Using the method of time-domain spectroscopy, we measure the far-infrared absorption and dispersion from 0.2 to 2 THz of the crystalline dielectrics sapphire and quartz, fused silica, and the semiconductors silicon, gallium arsenide, and germanium. For sapphire and quartz, the measured absorptions are consistent with the earlier work below 0.5 THz. Above 1 THz we measure significantly more absorption for sapphire, while for quartz our values are in reasonable agreement with those of the previous work. Our results on high-purity fused silica are consistent with those on the most transparent fused silica measured to date. For the semiconductors, we show that many of the previous measurements on silicon were dominated by the effects of carriers due to impurities. For high-resistivity, 10-kΩ cm silicon, we measure a remarkable transparency together with an exceptionally nondispersive index of refraction. For GaAs our measurements extend the precision of the previous work, and we resolve two weak absorption features at 0.4 and 0.7 THz. Our measurements on germanium demonstrate the dominant role of intrinsic carriers; the measured absorption and dispersion are well fitted by the simple Drude theory.

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Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors

Terahertz fields and applications

Ultrashort carrier lifetimes were experimentally observed in a variety of As-based III-V crystalline semiconductor layers grown by molecular beam epitaxy over a range of very low substrate temperatures. Time-resolved optical absorption, reflection, and photoconductive switching measurements with femtosecond resolution have been used to directly measure the ultrafast carrier dynamics in these materials. GaAs and In/sub 0.52/Al/sub 0.48/As grown at temperatures of 200 and 150 degrees C, respectively, exhibited subpicosecond carrier lifetimes, while lattice-mismatched In/sub x/Ga/sub 1-x/As on GaAs displayed a 7-ps carrier lifetime in material grown at 200 degrees C. A distinct decrease in carrier lifetime with decreasing growth temperature is documented for all three material systems. For epilayers annealed at high-temperature, resistivities are high, and thus these materials were found to be ideally suited for a number of ultrafast optoelectronic applications. A number of applications are reviewed and discussed. >

Ultrafast carrier dynamics in III-V semiconductors grown by molecular-beam epitaxy at very low substrate temperatures

We have significantly improved the emission and detection of electromagnetic beams of single‐cycle 0.5 THz pulses, through the use of new dipolar antenna structures. The frequency response was extended to well beyond 1 THz, and the beam power was increased by more than 15 times. The antennas were located at the foci of sapphire lenses and were photoconductively driven by ultrafast laser pulses. An additional collimation by a paraboloidal mirror produced a beam with a 25 mrad divergence, and subsequent focusing by a second identical mirror improved the coupling between the transmitting and receiving antenna by orders of magnitude.

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High‐brightness terahertz beams characterized with an ultrafast detector

Mode-locked laser systems generate extremely short optical pulses and are widely used for measurements of ultrafast phenomena. In these experiments, the effect of laser pulse timing fluctuations (jitter) is suppressed by exciting the system under test by the pulsed laser and probing with a delayed portion of the same optical pulse. If the experiment cannot be laser-excited or must be excited and probed by different lasers, timing jitter degrades the time resolution and introduces noise.

Subpicosecond laser timing stabilization

By reanalyzing an earlier experiment to generate subpicosecond pulses using photoconductive switches (M.B. Ketchen, et al., Appl. Phys. Lett., vol.48, pp.751-753, 1986), it is shown that to first order, the sliding-contact generation site has no capacitance. This conclusion is further supported by a double sliding-contact experiment where, to first order, neither the generation nor the detection site has any capacitance. This result removes the parasitic capacitance of the electrical circuit as one of the major difficulties to short electrical pulse generation using photoconductive switches. >

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Capacitance free generation and detection of subpicosecond electrical pulses on coplanar transmission lines

The authors have replaced the electron gun and beam blanking system of a conventional voltage contrast scanning electron microscope by a pulse-laser/photocathode combination, resulting in a source producing electron pulses of order 1 ps in duration at a 100 MHz repetition rate and with a peak brightness of 3 10/sup 8/ A/cm/sup 2/ sr at 1.8 keV. This novel instrument has demonstrated stroboscopic noncontact waveform measurements on metal interconnect lines in different environments with a temporal resolution between than 5 ps, a voltage resolution of 3 mV/(Hz)/sup 1/2/, and a spatial resolution of 0.1 mu m. These measurements are achieved with extraction fields above the sample of about 1 kV/mm. >

Noncontact high-speed waveform measurements with the picosecond photoelectron scanning electron microscope

Picosecond electrical pulses with well resolved frequency components into the terahertz region are ideally suited for the characterization of VLSI electronics, both for the devices themselves and for the interconnect wiring. In this paper, we describe the technology we employ to photoconductively generate and measure these ultrafast electrical signals./1/ A particular emphasis was given to the compatibility with existing VLSI device technology. Although other measurement techniques /2,3/ have shown very good sensitivity and time resolution, the photoconductive sampling technique is particularly attractive for silicon device characterization with a comparable sensitivity and time resolution /4/ and a better amplitude calibration and reproducibility as the sampling geometry is defined by the lithography itself. Impedance matching is facilitated while keeping identical geometries between the device site and the generating and sampling sites. We also present results on the optoelectronic characterization of a 1μm gate GaAs MESFET. We will not address in this paper the problem of circuit testing which has some stringent additional requirements, in particular that of submicron spatial resolution and of wide field addressing capability. These imperatives may ultimately limit the use of purely optical techniques for the functional testing of high speed circuits at their internal nodes.

P.G. May

Papers

Capacitance free generation and detection of subpicosecond electrical pulses on coplanar transmission lines

Noncontact high-speed waveform measurements with the picosecond photoelectron scanning electron microscope

Picosecond Electrical Pulse for VLSI Electronics Characterization