The effect of SixNy interlayer on the quality of GaN epitaxial layers grown on Si(1 1 1) substrates by MOCVD

In the present paper, the effects of nitridation on the quality of GaN epitaxial films grown on Si(1 1 1) substrates by metal–organic chemical vapor phase deposition (MOCVD) are discussed. A series of GaN layers were grown on Si(1 1 1) under various conditions and characterized by Nomarski microscopy (NM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and room temperature (RT) photoluminescence (PL) measurements. Firstly, we optimized LT-AlN/HT-AlN/Si(1 1 1) templates and graded AlGaN intermediate layers thicknesses. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.2 μm. Secondly, the effect of in situ substrate nitridation and the insertion of an Si_xN_y intermediate layer on the GaN crystalline quality was investigated. Our measurements show that the nitridation position greatly influences the surface morphology and PL and XRD spectra of GaN grown atop the Si_xN_y layer. The X-ray diffraction and PL measurements results confirmed that the single-crystalline wurtzite GaN was successfully grown in samples A (without Si_xN_y layer) and B (with Si_xN_y layer on Si(1 1 1)). The resulting GaN film surfaces were flat, mirror-like, and crack-free. The full-width-at-half maximum (FWHM) of the X-ray rocking curve for (0 0 0 2) diffraction from the GaN epilayer of the sample B in ω-scan was 492 arcsec. The PL spectrum at room temperature showed that the GaN epilayer had a light emission at a wavelength of 365 nm with a FWHM of 6.6 nm (33.2 meV). In sample B, the insertion of a Si_xN_y intermediate layer significantly improved the optical and structural properties. In sample C (with Si_xN_y layer on Al_0.11Ga_0.89N interlayer). The in situ depositing of the, however, we did not obtain any improvements in the optical or structural properties.     

Thermal stability of the Co/β-Si3N4/Si(111) interface: A photoemission study

The thermal stability of the Co/β-Si₃N₄/Si(111) interface has been studied by high-resolution photoemission spectroscopy in a temperature range extending from room temperature to 650 °C. It is demonstrated the ability of a very thin crystalline buffer layer of silicon nitride to prevent the interfacial reaction between cobalt and silicon at room temperature. The behaviour of the interface at higher temperature shows the formation of cobalt silicides already at 300 °C. Moreover, the presence of new components in the decomposition of the photoemission spectra is discussed in the light of the existing literature.     

Structural,electronic and energetic properties of water adsorbed on β-Si3N4 (0 0 0 1) surface: First-principles calculations

Structural, energetic and electronic properties of water molecules adsorbed on β-Si₃N₄ (0 0 0 1) surface, at various coverages, are investigated using density functional theory. At low coverages (θ ? 0.5), it is found that all H₂O molecules undergo spontaneous dissociation forming hydroxyl (OH) and imino (NH) groups where the reactive sites are identified, a result shown for the first time using ab initio theory. For higher coverages (θ > 0.5), only partial dissociation takes place where some of the molecules stay intact being bound via H-bond in good agreement with experimental findings. The driving force for the water dissociation has been identified to be dangling bonds on lower coordinated N and Si surface atoms showing that not all surface atoms are reactive corroborating with previous experimental findings.     

Effect of surface orientation and thickness on the magnetization of anisotropic FCC ferromagnetic films

The dependence on temperature of the layer magnetization of a Heisenberg ferromagnetic ultrathin film in presence of magnetocrystalline single-ion anisotropy was theoretically investigated in the framework of a Green's function approach using the random phase approximation (RPA). The effect of surface orientation and of film thickness N on the Curie temperature T_C was carefully investigated in the case of face centered cubic (FCC) films: the steepest increase of T_C(N) was found in the case of the FCC(1 1 1) orientation and the smoothest in the FCC(1 1 0) one. Our results for T_C(N) were successfully fitted by a finite-size scaling relation [T_C(∞)−T_C(N)]/T_C(N)=(N/N₀)^−λ, giving a shift exponent λ≃1.5, irrespectively of the surface orientation. Finally, the temperature evolution of the magnetization profile was analyzed, as well as its limiting shape at T_C.     

