Modeling of X-ray excited luminescence intensity dependence on the nanoparticle size

The thermalization length distribution of electrons over their kinetic energy in a conduction band is calculated on the basis of the data on the electron effective mass, density of states in conduction band, dielectric permittivity and energy of longitudinal optical phonons. The method of modeling of a recombinational luminescence intensity dependence on the nanoparticle size is proposed on the basis of the assumption that the contribution to a recombinational luminescence gives only those charge carriers which in the result of thermalization did not reach a near-surface layer of nanoparticles. Using such the approach the theoretical dependence of recombinational luminescence intensity on the nanoparticle size for LaPO₄ and LuPO₄ are calculated. The revealed correlation of experimental and theoretical dependences confirms that the commensurability of electron thermalization length with nanoparticle size is the main reason of the sharp decrease of X-ray excited luminescence intensity when the nanoparticle size decreases.     

A study of core and valence levels in β-PbO2 by hard X-ray photoemission

The core and valence levels of β-PbO₂ have been studied using hard X-ray photoemission spectroscopy (hν = 6000 eV and 7700 eV). The Pb 4f core levels display an asymmetric lineshape which may be fitted with components associated with screened and unscreened final states. It is found that intrinsic final state screening is suppressed in the near-surface region. A shift in the O 1s binding energy due to recoil effects is observed under excitation at 7700 eV. It is shown that conduction band states have substantial 6s character and are selectively enhanced in hard X-ray photoemission spectra. However, the maximum amplitude in the Pb 6s partial density of states is found at the bottom of the valence band and the associated photoemission peak shows the most pronounced enhancement in intensity at high photon energy.     

Thermo-and photostimulated luminescence of CaI2: Tl and CaI2: Pb crystals

Thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb scintillation crystals under optical and X-ray excitation is studied. It is shown on the basis of the results obtained with account for the data of studies of photo-and X-ray-luminescent properties of these scintillators that Tl⁺ and Pb²⁺ ions form complex capture centers with host defects. These centers are responsible for the thermostimulated luminescence in the temperature range of 150–295 K, and the centers of charge carrier trapping are spatially separated from the centers of recombination emission. An assumption is made that thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb crystals under optical excitation is observed mainly due to the delocalization of charge carriers from hydrogen-containing centers responsible for the excitation band at 236 nm and the photoluminescence of CaI₂ with a maximum at 395 nm. The luminescence of CaI₂: Tl crystals in the 510-nm band and CaI₂: Pb crystals in the 530-nm band is determined by the radiative decay of near-activator excitons.     

压缩真空态的激发态及其在非耗散介观含源电路的应用

借助逆算符的性质,提出了压缩真空态激发态的概念,作为应用,导出了非耗散介观含源电路在该激发态下其电荷和电流的量子零点涨落,数值计算表明,在不同的激发量子数下,电荷和电流均存在压缩效应,它们的不确定度之积分别趋向于一个最小值。     

Pulsed nanosecond annealing of magnesium-implanted silicon

Single-crystalline silicon is implanted by magnesium ions at room temperature and then subjected to pulsed ion-beam annealing. The surface morphology, crystallinity, and optical properties of the implanted silicon are studied before and after annealing. It is shown that ion implantation makes a near-surface layer of silicon about 0.1 m thick amorphous. Pulsed nanosecond ion-beam annealing results in silicon recrystallization and the formation of crystalline magnesium silicide precipitates. Optimal values of the implantation dose and pulse energy density for the formation of magnesium silicide precipitates in the near-surface layer of silicon are found.     

Intensity-modulating characteristics of a laser diode subjected to optical injection

We have found that the optical power of a laser diode (LD) does not change with the injected light intensity that is modulated when its injection current is at some specific values. The amplitude of optical power change of the LD varies periodically with the increase of the injection current. It is made clear through theoretical analysis that these phenomena are caused by gain compression and interband carrier absorption of the LD that depend on longitudinal mode competition, bandgap-shrinkage effects, thermal conduction, and so on. Our experimental results make it easy to eliminate optical power change of LDs. We only need to choose a proper value of the injection current.     

Radiation-induced conductivity of alkali halide crystals in strong electric fields under X-ray and photoexcitation

A study is undertaken into radiation-induced conductivity of alkali halide crystals under X-ray excitation and sequential excitation with X-ray and laser pulses within the absorption band of F-and F ⁻-centers. The basic conduction parameters (concentration and lifetime of carriers upon X-ray and photoexcitation) are estimated. The possible processes responsible for the nonlinearity of the current-voltage characteristics are discussed. It is shown that an increase in the conductivity in strong electric fields may be due to a decrease in the spatial localization of electrons in the conduction band of the insulator.     

