Growth Condition Dependence Research Articles

Study on chemical synthesis of SnSSe nanosheets and nanocrystals

Two kinds of raw material combinations for the hot injection method were investigated for the chemical synthesis of SnSSe nanosheets and nanocrystals, which are low-toxic optoelectronic materials. When SnSe quantum dots were synthesized by mainly using oleic acid as the Se precursor solvent, the quantum dots changed from spherical to cubic as the size increased. The growth condition dependence of the nanocrystal formation process was discussed. When SnSSe nanocrystals were synthesized by mainly using trioctylphosphine as the S Se precursor solvent, it was found that the nanocrystal shape changed from dot to rod or sheet by reducing the proportion of S. The bandgap energy did not simply depend on the composition ratio of S but was affected by the change in the nanocrystal shape depending on the quantum confinement effect.

Effect of step edges on the adsorption behavior on vicinal AlN(0001) surface during metal-organic vapor phase epitaxy: An ab initio study

We present systematic theoretical investigations for the adsorption behavior of Al and N adatoms on vicinal AlN(0001) surface under metal–organic vapor phase epitaxy (MOVPE) growth condition by means of ab initio electronic structure calculations. Prior to the investigation adatoms, we reveal the growth condition dependence of stable structure for bilayer step edges during the MOVPE. The vicinal surface with hydrogen terminated N atom and NH2 (N-H + Al-NH2) is found to be the most stable under N-rich limit whereas the surface with hydrogen terminated N atom (N-H + Al-H) with NH2 at the step edge is favorable over the wide range of Al chemical potential. This suggests that N-H + Al-NH2 and N-H + Al-H with NH2 at the step edge appear at high and low temperatures, respectively. For both surface models, we also elucidate that the adsorption energy of Al adatoms at the step edge is much lower than that in the terrace region, indicating that Al adatoms are easily incorporated into the step edge. The estimated values of Ehrlich-Schwöbel barrier (ESB) of Al adatoms for N-H + Al-NH2 and N-H + Al-H with NH2 at the step edge are −1.4 and −1.9 eV, respectively. The relationship between the adsorption behavior and surface morphology during the MOVPE is discussed on the basis of calculated ESB as well as adsorption energies and migration barriers.

Growth condition dependence of unintentional oxygen incorporation in epitaxial GaN

Growth conditions have a tremendous impact on the unintentional background impurity concentration in gallium nitride (GaN) synthesized by molecular beam epitaxy and its resulting chemical and physical properties. In particular for oxygen identified as the dominant background impurity we demonstrate that under optimized growth stoichiometry the growth temperature is the key parameter to control its incorporation and that an increase by 55 °C leads to an oxygen reduction by one order of magnitude. Quantitatively this reduction and the resulting optical and electrical properties are analyzed by secondary ion mass spectroscopy, photoluminescence, capacitance versus voltage measurements, low temperature magneto-transport and parasitic current paths in lateral transistor test structures based on two-dimensional electron gases. At a growth temperature of 665 °C the residual charge carrier concentration is decreased to below 1015 cm−3, resulting in insulating behavior and thus making the material suitable for beyond state-of-the-art device applications.

Growth condition dependence of photoluminescence polarization in (100) GaAs/AlGaAs quantum wells at room temperature

We conducted systematic measurements on the carrier lifetime (τc), spin relaxation time (τs), and circular polarization of photoluminescence (Pcirc) in (100) GaAs/AlGaAs multiple quantum wells grown by molecular beam epitaxy (MBE). The τc values are strongly affected by MBE growth conditions (0.4–9 ns), whereas the τs are almost constant at about 0.13 ns. The result suggests that spin detection efficiency [τs/(τc + τs)], which is expected to be proportional to a steady-state Pcirc, is largely dependent on growth condition. We confirmed that the Pcirc has similar dependence on growth condition to those of τs/(τc + τs) values. The study thus indicates that choosing the appropriate growth condition of the QW is indispensable for obtaining a high Pcirc from a spin-polarized light-emitting diode (spin-LED).

