Bistable amphoteric centers in semiconductors

It is shown that, at thermodynamic equilibrium, the release of charge carriers from the localized states of bistable amphoteric centers into quasi-free states depends on the degree of compensation. This brings about different functional dependences of the concentration of free charge carriers on temperature. It is found that, in uncompensated semiconductors, the concentration of free charge carriers follows the same dependence in the case of bistable amphoteric centers and bistable amphoteric U ^? centers, although the distributions of charge carriers over the charge states and configurations are different for these types of centers. The results can be used for interpreting various experimental data insufficiently explained in the context of the traditional approach.     

Characterization of defects in hydrogenated amorphous silicon devices using charge collection scanning electron microscopy

Electron-beam-induced current (EBIC) and secondary electron image (SEI) modes of a scanning electron microscope (SEM) are utilized for characterization of charge collection inhomogeneities in hydrogenated amorphous silicon devices. These inhomogeneities are due to such fabrication defects as substrate surface roughness, pin holes, blistering and lift-off. SEM observations are correlated with the electrical properties of the devices. Electronirradiation-induced damage in these devices is also investigated by measuring the EBIC time decay at continuous electron irradiation as a function of both the electron -beam energy and current. This decay mechanism is based on the formation of electron-irradiation-induced microscopic defects that act as recombination centers and reduce the lifetime of carriers.     

Effect of the levels of intrinsic defects in the CdP2 band gap on electrical characteristics of corresponding structures with the Schottky barrier

The electrical characteristics of Schottky barriers formed on n-type cadmium diphosphide are studied. It is established that the space-charge region at the metal-semiconductor interface represents in fact a Schottky layer formed owing to a high concentration of deep-level centers. The charge transport in the conducting direction for these structures is related to the above-barrier emission of electrons and is consistent with the diffusion theory for one or two types of charge carriers. The high concentration of ionized centers in the space-charge region gives rise to the tunneling mechanism of breakdown in the blocking direction. The frequency dependences of the complex conductance are governed by the exchange of charge carriers between the conduction band and donors that specify the conductivity type of the material and also by the recharing of the centers with a large depth of levels. Good agreement between the reported results and the theory is obtained.     

Intragap-trapped-carriers enhancement of the low-temperature delayed phosphorescence in Alq3

Time resolved electroluminescence of Alq₃ based organic light emitting devices was measured on a millisecond time scale. The millisecond delayed electroluminescence (DEL) was detected at 80 K and 26 K, while no long lived electroluminescence was observed at room temperature. The DEL spectrum consists of two components centered at ∼2.3 and ∼1.9 eV. The shape of the high energy band is similar to the prompt fluorescence emission peak, although a systematic red shift of about 30 meV is reproducibly found. The low-energy band position corresponds to the phosphorescence of Alq₃. The time decay of both bands is nonexponential, with the low energy band decaying slower. To explain dependencies of intensities of both components on the delay and the injection-current density a mechanism based on enhancement of the triplet-exciton radiative decay by carriers trapped in deep intragap traps is proposed.     

Mechanism of the effect of the electric field of a surface acoustic wave on the low-temperature photoluminescence kinetics in type-II GaAs/AlAs superlattices

Kinetics of the low-temperature photoluminescence (PL) of type-II GaAs/AlAs superlattices under the effect of the electric field of a surface acoustic wave is studied experimentally. It is found that application of electric-field pulses results in speeding up the PL kinetics of free and localized excitons and phonon replicas irrespective of the pulse duration. Analysis of the experimental data demonstrates that acceleration of the PL kinetics is related to the transport of excitons towards nonradiative recombination centers; this transport is promoted by the interaction of excitons with hot free charge carriers ejected from localized states.     

Edge photoluminescence of single-crystal silicon with a <Emphasis Type="Italic">p-n</Emphasis> junction: Structures produced by high-efficiency solar cell technology

The systematic features and kinetics of edge photoluminescence of silicon structures produced by the high-efficiency solar cell technology is studied at different voltages applied to the p-n junction. It is shown that the effect of modulation of the edge photoluminescence intensity by a dc voltage applied to the p-n junction is qualitatively similar to the effect induced by excitation of photoluminescence by laser radiation at the wavelengths 658 and 980 nm. The possibility of modulating the edge photoluminescence power by varying the resistance parallel to the p-n junction is demonstrated. It is found that, at zero voltage, the rise time constant of the photoluminescence intensity far exceeds the decay time constant. However, as the dc forward current is increased, the decay time constant approaches the rise time constant. To interpret the results, the concepts of the second, more efficient saturable recombination channel coexisting with the common Shockley-Read-Hall recombination channel in the structure are developed. The study extends the functional capabilities of the luminescence technique in determining the effective lifetimes of charge carriers.     

