GaAs-AlGaAs Heterojunction Research Articles

Superconductivity in AuNiGe Ohmic contacts to a GaAs-based high mobility two-dimensional electron gas

To cool a high mobility two-dimensional electron gas (2DEG) at a GaAs–AlGaAs heterojunction to milliKelvin temperatures, we have fabricated low resistance Ohmic contacts based on alloys of Au, Ni, and Ge. The Ohmic contacts have a typical contact resistance of RC≈0.8 Ω at 4.2 K, which drops to 0.2 Ω below 0.9 K. Scanning electron microscope images establish that the contacts have the same inhomogeneous microstructure that has been observed in previous studies. Measurements of the contact resistance RC, the four-terminal resistance along the top of a single contact, and the vertical resistance RV all show that there is a superconductor in the Ohmic contact, which can be turned completely normal with a magnetic field of 0.15 T. We briefly discuss how this superconductivity may be affecting the electrical transport measurements of 2DEGs, especially how it may hinder the cooling of electrons in a 2DEG below 0.1 K.

Distinguishing XY from Ising electron nematics

At low temperatures in ultraclean GaAs-AlGaAs heterojunctions, high fractional Landau levels break rotational symmetry, leading to increasingly anisotropic transport properties as temperature is lowered below $\sim$150mK. While the onset of transport anisotropy is well described by an XY model of an electron nematic in the presence of a weak uniform symmetry-breaking term, the low temperature behavior deviates significantly from this model. We find that inclusion of interactions between the electron nematic and the underlying crystalline lattice in the form of a 4-fold symmetry breaking term is sufficient to describe the entire temperature dependence of the transport anisotropy. This implies that the quantum Hall electron nematic is in the Ising universality class. We propose new experimental tests that can distinguish whether any two-dimensional electron nematic is in the XY or Ising universality class.

Electron dynamics at GaAs-AlGaAs heterojunction studied by ultrafast spectroscopy

In this letter the electron and spin dynamics at GaAs/AlGaAs heterojunction was studied by ultrafast spectroscopy techniques (photon echo and transient grating studies). Relaxation times and diffusion coefficients of photoexcited electrons and spins were obtained using pure optical setup. The estimated spin diffusion coefficient value of 160 cm2/s is relatively high and comparable to the electron diffusion coefficient of 200 cm2/s. This feature makes GaAs/AlGaAs heterosructure a promising material for practical application in semiconductor spintronics.

Monolithic integration of low-temperature-grown GaAs with a two-dimensional electron gas

A scheme is presented for the monolithic integration of low-temperature- grown gallium arsenide (LT-GaAs) with a two-dimensional electron gas. Pulsed terahertz (THz) emission (with a 3 THz bandwidth) is demonstrated from a LT-GaAs layer grown epitaxially above a GaAs–AlGaAs heterojunction containing a two-dimensional electron gas, using photoconductive antennae patterned on the LT-GaAs, and excited with 800 nm, 12 fs laser pulses. Electrical transport measurements of the two-dimensional electron gas at cryogenic temperatures, and in high magnetic fields, reveal quantum Hall behaviour. The THz emission characteristics of the LT-GaAs, and the transport properties of the two-dimensional electron gas demonstrate that this integrated approach compromises neither the quality of the LT-GaAs nor the two-dimensional electron gas, and indicate the potential of this monolithic integration for sub-picosecond electronic measurements of mesoscopic systems.

Thermodynamics Properties of Mesoscopic Quantum NanowireDevices

We investigate the thermodynamics properties of mesoscopicquantum nanowire devices, such as the effect of electron-phononrelaxation time, Peltier coefficient, carrier concentration,frequency of this field, and channel width. The influence oftime-varying fields on the transport through such device has been takeninto consideration. This device is modelled as nanowires connecting totwo reservoirs. The two-dimensional electron gas in a GaAs–AlGaAsheterojunction has a Fermi wave length which is a hundred times largerthan that in a metal. The results show the oscillatory behaviour ofdependence of the thermo power on frequency of the induced field. Theseresults agree with the existing experiments and may be important forelectronic nanodevices.

