Electroluminescence from Si／SiO2 films deposited on p—Si substrates

The structure of Au/Si/SiO₂/p-Si has been fabricated using the magnetron sputtering technique. It has a very good rectifying behaviour. Visible electroluminescence (EL) has been observed from the Au/Si/SiO₂/p-Si structure at a forward bias of 5V or larger. A broad band with one peak around 650－660 nm appears in all the EL spectra of the structure. The effects of the thickness of the Si layer in the Si/SiO₂ films and of the input electrical power on EL spectra are studied systematically.     

Comparative Study of Properties of ZnO／GaN／Al203 and ZnO／Al203 Films Grown by Low－Pressure Metal Organic Chemical Vapour Deposition

ZnO films were deposited by low-pressure metal organic chemical vapour deposition on epi-GaN/Al203 films and c-Al203 substrates. The structure and optical properties of the ZnO/GaN/Al203 and ZnO/Al203 films have been investigated to determine the differences between the two substrates. ZnO films on GaN/Al203 show very strong emission features associated with exciton transitions, just as ZnO films on Al2O3, while the crystalline structural qualities for ZnO films on GaN/Al2O3 are much better than those for ZnO films directly grown on Al203 substrates. Zn and O elements in the deposited ZnO/GaN/Al2O3 and ZnO/Al2O3 films are investigated and compared by x-ray photoelectron spectroscopy. According to the statistical results, the Zn/O ratio changes from Zn-rich for ZnO/Al2O3 films to O-rich for ZnO/GaN/Al2O3 films.     

Effect of Metallic Film in Diamond Growth from an Fe—Ni—C System at High Temperature and High Pressure

The metallic film surrounding a diamond single crystal,which plays an important role the diamond growth from and Fe-Ni-C system,has been successfully investigated by using transmission-electron microscopy(TEM),Raman layer(near diamond)of the film by TEM and Raman spectroscopy,but a parallel relationship exists between the(111) plane of γ-(Fe,Ni) and the (100) plane of (Fe2Ni)3C in this region.Compared with that of the film has an increase of 0.9V.According to the microstructures on the film obtained by the TEM.Raman spectra,and XPS,the catalytic mechanism of the film may be assumed as follows.In the surface layer of the film,iron and nickel atoms in the γ-(Fe,Ni) lattice can absorb carbon atoms in the (Fe,Ni)3C lattice and make them transform to an sp^3-like state.Then carbon atoms with the sp^3-like structure are separated from the (Fe,Ni)3C and stack on the growing diamond crystal.This study provides a direct evidence for the diamond growth from a metallic catlayst-graphite system under high temperature and high pressure.     

Structures of surface—and bulk—dispersion phases of NiO／Al2O3

The structural features of surface-disperison and bulk-dispersion states of NiO on/in γ-Al2O3 have been studied by x-ray absorption fine structure (XAFS) in combination with x-ray diffraction(XRD) and XPS.The results obtained from XRD and XAFS show that surface and buld dispersions of Ni^2 ions need different conditions and have different diffraction features and different corrdination structures.In both the surace-dispersion phase and the solid solution,Ni^2 ions seem to prefer to keep the coordination mode in NiO,which is quite different from that in the stoichioetric spinel.The Ni/Ai intensity ratios of XPS at different etching depths of samples have a maximum and the distribution of Ni^2 ions in the solid solution is not homogeneous.Both it and the coordination features provide evidence of the transition process of dispersing from surface to bulk.Copyright 2001 John Wiley and Sons,Ltd.     

Vertical and Smooth, etching of InP by Cl2／CH4／Ar Inductively Coupled Plasma at Room Temperature

We study the room-temperature dry, etching of InP by inductively coupled plasma (ICP) using C12/CH4/Ar mixtures. Etches were characterized in terms of anisotropy and surface roughness by scanning electron microscopy and atomic force microscopy, respectively. It is found that the flow ratio between C12 and Ctt4, ICP power, rf chuck power, and table temperature can greatly influence the, etching results. By adjusting, etching parameters,vertical sidewall and smooth surface can be obtained simultaneously, together with a moderate, etch rate and a good select ratio. The root-mean-square surface roughness is measured to be as low as 0.27nm. To the best of our knowledge, this is the best result for InP to date. The, etch rate is 855nm/min, and the selectivity ratio overSi02 is estimated to be higher than 15:1. The stoichiometry of the, etched surface has also been investigated by Auger electron spectroscopy. The, etched surface is found to manifest a slight P deficiency, and the ratio between P and In reaches the stoichiometric value within about 0.75nm depth into the surface.     

