Kimberley C. Hall

Non-magnetic semiconductor spin transistor

We propose a spin transistor using only nonmagnetic materials that exploits the characteristics of bulk inversion asymmetry~BIA ! in ~110! symmetric quantum wells. We show that extremely large spin splittings due to BIA are possible in ~110! InAs/GaSb/AlSb heterostructures, which together with the enhanced spin decay times in ~110! quantum wells demonstrates the potential for exploitation of BIA effects in semiconductor spintronics devices. Spin injection and detection is achieved using spin-dependent resonant interband tunneling and spin transistor action is realized through control of the electron spin lifetime in an InAs lateral transport channel using an applied electric field ~Rashba effect !. This device may also be used as a spin valve, or a magnetic field sensor. ©2003 American Institute of Physics. @DOI: 10.1063/1.1609656 #

Performance of a spin-based insulated gate field effect transistor

Fundamental physical properties limiting the performance of spin field effect transistors are compared to those of ordinary (charge-based) field effect transistors. Instead of raising and lowering a barrier to current flow these spin transistors use static spin-selective barriers and gate control of spin relaxation. The different origins of transistor action lead to distinct size dependences of the power dissipation in these transistors and permit sufficiently small spin-based transistors to surpass the performance of charge-based transistors at room temperature or above. This includes lower threshold voltages, smaller gate capacitances, reduced gate switching energies, and smaller source-drain leakage currents.

Ultrafast electron capture into p-modulation-doped quantum dots

Electron and hole relaxation kinetics are studied in modulation-doped InAs quantum dots using femtosecond time-resolved photoluminescence experiments. We demonstrate that, as a result of doping, carrier relaxation from the barrier layers to the quantum dot ground states is strongly enhanced due to rapid electron–hole scattering involving the built-in carrier population. Results for p-doped quantum dots reveal a threefold decrease in the room-temperature electron relaxation time relative to corresponding undoped quantum dots. Our findings are promising for the development of high-speed, GaAs-based quantum dot lasers with modulation speeds in excess of 30GHz.Electron and hole relaxation kinetics are studied in modulation-doped InAs quantum dots using femtosecond time-resolved photoluminescence experiments. We demonstrate that, as a result of doping, carrier relaxation from the barrier layers to the quantum dot ground states is strongly enhanced due to rapid electron–hole scattering involving the built-in carrier population. Results for p-doped quantum dots reveal a threefold decrease in the room-temperature electron relaxation time relative to corresponding undoped quantum dots. Our findings are promising for the development of high-speed, GaAs-based quantum dot lasers with modulation speeds in excess of 30GHz.

Spin relaxation in [110] and [001] InAs/GaSb superlattices

A 25 times enhancement of the electron spin lifetime is observed in a [110] InAs/GaSb superlattice relative to the corresponding [001] superlattice, an effect that is primarily attributed to suppression of native interface asymmetry.

Room-temperature electric-field controlled spin dynamics in (110)InAs quantum wells

We report the demonstration of room temperature gate control over the electron spin dynamics using the Rashba effect in a (110) InAs∕AlSb two-dimensional electron gas. Our calculations predict that the strong spin–orbit interaction in this system produces pseudomagnetic fields exceeding 1 T when only 140 mV is applied across a single quantum well. Using this large pseudomagnetic field, we demonstrate low-power spin manipulation on a picosecond time scale. Our findings are promising for the prospect of nonmagnetic low-power, high-speed spintronics.

Electron and hole spin dynamics in semiconductor quantum dots

We report direct measurement of the spin dynamics of electrons and holes in self-assembled InAs quantum dots (QDs) through polarization-sensitive time-resolved photoluminescence experiments on modulation-doped quantum dot heterostructures. Our measured hole spin decay time is considerably longer than in bulk and quantum well semiconductor systems, indicating that the removal of near degenerate hole states with different spin quantization axes through three-dimensional confinement slows hole spin relaxation in semiconductors. The electron and hole spin decay times we observe (electrons: 120ps; holes: 29ps) are consistent with spin relaxation via phonon-mediated virtual scattering between the lowest two confined levels in the QDs, which have a mixed spin character due to the spin–orbit interaction.

Efficient electron spin detection with positively charged quantum dots

Using polarization sensitive time-resolved photoluminescence upconversion experiments, we demonstrate that positively charged quantum dots act as a highly efficient detector for spin polarized electrons in semiconductor heterostructures.

Subpicosecond spin relaxation in GaAsSb multiple quantum wells

Spin relaxation times in GaAsxSb1−x quantum wells are measured at 295 K using time-resolved circular dichroism induced by 1.5 μm, 100 fs pulses. Values of 1.03 and 0.84 ps are obtained for samples with x=0 and 0.188, respectively. These times are >5 times shorter than those in InGaAs and InGaAsP wells with similar band gaps. The shorter relaxation times are attributed to the larger spin-orbit conduction-band splitting in the Ga(As)Sb system, consistent with the D’yakonov–Perel theory of spin relaxation [M. I. D’yakonov and V. I. Perel, Sov. Phys. JETP 38, 177 (1974)]. Our results indicate the feasibility of engineering an all-optical, polarization switch at 1.5 μm with response time <250 fs.

Ultrafast optical control of coercivity in GaMnAs

Femtosecond optical control of the magnetization and coercive field is demonstrated in GaMnAs using time-resolved magneto-optical Kerr effect techniques. These experiments reveal a near-complete, subpicosecond collapse of the hysteresis loop, consistent with femtosecond demagnetization. On longer time scales (∼300ps) an increase in coercivity is observed, attributed to hole-mediated enhancement of the domain wall energy.

Simultaneous observation of free and defect-bound excitons in CH 3 NH 3 PbI 3 using four-wave mixing spectroscopy

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2–55 meV) reported in recent years.