Low-pressure Chemical Vapor Research Articles

Top-emitting 940-nm thin-film VCSELs transferred onto aluminum heatsinks

Thin-film vertical cavity surface emitting lasers (VCSELs) mounted onto heatsinks open up the way toward low-power consumption and high-power operation, enabling them to be widely used for energy saving high-speed optical data communication and three-dimensional sensor applications. There are two conventional VCSEL polarity structures: p-on-n and n-on-p polarity. The former is more preferably used owing to the reduced series resistance of n-type bottom distributed Bragg reflection (DBR) as well as the lower defect densities of n-type GaAs substrates. In this study, the p-on-n structures of thin-film VCSELs, including an etch stop layer and a highly n-doped GaAs ohmic layer, were epitaxially grown in upright order by using low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The p-on-n structures of thin-film VCSELs were transferred onto an aluminum heatsink via a double-transfer technique, allowing the top-emitting thin-film VCSELs to keep the p-on-n polarity with the removal of the GaAs substrate. The threshold current (Ith) and voltage (Vth) of the fabricated top-emitting thin-film VCSELs were 1 mA and 2.8 V, respectively. The optical power was 7.7 mW at a rollover point of 16.1 mA.

Growth model for high-Al containing CVD TiAlN coatings on cemented carbides using intermediate layers of TiN

This work concerns high Al-containing TixAl1-xN coatings prepared using low pressure-chemical vapour deposition (LP-CVD). The coatings were examined using electron microscopy techniques, such as scanning transmission electron microscopy (STEM), energy dispersive X-ray analysis (EDX) and transmission Kikuchi diffraction (TKD). An intermediate TiN-layer with a 〈211〉 texture consisting of twinned, needle-shaped grains influences the subsequent growth of the TiAlN layer. The TiAlN grains were columnar with a texture of 〈211〉. As the grains grow along 〈111〉, with {001} facets, this led to a tilted pyramid surface morphology. The grains developed an internal periodic epitaxial nanolamella structure. The thicknesses were 2 nm for the low (x = 0.6) and 6 nm for the high (x = 0.9) Al-containing lamellae. The TiAlN layer growth could be described by a “two-wing” model, where two TiAlN grains with a twin-related orientation grow on a twinned TiN grain, where the two TiAlN grains gradually switch sides, making the appearance of two wings of columnar grains. In general, this work shows that it should be possible to control the growth of TiAlN layers by controlling the texture and morphology of an intermediate layer, such as TiN.

Effects of Thermal Annealing on Optical Properties of Be-Implanted GaN Thin Films by Spectroscopic Ellipsometry

Wide bandgap III-V compounds are the key materials for the fabrication of short-wavelength optical devices and have important applications in optical displays, optical storage devices and optical communication systems. Herein, the variable-angle spectroscopic ellipsometry (SE) measurements are performed to investigate the thickness and optical properties of beryllium-implanted gallium nitride thin films that have been deposited on (0001) sapphire substrates by using low-pressure metalorganic chemical vapor deposition (LPMOCVD). The film layer details are described by using Parametric Semiconductor oscillators and Gaussian oscillators in the wavelength range of 200–1600 nm. The thickness, refractive indices and extinction coefficients of the Be-implanted films are determined at room temperature. Analysis of the absorption coefficient shows that the optical absorption edge of Be-implanted films changes from 3.328 eV to 3.083 eV in the temperature range of 300–850 K. With the variable temperature, Eg is demonstrated to follow the formula of Varshni. A dual-beam ultraviolet–visible spectrophotometer (UV–VIS) is used to study the crystal quality of samples, indicating that the quality of rapid thermal annealing (RTA) sample is better than that unannealed sample. By transport of ions in matter (TRIM) simulation and SE fitting the depths of Be implanted gallium nitride (GaN) films are estimated and in good agreement. The surface and cross-section morphologies are characterized by atomic force microscopy (AFM) and scanning electron microscope (SEM), respectively. The surface morphologies and thickness measurements of the samples show that RTA can improve crystal quality, while increasing the thickness of the surface roughness layer due to partial surface decomposition in the process of thermal annealing.

