Constant Growth Temperature Research Articles

Non–steady state solidification of superconducting oxides

The non–steady state solidification of RE123 (RE = Y, Nd) superconductive oxides was investigated by using an undercooling growth method. In both the Y– and Nd–systems, RE123 crystals grew steadily in the initial growth stage, but the growth rates decreased gradually in the later stage under the constant growth temperature. In the case of Nd–system, the substitution ratio of Hd/Ba in the grown Nd123 crystal was found to be gradually changed. The liquid compositions revealed that the liquid composition was close to the ternary equilibrium point after the growth rates decreased. This non-steady growth of Nd123 crystal is intrinsically caused since the composition of grown Nd123 solid solution is not on the 422–123 line and the residual liquid composition gradually shifts to the ternary equilibrium point to compensate for the mass balance during 123 growth. On the other hand, in the case of the Y–system, the evaluation of the volume fraction of Y211 particles showed that the volume fractions in the grown crystal were lower than those expected from the initial composition. On the contrary, those in liquid were higher and had similar valuesirrespective of the initial compositions and growth conditions. This accumulation of 211 phase particles near the 123/liquid interface would reduce the diffusion of the solute elements, which have the partition ratio of less than unity, away from the 123 interface. Therefore, such a large volumefraction would enhance the compositional shift close to the growth interface and cause locally a final transient to decrease the growth rate of the 123 crystal, if there is a compositional shift from the line connecting 123 and 211 compositions.

Phase diagram and crystal growth of RE123 superconductive oxides

The non-steady-state solidification of Y123 superconductive oxides were investigated by using an undercooling growth method. Y123 crystals grew steadily in the initial growth stage, but the growth rates decreased gradually in the later stage under the constant growth temperature. The Y211 particles were found to be pushed and accumulated near the growth interface. These results suggest that the large volume fraction of Y211 particles in liquid would act as the obstruction of solute diffusion away from the Y123 interface. This will enhance the compositional shift to the ternary equilibrium point in the boundary layer and cause a final transient locally to reduce the growth rate when the initial composition is not on the line connecting 211 and 123. Based on this idea, we developed a simple non-steady solidification model in the diffusion boundary layer, which can describe the local final transient phenomenon. The calculation results show that the final transient will be easily caused in the boundary layer at the same temperature condition from a slight deviation of initial composition out of the 211–123 tie-line.

Growth mode of AlN epitaxial layers on 6H-SiC by plasma assisted molecular beam epitaxy

AlN layers were grown on 3.5° off 6H-SiC substrate by plasma-assisted molecular beam epitaxy (MBE). A study of the growth mechanism has been carried out by varying the growth time and the III/V ratio, at a constant growth temperature of 800°C. The layers were characterized by means of Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that nitrogen-rich conditions always give rise to a three-dimensional growth with spotty RHEED patterns and rough surface. In Al-rich conditions, the growth is much more two dimensional like with streaky RHEED patterns. By AFM, step bunching growth is observed. The size of the steps depends on the thickness of the layer as well as on the III/V ratio. All the samples grown in Al-rich conditions showed better crystalline quality than those grown in N-rich conditions.

A comparative study of GaAs- and InP-based HBT growth by means of LP-MOVPE using conventional and non gaseous sources

Low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) growth of carbon doped (InGa)P/GaAs and InP/(InGa)As heterojunction bipolar transistors (HBT) is presented using a non-gaseous source (ngs-) process. Liquid precursors TBAs/TBP for the group-V and DitBuSi/CBr 4 for the group-IV dopant sources are compared to the conventional hydrides AsH 3/PH 3 and dopant sources Si 2H 6/CCl 4 while using TMIn/TEGa in both cases. The thermal decomposition of the non gaseous sources fits much better to the need of low temperature growth for the application of carbon doped HBT. The doping behavior using DitBuSi/CBr 4 is studied by van der Pauw Hall measurements and will be compared to the results using Si 2H 6/CCl 4. Detailed high resolution X-ray diffraction (HRXRD) analysis based on 004 and 002 reflection measurements supported by simulations using BEDE RADS simulator enable a non-destructive layer stack characterization. InGaP/GaAs HBT structures designed for rf-applications are grown at a constant growth temperature of T gr=600°C and at a constant V/III-ratio of 10 for all GaAs layers. P-type carbon concentrations up to p = 5·10 19cm −3 and n-type doping concentrations up to n = 7·10 18cm −3 are achieved. The non self-aligned devices (A E = 3·10 μm 2)_show excellent performance, like a dc-current gain of B max = 80, a turn on voltage of V offset = 110 mV (Breakdown Voltage V CEBr,0 > 10 V), and radio frequency properties of f T/f max = 65 GHz/59 GHz. In the non-gaseous source configuration the strong reduction in the differences of V/III-ratios and temperatures during HBT structure growth enable easier LP-MOVPE process control. This is also found for the growth InP/InGaAs HBT where a high dc-current gain and high transit frequency of f T= 120 GHz are achieved.

