Modified Growth Method Research Articles

Thermodynamics of Ga2O3 Heteroepitaxy and Material Growth Via Metal Organic Chemical Vapor Deposition.

Heteroepitaxy of gallium oxide (Ga2O3) is gaining popularity to address the absence of p-type doping, limited thermal conductivity of Ga2O3 epilayers, and toward realizing high-quality p-n heterojunction. During the growth of β-Ga2O3 on 4H-SiC (0001) substrates using metal-organic chemical vapor deposition, we observed formation of incomplete, misoriented particles when the layer was grown at a temperature between 650 °C and 750 °C. We propose a thermodynamic model for Ga2O3 heteroepitaxy on foreign substrates which shows that the energy cost of growing β-Ga2O3 on 4H-SiC is slightly lower as compared to sapphire substrates, suggesting similar high-temperature growth as sapphire, typically in the range of 850 °C-950 °C, that can be used for the growth of β-Ga2O3 on SiC. A two-step modified growth method was developed where the nucleation layer was grown at 750 °C followed by a buffer layer grown at various temperatures from 920 °C to 950 °C. 2θ-ω scan of X-ray diffraction (XRD) and transmission electron microscope images confirm the β-polymorph of Ga2O3 with dominant peaks in the (-201) direction. The buffer layer grown at 950 °C using a "ramp-growth" technique exhibits root-mean-square surface roughness of 3 nm and full width of half maxima of XRD rocking curve as low as 0.79°, comparable to the most mature β-Ga2O3 heteroepitaxy on sapphire, as predicted by the thermodynamic model. Finally, the interface energy of an average Ga2O3 island grown on 4H-SiC is calculated to be 0.2 J/m2 from the cross-section scanning transmission electron microscope image, following the Wulff-Kaishew theorem of the equilibrium island shape.

Controlled growth of large-scale uniform 1T' MoTe2 crystals with tunable thickness and their photodetector applications.

The monoclinic-phase 1T' MoTe2 crystal exhibits inversion symmetry as an anisotropic semi-metal, dictating its interesting quantum transport phenomenon and other novel physical properties. However, large-scale controllable growth of uniform MoTe2 crystals still remains a great challenge, hindering its further fundamental research and applications for novel devices. Herein, we report a modified growth method for synthesizing few-layer 1T' MoTe2 crystals with large-scale uniformity with the assistance of molecular sieves. The theoretical simulations demonstrated that due to the temperature-dependent formation energies of different edges, the edge of (010) orientation shows a higher thermodynamic stability than that of (100) orientation, and results in the anisotropic growth behavior of 1T' MoTe2 crystals while the temperature changes. The photoresponse of tri-layer 1T' MoTe2-based devices shows a broad-spectrum response from 532 nm to 1550 nm. The photo-response time of 1T' MoTe2 crystals demonstrates that it supposes to be the synergistic mechanism of photo-conductive and photo-radiation effects. Our findings not only provide a method for the controllable growth of anisotropic two-dimensional materials at a wafer scale, but also explore a broad-spectrum photodetector with the MoTe2-based device.

Anion modulation epitaxy (AME), an alternative growth strategy for group III‐nitrides

AbstractIn this paper we report a new strategy for growth of group III‐nitrides by Plasma‐Assisted Molecular Beam Epitaxy (PA‐MBE), which we term Anion Modulation Epitaxy (AME). In this modified growth method, the nitrogen flux is interrupted periodically to build up a group III surface concentration. This leads to enhanced mobility and thus improved morphology compared to growth using conventional PA‐MBE. Direct comparison of GaN samples grown at equivalent temperature by PA‐MBE and AME show the advantages of this improved growth procedure (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Modified Growth Method on the Structural and Optical Properties of InAs/GaAs Quantum Dots for Controlling Density

We introduce a thermal treatment, namely, a growth technique to form low-density quantum dots (QDs) with homogeneous InAs deposition, and study the structural and optical properties of InAs/GaAs QDs during the formation of dots by the thermal treatment. The structural and optical properties are studied by atomic force microscopy and photoluminescence (PL). We achieve a wide range of dot densities from 1010 to 106 per cm2 by adjusting the thermal treatment temperature. The uniformity of InAs dots improves as the thermal treatment temperature increases. Comparing PL spectra with those in the literature, we confirm that the dots in this work are not QDs but small quasi-three-dimensional (Q3D) clusters. Q3D clusters are left after the thermal treatment. This behavior of Q3D is different from that of the QDs reported in the literature. We conclude that three phenomena occur during the thermal treatment: 1) In segregation, 2) In re-evaporation, and 3) the intermixing of InAs. As a result, we conclude that the thermal treatment is a very useful method for controlling the dot density.

