Atomic layer deposition and characterization of Ga-doped ZnO thin films

Low-resistivity n-type ZnO thin films were grown by atomic layer deposition (ALD) using diethylzinc (DEZ) and H₂O as Zn and O precursors. ZnO thin films were grown on c-plane sapphire (c- Al₂O₃) substrates at 300 C. For undoped ZnO thin films, it was found that the intensity of ZnO () reflection peak increased and the electron concentration increased from 6.8×10¹⁸ to 1.1×10²⁰ cm⁻³ with the increase of DEZ flow rate, which indicates the increase of O vacancies () and/or Zn interstitials (Zn_i). Ga-doping was performed under Zn-rich growth conditions using triethylgallium (TEG) as Ga precursor. The resistivity of 8.0×10⁻⁴ Ω cm was achieved at the TEG flow rate of 0.24 μmol/min.     

Atomic layer deposition of PbZrO3 thin films

In this paper, we report on the preparation of lead zirconate films for the first time using atomic layer deposition in an attempt to investigate some of the film properties and also to evaluate possible use of the precursor combination to prepare more complex lead titanate zirconate. In the depositions tetraphenyl lead (Ph₄Pb) was used as the lead and zirconium 2,2,6,6-tetramethyl-3,5-heptadionato (Zr(thd)₄) as the zirconium precursor, while ozone was used as the oxygen source. Film growth, stoichiometry and quality were studied using different pulsing ratios at deposition temperatures of 275 and 300 °C. According to X-ray diffraction, the crystalline perovskite phase was observed when films deposited on SrTiO₃(1 0 0) were annealed at 600 °C. Surface roughness was reduced for lead deficient films as well as in annealed samples.     

Atomic layer deposition of titanium dioxide thin films from tetraethoxytitanium and water

The structure and chemical composition of the titanium dioxide thin films formed by atomiclayer deposition (ALD) from tetraethoxytitanium and water precursors were studied by Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, Fourier transform infrared spectroscopy, and atomic force microscopy. The deposited films were demonstrated to have good stoichiometry and anatase type polycrystalline structure. The growth per cycle of titanium dioxide was calculated by an ALD model taking into account the sizes and number of ligands in reactant molecules.     

Preparation of tantalum oxide thin films by photo-assisted atomic layer deposition

Tantalum oxide thin films were prepared by photo-assisted atomic layer deposition (Photo-ALD) in the substrate temperature range of 170–400 °C using Ta(OC₂H₅)₅ and H₂O as precursors. The constant growth rates of 0.42 and 0.47 Å per cycle were achieved for the films grown by normal ALD and Photo-ALD, respectively. The increased growth rate in Photo-ALD is probably due to the reactive surface by photon energy and faster surface reaction. In Photo-ALD, however, the constant growth rate started at lower temperature of 30 °C and one cycle time shortened up to 5.7 s than that of normal ALD. The films grown by normal ALD and Photo-ALD were amorphous and very smooth (0.21–0.35 nm) as examined by X-ray diffractometer and atomic force microscopy, respectively. Also, the refractive index was found to be 2.12–2.16 at the substrate temperature of 190–300 °C, similar to that of the film grown by normal ALD. However, the remarkably low leakage current density of 0.6×10⁻⁶ A/cm² to 1×10⁻⁶ A/cm² at applied field of 1 MV/cm is several order of magnitude smaller than that of normal ALD, probably due to the presence of reactive atom species.     

Influence of oxygen depletion layer on the properties of tin oxide gas-sensing films fabricated by atomic layer deposition

In this paper we report on the influence of film thickness on the electrical and gas-sensing properties of tin oxide thin films grown by atomic layer deposition (ALD) technique. The nature of the carrier and post-flow gases used in ALD was found to have a dramatic influence on the electrical conductance of the deposited films. Up to a film thickness of 50 nm the sheet conductance of the films increased with the thickness, and above 50 nm the sheet conductance was not significantly influenced by the film thickness. This effect was attributed to oxygen depletion at the film surface. When the depth of oxygen depletion (d _dep) was greater than or equal to the film thickness (t), the sheet conductance was thickness dependant. On the other hand, when d _dep≤t, the sheet conductance was independent of the film thickness but depended on the depth of the oxygen depletion. This proposed explanation was verified by subjecting the films to different lengths of post-annealing in an oxygen depleted atmosphere. Gas-sensing functionality of the films with various thicknesses was examined. It was observed that the film thickness had a significant influence on the gas-sensing property of the films. When the thickness was greater than 40 nm, the sensitivity of the films to ethanol was found to follow the widely reported trend, i.e., the sensitivity decreases when the film thickness increases. Below the film thickness of 40 nm the sensitivity decreases as film thickness decreases, and we propose a model to explain this observation based on the increase in resistance due to multiple grain boundaries.     

