Intervalley Electron Transfer Research Articles

Photoinduced Negative Differential Resistivity and Gunn Oscillations in SrTiO3.

SrTiO3 , a perovskite oxide, holds significant potential for application in the field of oxide electronics. Notably, its photoelectric activity in the low temperature regime, which overlaps with the quantum paraelectric state, exhibits remarkable characteristics. In this study, it is demonstrated that when photo-excited with above band gap energy photons, SrTiO3 exhibits non-linear transport of photocarriers and voltage-controlled negative resistance, resulting from an intervalley transfer of photo-induced electrons. As a consequence of the negative resistance, the photocurrent becomes unstable and spontaneously gives rise to low frequency Gunn-like oscillations.

Twist Angle-Dependent Intervalley Charge Carrier Transfer and Recombination in Bilayer WS2.

A twist angle at a van der Waals junction provides a handle to tune its optoelectronic properties for a variety of applications, and a comprehensive understanding of how the twist modulates electronic structure, interlayer coupling, and carrier dynamics is needed. We employ time-dependent density functional theory and nonadiabatic molecular dynamics to elucidate angle-dependent intervalley carrier transfer and recombination in bilayer WS2. Repulsion between S atoms in twisted configurations weakens interlayer coupling, increases the interlayer distance, and softens layer breathing modes. Twisting has a minor influence on K valleys while it lowers Γ valleys and raises Q valleys because their wave functions are delocalized between layers. Consequently, the reduced energy gaps between the K and Γ valleys accelerate the hole transfer in the twisted structures. Intervalley electron transfer proceeds nearly an order of magnitude faster than hole transfer. The more localized wave functions at K than Q values and larger bandgaps result in smaller nonadiabatic couplings for intervalley recombination, making it 3-4 times slower in twisted than high-symmetry structures. B2g breathing, E2g in-plane, and A1g out-of-plane modes are most active during intervalley carrier transfer and recombination. The faster intervalley transfer and extended carrier lifetimes in twisted junctions are favorable for optoelectronic device performance.

GaN hot electron transistors: From ballistic to coherent

Previously we demonstrated high-current, high-gain III-nitride hot electron transistors (HETs) with collimated electron injection into an undoped base region, reaching collector current density of ∼0.44 MA/cm2 and Gummel gain β ∼ 2 at 300 K. Here, we compare the behavior of these devices from 300 to 77 and 4.2 K to elucidate the role of hot electron scattering in limiting the performance. Under cryogenic operation and Gummel biasing, we obtain a maximum current gain of 3.5 at a collector current density of 1.35 MA/cm2, limited by the onset of intervalley electron transfer, and common-emitter current gain β > 20. Our results point to the promise of nitride HETs for realizing the long-proposed coherent transistor.

Pulsed studies of intervalley transfer in Al0.35In0.65As : A paradigm for valley photovoltaics

${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$ is a direct semiconductor whose multivalley conduction-band structure has been proposed for use in so-called valley photovoltaics. In such hot-carrier solar cells, energetic (hot) photocarriers are stored in satellite valleys away from the \ensuremath{\Gamma} point, allowing them to be extracted prior to thermalization and to thereby increase power-conversion efficiency. While prior theoretical work has highlighted the potential of ${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$---a widely used barrier material in electronic and optoelectronic devices, for use in valley photovoltaics---surprisingly little is known about its electrical properties, especially how these are impacted by the application of high fields. In this work, we therefore undertake a detailed characterization of the electrical properties of Te-doped ($n$-type) ${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$, over wide ranges of temperature (3--400 K) and electric field (50 kV/cm). Using pulsed measurements to suppress the influence of Joule heating, we reveal the presence of clear negative-differential conductance in the current-voltage characteristics of the films, suggestive of the intervalley transfer of hot electrons. This conclusion is supported by the results of ensemble Monte Carlo simulations of the hot-carrier action, which confirm the connection of the observed negative-differential conductance to hot-electron transfer from the conduction-band (\ensuremath{\Gamma}) minimum, to the side valleys at the L point. The quantitative features of the experimentally determined velocity-field curves are found to be in good agreement with the results of these calculations, providing further confidence in the role of the implied intervalley transfer mechanism. Overall, these results confirm the excellent potential of ${\mathrm{Al}}_{0.35}{\mathrm{In}}_{0.65}\mathrm{As}$ for use as the absorber material in hot-carrier solar cell technology.

