Characteristics of Ni films deposited on SiO2/Si(1 0 0) and MgO(0 0 1) by direct current magnetron sputtering system with the oblique target

280 nm-thick Ni films were deposited on SiO₂/Si(1 0 0) and MgO(0 0 1) substrates at 300 K, 513 K and 663 K by a direct current magnetron sputtering system with the oblique target. The films deposited at 300 K mainly have a [1 1 0] crystalline orientation in the film growth direction. The [1 1 0]-orientation weakens and the [1 1 1]- and [1 0 0]-orientations enhance with increasing deposition temperature. The lattice constant of the Ni films is smaller than that of the Ni bulk, except for the film grown on MgO(0 0 1) at 663 K. Furthermore, as the deposition temperature increases, the lattice constant of the films grown on the SiO₂/Si(1 0 0) decreases whereas that of the films grown on the MgO(0 0 1) increases. The films deposited at 300 K and 513 K grow with columnar grains perpendicular to the substrate. For the films deposited at 663 K, however, the columnar grain structure is destroyed, i.e., an about 50 nm-thick layer consisting of granular grains is formed at the interface between the film and the substrate and then large grains grow on the layer. The Ni films deposited at 300 K consist of thin columnar grains and have many voids at the grain boundaries. The grains become thick and the voids decrease with increasing deposition temperature. The resistivity of the film decreases and the saturation magnetization increases with increasing deposition temperature.     

Growth of thin Fe(0 0 1) films for terahertz emission experiments

The electrical and magnetic properties of thin iron (Fe) films have sparked significant scientific interest. Our interest, however, is in the fundamental interactions between light and matter. We have discovered a novel application for thin Fe films. These films are sources of terahertz (THz) radiation when stimulated by an incident laser pulse. After intense femtosecond pulse excitation by a Ti:sapphire laser, these films emit picosecond, broadband THz frequencies. The terahertz emission provides a direct measure of the induced ultrafast change in magnetization within the Fe film. The THz generation experiments and the growth of appropriate thin Fe films for these experiments are discussed. Several criteria are used to select the substrate and film growth conditions, including that the substrate must permit the epitaxial growth of a continuous, monocrystalline or single crystal film, yet must also be transparent to the emitted THz radiation. An Fe(0 0 1) film grown on the (0 0 1) surface of a magnesium oxide (MgO) substrate makes an ideal sample. The Fe films are grown by physical vapor deposition (PVD) in an ultrahigh vacuum (UHV) system. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) are used to characterize the Fe(0 0 1) films. Two substrate surface preparation methods are investigated. Fe(0 0 1) films grown on MgO(0 0 1) substrates that are used as-received and films grown on MgO(0 0 1) substrates that have been UV/ozone-cleaned ex vacuo and annealed in vacuo produce the same results in the THz generation experiments. Either substrate preparation method permits the growth of samples suitable for the THz emission experiments.     

X-ray multiple diffraction in the characterization of TiNO and TiO2 thin films grown on Si(0 0 1)

TiO₂ and TiN_xO_y thin films grown by low pressure metal-organic chemical vapor deposition (LP-MOCVD) on top of Si(0 0 1) substrate were characterized by X-ray multiple diffraction. X-ray reflectivity analysis of TiO₂[1 1 0] and TiNO[1 0 0] polycrystalline layers allowed to determine the growth rate (−80 Å/min) of TiO₂ and (−40 Å/min) of TiNO films. X-ray multiple diffraction through the Renninger scans, i.e., ?-scans for (0 0 2)Si substrate primary reflection is used as a non-conventional method to obtain the substrate lattice parameter distortion due to the thin film conventional deposition, from where the information on film strain type is obtained.     

