Isotopic labeling study of oxygen diffusion in amorphous LaScO3 high-κ films on Si(100) and its effects on the electrical characteristics

The influence of post-deposition oxygen anneals on the properties of amorphous LaScO₃ films on Si(100) is reported. The use of an isotopically (¹⁸O₂) enriched atmosphere allowed to investigate the ¹⁶O–¹⁸O exchange and the oxygen diffusion across the dielectric layer. Such effects are connected to the formation of an interfacial layer. Oxygen annealing leads to nearly ideal capacitance–voltage curves, lower leakage currents and interface trap densities, as well as to κ-values up to 33 for the LaScO₃ films. These results are attributed to the suppression of oxygen-related trap centers and the achievement of a stoichiometric oxygen content.     

纳米硅上的弯曲表面效应及其特征发光

纳米硅结构使能带的带隙展宽,并形成准直接能带带隙结构.弯曲表面上的某些键合可以在带隙中产生局域电子态,计算表明:纳米硅弯曲表面上的Si-N,Si=O和Si-O-Si键合能够分别在带隙中2.02 eV,1.78 eV和2.03 eV附近形成局域态子带,对应了实验光致荧光谱(PL)中605 nm处的LN线、693 nm处的LO1线和604 nm处的LO2线特征发光.特别是,Si-Yb键合在纳米硅弯曲表面上可以将发光波长调控到光通信窗口,在1310 nm到1600 nm范围形成LYb线特征发光.     

Silica-based composite and mixed-oxide nanoparticles from atmospheric pressure flame synthesis

Binary TiO₂/SiO₂ and SnO₂/SiO₂ nanoparticles have been synthesized by feeding evaporated precursor mixtures into an atmospheric pressure diffusion flame. Particles with controlled Si:Ti and Si:Sn ratios were produced at various flow rates of oxygen and the resulting powders were characterized by BET (Brunauer–Emmett–Teller) surface area analysis, XRD, TEM and Raman spectroscopy. In the Si–O–Ti system, mixed oxide composite particles exhibiting anatase segregation formed when the Si:Ti ratio exceeded 9.8:1, while at lower concentrations only mixed oxide single phase particles were found. Arrangement of the species and phases within the particles correspond to an intermediate equilibrium state at elevated temperature. This can be explained by rapid quenching of the particles in the flame and is in accordance with liquid phase solubility data of Ti in SiO₂. In contrast, only composite particles formed in the Sn–O–Si system, with SnO₂ nanoparticles predominantly found adhering to the surface of SiO₂ substrate nanoparticles. Differences in the arrangement of phases and constituents within the particles were observed at constant precursor mixture concentration and the size of the resultant segregated phase was influenced by varying the flow rate of the oxidant. The above effect is due to the variation of the residence time and quenching rate experienced by the binary oxide nanoparticles when varying the oxygen flow rate and shows the flexibility of diffusion flame aerosol reactors.     

The azimuthal dependent oxidation process on Cu(110) by energetic oxygen molecules

We report a study on the oxidation process induced by a hyperthermal oxygen molecular beam (HOMB) on Cu(110) using X-ray photoemission spectroscopy in conjunction with a synchrotron radiation source. The oxidation process induced by energetic O₂ beams on Cu(110), depending on the azimuthal angle of incidence, suggests that the –Cu–O– added row structure has a role in inhibiting adsorption as a steric obstacle for incident O₂ molecules.     

Confined Ge–Pt states in self-organized Pt nanowire arrays on Ge(001)

By means of band structure calculations within the density functional theory and the generalized gradient approximation, we investigate the electronic structure of self-organized Pt nanowires on the Ge(001) surface. In particular, we deal with a novel one-dimensional surface state confined in the nanowire array and clarify its origin. Due to large Pt contributions, the novel state is rather a mixed Ge–Pt hybrid state than a confined Ge surface state. Moreover, we compare our results to data from scanning tunneling microscopy.     

