Usual Growth Temperature Research Articles

Low-temperature grown MnAs/InAs/MnAs double heterostructure on GaAs (111)B by molecular beam epitaxy

We used MBE to grow a double heterostructure of MnAs/InAs/MnAs on GaAs(111)B at low temperature for vertical spin FET applications. To confirm the ideal development environment, we primarily prepared single InAs thick layer (∼1.2 μm) at low growth temperature (∼250 °C) with varied V/III ratio (As/In = 2, 10, 20) due to the challenge of growing InAs at much lower temperature than its usual growth temperature (∼480 °C). We measured their structural and electrical properties and found an optimum condition at V/III ratio of 10. Afterwards, we prepared the double heterostructure at low temperature (∼250 °C), again varying the As/In beam equivalent pressure ratio to find its influence on the overall quality of the structure. Using atomic force microscopy, we observed the surface roughness variation corresponding to V/III ratio variation of InAs. We confirmed the growth of three individual thick layers of MnAs and InAs by cross-sectional analysis using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Using a superconducting quantum interference device magnetometer, we found in-plane easy magnetization and observed the effect of top and bottom MnAs layers on the hysteresis curve. We also found the existence of ferromagnetic behavior of MnAs layers at RT MH measurements. The MnAs/InAs/MnAs double heterostructure on GaAs(111)B, in our opinion, has potential as a structure for spin FETs.

Growth kinetics and substrate stability during high-temperature molecular beam epitaxy of AlN nanowires

We study the molecular beam epitaxy of AlN nanowires between 950 °C and 1215 °C, well above the usual growth temperatures, to identify optimal growth conditions. The nanowires are grown by self-assembly on TiN(111) films sputtered onto Al2O3. Above 1100 °C, the TiN film is seen to undergo grain growth and its surface exhibits {111} facets where AlN nucleation preferentially occurs. Modeling of the nanowire elongation rate measured at different temperatures shows that the Al adatom diffusion length maximizes at 1150 °C, which appears to be the optimum growth temperature. However, analysis of the nanowire luminescence shows a steep increase in the deep-level signal already above 1050 °C, associated with O incorporation from the Al2O3 substrate. Comparison with AlN nanowires grown on Si, MgO and SiC substrates suggests that heavy doping of Si and O by interdiffusion from the TiN/substrate interface increases the nanowire internal quantum efficiency, presumably due to the formation of a SiN x or AlO x passivation shell. The outdiffusion of Si and O would also cause the formation of the inversion domains observed in the nanowires. It follows that for optoelectronic and piezoelectric applications, optimal AlN nanowire ensembles should be prepared at 1150 °C on TiN/SiC substrates and will require an ex situ surface passivation.

Step-flow growth of Al droplet free AlN epilayers grown by plasma assisted molecular beam epitaxy

We have investigated an Al modulation epitaxy (AME) method to obtain step-flow growth of Al droplet free AlN layers by plasma assisted molecular beam epitaxy (MBE). At the usual growth temperature of (Al)GaN/AlN heterostructures, Al atom migration and desorption rate are very low and consequently it is very difficult to avoid the formation of Al droplets on AlN growth front by conventional MBE growth method. By adopting the AME growth method, such a difficulty has been effectively overcome and step flow growth mode of AlN has been clearly observed. By optimizing the AME growth time sequence, namely, AlN growth time and N radical beam treatment time, Al droplet free AlN layers with step flow growth characteristics have been obtained, with atomic flat surfaces and an average atomic step width of ∼118 nm at 970 °C–1000 °C, which is still suitable to grow (Al)GaN/AlN heterostructures by MBE.

Identification of a large amount of excess Fe in superconducting single-layer FeSe/SrTiO3 films

The single-layer FeSe films grown on SrTiO3 (STO) substrates have attracted much attention because of its record high superconducting critical temperature (Tc). It is usually believed that the composition of the epitaxially grown single-layer FeSe/STO films is stoichiometric, i.e., the ratio of Fe and Se is 1:1. Here we report the identification of a large amount of excess Fe in the superconducting single-layer FeSe/STO films. By depositing Se onto the superconducting single-layer FeSe/STO films, we find by in situ scanning tunneling microscopy (STM) the formation of the second-layer FeSe islands on the top of the first layer during the annealing process at a surprisingly low temperature ($\sim$150{\deg}C) which is much lower than the usual growth temperature ($\sim$490{\deg}C). This observation is used to detect excess Fe and estimate its quantity in the single-layer FeSe/STO films. The amount of excess Fe detected is at least 20% that is surprisingly high for the superconducting single-layer FeSe/STO films. The discovery of such a large amount of excess Fe should be taken into account in understanding the high-Tc superconductivity and points to a likely route to further enhance Tc in the superconducting single-layer FeSe/STO films.

