Role of charged defects in the photoconductivity of Se95As5 chalcogenide glassy semiconductor with the EuF3 impurity

The energy spectrum of local states associated with charged defects D ^? and D ⁺ playing a significant role in carrier generation and recombination in the chalcogenide glassy semiconductor system Se₉₅As₅ containing EuF₃ impurities is proposed based on the study of the temperature dependence of the dark conductivity and steady-state photoconductivity, current-luminance characteristic, and the spectral distribution of the photocurrent. It is shown that EuF₃ impurities nonmonotonically change the concentrations of these states. Low concentrations form chemical compounds with selenium and arsenic due to the chemical activity of the rare-earth element and fluorine ions, which result in a decrease in the concentration of initial intrinsic defects. High concentrations, according to the charged-defect model, lead to a decrease in the concentration of D ⁺ centers and an increase in the concentration of D ^? centers due to the presence of Eu³⁺ ions. Some parameters of the charged-defect model are estimated, in particular the effective correlation energy U _eff (0.6 eV) and the polaron relaxation energy (W ⁺ = 0.4 eV, W ^? = 0.45 eV).     

Conductivity of Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals in high electric fields

The effect of electric field and temperature on the conductivity of bulk Hg₃In₂Te₆ crystals is investigated. It is shown that the I–V characteristics in high electric fields are of the S type with the effect of switching into a low-resistance state. The critical voltage of transition from the Ohm law to the exponential dependence of the current (I) on the voltage (U) and the threshold voltage of transition into the region of negative differential resistance dU/dI = s< 0 linearly depend on the sample thickness. The activation energies of conductivity in low and high electric fields are determined. It is established that the superlinear portion of the I–V characteristic with dU/dI > 0 is described by the dependence of the type I = I ₀ exp(U/U ₀) and caused by the electron transitions from the local centers with the energy level E _t = 0.19 eV.     

Superconducting properties of silicon nanostructures

Superconducting properties of silicon sandwich nanostructures on the n-Si (100) surface, which represent the ultra-narrow p-type silicon quantum wells confined by heavily boron-doped δ barriers, manifest themselves in the measurements of the temperature and field dependences of resistivity, thermopower, heat capacity, and static magnetic susceptibility. The cyclotron-resonance, scanning-tunneling-microscopy, and ESR data identify the presence of the single trigonal negative-U dipole boron centers in nanostructured δ barriers B ⁺-,B ^?, which are formed due to the reconstruction of shallow boron acceptors, 2B ⁰ ? B ⁺ + B ^?. The obtained results indicate that these negative-U centers are responsible for the transport of small-radius hole bipolarons, which is likely the basis of the mechanism of high-temperature superconductivity with T _C = 145 K. The superconductor-gap value of 0.044 eV determined from the measurements of the critical temperature using the above techniques is almost identical to the data on the tunneling spectroscopy and direct record of tunneling I–V characteristics. The quantization of the superconductive characteristics for silicon sandwich nanostructures manifests itself in the temperature and field dependences of the heat capacity and static magnetic susceptibility, which show the oscillations of the second critical field and critical temperature arising due to the supercurrent quantization.     

On the detection of U − centers in g-As2Se3 films by thermal cycling measurements of electrical conductivity

Pinning of the Fermi level near the midgap is one of the basic properties of chalcogenide glassy semiconductors. It is assumed in most models that the pinning is due to the presence of charged defects (U ^? centers) that strongly polarize the lattice and have energy levels close to the Fermi level. U ^? centers are detected in this study by the thermal cycling method, which made it possible to markedly extend the time during which the charge released by these centers is recorded. The results of measurements of the electrical conductivity of As₂Se₃ films in successive cycles of sample cooling and heating are presented. This cycling leads to the appearance and evolution of two discrete peaks. A conclusion is made that at least one of these peaks is associated with negatively charged U ^? centers with levels near the Fermi level.     

Influence of charged defects on detection of electron spin resonance in vitreous chalcogenide semiconductors

A relationship between the concentrations of induced spins and native U^? centers is established on the basis of taking electronic processes accompanying band-to-band excitation into account.     

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I₂ iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I₂ molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ～10¹⁸?10¹⁹ cm^?3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P _b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I₂ molecules.     