High fluence deposition of polyethylene glycol films at 1064 nm by matrix assisted pulsed laser evaporation (MAPLE)

Matrix assisted pulsed laser evaporation (MAPLE) has been applied for deposition of thin polyethylene glycol (PEG) films with infrared laser light at 1064 nm. We have irradiated frozen targets (of 1 wt.% PEG dissolved in water) and measured the deposition rate in situ with a quartz crystal microbalance. The laser fluence needed to produce PEG films turned out to be unexpectedly high with a threshold of 9 J/cm², and the deposition rate was much lower than that with laser light at 355 nm. Results from matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis demonstrate that the chemistry, molecular weight and polydispersity of the PEG films were identical to the starting material. Studies of the film surface with scanning electron microscopy (SEM) indicate that the Si-substrate is covered by a relatively homogenous PEG film with few bare spots.     

ZnO thin films on Si(1 1 1) grown by pulsed laser deposition from metallic Zn target

ZnO thin films with highly c-axis orientation have been fabricated on p-type Si(1 1 1) substrates at 400 °C by pulsed laser deposition (PLD) from a metallic Zn target with oxygen pressures between 0.1 and 0.7 mbar. Experimental results indicate that the films deposited at 0.3 and 0.5 mbar have better crystalline and optical quality and flatter surfaces than the films prepared at other pressures. The full width at half maximum (FWHM) of (0 0 0 2) diffraction peak decreases remarkably from 0.46 to 0.19° with increasing annealing temperature for the film prepared at 0.3 mbar. In photoluminescence (PL) spectra at room temperature, the annealed film at 700 °C exhibits a smaller ultraviolet (UV) peak FWHM of 108 meV than the as-grown film (119 meV). However, an enhanced deep-level emission is observed. Possible origins to above results are discussed.     

Epitaxial growth of Nd2Hf2O7(1 1 1) thin films on Ge(1 1 1) substrates by pulsed laser deposition

Nd₂Hf₂O₇ (NHO) thin films have been epitaxially grown by pulsed laser deposition (PLD) on Ge(1 1 1) substrates. In situ reflection high-energy electron diffraction (RHEED) evolution of the (1 1 1)-oriented NHO during the deposition has been investigated and shows that the epilayer has a twin-free character with type-B stacking. Interfacial structure of NHO/Ge has been examined by high-resolution transmission electron microscopy (HRTEM). The results indicate a highly crystalline film with a very thin interface, and the orientation relationship between NHO and Ge can be denoted as (1 1 1)_NHO//(1 1 1)_Ge and . Finally, twin-free epitaxial growth of NHO with type-B orientation displays temperature dependence and the type-B epitaxy is favored at high temperature.     

Formation of Al-N co-doped p-ZnO/n-Si (1 0 0) heterojunction structure by RF co-sputtering technique

Al-N co-doped ZnO (ZnO:Al-N) thin films were grown on n-Si (1 0 0) substrate by RF co-sputtering technique. As-grown ZnO:Al-N film exhibited n-type conductivity whereas on annealing in Ar ambient the conduction of ZnO:Al-N film changes to p-type, typically at 600 °C the high hole concentration of ZnO:Al-N co-doped film was found to be 2.86 × 10¹⁹ cm⁻³ and a low resistivity of 1.85 × 10⁻² Ω-cm. The current-voltage characteristics of the obtained p-ZnO:Al-N/n-Si heterojunction showed good diode like rectifying behavior. Room temperature photoluminescence spectra of annealed co-doped films revealed a dominant peak at 3.24 eV.     

A route to continuous ultra-thin cerium oxide films on Cu(1 1 1)

The growth and morphology of ultra-thin CeO₂(1 1 1) films on a Cu(1 1 1) substrate were investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The films were grown by physical vapor deposition of cerium in an oxygen atmosphere at different sample temperatures. The preparation procedure is based on a modification of a previous method suggested by Matolin and co-workers [1], involving growth at elevated temperature (520 K). Here, LEED shows good long range ordering with a “(1.5 × 1.5)” superstructure, but STM reveals a three-dimensional growth mode (Vollmer-Weber) with formation of a closed film only at larger thickness. Using a kinetically limited growth process by reactive deposition at low sample temperatures (100 K) and subsequent annealing, we show that closed layers of ceria with atomically flat terraces can be prepared even in the regime of ultra-thin films (1.5 ML). Closed and atomically flat ceria films of larger thickness (3 ML) are obtained by applying a multistep preparation procedure, in which successive ceria layers are homoepitaxially grown on this initial film. The resulting overlayers show strong similarities with the morphology of CeO₂(1 1 1) single crystal surfaces, suggesting the possibility to model bulk ceria by thin film systems.     

