Michel Marso

Sensitivity Enhancement of Metal– Semiconductor –Metal Photodetectors on Low-Temperature-Grown GaAs Using Alloyed Contacts

We have fabricated and characterized metal- semiconductor-metal (MSM) photodetectors based on low- temperature-grown GaAs with alloyed (i.e., ohmic-type) contacts. The annealed contacts optimize the electric field distribution inside the photodetector structure which results in an up-to-200% responsivity increase of the devices, compared to conventional MSM detectors with standard nonalloyed (Schottky-type) metallization fabricated on identical material. The improved MSM device with alloyed contacts shows more than three times larger output amplitude at illumination with a 100-fs Ti: sapphire laser, compared to the nonalloyed devices, without degradation of detector speed.

Output Power Improvement in MSM Photomixers by Modified Finger Contacts Configuration

A novel type of metal-semiconductor-metal (MSM) photomixers (PMs) with an improvement of output power and frequency bandwidth was fabricated and tested. The device active area with interdigitated MSM structure is divided into two smaller ones connected in series (ldquotwinrdquo configuration) and thus the capacitance is reduced by factor four while the responsivity is reduced only by factor two compared to the standard MSM design with the same area. This was verified by responsivity and transient photoresponse measurements on low-temperature-grown GaAs devices. The photomixing measurements yielded (2-3)-times higher output power of the twin PM than the power of the standard one in the whole frequency range from 200 GHz to 1.2 THz, which is in agreement with the calculations.

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

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.

A novel two-color photodetector based on an InAlAs-InGaAs HEMT Layer structure

The spectral responsivity of an InAlAs-InGaAs metal-semiconductor-metal diode above a two-dimensional electron gas (2DEG) is investigated as a function of the applied bias. At low voltages, only the InAlAs layer above the 2DEG contributes to the photocurrent, while the InGaAs channel layer is activated at higher bias. This results in a voltage-dependent spectral response of the photodetector. The ratio of the responsivities at 1300 and 850 nm changes from 0.03- at 1-V to 0.44- at 1.6-V bias. This property makes the device a candidate suitable to detect and to separate optical information originated both from the GaAs (850 nm) and in the InGaAs (1300, 1550 nm)-based optoelectronic technology.