Ga Deposition Research Articles

Reaction Pathways of GaN (0001) Growth from Trimethylgallium and Ammonia versus Triethylgallium and Hydrazine Using First Principle Calculations

Gallium nitride (GaN) is a wide bandgap semiconductor with many important applications in optoelectronics, photonics, and both high power and high temperature operation devices. Understanding the surface deposition mechanisms and energetics for different precursors is essential to improving thin-film crystalline quality and growth process requirements for extended engineering applications. Here, we use ab initio calculations to study the reaction mechanisms of GaN thin film growth on (0001) surface from ammonia (NH_3) and hydrazine (N_2H_4), nitrogen precursors, and trimethylgallium (TMG) and triethylgallium (TEG), gallium precursors. We find that the initial dehydrogenation of N_2H_4 is more facile than that of NH_3, at 1.15 versus 13.61 kcal/mol, respectively, and that neighboring adsorbed surface hydrogens reduce the barriers for further decomposition of NH_2 and NH. We also find that the growth of nitrogen layers is a reaction-limited process rather than diffusion-limited at low adsorbate coverage. On the other hand, the deposition of Ga on a nitrogen rich surface via TMG is limited by the abstraction reaction of the second methyl (CH_3) group in TMG, with a barrier of 42.54 kcal/mol. The mechanisms of adsorption of TMG and TEG are different, whereas TMG dissociatively chemisorbs releasing one methane group, the beta-hydrate elimination (C_2H_4 + H) in TEG is favored through surface interactions (without chemisorption) at a comparable energy barrier to the first CH3 dissociation in TMG. This does not suggest a more favorable thermodynamic route to low-temperature growth, but it does favor TEG for avoiding the explicit abstraction or insertion of C groups during metalorganic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) techniques.

GaSb/GaAs quantum-ring-with-dot structures grown by droplet epitaxy

We have studied the growth of GaSb/GaAs nanostructures by droplet epitaxy with the variation of Ga deposition temperature which is one of the key parameters. With the use of low Sb flux, GaSb quantum rings (QRs) were formed as a result of the outward diffusion of Ga atoms from the droplets during crystallization. An increase of the deposition temperature results in the larger QR size and the lower QR density due to the longer diffusion length of Ga atoms, which gives rise to the larger initial droplets. Interestingly, some portion of QR lobe breaks up and transforms into a quantum dot (QD) so as to reduce the mismatch strain. A nanostructure containing both QR and QD is called a quantum-ring-with-dot structure (QRDS). As the deposition temperature increases, the nanostructure height distribution changes from unimodal to bimodal behaviors owing to the significant difference between QR and QD heights, whereas the nanostructure diameter still exhibits a unimodal distribution. The bimodal height distribution strongly affects the optical properties and the dynamic of thermal-excited carriers.

Extremely High- and Low-Density of Ga Droplets on GaAs{111}A,B: Surface-Polarity Dependence

Formation processes of Ga droplets on polar (111)A and (111)B surfaces of GaAs have been investigated. A single Ga atom forms a stable nucleus on the (111)A surface, so that the formation of extremely high-density of Ga droplets is achieved (2.8 × 1012 cm–2). On the (111)B surface, the initial Ga deposition on both As-rich (2 × 2) and Ga-rich (√19 × √19) reconstructions leads to the formation of a two-dimensional GaAs layer having a more Ga-rich (3 × 2) reconstruction. The Ga droplets are formed on the (3 × 2) surface with their densities being 4 orders of magnitude lower than those for the (111)A orientation.

Adaptation of the surface-near Ga content in co-evaporated Cu(In,Ga)Se2 for CdS versus Zn(S,O)-based buffer layers

