Characteristics of tunneling and impact ionization in ZnS:Mn-based thin-film electroluminescent structures

A method for measuring the characteristics of tunneling and impact ionization in thin-film electroluminescent emitters is suggested. This method makes it possible to find time dependences of the space-charge layer thickness near the anode and the length of the impact ionization region, to determine more exactly the time dependence of the field in the potential barrier at the cathode interface, the maximum depth of the surface states from which electron tunneling occurs, the minimum thickness of the barrier, and the electron tunneling probability, as well as the impact ionization rate for the deep centers related to structural defects of the phosphor layer.     

Characteristics of surface states at the insulator-semiconductor interface in the thin-film electroluminescent structures based on ZnS:Mn

Distributions of the density of occupied surface electron states at the cathode interface between the insulator and phosphor in thin-film electroluminescent emitters are simulated in relation to the energy on the basis of experimental data. The dependences of the above distributions on the conditions of excitation of emitters are obtained. It is shown that these distributions shift to deeper levels of surface states as the frequency of excitation voltage is decreased and the pause between two neighboring switched-on states of emitters is increased, which corresponds to the cascade mechanism of relaxation of electrons captured by surface states. The coefficient of cascade capture of electrons prior to relaxation ((4−5) × 10⁻¹² cm²/s), instantaneous lifetime of electrons prior to relaxation (0.2–0.25 s), cross section for capture of electrons to deeper levels of surface states (in excess or on the order of (6.7−8.3) × 10⁻²¹ cm²), largest values of the densities of occupied surface states at the cathode boundary from which electron tunnel (∼2.5 × 10¹³ cm⁻²), and energy density of above-specified surface states (7 × 1014 cm⁻² eV⁻¹) have been determined. The values of the quasi-equilibrium Fermi level at the surface in the course of the activity of electroluminescent emitters varies in the range from 0.9 to 1.35 eV, depending on conditions of excitation.     

Theory of steady-state plane tunneling-assisted impact ionization waves

The effect of band-to-band and trap-assisted tunneling on the properties of steady-state plane ionization waves in p ⁺-n-n ⁺ structures is theoretically analyzed. It is shown that such tunneling-assisted impact ionization waves do not differ in a qualitative sense from ordinary impact ionization waves propagating due to the avalanche multiplication of uniformly distributed seed electrons and holes. The quantitative differences of tunneling-assisted impact ionization waves from impact ionization waves are reduced to a slightly different relation between the wave velocity u and the maximum field strength E _M at the front. It is shown that disregarding impact ionization does not exclude the possibility of the existence of tunneling-assisted ionization waves; however, their structure radically changes, and their velocity strongly decreases for the same E _M . A comparison of the dependences u(E _M ) for various ionization-wave types makes it possible to determine the conditions under which one of them is dominant. In conclusion, unresolved problems concerning the theory of tunneling-assisted impact ionization waves are discussed and the directions of further studies are outlined.     

Avalanche injection in high-speed thyristors

The aim of the present paper is to show the possibility of designing high-power semiconductor switches the turn-on time of which is as short as that of hydrogen thyratrons. These devices switch pulses of 10⁵W with turn-on times in the nanosecond range. In the OFF state all the voltage applied to a semiconductor switch is concentrated on the space-charge region (SCR) where there are assumed to be no free carriers. The process of switching into the ON state in a conventional thyristor switching mechanism is to fill the SCR with free electrons and holes injected from emitters by diffusion through base regions. The generation of carriers due to impact ionization in the SCR during the whole transition process accelerates switching. The avalanche injection (AI) suggested by Gunn [1] in diodes is the process providing impact ionization despite the voltage decrease in the device during its switching. At first we consider simplified AI processes and their potentialities in three-layer structures. Then the results are extended to more complex four- and five-layer structures by including the gate current. At the end the experimental data are given.     

