Electron-hole liquid and excitonic molecules in quasi-two-dimensional SiGe layers of Si/SiGe/Si heterostructures

The electron-hole liquid (EHL) in SiGe layers of Si/Si_{1 − x} Ge _x /Si quantum-confinement heterostructures is discovered. It is composed of quasi-two-dimensional holes in the quantum well formed by the SiGe layer and quasi-three-dimensional electrons, which occupy a wider region of space centered on this layer. The densities of electrons and holes in the EHL are determined to be p ₀ ≈ 8.5 × 10¹¹ cm⁻² and n ₀ ≈ 4.8 × 10¹⁸ cm⁻³, respectively. It is demonstrated that the gas phase consists of excitons and excitonic molecules. The conditions on the band parameters of the structure under which the formation of the EHL of this kind and biexcitons is possible are formulated.     

DLTS and in situ C–V analysis of trap parameters in swift 50 MeV Li3+ ion-irradiated Ni/SiO2/Si MOS capacitors

Ni/SiO₂/Si MOS structures were fabricated on n-type Si wafers and were irradiated with 50 MeV Li³⁺ ions with fluences ranging from 1×10¹⁰ to 1×10¹² ions/cm². High frequency C–V characteristics are studied in situ to estimate the build-up of fixed and oxide charges. The nature of the charge build-up with ion fluence is analyzed. Defect levels in bulk Si and its properties such as activation energy, capture cross-section, trap concentration and carrier lifetimes are studied using deep-level transient spectroscopy. Electron traps with energies ranging from 0.069 to 0.523 eV are observed in Li ion-irradiated devices. The dependence of series resistance, substrate doping and accumulation capacitance on Li ion fluence are clearly explained. The study of dielectric properties (tan δ and quality factor) confirms the degradation of the oxide layer to a greater extent due to ion irradiation.     

The effect of excess surfactants on the adsorption of iron oxide nanoparticles during a dip-coating process

The effect of excess surfactants (oleic acids) in a colloidal solution on the adsorption behavior of 9.5-nm-sized, sterically stabilized iron oxide (γ-Fe₂O₃) nanoparticles on hydrogen terminated Si (Si:H) substrates during a dip-coating process is examined. While the particle coverage follows a type of Langmuir adsorption isotherm as initially increasing and subsequently saturating with increasing particle concentration, it also critically depends on the excess surfactant concentration in the solution. For instance, it is noted that by adding the oleic acids from 0.06 to 2.80 × 10¹⁸ ml⁻¹ in the solution with 4.65 × 10¹³ ml⁻¹ particle concentration, the coverage is gradually reduced from 0.42 to 0.25. In addition, increasing surfactant concentration distinctly changes the morphology of a self-assembled particle layer from densely distributed smaller clusters to sparsely connected, larger ones with enlarged space. The reduced coverage and enlarged cluster size with increasing oleic acid concentration are explained by the reduced interaction energy between particle and substrate and the increased capillary force between particles.     

Radiation defects introduced by MeV electrons in argon implanted MOS structures

The influence of MeV electron irradiation on the interface states of argon implanted thin oxide MOS samples has been studied by the thermally stimulated current (TSC) method. The oxide thickness of the structures is 18 nm. Two groups of n-type MOS structures non-implanted and implanted with 20 keV Ar⁺ ions and a dose of 5×10¹² cm⁻² are examined. Both groups are simultaneously irradiated by 23 MeV electrons with doses of 1.2×10¹⁶, 2.4×10¹⁶ or 6.0×10¹⁶ el/cm². The energy position and density of the interface states (generated by electron irradiation, ion implantation or both treatments of the samples) are determined. It is shown that MeV electron irradiation decreases the concentration of interface states (like an oxygen-vacancy and di-vacancy) slightly and creates additional interface states (like an impurity-vacancy) at the Si–SiO₂ interface of argon implanted MOS structures.     

