The film growth process in the bias sputtering of superconducting Nb3Ge

Metastable A15 Nb₃Ge films were sputter deposited onto sapphire substrates under thermalizing conditions. Simultaneously the configuration of the electric potential was varied by the application of a negative bias to the tantalum substrate holder or to both the holder and the growing Nb-Ge film. Films deposited with no bias showed an initial nucleation with disordered crystallites of low super-conducting transition temperature T_c and a subsequent “homoepitaxial” growth process towards a well-ordered state of high T_c. In contrast, bias sputtering led to well-ordered A15 crystallites at an earlier growth stage, which was then succeeded by a non-homoepitaxial growth process. These results will be discussed in the context of ion bombardment of the growing film surface and of the tantalum holder.     

Impact of the Se evaporation rate on the microstructure and texture of Cu(In,Ga)Se2 thin films for solar cells

Coevaporated Cu(In,Ga)Se₂ layers on Mo-coated soda-lime glass substrates were produced by a three-stage process using various Se overpressure conditions during the three stages. Cross-sections of these samples were analyzed by electron backscatter diffraction (EBSD) in a scanning electron microscope in order to reveal the microstructures in the Cu(In,Ga)Se₂ layers. In addition, the preferential orientations of these Cu(In,Ga)Se₂ layers were studied by plan-view EBSD measurements. It was found that Cu(In,Ga)Se₂ exhibits a texture in 110 orientation for Se/(Cu + In + Ga) atomic flux ratios R which are sufficiently large (≥ 4). In one Cu(In,Ga)Se₂ layer produced with approximately R = 4, a large density of (near) Σ3 (twin) boundaries were detected which are oriented preferentially perpendicular to the substrate. By comparison of the local textures of neighboring grains and the theoretically possible changes in orientation by twinning, it is possible to retrace how the twinning occurred.     

Annealing effect on crystallization behavior of cubic MgxZn1−xO films on A-plane and M-plane sapphire

Cubic Mg_xZn_1−xO thin films with Mg composition around 70% were deposited on A-plane and M-plane sapphire substrates by rf-reactive magnetron sputtering. Measured structural and optical properties of these thin films indicated an optimal annealing temperature of 700 °C which produced high quality cubic MgZnO thin films on both substrates. Moreover, when the annealing temperature exceeded 750 °C, a much rougher surface resulted, and several large mosaic particles on the surface of the annealed films appeared. From EDX results, the Mg composition was lower than that found in other sections of the annealed films. We attributed this to thermally induced reconstruction of the crystallites. This phenomenon was more obvious for annealed MgZnO films on A-plane sapphire than that on M-plane sapphire. Thermal expansion mismatch with the substrate is the principal reason.     

Single-crystal heteroepitaxial growth of PbxSn1−xTe films on germanium substrates by r.f. sputtering

Single-crystal heteroepitaxial growth of Pb_xSn_1?xTe films was achieved by r.f. sputtering deposition onto single crystals of germanium (both 〈100〉 and 〈111〉 orientations), although there is a large mismatch (～12%) in the lattice parameters between the grown film and the substrate.The structures of the r.f. sputtered films were investigated by high energy electron diffraction microscopy, electron surface replicas and scanning electron microscopy.Differences in the substrate temperature needed for the epitaxial growth on 〈100〉 or 〈111〉 orientations are attributed to the tendency of Pb_xSn_1?xTe films to grow parallel to the (100) plane, which is the cleavage plane of the NaCl structure. This causes a mixed (100)-oriented phase in addition to the (111) orientation expected from the symmetry of the substrate surface in the case of (111) germanium substrates.Examples of the effects on the heterogeneous nucleation of the substrate temperature and deposition rate are shown for both orientations.The first stages of growth (initial reactions with the substrates) and the thickness and concentration profiles were investigated by ion beam backscattering analysis.Channelling techniques confirmed the high degree of order in the single- crystal structure of films deposited on large areas (～1 cm²).     