Performance Improvement of GaN Based Schottky Barrier Ultraviolet Photodetector by Adding a Thin A1GaN Window Layer

We propose a new structure of GaN based Schottky barrier ultraviolet photodetector, in which a thin n-type A1GaN window layer is added on the conventional n^--GaN/n^＋-GaN device structure. The performance of the Schottky barrier ultraviolet photodetector is found to be improved by the new structure. The simulation result shows that the new structure can reduce the negative effect of surface states on the performance of Schottky barrier GaN photodetectors, improving the quantum efficiency and decreasing the dark current. The investigations suggest that the new photodetector can exhibit a better responsivity by choosing a suitably high carrier concentration and thin thickness for the A1GaN window layer.     

Removal of GaN film over AlGaN with inductively coupled BCl3/Ar atomic layer etch

Atomic layer etching(ALE)of thin film GaN(0001)is reported in detail using sequential surface modification by BCl₃ adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a reactive ion etching system.The estimated etching rate of GaN is~0.74 nm/cycle.The GaN is removed from the surface of AlGaN after 135 cycles.To study the mechanism of the etching,the detailed characterization and analyses are carried out,including scanning electron microscope(SEM),x-ray photoelectron spectroscopy(XPS),and atomic force microscope(AFM).It is found that in the presence of GaCl_x after surface modification by BCl₃,the GaCl_x disappears after having exposed to low energy Ar plasma,which effectively exhibits the mechanism of atomic layer etch.This technique enables a uniform and reproducible fabrication process for enhancement-mode high electron mobility transistors with a p-GaN gate.     

Determination of Oxidation States in Magnetic Multilayers

X-ray photoelectron spectroscopy has been used to characterize the oxidation states in Ta/NiOx/Ni81Fe19/Ta magnetic multilayers prepared by rf reaction and dc magnetron sputtering.The exchange coupling field and the coercivity of NiOx/Ni81Fe19 are studied as a function of the ratio of Ar to O2 during the deposition process.The chemical states of Ni atoms in the interface region of NiOx/NiFe have also been ivestigated by x-ray photoelectron spectroscopy and the peak decomposition technique.The results show that the ratio of Ar to O2 has a great effect on the chemical states of nickel in NiOx films.Thus the exchange coupling field and the coercivity of Ta/NiOx/Ni81Fe19/Ta are seriously affected.Also,the experiment shows that x-ray photoelectron spectroscopy is a powerful tool in characterizing magnetic multilayers.     

Shock-Assisted Superficial Hexagonal-to-Cubic Phase Transition in GaN/Sapphire Interface Induced by Using Ultra-violet Laser Lift-Off Techniques

Ultra-violet （KrF excimer laser,λ =248 nm） laser lift-off （LLO） techniques have been operated to the GaN/sapphire structure to separate GaN from the sapphire substrate. Hexagonal to cubic phase transformation induced by the ultra-violet laser lift-off （UV-LLO） has been characterized by micro-Raman spectroscopy, miero-photoluminescence, along with high-resolution transmission electron microscopy （HRTEM）. HRTEM indicates that UV-LLO induced phase transition takes place above the LLO interface, without phase transition under the LLO interface. The formed cubic GaN often exists as nanocrystal grains attaching on the bulk hexagona/GaN. The half-loop-clusterlike UV-LLO interface indicates that the LLO-indueed shock waves has generated and played an assistant role in the decomposition of the hexagonal GaN and in the formation of cubic GaN grains at the LLO surface.     

A Novel Ni/Ag/Pt Ohmic Contact to P-Type GaN for Flip-Chip Light-Emitting Diodes

We present a high-quality Ni/Ag/Pt Ohmic contact to p-type GaN. After the sample is annealed at 500℃ in 02 ambient for 3min, a specific contact resistance as low as 2.6 × 10^-5 Ωcm^2 and an optical reflectivity of 82% at 460 nm are obtained. The Auger electron spectroscopy analysis shows that the Pt layer can improve the surface morphology and thermal reliability of the annealed Ag-based electrode, Ag plays a key role in achieving good ohmic contact due to the outdiffusion of Ga into Ag forming Ga vacancies which increase the hole concentration, while the surface contamination of p-type GaN is reduced by Ni.     

