MBE growth and characterization of buried silicon oxide films on Si(100)

Silicon molecular beam epitaxy (Si-MBE) has been used to produce silicon oxide (SiO_x) films by evaporating Si on a heated Si(100) substrate in an ultra high vacuum system with an O₂ pressure of 10⁻⁶ to 10⁻⁴ mbar. Then the SiO_x films were overgrown with pure Si. The influence of the substrate temperature, the O₂ pressure and the Si deposition rate on the oxygen content in the SiO_x films and on the crystalline quality of the Si top-layer was investigated by Rutherford backscattering spectrometry and ion channeling. Epitaxial growth of the Si top-layer was observed up to a maximum concentration of ≈20 at.% oxygen content in the SiO_x film. Cross-sectional transmission electron microscopy shows that the structure of the SiO_x film changes duringa subsequent annealing procedure. Electron energy loss spectrometry proves that amorphous SiO₂ is formed and the development of holes indicates that the density of the as-grown SiO_x film is much lower than that of SiO₂. The specific for the as-grown SiO_x films was determined by I–V measurements.     

Mechanical properties of a-C:H and a-C:H/SiOx nanocomposite thin films prepared by ion-assisted plasma-enhanced chemical vapor deposition

a-C:H and a-C:H/SiO_x nanocomposite thin films were deposited on silicon, aluminum and polyimide substrates at 25 °C in an asymmetric 13.56 MHz r.f.-driven plasma reactor under heavy ion bombardment. Fourier transform infrared spectra of the films indicate that the nanocomposite filmsappears to consist of an atomic scale random network of a-C:H and SiO_x. Raman spectroscopy revealed that the sp² carbon fraction in the nanocomposite film was reduced compared with the a-C:H film. The intrinsic stress of both films increased with increasing negative bias voltage (−V_dc) at the substrate. However, the nanocomposite films exhibited lower intrinsic stress compared w with a-C:H-only films. Especially, a thin SiO_x-rich interlayer was very effective in reducing the film stress and enhancing the bonding strength at the interface. The interlayer allowed deposition of thick films of up to 5 μm. Also, the nanocomposite films were stable in 0.1 M NaOH solution and showed good microhardness.     

Structure and conducting properties of La0.5Sr0.5CoO3−δ films on YSZ

La_0.5Sr_0.5CoO_3−δ (LSCO) thin films were deposited on yttria stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell electrodes. During the deposition, the substrate temperature was varied from 450 to 750°C, and the oxygen pressure in the chamber was varied from 80 to 310 mTorr. Films deposited at 650°C and an oxygen background pressure of 150 mTorr were mostly (100) oriented. Deposition at higher temperatures or under lower oxygen pressures lead to mostly (110) oriented films. Films with low electrical resistivity of 10⁻³ Ω·cm were obtained.     

Investigation of stress behaviors and mechanism of void formation in sputtered TiSix films

We have investigated the stress behaviors and a mechanism of void formation in TiSi_x films during annealing. TiSi_x thin films were prepared by DC magnetron sputtering using a TiSi_2.1 target in the substrate temperature range of 200–500 °C. The as-deposited TiSi_x films at low substrate temperature (<300 °C) have an amorphous structure with low stress of 1×10⁸ dynes/cm². When the substrate temperature increases to 500 °C, the as-deposited TiSi_x film has a mixture of C49 and C54 TiSi₂ phase with stress of 8×10⁹ dynes/cm². No void was observed in the as-deposited TiSi_x film. Amorphous TiSi_x film transforms to C54 TiSi₂ phase with a random orientation of (311) and (040) after annealing at 750 °C. The C49 and C54 TiSi₂ mixture phase transforms to (040) preferred C54 TiSi₂ phase after annealing over 650 °C. By increasing substrate temperature, the transformation temperature for C54 TiSi₂ can be reduced, resulting in relieved stress of TiSi₂ film. The easy nucleation of the C54 phase was attributed to an avoidance of amorphous TiSi_x phase. We found that amorphous TiSi_x→C54 TiSi₂ transformation caused higher tensile stress of 2×10¹⁰ dynes/cm², resulting in more voids in the films, than C49→C54 transformation. It was observed that void formation was increased with thermal treatment. The high tensile stress caused by volume decreases in the silicide must be relieved to retard voids and cracks during C54 TiSi₂ formation.     

