Ga Flux Research Articles

Wurtzite-to Amorphous-to Cubic Phase Transition of GaN<sub>1-X</sub>As<sub>x</sub> Alloys with Increasing as Content

This paper describes Transmission Electron Microscopy studies of the structural changes of GaN1-xAsx alloys grown by Molecular Beam Epitaxy at low temperatures on Al2O3 substrate. We found that by lowering the growth temperature increasing amount of As can be incorporated in GaN1-xAsx forming a single phase alloy. For the low As content a columnar growth of wurtzite structure is observed but for increasing As in the range of 0.170.75 the layer becomes amorphous. Increase in Ga flux at low growth temperature (about 200°C) leads to columnar alloys with As content >75% with a cubic structure. In addition to the structural changes monotonic change of the band gap is also observed with the As content in the alloy. The amorphous alloy is stable up to annealing at temperatures not higher than 600°C. Annealing at higher temperature leads to phase separation of GaAs:N and GaN:As confirmed by Z-contrast electron microscopy.

Synthesis and structural characterization of the ternary Zintl phases AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As)

Ten new ternary phosphides and arsenides with empirical formulae AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) have been synthesized using molten Ga, Al, and Pb fluxes. They have been structurally characterized by single-crystal and powder X-ray diffraction to form with two different structures—Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4 crystallize with the Ca3Al2As4 structure type (space group C2/c, Z=4); Ba3Al2P4 and Ba3Al2As4 adopt the Na3Fe2S4 structure type (space group Pnma, Z=4). The polyanions in both structures are made up of TrPn4 tetrahedra, which share common corners and edges to form [TrPn2]3–∞2 layers in the phases with the Ca3Al2As4 structure, and [TrPn2]3–∞1 chains in Ba3Al2P4 and Ba3Al2As4 with the Na3Fe2S4 structure type. The valence electron count for all of these compounds follows the Zintl–Klemm rules. Electronic band structure calculations confirm them to be semiconductors.

Photoluminescence and Raman scattering studies of GaN nanowires obtained by top-down and bottom-up approaches

ABSTRACTWe present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.

Effect of growth conditions on magnetic and structural properties in Gd‐doped GaN layers grown by plasma‐assisted molecular beam epitaxy

AbstractEffects of growth conditions on structural and magnetic properties in Gd‐doped GaN layers grown by plasma‐assisted molecular beam epitaxy have been investigated. It is found that the growth of Gd‐doped GaN layers with atomically flat surface morphologies requires relatively high Ga fluxes compared with the growth of GaN. For these Gd‐doped GaN layers grown under Ga‐rich conditions, (0002) diffraction spots of GaGdN appear and shift toward low scattering angle side depending on the concentration of Gd, indicating the expansion of the c‐axis lattice constant due to the incorporation of Gd. In contrast, for the Gd‐doped GaN layers grown under N‐rich condition, just the (0002) diffraction spot of GaN is observed irrespective of the concentration of Gd. Although all the samples show hysteresis in the M‐H curves at room temperature, the samples grown under the Ga‐rich conditions have higher coercevities and larger saturation magnetizations than the samples grown under the N‐rich conditions. Based on the findings, we will discuss the origin of ferromagnetic behaviour observed in Gd‐doped GaN (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and transport properties of p-type GaNBi alloys

Abstract

Texture and morphology variations in (In,Ga)2Se3 and Cu(In,Ga)Se2 thin films grown with various Se source conditions

ABSTRACTTexture and morphology variations in co‐evaporated (In,Ga)2Se3 and Cu(In,Ga)Se2 (CIGS) films grown with various Se source conditions during growth were studied. The Se species of simply evaporated, large molecular Se (E‐Se, low‐sticking coefficient), and RF‐plasma cracked atomic Se (R‐Se, high sticking coefficient) were used in the present work. (In,Ga)2Se3 precursor films, which were prepared during the first stage of CIGS film growth by the three‐stage process, showed systematic variations in texture and Na distribution profile with varying evaporative Se (E‐Se) flux. The properties of CIGS films and solar cells also showed systematic variations, and the open‐circuit voltage (Voc) and fill factor were found to be especially sensitive to the E‐Se flux. R‐Se grown (In,Ga)2Se3 precursor films featured granular morphology with strong (105) and (301) peaks in the diffraction pattern, and the texture was very similar to an E‐Se grown film fabricated with a Se to group III metal (In + Ga) flux ratio (P[Se]/[In + Ga]) of about 6, although the nominal P[Se]/[In + Ga] used for an R‐Se source was very small and less than 0.5. The R‐Se grown CIGS films displayed, however, highly dense surfaces and larger grain sizes than E‐Se grown CIGS films. The controllability of film morphology and the Na diffusion profile in (In,Ga)2Se3 and CIGS films with various Se source conditions are discussed. Copyright © 2011 John Wiley & Sons, Ltd.