On increasing power of short InGaPAs graded-gap Gunn diodes

Background. The problem of development of the terahertz range with solid state devices remains relevant today. Gunn diodes, IMPATT diodes, resonant tunneling diodes and others are used as active elements. At frequencies above 100 GHz, these devices have a number of physical problems that limit maximum operation from above. One of the possibilities of creating high-frequency Gunn diodes is the use of various graded-gap multicomponent semiconductor compounds. Gunn diodes based on such compounds have higher generation efficiency and, accordingly, output power. Objectives. Multicomponent semiconductors, the fractional composition of which varies in space, can improve the interaction of the electric field and electrons in devices operating on the effect of intervalley electron transfer. To achieve the best effect, such a semiconductor should have an optimal coordinate dependence between the nonequivalent valleys of the conduction band. Therefore, the aim of the work is to investigate the dependences of the effective generation of current oscillations in the terahertz range based on a graded-gap semiconductor Ga1-x(z)Iny(z)Py(z)As1-y(z). Materials and methods. Using mathematical modeling n+ - n - n+ Gunn diodes based on a graded-gap semiconductor Ga1-x(z)Iny(z)Py(z)As1-y(z) with active region length being 1,0 µm and the concentration of ionized impurities in it being 9×1016 cm–3 are considered. The study was carried out based on the solving Boltzmann kinetic equation for a three-level Г–L–X model of intervalley electron transfer. The resulting system of equations is solved numerically and allows one to get the dynamic distribution of the concentration of charge carriers, their energy, current density, electric field strength, and the voltage drop across the diode. Results. It is shown that in graded-gap diodes based on Ga1-x(z)Iny(z)Py(z)As1-y(z) some domain current instability mode can be implemented. Unlike similar devices based on homogeneous semiconductors, such as GaAs, InP or Ga0,5In0,5As, in diodes based on graded-gap Ga1-x(z)Iny(z)Py(z)As1-y(z) undamped current oscillations occur. The maximum power of the main mode of such oscillations is 19 mW at a frequency of 95 GHz. Higher harmonics are present in the oscillations spectrum: the power of the second harmonic is 1,6 mW, and that of the third is 0,3 mW. The frequency and power of self-oscillations in graded-gap diodes depends on the composition of the semiconductor compound both in the anode and in the cathode and is observed at optimal values of the applied voltage. Conclusions. Graded-gap Gunn diodes based on Ga1-x(z)Iny(z)Py(z)As1-y(z) with active region length being 1,0 µm and the concentration of ionized impurities in it being 9×1016 cm–3 are able to generate continuous current oscillations in a fairly wide frequency range due to efficient operation at the main, second and third harmonics. The research results can be used in the development of high-frequency devices for various scientific research.

About bond model of S-type negative differential resistance in GaP LEDs

The bond models are presented that explain the S-type anomaly of GaP LEDs’ electrical characteristics at temperatures Т ≤ 120 K. A possible mechanism of negative differential resistance appearing in current-voltage characteristics is proposed, based on the features of the gallium phosphide complex band structure. The conductive zone absolute minimum in this crystal is near the Brillouin zone end. Due to the positive internal bond, controlled by the current, intervalley electron transfer occurs from the side valley to the higher one with the smaller effective electron mass. While the applied voltage is increased, electrons move from the lateral valley to the direct conductive zone bottom and an S-type negative differential resistance region appears.

Negative differential resistance in the I–V curves of Al2O3/AlGaN/GaN MIS structures

Negative differential resistance is firstly observed in I–V characteristic of GaN MIS structure, which is induced by intervalley electron transfer.

Gate-Tunable Electron Transport Phenomena in Al-Ge⟨111⟩-Al Nanowire Heterostructures.

Electrostatically tunable negative differential resistance (NDR) is demonstrated in monolithic metal–semiconductor–metal (Al–Ge–Al) nanowire (NW) heterostructures integrated in back-gated field-effect transistors (FETs). Unambiguous signatures of NDR even at room temperature are attributed to intervalley electron transfer. At yet higher electric fields, impact ionization leads to an exponential increase of the current in the ⟨111⟩ oriented Ge NW segments. Modulation of the transfer rates, manifested as a large tunability of the peak-to-valley ratio (PVR) and the onset of impact ionization is achieved by the combined influences of electrostatic gating, geometric confinement, and heterojunction shape on hot electron transfer and by electron–electron scattering rates that can be altered by varying the charge carrier concentration in the NW FETs.

Статический домен в приборе с междолинным переносом электронов на основе варизонного AlGaAs

Статический домен в приборе с междолинным переносом электронов на основе варизонного AlGaAs

Двухканальный переключатель СВЧ-мощности на основе электрически активных полупроводниковых структур

When constructing the fast-acting two-channel microwave switch, it is difficult to use p–i–n-diodes due to inertia of processes in such structures at change of control voltage polarity for providing of deep modulation. Under the practical realization of the microwave switches on p—i—n-diodes, the requirements to the operating speed of the output signal and to the frequency range are in conflict with each other. The optimum decision may be to use the bulk (without p—n-junctions) two-electrode semiconductor structures based on the effect of intervalley transfer of electrons (TEDs) and chalcogenide-glass-semiconductors (CGS-diodes) with high operating speed and stability at considerable power levels in the wide frequency band. The paper presents the construction of the two-channel microwave switch in the three-centimeter range of wave lengths based on bulk semiconductor structures having negative differential conductivity (NDC) of N- and S-type, and realizing the functions of peak power modulator on a TED-diode and the switch on a CGS-diode respectively.

Monte Carlo study of pulsed terahertz emission from multilayer GaAs/AlGaAs structure

Terahertz emission from the freestanding multilayer GaAs/AlGaAs δ-doped heterostructure illuminated by a femtosecond optical pulse is investigated by ensemble Monte Carlo simulations. It is shown that the amplitude of transient photocurrent and the energy of terahertz emission from the multilayer δ-doped heterostructure strongly depend on the thickness of GaAs absorption layers. The emitted energy increases as the thickness of the GaAs absorption layers decreases. The conduction band nonparabolicity and the electron intervalley transfer into upper valleys reduce the energy of terahertz emission. The main factor limiting the intensity of terahertz emission at a very small thickness of GaAs absorption layers is identified.

Quantized space-charge waves in quasi-2D

Hot-electron transport theory of standard semiconductor layered structures predicts the occurrence of negative differential resistance (NDR) associated with inter-valley electron transfer, negative effective mass, and real-space transfer. An analysis of the growth of quantized space-charge waves in single and double heterostructures in which no real-space transfer occurs is presented. It is shown that, in contrast to the situation in bulk material, growth is a complicated function of wave-vector because of quantization effects. As a consequence, growth is limited by quantization effects and diffusion to small wave-vectors. To illustrate the effect of quantization clearly, the analysis is limited to growth within the lowest sub-band. In spite of quantization differences, a quantitative comparison can be made by a simple rescaling. Growth is shown to be severely limited to small wave-vectors and its rate turns out to be independent of the details of quantization, being that for extreme confinement. The rate maximizes for a wave-vector of about 4 × 105 cm−1 corresponding to a frequency of 700 GHz. The criterion for domain formation in GaN systems becomes n02L≥1020cm-3, where n0 is the areal density of electrons and L is the sample length. As far as growth rate is concerned, the restriction to the lowest sub-band represents a worst-case scenario.

Temperature and doping dependencies of hot electron transport properties in bulk GaP, InP and Ga0.5In0.5P

An ensemble Monte Carlo simulation has been carriedout to study electron transport properties in GaP, InP and Ga 0.5 In 0.5 P materials. The simulation results show that intervalley electron transfer plays a dominant role in higher electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. In addition, the electron velocity in GaP is less sensitive to temperature than other group III-V semiconductors like InP and Ga 0.5 In 0.5 P. So GaP devices are expected to be more tolerant to self-heating and high ambient temperature device modeling.

Optical injection and detection of ballistic pure spin currents in Ge

Ballistic pure spin currents are injected into Ge at 295 K using quantum interference between one and two photon absorption processes for 1786 and 893 nm, 200 fs optical pulses. The spin currents are spatially and temporally detected using polarization- and phase-dependent differential transmission techniques with nanometer spatial and femtosecond temporal resolution. We interpret the dynamics in terms of the fast spin relaxation of the holes and intervalley transfer of electrons.

TEMPERATURE DEPENDENCE OF HIGH FIELD ELECTRON TRANSPORT PROPERTIES IN WURTZITE PHASE GaN FOR DEVICE MODELING

An ensemble Monte Carlo simulation has been used to model bulk electron transport at room and higher temperatures as a function of high electric fields. Electronic states within the conduction band valleys at the Γ1, U, M, Γ3 and K are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The simulation shows that intervalley electron transfer plays a dominant role in GaN in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. Our simulation results have also shown that the electron velocity in GaN is less sensitive to temperature than in other III-V semiconductors like GaAs . So GaN devices are expected to be more tolerant to self-heating and high ambient temperature device modeling. Our steady state velocity-field characteristics are in fair agreement with other recent calculations.

Temperature and doping dependencies of electron mobility in InAs, AlAs and AlGaAs at high electric field application

Temperature and doping dependencies of electron mobility in InAs, AlAs and AlGaAs structures have been calculated using an ensemble Monte Carlo simulation. Electronic states within the conduction band valleys at the G, L and X are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The simulation shows that intervalley electron transfer plays a dominant role in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. Our simulation results have also shown that the electron velocity in InAs and AlAs is less sensitive to temperature than in other III-V semiconductors like GaAs and AlGaAs. So InAs and AlAs devices are expected be more tolerant to self-heating and high ambient temperature device modeling. Our steady-state velocity-field characteristics are in fair agreement with other recent calculations.

Strong Spatial Dependence of Electron Velocity, Density, and Intervalley Scattering in an Asymmetric Nanodevice in the Nonlinear Transport Regime

Using a 2-D ensemble Monte Carlo method, we have studied the electron transport in a self-switching device, which is a semiconductor rectifier consisting of an asymmetric nanochannel. Apart from obtaining a good agreement between the theoretical and experimental current-voltage characteristics, the focus is to study the detailed electron transport inside the nanochannel. Our simulations reveal a drastic spatial dependence of the electron velocity, density, and intervalley scatterings along the channel direction because of the strongly nonlinear transport combined with the asymmetric device geometry. We show that pronounced negative differential electron velocity actually occurs in certain regions inside the channel and changes under different bias conditions. Electron intervalley transfer is also found to depend strongly on the sign of the bias voltage as well as the spatial location in the channel. Moreover, we find that it can take a distance of up to 1 mum for the hot electrons to relax their energy after passing through the nanochannel at high biases. The implications on device operating speed and integration are discussed.

Impact of High-Field Charge Transport on Terahertz Emission From Semiconductor Devices

We present an experimental study that provides insight into the charge carrier dynamics that lead to terahertz (THz) emission. We perform time-resolved THz experiments on semiinsulating GaAs structures with periodically poled surface electrodes. The individual charge carrier dynamics that contribute to the THz emission process are identified by their dependence on the applied field. Electron transport in inhomogeneous fields is found to dominate the emission of THz radiation. Other dynamics that become visible by their specific THz emission are the Dember effect, the intervalley transfer of electrons, and hole currents at high field strengths.

Frequency characteristics of diodes with intervalley electron transfer that are based on variband In x(z)Ga1 − x(z)As with various cathode contacts

A two-level model of intervalley electron transfer in a variband semiconductor is used to study the operation of a Gunn diode based on variband In x(z)Ga1 − x(z)As with n +-n cathodes and n +-n −-n cathodes for different lengths of the active region and different thicknesses of the variband layer. It is demonstrated that the critical frequency of a GaAs-In0.4Ga0.6As diode (280–290 GHz) is higher than the critical generation frequencies of GaAs, In0.4Ga0.6As, and In0.2Ga0.8As diodes.

Transient photoconductivity measurements of carrier lifetimes in an InAs∕In0.15Ga0.85As dots-in-a-well detector

A pulsed midinfrared photoconductivity study of electron recapture in dot-in-a-well infrared photodetectors yields bias-dependent electron-capture lifetimes in the range of 3–600ns and photoconductive gain factors of ∼104–105. The dependence of the lifetimes on temperature and electric field argues for these surprisingly long values being due to electron intervalley transfer. Under normal device operating conditions, photoexcited electrons transfer efficiently out of the central GaAs Γ minimum into the high energy L and X valleys, where they couple only weakly to the Γ-like confined states in the InAs dots.