Thermally Induced Transient Absorption of Light by Poly(3,4‐ethylenedioxythiophene):Poly(styrene sulfonic acid) (PEDOT:PSS) Films: A Way to Probe Charge‐Carrier Thermalization Processes

Infrared‐induced transient absorptions in the millisecond and sub‐picosecond time domains have been used to study the dynamics of charge carriers of a conducting polymer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS). On the millisecond timescale, the transient absorption is ascribed to a thermal effect induced by absorbed infrared light. The decay of the transient absorption is limited by the transport of heat from the polymer film to the substrate and corresponds to the decay kinetics of infrared‐induced changes in the resistivity of the material. Near 1.5 eV, the infrared‐induced absorption can be modeled in terms of an interband transition. The assignment of the optical transients in terms of carrier heating opens the possibility to study charge carrier thermalization processes using short laser pulses. Pump‐probe spectroscopy on a sub‐picosecond timescale shows that the initial thermalization of the excited charge carriers occurs with a time constant of less than 500 fs, i.e., faster than for noble metals.     

Effect of surface recombination on the transient decay of excess carriers produced by short wavelength laser pulses

Effect of surface recombination on the transient decay of excess carriers injected very near to the surface region has been analysed for a semi-infinite semiconductor sample. The one dimensional continuity equation for the excess minority carriers has been solved analytically assuming that the surface recombination is important only on the front surface from which the excess carriers are injected. The decay of total excess carrier charge as a function of time is calculated for various values of the surface recombination velocities. It is found that values of S lower than 10³ cm/sec. have little effect on the decay of total excess charge and the decay is exponential. For values of S larger than 10⁴ cm/sec. the initial decay of excess charge is much faster and is dominated by the surface recombination. However, if one waits long enough for the excess carrier to diffuse into the semiconductor the decay becomes exponential when the excess charge decays to about 5% of its initial value. This exponential decay can be used to determine the excess carrier lifetime fairly accurately. A source of error in such experiments may arise from the heating of the silicon sample by the laser pulse. This effect, however is negligible for short duration pulses of low average power.     

Kinetics of electroluminescence in an efficient silicon light-emitting diode with thermally stable spectral characteristics

The kinetics of the rise and decay of electroluminescence in an efficient silicon light-emitting diode that is formed by diffusion and features a high thermal stability of spectral parameters of electroluminescence in the region of band-to-band transitions is studied for various injection currents. The parameters of the electroluminescence kinetics are compared with effective lifetimes of minority charge carriers and characteristics of the current kinetics. It is found that the onset of a sharp rise in the electroluminescence intensity is delayed with respect to the instant when the voltage is applied to the diode; the results of measuring the emission distribution over the diode area are reported.     

The role of underbarrier transitions in processes of annihilation of excess charge carriers in II–VI semiconductors

The loss kinetics of charge carriers generated by the emission pulses of nitrogen laser in II–VI semiconductor compounds, namely, n- and p-CdTe and CdS, is studied by the method of microwave photoconductivity. The decays of photoresponse after switching off the light consisted of fast (manifesting itself at times <30 ns) and slow (at times >50 ns) components. The shape of decay of a slow component was virtually independent of temperature, and at long times, the decay law corresponded to the linear dependence of the photoresponse on the logarithm of time. The assumption follows from the results that the slow component of the photoresponse decay reflects the loss process of charges captured in the traps due to the tunnel recombination.     

Bulk and interface imperfections in semiconductors

Charges trapped at imperfection centers can be detected by observing the slow time dependences of the high-frequency small-signal capacitance and the current of a space charge region which is depleted of carriers. Detection of as low as 10¹⁰ to 10¹¹ trapped electron charges per cm³ in a volume of 10⁻³ cm × 10⁻³ cm², or 10⁵ to 10 electron charges, can be achieved. This technique has been used to study the temperature, electric field and photon energy dependences of the thermal, optical and Auger-impact emission and capture rates of electrons and holes at impurity and defect centers in depleted bulk and surface space charge regions in semiconductors, as well as the generation, annealing and diffusion kinetics of these centers. It has also been used to monitor the silicon integrated circuit fabrication processes. The principle of this technique is described and experimental examples are given to illustrate the applications of the technique.     

