Electron-initiated Impact Ionization Research Articles

Features of impact ionization occuring in semiconductor compaunds InGaN and InAlN

Background. The unique spectral position of terahertz range determines the difficulties of developing compact solid-state sources of terahertz radiation. In most cases, the operating frequencies of existing devices are displaced in the terahertz part of the spectrum. It is known that impact ionization, especially in its initial stage, is a rather fast process that can be used to improve the devices frequency properties. The temporal and spatial restrictions of impact ionization in InGaN and InAlN compounds need to be evaluated to determine the prospects of its use for the terahertz range devices design. Purpose of Work. The aim of this work is to determine the values of time and space delays in the development of electron-initiated impact ionization in the InGaN and InAlN semiconductor compounds at the initial stage of impact ionization. Techniques and Methodology. The modeling of electronic transport was performed using the Ensemble Monte Carlo technique. It takes into account the all actual mechanisms of scattering іincluding the alloy potential scattering and impact ionization. The constant electric field approximation has been considered. The homogeneous materials and the materials with composition depending on coordinate were considered. The spatial distributions of impact ionization acts for a charge carriers ensemble were analyzed to determine a characteristic mean distance a carrier travels before ionizing ("dead space") and a delay time of impact ionization appearing. Results. The delay times of impact ionization in InGaN and InAlN compounds if electric field strengths greater than 100 kV/cm are less than 2 ps. Delay times can be an order of magnitude lower when Ga and Al content less than 60%, respectively in InGaN and InAlN. The mean distance a carriers acquire enough energy to impact ionize for this case are about 100-200 nm. This distance decrease with increasing electric field strength and may be less than 50 nm in the case of the InAlN. The dead space length can be changed by using a graded band layer in which the band gap decreases towards the anode. But the smallest values of the "dead space" correspond to homogeneous materials. Conclusions: Thus, impact ionization at the initial stage can be used in ultrahigh-frequency devices in the terahertz range, in particular, as a mechanism of energy relaxation.

Detection of single holes generated by impact ionization in silicon

We propose a method for the observation of the electron-initiated impact ionization process in Si, which is potentially capable of detecting individual impact ionization events. This method detects holes generated by the impact ionization with single charge sensitivity. We demonstrate the method at 8 K by detecting the chain of single holes generated for a constant electron-injection current with the injection-energy threshold close to the Si bandgap energy.We propose a method for the observation of the electron-initiated impact ionization process in Si, which is potentially capable of detecting individual impact ionization events. This method detects holes generated by the impact ionization with single charge sensitivity. We demonstrate the method at 8 K by detecting the chain of single holes generated for a constant electron-injection current with the injection-energy threshold close to the Si bandgap energy.

Microcorona Discharge-Mediated Nonthermal Atmospheric Plasma for Seed Surface Modification

By exploiting the physical effect of a highly nonuniform localized electric field and electron-initiated impact ionization on space charge, we generated homogeneous nonthermal plasma under ambient atmosphere. We evaluated the physical characteristics and evolution of microcorona discharge-induced nonthermal atmospheric plasma (NAP) based on a point-to-plane electrode in open air with two distinct configurations. High-voltage pulses were employed as the primary power source of corona discharge generation to reveal the fundamental mechanism, polarity effect and feasibility of using NAP for organic surface modification. Consequently, we employed NAP to modify the surface of rice (Oryza sativa L.) seeds to improve their wettability. The surface modification of the rice seeds was investigated via water apparent contact angle (ACA) and water imbibition (WI) measurements. The ACA and WI measurements revealed not only the improvement in the wetting properties but also the mutual relationships between and limitations of ACA and WI analysis. We found that the WI time reached saturation after a certain treatment time, called the threshold treatment time. Because vacuum conditions are not required, well-established NAP technology will garner interest in many fields, ranging from the life, environmental, and biomedical sciences to solid-state electronics applications.

Multi-Gain-Stage InGaAs Avalanche Photodiode With Enhanced Gain and Reduced Excess Noise

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise and stable linear-mode operation at much higher avalanche gain than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1000.

