We demonstrate the presence of a resonant interaction between a pair of coupled quantum wires, which are realized in the ultra-high mobility two-dimensional electron gas of a GaAs/AlGaAs quantum well. Measuring the conductance of one wire, as the width of the other is varied, we observe a resonant peak in its conductance that is correlated with the point at which the swept wire pinches off. We discuss this behavior in terms of recent theoretical predictions concerning local spin-moment formation in quantum wires.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Hu, Qing; Williams, Benjamin S.; Kumar, Sushil; Callebaut, Hans; Reno, J.L.; Roskos, H.; Faist, J.; Smith, G.
We report our development of terahertz (THz) quantum cascade lasers (QCLs), in which the depopulation of the lower radiative level is achieved through resonant longitudinal optical (LO) phonon scattering. This depopulation mechanism, similar to that implemented in all the QCLs operating at mid-infrared frequencies, is robust at high temperatures and high injection levels. The unique feature of resonant LO-phonon scattering in our THz QCL structures allows a highly selective depopulation of the lower radiative level with a sub-picosecond lifetime, while maintaining a relatively long upper level lifetime (more than 5 ps) that is due to upper-to-ground-state scattering. The first QCL based on this mechanism achieved lasing at 3.4 THz (λ ≈ 87 μm) up to 87 K for pulsed operations, with peak power levels exceeding 10 mW at ca. 40 K. Using a novel double-sided metal waveguide for mode confinement, which yields a unity mode confinement factor and therefore a low total cavity loss at THz frequencies, we have also achieved lasing at wavelengths longer than 100 μm.
A voltage tunable two-color detector that contained multiple periods of AlGaAs/GaAs superlatice pairs was studied. It was found that the peak wavelength switched from 9.5 μm under large positive bias to 6 μm under negative bias. The background-limited temperature was found to be 55 K for 9.5 μm detection and 80 K for 6 μm detection. It was demonstrated that corrugated Quantum-well infrared photodetectors (C-QWIP) were capable of coupling normally incident light for this two color application.
The operation of a tetrahertz quantum-cascade laser at 3.8 THz (λ= 79 μm) up to a heat sink temperature of 137 K was presented. A resonant phonon depopulation design was used with a low-loss metal-metal waveguide, providing a confinement factor of nearly unity. A threshold current density of 625 A/cm 2 was found in pulsed mode at 5 K. It was found that a thicker barrier reduced the coupling between the injector state and the excited state in the wide well of the module, which was caused by the parasitic current channel.
A report lasing at ∼3.0 THz (λ≈98-102 μm) in which mode confinement was provided by a double-sided metal waveguide was presented. The waveguide consists of metallic films placed above and below the 10-μm-thick multiple-quantum-well gain region, which gives low losses and a modal confinement factor of nearly unity. The one-dimensional mode patterns for the two waveguides were calculated using a Drude model solver.
Terahertz lasing in a quantum cascade structure in pulsed mode up to a temperature of 87K at a wavelength of λ= 88 μm (3.4 THz) has been obtained. At 5K, a peak power of approximately 3.4 mW is observed, with still ∼1 mW at 78K.
Nonlocal resonant interaction between coupled quantum wires was studied. A resonant peak in the conductance that was correlated to the point at which the other wire pinches off was observed by measuring the conductance of one of the wires, as width of the other was varied. A micrograph of the device structure and schematics which illustrated the different measurements was showed.
Coupled double quantum well field-effect transistors with a grating gate exhibit a terahertz ({approx}600 GHz) photoconductive response that resonates with standing two dimensional plasma oscillations under the gate and may be the basis for developing a fast, tunable terahertz detector. The application of a precisely aligned in-plane magnetic field produces no detectable change in the device DC conductance but produces a dramatic inversion, growth of the terahertz photoconductive response and frequency shift of the standing plasmon resonances. The frequency shift can be described by a significant mass increase produced by the in-plane field. The mass increase is substantially larger than that calculated from a single well and we presume that a proper treatment of the coupled double quantum well may resolve this discrepancy.
Many MEMS-based components require optical monitoring techniques using optoelectronic devices for converting mechanical position information into useful electronic signals. While the constituent piece-parts of such hybrid opto-MEMS components can be separately optimized, the resulting component performance, size, ruggedness and cost are substantially compromised due to assembly and packaging limitations. GaAs MOEMS offers the possibility of monolithically integrating high-performance optoelectronics with simple mechanical structures built in very low-stress epitaxial layers with a resulting component performance determined only by GaAs microfabrication technology limitations. GaAs MOEMS implicitly integrates the capability for radiation-hardened optical communications into the MEMS sensor or actuator component, a vital step towards rugged integrated autonomous microsystems that sense, act, and communicate. This project establishes a new foundational technology that monolithically combines GaAs optoelectronics with simple mechanics. Critical process issues addressed include selectivity, electrochemical characteristics, and anisotropy of the release chemistry, and post-release drying and coating processes. Several types of devices incorporating this novel technology are demonstrated.
The temperature dependence of electron transfer between coupled quantum wells in a voltage tunable two-color quantum-well infrared photodetector (QWIP) was reported. It was shown that the electron transfer process is efficient only when interwell tunneling dominates. It was found that when thermoionic emission is substantial, electrons tend to accumulate in the shorter wavelength quantum well.
We present a self-consistent modeling of a 3.4-THz intersubband laser device. An ensemble Monte Carlo simulation, including both carrier-carrier and carrier-phonon scattering, is used to predict current density, population inversion, gain, and electron temperature. However, these two scattering mechanisms alone appear to be insufficient to explain the observed current density. In addition, the insufficient scattering yields a gain that is slightly higher than inferred from experiments. This suggests the presence of a non-negligible scattering mechanism which is unaccounted for in the present calculations.
The goal of this LDRD was to engineer further improvements in a novel electron tunneling device, the double electron layer tunneling transistor (DELTT). The DELTT is a three terminal quantum device, which does not require lateral depletion or lateral confinement, but rather is entirely planar in configuration. The DELTT's operation is based on 2D-2D tunneling between two parallel 2D electron layers in a semiconductor double quantum well heterostructure. The only critical dimensions reside in the growth direction, thus taking full advantage of the single atomic layer resolution of existing semiconductor growth techniques such as molecular beam epitaxy. Despite these advances, the original DELTT design suffered from a number of performance short comings that would need to be overcome for practical applications. These included (i)a peak voltage too low ({approx}20 mV) to interface with conventional electronics and to be robust against environmental noise, (ii) a low peak current density, (iii) a relatively weak dependence of the peak voltage on applied gate voltage, and (iv) an operating temperature that, while fairly high, remained below room temperature. In this LDRD we designed and demonstrated an advanced resonant tunneling transistor that incorporates structural elements both of the DELTT and of conventional double barrier resonant tunneling diodes (RTDs). Specifically, the device is similar to the DELTT in that it is based on 2D-2D tunneling and is controlled by a surface gate, yet is also similar to the RTD in that it has a double barrier structure and a third collector region. Indeed, the device may be thought of either as an RTD with a gate-controlled, fully 2D emitter, or alternatively, as a ''3-layer DELTT,'' the name we have chosen for the device. This new resonant tunneling transistor retains the original DELTT advantages of a planar geometry and sharp 2D-2D tunneling characteristics, yet also overcomes the performance shortcomings of the original DELTT design. In particular, it exhibits the high peak voltages and current densities associated with conventional RTDs, allows sensitive control of the peak voltage by the control gate, and operates nearly at room temperature. Finally, we note under this LDRD we also investigated the use of three layer DELTT structures as long wavelength (Terahertz) detectors using photon-assisted tunneling. We have recently observed a narrowband (resonant) tunable photoresponse in related structures consisting of grating-gated double quantum wells, and report on that work here as well.
A new class of magneto-oscillations in a high-density 2DEG was obtained and interpreted as the magnetophonon resonance with leaky interface-acoustic phonons. It was shown that owing to their 2D characteristics, the leaky interface modes play a key role in the scattering of 2D electrons in GaAs-AlGa-As heterostructures and quantum wells.
A novel planar resonant tunneling transistor is demonstrated. The growth structure is similar to that of a double-barrier resonant tunneling diode (RTD), except for a fully two-dimensional (2D) emitter formed by a quantum well. Current is fed laterally into the emitter, and the 2D--2D resonant tunneling current is controlled by a surface gate. This unique device structure achieves figures-of-merit, i.e. peak current densities and peak voltages, approaching that of state-of-the-art RTDs. Most importantly, sensitive control of the peak current and voltage is achieved by gating of the emitter quantum well subband energy. This quantum tunneling transistor shows exceptional promise for ultra-high speed and multifunctional operation at room temperature.
The authors report on their recent experimental studies of vertically-coupled quantum point contacts subject to in-plane magnetic fields. Using a novel flip-chip technique, mutually aligned split gates on both sides of a sub micron thick double quantum well heterostructure define a closely-coupled pair of ballistic one-dimensional (1D) constrictions. They observe quantized conductance steps due to each quantum well and demonstrate independent control of each ID constriction width. In addition, a novel magnetoconductance feature at {approximately}6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
The effect of the nuclear hyperfine interaction on the dc conductivity of 2D electrons under quantum Hall effect conditions at filling factor v= 1 is observed for the first time. The local hyperfine field enhanced by dynamic nuclear polarization is monitored via the Overhauser shift of the 2D conduction electron spin resonance in AlGaAs/GaAs multiquantum-well samples. The experimentally observed change in the dc conductivity resulting from dynamic nuclear polarization is in agreement with a thermal activation model incorporating the Zeeman energy change due to the hyperfine interaction. The relaxation decay time of the dc conductivity is, within experimental error, the same as the relaxation time of the nuclear spin polarization determined from the Overhauser shift. These findings unequivocally establish the nuclear spin origins of the observed conductivity change.
Polarized magneto-photoluminescence (MPL) measurements on a high mobility {delta}-doped GaAs/AlGaAs single quantum well from 0--60 T at temperatures between 0.37--2.1 K are reported. In addition to the neutral heavy hole magneto-exciton (X{sup 0}), the singlet (X {sub s}{sup {minus}}) and triplet (X {sub t}{sup {minus}}) states of the negatively charged magneto-exciton are observed in both polarizations. The energy dispersive and time-resolved MPL data suggest that their development is fundamentally related to the formation of the neutral magneto-exciton. At a magnetic field of 40 T the singlet and the triplet states cross as a result of the role played by the higher Landau levels and higher energy subbands in their energetic evolution, confirming theoretical predictions. The authors also observed the formation of two higher energy peaks. One of them is completely right circularly polarized and its appearance can be considered a result of the electron-hole exchange interaction enhancement with an associated electron g-factor of 3.7 times the bulk value. The other peak completely dominates the MPL spectrum at fields around 30 T. Its behavior with magnetic field and temperature indicates that it may be related to previous anomalies observed in the integer and fractional quantum Hall regimes.
The microstructure of lateral composition modulation in InAs/AlAs superlattices grown by MBE on InP is examined. The use of x-ray diffraction, TEM, AFM, and STEM to characterize the modulations is discussed. Combining the information from these techniques gives increased insight into the phenomenon and how to manipulate it. Diffraction measures the intensity of modulation and its wavelength, and is used to identify growth conditions giving strong modulation. The TEM and STEM analyses indicate that local compositions are modulated by as much as 0.38 InAs mole fraction. Plan-view images show that modulated structures consists of short ({approx_lt}0.2 {micro}m) In-rich wires with a 2D organization in a (001) growth plane. However, growth on miscut substrates can produce a single modulation along the miscut direction with much longer wires ({approx_gt}0.4 {micro}m), as desired for potential applications. Photoluminescence studies demonstrate that the modulation has large effects on the bandgap energy of the superlattice.