Effect of the substrate on the electrodeposition of Bi2Te3−ySey thin films

The potentiostatic electrodeposition of n-type Bi₂Te_3−ySe_y thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi₂Te_3−ySe_y films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi₂Te_3−ySe_y films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H₂SeO₃ in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO₂⁺ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. On stainless steel substrates, HTeO₂⁺ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H₂SeO₃ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates.     

Fabrication of Cu(In,Ga)Se2 thin films by ion beam sputtering deposition from a quaternary target at different substrate temperatures

Cu(In, Ga)Se₂ (CIGS) thin films were fabricated by ion beam sputtering deposition from a single quaternary target at different substrate temperatures (T _sub). The thin films were characterized with X-ray diffractometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy and four-point probe technique to study the microstructures, surface morphology, composition and electrical properties, respectively. The results show that the films grown above 400?°C are of chalcopyrite structure. Cu(In_0.7Ga_0.3)Se₂ thin film was obtained when T _sub is 550?°C. The Cu and Se atomic percentage when T _sub is above 500?°C is higher than when T _sub is below 500?°C. With the increase in T _sub, the surfaces morphology of the films is denser and the resistivity of the films decreases.     

Formation of three-dimensional lead telluride nanoislands on strained barium fluoride (111) surface

The nucleation and growth of three-dimensional lead telluride (PbTe) nanoislands from vapor phase on BaF₂(111) substrates strained by an external load under conditions close to thermodynamic equilibrium have been studied. Evolution of the shape of nanoislands was traced and their size distribution was statistically analyzed as dependent on the character of straining of the substrate. Homogeneous arrays of faceted PbTe nanoislands with an ultrahigh surface number density (above 10¹¹ cm^?2) and an average height of h ₁ ～ 3.5 nm (at a 8–9% rms deviation) are obtained for the first time on elastoplastically strained BaF₂(111) substrates under the conditions of small vapor supersaturation in the condensation zone. The formation of nanoisland arrays with such parameters is possible due to (i) the heterogeneous nucleation of PbTe nanoislands at the surface monatomic slip steps aligned in the 〈110〉 direction and (ii) the influence of straining on the kinetic processes on the substrate surface.     

Radio frequency sputtering of CdSe films

We have studied the influence of various substrate and sputtering parameters on the electrical and structural properties of CdSe films. The most important deposition parameters are the substrate temperature and the plasma power density. The condensation rate is determined by both parameters. The power density not only determines the arrival rate of the condensing species on the substrate but (which is even more important for the film quality) it also controls the substrate bombardment by energetic particles (secondary electrons and ions). This bombardment apparently has a big influence on the condensation rate and the structural ordering of the growing film.The film growth was investigated on two different substrates, namely basal- plane-oriented sapphire and silica glass. At substrate temperatures below 400°C the film properties on both kinds of substrates are approximately equal. These films are polycrystalline with a large degree of preferred (0001) orientation (the c axis perpendicular to the substrate). At substrate temperatures above 400°C the mechanism of film growth on the two different substrates is vastly different. The CdSe films on amorphous silica glass become increasingly amorphous with increasing substrate temperature or plasma power density. On the other hand the films on single-crystal sapphire become epitaxial with increasing substrate temperature. The epitaxial relation between substrate and film is as expected, namely (0001) CdSe_∥ on (0001) Al₂O₃.     

Non-toxic selenization using thermal annealing for CuGa/In bi-layer precursors deposited by sputtering

Cu(In_xGa_1?x)Se₂ (CIGS) thin films were produced using a two-step sputtering process consisting of precursor formation and selenization. In the first stage, we prepared Cu_0.75Ga_0.25/In bi-layer precursors by direct current sputtering on Mo/soda-lime glass substrates. In the second stage, the stacked precursors were selenized using non-toxic Se pellets in a graphite box in which the temperature was controlled at 475–680 °C during rapid thermal annealing. We investigated the effect of thermal annealing temperature on Ga distribution and the crystallinity of the Cu(In_xGa_1?x)Se₂ films. Thermal annealing significantly affected the distribution of Ga atoms. At low temperatures, segregation of Ga atoms into the CIGS/Mo interface and an absence of Ga content on the surface were observed. In addition, a phase-separated CuInSe₂/CuGaSe₂ structure and incomplete selenization phases were observed. At high temperatures, CIGS films were formed with the proper distribution of Ga content.     

One-step RF magnetron sputtering method for preparing Cu(In,Ga)Se<Subscript>2</Subscript> solar cells

Cu(In, Ga)Se₂ (CIGS) solar cell is one of the most promising thin film solar cells. However the marketization of the CIGS solar cells is hindered by the uncertainty of the element ratios. Traditional sputtering with post selenization is one of the most widespread methods to produce the CIGS solar cells. Nevertheless, the post selenization process is the most difficult part of this technique, which could lead to element mismatch and heterogeneous. To simplify the preparing process, Cu(In, Ga)Se₂ (CIGS) solar cells were prepared without post-selenization process by RF sputtering CIGS target with abundant Se element. We focus on the effect of working pressure, substrate temperature and sputtering power on the properties of CIGS solar cells. When CIGS thin film was deposited at 580 °C, 0.8 Pa working pressure and 160 W sputtering power, the solar cell showed the highest power conversion efficiency (PCE) of 5.77%, which is only 0.64% lower than that of the solar cell prepared by traditional sputtering with post selenization method, and the two kinds of solar cells have same structure without MgF₂ antireflection layer, but the one-step sputtering method could greatly simplify the manufacture process of the CIGS solar cells. Our work makes clear that element Se would run off almost half during the sputtering process. And the element atomic ratios and the photovoltaic properties could be controlled by changing the sputtering parameters.     

硅(100)衬底表面快速热退火制备硒纳米晶薄膜的结晶动力学

采用超高真空气相沉积系统在Si(100)衬底上制备非晶硒(Se)薄膜,然后快速热退火制得Se纳米晶薄膜。SEM观察结果表明,当热退火温度高于140℃,薄膜表面形貌从条状裂纹逐渐变成孤立的六角块状结构。Raman和XRD测试分析发现,退火后的Se纳米晶均为三角晶型结构,当退火温度高于140℃时,Se开始沿(100)方向择优取向结晶。分析得出,在Si(100)衬底上的Se晶粒(100)晶面的激活能比(101)晶面的激活能低,因而在(100)面上的结晶速率比(101)面上的结晶速率大,使得Se在(100)方向择优结晶。笔者认为这是因为Si(100)衬底对Se的结晶具有诱导作用,致使硒的结晶具有各向异性。     

On the zinc nitride properties and the unintentional incorporation of oxygen

Zinc nitride films were prepared by radio-frequency magnetron sputtering in N₂/Ar ambient using different substrates (glass and thermally-oxidized-Si) and buffer layers (low-temperature Zn₃N₂ and ZnO). Resonant Rutherford backscattering (RBS) allowed determining Zn_xN_y stoichiometry and thickness. Despite the sputtering system was operated in high vacuum conditions, unintentional oxygen incorporation during growth was detected. Calculations of the relative oxygen concentration showed that the oxygen content was very dependent on the growth rate. Ex-situ oxidation was also analyzed by resonant RBS and compared with the results of as-grown layers. Scanning electron microscopy and X-ray diffraction revealed that surface morphology and crystal orientation were strongly dependent on the substrate temperature (T_s). In addition, optical transmission measurements show a reduction of the optical energy band gap from 1.46 to 1.25 eV as T_s increased. The electrical properties were examined as a function of growth rate, total working gas flux and T_s aiming to maximize electron mobility. From those studies, it was found that Hall mobility increased significantly as the growth rate decreased. A maximum mobility of 100 cm²/Vs and a minimum carrier concentration of 3.2 × 10¹⁸ cm⁻³ were achieved at a T_s of 423 K and a growth rate of 4.44 nm/min.     

