Dr. Roman Adam Profile

Dr. Roman Adam

at Forschungszentrum Jülich GmbH

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Publications (3)

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Proceedings Article | 16 June 2004 Paper

Fabrication and performance of hybrid photoconductive devices based on freestanding LT-GaAs

Roman Adam, Martin Mikulics, Shuai Wu, Xuemei Zheng, Michel Marso, I. Camara, F. Siebe, R. Güsten, Arno Foerster, Peter Kordos, Roman Sobolewski

Proceedings Volume 5352, (2004) https://doi.org/10.1117/12.526429

KEYWORDS: Gallium arsenide, Silicon, Photodetectors, Positron emission tomography, Electro optics, Switches, Superconductors, Optoelectronics, Sapphire, Thin films

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We report on fabrication and high-frequency performance of our photodetectors and photomixers based on freestanding low-temperature-grown GaAs (LT-GaAs). In our experiments, the LT-GaAs/AlAs bilayers were grown on 2-inch diameter, semi-insulating GaAs wafers by a molecular beam epitaxy. Next, the bilayer was patterned to form 10×10 μm² to 150×150 μm² structures using photolithography and ion beam etching. The AlAs layer was then selectively etched in diluted HF solution, and the LT-GaAs device was lifted from its substrate and transferred on top of a variety of substrates including Si, MgO/YBaCuO, Al₂O₃, and a plastic foil. Following the transfer, metallic coplanar transmission lines were fabricated on top of the LT-GaAs structure, forming a metal-semiconductor-metal photodetectors or photomixer structures. Our freestanding devices exhibited above 200 V breakdown voltages and dark currents at 100 V below 3×10^-7 A. Device photoresponse was measured using an electro-optic sampling technique with 100-fs-wide laser pulses at wavelengths of 810 nm and 405 nm as the excitation source. For 810-nm excitation, we measured 0.55 ps-wide electrical transients with voltage amplitudes of up to 1.3 V. The signal amplitude was a linear function of the applied voltage bias, as well as a linear function of the laser excitation power, below well-defined saturation thresholds. Output power from the freestanding photomixers was measured with two-beam laser illumination experimental setup. Reported fabrication technique is suitable for the LT-GaAs integration with a range of semiconducting, superconducting, and organic materials for high-frequency hybrid optoelectronic applications.

Proceedings Article | 8 August 2003 Paper

Ultrafast photoresponse and fabrication of freestanding LT-GaAs photoconductive devices

Xuemei Zheng, S. Wu, R. Adam, M. Mikulics, A. Foerster, J. Schelten, M. Siegel, P. Kordos, Roman Sobolewski

Proceedings Volume 5123, (2003) https://doi.org/10.1117/12.517006

KEYWORDS: Switches, Gallium arsenide, Diodes, Ultrafast phenomena, Silicon, Crystals, Photons, Modulation, Semiconducting wafers, Terahertz radiation

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We report on fabrication and ultrafast photoresponse of novel, freestanding low-temperature-grown GaAs (LT-GaAs) photoconductive (PC) devices. 1-μm-thick, LT-GaAs single-crystal films were grown by molecular beam epitaxy at the temperature range of 200°C to 250°C. Next, the films were patterned to the desired device sizes, lifted-off from their host substrates, and placed on predetermiend places on either SiO₂/Si or MgO wafers. Our freestnding LT-GaAs devices consisted of either approximately 20-μm by 20-μm PC switches, or 150-μm by 150-μm metal-semiconductor-metal (MSM) interdigitated structures with Ti/Au fingers patterned directly on top of the LT-GaAs film. For testing purposes, our devices were integrated with Ti/Au coplanar striplines, fabricated directly on SiO₂/Si and MgO substrates. The test structures were illuminated with 100-fs-wide optical pulses and their time-resolved photoresponse was measured with an electro-optic sampling system, characterized by 200-fs time resolution and sub-millivolt sensitivity. Using 810-nm optical excitation, we recorded as narrow as 360-fs-wide electrical signals (1.25 THz, 3-dB bandwidth) for PC switches, resulting in 155 fs carrier lifetime in our freestanding LT-GaAs. For both types of devices, the photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the laser excitation power, below well-defined saturation thresholds. Our freestanding photo-switches are robust and very reproducible. They are best suited for applications in hybrid optoelectronic and ultrafast electronic systems, since they can be placed at virtually any point on a test circuit.

Proceedings Article | 6 September 2000 Paper

Subpicosecond dynamics of the switching process in Y-Ba-Cu-O Josephson junctions

Roman Adam, Roman Sobolewski, Marian Darula

Proceedings Volume 4058, (2000) https://doi.org/10.1117/12.397842

KEYWORDS: Switching, Superconductors, Picosecond phenomena, Digital electronics, Logic, Solids, Signal detection, Sapphire lasers, Electro optics, Device simulation

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We present comprehensive experimental and numerical studies of the subpicosecond switching dynamics of YBa₂Cu₃O_7-x (YBCO) grain-boundary Josephson junctions, excited by single-picosecond electrical pulses. The test structures were patterned in 100-nm- thick YBCO films grown by pulsed laser deposition on (100) MgO bicrystal substrates. Each sample consisted of a coplanar strip (CPS) transmission line, a microbridge acting as the electrical pulse generator, and a single Josephson junction positioned between the CPS lines about 100 micrometer away from the bridge. The junctions were characterized by the nonhysteretic current-voltage characteristics with the characteristic voltage approximately equal to 2.0 mV at 20 K (temperature of our experiments). A train of 100- fs-wide optical pulses from a Ti:sapphire laser photo-excited the microbridge and generated 2-ps-wide electrical pulses, which were then applied to switch the junction. In addition to the input pulse, the junction was dc-biased at +0.7 I_c, -0.7 I_c, +1.5 I_c, -1.5 I_c, and zero-I_c, where I_c is junction critical current. Time-resolved dynamics of the junction response was studied with the help of our cryogenic electro-optic sampling system, which can be regarded as a sampling oscilloscope featuring < 200-fs time resolution and < 150-(mu) V voltage sensitivity. We obtained 0.7-ps-wide single-flux-quantum (SFQ) pulses generated as a result of the junction switching process. The measurements were compared to numerical computations based on the equivalent circuit containing a resistively shunted Josephson junction, and we have found a satisfactory agreement between our simulations and experimental data. We believe our findings provide experimental confirmation of the potential of YBCO Josephson gattes as building blocks of ultrafast (sub-THz) digital electronics.

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