Abstract

GaAs grown at low substrate temperatures (200°C) (LT-GaAs) has shown to be a photoconductor with a carrier recombination time of ~1 ps.¹ We fabricated metal-semiconductor-metal (MSM) photodiodes with 0.75- μm wide fingers and 0.75μm distance between them by electron beam lithography on this material, using Ti/Pt/Au as electrical contacts. Excitation and EO sampling of the photocurrent response of the diode were performed using 100-fs optical pulses (120-MHz repetition rate) from a dispersion compensated colliding-pulse mode-locked (CPM) dye laser. Figure 1 shows the response of a MSM-diode on LT-GaAs (solid line) with 2.4- pj pulse energy (focused to ≈ 100 μm²) and 4- V bias. We found a rise time of 1.3 ps,a 1/e falltime of 2.5 ps for the first rapid decay and a full width at half maximum (FWHM) of 3.3 ps corresponding to 70-GHz bandwidth (residual tail neglected). The fastest response we got from a MSM diode fabricated on GaAs grown at 700 °C (HT-GaAs) is shown for comparison by the dotted line in Fig. 1. The latter diode had a finger distance of 0.5 μm, we used a pulse energy of 0,3 pJ and 3-V bias. The rise time was 2.2 ps, the 1/e time for the first decay 4.8 ps, and the FWHM 6 ps. The reduction of the FWHM of the pulse from the LT-GaAs diode compared with that of the HT-GaAs diode originates from efficient trapping of the carriers in the LT-GaAs on a time scale shorter than the carrier transit time.

© 1992 Optical Society of America