Scanning Hall probe measurements on single- and double-sided sputtered YBCO films for microwave applications

We have investigated the quality and the homogeneity of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) films up to /spl phi/=2" diameter and t=360 nm in thickness with a scanning Hall probe. The YBCO films were grown by high oxygen pressure sputtering with heater temperature compensation up to T=1020/spl deg/C, resulting in a constant growth temperature for both film sides. Typical /spl phi/=1" double-sided films on LaAlO/sub 3/ substrates revealed inductively T/sub c/=87.8(88.2) K and J/sub c/=4(4.5) MA/cm/sup 2/ for the first (second) deposited side. Surface resistance measurements at 87 GHz resulted in R/sub s/(4.2 K)=2.6 (1.6) m/spl Omega/. At 19 GHz, R/sub s/(4.2 K)=0.2 m/spl Omega/ with moderate field dependence up to B/sub s/=15 mT was obtained for both sides. The scanning Hall probe measurements have been carried out after cooling the film in an external magnetic field and then switching it off. The local J/sub c/ values deduced from the measured remanent induction B were in good agreement with inductive data taken at corresponding positions. Different kinds of defects and inhomogeneities were investigated with a spatial resolution of 1 mm.

Modeling seedling johnsongrass (Sorghum halepense) emergence as influenced by temperature and burial depth

Research was conducted to formulate a seedling johnsongrass emergence model as influenced by temperature and burial depth using the poikilotherm rate equation. A series of constant-temperature growth chamber experiments with johnsongrass seed buried at various depths in fritted clay was conducted to develop a temperature/burial emergence database. The poikilotherm rate equation was fit to the emergence data from burial depths of 0 to 2.5 cm at constant temperatures between 20 and 44 C. These data were then combined to formulate a single poikilotherm rate equation to model the emergence of seedling johnsongrass from 0 and 2.5 cm deep and 20 to 44 C. This combined model was validated against two independent emergence data sets with good results.

Germination Response of `Genesis' Triploid Watermelon [Citrullus lantaus (Thunb.) Matsum & Nakai] to Hydrogen Peroxide and Seed Coat Alteration

Triploid watermelons are fast becoming a highly desirable specialty crop. Poor germination and nonuniform emergence of the relatively expensive seed prohibits to many potential growers. Improvement of germination and emergence of triploid watermelons would reduce overall risk to a grower, thus increasing the crops market prominence. Seeds of `Genesis' triploid watermelon were subjected to three treatments: 1) seed coat removal; 2) clipping the seed coat opposite the radicle end, and 3) no seed coat alteration. Seeds from each treatment were germinated on agar alone or in the presence of 5 ml of a 1%, 2%, 4%, or 8% solution of H2O2. Test were conducted in a constant temperature (28 °C) growth chamber in the dark. Seed coat removal, clipping, and all levels of H2O2 increased final germination percentages relative to the control. Germination differences as great as 70% occurred. Hydrogen peroxide levels greater than 2% resulted in severe injury to germinating seeds.

Characterization of self-assembled Ge islands on Si(100) by atomic force microscopy and transmission electron microscopy

We present an alternative starting point of the fabrication of nanostructures for electronic devices by using self-assembling structures. One way for the growth of self-assembling structures as quantum dot (QD) arrays is based on the formation of coherently strained Ge islands on Si and requires controlling of a defined island growth (Stranski–Krastanov). For this reason we carried out systematic quantitative investigations of the growth of Ge islands. Stacks of two layers of Ge islands with a Si spacer were grown on a Si buffer and characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). In the first series the Ge layer thickness was varied at a constant growth temperature and in the second series the growth temperature was varied for a constant Ge layer thickness. Many of the results described in this paper confirm the expected growth behavior of Ge islands, i.e. the Stranski–Krastanov growth mode and the increasing island density with decreasing growth temperature. However, two new aspects of the island growth were found. Small Ge islands are already formed from 2.15 monolayers (ML) of Ge at high growth temperatures. At lower growth temperatures (≤645°C, 6.2 ML coverage), few large islands with defects and a high density of small coherent islands are observed simultaneously. TEM studies of cross-sectional and plan-view samples reveal that the small islands are elongated along (100) directions. Reasons for the formation of the two kinds of islands are discussed.

