The physical origin of the yellow luminescence in MBE-grown GaN co-doped with C and Si was investigated. Deep level optical spectroscopy (DLOS), deep level transient spectroscopy (DLTS), and photoluminescence (PL) were used to study the deep level spectrum as a function of C incorporation. In the absence of C co-doping, samples were n-type and demonstrated a weak yellow luminescence band, likely related to VGa. For increasing C co-doping, samples became semi-insulating concurrent with increased intensity of the yellow luminescence and the concentration of C-related deep acceptors. The DLOS results were used to develop a configuration-coordinate model for a C-related deep level with optical ionization energy of 3.0 eV and Franck-Condon shift of 0.4 eV that is consistent with the observed yellow luminescence and DLTS results. From these findings, a general model for independent mechanisms of the yellow luminescence related to VGa for n-type GaN and C for n-type and semi-insulating GaN:C:Si is discussed.

Dual Multi-Quantum Well (MQW) region light emitting diodes (LEDs) for efficient pumping of multiple phosphors have been grown by Metal Organic Chemical Vapor Deposition (MOCVD), for use in broadband, white solid state light sources. Blue (460 nm) and Violet/UV (∼400–420 nm) emitting MQW regions were incorporated into one device and show recombination mechanisms similar to single MQW region devices. The introduction of a decoupling region successfully separated the electroluminescent emissions, and two distinct emission peaks were observed. These devices can be used to pump a variety of phosphors designed for blue and UV sources, allowing for more design flexibility in color rendering and color temperature attributes of solid state light sources.

Forward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters τp = 758 ± 44 ns, Lp = 588 ± 45 nm, and μp = 0.18 ± 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of sub-bandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of τp = 3.0 μs was observed, and the diffusion length was shorter, Lp ≈ 280 nm. Mobility was an order of magnitude smaller than in GaN, μp ≈ 10−2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects.

New substrate materials; ZrB2,(3038) 4H-SiC and r-faced sapphire, are evaluated for use in nitride-based light-emitting diodes (LEDs). They are thought to be suitable for low-cost, short-wavelength, and highly efficient light-emitting devices. The ZrB2 substrate presents a high-quality nitride layers and a blue-violet LED with an excellent L-I curve. A vertical electrical transport is also possible due to its preferable work function and high electrical conductivity. Nitride layers tilted 54.7° from the c-plane can be grown on (3038) 4H-SiC. The LED grown on the (3038) 4H-SiC substrate shows a smaller blue shift of the EL peak wavelength with increasing driving current. Although the crystalline quality of nitride layers on r-faced sapphire was improved with the optimization of the misorientation angle, the performance of blue-violet LEDs is still low. Further work is needed to enable the practical application of r-faced sapphire to nitride-based LEDs.

We report on the novel normally-off GaN-based heterojunction field effect transistors (HFETs) on a Si substrate. The AlGaN/AlN/GaN heterostructure was grown using a metalorganic chemical vapor deposition (MOCVD). The HFET for a normally-off operation was fabricated using a precisely controlled thin-AlGaN layer as an electron supply layer. As a result, the HFET was operated at the condition of the positive gate bias. We also characterized the enlarged gate-width devices. The breakdown voltage of FET was over 300 V. A normally-off operation using GaN based HFETs with a thin-AlGaN/AlN/GaN heterostructure on the silicon substrate were thus confirmed for the first time.

In this paper we report the growth by MBE of GaN quantum dot superlattices (QDSLs) with AlN barriers on (0001) sapphire substrates at relatively high temperatures (770 °C) by the modified Stranski-Krastanov method. TEM studies indicate that the GaN QDs are truncated pyramids. We find that the height distribution of the dots depends strongly on the number of GaN monolayer coverage on the top of AlN. Specifically, we find that the height distribution consists of two Gaussian distributions (bimodal) for coverage of 3 and 4 MLs, and becomes single Gaussian distribution for 5 and 6 MLs of coverage. Furthermore, we find that the density of quantum dots increases with the degree of coverage and saturates at 2×1011 dots/cm2. The number of stacks in the superlattice structure was also found to lead to bimodal height distribution of the QDs. Ordering of the quantum dots was accomplished by thermal annealing of the sapphire substrates at 1400 °C prior to the growth of GaN QDs. The annealing process reveals the vicinal steps due to the miscut of the substrates and the GaN QDs were found to line up along those steps. Photoluminescence studies show a broad luminescence spectrum centered at 3 eV which is red shifted with respect to that of bulk GaN and is consistent with internal fields due to polarization (Quantum Confined Stark Effect). Furthermore, we find that the luminescence intensity increases with the number of stacks in the superlattice structure due to higher spatial density of QDs.

