The three-dimensional confinement inherent in InAs self-assembled quantum dots (SAQDs) yields vastly different optical properties compared to one-dimensionally confined quantum well systems. Intersubband transitions in quantum dots can emit light normal to the growth surface, whereas transitions in quantum wells emit only parallel to the surface. This is a key difference that can be exploited to create a variety of quantum dot devices that have no quantum well analog. Two significant problems limit the utilization of the beneficial features of SAQDs as mid-infrared emitters. One is the lack of understanding concerning how to electrically inject carriers into electronic states that allow optical transitions to occur efficiently. Engineering of an injector stage leading into the dot can provide current injection into an upper dot state; however, to increase the likelihood of an optical transition, the lower dot states must be emptied faster than upper states are occupied. The second issue is that SAQDs have significant inhomogeneous broadening due to the random size distribution. While this may not be a problem in the long term, this issue can be circumvented by using planar photonic crystal or plasmonic approaches to provide wavelength selectivity or other useful functionality.
We have investigated the physics of Bloch oscillations (BO) of electrons, engineered in high mobility quantum wells patterned into lateral periodic arrays of nanostructures, i.e. two-dimensional (2D) quantum dot superlattices (QDSLs). A BO occurs when an electron moves out of the Brillouin zone (BZ) in response to a DC electric field, passing back into the BZ on the opposite side. This results in quantum oscillations of the electron--i.e., a high frequency AC current in response to a DC voltage. Thus, engineering a BO will yield continuously electrically tunable high-frequency sources (and detectors) for sensor applications, and be a physics tour-de-force. More than a decade ago, Bloch oscillation (BO) was observed in a quantum well superlattice (QWSL) in short-pulse optical experiments. However, its potential as electrically biased high frequency source and detector so far has not been realized. This is partially due to fast damping of BO in QWSLs. In this project, we have investigated the possibility of improving the stability of BO by fabricating lateral superlattices of periodic coupled nanostructures, such as metal grid, quantum (anti)dots arrays, in high quality GaAs/Al{sub x}Ga{sub 1-x}As heterostructures. In these nanostructures, the lateral quantum confinement has been shown theoretically to suppress the optical-phonon scattering, believed to be the main mechanism for fast damping of BO in QWSLs. Over the last three years, we have made great progress toward demonstrating Bloch oscillations in QDSLs. In the first two years of this project, we studied the negative differential conductance and the Bloch radiation induced edge-magnetoplasmon resonance. Recently, in collaboration with Prof. Kono's group at Rice University, we investigated the time-domain THz magneto-spectroscopy measurements in QDSLs and two-dimensional electron systems. A surprising DC electrical field induced THz phase flip was observed. More measurements are planned to investigate this phenomenon. In addition to their potential device applications, periodic arrays of nanostructures have also exhibited interesting quantum phenomena, such as a possible transition from a quantum Hall ferromagnetic state to a quantum Hall spin glass state. It is our belief that this project has generated and will continue to make important impacts in basic science as well as in novel solid-state, high frequency electronic device applications.
The purpose of this work was to create a THz component set and understanding to aid in the rapid analysis of transient events. This includes the development of fast, tunable, THz detectors, along with filter components for use with standard detectors and accompanying models to simulate detonation signatures. The signature effort was crucial in order to know the spectral range to target for detection. Our approach for frequency agile detection was to utilize plasmons in the channel of a specially designed field-effect transistor called the grating-gate detector. Grating-gate detectors exhibit narrow-linewidth, broad spectral tunability through application of a gate bias, and no angular dependence in their photoresponse. As such, if suitable sensitivity can be attained, they are viable candidates for Terahertz multi-spectral focal plane arrays.
Split grating-gate field effect transistors (FETs) detectors made from high mobility quantum well two-dimensional electron gas material have been shown to exhibit greatly improved tunable resonant photoresponse compared to single grating-gate detectors due to the formation of a 'diode-like' element by the split-gate structure. These detectors are relatively large for FETs (1mm × 1mm area or larger) to match typical focused THz beam spot sizes. In the case where the focused THz spot size is smaller than the detector area, we have found evidence, through positional scanning of the detector element, that only a small portion of the detector is active. To further investigate this situation, detectors with the same channel width (1mm), but various channel lengths, were fabricated and tested. The results indicate that indeed, only a small portion of the split grating gated FET is active. This finding opens up the possibility for further enhancement of detector sensitivity by increasing the active area.
