H2 Carrier Gas Research Articles

Role of growth parameters in equalizing simultaneous growth of N‐ and Ga‐polar GaN by MOCVD

AbstractIn preparing periodically‐oriented gallium nitride (PO GaN) samples for optical applications, smooth surfaces are essential. Although post‐growth processing of these surfaces is possible, it is challenged by significantly different reactivity of the two polar surfaces. To this end, it would be highly desirable to grow PO GaN structures with smooth surfaces. In this paper, the influence of growth parameters on the morphology of PO GaN structures is investigated. The three primary parameters investigated include the V/III ratio of the gas phase precursors, the nature of the carrier gas, and the geometry of the structure. The V/III ratio was varied from 1100 to 5500. Substitution of N2 for the standard H2 carrier gas is investigated. Finally, the effects of changing the structure periodicity from 2 to 100 mm are studied. Smoother structures (resulting from more equal growth rates of the two polarities) are achieved with increases in V/III ratio, substitution of H2 with N2, and by reducing the stripe width. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermopower Enhancement of Bi2Te3 Films by Doping I Ions

The thermoelectric properties of I-doped Bi2Te3 films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi2Te3 films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi2Te3 films was easily controlled by the variation in a flow rate of H2 carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi2Te3 films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi2Te3 films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi2Te3 films. Through doping I ions into Bi2Te3, the thermopower was also enhanced in Bi2Te3, and a high power factor of 5 × 10−3 W K−2 m−1 was achieved.

Controlled Growth of High-Quality Monolayer WS2 Layers on Sapphire and Imaging Its Grain Boundary

Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm(2). Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions.

The MOVPE growth mechanism of catalyst-free self-organized GaN columns in H2 and N2 carrier gases

Columnar structures of III–V semiconductors recently attract considerable attention because of their potential applications in novel optoelectronic and electronic devices. In the present study, the mechanisms for the growth of catalyst-free self-organized GaN columns on sapphire substrate by metal organic vapor phase epitaxy have been thoroughly investigated. The growth behaviours are strongly affected by the choice of carrier gas. If pure nitrogen is used, Ga droplets are able to accumulate on the top of columns during growth, and they are converted into a high quality GaN layer during the cool down phase due to nitridation. Hydrogen as the carrier gas can improve the optical quality of the overall GaN columns substantially, and in addition increase the vertical growth rate. In this case, no indication of Ga droplets could be detected. Furthermore, silane doping during the growth promotes the vertical growth in both cases either pure nitrogen or pure hydrogen as the carrier gas.

Epitaxial Growth of Ge1-xSnx by Reduced Pressure CVD Using SnCl4 and Ge2H6

The epitaxial growth of Ge and GeSn on Si(100) is studied in the low temperature regime by reduced pressure chemical vapor deposition using showerhead technology. Most emphasis is placed on the growth kinetics in the low temperature regime of 375oC to 500oC which is characterized by surface limited reactions. Using Ge2H6 precursor low activation energy of 0.7 eV was determined for H2 carrier gas which further decreases to 0.5 eV if N2 is used. GeSn layers with 10 and 6.5 at.% Sn are obtained at growth temperatures of 375oC and 400oC respectively, using Ge2H6 and SnCl4 as precursors. All Ge and GeSn layers are of high crystalline quality.

Reduced Pressure CVD Growth of Ge and Ge1−xSnx Alloys

The epitaxial growth of Ge and GeSn alloys on Si(100) by Reduced Pressure Chemical Vapor Deposition is discussed. Particular emphasis is placed on the growth kinetics in the temperature window between 375°C and 475°C. Using Ge2H6 precursor and H2 carrier gas a low activation energy of 0.7 eV was found for the Ge growth which further decreases to 0.5 eV for N2 carrier gas. Thin Ge layers with high crystalline quality are aimed which may be used as buffers for the epitaxial growth of GeSn layers. Furthermore, GeSn alloys with up to 10 at.% Sn are synthesized on Si(100) at growth temperatures of 375°C employing Ge2H6 and SnCl4 precursors. All Ge and GeSn layers exhibit high crystalline quality as revealed by X-ray reciprocal space mapping and are nearly fully strain relaxed.

Effects of carrier gas ratio and growth temperature on MOVPE growth of AlN

AbstractThe strong demand for high‐efficiency light‐emitting diodes in the deep‐ultraviolet region and for sensors has led to renewed interest in the growth of high‐quality AlN. However, the growth of high‐quality AlN is extremely difficult owing to the low surface migration rate of Al adatoms. In this study, the effects of the carrier gas ratio and growth temperature on the growth rate of AlN are examined with the aim of optimizing the growth conditions of high‐temperature AlN. AlN epilayers were grown on sapphire substrates by metal‐organic vapor phase epitaxy (MOVPE) at a growth temperature of 1430 °C with a medium‐temperature AlN layer grown at 1100 °C. A combination of H2 and N2 was used as the carrier gas in the growth of AlN, and the ratio of H2 to N2 in the carrier gas was changed to examine its effect on the growth of AlN. Under all growth conditions, crack‐free and smooth surfaces at the atomic level with an extremely small tilt were obtained. The growth rate decreased with increasing growth temperature and ratio of H2 in the carrier gas. This suggests that the high temperature and the H2 carrier gas inhibit the adsorption of atoms. Also, with increasing ratio of N2 in the carrier gas, the radius of curvature of the AlN layer increased and the compressive strain decreased. The reduction of compressive strain and increase in curvature of the AlN layer were due to an increase in the number of dislocations. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier gas and ion beam parameter effects on the structure and properties of a-C:H/SiOx films deposited employing closed drift ion beam source

In the present study closed drift ion beam source was used to deposit SiOx containing amorphous hydrogenated carbon films (a-C:H/SiOx) employing hexamethyldisiloxane and H2 or He carrier gas. The structure, optical and mechanical properties of a-C:H/SiOx films deposited using different ion beam energies (300–800eV) and ion beam current densities (20–80μA/cm2) were analyzed. Raman spectroscopy has shown that the structure of a-C:H/SiOx films deposited using different carrier gas differs. In the case of H2 carrier gas I(D)/I(G) ratio decreased from 1.1 to 1 with the increase of ion beam energy from 300eV to 500eV. It is shown that the increase tendency observed for I(D)/I(G) ratio dependence on the ion beam current density was influenced by the structural changes (Si–H bonds formation) observed in FTIR analysis. The films with maximum hardness (12.8GPa for He and 11.9GPa for H2 carrier gas) were formed at 500eV ion beam energy for both carrier gas. The band gap and B parameter of the films (formed at 500eV with H2 carrier gas) increase almost linearly with the ion beam current density.

Nitrogen and copper doping in MgxZn1−xO films and their impact on p-type conductivity

We report on nitrogen (N) and copper (Cu) doping in MgxZn1−xO films grown via remote-plasma-enhanced metalorganic chemical vapor deposition and their impact on p-type conduction. The MgxZn1−xO films showed a transformation of conductivity from n-type to p-type with increasing Mg content (x), which occurred at x = 0.078 with N2 carrier gas and x = 0.106 with H2 carrier gas. A comparison of the temperature dependent photoluminescence and the Hall effect measurement data revealed that the transformation of the conduction type with the increasing Mg content of MgxZn1−xO films is due to a conduction band up shift that causes the enhancement of the activation energy of donor states and the reduction of n-type residual electron concentrations. The p-type conductivity of MgxZn1−xO films with a low value of Mg content with N2 carrier gas was due to the contribution from the (N)O acceptor. Though N doped MgxZn1−xO (MgxZn1−xO:N) films showed p-type conductivity for x > 0.078, the N and Cu co-doped MgxZn1−xO (MgxZn1−xO:Cu,N) films showed n-type conduction with larger electron concentrations. Despite being a good acceptor, Cu doped MgxZn1−xO films had an n-type conductivity that was due to the contribution from shallow donors such as Cui and CusZni. Moreover, the reduction of the Mg content of MgxZn1−xO:Cu,N films led to an increase in the residual electron concentrations and contributed to the n-type conductivity.

