Sotiris Alexandrou

Ultrafast nanoscale metal-semiconductor-metal photodetectors on bulk and low-temperature grown GaAs

Metal‐semiconductor‐metal photodetectors of finger spacing and width as small as 100 nm have been fabricated on bulk and low‐temperature grown GaAs, and tested using a femtosecond pulse laser and high‐speed electro‐optic sampling. The fastest photodetectors have a measured full width at half maximum impulse response and a 3‐dB bandwidth of 0.87 ps and 510 GHz, respectively, for low‐temperature grown GaAs limited by carrier recombination time; and of 1.5 ps and 295 GHz for bulk GaAs, limited by the RC time constant. To our knowledge, they are the fastest detectors of their kinds reported to date.

A 75 GHz silicon metal‐semiconductor‐metal Schottky photodiode

The ultrafast characteristics of crystalline‐silicon metal‐semiconductor‐metal (MSM) photodiodes with 300 nm finger width and spacing were measured with a subpicosecond electro‐optic sampling system. Electrical responses with full width at half maximum as short as 5.5 and 11 ps, at corresponding 3 dB bandwidths of 75 and 38 GHz, were generated by violet and red photons, respectively. The difference is attributed to the photon penetration depth which is much larger than the diode finger spacing at red, but smaller at violet. Light‐intensity dependence was also examined at different wavelengths, indicating a linear relation and a higher sensitivity in the violet. These results not only demonstrated the fastest silicon photodetector reported to date, but also pinpointed the dominant speed‐limiting factor of silicon MSM photodiodes. A configuration is suggested to improve the speed of these detectors at long wavelengths.

Generation of subpicosecond electrical pulses by nonuniform illumination of GaAs transmission-line gaps

We present a detailed study of subpicosecond pulse generation by nonuniform illumination of transmission-line gaps on semiinsulating GaAs. The dependence of such pulses on bias voltage, light intensity, and wavelength was examined in detail with the aid of a subpicosecond electrooptic sampling system. A complete spatial mapping of the excitation area indicates that the pulse generation is due to the depletion of the electrical field in the illuminated section of the gap. A comparison of uniform and nonuniform gap excitation schemes pinpointed the physical differences between the two processes of electrical-transient generation. Picosecond pulses were also generated by nonuniform illumination of a photoconductive gap placed in series with a coplanar waveguide for the first time and were found to contain balanced, odd modes only. >

Comparison of the picosecond characteristics of silicon and silicon‐on‐sapphire metal‐semiconductor‐metal photodiodes

The picosecond characteristics of silicon‐based metal‐semiconductor‐metal (MSM) diodes with submicrometer finger spacing and width were studied. Diodes made on both bulk silicon and silicon‐on‐sapphire (SOS) substrates were measured by a subpicosecond electro‐optic sampling system. The response of bulk‐silicon MSM diodes was strongly dependent on the wavelength of the excitation light because of the change in penetration depth. The response of SOS diodes, on the other hand, had a weak dependence on wavelength since the thickness of the silicon layer limits the depth of photogenerated carriers. The response of a 200 nm SOS diode has a full‐width at half‐maximum of 4.5 and 5.7 ps with blue‐ and red‐light excitations. The external quantum efficiency of SOS diodes was also determined at several selected wavelengths.

Attenuation characteristics of coplanar waveguides at subterahertz frequencies

We present experimental and simulation data of subterahertz attenuation of coplanar waveguides (CPWs) with wide and narrow ground planes. Experimental data are obtained by using a subpicosecond measurement technique based on electrooptic sampling. While time-domain data qualitatively describe the attenuation characteristics, they are converted to the frequency domain by a Fourier transform to give a quantitative interpretation. Simulation data are obtained by full-wave analysis and compared with the experimental results. It is shown that the simulation results consistently agree with experimental results. Furthermore, in the case of CPWs with wide ground planes, both of them are consistent with analytical theory. While CPWs with narrow ground planes have not been analytically studied, our results show that they suffer considerably lower attenuation, which we attribute to a reduced coupling between the CPW mode and substrate modes. The effects of ground-plane width and line dimensions on the attenuation characteristics are discussed in detail.

Ultrafast, all-silicon light modulator

An ultrafast, all-silicon light-intensity modulator is proposed. The carrier-refraction effect is used to modulate the refractive index of silicon. An electric-field-induced Bragg reflector on a silicon-on-insulator optical waveguide efficiently converts the small modulation of the index of refraction into light-intensity modulation. A modulator with 300-microm interaction length is expected to have a modulation depth of ~40% with 5-V bias. Being based on free-carrier depletion, this modulator is expected to have a bandwidth limited only by the RC time constant, which is calculated for a sample device to be ~40 GHz.

MONTE CARLO INVESTIGATION OF THE INTRINSIC MECHANISM OF SUBPICOSECOND PULSE GENERATION BY NONUNIFORM ILLUMINATION

The intrinsic mechanism of the generation of subpicosecond electrical pulses by nonuniform illumination of GaAs transmission‐line gaps is investigated using a self‐consistent Monte Carlo approach. It is attributed to the photocarrier‐induced field redistribution, which results in a displacement current pulse that is independent of the carrier lifetime. Partial‐gap illumination and high dark resistivity are the prerequisites for this pulse generation technique. The pulse dependence on light intensity, excitation wavelength, bias voltage, and the asymmetric response to bias polarity are discussed and clarified. It is predicted that this mechanism should also be observable in other semiconductors such as silicon.

110 GHz Si MSM photodetectors

Summary form only given. The authors report an Si MSMPD (metal-semiconductor-metal photodetector) with a 3.7-ps response time and 110-GHz bandwidth. They also present an experimental and theoretical study of the factors that are important to the speed of Si MSMPDs, such as the photon absorption length, carrier diffusion length, carrier mean free path, finger spacing, and finger width. They also discuss new possibilities for even faster ( approximately 400 GHz) Si-based high-speed MSMPDs. >

Picosecond silicon metal-semiconductor-metal photodiode

The ultrafast characteristics of crystalline-silicon metal-semiconductor-metal (MSM) photodiodes with finger widths and spacings down to 200 nm, subjected to femtosecond optical pulse excitations, was measured with a subpicosecond electro-optic sampling system. Electrical responses with fullwidth at half-maximum (FWHM) as short as 3.7 ps, at a corresponding 3 dB bandwidth of 110 GHz, were generated by violet-light excitation. These diodes are the fastest silicon photodetectors reported to date. Detailed bias and light-intensity dependence of the diode response has been measured. These results are used to obtain the velocity-field relation of electrons in silicon and to demonstrate the ideal transit-time-limited response of the diodes.

Nanoscale metal–semiconductor–metal photodetectors with subpicosecond response time fabricated using electron beam lithography

Metal–semiconductor–metal photodetectors (MSM PDs) with finger spacing and width as small as 25 nm have been fabricated using high‐resolution electron beam lithography. Measurements using an electro‐optic sampling system show that the fastest detector has a full width at half‐maximum response time of 0.87 ps and a 3 dB bandwidth of 510 GHz. Monte Carlo simulation of detector response time is studied and compared with experimental data. Finally, scaling rules for high‐speed MSM PDs are proposed.