Migration Of Ga Research Articles

Roles of Atomic Nitrogen/Hydrogen in GaN Film Growth by Chemically Assisted Sputtering with Dual Plasma Sources.

The growth of sputtered GaN at low temperature is strongly desired to realize the dissemination of low-cost GaN high electron mobility transistor devices for next-generation communication technology. In this work, the roles of atomic nitrogen (N)/hydrogen (H) in GaN film growth on AlN/sapphire substrates by chemically assisted dual source sputtering are studied at a low growth temperature of 600 °C under a pressure of 2 Pa using vacuum ultraviolet absorption spectroscopy. The lateral growth was strongly enhanced with an appropriate H/N flux ratio of 1.9 at a GaN growth rate of ∼1 μm h–1. X-ray photoelectron spectroscopy measurements indicated that N removal from the grown GaN surface by atomic hydrogen promoted the migration of Ga. A smooth GaN surface was achieved at a suitable N/Ga supply ratio of 53 and a H/N ratio of 1.9 with the addition of 0.5% chlorine to the Ar sputtering gas.

Recovery and accumulation of ion irradiation damage leading to dose rate dependence in GaAs

Competing dynamics was analyzed for generation, recovery and accumulation of ion irradiation damage using three methods including He-ion channeling backscattering. The in situ tracing of electrical conductance after pulsed 100 keV proton irradiations at 23 °C–92 °C revealed that the subsequent recovery process was a second-order reaction with an activation energy of 0.74 eV in n-type and 0.31 eV in p-type epitaxial layers arising from long-range migration of both Ga and As interstitials. The amorphized depth by pulsed irradiations exhibited that the recovery was suppressed by the incidence of two ions within a lateral separation, which increased from ∼5 to ∼20 nm with the ion mass for 80 keV Ne+, 150 keV Ar+, and 300 keV Kr+ ions of an identical projected range, 115 nm. From the comparison with ion-collision simulations, it was concluded that if two collision cascades overlap before the recovery, it enhances interstitial clustering while suppressing the annihilation and causes the dose rate dependence.

Stabilization of ε−Fe2O3 epitaxial layer on MgO(111)/GaN via an intermediate γ -phase

In the present study we have demonstrated epitaxial stabilization of the metastable magnetically-hard $\varepsilon$-Fe$_2$O$_3$ phase on top of a thin MgO(111) buffer layer grown onto the GaN (0001) surface. The primary purpose to introduce a 4\,nm-thick buffer layer of MgO in between Fe$_2$O$_3$ and GaN was to stop thermal migration of Ga into the iron oxide layer. Though such migration and successive formation of the orthorhombic GaFeO$_3$ was supposed earlier to be a potential trigger of the nucleation of the isostructural $\varepsilon$-Fe$_2$O$_3$, the present work demonstrates that the growth of single crystalline uniform films of epsilon ferrite by pulsed laser deposition is possible even on the MgO capped GaN. The structural properties of the 60\,nm thick Fe$_2$O$_3$ layer on MgO / GaN were probed by electron and x-ray diffraction, both suggesting that the growth of $\varepsilon$-Fe$_2$O$_3$ is preceded by formation of a thin layer of $\gamma$-Fe$_2$O$_3$. The presence of the magnetically hard epsilon ferrite was independently confirmed by temperature dependent magnetometry measurements. The depth-resolved x-ray and polarized neutron reflectometry reveal that the 10\,nm iron oxide layer at the interface has a lower density and a higher magnetization than the main volume of the $\varepsilon$-Fe$_2$O$_3$ film. The density and magnetic moment depth profiles derived from fitting the reflectometry data are in a good agreement with the presence of the magnetically degraded $\gamma$-Fe$_2$O$_3$ transition layer between MgO and $\varepsilon$-Fe$_2$O$_3$. The natural occurrence of the interface between magnetoelectric $\varepsilon$- and spin caloritronic $\gamma$- iron oxide phases can enable further opportunities to design novel all-oxide-on-semiconductor devices.

