Peter W. Deelman

Coherent singlet-triplet oscillations in a silicon-based double quantum dot

Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.

GaN double heterojunction field effect transistor for microwave and millimeterwave power applications

We report development of a novel AlGaN/GaN/AlGaN double heterojunction field effect tansistor (DHFET) with improved device performance over the conventional single heterojunction GaN FET (SHFET). The GaN DHFETs with low Al content Al/sub 0.04/Ga/sub 0.96/N buffer layer exhibit three orders of magnitude lower subthreshold drain leakage current and almost three orders of magnitude higher buffer isolation than corresponding SHFET devices (600 M/spl Omega//sq. vs. 1 M/spl Omega//sq.). In GaN DHFETs with 0.15 /spl mu/m conventional T-gates we observed 30% improvement in saturated power density and 10% improvement in PAE at 10 GHz over a corresponding SHFET device.

Sub-Micron Area Heterojunction Backward Diode Millimeter-Wave Detectors With 0.18

InAs/AlSb/AlGaSb heterojunction backward diodes are promising detectors for millimeter-wave imaging applications due to their high sensitivity, low noise, and high cutoff frequency. By using a device heterostructure with a thin (11 Å) barrier layer, δ-doped cathode, and optimized Al_xGa_1-xSb anode composition (x=12%), in conjunction with submicron (0.4×0.4 μm²) active area, fabricated detectors have demonstrated DC curvatures of -47 V^-1 and record unmatched sensitivities of 4600 V/W at 94 GHz. Impedance-matched sensitivities of 49,700 V/W at 94 GHz are projected from on-wafer S-parameter and sensitivity measurements. These detectors have measured junction resistances of 814 Ω·μm² and capacitances of 15 fF/μm². A record low NEP_min of 0.18 pW/Hz^1/2 has been projected under conjugate matching conditions. This study demonstrates the potential of Sb-heterostructure backward diodes as ultra-low-noise millimeter-wave direct detectors.

{\rm pW/Hz}^{1/2}

InAs/AlSb/GaSb backward diodes are used for millimeter-wave square-law power detection. A new heterostructure design with low capacitance, low resistance, and high curvature coefficient as compared to previous designs is presented. Voltage sensitivity, which is directly proportional to curvature coefficient, is improved by 31% as compared to prior reports of devices with similar barrier thicknesses. The junction capacitance is also reduced by 24% to 13 . The improved sensitivity and decreased junction capacitance originate from the incorporation of a p-type-doping plane with sheet concentration of in the n-InAs cathode layer. The combination of low resistance (and thus Johnson noise) and high sensitivity results in an estimated noise equivalent power of 0.24 at 94 GHz for a conjugately matched source, whereas the reduced capacitance facilitates wideband matching and increases the detector cutoff frequency. These modified Sb-based detectors have promise for improving the performance of passive millimeter-wave and submillimeter-wave imaging systems.

Noise Equivalent Power

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time. Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking.

Sb-Heterostructure Millimeter-Wave Detectors With Reduced Capacitance and Noise Equivalent Power

Most recent GaN-based HEMT technology has been focused toward microwave power applications. In this work, we report DC and RF characteristics of the first E-mode AlGaN/GaN HEMTs fabricated down to 0.2 /spl mu/m gatelength, and having an f/sub t/ reaching 25 GHz. Further improvement of E-mode GaN HEMTs could open potential applications for mixed-signal ICs with a high dynamic range.

Isotopically enhanced triple-quantum-dot qubit

We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si.

Submicron enhancement-mode AlGaN/GaN HEMTs

Nearly lattice-matched InAs/AlSb/GaSb-based heterostructure backward diodes for zero-bias millimeter wave detection were fabricated and measured. A record-high curvature, /spl gamma/=39.1 V/sup -1/, at zero bias was measured. On-wafer sensitivity measurements from 1 to 110 GHz gave a record-high average sensitivity of 3687 V/W for zero-bias operation. Further enhancement of detector sensitivity was observed with applied dc bias, with a sensitivity of 7996 V/W obtained for a 0.9 /spl mu/A bias. Extrapolating the conjugately-matched measured sensitivity suggests that 1000 V/W should be achievable at a record-high 541 GHz. The temperature dependence of detector sensitivity was evaluated from measured dc current-voltage characteristics and gave expected sensitivities ranging from 3910 V/W at 293 K to 7740 V/W at 4.2 K.

Pauli spin blockade in undoped Si/SiGe two-electron double quantum dots

We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N =2 ground state at low magnetic field and therefore the two-fold valley degeneracy is lifted. The valley splittings in these two devices were 120 and 270 {\mu}eV, suggesting the presence of atomically sharp interfaces in our heterostructures.

Bias and temperature dependence of Sb-based heterostructure millimeter-wave detectors with improved sensitivity

We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a dominant and unique function-control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the [Formula: see text] charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage.