Intervalley Electron Transfer Research Articles

Negative differential resistance in ultrathin Ge-on-insulator FETs

In this paper we report on the fabrication of all-epitaxial ultrathin germanium-on-crystalline lanthanum–yttrium-oxide field effect transistors. The oxide is lattice matched to Si (1 1 1) and has a dielectric constant of ∼18. The transistors show ambipolar behaviour, both N- and P-channel operations on the same device at any temperature. The transistor characteristics have an ON/OFF ratio of 102 at room temperature, while at cryogenic temperatures negative differential resistance (NDR) is observed in the channel. The NDR effect, which is stronger for lower temperatures, can be attributed to a number of reasons from intervalley or intersubband electron transfer to a heavy transport effective mass (low mobility) band or to charge trapping. A germanium-on-insulator technology, provided the material (Ge and oxide) quality improves, may be useful for building circuits that combine conventional FETs with unconventional ambipolar or NDR devices all on the same substrate.

Technology aspects of GaN-based diodes for high-field operation

Abstract This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.

Influence of the variband-layer thickness on the energy and frequency characteristics of In x(z)Ga1 − x(z)As Gunn diodes

The operation of variband-Inx(z)Ga1 − x(z)As Gunn diodes with an active-region length of 2.5 µm and an n+-n cathode contact is studied by using a two-temperature model of electron intervalley transfer in a varib-and semiconductor. It is established that, in diodes, dipole domains or accumulation layers may be formed depending on the variband-layer thickness. The use of variband Inx(z)Ga1 − x(z)As in the active region with an appropriate variband-layer thickness allows one to enhance the output power and the generation efficiency by a factor of approximately 1.5 and to increase the width of the frequency range of the diode operation approximately twofold as compared to that of an In0.2Ga0.8As-based diode.

Monte Carlo study of steady‐state and transient transport in wurtzite InN

We investigate the steady-state and transient transport in bulk wurtzite InN at different temperatures by the ensemble Monte Carlo method. All relevant phonon scattering mechanisms are included in the Monte Carlo simulations. Impact ionization is treated in the frame of the Keldysh approach. For the steady-state transport, the drift velocity attains a peak value of ∼5.3 × 105 m/s at an electric field strength of 32 kV/cm that considerably exceeds the steady-state saturation level at higher electric fields. The onset of the negative differential mobility (NDM) region is primarily related to the transport properties of electrons in the central valley rather than to the intervalley electron transfer. The transient behavior of the drift velocity demonstrates a strongly pronounced overshoot effect. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron inter-valley transfer dynamics in germanium studied by the optical pump - terahertz probe technique

Electron inter-valley transfer dynamics in germanium studied by the optical pump - terahertz probe technique

Nonparabolic multivalley balance-equation approach to high-field electron transport and impact ionization in ZnS: Comparison with full-band Monte Carlo simulations

Extended nonparabolic multivalley balance equations including impact ionization (II) process are presented and are applied to study electron transport and impact ionization in ZnS with a $\ensuremath{\Gamma},$ X, and L conduction-band structure at high electric field up to 2000 kV/cm. Hot-electron transport properties, such as electron drift velocity, electron energy, intervalley electron transfer, and electron-hole pair generation, are calculated by taking account of the scattering from polar optical, deformation potential, and intervalley interactions. Quantitative agreement is obtained among the results for drift velocity and II coefficient derived from the present calculations, from the recent full-band Monte Carlo simulations, and from experiments, thus confirming the validity of the present multivalley nonparabolic balance-equation approach.

High-field autosolitons in an electron-hole plasma in p-Ge

The formation of high-field thermodiffusion autosolitons was studied experimentally in a photogenerated electron-hole plasma heated up by an electric field in p-Ge samples oriented along the 〈111〉 axis at T=77 K. Measurements of the current-voltage characteristics, electric field distributions along the samples, and IR emission in the wavelength range λ=1.65–10 μm showed that the arising of an autosoliton was accompanied by the appearance of N-shaped current-voltage characteristic regions. Autosolitons were formed at electron-hole plasma concentrations n≥1×1014 cm−3 and field strengths E≥500 V/cm. They manifested themselves as static, moving, and pulsating strata with field strengths Eas=1000–20000 V/cm and carrier temperatures Te≥1000 K. We also observed a turbulent electron-hole plasma state when autosolitons chaotically appeared and disappeared in the samples. The multivalley band structure of germanium influenced the dynamics of autosoliton formation; intervalley transfer of electrons in the strong field of autosolitons caused a three-step autosoliton field growth.

Intervalley scattering in GaAs–AlAs quantum cascade lasers

We have investigated the importance of intervalley (Γ–Χ) electron transfer between Γ-point quantum well states and X-point barrier states in GaAs-based quantum cascade lasers with indirect band gap AlAs barriers. A series of samples has been studied in which the energy separation between the coupled injector/upper laser levels and the lowest confined X state in the injection barrier is varied. We demonstrate that for lasing to occur, electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm) for the present laser design is overcome by utilizing a double injection barrier to achieve lasing at λ=7.2 μm.

Resonant cavity-enhanced Schottky photodiode - modellingand analysis

The theoretical analysis of the linear pulse responses of homostructure GaAs conventional and resonant cavity-enhanced(RCE) Schottky photodiodes is presented in this paper. Theresults have been obtained by numerical simulation of thecomplete phenomenological model for a two-valley semiconductor.Besides the drift, as the dominant mechanism of the transport,the model takes into account the diffusion, the influence ofthe electric field due to the spatial and temporalredistribution of the photogenerated carriers within theabsorption layer, the influence of the parasitic time constantand the electron intervalley transfer. The behaviour of thebandwidths and bandwidth-quantum efficiency products when theactive area, the width of the absorption layer and the reversebias voltage change has been investigated. It is shown that conventional and RCE Schottky photodiodes with exactly the sameoperating parameters have practically identical frequencyresponses, i.e. bandwidths. For certain thicknesses of theabsorption layer the bandwidth-quantum efficiency products havebeen increased more than fivefold. In comparison with the RCEP-i-N photodetectors having the same operating parameters, theRCE Schottky photodiodes exhibited approximately twofoldincreased bandwidths.

Hot electron intervalley transfer in GaAs–AlAs MQWs: population inversion and possibility of intraband lasing

Discussion of different ways to achieve population inversion and IR intraband lasing in the hot carrier systems in MQW structures under lateral transport and intervalley and (or) real-space transfers is given. Reconsideration of the negative differential conductivity under the hot electron intervalley transfer in GaAs in terms of population inversion and indirect low-frequency optical transitions is presented. Evaluation via the Monte-Carlo simulation of the hot electron intervalley transfer in GaAs–AlAs MQW structures designed for the amplification at wavelength λ=10 μm and 15 μm is given. For the structure designed for λ=15 μm the amplification coefficient over 200 cm −1 at electric field E=8 kV/cm and 80 K is found.

Negative dynamic differential conductivity at the cyclotron frequency in Ga1−x AlxAs under conditions of ballistic intervalley electron transfer

It is theoretically shown that under certain conditions a cyclotron resonance maser can be fabricated based on n-Ga1−xAlxAs-type materials. Low temperatures and strong crossed fields (E⊥H) are considered. In these fields the electrons in the lower (light) valley of the conduction band dynamically (ballistically) transit the band up to the onset energy of intervalley scattering ɛ0. Studies were carried out on solid solutions with composition 0<x<0.39 (ɛ0=(2–17)ℏω*, where ℏω* is the intervalley phonon energy). The magnitudes of the E and H fields were varied within the limits E=5–20 kV/cm and H=6–40 kOe. This produced a smooth change in the transit conditions in the passive region (ɛ<ɛ0), which makes it possible to obtain the desired frequency dependence of the differential conductivity (DC), σ(ω). As our studies show, previously unknown, interesting peculiarities of the hot electron system appear under these conditions.

Pressure and strain sensors based on intervalley electron transfer in AlGaAs

A novel sensor, based on the intervalley transfer of electrons in AlxGa1−xAs (x∼0.3) subjected to hydrostatic pressure and/or strain is proposed. The operation characteristics are evaluated, optimized theoretically, and demonstrated experimentally.

Intersubband all-optical limiter based on thermally induced intervalley transfer

We propose an all-optical limiter based on the thermally induced intervalley transfer of electrons from Γ-valley states with forbidden normal-incidence intersubband interactions to L-valley states which absorb strongly. Detailed modeling of the device performance in the short-pulse regime (⩽100 ns) yields that the output intensity for a limiter with only 10% insertion loss at low excitation levels will remain clamped over a dynamic range of 25–40.

Analysis of transfer function of metal-semiconductor-metal photodetector equivalent circuit

The response of GaAs metal-semiconductor-metal photodetectors (MSM-PD) is simulated using a phenomenological model of transport equations, which takes into account the electron intervalley transfer. Calculations are performed for intrinsic devices with 1 μm and 0.5 μm interdigital spacing with the corresponding equivalent (parasitic) circuit of the photodetector. It is pointed out that parasitic effects may induce a significant distortion, a delay of intrinsic response, and increase bit error rate. Analysis of the equivalent or parasitic circuit of the MSM-PD transfer function shows that the distortion and delay of the intrinsic signal can be minimized by choosing the optimal photodetector capacitance.

The influence of electron intervalley transfer on response pulse width of the MSM photodetector

This paper analyses the influence of nonstationary effects on response pulswidth of Metal-Semiconductor-Metal Photodetector (MSM-PD) made of a two-valley semiconductor. A phenomenological model is used for 2D numerical simulation of carrier transport and intervalley transfer. The analysis of the pulsewidth response of the MSM-PD has been made for various micron and submicron structures, bias voltages and incident light wavelengths.

Green's-function study of the electron tunneling in a double-barrier heterostructure.

The quasi-one-dimensional aspects of the electron tunneling through AlAs/GaAs/AlAs (001) double-barrier structures have been investigated using the tunneling formalism based on the nonequilibrium Green's function developed by Keldysh. In describing the electronic structure of a double-barrier heterostructure, Green's-function formulation of scattering theory incorporated with the multiorbital tight-binding model is used, so as to take into account the effects of indirect band gap, band nonparabolicity, and multiple orbitals in the aperiodic layered structure. The effect of high external bias on the junction is solved exactly within this model. The electron-tunneling situation and the effect of the X-valley originated barrier confined states on the electron tunneling are analyzed with the spectral local density of states of the double-barrier structure in the [001] direction under the external bias. The calculated current density J(${\mathbf{k}}_{\mathrm{\ensuremath{\parallel}}}$=0) versus voltage curve exhibits the structures for the interlayer intervalley electron transfer, which are manifested as the electron propagation through \ensuremath{\Gamma}-valley originated states and through X-valley barrier confined states. The smaller current peak-to-valley ratios are found in the double-barrier structures with thicker barriers, which is consistent with the experimental results. The enhancement of \ensuremath{\Gamma}-X interlayer intervalley electron transfer and reduction of the \ensuremath{\Gamma}-\ensuremath{\Gamma} resonant electron interlayer transfer is found with the increase of the phenomenological imaginary potential term.

Monte Carlo simulation of electron transport in gallium nitride

The results of an ensemble Monte Carlo simulation of the electron transport in gallium nitride (GaN) are presented. The calculation 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. An analytic expression for the polar optical momentum relaxation time for phonon energies larger than the thermal energy is also derived. This expression applies to many wide-gap semiconductors, such as GaN and SiC, at room temperature since these semiconductors have large polar optical-phonon energies (on the order of 100 meV). The calculated mobility agrees well with the results of the Monte Carlo calculation.

Nonstationary and nonlinear response of a p-i-n photodiode made of a two-valley semiconductor

The influence of nonstationary and nonlinear effects on carrier transport in a p-i-n photodiode made of a two-valley semiconductor is analyzed. A phenomenological model based on a two-valley conduction-band model is used, incorporating transition rates for intervalley electron transfer into the continuity equations and the drift-diffusion equations for carrier transport. A large optical power absorption in a small device can perturb, via the space-charge potential, the electric field which governs the carrier transport, and thus give rise to a nonlinear electrical response. The influence of these effects on photodiode response to pulsed light stimulation under various reverse-bias voltages is calculated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Negative resistance in ZrSe 2 single crystals

Voltage-controlled negative resistance (VCNR) has been observed in the semiconducting layered compound ZrSe 2 at a field of ∼ 50–100 V cm −1, at room temperature. The occurence of VCNR is attributed to the intervalley transfer of electrons in the conduction band.

A novel approach to a transport model for ultra-small-scale compound semiconductor devices

The concepts of the previously developed macrokinetic distribution function and the ensemble average are used to develop a transport model that can account for effects of intervalley transfer, anisotropic scattering, and nonequilibrium dynamics of electrons in compound semiconductors. The developed transport model is rather simple and can be used for modeling of ultra-small-scale transport behavior of electrons in compound semiconductor devices. The developed model is applied to various multivalley semiconductor models. The study shows that the energy-dependent macroscopic effective mass of electrons in nonparabolic bandstructure, that is usually assumed for modeling of devices, is inadequate in situations where strong anisotropic scattering is involved and velocity overshoot is pronounced. It is found that the momentum dependence of the macroscopic effective mass is also significant during the interval of pronounced velocity overshoot.