Electric field effect on the mechanisms of recombination in KI doped with divalent ions

In KI crystals doped with divalent ions (Eu²⁺, Sr²⁺, Mn²⁺) a strong influence of the electric field is observed, after irradiation, on the carriers (electrons and holes) recombination kinetics. The phenomena are similar whether the electrons, distributed on traps bound to divalent ions, are excited by IR at 4 K, and recombine with trapped holes (V_k centers) or whether the holes are made thermally mobile at T>77 K. It is suggested that this is due to the recombination mechanism: the kinetics are simultaneously controlled by diffusion and tunneling. The tunneling range is a function of the applied field.     

A Tunneling Model for Afterglow Suppression in CsI:Tl,Sm Scintillation Materials

Combined radioluminescence, afterglow and thermoluminescence experiments on single-crystal samples of co-doped CsI:Tl,Sm suggest that samarium electron traps scavenge electrons from thallium traps and that electrons subsequently released by samarium recombine non-radiatively with trapped holes, thus suppressing afterglow. Experiments on single crystals support the inference that electrons tunnel freely between samarium ions and are trapped preferentially as substitutional Sm⁺ near V_KA(Tl⁺) centers where non-radiative recombination is the rate-limiting step. Afterglow in microcolumnar films of CsI:Tl,Sm is enhanced by inhomogeneities which impede tunneling between samarium ions, but is partly suppressed by annealing.     

A wire trap for neutral atoms

We present new ways of trapping a neutral atom with static electric and magnetic fields. We discuss the interaction of a neutral atom with the magnetic field of a current carrying wire and the electric field of a charged wire. Atoms can be trapped by the 1/r magnetic field of a current-carrying wire in a two-dimensional trap. The atoms move in Kepler-like orbits around the wire and angular momentum prevents them from being absorbed at the wire. Trapping was demonstrated in an experiment by guiding atoms along a 1 m long current-carrying wire. Stable traps using the interaction of a polarizable atom with the electric field of a charged wire alone are not possible because of the 1/r ² form of the interaction potential. Nevertheless, we show that one can build a microscopic trap with a combination of a magnetic field generated by a current in a straight wire and the static electric field generated by a concentric charged ring which provides the longitudinal confinement. In all of these traps, the neutral atoms are trapped in a region of maximal field, in theirhigh-field seeking state.Dedicated to H. Walther on the occasion of his 60th birthday     

动态应力下超薄栅氧化层经时击穿的可靠性评价

实验发现动态电压应力条件下,由于栅氧化层很薄,高电平应力时间内隧穿入氧化层的电子与陷落在氧化层中的空穴复合产生中性电子陷阱,中性电子陷阱辅助电子隧穿.由于每个周期的高电平时间较短(远远低于电荷的复合时间),隧穿到氧化层的电子很少,同时低电平应力时间内一部分电荷退陷,形成的中性电子陷阱更少.随着应力时间的累积,中性电子陷阱达到某个临界值,栅氧化层突然击穿.高电平时形成的陷阱较少和低电平时一部分电荷退陷,使得器件的寿命提高. 关键词： 超薄栅氧化层 斜坡电压 经时击穿     

Loss estimation of electrostatically trapped ND3 molecules

Losses of electrostatically trapped molecules are caused by the Majorana transition or the inelastic collisions between trapped molecules. The loss rate of electrostatically trapped ND₃ molecules in the (J=1,K=1,M=1, A) state was estimated: ND₃ molecules in this state have actually been trapped. When trapping cold molecules, using ¹⁵ND₃ molecules (fermion) with a minimum electric field higher than 10 kV/cm yields high stability.     

EPR,thermoluminescence, and thermally stimulated conductivity in BaWO4 crystals

In BaWO₄ crystals electrons and holes trapped at WO₄ complexes are identified by electron paramagnetic resonance (EPR) after X-irradiation at T = 80 K. The thermal decay of the intrinsic hole centres at about 100 K is accompanied by a simultaneous decrease of electron traps (WO₄)^3- and glow maxima of thermoluminescence (TL) and of thermally stimulated conductivity (TSC). This connection is explained by a thermally activated hopping of the $\text{(WO}{}_4\text{)}\text{3-}\text{2}$ hole centres followed by radiative recombination with electron traps (WO₄)^3-. A qualitative kinetic calculation based upon EPR data and the shift between TL and TSC glow peak confirms this model.     

