Efficiency Of Carbon Incorporation Research Articles

First Demonstration of Extrinsic C-Doped Semi-Insulating N-Polar GaN Using Propane Precursor Grown on Miscut Sapphire Substrate by MOCVD

In this study, carbon-doped semi-insulating N-polar GaN on a sapphire substrate was prepared using a propane precursor. Controlling the deposition rate of N-polar GaN helped to improve the carbon incorporation efficiency, providing a semi-insulating behavior. The material quality and surface roughness of the N-polar GaN improved with modified deposition conditions. C-doping using 1.8 mmol/min of propane gave an abrupt doping profile near the GaN/sapphire interface, which was useful for obtaining semi-insulating N-polar GaN grown on sapphire. This study shows that further development of the deposition process will allow for improved material quality and produce a state-of-the-art N-polar semi-insulating GaN layer.

Dynamic utilization of low-molecular-weight organic substrates across a microbial growth rate gradient.

Low-molecular-weight organic substances (LMWOSs) are at the nexus between micro-organisms, plant roots, detritus, and the soil mineral matrix. The nominal oxidation state of carbon (NOSC) has been suggested as a potential parameter for modelling microbial uptake rates of LMWOSs and the efficiency of carbon incorporation into new biomass. In this study, we assessed the role of compound class and oxidation state on uptake kinetics and substrate-specific carbon use efficiency (SUE) during the growth of three model soil micro-organisms, a fungal isolate (Penicillium spinulosum) and two bacterial isolates (Paraburkholderia solitsugae, and Ralstonia pickettii). Isolates were chosen that spanned a growth rate gradient (0.046-0.316 h-1 ) in media containing 34 common LMWOSs at realistically low initial concentrations (25 μM each). Clustered, co-utilization of LMWOSs occurred for all three organisms. Potential trends (p < 0.05) for early utilization of more oxidized substrates were present for the two bacterial isolates (P. solitsugae and R. pickettii), but high variability (R2 < 0.15) and a small effect of NOSC indicate these relationships are not useful for prediction. The SUEs of selected substrates ranged from 0.16 to 0.99 and there was no observed relationship between NOSC and SUE. Our results do not provide compelling population-level support for NOSC as a predictive tool for either uptake kinetics or the efficiency of use of LMWOS in soil solution. Metabolic strategies of organisms are likely more important than chemical identity in determining LMWOS cycling in soils. Previous community-level observations may be biased towards fast-responding bacterial community members.

Properties of carbon-doped GaN using isobutane as a dopant

Carbon doping is an effective method to obtain semi-insulating GaN buffer, which is a necessity to prevent current leakage, in the high-electron-mobility-transistor device structure. The properties of intentionally carbon-doped GaN using isobutane gas as a dopant has been studied in detail. The carbon incorporation efficiency has been measured by secondary ion mass spectrometry. It is found that the carbon concentration could be directly controlled by the flow rate of isobutane precursor. The surface morphology of carbon-doped gallium nitride epitaxial layers has been investigated by optical microscopy and atomic force microscopy. The growth mode of GaN layers changes from step-flow to island growth, when the incorporated carbon concentration is higher than 1×1019 cm-3. In order to evaluate the structural quality of intentionally carbon-doped GaN, the full-width-at-half-maximum values are extracted from the rocking curves in six different reflections measured by high resolution X-ray diffraction. Raman spectroscopy is utilized to evaluate the physical properties of the carbon-doped GaN epitaxial layer.

Effects of intentional oxygen and carbon doping in MOVPE‐grown GaN layers on photoelectric properties

In order to qualitatively and quantitatively analyze the effects of N source impurity species and concentrations on the properties of epitaxial GaN layers, we intentionally introduced the oxygen‐containing species CO and CO2 into GaN layers during growth. Subsequently, we determined the impurities or elements that had a strong effect on the photoelectric properties of the GaN layers and investigated the doping mechanism. The results of this study show that changes in the carrier density are more sensitive to CO2 than to CO. Oxygen as the main donor was found to affect the carrier density and photoluminescence (PL) emission intensities of CO and CO2‐doped GaN, and the carbon incorporation efficiency of these molecules was found to be lower than the oxygen incorporation efficiency. In order to obtain ideal epitaxial GaN layers with a reduced carrier density of 1013 cm−3, the CO and CO2 concentrations in the ammonia source should be less than 100 ppb (CO) and 10−1 ppb (CO2).