B incorporation and hole transport in fully strained heteroepitaxial Si1 − xGex grown on Si(0 0 1) by gas-source MBE from Si2H6, Ge2H6, and B2H6

B-doped Si 0.95 Ge 0.05 (0 0 1) films were grown on Si(0 0 1) by gas-source molecular beam epitaxy in the surface-reaction-limited regime using Si 2 H 6 , Ge 2 H 6 , and B 2 H 6 . Incorporated B concentrations C B (5 × 10 16 − 3 × 10 19 cm −3 ) were found to increase linearly with increasing B 2 H 6 flux J B 2 H 6 (6 × 10 12 −2 × 10 15 cm −2 s −1 ) at constant film growth temperature T s , and to decrease exponentially with 1/ T s at constant J B 2 H 6 . The B 2 H 6 reactive sticking probability ranged from ⋍ 3.3 × 10 −4 at T s = 600°C to 8.2 × 10 −4 at 700°C and film growth rates were independent of J B 2 H 6 . Structural analysis by in-situ reflection high-energy electron diffraction combined with post-deposition high-resolution plan-view and cross-sectional transmission electron microscopy, high-resolution X-ray diffraction, and reciprocal lattice mapping showed that all films were fully strained, with measured relaxations less than the detection limit, ⋍ 3 × 10 −5 , and exhibited no evidence of dislocations or other extended defects. Room-temperature Si 0.95 Ge 0.05 : B conductivity mobilities were equal to theoretical values and a factor of ⋍ 2 higher than corresponding results for bulk Si. μ c varied from 410 cm 2 V −1 s −1 with C B = 5 × 10 16 cm −3 to 60 cm 2 V −1 s −1 with C B = 3 × 10 19 cm −3 .

Growth kinetics of GaN grown by gas-source molecular beam epitaxy

We report on the incorporation kinetics of gallium during gas-source molecular beam epitaxy (GSMBE) of GaN using elemental Ga and NH 3 gas as source materials. Desorption mass spectrometry (DMS) is used to perform in situ quantitative measurements of GaN formation, Ga desorption and Ga surface accumulation during growth. The rate of formation of GaN is reported as a function of incident Ga flux (0.1–0.75 ML/s) and incident NH 3 flux (1−300 × 10 −7 Torr beam-equivalent pressure) at a constant growth temperature of 725°C. Three distinct growth regimes are observed: (1) GaN is formed where all of the incident Ga flux is consumed, (2) part of the incident Ga is consumed to form GaN while the excess is desorbed from the surface, and (3) GaN formation coexists with desorption and surface accumulation of Ga. It is generally observed that the Ga surface accumulation is inhibited by increasing the rate of incidence of NH 3 and/or by decreasing the rate of incidence of Ga at this temperature. In addition, the order of the reaction between Ga and NH 3 is determined to be unity, supporting the validity of the Ga + NH 3 → GaN + 3 2 H 2 reaction.

Modeling weed emergence as influenced by burial depth using the Fermi-Dirac distribution function

Research was conducted to determine the suitability of the Fermi-Dirac distribution function for modeling the seedling emergence of downy brome, johnsongrass, and round-leaved mallow, as influenced by burial depth. Six sets of previously published emergence data were used to formulate the model and test its adequacy. Two independent johnsongrass emergence data sets were used to validate the model. Constant temperature growth chamber studies were conducted to evaluate the effects of temperature and moisture on the model parameters. The Fermi-Dirac distribution function was found to adequately describe the seedling emergence of downy brome, johnsongrass, and round-leaved mallow as indicated by a good visual data fit, narrow confidence intervals for the model parameters, and regression analysis of observed vs. modeled data. Although this function is a model used in physical science, its parameters can be related to abiotic factors such as soil texture, temperature, and moisture.

