Temperature dependent responsivity of quantum dot infrared photodetectors

Temperature dependent behavior of the responsivity of InAs/GaAs quantum dot infrared photodetectors was investigated with detailed measurement of the current gain. The current gain varied about two orders of magnitude with 100 K temperature change. Meanwhile, the change in quantum efficiency is within a factor of 10. The dramatic change of the current gain is explained by the repulsive coulomb potential of the extra carriers in the QDs. With the measured current gain, the extra carrier number in QDs was calculated. More than one electron per QD could be captured as the dark current increases at 150 K. The extra electrons in the QDs elevated the Fermi level and changed the quantum efficiency of the QDIPs. The temperature dependence of the responsivity was qualitatively explained with the extra electrons.     

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.     

The effect of excited atomic states on the harmonic generation in a photoionized plasma

Owing to the conservation by the electrons of the memory of their intra-atomic states, a photoionized plasma arising in the mode of suppression of the ionization barrier exhibits a pronounced dependence of the efficiency of generation of the third and fifth harmonics of the pumping field on such states of electrons. For the maximal values of generation efficiency, this dependence is directly proportional to the tenth power of the principal quantum number of excited ns states of electrons.     

Performance enhancement of InGaN light-emitting diodes with a leakage electron recombination quantum well

An InGaN light-emitting diodes with a leakage electron recombination (LER) quantum well have been proposed and investigated numerically by using the APSYS simulation software. The simulation results indicate that the AlGaN electron blocking layer inserted between the last two quantum wells changed the carrier concentrations distribution, and the leakage electrons can be further recombined with holes in the LER quantum well which can decrease the electrons that spill out from active region. As a result, the internal quantum efficiency and light output power are markedly improved attributed to LER quantum well.     

Influence of varied doping structure on photoemissive property of photocathode

The built-in electric fields within a varied doping GaAs photocathode may promote the transport of electrons from the bulk to the surface,thus the quantum efficiency of the cathode can be enhanced remarkably. But this enhancement,which might be due to the increase in either the number or the energy of electrons reaching the surface,is not clear at present. In this paper,the energy distributions of electrons in a varied doping photocathode and uniform doping photocathode before and after escaping from the cathode surface are analysed,and the number of electrons escaping from the surface in different cases is calculated for the two kinds of photocathodes. The results indicate that the varied doping structure can not only increase the number of electrons reaching the surface but also cause an offset of the electron energy distribution to high energy. That is the root reason for the enhancement of the quantum efficiency of a varied doping GaAs photocathode.     

Anregung von Zinkoxyd durch langsame Elektronen

Under excitation with monochromatic electrons luminescence of a ZnO phosphor starts with a screen voltage of 2.8 volts (contactpotential 2.4 v) i.e. with an impact energy of 0.4 eV. A flattening was found in the rise of the fluorescence for screen voltages between 7 and 10 volts. This can be considered as an interaction of the incoming electron with the lattice electrons. The quantum efficiency of electrons having an energy of 10 eV was about one light quantum per 200 electrons. The work function of ZnO was found to be 4 eV.     

The variably spaced superlattice energy filter

A new superlattice device concept which provides for high energy injection of electrons into a semiconductor layer is presented. The device is based on resonant tunneling of electrons between adjacent aligned quantum well levels in a variably spaced superlattice structure. By a judicial choice of well and barrier widths the energy levels under reverse bias become aligned such that resonant tunneling of electrons through the structure can occur. Thus, electrons are injected into a semiconductor layer at an energy corresponding to the energy of the first subband in the last quantum well. This structure has significant advantages over the conventional method of producing hot electrons in that a nearly monoenergetic high-energy electron distribution is created at low reverse bias and with high efficiency, since energy loss to phonons is inhibited as a consequence of the channeling of electrons through a narrow band of quantum states. Applications of the VSSEF structure to avalanche photodiodes, IMPATT diodes and electroluminescent devices are discussed.     

