Kevin D. Maranowski

Coherent branched flow in a two-dimensional electron gas.

Semiconductor nanostructures based on two-dimensional electron gases (2DEGs) could form the basis of future devices for sensing, information processing and quantum computation. Although electron transport in 2DEG nanostructures has been well studied, and many remarkable phenomena have already been discovered (for example, weak localization, quantum chaos, universal conductance fluctuations), fundamental aspects of the electron flow through these structures have so far not been clarified. However, it has recently become possible to image current directly through 2DEG devices using scanning probe microscope techniques. Here, we use such a technique to observe electron flow through a narrow constriction in a 2DEG—a quantum point contact. The images show that the electron flow from the point contact forms narrow, branching strands instead of smoothly spreading fans. Our theoretical study of this flow indicates that this branching of current flux is due to focusing of the electron paths by ripples in the background potential. The strands are decorated by interference fringes separated by half the Fermi wavelength, indicating the persistence of quantum mechanical phase coherence in the electron flow. These findings may have important implications for a better understanding of electron transport in 2DEGs and for the design of future nanostructure devices.Semiconductor nanostructures based on two dimensional electron gases (2DEGs) have the potential to provide new approaches to sensing, information processing, and quantum computation. Much is known about electron transport in 2DEG nanostructures and many remarkable phenomena have been discovered (e.g. weak localization, quantum chaos, universal conductance fluctuations)1,2 - yet a fundamental aspect of these devices, namely how electrons move through them, has never been clarified. Important details about the actual pattern of electron flow are not specified by statistical measures such as the mean free path. Scanned probe microscope (SPM) measurements allow spatial investigations of nanostructures, and it has recently become possible to directly image electron flow through 2DEG devices using newly developed SPM techniques3-13. Here we present SPM images of electron flow from a quantum point contact (QPC) which show unexpected dynamical channeling - the electron flow forms persistent, narrow, branching channels rather than smoothly spreading fans. Theoretical study of this flow, including electron scattering by impurities and donor atoms, shows that the channels are not due to deep valleys in the potential, but rather are caused by the indirect cumulative effect of small angle scattering. Surprisingly, the channels are decorated by interference fringes well beyond where the simplest thermal averaging arguments suggest they should be found. These findings may have important implications for 2DEG physics and for the design of future nanostructure devices.

High mobility two-dimensional electron gas in AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

High quality AlGaN/GaN heterostructures have been grown by radio-frequency plasma-assisted molecular beam epitaxy on n-type GaN templates grown on sapphire by metal organic chemical vapor deposition. The unintentionally doped Al0.12Ga0.88N/GaN heterostructure exhibits a 77 K Hall mobility of 14 500 cm2/Vs and a 12 K mobility of 20 000 cm2/Vs (ns=5.0×1012 cm−2). A room temperature mobility of 1860 cm2/Vs (ns=4.8×1012 cm−2) was calculated for the two-dimensional electron gas channel using a two layer model from the measured mobility for the whole structure (template plus heterostructure). Magnetoresistance measurements at 4.2 K showed well-resolved Shubnikov–de Haas oscillations, which began at 2.6 T.

GaAs/AlGaAs self-sensing cantilevers for low temperature scanning probe microscopy

We have fabricated scanning probe microscope cantilevers with dimensions 65×11.4×0.25 μm3 and 3×2×0.129 μm3 from GaAs/Al0.3Ga0.7As heterostructures containing two-dimensional electron gases. Deflection is measured by an integrated field-effect transistor (FET) that senses strain via the piezoelectric effect and provides a low noise, low power displacement readout. We present images of a 200 nm mica grating taken with the large cantilever having a deflection (force) noise 10 A/√Hz (19 pN/√Hz) at T=2.2 K. The small cantilever has a resonant frequency of 11 MHz, a FET gate charge noise of 0.001 e/√Hz, and is projected to have a deflection (force) noise of 0.002 A/√Hz (1 pN/√Hz) at T=4.2 K.

