Decreasing Substrate Temperature Research Articles

Optical properties of CdIn 2O 4 thin films prepared by DC reactive magnetron sputtering

Transparent conducting oxides CdIn 2O 4 thin films were prepared by direct-current (DC) reactive magnetron sputtering from a Cd–In alloy target in Ar+O 2 atmosphere. The thickness of the films prepared in different oxygen concentration, substrate temperature and annealing treatment was calculated by an envelope method from transmission spectra. Meanwhile, the optical constants and the optical band gap of these films have been determined. It is found that the thickness of the films increases with the decrease of oxygen concentration or substrate temperature, and the optical band gap increases with the decrease of oxygen concentration or the increase of substrate temperature. After annealing treatment, the thickness decreases sharply and the band gap widens. The optical properties are sensitive to the preparing conditions and annealing treatment, which is due to the “Burstein–Moss” shift and many-body effect. The width of localized states E 0 is determined and the effect of band tailing is substantial.

Initiated Chemical Vapor Deposition of Poly(1H,1H,2H,2H-perfluorodecyl Acrylate) Thin Films

A solvent-free initiated chemical vapor deposition (iCVD) process was used to create low surface energy poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) thin films at deposition rates as high as 375 nm/min. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy showed full retention of the fluorine moieties, and no measurable cross-linking was detected. Additionally, the FTIR studies support the hypothesis that film deposition results from vinyl polymerization. For all iCVD PPFDA films, the static contact angle was found to be 120.8 +/- 1.2 degrees. The roughness of the films was found to be between 14.9 and 19.8 nm RMS, and the refractive index of the films was found to be between 1.36 and 1.37. The deposition rate was studied as a function of the substrate temperature and the partial pressure of the monomer. It was found that the deposition rate increases with decreasing substrate temperature and increasing monomer partial pressure. It was also found that the molecular weight increases with decreasing substrate temperature and increases with increasing monomer partial pressure. The highest molecular weight measured was 177 300 with a polydispersity of 2.27. Quartz crystal microbalance (QCM) measurements showed that these effects correlated with an increased monomer concentration at the surface. The deposition rate data and the QCM data were quantitatively analyzed to find the rate constants of the process using a previously published model for the iCVD process involving nonfluorinated monomers. The determined values of the rate constants of the surface polymerization were found to be similar to the rate constants measured in liquid-phase free radical polymerization. The kinetic data found in this paper can now be used to study iCVD deposition onto substrates with more complex geometries.

Room-Temperature Sputter-Deposited Gate SiO2 Films for High Quality Poly-Si TFTs

Gate SiO2 films in poly-Si TFTs were fabricated by sputtering at substrate temperature from 300°C down to room-temperature. It was found that poly-Si TFT characteristics of mobility, threshold voltage and subthreshold slope were greatly improved by decreasing substrate temperature. Mobility of the poly-Si TFT for room-temperature was 90 cm2/V.s, much higher than 50 cm2/V.s for 300°C. Moreover, sputtered gate SiO2 poly-Si TFTs showed high reliability against hot carrier effect even for room-temperature-deposition.

A Scalable Method for the Synthesis of Metal Oxide Nanowires

We report a novel, scaleable and versatile method for large scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates including amorphous quartz, fluorinated tin oxide, etc. The synthesis concept uses the chemical vapor transport of metal oxide vapor phase species onto substrates using air or oxygen flow over hot-filament sources. The hot-filaments were designed to provide uniform heating and gas phase composition over large substrate areas. The results show that the nucleation densities could be varied from 106/cm2 to 1010/cm2 with decreasing substrate temperature. A thermodynamic model for determining the nucleation density and nanowire diameter is presented. The analysis indicates that the nucleation for nanowires occurs through condensation of sub-oxide, WO2 species. This is in contrast to WO3 species condensation into WO3 solid phase for nanoparticle formation. The dispersion behavior of as- synthesized nanowires in aqueous and organic solvents is also studied.

