Continuous Growth Mode Research Articles

Flow modulation metalorganic vapor phase epitaxy of GaN at temperatures below 600 ºC

Heterogeneous integration of materials systems for devices and circuits is becoming of increasing interest, particularly for the wide bandgap nitrides in both optoelectronic and electronic applications. However, typical high growth temperatures of GaN by metalorganic vapor phase epitaxy prevent the growth of GaN on wafers with temperature sensitive materials. This work presents a flow modulation epitaxy (FME) growth scheme which allows for step-flow growth of GaN at temperatures below 600 °C. The utilization of a pulsed growth scheme such as FME allows for the mitigation of challenges associated with limited motion of adatoms at lower growth temperatures. Factors such as temperature, precursor flow rate, cycle time, and film thickness were explored. Under optimized conditions, step-flow growth of GaN films at 550 °C was demonstrated. In addition to an improved surface morphology over layers grown in the conventional continuous growth mode, the carbon and oxygen residual impurity concentrations in the FME grown layers were significantly lower, with carbon and oxygen contents of 9 × 1017 cm−3 and 8 × 1016 cm−3, respectively. Although the growth rate of the low temperature growth was very slow, ∼ 0.006 Å/s, the flow modulation growth scheme developed here provides a pathway towards further integration of GaN with temperature sensitive materials.

Effect of magnetic field on sodium arsenate metastable zone width and crystal nucleation kinetics for crystallization

AbstractSodium arsenate, the main component of arsenic‐containing solid waste pollutants, causes serious environmental health threats. Crystallization is one of the effective methods for separating and purifying sodium arsenate from arsenic‐alkali residue lixivium. However, the crystallization process is limited for its low observability and the lack of separation and purification data. In this work, a laser detection system with a magnetic field generator was designed, and the solubility, metastable zone width, interfacial tension, interfacial entropy factor, crystal nucleation, and growth rate of sodium arsenate were investigated in a constant composition environment. The results showed that the solubility, metastable zone width, interfacial tension, and interfacial entropy factor decreases with the presence of a magnetic field. The magnetic field shortened the crystallization induction time and changed the nucleation and growth rate of sodium arsenate. Under the magnetic field, the nucleation rate increased from 2.43 × 1016 to 8.98 × 1017 (s m3)−1, and the growth rate decreased from 4.94 × 10−8 to 2.73 × 10−8 (s m3)−1, the growth mechanism of sodium arsenate as a continuous growth mode was unchanged. In addition, the X‐ray diffraction and infrared showed that the crystal structure of sodium arsenate is unaffected by the magnetic field, indicating that the enhancement of the crystallization process of sodium arsenate with the magnetic field could be a feasible method in engineering application.

Controlled synthesis of nonpolar GaInN/GaN multiple-quantum-shells on GaN nanowires by metal-organic chemical vapour deposition

Nonpolar GaInN/GaN multiple-quantum shells (MQSs) on nanowires (NWs) were investigated for high-efficiency light-emitting diodes (LEDs). The growth conditions of NWs were systematically optimized via a continuous growth mode in metal-organic chemical vapour deposition (MOCVD). The In incorporation rate on the m-planes decreased as the growth temperature elaborated, whereas the crystalline quality is improved. The cathodoluminescence (CL) results revealed that longer growth time of the GaInN well can induce additional In-rich droplets and degrade the emission properties of MQSs. The CL emission intensity and the peak wavelength increased as the number of MQS pairs increased from one to three pairs, which was attributed to the increased In incorporation as the diameter enhanced. The linearly enhanced CL emission intensity with barrier thickness was ascribed to the increase of the electron-hole states from the GaN barrier to the wells, resulting in a larger recombination probability. The scanning transmission electron microscopy (STEM) results demonstrated that a thicker barrier shell can suppress the formation of In-rich droplets. Overall, the feasibility of obtaining high-quality m-plane coaxial GaInN/GaN MQSs structures are promising for NW-based white and micro LEDs.

