The authors have developed two versions of a flexible fabrication technique known as membrane projection lithography that can produce nearly arbitrary patterns in '212 D' and fully three-dimensional (3D) structures. The authors have applied this new technique to the fabrication of split ring resonator-based metamaterials in the midinfrared. The technique utilizes electron beam lithography for resolution, pattern design flexibility, and alignment. The resulting structures are nearly three orders of magnitude smaller than equivalent microwave structures that were first used to demonstrate a negative index material. The fully 3D structures are highly isotropic and exhibit both electrically and magnetically excited resonances for incident transverse electromagnetic waves.
3-D cubic unit cell arrays containing split ring resonators were fabricated and characterized. The unit cells are {approx}3 orders-of-magnitude smaller than microwave SRR-based metamaterials and exhibit both electrically and magnetically excited resonances for normally incident TEM waves in addition to showing improved isotropic response.
We describe a time-domain spectroscopy system in the thermal infrared used for complete transmission and reflection characterization of metamaterials in amplitude and phase. The system uses a triple-output near-infrared ultrafast fiber laser, phase-locked difference frequency generation and phase-matched electro-optic sampling. We will present measurements of several metamaterials designs.
LDRD Project 139363 supported experiments to quantify the performance characteristics of monolithically integrated Schottky diode + quantum cascade laser (QCL) heterodyne mixers at terahertz (THz) frequencies. These integrated mixers are the first all-semiconductor THz devices to successfully incorporate a rectifying diode directly into the optical waveguide of a QCL, obviating the conventional optical coupling between a THz local oscillator and rectifier in a heterodyne mixer system. This integrated mixer was shown to function as a true heterodyne receiver of an externally received THz signal, a breakthrough which may lead to more widespread acceptance of this new THz technology paradigm. In addition, questions about QCL mode shifting in response to temperature, bias, and external feedback, and to what extent internal frequency locking can improve stability have been answered under this project.
Plasmonic structures open up new opportunities in photonic devices, sometimes offering an alternate method to perform a function and sometimes offering capabilities not possible with standard optics. In this LDRD we successfully demonstrated metal coatings on optical surfaces that do not adversely affect the transmission of those surfaces at the design frequency. This technology could be applied as an RF noise blocking layer across an optical aperture or as a method to apply an electric field to an active electro-optic device without affecting optical performance. We also demonstrated thin optical absorbers using similar patterned surfaces. These infrared optical antennas show promise as a method to improve performance in mercury cadmium telluride detectors. Furthermore, these structures could be coupled with other components to lead to direct rectification of infrared radiation. This possibility leads to a new method for infrared detection and energy harvesting of infrared radiation.
We designed, fabricated and measured the performance of nanoelectromechanical (NEMS) switches. Initial data are reported with one of the switch designs having a measured switching time of 400 ns and an operating voltage of 5 V. The switches operated laterally with unmeasurable leakage current in the 'off' state. Surface micromachining techniques were used to fabricate the switches. All processing was CMOS compatible. A single metal layer, defined by a single mask step, was used as the mechanical switch layer. The details of the modeling, fabrication and testing of the NEMS switches are reported.
There is significant interest in forming quantum bits (qubits) out of single electron devices for quantum information processing (QIP). Information can be encoded using properties like charge or spin. Spin is appealing because it is less strongly coupled to the solid-state environment so it is believed that the quantum state can better be preserved over longer times (i.e., that is longer decoherence times may be achieved). Long spin decoherence times would allow more complex qubit operations to be completed with higher accuracy. Recently spin qubits were demonstrated by several groups using electrostatically gated modulation doped GaAs double quantum dots (DQD) [1], which represented a significant breakthrough in the solid-state field. Although no Si spin qubit has been demonstrated to date, work on Si and SiGe based spin qubits is motivated by the observation that spin decoherence times can be significantly longer than in GaAs. Spin decoherence times in GaAs are in part limited by the random spectral diffusion of the non-zero nuclear spins of the Ga and As that couple to the electron spin through the hyperfine interaction. This effect can be greatly suppressed by using a semiconductor matrix with a near zero nuclear spin background. Near zero nuclear spin backgrounds can be engineered using Si by growing {sup 28}Si enriched epitaxy. In this talk, we will present fabrication details and electrical transport results of an accumulation mode double top gated Si metal insulator semiconductor (MIS) nanostructure, Fig 1 (a) & (b). We will describe how this single electron device structure represent a path towards forming a Si based spin qubit similar in design as that demonstrated in GaAs. Potential advantages of this novel qubit structure relative to previous approaches include the combination of: no doping (i.e., not modulation doped); variable two-dimensional electron gas (2DEG) density; CMOS compatible processes; and relatively small vertical length scales to achieve smaller dots. A primary concern in this structure is defects at the insulator-silicon interface. The Sandia National Laboratories 0.35 {micro}m fab line was used for critical processing steps including formation of the gate oxide to examine the utility of a standard CMOS quality oxide silicon interface for the purpose of fabricating Si qubits. Large area metal oxide silicon (MOS) structures showed a peak mobility of 15,000 cm{sup 2}/V-s at electron densities of {approx}1 x 10{sup 12} cm{sup -2} for an oxide thickness of 10 nm. Defect density measured using standard C-V techniques was found to be greater with decreasing oxide thickness suggesting a device design trade-off between oxide thickness and quantum dot size. The quantum dot structure is completed using electron beam lithography and poly-silicon etch to form the depletion gates, Fig 1 (a). The accumulation gate is added by introducing a second insulating Al{sub 2}O{sub 3} layer, deposited by atomic layer deposition, followed by an Al top gate deposition, Fig. 1 (b). Initial single electron transistor devices using SiO{sub 2} show significant disorder in structures with relatively large critical dimensions of the order of 200-300 nm, Fig 2. This is not uncommon for large silicon structures and has been cited in the literature [2]. Although smaller structures will likely minimize the effect of disorder and well controlled small Si SETs have been demonstrated [3], the design constraints presented by disorder combined with long term concerns about effects of defects on spin decoherence time (e.g., paramagnetic centers) motivates pursuit of a 2nd generation structure that uses a compound semiconductor approach, an epitaxial SiGe barrier as shown in Fig. 2 (c). SiGe may be used as an electron barrier when combined with tensilely strained Si. The introduction of strained-Si into the double top gated device structure, however, represents additional fabrication challenges. Thermal budget is potentially constrained due to concerns related to strain relaxation. Fabrication details related to the introduction of strained silicon on insulator and SiGe barrier formation into the Sandia National Laboratories 0.35 {micro}m fab line will also be presented.
The authors have developed a treatment process to improve the etch resistance of an electron beam lithography resist (ZEP 520A) to allow direct pattern transfer from the resist into a hard mask using plasma etching without a metal lift-off process. When heated to 90 C and exposed for 17 min to a dose of approximately 8 mW/cm{sup 2} at 248 nm, changes occur in the resist that are observable using infrared spectroscopy. These changes increase the etch resistance of ZEP 520A to a CF{sub 4}/O{sub 2} plasma. This article will document the observed changes in the improved etch resistance of the ZEP 520A electron beam resist.
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.
We describe the development of a novel silicon quantum bit (qubit) device architecture that involves using materials that are compatible with a Sandia National Laboratories (SNL) 0.35 mum complementary metal oxide semiconductor (CMOS) process intended to operate at 100 mK. We describe how the qubit structure can be integrated with CMOS electronics, which is believed to have advantages for critical functions like fast single electron electrometry for readout compared to current approaches using radio frequency techniques. Critical materials properties are reviewed and preliminary characterization of the SNL CMOS devices at 4.2 K is presented.