Ferroelectric/superconductor heterostructures

This review covers the fabrication and characterization of ferroelectric/superconductor heterostructures such as Pb(Zr_xTi_1−x)O₃/YBa₂Cu₃O_7−δ (YBCO), BaTiO₃/YBCO and Ba_xSr_1−xTiO₃/YBCO etc. on various single crystal substrates. Pulsed laser deposition, laser molecular beam epitaxy, and magnetron-sputtering methods are compared. This report shows that pulsed laser deposition equipped with in situ reflection high-energy electron diffraction is a good method to control the growth mode of YBCO thin films. Furthermore, laser molecular beam epitaxy is a superb method for research of complex oxide films and their superlattices. Atomic force microscopy and transmission electron microscopy showed the ferroelectric films grown on the rough surface of the YBCO films produced high-density planar defects in the film and is detrimental to the ferroelectric/dielectric properties of the heterostructures. Therefore, for device usage, it is more advantageous to use SrRuO₃ than YBCO as the bottom electrode material. For growing atomically smooth surface films step-flow mode is highly recommended. Prospects of microwave device application of the ferroelectric/superconductor heterostructures are discussed, and proposed the BSTO films as the best candidate for passive microwave components.     

Millisecond-timescale electrodeposition of platinum atom-doped molybdenum oxide as an efficient electrocatalyst for hydrogen evolution reaction

We present a straightforward method for one-pot electrodeposition of platinum atoms-doped molybdenum oxide (Pt·MoO_3−x) films and show their superior electrocatalytic activity in the hydrogen evolution reaction (HER). A ~15-nm-thick Pt·MoO_3−x film was prepared by one-pot electrodeposition at −0.8 V for 1 ms. Due to considerably different solute concentrations, the content of Pt atoms in the electrodeposited composite electrocatalyst is low. No Pt crystals or islands were observed on the flat Pt·MoO_3−x films, indicating that Pt atoms were homogeneously dispersed within the MoO_3−x thin film. The catalytic performance and physicochemical features of Pt·MoO_3−x as a HER electrocatalyst were characterized. The results showed that our Pt·MoO_3−x film exhibits 23- and 11-times higher current density than Pt and MoO_3−x electrodeposited individually under the same conditions, respectively. It was found that the dramatic enhancement in the HER performance was principally due to the abundant oxygen defects. The use of the developed one-pot electrodeposition and doping method can potentially be extended to various catalytically active metal oxides or hydroxides for enhanced performance in various energy storage and conversion applications.     

Spectroscopic and structural characterisation of a VOx (x≈1) ultrathin epitaxial film on Pt (111)

VO_x ultrathin epitaxial films (0.8≤x≤1.3), grown on Pt(111) by evaporating vanadium in a controlled water background (1×10⁻⁷ Pa), have been chemically characterised by X-ray photoelectron spectroscopy (XPS) and X-ray-excited Auger electron spectroscopy (AES), which confirm the presence of V(II). The VO film shows a NaCl-type structure exposing the (111) plane, as proven by XPD. Multiple scattering calculations are compatible with an O-terminated surface and a surface relaxation of the outermost atomic layers, which leads to a V---O bond length contraction amounting to 7%.     