Indigenous microalgae biomass cultivation in continuous reactor with anaerobic effluent: effect of dilution rate on productivity, nutrient removal and bioindicators

ABSTRACTEffluents from municipal wastewater treatment have been long recognized as suitable media for the cultivation of microalgae biomass. However, few studies report data concerning biomass productivity in continuous reactors using unsterilized wastewater effluents. This study focuses on indigenous microalgae strains that grow with native bacteria and are applicable for biomass production and tertiary wastewater treatment in continuous growth mode. Initially, five Chlorophyta strains were isolated and grown in batch mode to single out a potential inoculum for the experiments in continuous growth mode. The isolate Chlorella sp. L06 was selected and evaluated based on five dilution rates from 0.1 to 0.5 day-1 on continuous growth reactor using unsterilized secondary effluent as culture medium. Maximal volumetric biomass productivity of 283 mg L-1 day-1 was achieved at 0.3 day-1 without CO2 addition or air bubbling. Carbohydrates were the major fraction of the dried biomass, followed by proteins and then lipids. The highest removal rates of total nitrogen and phosphorus from the liquid phase were 13.0 and 1.4 mg L-1 day-1, respectively, and were obtained at 0.4 day-1. The maximal decay rate for E. coli (2.9 day-1) was achieved both at 0.3 and 0.4 day-1. Conclusively, Chlorella sp. L06 cultivation in unsterilized secondary effluent can be adjusted depending on the objective: for biomass production, a dilution rate of approximately 0.3 day-1 is recommended; and for tertiary treatment a rate of 0.4 day-1 is suggested.

In-situ growth mode control of AlN on SiC substrate by sublimation closed space technique

For the growth of AlN single crystal with large diameter and low dislocation density on SiC substrate by physical vapor transport (PVT), a dislocation blocking buffer layer (DBBL) has been simply developed by optimizing the AlN growth parameters such as temperature gradient (ΔT), substrate temperature (Tsub), and N2 partial pressure (PN2) at the initial growth stage. Increase in ΔT resulted in the formation of an abrupt AlN/SiC interface due to the suppression of inhomogeneous thermal decomposition at the interface and the subsequent AlN unstable island growth. The well-defined AlN/SiC interface played an important role in controlling the two kinds of different AlN growth mode in-situ as functions of Tsub and PN2. One is a continuous step-flow growth mode, and the other is a discontinuous platelet-like growth. The discontinuous AlN layer, consisting of thin AlN platelets and air-gaps inserted between the two adjacent platelets, acted as the DBBL. By introducing the DBBL at the initial growth stage, followed by the step-flow growth, continuous AlN layer with dislocation density of 1.7 × 106 cm−2 was achieved at a total growth thickness of 60 μm, which is two orders of magnitude lower than the previously reported value.

Yellow-red light-emitting diodes using periodic Ga-flow interruption during deposition of InGaN well.

We report a possible way to extend the emission wavelength of InyGa1-yN/InxGa1-xN quantum-well (QW) light-emitting diodes (LEDs) to the yellow-red spectral range with little degradation of the radiative efficiency. The InyGa1-yN well with high indium (In) content (HI-InyGa1-yN) was realized by periodic Ga-flow interruption (Ga-FI). The In contents of the HI-InyGa1-yN well and the InxGa1-xN barrier were changed to manipulate the emission wavelength of the LEDs. An In0.34Ga0.66N/In0.1Ga0.9N-QW LED, grown by continuous growth mode (C-LED), was prepared as a reference. The photoluminescence (PL) wavelengths of the HI-InyGa1-yN/InxGa1-xN QW LEDs were changed from 556 to 597 nm. The PL intensity of the HI-InyGa1-yN/InxGa1-xN LED with a peak wavelength of 563 nm was 2.7 times stronger than that of the C-LED (λ = 565 nm). The luminescence intensity for the HI-InyGa1-yN/InxGa1-xN QW LED emitting at 597 nm was stronger than those of the other LED samples with shorter wavelengths. Considering the previous works on degradation in crystal quality and increase in the quantum-confined Stark effect with increasing In content in InGaN, the approach in this work is very promising for yellow-red InGaN LEDs.

Interface morphology and DFT computation of l-valinium fumarate

Based on crystal surface observation from AFM, the growth mechanism of l-valinium fumarate (LVF) crystal is in agreement with continuous growth mode. It is suggested that the microcrystals and pits may lead to the formation of inclusion macrodefects during growth. In addition, quantum chemical computations based on density functional theory (DFT) have been performed on LVF. The optimized molecular geometry, natural bond orbital analysis, Mulliken’s net charges and frontier molecular orbital analysis have been calculated. The linear polarizability and first hyperpolarizability have also been computed.