The polarity control of GaN films grown on c-plane sapphire substrates by low pressure metal organic chemical vapor deposition (MOCVD) was achieved by using N2 as a diluent and transport gas. The type of polarity was governed by the substrate treatment prior to the GaN growth. N-face (-c) GaN films were only obtained by pre-nitridation of the sapphire substrate after a H2 anneal, while Ga-face (+c) GaN films were grown directly on the substrates or on properly annealed AlN buffer layers. In addition, GaN films on improperly annealed AlN buffer layers, that is, under- or over-annealed buffer layers, yielded films with mixed polarity. Smooth N-face GaN films with 2.5 nm RMS roughness, as determined by atomic force microscopy (AFM), were obtained with shorter nitridation times (less than 2 min). Wet chemical etching in an aqueous solution of potassium hydroxide (KOH) was used to determine the polarity type.

The Structural properties of Europium (Eu) doped GaN and its relation with optical properties were studied. Concentration quenching of the intensity of the Eu related luminescence observed when Eu concentration exceeds 3 at.%. In situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were carried out to study this luminescence quenching and it was discovered that there is close relationship between the luminescence intensity at 622 nm and structural properties. The cause of the concentration quenching is likely related to the polycrystalline growth as well as to the EuN formation.

We have performed investigations of resonance effects inside a gallium nitride one-dimensional photonic crystal slab in order to enhance the second-harmonic generated from an beam incident on the surface of the slab. Convenient conditions on the incident beam propagation direction and polarization are first identified by experimental or theoretical linear optical studies. Giant enhancements in the second-harmonic conversion have been obtained by comparison with the unpatterned GaN layer. The combined role of the resonant coupling of the fundamental field and of the second-harmonic field has been observed by rotating the polarization of the fundamental beam.

The periodic silane burst technique was employed during metalorganic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111). Periodic silicon delta doping during growth of both the AlN and GaN layers led to growth of GaN films with decreased tensile stresses and decreased threading dislocation densities, as well as films with improved quality as indicated by x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. The possible mechanism of the reduction of tensile stress and the dislocation density is discussed in the paper.

The behavior of the E2 and A1(LO) optical phonons in Inx Ga1-x N has been analyzed by Raman scattering over the whole composition range. The frequencies of the E2 and A1(LO) modes decrease with increasing InN fraction. These modes display a significant broadening for an InN fraction of ≈ 60% and their linewidth decreases towards both ends of the composition range as a consequence of reduced cation disorder. Our results show a one-mode behavior for both E2 and A1(LO) modes of InGaN.

Structural defects affecting the lifetime of GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers have been investigated. Almost all of the threading dislocations that appeared in the wing regions have edge character, whereas the dislocations at the coalescence boundaries have both edge character and mixed character. The origins of the threading dislocations in the wing regions are the lateral extension of dislocations from the seed regions that contingently bend upwards to the epi-surface. Thus, edge dislocations are most considerable threading dislocations in GaN-based LDs on ELO GaN layers, since the laser stripes are fabricated in the wing regions. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations were observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. Although the threading dislocations in the LD stripes do not multiply during device operation, our degradation experiments revealed that the lifetime of the GaN-based LDs depends on the dislocation density. The degradation rate was almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.

The growth of InGaN on GaN (0001) by plasma-assisted molecular beam epitaxy was investigated with special focus on the dynamics of the formation of the dots. A metastable 2D growth regime, where the surface changes from smooth to rough by thermal treatment during growth interruption, existed previous to the 2D-3D transition. Both small regular-shaped dots and large irregular-shaped islands were observed. The large islands were suppressed by choosing correct growth conditions. The critical thickness for the transition from 2D to 3D growth also depended on the growth conditions. The growth of GaN capping layer to cover InGaN dot-structure was also attempted.

Bulk GaN crystals of dimensions 8.5 mm × 8.5 mm were grown at growth rates greater than 200μm/hr using Gallium Vapor Transport technique. GaN powder and Ammonia were used as the precursors for growing bulk GaN. Nitrogen is used as the carrier gas to transport the Ga vapor that was obtained from the decomposition of GaN powder. During the process, the source GaN powder was kept at 1155°C and the seed at 1180°C. Using this process, it was possible to achieve growth rates of above 200 microns/hr. The GaN layers thus obtained were characterized using X-Ray diffraction [XRD], scanning electron microscopy [SEM], and atomic force microscopy [AFM]. X-ray diffraction patterns showed that the grown GaN layers are single crystals oriented along c direction. AFM studies indicated that the dominant growth mode was dislocation mediated spiral growth. Electrical and Optical characterization were also performed on these samples. Hall mobility measurements indicated a mobility of 550 cm2/V.s and a carrier concentration of 6.67 × 1018/cm3