Red organic light-emitting diodes with high efficiency,low driving voltage and saturated red color realized via two step energy transfer based on ADN and Alq3 co-host system

We demonstrated efficient red organic light-emitting diodes based on a wide band gap material 9,10-bis(2-naphthyl)anthracene (ADN) doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10(2-benzothiazolyl)quinolizine-[9,9a,1gh]coumarin (C545T) as an assistant dopant. The typical device structure was glass substrate/ITO/4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA)/N,N′-bis(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/[ADN:Alq₃]:DCJTB:C545T/Alq₃/LiF/Al. It was found that C545T dopant did not by itself emit but did assist the energy transfer from the host (ADN) to the red emitting dopant via cascade energy transfer mechanism. The OLEDs realized by this approach significantly improved the EL efficiency. We achieved a significant improvement regarding saturated red color when a polar co-host emitter (Alq₃) was incorporated in the matrix of [ADN:Alq₃]. Since ADN possesses a considerable high electron mobility of 3.1 × 10⁻⁴ cm²  V⁻¹ s⁻¹, co-host devices with high concentration of ADN (>70%) exhibited low driving voltage and high current efficiency as compared to the devices without ADN. We obtained a device with a current efficiency of 3.6 cd/A, Commission International d’Eclairage coordinates of [0.618, 0.373] and peak λ_max = 620 nm at a current density of 20 mA/cm². This is a promising way of utilizing wide band gap material as the host to make red OLEDs, which will be useful in improving the electroluminescent performance of devices and simplifying the process of fabricating full color OLEDs.     

Ultrathin Y2O3(111) films on Pt(111) substrates

The growth of ultrathin films of Y₂O₃(111) on Pt(111) has been studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED). The films were grown by physical vapor deposition of yttrium in a 10^? 6 Torr oxygen atmosphere. Continuous Y₂O₃(111) films were obtained by post-growth annealing at 700 °C. LEED and STM indicate an ordered film with a bulk-truncated Y₂O₃(111)–1 × 1 structure exposed. Furthermore, despite the lattices of the substrate and the oxide film being incommensurate, the two lattices exhibit a strict in-plane orientation relationship with the [11?0] directions of the two cubic lattices aligning parallel to each other. XPS measurements suggest hydroxyls to be easily formed at the Y₂O₃ surface at room temperature even under ultra high vacuum conditions. The hydrogen desorbs from the yttria surface above ~ 200 °C.     

Effective demagnetizing factors in ferromagnetic resonance equations

A simple method of deriving effective demagnetizing factors (N_x^e,N_y^e) for the use of Kittel's ferromagnetic resonance formula is reported. The effective demagnetizing factors are expressed by an anisotropy energy (G) and a static field direction (θ,φ). By this derivation method, the resonance equations of thin films having a uniaxial or a four-fold anisotropy are obtained when a static field is rotated in the film plane. Six arrangements are calculated: (1) perpendicular anisotropy, (2) in-plane anisotropy, (3) cubic-crystal (0 0 1) face, (4) cubic-crystal (0 1 1) face, (5) cubic-crystal (1 1 1) face, and (6) oblique anisotropy films.     

Energy storage properties of antiferroelectric 0.92NaNbO3-0.08SrZrO3 film on (001)SrTiO3 substrate

The present study demonstrates the fabrication of an antiferroelectric 0.92NaNbO₃-0.08SrZrO₃ film deposited on a SrRuO₃ coated (001)SrTiO₃ single crystal substrate by pulsed laser deposition. In the 0.92NaNbO₃-0.08SrZrO₃ film, the domain with its c-axis aligned with the out-of-plane direction contributed to the stabilization of an antiferroelectric phase under the high electric field. The film had an energy storage density of 2.9 J cm⁻³ and storage efficiency of 67% at room temperature, which kept at 2.5 J cm⁻³ and 50% at high temperature of 150 °C.     