Epitaxial growth of zinc oxide by the method of atomic layer deposition on SiC/Si substrates

For the first time, zinc oxide epitaxial films on silicon were grown by the method of atomic layer deposition at a temperature T = 250°C. In order to avoid a chemical reaction between silicon and zinc oxide (at the growth temperature, the rate constant of the reaction is of the order of 10²²), a high-quality silicon carbide buffer layer with a thickness of ~50 nm was preliminarily synthesized by the chemical substitution of atoms on the silicon surface. The zinc oxide films were grown on n- and p-type Si(100) wafers. The ellipsometric, Raman, electron diffraction, and trace element analyses showed that the ZnO films are epitaxial.     

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO_x, 0 < x < 2) films by thermal annealing in high vacuum. The SiO_x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N₂O/SiH₄) flow ratios. The as-deposited films show increased Si concentration with decreasing N₂O/SiH₄ flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N₂O/SiH₄ reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO_x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.     

Pulsed laser deposition of oxide thin films

The pulsed laser deposition (PLD) process was studied in detail during oxide thin film growth by constructing a sophisticated PLD chamber combined with an energy-stable excimer laser. It was revealed that the transmitting laser energy at the entrance view port decreased exponentially with deposition runs, roughly by A e^−0.02x%, where A is the original transmission and x is the run number, and transmission loss could become considerable (up to 90%) in 100 deposition runs. In addition, area of the focused spot on the target was found a function of charging high voltage, which is a parameter of excimer laser operation, even through a slit forming rectangular sharp-edge beam profile. Laser energy density is one of the most important parameter governing grown film properties, and therefore accurate laser energy and spot area calibration is vital for reproducible film growth. In course of this study, the importance of spot area as well as the energy density is discussed in the view of deposition rate.     

Synchrotron radiation assistant MOCVD deposition of ZnO films on Si substrate

The growth of ZnO film on Si(1 0 0) substrate has been studied with synchrotron radiation (SR) assisted MOCVD method. The diethylzinc (DEZn) and CO₂ are used as source materials, while Nitrogen is employed as a carrier gas for DEZn. With the assistance of SR the ZnO film can be deposited even at room temperature. XRD, SEM and photoluminescence (PL) studies show that the crystal quality of ZnO films grown with the assistance of SR is higher than that of those without SR assistance. The growth mechanism of ZnO film with the SR assistant MOCVD system is primarily discussed.     

Atomic layer deposition of TiO2 and Al‐doped TiO2 films on Ir substrates for ultralow leakage currents

TiO₂ and Al‐doped TiO₂ (ATO) films were grown on Ir substrates by atomic layer deposition using O₃ as the oxygen source. With increasing O₃ feeding time, the crystalline structure of the TiO₂ films was transformed from anatase to rutile. Above an O₃ feeding time of 35 s, the films crystallized as only rutile due to the formation of IrO₂ layer at the interface. The TiO₂ and ATO films showed higher dielectric constants of 78 and 51, respectively. The films on Ir showed superior leakage properties compared to the films on Ru due to the high work‐function of Ir. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ag纳米颗粒对富Ag二氧化硅薄膜电致发光谱的影响

利用磁控溅射和热退火在硅衬底上制备了Ag纳米颗粒镶嵌的氧化硅薄膜(SiO2∶ Ag),制作了电致发光结构ITO/SiO2∶Ag/p-Si,观测到了可见区的电致发光.发现薄膜中的Ag纳米颗粒不仅成倍地提高器件的发光强度,还明显地移动电致发光的峰位.Ag含量越高,颗粒越大,发光峰位越红移.氧化硅中的发光中心与纳米Ag间的电...     

Pulsed laser deposition of molybdenum oxide thin films

Thin films of molybdenum oxide were deposited in vacuum by pulsed laser ablation using a xenon fluoride (351 nm) and a krypton fluoride (248 nm) excimer lasers. The films were deposited on unheated substrates and were post-annealed in air in the temperature range 300–500°C. The structural, morphological, chemical, and optical properties of the films were studied. As-deposited films were found to be dark. The transparency of the films was improved with annealing in air. The films were polycrystalline with diffraction peaks that belong to the orthorhombic phase of MoO₃. The surface morphology of the films showed a layered structure. Both the grain size and surface roughness increased with annealing temperature. The stoichiometry of the films improved upon annealing in air, with the best stoichiometry of MoO_2.95 obtained for films deposited by the XeF laser and annealed at 400°C. Similarly, the best transparency, with a transmittance exceeding 80%, was obtained with the films annealed in the temperature range 400–450°C.     

Atomic layer deposition coating of ZnO shell for GaN-ZnO core-sheath heteronanowires

We have synthesized GaN-core/ZnO-shell nanowires and investigated effects of the ZnO coating. The X-ray diffraction pattern showed that as-synthesized samples are composed of GaN and ZnO. Transmission electron microscopy indicated that the deposited ZnO shell layer is poly-crystalline. The photoluminescence (PL) spectrum of GaN has been changed by the ZnO coating, where emission bands centered at roughly 1.9 eV, 2.5 eV, and 3.3 eV were newly added to the emissions from core GaN nanowires. We found that overall PL intensity has been significantly increased by coating the ZnO shell layers.     

