Control of a- and c-plane preferential orientations of ZnO thin films

We report orientation-controllable growth of ZnO thin films and their orientation-dependent electrical characteristics. ZnO thin films were deposited on single-crystalline (1 0 0) LaAlO₃ and (1 0 0) SrTiO₃ substrates using pulsed laser deposition (PLD) at different substrate temperatures (400-800 °C). It was found that the orientation of ZnO films could be controlled by using different substrates of single-crystalline (1 0 0) LaAlO₃ and (1 0 0) SrTiO₃. The a-plane () and c-plane (0 0 0 2) oriented ZnO films are formed on LaAlO₃ and SrTiO₃, respectively. In both cases, the degree orientation increased with increasing deposition temperature T_s. Both the surface free energy and the degree of lattice mismatch are ascribed to play an important role for the orientation-controllable growth. Further characterization show that the grain size of the films with both orientations increases for a substrate temperature increase (i.e. from T_s = 400 °C to T_s = 800 °C), whereas the electrical properties of ZnO thin films depend upon their crystalline orientation, showing lower electrical resistivity values for a-plane oriented ZnO films.     

Pulsed-laser crystallization and epitaxial growth of metal-organic films of Ca-doped LaMnO3 on STO and LSAT substrates

Ca-doped LaMnO₃ (LCMO) thin films have been successfully prepared on SrTiO₃ (STO) and [(LaAlO₃)_0.3-(SrAlTaO₆)_0.7] (LSAT) substrates using the excimer laser assisted metal-organic deposition (ELAMOD) process. The crystallization and the epitaxial growth of the amorphous metal-organic LCMO thin films have been achieved using a KrF excimer laser irradiation while the substrates were kept at constant temperature of 500 °C. Epitaxial films were obtained using laser fluence in the interval of 50-120 mJ/cm². The microstructure of the LCMO films was studied using cross-section transmission electron microscopy. High quality of LCMO films having smooth surfaces and sharp interfaces were obtained on both the STO and the LSAT substrates. The effect of the laser fluence on the temperature coefficient of resistance (TCR) was investigated. The largest values of TCR of the LCMO grown on the LSAT and the STO substrates of 8.3% K⁻¹ and 7.46% K⁻¹ were obtained at different laser fluence of 80 mJ/cm² and 70 mJ/cm², respectively.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Microstructure of epitaxial SrRuO3 thin films on MgO substrates

SrRuO₃ thin films have been grown on singular (1 0 0) MgO substrates using pulsed laser deposition (PLD) in 30 Pa oxygen ambient and at a temperature of 400-700 °C. Ex situ reflection high-energy electron diffraction (RHEED) as well as X-ray diffraction (XRD) θ/2θ scan indicated that the films deposited above 650 °C were well crystallized though they had a rough surface as shown by atom force microscopy (AFM). XRD Φ scans revealed that these films were composed of all three different types of orientation domains, which was further confirmed by the RHEED patterns. The heteroepitaxial relationship between SrRuO₃ and MgO was found to be [1 1 0] SRO//[1 0 0] MgO and 45°-rotated cube-on-cube [0 0 1] SRO//[1 0 0] MgO. These domain structures and surface morphology are similar to that of ever-reported SrRuO₃ thin films deposited on the (0 0 1) LaAlO₃ substrates, and different from those deposited on (0 0 1) SrTiO₃ substrates that have an atomically flat surface and are composed of only the [1 1 0]-type domains. The reason for this difference was ascribed to the effect of lattice mismatch across the film/substrate interface. The room temperature resistivity of SrRuO₃ films fabricated at 700 °C was 300 μΩ cm. Therefore, epitaxial SrRuO₃ films on MgO substrate could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Investigation of Ni/Ta contacts on 4H silicon carbide upon thermal annealing

Nickel and Tantalum thin films with 3:5 thickness ratios were deposited in succession onto 4H-SiC substrate at room temperature. The samples were then heated in situ in vacuum at 650, 800 or 950 °C for 30 min. Glancing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES) and current-voltage (I-V) technique were used for characterising the interfacial reactions and electrical properties. Amorphous Ni-Ta can be formed by solid-state reaction at 650 °C. The minor dissolved Ni in the Ta metal promotes the reaction between Ta and SiC. With increasing annealing temperature up to 950 °C, the dominant carbide changes from Ta₂C to TaC and a layer structure is developed. Electrical measurements show that ohmic contact is formed after annealing at or above 800 °C.     

