Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

Coexistence properties of phase separation and CuPt-ordering in InGaAsP grown on GaAs substrates by organometallic vapor phase epitaxy

CuPt-ordering and phase separation were directly investigated in In_1-xGa_xAs_yP_1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.     

Influence of substrate temperature on nitridation of (0 0 1) GaAs using RF-radical source

The mechanism of nitridation of (0 0 1) GaAs surface using RF-radical source was systematically studied with changing substrate temperature, nitridation time and supplying As molecular beam. It was found from atomic forth microscopy (AFM) measurements that supplying As is very important to suppress the re-evaporation of As atoms and to keep the surface smooth. Reflection high-energy electron diffraction (RHEED) measurements shows that surface lattice constant (SLC) of GaAs of 0.565 nm decreases with increasing the substrate temperature and that it finally relaxes to the value of c-GaN of 0.452 nm, at 570 °C in both [1 1 0] and [1¯ 1 0] directions without concerning with the supply of As molecular beam. But, in the medium temperature range (between 350 and 520 °C), SLC of [1 1 0] direction was smaller than that of [1¯ 1 0] direction. This suggests a relation between the surface structure and the relaxing mechanism of the lattice. The valence band discontinuity between the nitridated layer and the GaAs layer was estimated by using X-ray photoemission spectroscopy (XPS). It was between 1.7 and 2.0 eV, which coincides well with the reported value of c-GaN of 1.84 eV. This suggests that the fabricated GaN layer was in cubic structure.     

Structure and growth morphology of Gd2O3-doped CeO2 thin films

Gd₂O₃-doped CeO₂ (Gd_0.1Ce_0.9O_1.95, GDC) thin films were synthesized on (1 0 0) Si single crystal substrates by a reactive radio frequency magnetron sputtering technique. Structures and surface morphologies were characterized by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and one-dimensional power spectral density (1DPSD) analysis. The XRD patterns indicated that, in the temperature range of 200–700 °C, f.c.c. structured GDC thin films were formed with growth orientations varying with temperature—random growth at 200 °C, (2 2 0) textures at 300–600 °C and (1 1 1) texture at 700 °C. GDC film synthesized at 200 °C had the smoothest surface with roughness of R_rms=0.973 nm. Its 1DPSD plot was characterized with a constant part at the low frequencies and a part at the high frequencies that could be fitted by the f^−2.4 power law decay. Such surface feature and scaling behavior were probably caused by the high deposition rate and random growth in the GDC film at this temperature. At higher temperatures (300–700 °C), however, an intermediate frequency slope (−γ₂≈−2) appeared in the 1DPSD plots between the low frequency constant part and the high frequency part fitted by f⁻⁴ power law decay, which indicated a roughing mechanism dominated by crystallographic orientation growth that caused much rougher surfaces in GDC films (R_rms>4 nm).     

Growth of InxGa1−xN quantum dots by nitridation of nano-alloyed droplet method using MOCVD

In_xGa_1−xN quantum dots (QDs) were grown on GaN/sapphire (0 0 0 1) substrates by employing nitridation of nano-alloyed droplet (NNAD) method using metal-organic chemical vapor deposition (MOCVD). In+Ga alloy droplets were initially formed by flowing the precursors TMIn and TMGa. Density of the In+Ga alloy droplets was increased with increasing precursors flow rate; however, the droplet size was scarcely changed in the range of about 100–200 nm. Two cases of In_xGa_1−xN QDs growth were investigated by varying the nitridation time and the growth temperature. It was observed that the In_xGa_1−xN QDs size can be easily changed by controlling the nitridation process at the temperature between 680 and 700 °C for the time of 5–30 min. Self-assembled In_xGa_1−xN QDs were successfully grown by employing NNAD method.     