Thickness and growth condition dependence of crystallinity in semipolar (20–21) GaN films grown on (22–43) patterned sapphire substrates

We have investigated the crystalline morphology such as lattice plane curvature and lattice plane tilting in (20–21) GaN films on (22–43) patterned sapphire substrates (PSS) by using X‐ray rocking curve (XRC) measurements. The results of the symmetric XRC for the GaN (20–21) plane showed anisotropic lattice plane curvature between the [−1014] and [−12–10] directions. This is due to the large difference of the thermal expansion coefficient in the [0001] direction between GaN and sapphire. From the results of asymmetric XRC for the GaN (20–22) and (12–31) diffractions, it is found that lattice plane tilting is highly dependent on the film thickness and the growth condition. The −c‐plane‐suppressed growth process with tuning the V/III ratio for the (20–21) GaN films on the (22–43) PSS is very effective to suppress the generation of basal plane stacking faults as confirmed by TEM analysis.

Thickness and growth-condition dependence of in-situ mobility and carrier density of epitaxial thin-film Bi2Se3

Bismuth selenide Bi2Se3 was grown by molecular beam epitaxy, while carrier density and mobility were measured directly in situ as a function of film thickness. Carrier density shows high interface n-doping (1.5 × 1013 cm−2) at the onset of film conduction and bulk dopant density of ∼5 × 1011 cm−2 per quintuple-layer unit, roughly independent of growth temperature profile. Mobility depends more strongly on the growth temperature and is related to the crystalline quality of the samples quantified by ex-situ atomic force microscopy measurements. These results indicate that Bi2Se3 as prepared by widely employed parameters is n-doped before exposure to atmosphere, the doping is largely interfacial in origin, and dopants are not the limiting disorder in present Bi2Se3 films.

In situ measurement of CuPt alloy ordering using strain anisotropy

The optical and electrical properties of many III-V alloys change with the degree of CuPt atomic ordering, which is very sensitive to growth conditions. The bulk ordered alloy is elongated along the normal to the ordered planes, and is asymmetrically strained when coherent to a cubic substrate. Here, we demonstrate in situ measurement of the anisotropic strain due to ordering using two-dimensional wafer curvature. The measurement is sensitive to bulk anisotropies, and so is complementary to other in situ measurements that are sensitive to surface anisotropies. Using ab initio calculations, we determine a maximum strain anisotropy of 0.27% between [110] and [1¯10] when perfectly ordered single-variant GaInP2 is coherent to a (001) cubic substrate. We relate the in situ measurement of strain anisotropy on various GaInP2 samples to ex situ measurements of the order parameter to validate the measurement and confirm the capability to predict material properties. The measurement monitors change in ordering during growth, useful for quickly determining the growth condition dependence of ordering or monitoring order-disorder transitions. More generally, this measurement technique could, in principle, be used to monitor phase changes in any epitaxial system for which the strain anisotropy of the two phases differs.

Unintentional Ga incorporation in metalorganic vapor phase epitaxy of In-containing III-nitride semiconductors

We prepared InAlN barrier layer films on GaN buffer layers using the metalorganic vapor phase epitaxy (MOVPE) method and investigated the InAlN/GaN heterointerfaces. Secondary ion spectroscopy experiments revealed that a quaternary alloy of InAlGaN is grown on GaN even when trimethylindium (TMIn) and trimethylaluminum (TMAl) are exclusively supplied as group-III precursors, indicating that Ga is unintentionally incorporated into the InAlN layers. This Ga incorporation is also observed in InGaN/GaN heterostructures. Our systematic investigations of the growth condition dependence, such as the TMIn flow rate, indicate that the Ga is supplied by a transmetalation reaction between TMIn and residual Ga on the flow distributor in the reactor. Here, we show that the Ga incorporation can be eliminated by adopting an elaborate growth sequence, including reactor cleaning and regrowth processes. This study provides guides for designing the MOVPE reactor configuration, as well as the growth sequences, for the growth of device structures with In-containing nitride layers.

