Ultrahigh-vacuum Deposition Research Articles

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.

In situ observation of the strain of Co and Ni deposited on Pd (111) by reflection high energy electron diffraction

Co/Pd(111) and Ni/Pd(111) bilayer films were grown epitaxially on single crystal MgO (111) substrates by ultrahigh vacuum deposition. In situ observation of the strain of Co and Ni deposited on Pd (111) plane was performed by reflection high energy electron diffraction (RHEED) during fabrication. From RHEED patterns it was confirmed that the lattice constant of a Co film of 2 Å in thickness deposited on Pd was about 3.7 Å, which was different from that of Pd (3.89 Å). The strain (εCo) of the Co layer was rapidly released during further Co deposition. The strain εCo of the third Co monatomic layer on Pd is nearly equal to zero. The large strain of the Co layer is localized at about one or two monatomic layers of the interface between Co and Pd. On the other hand, a Ni film deposited on Pd was not strained from the beginning of the deposition. The relationship between the perpendicular magnetic anisotropy of Co/Pd and Ni/Pd compositionally modulated multilayer films and the strain of Co and Ni is discussed.

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.

Preparation and Magnetic Properties of V Monolayers in V/Ag Multilayered Films

V/Ag multilayered films were prepared by alternate deposition in ultrahigh vacuum. We measured for the first time the magnetization of ultrathin V layers sandwiched in Ag layers with a SQUID magnetometer. The films consist of fixed Ag layers (50 A) and V layers with thickness from 1 A to 50 A. The magnetization obeyed well with Curie-Weiss law. For the films with V layers thinner than 10 A, a remarkable enhancement of magnetic moment was observed. The effective magnetic moment per V atom reaches to 0.3µ B ∼0.4µ B for the films with V monolayers and submonolayers. This enhancement results from the change of V structure due to the epitaxy with Ag layers.

Synchrotron radiation study of silver and chromium particles supported on graphite

Silver and chromium particles in the size range 10 nm to 25 nm formed by deposition in ultra-high vacuum on graphite have been studied using synchrotron radiation. We have determined the island size by measuring the spectral dependence of the reflectivity of each sample using light in the photon energy range 35 eV to 190 eV. Comparing this measurement to a calculation of the reflectivity modified by the size-dependent extinction, determined using Mie theory, gives the average particle radius. Using this sizing technique we find that, for the growth of silver at room temperature, the average radius increases linearly with the cube root of evaporation time. Deposition with the substrate at elevated temperatures produces, for similar coverages, an increase in the average size for temperatures between 200°C and 225°C followed by a rapid decrease above 225°C.

Structural and magnetic studies of ultrathin epitaxial Co films deposited on a Pd(111) single crystal

We have studied the structure and magnetic properties of ultrathin epitaxial Pd/Co films deposited by vapor deposition in ultrahigh vacuum on a Pd(111) single crystal. The film crystal structure was characterized by LEED and RHEED and the magnetic properties were studied by Kerr hysteresis loops, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) at 10 and 34GHz. Well-defined RHEED and LEED patterns were present at all thickness proving that there was always a high degree of crystal order in the films despite the large lattice mismatch of 9%. Lattice spacing measurements using RHEED revealed that the Co relaxes to the bulk in-plane spacing in the first monolayer. FMR, Kerr and VSM measurements showed that Co films thinner than ≈ 20Å had an easy magnetization axis perpendicular to the film plane. We deduced that the surface anisotropy constant was (0.80 ±0.05)MJ/m 2.

Quasi-two-dimensional ferromagnetism in polycrystalline Fe.

Ultrathin polycrystalline Fe films of an average thickness 5 \AA{} are prepared on a Ta substrate by ion-beam sputter deposition in ultrahigh vacuum. The Ta itself is deposited on top of a three-dimensional (3D) ferromagnet, and it acts as a nonmagnetic metallic spacer layer through which exchange fields ${\mathit{H}}_{\mathrm{ex}}$ of various strengths are transferred from the magnetic substrate into the overlayer ultrathin film. The temperature dependence of the magnetization M(T) of the overlayer film is obtained from measurement of the spin polarization of the low-energy cascade electrons. M(T) depends critically on the thickness x of the Ta nonmagnetic spacer layer. The magnetic transition of the overlayer film is increasingly smeared out as ${\mathit{H}}_{\mathrm{ex}}$ increases, yet the transition temperature ${\mathit{T}}_{\mathit{C}}$ is clearly defined down to x=3 \AA{}. In the context of crossover from 3D to 2D magnetism, ${\mathit{T}}_{\mathit{C}}$ increases with decreasing x in a manner consistent with the theory of phase transitions in quasi-2D systems. Adsorption of residual gas molecules such as CO changes the shape of M(T) but does not affect ${\mathit{T}}_{\mathit{C}}$. The nature of the changes suggests that adsorbed residual gas molecules act as surfactants that spread the film slightly on the substrate, thereby converting it from the superparamagnetic to the ferromagnetic state.

Magnetism and Magnetoresistance of Fe/Cr Composite Multilayers

AbstractFe/Cr multilayers with composite layered structures were prepared onto MgO (100) and glass substrates by alternate deposition in ultrahigh vacuum. A multilayer with two thicknesses of Fe layers shows a larger MR change, 6.2 % at RT and 13.5 % at 77 K, than a simple type ones with the applied field upto 3 k0e. This Is due to the ferrimagnetically coupled Fe layers. For the multilayers of Cr-composite type, the MR ratio could be enhanced, 12.9 % at RT with the field of 2 T.

Ｆｅ島状膜の磁気光学効果と電気抵抗のその場観察

Magneto-optical Kerr effect and electric resistance of Fe deposited on fused quartz substrates were observed during ultra-high vacuum deposition. Resistance measurements show that Fe makes islands on the quartz substrates and those island-like films turn to a continuous film at about 115 A in thickness at room temperature, while, magneto-optical Kerr effect is observable from 16 A and remanence magnetization appears from 21 A.

Structural Properties of Epitaxial Co-Cr Superlattices

AbstractWe demonstrate the growth of epitaxially ordered Cobalt-Chromium (Co-Cr) superlattices by ultrahigh-vacuum deposition techniques. The superlattices, consisting of hcp Co (00.1) and hcp Cr (00.1) layers with their [11.0] crystallographic axes parallel to GaAs [110] have a hexagonal closed packed (hcp) structure with the c-axis normal to the film plane.

I n s i t u measurement of stress in Co/Cu, Co/Pd, and Co/Au compositionally modulated multilayer films

Temporal changes of sign and magnitude of the intrinsic stress during ultra-high vacuum deposition of Co/Cu, Co/Pd, and Co/Au compositionally modulated multilayer films (CMF’s) were investigated by means of a cantilever technique. The intrinsic stress corresponding to monatomic layers could be detected during deposition. The intrinsic stress in Co layers deposited on Cu, Pd, and Au layers is tensile and the stress of Cu, Pd, and Au layers on Co is compressive. The intrinsic stress of a Co layer on Cu in Co/Cu CMF approaches near to the ideal critical shear stress of the perfect crystal, 1×1011 dyn/cm2.

Hydrogen annealing of PtSi-Si Schottky barrier contacts

Schottky barrier PtSi-Si diodes formed by ultrahigh vacuum deposition and annealing of 1-nm-thick Pt films on n- and p-type (100) Si substrates were characterized by current-voltage measurements at liquid-nitrogen temperature. The diodes exhibited nearly ideal characteristics, with barrier heights of 0.914 and 0.197 eV, respectively, for typical n- and p-type devices. Subsequent annealing in hydrogen increased the barrier height by 0.013 eV for the n-type devices and decreased it by the same amount for the p-type devices. Vacuum annealing of H2-annealed devices restored the barrier heights to approximately their initial values. These results can be attributed to the presence of Si interface defects that are passivated by hydrogen incorporation and subsequently reactivated by vacuum annealing to remove the hydrogen.