Space charge limited and emitter current limited injections in space charge region of semiconductors

If, in a space charge region of a semiconductor device, a unipolar drift current is a main component of the total current, an analogy with motion of electrons in vacuum exists. In the space charge limited emission, injection of minority carriers is limited by space charges, as in a punched-through p-n-p diode. On the other hand, in a transistor, minority carriers are injected into the depletion layer between the base and the collector, after diffusion in the neutral base region. This injection is limited by emitter current, and this mode of injection corresponds to a temperature limited emission in a vacuum tube. In this paper, equations of minority carrier impedances will be calculated in both these cases, and negative resistances can be expected at some transit angles. Then a generalized equation, which includes these two modes of injections as special cases, will be obtained. This generalized equation corresponds to the Llewellyn-Peterson equation of vacuum tubes, which include the space charge limited emission and the temperature limited emission as special cases.     

Effect of irradiation on the luminescence properties of low-dimensional SiGe/Si(001) heterostructures

This study is concerned with the effect of irradiation on the luminescence properties of low-dimensional Si/Ge heterostructures with different degrees of spatial localization of charge carriers. It is shown that the radiation stability of Si/Ge heterostructures is improved with increasing efficiency of localization of charge carriers in the structures. The spatial localization of charge carriers in the SiGe nanostructures decreases the probability of nonradiative recombination of charge carriers at radiation defects produced in the Si matrix. It is demonstrated that, among the structures explored in the study, the highest radiation stability of luminescence properties is inherent in the multilayered structures containing self-assembled Ge(Si) nanoislands, in which the most efficient spatial localization of charge carriers is attained. In this case, the localization is three- and two-dimensional, correspondingly, for holes in the islands and for electrons in the Si layers that separate neighboring layers containing the islands.     

Specific features of the spectra of nonlinear optical absorption in nonstoichiometric and Ni doped GaSe single crystals

The method of two radiation sources is used to measure the spectra of nonlinear optical absorption in the GaSe, GaSe:Ni, Ca_1.05Se_0.95, and Ca_0.95Se_1.05 single crystals. The levels with the depths of 1.38, 1.45, 1.55, 1.65, and 1.80 eV are detected. Absorption by nonequilibrium charge carriers, induced absorption, and induced transmission caused by a variation in the occupation of impurity centers due to laser radiation are observed in the spectra of nonlinear optical absorption.     

Energy characteristics of excitons in structures based on InGaN alloys

Samples containing ultrathin InGaN layers that emit radiation in the spectral range from the ultraviolet to yellow region are studied. The samples are grown by metal-organic vapor-phase epitaxy. The Urbach energy, the localization energy of excitons, and the activation energy of charge carriers are determined to characterize radiative and nonradiative processes in the quantum dots and barriers of the structures. It is shown that these energy parameters are linearly dependent on the photon energy in the range from 3.05 to 2.12 eV. It is established that temperature variations in the emission intensity are due to the increase in the number of charge carriers thermally activated from the quantum wells into barriers as well as due to the enhancement of scattering of free excitons at defects.     

Comparison of the Operation of Polymer/Fullerene,Polymer/Polymer,and Polymer/Nanocrystal Solar Cells: A Transient Photocurrent and Photovoltage Study

We utilize transient techniques to directly compare the operation of polymer/fullerene, polymer/nanocrystal, and polymer/polymer bulk heterojunction solar cells. For all devices, poly(3‐hexylthiophene) (P3HT) is used as the electron donating polymer, in combination with either the fullerene derivative phenyl‐C₆₁‐butyric acid methyl ester (PCBM) in polymer/fullerene cells, CdSe nanoparticles in polymer/nanocrystal cells, or the polyfluorene copolymer poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2,2‐diyl) (F8TBT) in polymer/polymer cells. Transient photocurrent and photovoltage measurements are used to probe the dynamics of charge‐separated carriers, with vastly different dynamic behavior observed for polymer/fullerene, polymer/polymer, and polymer/nanocrystal devices on the microsecond to millisecond timescale. Furthermore, by employing transient photocurrent analysis with different applied voltages we are also able to probe the dynamics behavior of these cells from short circuit to open circuit. P3HT/F8TBT and P3HT/CdSe devices are characterized by poor charge extraction of the long‐lived carriers attributed to charge trapping. P3HT/PCBM devices, in contrast, show relatively trap‐free operation with the variation in the photocurrent decay kinetics with applied bias at low intensity, consistent with the drift of free charges under a uniform electric field. Under solar conditions at the maximum power point, we see direct evidence of bimolecular recombination in the P3HT/PCBM device competing with charge extraction. Transient photovoltage measurements reveal that, at open circuit, photogenerated charges have similar lifetimes in all device types, and hence, the extraction of these long‐lived charges is a limiting process in polymer/nanocrystal and polymer/polymer devices.     