Plasma-assisted atomic layer deposition of Al2O3 moisture permeation barriers on polymers

Thin Al2O3 films of different thicknesses (10–40nm) were deposited by plasma-assisted atomic layer deposition on substrates of poly(2,6-ethylenenaphthalate) (PEN), and the water vapor transmission rate (WVTR) values were measured by means of the calcium test. The permeation barrier properties improved with decreasing substrate temperature and a good WVTR of 5×10−3gm−2day−1 (WVTRPEN=0.5gm−2day−1) was measured for a 20nm thick Al2O3 film deposited at room temperature using short purging times. Such ultrathin, low-temperature deposited, high-quality moisture permeation barriers are an essential requirement for the implementation of polymeric substrates in flexible electronic and display applications.

Characterization of reactively evaporated TiO2thin films as high and medium index layers for optical applications

Reactively evaporated TiO2 thin films were deposited on the glass substrate by electron beam heating technique. In the first stage evaporation rates are varied keeping substrate temperature fixed at 200 °C and oxygen flow rate at 9.0 sccm for all the samples. The deposition rates were varied from 0.12–0.20 nm/s with a step of 0.02 to optimize the optical constants for the film in the wavelength band of 380–850 nm as a high index material for multilayer optical coatings and filters. The films prepared at higher deposition rates have shown low refractive index values and comparatively higher extinction coefficients for the given substrate temperature in the desired spectral range. The best optical constants were observed for the sample prepared at 0.12 nm/s (at λ = 550 nm: n: 2.39 and k: of the order of 10−9). However, the variation in refractive index for all the samples was found to be in the range of 2.39–2.21 at 550 nm. In the second stage, films were prepared at a fixed deposition rate of 0.12 nm/s at different substrate temperature (50 °C, 100 °C, 150 °C and 200 °C) to study the optical properties of the films as a function of substrate temperature in order to investigate the potential of TiO2 film application as medium index layer. It was observed that refractive index decreases with decreasing substrate temperature. However, it was not found true for absorption. Instead, at Ts = 50 °C extremely low absorption (of the order of 10−5) has been observed with refractive index of 1.83, which increased sharply for next higher substrate temperature. Variation in absorption characteristics is found with varying substrate temperatures. The result shows that TiO2 films can be used as high and medium index layer in optical coating by varying the deposition parameters during growth. Effectively, one can achieve performance of two materials with only one. This effect could be utilized to produce variable index layers and coatings. Two samples with best high and medium optical constant values, were further investigated using Atomic force microscopy (AFM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) for surface and structure analysis. The results show that the particle size for the film is very small (10–20 nm), and the surface appears to be very compact, smooth and free of pinholes with roughness in angstrom range, extremely suitable for optical applications in single and multilayer structures.

Growth of honeycomb-symmetrical Mn nanodots arrays onSi(111)-7 × 7 surfaces

The growth of well-ordered Mn nanodots arrays onSi(111)-7 × 7 reconstructed surfaces was investigated by means of scanning tunnelling microscopy (STM)as well as kinetic Monte Carlo (KMC) simulation. Mn atoms deposited slowly onto elevatedsubstrates were observed to occupy preferentially on the faulted half unit cells (FHUCs) of theSi(111)-7 × 7 surface. The preference occupancy in the FHUCs,PF, defined as the ratio of the number of FHUCs occupied by Mn nanodots to the number of alloccupied in the two halves, decreases with increasing deposition rate as well as decreasingsubstrate temperature. The KMC simulations, which are in good agreement with theexperimental results, were employed to optimize the growth conditions, including depositionrate and substrate temperature, for the self-organized growth of Mn nanodots arrays onSi(111)-7 × 7 reconstructed surfaces. By adjusting the deposition rate, one can control the growth ofwell-ordered and uniform Mn nanodots arrays to form either a triangular symmetry or ahoneycomb one.

