xNx Alloys Research Articles

Unusual carrier thermalization in a dilute GaAs1−xNx alloy

Photoluminescence (PL) properties of the E0, E0+Δ0, and E+ bands in an x=0.62% GaAs1−xNx alloy were investigated in detail, including their peak position, linewidth, and line shape dependences on the excitation energy, excitation power, and temperature, using micro-PL. The hot electrons within the E+ band are found to exhibit highly unusual thermalization, which results in a large blueshift in its PL peak energy by >2kBT, suggesting peculiar density of states and carrier dynamics of the E+ band.

Computational study of GaAs1−xNx and GaN1−yAsy alloys and arsenic impurities in GaN

We have studied the structural and electronic properties of As-richGaAs1−xNx andN-rich GaN1−yAsy alloys in a large composition range using first-principles methods. We have systematicallyinvestigated the effect of the impurity atom configuration near both GaAs and GaN sidesof the concentration range on the total energies, lattice constants and bandgaps. The N(As) atoms, replacing substitutionally As (N) atoms in GaAs (GaN), cause thesurrounding Ga atoms to relax inwards (outwards), making the Ga–N (Ga–As) bondlength about 15% shorter (longer) than the corresponding Ga–As (Ga–N) bondlength in GaAs (GaN). The total energies of the relaxed alloy supercells and thebandgaps experience large fluctuations within different configurations and thesefluctuations grow stronger if the impurity concentration is increased. Substituting Asatoms with N in GaAs induces modifications near the conduction band minimum,while substituting N atoms with As in GaN modifies the states near the valenceband maximum. Both lead to bandgap reduction, which is at first rapid but laterslows down. The relative size of the fluctuations is much larger in the case ofGaAs1−xNx alloys. We have also looked into the question of which substitutional site (Ga or N)As occupies in GaN. We find that under Ga-rich conditions arsenic prefers thesubstitutional N site over the Ga site within a large range of Fermi level values.

Resonant Raman scattering with the E+ band in a dilute GaAs1−xNx alloy (x=0.1%)

Resonant Raman scattering has been applied to a dilute GaAs1−xNx alloy with 0.1% N. The Raman lines of GaAs and GaN related modes, their combinations, and multiple order replicas of GaAs-like longitudinal-optical modes have been observed with a lower N composition than those studied previously. All these Raman features are found to be strongly enhanced with excitations in resonance with a broad photoluminescence band that is associated with the so-called E+ transition. This study provides additional insights into how the GaAs host conduction band states are perturbed and thus the electron-phonon interaction is affected by the N doping.

Pressure and composition dependence of the electron effective mass in GaAs1–xNx

The electronic band structures of GaAs1–xNx alloys are calculated ab initio using a supercell approach in connection with the full-potential linear muffin-tin-orbital method. Alloying GaAs with small amounts of GaN modifies substantially the conduction bands. The lowest conduction band exhibits a strong nonparabolicity, which is reflected in the dependence of the electron effective mass on the k-vector. The calculations indicate that for x above 0.01 the alloys have electron effective masses, which are significantly larger than that of pure GaAs. In addition, the effective mass decreases with x. Hydrostatic pressure is found to strongly influence the electron effective masses. The pressure coefficient of the mass at the conduction band minimum varies non-linearly with x. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence from the nitrogen-perturbed above-bandgap states in dilute GaAs1−xNx alloys: A microphotoluminescence study

Using microphotoluminescence $(\ensuremath{\mu}\text{\ensuremath{-}}\mathrm{PL})$, in dilute N $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ alloys, we observe a PL band far above the bandgap ${E}_{0}$ with its peak energy following the so-called ${E}_{+}$ transition, but with contribution from perturbed GaAs host states in a broad spectral range $(>100\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$. This finding is in sharp contrast to the general understanding that ${E}_{+}$ is associated with a well-defined conduction band level (either ${L}_{1c}$ or ${\mathrm{N}}_{x}$). Beyond this insight regarding the strong perturbation of the GaAs band structure caused by N incorporation, we demonstrate that a small amount of isoelectronic doping in conjunction with $\ensuremath{\mu}\text{\ensuremath{-}}\mathrm{PL}$ allows direct observation of above-bandgap transitions that are not usually accessible by PL.

