Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.     

Zinc-blende MnAs thin films directly grown on InP (001) substrates as possible source of spin-polarized current

We have directly grown zinc-blende (zb)-type MnAs thin films on InP (001) substrates without the aid of any buffer layer using molecular beam epitaxy (MBE). From the High-resolution X-ray diffraction (XRD) data, assuming face-centered cubic (fcc) MnAs structure, the average lattice constants values were calculated to be 6.068 and 6.060 Å for growth temperatures of 250 and 300 °C, respectively. High-resolution transmission electron microscopy (TEM) investigations and selected-area electron-diffraction (SAD) verified the successful growth of zb-type cubic MnAs coexisting with the NiAs-type hexagonal MnAs. The saturation magnetization was estimated to be 300 emu/cm³ determined from the magnetic field dependence of the magnetization curves. From the temperature dependence of magnetization, the Curie temperature was found to be approximately 308 K. Success in the growth of zb-type MnAs thin films could be reasonably explained by the existence of a monolayer of InAs at the interface between the MnAs and InP substrates.     

Effects of substrate orientation on aluminum grown on MgAl2O4 spinel using molecular beam epitaxy

Al thin films have been grown on single-crystal MgAl₂O₄ spinel substrates using solid source molecular beam epitaxy. The structural properties of Al layers were systematically investigated as a function of substrate orientation. X-ray diffraction reveals that Al layers are coherently grown on both (0 0 1)- and (1 1 1)-oriented spinel substrates. However, scanning electron microscopy and atomic force microscopy show that Al layers on (0 0 1) spinel substrates display smoother surface morphology than those grown on (1 1 1) spinel substrates. Additionally, electron backscatter diffraction and transmission electron microscopy demonstrate the presence of a high density of twin domain structures in Al thin films grown on (1 1 1) spinel substrates.     

Uniformity improvement of selectively-grown InGaAs micro-discs on Si

Dislocation-free InGaAs micro-disks, having a diameter of approximately 5 μm and a thickness of approximately 0.2 μm, have been grown on Si(111) substrate patterned with SiO₂, with excellent uniformity in shape. This shape uniformity was dependent on the initial growth of InAs on Si windows. For the growth of well-controlled lateral islands, it is mandatory to cover the openings completely with a single-domain InAs crystal prior to the subsequent growth of InGaAs, the Ga content of which is essential for lateral growth. Moreover, the shape uniformity of InAs islands strongly affected the uniformity of the InGaAs islands' shape. On the basis of these observations, the diameter of Si openings was reduced to 1 μm to make it easier to fill the openings with InAs without failure. A two-step growth of InAs was devised to achieve void-less nucleation and lateral growth at the same time. It was also found that the recess at the boundary between Si and the SiO₂ mask played a vital role to limit the lateral size of InAs islands: when an InAs island hit that boundary, the lateral growth was suppressed and a stable {?110} facet emerged, and the growth seemed to wait for all the {?110} sidewalls to emerge. This mechanism compensated variation in the progress of InAs growth due to scattered timing of InAs nucleation. Therefore, for the purpose of improving the shape uniformity of InGaAs micro-disks, transition from InAs to InGaAs growth should be attempted after all the sidewalls are covered with {?110} facets and before vertical growth starts.     

Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces

Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O₂ system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbO_x films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O₂ fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase.     