Deposition conditions in tailoring the morphology of highly porous reticular films prepared by electrostatic spray deposition (ESD) technique

Spongy-like reticular structure is a unique morphology fabricated by electrostatic spray deposition (ESD) technique. The effects of solvent, substrate temperature, precursor feeding rate, static electric field strength, and deposition time on tailoring the reticular structure were investigated. Scanning electron microscopy was used to observe the film morphology. MnO x or LiMn 2O 4 were selected as the model materials. It is found that in addition to the conventional solvent butyl carbitol, other kinds of solvents such as ethylene glycol and propylene glycol can also be used to obtain reticular films at a suitable substrate temperature. Porous films with a low cross-linking degree pore structure can be prepared by increasing precursor feeding rate or decreasing substrate temperature. Increasing the deposition time or the electric field strength helps to obtain reticular films with more homogeneous pore size distribution. In addition, the addition of a high boiling-point solvent in mixed alcohol solvent results in the increase of proper substrate temperature. It is concluded that the fluidity of the spray droplets on the surface of a hot substrate is an important factor to form a reticular film.

Investigation of room temperature electrical resistivities of LaNiO3−δ thin films deposited by rf magnetron sputtering and high oxygen-pressure processing

Highly (100)-oriented electrically conductive LaNiO3−δ (LNO) thin film with perovskite-type structure was deposited on Si(100) substrates by rf magnetron sputtering at substrate temperatures of 200, 300, 450, and 600°C with a series of 0%, 20%, 40%, and 60% oxygen partial pressures, respectively. The room temperature (RT) resistivity of LNO films decreases with decreasing substrate temperature at a fixed oxygen partial pressure and with increasing oxygen partial pressure at a fixed substrate temperature. The lowest RT resistivity of as-sputtered LNO thin films was about 5.3×10−4Ωcm. This value could be as low as ∼1.55×10−4Ωcm by postprocessing called high oxygen-pressure processing at 8MPa and is comparable to the lowest one, 1.5×10−4Ωcm, of epitaxial LNO thin film deposited on lattice-matched SrTiO3, LaAlO3, or sapphire single-crystal substrates.

Precise Control of Vertical-Cavity Surface-Emitting Laser Structural Growth Using Molecular Beam Epitaxy In Situ Reflectance Monitor

A vertical-cavity surface-emitting laser (VCSEL) was fabricated using a in situ reflectance monitor by molecular beam epitaxy (MBE). Both the center wavelength of the stop band of the distributed Bragg reflector (DBR) and the resonant wavelength of the optical cavity were successfully controlled using the monitor. However, these wavelengths shifted with decreasing substrate temperature after the growth, which could be reasonably explained by the temperature dependence of refractive index. Therefore, it is necessary to set a target wavelength at a growth temperature, considering the change. The desirable laser performance of the VCSEL fabricated from the wafer indicates marked increases in the controllability and reproducibility of growth with the aid of the in situ reflectance monitor. Since it can directly measure the optical properties of the grown layers, the reflectance monitor greatly helps in the fabrication of a structure with the designed optical performance.

Structural and chemical characteristics of (Ti,Al)N films prepared by ion mixing and vapor deposition

The characterization of titanium aluminum nitride (Ti,Al)N films prepared by ion mixing and vapor deposition (IVD) technique has been performed using several analytical techniques. In this study, the phase diagram of the films with various evaporation ratios Al/Ti was summarized successfully. The phase transition from single-phase NaCl to double-phase (NaCl+wurtzite) structure occurs with an increase of Al/Ti and/or with a decrease of substrate temperature. The (Ti,Al)N films with two-phase structure have a high performance in hardness. They are also highly resistant to oxidation. Consequently, the results suggest that the Al oxide layers formed on the top of (Ti,Al)N films during elevated temperature oxidation tests protect the films from further oxidation.