Luminescence and conduction charge in thin-film electroluminescent devices

The luminescence and conduction currents of a doubly-insulated thin-film electroluminescent device were studied under various excitation conditions. The conduction current waveform was calculated numerically from the luminescence waveform. It was composed of two components: a fast transient component j_fc, which rises and decays rapidly when a pulse is applied, and a dc-like component j_dc, which maintains a constant level during the duration of pulse. The ratio of these components varied considerably with applied voltage. Both the slow response of luminescence to voltage change and the change in j_fc when the excitation conditions were changed from repetitive pulses to a single pulse suggest an accumulation of charges inside the electroluminescence layer. This accumulation causes a space charge effect which explains the dominance of j_fc in the high-brightness region. The fact that luminescence intensity in this region is not related to the amplitude of individual pulses, but rather to the average amplitude of the pulse train also indicates that the space-charge-induced internal electric field is the main factor in accelerating the carriers which excite the luminescence centers. It was found that the two conduction current components showed similar characteristics in singly-insulated devices.     

Charge accumulation layer on the <Emphasis Type="Italic">n</Emphasis>-GaN (0001) surface with ultrathin Ba coatings

The adsorption of Ba on the n-type GaN(0001) surface is studied. It is found that submonolayer Ba coatings induce cardinal changes in the electronic properties of the surface with the formation of a charge accumulation layer in the region of the near-surface band bending. The excitation of the Ba/n-GaN system by light from the region of GaN transparency results in photoemission. The lowest value of the work function corresponds to ～1.90 eV at a Ba coverage of ～0.4 ML. Two surface bands induced by Ba adsorption are found in the surface photoemission spectra.     

Preparation and electronic structure of phase pure K3C60

Phase pure K₃C₆₀ films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.     

As(0001) surfaces: Auger,loss, and photoelectron spectroscopic studies

The (0001) surfaces of arsenic single crystals have been characterized by Auger, loss, and X-ray photoelectron spectroscopy. A comparison of the results leads to an unusual suggestion for excitation, conduction, and emission modes in the low-energy region. It is proposed that the dominant 20 eV near-elastic loss is the excitation of N₁(4s) electrons to a conduction level 2 eV above the instrumental vacuum level. It is also proposed that two low-energy “Auger-like” peaks at 12 eV and 1.5 eV are electrons emitted at the surface from permitted bands in the bulk. A strong correlation among Auger and loss signals, integrated secondary-electron emission, elastic peak heights and the Kikuchi display is also reported. A study of the Kikuchi correlation of the 12 eV peak suggests that the surface Debye-Waller effect on As(0001) is a long-range, collective oscillation rather than short-range, individual atomic disorder.     

Small-polaron versus band conduction in some transition-metal oxides

In this paper an attempt is made to establish the nature of free charge carriers and of charge carriers bound to centres in p-type NiO, CoO and MnO and in n-type MnO and α-Fe₂O₃.

For free charge carriers, d.c. conductivity, Seebeck coefficient and Hall effect are considered. Effects arising from inhomogeneous conduction and impurity conduction are discussed. Impurity conduction appears to have a strong influence on transport properties in the case of α-Fe₂O₃, less so in NiO, whereas no influence of this effect has been found in CoO and MnO. It is shown that NiO and CoO do not exhibit the features characteristic of small-polaron conductors but rather can be consistently conceived of as large-polaron band semiconductors. It is suggested that magnetic resistance due to exchange coupling between charge-carrier spin and cation spins plays an important role. The anomalous behaviour of the Hall effect in NiO and α-Fe₂O₃ is extensively discussed. In contradistinction to NiO, CoO and n-type MnO, free charge carriers in p-type MnO seem to have small-polaron character.

For charge carriers bound to centres, dielectric loss, high-frequency conduction and optical absorption are considered. The dielectric loss data relate to Li or Na centres in NiO, CoO and MnO and to Ti, Zr, Sn, Ta, Nb and presumably oxygen vacancy centres in α-Fe₂O₃. It is concluded from the dependence of dielectric loss on frequency and temperature that bound charge carriers are small polarons. It is shown for the cases of NiO and α-Fe₂O₃ that apart from small-polaron effects, disorder due to locally varying electric fields determines the nature of dielectric loss. The small-polaron character of bound charge carriers in NiO is corroborated by the behaviour of high-frequency conduction and optical absorption due to centres and also by the magnitude of impurity conduction.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