Electron dominated impact ionization and avalanche gain characteristics in InAs photodiodes

An experimental investigation into impact ionization in InAs photodiodes is presented. Photomultiplication measurements on p-i-n and n-i-p diodes show that while high avalanche gains can be obtained for electron initiated multiplication, there is virtually no gain with hole initiated multiplication. This indicates that the electron ionization coefficient is significantly greater than the hole ionization coefficient raising the possibility of extremely low noise InAs avalanche photodiodes when gain is initiated by electrons. The onset of electron initiated impact ionization was detectable at electric fields below 10kVcm−1 with useful gain observed at biases below 10V.

Exciton impact-ionization dynamics modulated by surface acoustic waves in GaN

The quenching of the photoluminescence induced by the fundamental and guided high-order Rayleigh-type modes propagating in the $\mathrm{Ga}\mathrm{N}∕6\mathrm{H}\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(0001)$ heterostructure is compared. Two material types have been considered: conventional GaN and GaN underneath the Ga droplets formed during the molecular-beam-epitaxy growth. The different piezoelectric field-depth profile of each surface acoustic wave (SAW) is used to analyze their characteristics. A coupled-rate-equations model based on the impact ionization of the free and donor-bound excitons under the SAW-generated piezoelectric fields is presented, which satisfactorily reproduces the experimental results. The exciton impact-ionization rate reveals two regimes when the SAW power is increased, each of which seems to be plausibly dominated by the hole- and electron-initiated impact ionization. The SAW-induced impact-ionization model is also indicated to be valid for other wide direct band-gap piezoelectric semiconductors, such as CdS, ZnO, and AlN, where the field ionization requires an even larger electric field than in GaN.

Full band Monte Carlo study of high field transport in cubic phase silicon carbide

A full band Monte Carlo study of the electron transport in 3C–SiC is presented based on an ab initio band structure calculation using the local density approximation to the density functional theory. The scattering rates and impact ionization transition rates have been calculated numerically from the ab initio band structure using both energy dispersion and numerical wave functions. This approach reduces the number of empirical parameters needed to a minimum. The two empirical coupling constants used have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The peak velocity was found to be approximately 2.2×107 cm/s with a clear negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2–10 times stronger than the reported values for the hole initiated impact ionization.

Estimates of Impact Ionization Coefficients in Superlattice-Based Mid-Wavelength Infrared Avalanche Photodiodes

ABSTRACTWe describe band engineering strategies to either enhance or suppress electron-initiated impact ionization relative to hole-initiated impact ionization in type II superlattice mid-wavelength infrared avalanche photodiodes. The strategy to enhance electron-initiated impact ionization involves placing a high density of states at approximately one energy gap above the bottom of the conduction band and simultaneously removing valence band states from the vicinity of one energy gap below the top of the valence band. This gives the electrons a low threshold energy and the holes a high one. The opposite strategy enhances hole-initiated impact ionization. Estimates of the electron (α) and hole (β) impact ionization coefficients predict that α/β>>1 in the first type of superlattice and α/β<<1 in the second type.

Monte Carlo study of high-field carrier transport in 4H-SiC including band-to-band tunneling

A full-band ensemble Monte Carlo simulation has been used to study the high-field carrier transport properties of 4H-SiC. The complicated band structure of 4H-SiC requires the consideration of band-to-band tunneling at high electric fields. We have used two models for the band-to-band tunneling; one is based on the overlap test and the other on the solution of the multiband Schrödinger equations. The latter simulations have only been performed for holes in the c-axis direction, since the computer capacity requirement are exceedingly high. Impact-ionization transition rates and phonon scattering rates have been calculated numerically directly from the full band structure. Coupling constants for the phonon interaction have been deduced by fitting of the simulated low-field mobility as a function of lattice temperature to experimental data. Secondary hot electrons generated as a consequence of hole-initiated impact ionization are considered in the study for both models of band-to-band tunneling. When the multiband Schrödinger equation model is used for holes in the c-axis direction, a significant change in the electron energy distribution is found, since the hole impact-ionization rate is very much increased with this model. The secondary electrons increase the average energy of the electron distribution leading to a significant increase in the electron-initiated impact-ionization coefficients. Our simulation results clearly show that both electrons and holes have to be considered in order to understand electron-initiated impact ionization in 4H-SiC.

Band structure engineering of superlattice-based short-, mid-, and long-wavelength infrared avalanche photodiodes for improved impact ionization rates

We study the effects of the electronic band structure on the hole- and electron-initiated impact ionization in Sb-based superlattice avalanche photodiodes. Earlier calculations have revealed that bulk alloy AlGaSb avalanche photodiodes with alloy composition near the resonance between the energy gap and the spin-orbit splitting have an enhanced hole to electron impact ionization coefficient ratio for a low electric field but not for a high electric field. The absence of an enhancement under high fields is due to carrier heating spreading the hole distribution in the splitoff band. A strategy to extend this type of enhancement to high fields in a superlattice involves band engineering the superlattice to place flat bands at approximately one energy gap below the top of the valence band. This prevents holes from spreading in energy and hence gives rise to strong hole-initiated impact ionization. Quantitative results are presented for short-infrared AlAs/InGaAs/AlAs/InGaSb, midinfrared InAs/InGaSb/AlSb, and long-infrared InAs/InGaSb/AlSb superlattices.

Full-Band Monte Carlo Simulation of Electron Transport in 3C-SiC

A full band Monte Carlo study of the electron transport in 3C-SiC is presented based on ab initio band structure calculation using the Local Density Approximation (LDA) to the Density Functional Theory (DFT). The scattering rates and impact ionization transition rates have been calculated numerically from the band structure using both energy dispersion and wave functions. Coupling constants for the phonon interactions have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The saturation velocity was found to be approximately 2.2*107 cm/s with a visible negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2 to 10 times stronger than reported values for the hole initiated impact ionization

Gate polarity dependence of impact ionization probabilities in metal-oxide-silicon structures under Fowler–Nordheim stress

A theoretical investigation on the gate polarity dependence of Fowler–Nordheim (FN) tunneling electron initiated impact ionization probabilities in the bulk silicon dioxide (SiO 2) films as well as hole injection from the anode material (n + poly-Si gate or silicon substrate) in metal-oxide-silicon devices is presented. Our theoretical results of the gate polarity dependence of the probabilities of various impact ionization processes under constant current FN stress correlate the experimentally observed positive charge generation/trapping and stress-induced trap creation in thick to thin SiO 2 films.

Modelling of the hole-initiated impact ionization current in the framework of hydrodynamic equations

Several research papers have shown the feasibility of the hydrodynamic transport model to investigate impact ionization in semiconductor devices by means of mean-energy-dependent generation rates. However, the analysis has been usually carried out for the case of the electron-initiated impact ionization process and less attention has been paid to the modelling of the generation rate due to impact ionization events initiated by holes. This paper therefore presents an original model for the hole-initiated impact ionization in silicon and validates it by comparing simulation results with substrate currents taken from p-channel transistors manufactured in a 0.35 μm CMOS technology having three different channel lengths. The experimental data are successfully reproduced over a wide range of applied voltages using only one fitting parameter. Since the impact ionization of holes triggers the mechanism responsible for the back-bias enhanced gate current in deep submicron nMOS devices, the model can be exploited in the development of non-volatile memories programmed by secondary electron injection.

Ensemble Monte Carlo Study of Electron Transport in Bulk Indium Nitride

Ensemble Monte Carlo calculations of electron transport at high applied electric field strengths in bulk, wurtzite phase InN are presented. The calculations are performed using a full band Monte Carlo simulation that includes a pseudopotential band structure, all of the relevant phonon scattering agents, and numerically derived impact ionization transition rates. The full details of the first five conduction bands, which extend in energy to about 8 eV above the conduction band minimum, are included in the simulation. The electron initiated impact ionization coefficients and quantum yield are calculated using the full band Monte Carlo model. Comparison is made to previous calculations for bulk GaN and ZnS. It is found that owing to the narrower band gap in InN, a lower breakdown field exists than in either GaN or ZnS.

Ensemble Monte Carlo Study of Electron Transport in Bulk Indium Nitride

AbstractEnsemble Monte Carlo calculations of electron transport at high applied electric field strengths in bulk, wurtzite phase InN are presented. The calculations are performed using a full band Monte Carlo simulation that includes a pseudopotential band structure, all of the relevant phonon scattering agents, and numerically derived impact ionization transition rates. The full details of the first five conduction bands, which extend in energy to about 8 eV above the conduction band minimum, are included in the simulation. The electron initiated impact ionization coefficients and quantum yield are calculated using the full band Monte Carlo model. Comparison is made to previous calculations for bulk GaN and ZnS. It is found that owing to the narrower band gap in InN, a lower breakdown field exists than in either GaN or ZnS.

Calculation of the electron initiated impact ionization transition rate in cubic and hexagonal phase ZnS

The wave-vector dependent, electron initiated, impact ionization transition rates for the cubic and hexagonal phases of ZnS have been numerically determined using a pseudopotential calculated band-structure and wave-vector dependent dielectric function. The rates for both phases show a strong dependence on the relative band structures. A detailed band-by-band analysis of the transition rate in the first Brillouin zone has been performed to determine the wave-vector dependence of the rate. The energy dependent rates have also been computed by averaging the transition rate over energy. It is found that the hexagonal phase of ZnS exhibits a higher ionization transition rate than the cubic phase. A preliminary estimation of the nature of the threshold has been made using a previously published phonon scattering rate.

Threshold and probability of impact ionization by electrons in narrow-gap p-type semiconductors with highly degenerate holes

The process of electron-initiated impact ionization of states in the heavy-hole band is investigated theoretically for a p-type narrow-gap semiconductor with energy bands governed by the Kane dispersion law, subject to the condition that the Fermi level of the holes lies in the valence band. The dependence of the minimum electron energy for ionization of a state at the Fermi level in the valence band on the heavy-hole Fermi momentum is determined. The probability of impact ionization for electrons with near-threshold energies is calculated for the case in which the heavy-hole Fermi momentum exceeds the hole threshold momentum for the given ionization process. Relations between the temperatures of holes and electrons with energies of the order of the threshold value are found, thereby establishing the validity domain of the final results.

Monte Carlo calculation of electron initiated impact ionization in bulk zinc-blende and wurtzite GaN

Calculations of the high-field electronic transport properties of bulk zinc-blende and wurtzite phase gallium nitride are presented focusing particularly on the electron initiated impact ionization rate. The calculations are performed using ensemble Monte Carlo simulations, which include the full details of the band structure derived from an empirical pseudopotential method. The model also includes the numerically generated electron impact ionization transition rate, calculated based on the pseudopotential band structures for both crystallographic phases. The electron initiated impact ionization coefficients are calculated as a function of the applied electric field. The electron distribution is found to be cooler and the ionization coefficients are calculated to be lower in the wurtzite phase as compared to zinc-blende gallium nitride at compatable electric-field strengths. The higher electron energies and the resulting larger impact ionization coefficients in zinc-blende gallium nitride are believed to result from the combined effects of a lower density of states and phonon scattering rate for energies near and below 3 eV above the conduction-band minimum, and a somewhat higher ionization transition rate compared to the wurtzite phase. The nature of the impact ionization threshold in both phases of gallium nitride is predicted to be soft. Although there is considerable uncertainty in the knowledge of the scattering rates and the band structure at high energies which lead to uncertainty in the Monte Carlo calculations, the results presented provide a first estimate of what the electron initiated impact ionization rate in GaN can be expected to be.

Theoretical Study of Electron Initiated Impact Ionization Rate in Bulk GaN using a Wave Vector Dependent Numerical Transition Rate Formulation

ABSTRACTIn this paper, we present ensemble Monte Carlo based calculations of electron initiated impact ionization in bulk zincblende GaN using a wavevector dependent formulation of the interband impact ionization transition rate. These are the first reported estimates, either theoretical or experimental, of the impact ionization rates in GaN. The transition rate is determined from Fermi’s golden rule for a two-body screened Coulomb interaction using a numerically determined dielectric function as well as by numerically integrating over all of the possible final states. The Monte Carlo simulator includes the full details of the first four conduction bands derived from an empirical pseudopotential calculation as well as all of the relevant phonon scattering mechanisms. It is found that the ionization rate has a relatively "soft" threshold.

Threshold energies for impact ionization by electrons and holes in InP

The threshold conditions for impact ionization in InP at 300 K have been calculated from the electronic band structure along the three major symmetry axes. These calculations show that conditions for electron-initiated impact ionization in InP are much more favorable than in GaAs.