Crystal growth, structure, and physical properties of Ln 2PdGa 12 (Ln = La, Pr, Nd, and Sm)

Single crystals of Ln 2PdGa 12 (Ln = La, Pr, Nd, Sm) were synthesized with a molten Ga flux. The intermetallic phases are isostructural to Sm 2NiGa 12 with cell dimensions of a ∼ 6.1 Å and c ∼ 15.5 Å. Pr 2PdGa 12, Nd 2PdGa 12, and Sm 2PdGa 12 order antiferromagnetically at 18, 7.5, and 7.5 K, respectively, and magnetic properties of single crystals oriented parallel and perpendicular to the magnetic field are presented. Fitting the low temperature heat capacity of Pr 2PdGa 12 gives a Sommerfeld coefficient ( γ) of 250 mJ mol −1 K −2 and indicates the possibility of heavy fermion behavior.

Doping of GaN1−xAsx with high As content

Recent work has shown that GaN1−xAsx can be grown across the entire composition range by low temperature molecular beam epitaxy with intermediate compositions being amorphous, but control of the electrical properties through doping is critical for functionalizing this material. Here we report the bipolar doping of GaN1−xAsx with high As content to conductivities above 4 S/cm at room temperature using Mg or Te. The carrier type was confirmed by thermopower measurements. Doping requires an increase in Ga flux during growth resulting in a mixed phase material of polycrystalline GaAs:N embedded in amorphous GaN1−xAsx.

Surface reconstructions during growth of GaAs 1− xBi x alloys by molecular beam epitaxy

An in-situ electron diffraction study of surface reconstructions on (001) oriented GaAs 1− x Bi x films has been carried out during growth by molecular beam epitaxy on GaAs substrates in the temperature range from 250 °C to 400 °C. We observe (1×3), (2×3) and (2×4) reconstructions on both GaAs and GaAs 1− x Bi x surfaces. A (2×1) surface reconstruction is observed in the presence of Bi at low As 2:Ga flux ratios. Higher Bi incorporation and stronger photoluminescence were observed for GaAs 1− x Bi x films grown on (2×1) reconstructed surfaces, compared to samples grown on (1×3) surfaces. The location of the various surface phases has been mapped out as a function of temperature, Bi flux and As 2:Ga flux ratio.

Nitrogen flux induced GaN nanostructure nucleation at misfit dislocations on Al2O3(0001)

The work demonstrates the dominant role of nitrogen flux rate on GaN nanostructure formation on bare Al2O3(0001). In nitrogen rich conditions, wurtzite c-oriented GaN nanowall honeycomb network is formed as strain relaxation pathway of nucleation at edge dislocations. A specific nitrogen flux rate in a plasma assisted molecular beam epitaxy growth is necessary for fixed Ga flux and substrate temperature to form columnar self assembled nanostructures. It is argued that kinetically hindering diffusion of Ga adatoms and the low sticking coefficient of r and m planes of nanowalls promote 1-dimension nanocolumn formation at screw dislocations formed at the GaN-Sapphire interface.

High temperature step-flow growth of gallium phosphide by molecular beam epitaxy and metalorganic chemical vapor deposition

Post-growth surface morphologies of high-temperature homoepitaxial GaP films grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have been studied. Smooth, stepped surface morphologies of MBE-grown layers, measured by atomic force microscopy, were found for a wide range of substrate temperatures and P2:Ga flux ratios. A MOCVD-based growth study performed under similar conditions to MBE-grown samples shows a nearly identical smooth, step-flow surface morphology, presenting a convergence of growth conditions for the two different methods. The additional understanding of GaP epitaxy gained from this study will impact its use in applications that include GaP-based device technologies, III-V metamorphic buffers, and III-V materials integration with silicon.

