Increase in the rate and discretization of the kinetics of isothermal surface generation of minority charge carriers in metal-insulator-semiconductor structures with a planar-inhomogeneous insulator

Surface generation of minority charge carriers in silicon metal-oxide-semiconductor (MOS) structures is efficient only at the initial recombinationless stage. Quasi-equilibrium between surface generation centers and the minority-carrier band is established in a time t ～ 10^?5 s. In the absence of other carrier generation channels, an equilibrium inversion state at 300 K would need t = t_∞ > 10³ years to become established. In fact, the time t ∞ is much shorter, due to excess-carrier generation via centers located at the SiO₂/Si interface over the gate periphery. This edge-related generation can easily be simulated in an MOS structure with a single gate insulated from Si by oxide layers of various thicknesses. At gate depleting voltages V _g , the role of the periphery is played by a shallow potential well under a thicker oxide, and the current-generation kinetics becomes unconventional: two discrete steps are observed in the dependences I(t), and the duration and height of these steps depend on V _g . An analysis of the I(t) curves allows determination of the electric characteristics of the Si surface in the states of initial depletion (t = 0) and equilibrium inversion (t = t_∞), as well as the parameters of surface lag centers, including their energy and spatial distributions. The functionally specialized planar inhomogeneity of a gate insulator is a promising basis for dynamic sensors with integrating and threshold properties.     

Distribution of charge carriers in dissipative semiconductor structures

It was experimentally shown that redistribution of the charge-carrier concentration occurred in nonequilibrium electron-hole plasma during formation and excitation of the dissipative structure by a strong electric field in the bulk of Te and InSb single crystals. In this case, if there are only longitudinal autosolitons in the dissipative structure, the carrier concentration decreases outside the autosolitons. The charge-carrier concentration increases outside autosolitons if the transverse autosolitons are present. It is suggested that the longitudinal autosolitons formed in the nonequilibrium electron-hole plasma developed by the Joule heating are “cold” and the transverse autosolitons are “hot”.     

Model-free determination of the dependence of charge density in accumulation and inversion semiconductor layers on the semiconductor surface potential from the voltage-capacitance characteristics of metal-insulator-semiconductor structures

A method for the analysis of the quasi-static voltage-capacitance characteristics of metal-insulator-semiconductor structures in the majority-carrier depletion region of the semiconductor surface is developed. The method provides for the quantitative evaluation of the doping concentration, the flat-band voltage, and the effective values of the capacitance and thickness of the gate insulator. The dependences of the surface potential and surface charge in the semiconductor on the gate-electrode potential are obtained in the absence of a priori data on the state of the electron gas during high enrichment or strong inversion. It is shown that these experimental dependences can be used as a criterion for the validity of the theory of the space-charge region in a semiconductor when the effects of degeneracy and dimensional quantization of electron gas are considered.     

Specific features of the field generation of minority charge carriers in Si-SiO2 structures

The specific features of generation processes occurring in Si-SiO₂ structures in the presence of a strong electric field in the oxide layer (E _ox ≈ 10 MV cm^?1) are considered. It is shown that, after a certain threshold electric-field strength is reached, an additional mechanism of minority charge-carrier generation, associated with the development of electroluminescence processes in the oxide layer, becomes operative.     

Determination of the diffusion length of charge minority carriers using digital oscillography of surface photovoltage

A procedure for determining the diffusion length of charge minority carriers (CMCs), which uses digital oscillography to detect surface photovoltage (SPV), is considered. It is shown by experiments that the shape of pulses of SPV, excited by rectangular IR pulses, depends significantly on both their intensity and wavelength. It is shown that it is not valid to use a synchronous detector to measure the quasi-stationary SPV when determining the diffusion length of CMCs in semiconductors, because, in a number of cases, it results in an uncontrollable error that reaches 100% and more. A technique is developed for determining the diffusion length in silicon wafers and epitaxial structures with the specific resistance range 0.01–12 Ω cm, the diffusion length range 5–500 μm, and an error of not more than 8%.     

Temperature delocalization of charge carriers in semiconductor lasers

The temperature dependences of emission characteristics are investigated for laser diodes based on asymmetric separate-confinement heterostructures with a broadened waveguide. It is established that an increase in the charge-carrier concentration in the waveguide layer is the basic mechanism of saturation in the light-current characteristic with increasing temperature in the CW mode. It is experimentally shown that the temperature delocalization of charge carriers leads to increasing internal optical losses and decreasing external differential quantum efficiency. It is shown that the degree of delocalization of charge carriers depends on the charge-carrier temperature distribution, the threshold concentration, and the quantum-well depth. The effect of thickness and energy depth of the quantum well on the temperature sensitivity of the threshold current and output optical power is considered.     

