Carrier Temperatures Research Articles

Phonon emission by hot 2D electrons and holes in a Gallium Arsenide heterojunction: The influence of screening and acoustic anisotropy

The process of energy relaxation by phonon emission of hot two-dimensional electrons and holes in GaAs/AlGaAs heterojunctions has been studied using the heat-pulse technique for carrier temperatures below 40 K. To explain the experimental findings and to understand fully the process of electron– (or hole–) phonon coupling, theoretical calculations of the angular and polarization dependence of the emitted acoustic phonons were made. Phonon emission studies give detailed information concerning the phonon wave vector and polarization dependence of the emission process. This allows, in connection with the corresponding numerical simulations, interesting conclusions regarding the properties of the 2D carrier system itself (e.g. strength of carrier–phonon coupling constants, confinement of the 2D carriers perpendicular to the 2D system) to be reached.

Observation of Anomalous Negative DifferentialResistance in Diode Breakdown Simulation Using CarrierTemperature Dependent Impact Ionization

When carrier temperatures are used to model impact ionization with self‐consistent cooling effects, anomalous negative differential resistance (NDR) was found in diode breakdown simulation. Possible mechanisms responsible for the NDR are analyzed.

Electrical properties of SOI n-MOSFETs under nonisothermal lattice temperature

A physically rigorous device model for both carrier and lattice heat transports has been incorporated into a general purpose device simulation program, SNU-2D. In the model, the carrier mobility is explicitly expressed as a separate function of lattice and carrier temperatures. As an example for a nonisothermal problem, SOI MOSFETs have been considered. The negative slope and kink effect in the I DS − V DS characteristics as well as hysteresis in the I DS - V GS characteristics are well predicted by the developed model including the temperature dependence of the electron saturation velocity and degradation of the temperature dependence mobility.

Ultrashort-pulse laser heating of silicon to reduce microstructure adhesion

A technique to remove moisture from microelectronic devices and improve device yield in microelectromechanical systems by reducing microstructure surface adhesion is proposed. Ultrashort-pulse laser radiation is used to create excited carriers in, and consequently desorb water from, silicon micro-structures. A theoretical model for ultrashort-pulse laser heating of silicon is presented. Calculated carrier temperatures show significant increases at short time scales, while the lattice temperatures remain almost constant, indicating the possibility for water desorption without significant device heating. A preliminary experiment confirming the feasibility of using the technique to decrease microstructure adhesion is discussed.

Temperature- and carrier-density-dependent electron tunneling kinetics in (Ga,In)As/(Al,In)As asymmetric double quantum wells

We present a detailed investigation of electron tunneling in (Ga,In)As/(Al,In)As asymmetric double quantum wells as a function of different excitation and temperature conditions. We show that tunneling dynamics depend strongly on the initial carrier temperature and momentum. For example, electron and hole tunneling out of the narrow well is complete at low temperature. However at room temperature carriers do not exhibit any tunneling kinetics. We propose a simple kinetic model which describes the observed population dynamics at different carrier densities, temperatures, and excitation conditions.

Unified bulk mobility model for low- and high-field transport in silicon

An analytical bulk mobility model for hydrodynamic transport equations is developed from a microscopic level and designed for silicon device simulation. Applying Kohler’s variational method extended to the regime of nonlinear transport yields the general expression for the mobility as function of carrier temperature, lattice temperature, and doping. Assuming a nonparabolic, isotropic band model and a heated Maxwellian allows for the analytical calculation of the collision integrals. A nonelastic approximation for intravalley acoustic-phonon scattering is proposed, which improves the model in the low-temperature range. Intervalley scattering is treated in a one-mode, equipartition approximation. Here, an accurate analytical approach for all carrier temperatures is derived. For impurity scattering the Brooks-Herring theory is used including Fermi statistics and the effect of dispersive screening. The influence of other effects like anisotropic valleys and perturbation of the density of states by heavy doping are discussed quantitatively. Despite the oversimplified band structure, all essential features of the measured mobility in silicon can be reproduced except in the heavy doping range. The adjusted deformation potentials coincide with estimated sums of the corresponding sets used in full-band Monte Carlo simulation. The method has the potential of an extension to the Si−SiO2 system.

High-frequency noise of bipolar devices in consideration of carrier heating and low temperature effects

We derive an expression for the high-frequency spectrum of thermal current noise in bipolar devices from quantum mechanical linear response theory. This expression relates the noise spectrum to local small-signal quantities. It is independent of geometry and holds for arbitrary space-dependent carrier temperatures including low ones. The general result is applied to pn-diodes and bipolar transistors. In the isothermal case and for not too low temperatures the expressions known from the literature can be reproduced. We apply our approach to Si 1− x Ge x heterojunction bipolar transistors in order to calculate the minimum noise figure at high frequencies. The electron heating in the collector is shown to have a noticeable influence on the noise figure at very high frequencies.

Optical and electrical properties of quantum wells with electrically tunable two-dimensional electron density by selective contacts

We report on photoluminescence and absorption measurements in type-I hetero n-i-p-i structures. The electron density in the pseudomorphic InGaAs/GaAs quantum wells is tunable between zero and more then 5 · 1012 cm-2. This electrical tuning of the subband filling is achieved by a variable voltage applied between selective n-and p-contacts fabricated by epitaxial shadow mask MBE. One of the advantages of having selective contacts to the n- and p-layers is to get reliable information about the electron density, independent of measured absorption and luminescence spectra. This allows a more rigorous analysis of the data on bandgap renormalization, bandfilling and k∥-conservation. Moreover, the experiments can be performed at low optical power and low carrier temperatures. In the present investigation a bandgap renormalization of -20 meV and a bandfilling induced shift of the absorption edge as large as +50 meV was observed for a sheet electron density of 5 · 1012 cm-2.

Recombination kinetics and intersubband relaxation in semiconductor quantum wires

We present a detailed and systematic investigation of carrier capture, relaxation, cooling and radiative recombination in a one-dimensional semiconductor quantum wire of high structural perfection and optical quality over a large range of excitation (carrier) densities. Experimental evidence for a complete lack of 1D bandgap renormalization is found. Even up to high carrier densities, >106 cm-1, where strong band filling is already present and directly visible in the luminescence, no shift of bandgap to low energy is found. The carrier cooling in 1D is appreciably slower than in comparable 2D structures, thus leading to high carrier temperatures. This confirms theoretical predictions of reduced phonon scattering probability in one-dimensional structures. The temperature dependence of the radiative lifetime of the 1D carriers is investigated. The theoretically predicted T dependence is not found. On the contrary an empirical tau rad=0.02 T ns K-1 law is fulfilled.

Ultrafast relaxation of highly photoexcited carriers in p-type and intrinsic GaAs.

The thermalization and energy relaxation of highly photoexcited carriers in p-type and intrinsic GaAs is investigated at room temperature using an ensemble Monte Carlo approach. We present an alternative model for treating carrier-carrier scattering in Monte Carlo simulations. The model takes into account the wave vector and frequency dependence of the dielectric function by using a screened Coulomb potential in q space due to a charge moving at the velocity of the center of mass of the interacting particles. In comparison to the statically screened model, the calculated effective carrier temperatures for an excitation density of 2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ are in good agreement with the experimental results for both p-type and intrinsic GaAs. At the lower doping and excitation density of 2.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, the dynamic and the static screening models result in a comparable time evolution of the carrier energies and band populations.

Monte Carlo simulation of minority carrier transport and light emission phenomena in GaAs devices

We present a theoretical study of hot-carrier-induced light emission in III-V semiconductor devices. Carrier heating in the intense electric fields present under high-bias conditions is studied using self-consistent Monte Carlo simulations. The carrier distribution functions obtained from the simulation are then incorporated into a pseudopotential algorithm that describes the direct optical transitions and calculates the corresponding spectra. We show that, in general, no straightforward extrapolation of 'carrier temperatures' from the slope of the measured spectra is possible. Our analysis indicates that light emission due to hot carriers is made up by direct radiative interband transitions within the conduction and valence bands as well as between them.

Different carrier temperatures in the wells and in the barriers of InGaAs/GaAs single quantum well structures

Optical carrier heating by quasi-CW excitation has been studied in an InGaAs/GaAs single quantum well structure. At lower excitation intensities, until the quantum well is filled with the photoexcited carriers, the carrier temperature in the well is much higher than in the barriers. At higher excitation intensities, additional carrier heating in the barriers by the hot carriers located in the well is observed. The difference between carrier temperatures in the well and in the barriers is discussed comparing the carrier-LO phonon and carrier-carrier scattering times.

Far-infrared emission from two-dimensional electron and hole gases in GaAs/(AlGa)As heterojunctions

An investigation has been made of the far-infrared emission (FIR) from two-dimensional electron and hole gases (2DEGs and 2DHGs) in GaAs/(AlGa)As heterostructures as a function of source-drain current and magnetic field. The results on 2DEGs indicate that the proportion of FIR coming from the bulk of the 2DEG, rather than from the diagonally opposite corners near the contacts where a significant amount of the dissipation takes place, varies with both current and magnetic field. The results on PDHGs suggest that the carrier temperatures are very much lower than those in 2DEGs for the same source-drain current.

Dynamically screened electron-hole interaction effects on initial thermalization of photoexcited carriers in p+-GaAs

The thermalization and energy relaxation of highly photoexcited carriers in p+-GaAs is investigated at room temperature using an ensemble Monte Carlo approach. We present an alternative model for treating carrier-carrier scattering in Monte Carlo simulations. The model takes into account the wavevector and frequency dependence of the dielectric function by using a screened Coulomb potential in q-space due to a charge moving at the velocity of the centre of mass. In comparison with the statically screened model, the calculated effective carrier temperatures for an excitation density of 2*1018cm-3 and background p-doping of the same value are in good agreement with the experimental results.

Many body effects in transient luminescence spectra of a homogeneous electron-hole plasma in CdTe/CdMnTe quantum wells

Using time resolved photoluminescence spectroscopy, we have investigated many body effects and the dynamics of a high density electron-hole plasma in strained CdTe/CdMnTe quantum wells. Great care has been taken to achieve a spacially homogeneous plasma by mesa etching of single quantum well structures. Our experimental data show strong band filling effects up to the barrier energy due to the Pauli principle within a few tenths of ps. A lineshape analysis yields plasma densities up to 2 × 10 12 cm -2 and carrier temperatures up to 335 K (bath temperature 2 K). A band gap renormalization of the emission spectrum up to around 23 meV as compared to a low excitation reference spectrum is observed. The reduction of the carrier density due to recombination causes an increase of the emission intensity at the 1e-1hh exciton transition, indicating an enhancement of the oscillator strength below the Mott density.

A NEW FINITE‐ELEMENT DISCRETIZATION TECHNIQUE FOR THE HYDRODYNAMIC FORMULATION OF ENERGY BALANCE EQUATIONS

This paper presents a new discretization technique of the hydrodynamic energy balance model based on a finite‐element formulation. The concept of heat source lumping is introduced, and the thermal conductivity model includes the effect of varying both carrier concentrations and temperatures. The energy balance equation is formulated to account for kinetic energy as a convective flow. The new discretization method has the advantage that it allows for assembling the functions out of elementary variables available over elements instead of along element links. Therefore, theoretically, calculation of the Jacobian should be three times faster than by the classic method. Results are given for three examples. The method suffers from mathematical instabilities, but provides a good basis for future work to solve these problems.

Recombination of correlated electron-hole pairs in two-dimensional semiconductors.

The Coulomb interaction of electrons and holes in a two-dimensional semiconductor is shown to affect radiative and nonradiative recombination processes strongly. Our detailed calculations based on a many-body theory give the Coulomb correlation factors for a wide range of carrier densities and temperatures. For radiative recombination, a continuous transition between an excitonlike and a free-carrier-like behavior is revealed, which strongly influences the temperature dependence of the radiative lifetime. Auger recombination is strongly enhanced and its kinetic behavior is modified to look like classical radiative recombination up to fairly high temperatures.

Carrier relaxation in intermixed GaAs/AlxGa1-xAs quantum wires.

Time-resolved investigations on the photoluminescence of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wires as a function of the wire width and the potential well depth indicate a reduction of the energy relaxation in quasi-one-dimensional (1D) systems. The systematic change of the wire width and potential well depth of the quantum wires with mask widths down to 40 nm were realized by ion-implantation-induced intermixing of quantum wells. Lifetime measurements on the high-energy side of the quantum wire emission yield increased decay times for the smallest wires. This is consistent with our observation of increased carrier temperatures and slowed cooling in the quantum wires with increasing carrier confinement. We explain the reduction of the relaxation with the decreased possibility for the scattering particles to fulfill both energy and momentum relaxation in wires with gradually increasing 1D behavior.

Precise modeling of hot carrier gate current in short-channel MOSFET's

A powerful model which considers the fact that the values of the channel and carrier temperatures T and T/sub c/ vary with position in the bulk and channel is considered. It reveals that the energy distribution of hot carriers deviates from the well-known Maxwellian distribution by a small but nonnegligible perturbation and evaluates the dependence of this deviation of the device technology, geometry, and biasing conditions. The model helps to remove important discrepancies between the old hot-carrier models and measurements. >

Extraction of gold from aqueous solution by carrier‐mediated process using kryptofix 22 DD

AbstractReactive extraction of gold from alkaline aqueous cyanide solution into n‐decanol in the presence of Kryptofix 22 DD as complexing agent was studied using a stirred transfer cell. The study investigated the effects of carrier concentration and temperature on mass transfer rates. A two‐film model was proposed by considering film mass transfer resistances at the aqueous and organic phases. The model predicted adequately the experimental time‐concentration data at different carrier concentrations and temperatures. Experiments have also been carried out to extract gold in an emulsion system using the same carrier and a mixed solvent of kerosene and decanol.