Solvated behavior and crystal growth mechanism of erythromycin in aqueous acetone solution

The solubility of erythromycin acetone solvate and dihydrate was experimentally determined in aqueous acetone mixtures at different temperature. It has been demonstrated that solubility curves of the two solvates intersected at given solvent composition at various temperature, suggesting a transition behavior between two solvates. The induction period of acetone solvate at different supersaturation was measured by the laser monitoring observation technique. Based on classical homogeneous nucleation theory, the solid-liquid interfacial tension and surface entropy factor were calculated from the induction period data. From the surface entropy factor values calculated, together with surface morphology observation by the atomic force microscopy (AFM), the growth mechanism of erythromycin acetone solvate is consistent with continuous growth mode.

Growth of AlN by pulsed and conventional MOVPE

Aluminium nitride was grown on c-plane sapphire by metal organic vapor phase epitaxy (MOVPE) at temperatures of 1070°C by a pulsed growth method and in continuous growth mode at temperatures up to 1270°C. For both methods the V/III ratio was varied and different approaches for the growth start were investigated. The crystal quality was mainly characterized by scanning electron microscopy and high resolution X-ray diffraction which showed unusual line shape for certain samples. Both growth methods enabled the growth of more than 1μm thick, atomically flat, Al-polar layers with edge type dislocation densities in the order of 3×1010cm−2 for pulsed samples and 5×109cm−2 for conventionally grown samples.

Continuous-Flow MOVPE of Ga-Polar GaN Column Arrays and Core–Shell LED Structures

Arrays of dislocation free uniform Ga-polar GaN columns have been realized on patterned SiOx/GaN/sapphire templates by metal organic vapor phase epitaxy using a continuous growth mode. The key parameters and the physical principles of growth of Ga-polar GaN three-dimensional columns are identified, and their potential for manipulating the growth process is discussed. High aspect ratio columns have been achieved using silane during the growth, leading to n-type columns. The vertical growth rate increases with increasing silane flow. In a core–shell columnar LED structure, the shells of InGaN/GaN multi quantum wells and p-GaN have been realized on a core of n-doped GaN column. Cathodoluminescence gives insight into the inner structure of these core–shell LED structures.

Desulfovibrio vulgaris Hildenborough prefers lactate over hydrogen as electron donor

Desulfovibrio vulgaris can use lactate as an electron donor and accumulate hydrogen. Hydrogen can also be consumed as an electron donor when lactate is depleted or absent. The aim of this study was to determine whether D. vulgaris has an electron donor preference system between lactate and hydrogen and how this system is regulated. In order to be sure that D. vulgaris was grown under the same conditions except for electron donors, continuous growth mode was conducted and the optical density (600 nm) was kept constant. When 20 mmol/l lactate was the sole electron donor, it was depleted after 9 h of incubation while hydrogen was accumulated to 1,500 ppm. After that, the hydrogen level was decreased to and maintained at 400 ppm. When 1,200 ppm hydrogen was provided as the electron donor, the culture reached an OD of 0.2 after 24 h incubation and hydrogen was consumed to 600 ppm. When 1,200 ppm hydrogen and 20 mmol/l lactate were both present, the lactate was consumed during the first 9 h incubation and hydrogen was accumulated to 1,800 ppm. D. vulgaris used hydrogen as an electron donor after the lactate was depleted and the hydrogen level was decreased to 600 ppm. D. vulgaris has both pathways to utilize lactate and hydrogen as electron donors. It prefers lactate over hydrogen and the system is regulated by lactate starvation.

Solidification behavior of highly supercooled polycrystalline silicon droplets

Polycrystalline silicon balls are popularly used for solar cells to lower cost and improve their light collection capability. This article investigates on the microstructure evolution during solidification of polycrystalline silicon. The uniform droplet spray process, a controlled capillary jet break-up process, which enables stringent control of the nucleation and microstructure evolution during solidification of alloy droplets in a thermal spray process, was used to produced mono-sized silicon droplets. The experimental parameters for production of silicon droplets were established and the solidification behavior of silicon droplets was investigated using the modification of the free dendritic growth model and the dendritic fragmentation model. This enabled us to correctly establish the transition supercooling for transformation from lateral growth mode to continuous growth mode to be between 81K and 172K. The model was used to predict the microstructure of polycrystalline silicon droplets for solar cells produced by a droplet based manufacturing method, enabling greater control over process parameters and its relation to the final microstructure.

