Ga Flux Research Articles

Accurate Determination of Acceptor Densities and Acceptor Levels in Undoped InGaSb from Temperature Dependence of Hole Concentration

Without any assumptions regarding residual impurity species and intrinsic defects in an undoped semiconductor, it is experimentally demonstrated that the densities and energy levels of impurities and defects can be precisely determined by a graphical peak analysis method based on Hall-effect measurements, referred to as free-carrier-concentration spectroscopy (FCCS). By FCCS, the number of acceptor species in p-type undoped In0.2Ga0.8Sb epilayers is determined, and the densities and energy levels of these acceptor species are accurately estimated. Two acceptor species, whose acceptor levels are EV+25 meV and EV+86 meV, are detected, where EV is the valence band maximum. The density of the EV+25 meV acceptor increases with Sb4/(In+Ga) flux beam equivalent pressure (BEP) ratio, whereas the density of the EV+86 meV acceptor decreases with increasing BEP ratio. These observations are not consistent with the conventional assumption that these acceptor species are VSb+ and VSb2+ in GaSb-based semiconductors, where VSb is the Sb vacancy.

Preparation and Characterization of Ba8Ga16Ge30/Sr8Ga16Ge30 Core−Shell Single Crystals

Ba- and Sr-filled type I Ge clathrate single crystals were synthesized by the Ga flux method, using high purity elemental Ba, Sr, Ga, and Ge as starting materials. Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and Raman spectroscopy were used to characterize the cross-section of a single crystal. The results indicate an inhomogeneous composition distribution of the crystal: the core of the crystal is composed of Ba 8 Ga 16 Ge 30 and the shell is composed of Sr 8 Ga 16 -Ge 30 with a transition layer in between. This inhomogeneous composition of the crystal is confirmed by a locally resolving Seebeck coefficient measurement and thermal analysis by differential scanning calorimeter.

Carrier-tuning of single-crystalline Ba 8Ga 16Ge 30

Despite the fact that the only practical route to self-flux single crystal growth of Ba 8Ga 16Ge 30 clathrates is through excess Ga flux, we have developed a method which results in p-type or n-type samples, depending on the relative Ge content in the starting mixture, and thus allows us to gain effective control over the charge carrier nature and density. An initial Ba:Ga:Ge ratio of 8:38:30 results in p-type crystals with average composition Ba 8Ga 16.6Ge 29.4, while those grown from 8:38:34 initial ratio are n-type with average composition Ba 8Ga 15.9Ge 30.1. More importantly, the thermopower S( T), resistivity ρ( T) and thermal conductivity κ( T) show radically different behaviors. The p-type crystals show: (i) monotonically increasing S( T) up to S(300 K)≈+200 μV/K, (ii) semiconducting ρ( T) (which decreases two orders of magnitude between 10 and 300 K), and (iii) a glass-like low-temperature plateau in κ( T). In contrast, the n-type crystals show: (i) monotonically decreasing S( T) down to S(300 K)≈−200 μV/K, (ii) metallic ρ( T) in the 1–3 mΩ cm range, and (iii) a low-temperature crystalline peak in κ( T).

Ga adsorbate on (0001) GaN: In situ characterization with quadrupole mass spectrometry and reflection high-energy electron diffraction

We have investigated the adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The in situ QMS and RHEED desorption transient measurements demonstrate the Ga flux dependent accumulation of the theoretically predicted laterally contracted Ga bilayer [J. E. Northrup et al., Phys. Rev. B 61, 9932 (2000)] under conditions similar to those used during GaN growth by rf-plasma molecular beam epitaxy. We correlated bioscillatory RHEED desorption transients [C. Adelmann et al., J. Appl. Phys. 91, 9638 (2002)] to QMS-measured Ga-adsorbate coverage and found both to be consistent with layer-by-layer desorption of the Ga-adsorbate bilayer. The QMS-measured steady-state Ga-adlayer coverage exhibited a continuous increase from 0 to 2.4 ML (monolayer) with respect to impinging Ga flux at substrate temperatures of 640–700°C. We observed an exponential dependence of the Ga flux corresponding to 1.0 ML Ga-adsorbate coverage on substrate temperature and we measured an apparent activation energy of 2.43±0.11eV and an attempt prefactor of 6.77×1012nm∕min (4.36×1011Hz) for this transition.

