Laser Deposition In Ultrahigh Vacuum Research Articles

Buffer-layer-enhanced growth of a single-domain LaB 6 (1 0 0) epitaxial thin film on a MgO (1 0 0) substrate via pulsed laser deposition

Epitaxial growth of LaB 6 (1 0 0) thin films was examined on MgO (1 0 0) and sapphire (α-Al 2O 3) (0 0 0 1) substrates with insertion of a 2–3 nm-thick epitaxial SrB 6 buffer layer by pulsed laser deposition in ultra-high vacuum. Reflection high-energy electron diffraction, X-ray diffraction, and the Raman scattering spectroscopy measurements proved that the heteroepitaxial structure of the LaB 6 (1 0 0)/SrB 6 (1 0 0)/MgO (1 0 0) substrate has a single-domain, while and that of the LaB 6 (1 0 0)/SrB 6 (1 0 0)/sapphire (0 0 0 1) substrate have three domains. LaB 6 thin films grown without the buffer layer were not epitaxial; instead, they developed as polycrystalline films with a random in-plane configuration. The buffer layer greatly affected the initial growth of the LaB 6 thin films. Epitaxial LaB 6 thin films exhibited metallic behavior with almost constant resistivities in the temperature range 12–300 K. At room temperature, the resistivities of single-domain LaB 6 (1 0 0) epitaxial thin films on MgO substrates were about 5 times smaller than those of the three-domain LaB 6 (1 0 0) epitaxial thin films on the sapphire substrates.

Electrical and optical properties of epitaxial transparent conductive BaCuTeF thin films deposited by pulsed laser deposition

Transparent p-type conductive BaCuTeF thin films are reported. Epitaxial BaCuTeF films are obtained in situ by pulsed laser deposition in ultra-high vacuum and in Ar ambient up to 1 × 10 −3 Torr on single-crystal MgO (100) substrates above 500 °C. Nominally undoped films exhibit hole carrier mobilities of 4–8 cm 2/V s with a maximum conductivity of 167 S/cm. Polycrystalline films deposited under similar conditions on amorphous SiO 2 substrates have hole mobilities of 0.2 cm 2/V s and conductivities of 0.4–7 S/cm. The largest change in optical absorption occurs near 3 eV, with a weak feature near 2.4 eV in epitaxial films.

Room-temperature growth of AlN/TiN epitaxial multi-layer by laser molecular beam epitaxy

We have fabricated epitaxial AlN thin films at room temperature on sapphire (0001) substrates with a TiN (111) epitaxial buffer layer by pulsed laser deposition in ultra-high vacuum (laser molecular beam epitaxy method). The TiN buffer layers were also fabricated at room temperature. Four-circle X-ray diffraction analysis and reflection high-energy electron diffraction results indicate the heteroepitaxial structure of AlN (0001)/TiN (111)/sapphire (0001) with the epitaxial relationship of AlN [10–10]||TiN [11–2]||sapphire [11–20]. The surface of the room-temperature grown AlN film was found to be atomically flat, reflecting the nano-stepped surface of ultrasmooth sapphire substrates. Then, we could achieve the room-temperature epitaxial growth of [AlN/TiN] multi-layer. The temperature dependence of resistivity of the AlN/TiN multi-layer film was also measured.

Variation of the mechanical properties of pulsed laser deposited PMMA films during annealing

The structural and mechanical properties of polymethyl methacrylate (PMMA) thin films produced by pulsed laser deposition in ultra-high vacuum were investigated by infrared spectroscopy, electron microscopy, size-exclusion chromatography and microhardness measurements. After preparation, the film hardness and elastic modulus are found to be increased. This is caused by a reduced chain length of the polymer film to values of 5800 g/mol and by cross links of an amount of about 20% of the polymer, while the chemical structure of the deposited films is comparable to the target material. During annealing, relaxation processes lead to a reduction of the film hardness, without changing the cross-linked part in the samples.

Quantification of resputtering during pulsed laser deposition

During pulsed laser deposition in ultrahigh vacuum, the deposited material consists of a large fraction of ions with kinetic energies in the range of 100 eV. In many cases, these energetic particles induce resputtering at the film surface and lead to composition deviations. For Fe–Ag the resputtering effects are quantified by monitoring the deposition rate during growth of elementary and multilayer films. It is shown that preferential resputtering and, therefore, composition deviations of alloy films can be reduced at laser fluences close to the ablation threshold or even better under Ar gas pressure of about 0.05 mbar. The experimental results are described by a model, which includes atom deposition on the film surface, implantation of energetic ions below the surface and resputtering of atoms from the top monolayer.

OXYGEN CONTROL FOR EPITAXIAL GROWTH OF CUPRATES IN ULTRAHIGH VACUUM SYSTEM

The oxidation ability of various oxidants for Bi2Sr2CaCuOx and related cuprates were evaluated both by experiments and by a thermodynamical calculation. When Bi2Sr2CaCu2Ox sputtered films were treated at 400°C with such oxidants as photo-activated O2, NO2, O3, and N2O, significant Tc change was observed. This Tc change was found in parallel with the variation of c-lattice parameter and verified to originate from oxygen nonstoichiometry in the films by the Hall coefficient measurement. The highly oxidative property of NO2, ozone, and atomic oxygen derived from this experiment was supported by the thermodynamical calculation for the oxidation of Cu2O to CuO. The results were further utilized to grow alkaline earth cuprate films by pulsed laser deposition in ultrahigh vacuum (laser MBE). In situ RHEED and XPS analyses clearly indicated that alkaline earth cuprates could be grown epitaxially and laterally under such conditions as 700°C and 1×10-7 NO2pressure.

Laser MBE layered [( AE) 2+/( CuO2) 2-] n (AE = Sr or Ba) latticess

Pulsed laser deposition in ultrahigh vacuum was verified to be a promising method to deposit epitaxial films composed of bi-layered [( AE) 2+/( CuO 2) 2-] n (AE = Sr or Ba) lattices. As-grown films deposited on SrTiO 3(001) substrates under NO 2 flushing exhibited a metallic conduction.

Preparation of SrCuO y film in ultra-high vacuum system

SrCuO 2 crystal contains two-dimensional CuO 2 planes like those constituting supercurrent path in high T c superconducting cuprates. We have investigated on epitaxial SrCuO y film growth on SrTiO 3(001) substrate by ArF pulsed laser deposition in ultra-high vacuum. In situ RHEED and XPS diagnoses of the film growth process under a very low NO 2 pressure indicated that there were slight fluctuations in surface and electronic states of the initial 1 nm thick film growth. Beyond this initial stage, the film growth gave streaky RHEED pattern with its intensity oscillated at a periodicity corresponding well to the c-axis length of SrCuO 2, verifying the layer-by-layer growth. The as-grown film was electrically conductive.

Fabrication and anomalous conducting behavior of atomically regulated superlattices

Abstract Fine streak patterns and intensity oscillations were observed in RHEED diagnoses of the growth of perovskite-type SrVO3-x and SrTiO3-y films by pulsed laser deposition in ultrahigh vacuum. The oscillation periodicity coincided with the a-axis length of the perovskite oxide, indicating that the films were grown not only epitaxially but also laterally, i.e. in a layer-by-layer mode. This enabled us to fabricate ( SrVO 3−x ) ( SrTiO 3-y ) superlattices with digital control of each layer thickness by counting the peak numbers of RHEED intensity oscillation. All of the as-grown SrVO3-x, SrTiO3-y, and superlattice films were highly conductive, but the temperature-dependent resistivity of the superlattice exhibited a behavior quite different from those of SrVO3-x and SrTiO3-y films. A metal-semiconductor transition observed in the superlattice at about 100K suggests a phenomenon originating from the two dimensionality of conducting layers.

Ceramic layer epitaxy by pulsed laser deposition in an ultrahigh vacuum system

Ceramic layer epitaxy, defined as the epitaxial growth of ceramic thin films with thicknesses regulated on an atomic scale, was verified by a new method using pulsed laser deposition in ultrahigh vacuum (UHV). The intensity oscillation of reflection high-energy electron diffraction (RHEED) was observed in an ArF excimer laser deposition of CeO2 and SrTiO3 films on Si(111) and SrTiO3(001) substrates, respectively, at 650–750 °C under 5×10−7 Pa. The oscillation periodicities corresponded well to interplane distances of CeO2(111) and SrTiO3(001). This first observation of RHEED oscillation in laser deposition of ceramic films suggests that UHV laser deposition is a promising method for producing atomically regulated ceramic layers inclusive of possible new high Tc superconductors.