Precursors for carbon doping of GaN in chemical vapor deposition

Methane (CH4), ethylene (C2H4), acetylene (C2H2), propane (C3H8), iso-butane (i-C4H10), and trimethylamine [N(CH3)3] have been investigated as precursors for intentional carbon doping of (0001) GaN in chemical vapor deposition. The carbon precursors were studied by comparing the efficiency of carbon incorporation in GaN together with their influence on morphology and structural quality of carbon doped GaN. The unsaturated hydrocarbons C2H4 and C2H2 were found to be more suitable for carbon doping than the saturated ones, with higher carbon incorporation efficiency and a reduced effect on the quality of the GaN epitaxial layers. The results indicate that the C2H2 molecule as a direct precursor, or formed by the gas phase chemistry, is a key species for carbon doping without degrading the GaN quality; however, the CH3 species should be avoided in the carbon doping chemistry.

Epitaxy of Si:C on Si(001) via methyldichlorosilane

Methyldichlorosilane (CH3SiHCl2) is compared to methylsilane (CH3SiH3) for vapor phase epitaxy of cubic silicon - carbon (NO :) (Si:C) alloys on Si (001). Parameters of interest are growth rate, percent carbon and crystallinity. Carbon incorporation efficiency and experimental window for epitaxial growth are similar for both precursors. The two precursors differ in Si contribution to the film growth rate.

Decomposition of Mytilus edulis: The effect on sediment nitrogen and carbon cycling

The benthic degradation of mussel tissue ( Mytilus edulis) was studied in a continuous flow-through system over a 32 day incubation period. Sediment chambers without mussels served as controls. The inflowing artificial seawater and the outflow water were analyzed for dissolved organic nitrogen (DON), short chain fatty acids (SCFA), dissolved inorganic nitrogen (DIN), ∑CO 2 and O 2 during the course of incubation. Sediment profiles of particulate organic carbon (POC), particulate organic nitrogen (PON), total hydrolyzable amino acids (THAA), pore water concentrations of DON and DIN and turnover rate of dissolved free amino acids (DFAA) were measured at four different times during the 32 day experiment. Immediately after the addition of mussel tissue, the chambers became completely anoxic and there was an increase in carbon oxidation and the efflux of DON, SCFA and NH 4 + from the sediment + mussel layer to the overlaying water. During the first 9 days there was a net buildup of DON, and NH 4 + in the sediment followed by a net consumption of the respective N-species during the remainder of the experiment. During the course of incubation 41% of the organic content of the added mussel tissue was released from the sediment as DON, whereas most of the other mussel-N effluxed the sediment as NH 4 +. Only 8% of the added mussel-N remained by the end of the experiment. There were indications of stimulated bacterial growth in both the mussel amended and the unamended sediments. This was measured as a net increase in THAA, which could only be explained by net bacterial growth and/or protein synthesis. During mussel decomposition both the estimated bacterial carbon incorporation efficiency and the C:N ratio of the substrates used by the bacteria were low. This resulted in a low bacterial nitrogen demand. As a consequence, almost all of the nitrogen mineralized within the sediment was released to the water column as NH 4 +.

Incorporation efficiency of carbon in GaAs using carbon tetrabromide in solid source molecular beam epitaxy

The incorporation efficiency of carbon in GaAs using carbon tetrabromide (CBr4) was studied. A series of carbon-doped GaAs samples were grown at different CBr4 pressures and V/III ratios using solid source molecular beam epitaxy (SSMBE). The results showed a high carbon incorporation efficiency of ∼30% within the CBr4 pressure range up to 2×10−7 Torr, which resulted in carbon doping concentration of up to 2×1020 cm−3. However, an increase in V/III ratio was found to reduce the carbon incorporation efficiency. Based on our experimental results, a kinetic model was developed to explain the effect of carbon incorporation in GaAs from CBr4 in SSMBE growth. The model incorporates the effects of different CBr4 decomposition rates on the As-covered and Ga-covered surface.