Improved Surface Characteristics and Contact Performance of Epitaxial p-AlGaN by a Chemical Treatment Process

The comparative study of epitaxial 380-run-thick p-Al0.091 Ga0.909 N materials without and with special surface chemical treatment is systematically carried out. After the treatment process, the deep level luminous peak in the 10 K photoluminescence spectrum is eliminated due to the decrease of surface nitrogen vacancy VN related defective sites, while the surface root-mean-square roughness in atomic force microscopy measurement is decreased from 0.395nm to 0.229nm by such a surface preparation method. Furthermore, the performance of surface contact with Ni/Au bilayer metal fihns is obviously improved with the reduction of the Schottky barrier height of 55meV. The x-ray photoelectron spectroscopy （XPS） results show a notable surface element content change after the treatment which is considered to be the cause of the above-mentioned surface characteristics improvement.     

ELECTRONIC STRUCTURE STUDIES OF THE C60 FILM CONDENSED ON 2H-MoS2(0001) SURFACE

The electronic structure and vibrational spectrum of the C₆₀ film condensed on a 2H- MoS₂(0001) surface have been investigated by X-ray photoelectron spectroscopy (XPS), ul-traviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES) and infrared high-resolution electron-energy-loss spectroscopy (HREELS). AES analysis showed that at low energy side of the main transition, C₆₀ contains a total of three peaks just like that of graphite. However, the energy position of the KLL main Auger transition of C₆₀ looks like that of diamond, indicating that the hybridization of the carbon atoms in C₆₀ is not strictly in sp²- bonded state but that the curvature of the molecular surface introduces some sp²pz- bonded character into the molecular orbitals. XPS showed that the C 1s binding energy in C₆₀ was 285.0eV, and its main line was very symmetric and offered no indication of more than a single carbon species. In UPS measurement the valence band spectrum of C₆₀ within 10eV below the Fermi level (E_F) shows a very distinct five-band structure that character-izes the electronic structure of the C₆₀ molecule. HREEL results showed that the spectrum obtained from the C₆₀ film has very rich vibrational structure. At least, four distinct main loss peaks can be identified below 200 meV. The most intense loss was recorded at 66 meV, and relatively less intense losses were recorded at 95, 164 and 197meV at a primary energy of electron beam E_P = 2.0eV. The other energy-loss peaks at 46, 136, 157 and 186meV in HREEL spectrum are rather weak. These results have been compared to infrared spectrum data of the crystalline solid C₆₀ taken from recent literatures.     

Growth and Optical Properties of Double Heterostructure GaN／InGaN／GaN Films with Large Composition

The double heterostructure GaN/InGaN/GaN films with different thicknesses of the InGaN layer were grown at 780℃ or 800℃ by metal-organic chemical vapour deposition.The samples were investigated using x-ray diffraction (XRD),room-temperature photoluminescence (PL) and Raman scattering.The dependences of the samples on both the growth temperature and the thickness of the InGaN layer were studied.The composition of InGaN was determined by the results of XRD,and the bowing parameter of InGaN was calculated in terms of the PL spectra.When the thickness of the InGaN layer was reduced,the phase separation of InGaN was found in some samples.The raman frequency of the A1(LO) and E2(low) modes in all the samples shifted and did not agree with Vegard‘s law.     

Nitrogen Adatom Diffusion on a Ga－Rich GaN （0001） Surface

The diffusion of N adatoms on a Ga-rich GaN(O001) surface has been studied using density-functional theory.The configuration of Ga adatoms on a Ga-rich GaN surface has been identified. The first adlayer Ga adatoms are on top of the terminating substrate Ga atoms, and the outmost adlayer Ga adatoms exist randomly at the T4 or H3 sites. A very different diffusivity of N adatoms on a Ga-rich GaN(0001) surface has been found. The excess Ga adatoms on a GaN(0001) surface reduce the diffusion barrier by 0.75eV and influence the migration path. It seems that bilayer Ga adatoms are helpful for N atom diffusion.     

Analysis of Second-Harmonic Generation from CuttbPc LB Film／Metal Interface

Second-harmonic generation signals from a CuttbPc LB film deposited on metal (Al or Au)-glass substrates were investigated. It was observed that there were two second-harmonic peaks at the wavelength of 1060 and 1250nm in the CuttbPc/Al film, but only one peak at 1050nm in the CuttbPc/Au film. Meanwhile the surface electric potentials (SEP) at the interfaces of LB film/metals were also measured using a Kelvin probe. The SEP in the CuttbPc/Al decreases and eventually approaches a saturated value of -1.0V as the film thickness increases, while the SEP in the CuttbPc/Au is nearly zero. Based on the experimental results and theoretical analysis, it was considered that the space-charge-induced electric field makes a main contribution to the second-harmonic generation at 1250nm in the CuttbPc/Al film.     