Ga assisted oxide desorption on GaAs(0 0 1) studied by scanning tunnelling microscopy

Native oxide removal on GaAs wafers under conventional thermal desorption causes severe surface degradation. Recently a new method of Ga assisted oxide removal has reported improved initial surface conditions. A precise dosing of Ga is required to optimise the oxide removal, however the effects of alternate temperatures on the desorption process effects the reaction kinetics. By using selected bias imaging, scanning tunnelling microscopy (STM) can be used to probe the underlying bulk whilst the native oxide is still present. Hence the effects of oxide removal on the surface can be identified during the native oxide desorption. By comparing Ga assisted oxide removal on both vicinal and off cut samples, the Ga adatom kinetics are shown to underpin the oxide removal process and a sample temperature in excess of 500 °C is necessary to optimise the procedure.     

Hydrothermal crystal growth of fresnoite

The growth of fresnoite, Ba₂TiSi₂O₈, by hydrothermal synthesis has led to spontaneous generation of large, (4-5 mm) optically clear crystals from 6 M KF mineralizer solutions. Growth was achieved at relatively low synthesis temperatures (575 °C) comparative to fresnoite synthesis by Czochralski or flux methods. Bulk crystal growth possibilities were explored by transport reactions performed in both fluoride and hydroxide mineralizers with 25-45 °C temperature gradients. Growth rates of 0.14×0.19×0.22 mm³/week were established in 6 M KOH, which is significantly slower than standard hydrothermal rates of 1 mm/week. Although relatively slow, the hydrothermal method has been demonstrated as a synthesis route to high quality single crystals of fresnoite.     

Effect of arsenic species on the kinetics of GaAs nanowires growth by molecular beam epitaxy

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As₂ or As₄ molecules. In a wide range of growth temperatures, As₄ leads to growth rates twice faster than As₂. The shape of the NWs also depends on the arsenic species: with As₄, regular rods can be obtained, while pencil-like shape results from growth with As₂. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As₂ flux as compared to that under As₄ flux. It follows that As₂ flux is favourable to the formation of radial heterostructures, whereas As₄ flux is preferable to maintain pure axial growth.     

A low silica, barium borate glass-ceramic for use as seals in planar SOFCs

A low silica, barium borate glass-ceramic for use as seals in planar SOFCs containing 64 mol%BaO, 3 mol%Al₂O₃ and 3 mol%SiO₂ was studied. Coefficient of thermal expansion (CTE) between 275-550 °C, glass transition temperature (T_g), and dilatometric softening point (T_s) of the parent glass were 11.9 × 10⁻⁶ °C⁻¹, 552 °C, and 558 °C, respectively. Glass-ceramic was produced by devitrification heat treatment at 800 °C for 100 h. It was found that nucleation heat treatment, seeding by 3 wt%ZrO₂ as glass-composite and pulverization affected the amount, size and distribution of crystalline phases. SEM-EDS and XRD results revealed that crystalline phases presented in the devitrified glass-ceramic were barium aluminate (BaAl₂O₄), barium aluminosilicate (BaAl₂Si₂O₈) possibly with boron associated in its crystal structure, and barium zirconate (BaZrO₃). CTE of the devitrified glass-ceramic was in the range of (10.1-13.0) × 10⁻⁶ °C⁻¹. Good adhesion was obtained both in the cases of glass and devitrified glass-ceramic with YSZ and AISI430 stainless steel. Interfacial phenomena between these components were discussed.     

Preparation of anhydrous dicalcium phosphate, DCPA, through sol-gel process, identification and phase transformation evaluation

Anhydrous dicalcium phosphate, CaHPO₄, DCPA, as a member of the series of calcium phosphates, has considerable biological importance to the biomineralization processes in bones and teeth, and has found practical applications in dental cements and restorative materials. This compound, can reconstruct hard tissues because of high solubility in vivo in comparison with many of the other calcium phosphate compounds. In this work, Ca-deficient DCPA was synthesized through sol-gel process as a new method. Ca(NO₃)₂ · 4H₂O and H₃PO₄ were employed as precursors and identified by XRD, FTIR and SEM spectroscopic techniques. The later technique makes the elemental analysis possible. The measured Ca/P molar ratio suggests, Ca_1−x(H₂PO₄)_2x(H₂PO₄)_1−2x (x = 0.02, 0.04 and 0.07) formula for the synthesized compounds. The effect of aging time and heat treatment (sintering stage) on phase purity and phase transformations of products were systematically studied. The minimum necessary aging time for Ca-deficient DCPA preparation was 24 h. From X-ray patterns of powders sintered at different temperatures, the sintering temperature of Ca-deficient DCPA was confirmed to be 300 °C. According to the experimental results, varying the temperature from 300 to 1000 °C gave different stable phases as products: Ca-deficient DCPA at 300 °C, hydorxyapatite (Ca₅(PO₄)₃OH, HA) at 600 °C and β-tri calcium phosphate (β-Ca₃(PO₄)₂, β-TCP) at 1000 °C.     