RBS/C and TEM characterization of GaN nanolayer obtained by IBS on (001) GaAs

Rutherford Backscattering Spectrometry in random (RBS) and aligned (RBS/C) to GaAs 001-channel, and Transmission Electron Microscopy (TEM) techniques were employed to characterize the GaN nanolayer obtained by Ion Beam Synthesis (IBS). N+ ions at 50 keV were implanted up to a fluence of 3 × 1017 cm−2 into a 001 GaAs substrate capped by 125 nm Si3N4 layer obtained by sputtering deposition. N+ implantation was performed with samples held at 450 °C, which were subsequently annealed in a temperature range between 550 and 1000 °C for 5 min by Rapid Thermal Annealing (RTA) under N2 flow. TEM demonstrated a layered structure on the as-implanted sample, with N bubbles formed at around Si3N4/GaAs interface. RBS/C analysis carried out on samples annealed at distinct temperatures revealed a threshold temperature of about 850 °C to fully convert the as-implanted layered structure into a GaN nanolayer. At this temperature and above, the GaAs implanted side of the former Si3N4/GaAs interface was converted into GaN. As a result, an underneath voids structure was also formed indicating an intense migration of Ga from deeper areas of the GaAs substrate. RBS showed results compatible with a GaN nanolayer rich in N for the 850 °C annealing, whose sample also offered canalization (χmin = 63%) along the GaAs 001-channel. However, a regression in the crystalline quality (χmin = 95%) was observed for the 1000 °C anneal.

Shape Evolution of Highly Lattice-Mismatched InN/InGaN Nanowire Heterostructures

We have investigated the structure and shape of GaN-based nanowires grown on (001) Si substrates for optoelectronic device applications. The nanowire heterostructures contained InN disks and In0.4Ga0.6N barrier layers in the active region. The resulting nanowire array comprised two differently shaped nanowires: shorter pencil-like nanowires and longer bead-like nanowires. The two different nanowire shapes evolve due to a variation in the In incorporation rate, which was faster for the bead-like nanowires. Both types of nanowires exhibited evidence of significant migration of both Ga and In during growth. Ga tended to diffuse away and down along the sidewalls, resulting in a Ga-rich shell for all nanowires. Despite the complex structure and great variability in the In composition, the optical properties of the nanowire arrays were very good, with strong luminescence peaking at ∼ 1.63 μm.

Distribution, occurrence, and enrichment of gallium in the Middle Jurassic coals of the Muli Coalfield, Qinghai, China

Gallium (Ga) is known as the backbone of the electronics industry, but its independent mineralization is rarely found in nature. The discovery of large Ga-bearing coal deposits suggests that the coal-bearing series can be used as the main source for the future development and utilization of Ga, and the organic matter of coal may play an important role in the aggregation of Ga. Based on the analysis of geological prospecting and study of coal samples, this paper discussed the distribution, occurrence, and enrichment of Ga in the Middle Jurassic coals of the Muli coalfield. The results demonstrate that coals from eastern mines are enriched in Ga, where the Ga concentration reaches the propositional cut-off grade (30μg/g). The Ga concentration increases along with an increasing distance from the sediment source; the areas with the highest Ga concentration are the deposit-center and the location with the highest content of mudstone. Sediments were derived from the acid and intermediate magmatic rocks of the Tuolai Mountain in the northeastern Muli coalfield, partly from the crust weathering of the Upper Triassic formation. Ga was first transported in the form of colloids or solutions and finally deposited in authigenic kaolinite. In addition, the positive correlation between the Ga concentration and the inertinite constituents of coal indicates that fibrous tissue might be the dominant component absorbing Ga and that organic matter had a positive impact on the migration of Ga to peat swamps. Ga is prone to deposition in partially oxidized, acidic coal-forming environments with strong hydrodynamic forces. Hydrothermal activity might also stimulate the enrichment of Ga in coal.

