Some physical properties of ZnO sputtered films

The secondary electron emission (SEE) yield δ of ZnO films was investigated. The films were deposited in an r.f. sputtering system using the r.f. power W, the argon pressure p, the O₂ partial pressure p_O2 and the substrate temperature T_s as parameters. Complementary measurements of the electrical resistivity and the optical absorption were performed. The ratio x of oxygen to zinc is an essential factor which determines the values of δ, and for the ZnO films.

Auger analyses showed that excess (overstoichiometric, x =1) oxygen is present in ZnO films obtained at room temperature. For x =1 the values of , the maximum SEE yield δ_m and the energy band gap E_g (determined from ) were found to be higher than those for stoichiometric ZnO (obtained at T_s200 °C). The highest values of (10⁴ Ω m), δ_m (4.4) and E_g (3.44 eV) were obtained for films with x = 1.7.     

(Fe,Ti) 16N2 films with high saturation magnetization prepared by facing target sputtering

(Fe,Ti)-N films with a Ti concentration of 10 at.% were prepared on Si(100) and NaCl substrates by facing targets sputtering. The effects of the nitrogen pressure (P_N) and the substrate temperature (T_s) on the formation of various (Fe,Ti)-N phases and their microstructures were investigated in detail. X-ray diffractometer and transmission electron microscope provided complete identification of the phases present in the films and the characterization of their microstructures. Films deposited at a lower P_N = 1 3 × 10⁻² Pa or a lower T_s = RT consist of mainly -phase. Films deposited at a higher P_N = 1.3 2 × 10⁻¹ Pa or a higher T_s = 200 °C contain a great many γ' and Fe₂N phases with a higher nitrogen content. When P_N = 4 7 × 10⁻² Pa and T_s = 100 150 °C, it is advantageous to the formation of ′' phase. These films exhibit a high saturation magnetization (M_s) up to the range of 2.3 2.5 T, which is larger than that of pure iron.     

Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation

Highly conducting and transparent indium tin oxide (ITO) thin films were prepared on SiO₂ glass and silicon substrates by pulsed laser ablation (PLA) from a 90 wt.% In₂O₃-10 wt.% SnO₂ sintered ceramic target. The growths of ITO films under different oxygen pressures (P_O2) ranging from 1×10⁻⁴–5×10⁻² Torr at low substrate temperatures (T_s) between room temperature (RT) and 200°C were investigated. The opto-electrical properties of the films were found to be strongly dependent on the P_O2 during the film deposition. Under a P_O2 of 1×10⁻² Torr, ITO films with low resistivity of 5.35×10⁻⁴ and 1.75×10⁻⁴ Ω cm were obtained at RT (25°C) and 200°C, respectively. The films exhibited high carrier density and reasonably high Hall mobility at the optimal P_O2 region of 1×10⁻² to 1.5×10⁻² Torr. Optical transmittance in excess of 87% in the visible region of the solar spectrum was displayed by the films deposited at Po₂≥1×10⁻² Torr and it was significantly reduced as the P_O2 decreases.     

Doping effects on the optical properties of evaporated a-Si:H films

Thin films of a-Si:H are deposited on substrates at 300°C by a conventional thermal evaporation technique. The electrical conductivity of these films is modified by the addition of antimony giving n-type films. The optical properties of the films are investigated using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 200–3000 nm. Both the refractive index n and the absorption coefficient increase when the Sb content is increased. The absorption edge shifts to lower energies for doped films. The optical gap E_g is evaluated using three different plots for comparison, namely; (hν)^1/2, (/hν)^1/2 and (hν)^1/3. The value of E_g decreases with doping for the three expressions. The Urbach parameter E₀ is calculated and found to increase with doping from 74 meV for the undoped film to 183 meV for concentrations of 9.4 at.% Sb.     