Electrical and optical properties of In2Se3 thin films

In₂Se₃ thin films were grown with good stoichiometry at a substrate temperature around 460 K in the α phase and were shown to remain in the β phase above 480 K. The Hall coefficient and the d.c. conductivity of polycrystalline In₂Se₃ films grown on Pyrex and mica substrates were studied in the temperature range 77–530 K. After thermal treatment above 473 K of the α phase thin film, the electrical resistivity decreased and the sample remained in an irreversible phase. This is explained in terms of structural changes at high temperatures. The mobility behaviour of the β phase annealed thin films is illustrated. We use the Petritz barrier model to explain the activation energy of the mobility as due to the grain boundaries of the polycrystallites.The optical properties (refractive index and absorption coefficient) are also reported. The direct band gaps of In₂Se₃ thin films are 1.43 eV and 1.55 eV for the α phase and β phase respectively. These values are obtained from transmission measurements and are confirmed through photoconductivity measurements.     

Physical properties of Bi2 (Te,Se)3 and Bi2Se1.2Te1.8 prepared using solid-state microwave synthesis

Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 bulk products were synthesised using standard solid-state microwave synthesis. The Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 were then deposited thermally onto glass substrates at a pressure of 10^? 6 Torr. The structure of the samples was analysed using X-ray diffraction (XRD), and the powders and thin films were observed to be polycrystalline and rhombohedral in structure. The surface morphology of the samples was determined using scanning electron microscopy (SEM). From the measurements of optical properties, the energy gap values for the Bi₂Te₃, Bi₂Se₃, and Bi₂Se_1.2Te_1.8 thin films were 0.43, 0.73, and 0.65 eV, respectively.     

Synthesis of c-axis oriented AlN thin films on different substrates: A review

Highly c-axis oriented AlN thin films have been deposited by reactive sputtering on different substrates. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on a variety of substrates, have been examined. Aluminum nitride thin films have been deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N₂ gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the rocking curve Full-Width at Half Maximum (FWHM) of the AlN-(0 0 0 2) peak. The relationship between the substrate, the synthesis parameters and the crystallographic orientation of the AlN thin films is discussed. A guide is provided showing how to optimize these conditions to obtain highly c-axis oriented AlN thin films on substrates of different nature.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Flexible Cu(In,Ga)Se2 on Al foils and the effects of Al during chemical bath deposition

Cu(In,Ga)Se₂ (CIGS) solar cells on aluminum foils offer the advantage to be flexible, lightweight and, because of the low cost substrate, can be used for several applications, especially in buildings, where aluminum is already commonly used. There are reports of a-Si solar cells on Al foil, but to our knowledge development of CIGS solar cells on Al foils has not been reported. We have developed CIGS solar cells on coated Al-foil samples. When using Al as substrate, CIGS layers of suitable structural and opto-electronic properties should be grown at low (< 450 °C) deposition temperatures, because of the difference in the thermo-physical properties of layers and substrates. We have grown CIGS layers by evaporation of elemental Cu, In, Ga, and Se at different substrate temperatures and investigated the properties of these CIGS layers by different methods (SEM, SIMS, and EDX). The photovoltaic properties of small area solar cells were characterized with I-V and quantum efficiency measurements. An efficiency of 6.6% has been achieved. We have also observed that some Al from the foil dissolves during chemical bath deposition (CBD) of CdS. The presence of Al in the bath seems, in some cases, to be beneficial for the electrical properties of the CIGS solar cells. Thinner and more homogenous CdS layers are obtained. Elastic Recoil Detection Analysis (ERDA) and SIMS measurements have shown incorporation of Al in the CdS.     

Nucleation of AuAg binary alloys from the vapour phase on KBr single crystals

The nucleation, growth and composition of particulate AuAg alloy deposits on a KBr(100) crystal cleavage surface was investigated by coevaporation of the metals in ultrahigh vacuum. Series of specimens were prepared by increasing the deposition time at constant fluxes of the vapour beams. Analysis of the nucleation rates by transmission electron microscopy revealed that they are mainly determined by the nucleation of gold. Correspondingly, the preferred condensation of gold was found in the early stages of particle growth, as measured by quantitative X-ray fluorescence analysis. During further deposition the composition approached that of the vapour beam, while the substrate coverage increased. The results were interpreted by means of an extended kinetic nucleation model in which it is assumed that the nucleation and growth of alloy particles is mainly governed by the differences in the adatom adsorption energies E_a, in the activation energies E_d for surface diffusion and in the pre-exponential constants of the components. Evaluation of the experimental data resulted in E_{a, Au} ? E_{a, Ag} = 0.03 eV and E_{d, Au} ? E_{d, Ag} = -0.02 eV, in close agreement with data for AuAg on NaCl reported in earlier work. As small variations of these values were found to result in large deviations in the condensation behaviour, particularly in the composition in the nucleation phase, the error limits were only about ±0.02 eV. The ratio of the pre-exponential constants was found to be about unity.     

Effect of deposition angle on the structure and properties of pulsed-DC magnetron sputtered TiAlN thin films

This article reports the comparison of structure and properties of titanium aluminum nitride (TiAlN) films deposited onto Si(100) substrates under normal and oblique angle depositions using pulsed-DC magnetron sputtering. The substrate temperature was set at room temperature, 400 °C and 650 °C, and the bias was kept at 0, − 25, − 50, and − 80 V for both deposition angles. The surface and cross-section of the films were observed by scanning electron microscopy. It was found that as the deposition temperature increases, films deposited under normal incidence exhibit distinct faceted crystallites, whereas oblique angle deposited (OAD) films develop a kind of “tiles of a roof” or “stepwise structure”, with no facetted crystallites. The OAD films showed an inclined columnar structure, with columns tilting in the direction of the incident flux. As the substrate temperature was increased, the tilting of columns nearly approached the substrate normal. Both hardness and Young's modulus decreases when the flux angle was changed from α = 0° to 45° as measured by nanoindentation. This was attributed to the voids formed due to the shadowing effect. The crystallographic properties of these coatings were studied by θ-2θ scan and pole figure X-ray diffraction. Films deposited at α = 0° showed a mixed (111) and (200) out-of-plane orientation with random in-plane alignment. On the other hand, films deposited at α = 45° revealed an inclined texture with (111) orientation moving towards the incident flux direction and the (200) orientation approaching the substrate normal, showing substantial in-plane alignment.     

Residual compressive stress in sputter-deposited TiC films on steel substrates

Polycrystalline TiC films with thickness between 0.1 and 2.8 microm were deposited by r.f. sputtering onto 1010 steel and borosilicate glass substrates at 200°C. All films were found to be in a state of compression. For a film grown under a given set of deposition conditions, the incremental compressive stress, i.e. the average stress in the uppermost deposited layer, was generally found to be largest near the film-substrate interface and to become constant with film thickness t_f for t_f ? 0.3 microm. However, for a given t_f the incremental stress increased with a decrease in the argon sputtering pressure P_Ar. Experimental results showed that the incremental compressive stress in bulk films could be directly related to the trapped argon concentration. Argon incorporation is due to the burial of energetic species incident on the growing film surface from two primary sources: energetic neutrals produced by Ar⁺ ions scattered off the target in binary collisions and Ar⁺ ions accelerated to the substrate owing to its induced negative potential with respect to the positive space charge region in the r.f. discharge. The trapped argon concentration from both contributions increased with decreasing P_Ar. All films grown on steel substrates exhibited good adhesion as indicated by indentation and diamond stylus scratch tests. The residual compressive stress in the films was found to be beneficial for wear-related applications in which the film was subjected to a large tensile stress.