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We present a study of room-temperature, ultrafast photoconductivity associated with a strong, sub-bandgap, resonant absorption around λ=1550nm in three MBE-grown GaAs epitaxial layers heavily doped with Er at concentrations of ≈2.9×1018 (control sample), 4.4×1020, and 8.8×1020 cm-3, respectively. Transmission-electron microscopy reveals lack of nanoparticles in the control sample, but abundant in the other two samples in the 1.0-to-3.0-nm-diameter range, which is consistent with the previously known results. We measure very high photoelectron (Hall) mobility (2.57×103 cm2/V-s) and terahertz power (46 μW average) in the 4.4×1020 sample, but then, an abrupt decay in these properties as well as the dark resistivity is seen as the Er doping is increased just 2 times. The Er doping has little effect on the picosecond-scale, 1550-nm photocarrier lifetime. [ABSTRACT FROM AUTHOR]

In this article we examine tunneling through MnAs particles at a GaAs p+n+ junction. We grew the device structures by molecular beam epitaxy on semi-insulating GaAs (001) substrates, with the n+(5×1018 cm-3 Si) and p+(2×1019 cm-3 Be) layers grown at 580 °C. At the p+n+ junction, we grew a 30 nm layer of random alloy Ga1-xMnxAs at 250 °C. In situ annealing the Ga1-xMnxAs transforms to thermodynamically stable MnAs particles in a GaAs matrix. Magnetization measurements show that the MnAs particles are superparamagnetic with a distribution of blocking temperatures that depends on the annealing protocol. The MnAs particles at the interface are imaged using atomic force microscopy of selectively etched, MnAs-topped nanocolumns. Current-voltage (IV) scans show that the presence of particles increases the forward bias current density. Low-temperature current-voltage (IV) scans confirm an increase in the forward bias current density due to tunneling through MnAs particles. [ABSTRACT FROM AUTHOR]

This paper describes a compact, self-complementary square-spiral antenna on a GaAs substrate with a broadside high-directivity (∼29 dB) frequency-independent pattern when coupled through a silicon hyperhemisphere. The driving-point resistance undulates between ∼100 and 300Ω from 200 GHz to 1 THz—much higher than the 72Ω value from Booker's modified formula, but quite beneficial for coupling to high-impedance broadband devices. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 524–529, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21398 [ABSTRACT FROM AUTHOR]

An Er:GaAs-based 1550-nm CW photomixer is demonstrated. The related mechanism is extrinsic photoconductivity with optical absorption between the localized deep levels created by the Er and the extended states above the conduction band edge of GaAs. With the power boost made possible by a fiber-coupled erbium-doped-fiber amplifier, the Er:GaAs photomixers, operating at 1550 nm, radiate THz power levels easily measured by a Golay cell, and display a power spectrum having a --3 dB roll-off frequency of 307 GHz. This corresponds to a photocarrier lifetime of 520 fs, in good agreement with a previous measurement of the bandwidth of the same material in a photoconduc-tive switch. [ABSTRACT FROM AUTHOR]

We examine the terahertz (THz) performance of an ErAs:GaAs photoconductive switch under varying bias conditions and optical drive power. Despite THz power up to 287 μW, saturation effects were not seen. In addition, the THz power spectra were measured with a Fourier transform infrared spectrometer, and the roll-off was found to be invariant to bias voltage and consistent with a THz pulsewidth of 1.59 ps and a peak power of 3.1 W. These results are confirmed by a large-signal, high-frequency circuit model that suggests that further increase in THz power and efficiency are possible through an increase in the mode-locked laser power and reduction in its pulse width. The model is useful in designing both the laser and photoconductive switches to maximize available power and efficiency. [ABSTRACT FROM AUTHOR]

We report room-temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1-nm-thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound-state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak-to-valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak-to-valley ratio, should allow oscillations up to about 1 THz. [ABSTRACT FROM AUTHOR]

Resonant slot and dipole antennas coupled to low-temperature-grown GaAs photomixers have been fabricated and tested at terahertz operating frequencies. Enhanced output power is seen from the resonant structures compared to mixers coupled to broadband self-complementary spiral antennas. Driving point impedances as high as 300 Ω are attained at the resonant frequencies. These devices will be useful as fixed frequency local oscillators for submillimeter heterodyne receivers. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]