Interband scattering effects on secondary ionization coefficients in GaAs

The ionization rates of holes and electrons in GaAs were measured experimentally over a wide doping range covering field values from 2.2×10⁵ V/cm to 4.7×10⁵ V/cm. As opposed to most experimemental measurements in GaAs, no assumption of equal ionization rates of the two carriers has been made. By using the conventional theoretical relationship between carrier ionization coefficients and multiplication data, the effective α is observed to be larger than β in lightly doped diodes while β is larger than α in heavily doped diodes. Previous theories of ionization rates utilizing just the normal conduction and valence bands do not show any possibility of such a crossover. It is suggested that electron transitions to higher conduction bands, which effectively increase the equilibration time of the electron distribution function, offer a resolution of this difficulty. The dependence of the effective electron ionization rate on doping can be explained by the requirement that electrons must make an interband transition before reaching the ionization threshold energy. This interband transition time estimated by this experiment is of the order of 10^?13 sec and is comparable to the transit time of electrons in the avalanching region. The breakdown voltages extrapolated from the measured α and β are consistent with those observed experimentally.     

Simulation of special features of the drift-mobility saturation in submicrometer silicon structures

Special characteristics of the high-field drift of electrons in submicrometer n⁺-n-n⁺ structures are studied by mathematical simulation methods in the quasi-hydrodynamic approximation. Alternative dependences of the mobility and energy-relaxation time on the electron temperature are used to calculate the profiles of the potential, temperature, drift mobility, and density of the thermal-energy flux of electrons. It is shown that, in a submicrometer configuration, a large part of the thermal energy acquired by an electron in a high-resistivity n-type region is dissipated in a low-resistivity n⁺-type contact. This effect reduces the rate of increase in the electron temperature in the drift region as the voltage increases, brings about an increase in the effective mobility, and prevents saturation of the drift velocity, as is shown by the calculated current-voltage characteristics.     

Electrical Breakdown in Pure <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-Si

The results of calculations of the dependences of the kinetic coefficients of impact ionization and thermal recombination on an electric field in pure silicon are presented. By analogy with germanium, the dependences of the breakdown field Е_br on the material compensation ratio K are calculated. The validity of such calculation is justified in detail. The Е_br(K) curves are presented and compared with experimental data in the weak-compensation region. Matching with experimental results at which satisfactory agreement between theory and experiment is observed is performed.     

Negative resistance in p-n junctions under avalanche breakdown conditions, part II

The small-signal impedance of the space-charge region of p-n junctions under avalanche breakdown conditions is calculated using reasonably realistic dependences of electron and hole ionization rates and drift velocities upon electric field. Two structures are analyzed: one is p⁺νn⁺structure which has a fairly uniform distribution of avalanche multiplication, and the other is a singly diffused junction which is a hybrid of an abrupt and a linear graded junction. Both structures show negative resistance when the transit time of carriers becomes appreciable. A computer program was evolved which requires, as input, the impurity profile and field dependences of ionization rates and drift velocities. The program first calculates the dc field and electron and hole currents and then solves the ac small-signal problem. Both the ac small-signal impedance and theQof the diode are calculated.     