A Theoretical Investigation of the Miscibility and Structural Properties of InxAlyGa1−x−yN Alloys

In this theoretical investigation of the miscibility in quaternary alloys, we stay in the regular solution model and determine the interaction parameters from the enthalpy of mixing calculated using a modified Stillinger–Weber (SW) potential for III‐nitride ternary alloys. From our calculation, the values of the interaction parameters are 6.605, 10.523, and 0.677 kcal mol−1, respectively, for InGaN, InAlN, and AlGaN at x = 0.5 (where x is the In and Al fraction in InGaN, InAlN, and AlGaN, respectively). These values are in good agreement with those predicted by the improved delta lattice parameter (DLP) model for nitride ternary alloys. To determine the unstable regions characterized by the spinodal decomposition of the quaternary alloy InxAlyGa1−x−yN, we consider the composition ranges of 0.06 ≤ x ≤ 1 and 0 ≤ y ≤ 0.94. Our calculations show that the phase separation temperature increases with the increase of indium concentration. For In concentration lower than 9.4%, the quaternary alloy is miscible independent of the Al and Ga compositions in the usual growth temperature range for metalorganic vapor phase epitaxy.

Influence of structure and thermodynamic stability on electronic properties of two-dimensional SiC, SiGe, and GeC alloys

The energetics and thermodynamic properties of two-dimensional binary graphene-like alloys made from graphene, silicene, or germanene are investigated by combining first-principles total energy calculations, and a statistical approach to account for disorder and composition effects. For the electronic properties the calculations are performed within the GGA-1/2 approach for an approximate quasiparticle bands. We derive lattice constants, first-neighbor distances, and buckling parameters as a function of composition $x$. The ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ system is the only stable random alloy at usual growth temperatures. For ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$, we observe strong distortions of the lattice making the random configurations less favorable and leading to a pronounced tendency for phase separation. The situation for ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ alloys is completely different. An ordered structure with composition $x=0.5$ is stable up to $T\ensuremath{\approx}1000$ K, while intermediate compositions are mainly realized by silicongraphene and graphene or silicene. The ordering and decomposition effects have a strong influence on the average fundamental energy gap versus composition. Whereas large gaps appear for ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ systems they almost vanish for ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ and ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$. Moreover, the dependence of the ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ energy gap on growth temperature is also obtained. The results can be very useful for chemical vapor deposition growth of these materials.

Cold-tolerant strain of Haematococcus pluvialis (Haematococcaceae, Chlorophyta) from Blomstrandhalvøya (Svalbard)

A new cold-adapted Arctic strain of Haematococcus pluvialis from Blomstrandhalvya Island (Svalbard) is described. This strain is predominantly always in non-motile palmelloid stage. Transmission electron microscopy showed the presence of very thick cell wall and abundant lipid vesicles in the palmelloids, including red and green cells. The external morphology of the non-motile palmelloid and motile bi-flagellated cells of our strain is similar to H. pluvialis; however it differs from H. pluvialis in physiology. Our strain is adapted to live and produce astaxanthin in the low temperature (), whilst the usual growth temperature for H. pluvialis is between . Phylogenetic analysis based on 18S rRNA gene data showed that our strain nested within the Haematococcus group, forming a sister relationship to H. lacustris and H. pluvialis, which are considered synonymous. Therefore, we identified our Arctic strain as H. pluvialis.