Atomic assembly during GaN film growth: Molecular dynamics simulations

Molecular dynamics simulations using a recently developed Ga-N Tersoff type bond order interatomic potential have been used to investigate the growth mechanisms of 0001 wurtzite GaN films from thermalized atomic gallium and nitrogen fluxes. The crystallinity and stoichiometry of the deposited wurtzite lattice structures were determined as a function of growth temperature and N:Ga flux ratio. The lattice perfection was found to improve as the growth temperature was increased to 500 K. At a fixed growth temperature, the lattice quality and stoichiometry both reached optimum as the N:Ga ratio approached a value between two and three. The optimum flux ratio increased with increasing growth temperature. These three observations are consistent with experimental studies of growth on wurtzite phase promoting substrates. The atomic assembly mechanisms responsible for these effects have been explored using time-resolved atom position images. The analysis revealed that high quality crystalline growth only occurred when off-lattice atoms which are usually associated with amorphous embryos or defect complexes formed during deposition were able to move to unoccupied lattice sites by thermally activated diffusion processes. The need for a high N:Ga flux ratio to synthesize stochiometric films arises because many of the nitrogen adatoms that impact N-rich 0001 GaN surfaces are re-evaporated. Reductions of the substrate temperature reduce this reevaporation and as a result, the optimum N:Ga ratio for the stoichiometric film formation and best lattice perfection was reduced as the growth temperature was decreased.

Tunable charge carriers and thermoelectricity of single-crystalBa8Ga16Sn30

We have grown single crystals of the type-VIII intermetallic clathrateBa8Ga16Sn30 from both Sn and Ga fluxes, evaluated their compositions through electron microprobeanalysis and studied their transport properties through measurements on the temperaturedependent resistivity, thermopower and Hall coefficient. Crystals grown in Sn fluxshow n-type carriers and those from Ga flux show p-type carriers, whereas allmeasured compositions remain very close to the stoichiometric 8:16:30 proportion ofBa:Ga:Sn, expected from charge-balance principles. Our results indicate a very highsensitivity of the charge carrier nature and density with respect to the growthconditions, leading to relevant differences in transport properties which point tothe importance of tuning this material for optimal thermoelectric performance.

Growth and formation of InGaN and GaN nano-structures studied by STM

Growth and morphology of metal organic vapour phase epitaxy (MOVPE) deposited InGaN nano-islands and molecular beam epitaxy (MBE) grown GaN films on GaN(0001) template layers on sapphire substrates have been investigated using scanning tunneling microscopy. For MOVPE InGaN growth, the nucleation of self-organized nano-structures can be achieved by a careful choice of the growth temperature, the In partial pressure, the growth rate and V/III flux ratio. For growth at 650°C, large spiral disc-like islands are found, preferentially nucleating at GaN substrate defects. At 600°C, islands of smaller average size are observed. Lowering the In flux at this temperature, a homogeneous nucleation of small quantum dot like islands with a density of 1012/cm2 is found. For homoepitaxial MBE growth of thin GaN layers on GaN templates, a layer-by-layer growth mode is observed for Ga rich growth conditions. For growth at 750°C, an atomically resolved 4×4 surface reconstruction with a high defect density is found in the initial growth stage. However, subsequent growth at 790°C leads to the formation of one dimensional nanoclusters of about 3 nm lateral spacing. For GaN growth at a lower Ga-flux, a rougher surface morphology and three dimensional growth is observed. Independent on the Ga flux, one-dimensional nanostructures appear after prolonged growth at higher temperature, which are attribute to the impact of ions emerging from the N-plasma. [DOI: 10.1380/ejssnt.2006.90]

Magnetic and Optical Properties of Eu-doped GaN

ABSTRACTGaN films were doped with Eu to a concentration of ∼0.12 at. % during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were post-annealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622 nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3-5.4×10−7 Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7 Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼0.1-0.45 emu/cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all of our growth conditions but these cannot account for the observed magnetic properties.

A comparison of the structure and localized magnetism in Ce 2PdGa 12 with the heavy fermion CePdGa 6

Single crystals of Ce 2PdGa 12 have been synthesized in Ga flux and characterized by X-ray diffraction. This compound crystallizes in the tetragonal P 4 / nbm space group, Z = 2 with lattice parameters of a = 6.1040 ( 2 ) Å and c = 15.5490 ( 6 ) Å . It shows strongly anisotropic magnetism and orders antiferromagnetically at T N∼11 K. A field-induced metamagnetic transition to the ferromagnetic state is observed below T N. Structure–property relationships with the related heavy-fermion antiferromagnet CePdGa 6 are discussed.