Second Harmonic Generation Indicates a Better Si/Ge Interface Quality for Higher Temperature and With $\hbox{N}_{2}$ Rather Than With $\hbox{H}_{2}$ as the Carrier Gas

In order for germanium (Ge) to replace silicon in advanced MOSFET channels, proper passivation of Ge is required. For this purpose, an ultrathin epitaxial Si cap was grown on Ge(001), and we applied second harmonic generation (SHG) in order to probe the Si/Ge interface quality. SHG indicates a better interface quality for a growth temperature of 500°C rather than 450°C. Similarly, a better quality of the interface is observed upon replacing the conventional H2 carrier gas with N2. Additionally, from the SHG signal, we were able to extract both the thickness of the native SiO2 layer (~4 monolayers (MLs)] and the thickness of the strained Si layer (relaxation at ~12 MLs). These results are important for building Ge-based electronic components.

Effect of H2 Carrier Gas on the Growth of GaN Nanowires on Si(111) Substrates by Metalorganic Chemical Vapor Deposition

We report on the morphological changes of GaN nanowires (NWs) induced by varying H2 carrier gas flow rate. The GaN NWs were grown on Au-coated silicon (111) substrates by metalorganic chemical vapor deposition (MOCVD). The surface morphology and optical characterization of the grown GaN NWs were studied by field-emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) and cathodoluminescence (CL) measurements, respectively. The GaN NWs with uniform diameters from bottom to top sizes ranging from 60 to 100 nm and lengths up to 2–3 µm were obtained. Energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the presence of gallium and nitrogen in the grown GaN NWs. It was observed that the lateral growth behavior of the GaN NWs prevailed in the absence of H2 carrier gas. On the other hand, the vertically aligned growth tendency of the GaN NWs was induced by the supply of H2 carrier gas.

Development of a MEMS Gas Flow Sensor Unified with an Impurity-Gas Concentration Sensor

Thermal conduction type gas flow sensor unified with an impurity-gas concentration sensor using the MEMS processing technique is proposed. This unified sensor has a pair of returning groove channels, and each channel has a sensor-unit composed of a central thermocouple and a pair of thermocouples formed by both sides of the central one. The central thermocouple is working as both a heater and a temperature sensor working as a current detection type thermocouple. This unified sensor can measure both carrier gas (H2) flow and impurity-gas concentration. The impurity-gas is mixed on the way into the H2 carrier gas. The extremely compact sensor with high sensitivity is able to realize. The carrier gas flow is measured using one of sensor-units, and the impurity-gas concentration is obtained from measurement of differential temperature difference between two central thermocouples of sensor-units in the cooling process after cease of thermocouple-heating during stopping the gas flow.

Gas Chromatograph-Combustion System for 14C-Accelerator Mass Spectrometry

A gas chromatograph-combustion (GC-C) system is described for the introduction of samples as CO(2) gas into a (14)C accelerator mass spectrometry (AMS) system with a microwave-plasma gas ion source. Samples are injected into a gas chromatograph fitted with a megabore capillary column that uses H(2) as the carrier gas. The gas stream from the outlet of the column is mixed with O(2) and Ar gas and passed through a combustion furnace where the H(2) carrier gas and separated components are quantitatively oxidized to CO(2) and H(2)O. Water vapor is removed using a heated nafion dryer. The Ar carries the CO(2) to the ion source. The system is able to separate and oxidize up to 10 microg of compound and transfer the products from 7.6 mL/min of H(2) carrier gas into 0.2-1.0 mL/min of Ar carrier gas. Chromatographic performance and isotopic fidelity satisfy the requirements of the (14)C-AMS system for natural abundance measurements. The system is a significant technical advance for GC-AMS and may be capable of providing an increase in sensitivity for other analytical systems such as an isotope-ratio-monitoring GC/MS.