Giant and composition-dependent optical band gap bowing in dilute GaSb1−xNx alloys

Ab initio pseudopotential plane wave calculations and large 64-atom relaxed supercells are used to investigate the structural and electronic properties of GaNxSb1−x dilute alloys. While the band gaps of conventional III-V semiconductors have a simple and weak dependence on composition, this work illustrate a violation of this expected behavior. We show that the band gap decreases rapidly with increasing compositions of N and that GaNxSb1−x show an abnormal giant gap reduction. As a consequence, the optical band gap bowing is found to be giant and composition dependent as found for other mixed anion III-V–N systems.

Dependence of electronic properties on nitrogen concentration in GaAs1−xNx dilute alloys

Based on the pseudopotential scheme, the effect of nitrogen concentration on electronic properties of zinc-blende GaAs1−xNx alloys has been investigated for small amounts of N. The agreement between our calculated electronic band parameters and the available experimental data is generally reasonable. In agreement with recent experiment, we find that the incorporation of a few percent of N in the material of interest reduces substantially the fundamental band-gap energy and narrows the full valence band width. The electron and heavy hole effective masses are found to decrease rapidly when adding a concentration of nitrogen less than 0.005 in GaAs. This may increase the mobility of electrons and heavy holes providing new opportunities regarding the transport properties. The information derived from the present study shows that GaAs1−xNx (0⩽x⩽0.05) properties may have an important optoelectronic applications in infrared and mid-infrared spectral regions.

Direct Observation of NN Pairs Transfer in GaP1−xNx(x = 0.12%)

Time-resolved photoluminescence (TRPL) was applied to investigate thetransient process in GaP1−xNx (x = 0.12%) alloy. The filling,transferring and decay processes among nitrogen pairs are directlyobserved. The NN4 pair, either not present or only a small obscurepeak under a proper excitation condition in the steady-statephotoluminescence spectrum, is well resolved by TRPL.

Stress evolution in GaAsN alloy films

We have investigated stress evolution in dilute nitride GaAs1−xNx alloy films grown by plasma-assisted molecular-beam epitaxy. For coherently strained films (x<2.5%), a comparison of stresses measured via in situ wafer curvature measurements, with those determined from x-ray rocking curves using a linear interpolation of lattice parameter and elastic constants, suggests significant bowing of the elastic properties of GaAsN. The observed stress differences are used to quantify the composition-dependent elastic constant bowing parameters. For films with x>2.5%, in situ wafer curvature measurements reveal a signature for stress relaxation. Atomic force microscopy and transmission electron microscopy measurements indicate that stress relaxation occurs by a combination of elastic relaxation via island formation and plastic relaxation associated with the formation of stacking faults.

Carrier recombination processes in GaAsN: from the dilute limit to alloying

Low-temperature time-resolved photoluminescence (TR–PL) experiments were used to study the dependence on nitrogen composition of the nature, the energy and the dynamics of radiative carrier recombinations in GaAs1−xNx alloys. Epitaxial layers were grown on (001) GaAs substrates by molecular beam epitaxy using solid sources for group-III and As elements, and a radiofrequency plasma source for N. The nitrogen content, measured by secondary ion mass spectroscopy, was in the range 5 × 10-5 – 7 × 10-2. PL spectra are dominated by the emission from the discrete states of nitrogen-based excitonic complexes for the very low x regime (around 5 × 10-5), showing exponential time decays. For x larger than 5 × 10-4, PL spectra are rather dominated by emission from a continuum of states originating from nitrogen clusters. In this case, nonexponential time decays are obtained. It is shown that these PL decays involve complex carrier transfers between the various available states. Changing the nitrogen content changes the recombination mechanism by changing the distance between the clusters.

