As-doped GaN Research Articles

Density functional theories study on optoelectronic properties of arsenic-doped GaN nanowires

The electronic and optical properties of As-doped GaN nanowires have been investigated using density functional theory. The energy gaps of As-doped GaN nanowires with different concentrations exhibit indirect band gaps, and the band gaps are decreasing with the increasing doping concentration. The results predicate that alloys exhibit a typical semiconductivity. Moreover, the optical properties, including the complex dielectric function, optical refractive index, energy-loss function, reflectivity, and absorption coefficient are discussed for radiation up to 30 eV. With the increase of the As-fraction, the material gradually exhibits noticeable anisotropy in the photon energy range of 0–15 eV. Quickly increases the static dielectric constant and obviously red-shifts the absorption edge. The results are in good agreement with experimental data reported previously. Furthermore, the work gives a theoretical guidance for the preparation of As-doped GaN optoelectronic nanodevices.

Raman scattering study of undoped and As-doped GaN grown with different III/V ratios

We have used Raman scattering to investigate a series of undoped and As-doped GaN epilayers grown on sapphire substrates by plasma-assisted molecular beam epitaxy with different Ga-to-N ratios. We find that the frequency of the E2h phonon mode of GaN is very sensitive to the Ga-to-N ratio used to grow both types of samples, which may be attributed to strain effects arising from the different growth conditions. We observe Raman signatures corresponding to the formation of submicron cubic domains in some of the spectra of the As-doped samples. We discuss the origin of a broad band located at 232 cm−1 that appears in both the undoped and the As-doped epilayers. We speculate that this band, reported in previous Raman studies on As-doped and Bi-doped GaN, may be attributed to the formation of Ga clusters.

Study of photoluminescence from self-formed GaAs nanocrystallites in As-doped GaN grown by molecular beam epitaxy

We report on the results of the investigation of the photoluminescence (PL) from GaAs nanocrystallites embedded in a GaN host crystal and formed during the growth of GaN(As) by molecular beam epitaxy. The low-temperature PL spectra reveal two emission bands with maxima at 1.20 and 1.43 eV. The emission intensities of both PL bands show a characteristic 2/3 power-law dependence upon the optical excitation intensity, suggesting a strong contribution of Auger recombination in the total recombination rate of non-equilibrium carriers inside the GaAs nanocrystallites. The integral PL intensity demonstrates a sharp maximum in the growth temperature dependence at ∼780 °C. This can be explained by the competition of several temperature-dependent processes, which influence the formation of the GaAs nanocrystallites.

Blue emission from As-doped GaN films grown by molecular beam epitaxy on GaN templates

We have investigated the influence of the growth conditions on the intensity of the blue emission from arsenic doped GaN grown by plasma-assisted molecular beam epitaxy on GaN templates. We have shown that to achieve blue emission from As-doped GaN layers on GaN templates we need to grow layers under extremely Ga-rich conditions.

The Influence of Substrate Polarity on the Blue Emission from As-doped GaN Layers Grown by Molecular Beam Epitaxy

ABSTRACTThe influence of sample orientation and polarity on the blue emission from As-doped GaN layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated. Arsenic doped GaN layers were grown under identical PA-MBE conditions on several types of substrates including c-plane (0001) sapphire and polar and non-polar GaN templates grown by metal-organic vapour phase epitaxy (MOVPE). Non-polar GaN MOVPE templates were grown on aplane (11–20) sapphire and LiAlO2 (100). The orientation and polarity have a strong influence on the morphology and the optical properties of As-doped GaN layers. Strong blue emission from As-doped GaN was observed only in the case of (000–1) oriented N-polarity layers.

Bismuth a new dopant for GaN films grown by molecular beam epitaxy—surfactant effects, formation of GaN 1− xBi x alloys and co-doping with arsenic

We have investigated the influence of an additional bismuth flux during growth on the properties of GaN films prepared by plasma-assisted molecular beam epitaxy (MBE). A wide range of bismuth fluxes have been used, the highest Bi flux was larger than the flux of Ga. We have demonstrated that using a Bi flux during the growth of GaN by MBE at a temperature ∼800°C improves the surface morphology of the films and decreases the deep emission. We have demonstrated for the first time the growth of GaN 1− x Bi x alloys by MBE, the Bi concentration was small and increased with decreasing growth temperature. We have studied the influence of an additional bismuth flux on the growth of As-doped GaN layers and observed an increase of blue emission from the layers at some optimum range of Bi fluxes. All the results allow us to conclude that Bi may be a potential new dopant for the growth of GaN by MBE.