Rapid-thermal-anneal-based internal gettering for?germanium-doped Czochralski silicon

The internal gettering (IG) effects involved with a rapid thermal anneal (RTA) in germanium-doped Czochralski silicon (GCz-Si) wafer have been investigated. It was found that germanium doping could enhance the oxygen precipitation in bulk while shrinking the denuded zone width near the surface through pre-RTA at high temperature plus low–high temperature conventional furnace anneals. Rapid cooling rate after RTA was clarified to be beneficial for oxygen precipitation for GCz-Si wafer. It was suggested that the germanium doping could increase the vacancy concentration in Cz-Si during RTA by forming the germanium–vacancy complexes. In contrast to that in Cz-Si wafer, the smaller-sized higher-density oxygen precipitates were presented in the nucleation anneals, then followed RTA pretreatment while more oxygen precipitates survived during ramping processes after nucleation anneals in the GCz-Si wafer. Enhanced heterogeneous nucleation and reduced critical radius of precipitates associated with the germanium–vacancy complexes have been proposed for the oxygen precipitation enhancement.     

Kinetics of laser-induced oxidation of silicon near room temperature

The growth of ultra-thin (<6 nm) silicon-dioxide films on Si(100):H, Si(111):H, and a-Si:H surfaces in a dry oxygen atmosphere (0.1–10 Pa) at low temperatures (35–200 °C) was investigated. Oxidation was induced by pulsed F₂-laserradiation at 157 nm. The thickness and composition of the growing films were monitored in real time by spectroscopic ellipsometry in the photon energy range of 1.15–4.75 eV. The kinetics of low-temperature oxidation was similar for the Si surfaces investigated and differs from that of high-temperature thermal oxidation (900–1200 °C) that can be described by the Deal–Grove model. To explain the faster growth at the initial stage, it is proposed that oxidation occurs by diffusion of oxygen atoms O and/or ions O^-rather than oxygen molecules. The recombination of diffusive species to oxygen molecules limits their penetration into the bulk. A diffusion model is developed for low-temperature oxidation which takes into account the recombination process of the diffusive species. Good agreement between theory and experiment is found. The activation energy of diffusion of the active species was found to be 0.15 eV, in agreement with previous results and recent calculations for O^- ions. PACS 82.65.+r; 07.60.Fs; 81.65.Mq; 82.50.Hp     

Luminescence properties of thin films prepared by laser ablation of Nd-doped potassium gadolinium tungstate

Optically active thin films on Si substrates have been produced by laser ablation of a Nd-doped potassium gadolinium tungstate (Nd:KGW) single crystal. Films grown at low oxygen pressures (<0.6 mbar) are potassium-deficient and appear to be mainly disordered. They show a poor photoluminescence (PL) performance that improves upon annealing in air at temperatures in the range 700–1000 °C. Films grown at high oxygen pressure (1 mbar) show instead good stoichiometry and the presence of a dominant textured gadolinium-tungstate phase compared to KGW. These films have low absorption, a refractive index close to that of bulk KGW and good PL performance, the emission lifetimes being longer (τ>150 μs) under certain conditions than those measured in the single-crystal material. Received: 25 July 2001 / Accepted: 26 July 2001 / Published online: 17 October 2001     

Silicon–germanium nanostructures for on-chip optical interconnects

Silicon–germanium epitaxially grown on silicon in the form of two-dimensional (quantum wells) and three-dimensional (quantum dots) nanostructures exhibits photoluminescence and electroluminescence in the technologically important spectral range of 1.3–1.6 μm. Until recently, the major roadblocks for practical applications of these devices were strong thermal quenching of the luminescence quantum efficiency, and a long carrier radiative lifetime. This paper summarizes recent progress in the understanding of carrier recombination in Si/SiGe nanostructures and presents a potential new route toward CMOS compatible light emitters for on-chip optical interconnects.     

First principle study of the magnetism of Sr2CoMoO6-δ (δ = 0, 1/2) double perovskites

We investigate the electronic structure of bulk Sr₂CoMoO_6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results — AFM order for δ= 0 and FM order for δ= 1/2 — is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used.     