Increase in the rate and discretization of the kinetics of isothermal surface generation of minority charge carriers in metal-insulator-semiconductor structures with a planar-inhomogeneous insulator

Surface generation of minority charge carriers in silicon metal-oxide-semiconductor (MOS) structures is efficient only at the initial recombinationless stage. Quasi-equilibrium between surface generation centers and the minority-carrier band is established in a time t ～ 10^?5 s. In the absence of other carrier generation channels, an equilibrium inversion state at 300 K would need t = t_∞ > 10³ years to become established. In fact, the time t ∞ is much shorter, due to excess-carrier generation via centers located at the SiO₂/Si interface over the gate periphery. This edge-related generation can easily be simulated in an MOS structure with a single gate insulated from Si by oxide layers of various thicknesses. At gate depleting voltages V _g , the role of the periphery is played by a shallow potential well under a thicker oxide, and the current-generation kinetics becomes unconventional: two discrete steps are observed in the dependences I(t), and the duration and height of these steps depend on V _g . An analysis of the I(t) curves allows determination of the electric characteristics of the Si surface in the states of initial depletion (t = 0) and equilibrium inversion (t = t_∞), as well as the parameters of surface lag centers, including their energy and spatial distributions. The functionally specialized planar inhomogeneity of a gate insulator is a promising basis for dynamic sensors with integrating and threshold properties.     

Electric-field-induced modification of the optical properties of CuI crystals

The study is concerned with the effect of a dc electric field applied to copper iodide crystals on the resistivity of the crystals and their transparency and luminescence in the visible and ultraviolet spectral regions. It is shown that the conductivity, transparency, and edge luminescence intensity of the crystals decrease upon application of an electric field at room temperature with characteristic times of about 10 min. The reversibility of these processes, associated with the formation and diffusion of donor- and acceptor-type intrinsic defects (Cu _i and V _Cu) in the crystal lattice of the CuI compound, is established.     

Cooperative effects between arsenic and boron in silicon during simultaneous diffusions from ion implanted and chemical source predepositions

Simultaneous diffusions of As and B from predeposited layers (chemical source or ion implantation) have been used in order to fabricate the emitter and base regions, respectively, of microwave transistors. Mathematical simulations of the doping profiles in these transistors have shown that the cooperative diffusion effects that occur in sequentially diffused As-B structures (chemical sources) are noticeably absent in the predeposited-diffused structures. The result is that transistors that are fabricated via this technique show no base retardation, and the active base doping concentrations are higher than predicted by previously established diffusion equations. In order to determine the extent to which cooperative effects are important, transistor doping profiles were measured and compared with calculated profiles. By including the electric field interaction, the vacancy undersaturation condition due to [V_SiAs₂] complex formation, and ion pairing, it was possible to estimate the significance of each effect upon the B diffusion. It is shown that the electric field interaction is two- to three-times smaller in a predeposited-diffused structure than in a constant surface concentration diffusion (non-depleting source). More importantly, a negligible undersaturation of vacancies occurs during simultaneous As-B diffusions from predeposited layers. In the case of a chemical source As predeposition, this is due to the fact that a quasi-equilibrium concentration of [V_SiAs₂] complexes is achieved during the predeposition (before the simultaneous diffusion with B). In the case of an As implantation predeposition, complexes appear to be formed either during implantation or very rapidly during annealing for doses ? 3 × 10¹⁵ cm^?2. Data is presented which suggests that no inactive As complexes are formed in As implanted-annealed structures in which the maximum solubility of As⁺ ions is approached (3·8 × 10²⁰ atoms/cm³ at 1000°C, or a dose of 5–8 × 10¹⁵ cm^?2). This result pertains to the As dose used in this study. Since this result has not been observed in As-doped layers that were diffused from chemical sources, further work is needed in order to explain this anomaly.     

Origin of an absorption band peaked at 5560 cm−1 and related to divacancies in Si1−x Gex

It is found that two types of centers are formed in Si_1?x Ge_x single crystals as a result of irradiation with fast electrons: divacancies (V ₂) characteristic of silicon and the V ₂ ^* centers; the latter are complexes of divacancies V ₂ with germanium atoms (V ₂Ge). It is shown that an absorption band peaked at about 5560 cm^?1 is a superposition of two absorption bands that correspond to the above centers. The V ₂ divacancies diffuse during isochronous heat treatment and interact with germanium atoms, thus giving rise to additional V ₂ ^* centers. The latter have a higher thermal stability than the V ₂ centers do, and their annealing temperature increases with increasing content of germanium.     