Characteristics of heteroepitaxial Cu2−xMnxO/Nb–SrTiO3 p–n junction

Mn-doped cuprous oxide Cu_2?xMn_xO (CMO), where x = 0.03, is a p-type diluted magnetic semiconductor (DMS) with Curie temperature above room temperature [M. Wei, N. Braddon, et al., Appl. Phys. Lett. 86 (2005) 0725141; Y.L. Liu, S. Harrington, et al., Appl. Phys. Lett. 87 (2005) 222108]. We have grown CMO (x = 0.03) thin films of about 200 nm thick on n-type semiconducting (0 0 1)Nb–SrTiO₃(NSTO) single crystal substrates by pulsed laser deposition. Cubic crystalline phases of CMO layers were obtained in a narrow deposition pressure window of about 20 mTorr at growth temperature of 650 °C. X-ray diffraction and TEM studies of these heterostructures reveal a cube-on-cube epitaxial relationship of [CMO]₀₀₁/[NSTO]₀₀₁. All the oxide p–n junctions with the size of 500 × 500 μm were fabricated by the shadow masking technique. These junctions show highly asymmetric I–V characteristics. The rectification ratio at room temperature is about 10³ at ±2 V. Leakage current density of 10^?4 A cm^?2 at ?1 V is observed. No apparent junction breakdown is recorded at reverse bias voltages down to ?5 V. From the 1/C²–V plots, the forward bias turn on voltage is ～1.4 V. Clear junction current rectifying property is maintained at up to 200 °C. Our results have demonstrated that epitaxial CMO films can be fabricated on lattice matched cubic substrates. They are suitable DMS for above room temperature spintronic junction applications.     

Platinum-catalyst-assisted metalorganic vapor phase epitaxy of InN

This paper reports the first attempt of the Pt-catalyst-assisted MOVPE growth of InN. In order to enhance NH₃ decomposition at a relatively low growth temperature (～550 °C), Pt is used as a catalyst. The catalyst is installed in the NH₃ introduction tube in the MOVPE reactor and the tube is located just above the susceptor to be heated. Compared with InN films grown without the catalyst, the samples grown with Pt catalyst show improved electrical properties; a carrier concentration in the order of 10¹⁸ cm^?3 and a Hall mobility as high as 1350 cm²/Vs are obtained. The crystalline quality is also improved by employing the catalyst and a tilt fluctuation as low as 8.6 arcmin is obtained for a sample grown on a GaN/sapphire template. It is confirmed that for InN films grown at 550 °C with Pt catalyst, the electrical and crystallographic properties are also improved with increase in thickness. These results indicate that the growth at around 550 °C with the Pt catalyst is performed under the circumstances where NH₃ is effectively decomposed, whereas the deterioration of InN during growth is significantly suppressed.     

MOCVD and characterization of Hf-silicate thin films using HTB and TEMAS

Hafnium silicate (HfSi_xO_y) films were deposited by metal-organic chemical vapor deposition (MOCVD) using a combination of precursors: hafnium tetra-tert-butoxide [Hf(OC(CH₃)₃)₄, HTB] and tetrakis-ethylmethylamino silane [Si(N(C₂H₅)(CH₃))₄, TEMAS]. The activation energy was independent on the ratio of precursor amounts in the surface reaction regime. The grown films showed Hf-rich characteristics and the impurity concentrations were less than 1 at.% (below detection limits). Hafnium silicate films were amorphous up to 700 °C annealing. Hf/(Hf + Si) composition ratio and dielectric constant (k) of the Hf-silicate films decreased by increasing the growth temperature above 270 °C.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Structural features of epitaxial NiFe2O4 thin films grown on different substrates by direct liquid injection chemical vapor deposition

NiFe₂O₄ (NFO) thin films are grown on four different substrates, i.e., Lead Zinc Niobate–Lead Titanate (PZN–PT), Lead Magnesium Niobate–Lead Titanate (PMN–PT), MgAl₂O₄ (MAO) and SrTiO₃ (STO), by a direct liquid injection chemical vapor deposition technique (DLI-CVD) under optimum growth conditions where relatively high growth rate (～20 nm/min), smooth surface morphology and high saturation magnetization values in the range of 260–290 emu/ cm³ are obtained. The NFO films with correct stoichiometry (Ni:Fe=1:2) grow epitaxially on all four substrates, as confirmed by energy dispersive X-ray spectroscopy, transmission electron microscopy and x-ray diffraction. While the films on PMN–PT and PZN–PT substrates are partially strained, essentially complete strain relaxation occurs for films grown on MAO and STO. The formations of threading dislocations along with dark diffused contrast areas related to antiphase domains having a different cation ordering are observed on all four substrates. These crystal defects are correlated with lattice mismatch between the film and substrate and result in changes in magnetic properties of the films. Atomic resolution HAADF imaging and EDX line profiles show formation of a sharp interface between the film and the substrate with no inter-diffusion of Pb or other elements across the interface. Antiphase domains are observed to originate at the film–substrate interface.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Role of oxygen in structural properties of annealed CuAlO2 films