On the theory of quantum efficiency in germanium

A system of functional integral equations describing multiple impact ionization in semiconductors due to hot electrons and holes generated by the absorption of photons is solved. Two relaxation processes, namely interband Auger transitions and phonon emission, are analyzed theoretically to explain the spectral dependence of the quantum efficiency of the internal photo-electric effect. The dependence of the quantum efficiency in Ge on the energy of photons absorbed is calculated and compared with experiment.     

Quantum efficiency and overlap integrals in InSb

The paper deals with the problem of the enhanced quantum efficiency in semiconductors. An expression is derived for the spectral dependence of quantum efficiency in InSb in the low energy region. The probabilities both of the primary absorption process and of secondary relaxation processes, i.e. impact ionization (inter-band Auger transitions) and the thermalization of hot electrons, are calculated. The behaviour of overlap integrals in InSb is discussed in connection with these processes. The calculated spectral dependence of the quantum efficiency is compared with the experimental measurements.     

Magneto-optics of zero-dimensional electron systems

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems (0DESs) in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by both reactive-ion etching and strain-confining GaAs heterojunctions with a -layer of Be present in the GaAs, in order to improve luminescence efficiency. For the etched quantum dots we show that the low magnetic field dispersion T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground-state energy of interacting electrons in the quantum dots. For the strain-confined 0DESs we have mapped the magneto-dispersion of the quantum confined electron states to reveal 15 electrons per dot.     

Laser cooling of electrons and X-ray generation in a relativistic quantum heat engine

We consider a relativistic quantum heat engine that goes through a thermodynamical cycle consisting of stages involving laser-assisted cooling of electrons and the generation of X-ray radiation. Quantum treatment of the processes makes it possible to obtain the necessary condition and the amount of work extracted from the interaction ingredients, as well as the efficiency of the heat engine. We have also found that the efficiency of the relativistic engine is less than the one for the nonrelativistic case for the same momenta. The obtained results set the limits to the cooling, as well as the intensity of X-ray radiation, in the quantum regime of the interaction of electrons with laser fields.     

Crystallization and quantum melting of few electron system in a spherical quantum dot: quantum Monte Carlo simulation

Spherical quantum dots with a few charged Fermi particles (electrons or holes) are studied for different total spins. Simulation by quantum path integral Monte Carlo method is performed. The dependence of the electron correlations in the quantum dot is studied at different mean interelectron separation controlled by number of electrons in the quantum dot and by steepness of electron confinement (the latter parameter can be changed by the gate voltage). The ‘cold’ melting—quantum transition from Wigner crystal-like state (i.e. from regime of strongly correlated electrons) to a Fermi liquid-like state—driven by the steepness of electron confinement is studied. The pair correlation function and radial function characterizing electron quantum delocalization are analyzed.     

量子阱结构对有机电致发光器件效率的影响

实验中共制备了五种有机量子阱结构电致发光器件,分别对这五种量子阱结构器件的电致发光特性进行了研究,分析了量子阱结构的周期数和势垒层的厚度对器件电学性能的影响.实验结果表明适当周期数的量子阱结构器件的亮度和电流效率比传统的三层结构器件的要大,主要原因是量子阱结构对电子和空穴的限制作用,这种限制作用提高了电子和空穴在发光层中形成激子和复合的概率,从而提高了发光的亮度和效率.当改变阱结构器件中势阱层的厚度时,也会对器件的亮度和效率产生影响,采用适当的势阱层厚度能够提高器件的亮度和效率. 关键词： 量子阱结构 电致发光 电流效率 光谱     

Control of electron correlations in a spherical quantum dot

Spherical quantum dots containing several electrons are considered for different values of the total spin. Numerical calculations are carried out using the quantum path-integral Monte Carlo method. The dependence of the electron correlations on the dimensionless control quantum parameter q associated with the steepness of the confinement potential is studied. The quantum transition from a Wigner crystal-like state (i.e., from the regime of strongly correlated electrons) to a Fermi-liquid state (“cold” melting) driven by the parameter q is studied in detail. The behavior of the radial and pair correlation functions, which characterize quantum delocalization of the electrons, is considered.     