Surface reconstructions and growth mode transitions of AlAs(100)

The surface reconstructions of AlAs(100) layers grown by molecular beam epitaxy (MBE) on GaAs(100) were mapped as a function of substrate temperature and arsenic flux. Three main reconstructions were observed - a c(4×4) at lower temperatures and higher arsenic fluxes, a (2×4) at middle temperatures, and a (3×2) at higher temperatures and lower arsenic fluxes. Growth of AlAs on AlAs(100) is layer-by-layer for the high temperature and low temperature reconstructions. In the mid-temperature region, AlAs grows rough on (2×4) reconstructed AlAs(100) as indicated by rapidly damped reflection high-energy electron diffraction (RHEED) intensity oscillations and the appearance of three-dimensional (3D) features. The addition of fractional layers of Ga enhances the smooth growth of AlAs. A metastable (5×2) reconstruction was observed when a fraction of a layer of Ga was present on the surface. The results indicate that Ga segregates during the growth of AlAs on GaAs(100) at temperatures at least as low as 500°C, and that annealing at temperatures above 700°C removes most of the Ga from the surface.     

Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500 °C by reactive rf sputtering of GaAs target in argon-nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAs_xN_1−x (x ≈ 0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.     

MOVPE growth of InN films using 1,1-dimethylhydrazine as a nitrogen precursor

InN films have been successfully grown on sapphire substrates by MOVPE using trimethylindium (TMIn) and 1,1-dimethylhydrazine (DMHy) with N₂ carrier. DMHy is an advantageous precursor of N as it decomposes efficiently at relatively low temperature (T₅₀=420 °C) compatible with the InN growth. The reactor is specially designed so as to avoid parasitic reaction between TMIn and DMHy occurring at room temperature. The growth feature was studied by varying growth temperature, V/III ratio, TMIn flow and reactor pressure. The InN films were obtained at 500–570 °C and 60–200 Torr with a V/III ratio optimized to 100–200. The In droplets are seen on the grown surfaces, indicating an excess supply of TMIn. It is demonstrated that the InN films grows on the sapphire substrate in a single domain with an epitaxial relationship, [1 01¯ 0]_InN//[1 1 2¯ 0]_sapphire.     

The growth of GaAs and InAs dots on etched mesas: The effect of substrate temperature on mesa profile and surface morphology on dot distribution

The molecular beam epitaxy (MBE) growth of GaAs and InAs quantum dots on etched mesas has been studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The [0 1 1]-oriented mesas are etched into (1 0 0) GaAs substrates, exposing (5 3 3)B sidewall facets. At a substrate temperature of 610 °C a top (1 0 0) plane is seen to evolve on a ridge mesa structure. Alternatively, if the overgrowth is carried out at 630 °C no such facet is seen, and the top ridge remains unchanged during GaAs growth. By controlling the mesa shape, either ordered lines of dots can be grown or the dot density can be varied from <5×10⁸ cm⁻² to >1×10¹¹ cm⁻² on the same substrate in pre-defined regions. The dot distribution observed on the mesa sidewalls and top is discussed in terms of net migration of adatoms from different facets, underlying step density, step height and surface curvature of the mesa top.     