Effects of In0.82Ga0.18As/InP Double Buffers Design on the Microstructure of the In0.82G0.18As/InP Heterostructure

In order to reduce the dislocation density and improve the performance of high indium content In0.82Ga0.18As films, the design of double buffer layers has been introduced into the In0.82Ga0.18As/InP heterostructure. Compared with other buffer layer structures, we introduce an InP thin layer, which is the same as the substrate, into the In0.82Ga0.18As/InP heterostructure. The epitaxial layers and buffer layers were grown by the low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method. In this study, the surface morphology and microstructures of the heterostructure were investigated by SEM, AFM, XRD and TEM. The residual strains of the In0.82Ga0.18As epitaxial layer in different samples were studied by Raman spectroscopy. The residual strain of the In0.82Ga0.18As epitaxial layer was decreased by designing double buffer layers which included an InP layer; as a result, dislocations in the epitaxial layer were effectively suppressed since the dislocation density was notably reduced. Moreover, the performance of In0.82Ga0.18As films was investigated using the Hall test, and the results are in line with our expectations. By comparing different buffer layer structures, we explained the mechanism of dislocation density reduction by using double buffer layers, which included a thin InP layer.

Incorporation of pervasive impurities on HVPE GaN growth directions

High crystallinity thick films with low free carrier concentration (≤1×1015/cm3) and low compensation are required for many GaN-based electronic device applications. It has been demonstrated that low pressure chemical vapor and molecular beam epitaxy techniques can reproducibility deposit homoepitaxial films with low residual impurity concentrations. However, their typical slow growth rates prevent their utilization for thick film growth. Presently, hydride vapor phase epitaxy is the sole method that can deposit films with residual impurity concentrations ≤5×1016/cm3 at hundreds of microns per hour growth rate. It is crucial to verify if this method can reproducibly deliver thick free-standing GaN films of high crystalline quality with exceptionally low and uniform free carrier concentration. X-ray diffraction, Raman scattering, and low temperature photoluminescence experiments were carried out on a number of samples prepared by dicing a free-standing wafer into several pieces perpendicular and parallel to the major growth directions; namely, c-plane {0001}, a-plane {11−20}, and m-plane {1−100}. SIMS depth profiles were employed to identify and quantify the concentration of the pervasive impurities. Spatial maps of a Raman line sensitive to free-carrier concentration were measured to determine the spatial distribution of the net impurity concentration. The reduced concentration of un-compensated shallow donors was also verified by low temperature electron paramagnetic resonance.

Thermodynamic analysis on deposition of Si[sbnd]B[sbnd]C[sbnd]N ceramic by low pressure chemical vapor deposition/infiltration from SiCH3Cl3[sbnd]BCl3[sbnd]NH3[sbnd]H2[sbnd]Ar system

In order to guide the kinetic deposition experiments of siliconboron carbonitride ceramics (SiBCN) from SiCl3CH3NH3BCl3H2Ar system, the thermodynamic calculation was undertaken using Factsage software. Predominant condensed phases at equilibrium were SiC, Si3N4, BN, B4C and C. The area of phase regions was affected by deposition parameters, such as temperature, total pressure, dilute H2 ratio, nCH3SiCl3/(nNH3+nBCl3) and nNH3/(nNH3+nBCl3). The results showed that SiBCN ceramics with desired phases, such as C+SiC+BN, could be obtained via this system by controlling above parameters theoretically. Kinetic verification at 900°C demonstrated that SiBCN ceramic could be obtained by low pressure chemical vapor deposition/infiltration from this system. The deposition was amorphous from XRD spectra and mainly constituted by CC, SiN and SiC bonds from XPS analysis, which was in agreement with the results of thermodynamic calculation in general.

