High-temperature low-pressure chemical vapor deposition of β-Ga2O3

Control of diameter of ZnO nanorods grown by chemical vapor deposition with laser ablation of ZnO

Phase formation of hexagonal wurtzite ZnO through decomposition of Zn(OH)2 at various growth temperatures using CBD method

Temperature dependency of the LPCVD growth of ZrC with the ZrCl 4–CH 4–H 2 system

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability up to 1400 oC. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). It exhibits high transparency in the deep ultraviolet (DUV) and visible wavelength region due to its very large bandgap. The monoclinic b-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ε). The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques such as edge-defined film-fed growth (EFG), floating zone (FZ) and czochralski methods. For β-Ga2O3 thin film synthesis, both molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have been demonstrated to produce high quality and controllable doping films but with slow growth rates (2-10 nm/min). Halide vapor phase epitaxy (HVPE) using chloride precursors was demonstrated to grow β-Ga2O3 films with fast growth rates (>5 μm/hr). Recently, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality β-Ga2O3 thin films on both native Ga2O3and c-sapphire substrates with controllable doping and fast growth rates up to 10 μm/hr. In this talk, we present a study on the defects in β-Ga2O3 thin films grown via LPCVD. The β-Ga2O3 thin films were grown on native β-Ga2O3 substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. The β-Ga2O3 thin films were characterized by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM images were taken with Helios 650. High resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were taken using a FEI Tecnai F30 at 300 kV. Bright field, dark field, combined with two-beam condition TEM were used to characterize the defects in the as-grown films. From our studies, β-Ga2O3 thin films grown on (010), (001) and (-201) β-Ga2O3 substrates have shown different growth rates as well as different properties of interfacial defects. Improved LPCVD growths to suppress defects in β-Ga2O3thin films grown on different substrates will be discussed. Room temperature Hall measurements will be performed to understand the dependence of carrier transport on defects. In summary, the fundamental defect studies are performed on β-Ga2O3 thin films grown via LPCVD on Ga2O3 and sapphire substrates. The results from this study are important knowledge for power device applications.

Monolayer and few-layer molybdenum disulfide (MoS2) has attracted tremendous attention owing to the unique structure as well as excellent semiconducting and optical properties. However, the synthesis of large area few-layer MoS2 with high crystalline quality and highly uniform remains a challenge. In this work, the synthesis of MoS2 nanosheets via chemical vapor deposition (CVD) method was investigated by controlling the heating temperature of MoO3 powder. The results show that the growth temperature is a crucial parameter to adjusting morphology and crystalline quality of MoS2 nanosheets. At a low growth temperature below 730 °C, MoS2 nanosheets tend to lateral growth and be triangular in shape. While at a growth temperature over 730 °C, MoS2 nanosheets turn to longitudinal growth and the morphology translates into hexagonal shapes. Large-size monolayer triangular MoS2 nanosheets with high homogeneity and high crystalline quality were synthesized at the temperature of 700 °C. Moreover, the higher growth temperature is favorable for increasing crystalline quality of MoS2.

Metalorganic chemical vapor deposition (MOCVD) growths of β-Ga2O3 on on-axis (100) Ga2O3 substrates are comprehensively investigated. Key MOCVD growth parameters including growth temperature, pressure, group VI/III molar flow rate ratio, and carrier gas flow rate are mapped. The dependence of the growth conditions is correlated with surface morphology, growth rate, and electron transport properties of the MOCVD grown (100) β-Ga2O3 thin films. Lower shroud gas (argon) flow is found to enhance the surface smoothness with higher room temperature (RT) electron Hall mobility. The growth rate of the films decreases but with an increase of electron mobility as the VI/III molar flow rate ratio increases. Although no significant variation on the surface morphologies is observed at different growth temperatures, the general trend of electron Hall mobilities are found to increase with increasing growth temperature. The growth rates reduce significantly with uniform surface morphologies as the chamber pressure increases. By tuning the silane flow rate, the controllable carrier concentration of (100) β-Ga2O3 thin films between low-1017 cm−3 and low-1018 cm−3 was achieved. Under optimized growth condition, an (100) β-Ga2O3 thin film with RMS roughness value of 1.64 nm and a RT mobility of 24 cm2/Vs at a carrier concentration of 7.0 × 1017 cm−3 are demonstrated. The mobilities are primarily limited by the twin lamellae and stacking faults defects generated from the growth interface. Atomic resolution scanning transmission electron microscopy reveals the formation of twin boundary defects in the films, resulting in the degradation of crystalline quality. Results from this work provide fundamental understanding of the MOCVD epitaxy of (100) β-Ga2O3 on on-axis Ga2O3 substrates and the dependence of the material properties on growth conditions. The limitation of electron transport properties of the (100) β-Ga2O3 thin films below 25 cm2/Vs is attributed to the formation of incoherent boundaries (twin lamellae) and stacking faults grown along the on-axis (100) crystal orientation.

Scanning tunneling microscopy (STM) connected to molecular beam epitaxy (MBE) has been used to investigate InAs islands and wetting layers (WLs) on GaAs[001] substrates. STM results reveal that the size and density of InAs islands strongly depend on the growth temperature. The density of InAs islands decreases and their size increases with increasing growth temperature. From photoluminescence (PL) results, a PL peak shift is observed with increasing InAs growth temperature. This shift agrees with the results of the STM observation that larger islands are grown at higher growth temperature.

Effects of growth temperature and thickness on structure and optical properties of Ga2O3 films grown by pulsed laser deposition

Epitaxial thin GaN films (∼60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500–700 °C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 °C grown GaN film are 1.09° and 1.08°, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30–60 to 70–125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractive index value for the dense GaN film grown at 600 °C is obtained to be ∼2.19 at the wavelength of 632 nm as deduced by spectroscopic ellipsometry. Photoluminescence spectroscopy confirmed that the epitaxial GaN layers grown on a-sapphire at 600–700 °C possess near band edge emission at ∼3.39 eV, close to bulk GaN. The GaN growth at 700 °C without a buffer still produced films with better crystalline and optical properties, but their surface morphology and coverage were inferior to those of the films grown with LT buffer. The results show that the growth temperature strongly influences the structural and optical quality of LMBE grown epitaxial GaN thin films on a-plane sapphire, and a growth temperature of >600 °C is necessary to achieve good quality GaN films.

Stimulation of photosynthesis in response to elevated carbon dioxide concentration [CO2] in the short-term (min) should be highly temperature dependent at high photon flux. However, it is unclear if long-term (days, weeks) adaptation to a given growth temperature alters the temperature-dependent stimulation of photosynthesis to [CO2]. In velveltleaf (Albutilon theophrasti), the response of photosynthesis, determined as CO2 assimilation, was measured over a range of internal CO2 concentrations at 7 short-term measurement (12, 16, 20, 24, 28, 32, 36 degrees C) temperatures for each of 4 long-term growth (16, 20, 28 and 32 degrees C) temperatures. In vivo estimates of VCmax, the maximum RuBP saturated rate of carboxylation, and Jmax, the light-saturated rate of potential electron transport, were determined from gas exchange measurements for each temperature combination. Overall, previous exposure to a given growth temperature adjusted the optimal temperatures of Jmax and VCmax with subsequently greater enhancement of photosynthesis at elevated [CO2] (i.e., a greater enhancement of photosynthesis at elevated [CO2] was observed at low measurement temperatures for A. theophrasti grown at low growth temperatures compared with higher growth temperatures, and vice versa for plants grown and measured at high temperatures). Previous biochemical based models used to predict the interaction between rising [CO2] and temperature on photosynthesis have generally assumed no growth temperature effect on carboxylation kinetics or no limitation by Jmax. In the current study, these models over predicted the temperature dependence of the photosynthetic response to elevated [CO2] at temperatures above 24 degrees C. If these models are modified to include long-term adjustments of Jmax and VCmax to growth temperature, then greater agreement between observed and predicted values was obtained.

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic beta-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases . The breakdown field of beta-Ga2O3 is estimated to be 6-8 MV/cm, which is much larger than that of the 4H-SiC and GaN. These unique properties make beta-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal beta-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Different from the molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) growth techniques, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality beta-Ga2O3 thin films on both native Ga2O3 and c-sapphire substrates with controllable doping and fast growth rates up to 10 um/hr. In this talk, we present the growth, material characterization and device demonstration of beta-Ga2O3 thin films grown via LPCVD. The beta-Ga2O3 thin films were grown on native beta-Ga2O3 (010), (001) and (-201) substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. Fundamental material properties including temperature dependent Hall measurements and device demonstration based on vertical Schottky barrier diodes will be discussed.