Scalable Microstructured Photoconductive Terahertz Emitters

The development of scalable emitters for pulsed broadband terahertz (THz) radiation is reviewed. Their large active area in the 1 – 100 mm² range allows for using the full power of state-of-the-art femtosecond lasers for excitation of charge carriers. Large fields for acceleration of the photogenerated carriers are achieved at moderate voltages by interdigitated electrodes. This results in efficient emission of single-cycle THz waves. THz field amplitudes in the range of 300 V/cm and 17 kV/cm are reached for excitation with 10 nJ pulses from Ti:sapphire oscillators and for excitation with 5 μJ pulses from amplified lasers, respectively. The corresponding efficiencies for conversion of near-infrared to THz radiation are 2.5 × 10^-4 (oscillator excitation) and 2 × 10^-3 (amplifier excitation). In this article the principle of operation of scalable emitters is explained and different technical realizations are described. We demonstrate that the scalable concept provides freedom for designing optimized antenna patterns for different polarization modes. In particular emitters for linearly, radially and azimuthally polarized radiation are discussed. The success story of photoconductive THz emitters is closely linked to the development of mode-locked Ti:sapphire lasers. GaAs is an ideal photoconductive material for THz emitters excited with Ti:sapphire lasers, which are widely used in research laboratories. For many applications, especially in industrial environments, however, fiber-based lasers are strongly preferred due to their lower cost, compactness and extremely stable operation. Designing photoconductive emitters on InGaAs materials, which have a low enough energy gap for excitation with fiber lasers, is challenging due to the electrical properties of the materials. We discuss why the challenges are even larger for microstructured THz emitters as compared to conventional photoconductive antennas and present first results of emitters suitable for excitation with ytterbium-based fiber lasers. Furthermore an alternative concept, namely the lateral photo-Dember emitter, is presented. Due to the strong THz output scalable emitters are well suited for THz systems with fast data acquisition. Here the application of scalable emitters in THz spectrometers without mechanical delay stages, providing THz spectra with 1 GHz spectral resolution and a signal-to-noise ratio of 37 dB within 1 s, is presented. Finally a few highlight experiments with radiation from scalable THz emitters are reviewed. This includes a brief discussion of near-field microscopy experiments as well as an overview over gain studies of quantum-cascade lasers.     

Far infrared response of thin film Bi2Sr2CaCu2O8 using a free electron laser

The far infrared response of granular thin-film Bi₂Sr₂CaCu₂O₈ superconductor has been investigated using long (≈5 μs) but sharply truncated free electron laser pulses in the frequency range between 50 cm^?1 and 125 cm^?1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (≈ 200 cm^?1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model.     

Impact ionization of excitons in single-crystal silicon and its effect on the exciton concentration and luminescence near the fundamental absorption edge

The effect of impact ionization of excitons by free charge carriers on the exciton concentration in single-crystal silicon (c-Si) at room temperature and at a high injection level is investigated. At sufficiently high concentrations of free electrons (n), the impact ionization dominates over thermal ionization. At such n, the effect results in much lower exciton concentrations (n _ex) compared to those with disregard of it and linear or almost linear portions in the dependences n _ex(n) and the dependences of the near-band-edge luminescence intensity in c-Si on the intensity of its excitation. The proposed technique for calculating n _ex can be developed for other semiconductors at other temperatures.     

Monte Carlo simulation of impact ionization rates in InAlAs-InGaAssquare and graded barrier superlattice

The Monte Carlo method is used to analyze impact ionization rates for electrons and holes in a 〈100〉 crystal direction In_0.52Al_0.48As-In_0.53Ga_0.47 As square and graded barrier superlattice. The calculated impact ionization rate ratio α/β is enhanced to more than 10 in a wide barrier and narrow-well square barrier superlattice. This is because the hole ionization rate β is greatly reduced in the narrower well superlattice, while the electron ionization rate α is less sensitive to well and barrier layer thickness. These results are explained by a combination of the ionization dead space effect for the barrier layer and the electron ionization rate enhancement in the well layer due to large conduction band edge discontinuity. Furthermore, it is found that in a graded barrier superlattice, the impact ionization rate ratio α/β is smaller than that for a square barrier superlattice having the same barrier and well thickness. This is due to the occurrence of hole ionization in the narrow bandgap region in graded barriers. The band structure effects on hot carrier energy distribution, as well as impact ionization, are also discussed     

Giant burst of impact ionization in a <Emphasis Type="Italic">p-n</Emphasis> junction of the 6<Emphasis Type="Italic">H</Emphasis>-SiC polytype

A study of the electron component of impact ionization in the p ⁺-n ^?-n ⁺ junction in the 6HSiC polytype made it possible to detect a giant burst of impact ionization and origination of an extra early avalanche breakdown. The electric field of this breakdown is lower by ～20% than the electric field of the breakdown arising as a result of a steady development of the impact ionization. It is of interest that this phenomenon occurs abruptly, without any apparent causes, in particular, without an increase in the dark current characteristic of a prebreakdown state of the p-n junction. Conditions for origination of an unusual breakdown and its properties made it possible to assume that there are nonlinear processes that give rise to a streamer. In the p-n junction plane, the anomalous breakdown is seen as a narrow glowing track with a width of ≈10 μm. This effect takes place in the conditions of the Wannier-Stark ladder of states. The latter can stimulate a local accumulation of charge and formation of a streamer structure.     