A superstructural 2D-phase diagram for Ga on the Si(111)- 7x7 system

The structural modifications of an Si(111)- 7x7 reconstructed surface and the evolution of growth induced by Ga adsorption in the submonolayer regime at various substrate temperatures ranging from room temperature (RT) to 600 °C, with a low Ga flux rate of 0.1 ML/min (1 ML∼6.8×10 14 atoms/cm 2) have been studied in-situ in Ultra High Vacuum (UHV) using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS) as characterization probes. Ga grows in the Stranski–Krastanov (SK) growth mode for temperatures ranging from RT to 350 °C, where 3D-islands form after one and two flat monolayers of Ga adsorption, while for higher temperatures ranging from 450 to 550 °C, Ga grows in the Volmer–Weber (VW) growth mode. A comprehensive 2D-phase diagram for this Ga/Si(111) system for adsorption, which provides pathways to attain the observed superstructural phases, viz., √3x√3-R30°, 6.3x6.3, 6.3√3x6.3√3-R30° and 11x11, has been investigated. The characteristic EELS spectrum for each superstructural phase is also reported in this study.

Reduction of oxygen impurity at GaN/β-Si 3N 4/Si interface via SiO 2 to Ga 2O conversion by exposing of Si surface under Ga flux

The removal of native oxide from Si (1 1 1) surfaces was investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectra (SIMS) depth profiles. Two different oxide removal methods, performed under ultrahigh-vacuum (UHV) conditions, were carried out and compared. The first cleaning method is thermal desorption of oxide at 900 °C. The second method is the deposition of metallic gallium followed by redesorption. A significant decrease in oxygen was achieved by thermal desorption at 900 °C under UHV conditions. By applying a subsequent Ga deposition/redesorption, a further reduction in oxygen could be achieved. We examine the merits of an alternative oxide desorption method via conversion of the stable SiO 2 surface oxide into a volatile Ga 2O oxide by a supply of Ga metals. Furthermore, ultra thin films of pure silicon nitride buffer layer were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma followed by GaN growth. The SIMS depth profile shows that the oxygen impurity can be reduced at GaN/β-Si 3N 4/Si interfaces by applying a subsequent Ga deposition/redesorption.

Ga-induced restructuring of Si(5 5 12) − 2 × 1 reconstructed surface at room temperature

Motivated by the need to form 1D-nanostructured dopants on silicon surfaces, we have attempted to grow Ga on the high index Si(5 5 12) surface which has a highly trenched (1D) morphology. The evolution of the interface with Ga adsorption in the monolayer regime has been probed by in situ AES, LEED and EELS. Controlling the kinetics by changing the Ga flux rates shows an interesting difference in the 1.0 to 1.5 ML region. The low flux rate (0.03 ML/minute) results in a Frank van der Merwe (layer by layer) growth mode up to 2 ML, while the higher flux rate (0.1 ML/minute) shows a transient island formation after the completion of 1 ML. The low rate shows the formation of 2 × (3 3 7) and (2 2 5) superstructures, while only the 2 × (3 3 7) is observed in a wide coverage range for the higher rate. The results demonstrate the ability to kinetically control the surface phases with different electronic properties of this technologically important interface.

Mg doping of GaN by molecular beam epitaxy

We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga : N flux ratio and Mg : Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga : N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga : N ratio, an optimum Mg : Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN : 0.31% < Mg : Ga < 5.0%. A lowest resistivity of 0.98 Ω cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 × 1017 cm−3 and a mobility of 15 cm2 V−1 s−1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100 meV for a hole concentration of 5.5 × 1017 cm−3. The corresponding Mg concentration is 5 × 1019 cm−3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800 °C or higher. This result is thus in contrast to previously reported GaN : Mg doping at much lower growth temperatures of 500 °C.

Optical and structural characterization of GaN thin films at different N to Ga flux ratios

GaN films were grown on Si(111) substrates under various beam equivalent pressure (BEP) ratios by plasma-assisted molecular beam epitaxy. The optical properties for the grown samples were studied over a wide spectral range from 200 to 3300 nm using the reflectance spectrum only. It was found that increasing the N/Ga BEP ratio from 17.9 to 46.1 increases the refractive index (n) from 2.05 to 2.38 at wavelength 630 nm (for example), while the optical energy gap (Eg) were found to be in the range between 3.325 to 3.35 eV with no specific trend. The structural properties for the grown films were studied through two types of rocking curve measurements; normal rocking curve (ω-scan) and triple axis rocking curve (ω/2θ-scan). It was found that with decreasing the N/Ga ratio from 46.1 to 17.9 the full width at half maximum decreases from 0.62° to 0.58° for ω-scan and from 0.022° to 0.021° for ω/2θ-scan. Thus, our results showed a clear correlation between the optical-structural parameters and the BEP ratios of N and Ga.