Atomistics of Ge deposition on Si(100) by atomic layer epitaxy

Chlorine termination of mixed Ge/Si(100) surfaces substantially enhances the contrast between Ge and Si sites in scanning tunneling microscopy observations. This finding enables a detailed investigation of the spatial distribution of Ge atoms deposited on Si(100) by atomic layer epitaxy. The results are corroborated by photoemission measurements aided by an unusually large chemical shift between Cl adsorbed on Si and Ge. Adsorbate-substrate atomic exchange during growth is shown to be important. The resulting interface is thus graded, but characterized by a very short length scale of about one monolayer.     

Atomic layer deposition of Cr2O3 thin films: Effect of crystallization on growth and properties

Atomic layer deposition of Cr₂O₃ thin films from CrO₂Cl₂ and CH₃OH on amorphous SiO₂ and crystalline Si(1 0 0) and -Al₂O₃() substrates was investigated, and properties of the films were ascertained. Self-limited growth with a rate of 0.05–0.1 nm/cycle was obtained at substrate temperatures of 330–420 °C. In this temperature range epitaxial eskolaite was formed on the -Al₂O₃() substrates. The predominant crystallographic orientation in the epitaxial films depended, however, on the growth temperature and film thickness. Sufficiently thick films grown on the SiO₂ and Si(1 0 0) substrates contained also the eskolaite phase, but thinner films deposited at 330–375 °C on these substrates were amorphous. The growth rate data of films with different phase composition allowed a conclusion that the crystalline phase grew markedly faster than the amorphous phase did. The amorphous, polycrystalline and epitaxial films had densities of 4.9, 5.1 and 5.1–5.3 g/cm³, respectively.     

Reactive pulsed laser deposition of zinc oxide thin films

Zinc oxide thin films have been obtained by reactive pulsed laser ablation of a Zn target in O₂ atmosphere (gas pressure 2 Pa) using a doubled frequency Nd:YAG laser (532 nm) which was also assisted by a 13.56 MHz radiofrequency (rf) plasma. The gaseous species have been deposited on Si(100) substrates positioned in on-axis configuration and heated from RT up to 500 °C. The obtained thin films have been compared to those produced in the same conditions by ablation of a ZnO target. The deposited thin films have been characterized by scanning electron microscopy, X-ray diffraction, Raman and infrared spectroscopy techniques. The influence of the rf plasma on the morphological and structural characteristics of these thin films is also briefly discussed. PACS 81.15.Fg; 68.55.Jk; 78.30.j     

Nanotribological behavior of ZnO films prepared by atomic layer deposition

We used atomic layer deposition to form ZnO thin-film coatings on Si substrates and then evaluate the effect of pile-up using the nanoscratch technique under a ramped mode. The wear volume decreased with increasing annealing temperature from room temperature to 400 °C for a given load. Elastic-to-plastic deformation occurred during sliding scratch processing between the groove and film for loading penetration of 30 nm. The onset of non-elastic behavior and greater contact pressure were evident for loading penetration of 150 nm; thus, full plastic deformation occurred as a result of a substrate effect. We suspect that elastic–plastic failure events were related to edge bulging between the groove and film, with elastic–plastic deformation attributable to adhesion discontinuities and/or cohesion failure of the ZnO films.     

Titanium oxide nanostructured films by reactive pulsed laser deposition

Nanostructured titanium oxide thin films have been grown by nanosecond UV pulsed laser deposition (PLD) performed in a reactive background atmosphere. We exploited laser ablation of a Ti target at different pressures of pure oxygen and Ar:O₂ mixtures to show that film growth can be tuned at the nanoscale from compact and dense to columnar and to porous, leading to different morphology, density and structure (oxidized fraction and degree of crystallinity). We observed that the position of the substrate relative to the time integrated visible plume front is fundamental in the determination of film structure and morphology. Film growth and film properties can be related to a non-dimensional parameter L which is the ratio between the target-to-substrate distance and the visible plume length. In particular, surface morphology and degree of structural order are strictly related to L irrespective of the oxygen content, while the latter mainly affects the oxidized fraction in the film.     

Effects of thermal annealing on titanium oxide films prepared by ion-assisted deposition

Titanium oxide thin films are prepared at a substrate temperature of 250 °C by electron-beam evaporation and ionassisted deposition. The effects of thermal annealing temperatures from 100 to 450 °C on the optical and mechanical properties are studied. The optical and mechanical properties include refractive indices, extinction coefficients, residual stress, surface roughness and crystallization. Experimental results show these properties of titanium oxide films clearly depend on the thermal annealing process.