Fabrication and characterization of GaAs quantum well buried in AlGaAs/GaAs heterostructure nanowires

We developed a growth method for forming a GaAs quantum well contained in an AlGaAs/GaAs heterostructure nanowire using selective-area metal organic vapor phase epitaxy. To find the optimum growth condition of AlGaAs nanowires, we changed the growth temperature between 800 and 850 °C and found that best uniformity of the shape and the size was obtained near 800 °C but lateral growth of AlGaAs became larger, which resulted in a wide GaAs quantum well grown on the top (1 1 1)B facet of the AlGaAs nanowire. To form the GaAs quantum well with a reduced lateral size atop the AlGaAs nanowire, a GaAs core nanowire about 100 nm in diameter was grown before the AlGaAs growth, which reduced the lateral size of AlGaAs to roughly half compared with that without the GaAs core. Photoluminescence measurement at 4.2 K indicated spectral peaks of the GaAs quantum wells about 60 meV higher than the acceptor-related recombination emission peak of GaAs near 1.5 eV. The photoluminescence peak energy showed a blue shift of about 15 meV, from 1.546 to 1.560 eV, as the growth time of the GaAs quantum well was decreased from 8 to 3 s. Transmission electron microscopy and energy dispersive X-ray analysis of an AlGaAs/GaAs heterostructure nanowire indicated a GaAs quantum well with a thickness of 5−20 nm buried along the 〈1 1 1〉 direction between the AlGaAs shells, showing a successful fabrication of the GaAs quantum well.     

Growth of embedded ErAs nanorods on (4 1 1)A and (4 1 1)B GaAs by molecular beam epitaxy

The low solubility of Er in GaAs results in the formation of ErAs nanostructures when GaAs is grown with 5–6 at% Er/Ga ratio by molecular beam epitaxy on GaAs surfaces. For growth on the (4 1 1)A GaAs surface, cross-sectional scanning transmission electron microscopy images show the presence of ErAs nanorods embedded in a GaAs matrix extending along the [2 1 1] direction with a spacing of roughly 7 nm and a diameter of roughly 2 nm. Growth on the GaAs (4 1 1)B surface resulted in only nanoparticle formation. Variation of the polarized optical absorption with in-plane polarization angle is consistent with coupling to surface plasmon resonances of the semimetallic nanostructures.     

Hydride-assisted growth of GaN nanowires on Au/Si(0 0 1) via the reaction of Ga with NH3 and H2

High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 μm have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH₃ and N₂:H₂ at 900 °C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH₃, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of β-Ga₂O₃ NWs with diameters of ≤50 nm and lengths up to 10 μm were obtained, however, via the reaction of Ga with residual O₂ under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H₂ dissociates on the Au nanoparticles and reacts with Ga giving Ga_xH_y thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH₃ near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer β-Ga₂O₃ NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation.     

Ga and In incorporation rates in Ga1−xInxAs growth by chemical beam epitaxy

GaAs, InAs and Ga_1?xIn_xAs 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?xIn_xAs 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.     

Formation of GaN nanodots on Si (1 1 1) by droplet nitridation

GaN nanodots (NDs) are obtained by Ga metallic droplet formation on Si (1 1 1) substrates followed by their nitridation. The size and density of Ga droplets and GaN NDs can be controlled by varying the growth temperature within the range 514–640 °C. Atomic force microscopy (AFM) investigation of Ga droplets shows an increase in the average diameter with temperature. The average diameter of GaN NDs increases with growth temperature while their density decreases more than one order of magnitude. In addition, the formation of a GaN crystallite rough layer on Si, in-between NDs, indicates that a spreading mechanism takes place during the nitridation process. High-resolution transmission electron microscopy (HRTEM) is used for the investigation of shape, crystalline quality and surface distribution of GaN dots. X-ray photoelectron spectroscopy (XPS) results confirm that Ga droplets that are transformed into GaN NDs spread over the sample surface during nitridation.     

Crystal growth of KInO2 from a hydroxide flux

Single crystals of KInO₂ were obtained from a reactive potassium hydroxide flux at 700 °C. KInO₂ crystallizes in the R-3m crystal system with a=3.2998(10) Å, c=18.322(10) Å and V=172.78(12) Å³. The crystal structure is isotypic with that of α-NaFeO₂ and consists of the (1 1 1) layers being occupied alternately by KO₆ and InO₆ octahedra. Three different AInO₂ structure types are discussed.