Jeng-Chung Chen

Transition between Quantum States in a Parallel-Coupled Double Quantum Dot

Strong electron and spin correlations in a double quantum dot (DQD) can give rise to different quantum states. We observe a continuous transition from a Kondo state exhibiting a single-peak Kondo resonance to another exhibiting a double peak by increasing the interdot coupling (t) in a parallel-coupled DQD. The transition into the double-peak state provides evidence for spin entanglement between the excess electrons on each dot. Toward the transition, the peak splitting merges and becomes substantially smaller than t because of strong Coulomb effects. Our device tunability bodes well for future quantum computation applications.

Infrared phototransistor using capacitively coupled two-dimensional electron gas layers

A narrow-band infrared phototransistor (14.8μm) is designed and realized based on a GaAs∕AlGaAs double-layer structure. An isolated island formed from the first quantum well (QW) works as a gate, which is capacitively coupled to the remote two-dimensional electron gas (2DEG) layer working as the source/drain channel. Incident radiation excites the intersubband transition within the isolated QW island. Excited electrons tunnel out of the QW causing it to positively charge up. This affects the conductance of the remote 2DEG channel, yielding detectable photosignals. The present detection mechanism makes it possible to design semiconductor infrared detectors with higher sensitivities along with custom designed tunability. The mechanism also holds potentiality of single-photon detection in the infrared region.

Characteristics of a sensitive micro-Hall probe fabricated on chemical vapor deposited graphene over the temperature range from liquid-helium to room temperature

We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated on chemical vapor deposited (CVD) graphene, from 300 K to 4.2 K. The field sensitivity of the Hall probe was tunable within ∼0.031-0.12 Ω/G, while the field resolution could reach ∼0.43-0.09 G/Hz1/2 at room temperature. The characteristics of graphene Hall probes (GHPs) were found to be comparable to present Hall sensors. Our results indicate that the fundamental limitation of the field sensitivity and the field resolution are respectively restricted by intrinsic and extrinsic defects. Our study paves the way for the use of CVD GHPs for scanning Hall probe with high field sensitivity and submicron spatial resolution at room temperature.

A sensitive double quantum well infrared phototransistor

An infrared phototransistor (∼14.5μm) on a GaAs∕AlGaAs double quantum well (QW) heterostructure is studied. A confined upper QW behaves as a photoactive gate to a conducting channel formed by the lower QW. By properly biasing the narrow gates for isolating the upper QW island, the lateral tunneling rate of cold electrons on upper QW can be tuned and hence the lifetime of photocarriers on the QW island can be controlled. Associated with this controllable lifetime, photoresponse takes a sharp maximum, which reaches as high as ∼103A∕W. Analysis in terms of a simple model suggests that the peak response originates from the interplay∕trade-off between the lifetime of photocarriers and the efficiency of photodetection process. The photodetection efficiency substantially varies as a consequence of large band bending induced by the 300K thermal background radiation. The long (approximately millisecond order) and controllable lifetime in our device paves the way for future development of photon counters in the lon...

The growth of pinhole-free epitaxial DySi2−x films on atomically clean Si(111)

The growth of pinhole-free epitaxial DySi2−x films on atomically clean Si(111) has been achieved by depositing a 2-nm-thick Dy layer onto Si(111) with a 1.5-nm-thick capping amorphous Si (a-Si) layer at room temperature followed by annealing at 700 °C in ultrahigh vacuum. The thickness of the a-Si was selected to be such that the consumption of Si atoms from the substrate is minimized by taking into account the formation of an amorphous interlayer at the Dy/Si(111) interface. Based on our experimental findings, a new mechanism for the formation of pinhole is proposed. The Stranski–Krastanov growth behavior of epitaxial DySi2−x on Si(111) by solid phase epitaxy leads to the apparently random formation of a high density of recessed regions at the initial stage of silicidation. Polycrystalline DySi2−x was found to be present at the areas inside and epitaxial DySi2−x outside the recessed regions. Large numbers of Si atoms from the substrate can therefore diffuse through the recessed regions. As a result, the ...

Effects of impurity scattering on the quantized conductance of a quasi-one-dimensional quantum wire

We report an experimental observation of how the presence of an impurity in a quasi-one-dimensional wire influences the quantized conductance. The impurity is a chemically etched nanohole, relative to which the walls of the wire can be tuned via external gate voltages. Depending on the positions of the sidewalls, resonance features are observed in the quantized conductance due to either the multiple scatterings between the impurity and the wall of wire or the channel interference. Meanwhile, the differential conductance exhibits the well-known half-plateau features in a single channel wire or saturates in a wire with coupled two channels.

The Kondo effect in coupled-quantum dots

We discuss Kondo systems in coupled-quantum dots, with emphasis on the semiconductor quantum dot system. The rich variety of behaviors, such as distinct quantum phases, non-Fermi-liquid behavior and associated quantum phase transitions and cross-over behaviors are reviewed. Experimental evidence for such novel characteristics is summarized. The observed behaviors may provide clues as to the relevance of the 2-impurity Kondo (2IK) effect and the 2-channel Kondo (2CK) effect to the unusual characteristics in strongly correlated systems, such as the heavy fermion system.

Interfacial reactions of Gd thin films on (111) and (001)Si

Abstract Interfacial reactions of Gd on both (111) and (001)Si have been investigated. An amorphous interlayer (a-interlayer) is formed at the initial stage of reactions of RE metal thin films on Si systems. The growth of amorphous phase was followed by the nucleation of the crystalline silicide at the a-interlayer/Si interface. Both hexagonal GdSi2−x (h-GdSi2−x) and orthorhombic GdSi2 (o-GdSi2) were observed to form in (001) and (111) samples. Crystalline defects such as vacancy ordering and stacking faults were observed and analyzed.

Graphene-GaAs/AlxGa1−xAs heterostructure dual-function field-effect transistor

We have integrated chemical vapor-deposited graphene and GaAs/AlxGa1−xAs heterostructure into a hybrid field effect transistor (FET). Depending on the operation scheme, graphene can be utilized either as a gate electrode for a GaAs-based high electron mobility transistor (HEMT) or as a channel material gated by two dimensional electron gas (2DEG) formed in the interface of a heterojunction. Our studies reveal that 2DEG can function as an effective back-electrode to tune the ambipolar effect of graphene. The performance of graphene FET (GFET) is limited by the interface band bending of the heterojunction associated with the gating voltages and the intrinsic surface morphology of GaAs substrate. Our results bode a way to implement HEMT/GEFT-based bi-FET integrated circuits.

Formation of Gd oxide thin films on (111)Si

Abstract Formation of Gd oxide thin films on (111)Si has been investigated. A complicated multilayered structure was formed in Gd thin films on (111)Si annealed in O2 ambient. On the other hand, a uniform oxide layer was formed in GdSi2−x films on (111)Si oxidized in O2 ambient. Oxidation mechanisms are discussed.