Compensating defect centres in semi-insulating 6H-SiC

Photoinduced transient spectroscopy (PITS) has been applied to study electronic properties of point defects associated with charge compensation in semi-insulating (SI) 6H-SiC substrates. The photocurrent relaxation waveforms were digitally recorded in a wide temperature range of 20–800 K and in order to extract the parameters of defect centres, a two-dimensional analysis of the waveforms as a function of time and temperature has been implemented. As a result, the processes of thermal emission of charge carriers from defect centres were seen on the spectral surface as the folds, whose ridgelines depicted the temperature dependences of emission rate for detected defect centres. The new approach was used to compare the defect levels in vanadium-doped and vanadium-free (undoped) SI 6H-SiC wafers.     

Chromium(III) complexes as intermolecular probes

Metal ion complexes provide flexible paramagnetic centers that may be used to define intermolecular contacts in a variety of solution phase environments because both the charge and electronic relaxation properties of the complex may be varied. For most complex ions, there are several proton equilibria that may change the effective charge on the complex as a function of pH which in turn affects the efficacy of application for defining the electrostatic surfaces of co-solute molecules. We report here spectrophotometric and nuclear spin relaxation studies on aqueous solutions of chromium(III) complexes of EDTA, DTPA, and bis-amides of both. The effective charges available from these paramagnetic centers range from -3 to +1 and we report the pH ranges over which the effective charge is defined with confidence for application in magnetic relaxation experiments.     

Spin transport and relaxation in graphene

We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications.     

Dynamic-mechanical study of molecular motions in solid polyoxymethylene copolymers from 4 to 315°K

Variations in four mechanical relaxations in polyoxymethylene copolymers in the temperature range from 4 to 315°K are described as a function of comer size and amount. The 6 relaxation at 50°K is interpreted as being due to limited motions in defect crystals; the γ₂ relaxation, at ca. 175°K, is also associated with defect motion in the crystal; the γ₁ relaxation, at ca. 205°K, is suggested to involve motion of short segments in disordered regions; aad the β relaxation, at ca. 270°K, involving motion of long segments, can be considered to be the glass transition.     

Convergence properties of the supercell approach in the study of local defects in solids

The “supercell” scheme is applied to the study of local defects in MgO (Ca substitution, cation and anion vacancies) and bulk silicon (carbon substitution). The trend of the quantities of interest (defect formation energy, geometrical relaxation, charge distribution around the defect) as a function of the supercell size is explored; when neutral defects are considered, supercells containing 50 to 100 atoms are large enough to allow for most of the nuclear and electronic relaxation and to produce a negligible interaction between defects in different cells. These conclusions apply both to ionic and covalent host crystals. Present day ab initio quantum mechanical periodic computer programs can handle cells of such a size at a relatively low cost and high numerical accuracy.

When charged defects are considered (vacancies in MgO), the supercell scheme must be modified in order to avoid Coulomb divergencies, but the usually adopted correction, which consists in introducing a compensating uniform background of charge, generates spurious higher order electrostatic interactions, which are far from being negligible. The resulting defect formation energies show very slow, if any, convergence trends and “a posteriori” semiclassical corrections proposed in the literature do not represent a general solution to the problem. On the other hand, other properties, such as atom relaxation and charge distribution, show a much faster convergence than energy with respect to the cell size.     

DC conduction processes and electrical parameters of the organic semiconducting zinc phthalocyanine, ZnPc, thin films

Electronic conduction in thermally evaporated thin films of organic semiconductor zinc phthalocyanine (ZnPc) has been investigated in a broad temperature range using gold Ohmic contacts. Electronic conduction by charge carrier hopping was dominated at low temperatures and for all applied voltages. At higher temperatures and at voltages just below 2 V conduction was found to obey Ohm's law, while at higher voltages space-charge-limited conduction (SCLC) was the dominated mechanism, which was controlled by hole-trapping states distributed exponentially within the band gap.In freshly prepared samples adsorbed oxygen was responsible for lower hole mobility and higher charge carrier concentrations. Prolonged heating of ZnPc films at 425 K resulted in lower defect state density, and thus reduced trap concentration and higher charge carrier mobility.     