The form of the profile of heterointerfaces in (311)Ga GaAs/AlAs structures

The steady-state photoluminescence and kinetics of photoluminescence of the (100)-oriented and (311)Ga-oriented type II GaAs/AlAs superlattices are studied under the effect of the electric field of the surface acoustic wave. It is found that, in the (100)-oriented structures, the drop of intensity of steady-state photoluminescence and acceleration of photoluminescence kinetics are independent of the direction of the electric field of the surface acoustic wave with respect to crystallographic directions, while in the (311)Ga-oriented structures these effects are anisotropic. It is shown that all variations in the steady-state photoluminescence and in kinetics of photoluminescence of (100)-oriented and (311)Ga-oriented structures under the effect of the electric field of the acoustic wave are associated with transfer and capture by the nonradiative recombination centers of nonequilibrium charge carriers, which are initially localized in wide quantum wells formed by fluctuations of the thickness of the layers of the structures. From the obtained experimental data, the parameters of the profile of heterointerfaces of the (311)Ga GaAs/AlAs superlattices are determined. It is established that the lateral sizes of microgrooves in the [011] direction on the direct and inverse heterointerfaces of the (311)Ga superlattices exceed 3.2 nm, while the modulation of the thickness of the AlAs layers is from 0.8 to 1.2 nm.     

Electronic properties of Ge nanocrystals for non volatile memory applications

Charge and discharge phenomena of Germanium nanocrystals fabricated by low pressure chemical vapor deposition are investigated by means of Capacitance–Voltage and capacitance decay measurements. The study shows fast programming and erasing times as compared with conventional devices. It is shown that the charge saturation depends on the gate voltage stress in the low electric field regime. For high gate voltages, a saturation of the stored charge is obtained, indicating that the density of trapped carriers in Ge nanocrystals is limited and depends only on the dots size. Capacitance decay measurements exhibits a very long retention time for holes as compared with silicon nanocrystal memories. This is mainly due to the barrier height for holes at the nc-Ge/ ₂ interface. A model for simulation of the retention kinetics has been developed and allows to extract the band alignment of the nc-Ge/SiO₂/Si system. The simulation results are then used to determine the band gap energy of Ge nanocrystals. Finally, it is shown that Ge nanocrystals are very good candidates for P-type Metal Oxide Semiconductor nonvolatile memories.     

The shape of the signals of transient photoconductivity in silicon doped with gold or sulfur

Experimental results of studying the photoconductivity kinetics in silicon doped with either gold or sulfur are reported. The nonequilibrium charge-carrier concentration was generated by the effect of pulsed laser radiation. The time dependence of nonequilibrium conductivity was determined by contactless measurements from a variation in the reflected power of the microwave field. The long-term photoconductivity with the time constant of τ ≈ 1.6 ms at room temperature is observed in the silicon samples overcompensated with sulfur. Conventional short-term processes (τ ≈ 2–3 μs) characteristic of the centers with deep levels are observed in the silicon samples doped with gold. These differences are accounted for using the known data on the energy characteristics of the gold and sulfur levels in the silicon band gap and their charge states. Original Russian Text ? A.D. Kiryukhin, V.V. Grigor’ev, A.V. Zuev, V.V. Zuev, N.A. Korolev, 2007, published in Fizika i Tekhnika Poluprovodnikov, 2007, Vol. 41, No. 3, pp. 269–272.     

Comparative analysis of radiation effects on the electroluminescence of Si and SiGe/Si(001) heterostructures with self-assembled Islands

The effect of neutron radiation on the electroluminescence of the Si p-i-n diode containing a multilayered Ge/Si heterostructure with self-assembled nanoislands is studied. In comparison with bulk Si, the diodes containing Ge(Si) nanoislands exhibit a higher radiation hardness of the electroluminescence signal, which is attributed to spatial localization of charge carriers in the Ge/Si nanostructures. The spatial localization of charge carriers impedes their diffusion to radiation defects followed by nonradiative recombination at the defects. The results show the possibilities of using Ge/Si heterostructures with self-assembled nanoislands for the development of optoelectronic devices resistant to radiation.