Growth of heavily C-doped MQW structures by MOVPE for 2–3 μm normal incidence photodetectors

The growth and intersubband optical properties of high quality heavily doped p-type GaAs AlGaAs multiple quantum well (MQW) structures are reported. The MQWs were fabricated by the atmospheric pressure metalorganic vapor phase epitaxy process using liquid CCl 4 to dope the wells with C acceptors ( N a ≈ 2 × 10 19 cm −3 ). A constant growth temperature was maintained for the entire structure while different V III ratios were used for the well and barrier regions. By this process it is possible to achieve both high C doping densities in the wells and to simultaneously obtain good quality AlGaAs barriers. Fourier transform infrared spectroscopy measurements on heavily doped 10-period MQW structures reveal a new absorption peak at ∼ 2 μ m with an effective normal incidence absorption coefficient of 4000 cm −1 . Photocurrent measurements on mesa-shaped diodes show a corresponding peak at 2.1 μm. The photodiodes exhibit a symmetrical current-voltage characteristic and a low dark current, which are indicative of a high quality MQW structure and a well-controlled C doping profile. The 2 μm absorption represents the shortest wavelength ever reported for any GaAs AlGaAs or InGaAs AlGaAs MQW structure and should be very useful for implementing multicolor infrared photodetectors.

The effect of arsenic overpressure on the structural properties GaAs grown at low temperature

The structural properties of GaAs grown by molecular-beam epitaxy at low temperatures have been investigated by scanning electron microscopy, transmission electron microscopy, and high-resolution x-ray double-crystal rocking curves as a function of arsenic overpressure during growth. It was found that surface smoothness and excess arsenic incorporation both depend strongly on growth temperature and on As/Ga flux ratio. For each growth temperature there is a ‘‘window’’ in the flux ratio which results in smooth surfaces. As-grown layers have an increased lattice constant in the growth direction. This relative lattice expansion increases with flux ratio at a constant growth temperature and eventually saturates. Transmission electron micrographs have revealed the presence of arsenic precipitates in material annealed at 600 °C. Increasing the As4 pressure during growth results in increases in precipitate diameter by almost 50% while their density and shape remain constant. Based on these observations a model has been developed to explain the lattice expansion dependence on arsenic overpressure.

B incorporation in Ge(001) grown by gas-source molecular-beam epitaxy from Ge2H6 and B2H6

Secondary-ion-mass spectrometry (SIMS) was used to determine the concentration and depth distribution of B incorporated into Ge(001)2×1 films grown on Ge(001) substrates by gas-source molecular-beam epitaxy using Ge2H6 and B2H6. B concentrations CB (3×1016–4×1019 cm−3) were found to increase linearly with increasing flux ratio JB2H6/JGe2H6 (8.2×10−3–1.7) at constant film growth temperature Ts (300–400 °C) and to increase exponentially with 1/Ts at constant JB2H6/JGe2H6 ratio. The difference in the overall activation energies for B and Ge incorporation over this growth temperature range is ≂0.22 eV while B2H6 reactive sticking probabilities ranged from 8×10−4 at 300 °C to 2×10−5 at 400 °C. SIMS depth profiles from B modulation-doped samples and two-dimensional δ -doped samples grown at Ts<350 °C were abrupt to within instrumental resolution with no indication of surface segregation. Structural analysis by in situ reflection high-energy electron diffraction combined with postdeposition high-resolution plan-view and cross-sectional transmission electron microscopy showed that all films were high-quality single crystals with no evidence of dislocations or other extended defects. B doping had no measurable affect on Ge deposition rates.

Modern epitaxial techniques for HBT structures

This paper reviews the different specifications required for a high yield and low cost epitaxial process of heterojunction bipolar transistor (HBT) structures on GaAs and InP substrate. For these two semiconductor systems, it is shown that arsenide/phosphide heterostructures and carbon doping of the npn HBT base layer lead to both high device reliability and simple and reproducible HBT fabrication process. Other requirements concerning high growth rate, low and constant growth temperature, high growth uniformity on several wafers as well as low surface defect density are also identified. It is shown that chemical beam epitaxy (CBE) and metal-organic vapor phase epitaxy (MOVPE) techniques fit all these requirements while molecular beam epitaxy (MBE) must be improved on specific points.

Quantitative correlation between oxygen impurity and current gain β of AlGaAs/GaAs heterojunction bipolar transistors grown by molecular beam epitaxy

The experimental correlation between current gain β of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) and oxygen in the n-AlGaAs emitter layer is discussed quantitatively. It has been observed that β decreases monotonically with an increase of the oxygen concentration in the n-AlGaAs layer. It has been found that the growth rate is one of the dominant factors which determine the crystal quality including the oxygen concentration, and that the optimum growth rate for the AlGaAs/GaAs HBT structure is around 0.7 μm/h under the constant growth temperature used. It has been thought that the variation of β is mainly ascribed to the recombination center at the emitter/base junction due to the oxygen impurity. The value of 3 × 10 17 cm −3 is consequently obtained as the upper limit of the oxygen concentration for high and stable β by extrapolating the linear relation between the oxygen concentration and 1/β Brs (the reciprocal of the recombination component of β at the emitter/base junction).