Lifetime assessment of engineering thermoplastics

The life expectancy of thermoplastics in durable applications varies from about 10years to 50 and even 100years in certain cases. It calls for an accelerated testing of material and structures. The challenges of accelerated testing for lifetime are (a) to reproduce the mechanisms of field failures and (b) to develop a reliable procedure for extrapolation of a relatively short test data into long-term service conditions. Acceleration of fracture by high stress level turns to be inadequate, since the fracture mechanisms change with stress level. Acceleration of testing for lifetime by elevated temperature is the most widely used technique at the present. This paradigm, however, faces a problem associated with the changes in the mechanism and kinetics of slow crack growth (SCG). At a certain combination of load and temperature, a transition from a continuous SCG to discontinuous, stepwise crack propagation has been recorded. Optical and scanning electron microscopy observations suggest that the change of SCG mechanisms is closely related to the material ability to form in front of the growing crack a stable process zone that consists of single or multiple crazes and/or shear bands. The crack acceleration in the continuous growth mode is observed to be significantly higher than that in stepwise propagation. Such changes in the mechanism and kinetics of SCG are associated with a transition from a ductile to brittle behavior of microfibers within the process zone. It is referred to as ductile–brittle transition of the second kind (DBT2) based on a resemblance with well-known ductile–brittle transition in dynamic impact resistance. DBT2 is presented in form of SCG mechanisms map in temperature–stress intensity factor coordinates. SCG mechanism map implies certain limitations for extrapolation of conventional temperature accelerated test data to the service conditions of plastic components. An alternative to conventional accelerated testing approach to evaluate lifetime of plastics structures is proposed in this paper. It consists of three steps. The first is a characterization of the defects population that may be responsible for fracture initiation. Formulation of constitutive equations of fracture process based on specially designed tests is the second step. Numerical simulation of fracture process using constitutive equations developed within the second step and evaluation of the lifetime of plastic structure is the third step. A validation testing of the proposed program is required.

GaAs nanowires grown by MOVPE

AbstractGaAs nanowire (NW) growth was studied by metal‐organic vapour phase epitaxy (MOVPE). The vapour–liquid–solid (VLS) mechanism with gold‐based alloy particles and the selective‐area growth (SAG) mechanism on electron beam lithographically prepared SiNx/GaAs mask structures were applied. A special focus is set on thermodynamic aspects of the VLS process. The alloy particle formation and the influence of MOVPE growth parameters on the growth rate and the GaAs NW morphology are examined. Furthermore, the improvement of the real structure with particular interest on the twin formation is studied. Besides the commonly used continuous VLS growth mode also a pulsed VLS growth mode with alternating precursor supply is reported. Based on photoluminescence measurements the effect of strain in core/shell NW structures is confirmed. For the SAG mechanism the MOVPE growth parameters are determined and the real structure is described. magnified image

A Hypothesis for Cast Iron Microstructures

The various microstructures of cast irons are reviewed, including carbidic and graphite forms (flake, compacted, spheroidal, and undercooled, etc.), exploring whether the presence of externally introduced defects in the form of oxide double films (bifilms) in suspension in melts seem to provide, for the first time, a uniform explanation for all the structures and their properties. Silica-rich oxide bifilms provide the substrates on which oxysulfide particles form, nucleating graphite. The presence of the film provides the favored substrate over which graphite grows, which leads to the development of flake graphite. The addition of limited Mg to form compacted graphite destroys all but a remnant of the silica-rich bifilms. The oxide film remnant is stabilized by the presence of the graphite nucleus, which causes the graphite to grow unidirectionally in a filamentary form. The addition of excess Mg destroys all traces of the oxide bifilms, leaving only the original nuclei, around which graphite is now free to entirely enclose, initiating the spherical growth mode. Undercooled graphite is the true coupled growth form, nucleated at even lower temperatures in the absence of favorable film substrates in suspension; the graphite adopts a continuous growth mode in a matrix of austenite. Carbides in mottled and white irons form on the oxide bifilms that often lie along grain and interdendritic boundaries, which explains the apparent brittleness of these strong, hard phases. In most cases of nonspheroidal growth modes (flake and misshaped spheroids), it is proposed that the impairment of the mechanical properties of irons is not strongly determined by graphite morphology but by the presence of oxide bifilms. Spheroidal graphite iron has the potential for high properties because of the absence of bifilms.