The purpose of this work was to develop a wavelength tunable detector for Terahertz spectroscopy and imaging. Our approach was to utilize plasmons in the channel of a specially designed field-effect transistor called the grating-gate detector. Grating-gate detectors exhibit narrow-linewidth, broad spectral tunability through application of a gate bias, and no angular dependence in their photoresponse. As such, if suitable sensitivity can be attained, they are viable candidates for Terahertz multi-spectral focal plane arrays. When this work began, grating-gate gate detectors, while having many promising characteristics, had a noise-equivalent power (NEP) of only 10{sup -5} W/{radical}Hz. Over the duration of this project, we have obtained a true NEP of 10{sup -8} W/{radical}Hz and a scaled NEP of 10{sup -9}W/{radical}Hz. The ultimate goal for these detectors is to reach a NEP in the 10{sup -9{yields}-10}W/{radical}Hz range; we have not yet seen a roadblock to continued improvement.
The potential for implementing quantum coherence in semiconductor self-assembled quantum dots has been investigated theoretically and experimentally. Theoretical modeling suggests that coherent dynamics should be possible in self-assembled quantum dots. Our experimental efforts have optimized InGaAs and InAs self-assembled quantum dots on GaAs for demonstrating coherent phenomena. Optical investigations have indicated the appropriate geometries for observing quantum coherence and the type of experiments for observing quantum coherence have been outlined. The optical investigation targeted electromagnetically induced transparency (EIT) in order to demonstrate an all optical delay line.
For several years now quantum computing has been viewed as a new paradigm for certain computing applications. Of particular importance to this burgeoning field is the development of an algorithm for factoring large numbers which obviously has deep implications for cryptography and national security. Implementation of these theoretical ideas faces extraordinary challenges in preparing and manipulating quantum states. The quantum transport group at Sandia has demonstrated world-leading, unique double quantum wires devices where we have unprecedented control over the coupling strength, number of 1 D channels, overlap and interaction strength in this nanoelectronic system. In this project, we study 1D-1D tunneling with the ultimate aim of preparing and detecting quantum states of the coupled wires. In a region of strong tunneling, electrons can coherently oscillate from one wire to the other. By controlling the velocity of the electrons, length of the coupling region and tunneling strength we will attempt to observe tunneling oscillations. This first step is critical for further development double quantum wires into the basic building block for a quantum computer, and indeed for other coupled nanoelectronic devices that will rely on coherent transport. If successful, this project will have important implications for nanoelectronics, quantum computing and information technology.
Macroscopic quantum states such as superconductors, Bose-Einstein condensates and superfluids are some of the most unusual states in nature. In this project, we proposed to design a semiconductor system with a 2D layer of electrons separated from a 2D layer of holes by a narrow (but high) barrier. Under certain conditions, the electrons would pair with the nearby holes and form excitons. At low temperature, these excitons could condense to a macroscopic quantum state either through a Bose-Einstein condensation (for weak exciton interactions) or a BCS transition to a superconductor (for strong exciton interactions). While the theoretical predictions have been around since the 1960's, experimental realization of electron-hole bilayer systems has been extremely difficult due to technical challenges. We identified four characteristics that if successfully incorporated into a device would give the best chances for excitonic condensation to be observed. These characteristics are closely spaced layers, low disorder, low density, and independent contacts to allow transport measurements. We demonstrated each of these characteristics separately, and then incorporated all of them into a single electron-hole bilayer device. The key to the sample design is using undoped GaAs/AlGaAs heterostructures processed in a field-effect transistor geometry. In such samples, the density of single 2D layers of electrons could be varied from an extremely low value of 2 x 10{sup 9} cm{sup -2} to high values of 3 x 10{sup 11} cm{sup -2}. The extreme low values of density that we achieved in single layer 2D electrons allowed us to make important contributions to the problem of the metal insulator transition in two dimensions, while at the same time provided a critical base for understanding low density 2D systems to be used in the electron-hole bilayer experiments. In this report, we describe the processing advances to fabricate single and double layer undoped samples, the low density results on single layers, and evidence for gateable undoped bilayers.
We show theoretically that an electric current in a high-mobility quasi-two-dimensional electron layer induces a significant drift of excitons in an adjacent layer through interlayer Coulomb interaction. The exciton gas is shown to drift with a velocity which can be a significant fraction of the electron drift velocity at low temperatures. The estimated drift length is of the order of micrometers or larger during the typical exciton lifetime for GaAs/Al{sub x}Ga{sub 1-x} double quantum wells. A possible enhancement of the exciton radiative lifetime due to the drift is discussed.
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.