Stephen J. Allen

Simulations of electric field domain suppression in a superlattice oscillator device using a distributed circuit model

We present a method for simulating static domain formation in distributed negative differential resistance devices using a distributed circuit array model coupled with quantum transport simulations. This simulation method is applied to the case of a superlattice Bloch oscillator to ascertain the efficacy of electric field domain wall suppression by micro shunt side walls. Two independent simulation mechanisms using the same basic distributed circuit model are employed to separate simulation artifacts from true physical trends. Simulations are presented, suggesting that the presence of the micro shunt can suppress domain formation above a critical device bias voltage. The simulated dependence of this critical voltage on macroscopic device parameters is presented.

Terahertz frequency response of an In0.53Ga0.47As/AlAs resonant‐tunneling diode

We have measured the room‐temperature, broad‐band, terahertz response of a high‐speed In0.53Ga0.47As/AlAs resonant‐tunneling diode from 120 GHz to 3.9 THz using the free‐electron lasers at UCSB. The ‘‘rectified’’ response is measured with a conventional probe station by using the tungsten probe tip as a whisker antenna. Normalizing the rectified response in the resonant‐tunneling regime with the off‐resonant response we remove the extrinsic frequency dependence controlled by the antenna and the RC time constant and measure an intrinsic relaxation time.

Terahertz response of resonant tunneling diodes

Abstract We have measured the broad-band terahertz response of state of the art InGaAs/AlAs and InAs/AlSb resonant tunneling diodes from 180 GHz to 3.6 THz using the free-electron lasers at UCSB. A tungsten whisker antenna in a conventional probe station is used to couple the far-infrared radiation into the device. Normalizing the resonant tunneling response with the off-resonant response allows us to circumvent the much slower RC time constant of the device and consequently enables a measurement of the relaxation time due to the quantum inductance.

Superconductivity and the Josephson effect in a periodic array of NbInAsNb junctions

We have applied high-frequency radiation to a one-dimensional array of superconducting-normal-superconducting junctions, comprised of Nb2DInAsNb, and observed Shapiro steps in the I–V curve which are dominated by a step at V = hv/4e, half the voltage of the usual AC Josephson effect. This result is discussed in view of a coupling between the Nb stripes that differs from the usual jc sin Φ form. The zero-bias resistance of the sample is finite and increases exponentially with temperature. The Shapiro steps, however, persist up to the Nb transition temperature. These results imply that the finite resistance of the sample originates from excitations in the superconducting state.

Far-infrared studies of induced superconductivity in quantum wells

Recent work on peridic SNS microstructures, in which a superconductor grating contacts an underlying InAs quantum well, has demonstrated that supercurrents can flow across the strips of quasi-two-dimensional electron gas between the superconductor grating lines. Here we show that the loss of DC resistance is accompanied by the emergence of striking features in the samples excitation spectra, for energies below ∼ 2†. Although such series weak-link structures may display a Josephson plasmon resonance, the changes in the samples far-infrared transmission in an applied H field appear to be inconsistent with the behavior expected for this mode. The transmission spectra also differ from the form expected for an energy gap in the quantum well strips between adjacent superconductor lines. We speculate that the observed absorption features instead arise from transitions between quasi-particle states produced by Andreev reflections at the superconductor/InAs interfaces.

Terahertz response of an InGaAs/AlAs resonant tunnelling diode

We have measured the broad-band terahertz response of a state-of-the-art InGaAs/AlAs resonant tunnelling diode from 120 GHz to 3.9 THz using the free-electron lasers at the University of California, Santa Barbara. A tungsten whisker antenna in a conventional probe station is used to couple the far-infrared radiation into the device. By normalizing the rectified response in the resonant tunnelling regime with the off-resonant response we are able to remove the antenna frequency effects and the frequency dependence controlled by the much slower RC time constant and measure the relaxation time due to the quantum inductance.

Regrowth of InAs on the sides of in-situ etched InAs/AlSb superlattices

We propose to bias uniformly a superlattice by etching mesas of an InAs/AlSb superlattice in-situ and subsequently regrowing InAs on the sidewalls. A uniformly biased semiconductor superlattice produces a Stark ladder which can support gain for frequencies just below the Stark splitting. Since the Stark ladder can be easily tuned with the applied bias, the superlattice can be the basis for a terahertz frequency solid state oscillator. Unfortunately, charge accumulation, electric field instabilities, and electric field domain formation occur in superlattices. Circuit models of the domain formation suggest that a thin (4-40 nm) layer of InAs on the sidewall of a /spl sim/1 /spl mu/m wide InAs/AlSb mesa could suppress domain formation without compromising the gain at terahertz frequencies.

Intrinsic response times of double-barrier resonant tunneling diodes at terahertz frequencies

We have measured the broad band terahertz response of state of the art InGaAs/AlAs and InAs/AlSb resonant tunneling diodes from 180 GHz to 3.6 THz using the free-electron lasers at UCSB. A tungsten whisker antenna in a conventional probe station is used to couple the far- infrared radiation into the device. Normalizing the resonant tunneling response with the off- resonant response allows us to circumvent the much slower RC time constant of the device and consequently enables a measurement of the relaxation time due to the quantum inductance.