Growth of cubic InN films with high phase purity by pulsed laser deposition

Cubic InN films have been grown on MgO (1 0 0) substrates with cubic GaN buffer layers by pulsed laser deposition (PLD). It has been found that cubic InN (1 0 0) films grow on the GaN (1 0 0)/MgO (1 0 0) structure with an in-plane epitaxial relationship of [0 0 1]_InN∥[0 0 1]_GaN∥[0 0 1]MgO. The phase purity of a cubic InN film grown at 440 °C was as high as 99% that can probably be attributed to the enhanced surface migration of film precursors in case of PLD. These results indicate that PLD is a suitable technique for the growth of high-quality cubic InN films, and will makes it possible to fabricate optical and electron devices based on cubic InN films.     

Texture and surface analysis of thin-film GaAs on glass formed by pulsed-laser deposition

Thin-film GaAs on glass was formed by ablating n-type GaAs with nano-second pulses at 532 nm. The deposition was done in the most straightforward way without heating the substrate. The texture of films has been investigated with X-ray measurements, spatially resolved micro-Raman spectroscopy, and atomic force microscopy. The results reveal that the film texture is of multi-phase nature consisting of randomly oriented GaAs microcrystallites, amorphous parts, and (1 1 1) zincblende migrations in the nano-regime.     

Epitaxial growth of InN films on lattice-matched EuN buffer layers

Indium nitride (InN) films have been grown on lattice matched europium nitride (EuN) buffer layers by pulsed laser deposition (PLD) and their structural properties have been investigated. It has been revealed that the growth of EuN films on Al₂O₃ (0 0 0 1) substrates leads to the formation of polycrystalline EuN films, whereas epitaxial EuN (1 1 1) films grow on MgO (1 1 1) substrates at 860 °C. By using the EuN (1 1 1) films as buffer layers for InN growth, we have succeeded in the epitaxial growth of InN (0 0 0 1) films at 490 °C with an in-plane epitaxial relationship of [1 1 2¯ 0]_InN∥[1 1¯ 0]_EuN∥[1 1¯ 0]_MgO, which minimized the lattice mismatch between InN and EuN.     

Study of microstructural evolutions in phosphorus-doped ZnO films grown by pulsed laser deposition

The microstructure of P-doped ZnO films grown on the c-plane sapphire substrate by pulsed laser deposition (PLD) was investigated. ZnO films were highly textured along c-axis with two different in-plane orientations. The textured domain was surrounded by the threading dislocations, resulting in the formation of low-angle grain boundary. It was found that the degree of texture and crystalline quality of P-doped ZnO films decreased with increasing the phosphorus atomic percent. For the microstrain study, X-ray diffraction line profile analysis (LPA) was performed. The 0.5 at% P-doped ZnO film showed much higher microstrain than the 1.0 at% P-doped ZnO film as well as as-grown film, which indicated that the phosphorus in former film was effectively incorporated into ZnO film. X-ray photoelectron spectroscopy (XPS) results showed that the phosphorus in 1.0 at% P-doped ZnO film tended towards segregation, which was well consistent with XRD results.     

The effect of oxygen pressure on the microstructures of Co-doped rutile TiO2 thin films grown by pulsed laser deposition

Comprehensive microstructures of 7% cobalt-doped rutile TiO₂ thin films grown on c-plane sapphire by pulsed laser deposition were characterized using transmission electron microscopy (TEM). The effects of oxygen pressure during growth on the Co distribution inside the films were investigated, and the detailed growth mechanism of both TiO₂ and TiO₂+Co was discussed. The similar oxygen sublattices and low mismatch between (1 0 0) rutile and c-plane sapphire favors the rutile phase. However, the three-fold symmetry of the substrate surface resulted in three rutile domain orientation variants, and they grow adjacent to each other. Cobalt was found to precipitate out as nanocrystals inside the TiO₂ matrix as the growth pressure of oxygen was decreased. At 0.05 mTorr oxygen pressure, almost all of the Co segregates into crystallographically aligned nanocrystals with a particle size of 4.4±0.15 nm. All the samples have magnetic coercivity at room temperature. The magnetic moment per Co atom increased with decreased oxygen pressure, suggesting that the Co that replaced the Ti²⁺ in the TiO₂ lattice does not have a large magnetic moment.     