Structural control of vacuum-deposited thin films by the use of a Langmuir-Blodgett multilayer

An amphiphilic diacetylene of heptacosa-10,12-diynoic acid (14-8C) was vacuum deposited on a commercially available glass plate and on 10-layer LB films of both arachidic acid (C₂₀C) and 14-8C at various substrate temperatures (T_s). At a T_s of 22°C, 14-8C molecules on the glass substrate were oriented randomly with respect to the substrate, whereas those on 14-8C LB film at the same T_s were all oriented perpendicularly. At the higher T_s of 50°C, 14-8C molecules were randomly oriented even on the LB film because of the disordering of molecules in LB films.

In the process of structural characterization we have succeeded for the first time in the direct observation of lattice images of LB films by use of a liquid-helium- cooled cryo-electron microscope.     

Properties of tantalum oxide thin films grown by atomic layer deposition

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl₅ and H₂O at temperatures in the range 80–500 °C. The films deposited at temperatures below 300 °C were predominantly amorphous, whereas those grown at higher temperatures were polycrystalline containing the phases TaO₂ and Ta₂O₅. The oxygen to tantalum mass concentration ratio corresponded to that of TaO₂ at all growth temperatures. The optical band gap was close to 4.2 eV for amorphous films and ranged from 3.9 to 4.5 eV for polycrystalline films. The refractive index measured at λ = 550 nm increased from 1.97 to 2.20 with an increase in growth temperature from 80 to 300 °C. The films deposited at 80 °C showed low absorption with absorption coefficients of less than 100 cm⁻¹ in the visible region.     

Highly conducting doped poly-Si deposited by hot wire CVD and its applicability as gate material for CMOS devices

Highly conducting p- and n-type poly-Si:H films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH₄+H₂+B₂H₆ and SiH₄+H₂+PH₃ gas mixtures, respectively. Conductivity of 1.2×10² (Ω cm)⁻¹ for the p-type films and 2.25×10² (Ω cm)⁻¹ for the n-type films was obtained. These are the highest values obtained so far by this technique. The increase in conductivity with substrate temperature (T_s) is attributed to the increase in grain size as reflected in the atomic force microscopy results. Interestingly conductivity of n-type films is higher than the p-type films deposited at the same T_s. To test the applicability of these films as gate contact Al/poly-Si/SiO₂/Si capacitor structures with oxide thickness of 4 nm were fabricated on n-type c-Si wafers. Sputter etching of the poly-Si was optimized in order to fabricate the devices. The performance of the HWCVD poly-Si as gate material was monitored using C–V measurements on a MOS test device at different frequencies. The results reveal that as deposited poly-Si without annealing shows low series resistance.     

Transparent conducting F-doped SnO2 thin films grown by pulsed laser deposition

Transparent conducting fluorine-doped tin oxide (SnO₂:F) films have been deposited on glass substrates by pulsed laser deposition. The structural, electrical and optical properties of the SnO₂:F films have been investigated as a function of F-doping level and substrate deposition temperature. The optimum target composition for high conductivity was found to be 10 wt.% SnF₂ + 90 wt.% SnO₂. Under optimized deposition conditions (T_s = 300 °C, and 7.33 Pa of O₂), electrical resistivity of 5 × 10^− 4 Ω-cm, sheet resistance of 12.5 Ω/□, average optical transmittance of 87% in the visible range, and optical band-gap of 4.25 eV were obtained for 400 nm thick SnO₂:F films. Atomic force microscopy measurements for these SnO₂:F films indicated that their root-mean-square surface roughness ( 6 Å) was superior to that of commercially available chemical vapor deposited SnO₂:F films ( 85 Å).     