Femtosecond pulsed laser deposition of Ge quantum dot growth on Si(1 0 0)-(2 × 1)

Ge quantum dots were grown on Si(1 0 0)-(2 × 1) by femtosecond pulsed laser deposition at various substrate temperatures using a femtosecond Ti:sapphire laser. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy were used to analyze the film structure and morphology. The morphology of germanium islands on silicon was studied at different coverages. The results show that femtosecond pulsed laser deposition reduces the minimum temperature for epitaxial growth of Ge quantum dots to ∼280 °C, which is 120 °C lower than previously observed in nanosecond pulsed laser deposition and more than 200 °C lower than that reported for molecular beam epitaxy and chemical vapor deposition.     

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He⁺ scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe-Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe-Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.     

Characterization of bicrystalline epitaxial LaNiO3 films fabricated on MgO (1 0 0) substrates by pulsed laser deposition

A series of metallic LaNiO₃ (LNO) thin films were deposited on MgO (1 0 0) substrates by pulsed laser deposition (PLD) under the oxygen pressure of 20 Pa at different substrate temperatures from 450 to 750 °C. X-ray diffraction (XRD) was used to characterize the crystal structure of LNO films. θ-2θ scans of XRD indicate that LNO film deposited at a substrate temperature of 700 °C has a high orientation of (l l 0). At other substrate temperatures, the LNO films have mixed phases of (l l 0) and (l 0 0). Furthermore, pole figure measurements show that LNO thin films, with the bicrystalline structure, were epitaxially deposited on MgO (1 0 0) substrates in the mode of LNO (1 1 0)//MgO (1 0 0) at 700 °C. Reflection high-energy electric diffraction (RHEED) and atomic force microscopy (AFM) were also performed to investigate the microstructure of LNO films with the high (l l 0) orientation. RHEED patterns clearly confirm this epitaxial relationship. An atomically smooth surface of LNO films at 700 °C was obtained. In addition, bicrystalline epitaxial LNO films, fabricated at 700 °C, present a excellent conductivity with a lower electrical resistivity of 300 μ Ω cm. Thus, the obtained results indicate that bicystalline epitaxial LNO films could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Growth and characterization of thin PTCDA films on 3C-SiC(0 0 1)c(2 × 2)

The growth of thin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films on a 3C-SiC(0 0 1)c(2 × 2) substrate has been studied by means of photoelectron spectroscopy (PES) and atomic force microscopy (AFM). In the first monolayer the molecules interact with the substrate mainly through the O atoms in the end groups of the molecule. The O atoms have a higher binding energy in the first molecular layer compared to the following layers. No chemical shifts are observed in the Si 2p spectra or in the C 1s spectra from the perylene core of the molecules. From the VB spectra and LEED pattern we conclude that the substrate remains in the c(2 × 2) reconstruction after PTCDA deposition. For thicker films a Stranski-Krastanov film growth was observed with flat lying molecules relative to the substrate.     