Contact potential of p-Al0.5Ga0.5As/n-GaAs structures

The built-in potential of Al_0.5Ga_0.5As (10¹⁸ cm^?3) gates on n-type (10¹⁷ cm^?3) GaAs channel layers in the case of heterojunction normally-off FETs has been measured via C-V and forward I-V methods. A built-in potential of 1.39 eV from C-V measurements and 1.36 eV from forward I-V characteristics which compare well with the theory (1.4 eV) have been deduced. The ideality coefficient is found to be 1.56.     

Channel width effect for organic thin film transistors using TIPS-pentacene employed as a dopant of poly-triarylamine

The effects of the physical channel width on the characteristics of organic thin film transistors (OTFTs), made with 6,13-bis(triisopropyl-silylethynyl)-pentacene (TIPS-pentacene) embedded into poly-triarylamine (PTAA, hole conductor within an active channel), have been examined in this paper. The devices are estimated by measuring the drain-source current (I_DS) for different contact metals such as Au and Ag, at fixed gate and drain voltages. The results show that the threshold voltage (V_T) and I_DS increase with increasing channel width. Furthermore, it has been observed that the field effect mobility is dependent on V_T, which is influenced by the channel width. The OTFTs, produced using Au and Ag contacts, exhibited the highest values of mobility in the saturation regime, namely 5.44 × 10^?2 and 1.33 × 10^?2 cm²/Vs, respectively.     

Thermal capture of electrons and holes at zinc centers in silicon

The thermal capture rates of electrons and holes at zinc centers in silicon are obtained in reversed biased N⁺P and P⁺N diodes from junction capacitance transients due to capture of photogenerated and injected carriers. The electric field dependence of the thermal capture rates of holes at singly ionized zinc centers is E^?1·1, while that at doubly ionized zinc centers is exp(?E/E₀). The temperature dependence of the thermal capture rate of holes at doubly ionized zinc centers is small. The thermal capture rate of electrons at neutral zinc centers is 2·0×10^?9cm³/sec with very little field dependences, while that at singly ionized zinc centers is 5·0×10^?11cm³/sec with very little field and temperature dependence below 170°K. Auger-impact emission rate of electrons trapped at doubly ionized zinc centers by electrons is 3·5×10^?10cm³/sec with very little field and temperature dependence below 170°K.     

Energy parameters of two-electron tin centers in PbSe

Relations that make it possible to use an experimentally measured temperature dependence of carrier concentration to determine the Hubbard energy U and temperature dependence of the Fermi level F for two-electron tin centers in lead selenide are derived. A study of Pb_1?x?ySn_xNa_ySe solid solutions shows that their Fermi level in the temperature region 100–600 K lies below the valence band top E _v and that their F(T) dependences are linear, with extrapolation to T = 0 yielding E _V ?F = 210±10 meV. The Hubbard energy of the two-electron tin centers in PbSe is found to be U = ?80±20 meV.     

Realizing high breakdown voltage for a novel interface charges islands structure based on partial-SOI substrate

A novel interface charge islands partial-SOI (ICI PSOI) high voltage device with a silicon window under the source and its mechanism are studied in this paper. ICI PSOI is characterized by a series of equidistant high concentration n⁺-regions on the bottom interface of top silicon layer. On the condition of high-voltage blocking state, inversion holes located in the spacing of two n⁺-regions effectively enhance the electric field of the buried oxide layer (E_I) and reduce the electric field of the silicon layer (E_S), resulting in a high breakdown voltage (V_B). It is shown by the simulations that the enhanced field ΔE_I and reduced field ΔE_S by the accumulated holes reach to 449 V/μm and 24 V/μm, respectively, which makes V_B of ICI PSOI increase to 663 V from 266 V of the conventional PSOI on 5 μm silicon layer and 1 μm buried oxide layer with the same silicon window length. On-resistance of ICI PSOI is lower than that of the conventional PSOI. Moreover, self-heating-effect is alleviated by the silicon window in comparison with the conventional SOI at the same power of 1 mW/μm.     

