Influence of substrate crystallography on the room temperature synthesis of AlN thin films by reactive sputtering

A pulsed DC reactive ion beam sputtering system has been used to synthesize aluminium nitride (AlN) thin films at room temperature by reactive sputtering. After systematic study of the processing variables, high-quality polycrystalline films with preferred c-axis orientation have been grown successfully on silicon and Au/Si substrates with an Al target under a N₂/(N₂ + Ar) gas flow ratio of 55%, 2 mTorr processing pressure and keeping the temperature of the substrate holder at room temperature. The crystalline quality of the AlN layer as well as the influence of the substrate crystallography on the AlN orientation has been characterized by high-resolution X-ray diffraction (HR-XRD). Best ω-FWHM (Full Width at Half Maximum) values of the (0 0 0 2) reflection rocking curve in the 1 μm thick AlN layers are 1.3°. Atomic Force Microscopy (AFM) measurements have been used to study the surface morphology of the AlN layer and Transmission Electron Microscopy (TEM) measurements to investigate the AlN/substrate interaction. AlN grew off-axis from the Si substrate but on-axis to the surface normal. When the AlN thin film is deposited on top of an Au layer, it grows along the [0 0 0 1] direction but showing a two-domain structure with two in-plane orientations rotated 30° between them.     

Room temperature growth and properties of ZnO films by pulsed laser deposition

ZnO thin films were prepared by pulsed laser deposition at room temperature on glass substrates with oxygen pressures of 10-30 Pa. The structural, electrical, and optical properties of ZnO films were studied in detail. ZnO films had an acceptable crystal quality with high c-axis orientation and smooth surface. The resistivity was in the 10² Ω cm order for ZnO films, with the electron concentration of 10¹⁶-10¹⁷ cm⁻³. All the films showed a high visible transmittance ∼90% and a high UV absorption about 90-100%. The UV emission ∼390 nm was observed in the photoluminescence spectra. The oxygen pressures in the 10-30 Pa range were suitable for room temperature growth of high-quality ZnO films.     

On the use of response surface methodology to predict and interpret the preferred c-axis orientation of sputtered AlN thin films

This paper deals with experimental design applied to response surface methodology (RSM) in order to determine the influence of the discharge conditions on preferred c-axis orientation of sputtered AlN thin films. The thin films have been deposited by DC reactive magnetron sputtering on Si (1 0 0) substrates. The preferred orientation was evaluated using a conventional Bragg-Brentano X-ray diffractometer (θ-2θ) with the CuKα radiation. We have first determined the experimental domain for 3 parameters: sputtering pressure (2-6 mTorr), discharge current (312-438 mA) and nitrogen percentage (17-33%). For the setup of the experimental design we have used a three factors Doehlert matrix which allows the use of the statistical response surface methodology (RSM) in a spherical domain. A four dimensional surface response, which represents the (0 0 0 2) peak height as a function of sputtering pressure, discharge current and nitrogen percentage, was obtained. It has been found that the main interaction affecting the preferential c-axis orientation was the pressure-nitrogen percentage interaction. It has been proved that a Box-Cox transformation is a very useful method to interpret and discuss the experimental results and leads to predictions in good agreement with experiments.     

Magnetic properties and characterization of CoCrM/TiCr/Si films produced by pulsed laser deposition

A series of nanogranular CoCrM/TiCr thin films have been fabricated by pulsed-laser deposition on Si(1 1 1) substrates at 450–500 °C. The crystal structure and magnetic properties of these films were investigated. The transmission electron microscope images with selected area diffraction and X-ray diffraction showed that the structure of as-prepared films is dependent on laser energy and deposition temperature. It was found that the microstructure of CoCrM/TiCr films consisted of fine dispersive crystal grains, while the preferential c-axis orientation of films deteriorated when the thickness of CoCr-alloy layer increased along with metal doping into the CoCr films. The magnetic properties of CoCrM/TiCr films formed on Si are strongly dependent on the thickness of magnetic layer, grain structure, and grain shape. Enhancement of coercivity and squareness of the laser-deposited film is probably due to the improvement in the magnetocrystalline anisotropy energy and the reduction in the thickness of magnetic layer.     