Spatial Entanglement of Fermions in One-Dimensional Quantum Dots

The time-dependent quantum Monte Carlo method for fermions is introduced and applied in the calculation of the entanglement of electrons in one-dimensional quantum dots with several spin-polarized and spin-compensated electron configurations. The rich statistics of wave functions provided by this method allow one to build reduced density matrices for each electron, and to quantify the spatial entanglement using measures such as quantum entropy by treating the electrons as identical or distinguishable particles. Our results indicate that the spatial entanglement in parallel-spin configurations is rather small, and is determined mostly by the spatial quantum nonlocality introduced by the ground state. By contrast, in the spin-compensated case, the outermost opposite-spin electrons interact like bosons, which prevails their entanglement, while the inner-shell electrons remain largely at their Hartree–Fock geometry. Our findings are in close correspondence with the numerically exact results, wherever such comparison is possible.     

Nonideal effects in quantum field-effect directional coupler

The nonideal effects in a quantum field-effect directional coupler where two quantum wires are coupled through a finite potential barrier are studied by adopting the lattice Green function method. The results show that the electron energy distribution, asymmetric geometry and finite temperature all have obvious influence on the electron transfer of the coupler. Only for the electrons with energies in a certain region, can the complete periodic transfer between two quantum wires take place. The conductance of these electrons as a function of the barrier length and potential height exhibits a fine periodic or quasi-periodic pattern. For the electrons with energies beyond the region, however, the complete periodic transfer does not hold any more since many irregular oscillations are superimposed on the conductance profile. In addition, the finite temperature and asymmetric geometry both can reduce the electron transfer efficiency.     

Optimizing GaN photocathode structure for higher quantum efficiency

To improve the quantum efficiency of GaN photocathode, we optimized the photocathode's structure in three aspects. We use AlN replacing GaN as the buffer layer, which can act as potential barrier to reflect electrons back to surface. The optimal thickness of emission layer is calculated as 162.5 nm, and considering the graded doping profile, we optimized the thickness as 180 nm. Three built-in electric fields are introduced by Mg graded dope, and the intensities of the high fields are calculated to give the quantitive results of their influence. After surface cleaning and activation, quantum efficiency of the optimized sample was increased and the highest value of 56% was achieved at 240 nm. More quantum efficiency enhancement is possible by further optimizing the photocathode structure.     

High transparency of a two-photon scattering channel in triple-barrier structures

An analytical solution of the Schrödinger equation with open boundary conditions in all scattering channels has been found for asymmetric triple-barrier resonance-tunneling structures with thin high barriers. This solution describes resonance transitions between three quantum levels in a high rf electric field. It is found that, under certain conditions, most electrons incident on the upper resonance level can emit two photons and leave a structure through the lower level without intermediate interaction with phonons. The structure appears to be almost absolutely transparent in a wide range of the rf field amplitude. This behavior fundamentally distinguishes the multiphoton scattering process from previously considered single-photon scattering processes and the quantum efficiency of such transitions can be twice as high as the maximum quantum efficiency of the transitions between neighboring levels and can reach a value of 160% in the limiting case.     

Energy filtration effect and the possibility of the generation of terahertz radiation in resonant tunneling structures with several quantum wells

The properties of a high-frequency response in resonant tunneling double-well nanostructures have been considered for various energies of electrons arriving to a structure of electrons, various frequencies of the external electromagnetic field, and various features associated with the interaction of electronic states in neighboring quantum wells in double-well nanostructures. The energy filtration effect that is caused by the breaking of the symmetry of the high-frequency response in double-well nanostructures in a static electric field has been revealed. This effect leads to a sharp increase in the gain under conditions of the quantum amplification regime and opens real prospects of a significant increase in the efficiency of solid amplifying and generating devices based on resonant tunneling double-well nanostructure in the subterahertz and terahertz frequency ranges.     

Thermal and non-radiative transitions of electrons at imperfections in ionic crystals

The theory of non-radiative and thermal transitions of electrons at imperfections in ionic crystals is elaborated. The results obtained are used to confirm Mott's and Gurney's theories on the detachment process in two steps of electrons from F centres during the absorption of light in the F band and to explain theoretically the temperature dependence of quantum efficiency. Good agreement with experiment was obtained. The possibility of the luminescence of alkali halide crystals containing F centres at sufficiently low temperatures was also determined.