Nano-graphene structures deposited by N-IR pulsed laser ablation of graphite on Si

Thin nano-structured carbon films have been deposited in vacuum by pulsed laser ablation, from a rotating polycrystalline graphite target, on Si 〈1 0 0〉 substrates, kept at temperatures ranging from RT to 800 °C. The laser ablation was performed by a Nd:YAG laser, operating in the near IR (λ = 1064 nm).X-ray diffraction analysis, performed at grazing incidence angle, both in-plane (ip-gid) and out-of-plane (op-gid), has shown the growth of oriented nano-sized graphene particles, characterised by high inter-planar stacking distance (d_? ∼ 0.39 nm), compared to graphite. The film structure and texturing are strongly related both to laser wavelength and substrate temperature: the low energy associated to the IR laser radiation (1.17 eV) generates activated carbon species of large dimensions that, also at low T (∼400 °C), easy evolve toward more stable sp² aromatic bonds, in the plume direction. Increasing temperature the nano-structure formation increases, causing a further aggregation of aromatic planes, voids formation, and a related density (by X-ray reflectivity) drop to very low values. SEM and STM show for these samples a strongly increased macroscopic roughness. The whole process, mainly at higher temperatures, is characterised by a fast kinetic mode, far from equilibrium and without any structural or spatial rearrangement.     

Spray-pyrolysis deposition of LaMnO3 and La1−xCaxMnO3 thin films

Ethylene glycol solutions of La-Mn(II) and La-Ca-Mn(II) citric complexes has been used as a starting material for spray-pyrolysis deposition of LaMnO₃ and La_1−xCa_xMnO₃ thin films on β-quartz, fused quartz, Si(0 0 1) and SrTiO₃(1 0 0) substrates heated during the deposition at 380 °C. At suitable post-deposition heating conditions highly uniform films, 0.1-1 μm in thickness, with good crystal structure were obtained. Highly textured LaMnO₃ films are obtained on SrTiO₃(1 0 0) substrate. Interaction between the layer and Si-containing substrates is observed during the post-deposition heating in static air.     

Preparation of the La0.8Sr0.2MnO3 films on STO and LAO substrates by excimer laser-assisted metal organic deposition using the KrF laser

La_0.8Sr_0.2MnO₃ films were prepared on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates using excimer laser-assisted metal organic deposition (ELAMOD). For the LAO substrate, no epitaxial La_0.8Sr_0.2MnO₃ film was obtained by laser irradiation in the fluence range from 60 to 110 mJ/cm² with heating at 500 °C. On the other hand, an epitaxial La_0.8Sr_0.2MnO₃ film on the STO substrate was formed by laser irradiation in the fluence range from 60 to 100 mJ/cm² with heating at 500 °C. To optimize the electrical properties for an IR sensor, the effects of the laser fluence, the irradiation time and the film thickness on the temperature dependence of the resistance and temperature coefficient of resistance (TCR: defined as 1/R·(dR/dT)) of the LSMO films were investigated. An LSMO film on the STO substrate that showed the maximum TCR of 3.9% at 265 K was obtained by the ELAMOD process using the KrF laser.     

Study on effects of substrate temperature on growth and structure of alignment carbon nanotubes in plasma-enhanced hot filament chemical vapor deposition system

Alignment carbon nanotubes (ACNTs) were synthesized on silicon substrate coated with Ni catalyst film and Ta buffer layer by plasma-enhanced hot filament chemical vapor deposition using CH₄, NH₃, and H₂ as the reaction gas, and they were investigated by scanning electron microscopy and transmission electron microscopy. It is found that the diameter of the bamboo-structured ACNTs is increased from 62 to 177 nm when the substrate temperature was changed from 626 to 756 °C. Their growth rate is enhanced by the substrate temperature in a range of 626-683 °C and it is reversely reduced with the substrate temperature after the substrate temperature is over 683 °C. Beginning with wetting phenomenon, the effects of the substrate temperature on the structure and growth rate of the ACNTs are analyzed.     

Effect of growth temperature on the morphology and bonded states of SnO2 nanobaskets

The nanobaskets of SnO₂ were grown on in-house fabricated anodized aluminum oxide pores of 80 nm diameter using plasma enhanced chemical vapor deposition at an RF power of 60 W. Hydrated stannic chloride was used as a precursor and O₂ (20 sccm) as a reactant gas. The deposition was carried out from 350 to 500 °C at a pressure of 0.2 Torr for 15 min each. Deposition at 450 °C results in highly crystalline film with basket like (nanosized) structure. Further increase in the growth temperature (500 °C) results in the deterioration of the basket like structure and collapse of the alumina pores. The grown film is of tetragonal rutile structure grown along the [1 1 0] direction. The change in the film composition and bonded states with growth temperature was evident by the changes in the photoelectron peak intensities of the various constituents. In case of the film grown at 450 °C, Sn 3d_5/2 is found built up of Sn⁴⁺ and O-Sn⁴⁺ and the peaks corresponding to Sn²⁺ and O-Sn²⁺ were not detected.     

An in situ transport measurement of interfaces between SrTiO3(1 0 0) surface and an amorphous wide-gap insulator

We have explored the transport properties of the interface between a SrTiO₃(1 0 0) single crystal and a CaHfO₃ wide-gap insulator layer deposited by pulsed laser deposition. The electrical transport measurements were done in situ during the heterojunction fabrication and consistently showed an enhancement of interface conductivity. A conducting interface was always obtained, independent of deposition parameters (laser pulse rate, laser fluence, oxygen pressure, and substrate termination). The conduction was attributed to plume-induced photocurrent in SrTiO₃. The current decay rate after insulator film fabrication was strongly influenced by substrate termination. An exponential relaxation-type photocurrent decay was clearly seen on SrO-termination, whereas a nearly constant conductivity was seen for up to 24 h on TiO₂-terminated surfaces.     

Femtosecond Z-scan measurement of third-order optical nonlinearities in anatase TiO2 thin films

Anatase phase TiO₂ films have been grown on fused silica substrate by pulsed laser deposition technique at substrate temperature of 750 °C under the oxygen pressure of 5 Pa. From the transmission spectra, the optical band gap and linear refractive index of the TiO₂ films were determined. The third-order optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. The real and imaginary parts of third-order nonlinear susceptibility χ⁽³⁾ were determined to be −7.1 × 10⁻¹¹esu and −4.42 × 10⁻¹²esu, respectively. The figure of merit, T, defined by T=βλ/n₂, was calculated to be 0.8, which meets the requirement of all-optical switching devices. The results show that the anatase TiO₂ films have great potential applications for nonlinear optical devices.     

Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition

Well-crystallized 250 nm-thick SrTiO₃ thin films on fused-quartz substrate were prepared by pulsed laser deposition. The band-gap of SrTiO₃ thin film by transmittance spectra is equal to 3.50 eV, larger than 3.22 eV for the bulk crystal. The nonlinear optical properties of the films were examined with picosecond pulses at 1.064 μm excitation. A large two-photon absorption (TPA) with absorption coefficient of 87.7 cm/GW was obtained, larger than 51.7 cm/GW for BaTiO₃ thin films. The nonlinear refractive index n₂ is equal to 5.7×10⁻¹⁰ esu with a negative sign, larger than 0.267×10⁻¹¹ esu for bulk SrTiO₃. The large TPA is attributed to intermediate energy levels introduced by the grain boundaries, and the optical limiting behaviors stemming from both TPA and negative nonlinear refraction were also discussed.     

Effects of annealing temperature and method on structural and optical properties of TiO2 films prepared by RF magnetron sputtering at room temperature

TiO₂ thin film was deposited on non-heated Si(1 0 0) substrate by RF magnetron sputtering. The as-deposited films were annealed by a conventional thermal annealing (CTA) and rapid thermal annealing (RTA) at 700 and 800 °C, and the effects of annealing temperature and method on optical properties of studied films were investigated by measuring the optical band gaps and FT-IR spectra. And we also compared the XRD patterns of the studied samples. The as-deposited film showed a mixed structure of anatase and brookite. Only rutile structures were found in samples annealed above 800 °C by CTA, while there are no special peaks except the weak brookite B(2 3 2) peak for the sample annealed at (or above) 800 °C by RTA. FT-IR spectra show the broad peaks due to Ti-O vibration mode in the range of 590-620 cm⁻¹ for the as-deposited film as well as samples annealed by both annealing methods at 700 °C. The studied samples all had the peaks from Si-O vibration mode, which seemed to be due to the reaction between TiO₂ and Si substrate, and the intensities of these peaks increased with increasing of annealing temperature. The optical band gap of the as-deposited film was 3.29 eV but it varied from 3.39 to 3.43 eV as the annealing temperature increased from 700 to 800 °C in the samples annealed by CTA. However, it varied from 3.38 to 3.32 eV as the annealing temperature increased from 700 to 800 °C by RTA.     

Epitaxial growth of InN on nearly lattice-matched (Mn,Zn)Fe2O4

We have grown InN films on nearly lattice-matched (Mn,Zn)Fe₂O₄ (111) substrates at low temperatures by pulsed laser deposition (PLD) and investigated their structural properties. InN films grown at substrate temperatures above 400 °C show poor crystallinity, and their in-plane epitaxial relationship is [10-10]InN//[11-2](Mn,Zn)Fe₂O₄, which means that their lattice mismatch is quite large (11%). By contrast, high quality InN films with flat surfaces can be grown at growth temperatures lower than 150 °C with the ideal in-plane epitaxial relationship of [11-20]InN//[11-2](Mn,Zn)Fe₂O₄, which produces lattice mismatches of as low as 2.0%. X-ray reflectivity measurements have revealed that the thickness of the interfacial layer between the InN and the substrates is reduced from 14 to 8.4 nm when the growth temperature is decreased from 400 °C to room temperature. This suppression of the interface reactions by reducing the growth temperature is probably responsible for the improvement in crystalline quality. These results indicate that the use of (Mn,Zn)Fe₂O₄ (111) substrates at low growth temperatures allows us to achieve nearly lattice matched epitaxial growth of InN.     

Structural, electrical, and surface characteristics of La0.5Sr0.5CoO3 thin films prepared by pulsed-laser deposition

A parametric study of the growth of La_0.5Sr_0.5CoO₃ (LSCO) thin films on (100) MgO substrates by pulsed-laser deposition (PLD) is reported. Films are grown under a wide range of substrate temperature (450–800 °C), oxygen pressure (0.1–0.9 mbar), and incident laser fluence (0.8–2.6 J/cm²). The optimum ranges of temperature, oxygen pressure, and laser fluence to produce c-axis oriented films with smooth surface morphology and high metallic conductivity are identified. Films deposited at low temperature (500 °C) and post-annealed in situ at higher temperatures (600–800 °C) are also investigated with respect to their structure, surface morphology, and electrical conductivity. Received: 20 November 1998 / Accepted: 6 July 1999 / Published online: 21 October 1999     

Microstructure and dielectric properties of La2O3 doped amorphous SiO2 films as gate dielectric material

As a potential gate dielectric material, the La₂O₃ doped SiO₂ (LSO, the mole ratio is about 1:5) films were fabricated on n-Si (0 0 1) substrates by using pulsed laser deposition technique. By virtue of several measurements, the microstructure and electrical properties of the LSO films were characterized. The LSO films keep the amorphous state up to a high annealing temperature of 800 °C. From HRTEM and XPS results, these La atoms of the LSO films do not react with silicon substrate to form any La-compound at interfacial layer. However, these O atoms of the LSO films diffuse from the film toward the silicon substrate so as to form a SiO₂ interfacial layer. The thickness of SiO₂ layer is only about two atomic layers. A possible explanation for interfacial reaction has been proposed. The scanning electron microscope image shows the surface of the amorphous LSO film very flat. The LSO film shows a dielectric constant of 12.8 at 1 MHz. For the LSO film with thickness of 3 nm, a small equivalent oxide thickness of 1.2 nm is obtained. The leakage current density of the LSO film is 1.54 × 10⁻⁴ A/cm² at a gate bias voltage of 1 V.     

Parallel syntheses and thermoelectric properties of Ce-doped SrTiO3 thin films

Thermoelectric properties of single crystalline Ce_xSr_1−xTiO₃ films (0 ≤ x ≤ 0.5) have been studied by using combinatorial pulsed-laser deposition. Temperature gradient method was used for identifying an optimum growth temperature for SrTiO₃ homoepitaxial growth, at which both oxygen stoichiometry and persisting layer-by-layer growth mode could be accomplished. Electrical conductivity (σ) and Seebeck coefficient (S) were measured at room temperature for the composition-spread films grown at the optimized temperature and found to be considerably higher than those reported for bulk poly-crystalline compounds. Hall measurement revealed that carrier density linearly increased with increasing x, suggesting that a trivalent Ce ions substituted divalent Sr ions to supply electrons. A maximum power factor (S²σ) was obtained for the x = 0.2 film, being 7 and 14 μW/K² cm at 300 and 900 K, respectively.     

Stress dependence of growth mode change of epitaxial layered cobaltite γ-Na0.7CoO2

We report stress dependence of growth characteristics of epitaxial γ-Na_0.7CoO₂ films on various substrates deposited by pulsed laser deposition method. On the sapphire substrate, the γ-Na_0.7CoO₂ thin film exhibits spiral surface growth with multi-terraces and highly crystallized texture. For the γ-Na_0.7CoO₂ thin film grown on the (1 1 1) SrTiO₃ substrate, the nano-islands of ∼30 nm diameter on the hexagonal grains are observed. These islands indicate that the growth mode changes from step-flow growth mode to Stranski-Krastanow (SK) growth mode. On the (1 1 1) MgO substrate, the large grains formed by excess adatoms covering an aperture between hexagonal grains are observed. These experimental demonstrations and controllability could provide opportunities of strain effects of Na_xCoO₂, physical properties of thin films, and growth dynamics of heterogeneous epitaxial thin films.     

Growth and structure of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 thin films prepared by pulsed laser deposition technique

Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ (BNKLT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition (PLD) technique. The films prepared were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of the processing parameters, such as oxygen pressure, substrate temperature and laser power, on the crystal structure, surface morphology, roughness and deposition rates of the thin films were investigated. It was found that the substrate temperature of 600 °C and oxygen pressure of 30 Pa are the optimized technical parameters for the growth of textured film, and all the thin films prepared have granular structure, homogeneous grain size and smooth surfaces.