Ultrasonic attenuation in the Gd-doped superconductor LaAl2

The ultrasonic attenuation (10–250MHz) in (LaGd)Al₂ with 0.23at% Gd has been measured between 293 K and 0.5 K. Below 10K the absorption is caused by superposition of phonon/electron absorption of Pippard type and a mechanical relaxation process which is attributed to a lattice defect of either vacancy, dislocation or impurity type. The relaxation takes place by interaction of the defect with both conduction electrons and lattice phonons with a mixed relaxation time. Superconductivity offers the unique chance to separate both relaxation mechanisms since during superconducting transition relaxation into the electronic system vanishes and leaves the phonon process alone to determine the defect dynamics. The temperature dependence of the individual relaxation times has been derived.     

Specific features of photo-and electrical conduction in manganese germanium garnets

The specific features of photo-and electrical conduction in manganese germanium garnet crystals are investigated in the temperature range 4.2–370 K for the first time. Under exposure of samples with ohmic contacts to visible light, the photocurrent in these samples is observed only at high temperatures. The characteristic times of the photocurrent rise differ from those of photocurrent relaxation after the light is switched off. The inference is made that the photo-and electrical conduction is determined by the electrical recharging of manganese ions. The generation and transport of charge carriers are controlled by centers with electrical inhomogeneities and shallow attachment levels.     

Role of coherence in resistance quantization

The quantization of resistances in the quantum Hall effect and ballistic transport through quantum point contacts is compared with the quantization of the charge relaxation resistance of a coherent mesoscopic capacitor. While the former two require the existence of a perfectly transmitting channel, the charge relaxation resistance remains quantized for arbitrary backscattering. The quantum Hall effect and the quantum point contact require only local phase coherence. In contrast quantization of the charge relaxation resistance requires global phase coherence.     

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O⁺ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O⁺ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1491–1497 (August 1997)     

On anomalous drift mobility results in α-silicon alloys

Standard carrier drift experiments using a nano second pulsed laser have been performed on near micron thick α-Si alloys sandwiched between blocking metal and injecting contacts. Our results show that the transient response is only partially due to the drifting carriers. Another major contribution comes from the previously neglected time-dependent relaxation of the Schottky barrier space charge layer after excitation. The accompanying current resembles a slow dispersive drift of carriers. Consequently, generally accepted estimates for the drift mobility which ignored this extra current may therefore be unreliable.     

Effect of injecting electrodes on TSD behaviour of vacuum deposited PVF films

Depolarization behaviour of vacuum deposited polyvinyl fluoride (PVF) films has been studied as a function of nature of the electrode materials used during polarization such as copper, silver, aluminium and indium using the thermally stimulated discharge current (TSD) technique. TSD spectra of these films show a single relaxation peak centered around 430 ± 1 K with activation energies of ∼ 0.65 eV. The peak current and the charge associated with the relaxation peak depend strongly on the electrode material used. This has been attributed to the electrode-polymer interface barrier controlling the injection of the charge carriers into PVF films that results into space charge effects by subsequent trapping of the injected charge carriers at macroscopic distances. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Charge carrier concentration in glasses. dependence on composition

Admittance spectra of the ion conducting glasses xNa₂O(1−x)SiO₂ and xK₂O(1−x)SiO₂,(x=0,1–0,3) have been studied on small signal conditions from room temperature to 713 K. Conductivity relaxation in the bulk and space charge relaxation due to drift and diffusion near the electrodes were found in separated parts of the frequency range 10⁻⁴–10⁶ Hz. The data show Arrhenius behaviour for dc conductivity and conductivity relaxation. The determination of the charge carrier concentration is based on the analysis of the beginning of space charge relaxation. The free carrier concentration, n₀, were evaluated to be of the order of 10²³ m⁻³ at temperatures 400 – 700 K and show a weak dependence on composition. The dominant factor determining conductivity was mobility in these glasses. The strong temperature dependence of n₀ below 400 K indicates changed conditions for the development of the space charge relaxation which are discussed. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994.     