Integrated micromechanical cantilever magnetometry of Ga1−xMnxAs

We have developed a technique for fabricating submicron GaAs micromechanical cantilevers into which lithographically patterned samples grown by molecular beam epitaxy or evaporative deposition are integrated. The torque sensitivity of the 100-nm-thick cantilevers makes them ideal for torsional magnetometry of nanometer-scale, anisotropic samples. We present measurements on samples of the ferromagnetic semiconductor Ga1−xMnxAs at temperatures from 350 mK to 65 K and in fields from 0 to 8 T. By measuring the shift in the resonant frequency of the cantilevers, we demonstrate a moment sensitivity of 3×106 μB at 0.1 T, an improvement of nearly five orders of magnitude upon existing torsional magnetometers.

Strongly capacitively coupled quantum dots

Double quantum dots were formed in a gated GaAs/AlGaAs heterostructure with negligible interdot tunneling; strong capacitive coupling was provided by a floating interdot capacitor. The interdot capacitance was measured to be 0.28CΣ, where CΣ is the single-dot capacitance. Coulomb blockade conductance images for both dots versus side gate voltages at 70 mK show a hexagonal pattern of peaks; the double dot acts as a single-electron current switch. For weak tunneling, the conductance peaks of both dots fit thermally broadened line shapes. Charge fluctuations produced by strong tunneling on one dot are induced on the second, filling in its peak splitting.

FAR-INFRARED EMISSION FROM PARABOLICALLY GRADED QUANTUM WELLS

We have observed grating coupled far infrared (FIR) emission from parabolically graded quantum wells (PQWs) by the application of an in‐plane electric field. The peak emission frequency from different wells matches the designed harmonic oscillator frequency for each well, as determined by the curvature of the PQWs. This is a confirmation that the generalized Kohn theorem applies for emission of FIR radiation.

Real‐time simultaneous optical‐based flux monitoring of Al, Ga, and In using atomic absorption for molecular beam epitaxy

We have developed a multichannel atomic absorption measurement system for real‐time simultaneous monitoring of Al, Ga, and In molecular beam fluxes. In our configuration, distinct atomic emission lines from three hollow cathode lamps are combined into one beam, thus requiring only one pair of through view ports for the optical probe beam. Based on the dual beam optical configuration, the reference arm compensates for intensity drift of the light sources. In this work, we demonstrate the use of reflection high energy electron diffraction oscillations for calibrating the absorption signal.

Imaging interedge-state scattering centers in the quantum Hall regime

After years of study and two Nobel prizes, the quantum Hall effect continues to provide important challenges to both experimentalists and theorists. Many of the most interesting questions concern the nonuniform spatial structures that can occur within a two-dimensional electron gas ~2DEG! in high magnetic fields. These structures arise from competition between the effects of Landau level ~LL! quantization, Coulomb interactions, and external potentials and include striped phases 1 and insulating phases in the bulk 2 as well as conducting states localized at the edges of the sample ~edge states!. 3,4 Scanned probe techniques offer a new approach to investigate these structures directly. They have recently been used to probe the Hall voltage profile and the properties of the insulating state within a quantum Hall plateau. 5‐9 Here we use a scanned probe to investigate the microscopic effects of the spatial structure in a 2DEG on electron transport by examining the nature of the scattering between edge states in a quantum Hall conductor. Transport in the integer quantum Hall regime is now well understood in terms of transport through both quasi-1D edge

Temperature dependence of far-infrared electroluminescence in parabolic quantum wells

We have measured the far-infrared emission from parabolically graded quantum wells driven by an in-plane electric field in the temperature range from 20 to 240 K. The peak emission corresponds to the intersubband plasmon in the parabolic potential. Its photon energy (6.6/9.8 meV) remains rather unaffected by temperature variations, the full-width at half-maximum ranges from 1 (T=20 K) to 2 meV (T=240 K). The reduction of emission efficiency with increasing temperature is attributed to the change in the nonradiative lifetime.

Fabrication and characterization of 100‐nm‐thick GaAs cantilevers

We describe a new process for making submicron, micromechanical cantilevers out of GaAs epilayers grown by molecular beam epitaxy. The extremely high aspect ratios of these cantilevers (typically 100 nm thick and 100 μm long) give spring constants as low as 10−4 N/m. We present characterizations of the cantilevers’ resonant frequencies, quality factors, and spring constants. The ability to fabricate III–V GaAs‐based mechanical microstructures offers new opportunities for integration with electronics for strain‐sensitive force detection.