Thin TiO2 grown by metal–organic chemical vapor deposition on (NH4)2S x -treated InP

The electrical characteristics of thin TiO₂ films prepared by metal–organic chemical vapor deposition grown on a p-type InP substrate were studied. For a TiO₂ film of 4.7 nm on InP without and with ammonium sulfide treatment, the leakage currents are 8.8×10⁻² and 1.1×10⁻⁴ A/cm² at +2 V bias and 1.6×10⁻¹ and 8.3×10⁻⁴ A/cm² at −2 V bias. The lower leakage currents of TiO₂ with ammonium sulfide treatment arise from the improvement of interface quality. The dielectric constant and effective oxide charge number density are 33 and 2.5×10¹³ cm², respectively. The lowest mid-gap interface state density is around 7.6×10¹¹ cm⁻² eV⁻¹. The equivalent oxide thickness is 0.52 nm. The breakdown electric field increases with decreasing thickness in the range of 2.5 to 7.6 nm and reaches 9.3 MV/cm at 2.5 nm.     

Degradation in polymer Ni-MH battery

The alkaline solid polymer electrolyte system with the conductivity of (8.5±0.2) × 10⁻⁴ Scm⁻¹ composes of polyvinyl alcohol (PVA) and potassium hydroxide (KOH) have been used in fabrication of Mg₂Ni/Ni(OH)₂ nickel metal hydride cell. The self-dehydriding of the cell during storage was presented by open circuit voltage study. Cell degradation after charge-discharge cycling is characterized by means of X-ray diffraction and scanning electron microscopic analyses. The failure of the cell can be ascribed by a small formation of Mg(OH)₂ oxide layer and surface resistance between electrode-electrolyte.     

Interactions of oxygen with yttria-stabilized zirconia at room temperature

This paper reports the results of work function and EPR studies of yttria-stabilized zirconia (10Y-ZrO₂). The experimental data are considered in terms of the formation of oxygen chemisorbed species and subsequent oxygen incorporation as well as related charge transfer. It is concluded that oxygen chemisorption on 10Y-ZrO₂ at room temperature in its initial stage results in the formation of O ₂ ⁻ species. These species are then transformed into O ₂ ²⁻ , O⁻ species and, finally, are slowly incorporated into the oxide lattice. These processes take place without presence of Pt or any other electrode material.     

High resistance modulation by the electric field based on La0.9Sr0.1MnO3/SrTiO3/Si structure

A field-effect configuration based on La_0.9Sr_0.1MnO₃/SrTiO₃/Si structure is fabricated on Si substrate by laser molecular-beam epitaxy. The resistance modulation by electric field of the La_0.9Sr_0.1MnO₃/SrTiO₃/Si structure is investigated in detail. An evident resistance modulation effect is observed at 80 K. The channel resistance modulation by field effect reaches 1.4×10⁷% and 2.6×10⁶% when V _DS are −2 and −6.5 V, respectively. The ON/OFF ratio of approximately 4000 is obtained. The present results are worthy of further investigations for potential applications of resistance modulation by electrostatic field in the heterostructures consisting of perovskite oxides and Si. Supported by the National Natural Science Foundation of China (Grant No. 50672120) and the National Basic Research Program of China (Grant No. 2004CB619004)     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Memory element based on Si/CaF<Subscript>2</Subscript> periodic nanostructures

A memory element based on a Si/CaF₂ periodic nanostructure is proposed. In this element, information is recorded through charge capture by trap states in a CaF₂ dielectric. The high and low signal levels correspond to the current in the maximum and minimum of the negative differential resistance region, which forms as a result of the resonant tunnel distribution of charge carriers over trap levels in the dielectric. The speed of such logical elements depends on the rate of activation carrier trapping and the rate of tunnel carrier transfer from one state to another. It is shown that both Si/CaF₂-based logical elements and memory elements proposed operate at temperatures from 77 to 300 K, have a switching time of 10⁻¹²–10⁻¹⁰ s, and are compatible with silicon IC technology.     

Thermal stability and radiation hardness of SiC-based schottky-barrier diodes

Pt/W/Cr/SiC Schottky-barrier diodes that retain good electrophysical parameters up to 450°C are studied. With the Auger electron spectroscopy (AES) method, it is shown that the thermal stability is provided by using a multilayer metal composition that ensures the metal/SiC interface stability. The surface-barrier structures obtained are tested for radiation hardness. They are irradiated by fast neutrons with a fluence of 4.42×10¹⁵ n/cm² and attendant γ radiation with a dose of 8.67×10⁵ R in the concentration range of N _d-N _a=10¹⁶−5×10¹⁷ cm⁻³. Irreversible modifications of the structures at N _d-N _a≤8×10¹⁶ cm⁻³ are found. The degradation of the parameters is inversely proportional to the doping level.     