Empirical tight-binding calculations of the electronic structure of dilute III–V–N semiconductor alloys

We present empirical tight-binding (TB) calculations of the electronic structure of GaP1−xNx and InyGa1−yAs1−xNx alloys with low nitrogen content (x<0.05) over the entire Brillouin zone. Following the method recently developed for GaAs1−xNx [Shtinkov et al., Phys. Rev. B 67, 081202 (2003)], we add to the TB basis an additional anion s orbital (sN) in order to account for the N-induced change of the electronic structure. The band structures of GaP1−xNx and InyGa1−yAs1−xNx are calculated using an sp3d5s*sN TB parametrization. Our TB results are in excellent agreement with experimental and other theoretical data without introducing any additional fitting parameters, demonstrating that the developed method is a promising tool for modeling a wide range of dilute nitride materials and heterostructures.

Organometallic vapor phase epitaxy of GaAs1−xNx alloy layers on GaAs(001): Nitrogen incorporation and lattice parameter variation

Epitaxial GaAs1−xNx alloy layers, nominally 200-nm-thick, with x up to 0.0375 were grown on GaAs(001) at temperatures Ts varying from 500 to 650 °C to investigate nitrogen incorporation and lattice parameter variations during organometallic vapor phase epitaxy from trimethylgallium, tertiarybutylarsine, and 1,1-dimethylhydrazine. Quantitative secondary ion mass spectrometry measurements (SIMS) indicate that N incorporation decreases systematically with increasing Ts to become almost negligible at 650 °C. All films are coherent with the substrate as judged by high-resolution x-ray reciprocal lattice mapping although atomic force microscopy and cross-sectional transmission electron microscopy reveal the presence of cracks in films with x>0.02. High-resolution x-ray diffraction measurements combined with SIMS analyses indicate that the lattice constant decreases linearly with increasing x following closely the predictions of Vegard’s rule for x<0.03. At higher concentrations, the lattice constant decreases more rapidly as a significant fraction of N atoms becomes incorporated in nonsubstitutional sites as demonstrated by nuclear reaction analysis.

Improvement in hard magnetic properties of FePt films by N addition

The structure and magnetic properties of (Fe56Pt44)100−xNx alloy films with x=0–15 at. % prepared by rf magnetron sputtering were investigated. Compared to an Fe56Pt44 binary film, (Fe56Pt44)100−xNx alloy films showed great enhancement in hard magnetic properties. A coercivity Hc of 1027 kA/m, a remanence Mr of 1.24 T, and a maximum energy product (BH)max of 260 kJ/m3 were obtained for an (Fe56Pt44)93N7 film annealed at 600 °C for 10 min, whereas the Fe56Pt44 film annealed at the same conditions gave Hc=682 kA/m, Mr=0.87 T, and (BH)max=132 kJ/m3. Structural analysis revealed that N addition influenced the phase structure and microstructure of FePt films. N atoms incorporated in the disordered FePt phase in as-deposited films, they released rapidly out of the FePt phase by vacuum annealing. It is suggested that the rapid release of N atoms from the FePt phase promotes the transformation to the ordered phase, results in a higher coercivity of the film. The high remanence of the (Fe56Pt44)93N7 film is attributed to enhanced exchange coupling of the soft iron nitride phase and the hard FePt phase.

Photoluminescence excitonic linewidth in GaAsN alloys

We have calculated the variation of the photoluminescence excitonic linewidth, defined as the full width at half maximum, due to composition fluctuations as a function of nitrogen composition in completely random GaAs1−xNx alloys. We have considered the effects of a constant and of the composition dependent bowing parameter (and, thus, the energy band gap) on the excitonic linewidth. We have used two different recently proposed variations of the conduction electron mass with nitrogen composition. We have considered both the free and localized excitons. We find that the behavior of excitonic linewidth as a function of nitrogen composition, using two different bowing parameters, is qualitatively and quantitatively quite different.