We report a new defect-related photoluminescence (PL) band peaking at 2.56 eV at 15 K in undoped GaN layers grown by molecular beam epitaxy (MBE) on GaN templates. The maximum of the aquamarine luminescence (AL) band shifts to higher energies by about 100 meV with increasing temperature from 15 to 300 K. In spite of the fact that the shape and the peak position of the AL band are close to the characteristics of the green luminescence band in a freestanding GaN template, we were able to delineate these two bands for their markedly different behavior with temperature and excitation intensity. The origin of defect responsible for this PL band remains unknown.

GaN epilayers have been grown by plasma-assisted molecular-beam epitaxy on ZrB2 substrates with close in-plane lattice match. Growth processes utilizing both low-temperature GaN (LT-GaN) and AlN nucleation layers were investigated. The x-ray ω-scan widths for the optimized LT-GaN nucleation process were 400 and 750 arcsec for symmetric and asymmetric reflections, respectively. When using LT-GaN nucleation layers, the chemical incompatibility of ZrB2 results in a high dislocation density despite the in-plane lattice match. The epilayer polarity was N-polar for LT-GaN nucleation layers under all conditions investigated. For AlN nucleation layers, Ga-polar epilayers were obtained under suitable conditions (Al-rich, lower nucleation temperatures) for nominal AlN thickness as low as 1 nm. From RHEED analysis it appears that a psuedomorphic Al wetting layer forms on the ZrB2 surface, and that using AlN as the nucleation layer may offer promise for reducing the epilayer defect density.

We have grown InN quantum dots (QDs) on nitrogen-polarity (N-polarity) GaN under-layer by the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE), and systematically investigated growth mechanism of the InN dots. The InN QDs with the N-polarity could be grown at about 500°C, which was much higher than that of previous reports on InN dots grown by MBE. When the nominal coverage of InN became more than 1 mono-layer (ML), lattice relaxation of InN occurred and high density InN dots were uniformly formed. These results indicated that InN dots were formed by Stranski-Krastanov (S-K) growth mode. For the InN deposition above about 8ML, InN dots tended to coalesce and resulted in remarakable decrease of the dots density.

The optical and electrical properties of Si doped GaN and Al0.20Ga0.80N films irradiated with 1 MeV electrons at a fluence of 1×1017 cm−2 are investigated using cathodoluminescence (CL), variable-temperature Hall-effect, and deep level transient spectroscopy (DLTS) measurements. The CL spectra measured at 6 K show peak luminescence intensity of the near band edge decreases, on average, by 50% after electron irradiation, indicating the creation of non-radiative recombination centers which are stable at room temperature. At room temperature, the free carrier concentration decreases significantly in both the GaN and AlGaN samples following the irradiation, and the carrier removal rate depends strongly on the initial carrier concentration. DLTS measurements show three electron traps in the as-grown Al0.20Ga0.80N. Following 1 MeV electron irradiation of the Al0.20Ga0.80N sample, three additional electron traps labeled R2, R3, and R4 are observed. The first two traps appear to correspond to radiation-induced traps reported in GaN while the latter appears to be unique to AlGaN.

Stable discharging of pure nitrogen can be maintained even at atmospheric pressure when alternative pulsed voltage is applied between two parallel plate electrodes. From optical emission spectroscopy, strong emissions from the N2 2nd positive system, weak emissions from N2 Herman's infrared system and N2 1st positive system were observed. The emission intensities from the N2 2nd positive system and the N2 Herman's infrared system increase with increasing the nitrogen gas pressure, whereas the emission intensities from the N2+ 1st negative system and N2 1st positive system decrease. The thickness of the silicon nitride film fabricated at 500 Torr was 1.6 nm at a nitridation temperature as low as 25°C, regardless of the nitridation temperature and nitridation time. From these results, we conclude that N2(C3IIu) plays an important role for the excellent reactivity of the nitrogen plasma generated near atmospheric pressure.

In this paper we report the effect of low temperature annealing on the high field magnetotransport properties of epitaxial thin films of (Ga, Mn)As Dilute Magnetic Semiconductor (DMS) with low concentration (1.5 %) of Mn doping, which results in a ferromagnetic insulator. Annealing at an optimal temperature enhances the conductivity, carrier concentration, and ferromagnetic transition temperature. The field dependence of magnetoresistance is different below and above the ferromagnetic transition temperature. An attempt is made to analyze the data using a theoretical model proposed by Kaminski and Das Sarma [1].