Selective MOCVD of titanium oxide and zirconium oxide thin films using single molecular precursors on Si(1 0 0) substrates

Titanium oxide (TiO₂) and zirconium oxide (ZrO₂) thin films have been deposited on modified Si(1 0 0) substrates selectively by metal-organic chemical vapor deposition (MOCVD) method using new single molecular precursor of [M(OⁱPr)₂(tbaoac)₂] (M=Ti, Zr; tbaoac=tertiarybutyl-acetoacetate). For changing the characteristic of the Si(1 0 0) surface, micro-contact printing (μCP) method was adapted to make self-assembled monolayers (SAMs) using an octadecyltrichlorosilane (OTS) organic molecule which has -CH₃ terminal group. The single molecular precursors were prepared using metal (Ti, Zr) isopropoxide and tert-butylacetoacetate (tbaoacH) by modifying standard synthetic procedures. Selective depositions of TiO₂ and ZrO₂ were achieved in a home-built horizontal MOCVD reactor in the temperature range of 300-500 °C and deposition pressure of 1×10⁻³-3×10⁻² Torr. N₂ gas (5 sccm) was used as a carrier gas during film depositions. TiO₂ and ZrO₂ thin films were able to deposit on the hydrophilic area selectively. The difference in surface characteristics (hydrophobic/hydrophilic) between the OTS SAMs area and the SiO₂ or Si-OH layer on the Si(1 0 0) substrate led to the site-selectivity of oxide thin film growth.     

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe₂O₄ (CFO) thin films grown on single crystal Si (1 0 0) and c-cut sapphire (Al₂O₃) (0 0 0 1) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50 nm for CFO films grown on Si substrates, while it was fixed at 200 nm for CFO films grown on Al₂O₃ substrates. We observed that the 200 and 100 nm thick CFO-Si films grew in both (1 1 1) and (3 1 1) directions and displayed out-of-plane anisotropy, whereas the 50 nm thick CFO-Si film showed only an (1 1 1) orientation and an in-plane anisotropy. The 200 nm thick CFO film grown on an Al₂O₃ substrate was also found to show a complete (1 1 1) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.     

Atypical grain growth for (2 1 1) CdTe films deposited on surface reconstructed (1 0 0) SrTiO3 substrates

The (1 0 0) SrTiO₃ substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate's unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (1 0 0) SrTiO₃. The as-received substrate gives rise to a [1 1 1] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film's grain structure. For this case, a [2 1 1] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film's grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges.     

Dependence of the microstructural properties on the substrate temperature in strained CdTe (1 0 0)/GaAs (1 0 0) heterostructures

CdTe thin films were grown on GaAs (1 0 0) substrates by using molecular beam epitaxy at various temperatures. The bright-field transmission electron microscopy (TEM) images and the high-resolution TEM (HRTEM) images showed that the crystallinity of CdTe epilayers grown on GaAs substrates was improved by increasing the substrate temperature. The result of selected-area electron diffraction pattern (SADP) showed that the orientation of the grown CdTe thin films was the (1 0 0) orientation. The lattice constant the strain, and the stress of the CdTe thin film grown on the GaAs substrate were determined from the SADP result. Based on the SADP and HRTEM results, a possible atomic arrangement for the CdTe/GaAs heterostructure is presented.     

Structural and mechanical characteristics of (1 0 3) AlN thin films prepared by radio frequency magnetron sputtering

The (1 0 3)-oriented aluminum nitride (AlN) thin film is an attractive piezoelectric material for the applications in surface acoustic wave and film bulk acoustic wave resonator devices. In this work, we repot structural and mechanical characteristics of (1 0 3) AlN thin films deposited onto (1 0 0) Si substrates with radio frequency magnetron sputtering at different sputtering powers at 150, 250, and 350 W. Comparisons were made on their crystalline structures with X-ray diffraction, surface morphologies with atomic force microscopy, mechanical properties with nanoindentation, and tribological responses with nanoscratch. Results indicate that for the sputtering power of 350 W, a high-quality (1 0 3) AlN thin film, whose hardness is 18.91 ± 1.03 GPa and Young's modulus is 242.26 ± 8.92 GPa, was obtained with the most compact surface condition.     