CuAlO₂ films were sputtered on quartz substrates at different oxygen partial pressures (OPP) and carried out the annealing at 900 °C for 5 h in N₂ ambient. The structural properties of these films have been studied in detail by X-ray diffraction, Raman spectroscopy, and atomic force microscopy. Annealed CuAlO₂ films are grown along the (0 0 1) preferential orientation. The film deposited at 20% OPP demonstrates the excellent crystalline behavior and the smallest electrical resistivity (41.8 Ω cm). At higher OPP, the crystalline behavior begins to degenerate up to the amorphous state at 60% OPP, and some micro-caves presented in the film surface become larger and deeper with the increase in OPP. We believe that the negative thermal expansion behavior associated with excess oxygen atoms is the primary responsibility for the change in structural properties.     

Structure formation in Gd–Fe thin films

The structure, thermal stability and kinetics of phase transformations were explored for films of Gd₂Fe₁₇ compounds. The films were obtained by means of thermal evaporation in vacuum. Amorphous films were found to form at room temperature of substrates, amorphous-crystal condensates at T_s = 300–500 K, and polycrystalline films at T_s > 500 K. The crystal structure of condensates was determined at various temperatures and crystallization of amorphous films was found to be heterogeneous in character. Two phases, Gd₆Fe₂₃ and a-Fe, were observed in polycrystalline films, while three phases were found to exist in the films obtained at substrate temperatures >500 K: a hexagonal Gd₂Fe₁₇ phase of the Th₂Ni₁₇ structural type, a rhombohedral Gd₂Fe₁₇ phase of the Th₂Zn₁₇ structural type and a hexagonal GdFe₅ phase of the CaCu₅ structural type.     

Crystallization of bismuth titanate and bismuth silicate grown as thin films by atomic layer deposition

Bismuth silicate and bismuth titanate thin films were deposited by atomic layer deposition (ALD). A novel approach with pulsing of two Bi-precursors was studied to control the Si/Bi atomic ratio in bismuth silicate thin films. The crystallization of compounds formed in the Bi₂O₃–SiO₂ and Bi₂O₃–TiO₂ systems was investigated. Control of the stoichiometry of Bi–Si–O thin films was studied when deposited on Si(1 0 0) and crystallization was studied for films on sapphire and MgO-, ZrO₂- and YSZ-buffered Si(1 0 0). The Bi–Ti–O thin films were deposited on Si(1 0 0) substrate. Both Bi–Si–O and Bi–Ti–O thin films were amorphous after deposition. Highly a-axis oriented Bi₂SiO₅ thin films were obtained when the Bi–Si–O thin films deposited on MgO-buffered Si(1 0 0) were annealed at 800 °C in nitrogen. The full-width half-maximum values for 200 peak were also studied. An excess of bismuth was found to improve the crystallization of Bi–Ti–O thin films and the best crystallinity was observed with Ti/Bi atomic ratio of 0.28 for films annealed at nitrogen at 1000 °C. Roughness of the thin films as well as the concentration depth distribution were also examined.     

Hydrothermal synthesis of BaTiO3 thin films on nanoporous TiO2 covered Ti substrates

It is well known that there is an upper limit (<0.25 μm) for the thickness of hydrothermal thin films grown on Ti substrate in the 100–200 °C temperature range, even the reaction time is extended to several weeks. In this paper, BaTiO₃ thin films have been firstly hydrothermally synthesized on titanium substrates covered with a nanoporous TiO₂ layer. By using TiO₂ covered substrates, the thickness of BaTiO₃ films can easily reach ∼1.0 μm at 110 °C after only 2 h hydrothermal treatment. It is found that the large quantity of pores with size at the tens of nanometer range in the oxide layer served as easy paths for the diffusion of Ba²⁺ and OH⁻ and enabled the film grow thicker. SEM and XRD results show that the films are crack-free and in polycrystalline phase.     

