Negative FIR-photoconductivity in n-GaAs

Negative photoconductivity in n-GaAs has been observed in the far infrared spectral range between 20 and 29 cm^–1. Negative photoconductivity occurs when a magnetic field is applied to the samples and impact ionization of shallow donors by the electric bias field is the dominant mechanism of electron excitation to the conduction band. A conceivable model qualitatively explaining the experimental results is proposed, which involves optical transitions from the lowest Landau subband to higher bound states of shallow donors.     

Impact ionization induced negative far-infrared photoconductivity inn-GaAs

Far-infrared photoconductivity ofn-GaAs epitaxial layers showing impact ionization breakdown has been investigated by molecular lasers at photon energies below the 1s-2p shallow donor transition energy. Negative photoconductivity was observed if a magnetic field was applied to the crystals and if impact ionization of donors by the electric bias field was the dominant electron excitation mechanism. The experimental results are qualitatively explained on the basis of the generation-recombination kinetics of electrons bound to donors. Negative photoconductivity is attributed to optically induced free to bound transitions of electrons from theN=0 Landau band to donor levels shifted by the magnetic field above the low energy edge of the conduction band.     

Space charge effects on radiative ultrashort laser-plasma interactions: Relativistic fluid model

Ultrashort laser-gas interaction is a promising candidate for the intense broad band far-infrared radiation in which the gas ionization and the resultant plasma formation occur consequently. The electron current produced in the process is the most important influential parameter which affects the far-infrared radiation generation. Although the interacting forces of the process are the space charge electric and the laser electromagnetic forces, the effect of the former one, has not been investigated on the gas-plasma THz generation. It is noteworthy that the space charge electric force, due to its effect on the electron distribution, has potential influence on the produced electron current and its consequent emission. Here, a 2D relativistic fluid model is presented in which the ions and the resultant space charge field are incorporated. The model investigates the air ionization, electron-ion plasma formation, and the system's evolution, spatiotemporally. Moreover, as the model is based on the transient electron current, as the source for the electromagnetic pulse radiation, it gives the temporal profile of the radiated field in which the space charge effects are observable. Our results show that the space charge field affects the electron velocity and its resultant current. Therefore, the temporal profiles and amplitudes of the radiated field components are affected and their resemblance to the experimental data is enhanced. The results indicate that the amplitude of the radiated field increases in the presence of the space charge field. In addition, it is shown that the space charge effects become more pronounced with the laser intensity.     

Far-infrared photoconductivity spectra of quenched-in acceptors in Germanium: absorption and photo-thermal ionization lines of two new shallow acceptors

The far-infrared photoconductivity spectra of germanium crystals quenched from T>800C show three new sets of hydrogenic acceptor excitation lines, in addition to the 8.69 meV (SA^'₁) and 9.48 meV (SA^“₁) acceptor series reported earlier. The corresponding ground state binding energies are 13.89 meV (SA^'₂), 14.42 meV (SA^“₂), and 17.89 meV (SA₃). We tentatively attribute both the SA^'₂ and the SA^“₂ line series to one acceptor complex with a multiple ground state. At high acceptor concentrations the excitation lines are seen as sharp minima superimposed on a continuous photoconductivity background. With decreasing acceptor concentration, or increasing temperature, they gradually evolve into the familiar positive-going photo-thermal ionization peaks. This is explained by the different concentration and temperature dependence of the cross-sections for the absorption and photo-thermal ionization mechanisms.     

Deep impurity-center ionization by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E) ∝ exp(E ²/E _c ² ) (where E is the wave field, and E _c is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole-Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant. Fiz. Tverd. Tela (St. Petersburg) 39, 1905–1932 (November 1997)     

The kinetics of electron transitions for donors with excited states

A new theory of the kinetics of non-equilibrium phenomena involving shallow donors with excited levels is presented, applicable to experiments on photoconductivity, impact ionization, current-controlled negative resistance, and the like. For the steady-state case the solution obtained is rigorous. For the transient case a rigorous solution would not in general be feasible, but it is shown that under certain conditions of sample purity it reduces to a trivial modification of the steady-state solution. Numerical results are given for the requisite region of sample purity.     

Influence of electric current and magnetic field on terahertz photoconductivity in Pb1 − x Sn x Te(In)

Photoconductivity of Pb_{1 ? x} Sn _x Te(In) solid solutions in the terahertz spectral range is defined by a new type of local electron states linked to the quasi-Fermi level. The paper deals with investigation of the influence of electric current and magnetic field on the amplitude of the terahertz photoconductivity in Pb_{1 ? x} Sn _x Te(In) alloys of different composition. It is shown that the density of local electron states responsible for the positive persistent photoconductivity decreases with increasing electric current via a sample, as well as with transition to the hole conductivity in samples with a high content of tin telluride (x > 0.26). It is found that the magnetic field dependence of the positive photoconductivity is non-monotonous and has a maximum. The maximum position in magnetic field is proportional to the terahertz radiation quantum energy. Mechanisms responsible for the effects observed are discussed.     