Study of Sm-doped ZnO samples sintered in a nitrogen atmosphere and deposited on n-Si(1 0 0) by evaporation technique

The samarium doping zinc oxide (Zn_1-xSm_xO) with (x=0.0, 0.04, 0.05 and 0.17) polycrystalline thin films have been deposited on n-Si(1 0 0) substrate using thermal evaporation technique. Ceramic targets for deposition were prepared by the standard solid-state reaction method and sintered in nitrogen atmospheres. X-ray diffraction and scanning electron microscopy analyses show that the bulk and films features reveal wurtzite crystal structure with a preferential (1 0 1) crystallographic orientation and grows as hexagonal shape grains. According to the results of the Hall effect measurements, all the films show p-type conductivity, possibly a result of nitrogen incorporation into the Sm-doped ZnO samples. Magnetic measurements show that ferromagnetic behavior depends on the Sm³⁺ concentration. For a film with lower Sm₂O₃ contents (x=0.04), a phenomenon of paramagnetism has been observed. While, with further increase of Sm³⁺ contents (x=0.05) the ferromagnetic behavior has been observed at room temperature. However, at higher doping content of Sm³⁺, the ferromagnetic behavior was suppressed. The decrease of ferromagnetism with increasing doping concentration demonstrates that ferromagnetism observed at room temperature is an intrinsic property of Zn_1-xSm_xO films.     

Nucleation, growth and properties of Co nanostructures electrodeposited on n-Si(1 1 1)

In the present work, cobalt thin films deposited directly on n-Si(1 1 1) surfaces by electrodeposition in Watts bath have been investigated. The electrochemical deposition and properties of deposits were studied using cyclic voltammetry (CV), chronoamperometry (CA), ex situ atomic force microscopy (AFM), X-ray diffraction (XRD) and alternating gradient field magnetometer (AGFM) techniques. The nucleation and growth kinetics at the initial stages of Co studied by current transients indicate a 3D island growth (Volmer-Weber); it is characterized by an instantaneous nucleation mechanism followed by diffusion limited growth. According to this model, the estimated nucleus density and diffusion coefficient are on the order of magnitude of 10⁶ cm⁻² and 10⁻⁵ cm² s⁻¹, respectively. AFM characterization of the deposits shows a granular structure of the electrodeposited layers. XRD measurements indicate a small grain size with the presence of a mixture of hcp and fcc Co structures. The hysteresis loops with a magnetic field in the parallel and perpendicular direction and showed that the easy magnetization axis of Co thin film is in the film plane.     

Stability of MgO(1 1 1) films grown on 6H-SiC(0 0 0 1) by molecular beam epitaxy for two-step integration of functional oxides

Crystalline magnesium oxide (MgO) (1 1 1), 20 Å thick, was grown by molecular beam epitaxy (MBE) on hydrogen cleaned hexagonal silicon carbide (6H-SiC). The films were further heated to 740 °C and 650 °C under different oxygen environments in order to simulate processing conditions for subsequent functional oxide growth. The purpose of this study was to determine the effectiveness and stability of crystalline MgO films and the MgO/6H-SiC interface for subsequent heteroepitaxial deposition of multi-component, functional oxides by MBE or pulsed laser deposition processes. The stability of the MgO films and the MgO/6H-SiC interface was found to be dependent on substrate temperature and the presence of atomic oxygen. The MgO films and the MgO/6H-SiC interface are stable at temperatures up to 740 °C at 1.0 × 10⁻⁹ Torr for extended periods of time. While at temperatures below 400 °C exposure to the presence of active oxygen for extended periods of time has negligible impact, exposure to the presence of active oxygen for more than 5 min at 650 °C will degrade the MgO/6H-SiC interface. Concurrent etching and interface breakdown mechanisms are hypothesized to explain the observed effects. Further, barium titanate was deposited by MBE on bare 6H-SiC(0 0 0 1) and MgO(1 1 1)/6H-SiC(0 0 0 1) in order to evaluate the effectiveness of the MgO as a heteroepitaxial template layer for perovskite ferroelectrics.