Determination of the rate constant of oxygen quenching of the excited states of molecules from the singlet oxygen luminescence

We have pioneered a method of determining the rate constant of quenching of the excited electronic states of molecules by molecular oxygen from measurements of the kinetics of photosensitized luminescence of singlet molecular oxygen (^lδ_g). The method can be used in the case where the lifetime of the excited electronic state in an air-saturated solution is comparable with or larger than the luminescence time of the singlet molecular oxygen in the given solvent. It is shown that this situation is implemented on quenching, by molecular oxygen, of the excited triplet states associated with the biopolymers of tetrapyrrole molecules in aqueous (H₂O and D₂O) solutions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 401–404, May–June, 2000.     

Buffer layer ZnO-assistant fabrication of c-axis GaN films by using pulsed laser deposition on Si(111) substrate: annealing effects in ammonia ambience

ZnO buffer layers have been used to fabricate GaN thin films by using pulsed laser deposition on Si (111) substrates. c-axis GaN thin films were obtained by annealing in NH₃ atmosphere at 950°C for 15 min. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM) have been used for the characterizations of the crystalline quality, composition, and surface morphology of the films. The annealing in ammonia (NH₃) atmosphere markedly affects the preparation of GaN films and the least annealing time is 15 min under our experimental conditions. The mechanism of the effects of the ZnO buffer layers was studied. In the beginning, Zn–O bonds are destroyed in the interface of the films; a few O and Zn atoms depart from their positions, while N and Ga atoms fill in the empty positions and form a hexagonal structure of a special component. Many bonds (such as Ga–O bonds, Zn–N bonds) existed then. The number of Zn–O bonds decreases and the number of Ga–N bonds increases in the films with increasing of the annealing time. Many other bonds (such as Ga–O bonds, Zn–N bonds) also decreased and more Ga–N bonds formed with annealing time increasing. After having been annealed for 15 min under our experimental conditions, the quality of the hexagonal structure GaN films was markedly improved by the destroying of the Zn–O bonds during high-temperature annealing.     

Application of MEH-PPV/SnO<Subscript>2</Subscript> bilayer as hybrid solar cell

The effects of oxygen content in the sputtering gas on the crystallographic and optoelectronic properties of 210 nm-thick Zr–doped In₂O₃ (Zr–In₂O₃) films by rf magnetron sputtering were initially studied. The results of X-ray diffraction show that the Zr–In₂O₃ films grown on glass substrates exhibit mixed crystallographic orientations. Moreover, the Zr–In₂O₃ film grown in an Ar atmosphere promotes the appearance of crystallographic orientation of (222). The surface of the Zr–In₂O₃ film becomes rougher as the oxygen content in the sputtering gas decreases; the current images obtained by conductive atomic force microscopy reveal that the surfaces of the Zr–In₂O₃ films exhibit a distribution of coexisting conducting and nonconducting regions, and that the area of the nonconducting surface increases with the oxygen content in the sputtering gas. The resistivity is minimized to 3.51×10⁻⁴ Ω cm when the Zr–In₂O₃ film is grown in an Ar atmosphere and the average transmittance in the visible light region is ∼85%. The optical band gap decreases as the oxygen content in the sputtering gas increases.     

Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10⁻²–10⁷ Hz) over a large temperature interval (213–333K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.     

InTaO<Subscript>4</Subscript>-based nanostructures synthesized by reactive pulsed laser ablation

Nanostructured Ni-doped indium–tantalum–oxides (InTaO₄) were synthesized by a reactive pulsed laser ablation process, aiming at the final goal of direct splitting of water under visible sunbeam irradiation. The third harmonics beam of a Nd:YAG laser was focused onto a sintered In_0.9Ni_0.1TaO_4−δ target in pure oxygen background gases (0.05–1.00 Torr). Increasing the oxygen gas pressure, via thin films having nanometer-sized strong morphologies, single-crystalline nanoparticles were synthesized in the reactive vapor phases. The nanostructured deposited materials have the monoclinic layered wolframite-type structure of bulk InTaO₄, without oxygen deficiency.     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