In this work, we show that in order to optimize the efficiency of Cu(In1-x,Gax)Se2 (CIGS) solar cells with Cd-free Zn(S,O)-based buffer layers, the Ga concentration in the CIGS absorber layer towards the hetero-interface has to be adapted. We varied the In and Ga deposition rates in the last stage of our 3-stage co-evaporation process, leading to different compositional ratios xf=[Ga]/([Ga]+[In]) between 0.15 and 0.6 in the top 400nm of the absorber layer. All absorber layers were then completed with both CdS and Zn(S,O) buffer layers by chemical bath deposition. While cells with our standard grading of xf≈0.4 in the front region result in a best performance of 15% with a CdS buffer, similar efficiencies with a Zn(S,O) buffer layer are only obtained when the Ga content near the hetero-interface is reduced down to x≈0.25. The maximum efficiency for the CdS buffer layer coincides with the maximum open circuit voltage (Voc) and fill factor (FF). Interestingly, for the Zn(S,O) buffer layer, this is not the case: the Voc increases steadily for higher Ga ratios, while the FF is fairly constant for 0.25<x <0.5 and decreases drastically for more extreme values. The findings are explained by differences in the conduction band offsets which result from the conduction band shift close to the surface due to Ga content variations. The results illustrate the importance of the absorber layer adaptation for different buffer layers and are an important step on the way to Cd-free buffer layers.

Effect of interfacial oxygen on the microstructure of MBE-grown homoepitaxial N-polar GaN

We have investigated the microstructure of homoepitaxial N-polar GaN layers grown by rf-plasma-assisted molecular beam epitaxy on freestanding GaN substrates. The structural quality of the epitaxial layers improves when the sheet density of oxygen present on the substrate surface diminishes. An initial sheet density of oxygen of ~0.5 monolayer (ML) results in a highly defective epitaxial layer, while an epitaxial layer with no visible threading dislocations was grown on a substrate with an initial oxygen sheet density of ~0.08ML. The significant reduction in oxygen was achieved by using several cycles of Ga deposition and thermal desorption prior to the start of epitaxial growth combined with an initial ultrathin 15-Å AlN nucleation layer. These results indicate that reducing the density of oxygen on the surfaces of freestanding N-polar GaN substrates is vital for obtaining high quality homoepitaxial N-polar GaN layers.

Effect of Ga deposition rates on GaSb nanostructures grown by droplet epitaxy

We investigate the effect of Ga deposition rates on GaSb nanostructures grown by droplet epitaxy on GaAs (001) substrates. Ga deposition rate was varied to form the different size and density of Ga droplets. After the droplets were exposed to Sb flux, not only the GaSb ring structure but also the complex nanostructure like the GaSb ring structure surrounded by ring-shaped dot molecules were obtained. A simple descriptive model is proposed to describe formation mechanisms of these nanostructures. It is found that Ga droplet size, distance between Ga droplets and diffusion area of Ga atoms during crystallization with Sb flux are all the crucial factors which determine the shape of GaSb nanostructures. Due to a lattice mismatch between GaSb and GaAs, strain occurred during a crystallization process should also be taken into account. Photoluminescence was carried out to verify our model.

A new voltammetric strategy for sensitive and selective determination of gallium using cupferron as a complexing agent

This article describes a differential pulse adsorptive stripping voltammetric method for the trace determination of gallium in environmental water samples. It is based on the adsorptive deposition of the complex Ga(III)-cupferron at the hanging mercury drop electrode (HMDE) at −0.4 V (versus Ag/AgCl) and its cathodic stripping during the potential scan. The method was optimized as concerns the main electrochemical parameters that affect the voltammetric determination (supporting electrolyte, pH, cupferron concentration, deposition potential and time). The calibration graph is linear from 5 × 10−10 to 5 × 10−7 mol L−1 with a detection limit calculated as 1.3 × 10−10 mol L−1 for deposition time of 30 s. The influence of interfering substances such as surfactants and humic substances present in the matrices of natural water samples on the Ga(III) signal was examined and a satisfying minimization of these interferences was proposed. The procedure was applied to direct determination of gallium in environmental water samples.

Single CdS nanowire photodetector fabricated by FIB

In this paper, a photodetector based on single cadmium sulfide (CdS) nanowire was fabricated using focused ion beam (FIB) technique. The nanowire manipulation, transferring and electrode fabrication process by the FIB and its influence on the photodetector performance were studied. CdS nanowire was synthesized by Vapor–Phase–Transport (VPT) method. CdS powder was thermally evaporated at 950°C in the Argon (Ar) gas as the source material. High Resolution Transmission Electron Microscope (HRTEM) and X-ray Diffraction (XRD) experiment results showed that the nanowire presented wurtzite CdS structure and good crystal quality. The photoresponse time and recovery time of the photodetector were 150ms and 53ms, respectively. The photosensitivity was 2.83, and the low sensitivity was caused by the deposition of Ga on the surface of the nanowire and electrode gap during the FIB fabricating process.