One of the major challenges in β-Ga2O3-based high power and high frequency devices is anticipated to be related to the low thermal conductivity of the material which is on the order of 10–30 W/m K. The use of diamond (thermal conductivity ∼2000 W/m K) as a substrate can be one effective approach for achieving better thermal management in β-Ga2O3-based devices. In this work, low pressure chemical vapor deposition was used to grow β-Ga2O3 films on (100) oriented, single-crystalline diamond substrates. A two-step growth technique was employed to avoid the oxidation of the diamond surface at high temperatures. From x-ray diffraction measurements, the β-Ga2O3 films grew along the ⟨−201⟩ crystalline axis with the β-Ga2O3 (002) planes rotated by ±24.3–27° with respect to the diamond (111) planes. High-magnification scanning transmission electron microscopy imaging revealed an abrupt β-Ga2O3/diamond interface without any voids which is essential for the high rate of heat transfer across the interface. N-type electrical conductivity was measured in a Si-doped β-Ga2O3 film with 1.4 × 1019 cm−3 electron concentration and ∼3 cm2/V s electron mobility. This work demonstrates the feasibility of heteroepitaxy of β-Ga2O3 films on diamond substrates for potential device design and device fabrication with efficient thermal management.

Combined effects of rising [CO₂] and temperature on boreal forests: growth, physiology and limitations

Microencapsulation of lactic acid bacteria (LAB) via spray drying differs from that of common bioactive substances in that the intrinsic stress tolerance of cells can be modulated to improve cell survival. In this study, elevated growth temperatures that were 3–5 °C above the standard conditions were used to culture Lactococcus lactis subsp. cremoris, Lactobacillus rhamnosus GG (LGG), and Lactobacillus acidophilus for spray drying. The heat-adapted cultures showed lower bacterial population than the controls by 0.45 log at stationary growth phase and produced lactobacilli cells with elongated shape, while their metabolic activities were maintained similar to the controls. Heat-adapted L. cremoris and LGG demonstrated increases in survival by 0.7–1.5 log and 0.3 log, respectively, after heat treatment at 60 °C. The thermotolerance of L. acidophilus grown at 42 °C was dependent on growth phase, and the culture entered death phase within 24 h of incubation. The survival of heat-adapted L. cremoris and L. acidophilus after spray drying was increased by 21.0% and 13.7%, respectively, whereas the increase shown by LGG was relatively insignificant (9.9%). Spray-dried powders containing heat-adapted cells showed substantial reduction of viability at the first week of storage, reaching 1.03–1.23 log, compared to 0.87–0.90 log of reduction shown by the controls. The findings demonstrated that strain-specific cellular response toward variations in growth conditions is crucial to the intrinsic properties of LAB and to cell survival during spray drying and storage. Controlling cellular response is one of the key factors in developing a viable spray drying scheme for active LAB.

β-Ga2O3 films have been prepared on (0001) sapphire substrates with different off-axis angles toward $?> <112ˉ0> by sol–gel method and the solar-blind ultraviolet photodetectors (PDs) with metal-semiconductor-metal structure have been fabricated on these films. The effects of off-angled sapphire substrates on the surface morphology and crystal quality of β-Ga2O3 films have been systematically investigated. All the results indicate that to some extent the larger off-angled sapphire substrate could lead to the smoother surface morphology and higher crystal quality of β-Ga2O3 films. Reduction in the numbers of in-plane rotational domains for β-Ga2O3 films on off-angled substrates may result in the above-mentioned improvement, which have been confirmed by x-ray diffraction ϕ-scan. β-Ga2O3 film grown on ∼7° off-angled sapphire substrate has ( 400 ) preferred orientation with the single-domain growth mode. Besides, the performance of β-Ga2O3 PDs fabricated on different off-angled substrates show the same trend with changing the off-axis angles. The β-Ga2O3 PD fabricated on ∼7° off-angled sapphire substrate has excellent optical characteristics with large photocurrent to dark current ratio of 430.53 and high UV–visible rejection ratio of 1.42 × 103. These results would be helpful for improving the crystal-quality of hetero-epitaxy β-Ga2O3 films by using off-angled sapphire substrates and fabricating the cost-effective mass-producible solar-blind ultraviolet PDs by sol–gel method.