Growth,structure, and thermal stability of epitaxial BaTiO3 films on Pt(111)

The growth of epitaxial ultrathin BaTiO₃ films upon rf magnetron sputter deposition on a Pt(111) substrate has been studied by scanning tunnelling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. The BaTiO₃ films have been characterized from the initial stages of growth up to a film thickness of 4 unit cells. The deposited films develop a long-range order upon annealing at 1050 K in UHV. In the submonolayer regime a wetting layer is formed on Pt(111). Thicker films reveal a Stranski–Krastanov-like structure as observed with STM. By XPS a good agreement of the thin film stoichiometry with BaTiO₃ single crystal data is determined. Due to annealing at 1150 K BaTiO₃ forms large two-dimensional islands on the Pt(111) substrate. Different surface structures develop on the islands depending on the O₂ partial pressure during annealing.     

Electronic charge transfer between Au nano-particles and TiO2-terminated SrTiO3(0 0 1) substrate

The interaction between Au nano-particles and oxide supports is recently discussed in terms of the catalytic activities. This paper reports the electronic charge transfer between Au nano-particles and TiO₂-terminated SrTiO₃(0 0 1) substrate, which is compared with that for stoichiometric(S)-, pseudo-stoichiometric(S¹)- and reduced(R)-TiO₂(1 1 0) supports. We observed the photoelectron spectra of Au 4f, O 2s, Ti 3p, and Sr 4p lines and also measured the work functions for Au/oxides supports using synchrotron-radiation light. As the results, all the O 2s, Ti 3p, and Sr 4p lines for Au/SrTiO₃(0 0 1) show lower binding energy shifts in a quite same manner and abrupt increase in the work function is seen in an initial stage. This clearly evidences an electronic charge transfer from the substrate to Au probably due to a much larger work function of Au than SrTiO₃(0 0 1), which leads to an upward band bending (0.3 eV) just like a Schottky contact. Electronic charge transfers also take place at Au/S- and Au/S¹-TiO₂(1 1 0) and Au/R-TiO₂(1 1 0) interfaces, where electrons are transferred from Au to S- and S¹-TiO₂ and from R-TiO₂ to Au, as predicted by ab initio calculations.     

Kinetic Monte Carlo simulation of the growth of metal clusters on regular array of defects on insulator

A Kinetic Monte Carlo simulation of the nucleation and growth of Pd clusters on a nanostructured alumina substrate is presented. The new Monte Carlo simulation program allows to derive the 3D shape of the growing clusters without performing a full all atoms simulation. The simulation shows, like in previous pure 2D simulations, that clusters nucleate exclusively on the defects of the nanostructure in a limited range of substrate temperature. Around 300 K, the clusters have a compact faceted shape and they grow, at not too large coverage, layer by layer. These results are in agreement with previous studies of the nucleation and growth of Pd clusters on an ultrathin alumina film on Ni₃Al (1 1 1).     

Characterization of TiN thin films grown by low-frequency (60 Hz) plasma enhanced chemical vapor deposition

The characteristics of TiN thin films grown on glass substrates by very low frequency (60 Hz) PECVD were investigated along with the reactive plasma generated using a 60 Hz power source. The TiN film depositions were performed using a gaseous mixture of H₂, N₂ and TiCl₄ onto a substrate positioned between two electrodes using a floating substrate holder with a heating unit. The substrate is electrically floated to avoid sample damages due to ion bombardment. As-grown TiN films showed a NaCl-type fcc structure with a (200) crystallographic plane, low resistivity (～60 μΩ cm) and gold-like color. Crystallinity was improved, impurities such as O and Cl were reduced, and the atomic ratio of N/Ti became stoichiometric with the increase of substrate temperature. Particularly, no chlorine component was detected above 500 °C. Also, the N₂ partial pressure strongly affected the deposition rate and ratio of N/Ti. Otherwise, impurities and crystallinity barely changed with the change of N₂ pressure. The atomic ratio of N/Ti, impurities, and crystallinity of the films significantly affected the optical and electrical properties. Consequently, we produced stoichiometric Cl-free TiN films with golden color above 500 °C at 60 mTorr. The effects of temperature played an important role in controlling the film properties compared to the N₂ partial pressure.     