Growth and thermal stability of pseudomorphic Ge1−yCy/Ge superlattices on Ge(001)

High quality Ge/Ge1−yCy superlattices with nominal carbon contents of 1.2% and 2.1% were grown by molecular beam epitaxy on Ge(001). In transmission electron microscopy the layers are planar and perfectly pseudomorphic without any extended defects observable. The infrared absorption line at 529 cm−1 is attributed to the local vibrational mode of substitutional carbon in germanium. However, in contrast to Si1−yCy alloys where almost 100% of the C is substitutional under optimized growth conditions, x-ray diffraction measurements indicate that the efficiency of carbon incorporation onto substitutional sites is only about 30% for low temperature growth at TS=200 °C. It reduces further for higher growth temperatures to only about 10% at TS=300 °C. Post-growth annealing experiments indicate thermal stability up to 450 °C. Annealing at higher temperature results in a reduction of substitutional carbon content. As in the case of Si1−yCy alloys the built-in strain is relaxed by C diffusion and not by nucleation of misfit dislocation.

Epitaxial Si1−yCy alloys: The role of surface and gas phase reactions

We have studied substitutional carbon incorporation in Si1−yCy alloys grown on Si (100) by ultrahigh vacuum chemical vapor deposition using silane and methylsilane as silicon and carbon precursors. Carbon incorporation has been studied in the growth temperature range of 550–650 °C. The total carbon content of the alloys increases linearly with the methylsilane partial pressure and a methylsilane sticking coefficient approximately two times higher than that of silane was determined from the data. Substitutional carbon content in the alloys tends to saturate at higher methylsilane flows. Our results, together with other previous reports, provide new insight into the roles of gas phase and surface reactions in determining growth rates and carbon incorporation efficiency.

Influence of gallium sources on carbon incorporation efficiency into InGaAs grown by metalorganic chemical vapor deposition

Carbon incorporation into InGaAs has been systematically investigated using trimethylgallium (TMGa) and triethylgallium (TEGa) as gallium sources. Although the hole concentrations decrease with increasing In composition, the hole concentrations in layers grown with TMGa are found to be several times higher than those grown with TEGa. From Hall and secondary ion mass spectroscopy measurements, the hole concentration decrease with increasing In composition is revealed to be due to a decrease in the efficiency of C incorporation into InGaAs. Based on a numerical analysis where it was assumed the In atoms enhance the C source dissociation from the substrate surface, the higher surface step density in the kinetically limited growth mode region is proposed as the essential reason for the higher C incorporation efficiency into InGaAs grown with TMGa. This model is experimentally confirmed by the atomic force microscope observation and comparing higher surface step density substrates.

Carbon doped GaAs grown in low pressure-metalorganic vapor phase epitaxy using carbon tetrabromide

Carbon tetrabromide was used as carbon source for heavily p-doped GaAs in low pressure metalorganic vapor phase epitaxy (MOVPE). The efficiency of carbon incorporation was investigated at temperatures between 550 and 670°C, at V/III ratios from 1 to 50 and carbon tetrabromide partial pressures from 0.01 to 0.03 Pa. Hole concentrations from 8 × 1017 to 5 × 1019 cm−3 in as-grown layers were obtained. After annealing in nitrogen atmosphere at 450°C, a maximum hole concentration of 9 × 1019 cm−3 and a mobility of 87 cm2/Vs was found. At growth temperatures below 600°C, traces of bromine were detected in the layers. Photoluminescence mapping revealed an excellent doping homogeneity. Thus, CBr4 is found to be a suitable carbon dopant source in MOVPE.

P-Type GaAs doped by diiodomethane (CI 2H 2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI 2H 2) as a carbon source. In the low 10 19 cm −3 range, almost all carbon atoms are electrically activated and at 9 × 10 19 cm −3, 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.