Ab initio study of the diffusion mechanisms of gallium in a silicon matrix

We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si using ab initio techniques. Five stable neutral configurations for single and multi-atom defects are identified by density-functional theory (DFT) calculations within the local density approximation and using a localized basis set as implemented in the SIESTA package. Formation energy (E _F ) calculations on these stable structures show the most likely neutral single-atom defect to be the Ga substitutional, with an E _F of 0.7 eV in good agreement with previous work. Charge state studies show the Ga tetrahedral interstitial defect to be in a + 1 state for most doping conditions. They also indicate the possibility for a gallium substitutional-tetrahedral interstitial complex to act as a deactivating center for the Ga dopants except in n-doped regime, where the complex adopts a − 1 charge state. Migration pathway calculations using SIESTA coupled with the activation relaxation technique (ART nouveau) allow us to determine possible migration paths from the stable configurations found, under various charge states. In general, diffusion barriers decrease as the charge state becomes more negative, suggesting that the presence of Si self-interstitials can enhance diffusion through the kicking out of substitutional Si and by adding negative charge carriers to the system. An overall picture of a possible Ga diffusion and complex formation mechanism is presented based on these results.     

Ni/4H-SiC 肖特基势垒磁场下输运性质的分析

采用高真空电子束蒸发的方法将镍 (Ni)淀积在 4H SiC(0 0 0 1)面上 ,制备出良好的Ni/4H SiC肖特基接触 .研究了Ni/4H SiC肖特基势垒在强磁场和低温下的I -V特性 ,并以热电子发射理论为基础 ,结合弛豫近似玻尔兹曼方程对Ni/4H SiC肖特基势垒在磁场下的输运性质进行了分析和计算 ,发现电流的变化与磁场的平方和电压成线性关系 ,和温度成反比关系 ,与实验结果基本符合     

Frequency scales for current statistics of mesoscopic conductors

We calculate the third cumulant of current in a chaotic cavity with contacts of arbitrary transparency as a function of frequency. Its frequency dependence drastically differs from that of the conventional noise. In addition to a dispersion at the inverse RC time characteristic of charge relaxation, it has a low-frequency dispersion at the inverse dwell time of electrons in the cavity. This effect is suppressed if both contacts have either large or small transparencies.     

A thermoluminescence study of defect centres in MgO

Thermoluminescence glow curves are a fairly good representation of the different defect centres in MgO produced by γ or reactor irradiation. The glow peaks at 370, 440, 485 and 545 K have been found to be due to hole trapped centres. The 370 K peak has been found to be due to holes released from V_M centres (hole trapped at action vacancy associated with a neighbouring impurity ion). The 665 K peak in γ-irradiated MgO is due to charge transfer between the adsobred oxygen ions and surface defect centres. The F-type centres produced on reactor irradiation (neutron and γ) and on quenching from high temperatures before γ-irradiation are also found to give thermoluminescence peaks at temperatures higher than 560 K. It has also been found that reactor irradiation partially annihilates the trapping centres which are responsible for lower temperature glow peaks.     

Effects of surface adhesion of nonequilibrium charge carriers in the photoconductivity spectra of CdS crystals

The relaxation of low-temperature (T = 77 K) edge photoconductivity (PC) spectra of CdS crystals of the first group1 caused by the removal of the transverse electric field is investigated. A clearly manifested inversion of the fine (exciton) structure is found in the spectra during their relaxation to the initial form (existing before the application of the field). It is shown that the observed relaxation changes in the PC spectra of CdS are due to relaxation of the nonequilibrium surface charge appearing as a result of trapping of a part of the charge, induced by the transverse field into the sample, by slow surface states (adhesion levels). The experimental data on the effect of preliminary illumination by self-radiation on the edge PC spectra of the first-group CdS crystals indicating the existence of surface adhesion levels with a high density in these crystals are presented.     

Proposal of a new physical model for Ohmic contacts

Ohmic contacts are crucial for both device applications and the study of fundamental physics. In this study, we propose a new physical model for Ohmic contacts based on the detailed considerations of a metal/semiconductor interface, such as charge neutrality level concept, with which it is possible to describe the real situation precisely. Our proposed model contains many defect energy levels that originate from vacancies and impurities located in the vicinity of the metal/semiconductor interface, within the energy range of the Schottky barrier height. Moreover, we calculate the current–voltage characteristics based on our model. Our calculated results show that our model reveals linear Ohmic I–V characteristics and dense defect energy level distribution in the energy range of the Schottky barrier height is crucial for obtaining Ohmic I–V characteristics.