Study of S ion-assisted sulfurization of n-GaAs (1 0 0) surface

The chemical structure and site location of sulfur atoms on n-GaAs (1 0 0) surface treated by bombardment of S⁺ ions over their energy range from 10 to 100 eV have been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. The formation of Ga-S and As-S species on the S⁺ ion bombarded n-GaAs surface is observed. An apparent donor doping effect is observed for the n-GaAs by the 100 eV S⁺ ion bombardment. It is found that the S⁺ ions with higher energy are more effective in the formation of Ga-S species, which assists the n-GaAs (1 0 0) surface in reconstruction into an ordered (1 × 1) structure upon subsequent annealing. The treatment is further extended to repair Ar⁺ ion damaged n-GaAs (1 0 0) surface. It is found that after a n-GaAs (1 0 0) sample is damaged by 150 eV Ar⁺ ion bombardment, and followed by 50 eV S⁺ ion treatment and subsequent annealing process, finally an (1 × 1) ordering GaAs (1 0 0) surface with low surface states is obtained.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.     

Characterization of bicrystalline epitaxial LaNiO3 films fabricated on MgO (1 0 0) substrates by pulsed laser deposition

A series of metallic LaNiO₃ (LNO) thin films were deposited on MgO (1 0 0) substrates by pulsed laser deposition (PLD) under the oxygen pressure of 20 Pa at different substrate temperatures from 450 to 750 °C. X-ray diffraction (XRD) was used to characterize the crystal structure of LNO films. θ-2θ scans of XRD indicate that LNO film deposited at a substrate temperature of 700 °C has a high orientation of (l l 0). At other substrate temperatures, the LNO films have mixed phases of (l l 0) and (l 0 0). Furthermore, pole figure measurements show that LNO thin films, with the bicrystalline structure, were epitaxially deposited on MgO (1 0 0) substrates in the mode of LNO (1 1 0)//MgO (1 0 0) at 700 °C. Reflection high-energy electric diffraction (RHEED) and atomic force microscopy (AFM) were also performed to investigate the microstructure of LNO films with the high (l l 0) orientation. RHEED patterns clearly confirm this epitaxial relationship. An atomically smooth surface of LNO films at 700 °C was obtained. In addition, bicrystalline epitaxial LNO films, fabricated at 700 °C, present a excellent conductivity with a lower electrical resistivity of 300 μ Ω cm. Thus, the obtained results indicate that bicystalline epitaxial LNO films could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Solid phase epitaxy of Ge films on CaF2/Si(1 1 1)

At room temperature deposited Ge films (thickness < 3 nm) homogeneously wet CaF₂/Si(1 1 1). The films are crystalline but exhibit granular structure. The grain size decreases with increasing film thickness. The quality of the homogeneous films is improved by annealing up to 200 °C. Ge films break up into islands if higher annealing temperatures are used as demonstrated combining spot profile analysis low energy electron diffraction (SPA-LEED) with auger electron spectroscopy (AES). Annealing up to 600 °C reduces the lateral size of the Ge islands while the surface fraction covered by Ge islands is constant. The CaF₂ film is decomposed if higher annealing temperatures are used. This effect is probably due to the formation of GeF_x complexes which desorb at these temperatures.     

Fabrication of multilayered Ge nanocrystals embedded in SiOxGeNy films

Multilayered Ge nanocrystals embedded in SiO_xGeN_y films have been fabricated on Si substrate by a (Ge + SiO₂)/SiO_xGeN_y superlattice approach, using a rf magnetron sputtering technique with a Ge + SiO₂ composite target and subsequent thermal annealing in N₂ ambient at 750 °C for 30 min. X-ray diffraction (XRD) measurement indicated the formation of Ge nanocrystals with an average size estimated to be 5.4 nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode downward shifted to 299.4 cm⁻¹, which was caused by quantum confinement of phonons in the Ge nanocrystals. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (Ge + SiO₂) layers. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the ‘Z’ growth direction.     

Fabrication of nanostructured CuInS2 thin films by ion layer gas reaction method

CuInS₂ thin films were prepared by a two-stage ion layer gas reaction (ILGAR) process in which the Cu and In precursors were deposited on glass substrate by using a simple and low-cost dip coating technique and annealed in H₂S atmosphere at different temperatures. The influence of the annealing temperature (250-450 °C) on the particle size, crystal structure and optical properties of the CuInS₂ thin films was studied. Transmission electron microscopy revealed that the particle radii varied in the range 6-21 nm with annealing. XRD and SAED patterns indicated polycrystalline nature of the nanoparticles. The optical band gap (E_g) varied from 1.48 to 1.56 eV with variation of particle size. The variation of Urbach tail with temperature indicated higher density of the defects for the films annealed at lower temperature. From the Raman study, it was observed that the FWHM of the A₁ mode at ∼292 cm⁻¹ corresponding to the chalcopyrite phase of CuInS₂ decreased with increasing annealing temperature.     