Crystal growth and structure determination of the novel tetragonal compound Ce2RhGa12

Single crystals of Ce2RhGa12 have been synthesized using Ga flux and their structure was determined by single-crystal X-ray diffraction. Ce2RhGa12 crystallizes in the tetragonal space group P4/nbm (No. 125), and is isostructural to Ce2PdGa12, with Z = 2 and lattice parameters a = 6.0405 A and c = 15.706 A. Data were collected at the Swiss Norwegian Beam Line at the European Synchrotron Facility, Grenoble, France. Laue diffraction was carried out to confirm the quality of the single crystal and showed well-defined spots and tetragonal symmetry.

The presence of CuGaSe 2 interface layer in the growth of Cu-rich CuInSe 2/GaAs(0 0 1) epitaxial films

The Cu-rich CuInSe 2 (CIS) epitaxial films with y=[Cu]/[In]=1.65 are grown on GaAs(0 0 1) substrates using the MBE technique at substrate temperatures ranging from 400 to 600 °C. The unusual background and broad peaks in the XRD spectra are found at 2 θ between 67° and 69°, which are not accounted for any crystal planes of the CIS. They also cannot be removed using the KCN solution and are not observed in the stoichiometric ( y=1) CIS epitaxial films, suggesting that there exists some other relatively thin epitaxial structures at the interface of the CIS and the GaAs(0 0 1). The broad peaks are found to be in the vicinity of the (0 0 8) crystal plane of the CuGaSe 2 (CGS) even if the Ga flux is not allowed during the growth of CIS. The interface layer is believed to be CGS formed by the diffusion of the excess Cu 2− x Se and Ga atoms at the surface of the GaAs substrate. The formation of the CGS interface layer is directly verified by STEM images and the composition of Cu:Ga:Se obtained from the EDS measurement is approximately 1.1:1.0:2.0. The thickness of the CGS interface layer is dependent on the substrate temperature and found to occur when the substrate temperature is approximately higher than 430 °C.

Role of strain in growth kinetics of AlGaN layers during plasma-assisted molecular beam epitaxy

Comparative study of growth kinetics of the AlxGa1−xN (x=0–1) layers grown by plasma-assisted molecular beam epitaxy (PA MBE) under different growth conditions (group III to activated nitrogen and Al to Ga flux ratios) is presented. The strong influence of elastic stress on the surface morphology, growth rate and Al content in AlxGa1−xN (x=0–1) layers is most pronounced for the AlxGa1−xN films with high Al-content grown atop of the 2D-AlN buffer layer at near the unity flux ratio FIII/FN∼1. The use of strong Ga-rich growth conditions with FIII/FN∼1.6–2 for the growth of AlxGa1−xN/2D-AlN with high Al-content (x>0.25) allows one to reduce the strain effect as well as provide smooth surface morphology and precise control of Al content in the AlxGa1−xN (x=0–1) layers by employing a simple ratio x=FAl/FN.

Ga and In incorporation rates in Ga 1− xIn xAs growth by chemical beam epitaxy

GaAs, InAs and Ga 1− x In x As layers were grown by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and tertiarybutylarsine as precursors for Ga, In and As, respectively. The growth rate during the homoepitaxial growth of GaAs and InAs, deduced from the frequency of reflection high-energy electron diffraction intensity oscillations, was used to calibrate the incorporation rates for the III elements. The In content of the Ga 1− x In x As layers was measured by Rutherford backscattering spectrometry and compared with the value predicted from the above calibration data; while the measured In mole fraction is close to the predicted value for the samples grown for low In to Ga flux ratios ( x<0.2), the In incorporation is enhanced for larger values of this ratio. The results obtained on layers grown at different substrate temperatures show that In mole fraction is almost constant at growth temperatures in the range 400–500 °C, but a strong dependence on the substrate temperature has been found outside this range. The above results, not observed for samples grown by solid source molecular beam epitaxy, indicate that some interaction between Ga and In precursors at the sample surface could take place during the growth by CBE.