Synchrotron X-ray diffraction studies of heteroepitaxial ZnO films grown by pulsed laser deposition

Heteroepitaxial ZnO films were grown by pulsed laser deposition on various substrates such as GaN-buffered C-Al₂O₃, C-Al₂O₃, A-Al₂O₃, and R-Al₂O₃. The epitaxy nature of the films was investigated mainly by synchrotron X-ray diffraction. The results showed that the GaN interlayer plays a positive role in growing an unstrained, well-aligned epitaxial ZnO film on the basal plane of Al₂O₃. Importantly, the ZnO film grown on R-Al₂O₃ has two differently aligned domains. The dominant (1 1 0) oriented domain has much better alignment in the in-plane direction than the minor portion of (0 0 1) oriented domain, while in the out-of-plane direction the two domains have almost the same mosaic distribution.     

Growth and optical absorption spectra of ZnO films grown by pulsed laser deposition

ZnO films on Al₂O₃ substrate were grown by using a pulsed laser deposition method. Through photoluminescence (PL) and X-ray diffraction (XRD) measurements, the optimum growth conditions for the ZnO growth were calculated. The results of the XRD measurement indicate that ZnO film was strongly oriented to the c-axis of hexagonal structure and epitaxially crystallized under constraints created by the substrate. The full-width half-maximum for a theta curve of the (0 0 0 2) peak was 0.201°. Also, from the PL measurement, the grown ZnO film was observed to be a free exciton, which indicates a high quality of epilayer. The Hall mobility and carrier density of the ZnO film at 293 K were estimated to be 299 cm²/V sec and , respectively. The absorption spectra revealed that the temperature dependence of the optical band gap on the ZnO films was − .     

Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

Growth control of stoichiometry in LaMnO3 epitaxial thin films by pulsed laser deposition

We have studied structural, magnetic, and optical transport properties of LaMnO₃ (LMO) thin films grown on SrTiO₃. While the stoichiometric LMO is an insulating antiferromagnet, it tends to be a ferromagnetic insulator when grown as thin films. By exploring the majority of growth parameters, we have found that the bulk-like electronic and magnetic phases can be stabilized by growing thin films under reducing atmospheres and by using more energetic laser processes. These conditions are found to reduce the La deficiency in the film resulting in the greatly improved cation stoichiometry. Since oxides are prone to reduce the oxygen content and to alter the cation ratio under such growth conditions, it suggests that the cation and oxygen stoichiometries in complex oxide thin films can be improved by properly optimizing the growth parameters.     

Formation of ultrasmooth thin silver films by pulsed laser deposition

Ultrasmooth thin silver films have been formed on a quartz substrate with a buffer yttrium oxide layer by pulsed laser deposition. The dependence of the surface morphology of the film on the gas (N₂) pressure in the working chamber and laser pulse energy is investigated. It is found that the conditions of film growth are optimal at a gas pressure of 10^?2 Torr and lowest pulse energy. The silver films formed under these conditions on a quartz substrate with an initial surface roughness of 0.3 nm had a surface roughness of 0.36 nm. These films can be used as a basis for various optoelectronics and nanoplasmonics elements.     

Dielectric properties of Zr-Sn-Ti-O thin films prepared by pulsed laser deposition

Zr_0.26Sn_0.23Ti_0.51O₂ (ZSTO) films with a dielectric constant of about 40 have been prepared directly on silicon substrates by pulsed laser deposition at 600 °C. TEM observation showed that the as-deposited films are amorphous. Differential thermal analysis showed that the ZSTO films crystallize at about 620 °C. Capacitance-voltage (C-V) characteristics of metal-oxide-semiconductor (MOS) composed of Pt/ZSTO/Si prepared at different deposition temperature have been measured. The EOT of the MOS structures with the same ZSTO physical thickness increased slightly when the deposition temperature increased. The EOT is about 4.2 nm for the 40 nm ZSTO deposited at 600 °C. The leakage current characteristics of ZSTO films for the as deposited, post-annealed in oxygen ambient and post-annealed in nitrogen ambient by rapid thermal annealing have been studied comparatively. The films post-annealed in nitrogen ambient have the lowest leakage current and the as-deposited films have the largest leakage current characteristics. It is proposed that amorphous Zr-Sn-Ti oxide stabilized at 600 °C is a potential dielectric material for dynamic random access memory and high k dielectric gate applications.     

