Fabrication of epitaxial ZnO films by atomic-layer deposition with interrupted flow

We formed epitaxial ZnO thin films on a (0001) c-plane sapphire substrate through deposition of atomic layers (ALD) at 25-160 °C using diethylzinc (DEZn) and deionized water as precursors in combination with interrupted flow. High-resolution X-ray diffraction measurements were employed to characterize the microstructure of these films. With interrupted flow, we obtained ZnO thin films with an optimal growth window in a range of 40-160 °C, effectively decreasing the growth temperature by about 120 °C relative to a conventional method involving continuous flow. X-ray reflectivity measurements showed that the rate of growth increased also between 20 °C and 120 °C. The XRD results indicate that the stock time might extend the reaction of DEZn and water through an increased duration. All results show that a low temperature for growth improves the crystalline quality and is consistent with thermodynamically blocked self-compensation.     

Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition

We report on the self-limiting growth and characterization of aluminum nitride (AlN) thin films. AlN films were deposited by plasma-enhanced atomic layer deposition on various substrates using trimethylaluminum (TMA) and ammonia (NH₃). At 185 °C, deposition rate saturated for TMA and NH₃ doses starting from 0.05 and 40 s, respectively. Saturative surface reactions between TMA and NH₃ resulted in a constant growth rate of ~ 0.86 Å/cycle from 100 to 200 °C. Within this temperature range, film thickness increased linearly with the number of deposition cycles. At higher temperatures (≥ 225 °C) deposition rate increased with temperature. Chemical composition and bonding states of the films deposited at 185 °C were investigated by X-ray photoelectron spectroscopy. High resolution Al 2p and N 1s spectra confirmed the presence of AlN with peaks located at 73.02 and 396.07 eV, respectively. Films deposited at 185 °C were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction. High-resolution transmission electron microscopy images of the AlN thin films deposited on Si (100) and glass substrates revealed a microstructure consisting of nanometer sized crystallites. Films exhibited an optical band edge at ~ 5.8 eV and an optical transmittance of > 95% in the visible region of the spectrum.     

Thermal conductivity of InAs quantum dot stacks using AlAs strain compensating layers on InP substrate

The growth and thermal conductivity of InAs quantum dot (QD) stacks embedded in GaInAs matrix with AlAs compensating layers deposited on (1 1 3)B InP substrate are presented. The effect of the strain compensating AlAs layer is demonstrated through Atomic Force Microscopy (AFM) and X-ray diffraction structural analysis. The thermal conductivity (2.7 W/m K at 300 K) measured by the 3ω method reveals to be clearly reduced in comparison with a bulk InGaAs layer (5 W/m K). In addition, the thermal conductivity measurements of S doped InP substrates and the SiN insulating layer used in the 3ω method in the 20–200 °C range are also presented. An empirical law is proposed for the S doped InP substrate, which slightly differs from previously presented results.     

Effect of filament temperature and deposition time on the formation of tungsten silicide with silane

The effect of filament temperature and deposition time on the formation of tungsten silicide upon exposure to the SiH₄ gas in a hot wire chemical vapor deposition process was studied using the techniques of cross-sectional scanning electron microscopy and Auger electron spectroscopy. At a relatively low temperature of 1500 °C, the decomposition of WSi₂ phase and the diffusion of Si towards the silicide/W interface produce a thick W₅Si₃ layer. The diffusional nature leads to a parabolic rate law for silicide growth. An exponential decrease of silicide thickness with temperature between 1600 and 2000 °C illustrates the dominance of Si evaporation at higher temperatures (T ≥ 1600 °C) over the silicide formation.     

