Ultrafast Silicon Based Internal Photoemission Detectors
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Abstract We have studied different metal-silicon-metal (MSM) Schottky barrier photodiodes for the detection of visible and infrared light. We investigated the different Schottky barriers fromTi, Cr and Pt. At infrared wavelengths, the Schottky contacts provide electrons and holes by "internal photoemission" into the Si. The lowest Schottky barrier determines the long wavelength cutoff and the current noise. The temporal response was measured byultrashort (100 fs) laser pulses from a Ti:A120 3 laser, which were converted by an optical parametric oscillator to a wavelength of 1.1 to 1.6 pm. The measurements were performed between 30 K and room temperature. The best detectors show a pulse width of 3.2ps FWHM at 1.25 prm wavelength and room temperature. To our knowledge this is the fastest infrared response for silicon based diodes ever reported. Introduction
Silicon photodiodes are very popular detectors for the wavelength range from 0.4 to 1 pm. Generally, a p-n junction or a p-i-n structure is used. In reverse polarity, the dark current of these diodes is sufficiently low and photons create electron-hole pairs, which are separated in the high field region. They provide the electrical signal. Due to the indirect bandgap the absorption coefficient of silicon is low and a fairly large volume is needed for efficient absorption. The large volume is provided by the p-i-n structure, but a large volume always limits the response speed. In Si, both electrons and holes reach saturation drift velocities of V. = 107 cm/s (corresponding to v, = 0.lpm/ps) at an applied field of 10 V/pm. In order to realize a high homogeneous electrical field over a well defined, relatively short drift path, we prefer an MSM design, which consists of a tiny parallel-plate capacitor structure with silicon as the dielectric [1]. At both metal-silicon interfaces Schottky diodes are formed and one of them is always in reverse, the other one in forward polarity, if a voltage is applied.
167 Mat. Res. Soc. Symp. Proc. Vol. 558 0 2000 Materials Research Society
The speed of the electrical response is determined by the carrier transit time and also by the RC time constant, where R is the resistance of the metallic microstrip connection line and C the capacitance of the metallic electrodes. If the detector is excited by light of X < 1.1 pm, silicon. In this case the carriers are accelerated spacing L, the average carrier travel length is 0.5 - L/vs. For a gap of 0.4 pIm this leads tot bandwidth of 1/2 7Tt = 80 GHz. In the case of IR excitation at X > semiconductor-metal interfaces. They discussed in a forthcoming publication. spacing results in a faster carrier transit
electron-hole pairs are generated in the to the two electrodes. For an electrode L/2 and the average transit time T is = 2 ps, which corresponds to a - 3 dB
1.1 pm, carriers are emitted only at the internal show a different type of dynamics, which will be Under all circumstances, a reduction of the electrode time at the expense of an increased capacitance.
Au microstrip line Si02
Si
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