Reduction of the dark current in stabilized a-Se based X-ray detectors

The carrier ranges in vacuum deposited stabilized amorphous selenium films have been studied as a function of substrate temperature during deposition. While the electron range remains relatively unaffected, the hole range drops sharply with decreasing substrate temperature. At low substrate temperatures, the hole range becomes sufficiently shorter than the electron range, which means that such layers can replace the alkali doped hole trapping layers in the multilayer amorphous selenium based X-ray detector structures. We have produced double layer amorphous selenium based detector structures using the same starting material but varying the fabrication processes through the following steps: deposition of a thin cold hole trapping layer, a so called n-layer; annealing of the n-layer; and a much thicker layer that can transport both holes and electrons is grown at an elevated substrate temperature on the annealed n-layer. The structures produced by this technique have dark current densities that are similar to the ones reported for the presently commercialized amorphous selenium p–i–n-like structures (⩽10−10Acm−2 at 10Vμm−1).

Precise control of growth of DBR by MBE using in‐situ reflectance monitoring system

An in-situ reflectance monitor was used to improve the reproducibility and controllability of crystal growth by MBE (Molecular Beam Epitaxy). A DBR (Distributed Bragg Reflector) was grown using the in-situ monitor. The controllability of MBE has been remarkably improved and in-situ adjustment of the center wavelength of the stop band of DBR was also accomplished. The center wavelength, however, was found to shift 25 nm with decreasing substrate temperature after growth. This can be reasonably explained by the temperature change of the refractive indices. Therefore, it is necessary to set a target center wavelength at growth temperature, which is shifted form the designed center wavelength considering the temperature change of the refractive indices. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogen-plasma assisted molecular beam epitaxial growth of high-purity InAs

In this article we report the effect of H plasma on the purity and transport properties of molecular beam epitaxial (MBE) grown InAs, which has attracted considerable attention for high-speed and low power electronic devices in recent years. We observed that the native oxide on the InAs surface could be desorbed at temperatures as low as 300°C under H-plasma irradiation. The time required for oxide desorption increased with decreasing substrate temperature. Thick InAs was grown with H-plasma assistance on semi-insulating (SI) GaAs and on an AlAs0.16Sb0.84∕InAs insulating buffer layer to investigate the effect of H plasma on the transport properties of InAs. Significant improvement of the electron mobility was observed using H-plasma assisted growth. For 6μm InAs grown with H plasma on SI GaAs, the electron mobilities were as high as 98000cm2∕Vs at 77K and 19500cm2∕Vs at 300K, which represent the best published data for MBE grown InAs on GaAs. For thick InAs grown on AlAs0.16Sb0.84∕InAs, the electron mobilities were as high as 420000cm2∕Vs at 77K and 31000cm2∕Vs at 300K. The free carrier concentration of InAs grown with H-plasma assistance is larger than that of the sample grown without, indicating the H plasma yields a much lower compensation ratio γ(γ=(NA+ND)∕n), and moreover, the H plasma is more effective in passivating acceptors than donors in InAs. The mobility and free carrier concentration were both unchanged after thermal annealing, which are advantageous for device processing.

Point-defects-induced band edge displacements and band-gap narrowing in CdIn 2O 4 thin films

Transparent conducting oxides CdIn 2O 4 thin films were prepared by radio-frequency reactive sputtering from a Cd–In alloy target in Ar+O 2 atmosphere. By transmission spectrum and Hall measurement for different samples prepared at different substrate temperatures, it could be found that the carrier concentration would increase with the decrease of substrate temperature, but absorption edge showed an abrupt variation from a blue shift to a red shift. Theoretically, the paper formulated the effect of high-density point defects on band structures; it embodied the formation of band tailing, Burstein-Moss shift and band-gap narrowing. The density of holes will influence the magnitude of optical band gap and transmittance of light. Since extrapolation method does not fit degenerate semiconductor materials, a more accurate method of obtaining optical band gap is curve fitting. In addition, ionized impurities scattering is the main damping mechanism of the free electrons in CdIn 2O 4 films, the density of ionized impurities induced by altering substrate temperature will affect the carriers mobility.