Influence of the annealing temperature on a crystal phase of W/WC bilayers grown by pulsed arc discharge

The X-ray diffraction technique was used to study the influence of the temperature on a crystal phase of W/WC bilayer produced by the plasma-assisted pulsed arc discharge. In order to grow the films, a target of W with 99.9999% purity and stainless-steel 304 substrate were used. For the production of W layer, the reaction chamber was filled up with argon gas until reaching a 300 Pa and the discharge was performed at 270 V with 3 pulses. The WC layer was grown in a methane atmosphere at 300 Pa and 275 V discharge voltage with 4 pulses. The active and passive times of the pulsed discharge were 1 and 0.5 s, respectively. The influence of post-annealing temperature of their crystal phases was studied at the post-annealing temperatures up to 600 °C. As-grown layer comprised of mixed phases WC, W₂C and W. The post-annealed layer also comprised of the mixed phases of WC, W₂C and W at annealing temperatures below 600 °C. At the annealing temperature above 600 °C, XRD diffractograms showed only substrate and W peaks, and tungsten carbide peaks were not observed, but the presence of WO phases were detected for an annealing temperature of 600 °C. XPS analyses showed the presence of WC before the annealing process and the existence of C-C bond that is considered responsible for the high polycrystallinity of the material was also detected. The XPS showed the formation of WO₂ and WO₃ without the presence of WC for post-annealing at 600 °C.     

Characteristics of highly orientated BiFeO3 thin films on a LaNiO3-coated Si substrate by RF sputtering

BiFeO₃ (BFO) films were grown on LaNiO₃-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 °C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 °C and at 700 °C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 °C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 °C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 °C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present.     

Supermirror hard-x-ray telescope

The practical use of a grazing x-ray telescope is demonstrated for hard-x-ray imaging as hard as 40 keV by means of a depth-graded d-spacing multilayer, a so-called supermirror. Platinum-carbon multilayers of 26 layer pairs in three blocks with a different periodic length d of 3-5 nm were designed to enhance the reflectivity in the energy range from 24 to 36 keV at a grazing angle of 0.3 deg. The multilayers were deposited on thin-replica-foil mirrors by a magnetron dc sputtering system. The reflectivity was measured to be 25%-30% in this energy range; 20 mirror shells thus deposited were assembled into the tightly nested grazing-incidence telescope. The focused hard-x-ray image was observed with a newly developed position-sensitive CdZnTe solid-state detector. The angular resolution of this telescope was found to be 2.4 arc min in the half-power diameter.     

Energy transfer by radiation in a lumpy layer

Energy transfer by radiation in a layer of lumpy material is examined, A formula is derived for the effective coefficient of heat conduction of the layer for radiation heat transfer.Notation I, K opposing radiant fluxes, W/m² - a _c, r_c, d_c layer absorptivity, reflectivity and transmissivity - a _p.th, d₀ transmissivity for radiant energy passing directly through the layer and after reflection from the layer material - _c emissivity - r material reflectivity - L layer thickness, m - h distance between separate interlayers, m - T temperature, °K - q_r resultant radiant flux in the layer, W/m2 - _eff effective coefficient of radiation heat conduction of the layer, W/m · deg - angular coefficient from one base of the hole to the other - f₀ magnitude of the hole area in the plate, m²/m² - p porosity - E_{incident k} magnitude of incident radiant fluxes on the plate k Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 5, pp. 796–801, May, 1971.     

Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering

Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering were studied. The results indicated that the electrical properties were relative to the oxygen and other impurities rather than to growth orientation. As the sputtering power increases from 25 to 100 W, the preferred-growth orientation of Ta films changes from (200) to (202) and the oxygen and impurities content in the films decrease. The temperature coefficient of resistance also reduces from −289.79 to −116.65 ppm/°C. The O/Ta ratio decrease and grain size reduction related to a change of electrical resistivity were observed at substrate temperatures in the range 300-500 °C. At 650 °C, partial stable α-Ta associated with a sharp decrease of the electrical resistivity was also found.     

Synthesis and characterization of carbon/silica superhydrophobic multi-layer films

C/SiO₂ multi-layer films (3-layer films and 5-layer films) were obtained by sol-gel method and physical deposition on glass plates, and then heated at 500 °C for 1 h under a nitrogen atmosphere. The mechanical adhesive force with the substrate of the multi-layer films was sharply enhanced compared to the as-deposited amorphous carbon film. An absorption layer was formed on heat treated C/SiO₂ multi-layer films by modification of the surface with trimethylchlorosilane, and the wettability of the films changed from hydrophilic to super-hydrophobic. The structures of the physically deposited carbon and the multi-layer films were analyzed by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The experimental results showed that the 5-layer films had a concentric ring structure that caused the film to be superhydrophobic.     