First-Principles Calculations on Electronic,Chemical Bonding and Optical Properties of Cubic Hf_3N_4

Electronic,chemical bonding and optical properties of cubic Hf 3N4(c-Hf3N4) are calculated using the firstprinciples based on the density functional theory(DFT).The optimized lattice parameter is in good agreement with the available experimental and calculational values.Band structure shows that c-Hf3N4 has direct band gap.Densities of states(DOS) and charge densities indicate that the bonding between Hf and N is ionic.The optical properties including complex dielectric function,refractive index,extinction coefficient,absorption coefficient,and reflectivity are predicted.From the theory of crystal-field and molecular-orbital bonding,the optical transitions of c-Hf3N4 affected by the electronic structure and molecular orbital are studied.It is found that the absorptive transitions of c-Hf3N4 compound are predominantly composed of the transitions from NT22p valence bands to HfT2(dxy,dxz,dyz) conduction bands.     

Magnetic and electrical properties of zinc selenide crystals containing disorder

The electrical and magnetic properties of ZnSe single crystals containing disorder have been studied between temperatures 290K and 900K. The study of the magnetic properties has been extended to low temperatures (100K). Paramagnetism has been found to appear at high temperatures (460–900K). From the fact that this paramagnetism is proportional to e^?δE/kT, it is suggested that localized states of single occupancy are created by thermal excitation. The study of the magnetic properties has been of help in ascertaining the nature of the transport (band conduction or hopping conduction) and in finding the hopping energy and excitation energy separately. It has also been shown from this that both band conduction and hopping conduction exist simultaneously in the sample. A study of the thermo electric power (t.e.p.) shows that below 450K current is carried by electrons in the conduction band and above by hopping of holes.     

Multiplication of electronic excitations in AgCl crystals

The kinetics of pulse conduction in silver chloride single crystals is investigated in the temperature range 12–300 K upon picosecond excitation with x-ray bremsstrahlung. It is found that the experimentally observed concentration of conduction electrons is nearly twice as high as the concentration of electron-hole pairs generated by an exciting pulse. This effect is associated with the chain multiplication of band charge carriers.     

Radiation induced conductivity in α Al2O3 crystals

Radiation induced conductivity in α Al₂O₃ crystals is studied during excitation by subnanosecond pulses from an electron accelerator. At least three components are observed in the relaxation of the conduction current. A component of the conduction current which follows the exciting pulse without delay is not observed, as, for example, in NaCl and KCl crystals. This is followed by a subnanosecond decay that does not fit a simple pattern and then, to within the limits of the sensitivity of the apparatus, by at least two other relaxation components for the conduction current in the nanosecond range. It is assumed that the observed components of the conduction current relaxation, including the fastest, are caused by thermal release of carriers from traps and their subsequent redistribution, while the second and third components of the conduction current pulse appear to the associated with electrons. Fiz. Tverd. Tela (St. Petersburg) 39, 1995–1996 (November 1997)     

Optical investigations of Zn,Hg and Li doped GaN

The use of infrared quenching of photoluminescence to study the spectral dependence of the photoionization cross-section in GaN doped with Zn, Hg, and Li is reported. It is shown that these impurities produce deep level centres 0.48 eV, 0.41 eV, and 0.75 eV, respectively, above the valence band. In addition, excitation spectra are investigated for Zn and Li doped GaN, giving values of 3.17 eV and 2.86 eV at 78 K for the energy distance of these levels from the conduction band. Finally, from the temperature dependence of the excitation spectra, it is concluded that the levels are probably pinned to the valence band.     

衬底对PTCDA薄膜结构与电荷输运特性的影响

在不同导电衬底(Au,Al和ITO)上制备了PTCDA薄膜,用XRD和AFM技术研究了PTCDA薄膜的结构和表面形貌。结果表明,薄膜中的大部分PTCDA分子平面与衬底不平行,这表明薄膜垂直方向的电流传导将以电子传输为主;在ITO和Au衬底上生长的PTCDA薄膜晶粒排列规则,在薄膜垂直方向呈现出较好的电子传输性能;而在Al衬底上生长的PTCDA薄膜晶粒排列无序,电子传输性能差。通过制备单层结构有机薄膜器件,研究了PTCDA薄膜垂直方向的电子迁移率。综合应用金属-有机界面的热电子发射理论和有机层体内空间电荷限制传导理论,并考虑电场强度对迁移率变化的影响,对ITO/PTCDA/Al器件的电流密度-电压曲线进行拟合,得到ITO衬底上生长的PTCDA薄膜在垂直方向随电场强度变化的电子迁移率数值。     

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e²/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.