Electrophoretic deposition of Ga–Cu core–shell nanocomposites for CuGaS2 thin films

This work reports on the syntheses of copper-gallium bimetallic nanoparticles by ultrasound-assisted chemical reduction method in alcohol solutions to prepare CuGaS2 thin films. Gallium metal (Ga) in the form of pellet was dispersed in tetraethylene glycol by ultrasonic treatment. The chemical reduction of copper sulfate (CuSO4) into copper metal by NaBH4 at the surface of the Ga droplets enabled the formation of Ga–Cu core–shell structure. The Ga droplets with submicrometer size were homogenously coated with ~40-nm Cu nanoparticles. The exothermic transition of the Ga–Cu core–shell nanocomposite materials occurred at 485°C, which corresponds to the crystalline phase formation of γ-Cu9Ga4. Ga–Cu nanocomposites with a 1:1 ratio were deposited onto molybdenum substrate via electrophoretic deposition method as a precursor of CuGaS2, a p-type semiconductor. Mechanical pressing and sulfurization provided dense CuGaS2 thin films with good crystallinity and high phase purity.

Magnetic properties and large coercivity of MnxGa nanostructures

To investigate structure–property correlations, high-coercivity MnxGa nanoparticles were synthesized by the method of sequential deposition of Ga and Mn fluxes using molecular beam epitaxy. Spontaneous nanostructuring was assisted by the use of an Au precursor and thermal annealing, and the growth properties, structure and magnetic properties were characterized. Atomic force microscopy revealed average particle dimensions of 100nm and X-ray diffraction revealed a dominant tetragonal D022 crystal structure. Magnetic characterization at room temperature identified the presence of two magnetic phases, dominated by a high-coercivity (2.3T) component in addition to a low-coercivity component.

2.1–1.85 Ga tectonic events in the Yangtze Block, South China: Petrological and geochronological evidence from the Kongling Complex and implications for the reconstruction of supercontinent Columbia

This paper presents petrography, zircon U–Pb ages and Hf isotopic data as well as whole-rock Sm–Nd isotopic data for mafic granulites, metapelitic rocks and high-grade marble from the Kongling Complex in the Yangtze Block, South China. Petrographic observations indicate that these three types of rocks experienced high-pressure metamorphism. Their mineral assemblages and P–T conditions define a clockwise P–T path involving isothermal decompression following the peak high-pressure metamorphism, which is considered to record a continent–continent collisional event. This is systematic documentation of the tectonic evolution of the Kongling Complex from 2.1-2.0Ga deposition (constrained by youngest detrital zircon and metamorphic zircon) through ~2.0Ga collision (high-pressure metamorphism) and syn-collisional partial melting (S-type granite and migmatization of TTG gneiss) to ~1.85Ga post-collisional extension (A-type high-K granite and mafic dyke). These ages are broadly coincident with global collisional events (2.1–1.8Ga) that led to the assembly of the Palaeo-Mesoproterozoic Columbia (or Nuna) supercontinent. Therefore, this study provides strong evidence that the Yangtze Block in South China was a component of the Columbia supercontinent.

Growth characteristics and film properties of gallium doped zinc oxide prepared by atomic layer deposition

We carried out comprehensive studies on structural, optical, and electrical properties of gallium-doped zinc oxide (Ga:ZnO) films deposited by atomic layer deposition (ALD). The gallium(III) isopropoxide (GTIP) was used as a Ga precursor, which showed pure Ga2O3 thin film with high growth rate. Using this precursor, conductive Ga doped ZnO thin film can be successfully deposited. The electrical, structural and optical properties were systematically investigated as functions of the Ga doping contents and deposition temperature. The best carrier concentration and transmittance (7.2 × 1020 cm−3 and 83.5 %) with low resistivity (≈3.5 × 10−3 Ωcm) were observed at 5 at.% Ga doping concentration deposited at 250 °C. Also, low correlation of deposition temperature with the carrier concentration and film structure was observed. This can be explained by the almost same atomic radius of Ga and Zn atom.

Crystallization kinetics of Ga metallic nano-droplets under As flux

We present an experimental investigation of the crystallization dynamics of Ga nano-droplets under As flux. The transformation of the metallic Ga contained in the droplets into a GaAs nano-island proceeds by increasing the size of a tiny ring of GaAs which is formed just after the Ga deposition at the rim of a droplet. The GaAs crystallization rate depends linearly on the liquid–solid interface area. The maximum growth rate is set by the As flux impinging on the droplet, thus showing an efficient As incorporation and transport despite the predicted low solubility of the As in metallic Ga at the crystallization temperatures.