Adhesion and Cu Diffusion Barrier Properties of a MnOx Barrier Layer Formed with Thermal MOCVD

AbstractA thin-amorphous MnOx layer using self-forming barrier process with a Cu-Mn alloy shows good adhesion and diffusion barrier properties between copper and dielectric layer, resulting in excellent reliability for stress and electromigration. Meanwhile, chemical vapor deposition (CVD) can be employed for conformal deposition of the barrier layer in narrow trenches and vias for future technology node. In our previous research, a thin and uniform amorphous MnOx layer could be formed on TEOS-oxide by thermal metal-organic CVD (MOCVD), showing a good diffusion barrier property. In addition, a good adhesion strength is necessary between a Cu line and a dielectric layer not only to ensure good SM and EM resistance but also to prevent film delamination under mechanical or thermal stress conditions during fabrication process such as chemical mechanical polishing or high temperature annealing. To date, no information is available with regard to the adhesion property of CVD-MnOx. In this work, we report diffusion barrier property in further detail and adhesion property in PVD-Cu/CVD-MnOx/SiO2/Si. The temperature dependence of the adhesion property is correlated with the chemical composition and valence state of Mn investigated with SIMS and Raman spectroscopy.Substrates were p-type Si wafers having a plasma-TEOS oxide of 100nm in thickness. CVD was carried out in a deposition chamber. A manganese precursor was vaporized and introduced into the deposition chamber with H2 carrier gas. After the CVD, a Cu overlayer was deposited on some samples using a sputtering system in load lock chamber of the CVD machine. The diffusion barrier property of the MnOx film was investigated in annealed samples at 400 oC for 100 hours in a vacuum of better than 1.0×10-5 Pa. The Adhesion property of Mn oxide was investigated by Scotch tape test in the as-deposited and in the annealed Cu/CVD-MnOx/TEOS samples. The obtained samples were analyzed for thickness and microstructure with TEM, chemical bonding states of the MnOx layer with XPS, and composition of each layer with SIMS.In the CVD deposition below 300 °C, no Cu delamination was observed both in the as-deposited and in the annealed Cu/CVD-MnOx/SiO2 samples. On the other hand, in the CVD deposition at 400 °C, the Cu films were delaminated from the CVD-MnOx/TEOS substrates. The XPS peak position of Mn 2p and Mn 3s spectra indicated that the valence state of Mn in the as-deposited barrier layer below 400 °C was 2+. Composition analysis with SIMS as well as Raman also indicated the presence of a larger amount of carbon at 400 °C than at less than 300 °C. The good adhesion between Cu and MnO could be attributed to an amount of carbon inclusion in the CVD barrier layer.

Atomic arrangement, composition and orientation of hexagonal BCN films synthesized by radiofrequency plasma enhanced CVD

Oriented hexagonal BCN films were synthesized on Si (1 0 0) substrate by radiofrequency plasma enhanced chemical vapor deposition using tris-(dimethylamino)borane as a precursor. The formation of the crystalline hexagonal BCN hybrids was confirmed by FT-IR and XRD. CH4 + H2 carrier gas increased the C content in the films in comparison with N2 carrier gas. XPS measurement showed that the films were composed of a variety of B-N, B-C, C-N bonds to form the BCN atomic hybrids. The π* resonance peak in the B K-edge NEXAFS spectra also indicated the presence of the sp2 hybrid configuration of B-C-N bonds like BN3 in h-BN. The σ* resonance peaks suggested the diversity in the σ bonds in the films. Orientation of the h-BCN hybrids was suggested on the basis of the polarization dependence of NEXAFS.

H2 carrier gas dependence of Young's modulus and hardness of chemical vapor deposited polycrystalline 3C-SiC thin films

This paper presents the mechanical properties of poly (polycrystalline) 3C-SiC thin films according to 0%, 7%, and 10% carrier gas (H2) concentrations using nano-indentation. When H2 concentration was 10%, it has been proved that the mechanical properties, Young's modulus, and hardness of poly 3C-SiC films are the best of them. In the case of 10% H2 concentration, Young's Modulus and hardness were obtained as 367 and 36GPa, respectively. The surface roughness according to H2 concentrations was investigated by AFM (atomic force microscope). When H2 concentration was 10%, the roughness of 3C-SiC thins was 9.92nm, which is also the best of them. Therefore, in order to apply poly 3C-SiC thin films to MEMS (micro-electromechanical system) applications, H2 concentration's rate should increase to obtain better mechanical properties and surface roughness.