B-doped Si(001) grown by gas-source molecular-beam epitaxy from Si2H6 and B2H6:B incorporation and electrical properties

B-doped Si(001)2×1 films were grown on Si(001) substrates by gas-source molecular beam epitaxy using Si2H6 and B2H6. B concentrations CB (5×1016–5×1019 cm−3) were found to increase linearly with increasing flux ratio JB2H6/JSi2H6 (9.3×10−5–2.5×10−2) at constant film growth temperature Ts (600–950 °C) and to decrease exponentially with 1/Ts at constant JB2H6/JSi2H6 ratio. The B2H6 reactive sticking probability ranged from ≂6.4×10−4 at Ts=600 °C to 1.4×10−3 at 950 °C. The difference in the overall activation energies for B and Si incorporation at Ts=600–950 °C is ≂0.34 eV. A comparison of quantitative secondary-ion mass spectrometry (SIMS) and temperature-dependent Hall-effect measurements showed that B was incorporated into substitutional electrically active sites over the entire B concentration range investigated. SIMS B depth profiles from modulation-doped samples were abrupt with no indication of surface segregation to within the instrumental resolution limit and initial δ-doping experiments were carried out. Structural analysis by in situ reflection high-energy electron diffraction combined with post-deposition high-resolution plan-view and cross-sectional transmission electron microscopy showed that all films were high-quality single crystals with no evidence of dislocations or other extended defects. Temperature-dependent (20–300 K) hole carrier mobilities were equal to the best reported bulk Si:B values and in good agreement with theoretical maximum values.

Photosynthesis, lipids and proteins in the cyanobacteriumSynechocystis PCC 6803 as affected by temperature

The cyanobacteriumSynechocystis PCC 6803 was grown photoautotrophically in an inorganic medium at constant growth temperatures of 20, 38 (control) or 43°C for 9 h. The up and down-shift of cultivation temperature decreased the growth as measured by culture absorbance and chlorophylla content. However, high temperature slightly increased the oxygen evolution while temperature lower than control inhibited oxygen evolution during the whole incubation period. The protein synthesis studied by14C-labeled protein declined under low temperature by about 50%. The fatty acid pattern is characterized as lacking in C20/C22 acids but containing large amounts of C16 and C18 polyunsaturated fatty acids, 16:2 and 18:3 in particular. The lower temperature increased the percentage of monounsaturated fatty acids while higher temperature increased the saturated fatty acid content in total lipids and lipid classes studied.

Growth from solution-melt in KCl and properties of high-quality low-T c phase (2 2 0 1) single crystals

High Sr-deficient Bi 2+ x Sr 2−( x+ y ) Cu y O z (2 2 0 1)- phase) single crystals from solution-melt in KCl keeping constant both growth temperature and temperature gradient were obtained. Superconducting crystals were produced in the temperature range 830–845°C up to Bi/Sr ratio of 1.7 provided the Cu concentration was increased. The T c values measured by d.c. resistive and a.c. susceptibility methods are equal to 3−13 K. The best single crystals with T c0 = 3–5 K, ΔT c = 1 K have size up to 2 × 2 × 0.01 mm 3 and consist of blocks with a relative misorientation in the ab-plane no larger than three tenths of a degree. Measurements of supercell structure parameters have revealed that the in-plane and out-of-plane components of the modulation vector vary with opposite manner. The superlattice monoclinic angle of the superconducting crystals ranges from 123.5° to 129° whereas that of nonsuperconducting ones from 110° to 121°. It was found that the parameters of modulated structure can be successfully used to assess the crystal inhomogeneity.

Spontaneous Extinction of Twin Surface in GaAs on Si(100) Grown by One-Step Low-Pressure Metal-Organic Chemical Vapor Deposition

In heteroepitaxy of GaAs layers on Si(100) substrates by one-step low-pressure metal-organic chemical vapor deposition (LP-MOCVD), spontaneous extinction of twin crystals was confirmed. From the observation by reflection high-energy electron diffraction (RHEED), the {111} twin surface was found to coexist in the initial stage, and this twin surface was extinguished as deposition proceeded under constant growth temperature. The thickness required to extinguish the twin surface was dependent on both whether or not thermal cleaning was carried out prior to deposition and off-orientation angle toward the Si<011> direction. These suggest that the extinction mechanism of twin crystals is included in the heteroepitaxial growth of GaAs layers. Scanning electron microscopy (SEM) revealed that one-step heteroepitaxy by LP-MOCVD proceeds with embedding of the facet planes with high Miller index.