Spin depolarization in quantum wires polarized spontaneously in zero magnetic field

The conditions for a spontaneous spin polarization in a quantum wire positioned in a zero magnetic field are analyzed under weak population of one-dimensional subbands that gives rise to the efficient quenching of the kinetic energy by the exchange energy of carriers. The critical linear concentration of carriers above which the quasi one-dimensional gas undergoes a complete spin depolarization is determined by the Hartree-Fock approximation. The dependence of the critical linear concentration on the concentration of carriers is defined to reveal the interplay of the spin depolarization with the evolution of the 0.7 (2e2/h) feature in the quantum conductance staircase from the e2/h to 3/2 (e2/h) values. This dependence is used to study the effect of the hole concentration on the 0.7 (2e2/h) feature in the quantum conductance staircase of the quantum wire prepared inside the p-type silicon quantum well using the split-gate technique. The 1D channel is demonstrated to be spin-polarized at the linear concentration of holes lower than the critical linear concentration, because the 0.7 (2e2/h) feature is close to the value of 0.5 (2e2/h) that indicates the spin degeneracy lifting for the first step of the quantum conductance staircase. The 0.7 (2e2/h) feature is found to take however its normal magnitude when the linear concentration of holes attains the critical value corresponding to the spin depolarization. The variations in the height of the 0.7 (2e2/h) feature observed in the hole quantum conductance staircase that is revealed by the p-type silicon quantum wire seem to be related to the evidences of the quantum conductance staircase obtained by varying the concentration of electrons in the 1D channel prepared inside the GaAs-AlGaAs heterojunction.

Effects of Weak Localization and Electron–Electron Interaction in GaAs–AlGaAs Heterostructures

The influence of the processes of weak localization and electron–electron interaction in an inhomogeneous two-dimensional electron gas of a single GaAs–AlGaAs heterojunction on the low-temperature transport characteristics in the case of occupation of two quantum subbands has been investigated. The transport characteristics have been interpreted from the viewpoint of a two-layer model taking into account the existence of two bypass conduction channels corresponding to the two-dimensional and three-dimensional electron gas. Both the electrical and optical measurements point to the existence of large-scale fluctuations of the potential, which determine the dependence of the conduction and the Hall resistance of the heterostructures on the magnetic field. It has been established that the weak localization determines the charge transport in a weak magnetic field, and the electron–electron interaction determines this transport in a strong magnetic field.

Simulation of the thermal response of self-heating power transistors using a time step adaption method

An implementation of the Rosenbrock-Wanner scheme with embedded time-step adaption in the thermal simulation of AlGaAs-GaAs heterojunction bipolar transistors (HBT) is presented. The simulations are based on discrete 3-D models of the device using the finite integration technique for the solution of the thermal equation. The thermally coupled current response of the device in DC operation is obtained by a nonlinear EM/sub 3/ model. Several simulations featuring the thermal crosstalk and the current collapse instability demonstrates the utility of time-step adaption in simulating the alternating slow-fast transients in multifinger HBTs.

Classic ionic crystals and quantal Wigner electron solids: Role of electron correlation

AbstractMotivated by the pioneering studies of Löwdin on ionic crystals like NaCl and CsCl, and on electron correlation, recent work on the polarizability of the anion in such materials, and its relation to the free‐space polarizability, will first be reported. The essential role of electron correlation in treating negative ions will be emphasized. The Madelung energy, crucial for the stability of such classic ionic crystals, is then pointed out, within the framework of the jellium model of strongly interacting electrons, to lead to quantal Wigner electron crystals. Work will be summarized first on such an electron solid in three dimensions and in particular its melting curve will be discussed. Attention will then be focused on the freezing of the 2‐D liquid in a GaAs–AlGaAs heterojunction in a transverse magnetic field. Finally, it is stressed that electron–electron repulsion plus localization due to the applied magnetic field order the low‐density electrons into such a magnetically induced Wigner electron solid. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Classical Theory of Hot-Electron Transport in Electric and Magnetic Fields

Balance equation approach to the hot-electron transport in electric and magnetic fields is reformulated. The balance equations are re-derived from the Boltzmann equation. A new expression for the distribution function is reported in the present paper. It is homogeneous steady solution of the Boltzmann equation in constant relaxation time approximation. It holds when or . As an example, the mobility of 2D electron gas in the GaAs-AlGaAs heterojunction is computed as a function of electric field and magnetic field.

Planar Hall sensors for micro-Hall magnetometry

In this work we present a new method to fabricate planar Hall sensors from GaAs–AlGaAs heterojunctions, which can be used to examine the local stray field at a specific section of a micron-sized magnet. Instead of mesa etching we implanted oxygen ions with an energy of 1.5 keV which deplete the two-dimensional electron gas underneath the exposed areas but leave the wafer flat. Planar double Hall cross devices were employed to investigate 30 nm thick electroplated Ni rings with outer and inner diameters ranging from 1.2 to 2 μm and from 0.3 to 1.6 μm, respectively. By comparing the signals from both Hall crosses of the sensor, we can distinguish between local stray field variations and changes of the global magnetization pattern. A hysteresis loop measured at a temperature of 110 K suggests that magnetization reversal occurs via a magnetic vortex structure.

Ac electric and magnetic responses of nonconnected Aharonov-Bohm rings

Signature of phase coherence on the electric and magnetic response of non connected Aharonov-Bohm rings is measured by a resonant method at 350 MHz between 20 mK and 500 mK. The rings are etched in a GaAs-AlGaAs heterojunction. Both quantities exhibit a oscillating behaviour with a periodicity consistent with half a flux quantum $\Phi_0/2=h/2e$ in a ring. Electric screening is shown to be enhanced when time reversal symmetry is broken by magnetic field, leading to a positive magnetopolarisability, in agreement with theoretical predictions for isolated rings at finite frequency. Temperature and electronic density dependence are investigated. The dissipative part of the electric response, the electric absorption, is also measured and leads to a negative magnetoconductance. The magnetic orbital response of the very same rings is also investigated. It is consistent with diamagnetic persistent currents of 0.25 nA. This magnetic response is an order of magnitude smaller than the electric one, in qualitative agreement with theoretical expectations.

Noise characteristics of GaAs-AlGaAs heterojunction punch-through phototransistors

Novel GaAs-AlGaAs heterojunction phototransistors with a /spl delta/-doped base have been fabricated. Very high gain and low output noise have been measured. The measured noise is composed of shot noise associated with collector quiescent bias current and amplified shot noise due to collector leak current for nonpassivated devices. The high gain and low intrinsic noise characteristics of these transistors make them very promising in weak light detection.

Charge screening in the quantum Hall regime probed by the lateral photoelectric effect

Electrons are photoelectrically injected into the two-dimensional electron system (2DES) near a standard AlGaAs–GaAs heterojunction by a focussed light spot. The photovoltage in the plane of the 2DES exhibits quantum oscillations with magnetic field that are basically out of phase with respect to the oscillations of the conductivity σxx. This is evidence for the influence of screening effects on the injected charge. At low filling factors a dip is superimposed on the maxima, which points to a competition between screening properties and the expected diffusion effects.

Mobility of Two-Dimensional Electron Gas in GaAs–AlGaAs Heterojunctions (II)

It is argued that one should assume phonons to be in an equilibrium state at temperature Tp instead of T, the temperature of the heat bath. An equation for determining Tp is proposed. By introducing such a revision together with those revisions stated in the previous paper to the current balance equation method, the mobility versus the electric field curve is recomputed. The conformity between theory and experiment is satisfactory.

Mobility of Two-Dimensional Electron Gas in GaAs-AlGaAs Heterojunctions (II)

The computation of the mobility of a two-dimensional electron gas in GaAs-AlGaAs heterojunctions carried out by Lei et al. is revised by using a new expression for the distribution function instead of the displaced Fermi function, And also the bare phonon frequency in the theory of Lci et al. is replaced by the renormalized phonon frequency. Our result shows that the agreement between theory and experiment is greatly improved by these revisions.

A 2-GHz clocked AlGaAs/GaAs HBT byte-slice datapath chip

A byte-slice datapath for exploring multi-chip RISC processor development in AlGaAs-GaAs heterojunction bipolar transistor (HBT) technology has been designed, fabricated and tested. The circuits are implemented using differential current-mode logic (CML) and emitter-coupled logic (ECL) with signal swings of 250 mV. Each datapath chip contains a single slice, including an 8-bit by 32-word single-port register file with a 230-ps read access time, and an 8-bit carry-select adder with a 140-ps select path and a 380-ps ripple-carry path. Each unpackaged die was tested using an at-speed boundary scan test scheme. The register file and adder carry chain are also implemented in a special test chip for accurate performance characterization of these critical circuits.

Two-dimensional hot-electron transport in GaAs-AlGaAs heterojunctions

The mobility of a two-dimensional electron gas in GaAs-AlGaAs heterojunction is recomputed through solving numerically the balance equation. The peculiarity of the present study lies in that a new expression for the distribution function and the renormalized phonon frequency are used in the computation. Compared with those reported by previous authors, our result shows better agreement with the experimental data.

Mobility of Two-Dimensional Electron Gas in GaAs–AlGaAs Heterojunctions

Mobility of Two-Dimensional Electron Gas in GaAs–AlGaAs Heterojunctions

Mobility of Two-Dimensional Electron Gas in GaAs-AlGaAs Heterojunctions

The computation of the mobility of a two-dimensional electron gas in GaAs–AlGaAs heterojunctions carried out by Lei et al. is revised by using a new expression for the distribution function instead of the displaced Fermi function. And also the bare phonon frequency in the theory of Lei et al. is replaced by the renormalized phonon frequency. Our result shows that the agreement between theory and experiment is greatly improved by these revisions.