Spectral photoresistive effect of the field in CdS crystals at low temperatures

The influence of the surface electric field on the low-temperature (T=77 K) photoconductivity spectra of CdS crystals in the region of exciton and interband transitions is experimentally studied by the field-effect method. The photoconductivity spectra of a semiconductor are numerically calculated in the framework of a model allowing for the dependence of the surface recombination rate of nonequilibrium charge carriers on the surface electric field. It is demonstrated that the surface electric field plays a decisive role in the formation of the fine spectral structure associated with the excitons. A correlation between the type of fine structure and the surface bending of the energy bands is revealed. It is shown that the surface electric field can be evaluated from the shape of the low-temperature photoconductivity spectrum of the semiconductor.     

Isolated A+ states in boron doped silicon under high compressive stress

Long-wavelength photoconductivity spectra of isolated A⁺ states in B-doped Si under uniaxial [111] or [100] compressive stress have been studied at 1.5 K. Besides, far-infrared photoconductivity and adsorption spectra of acceptor states in the crystal have been measured to obtain the experimental ionization energy of the acceptor. The binding energy of the second hole in the A⁺ center is rather close to, and a tittle less than, the theoretical value of 0.055 of the effective Rydberg obtained with the effective mass approximation.     

基于里德堡原子的电场测量

里德堡原子具有大的极化率、低的场电离阈值和大的电偶极矩,对外部电磁场十分敏感,可以用来测量电场强度特别是微波电场的强度. 利用里德堡原子的量子干涉效应(电磁诱导透明和Autler-Townes效应)测量微波电场强度的灵敏度远高于传统采用偶极天线测量微波电场的灵敏度. 此外,里德堡原子电场计 可以溯源到标准物理量,不需要额外校准; 采用玻璃探头,对待测电场干扰少; 灵敏度也不依赖于探头的物理尺寸. 同时,该电场计还可以实现对微波电场的偏振方向的测量, 实现亚波长和近场区域电场成像与测量. 通过选择不同的里德堡能级,可以实现1-500 GHz超宽频段范围内微波电场强度的测量. 主要综述基于里德堡原子的电场精密测量研究, 详细介绍了里德堡原子电场计的原理与实验进展, 并简单讨论了其发展方向.     

High-temperature field evaporation and its connection with surface ionization

High-temperature field evaporation of metals and alloys and its connection with surface ionization are considered. The main parameters of the evaporation process (dependence of the evaporation rate on the emitter temperature and on the electric field at the emitter surface, the charge of the ions being evaporated and its temperature dependence, kinetic parameters of the evaporation process, as well as the state of the emitter surface under simultaneous action of high fields and temperatures) are analyzed. The similarity and the difference between field evaporation at high temperatures and surface ionization in a strong electric field are determined.     

Photoconductivity of erbium-doped germanium

We have applied the technique of Photo Thermal Ionization Spectroscopy (PTIS) to the study of an erbium-doped p-Ge epitaxial layer, grown by MBE on an undoped n-type germanium substrate. The Er-doped Ge layer shows continuum photoconductivity response in the far-infrared region extending from 70 cm^–1 to 900 cm^–1. This type of epitaxial Er-doped Ge layers is a potentially attractive system for photoconductivity detectors of far-infrared radiation. Below 900 cm^–1 three acceptor-like charged states can be distinguished with ionization energies of 9, 26.6 and 50 meV. Additionally, a study of the photoconductive response of the same sample for radiation from 1000 cm^–1 to 10000 cm^–1, i.e., for radiation energies well inside the forbidden gap to energies above it, shows a wealth of levels, some of which have previously been associated with erbium.On leave from: Instituto de Física, Universidad Autónoma de Puebla, Puebla, México     

电场中Eu原子电离阈移动的实验研究

提出了一种新方法, 精确确定了稀土Eu原子第一电离阈的位置. 先用脉冲电场对Eu原子的高激发Rydberg态进行延时场电离探测, 再通过反向静电场排除光电离和自电离等其他路径所产生的离子信号的干扰, 观察并研究了Eu原子第一电离阈随着电场移动的规律. 由此所确定的零场下第一电离阈的数值与采用其他方法所确定的文献值^{[1, 2]}相一致, 从而验证了该方法的可靠性.     