Effect of deposition temperature on orientation and electrical properties of (K0.5Na0.5)NbO3 thin films by pulsed laser deposition

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) thin films have been prepared on Pt/TiO₂/SiO₂/Si substrates by pulsed laser deposition process. The structures, crystal orientations and electrical properties of thin films have been investigated as a function of deposition temperature from 680 °C to 760 °C. It is found that the deposition temperature plays an important role in the structures, crystal orientations and electrical properties of thin films. The crystallization of thin films improves with increasing deposition temperature. The thin film deposited at 760 °C exhibits strong (0 0 1) preferential orientation, large dielectric constant of 930 and the remnant polarization of 8.54 μC/cm².     

Influence of target substrate distance on the properties of transparent conductive Si doped ZnO thin films

Si doped zinc oxide (SZO, Si3%) thin films are grown at room temperature on glass substrates under argon atmosphere, using direct current magnetron sputtering. The influence of the target substrate distances on structure, morphology, optical and electrical properties of SZO thin films is investigated. Experimental results show that the target substrate distances have a significance impact on the growth rate, crystal quality and electrical properties of the films, and have little impact on the optical properties of the films. SZO thin film samples grown on glasses are polycrystalline with a hexagonal wurtzite structure and have a preferred orientation along the c-axis perpendicular to the substrate. When the target substrate distance decreases from 76 to 60 mm, the degree of crystallization of the films increased, the grain size increases, and the resistivity of films decreases. However, when the distance continuously decreases from 60 to 44 mm, the degree of crystallization of the films decreased, the grain size decreases, and the resistivity of the films increases. SZO(3%) thin films deposited at a target substrate distance of 60 mm show the lowest resistivity of 5.53 × 10⁻⁴ Ω cm, a high average transmission of 94.47% in the visible range, and maximum band gap of 3.45 eV under 5 Pa of argon at sputtering power of 75 W for sputtering time of 20 min.     

Effects of deposition temperature on the crystallinity of Ga-doped ZnO thin films on glass substrates prepared by sputtering method

The microstructural characterization of Ga-doped (5 at.%) ZnO thin film was conducted by a transmission electron microscopy study. The atomic arrangement of Ga-doped ZnO having an wurtzite structure was identified by the experimental HRTEM and Fourier filtered images as well as the electron diffractions. As a result, we have revealed that the orientation and defect density of Ga-doped ZnO thin films were greatly influenced by the deposition temperature, resulting in the variation of electrical property. In other words, the tendency forming a c-axis oriented texture grows up and the defects such as dislocations and stacking faults decrease, as the temperature of sputtering deposition increases. Consequently, the electrical properties of Ga-doped ZnO thin films can be controlled by the deposition temperature directly related with the defect density.     

Enhanced ferroelectric properties of vanadium doped bismuth titanate (BTV) thin films grown by pulsed laser ablation technique

Bi_3.99Ti_2.97V_0.03O₁₂ (BTV) thin films were grown by pulsed laser deposition at substrate temperatures ranging between 650 and 750 °C. The structural phase, and orientation of the deposited films were investigated in order to understand the effect of the deposition parameters on the properties of the BTV films. As the substrate temperature was increased to 700 °C, the films started showing a tendency of assuming a c-axis preferred orientation, while at lower temperatures polycrystalline films were formed. The Au/BTV/Pt capacitor showed an interesting dependence of the remnant polarization (P_r) as well as dc leakage current values on the growth temperature. The film deposited at 675 °C showed a very large 2P_r of 42 μC cm⁻², which is the largest for BTV thin films among the values reported so far.     

Temperature-dependent growth mode of W-doped ZnO nanostructures

We report on the effects of glass substrate temperature on the crystal structure and morphology of tungsten (W)-doped ZnO nanostructures synthesized by pulsed-laser deposition. X-ray diffraction analysis data shows that the W-doped ZnO thin films exhibit a strongly preferred orientation along a c-axis (0 0 0 L) plane, while scanning electron and atomic force microscopes reveal that well-aligned W-doped ZnO nanorods with unique shape were directly and successfully synthesized at substrate temperature of 550 °C and 600 °C without any underlying catalyst or template. Possible growth mechanism of these nanorods is suggested and discussed.     

Effects of growth temperature on Li-N dual-doped p-type ZnO thin films prepared by pulsed laser deposition

Li-N dual-doped p-type ZnO (ZnO:(Li,N)) thin films have been prepared by pulsed laser deposition. The introduction of Li and N was confirmed by secondary ion mass spectrometry measurements. The structural, electrical, and optical properties as a function of growth temperature were investigated in detail. The lowest room-temperature resistivity of 3.99 Ω cm was achieved at the optimal temperature of 450 °C, with a Hall mobility of 0.17 cm²/V s and hole concentration of 9.12 × 10¹⁸ cm⁻³. The ZnO:(Li,N) films exhibited good crystal quality with a complete c-axis orientation, a high transmittance (about 90%) in the visible region, and a predominant UV emission at room temperature. The two-layer-structure p-ZnO:(Li,N)/n-ZnO homojunctions were fabricated on a sapphire substrate. The current-voltage characteristics exhibited the rectifying behavior of a typical p-n junction.     