Growth and characterization of ZnGeP 2 single crystals by the modified Bridgman method

A good quality ZnGeP 2 (ZGP) single crystal 15 mm in diameter and 40 mm in length was grown in a vertical three-zone tubular furnace by the modified Bridgman method, i.e. real-time temperature compensation technique (RTTCT) with descending ampoule. The starting material is high-pure and single-phase polycrystal of ZnGeP 2 synthesized by the single-temperature zone and mechanical oscillation method (STZMOM). The as-grown single crystal was characterized by various methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analyzer of X-ray (EDAX), and IR and UV spectrophotometers. It is found that there is a cleavage face of (1 1 2) and second-order XRD peaks of the {1 1 2} faces are observed, the IR transmittance of a sample of 2 mm thickness is above 55% in the range from 6000 to 800 cm −1, the absorption edge is near 612.5 nm and the band gap is about 2.02 eV. The absorption coefficient ( α) is within 0.015–0.022 cm −1 at the spectral region 2–8 μm. The crystal has a stoichiometry ratio Zn:Ge:P=1:1.12:1.91 which is close to the ideal stoichiometry ratio of 1:1:2. All results demonstrate that the modified growth method is a new and promising method for the ZGP single crystal and the quality of as-grown crystal is good.

Improved barrier function observed in cultured skin substitutes developed under anchored conditions

The current method of producing cultured skin substitutes (CSS) is focused on providing treatments for severe skin wounds/burns. We have developed a modified growth method to make them more suitable for in vitro product-testing/toxicity-testing purposes. CSS grown in Petri dishes were either transferred to Franz diffusion cells on day 5 (modified method) or left in the Petri dish (standard method) and maintained in these environments for the remainder of the growth phase. Mitochondrial metabolism (MTT assay) was measured on days 5, 10 and 14 and histology was studied on days 5, 10 and 14. Barrier function for all tissues was evaluated by transferring them to Franz cells (standard method) and measuring transepidermal water loss (TEWL), 3H2O penetration and 14C-niacinamide permeability on days 7, 14 and 21. CSS grown by the standard and modified methods showed comparable cell viability and tissue morphology. Barrier function, however, was markedly improved in CSS grown by the modified method. The average improvement at days 7 and 14 was 1.3-fold for TEWL, 2.1-fold for 3H2O penetration and 6.4-fold for 14C-niacinamide permeability. The barrier function of CSS grown by the modified method was still significantly lower than that of human cadaver skin tested by the same methods. CSS developed using the anchored multi-cell system showed similar cell viability and morphology and improved barrier function compared with CSS produced by the standard Petri dish method, thereby improving its potential as an in vitro skin permeability and toxicity model.

GROWTH DYNAMICS OF II–VI COMPOUND SEMICONDUCTOR QUANTUM DOTS EMBEDDED IN BOROSILICATE GLASS MATRIX

Wide bandgap II–VI semiconductor quantum dots embedded in glass matrix have shown great potential for opto-electronic device applications. The current problem is to achieve low size dispersion, high volume fraction, and better control over the size of the quantum dots in glass matrix. In this work, a modified growth method has been proposed to achieve a greater control over the size of quantum dots, to reduce their size dispersion and to increase their volume fraction. A theoretical model has been developed to quantitatively estimate the various parameters of the quantum dots. The effects of aging on various parameters of quantum dots in Semiconductor-Doped Glass (SDG) samples have also been discussed in the present work.

Twin-free and High-Quality DAST Crystals − Effected through Solutions of Lower Supersaturation Coupled with Isothermal Solvent Evaporation

Growth of twin-free and high-quality 4-N,N-dimethylamino-4‘-N‘-methyl stilbazolium tosylate (DAST) crystals, with a surface roughness in the range of 1.7−3.8 nm (<4 nm) for as-grown crystals, has been achieved using solutions of lower supersaturation coupled with isothermal solvent evaporation. In addition, these efficient organic−ionic nonlinear optical (NLO) crystals possessed well-developed 001 and 001 faces that facilitate direct utilization of these crystals for electrooptic (EO) and terahertz (THz) applications. Since this modified growth method necessitates a relatively lower amount (about 46%) of the expensive starting material as compared to that required for routine methods, it is also proven to be cost-effective.

Growth and characterization of CdSe single crystals by modified vertical vapor phase method

Good quality, large single crystals of CdSe were grown by the modified growth method (i.e., vertical unseeded vapor phase growth with multi-step purification of the starting material in the same quartz ampoule without any manual transfer between the steps). Lower temperature gradients (8–9°C/cm) at the growth interface were used for the crystal growth. As-grown CdSe crystals was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive analyzer of X-rays, high-resistance instrument measurement, and etch-pit observation. It is found that there are two cleavage faces of (1 0 0) and (1 1 0) orientations on the crystal, the resistivity is about 10 8 Ω cm, and the density of etch pits is about 10 3–4/cm 2. The crystal was cut into wafers and was fabricated into detectors. The detectors were tested using an 241Am radiation source. γ-ray spectra at 59.5 keV were obtained. The results demonstrated that the quality of the as-grown crystals was good. The crystals were useful for fabrication of room-temperature-operating nuclear radiation detectors. Therefore, the modified growth technique is a promising, convenient, new method for the growth of high-quality CdSe single crystals.