P-Type GaAs doped by diiodomethane (CI2H2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI 2 H 2 ) as a carbon source. In the low 10 19 cm −3 range, almost all carbon atoms are electrically activated and at 9 × 10 19 cm −3 , 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.

Diamond Homoepitaxy Kinetics: Growth, Etching, and the Role of Oxygen

ABSTRACTGrowth and etch rates for diamond homoepitaxy have been measured in situ using Fizeau interferometry. Experiments were conducted in a hot-filament reactor using hydrogen, methane, and oxygen feed gases at a reactor pressure of 25 torr. The substrate temperature dependence for growth on diamond(lOO) was studied for 0.5% and 1% CH4 and 0–0.44% O2. Apparent activation energies of 17 and 5 kcal/mol were determined for growth from 0.5% and 1% CH4 in hydrogen, over the ranges of 700 – 1000 °C and 800 – 1050 °C, respectively. When a minimal amount of Oxygen was added to the feedstock, the growth-rate behavior was similar for that with pure methane. With greater amounts of added oxygen, growth rates were higher than those without Oxygen at low temperatures, proceeded through a maximum, and then decreased until etching was observed at high temperatures. Similar behavior was observed for growth from 1% CH4 with and without oxygen. We also measured the temperature dependence for etching of homoepitaxial diamond films in hydrogen with 0–0.1% O2, and observed etch rates of 0.01 – 0.1 microns/hr in the range of 950 – 1150 °C. We propose that oxygen facilitates diamond growth at low temperatures by enhancing the removal of both sp2- and sp3-bonded “errors” and/or by increasing the efficiency of carbon incorporation by roughening the diamond surface, and that these etching processes become dominant at high temperatures.

Characterization of Pulsed Supersonic Cvd System For Diamond Growth

ABSTRACTAn ultra-high vacuum chemical vapor deposition system using pulsed, supersonic jets of halogenated hydrocarbon precursors has been constructed with the intention of growing diamond thin films. Experimental results from this system should determine whether the hyperthermal activation of precursors improves the carbon incorporation efficiency of film growth. The jets, produced by commercially available pulsed valves, were characterized with a fast ionization gauge. Parameters such as valve backing pressure and the pulse width were varied to optimize material flux and avoid the formation of boundary layers above the substrate that could reduce the kinetic energy of the precursors.

Effect of growth conditions on enzyme activities, intracellular kinetics and biomass yields of a new RuMP-type methylotroph (T15)

A new methylotrophic strain (T15), which employs the ribulose monophosphate (RuMP) cycle of formaldehyde assimilation, was isolated on the basis of high in vitro activities of formaldehyde and formate dehydrogenases (19 and 678 mU per mg protein, respectively). Serial subculturing of the strain in batch cultures, on 4 g/l CH3OH for 6 months, led to loss of substantial percentages of the NAD-linked formaldehyde (25%) and formate (98%) dehydrogenases. The activities of these two enzymes were partially recovered when cells were grown continuously at very low dilution rate (0.03 h−1). We found large variations (40 to 1000%) in the activities of other key enzymes of carbon-substrate oxidation (both linear and cyclic) and assimilation, in batch cultures with pure and mixed substrates, and in continuous cultures of different dilution rates. Key intracellular reaction rates, including those of the cyclic and linear substrate oxidation, were measured in vivo using a 14C-tracer technique in both continuous and batch cultures. The results indicate significant variations in these reaction rates, particularly those of linear and cyclic carbon oxidation. Overall, the cyclic oxidation appears to be employed to a larger (although not predominant) extent in strain T15 compared with another RuMP strain (L3) we have previously examined. T15 exhibits high biomass yields (up to 0.63 g cells per g CH3OH) and growth rates (up to 0.46 h−1) on CH3OH in batch cultures. CH3NH2 can also be utilized as a substrate. In continuous culture, T15 could be grown at dilution rates up to 0.36 h−1 with a corresponding biomass yield of 0.4. Examination of a large number of data on the biomass yields of strains T15 and L3 reveals that the large variations in yields derive from the variable branching of carbon flow between linear and cyclic oxidation and assimilation, rather than changes in the biosynthetic efficiency of carbon incorporation into biomass.