Ultraviolet emitting (Y1−xGdx)2O3−δ thin films deposited by radio frequency magnetron sputtering: Combinatorial modeling, synthesis, and rapid characterization

Gadolinium-doped, yttrium oxide thin films have been deposited on silicon (001) substrates by radio-frequency (RF) magnetron reactive sputtering that exhibit cathodoluminescence (CL) at ultraviolet frequencies. The maximum CL brightness occurred at λ314–315 nm characteristic of the ⁶P_3 / 2 → ⁸S (λ = 314 nm) transition observed in Gd-doped, yttrium oxide powders. The radiative recombination takes place at the rare earth activator Gd³⁺ site embedded in the Y₂O_3−δ host; the optical transition resides within the band gap of the Y₂O_3−δ host and the transition observed is characteristic of atomic gadolinium. A combinatorial approach to sputtering was used to deposit a film of variable composition from 1 to 23 at.% Gd in Y₂O_3−δ in order to rapidly discern the composition node of optimal CL brightness. A simulation was created for the purpose of predicting the film combinatorial composition for binary and ternary alloys prior to sputtering experiments in order to facilitate our combinatorial thin film synthesis technique. The model prediction varied from the real experimental composition profile by only 2.2 at.% Gd ± 1.6 at.% proving the predictor as a useful aide to complement combinatorial thin film experiments. A film of composition Y_1.56Gd_0.44O_3.25 (8.3 at.% Gd) yielded the maximum CL brightness. CL brightness increased continuously up to the 8.3 at.% Gd composition due to the increased number of activators present in the host. Beyond this composition the brightness drastically decreased. The oxygen composition in the combinatorial film was strongly dependent on the Gd composition; films were sub-stoichiometric δ > 0 below 6 at.% Gd and was over-stoichiometric δ < 0 beyond this composition.     

CuGaSe2 thin films for photovoltaic applications

Of the I-III-VI₂ group chalcopyrites, CuInSe₂ has already proved its suitability for thin film solar cells owing to its excellent optical and transport properties. CuGaSe₂ is expected to exhibit comparable properties from this point of view. With its band gap of 1.7 eV it is a candidate for use in photovoltaic tandem systems.

The preparation of CuGaSe₂ thin films by means of the vacuum evaporation of the constituent elements (four-temperature method) is described. The structural, electrical and optical properties of these films were investigated. Secondary electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction examination and measurements of the optical transmission, resistivity and thermoelectric power were used to determine the film properties relative to the preparation parameters and stoichiometry. The growth conditions were optimized for solar cell applications. Heterojunctions were prepared by the in situ evaporation of Zn_xCd_1−xS onto the CuGaSe₂ films. The characteristic data of the cells are a short-circuit current of 6 mA and an open-circuit voltage of 620 mV at an illumination at air mass 1.5 on an area of 1 cm².     

Investigation of the growth dynamics of pulsed laser-deposited RuO2 films using in situ resistance measurement and atomic force microscopy

RuO₂ thin films were grown on (001) LaAlO₃ utilizing the pulsed laser deposition technique. Atomic force microscopy was used to check the topography of films at different growth stages. The in situ resistivity measurement was used to monitor the resistance change during and post film growth with changes of resistivity associated with the change of film growth mode. Transmission electron microscopy was used to reveal film quality and crystalline information. The layer-plus-island Stranski–Krastanov growth mode is proposed according to above results. The ambient O₂ filled during growth is found to be the main oxygen source for the formed RuO₂.     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Preparation of B-doped a-Si1−xCx:H films and heterojunction p–i–n solar cells by the Cat-CVD method

B-doped a-Si_1−xC_x:H films for a window layer of Si thin film solar cells have been prepared by the Cat-CVD method. It is found that C is effectively incorporated into the films by using C₂H₂ as a C source gas, where an only little C incorporation is observed from CH₄ and C₂H₆ under similar deposition conditions. Using a-Si_1−xC_x:H films grown from C₂H₂, heterojunction p–i–n solar cells have been prepared by the Cat-CVD method. The cell structure is (SnO₂ Asahi-U)/ZnO/a-Si_1−xC_x:H(p)/a-Si:H(i)/μc-Si:H(n)/Al. The obtained conversion efficiency was 5.4%.     