Optimisation of the Blue Emission from As-Doped GaN Films Grown by Molecular Beam Epitaxy

We have investigated the influence of the growth conditions on the intensity of the blue emission from arsenic doped GaN films grown by plasma-assisted molecular beam epitaxy. We have demonstrated, that for fixed Ga, N and As fluxes there is an optimum growth temperature, at which the highest blue emission intensity is observed. Possible models explaining this effect were discussed.

The growth andproperties of GaN:As layers prepared by plasma-assistedmolecular beam epitaxy

We have studied the growth and properties of GaN:As layersprepared by molecular beam epitaxy, using a plasma source foractive nitrogen. We have demonstrated that arsenic dopingduring growth produces films showing blue emission at roomtemperature. The blue emission is centred at 2.6 eV and is morethan a decade stronger than the band edge emission. The filmsare predominantly wurtzite, but with a small cubic contentwhich exists mainly close to the substrate-epilayer interface.We have investigated the influence of growth conditions on theintensity of this blue emission. In films grown under optimumconditions, the blue emission is strong enough to be clearlyvisible under normal room lighting. We have also discussed the transition from As-doped GaN showing blue emission to theformation of GaN1-xAsx alloys. We have determinedthat for a fixed arsenic flux, increasing the N/Ga ratio leadsto the formation of alloy films. Our results suggest that thismaterials system may have potential applications in electronicand opto-electronic devices.

The transition from As-doped GaN, showing blue emission, to GaNAs alloys in films grown by molecular beam epitaxy

We have studied the transition from As-doped GaN showing strong blue emission (∼2.6 eV) at room temperature to the formation of GaN 1− x As x alloys for films grown by plasma-assisted molecular beam epitaxy. We have demonstrated that with increasing N-to-Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN 1− x As x alloy films. We present a model based on thermodynamic considerations, which can explain how this might occur.

Characterization of nitride thin films by electron backscatter diffraction.

Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90 degrees rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tiltedby 13 +/- 1 degrees towards [1010]GaN), where the tilt is due to the inclination of the sapphire substrate (cut off-axis by 10 degrees from (0001)sapphire towards (1010)sapphire). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As4 flux, respectively. Higher As4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled.

X-ray studies of As-doped GaN grown by plasma-assisted molecular beam epitaxy

We have used a range of X-ray diffraction techniques to study the structural properties of As-doped GaN samples grown on sapphire substrates by plasma-assisted molecular beam epitaxy (PA-MBE). The X-ray methods employed include simple ω−2 θ scans, 2D reciprocal space maps and pole figures. We have estimated the ratio of cubic to hexagonal GaN as a function of As flux and sample thickness. We show that the cubic content is only weakly dependent on the arsenic flux in contrast to the intensity of the photoluminescence peak at 3.2 eV, previously associated with cubic-phase GaN material. This suggests that the photoluminescence at 3.2 eV does not arise mainly from direct recombination in cubic phase material, but is probably associated with a donor–acceptor pair transition. By studying As-doped GaN films of different thicknesses, we are able to show that the cubic material is predominantly at the substrate–epilayer interface and that in thicker samples, the growth of hexagonal phase material is dominant. As a result of this study, we can conclude that in As-doped GaN samples grown at 800°C on sapphire by PA-MBE, arsenic does not appear to stimulate the growth of pure cubic phase material.

On the Origin of Blue Emission from As-Doped GaN

As-doped GaN films have been grown by plasma-assisted molecular beam epitaxy and their properties investigated using atomic force microscopy, X-ray diffraction and photoluminescence (PL) spectroscopy. The structural properties of the As-doped GaN films improve with increasing sample thickness. The room temperature PL is dominated by a strong blue emission band, exhibiting multiple peaks centered at 2.6 eV. The number of peaks increases monotonically with sample thickness. From this we conclude that the multiple peaks in the blue emission band of As-doped GaN samples arise mainly from optical interference effects. However, the possibility of several transitions involving As being responsible for the blue emission process cannot be excluded.

The Transition from Blue Emission in As-Doped GaN to GaNAs Alloys in Layers Grown by Molecular Beam Epitaxy

The transition from As-doped GaN showing strong blue emission (∼2.6 eV) at room temperature to the formation of GaN 1-x As x alloys for films grown by molecular beam epitaxy was investigated. This study demonstrates that with increasing N to Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN 1-x As x alloy films. Several possible models, which can explain how this might occur are presented.

The Transition from Blue Emission in As-Doped GaN to GaNAs Alloys in Layers Grown by Molecular Beam Epitaxy

The transition from As-doped GaN showing strong blue emission (∼2.6 eV) at room temperature to the formation of GaN1—xAsx alloys for films grown by molecular beam epitaxy was investigated. This study demonstrates that with increasing N to Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN1—xAsx alloy films. Several possible models, which can explain how this might occur are presented.

The Influence of Arsenic Incorporation on the Optical Properties of As-doped GaN Films Grown by Molecular Beam Epitaxy Using Arsenic Tetramers

We have studied the influence of the incorporation of As on the optical properties of As-doped GaN layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) using arsenic tetramers. The doping level of arsenic was determined by secondary ion mass spectrometry. The arsenic concentration is uniform throughout the layers. There is a sub-linear dependence of the arsenic incorporation on the flux with a log–log slope of about 0.1. The photoluminescence from the As-doped GaN films consists of UV excitonic emission at 3.4 eV, UV emission at 3.2 eV and a strong blue band centred at 2.6 eV. The intensity of the blue band centred at 2.6 eV increases more rapidly with arsenic flux than the concentration of arsenic in the bulk, and has a log–log slope of about 0.49. This suggests an approximately fourth-power dependence of the intensity of the blue emission on the concentration of arsenic in the GaN films.

Temperature Dependence of the Miscibility Gap on the GaN-Rich Side of the Ga-N-As System

We have investigated the temperature dependence of the transition from single phase films of GaN1—xAsx to phase separated layers, which show regions of hexagonal [0001] oriented GaN, cubic [111] oriented GaAs and hexagonal [0001] oriented GaN1—xAsx. We see a strong temperature dependence of the arsenic flux at which GaAs inclusions are first observed. Finally the intensity of blue emission observed in As-doped GaN samples decreases strongly with decreasing growth temperature.

Temperature Dependence of the Miscibility Gap on the GaN-Rich Side of the Ga–N–As System

We have investigated the temperature dependence of the transition from single phase films of GaN1—xAsx to phase separated layers, which show regions of hexagonal [0001] oriented GaN, cubic [111] oriented GaAs and hexagonal [0001] oriented GaN1—xAsx. We see a strong temperature dependence of the arsenic flux at which GaAs inclusions are first observed. Finally the intensity of blue emission observed in As-doped GaN samples decreases strongly with decreasing growth temperature.

The influence of arsenic incorporation on the optical properties of As-doped GaN films grown by molecular beam epitaxy using As 4 molecules

We have studied the influence of the incorporation of As on the optical properties of As-doped GaN layers grown by plasma-assisted molecular beam epitaxy using As 4 molecules. The arsenic concentration, determined by secondary ion mass spectrometry, is uniform throughout the layers. The incorporation of arsenic depends sub-linearly on the arsenic flux with a log–log slope of about 0.1. The photoluminescence from the As-doped GaN films consists of three main bands, UV excitonic emission at 3.4 eV, UV emission at 3.2 eV and a strong blue band centred at 2.6 eV. The intensity of the blue emission centred at 2.6 eV increases more rapidly with arsenic flux than the concentration of arsenic in the bulk, and has a log–log slope of about 0.49. This suggests that the intensity of the blue emission depends on the concentration of arsenic in the GaN layer approximately to the 4th power.

A study of the mechanisms responsible for blue emissionfrom arsenic-doped gallium nitride

We have investigated the influence of the growthconditions on the intensity of blue emission at roomtemperature from As-doped GaN samples grown by molecularbeam epitaxy. A series of As-doped GaN samples was grown at800 °C with constant fluxes of As and gallium, butwith different amounts of active nitrogen. Varying the N fluxallowed us to investigate films grown from strongly Ga-richconditions to more N-rich conditions. The blue emissionincreases monotonically with the nitrogen flux and is mostintense in the layers grown under the most nitrogen-richconditions. This fact suggests that As atoms incorporatedinto the Ga sub-lattice are responsible for the strong blueemission in As-doped GaN.

Isoelectronic As doping effects on the optical characteristics of GaN films grown by metalorganic chemical-vapor deposition

We have studied the As doping effects on the optical characteristics of GaN films by time-integrated photoluminescence and time-resolved photoluminescence. When As is incorporated into the film, the localized defect levels and donor–acceptor pair transition become less resolved. The recombination lifetime of neutral-donor-bound exciton (I2) transition in undoped GaN increases with temperature as T1.5. However, the I2 recombination lifetime in As-doped GaN first decreases exponentially from 98 to 41 ps between 12 and 75 K, then increases gradually to 72 ps at 250 K. Such a difference is related to the isoelectronic As impurities in GaN, which generate nearby shallow levels that dominate the recombination process.