Silicon nitride and silicon etching by CH3F/O2 and CH3F/CO2 plasma beams

Silicon nitride (SiN, where Si:N ≠ 1:1) films low pressure-chemical vapor deposited on Si substrates, Si films on Ge on Si substrates, and p-Si samples were exposed to plasma beams emanating from CH3F/O2 or CH3F/CO2 inductively coupled plasmas. Conditions within the plasma beam source were maintained at power of 300 W (1.9 W/cm3), pressure of 10 mTorr, and total gas flow rate of 10 sccm. X-ray photoelectron spectroscopy was used to determine the thicknesses of Si/Ge in addition to hydrofluorocarbon polymer films formed at low %O2 or %CO2 addition on p-Si and SiN. Polymer film thickness decreased sharply as a function of increasing %O2 or %CO2 addition and dropped to monolayer thickness above the transition point (∼48% O2 or ∼75% CO2) at which the polymer etchants (O and F) number densities in the plasma increased abruptly. The C(1s) spectra for the polymer films deposited on p-Si substrates appeared similar to those on SiN. Spectroscopic ellipsometry was used to measure the thickness of SiN films etched using the CH3F/O2 and CH3F/CO2 plasma beams. SiN etching rates peaked near 50% O2 addition and 73% CO2 addition. Faster etching rates were measured in CH3F/CO2 than CH3F/O2 plasmas above 70% O2 or CO2 addition. The etching of Si stopped after a loss of ∼3 nm, regardless of beam exposure time and %O2 or %CO2 addition, apparently due to plasma assisted oxidation of Si. An additional GeOxFy peak was observed at 32.5 eV in the Ge(3d) region, suggesting deep penetration of F into Si, under the conditions investigated.

Investigation on HT-AlN Nucleation Layers and AlGaN Epifilms Inserting LT-AlN Nucleation Layer on C-Plane Sapphire Substrate

In this study, we have investigated a high-temperature AlN nucleation layer and AlGaN epilayers on c-plane sapphire substrate by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), scanning electron microscope (SEM) and Raman scattering measurements have been exploited to study the crystal quality, surface morphology, and residual strain of the HT-AlN nucleation layer. These analyses reveal that the insertion of an LT-AlN nucleation layer can improve the crystal quality, smooth the surface morphology of the HT-AlN nucleation layer and further reduce the threading dislocation density of AlGaN epifilms. The mechanism of inserting an LT-AlN nucleation layer to enhance the optical properties of HT-AlN nucleation layer and AlGaN epifilm are discussed from the viewpoint of driving force of reaction in this paper.

DBD reactor design and optimization in continuous AP-PECVD from HMDSO/N2/N2O mixture

Dielectric barrier discharge (DBD) deposition of thin films is increasingly studied as a promising alternative to other non-thermal processes such as low-pressure plasma-enhanced chemical vapor deposition (PECVD) or wet-coating. In this paper we demonstrate how optimizing gas injection in the DBD results in an improvement in the reactor performance. We propose to confine the precursor gas close to the deposition substrate by an additional gas flow. The performance of this design is studied though simulation of mass transport. To optimize the deposited thickness, gas cost and reactor clogging, we assess the influence of the confinement, total gas flow rate and DBD length. The confinement is found to reduce reactor clogging, even for long DBD, and increase the deposit thickness. This increase in thickness requires a proportionate increase in the gas flow-rate, making the gas-cost the main limitation of the proposed design. We show, however, that by fine-tuning the operating conditions a beneficial compromise can be obtained between the three optimization objectives.

Surface phase transition on silicon crystals (100) with low-temperature microwave plasma chemical vapor deposition

Using the Langmuir model of adsorption from the precursor state, the phenomenon of self-organization of nanosized domains with low-pressure ethanol chemical vapor deposition of submonolayer carbonaceous coatings on the atomically-clean surface of silicon crystals (100) is investigated. The dependence of the substrate temperature and the electrical bias on the kinetics and the mechanism of the domain structuring, which is the phase transition on the silicon surface (100), is analyzed. It is shown that, in some chemically-active gaseous environments, the high etching selectivity of silicon and carbon makes it possible to use carbon islands being formed as nonlithographic masks to create nanosized integrated column structures in silicon crystals (100) by means of high-resolution plasma-chemical etching.