Structural, petrological and U–Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in the SW Rae craton, Canadian Shield

The Beaverlodge domain is a polydeformed Precambrian terrane in the southwestern Rae craton whose rocks variably record the effects of Archean tectonism and four major Paleoproterozoic tectonic events: the 2.5–2.3Ga Arrowsmith orogeny, the 1.99–1.93Ga Taltson orogeny, the ca. 1.90Ga Snowbird orogeny and the ca. 1.87–1.80Ga Hudsonian orogeny. Structural–metamorphic analysis and in situ U–Pb SHRIMP zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding: a lower level comprising mainly high-grade Archean (ca. 3.0 and 2.7–2.6Ga) metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca. 2.33 to <2.17Ga supracrustal rocks of the Murmac Bay Group at more variable metamorphic grade. Whereas the older package underwent thermal and/or metamorphic events in the Archean (ca. 2.57Ga) and early Paleoproterozoic (ca. 2.34Ga; Arrowsmith orogeny), rocks at higher structural levels bear no record of these events, recording only younger (1.94–1.90Ga) metamorphism interpreted to reflect two further stages of orogenesis. The first stage, correlated with Taltson orogeny, involved tight to isoclinal folding (D1/2) that led to development of the composite ESE-trending (S0/S1/S2) transposition foliation followed by more open folding (D3). The absence of Taltson-age plutonic rocks, except for low-volume crustal melts, suggests that this cycle was driven by crustal thickening, consistent with a clockwise P–T–t path derived for one sample. The high degree of strain along the contact between Archean rocks and the Murmac Bay Group, along with local structural discordance, indicates some degree of translation of cover rocks over basement during this stage. Pelitic rocks at higher structural levels (low- to medium-grade zone) underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels (high-grade zone) attained upper amphibolite facies conditions. Monazite data bracket prograde metamorphism (and related garnet growth) between 1939±6Ma and 1934±5Ma. The second stage, dated most precisely at 1906±7Ma, involved refolding of D1 to D3 structures about NE-trending axes (F4) and coeval dextral shearing and is correlated with Snowbird orogenesis. Rocks of the low- to medium-grade zone were held at roughly the same crustal level whereas rocks in the high-grade zone were uplifted via dextral–oblique displacement along domain-bounding shear zones, instigating decompression (garnet to cordierite) reactions. This study demonstrates that after <2.17Ga deposition of the upper Murmac Bay Group, rocks along the entire west-southwestern Rae margin, from the Northwest Territories/northern Alberta to the Snowbird tectonic zone, were affected by Taltson (1940–1930Ma) orogenesis. The Taltson tectono-metamorphic cycle then appears to have transitioned, as rocks underwent further shortening and uplift in a dextral transpressive regime, into Snowbird tectonism at ca. 1910–1900Ma. Snowbird orogenesis is therefore best viewed as part of a semi-continuous 1940–1900Ma cycle of tectonic burial and exhumation, with its inception at ca. 1910Ma triggered by a change in boundary conditions, as yet not fully understood, but consistent with arrival of the Hearne craton proposed by others. While the Beaverlodge domain has many features in common with the Tantato domain, future work needs to explore why rocks in the Tantato domain lack evidence for Arrowsmith orogeny and failed to record the cycle of 1.94–1.93Ga tectonic burial.

Change in the branch period of the step pattern formed by a moving linear source—initial coarsening and effect of an abrupt change in the velocity

We study pattern formation of a step induced by a moving linear source of adatoms, which is related to a step pattern during Ga deposition on Si(111), and possibly to a graphene film grown on SiC. Diffusion of adatoms released from the source in front of the step causes wandering instability of the step. Many small intrusions with branches appear, and the characteristic length of the pattern increases until it reaches a steady state. Coarsening process of the branch period is examined. In the first stage the period increases as λ∼tν1, with ν1≈1/4, increasing slowly with decreasing the velocity of the source. Competition between the intrusions results in a faster growth of the branch period with exponent ν2≈1/2. Change in the step pattern by an abrupt change in the source velocity is also studied. Branches adjust their period by terminating growth of some branches or increasing their number by tip-splitting. The latter is suppressed by a large stiffness and a metastable state with side branches is seen.