Fabrication and Optical Properties of Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/Thin Films on Si Substrates Using the PLD Method

Epitaxial 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) thin films with electro-optic effects were fabricated on (La_0.5Sr_0.5)CoO₃(LSCO)/CeO₂/YSZ-buffered Si(001) substrates using double-pulse excitation pulsed laser deposition (PLD) method with a mask placed between the target and the substrate. Epitaxial growth of PMN-PT thin films was undertaken using the two-step growth method of PMN-PT film. The PMN-PT seed layer was deposited at 500degC on the LSCO/CeO₂/YSZ/Si, which temperature was the same as that used for LSCO deposition. The PMN-PT thin films were deposited on the PMN-PT seed layer at 600degC, which enables growth of high-crystallinity PMN-PT films with smooth surfaces. We obtained optimum fabrication conditions of PMN-PT film with micrometer-order thickness. Resultant films showed high crystallinity with full width at half maximum (FWHM)=0.73 deg and 1.6 mum thickness. Electro-optic properties and the refractive index value were measured at 633 nm wavelength using the prism coupling method. The obtained refractive index was 2.59. The electro-optic coefficients r₁₃ and r₃₃ were determined by applying the electrical field between a semitransparent, thin top electrode of Pt and a bottom LSCO electrode. The electro-optic coefficient was r₁₃=17 pm/V at transverse electric field (TE) mode and r₃₃=55 pm/V at transverse magnetic field (TM) mode.     

Influence of substrate properties on the growth of titanium films: part III

The growth of thin Ti-oxide films (12 nm) on alumina substrate films formed by reactive evaporation of Ti in an oxygen atmosphere was studied by in situ internal stress measurements under ultra high vacuum conditions and transmission electron microscopy. Oxygen pressure and substrate temperatures were the varied parameters of the reactive evaporation. These Ti-oxide-films with different oxygen content (O₂/Ti-films) were then used as substrate films for the deposition of a clean titanium film. The growth stress of the titanium film on the as-deposited O₂/Ti-substrate films is comparable with that previously found for H₂O/Ti-substrates and indicates island growth and the formation of polycrystalline titanium films. Annealing (400°C, 20 min) of the as-deposited – amorphous – O₂/Ti-films gives rise to the formation of crystalline TiO₂. The amount of TiO₂ formed during annealing is strongly dependent on the oxygen content of the O₂/Ti-film. The oxygen content, in return, is dependent on oxygen partial pressure and substrate temperature during O₂/Ti-film deposition. The corresponding changes in the substrate film properties (oxygen content, crystallinity, etc.) are reflected in significant changes in the growth stress of the titanium film. The stress vs. thickness curve of these titanium films appears to indicate a superposition of the growth stress of two different growth modes, i.e. growth of a polycrystalline film with island growth on the as-deposited, amorphous oxide substrate and epitaxial growth of a quasi single crystalline film on the crystalline TiO₂-substrate.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

The formation of a SiOx interfacial layer on low-k SiOCH materials fabricated in ULSI application

The characterizations of SiOCH films using oxygen plasma treatment depends linearly on the O₂/CO flow rate ratio. According to the results of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses, it was found that the carbon composition decreases with increasing O₂/CO flow rate ratio, because more carbon in the Si–O–C and Si–CH₃ bonds on the film surface would be converted by oxygen radicals. It was believed that the oxygen plasma could oxidize the SiOCH films and form a SiO_x interfacial capping layer without much porosity. Moreover, the result of FTIR analysis revealed that there was no water absorbed on the film. A SiO₂-like capping layer formed at the SiOCH film by the O₂/CO flow rate ratio of 0.75 had nearly the same dielectric properties from the result of capacitance–voltage (C–V) measurement in our research.     