Reactor dependent starting transients of doping profiles in MOVPE grown GaN

The quality of heterostructures greatly depends on the accurate placement of dopants. However, doping transients remain as drawback in vapor-phase epitaxy. In this work starting transients of doping profiles of Si and Mg in GaN layers grown by low pressure metal-organic vapor-phase epitaxy (MOVPE) on sapphire have been investigated. Here we restrict the investigation to concentrations typically used for electrical or electro-optical devices which are in the range of 2×10 17 cm −3 to 1×10 19 cm −3 for Si and 3×10 19 cm −3 to 1×10 20 cm −3 for Mg. In the case of Mg possible diffusion was excluded by growing the doped layers at a lower growth temperature, far too low for device structures, where the profiles show essentially no sign of Mg back-diffusion. In the case of lower Si concentrations we did not observe diffusion at usual growth temperatures. The profiles were measured by secondary ion mass spectrometry. By comparing the transients from two different reactor types we show and quantitatively explain the influence of adsorption at the internal surfaces of the reactor on the dopant incorporation.

Co‐ordination alignments at the vicinity of the dopant Cr ions in AlN

AbstractLocal configuration around the Cr ion site in AlCrN is surveyed by the analysis of X‐ray absorption fine structure at the Cr K‐edge. It is found that the high concentration doping of the Cr ions at usual growth temperature (700 °C) for GaN lets the Cr ions to segregate as Cr metal nano‐clusters, which is different from the ordinary second phase found in GaCrN and InCrN with rather high concentration of the dopant, where the Cr ions are nucleated in the form of CrN clusters. On the other hand, the low temperature growth mode at 540 °C places the Cr ions onto the Al lattice site substitutionally. The Cr metal formation mechanism is qualitatively discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stability and band gaps of InGaP, BGaP, and BInGaP alloys: Density-functional supercell calculations

Energies and band gaps of (InGa)P, (BGa)P and (BInGa)P alloys, including the antisite boron substitutions, are calculated using supercells and the density-functional method. The influence of the substituent concentration and arrangement on the stability and band gaps is investigated and compared with experimental findings. Linearly corrected band-gap widths reproduce well experimental and GW data. The most stable structures of the ternary alloys are 200/211 arrangements of the substituents. The T–x phase diagram for (InGa)P provides no miscibility gap for usual growth temperatures. However, for (BGa)P alloys a large miscibility gap is obtained between about 2% and 99% boron, what confirms experimental results. In quaternary (BInGa)P, boron and indium atoms prefer the (220) but also the (110) position to each other. A 2:1 ratio is preferred for the indium to the boron content. Supercell with the most probable B1InnGa31−nP32 structure (n = 2).

Micronucleus‐Specific Bacterium Holospora elegans Irreversibly Enhances Stress Gene Expression of the Host Paramecium caudatum

The bacterium Holospora is an endonuclear symbiont of the ciliate Paramecium. Previously, we reported that paramecia bearing the macronuclear-specific symbiont Holospora obtusa survived better than symbiont-free paramecia, even under high temperatures unsuitable for growth. The paramecia with symbionts expressed high levels of hsp70 mRNAs even at 25 degrees C, a usual growth temperature. We report herein that paramecia bearing the micronuclear-specific symbiont Holospora elegans also acquire the heat-shock resistance. Even after the removal of the bacteria from the hosts by treatment with penicillin, the resulting aposymbiotic paramecia nevertheless maintained their heat shock-resistant nature for over 1 yr. Like symbiotic paramecia, these aposymbiotic paramecia also expressed high levels of both hsp60 and hsp70 mRNAs even at 25 degrees C. Moreover, analysis by fluorescent in situ hybridization with a probe specific for Holospora 16S rRNA revealed that the 16S rRNA of H. elegans was expressed around the nucleoli of the macronucleus in the aposymbiotic cells. This result suggests the possible transfer of Holospora genomic DNA from the micronucleus into the macronucleus in symbiotic paramecia. Perhaps this exogenous DNA could trigger the aposymbiotic paramecia to induce a stress response, inducing higher expression of Hsp60 and Hsp70, and thus conferring heat-shock resistance.

Comparative study of the adsorption and dissociation of vinylacetic acid and acrylic acid on silicon (001)

In this work, we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of acrylic acid (AAc) and vinylacetic acid (VAA) on the silicon surface. Our total energy calculations support the proposed experimental process, as it indicates that the chemisorption of the molecule is as follows: The gas phase VAA (AAc) adsorbs molecularly to the electrophilic surface Si atom and then dissociates into ${\mathrm{H}}_{2}\mathrm{C}\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}$ and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The activation energy for both processes correspond to thermal activations that are smaller than the usual growth temperature. In addition, the electronic structure, calculated vibrational modes, and theoretical scanning tunneling microscopy images are discussed, with a view to contribute to further experimental investigations.