The determination of impact ionization coefficients in ln0.2Ga0.8As/GaAs strained-layer superlattice mesa photodiodes

Sandia National Laboratories, Albuquerque, NM 87185 Photocurrent multiplication measurements have been performed on two different In_0.2Gao_0.8As/GaAs strained-layer superlattice (SLS)p ⁺n photodiode structures which are designed to permit simultaneous injection of electrons and holes. Initial devices were found to suffer from low quantum efficiencies produced by small electron diffusion lengths as well as mixed injection caused by lower than expected optical absorption coefficients in the SLSn ⁺ contact layers for hole injection conditions. Using a second device structure having a thicker n⁺ contact region, the electron multiplication factors are found to be larger than that of holes with a ratio of the electron to hole ionization coefficient of 1.4 for fields between 2.9 and 3.4 x 10⁵V/cm.     

Influence of radiation defects on electrical losses in silicon diodes irradiated with electrons

Silicon diodes with a p ⁺-n junction irradiated with 3.5-MeV electrons (the fluence ranged from 10¹⁵ to 4 × 10¹⁶ cm⁻²) have been studied. It is established that the dependence of the tangent of the angle of electrical losses tanδ on the frequency f of alternating current in the range f = 10²−10⁶ Hz is a nonmonotonic function with two extrema: a minimum and a maximum. Transformation of the dependences tanδ(f) as the electron fluence and annealing temperature are increased is caused by a variation in the resistance of n-Si (the base region of the diodes) as a result of accumulation (as the fluence is increased) or disappearance and reconfiguration (in the course of annealing) of radiation defects. The role of time lag of the defect recharging in the formation of tanδ(f) is insignificant.     

Superlinear electroluminescence in GaSb-based heterostructures with high potential barriers

The electroluminescence in isotype and anisotype light-emitting diode heterostructures grown by the method of liquid-phase epitaxy with large conduction-band offset ΔE _c at the heterointerface between a narrow-band active region and a wide-band layer is studied. Two types of electroluminescence peaks are observed in the range of photon energies 0.28–0.74 eV at temperatures T = 300 and 77 K; in this case, a super-linear increase in the intensity and optical power of emission by a factor of 1.5–2 is observed in the range of pump currents 20–220 mA. This effect is attributed to the formation of additional electron-hole pairs as a result of impact ionization by hot electrons heated as a result of the band offset ΔE _c in the conduction band at the n-AlGaAsSb/n-InGaAsSb and n-GaSb/n-InGaAsSb heteroboundaries. This effect can be used to increase the quantum efficiency of semiconductor emitters (light-emitting diodes, lasers) in the mid-infrared region.     

Scattering of conduction electrons at a spatially correlated system of charges in a heavily doped GaAs: Te semiconductor

The results of studying the absorption of infrared radiation by free charge carriers in the GaAs:Te single crystals, grown by the Czochralski method, had the electron concentration n ₀=5×10¹⁷?6×10¹⁸ cm^?3 are reported. An analysis of the spectral dependences of the absorption coefficient took into account the spatial correlation in the impurity-charge distribution. It is shown that the short-range correlation model makes it possible to account for the decrease in the absorption coefficient and a weakening of its spectral dependence, in the region of the impurity-mediated free-carrier absorption.     

Extremal dependence of the concentration of paramagnetic centers related to dangling bonds in si on ion-irradiation dose as evidence of nanostructuring

Dose dependences of the concentration of paramagnetic centers with g=2.0055 under irradiation of silicon with Ge⁺, Ar⁺, and Ne⁺ ions have been studied in detail. It is shown that, in all cases, the dependences are characterized by the previously unnoticed presence of maxima at doses corresponding to transition to complete amorphization. This feature is explained in terms of a model assuming an additional contribution to electron spin resonance from dangling bonds at the interface between nanocrystals and the amorphous matrix.