Structural characterisation of zinc-blende Ga 1−xMn xN epilayers grown by MBE as a function of Ga flux

Zinc-blende Ga 1− x Mn x N epilayers grown by plasma-assisted molecular beam epitaxy as a function of Ga flux are assessed using a variety of structural characterisation techniques. The Ga:N ratio was found to have a dominant impact on the zinc-blende Ga 1− x Mn x N epilayer growth rate and resultant composition, morphology and microstructure. A maximum growth rate and an improved microstructure are associated with growth under slightly Ga-rich conditions. A reduced growth rate and enhanced Mn incorporation are associated with growth under slightly N-rich conditions. α-MnAs inclusions and voids extending into the GaAs buffer layer were observed in all cases, however they are considered not to affect the layer electrical and magnetic properties.

Uniformity and control of surface morphology during growth of GaN by molecular beam epitaxy

A “modulated growth” technique has been introduced to achieve large area uniformity and surface morphology control during rf plasma assisted molecular beam epitaxy of GaN. The technique consists of modulating the surface coverage of Ga through short-period shuttering of Ga, N, or Ga and N together (e.g., 50–100Å growth periods), to achieve alternating high and low surface coverages of Ga on the (0001) GaN surface. The periods of growth with high Ga flux provide saturation coverage of Ga (∼2.5 ML Ga plus Ga droplets) over the full wafer, while the subsequent growth with low Ga flux facilitates a time-averaged Ga flux which is just below the crossover for droplet formation at the growth temperature. The growth transients in the Ga droplet regime are necessary to maintain smooth, pit-free surface morphologies, and the subsequent growth with low Ga flux suppresses droplet buildup over time. The process is monitored in situ utilizing reflection high energy electron diffraction and line-of-sight quadrupole mass spectroscopy. Results are presented which demonstrate that this approach is an effective means to achieve uniform surfaces over both 1∕4 and 2in. wafer sizes during nonbonded growth. The effects of temperature nonuniformities were mitigated and a surface free of both pitting and droplets has been realized over large areas.

Ln5Co4Si14 (Ln: Ho Er, Tm, Yb): Silicides Grown from Ga Flux Showing Exceptional Resistance to Chemical and Thermal Attack.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Tb4FeGe8 Grown in Liquid Gallium: trans‐cis Chains from the Distortion of a Planar Ge Square Net.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effects of hydrogen during molecular beam epitaxy of GaN

We study the effect of introducing hydrogen gas through the RF plasma source during plasma-assisted molecular beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present. But, the critical Ga flux for this transition is increased by the presence of H, and for sufficiently high H pressure a new 2 × 2 surface structure that is believed to be H-terminated is observed. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching on the surface, from which we argue that H selectively bonds to surface step and/or kink sites. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of hydrogen on the morphology and electrical properties of GaN grown by plasma-assisted molecular-beam epitaxy

We study the effect of introducing hydrogen gas through the rf-plasma source during plasma-assisted molecular-beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present, although the critical Ga flux for this transition increases. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching (facetting) on the surface. Conductive atomic force microscopy reveals that leakage current through dislocation cores is significantly reduced when hydrogen is present during the growth.

RE5Co4Si14 (RE = Ho, Er, Tm, Yb): Silicides Grown from Ga Flux Showing Exceptional Resistance to Chemical and Thermal Attack

The compounds RE5Co4Si14 (RE = Ho, Er, Tm, and Yb) have been synthesized from a Ga flux and characterized by X-ray diffraction. They can also be made by direct combination of the elements with arc melt alloying. All crystallize in the Lu5Co4Si14 structure type in the monoclinic space group P21/c with lattice parameters: a = 12.3765(15) A, b = 7.8142(9) A, c = 7.7330(9) A, β = 98.867(2)°; a = 12.337(2) A, b = 7.7990(14) A, c = 7.7144(15) A, β = 98.85(3)°; a = 12.3140(16) A, b = 7.778(1) A, c = 7.705(1) A, β = 98.815(2)°; and a = 12.255(3) A, b = 7.7537(16) A, c = 7.6730(15) A, β = 98.95(3)° for Ho5Co4Si14, Er5Co4Si14, Tm5Co4Si14, and Yb5Co4Si14, respectively. The compounds are composed of a dense Si/Co framework that is assembled from [Co4Si10] slabs and [Si4] zigzag chains. Crystals of the title compounds proved to be remarkably resistant to thermal oxidative attack (up to 1000 °C) as well as chemical attack from concentrated mineral acids and bases. The origin of this behavior can be traced to the detai...