Effect of substrate temperature on the properties of Ti-doped ZnO films by simultaneous rf and dc magnetron sputtering

The Ti-doped ZnO (ZnO:Ti) films were prepared by simultaneous radio frequency (rf) magnetron sputtering of ZnO and dc magnetron sputtering of Ti. The Ti oxides increased with the decrease in substrate temperature. Therefore, by decreasing the substrate temperature, X-ray peaks shifted towards a lower diffraction angle. When the substrate temperature increased from 50 to 150 °C, the crystallinity increased obviously, but it decreased slightly at 200 °C. The morphologies of films were significantly affected by the substrate temperature. The electrons of ZnO:Ti films may be originated from the oxygen vacancies and Ti donors. The mean free path of carriers were much shorter, therefore grain boundary scattering could be ruled out. The variation in mobility could be due to the ionized impurity scattering and surface roughness. The resistivity of ZnO:Ti film at 100 °C has a minimum of 9.69 × 10 −3 Ω cm, which is mainly due to the higher product of carrier concentration and mobility. At various substrate temperatures, the visible transmission of ZnO:Ti films was high.

Cryogenic deposition of carbon nitride thin solid films by reactive magnetron sputtering; suppression of the chemical desorption processes

Nanostructured fullerene-like carbon nitride (FL CNx) is commonly grown by reactive magnetron sputtering of carbon in a nitrogen-containing atmosphere. The film structure formation for this technique is presumably due to the existence of preformed molecular CxNy (x,y≤2) species in the deposition flux, which act as growth templates and enhance the selectivity of chemical desorption processes. In the present study, the extent of the desorption processes and the implications on the resulting film have been investigated in detail, addressing in particular the structure evolution and the origin of the incorporated nitrogen. This was studied by varying the N2-fraction in the discharge from 0 to 1 and the substrate temperature from 600 °C (873 K) down to minus 130 °C (143 K). The results show that the incorporation rate of carbon and nitrogen into the film increases substantially with an increased N2-content in the plasma and decreasing substrate temperature, thus indicating that the chemistry and magnitude of the arriving flux is substantially altered with the N2-fraction in the discharge. It is concluded that the chemically activated desorption in conjunction with the varying chemistry of the film-forming flux affects the sensitive structural balance, determined by incorporation and desorption of film forming CxNy (x,y≤2) species.

Preparation of LaNiO3 thin films by mist plasma evaporation from aqueous precursor

LaNiO3 thin films were prepared by mist plasma evaporation on n-Si(111) wafers using lanthanum nitrate and nickel nitrate aqueous solutions as precursor. The obtained LaNiO3 thin films possessed a perovskite structure with rhombohedral distortion. No impurity phase was detected in the LaNiO3 film deposited at 630°C. The grain size of the films decreased with the decreasing substrate temperature, and increased with the increase of deposition time and atomizing rate of precursor. The resistivity of the LaNiO3 films decreased with the decreasing substrate temperature. After annealing at 400°C for 2h, the room temperature resistivity of the LaNiO3 film deposited at 630°C for 20min decreased from 5.60 to 0.68mΩcm.

Low temperature growth of high- Jc Sm 1 + xBa 2 − xCu 3O y films

We fabricated Sm 1 + x Ba 2 − x Cu 3O y (SmBCO) films on a c-axis oriented thin SmBCO seed layer by low temperature growth technique. The orientation of the film without seed layer changed from c-axis to a-axis as decrease of substrate temperature ( T s), whereas the SmBCO film with seed layer showed the complete c-axis orientation at low- T s. This orientation mechanism was explained in view of adatom surface migration. In addition, the J c value of the film on seed layer increased with decreasing T s. From XRD φ-scan measurement, it is found that in-plane alignment of SmBCO film was improved by decreasing T s. We considered that this behavior in high J c SmBCO films was ascribed to improved in-plane alignment.

Microstructural analysis of Ga1−Mn N films grown by PEMBE

Abstract The microstructural changes of Ga 1− x Mn x N films grown at various substrate temperatures (350–700°C) by a PEMBE system have been investigated using transmission electron microscopy. Ga 1− x Mn x N film with low dislocation density was grown epitaxially at 700°C but dislocation density increased with the decrease of substrate temperature. Moreover, HCP and FCC structures coexisted in the vicinity of interface between GaN and Ga 1− x Mn x N films grown below 450°C.