Temperature dependence of the magnetization of Co/Pt multilayers (abstract)

Co/Pt multilayers with perpendicular anisotropy have recently been shown to be candidate materials for magneto-optic recording.1 In this application the Curie temperature has to be sufficiently low in order to switch the magnetization with the available laser powers. Apart from this technological importance, it is also of fundamental interest to investigate the effect of coupling between two-dimensional systems on their critical behavior. We therefore studied the influence of the Pt thickness on the temperature dependence of the magnetization for Co/Pt multilayers. The multilayers were prepared by vapor deposition in ultrahigh vacuum. X-ray-diffraction measurements confirmed the layered structure. The spontaneous magnetization was obtained by extrapolating the high-field part of the magnetization curves back to zero field. The experiments showed that the changes in the magnetization are reversible up to 450 K. Above 500 K we detected slight irreversible changes probably due to interdiffusion. The Curie temperature decreased with increasing Pt thickness up to about 27 Å, above which no dependence was found. This phenomenon is ascribed to a long-range interaction between the thin (4 or 6 Å) Co layers. The development of a model describing the Curie-temperature dependence on the Pt thickness is in progress. Some preliminary calculations will be given.

Role of i n s i t u rapid isothermal processing in the solid phase epitaxial growth of II-A fluoride films on (100) and (111) InP

Rapid isothermal processing based on incoherent sources of light is emerging as a reduced thermal budget processing technique for the fabrication of next generation of semiconductor devices and circuits. In this letter, we show that integration of the rapid isothermal processing unit and the ultrahigh vacuum deposition system provides an in situ rapid isothermal processing capability for the solid phase epitaxial growth of SrF2 and BaF2 films on (100) and (111)InP. We also show that neither as-deposited nor ex situ annealed films show solid phase epitaxial growth.

Angular resolved photoemission of thick chromium epitaxially grown on a FeSi (001) surface

We prepared ultrapure epitaxially grown chromium films by vapor deposition in ultrahigh vacuum on a FeSi (001) monocrystal maintained at room temperature. Prior to the deposition, the surface of the substrate was flashed at 800°C and exhibits a c(2 × 2) Si overstructure. The epitaxial system is studied by LEED, work function measurements, XPS and ARUPS. All data confirm the perfection and purity of the chromium (001) the literature are controversial. We obtain original results for the ARUPS with He I source along Δ. We clearly observe the band folding caused by antiferromagnetism, mainly on both Δ 5 one, according to the chromium band calculation of Asano and Yamashita, J PhySoc Japan, 23, 714 (1967). Furthermore, an intense surface state peak located at 0.2 eV below E F, which does not disperse, exists in the ARUPS data.

Epitaxial growth of single-crystal Si1−xGex on Si(100) by ion beam sputter deposition

The first results are reported on Si1−xGex epitaxial layers grown on Si(100) by ion beam sputter deposition in ultrahigh vacuum. Growth temperatures were varied from 300 to 700°C, for compositions in the range 0.05 < x < 0.5. The properties of the grown films, such as morphology and structure, were studied by scanning electron microscopy and reflection high-energy electron diffraction respectively. Results indicate good monocrystallinity for the entire range of deposition temperature used. For deposition temperatures over 300°C, the films have smooth surfaces for all compositions and thicknesses. The distorted lattice parameters of epitaxial Si0.7Ge0.3 layers were measured by double-crystal diffractometry and the tetragonal strain was calculated. Increasing deposition temperature results in strain relaxation. Layers 3000 A thick grown at 400°C still retain a larger strain than that measured in a similar layer grown by molecular beam epitaxy. All these results may show the effects of energetic bombardment on the growing film.

Thickness Dependence of Epitaxial TiSi2 on Si(111).

ABSTRACTThe epitaxy and morphology of TiSi2 on Si(l 11) are studied as a function of Ti thickness. Titanium thicknesses of 1–2 monolayers and Ti films with thicknesses of 50Å; were examined. The titanium silicide films were formed on atomically clean Si substrates by ultrahigh vacuum (UHV) deposition of Ti followed by in situ annealing. In situ LEED and AES measurements characterized the reaction process. For room temperature deposition of less than two monolayers of Ti the LEED pattern associated with the reconstructed substrate disappeared, and the 1×1 bulk pattern also disappeared completely. Annealing at 200°C resulted in a decrease in the Ti AES signal indicating interdiffusion. For annealing up to 500°C, a series of changes in the LEED patterns were observed, which indicated that the disordered layer transformed to an ordered surface layer. Annealing to higher temperature resulted in the reappearance of the diffraction pattern associated with the 7×7 reconstructed Si(111) surface. This indicated three dimensional island growth. For the TiSi2 formed by in situ annealing of 50Å of Ti on Si(111), three different types of island morphologies were observed and identified as the C49 phase of TiSi2. The C49 TiSi2 was proven to be an epitaxial titanium silicideby HRTEM and SAD. Two different orientation relationships of the TiSi2 islands are found: 1) [3 1 1] C49 TiSi2 // [112]Si and (130) C49 TiSi2 //(11 1) Si, and 2) [1 1 2] C49 TiSi2 // [110] Si and (021) C49 TiSi2 //(1 11) Si. The C49 nucleation and island morphologies are discussed in terms of the character of the epitaxial interface.

Surface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materials

Thin films of vacuum deposited trivalent and tetravalent metal phthalocyanines (Pc), and derivatives of perylenes (Pe), have been explored by a combination of ultrahigh vacuum deposition, surface spectroscopic characterization techniques, and measurement of photoconductivity and photocurrent spectral responses involving a) interdigitated array microcircuits (MC), and b) photoelectrochemical cells. Of particular interest has been the way adsorbed molecules such as O 2 and NH 3 affect the photoconductivity of these thin film materials, and the effect on photoactivity of the formation of a sharp interface between a phthalocyanine and a perylene. Multiple chemisorption sites are implicated for molecules such as O 2 and NH 3 on trivalent metal Pc thin film surfaces. These sites may either increase or decrease photoconductivity, depending on the partial pressure of the adsorbant, and the coverage of molecules competing for the same chemisorption sites. Photoelectrochemical modification of the Pc surface introduces submonolayer coverages of metals like Ag, providing a chemisorption site for molecules such as NH 3 , thereby improving the performance of these photoconductivity-based chemical sensors. Photoelectrochemical techniques can be used to characterize Pc/Pe bilayers, since the electrolyte provides a noncorrosive, optically transparent electrical contact. Photocurrent yield spectra, obtained from two different illumination directions, have been used to estimate the width of the interfacial region which is most active for exciton dissociation in the Pc/Pe bilayer. Transient photocurrent yields in Pc/Pe multilayers (up to 16 total layers), show that the photocurrent increases linearly with the number of Pc/Pe interfaces. These increases occur up to the point where the width of the individual layers becomes narrower than the interface resolution that our present vacuum deposition technologies can provide

Interface formation on n- and p-CdTe(110) surfaces with Cu and Au

The first stages of Cu and Au ultra-high vacuum deposition onto clean, in-situ cleaved (110) surfaces of n- and p-type CdTe single crystals at room temperature have been studied using low energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. The changes of the measured spectra as a function of metal coverage are well explained for both Cu and Au by the formation of metallic islands which are essentially two-dimensional up to a fraction of a monolayer before becoming more (for Au) or less (for Cu) three-dimensional at higher coverages. The corresponding band bending changes are tentatively evaluated, in spite of the inhomogeneous character of the surface. The presence of a Au induced shallow donor surface state is also recognized below monolayer coverage.