Characterization and microstructure of highly preferred oriented lead barium titanate thin films on MgO (100) by sol-gel process

Highly preferred oriented lead barium titanate (Pb_1−x,Ba_x)TiO₃ thin film, with particular emphasis on (Pb_0.5,Ba_0.5)TiO₃, can be obtained by spin-coating on MgO (100) substrate by using the precursor sol, which was synthesized from acetylacetone chelating with titanium isopropoxide and ethylene glycol as a solvent, in the sol-gel process. Film thickness, pyrolysis temperature and heating rate were studied systemically to investigate their influences on the formation of preferred oriented thin films. The highly preferred (001)/(100) oriented thin film could be obtained by the pyrolysis of wet film at 500 °C and annealing at 600 °C at a slow heating rate of 5 °C/min. It is confirmed that the tetragonal perovskite structure of the titanate ceramic decreases with an increase of Ba content in (Pb_1−x,Ba_x)TiO₃. The (001)/(100) oriented films were synthesized from all compositions between x = 0.2 and x = 0.8, at a crystallization temperature of 600 °C. In particular, for the Ba content in the range of x = 0.50.6, highly preferred (001)/(100) planes were observed.     

Growth of CNx/BN:C multilayer films by magnetron sputtering

Symmetric CN_x/BN:C multilayer thin films, with nominal compositional modulation periods of Λ=2.5, 5, and 9 nm were deposited by unbalanced dual cathode magnetron sputtering from C (graphite) and B₄C targets in an Ar/N₂ (60/40) discharge. The multilayers and single-layer of the constituent CN_x and BN:C compounds were grown to a total thickness of 0.5 μm onto Si(001) substrates held at 225°C and a negative floating potential of 30 V (E_i≈24 eV). Layer characterizations were performed by TEM, X-ray reflectivity, RBS, and nanoindentation measurements. Results show that CN_0.33 and BN:C (35, 50, and 15 at.% of B, N, and C, respectively) layers were prepared at the above conditions. It is suggested that all films exhibit a three-dimensional interlocked structure with a cylindrical texture in the film growth direction. The structure was continuous over relatively well defined and smooth CN_x/BN:C interfaces. All coatings exhibit extreme elasticity with elastic recoveries as high as 85–90% (10 mN maximum load) attributed to the observed structure. However, the multilayers were stiffer and more elastic compared to that of the single-layers and thus shows promise for improved protective properties.     

Effects of oxygen partial pressure on lead content of PLZT thin films produced by excimer laser deposition

Excimer laser ablation has been used to produce thin films of lanthanum-modified lead zirconate titanate (PLZT), or Pb_1−xLa_x(Zr_1−yTi_y)_1−x/4O₃. PLZT is an interesting class of materials since it has a wide range of compositionally dependent electro-optical properties and strong non-linear optical characteristics. PLZT thin films of 7/0/100, 28/0/100 and 0/0/100 compositions have been deposited onto crystalline Si100 and amorphous fused silica substrates. Effects of oxygen backfill pressure on the Pb:(Ti + La) ratios were investigated. The results indicate that controlling the oxygen backfill pressure during laser deposition strongly influences the stoichiometry and crystal structure of PLZT thin films.     

Structure and electrical properties of boron-added (Ba,Sr)TiO3 thin films fabricated by the sol-gel method

Thin films of Ba_xSr_1−xTiO₃ (BST, with x=0.5) were fabricated on a RuO₂/Ru/SiO₂/Si substrate by the spin coating of the multicomponent sol prepared using metal alkoxides. Boron alkoxide was intentionally introduced to establish a better microstructure and to reduce the leakage current. AFM indicated that a crack-free uniform microstructure having a smooth surface was gradually developed with increasing boron content. The relative dielectric permittivity of the 250-nm thick BST thin films fired at 700°C decreased with increasing content of boron, from 420 for the undoped film to 190 for the 10 mol% boron-added film at 1 MHz. This observation was interpreted in terms of a serial capacitance composed of the perovskite BST grain and the interfacial B₂O₃ glassy phase having a low dielectric permittivity. The leakage current density (J) also decreased with the amount of boron added. The leakage current for the applied voltage greater than 1 V showed a linear variation of logJ with E^1/2 at room temperature, suggesting that the interface-controlled Schottky emission was the dominant conduction process for the BST thin films fabricated on the RuO₂ electrode.