Investigations of electronic minority charge carrier conductivity in La0.9Sr0.1Ga0.8Mg0.2O2.85

The perovskite structured material LaGaO₃ doped with 10 mol-% strontium and 20 mol-% magnesium was prepared by two different wet-chemical synthesis routes. The total conductivity was measured in air and under an oxygen partial pressure of 10⁻²⁰ bar. There was a decrease by 10 % in 4 days when the atmosphere was changed from air to 10⁻²⁰ bar. This process is reversible. Hebb-Wagner measurements resulted in values for the electronic minority charge carrier conductivities in pure oxygen of log σ_h [S/cm]=−4.02 and log σ_e [S/cm]=−15.5 for the holes and electrons, respectively, at 600 °C. In the partial pressure range 10⁻³ bar≤p(O₂)≤1 bar, a slope of +1/4 was observed for d(log (σ_h)) / d(log (p(O₂)) at T=600, 650 and 700 °C. That is in agreement with the assumption of a large number of oxygen vacancies. The diffusion coefficient of the holes was evaluated from the relaxation curves to be 1.1*10⁻⁷ cm²/s at 600 °C. Degradation effects were observed under highly reducing conditions which are attributed to the formation of gallium-platinum alloys and the loss of gallium oxide if O₂ is available in the gas phase. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

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     

Sub-THz radiation room temperature sensitivity of long-channel silicon field effect transistors

Room temperature operating n-MOSFETs (n-type metal-oxide silicon field effect transistors) used for registration of sub-THz (sub-terahertz) radiation in the frequency range ν = 53−145 GHz are considered. n-MOSFETs were manufactured by 1-μm Si CMOS technology applied to epitaxial Si-layers (d ≈15 μm) deposited on thick Si substrates (d = 640 μm). It was shown that for transistors with the channel width to length ratio W/L = 20/3 μm without any special antennas used for radiation input, the noise equivalent power (NEP) for radiation frequency ν ≈76 GHz can reach NEP ∼6×10⁻¹⁰ W/Hz^1/2. With estimated frequency dependent antenna effective area S_est for contact wires considered as antennas, the estimated possible noise equivalent power NEP_pos for n-MOSFET structures themselves can be from ∼15 to ∼10³ times better in the specral range of ν ∼55–78 GHz reaching NEP_pos ≈10⁻¹² W/Hz^1/2.     

Alternating current surface photovoltage in thermally oxidized chromium-contaminated n-type silicon wafers

We investigated a variation of frequency-dependent alternating current (AC) surface photovoltages (SPVs) in thermally oxidized, chromium-contaminated, n-type silicon (Si) wafers. As previously reported, immediately after rinsing in Cr-contaminated solution, a Cr(OH)₃–Si contact causes a Schottky-barrier-type AC SPV on n-type Si. Upon oxidation at 373 K for 10 min, the Schottky barrier collapses and, with further oxidation, a metal-induced negative oxide charge, due to atomic bridging of (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks, definitely grows over time in SiO₂. For samples oxidized at temperatures between 823 and 1023 K for 30 min, the observed AC SPV gives evidence that the metal-induced negative oxide charge causes a strongly inverted state of the Si surface. At oxidation temperatures higher than 1023 K and /or for an oxidation time longer than 60 min, the level height of the AC SPV is reduced, implying that the strongly inverted state changes into a less depleted state, whilst, finally, the AC SPV disappears. In this case, the collapse of the (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks is anticipated, with a possible change into Cr₂O₃. The existence of the (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks has also been confirmed in p-type Si wafers.     