聚合物材料空间电荷陷阱模型及参数

采用电声脉冲测量技术研究了直流电场下低密度聚乙烯材料的电荷入陷和脱陷特征. 发现在不同电场周期下样品的电荷衰减呈现不同的特征, 为此提出了一个简单的基于两陷阱水平的入陷和脱陷模型, 并计算了相应的参数, 如陷阱能级和密度. 确定了不含任何添加剂的低密度聚乙烯样品中存在的两种水平的陷阱能级分别为: 较浅陷阱能级0.77–0.81 eV 对应的浅陷阱电荷密度为(1.168–1.553)× 10¹⁹ m^-3; 较深陷阱能级0.96–1.01 eV 对应的深陷阱电荷密度为(1.194–4.615)× 10¹⁸ m^-3. 最后初步验证了材料的深陷阱能级和对应的深陷阱电荷密度随老化而增加, 可考虑将模型中的两能级陷阱参数作为老化诊断特征参量. 关键词： 聚合物 空间电荷 陷阱 老化     

Analysis of the charge transfer mechanisms responsible for the current-voltage characteristics of Ca<Subscript>4</Subscript>Ga<Subscript>2</Subscript>S<Subscript>7</Subscript>: Eu<Superscript>3+</Superscript> single crystals

The current-voltage characteristics of Ca₄Ga₂S₇: Eu³⁺ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca₄Ga₂S₇: Eu³⁺ single crystals are high-resistance semiconductors with a resistivity of ～10⁹ Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×10¹⁴ to 2.7×10¹⁵ cm^?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×10³ V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×10⁴ V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level.     

Structure of paramagnetic defects in BaTiO3 and their light induced charge conversion

Abstract

We report the ESR of Nd³⁺ in BaTiO₃ and derive the electronic structure of its groundstate. – Among the light induced charge conversions found with ESR in nominally undoped BaTiO₃ crystals hole photoionization of Fe⁴⁺ is the most important. The process is increased by oxidizing the crystals. Some of the holes are trapped as O^? next to an unidentified acceptor. The trapping site of the remaining holes is ESR-silent.     

Low-temperature photoluminescence of MBE-grown GaAs subject to an electric field

Low-temperature photoluminescence of GaAs has been investigated in MBE-grown Al _x Ga_1–x As-GaAs single heterojunctions subject to an electric field. No peak energy shift is observed in the emission lines due to free excitons and excitons bound to isolated centers when the electric field is applied. In contrast, the excitonic lines arising from the previously described defect-induced bound exciton (DIBX) transitions exhibit a prominent low-energy shift when the electric field is increased. We attribute these lines to excitons bound to acceptor pairs. The excitons bound to distant pairs have smaller binding energies than those bound to closer pairs. They are, therefore, easily dissociated in a weak electric field. The electrons and holes thus dissociated may again be trapped by closer pairs, which results in a low-energy shift of the overall spectrum. The photocurrent measured as a function of the electric field supports Dingle's rule for the valence bandedge discontinuity.     

Hole traps and thermoluminescence in Li2B4O7:Be

A trapped-hole centre with a high thermal stability is studied in Li₂B₄O₇:Be. EPR spectrum demonstrates a hyperfine splitting due to interaction of the trapped hole with a ⁹Be nucleus. The hole is trapped at the common vertex (bridging oxygen) of two adjacent BO₄ tetrahedra when Be²⁺ substitutes for B³⁺ in one of them. The centre is stable up to ≈500 K. The high-temperature thermoluminescence peak at 510–575 K occurs when the released holes recombine with trapped electrons. This peak shows an ultraviolet exciton-like luminescence band and also luminescence bands of casual impurities.     

Radiation-induced electrical and optical processes in materials based on Al2O3

The irradiation of dielectrics induces electric charging of microscopic regions in the bulk, which is associated with concentration inhomogeneities in the system of traps and the differences in characteristic diffusion lengths of free electrons and holes that are produced in ion tracks and collision cascades. Experimental data on radiation-induced luminescence (RIL) give evidence of the existence of three states of oxygen vacancies in Al₂O₃: an optically inactive (electrically neutral) vacancy, its excited state (known as F ⁺ center), and a negatively charged vacancy (F center). The formation of negatively charged regions under irradiation increases the intensity of the 415-nm band of F centers of RIL of Al₂O₃ single crystals. In Al₂O₃:Cr³⁺ ceramics, a radiation-induced negative charging of grain boundaries with respect to the bulk of grains takes place, which manifests itself as an increase in the intensity of the 690-nm band of RIL of Cr³⁺ ions, whereas the intensity of this band in Al₂O₃:Cr³⁺ single crystals remains unchanged. Using data on RIL, the local-charge density in grains of Al₂O₃:Cr³⁺ ceramics and the field produced by this charge are estimated.     

Modelling the optical bleaching of the thermoluminescence of K2YF5:Pr3+

Optical bleaching of the thermoluminescence (TL) curve of K₂YF₅:Pr³⁺ has been observed after optically stimulated luminescence (OSL) readout of pre-irradiated crystals. The traps being responsible for the TL signal are not emptied completely by the optical stimulation. Furthermore, if the illumination time is increased a constant intensity level of the residual TL glow curve is eventually achieved. On the other hand, if the low temperature peak of the glow curve is thermally cleaned, no subsequent OSL is measured. This behavior has been successfully explained by assuming that part of the electrons in the trap being responsible for the low temperature glow peak of K₂YF₅:Pr³⁺ recombine with holes via localized transitions during optical stimulation. During TL all trapped electrons recombine via delocalized transitions. Simulations have been carried out in order to demonstrate the feasibility of the model.     