Electronic properties of the polycrystalline tin dioxide interface with conjugated organic layers

The results on the electronic structure of the unoccupied electronic states of the polycrystalline SnO₂ in the energy range from 5 eV to 25 eV above the Fermi level are presented. The modification of the electronic structure and of the surface potential upon deposition of the ultrathin films of copper phthalocyanine (CuPc) and of perylene tetracarboxylic acid dianhydride (PTCDA) film onto the SnO₂ surface were studied using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. A substantial attenuation of the TCS signal coming from the SnO₂ surface was observed upon formation of a 1.5–2 nm thick organic deposit layer while no new spectral features from the deposit were distinguishable. It was observed that the electronic structure typical for the organic films was formed within the organic deposit thickness range from 2 nm to 7 nm. The interfacial charge transfer was characterized by the formation of the polarization layer up to 5 nm thick in the organic films. The PTCDA deposition on SnO₂ was accompanied by the negative charge transfer onto the organic layer and to the 0.65 eV increase the surface work function. At the CuPc/SnO₂ interface, the negative charge was transferred to the SnO₂ surface and the overall surface work function decreased by 0.15 eV.     

Influence of the antireflection,window, and active layers on optical properties of exponential-doping transmission-mode GaAs photocathode modules

In order to well study the influence of the thickness and doping concentration on optical properties of transmission-mode GaAs photocathode, three exponential-doping and one uniform-doping photocathode modules were prepared by molecular beam epitaxy with a structure of glass/Si₃N₄/Ga_1 ? xAl_xAs/GaAs. By use of the spectrophotometer, the optical properties were separately measured including the reflectivity and transmissivity curves. Based on thin film optical principles, the optical properties and their integral values are calculated by matrix formula for the four-layer photocathode module. The result shows that the antireflection and window layers affect the peak and valley of the optical property curves and the active layer influences the absorptivity values of the transmission-mode cathode modules. The photocathode module has high absorptivity within the response waveband when the optimal module has the Si₃N₄ antireflection layer of 0.1 μm, the Ga_1 ? xAl_xAs window layer of more than 0.4 μm, and the GaAs active layer of 1.5 μm–2 μm and low average doping concentration.     

Reduction of thin-film ceria on Pt(111) by supported Pd nanoparticles probed with resonant photoemission

Local defects present in CeO_2 ? x films result in a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. Previous studies of the Ce 3d region with XPS have shown that depositing metal nanoparticles on ceria films causes further reduction, with an increase in Ce³⁺ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce³⁺ and Ce⁴⁺ in CeO_2 ? x films grown on Pt (111), and the variation of this concentration as a function of Pd deposition. Due to the nature of the electronic structure of CeO_2 ? x, resonant peaks are observed for the 4d–4f transitions when the photon energy matches the resonant energy; (hν = 121.0 eV) for Ce³⁺ and (hν = 124.5 eV) for Ce⁴⁺. This results in two discrete resonant photoemission peaks in valence band spectra. The ratio of the difference of these peaks with off-resonance scans gives an indication of the relative contribution of Ce³⁺. Results from RESPES indicate reduction of CeO_2 ? x on deposition of Pd, confirming earlier findings from XPS studies.     