Intrinsic stress of ultrathin epitaxial films

The present article focuses on the stress developing during the deposition of ultrathin epitaxial films in the thickness range of a few atomic layers. The studied systems exhibit the three well-known modes of film growth: Stranski–Krastanow mode [Ge/Si(001), Ge/Si(111), Ag/Si(111)], Frank–Van der Merwe mode [Fe/MgO(001)] and Volmer–Weber mode [Ag/mica(001), Cu/mica(001)]. The experimental results demonstrate the important role of the misfit strain as well as the contribution of surface stress effects as mechanisms for the stress in single atomic layers. Received: 26 April 1999 / Accepted: 25 June 1999 / Published online: 6 October 1999     

Electronic properties of Ge–Si nanoparticles

Using a parameterized density-functional tight-binding method we have calculated the electronic and structural properties of Ge–Si nanoparticles. Starting with a spherical part of a zinc-blende/diamond crystal (with the center of the sphere at the mid-point of a nearest-neighbour bond) we have constructed initial structures that subsequently were allowed to relax. Structures consisting solely of Ge atoms or solely of Si atoms were studied, together with core-shell structures for which one semiconductor forms a shell on the core of the other semiconductor. Moreover, homogeneous, ordered SiGe structures as well as structures with a semisphere of one semiconductor and a semisphere of the other were also considered. In analysing the results special emphasis is put on identifying particularly stable structures, on explaining the occurrence of those, on the spatial distribution of the frontier orbitals, and on the variation of the total energy with structure and composition.     

Production and characterization of Nd,Cr:GSGG thin films on Si(001) grown by pulsed laser ablation

Nd,Cr:Gd₃Sc₂Ga₃O₁₂ (GSGG) thin films have been produced for the first time. They were grown on Si(001) substrates at 650 °C by pulsed laser ablation at 248 nm of a crystalline Nd,Cr:GSGG target rod. The laser plume was analyzed using time-of-flight quadrupole mass spectroscopy, and consisted of elemental and metal oxide fragments with kinetic energies typically in the range 10 to 40 eV, though extending up to 100 eV. Although films deposited in vacuum using laser fluences of 0.8±0.1 J cm⁻² reproduced the Nd,Cr:GSGG bulk stoichiometry, those deposited using fluences above ≈3 J cm⁻² resulted in noncongruent material transfer and were deficient in Ga and Cr. Attempts to grow films using synchronized oxygen or oxygen/argon pulses yielded mixed oxide phases. Under optimal growth conditions, the films were heteroepitaxial, with GSGG(001)[100]∥Si(001)[100], and exhibited Volmer–Weber-type growth. Room-temperature emission spectra of the films suggest efficient non-radiative energy transfer between Cr³⁺ and Nd³⁺ ions, similar to that of the bulk crystal. Received: 1 October 1999 / Accepted: 15 October 1999 / Published online: 23 February 2000     

Open-system density matrix description of an STM-driven atomic switch: H on Si(100)

Previous experiments indicate that an STM (scanning tunnelling microscope) can be used to switch a hydrogen atom at a partially hydrogen-covered Si(100)-2×1 surface, from one Si atom of a Si dimer to a neighbouring, empty Si site [U.J. Quaade et al., Surf. Sci. 415, L1037, 1998]. It has been suggested that the switching occurs via a transient positive ion resonance state. In an earlier paper, we have examined the switching process for the “above threshold” regime when the bias is large enough to directly populate the positive ion resonance. In the present paper we study the “below threshold” regime instead, where the switching is more appropriately modelled as a ladder climbing over the barrier, in the ground electronic state. For this purpose we solve the Liouville–von Neumann equation in Lindblad form, describing a switching H atom on a Si dimer. STM-induced transition rates between vibrational levels are estimated from cluster calculations, assuming contributions both from a dipole and a resonance scattering mechanism. Vibrational relaxation is also included, as well as finite temperature and field effects. The switching rate in a current regime of about 1 to 10 nA scales highly non-linearly with current, and it is found to be governed by vibrational “ladder climbing” and subsequent tunnelling through the top of the ground state barrier. Multi-phonon processes also play a role. As a result of tunnelling, pronounced isotope effects are observed when replacing H with D. It is further argued that resonance-mediated inelastic scattering dominates over dipole excitation, and that the STM switch is stable also at room temperature.