Kinetics of the crystallization in amorphous NiTi thin films

The crystallization kinetics in Ni_50.54Ti_49.46 film was studied by differential scanning calorimetry through continuous heating and isothermal annealing. The activation energy for crystallization was determined to be 411 and 315 kJ/mol by Kissinger and Augis & Bennett method, respectively. In the isothermal annealing study, The Avrami exponents were in the range of 2.63–3.12 between 793 and 823 K, suggesting that the isothermal annealing was governed by diffusion-controlled three-dimensional growth for Ni_50.54Ti_49.46 thin films.     

Stability and phase changes in thin layers of rare-earth metals/iron and other binary compounds

The structure, thermal stability and phase transformations in thin Gd–Fe films have been studied. The films were obtained by means of thermal evaporation in vacuum. At room temperature of substrates (T_s) amorphous films were formed, at T_s = 300–500 K – amorphous–crystal condensates, and at T_s > 500K – polycrystalline films. The crystal structure of condensates was determined at various temperatures and crystallization of amorphous films was found to have a heterogeneous character. During crystallization of Gd₂Fe₁₇ amorphous films, formation of two phases, viz. Gd₆Fe₂₃ and a-Fe was observed. In the films obtained at substrates temperature > 500K, the presence of three phases was established: a hexagonal phase Gd₂Fe₁₇ of the Th₂Ni₁₇-type structure, a rhombohedral phase Gd₂Fe₁₇ of the Th₂Zn₁₇-type structure and a hexagonal phase GdFe₅ of the CaCu₅-type structure. GdFe₂ films had a face-centered cubic structure, expected in bulk GdFe₂.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Preparation and mechanical properties of a bulk icosahedral quasicrystalline Ti45Zr35Ni17Cu3 alloy

A bulk Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy, which consisted of the icosahedral quasicrystalline phase, was prepared by mechanical alloying(MA) and subsequent pulse discharge sintering. Ti₄₅Zr₃₅Ni₁₇Cu₃ amorphous powders (with particle size <50 μm) were obtained after mechanical alloying for more than 150 h from the mixture of the elemental powder. The transformation temperature range from amorphous phase to the quasicrystalline phase was from 400 K to 900 K. The mechanical properties of the bulk quasicrystalline alloy have been examined at room temperature. The Vickers hardness and compressive fracture strength were 620 ± 40 and 1030 ± 60 MPa, respectively. The bulk quasicrystalline alloy exhibited the elastic deformation by the compressive test. The fracture mode was brittle cleavage fracture.     

Low temperature epitaxial growth of Ge on CaF2 buffered cube-textured Ni

Quasi-single crystal Ge films were grown on cube textured Ni substrate at a temperature of 350 °C using an insulating buffer layer of CaF₂. A direct deposition of Ge on Ni at 350 °C was shown to alloy with Ni. From x-ray pole figure analysis, it was shown that Ge grew epitaxially with the same orientation as CaF₂ and the dispersions in the out-of-plane and in-plane directions were found to be 1.7±0.1° and 6±1°, respectively. In the out-of-plane direction, Ge[111]6CaF₂[111]6Ni[001]. In addition, the Ge consisted of four equivalent in-plane oriented domains such that two mutually orthogonal directions: Ge〈2?11〉 and Ge〈01?1〉 are parallel to mutually orthogonal directions: Ni〈1?10〉 and Ni〈1?1?0〉, respectively, of the Ni(001) surface. This was shown to originate from the four equivalent in-plane oriented domains of CaF₂ created to minimize the mismatch strain between CaF₂ and Ni in those directions.