Semi-analytic formulae of impedance spectroscopy in organic layers with Gaussian traps

A semi-analytic expression of impedance spectroscopy is derived as the mobility being a function of temperature, carrier density, and electric field. The formula is further extended to consider the trapped charges, series resistance to describe non-ideal contacts of polymer layers with electrodes, and parallel capacitance due to metallic electrodes. The numerical results are calculated and analyzed for two typical organic NRS-PPV and OC₁C₁₀-PPV diodes with parameters optimized by Pasveer et al. (Phys Rev Lett 94: 206601, 2005). It is shown that the shapes of impedance spectroscopy of both types of diodes are similar but very different in numerical amounts, and the influence of temperature and bias voltage to all curves is prominent. These tendencies have been qualitatively explained by that the mobility model of Pasveer et al. is an increasing function of temperature and electric field strength. The extended formula is applied to MEH-PPV, BDMOS-PPV, and AlQ₃ devices; the numerical results are in good agreement with experimental data and support the Gaussian DOS of traps.     

Charge-Transfer Processes in Doped Alkali Halides

Defects formation under UV-irradiation in the impurity-induced absorption bands at 4.2 K has been studied for crystals with and without traps for electrons (CsI:Pb and Eu^2?-doped alkali halides, respectively). In both cases the results have been explained by an electron transfer from the impurity-perturbed halogen ion states, resulting in the appearance of electrons and holes in the crystal. In CsI:Pb, the electrons are trapped by lead ions and the holes are self-trapped. In Eu^2?-doped crystals, the electrons and the holes recombine with the formation of excitons, whose decay results in the creation of Frenkel defects.     

Cr+ centres in mixed polytype ZnS single crystal

The glow peaks of mixed polytype ZnS: Cu, Cr, Cl crystals were identified as produced by three Cr⁺ centres observed by electron paramagnetic resonance (EPR): one cubic (C) and two axial (H₁ and H₂). The characteristics of these centres were partly found by the methods of interrupted thermoluminescence, infrared stimulation, “thermo-EPR” (variation of the EPR signal with heating of a previously excited crystal) and also by decay of glow areas and of the three Cr⁺EPR signals after excitation at different temperatures. Moreover, these measurements showed the important part played by the non-thermal emptying of the Cr⁺ traps. This emptying occurs through photoexcitation of the trapped electrons by the reabsorbed luminescence light or through recombination of these electrons with free holes. Both of these processes diminish the efficiency of phosphorescence when the crystal is cooled; so it was necessary to take them into account when analysing the experimental results. The percentage of H₁ and H₂ found by EPR and luminescence is in agreement with the percentage of the hexagonalicity as found from the birefringence measurements. The crystal field calculation gives account of three centres in such a mixed polytype crystal.     

A-188 V 7.2Ω·mm~2,P-channel high voltage device formed on an epitaxy-SIMOX substrate

This paper proposes a new n +-charge island (NCI) P-channel lateral double diffused metal-oxide semiconductor (LDMOS) based on silicon epitaxial separation by implantation oxygen (E-SIMOX) substrate.Higher concentration self-adapted holes resulting from a vertical electric field are located in the spacing of two neighbouring n +-regions on the interface of a buried oxide layer,and therefore the electric field of a dielectric buried layer (E I) is enhanced by these holes effectively,leading to an improved breakdown voltage (BV).The V B and E I of the NCI P-channel LDMOS increase to-188 V and 502.3 V/μm from 75 V and 82.2 V/μm of the conventional P-channel LDMOS with the same thicknesses SOI layer and the buried oxide layer,respectively.The influences of structure parameters on the proposed device characteristics are investigated by simulation.Moreover,compared with the conventional device,the proposed device exhibits low special on-resistance.     

Field-independence of thermal emission rate in Au-doped silicon

The thermal emission rate of holes, e^t_p, has been measured from dark leakage currents for the Au acceptor level in silicon diodes at different electric fields and found to be field independent for average electric fields below 10⁵V/cm.     

Copper as an electron trap in GaAs0.6P0.4

An electron trap having an energy level of 0.14 eV from the conduction band edge was found in the bulk of copper-diffused VPE-grown n-GaAs_0.6P_0.4 by conventional DLTS measurements and by pulse-duration dependent capacitance amplitude measurements. The capture cross section at room temperature is about 1.0×10^–21 cm² and has a weak temperature dependence. These properties are attributed to a non-repulsive center having a capturing mechanism which involves multiphonon emission processes with hardly any lattice relaxation. Evolution of the spatial distributions of the traps with time under junction electric field were studied. The results suggest that the trap is positively charged and has a high diffusivity under electric field. The center can thus be identified as positively charged interstitial copper ion rather than some form of copper complexes.     

Influence of an electric field on photostimulated states in NH4BPh4 films

The influence of an electric field on stable photostimulated triplet states of NH₄BPh₄ at a temperature of 77 K have been studied by EPR spectroscopy. It has been established that, on exposure to UV radiation, electron capture by traps in the band gaps takes place with formation of triplet state. After application of an electric field, triplet states are destructed because, with an increase in the applied voltage, a gradual inclination of energy bands takes place and electrons found in traps on different energy levels are released. The assumption that captured electrons are found in traps on different energy levels is confirmed by earlier studies of thermoluminescence spectra.