Structure and electrical characterization of amorphous ErSiO films deposited by rf magnetron sputtering on Si (001)

Amorphous ErSiO films have been fabricated on p-type Si (001) substrates using rf magnetron sputtering technique. X-ray diffraction, high-resolution transmission electron microscopy, and atomic force microscopy were employed to investigate the samples. It is found that ErSiO film exhibits a flat surface, a sharp interface and superior electrical properties after post-deposition annealing in O₂ ambience for 30 min at 450°C. The effective dielectric constant of the film is measured to be 14.2, and the effective oxide thickness reaches 1.9 nm, with a low leakage current density of 1.1×10⁻⁴ A/cm² at an electric field of 1 MV cm⁻¹ after annealing at 450°C. The obtained characteristics make the amorphous ErSiO films a promising substitute for SiO₂ as a high-k gate dielectric.     

Crystallization-water-stimulated dynamic conductivity

In water-molecule-doped barium peroxide and barium oxide, a step increase in the dynamic conductivity to ∼10⁻³ Ω⁻¹ cm⁻¹ was found. The increase is observed when water molecules are present in two nonequivalent states in the lattice, with concentrations of the molecules n _t of ≥2.2×10²¹ cm⁻³. At n>n _t , the conductivity does not depend on the number of molecules in the lattice but is temperature-dependent, obeying the law σ(T = C ₁exp(−E ₁/kT) + C ₂exp(−E ₂/kT. The run of the σ(n, T) curve is explained by trapping electrons that result from H₂O dissociation and by two sorts of carrier jumps between localized and delocalized states.     

Growth of tin oxide nanotubes by aerial carbothermal evaporation

One-dimensional nanostructures of tin oxide nanotubes were fabricated by carbothermal evaporation at 900°C in air. The synthesized film was grown on Au-coated (100) Si substrate. Heterogeneous catalysis by Au/Sn droplets assisted the formation of the tin oxide nanotubes of less than 40 nm diameter at Sn vapor pressures around 1.4×10⁻⁷ Pa. In order to reduce the nanotube diameter further, an increase in the Sn vapor pressure by changing the source materials’ ratio seemed viable.     

Passivation of defect states in Si-based and GaAs structures

Formation of defect states on semiconductor surfaces, at its interfaces with thin films and in semiconductor volumes is usually predetermined by such parameters as semiconductor growth process, surface treatment procedures, passivation, thin film growth kinetics, etc. This paper presents relation between processes leading to formation of defect states and their passivation in Si and GaAs related semiconductors and structures. Special focus is on oxidation kinetics of yttrium stabilized zirconium/SiO₂/Si and Sm/GaAs structures. Plasma anodic oxidation of yttrium stabilized zirconium based structures reduced size of polycrystalline silicon blocks localised at thin film/Si interface. Samarium deposited before oxidation on GaAs surface led to elimination of EL2 and/or ELO defects in MOS structures. Consequently, results of successful passivation of deep traps of interface region by CN⁻ atomic group using HCN solutions on oxynitride/Si and double oxide layer/Si structures are presented and discussed. By our knowledge, we are presenting for the first time the utilization of X-ray reflectivity method for determination of both density of SiO₂ based multilayer structure and corresponding roughnesses (interfaces and surfaces), respectively.     

The time evolution of the sol-gel process:29Si NMR study of the hydrolysis and condensation reactions of tetraethoxysilane

The structure and properties of the glass products prepared by the sol-gel process are strongly dependent on the experimental conditions of their preparation. The time dependence of the structural intermediate formation obtained during hydrolysis and condensation reactions in tetraethoxysilane, ethanol, H₂O, HCl (molar ratio 1:3:1.5:0.01) sol-gel reaction mixture was investigated with²⁹Si nuclear magnetic resonance spectroscopy at −75°C. It is clear that the substitution of the Si ethoxy group (OCH₂CH₃) by a hydroxyl (OH) shifts the resonance lines downfield. The dimer peaks are located about 9–10 ppm upfield from the corresponding monomers, and Si located in the middle of the trimers appeared at the chemical shift of 18–19 ppm from the corresponding monomers. The peaks of cyclic trimers, cyclic tetramers and the other higher oligomers are relatively broad and weak and exhibit a multiplet structure. The time dependence of the concentrations of the individual types of silicon in the reaction mixture was evaluated from the integral intensity of the corresponding Si atoms from the²⁹Si NMR spectra. The results obtained represent an input data to the various kinetic models of the complicated hydrolysis and condensation reactions.