A.J. Winser

Strong blue emission from As doped GaN grown by molecular beam epitaxy

Arsenic doped GaN grown by molecular beam epitaxy has been studied by room temperature photoluminescence. In addition to the wurzite band edge transition, luminescence from the cubic phase and very strong blue emission at ∼2.6 eV are observed. The intensities of the blue and the cubic band edge emissions have a power law dependence on the As2 flux. The formation of the cubic phase has been explained by the initial formation of GaAs before substitution of the As by the more reactive N. The intensity of the blue emission at room temperature of the As doped samples is more than an order of magnitude stronger than the band edge emission in undoped samples.

The growth and properties of GaN:As layers prepared by plasma-assisted molecular beam epitaxy

We have studied the growth and properties of GaN:As layers prepared by molecular beam epitaxy, using a plasma source for active nitrogen. We have demonstrated that arsenic doping during growth produces films showing blue emission at room temperature. The blue emission is centred at 2.6 eV and is more than a decade stronger than the band edge emission. The films are predominantly wurtzite, but with a small cubic content which exists mainly close to the substrate-epilayer interface. We have investigated the influence of growth conditions on the intensity of this blue emission. In films grown under optimum conditions, the blue emission is strong enough to be clearly visible under normal room lighting. We have also discussed the transition from As-doped GaN showing blue emission to the formation of GaN1-xAsx alloys. We have determined that for a fixed arsenic flux, increasing the N/Ga ratio leads to the formation of alloy films. Our results suggest that this materials system may have potential applications in electronic and opto-electronic devices.

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

We have investigated the influence of the growth conditions on the intensity of blue emission at room temperature from As-doped GaN samples grown by molecular beam epitaxy. A series of As-doped GaN samples was grown at 800 °C with constant fluxes of As and gallium, but with different amounts of active nitrogen. Varying the N flux allowed us to investigate films grown from strongly Ga-rich conditions to more N-rich conditions. The blue emission increases monotonically with the nitrogen flux and is most intense in the layers grown under the most nitrogen-rich conditions. This fact suggests that As atoms incorporated into the Ga sub-lattice are responsible for the strong blue emission in As-doped GaN.

Bismuth a new dopant for GaN films grown by molecular beam epitaxy—surfactant effects, formation of GaN1−xBix alloys and co-doping with arsenic

Abstract 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.

The influence of arsenic incorporation on the optical properties of As-doped GaN films grown by molecular beam epitaxy using As4 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 (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.

Carrier relaxation dynamics for As defects in GaN

Long decay times in the 50–150 ns range have been measured for the characteristic blue photoluminescence that peaks at 2.6 eV in GaN:As. We interpret these long decay times according to the theoretical predictions that this blue photoluminescence is caused by the incorporation of arsenic on the gallium site. The long decay times are characteristics of the large lattice relaxation for such a deep donor with a negative-U center behavior.

Bismuth a New Surfactant or Contact for GaN Films Grown by Molecular Beam Epitaxy

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. A wide range of bismuth fluxes have been used, at the highest Bi flux this was larger than the flux of Ga. X-ray and photoluminescence studies demonstrate that the structural quality and optical properties of the GaN films grown at ∼800 °C are practically unchanged. X-ray and Auger studies indicated that the concentration of Bi in the bulk of the films grown at ∼800 °C is rather small. We have demonstrated that it is possible to grow conductive metallic Bi films epitaxially onto GaN films in-situ after epitaxy by reducing the deposition temperature. Bismuth may thus potentially be a new material for in-situ contacts to GaN layers and device structures.

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 GaN 1-x As x to phase separated layers, which show regions of hexagonal [0001] oriented GaN, cubic [111] oriented GaAs and hexagonal [0001] oriented GaN 1-x As x . 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.

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.

Photoluminescence and optical quenching of photoconductivity studies on undoped GaN grown by molecular beam epitaxy

Deep levels in undoped GaN materials grown by modified molecular beam epitaxy (MBE) are investigated by photoluminescence (PL) and optical quenching of photoconductivity measurements. A broad band which extends from 2.1 to 3.0 eV with a maximum at about 2.7 eV is observed, and four prominent quenching bands were found located at 2.18, 2.40, 2.71, and 2.78 eV above the valence band, respectively. These levels are attributed to four holes trap levels existence in the material. The defects cannot be firmly identified at present