In situ Gravimetric Monitoring of Decomposition Rate on Surface of (1012) R-Plane Sapphire for High-Temperature Growth of Nonpolar AlN

The thermal stability and reaction with H2 of the surface of a (1012) R-plane sapphire substrate at high temperatures were investigated using an in situ gravimetric monitoring system. Although the R-plane sapphire surface was stable up to 1400 °C in an inert carrier gas, decomposition started at 1200 °C in H2 carrier gas. In addition, the activation energy for the decomposition of R-plane sapphire in H2 carrier gas changed at approximately 1300 °C, which indicates that the rate-limiting reaction for the decomposition of the R-plane sapphire substrate shifts near 1300 °C. These results agree with those for the stability of the C-plane sapphire substrate, as described previously. However, it was found that decomposition rate of the R-plane sapphire substrate was significantly greater than that for the C-plane substrate. The relationship between the decomposition rate and the surface orientation of sapphire and detailed mechanisms for the decomposition of R- and C-plane sapphire in H2 carrier gas are discussed.

In situ gravimetric monitoring of decomposition rate on the surface of (0001) c‐plane sapphire for the high temperature growth of AlN

AbstractThe thermal stability of (0001) sapphire was investigated at atmospheric pressure using the in situ gravimetric monitoring (GM) method. The weight change of a sapphire substrate was monitored at various hydrogen partial pressures in carrier gas (P) at temperatures over 1200 °C. Although the sapphire substrate was stable up to 1450 °C in an inert carrier gas (P = 0.0 atm), sapphire decomposition started to occur at 1200 °C in H2 carrier gas (P = 1.0 atm). Moreover the activation energy and order of reaction for sapphire surface decomposition changed at approximately 1300 °C. These results indicate that the rate‐limiting reaction for sapphire decomposition shifts near 1300 °C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical Investigation of the Gas-Phase Kinetics Active during the GaN MOVPE

Quantum chemistry investigations have been performed to study the gas-phase chemistry active during the MOVPE of GaN when Ga(CH3)(3) and NH3, diluted in a H2 carrier gas, are used as precursors. Optimized molecular geometries, energies, and transition-state structures of gas-phase species have been determined with density functional theory at the B3LYP/6-311+g(d,p) level. On the basis of the similarity with the soot formation mechanism active during hydrocarbon combustion, we propose that in this system a gas-phase chemistry is active and its reactivity is enhanced by a radical chain mechanism started from methyl radicals. Initiation reactions are surface processes or the pyrolysis of Ga(CH3)(3). A propagation mechanism composed of fast radical reactions, most of which without an activation energy, was identified, and kinetic constants were determined for each step. The proposed mechanism is able to describe the formation of large GaN adducts formed by up to three R-Ga-NH units. These molecules can give fast cyclization reactions that lead to the formation of six-membered cyclic species, which, similar to benzene for combustion, are thermodynamically stable in vast temperature and pressure ranges and can thus be considered as the first GaN nuclei. We also found that the presence of H2 as a carrier gas can greatly enhance the rate of formation of gas-phase particles because it is a major source of atomic hydrogen, a promoter of gas-phase reactivity.

Theoretical Study of the Pyrolysis of Methyltrichlorosilane in the Gas Phase. 1. Thermodynamics

Structures and energies of the gas-phase species produced during and after the various unimolecular decomposition reactions of methyltrichlorosilane (MTS) with the presence of H2 carrier gas were determined using second-order perturbation theory (MP2). Single point energies were obtained using singles + doubles coupled cluster theory, augmented by perturbative triples, CCSD(T). Partition functions were obtained using the harmonic oscillator-rigid rotor approximation. A 114-reaction mechanism is proposed to account for the gas-phase chemistry of MTS decompositions. Reaction enthalpies, entropies, and Gibbs free energies for these reactions were obtained at 11 temperatures ranging from 0 to 2000 K including room temperature and typical chemical vapor deposition (CVD) temperatures. Calculated and experimental thermodynamic properties such as heat capacities and entropies of various species and reaction enthalpies are compared, and theory is found to provide good agreement with experiment.