Photoelectric C-V profiling of majority charge carriers and effective lifetimes of minority charge carriers in gettered GaAs wafers

The procedure for photoelectrochemical C-V profiling of the concentration of majority charge carriers and effective lifetimes of minority charge carriers in high-resistivity thick (1.6 mm) GaAs wafers subsequently to their gettering is described. Gettering was performed by both one-side and two-side coating of the wafers with a Y film and subsequent thermal treatment at 700 and 800°C. It was demonstrated that the concentration profile N _d-N _a and the effective lifetime for minority charge carriers throughout the wafer are rather uniform in both cases. This procedure makes it possible to measure the charge carrier concentration as low as 10¹² cm^?3.     

Optical absorption and scattering at one-particle states of charge carriers in semiconductor quantum dots

A theory is developed for the interaction of an electromagnetic field with one-particle quantum-confined states of charge carriers in semiconductor quantum dots. It is shown that the oscillator strengths and dipole moments for the transitions involving one-particle states in quantum dots are rather large, exceeding the corresponding typical parameters of bulk semiconductor materials. In the context of dipole approximation it is established that the large optical absorption cross sections and attenuation coefficients in the quasi-zero-dimensional systems make it possible to use the systems as new efficient absorbing materials.     

半导体纳米颗粒载流子的超快弛豫过程

分析了纳米颗粒的能级结构,建立了载流子弛豫的简化模型,运用数值模拟方法讨论了激发密度、表面态密度及俘获态电子的弛豫率对弛豫过程的影响。讨论结果表明,激发密度的增大及表面态的减少都会导致表面态上电子的饱和,使导带上出现电子的积累,导带电子寿命增大;深俘获态电子的弛豫是影响材料响应速度的主要因素。最后应用此模型对近红外泵浦探测实验的结果进行分析,表明模型可望在实验结果分析上得到应用。     

Measurement of the diffusion length of minority charge carriers using real Schottky barriers

The characteristic features of the field dependence of the short-circuit photocurrent of real Schottky barriers based on strongly doped semiconductors under conditions of an oscillatory dependence of the light absorption coefficient a on the field intensity in the space-charge region and the photon energy (hν>E _g ) are analyzed. An analytical expression is obtained for the dependence of the photocurrent on the thickness W of the space-charge region when the condition αW≪1 is satisfied. An improved method is proposed for determining the diffusion length of minority charge carriers by analyzing the dependence I _p (W) in the spectral region satisfying the conditions of applicability of the expressions obtained. Some improvements are also made in the method for distinguishing the capacitance of the space-charge region from the high-frequency capacitance of a real Schottky barrier. The method is tested on structures Au-GaAs with N _d =4.5×10¹⁶–1×10¹⁸ cm⁻³. The method of determining L is checked independently on the basis of a theoretical descripion of the spectral dependence of the quantum efficiency of the structure. Fiz. Tekh. Poluprovodn. 31, 781–785 (July 1997)     

Free ion transport in the insulator layer and electron-ion exchange at an insulator-semiconductor phase boundary produced as a result of thermally stimulated ionic depolarization of silicon MOS structures

The nature of thermally stimulated depolarization currents in silicon MOS structures is investigated. The analysis is based on the experimentally established fact that the activation energy of the depolarization current in the initial section of growth is independent of the magnitude of the depolarizing voltage and on the previously discovered phenomenon of the formation of neutral associates ion + electron at the insulator-semiconductor interface. Transport of free ions in the insulator layer (the transit time is of a thermal activation character) and electron-ion exchange processes at the Si-SiO₂ phase boundary, which include tunneling ionization (decay) of neutral associates, are studied. The ion transfer in SiO₂ layers, found from the thermally stimulated depolarization curves using the developed theory, agrees well with data from independent experiments. Fiz. Tekh. Poluprovodn. 33, 962–968 (August 1999)     

Mobility of charge carriers in double-layer PbTe/PbS structures

The method of hot-wall epitaxy is used to grow epitaxial heterostructures of p-PbTe/n-PbS on BaF₂ substrates. Hall-effect measurements are analyzed in order to obtain the dependence of the effective carrier mobility on thickness and temperature in the ranges 0.1–2 μm and 100–300 K, respectively. It is found that this mobility depends on the thickness of the sample and on the individual thicknesses of its constituent layers. The effective mobility is calculated under the assumption that charge carriers are scattered by the surface of the structure and by dislocations that form at the heterojunction boundary. Fiz. Tekh. Poluprovodn. 32, 1064–1068 (September 1998)     

Investigation of the generation characteristics of metal-insulator-semiconductor structures

A method for determining the surface and volume generation-recombination parameters of MIS structures based on nonequilibrium C(V) and dC/dV(V) measurements using the linear voltage sweep technique is proposed. A theory is developed enabling simple treatment of experimental data and taking into account the dependence of the surface generation velocity, S_g, on the minority carrier concentration p_s. It is shown that the linearity of Zerbst plots can be obtained by using the trustworthy S_R(p_s) relation. Some experimental data concerning the generation properties of the Si---SiO₂ interface are presented. In particular, the experimental S_R(p_s) curves are shown to be in qualitative agreement with the theoretical predictions.     