Surface biofunctionalization of nanostructured GeSbSe chalcogenide glass thin films

Thin nanostructured chalcogenide films were grown using the oblique angle deposition (OAD) technique and subsequently polymerized with thin poly(amino-p-xylylene) (PPX) films. Our objective was twofold, i.e., to use deposited polymeric thin films to allow the attachment of biomolecules to chalcogenide glass thin films, and at the same time, to increase surface area by OAD to enhance surface functionality. The effectiveness of this approach was evaluated by Fourier transform infrared spectroscopy (FTIR), together with a combination of fluorescent protein immobilization and confocal microscopy characterization. It is shown that the presence of amine groups on the surface of the polymer coated chalcogenide thin films yield a notable increment of surface coverage with proteins at large evaporation oblique angles which is expected to enhance detection performance of the film in biosensor applications.     

Fluorinated silica glass ablated with ArF excimer laser at low fluence

Amorphous SiO₂ (a-SiO₂) was formed by liquid-phase deposition (LPD) at room temperature. As a result of one shot of ArF excimer laser irradiation, LPD-formed a-SiO₂ shows a threshold fluence for ablation of below than 200 mJ/cm², which is much lower than the threshold fluence (∼1 J/cm²) of a-SiO₂ formed by thermal oxidation of silicon. Raman scattering spectroscopy revealed that two sharp lines at 495 cm⁻¹ and 606 cm⁻¹, respectively, labeled D₁ and D₂, had disappeared, and the main band at 430 cm⁻¹ was sharpened in LPD-formed a-SiO₂. It is presumed that the fluorine broke the silica network, relaxing the Si-O-Si bond angle and dramatically reducing the threshold energy for ablation of a-SiO₂.     

Layer-by-layer growth and growth-mode transition of SrRuO3 thin films on atomically flat single-terminated SrTiO3 (1 1 1) surfaces

We report on growth-mode transitions in the growth of SrRuO₃ thin films on atomically flat Ti⁴⁺ single-terminated SrTiO₃ (1 1 1) substrates, investigated by reflection high-energy electron diffraction and atomic force microscopy. Over the first ~9 unit cells, the dominant growth mode changes from island to layer-by-layer for the growth rate of 0.074 unit cells/s and the growth temperature of 700 °C. Moreover, in the course of growing SrRuO₃ films, the governing growth mode of interest can be manipulated by changing the growth temperature and the growth rate, which change allows for the selection of the desired layer-by-layer mode. The present study thus paves the way for integrations of SrRuO₃ thin layers into (1 1 1)-orientated oxide heterostructures, and hence multi-functional devices, requiring control of the sharp atomic-level interfaces and the layer-by-layer growth mode.     

Characterization of GaInN/GaN layers for green emitting laser diodes

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c-axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm² of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm² we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m-axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.     

Epitaxial films of metals from the platinum group (Ir,Rh, Pt and Ru) on YSZ-buffered Si(1 1 1)

In the present work we have grown twin-free single crystal metal films of iridium (Ir), rhodium (Rh), platinum (Pt) and ruthenium (Ru) on silicon (1 1 1) substrates via an yttria-stabilized zirconia (YSZ) buffer layer. A prerequisite for the realisation of heteroepitaxial metal films without additional texture components was the twin-free deposition of the YSZ films by pulsed laser deposition (PLD). For the metal films on top, a novel two-step growth process was applied with an extremely low deposition rate for the first 20 nm. For all metals, a drastic texture improvement by up to a factor of 9 could be observed compared to the oxide buffer layer. Minimum values were 0.18° (Ir) and 0.12° (Rh) for tilt and twist, respectively. For all four metals investigated, twin-free epitaxial films could be grown on YSZ/Si(1 1 1) whereas the twinning problem for platinum films was solved by decoupling the Pt-YSZ interface via an additional iridium interlayer. The grown metal/YSZ/Si(1 1 1) multilayer samples offer the possibility to integrate a variety of interesting nanostructures and functional materials on silicon. They are now available in 4 in wafer size.     

Low-temperature deposition of crystalline silicon nitride nanoparticles by hot-wire chemical vapor deposition

The nanocrystalline alpha silicon nitride (α-Si₃N₄) was deposited on a silicon substrate by hot-wire chemical vapor deposition at the substrate temperature of 700 °C under 4 and 40 Torr at the wire temperatures of 1430 and 1730 °C, with a gas mixture of SiH₄ and NH₃. The size and density of crystalline nanoparticles on the substrate increased with increasing wire temperature. With increasing reactor pressure, the crystallinity of α-Si₃N₄ nanoparticles increased, but the deposition rate decreased.