The Influence of Overaluminizing on TGO Formation on Thermal Barrier Coatings Deposited by Low Pressure Plasma Spraying and Chemical Vapour Deposition Methods on Rene 80 Nickel Superalloy

The paper presents results of research into isothermal oxidation test of thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were deposited by overaluminizing of MeCrAlY–type coating deposited by LPPS method. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized.. The isothermal oxidation test at 1100oC for 1000h shown that thickness of the TGO layer in overaluminized bondcoat was significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.

Microstructure of Thermal Barrier Coatings Deposited by Low Pressure Plasma Spraying and Chemical Vapour Deposition Methods on Rene 80 Nickel Superalloy

The paper presents results of research into thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were formed by overaluminizing of an MeCrAlY–type coating deposited by low pressure plasma spraying. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized. Used as base material was Rene 80 nickel superalloy. The research has shown that as a result of aluminizing by the CVD method there was formed in the bondcoat a deposit zone built of the β-NiAl phase which protects from oxidation. Preserved below increased chromium content ensures resistance to hot corrosion. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process.

The Development of Polyethylene Naphthalate Films by Low-pressure High-frequency Plasma Chemical Vapor Deposition System with Advance Oxidations Process

AbstractThe low-pressure high-frequency plasma chemical vapor deposition (CVD) system was developed with non-thermal plasma process to study the Polyethylene naphthalate (PEN) surface characteristics. Plasma surface treatment by oxygen can improve the adhesive properties. A mixture of Ar and O

PEN Film Surface Development Using High-Frequency - Low Pressure Plasma Chemical Vapor Deposition System

Solar cell converts sunlight into electricity with no moving parts and environmental friendly. Although silicon based solar cell is currently more efficient, the dye-sensitized solar cell is considerably cheaper to manufacture because of its low cost materials and simplicity process of fabrication. In this paper, the development of plasma formed equipment for thin film material on flexible solar cell using low-pressure high frequency Plasma Chemical Vapor Deposition method on the surface of Polyethylene naphthalate (PEN) with the mixture of Ar gas and N2gas is presented. The results indicate that using this method can be possible for surface modification.

Influence of growth temperature on crystalline quality and Raman property of InAs0.6P0.4/InP

InAs0.6P0.4 epilayers grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) on InP (100) substrate are investigated. The influence of growth temperature on crystalline quality of InAs0.6P0.4 epilayer is characterized by scanning electron microscopy (SEM), Hall measurements, photoluminescence (PL) spectra, and the Raman properties are analyzed by Raman scattering spectrum. The characterization results show that the crystalline quality and Raman property of InAs0.6P0.4 epilayers have close relation to the growth temperature. It indicates that 530 °C is the optimum growth temperature to get good quality and properties of InAs0.6P0.4 epilayers.

Influences of Au nanoparticle density on the performance of GaAs solar cells

The influence of the Au nanoparticle density on the performance of single-junction GaAs solar cells has been investigated. The single-junction GaAs solar cells are grown on (100) GaAs substrates by using a low-pressure metalorganic chemical vapor deposition (MOCVD) system. Au nanoparticles are dispersed on the surfaces of the cells by micro-pipetted casting and the density of the dispersed nanoparticles varies from 4.7 × 109 to 4.3 × 1010 cm−2. The conversion efficiencies of the GaAs solar cells are observed to depend strongly on the Au nanoparticle densities. For Au nanoparticle densities lower than 2.8 × 1010 cm−2, the conversion efficiency improvement of the GaAs solar cells mainly originates from the antireflection coating effect. In contrast, for the sample with a Au nanoparticle density of 2.8 × 1010 cm−2, the antireflection effect decreases due to the shadow effect and the local field enhancement effect due to the Au nanoparticles starts to be effective. The GaAs solar cell with a Au nanoparticle density of 1.9 × 1010 cm−2 shows a maximum conversion enhancement compared to that of the reference sample.