Chlorinated precursor study in low temperature chemical vapor deposition of 4H-SiC

Low temperature (1300 °C) chemical vapor deposition (CVD) of SiC has gained interest in the last years for being less demanding in terms of reaction chamber lifetime, but also for allowing higher p-type dopant incorporation. Chloride-based CVD at low temperatures has been studied using chloromethane with tetrachlorosilane or silane, respectively and with or without controlled HCl addition. In this study we explore the use of methyltrichlorosilane (MTS) at growth temperatures (1300 °C) significantly lower than what is commonly used for homoepitaxial growth of SiC (1600 °C). MTS is a molecule containing all the needed precursor atoms; its effects are compared to the standard CVD chemistry, consisting of silane, ethylene, and HCl.Very different chemistries between the two precursor systems are proposed; in the case of MTS, C/Si ratios higher than 1 were required, however using the standard chemistry ratios lower than 1 were needed to obtain a defect-free epitaxial layer. We also demonstrate the need of using Cl/Si ratios as high as 15 to achieve a growth rate of 13 μm/h for 8° off-axis 4H-SiC epitaxial layers at 1300 °C. Limitations due to the low growth temperature are discussed in light of the experimental evidence on the growth mechanism as determined by the morphology degradation and the limited growth rate. Finally a comparison between the epilayers morphology obtained on 4H-SiC substrates with different off-cuts are presented, confirming the importance of lower C/Si ratios for 4° off-axis material and the inevitable growth of the cubic SiC polytype on on-axis substrates.     

Growth and characterization of tris(8-hydroxyquinoline)-aluminum molecular films

Various tris(8-hydroxyquinoline)-aluminum (Alq3) molecular solid films were grown on top of indium-tin-oxide (ITO) glass substrates using physical vapor deposition. The effect of changing the growth conditions on the properties of the films was studied. From scanning electron microscopy, an Alq3 planar layer over an ITO-substrate was observed at the initial period, and an Alq3 tubular structure (which becomes dominant at substrate temperature T_sub ≧ 90 °C) was found to nucleate from this layer. From X-ray diffraction, the Alq3 planar layer possesses an amorphous character while the Alq3 tubular layer has a triclinic α-phase structure. Based on an Arrhenius plot of the growth rate versus 1/T_sub, the growth behaviors in various T_sub-regions were discussed to be dominated by adhesion (for T_sub < 90 °C), steric effect (90 °C < T_sub < 150 °C), and re-evaporation (T_sub > 150 °C). Then, from optical transmission and photoluminescence spectra performed on the high crystalline Alq3 films, two signals associated with the optical-bandgap E_g absorption and the gap-state absorption were determined and discussed in terms of the optical properties of the constituent Alq3 molecules. Finally, from a fit of E_g(T) by an effective electron-phonon interaction model, the physical significance of these fitting parameters for the Alq3 molecular solid was investigated.     

Diffusion barrier characteristics and shear fracture behaviors of eutectic PbSn solder/electroless Co(W,P) samples

Diffusion barrier characteristics, activation energy (E_a) of IMC growth and bonding properties of amorphous and polycrystalline electroless Co(W,P) (termed as α-Co(W,P) and poly-Co(W,P)) to eutectic PbSn solder are presented. Intermetallic compound (IMC) spallation and an nano-crystalline P-rich layer were observed in PbSn/α-Co(W,P) samples subjected to liquid-state aging at 250 °C. In contrast, IMCs resided on the P-rich layer in PbSn/α-Co(W,P) samples subjected to solid-state aging at 150 °C. Thick IMCs neighboring to an amorphous W-rich layer was seen in PbSn/poly-Co(W,P) samples regardless of the aging type. α-Co(W,P) was found to be a sacrificial- plus stuffed-type barrier while poly-Co(W,P) is mainly a sacrificial-type barrier. The values of E_a's for PbSn/α-Co(W,P) and PbSn/poly-Co(W,P) systems were 338.6 and 167.5 kJ/mol, respectively. Shear test revealed the ductile mode dominates the failure in both α- and poly-Co(W,P) samples. Analytical results indicated the high P content in electroless layer might enhance the barrier capability but degrade the bonding strength.     