High power splitting of the cyclotron resonance induced photoconductivity in n-GaAs

Conclusion In summary our results show that photoconductivity and optical absorption under cyclotron resonance conditions in high purity n-GaAs are more complex than it has been assumed previously. As cyclotron resonance can only be measured with non zero free electron concentration, and because the 2p _– shallow donor level remains below the N=0 Landau level for all magnetic field strengths, a certain population of 2p _– states must be present and thus the interference between both absorption processes is unavoidable as long as 2p _–2p ₊ electric dipole transitions are activated by ionized impuritics. This interference gets most drastically manifest at high intensities causing an apparent splitting of the photoconductivity line. Even at lower intensities however when the dip in the photoconductivity line is not observable, all optical characteristics previously attributed to cyclotron resonance are affected by the shallow donor absorption. The energy separationE _2p+–E _2p– of shallow donors in a magnetic field is exactly equal to w _CR only for isolated impurities in the effective mass approximation. The same electric stray field of ionized impurities which cause the activation of 2p _–2p ₊ absorption may shift the donor energy levels due to stark effect. For magnetic field strengthsB<1 T a field dependent deviation ofE _{2 +}–E _{1p –} from the cyclotron resonance quantum energy was observed [15] amounting up to 8 pc. In this case we expect that the peak positions of photoconductivity and absorption do not spectrally coincide and do not occur at the resonance magnetic field strength of 01 Landau level transitions. Thus effective masses determined by standard cyclotron resonance methods at low magnetic fields may be incorrect by a few percent.     

Discrepancy between photoluminescence and photoresponse intensities in n-InAs/GaAs and InAs/n-GaAs quantum-dot heterostructures

We report a discrepancy between near-infrared photoluminescence (PL) and far-infrared photoresponse (PR) efficiencies in self-assembled InAs/GaAs quantum-dot (QD) heterostructures with silicon doping in either InAs QDs or GaAs barriers. The structure with n-GaAs barriers reveals a much higher PR intensity in spite of a weaker PL intensity in comparison with n-InAs QD structure. This discrepancy is explained by differences in the electron occupation of QD sublevel associated with the Fermi-level position and in the mean free path of photogenerated carriers in GaAs barriers due to impurity scattering.     

Ionization of an air-methane mixture in a near-critical electric field

The process of effective ionization at the early stages of development of an electric discharge in methane-air mixtures at moderate and high pressures is analyzed. The ionization in an external electric field with consideration given to background electrons, electron attachment to and detachment from atoms and oxygen molecule, recharging, and anion-neutral electron exchange is considered. The dependences of the rates of ion-molecular processes on the temperature and electric field strength are examined.     

Impurity photoconductivity in epitaxial layers of gallium arsenide

Impurity photoconductivity spectra in the range 0.5–1.5 eV are studied in epitaxial layers of n-GaAs grown on substrates of semiconductive GaAs withρ > 10⁶ Ω·cm in the system Ga-AsCl₃ -H₂. The effect of uncontrolled acceptor impurities on the impurity photoconductivity spectrum is evaluated.     

Optical, electrical, and photophysical properties of films of polycomplexes of azobenzene derivatives with cobalt

The absorption spectra, photoconductivity, and electrical conductivity of films of new polycomplexes of 4-methacyloyloxy-(4′-carboxy-3′-oxy)azobenzene and 4-methacryloyloxy-2-nitro-(4′-carboxy-3′-oxy)azobenzene with cobalt were investigated with switched-off and switched-on external electric fields. The effect of an external electric field on the transmission of linearly polarized light through films near the long-wavelength absorption edge is revealed. It is shown that the change in the light transmission under the action of an external electric field is due to the spatial reorientation of the dipole moments of azobenzene groups photoinduced by polarized light in an external electric field. The effect of an electric field on the light transmission is explained by the formation of forces acting on the dipole moments of azobenzene groups and on metal ions. The increase in the dipole moments of azobenzene groups as a result of introduction of nitro groups into their composition decreases the effect of cobalt ions on the electro-optical properties of polycomplex films.     

Air ionization in a near-critical electric field

A set of experimental dependences of the air ionization effective rate on the electric field strength is presented. The concept of the critical breakdown field is discussed. It is indicated that experimental data are quantitatively inconsistent with analytical results based on this concept. This inconsistency is eliminated if the ionization balance takes into account not only dissociative adhesion of electrons to oxygen molecules but also their detachment from the molecules that gained a charge during the charge exchange process. Based on the results obtained, a new physical interpretation of the critical field is suggested. A formula for the effective rate of air ionization in near-critical fields is derived.     

一种新的GaAs PHEMT器件可靠性评估方法研究

通过研究应力前后GaAs PHEMT器件电特性的测量，分析了GaAs PHEMT退化的原因，从实验中 得出高场下碰撞电离的电离率与器件沟道电场峰值的关系曲线.对高场下碰撞电离率的实验 曲线进行拟合，可以得到碰撞电离率与器件沟道电场峰值的量化关系，由此可以对GaAs PHE MT器件的电性能和可靠性进行改善和评估.进一步改进GaAs PHEMT的击穿电压，将需要严格 控制沟道中的碰撞电离. 关键词： 高电子迁移率晶体管 碰撞电离率 可靠性评估