Thermodynamic analysis of the growth of germanium-containing films from Ge(C2H5)4 + hydrogen mixtures

Low-pressure (1.33 and 13.3 Pa) chemical vapor deposition (CVD) of GeCx from tetraethylger-manium + hydrogen mixtures has been analyzed in a wide range of deposition temperatures, 300–1300 K, using thermodynamic modeling. In low-pressure CVD processes, films can be contaminated with impurities, in particular with oxygen, which may originate in the system in question from the residual air pressure in the reactor. Modeling results for the Ge-C-H and Ge-C-H-O-N systems indicate that the presence of oxygen adds significant complexity to the composition of condensed phases. At sufficiently high temperatures (T > 550 K), one can obtain GeCx films essentially free from oxygen impurities. We have identified deposition conditions (T > 650 K) under which the overall composition of the GeCx carbon-containing germanium layers remains constant. Experimental data are presented on the growth of germanium-containing films from a gas mixture with GeEt4: H2 = 1: 10 in the temperature range 573–823 K by plasma-enhanced CVD. At high deposition temperatures (T > 700 K), the composition of the films was near GeCx, in agreement with thermodynamic analysis results.

High Conductivity of Mg-Doped Al0.3Ga0.7N with Al0.4Ga0.6N/AlN Superlattice Structure

The highly conductance of Mg-doped Al0.3Ga0.7N layer using low-pressure metal organic chemical vapour deposition (MOCVD) on Al0.4Ga0.6N/AlN superlattice structure was reported. The rapid thermal annealing (RTA) has been employed for the effective activation and generation of holes, and a minimum p-type resistivity of 3 Ω·cm for p-type Al0.3Ga0.7N was achieved. The RTA annealing impacted on electrical, doping profile and morphological properties of Mg-doped Al0.3Ga0.7N with Al0.4Ga0.6N/AlN superlattice structure have been also discussed. The quality of Mg-doped Al0.3Ga0.7N with Al0.4Ga0.6N/AlN superlattice structure degraded after annealing from HRXRD. At appropriate annealing temperature and time, surface morphology of Mg-doped Al0.3Ga0.7N can be improved. A step-like distribution of [Mg] and [H] in p-type Al0.3Ga0.7N was observed, and thermal diffusion direction of [Mg] and [H] was also discussed.

Photocatalytic Activity of Hierarchically Structured TiO2Films Synthesized by Chemical Vapor Deposition

Hierarchically structured TiO2photocatalyst films were synthesized using low-pressure metal-organic chemical vapor deposition (LPMOCVD) method to examine their photocatalytic activity. The thickness of the TiO2films increased proportionally with increasing deposition time. The TiO2film synthesized at 773 K showed a hierarchical structure composed of vertically grown laminar (112)-oriented anatase crystals. With increasing deposition time, the grain became larger and the morphology became sharper. In the initial CVD stage, small particular crystals were formed, above which sequential growth of layers of columnars with increasing size took place, forming hierarchical structure. The hierarchically structured TiO2film exhibited much higher photocatalytic activity than unhierarchically structured TiO2film. The photocatalytic activity increased with increasing film thickness.

Performance of InGaN Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates with Modified Top-Tip Cone Shapes

InGaN light-emitting diodes (LEDs) were fabricated on cone-shaped patterned sapphire substrates (PSSs) by using low-pressure metalorganic chemical vapor deposition. To enhance the crystal quality of the GaN epilayer and the optoelectronic performance of the LED device, the top-tip cone shapes of the PSSs were further modified using wet etching. Through the wet etching treatment, some dry-etched induced damage on the substrate surface formed in the PSS fabrication process can be removed to achieve a high epilayer quality. In comparison to the LEDs prepared on the conventional sapphire substrate (CSS) and cone-shaped PSS without wet etching, the LED grown on the cone-shaped PSS by performing wet etching for 3 min exhibited 55% and 10% improvements in the light output power (at 350 mA), respectively. This implies that the modification of cone-shaped PSSs possesses high potential for LED applications.