Effect of Cu/(In+Ga) ratios on the secondary phases and the performance of Cu2InGa(S,Se)4 thin film solar cells

This study investigated the influence of Cu/(In+Ga) ratios on the formation of secondary phases, leakage current, and their effects on the performance in Cu2InGa(S,Se)4 (CIGSSe) thin film solar cells. An increase in Cu content in the CIGSSe absorber layer was shown to contribute to the growth of grains in that layer as well as the migration of Ga to the surface of the absorber, which increased the energy gap. Excess Cu content resulted in the formation of Cu2−xSe hexagonal nanoplates on the surface of the absorber layer. Conductive atomic force microscopy identified grain boundaries in the CIGSSe absorber layer as the main path involved in the transfer of current through the device. The formation of Cu2−XSe or InSe phases on the surface of the absorber layer expanded the region acting as a current pathway, leading to a breakdown in the P-N junction following the application of negative bias. Measurements of external quantum efficiency under negative bias revealed that secondary phases in the absorber layer act as a recombination center, resulting in a reduction in the Voc and Jsc of the device. CIGSSe thin film solar cells fabricated by sputtering in this study achieved a maximum conversion efficiency of 13.06%.

Degradation of external quantum efficiency of AlGaN UV LEDs grown by hydride vapor phase epitaxy

AbstractA comparative study of the degradation of HVPE‐grown 360 nm AlGaN/GaN UV and commercially available InGaN/GaN blue LED chips was performed. The common feature of the degradation of these two types of LEDs was found to be the increase of the conductivity of shunt paths under current injection. The paths (shunts) are localized in the extended defects system (EDS). It is proposed that the conductivity increase is due to defect formation under multiphonon carriers recombination in a part of the EDS enriched by Ga or In atoms. This process is accompanied by a local overheating and migration of Ga or In atoms. To increase the lifetime of the AlGaN/GaN UV LEDs to more than 2000 h it is necessary to improve their nano‐structural and nanoscale AlGaN composition ordering. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interplay of dopants and native point defects in ZnO

The controlled doping of ZnO required for a wide range of micro- and optoelectronic applications remains a major challenge. Recent studies of Li-doped ZnO and degenerately Ga-doped ZnO (GZO) reveal a systematic interplay between dopant incorporation and native point defects. Using the optical signature of Zn vacancies and their clusters to monitor their densities with growth conditions, processing and doping, we demonstrate that migration of Li and Ga into VZn sites plays a significant role in forming substitutional dopants and native point defects are actively involved in both donor and acceptor doping of ZnO.

Growth Interruption Effect on the Fabrication of GaAs Concentric Multiple Rings by Droplet Epitaxy

We present the molecular beam epitaxy fabrication and optical properties of complex GaAs nanostructures by droplet epitaxy: concentric triple quantum rings. A significant difference was found between the volumes of the original droplets and the final GaAs structures. By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

Role of Pt on PtGaZr/SiO 2 catalyst in light naphtha isomerization

The effect of Pt content on the light naphtha isomerization reaction was studied using PtGaZr/SiO 2-type catalysts. The catalysts were characterized by XPS, IR, pyridine, and CO adsorption, and by chemical reaction. The reaction was studied in a radial plug flow-type gas-phase reactor at three temperature levels, residence times, and hydrogen partial pressures. Other exploratory tests involved adding 20% of 1-pentene, cyclopentane, or isopentane to the feedstock. The results indicate that Pt is incorporated into the Ga–Zr and Ga–Si cluster, which is generated by migration of Ga and Zr to the surface of the silica. The Pt slightly modifies the number, activity, and selectivity of the acid sites, but its most important role is on catalyst stability. It was found that some Pt δ + species may be located in the border of the crystal, and Pt 2+ species may be found in the subsurface of the clusters. A mechanism of two isomerization acid sites proposed, and an apparent kinetics model for light naphtha isomerization is used to predict the isomer yields in a gas-phase radial reactor. The role of catalysts surface composition in chemical reactions is discussed.

Preparation and characterization of bifunctional catalysts of the Pd, Pt/H[Ga]MFI types

An MFI type galosilicate was synthesized and characterized employing XRD, N 2 adsorption at −196 °C, chemical analysis by ICP-AES, and XPS. This material was used in the preparation of Pd/H[Ga]ZSM5 and Pt/H[Ga]ZSM5 bifunctional catalysts, calcined under dry air for 6 h at 500 and 300 °C, respectively, and then reduced “in situ” under H 2 flow for 6 h at 500 °C. The bifunctional catalysts were also characterized by XRD, N 2 physisorption, TEM, chemical analysis, and XPS. The support possesses an MFI structure with excellent crystallinity, as shown by XRD. Bulk chemical analysis shows a (Si/Ga) ICP-AES atomic ratio of 16. The absence of Ga 2O 3 is suggested by both XRD and XPS (at surface level). The atomic ratio at the surface, (Si/Ga) XPS, is the same as that found by ICP-AES, suggesting that Ga is distributed homogeneously. XRD and N 2 physisorption results of the bifunctional catalysts suggest that metal incorporation does not affect sensibly the structure of the support. Dispersion of Pd and Pt in either catalyst was estimated from TEM analysis, indicating in both cases values of the order of 80%. The atomic ratios (Si/Ga) XPS at the surface were 16 for Pd/Galosilicate, and 4 for Pt/Galosilicate, suggesting that Pt promotes migration of Ga towards the external surface of the solid, a phenomenon that is not observed for the Pd/H[Ga]ZSM5 catalyst. Degaliation promoted by Pt could be explained assuming that Pt and Ga could form stable chemical species, as might be indicated by XPS results. The transformation of acetone to MIBK was employed as catalytic test. These results indicate very different behaviors for either bifunctional catalyst.

Self-Assembled GaAs Quantum Rings by MBE Droplet Epitaxy

Fabrication of semiconductor nanostructures such as quantum dots (QDs), quantum rings (QRs) has been considered as the important step for realization of solid state quantum information devices, including QDs single photon emission source, QRs single electron memory unit, etc. To fabricate GaAs quantum rings, we use Molecular Beam Epitaxy (MBE) droplet technique in this report. In this droplet technique, Gallium (Ga) molecular beams are supplied initially without Arsenic (As) ambience, forming droplet-like nano-clusters of Ga atoms on the substrate, then the Arsenic beams are supplied to crystallize the Ga droplets into GaAs crystals. Because the morphologies and dimensions of the GaAs crystal are governed by the interplay between the surface migration of Ga and As adatoms and their crystallization, the shape of the GaAs crystals can be modified into rings, and the size and density can be controlled by varying the growth temperatures and As/Ga flux beam equivalent pressures(BEPs). It has been shown by Atomic force microscope (AFM) measurements that GaAs single rings, concentric double rings and coupled double rings are grown successfully at typical growth temperatures of 200°C to 300°C under As flux (BEP) of about 1.0×10-6 Torr. The diameter of GaAs rings is about 30-50 nm and thickness several nm.

Degradation of blue LEDs related to structural disorder

AbstractThe degradation of blue light emitting diodes (LEDs) with different structural disorder based on MQW InGaN/GaN grown by MOCVD on sapphire has been investigated. New approach to analyze the degradation has been used. It takes into account the structural disorder determined by the extended defect system relaxation and related with poor coalescence of the mosaic structure domains. The results obtained leads to assumption that the migration and segregations of Ga on domain dislocation boundaries of the mosaic structure and the change of energy activation of Mg related centers are important reasons of degradation for all types of LEDs The fastest degradation for poor ordered LEDs was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the Supported Pt on the Migration of Structural Ga Toward the Surface of Catalysts of the Pt/H[Ga]ZSM5 Type