Glycine adsorption on silicon (001)

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the dissociative adsorption process of glycine on the silicon surface. Our total energy calculations indicate that the chemisorption of the molecule is as follow. The gas phase NH2-C2H2-OOH adsorbs molecularly to the electrophilic surface Si atom and then dissociates into NH2-C2H2-OO and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms respectively, with an energy barrier corresponding to a thermal activation that is smaller than the usual growth temperature, indicating that glycine molecules will be observed in their dissociated states at room temperature. This picture is further support by our calculated vibrational modes for the considered adsorbed species.

Monte Carlo simulations applied toAlxGayIn1−x−yXquaternary alloys(X=As,P,N): A comparative study

We develop a different Monte Carlo approach applied to the ${A}_{x}{B}_{y}{C}_{1\ensuremath{-}x\ensuremath{-}y}D$ quaternary alloys. Combined with first-principles total-energy calculations, the thermodynamic properties of the $(\mathrm{Al},\mathrm{Ga},\mathrm{In})X$ ($X=\mathrm{As}$, P, or N) systems are obtained and a comparative study is developed in order to understand the roles of As, P, and N atoms as the anion $X$ in the system ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{y}{\mathrm{In}}_{1\ensuremath{-}x\ensuremath{-}y}X$. Also, we study the thermodynamics of specific compositions in which AlGaInN, AlGaInP, and AlGaInAs are lattice matched, respectively, to the GaN, GaAs, and InP substrates. We verify that the tendency for phase separation is always towards the formation of an In-rich phase. For arsenides and phosphides this occurs in general for lower temperatures than for their usual growth temperatures. This makes these alloys very stable against phase separation. However, for nitrides the In and/or Al concentrations have to be limited in order to avoid the formation of In-rich clusters and, even for low concentrations of In and/or Al, we observe a tendency of composition fluctuations towards the clustering of the ternary GaInN. We suggest that this latter behavior can explain the formation of the InGaN-like nanoclusters recently observed in the AlGaInN quaternary alloys.

時間分解内殻準位光電子分光による半導体表面過程の解析

We describe the development of an equipment which can be employed for examining various surface phenomena in real-time by performing time-resolved core-level photoelectron spectroscopy. We demonstrate this unique capability by examining the desorption of Sb atoms occurring from GaSb(001) as well as Sb/GaAs(001) in the temperature range of 440oC∼580oC, which is the usual growth temperature. The time dependence of core-level photoelectron spectra revealed that the activation energy for Sb desorption varies from 0.7 eV to 2.5 eV due to the difference in local bonding structures. In addition, experiments on alternating growth of GaAs suggest that there are two Ga adsorption sites, the occupation of which is dependent on the Ga coverages. This finding is in agreement with theoretical studies. These results demonstrate the potential of the technique to elucidate the surface process by real-time analysis.

A comparative study of dissociative adsorption of NH3, PH3, and AsH3 on Si(001)–(2×1)

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH3, PH3, and AsH3 on the Si(001)–(2×1) surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH3 molecules considered in this work is as follows: the gas phase XH3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si–Si dimers that are (almost) symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH3 and XH2 systems retain the pyramidal geometry observed for the gas molecules. Our calculated vibrational spectra further support the dissociative model for the XH3 molecules considered here.