Hot-wire chemical vapor deposition of epitaxial film crystal silicon for photovoltaics

We have demonstrated that hot-wire chemical vapor deposition (HWCVD) is an excellent technique to produce high-quality epitaxial silicon at high rates, at substrate temperatures from 620 to 800 °C. Fast, scalable, inexpensive epitaxy of high-quality crystalline Si (c-Si) in this temperature range is a key element in creating cost-competitive film Si PV devices on crystalline seed layers on inexpensive substrates such as display glass and metal foil. We have improved both the quality and rate of our HWCVD Si epitaxy in this display-glass-compatible T range. We understand factors critical to high-quality epitaxial growth and obtain dislocation densities down to 6 × 10⁴ cm⁻² by techniques that reduce the surface oxygen contamination at the moment growth is initiated. We have also developed and validated a model of the HWCVD silicon growth rate, based on fundamentals of reaction chemistry and ideal gas physics. This model enables us to predict growth rates and calculate the sticking coefficient of the Si radicals contributing to film formation between 300 and 800 °C. We obtain efficiencies up to 6.7% with a 2.5-micron absorber layer grown on heavily-doped ‘dead’ Si wafers although these cells still lack hydrogenation and light trapping. Open-circuit voltages up to 0.57 V are obtained on 2-μm cells. Efficient film crystal silicon photovoltaics will require dislocation spacing more than 6 times the cell thickness, or else effective H passivation of the dislocations.     

Low-temperature preparation of phosphorus doped μc-Si:H thin films by low-frequency inductively coupled plasma assisted chemical vapor deposition

The phosphorus doped n-type hydrogenated microcrystalline silicon (n-μc-Si:H) thin films are prepared, at the two low substrate temperatures of room temperature and 200 °C, through a low-frequency inductively coupled plasma assisted chemical vapor deposition. The effect of the substrate temperature on the structural properties of the thin films, such as the X-ray Diffraction (XRD) patterns and the Raman spectra, is studied. The XRD measurements show that the diffraction orientations of the thin films present an obvious change when the radio frequency power is increased from 1300 W to 2300 W. The Raman spectra of the thin films deposited at room temperature unambiguously present a phase transition from the amorphous structure to microcrystalline structure whereas no structural phase transition is observed for the thin films deposited at 200 °C. The effect of the substrate temperature on the crystalline volume fraction of the thin films presents a large difference for the radio frequency power in the range of 1300 W-1700 W, while the difference becomes small when the power is increased from 1700 W to 2300 W. The deposition rate and the radio frequency power-sheet resistance curve of the thin films deposited at room temperature are obviously different from those of the thin films prepared at 200 °C. It is attributed to the joint effect of the radio frequency power and substrate temperature on the doping concentration. The electron energy distribution function of the species in the chamber is mainly distributed in a low energy range.     

Dielectric properties and noncontact damage detection of plain-woven fabric glass fiber reinforced epoxy matrix composites using millimeter wavelength microwave

Electromagnetic wave reflections from glass fiber reinforced epoxy matrix composites with 0°/90° and ±45° fiber oriented plain-woven glass fabric (PW-GFRP-0/90, PW-GFRP-±45) at incident angles of 30°, 40° and 50° were measured in the frequency range 50-75 GHz using a free-space reflection measurement system. The complex dielectric constants of both composites were calculated using a simple transmission line theory. The complex dielectric constants of PW-GFRP-0/90 and PW-GFRP-±45 are similar and were measured to be ε′ = 4.61 ± 0.01 and ε″ = 0.16 ± 0.002, respectively.The damage stored in PW-GFRP-0/90 and PW-GFRP-±45 was also evaluated by dielectric constant changes using the same system at an incident angle of 30°. For both composites, ε′ decreased with increasing applied stress and damage parameter. The dielectric constant change is effective for detecting the damage stored in composites and can be used to quantitatively evaluate the damage.     

