Tetsuya D. Mishima

InGaAs/GaAs three-dimensionally-ordered array of quantum dots

We report on the first fabrication of (In,Ga)As/GaAs quantum dots with both vertical and lateral ordering forming a three-dimensional array. An investigation of the photoluminescence spectra from the ordered array of quantum dots, as a function of both temperature and optical excitation intensity, reveals both a lateral and vertical transfer of excitation.

Interband cascade photovoltaic devices

A photovoltaic (PV) device based on an interband cascade (IC) structure is proposed for efficiently converting solar and thermal energy to electricity. These IC PV devices employ absorption and transport regions with characteristics that are favorable for achieving high open-circuit voltage and thus possibly improving conversion efficiency over conventional PV devices. Preliminary experiments carried out using IC infrared photodetectors (seven stages) and lasers (11 stages) showed open-circuit voltages that exceed the single-band gap voltage from these devices under infrared illumination. The observed open-circuit voltage demonstrates multiple stages operating in series and provides an initial proof of concept for IC PV devices.

Anisotropic structural and electronic properties of InSb/ AlxIn1-xSb quantum wells grown on GaAs (001) substrates

InSb quantum wells (QWs) with remotely doped Al/sub x/In/sub 1-x/Sb barriers are candidates for several novel device structures that rely on a long electron mean free path. Mesoscopic magnetoresistors that take advantage of the high electron mobility in InSb QWs at room temperature are currently being developed for read-head applications. The promise of InSb QWs for spin-transistor applications has been shown recently by experiments that demonstrate a large zero-field spin splitting and ballistic transport at temperatures as high as 185 K. Since a semi-insulating substrate is required for electronic applications, the InSb/Al/sub x/In/sub 1-x/Sb structures are grown on GaAs [001] substrates. The /spl sim/14% lattice mismatch between the epilayers and the substrate results in a high defect density that partially limits the electron mean free path. We will present a detailed characterization of these defects and elucidate their role in limiting electron mobility.

Single-step growth and low resistance interconnecting of gold nanowires

We present a single-step, electrochemical approach to the growth and low contact resistance interconnecting of gold nanowires with targeted points on lithographic electrodes. Electron diffraction studies indicate that these nanowires are composed of face-centred cubic crystalline gold, and that the crystal structure is invariant along the wire lengths. Four-point resistance determinations of these electrode?nanowire?electrode assemblies consistently yield resistances of <50??, and the contributions from the electrode?wire contacts are of the order of 10??. Atomic force microscopy was used to depict the structurally integrated nature of the electrode?wire contacts. This feature underlies the low electrode?wire contact resistances.

Do Pb1 centers have levels in the Si band gap? Spin-dependent recombination study of the Pb1 “hyperfine spectrum”

The electronic properties of the (001) Si/SiO2 Pb1 defect are the subject of considerable controversy. We present spin-dependent recombination results which indicate most strongly that the Pb1 centers have levels in the Si band gap. Our results suggest that the Pb1 correlation energy is smaller than that of the more widely studied (001) Si/SiO2 defect called Pb0.

Anisotropic photoconductivity of InGaAs quantum dot chains measured by terahertz pulse spectroscopy

We report results of time-resolved terahertz (THz) pulse spectroscopy experiments on laterally ordered chains of self-assembled InGaAs quantum dots photoexcited with 400nm, 100fs laser pulses. A large anisotropy in the transient photoconductive response is observed depending on the polarization of the THz probe pulse with respect to the orientation of the dot chains. Fast (3.5–5ps) and efficient carrier capture into the dots and one-dimensional wetting layers underneath the dot chains is observed below 90K. At higher temperatures, thermionic emission into the two-dimensional wetting layers and barriers becomes significant and the anisotropy in the photoconductive signal is reduced.

Highly sensitive micro-Hall devices based on Al0.12In0.88Sb∕InSb heterostructures

Micro-Hall devices based on modulation-doped Al0.12In0.88Sb∕InSb heterostructures are fabricated and studied in terms of sensitivity and noise. Extremely high supply-current-related magnetic sensitivities of 1800VA−1T−1 at 77K and 1220VA−1T−1 at 300K are reported and observed to be independent of the bias current. The detection limit of the devices studied at low and room temperature are at nanotesla values throughout the broad frequency range from 20Hzto20kHz. The low detection limit of 28nT at 300K and 18nT at 77K were found at high frequencies where the Johnson noise is dominant. A measured detection limit per unit device width of 630pTmmHz−1∕2 is reported indicating the potential for picotesla detectivity.

Dislocation filtering by AlxIn1−xSb∕AlyIn1−ySb interfaces for InSb-based devices grown on GaAs (001) substrates

Dislocation filtering by interfaces between AlxIn1−xSb and AlyIn1−ySb layers grown on a GaAs (001) substrate has been investigated. Transmission electron microscopy analysis shows that as many as 59% of threading dislocations (TDs) can be eliminated by such an interface. An interlayer sample that contains six Al0.12In0.88Sb∕Al0.24In0.76Sb interfaces has 6.0×108TDs∕cm2 at 1.6μm thickness. Compared with an Al0.12In0.88Sb epilayer without an interlayer, this TD density is a factor of ∼4 lower for the same thickness, and about the same as for a layer that is more than twice as thick. Our results suggest that AlxIn1−xSb∕AlyIn1−ySb interfaces can be used to improve the performance of any InSb-based device in which AlxIn1−xSb is used as a buffer, insulating, or barrier layer material.

Directed growth of single-crystal indium wires

Tailored electric fields were used to direct the dendritic growth of crystalline indium wires between lithographic electrodes immersed in solutions of indium acetate. Determination of the conditions that suppress sidebranching on these structures has enabled the fabrication of arbitrarily long needle-shaped wires with diameters as small as 370nm. Electron diffraction studies indicate that these wires are crystalline indium, that the unbranched wire segments are single-crystal domains, and that the predominant growth direction is near ⟨110⟩. This work constitutes a critical step towards the use of simply prepared aqueous mixtures as a convenient means of controlling the composition of submicron, crystalline wires.

Plasmon-Waveguide Interband Cascade Lasers Near 7.5

Broad-area plasmon-waveguide interband cascade lasers with emission wavelengths near 7.5 mu m were demonstrated at temperatures up to 121 K in continuous-wave mode. Their threshold current densities and voltages varied from 72 A/cm2 and 2.1 V at 84 K to 400 A/cm2 and 2.7 V at 121 K, showing very efficient use of bias voltage (e.g., voltage efficiency of about 90% at 84 K) at this long wavelength. These plasmon-waveguide lasers also operated in pulsed mode at temperatures up to 165 K with emission wavelengths near 7.6 mum and threshold current density of 1100 A/cm2.