Comparison of HgTe materials grown in (100), (110), (111), and (211) Orientations

We calculated energies required to remove atoms from various configurations on (111), (110), (100), and (211) HgTe surfaces. The excess pair energies for various species are then calculated and are used in a thermodynamic model to study the growth. All energies are obtained using a Green’s function method. The pair distributions are calculated from these energies in a generalized quasi-chemical approximation. The calculated critical temperatures for surface roughness transition are found to be considerably higher than the usual growth temperature of 185°C, so the growth on these surfaces is expected to be layer-by-layer with formation of two-dimensional islands. However, among the surfaces studied, only the (211) surfaces have an attractive binding energy for Hg, making those surfaces suited for better growth. The critical temperature for growth on (21 l)Hg is slightly higher than that for (211)Te, but we also find that Hg sticking coefficient on (21 l)Hg surface is considerably lower than that on (21 l)Te surface. These calculations are consistent with the observed higher growth rate of the (211)Te surface. Our calculations suggest that there will be fewer grown-in vacancies and Te antisites, at the expense of growth rate and sticking coefficient, for crystals grown on (211)Hg surface. We further calculated the Hg and Te vacancy formation energies as functions of surface orientations and layer depth. The cation vacancy formation energies from completed surface regions (islands) are higher than bulk values near anion terminated surfaces and smaller than bulk values near cation terminated surfaces.

Influence of kinetic roughening on the epitaxial growth of silicon

The low temperature homoepitaxial growth of silicon has been probed in situ by Reflection High Energy Electron Diffraction (RHEED) in a Molecular Beam Epitaxy (MBE) chamber in the temperature range 250 o C-400 o C. The continuous change in the RHEED patterns during the growth of thick films (several hundred angstroms) shows the progressive appearance of a surface roughness during and after the decay of RHEED oscillations. This is a clear evidence for kinetic roughening in the case of silicon epitaxial growth at low temperatures on the Si(111) face. The surface width, σ, measured as the film thickness, h, is increased, can be described by: σ(h, T)∼Ah exp(E f /kT) with E f ∼0.65 eV. This is in marked difference with the kinetic roughening behavior measured during the growth of a metal like iron by means of the same RHEED technique (see Chevrier et al., Europhys. Lett. 8 (1991) 737). Furthermore, an extrapolation of this behavior to epitaxial growths at higher temperatures (T∼600 o C-800 o C) suggests an effective influence of kinetic roughening in the determination of growth temperatures generally used for silicon MBE (i.e. the empirical usual growth temperatures Tepi ∼ 650 o C). During growth at substrate temperatures between 250 o C and 400 o C, the nucleation of misoriented silicon islands takes place following the occurrence of kinetic roughening at the surface. In this range of temperatures, this suggests that the loss of epitaxy occurs on a rough surface through the proliferation of defects and of misoriented crystals

Hg-rich liquid-phase epitaxy of Hg 1−xCd xTe

Over the past decade, liquid-phase epitaxy (LPE) has become an established technique for the growth of HgCdTe. This article reviews one of the successful LPE technologies developed for HgCdTe, specifically, “infinite-melt” vertical LPE (VLPE) from Hg-rich solutions. In spite of the relatively low solubility of Cd in Hg-rich solutions and the relatively high Hg pressure at the usual growth temperatures, this approach has been found to offer superior results for growth of HgCdTe suitable for various compositions and layer structures. An historical perspective and the current status of VLPE technology are presented. Particular emphasis is placed on the important role of the thermodynamic parameters (phase diagram), on control of stoichiometry (defect chemistry) and on impurity doping (distribution coefficient) for growth of HgCdTe layers from Hg solutions. Critical material characteristics, such as transport properties, minority-carrier lifetime, morphology and crystal structure, are also discussed.

Epitaxially grown semiconductor surfaces

The atomic distributions on surfaces of Si, GaAs, HgTe and CdTe are studied as functions of temperature and substrate orientation in the [100], [111] and [111] directions. Surface entropy is calculated within the quasichemical approximation and the pair interaction energies are obtained using the tight-binding Green's function method. In most cases considered, the interactions between the atoms are attractive and they tend to congregate into islands for submonolayer coverage at temperatures below the roughening transition temperatures. Free energy curves for double-layer growth are presented. We find that growth in this case is mostly layer by layer as is often observed in atomic layer or molecular epitaxial growth. However, in cases such as Ga terminated [111] GaAs surfaces and most CdTe surfaces, the in-plane interactions are predicted to be repulsive. The calculated order-disorder transition temperatures for these cases are often much larger than usual growth temperatures and, consequently, incompletely filled surfaces are temperatures for these cases are often much larger than usual growth temperatures and, consequently, incompletely filled surfaces are expected to have domains in which atoms and vacancies arrange themselves in superlattice patterns.