Ablation behavior and mechanism of 3D C/ZrC composite in oxyacetylene torch environment

Ablation property of three dimensional carbon fiber reinforced zirconium carbide composite (3D C/ZrC composite) was determined using oxyacetylene torch test with a heat flux of 4187 kW/m² and flame temperature of over 3000 °C. C/ZrC composite exhibited an excellent configurational stability with a surface temperature of over 2000 °C during 60-300 s period, while 3D C/SiC composite was perforated at 55 s. After ablation for 300 s, the composite showed a mass loss rate of 0.006 g/s and a linear recession rate of 0.004 mm/s. The formation of zirconia melt on the surface of the C/ZrC composite contributed mainly the ablation property improvement. The C/ZrC composite after ablation showed four different layers due to the temperature and pressure gradients: the melting layer, the loose tree-coral-like ZrO₂ layer, the undersurface oxidation layer, and the composite layer.     

Atomic layer deposition of ZnS via in situ production of H2S

Atomic layer deposition (ALD) of ZnS films utilizing diethylzinc and in situ generated H₂S was performed over a temperature range of 60 °C-400 °C. This method for generating H₂S in situ was developed to eliminate the need to store high pressure H₂S gas. The H₂S precursor was generated by heating thioacetamide to 150 °C in an inert atmosphere, producing acetonitrile and H₂S as confirmed with mass spectroscopy. ALD behavior was confirmed by investigation of growth behavior and saturation curves. The properties of the films were studied with X-ray diffraction, transmission electron microscopy, ellipsometry, atomic force microscopy, scanning electron microscopy, ultraviolet-visible spectroscopy, and X-ray photoelectron spectroscopy. The results show a growth rate that monotonically decreases with temperature, and films that are stoichiometric in Zn and S. The root mean square roughness of the films increases with temperature above 100 °C. A change in crystal phase begins at ∼ 300 °C. The band gap is dependent on the crystal phase and is estimated to be 3.6-4 eV.     

Grazing incidence X-ray study of Ge-nanoparticle formation in (Ge:SiO2)/SiO2 multilayers

We present the study of formation of Ge-nanoparticles (Ge-NP) in germanosilicate (Ge:SiO₂) multilayer (ML) films under thermal treatment. In anticipation of controllable formation of Ge-NP, ML films were prepared by magnetron deposition at room temperature as 20 bi-layer stacks, each bi-layer comprised of a 7 nm thick layer of (Ge + SiO₂) (molar ratio: 60:40) succeeded by a 7 nm thick layer of pure SiO₂, and then annealed for 1 h, up to T_a = 900 °C. Formation and morphology of Ge-NP were analyzed by combining the information obtained from the grazing incidence small angle X-ray scattering and X-ray diffraction. It was found that precipitation of Ge-NP starts at T_a = 600 °C, while high degree of in-plane confinement and lateral ordering of rather uniform precipitated particles is achieved at T_a =  700-800 °C range. At still higher annealing temperature T_a > 800 °C, volume fraction of precipitated Ge-NP in SiO₂ matrix diminishes due to the out-diffusion of Ge atoms from the film, while Ge-NP are no more well confined to (Ge + SiO₂) layers.     

High reflectivity multilayer for He-II radiation at 30.4 nm

SiC/Mg and B(4)C/Mo/Si multilayers were designed for He-II radiation at 30.4 nm. These multilayers were prepared by use of a direct current magnetron sputtering system and measured at the National Synchrotron Radiation Laboratory, China. The measured reflectivities were 38.0% for the SiC/Mg multilayer at an incident angle of 12 deg and 32.5% for the B(4)C/Mo/Si multilayer at 5 deg, respectively. A dual-function multilayer mirror was also designed by use of the aperiodic SiC/Mg multilayer. Annealing experiments were performed to investigate the thermal stability of the SiC/Mg multilayer. The interface of the SiC/Mg multilayer before and after annealing was studied by electron-induced x-ray emission spectra, which evidences the absence of thermal reaction products at the interfaces after annealing.