Lactic acid aided electrochemical deposition of c-axis preferred orientation of zinc oxide thin films: Structural and morphological features

Compact and homogeneous c-axis preferred orientation of zinc oxide (ZnO) films on indium tin oxide (ITO) coated glass have been prepared electrochemically at −1.2 V vs. Ag|AgCl in a weak acidic condition from 0.06 M Zn(NO₃)₂ with 3 mM lactic acid (LA) added. LA was found having strong influence on the electrodeposition of c-axis preferred orientation of zinc oxide films. Other experimental variables such as deposition temperature, potential, and precursor concentration were also conducted in this article. Among these variables, it was found that precursor concentration of zinc nitrate influenced significantly on growth direction and crystal diameter of zinc oxide. Cyclic voltammetry was used to observe the electrochemistry of the deposition. Crystallinities of the films were examined by X-ray diffractometer. The morphologies of zinc oxide films were observed with a field emitting scanning electron microscope. Optical characteristics of zinc oxide layers were measured with UV-vis spectrophotometer. The band gap of the deposited zinc oxide thin films was evaluated from the Tauc relationship of (αhν)² vs. hν, which was found to be 3.31 eV.     

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.     

Effects of co-substitution on the electrical properties of BiFeO3 thin films prepared by chemical solution deposition

Ferroelectric BiFeO₃ thin films with Nd-Cr (or Sm-Cr) co-substitution (denoted by BNdFCr and BSmFCr, respectively) were deposited on the Pt(2 0 0)/TiO₂/SiO₂/Si(1 0 0) substrates by a chemical solution deposition method. X-ray diffraction patterns revealed the formation of BNdFCr and BSmFCr thin films without any secondary phases. The co-substituted BNdFCr (or BSmFCr) thin films, which were annealed at 550 °C for 30 min in N₂ atmosphere, exhibited enhanced electrical properties compared to BFO thin films with the remanent polarization (2P_r) and coercive electric field (2E_c) of 196, 188 μC/cm² and 600, 570 kV/cm with the electric field of 800 kV/cm, respectively. The leakage current densities of BNdFCr and BSmFCr thin films measured at room temperature were approximately three orders of magnitude lower than that of BFO thin film, and the leakage current at room temperature of the thin films exhibited three distinctive conduction behaviors. Furthermore, the values of pulse polarizations [i.e., +(P*-P^{^}) or −(P*-P^{^})] of BNdFCr and BSmFCr thin films were reasonably unchanged up to 1.4 × 10¹⁰ switching cycles.     

Structural and optical properties of ZnO thin films deposited on quartz glass by pulsed laser deposition

ZnO thin films with typical c-axis (0 0 2) orientation were successfully deposited on quartz glass substrates by pulse laser ablation of Zn target in oxygen atmosphere at a relatively low temperature range of 100-250 °C. The structural and optical properties of the films were studied. In photoluminescence (PL) spectra at room temperature, single ultraviolet emission (without deep-level emission) was obtained from ZnO film deposited at the temperature of 200 °C. This was attributed to its low intrinsic defects.     

Relationship between the photoluminescence and conductivity of undoped ZnO thin films grown with various oxygen pressures

The pulsed laser deposition (PLD) technique is used to deposit undoped ZnO thin films on glass substrates at 150 °C with different oxygen pressures of 40, 80, 100 and 150 mTorr. X-ray diffraction (XRD) and atomic force microscopy (AFM) studies indicated that the obtained ZnO thin films were hexagonal wurtzite-type structures with strong (0 0 2) c-axis orientation. The relationship between photoluminescence and the conductivity of the ZnO thin films grown by pulsed laser deposition at various oxygen pressures was also discussed. The intensity of the deep-level-emission (DLE) and conductivity generally increased as the oxygen pressure decreased. The intensity of DLE peak was generally proportional to the conductivity. The band gap energy values, determined from transmittance spectra, were around 3.30-3.34 eV, and decreased when the oxygen pressure increased.     

Electrical and optical properties of Al-doped ZnO thin films by sol-gel process

Al-doped ZnO (AZO) thin films oriented along the (0 0 2) plane have been prepared by the sol-gel process and their electrical and optical properties with post-deposition heating temperature were investigated. The preferred c-axis orientation along the (0 0 2) plane was enhanced with increasing post-deposition heating temperature and the surface of the films showed a uniform and nano-sized microstructure. The electrical resistivity of the films decreased from 73 to 22 Ω cm as the post-deposition heating temperature increased from 500 to 650 °C; however, the film postheated at 700 °C increased greatly to 580 Ω cm. The optical transmittance of the films postheated below 650 °C was over 86%, but it decreased at 700 °C. The electrical and optical properties of the AZO films with post-deposition heating temperature are discussed.     

Thickness effect in Pb(Zr0.2Ti0.8)O3 ferroelectric thin films grown by pulsed laser deposition

Epitaxial Pb(Zr_,Ti)O₃ (PZT) thin films with thicknesses in the range of 50-200 nm and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD).The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0) SrTiO₃ (STON). SrRuO₃ (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the extended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance-voltage (C-V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT.     

Properties of barium hexa-ferrite thin films dependent on sputtering pressure

M-type barium ferrite thin films were deposited onto sapphire (0 0 l) substrates by radio frequency magnetron sputtering. An ultra-thin layer about 20 nm was deposited and annealed before continuous deposition of the films up to 500 nm under different sputtering pressures: 0.2, 0.5, 0.8 and 1.0 Pa, respectively. It was found that the atomic ratios of Fe to Ba increased from 9.3 to 15.0 with the increase of the pressure. The films sputtered at all pressures have c-axis normal to the film plane by a four circle X-ray diffractometer, which is an improvement of the films directly sputtered on the substrate. Needle-like grains were formed on the surface of the films under higher sputter pressure with bubble domains, which is originated from high magnetocrystalline anisotropy of the film. Magnetic hysteresis loops recorded by vibrating sample magnetometer agree with them, where in-plane and out-of-plane loops of the samples prepared under high sputtering pressures are quite different, while they are almost identical of the samples under low pressures. The influence of the sputtering pressure was understood by that with the increase of the pressure, resputtering of the films was increased. Nucleation with c-axis normal to the film plane was deteriorated. Thus samples prepared under high pressure have more needle-like crystallites which have c-axis parallel to the film plane.     

Influence of deposition temperature on the crystallinity of Al-doped ZnO thin films at glass substrates prepared by RF magnetron sputtering method

Al-doped ZnO (AZO) transparent conducting films were successfully prepared on glass substrates by RF magnetron sputtering method under different substrate temperatures. The microstructural, electrical and optical properties of AZO films were investigated in a wide temperature range from room temperature up to 350 °C by X-ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM), Hall measurement, and UV–visible meter. The nature of AZO films is polycrystalline thin films with hexagonal wurtzite structure and a preferred orientation along c-axis. The crystallinity and surface morphologies of the films are strongly dependent on the growth temperature, which in turn exerts a great effect on microstructural, electrical and optical properties of the AZO films. The atomic arrangement of AZO film having an wurtzite structure was indeed identified by the HRTEM as well as the Selected Area Electron Diffraction (SAED). The defect density of AZO film was investigated by HRTEM. The film deposited at 100 °C exhibited the relatively well crystallinity and the lowest resistivity of 3.6 × 10⁻⁴ Ω cm. The average transmission of AZO films in the visible range is all over 85%. More importantly, the low-resistance and high-transmittance AZO film was also prepared at a low temperature of 100 °C.     

Correlation between structural and mechanical properties of PbTiO3 thin films grown by pulsed-laser deposition

Tetragonal lead titanate (PbTiO₃, PT) thin films are grown on (1 0 0) MgO substrate by pulsed-laser deposition (PLD) for expected applications in integrated optics. The realisation of outstanding and reliable devices into integrated circuits requires sufficient mechanical resistance despite that the obtained PT films display interesting waveguiding properties associated with low optical losses. Two mechanical properties characteristic of elasticity and hardness of PT films are studied. The elastic modulus (E or Young's modulus) and the hardness (H) are measured by the nanoindentation technique. These mechanical properties are correlated to the crystalline quality of PT/MgO thin films. The films show epitaxial relationship with the MgO substrate and the orientation of crystallites perpendicularly to the surface substrate may be the consequence of a growth process along c-axis, a-axis or both. Differences on curves plotting hardness and elastic modulus as a function of indentation depth are observed as the curves are less dispersed for the films mainly c-axis oriented.