Tb4FeGe8 Grown in Liquid Gallium: Trans−Cis Chains from the Distortion of a Planar Ge Square Net

The ternary germanide Tb4FeGe8 was obtained from Ga flux reactions. The crystal structure studied with single-crystal X-ray diffraction revealed the existence of an orthorhombic average substructure (Cmcm, Z=1) with cell parameters a = 4.1118(14) A, b=15.844(5) A, and c=3.9885(15) A. The refinement [I > 2sigma(I)] converged to final residuals R1/wR2 = 0.0363/0.0893. The average structure (CeNiSi2-type) consists of a 3D [Fe1/4Ge2] framework where Ge atoms form a square net and Fe atoms reside alternatively above and below it with only 1/4 occupation probability. X-ray and electron diffraction studies showed a modulation in the Ge net. The modulated structure was refined based on a 4-fold monoclinic supercell (P2(1)/n) with parameters a = 5.7315(11) A, b = 15.842(3) A, c = 11.438(2) A, and beta = 91.724(4) degrees with R1/wR2 = 0.0643/0.1735 and uncovered a severe distortion of the Ge square net. The Ge atoms are displaced to form an array of cis-trans chains. The Ge-Ge distances within these chains are distinctively bonding, whereas those between the chains are nonbonding. Results of the electronic structure calculations and magnetic measurements are also reported. The structural distortions found in Tb4FeGe8 cast a doubt onto the correctness of many of the reported REM1-xGe2 disordered compounds and call for reinvestigation.

Continuous evolution of Ga adlayer coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

We present a study of the evolution of the Ga adlayer during plasma-assisted molecular-beam epitaxy of (0001) GaN as a function of both Ga flux and growth temperature. In situ quadrupole mass spectrometry was used to quantitatively determine the adsorbed Ga coverage by monitoring its subsequent desorption after GaN growth. Independent of the growth time, the Ga adlayer was found to form steady-state coverages that increase continuously from 0 to 2.5 monolayers when raising the Ga flux from N-rich to moderate Ga-rich growth conditions. At higher Ga fluxes or lower growth temperatures, macroscopic Ga droplets form on top of the Ga adlayer (Ga droplet regime). Based on the temperature dependency for the transition between the Ga adlayer and Ga droplet regime, we determined an apparent activation energy of 3.4eV, which is discussed with respect to previously reported values.

Large Area Cu(In,Ga)Se2 Films and Devices on Flexible Substrates Made by Sputtering

AbstractA reactive sputtering process was developed for the production of Cu(In,Ga)Se2 films on a moving stainless steel substrate, in simulation of the operation of a roll coater. Cu, In and Ga fluxes were provided through magnetron sputtering and were reacted in a flux of Se on the heated substrate. CdS films were deposited either by chemical bath deposition (CBD) or by sputtering. Devices of the type steel/Cr/Mo/CIGS/CdS/ZnO/Ag were completed by sputtering ZnO layers and by screen printing grid lines. We made devices with efficiency values above 9%. A uniformity study was performed on a CIGS film and on small area devices made from it. The target length was 12”. Targets of this size are expected to produce a uniformly thick deposit over a range of 6-8”. The film thickness was 2.54 μm over a range of 6” with a standard deviation ó of 0.04 μm. The film composition was uniform over a range of 16”. The values of Cu/III and Ga/III were 0.84 and 0.31, with ó values of 0.02 and 0.01, respectively. The efficiency of allsputtered devices was uniform over a range of 12”, well beyond the 6” wide range of constant CIGS film thickness. Their efficiency was 6.6% on average with ó=0.6%.

GaNAs(001) surface phases under growing condition

GaNAs∕GaAs(001) growth surfaces were investigated by reflectance difference (RD)∕anisotropy spectroscopy. In situ monitoring was carried out in a plasma-assisted molecular-beam epitaxy system during growth. In the growth temperature range of our experiment, the reflection high-energy electron diffraction pattern always showed a (2×4) reconstruction pattern, however, we observed three types of RD spectra. It was suggested that the GaNAs growth surface has three phases because RD spectra properly reflect surface electronic states. It is revealed that the phases are independent from nitrogen concentration, but dependent upon growth temperature and As4∕Ga flux ratio. In this article, we show the results of the RD observation, present a phase diagram, and discuss possible GaNAs growth surfaces.