Dependence of MOSFET noise parameters in n-channel MOSFETs on oxide thickness

The expression for the $\text{1}\text{f}$ noise resistance R_n of a MOSFET at low drain bias contains the factor I_d²/[g_m(V_g-V_T)]², which is unity in the elementary MOSFET theory but can be considerably larger than unity in practical units. The number fluctuation model characterizes R_n further by the parameter [N_T(E_f)]_eff/? and the mobility flucuation model introduces Hooge's parameter α. Measurements show that [N_T(E_f)]_eff/? varies as t^?2, α as t^?1 and the mobility μ varies as t, where t is the oxide thickness. The dependence of R_n upon T is chiefly determined by the t-dependence of the factor I_d²/[g_m(V_g-V_T)]². Since the drain noise spectrum S_Id(f) is practically independent of t, it is a very useful parameter for characterizing the noise. All data are now mutually consistent.     

Characterization of ZrTiO4 thin films prepared by sol–gel method

The electrical properties, memory switching behavior, and microstructures of ZrTiO₄ thin films prepared by sol–gel method at different annealing temperatures were investigated. All films exhibited ZrTiO₄ (111) and (101) orientations perpendicular to the substrate surface, and the grain size increased with increasing annealing temperature. A low leakage current density of 1.47×10^?6 A/cm² was obtained for the prepared films. The I–V characteristics of ZrTiO₄ capacitors can be explained in terms of ohmic conduction in the low electric field region and Schottky emission in the high electric field region. An on/off ratio of 10² was measured in our glass/ITO/ZrTiO₄/Pt structure with an annealing temperature of 600 °C. Considering the primary memory switching behavior of ZrTiO₄, ReRAM based on ZrTiO₄ shows promise for future nonvolatile memory applications.     

Dependence of carrier mobility on an electric field in gallium selenide crystals

The dependence of the mobility of charge carriers on voltage has been studied in undoped GaSe single crystals and crystals doped with gadolinium; the latter crystals have exhibited various values of dark resistivity (??_d.r ?? 10⁴?10⁸ ?? cm at 77 K) and of the doping level (N = 10^?5, 10^?4, 10^?3, 10^?2, and 10^?1 at %). It is established that the dependence of the charge-carrier mobility on the electric field applied to the sample E ?? 10² V/cm is observed in undoped high-resistivity GaSe crystals (??_d.r ?? 10⁴ ?? cm) and in lightly doped GaSe crystals (N ?? 10^?2 at %) in the region of T ?? 150 K. It is found that this dependence is not related to heating of the charge carriers by an electric field; rather, it is caused by elimination of drift barriers as a result of injection.     

Carrier transport in porous silicon

Carrier transport in porous silicon layers has been studied by the time-of-flight method in the strong injection mode at temperatures T=290–350 K and electric field strengths F=(1.5–7)×10⁴ V cm^?1. The electron and hole drift mobilities μ_e≈2×10^?3 cm² V^?1 s^?1 and μ_h≈6×10^?4 cm² V^?1 s^?1 were obtained at T=292 K and F=4×10⁴ V cm^?1. An exponential temperature dependence of drift mobility with activation energy of ～0.38 and ～0.41 eV for, respectively, electrons and holes was established. It is shown that the type of time dependences of the photocurrent associated with carrier drift and the superlinear dependence of the transit time on the reciprocal of the voltage applied to a sample allow use of the concept of space-charge-limited currents under the conditions of anomalous dispersive transport. The experimental data are accounted for in terms of the model of transport controlled by carrier trapping into localized states with energy distribution near the conduction and valence band edges described by an exponential function with a characteristic energy of ～0.03 eV.     

Nonmonotonic variations in the concentration of the donor-and acceptor-type radiation defects in silicon irradiated with low-intensity fluxes of β particles

Deep-level transient spectroscopy is used to study the dependence of the concentration of the donor-and acceptor-type radiation defects in silicon on the duration of irradiation with low-intensity fluxes of β particles (I ≈ 9 × 10⁵ cm^?2 s^?1). It is found that the concentrations of the defects C _i , C _i -C _s , and/or V-O in n-Si and the defects V-B, C _i -O _i , and/or V ₂-O-C in p-Si vary nonmonotonically.