Oxygen incorporation in homoepitaxial N-polar GaN grown by radio frequency-plasma assisted molecular beam epitaxy: Mitigation and modeling

We have investigated the effect of gallium deposition and desorption cycles and ultrathin (15 Å) AlN layers on the oxygen impurity concentrations in homoepitaxial N-polar GaN layers. Secondary ion mass spectroscopy (SIMS) indicates that three Ga deposition and desorption cycles reduce the total oxygen by 70%–80%, while ten cycles reduces the total oxygen by more than 90%. We present a model of surface segregation and incorporation which accurately captures the distribution of oxygen in these layers. By fitting the model to the SIMS data, we are able to determine the fraction of an oxygen layer adsorbed on a GaN surface which segregates upon initiation of epitaxial GaN growth. Under the conditions investigated, we find this fraction to be 80%.

Epitaxial GaN films by hyperthermal ion-beam nitridation of Ga droplets

Epitaxial GaN film formation on bare 6H-SiC(0001) substrates via the process of transformation of Ga droplets into a thin GaN film by applying hyperthermal nitrogen ions is investigated. Pre-deposited Ga atoms in well defined amounts form large droplets on the substrate surface which are subsequently nitridated at a substrate temperature of 630 °C by a low-energy nitrogen ion beam from a constricted glow-discharge ion source. The Ga deposition and ion-beam nitridation process steps are monitored in situ by reflection high-energy electron diffraction. Ex situ characterization by x-ray diffraction and reflectivity techniques, Rutherford backscattering spectrometry, and electron microscopy shows that the thickness of the resulting GaN films depends on the various amounts of pre-deposited gallium. The films are epitaxial to the substrate, exhibit a mosaic like, smooth surface topography and consist of coalesced large domains of low defect density. Possible transport mechanisms of reactive nitrogen species during hyperthermal nitridation are discussed and the formation of GaN films by an ion-beam assisted process is explained.

Successful growth of Cu2Se-free CuGaSe2 by migration-enhanced epitaxy

CuGaSe2 films were grown on GaAs (001) substrates by migration enhanced epitaxy, where Cu + Ga and Se are alternately deposited. The in situ reflection high energy electron diffraction observation during growth revealed that the segregation of Cu2Se on CuGaSe2 can be detected by anomalous diffraction patterns. When the Cu2Se segregation takes place, a distorted pattern appears in both Cu + Ga and Se deposition periods. By optimizing the flux ratios, the anomalous diffraction disappears, and we succeeded in growing high quality CuGaSe2 single crystal layers free from Cu2Se segregation on GaAs (001) substrates. The authors also found that, in the CuGaSe2/GaAs wafers with Cu2Se segregation, a high density of voids is often observed at the substrate surface. These voids disappeared when the Cu2Se-free growth conditions were employed. Ga atoms near the GaAs substrate surface were probably drawn out by excess Cu atoms in the growing layer to form CuGaSe2, leading to the creation of voids with fairly large size.

Characterisation of Ga-coated and Ga-brazed aluminium

This work is devoted to the brazing of aluminium using liquid gallium. Gallium was deposited on aluminium samples at ~ 50 °C using a liquid gallium “polishing” technique. Brazing was undertaken for 30 min at 500 °C in air. EDS (Energy Dispersive X-ray Spectroscopy) and AES (Auger Electron Spectroscopy) characterisation of Ga-coated samples has shown that the Ga surface layer thickness is of ten (or a few tens of) nanometres. Furthermore, aluminium oxide layer (Al2O3) was shown to be “descaled” during Ga deposition, which ensures good conditions for further brazing. Cross-section examination of Ga-coated samples shows that liquid gallium penetrates into the aluminium grain boundaries during deposition. The thickness of the grain boundary gallium film was measured using an original EDS technique and is found to be of a few tens of nanometres. The depth of gallium grain boundary penetration is about 300 μm at the deposition temperature. The fracture stress of the brazed joints was measured from tensile tests and was determined to be 33 MPa. Cross-section examination of brazed joints shows that gallium has fully dissolved into the bulk and that the joint is really autogenous.

Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700°C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5×1013 and 2×1014at.cm−2s−1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio.