P. F. Hopkins

New quantum structures

Structures in which electrons are confined to move in two dimensions (quantum wells) have led to new physical discoveries and technological applications. Modification of these structures to confine the electrons to one dimension (quantum wires) or release them in the third dimension, are predicted to lead to new electrical and optical properties. This article discusses techniques to make quantum wires, and quantum wells of controlled size and shape, from compound semiconductor materials, and describes some of the properties of these structures.

IMAGING AND MAGNETOMETRY OF SWITCHING IN NANOMETER-SCALE IRON PARTICLES

The reversal mechanisms in arrays of nanometer‐scale (<40 nm diameter) iron particles are studied by low‐temperature Hall magnetometry and room‐temperature magnetic force microscopy. Rotation of the net array magnetization at low temperatures (20 K) occurs by both reversible and irreversible modes, the latter revealed by Barkhausen jumps. Spatially resolved measurements at room temperature show the particles to be single domain with remanence and coercivity indicating they are not superparamagnetic. Individual particles are observed to switch irreversibly over a small field range (<10 Oe) between preferred magnetic directions parallel to the growth direction of the particles. Scaling of the arrays offers the possibility of magnetic storage at the 45 Gbit/in.2 level, nearly 50 times greater than current technology.

Far‐infrared pump‐probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled‐quantum well

We report pump‐and‐probe measurements of the electron intersubband lifetime (T1) in an AlGaAs/GaAs heterostructure using a picosecond pulsed far‐infrared laser. The subband spacing (11 meV) is less than the optical‐phonon energy. Time‐resolved measurements yield intersubband lifetimes ranging from T1=1.1±0.2 ns to T1=0.4±0.1 ns depending on measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities ≤1 W/cm2. The steady‐state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission.

Cryogenic field‐effect transistor with single electronic charge sensitivity

We have fabricated matched pairs of cryogenic field‐effect transistors with input charge sensitivity qn=0.01 e/√Hz at T=1.3 K, low input capacitance 0.4 pF, and extremely high input resistance in excess of 1015 Ω. Low leakage permits dc charge‐coupled operation for times up to ∼103 s. The channel noise is characterized by a flat spectrum at high frequencies, and 1/f noise below a corner frequency fc<1 kHz. These devices can resolve charge differences as small as qn√fc=0.4e.

Conductance fluctuations in a quantum dot in the tunneling regime: Crossover from aperiodic to regular behavior

Abstract We discuss magnetoconductance fluctuations in a ∼ 1 microm GaAS/AlxGa1−xAs quantum dot in the shape of a chaotic stadium billiard with point contact leads. Here we emphasize measurements carried out in the tunneling regime, where each lead carries less than one fully conducting channel. We discuss various crossover phenomena as the magnetoconductance fluctuations evolve from aperiodic structure at low fields (B ≲ 0.5 T) into periodic, Aharonov-Bohm-like oscillations at higher fields.

Far-infrared saturation spectroscopy of a single square well

We have performed saturation spectroscopy measurements of the lowest intersubband transition in a single 400 AA GaAs/Al0.3Ga0.7As modulation-doped square quantum well. We couple intense tunable far-infrared radiation from the Santa Barbara free electron laser into our sample using an edge-coupling technique and measure absorption as a function of frequency and intensity. Saturation and frequency shifts in the absorption line are clearly observed. We attribute the frequency shifts to reductions in the many-body depolarization shift. From our preliminary measurements, we estimate the intersubband relaxation time to be 600 ps to within a factor of three.

Materials science in the far-IR with electrostatic based FELs

Abstract A technology gap exists between ∼ 100 GHz and ∼ 10 THz. Free-electron lasers (FELs), driven by high quality, relatively low energy electron beams from electrostatic accelerators, and capable of generating kilowatts of coherent, tunable radiation, are ideally suited to explore the enabling science for future technology in this spectral range. We describe two experiments that use terahertz “optical rectification” to measure i) the intensity and temperature dependent energy relaxation in quantum wells and ii) the intrinsic relaxation of resonant tunneling diodes. Both benefit from the power and tunablilty of the UCSB FELs.

Chapter 2 Wide Graded Potential Wells

Publisher Summary Wide graded potential wells offer a means to extend the study of high-quality electron gases to new geometries with broader spatial extents than had previously been possible. The chapter discusses that growth is a substantially important issue in the formation of compositionally graded structures. Since the local effective positive background charge of a bare graded well is proportional to the local second derivative of the composition profile, a uniform constant charge density profile requires a precisely controlled composition profile. Small ripples or flat spots in the profile are expected to give strongly non-uniform charge densities. Molecular beam epitaxy provides a precisely controlled growth technique suitable for producing these closely controlled composition profiles. The discussion on the approach to the three-dimensional electron gas in wide parabolic quantum wells is perhaps the most advanced and probably the most promising approach presently available to observe the range of phenomena that are expected for the ideal three-dimensional electron system. Applications of the wide parabolic quantum wells are still in their infancy. Since the materials form resonant structures in the submillimeter and far infrared frequency ranges, it seems probable that they will find applications in these areas.

Logarithmically graded quantum well far‐infrared modulator

We have designed and fabricated a remotely doped ‘‘logarithmic’’ potential well intended to have a tunable, narrow band absorption at far‐infrared frequencies. A surface gate, epitaxially grown backgate, and contact to the electron gas in the quantum well allow independent control of the absorption frequency and the integrated absorption strength. The resonance frequency is dominated by the well curvature at the potential minimum and can be Stark shifted from ω/2πc=35 cm−1 to a frequency of 125 cm−1 by moving the electron gas through the asymmetric well.

Probing terahertz dynamics in semiconductor nanostructures with the UCSB free-electron lasers

Abstract The UCSB free-electron lasers provide kilowatts of continuously tunable radiation from 120 GHz to 4.8 THz. They have the most impact on terahertz science and technology that require a tunable, high power source to explore non-linear dynamics or that sacrifice incident power to recover the linear response of systems with very small cross-section. We describe three experiments that demonstrate the utility of these lasers in experiments on the terahertz dynamics of semiconductor nanostructures: (i) terahertz dynamics of resonant tunneling diodes, (ii) saturation spectroscopy of quantum wells and (iii) photon-assisted tunneling in superlattices.