Nitrogen-activated bowing of dilute InyGa1−yAs1−xNx based on photoreflectance studies

The dependence of the fundamental band gap and higher-lying critical-point energies of dilute-nitrogen Ga1−yInyAs1−xNx epilayers on nitrogen mole fraction (x), for x⩽0.0125, and temperature, from 20 to 300 K, was investigated by photoreflectance spectroscopy. The band gap, EG, was found to decrease with increasing x in a highly nonlinear manner. The bowing parameter (the second-order parameter b in a quadratic expression for the dependence of EG on x) was found to become less negative with increasing x; the value of b changed from −50 eV, at very low nitrogen fraction, to −20 eV, at x>0.01. These results strongly suggest that nitrogen-related impurity levels arise within the band gap of dilute-nitrogen Ga1−yInyAs1−xNx alloys.

Spectroscopic ellipsometry study on the dielectric functions of GaPN alloys

We have determined the complex dielectric functions of GaP1−xNx alloys using spectroscopic ellipsometry to clarify the electronic structure. With increasing N concentration, the E1-gap peak height for both e1 and e2 decreases and the E1-gap energy shifts to higher energies, showing that the electronic state at the L point is considerably changed due to the N incorporation. In the lower energy range, a broad peak appears near the fundamental band gap energy of GaP. With increasing N concentration, this peak becomes broader and the peak height increases. This indicates that the band-edge formation in GaP1−xNx alloys is extremely unique compared to that in conventional semiconductor alloys.

Measurement of charge-separation potentials in GaAs1−xNx

The ternary alloy GaAs1−xNx is interesting as a semiconductor that can be grown epitaxially on GaAs. As is well known, the band gap can be reduced by as much as 0.4 eV by changing the nitrogen concentration from 0% to 3%. We measured the spectral response and photoconductive lifetime of the alloys as a function of temperature. In this work, the films were grown by gas-source molecular-beam epitaxy on semi-insulating GaAs substrates. All measurements were made using the contactless, resonant-coupled photoconductive decay method. Our data show that the spectral response of GaAs1−xNx alloys consists of photoconductive band tails that extend well into the infrared (beyond the nominal band gap). For example, the photoconductive band tails extend to about 1900 nm for GaAs0.967N0.033 at room temperature. The primary photoconductive decay times are in the range of 200–300 ns at room temperature. At temperatures below about 200 K, the decay rate decreases substantially. By plotting the decay rate values on an Arrhenius plot, we calculate activation energies of 70–80 meV for GaAs1−xNx samples with x=1.1%–3.3%. These energies represent potential barriers that inhibit recombination.

Alloy states in dilute GaAs1−xNx alloys (x<1%)

A set of GaAs1−xNx samples with small nitrogen composition (x<1%) were investigated by continuous-wave photoluminescence (PL), pulse-wave excitation PL, and time-resolved PL. In the PL spectra, an extra transition located at the higher-energy side of the commonly reported N-related emissions was observed. By measuring the PL dependence on temperature and excitation power, the PL peak was identified as a transition of alloy band edge-related recombination in GaAsN. The PL dynamics further confirms its intrinsic nature as being associated with the band edge rather than N-related bound states.

Alloy states in dilute GaAs1−xNx alloys (x

Alloy states in dilute GaAs1−xNx alloys (x<1%)

Nitrogen–hydrogen complexes in GaAs and GaAs1–xNx alloys

AbstractWe investigate the atomic structure of various N–H complexes in GaAs and GaAs1–xNx alloys through first‐principles pseudopotential calculations within the local‐density‐functional approximation. For GaAs with low H concentrations, we suggest that nitrogen forms a N–H(BC) complex, where H is positioned at a bond‐center site between a substitutional N and one of the neighboring Ga atoms. For very high H concentrations, an optically inactive N–H2*(BC‐ABN) complex, which corresponds to a configuration of H2* in the vicinity of N, is suggested to be an energetically favorable structure. These atomic models explain many experimental features observed in GaAs and GaAs1–xNx alloys, which contain both N and H.