CADMIUM TELLURIDE NANOCRYSTALS: SYNTHESIS, GROWTH MODE AND EFFECT OF REACTION TEMPERATURE ON CRYSTAL STRUCTURES

A series of cadmium telluride (CdTe) nanocrystals were synthesized by a modified organometallic synthesis method at various reaction temperatures ranging from 130 to 250°C. In this method, octadecylamine (ODA) was introduced as an additional coordinating component to the mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP). CdO was used as a precursor. The prepared CdTe nanocrystals were studied by the absorption and emission spectra as well as the powder X-ray diffraction (XRD) patterns. The result shows that besides the traditional continuous-growth mode observed frequently at relatively high reaction temperature, a discontinuous-growth mode was confirmed at the initial growth stage of CdTe nanocrystals, arising from the change of the absorption spectra of CdTe nanocrystals with the reaction time at relatively low reaction temperature. The structures of CdTe nanocrystals, e.g., the cubic zinc blende structure at 160°C and the hexagonal wurtzite structure at 250°C, were characterized by XRD.

Epitaxial growth of GaInAs/AlGaAsSb quantum cascade lasers

Molecular beam epitaxial growth of quaternary barrier GaInAs/AlGaAsSb quantum cascade lasers grown lattice-matched on n -InP substrates is reported. By adjusting the As/Sb flux ratio as well as the group-III growth rate and the Al/Ga ratio for the AlGaAsSb layers, a growth technique has been optimized employing a continuous growth mode (i.e. no growth interruption) for the GaInAs/AlGaAsSb layer sequence of GaInAs/AlGaAsSb on InP quantum cascade structures. We found that the As/Sb incorporation ratio is determined by an interplay of arrival rate and chemical bonding of the involved species. The lasers operate in pulsed mode up to temperatures higher than 400 K, with a characteristic temperature of T 0 = 169 K .

Growth morphologies and mechanisms of non-equilibrium solidified MC carbide

Growth morphologies and mechanisms of the carbide of group IVB and VB elements (MC carbide), a typical faceted crystal, were studied with an estimated cooling rate from 102 to 105 K/s. Results showed that although the growth morphologies of the MC carbide vary remarkably with solidification cooling rate, the solid/liquid interface is always atomically smooth, and the growth mechanisms are always lateral growth. The growth mechanism transition from lateral to continuous growth mode, which was predicted by the classic crystal growth theory, was not observed for the TiC type MC carbide within the estimated cooling rate range of 102–105 K/s.

High-quality GaInAs/AlAsSb quantum cascade lasers grown by molecular beam epitaxy in continuous growth mode

Short wavelength ( λ<5 μm) quantum cascade lasers are of current interest as they operate in the technologically important atmospheric 3–5 μm transparency window. For this wavelength range the GaInAs/AlAsSb-on-InP material system offers the advantage of a large conduction band offset of about 1.6 eV over the well-established lattice-matched GaInAs/AlInAs-on-InP material combination with a band offset of only 0.5 eV. In this paper, we report on molecular beam epitaxial growth and subsequent structural and compositional analysis of GaInAs/AlAsSb quantum wells as well as quantum cascade laser structures. Special emphasis has been laid on establishing a growth procedure which allows the growth of these structures without growth interruption at the GaInAs/AlAsSb interfaces. The epitaxial layer sequences were analyzed by high-resolution X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy. Finally, mesa waveguide GaInAs/AlAsSb QC lasers were fabricated emitting around 4.5 μm, which could be operated in pulsed mode up to 400 K.

Growth of GaInNAs quaternaries using a digital alloy technique

Strain-compensated Ga0.92In0.08As/GaN0.03As0.97 and strained InAs/GaN0.023As0.977 short-period superlattices (SPSLs) were grown as a digital alloy of GaInNAs by gas-source molecular beam epitaxy. Material quality of GaInNAs was improved through the spatial separation of In and N by SPSL growth mode. The photoluminescence (PL) intensity of the strain-compensated Ga0.92In0.08As/GaN0.03As0.97 digital alloys is 3 times higher than that of random alloys at room temperature, and the improvement is even greater at low temperature, by a factor of about 12. Room-temperature PL intensity of the GaInNAs quantum well grown by the strained InAs/GaN0.023As short-period superlattices (SPSL) growth method is higher by a factor of 5 as compared to the continuous growth mode. The strained InAs/GaN0.023As SPSL growth method allows independent adjustment of the In-to-Ga ratio without group III competition since the In composition can be easily adjusted by controlling the InAs growth time.