Aligned ZnO Nanofibre Array Prepared by Vapour Transport in Air

An aligned zinc oxide nanofibre array has been fabricated by heating the mixture of ZnO, Ga2Oa, and graphite powders in atmosphere. The ZnO nanofibre showed a uniform size of about 150nm in diameter and 50μm in length. The nanofibres grew predominantly along one direction. Both x-ray diffraction (XRD) and Raman shift spectra show that the product is composed of ZnO with the typical hexagonal structure. The good crystallinity of these ZnO nanofibres has been verified by photoluminescence spectra with strong UV emission at 287nm and weak green band emission observed at room temperature. The component of the product was analysed by XRD,Raman shift spectrum, x-ray energy dispersion (EDX) and x-ray photoelectronic energy spectroscopy (XPS). The growth process and the characteristics can be interpreted by vapour-liquid-solid mechanism.     

Rectifying barrier at GaN/SiC hetero-junction studied with positron annihilation spectroscopy

Positron annihilation spectroscopy on GaN films grown on SiC substrate with MBE are presented. It is shown that the GaN/SiC interface is rectifying towards positrons, such that positrons can only travel from SiC to GaN and not vice versa. Potential steps seen by the positron at the GaN/SiC interface are calculated from experimental values of electron and positron work function. This “rectifying” effect has been successfully mimicked by inserting a thin region of very high electric field in the Variable Energy Positron Fit （VEPF） analysis. The built-in electric field is attributed to different positron affinities, dislocation and/or interface defects at the GaN/SiC interface.     

Performance comparison of Pt/Au and Ni/Au Schottky contacts on AlxGa1-x N/GaN heterostructures at high temperatures

In contrast with Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts,this paper systematically investigates the effect of thermal annealing of Au/Pt/Al 0.25 Ga 0.75 N/GaN structures on electrical properties of the two-dimensional electron gas in Al 0.25 Ga 0.75 N/GaN heterostructures by means of temperature-dependent Hall and temperature-dependent current-voltage measurements.The two-dimensional electron gas density of the samples with Pt cap layer increases after annealing in N 2 ambience at 600℃ while the annealing treatment has little effect on the two-dimensional electron gas mobility in comparison with the samples with Ni cap layer.The experimental results indicate that the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts reduce the reverse leakage current density at high annealing temperatures of 400-600℃.As a conclusion,the better thermal stability of the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts than the Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts at high temperatures can be attributed to the inertness of the interface between Pt and AlxGa1-xN.     

AlGaN/GaN HEMTs with an Insulated Gate Fabricated by Inductively Coupled Plasma Oxidization

Novel AlGaN/GaN high electron mobility transistors （HEMTs） with an insulated gate have been demonstrated by oxidizing the surface of an AlGaN layer using inductively coupled O2 plasma. X-ray photoelectron spectroscopy measurement reveals that O2 plasma treatment can produce a thin oxidized layer on the AlGaN surface. The insulated-gate devices are realized by plasma oxidization before gate metallization. The reverse gate leakage current of the fabricated HEMT has been reduced in two orders of magnitude, and the cut-off frequency increases from 5.4 GHz to 6.5 GHz.     

A structure study of copper oxide for monolayer dispersion of anatase supported

The monolayer dispersion of copper oxide on the surface of anatase and its effect on the properties have been studied by X-ray photoelectron spectroscopy(XPS) and X-ray extended absorption fine structure(EXAFS).XPS results give an utmost dispersion capacity of 7.2mg/gTiO2.Strong interactions between copper oxide and anatase can be seen from EXAFS results.The structure of the supported CuO species is strongly dependent on the amount of CuO loading.When the content of CuO loading is below the utmost dispersion capacity,the surface of CuO/TiO2 is dominated by the highly dispersed CuO species having no-Cu-O-Cu-chains,The copper ion is located in an octahedral coordination environment,and the Cu－O-coordination distance is much longer than that in pure crystalline CuO,when CuO loading is exceeds the utmost dispersion capacity,crystalline CuO is formed on the surface of CuO/TiO2,From the result of the sturcture study,it is Cu-O octahedral Coordination and coordination distance change in comparation with pure crystalline Cuo on the surface CuO/TiO2 that have catalytic activity.