Semiconducting and structural properties of CrSi2 A-type epitaxial films on Si(111)

Chromium disilicide (CrSi₂) films 1 000 Å thick have been prepared by molecular beam epitaxy on CrSi₂ templates grown on Si(111) substrate. The effect of the substrate temperature on the structural, electrical and optical properties of CrSi₂ films has been studied by transmission and scanning electron microscopies, optical microscopy, electrical resistivity and Hall effect measurements and infrared optical spectrometry. The optimal temperature for the formation of the epitaxial A-type CrSi₂ film have been found to be about 750°C. The electrical measurement have shown that the epitaxial A-type CrSi₂ film is p-type semiconductor having a hole concentration of 1 × 10¹⁷cm⁻³ and Hall mobility of 2 980 cm² V⁻¹ s⁻¹ at room temperature. Optical absorption coefficient data have indicated a minimum, direct energy gap of 0.34 eV. The temperature dependence of the Hall mobility (μ) in the temperature range of T = 180–500 K can be expressed as μ = 7.8 × 10¹⁰T⁻³cm²V⁻¹s⁻¹.     

Fabrication of robust Bi3.25La0.75Ti3O12 thin film resistant to hydrogen damage

Ferroelectric bismuth lanthanum titanate (Bi_3.25La_0.75Ti₃O₁₂; BLT) thin films were deposited on Pt/TiO₂/SiO₂/Si substrate by chemical solution deposition method. The films were crystallized in the temperature range of 600–700 °C. The spontaneous polarization (P_s) and the switching polarization (2P_r) of BLT film annealed at 700 °C for 30 min were 22.6 μC/cm² and 29.1 μC/cm², respectively. Moreover, the BLT capacitor did not show any significant reduction of hysteresis for 90 min at 300 °C in the forming gas atmosphere.     

Optical constants of polycrystalline CuGaTe2 films

Thin films of CuGaTe₂ with thicknesses in the range, 0.1–1.0 μm were deposited on Corning 7059 glass substrates by flash evaporation. The substrate temperatures, T_s, were maintained in the range 373–623 K. The transmittance of the films was recorded in the wavelength range 900–2500 nm. The dependence of the optical band gap, E_g, on substrate temperature showed that the value of E_g varied from 1.21 eV to 1.24 eV. The variation of refractive index and extinction coefficient with photon energy was studied from which the material properties such as the limiting value of dielectric constant, ε_∞, plasma frequency, ω_p, and hole effective mass, m_h^*, were evaluated as ε_∞ = 7.59, ω_p = 1.47 × 10¹⁴ and m_h^* = 1.25 m₀.     

Investigation of the nucleation mechanism in bias-enhanced diamond deposition on silicon and molybdenum

Polycrystalline diamond films were deposited on Si and Mo substrates in a microwave plasma-enhanced chemical vapour deposition reactor employing bias-enhanced nucleation. The deposition process was subdivided into two consecutive steps: the pretreatment (bias-enhanced nucleation) and the diamond growth step. To investigate the nucleation process we kept the deposition parameters during the diamond growth step constant and only changed the parameters during the pretreatment. The methods employed to analyze the deposited films after the pretreatment step were electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy.

The nucleation density (N_D) on Si following the complete deposition cycle (pretreatment and diamond growth step) increases considerably from 5 × 10⁸ cm⁻² to 5 × 10¹⁰ cm⁻² with an increase in the substrate temperature during the pretreatment (T_p) in the temperature range from 680 to 750 °C. For T_p ≥ 770 °C continuous films are formed. The structure of the pretreatment deposit undergoes likewise considerable changes: if T_p exceeds 770 °C the appearance of an intense diamond plasmon at 34 eV is observed, indicative of an increase in the concentration of diamond crystallites embedded in an otherwise amorphous carbon matrix. Our experiments suggest that diamond crystallites formed during the pretreatment serve as nucleation centres for the subsequent diamond growth.

The same deposition parameters which result in the formation of a continuous diamond film on Si, yield only low nucleation densities on Mo. An increase in N_D from 6 × 10⁶ cm⁻² to 2 × 10⁸ cm⁻² can be achieved by raising the methane concentration [CH₄] in the gas phase during the pretreatment from 5 to 50% (T_p = 820 °C). The carbon concentration at the surface for the pretreatment deposit, determined by XPS analysis, increases likewise with [CH₄]. According to the EELS analysis the structure of the pretreatment deposit is comparable with disordered graphite or a-C and no diamond plasmon is observed. The high [CH₄] is required to form the Mo-carbide interface and balance the diffusion of carbon into the metal before the a-C layer can be formed.

The formation of nucleation centres during the bias-enhanced nucleation seems under these deposition conditions to proceed via different pathways on Si and Mo. While the nucleation on Si appears to be linked to the formation of diamond nanocrystals during the pretreatment, this is not the case for Mo.     

Preparation and characterization of YBa2Cu3O7−x high Tc superconducting thin films deposited by S-gun sputtering

We have prepared YBa₂Cu₃O_7−x high T_c superconducting (HTS) thin films on (100) yttria-stabilized zirconia (YSZ) and LaAlO₃ (LAO) substrates, using a 2 kW S-gun in an off-axis mode. By varying the temperature of the substrates, films with a axis and c axis orientations were readily obtained. The X-ray diffraction pattern and Laue pattern confirmed that films with a axis orientation exhibited a single-crystal texture. All films had a good mirror-like surface. For films grown on YSZ substrates, scanning electron microscopy (SEM) revealed a clear distinction between the surfaces of the films grown at various temperatures (520–780°C). Films grown on LAO substrates exhibited even smoother and flatter surfaces. The SEM changes will be discussed in correlation with J_c. The best HTS thin films were obtained on LAO substrates at a temperature of 820°C, with T_c=89 K and J_c=1×10⁶ A cm^-2 (77 K).     

Surface and electrical studies of CuO:V2O5 thin films

Results from the studies of multicomponent CuO:V₂O₅ bulk material and thermally evaporated thin films of highly conducting bulk composition prepared at different substrate temperatures are thus compared and discussed. The electronic conductivity is enhanced on increase in the substrate temperature T_s and reaches a maximum value of 12.3 × 10⁻⁶Ω⁻¹ cm⁻¹ for T_s = 423 K. X-ray photoelectron spectroscopy studies indicate an increase in the reduced states of vanadium and copper ions in going from the bulk glass to the thin film. Dynamic secondary-ion mass spectroscopy studies on thin films over a depth of 3000 Å show a strong dependence of T_s on the Cu-to-V intensity ratio. Even though stoichiometric values for thin films are achievable by varying the T_s, the oxidation states of Cu in these films are predominantly monovalent. The electrical behaviors of these materials and their thin film counterparts are finally being discussed in relation to the surface analysis data.     

Stress variation in epitaxial GaAs films on silicon under thermal treatment

High quality epitaxial GaAs films of 1.8 and 6.3 μm thickness on silicon substrates were examined for lattice distortion, misalignment and curvature by X-ray diffraction (Bond method) at 20–400 °C. These films were deposited by the metal-organic chemical vapour deposition method on the (001) plane of silicon using a buffer layer produced at T_b = 370 or 460 °C. A top layer was then grown at T_t = 560 or 650 °C. The GaAs films contract more strongly on cooling than the substrate, which causes a curvature and a tetragonal distortion below a critical temperature T_c. This temperature varies on thermal treatment at 200–400 °C and approaches T_b, the growth temperature of the buffer layer. The tetragonal distortion can be stabilized, so that T_c approximates T_b, if the GaAs films are annealed for several days at 400 °C.     

Atmospheric plasma discharge chemical vapor deposition of SnOx thin films using various tin precursors

A series of 0.2–0.6 μm thick SnO_x films were deposited onto borosilicate and sodalime silica glass substrates by atmospheric plasma discharge chemical vapor deposition at 80 °C. SnO_x films deposited from monobutyltin trichloride contained a large percentage of SnCl₂:2H₂O, and therefore were partially soluble in water. SnO_x coatings deposited from tetrabutyltin were not soluble in water or organic solvents, had good adhesion even at growth rates as high as 2.3 nm/s, had high transparency of  90% and electrical resistivity of 10⁷ Ω cm. As-grown tin oxide coatings were amorphous with a small concentration of SnO₂, SnO and Sn crystalline phases as determined by grazing angle X-ray diffraction and X-ray photoelectron spectroscopy measurements. Upon annealing in air at 600 °C the resistivity of SnO_x films decreased to 5–7 Ω cm. Furthermore, optical and X-ray measurements indicated that SnO_x was converted into SnO₂ (cassiterite) with a direct band gap of 3.66 eV. Annealing of as-grown SnO_x films in vacuum at 340 °C led to formation of the p-type conductor SnO/SnO_x. The indirect band gap of SnO was calculated from the optical spectra to be 0.3 eV.     

Growth of GaN and AlN thin films by laser induced molecular beam epitaxy

Hexagonal GaN and AlN thin films were grown by laser induced molecular beam epitaxy using Al or Ga metal as target material and N₂as nitrogen source. The films were deposited on sapphire (0001) and SiC (0001) substrates. Epitaxial growth of GaN has been achieved at 730°C and 10⁻³ mbar N₂ pressure. The AlN films were polycrystalline with predominant (0001) orientation.     

Effect of N2O/SiH4 ratio on the properties of low-temperature silicon oxide films from remote plasma chemical vapour deposition

Silicon oxide films have been deposited with remote plasma chemical vapour deposition (RPCVD) at low temperatures (<300 °C) from SiH₄---N₂O. The effect of a gas-phase reaction on the SiO₂ film properties and Si/SiO₂ interface was investigated. As the partial pressure ratio was increased above N₂O/SiH₄ = 4, a gas-phase reaction with powder formation was observed, which degraded film properties, increased surface roughness, and decreased deposition rate. When N₂O/SiH₄ <4, there was no detectable IR absorption in the film associated with hydrogen-related bonds (Si---OH and Si---H) but when N₂O/SiH₄ >4, the incorporation of Si---OH bond became significant and Si¹⁺, intermediate state silicon at the interface, was more abundant. The oxide fixed charge, interface trap density, surface roughness and leakage current were increased when there was powder formation in the gas phase. High plasma power also favoured the powder formation in the gas phase. C---V and I---V characteristics were measured and it was shown that these electrical properties were directly related to the process condition and material properties of the oxide and the Si/SiO₂ interface.     

Growth, selectivity and adhesion of CVD-deposited copper from Cu (hexafluoroacetylacetonate trimethylvinylsilane) and dichlorodimethylsilane

Copper films have been deposited by low-pressure (1–20 mTorr) chemical vapour deposition using Cu¹⁺ (hexafluoroacetylacetonate) trimethylvinylsilane onto SiO₂ patterned substrates having seed layers of W, TiN and Al. Blanket deposition is observed for all growth temperatures in the range 140 °C ≤ T_g ≤ 240 °C. However, depending on the initial substrate seed layer and pre-treatment, the relative strength of the copper-oxide and copper-seed layer bond can be dramatically altered particularly when growth is carried out in the presence of dichloromethylsilane (DCDMS). The degree of selectivity as well as film morphology is also found to be sensitive to the initial pre-treatment, growth temperature and flow rate of DCDMS.     

Correlation of structure and optical properties of an annealed hydrogenated amorphous silicon-carbon alloy film

A hydrogenated amorphous silicon-carbon (a-Si_0.76C_0.24:H) film has been prepared via the glow discharge decomposition of SiH₄ and C₂H₄. The optical constants of this alloy film have been determined as a function of annealing temperature T_a for photon energies between 1.5 and 4.75 eV. The refractive index n and its imaginary part k show small but significant variation with annealing temperature. The optical energy gap E_opt also exhibits interesting variation with annealing temperature, decreasing with increasing annealing temperature up to T_a = 650°C and then increasing above this temperature. Further, E_opt is found to be correlated with the inverse of the full width at half-maximum of the Si---C and the Si---O IR stretch absorption mode, which seems to indicate that the changes in E_opt are structural in origin and that phonon order correlates well with electronic order.