Effects of annealing on laser-induced damage threshold of TiO2/SiO2 high reflectors

The mechanism of improving 1064 nm, 12 ns laser-induced damage threshold (LIDT) of TiO₂/SiO₂ high reflectors (HR) prepared by electronic beam evaporation from 5.1 to 13.1 J/cm² by thermal annealing is discussed. Through optical properties, structure and chemical composition analysis, it is found that the reduced atomic non-stoichiometric defects are the main reason of absorption decrease and LIDT rise after annealing. A remarkable increase of LIDT is found at 300 °C annealing. The refractive index and film inhomogeneity rise, physical thickness decrease, and film stress changes from compress stress to tensile stress due to the structure change during annealing.     

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Thermal annealing effect on the crystallization and optical dispersion of sprayed V2O5 thin films

Polycrystalline vanadium pentoxide (V₂O₅) thin films have been deposited by spray pyrolysis technique on preheated glass substrate. The influence of thermal annealing on the crystallization of V₂O₅ has been investigated. X-ray diffraction analysis (XRD) revealed that the films deposited at T_sub=350 °C were orthorhombic structures with a preferential orientation along 〈0 0 1〉 direction. Moreover, the degree of crystallinity was improved by thermal annealing. Optical properties of these samples were studied by spectrophotometer in the wavelength range 300-2500 nm. Some of the important optical absorptions such as optical dispersion energies E_o and E_d, dielectric constant ε, ratio between number of charge carriers and effective mass N/m^*, wavelength of single oscillator λ₀, plasma frequency ω_p, single resonant frequency ω₀ and the average of oscillator strength S_o, have been evaluated. In the annealing process, the dielectric properties have weak dependencies of film thickness and annealing time. Furthermore, a value of carrier concentration was obtained of 3.02×10²⁵ m⁻³ for the as-deposited film and slight changes with annealing time.     

Surface characterization of self-assembled N-Cu nanostructures

We report on the process of low energy N₂⁺ implantation and annealing of a Cu(0 0 1) surface. Through AES we study the N diffusion process as a function of the substrate temperature. With STM and LEIS we characterize the surface morphology and the electronic structure is analyzed with ARUPS. Under annealing (500 < T < 700 K) N migrates to the surface and reacts forming a Cu_xN compound that decomposes at temperatures above 700 K. LEIS measurements show that N locates on the four-fold hollow sites of the Cu(0 0 1) surface in a c(2 × 2) arrangement. Finally, a gap along the [0 0 1] azimuthal direction is determined by ARUPS. DFT calculations provide support to our conclusions.     

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO_x, 0 < x < 2) films by thermal annealing in high vacuum. The SiO_x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N₂O/SiH₄) flow ratios. The as-deposited films show increased Si concentration with decreasing N₂O/SiH₄ flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N₂O/SiH₄ reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO_x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.     

Nonlinear optical properties of Au/ZnO nanoparticle arrays

The triangular-shaped Au/ZnO nanoparticle arrays were fabricated on fused quartz substrate using nanosphere lithography. The structural characterization of the Au/ZnO nanoparticle arrays was investigated by atomic force microscopy. The absorption peak due to the surface plasmon resonance of Au particles at the wavelength of about 570 nm was observed. The nonlinear optical properties of the nanoparticle arrays were measured using the z-scan method at a wavelength of 532 nm with pulse duration of 10 ns. The real and imaginary part of third-order nonlinear optical susceptibility, Re χ⁽³⁾ and Im χ⁽³⁾, were determined to be 1.15 × 10⁻⁶ and −5.36 × 10⁻⁷ esu, respectively. The results show that the Au/ZnO nanoparticle arrays have great potential for future optical devices.     

The ferroelectric and ferromagnetic characterization of CoFe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 multilayered thin films

The multiferroic (PMN-PT/CFO)_n (n = 1,2) multilayered thin films have been prepared on SiO₂/Si(1 0 0) substrate with LNO as buffer layer via a rf magnetron sputtering method. The structure and surface morphology of multilayered thin films were determined by X-ray diffraction (XRD) and atom force microscopy (AFM), respectively. The smooth, dense and crack-free surface shows the excellent crystal quality with root-mean-square (RMS) roughness only 2.9 nm, and average grain size of CFO thin films on the surface is about 44 nm. The influence of the thin films thickness size, periodicity n and crystallite orientation on their properties including ferroelectric, ferromagnetic properties in the (PMN-PT/CFO)_n multilayered thin films were investigated. For multilayered thin films with n = 1 and n = 2, the remanent polarization Pr are 17.9 μC/cm² and 9.9 μC/cm²; the coercivity H_c are 1044 Oe and 660 Oe, respectively. In addition, the relative mechanism are also discussed.     