The growth of low resistivity, heavily Al-doped 4H–SiC thick epilayers by hot-wall chemical vapor deposition

Heavily Al-doped 4H–SiC thick epilayers (∼90μm) were grown on 3-in n+ 4H–SiC wafers by using the hot-wall CVD method. On the purpose of enhancing incorporated Al-dopant concentration, the growth condition dependence of the Al incorporation behavior in the heavy doping range near Al solubility limit in 4H–SiC was investigated by varying the growth parameters, i.e., growth rate, pressure, temperature and Al-dopant source flow rate. A series of thick epilayers possessing Al-dopant concentration from 9.6×1019 to 4.7×1020cm−3 were obtained. Among them, the epilayer with Al-dopant concentration of 3.5×1020cm−3 demonstrates a comparably low resistivity of 16.5mΩcm as that of commercial n+ 4H–SiC wafer. The incorporated Al-dopant concentration dependences on surface morphology, crystalline quality and crystal structures of the heavily Al-doped thick epilayers on n+ 4H–SiC substrates were characterized and discussed.

Syntheses and structural characterizations of CrSi2 nanostructures using Si substrates under CrCl2 vapor

Chromium disilicide (CrSi2) nanostructures were grown by the exposure of Si substrates to CrCl2 vapor at atmospheric pressure, and the growth condition dependence on the morphological and structural properties of CrSi2 was systemically investigated. The various structures of CrSi2, such as microrods, nanowire bundles along with microrods and dendritic nanowire structures, have been grown. It was found that the Si substrate temperature and the quantity of the CrCl2 source materials significantly affect the morphological and structural properties of the CrSi2 structures. The structural evolution of CrSi2 has been discussed on the basis of thermodynamic reactions, vapor deposition growth and dendritic growth phenomena. This growth process provides a simple and controllable method to grow novel nano-scaled structures of transition metal silicides for technological use.

Growth condition dependence of Ge-doped β-FeSi2 epitaxial film by molecular beam epitaxy

We have investigated the growth condition of Ge-doped β-FeSi2 on Si(111) substrates by molecular beam epitaxy (MBE). By the optimization of growth temperature (Ts) and Si/Fe flux ratio, the β-Fe(Si1−xGex)2 with a flat surface was grown at Ts=500°C and Si/Fe=0.5, which were different from the optimized condition of the undoped β-FeSi2 (Ts=670°C, Si/Fe=2.0). In the dependence of lattice constants on Ge concentration, all the lattice constants expanded in the range of x=0–11%. In the direct transition energies (Eg) measured by photoreflectance measurements, the Eg decreased with the increase of Ge concentration. These results revealed that β-Fe(Si1−xGex)2 was successfully grown on Si(111) substrate in the range of x=0−11%.

Growth condition dependence of direct bandgap in β-FeSi2 epitaxial films grown by molecular beam epitaxy

Direct bandgap energy (Eg) and lattice deformations were investigated in β-FeSi2 epitaxial films grown by molecular beam epitaxy (MBE) with different growth condition. As Si/Fe flux ratio during the MBE growth became smaller than Si/Fe = 2.0, the lattice constants deviated from those of β-FeSi2 single crystal, which indicated an enhanced lattice deformation at the lower Si/Fe ratio. In photoreflectance (PR) measurements, the PR spectra shifted to lower photon energy with the enhanced lattice deformation. These results revealed that the Eg of β-FeSi2 epitaxial film was modified by the lattice deformation depending on the growth condition.

Cd2Ge2O6 nanowires grown by a simple hydrothermal route

AbstractCd2Ge2O6 nanowires have been synthesized by a simple hydrothermal route in the absence of any surfactants. The diameter and length of the Cd2Ge2O6 nanowires with flat tips are 30‐300 nm and several dozens of micrometers, respectivley. X‐ray diffraction and high‐resolution transmission electron microscopy results show that the nanowires are composed of monoclinic Cd2Ge2O6 phase. The growth condition dependence results show that the formation of the Cd2Ge2O6 nanowires undergoes three morphological changes from spherical particles to nanorods, and finally to nanowires. The photoluminescence spectrum of the Cd2Ge2O6 nanowires exhibits three fluorescence emission peaks centered at 422 nm, 490 nm and 528 nm showing the potential application for optical devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Effects of Gallium Droplets on the Morphologies and Structures of <I>α</I>-Fe<SUB>2</SUB>O<SUB>3</SUB> Nanostructures Grown on Iron Substrates

Alpha-Fe2O3 nanowires and nanobelts were grown by the thermal oxidation of iron substrates with or without gallium droplets in the air. The nanowires and nanobelts show a bicrystal structure with the growth direction uniformly along [110]. The morphological and structural properties of the as-grown alpha-Fe2O3 nanostructures are described and the growth condition dependence of the alpha-Fe2O3 nanostructures is shown. The transformation from nanowires to nanobelts occurs with the increase of growth temperature and addition of gallium. In addition, the growth evolution is investigated with reference to the Fe surface diffusion and supersaturation.

Growth condition dependence of structural and electrical properties of Mg 2Si layers grown on silicon substrates

Mg 2Si layers were grown on Si substrates by thermal treatment of the substrates in a Mg vapor, and the growth condition dependence of the structural and electrical properties of the layers was investigated. The layers were grown by an interdiffusion process between the deposited Mg atoms and the substrates. The structural and electrical properties of the layers depend on the combinations of the Si substrate and Mg source temperatures during the heat treatment. Any deviation from the isothermal treatment conditions causes degradation of the structural and/or electrical properties of the Mg 2Si layers. It was confirmed that the layers with the optimum structural and electrical properties were obtained when the layers were grown under isothermal growth conditions.

Crystal growth condition dependence of local structure around Gd in GaN nanotods

Crystal growth condition dependence of local structure around Gd in GaN nanotods

Growth condition dependence of zinc oxide nanostructures on Si substrates in an electrochemical process

Zinc oxide nanostructures were synthesized in an aqueous solution of hexamine and zinc nitrate by an electrochemical process. The effects of growth conditions, including electrical potential, growth temperature and template size, on morphology and composition of the nanostructures were systematically investigated. A negative potential enhanced the growth of single crystalline ZnO nanowire arrays while a positive potential caused nano-disks of ZnO and ZnO2 composites to grow on the substrate. The applied negative potential also helped room temperature growth of ZnO nanowires with a reduced growth rate. Similar growth behavior was observed on a bare substrate and that with pre-defined polymer template.

Metalorganic Vapor Phase Epitaxial Growth Parameter Dependence of Phase Separation in Miscibility Gap of InGaAsP

The generation mechanism of the immiscibility of InGaAsP grown by metalorganic vapor phase epitaxy (MOVPE) is investigated. The growth condition dependence of the immiscibility is examined in detail using particular photoluminescence (PL) characteristics observed in the boundary composition between stable and unstable compositional regions. A lower growth rate or a larger V/III ratio is effective for suppressing phase separation. To explain this growth condition dependence, some factors related to surface energy should be taken into consideration.

Growth condition dependence of spin-polarized electroluminescence in Fe∕MgO∕light-emitting diodes

We compared the electroluminescence (EL) polarization of two Fe∕MgO∕light-emitting-diode (LED) structures grown at different substrate temperatures for MgO growth: room temperature and 400°C. Two spin-LED wafers were prepared on molecular beam epitaxy grown LEDs by e-beam evaporation: one was LED∕MgO (RT)∕Fe (RT)∕Au cap (RT), and the other was LED∕MgO (400°C)∕Fe (150°C)∕Au cap (90°C). Spin-polarized EL was clearly observed in the latter sample, while the EL polarization was hardly observed in the former sample. The reasons for the near absence of EL polarization in the former sample are considered to be the degradation of the tunneling junction resulting from the crystallinity and the As-rich surface of the LED.

Area-Selective Epitaxial Growth of GaAs on GaAs(111)A Substrates by Migration-Enhanced Epitaxy

We have carried out area-selective epitaxial growth of GaAs on GaAs(111)A substrates using migration-enhanced epitaxy and have investigated the pyramidal hillocks formed on the grown surface. Scanning electron microscopy and atomic force microscopy observations indicate that these hillocks can be classified into two types: hillocks with small top plateaus and those with large plateaus. Investigations on the growth condition dependence of surface morphology and plan-view transmission electron microscopy observation have revealed that both types of hillocks are caused by stacking faults. Hillock cores are found to be stacking fault tetrahedra, whose formation processes result in two types of hillocks. Additionally, we have succeeded in improving the surface flatness by increasing the surface arsenic pressure during growth.