The aim of this work is to find out how platinum supported over a gallosilicate of the MFI type influences the removal of Ga from the framework and affects the properties of the Pt/H[Ga]ZSM5 catalysts. In this regard, a gallosilicate and an aluminosilicate of MFI structure, with a Si/M ratio ≈15 (M = Ga, Al) were synthesized and characterized by XRD, nitrogen physical adsorption at −196 °C and chemical analysis by ICP-AES. Afterwards, they were used to prepare the following bifunctional catalysts: xPt/H[Ga]ZSM5 (x=0.10, 0.50 and 1.00 wt.% Pt) and 1.00%Pt/H[Al]ZSM5, which were characterized by ICP-AES, TEM, XPS, and the model reaction of toluene hydrogenation. The results found show dispersions within 50–80% for the supported platinum; the XPS analysis indicated a decrease in the Si/Ga ratio as a response to the increase of Pt content, suggesting a substantial influence of platinum on the migration of structural Ga toward the surface of the catalysts. This nonframework gallium remains oxidic and appears to strongly interact with Pt, which considerably affects the hydrogenating ability of these catalysts.

Publisher’s Note: Intrinsic defects in GaN. II. Electronically enhanced migration of interstitial Ga observed by optical detection of electron paramagnetic resonance [Phys. Rev. B69, 045208 (2004)

Publisher’s Note: Intrinsic defects in GaN. II. Electronically enhanced migration of interstitial Ga observed by optical detection of electron paramagnetic resonance [Phys. Rev. B<b>69</b>, 045208 (2004)

Intrinsic defects in GaN. II. Electronically enhanced migration of interstitial Ga observed by optical detection of electron paramagnetic resonance

Optical excitation at 1.7 K with 364-nm laser light produces partial annealing recovery of the damage produced in GaN by 2.5-MeV electron irradiation in situ at 4.2 K. Observed is a reduction in the irradiation-produced 0.95-eV photoluminescence (PL) band, recovery in the visible luminescence, and conversion between the two electron-paramagnetic-resonance (ODEPR) signals L5 and L6 associated with interstitial Ga. This is interpreted as resulting from electronically excited migration of the interstitial Ga allowing it to convert between lattice sites near the Ga vacancy from which it was ejected. The relative rates of conversion between the two sites are found to vary between different samples and upon electron irradiation fluence.

Chemical and structural aspects of annealed ZnSe/GaAs(001) heterostructures

The thermal evolution of a ZnSe epilayer grown by molecular beam epitaxy on GaAs(001) has been studied by high resolution x-ray diffraction as well as photoelectron and Raman spectroscopies. Sequential annealing of a relaxed epilayer reveals a fast migration of Ga towards the ZnSe cap layer with a significant accumulation of As atoms near the ZnSe-reacted interface. A Ga2Se3 compound appears as a predominant byproduct whereas Zn atoms are probably diffusing from the reacted interface into the GaAs substrate.

Control of GaN surface morphologies grown on 6H-SiC (0 0 0 1) using plasma-assisted molecular beam epitaxy

We report the structural and surface morphologies of GaN layer directly grown on 6H-SiC (0 0 0 1) substrates by plasma-assisted molecular beam epitaxy. The structural properties of the GaN layers grown under Ga-rich conditions have been strongly influenced by the surface oxide-desorption through Ga-metal deposition and flash-off at high temperature. The resulting AFM surface morphology exhibits atomic steps with <5 Å rms surface roughness. The control of Ga-adatom migration governs the surface morphology of the grown layers, the atomic step features are realized with sufficient surface migration of Ga for the available N-radicals. The excellent structural quality of these layers is manifested by the narrow full-width at half-maximum of X-ray rocking curve (<124 arcsec) measured along the ω (0 0 0 2) diffraction.

Characterization of Ga-substituted zeolite Beta by X-ray absorption spectroscopy

The incorporation of Ga into the framework of zeolite Beta has been evidenced by X-ray absorption spectroscopy. Analysis of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data reveals the atomic environment of Ga in zeolite Beta. Both techniques provide evidence that calcination and dehydration of the synthesized zeolites provokes changes in the Ga atomic environment, which can be understood as due to the migration of Ga from framework sites to extra-framework positions. The XANES results allow calculation of the atomic fraction at both positions.