The important role of Mn in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga_1−xMn_xN) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga_1−xMn_xN films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn³⁺ due to the thermal treatment. Mn³⁺, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn³⁺ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn³⁺ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn_xGa_y formation.     

Ta/Ni/Ta multilayered ohmic contacts on n-type SiC

The interface formation, electrical properties and the surface morphology of multilayered Ta/Ni/Ta/SiC contacts were reported in this study. It was found that the conducting behavior of the contacts so fabricated is much dependent on the metal layer thickness and the subsequent annealing temperature. Auger electron spectroscopy (AES) and X-ray diffraction analyses revealed that Ni₂Si and TaC formed as a result of the annealing. The Ni atoms diffused downward to metal/SiC interface and converted into Ni₂Si layer in adjacent to the SiC substrate. The released carbon atoms reacted with Ta atoms to form TaC layer. Ohmic contacts with specific contact resistivity as low as 3 × 10⁻⁴ Ω cm² have been achieved after thermal annealing. The formation of carbon vacancies at the Ni₂Si/SiC interface, probably created by dissociation of SiC and formation of TaC during thermal annealing, should be responsible for the ohmic formation of the annealed Ta/Ni/Ta contacts. The addition of Ta into the Ni metallization scheme to n-SiC restricted the accumulation of carbon atoms left behind during Ni₂Si formation, improving the electrical and microstructure properties.     

Effect of annealing temperature on the optical and electrical properties of amorphous As45.2Te46.6In8.2 thin films

The optical absorption of the As-prepared and annealed As_45.2Te_46.6In_8.2 thin films are studied. Films annealed at temperatures higher than 453 K show a decrease in the optical energy gap (E_o). The value of E_o increases from 1.9 to 2.43 eV with increasing thickness of the As-prepared films from 60 to 140 nm. The effect of thickness on high frequency dielectric constant (?_∞) and carrier concentration (N) is also studied. The crystalline structures of the As_45.2Te_46.6In_8.2 thin films resulting from heat treatment of the As-prepared film at different elevated temperatures is studied by X-ray diffraction. An amorphous-crystalline transformation is observed after annealing at temperatures higher than 453 K. The electrical conductivity at low temperatures is found due to the electrons transport by hopping among the localized states near the Fermi level. With annealing the films at temperatures higher than 473 K (the crystallization onset temperature) for 1 h, the electrical conductivity increases and the activation energy decreases, which can be attributed to the amorphous-crystalline transformations.     

Improving p-type contact characteristics by Ni-assisted annealing and effects on surface morphologic evolution of InGaN LED films grown on Si (1 1 1)

Ni thin layer was deposited to assist to activate p-GaN and then was removed. The process was named Ni-assisted annealing (NA). We investigate the surface morphology and p-type contact behaviors of InGaN LED films grown on Si (1 1 1) substrates. Compared with conventional thermal annealing (TA), NA can improve the p-type contact characteristic at lower anneal temperature and a smaller specific contact resistivity (ρ_c = 6.1 × 10⁻⁵ Ω cm²) employing nonalloy Pt electrode was obtained. A wet etching method using acid-hydrogen peroxide was adopted to boil films surface after activation. We found that some nano-pits appeared on surfaces while original surface step structure was still clearly visible, which shows a defect-selective etching characteristic. Otherwise, we demonstrated that the surface morphology could be affected by NA while independent to TA. Some mechanisms for experimental phenomena were also discussed in the letter.     

Characterization of optical waveguide in Nd: GdVO4 by triple-energy oxygen ion implantation

We report on the optical planar waveguide formation and modal characterization in Nd: GdVO₄ crystals by triple oxygen ion implantation at energies of (2.4, 3.0, and 3.6 MeV) and fluences of (1.4, 1.4, and 3.1)  × 10¹⁴ions/cm². The prism-coupling method is used to investigate the dark-mode property at wavelength of 632.8 nm. The refractive index profiles of the waveguide are reconstructed by an effective refractive index, n_eff method. The modal analysis shows that the fields of TE modes are well restricted in the guiding region, which means the formation of nonleaky waveguide in the crystal.     

Hydroxylation-induced modifications of the Al2O3/NiAl(0 0 1) surface at low water vapour pressure

STM, STS, LEED and XPS data for crystalline θ-Al₂O₃ and non-crystalline Al₂O₃ ultra-thin films grown on NiAl(0 0 1) at 1025 K and exposed to water vapour at low pressure (1 × 10⁻⁷-1 × 10⁻⁵ mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O_1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al³⁺ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlO_xOH_y with x + y ∼ 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H₂O) = 1 × 10⁻⁷ mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified.