to different nearest-neighbour configurations of the nitrogen is observed The blue shift after annealing is ascribed to a rearranging of the N-environments.

GaN x As 1−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.

GaNxAs1-x samples with x ranging from 0.043% to 2.8% were grown by MOVPE. Analysing low-temperature photoluminescence spectra taken under hydrostatic pressure and room temperature photomodulated reflectance spectra gives strong evidence that the transition from N acting as an isoelectronic impurity to forming N-induced bands takes place at a N-concentration of about 0.2%.

We have investigated the unusual band formation at the Gamma-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 Eo 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 E-0 band gap on N-content for x < 1% at-8 K is considerably stronger than at 300K. Furthermore, R spectra of the E-1 and E-1 + Delta(1) transitions show an uncommonly strong disorder-induced broadening with increasing N-content.