Electronic energy alignment at the PbSe quantum dots/ZnO(101̅0) interface

A number of solar energy conversion strategies depend on exciton dissociation across interfaces between semiconductor quantum dots (QDs) and other electron or hole conducting materials. A critical factor governing exciton dissociation and charge transfer in these systems is the alignment of electronic energy levels across the interface. We probe interfacial electronic energy alignment in a model system, sub-monolayer films of PbSe QDs adsorbed on single crystal ZnO(101?0) surfaces using ultraviolet photoemission spectroscopy. We establish electronic energy alignment as a function of quantum dot size and surface chemistry. We find that replacing insulating oleic-acid capping molecules on the QDs by the short hydrazine or ethanedithiol molecules results in pinning of the valence band maximum (VBM) of QDs to ZnO substrate states, independent of QD size. This is in contrast to similar measurements on TiO₂(110) where the alignment of the PbSe QD VBM to that of the TiO₂ substrate depends on QD size. We interpret these findings as indicative of strong electronic coupling of QDs with the ZnO surface but less with the TiO₂ surface. Based on the measured energy alignment, we predict that electron injection from the 1s_e level in photo-excited PbSe QDs to ZnO can occur with small QDs (diameter ? = 3.4 nm), but energetically unfavorably for larger dots (? = 6.7 nm). In the latter, hot electrons above the 1s_e level are necessary for interfacial electron injection.     

Surface electromigration of In on vicinal Si(0 0 1)

Surface mass transport of In film on vicinal Si(0 0 1) has been systematically investigated by a scanning Auger electron microscopy (SAM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). It was observed that the temperature dependence of the mass transport shows the critical phenomenon. Above a critical temperature T_c, surface electromigration of the In film toward the cathode side dominated the surface mass transport on the vicinal Si(0 0 1) surface. The LEED and AFM observations revealed that the In film surface on the vicinal Si(0 0 1) consists of 3×4 terraces and (3 1 0) facets. The area ratio of the facet to the terrace exhibited abrupt an increase at T_c. It is believed that the change of the mass transport is related to the abrupt change of the area ratio of the facet to the terrace. Both the critical temperature T_c and the spread due to the surface electromigration of the In film depended on the configuration of the DC current direction and the step edge.     

Effects of growth rate and buffer-layer on Bi2Sr2CaCu2Oy thin film fabrication

The effect of the growth rate on the Bi₂Sr₂CaCu₂O_y (Bi2212) thin film quality on MgO substrate is investigated at several growth rates from 0.175 to 3 nm/min. The maximal step height on the film surface is improved from about 100 to 6 nm by the reduction of growth rate to 0.5 nm/min and simultaneously the superconducting critical temperature attaining to a zero resistance T_c(R=0), is also improved from 50 to 63 K. The surface morphologies of the upmost Bi-superconducting thin films with the intermediate layers on MgO substrate is also studied in contrast to that deposited directly on the MgO substrate.     

Nitrogen physisorption and site blocking on single-walled carbon nanotubes

N₂ physisorption on Single-Walled Carbon Nanotubes (SWCNTs) was investigated by cryogenic thermal-desorption spectroscopy (cryo-TDS). TDS spectra revealed a desorption peak at 48 K (α) for as-purified SWCNTs and an additional desorption peak at 73 K (β) for air-oxidized SWCNTs. When N₂ and H₂ were coadsorbed on SWCNTs, H₂ adsorption was blocked by the N₂ preadsorption. By comparing the TDS data with and without the N₂ preadsorption, the α and β peaks were attributed to N₂ adsorbed on the groove site and inside of SWCNTs, respectively.     

Patterning of Aluminium thin film on polyethylene terephthalate by multi-beam picosecond laser

High speed patterning of a 30 nm thick Aluminium thin film on a flexible Polyethylene Terephthalate substrate was demonstrated with the aid of Computer Generated Holograms (CGH׳s) applied to a phase only Spatial Light Modulator. Low fluence picosecond laser pulses minimise thermal damage to the sensitive substrate and thus clean, single and multi-beam, front side thin film removal is achieved with good edge quality. Interestingly, rear side ablation shows significant Al film delamination. Measured front and rear side ablation thresholds were F_th=0.20±0.01 J cm⁻² and F_th=0.15±0.01 J cm⁻² respectively. With laser repetition rate of 200 kHz and 8 diffractive spots, a film removal rate of R>0.5 cm² s⁻¹ was demonstrated during patterning with a fixed CGH and 5 W average laser power. The effective laser repetition rate was f_eff~1.3 MHz. The application of 30 stored CGH׳s switching up to 10 Hz was also synchronised with motion control, allowing dynamic large area multi-beam patterning which however, slows micro-fabrication.