Study of surface charge in VMOS structures

The effect on the surface charge of VMOS capacitors of etchants most frequently used in anisotropic etching of silicon (100) has been investigated. The results show an increased density of fixed charge in the oxide after use of an etch system containing KOH. Singularities were observed in the energy distribution of the surface charge owing to the particular geometry of the investigated structures.     

Evidence of thermally activated transfer of excited carriers between CdSe/ZnSe quantum dots

Temperature dependent photoluminescence and cathodoluminescence of selfassembled CdSe/ZnSe quantum dots grown by metalorganic vapor phase deposition were investigated. We found an unusual large red shift and a narrowing of the photoluminescence peak with temperature increases. Cathodoluminescence studies of a small number of quantum dots showed that the broad peak observed in the photoluminescence spectra is, in fact, made up of a series of narrower peaks, coming from quantum dots of different sizes. While the intensity of luminescence from small dots drops monotonously with temperature rises, that from the large dots displays a peculiar behavior. It actually increases within the temperature range of 140–170 K, the same range in which the photoluminescence peak shows narrowing. The simultaneous increase of luminescence from some quantum dots and decrease from others are believed to be responsible for the red shift and narrowing of the observed photoluminescence peak. A simple analytically solvable rate equation model was used to understand the spectral data. We suggest that the unusual behaviors observed can be understood as resulting from a transfer of thermally activated carriers from small to large quantum dots.     

Two-dimensional crystal growth of thermally converted organic semiconductors at the surface of ionic liquid and high-mobility organic field-effect transistors

We report a novel solution-crystallization method to grow two-dimensional platelet-shaped single-crystals of well-known insoluble organic semiconductors via thermal conversion of their precursor molecules dissolved in ionic liquids (ILs). By optimizing conditions of the crystal growth regarding physical properties of ILs such as density and viscosity, we successfully and reproducibly obtained thin platelets of pentacene and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) single-crystals, with which nearly the best performing field-effect transistors are constructed for the two compounds. The prompt and simple technique has opened the way to use practically insoluble organic semiconductor materials for high-performance printed electronics, which enables mass-producible and large-area organic circuitry devices.     

Volume and surface recombination of injected carriers in cylindrical semiconductor ingots

An exact solution to the diffusion-recombination problem is obtained for the case of a sample in the form of a right circular cylinder with arbitrary bulk lifetime, arbitrary surface recombination velocity on the lateral curved surface of the sample, and infinite surface recombination velocity on the (lapped) plane end surfaces of the sample. The latter surfaces may be regarded as electrical contacts to the sample, and in such a case the geometry corresponds precisely to a very commonly used experimental arrangement for recombination measurements. Relations between bulk lifetime, surface recombination velocity and observed time constant are calculated and plotted with the height-radius ratio of the cylinder as parameter for the principal decay mode, and the higher-mode decay scheme is worked out for a few cases of practical interest. Examples Of simultaneous surface recombination and bulk lifetime measurement by observation of the higher-mode decay components are presented.     

Direct tunneling of electrons in Al-n+-Si-SiO2-n-Si structures under conditions of nonstationary depletion of the semiconductor surface of the majority charge carriers

Specific features of direct tunneling of electrons through an ultrathin (∼40 ?) oxide in metal-SiO₂-Si structures under nonstationary conditions of depletion of the semiconductor surface, in which case the potential relief in the insulator is only slightly perturbed by external electric fields, have been experimentally studied. Penetrability of the tunneling barrier is appreciably limited by a classically forbidden region in n-Si (this region is brought about by fixed negative charge in SiO₂). As the voltage drop across oxide is increased, the electrons localized within this oxide transfer to the semiconductor, which is accompanied by a drastic increase in the tunneling current. The values of coefficients linear rise in the logarithm of tunneling current as the voltage at the isolator is increased are determined from the experiment. These values are not consistent with the data calculated on the basis of a model of a rectangular barrier with parameters typical of “thick” oxides. It is shown that actual values of the effective mass are bound to be larger than 0.5m ₀, while the height of the barrier is bound to be lower than 3.1 eV.     

Determination of the density of surface states at the semiconductor-insulator interface in a metal-insulator-semiconductor structure

The temporal dependence of thermal generation of electrons from occupied surface states at the semiconductor-insulator interface in a metal-insulator-semiconductor structure is studied. It is established that, at low temperatures, the derivative of the probability of depopulation of occupied surface states with respect to energy is represented by the Dirac δ function. It is shown that the density of states of a finite number of discrete energy levels under high-temperature measurements manifests itself as a continuous spectrum, whereas this spectrum appears discrete at low temperatures. A method for processing the continuous spectrum of the density of surface states is suggested that method makes it possible to determine the discrete energy spectrum. The obtained results may be conducive to an increase in resolution of the method of non-stationary spectroscopy of surface states.