Planar interdigitated electrode MSM photodetectors offer high bandwidths and responsivity, and are therefore attractive for optoelectronic integrated circuits for optical communication, and for ultra fast optical sampling measurements. The planar structure leads to low parasitic capacitance, with consequent improvements in both bandwidth and receiver sensitivity and very easy integration with FETs such as MES FETs or HEMTs.
A typical device structure is shown in FIG. 1 and consists of two sets of interdigitated electrodes 2 and 3 formed by vapor deposition and lithography, on a semiconductor sample. The electrode sets are biased with respect to each other, so that alternate electrodes e.g. 2 and 3 of the sets are at bias +V,-V etc. The width of each electrode is w and the gap between the electrodes is d. To understand the present invention, it is necessary to draw a clear distinction between photodiodes and photoconductors. These are similar structures but operate in different ways.
In a photodiode, contacts are evaporated onto an undoped semiconductor with no subsequent annealing process, thereby forming Schottky or blocking contacts. As is well known in the art, Schottky contacts function as diodes, producing a depletion region in the semiconductor. In use, a reverse bias is applied so that charge carriers formed in the depletion region of the semiconductor by incident photons, are swept rapidly to the electrodes 2 and 3. The bandwidth and responsivity of the device are limited by the transit time for the carriers. The internal quantum efficiency (collection efficiency) is almost unity since the carrier lifetime (.tau..sub.r .apprxeq.10.sup.-9 s in GaAs) is much longer than the charge carrier transit times, for typical electrode separations of a few microns or less. Thus, the response time is determined by the transit time, which is .tau..sub.d .apprxeq.10 ps for a drift velocity of 10.sup.5 m.s.sup.-1 and contact separations=1 .mu.m. The properties of MSM photodiodes have been previously studied in detail. For a review of InGaAs MSM photodiodes for optical communications, see B. D. Soole and H. Schumacher, IEEE J. Quantum Electron. QE-27, 737 (1991) "InGaAs metal-semiconductor-metal photodetectors for long- wavelength communication", IEEE Trans. Electron Devices 37, 2285 1990! "Transit-time limited frequency response of InGaAs MSM photodetectors". A GaAs MSM photodiode with w=d=0.1 .mu.m showed a measured full-width-at-half-maximum (FWHM) response time of 1.5 ps, limited by the parasitic capacitance--S. Y. Chou, Y. Liu, W. Khali, T. Y. Hsiang & S. Alexandrou, Appl. Phys. Lett. 61, 819 1992! "Ultrafast nanoscale metal-semiconductor-metal photodetectors on bulk and low temperature GaAs".
In contrast, in a photoconductor, the response time is limited by the recombination time .tau..sub.r of the charge carrier pairs produced by incident light, rather than by the transit time .tau..sub.d as in the case of a photodiode. This situation occurs for an ultrafast photoconductive material where the carrier recombination time is shorter than the transit time between the electrodes, and is irrespective of whether the electrodes form Schottky (blocking) contacts or Ohmic (injecting) contacts. Carrier recombination lifetimes of a few picoseconds or less are required for recombination-limited behaviour in a GaAs device with 1 .mu.m electrode separation. The carrier recombination lifetime may be reduced to less than 1 ps by, for example, proton implantation or low-temperature growth of GaAs with As precipitates (LT GaAs or GaAs:As) see M. Lambsdorf, J. Kuhl, J. Rosenzweig, A.Axmann & Jo. Schneider, Appl. Phys. Lett. 58, 1881 1991!"Subpicosecond carrier lifetimes in radiation-damaged GaAs", and S.Gupta, M. Y. Frankel, J. A. Valdmanis, J. F. Whittaker, G. A. Mourou, F. W. Smith and A. R. Calewa, "Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures", Appl. Phys. Lett 59 3276 1991!. The responsivity is reduced by a factor (.tau..sub.r /.tau..sub.d) since only a portion of the carriers reach the contacts before recombining (the photoconductive gain is less than unity), giving a responsivity which scales with (1/d). The responsivity can therefore be increased by reducing d until .tau..sub.r .apprxeq..tau..sub.d (d.apprxeq.0.1 .mu.m for 1 ps carrier lifetime). Referring now to FIG. 2, this shows a section through the arrangement of FIG. 1, and in this discussion, is considered to be configured as a photodiode. The substrate 1 includes an absorption layer 4 of thickness h in the substrate 1 beneath the electrodes 2 and 3, the absorption layer 4 being the region in which incident optical radiation is absorbed to produce charge carrier pairs. The field between the adjacent electrodes 2 and 3 is shown in the absorption layer. The field is reduced far from the electrodes 2 and 3. For low applied fields, the steady-state velocity for charge carriers produced in the absorption layer 4, is proportional to the field, and hence the charge carrier velocity is decreased for carriers generated deep within the absorption layer. Also, the distance the carriers need to travel to the electrodes is increased. This can lead to reduced bandwidths for MSM photodiodes, giving a long tail in the response to a short duration light pulse. The absorption coefficient for above-bandgap illumination in direct gap III-V semiconductors is typically .apprxeq.10.sup.6 m.sup.-1, so that the absorption layer thickness h must normally be several .mu.m for high responsivity, whereas d must be &lt;1 .mu.m for picosecond transit times. Soole and Schumacher supra have investigated InGaAs MSM photodiodes where h was decreased from 3.mu.m to 0.5.mu.m, and the responsivity decreased and the bandwidth increased as expected. A prior method of improving bandwidth of MSM photodiodes will now be described with reference to FIG. 3. The absorption layer 4 is arranged as a thin layer confined between layers 5, 6 that define an optical cavity. Thus, light incident on the device, resonates between the layers 5 and 6 so as to produce multiple reflections of the light through the absorption layer. As a result, the absorption layer can be made thinner than the arrangement shown in FIG. 2, and close to the electrodes 2 and 3 so as to be disposed in the relatively high, uniform field. --see A. 25 Chi and T. Y. Chang, J. Vac. Sci. Technol. B 8, 399 1990!; K. Kishino, M. S. Unlu, J. Chyi, J. Reed, L. Arsenault and H. Morkoc, IEEE J.Quantum Electronics JQE-27, 2025 1991!; A. G. Dentai, R. Kuchibhotla, J. C. Campbell, C. Tsai and C. Lei, Electron Lett. 27, 2125 1991!; U. Prank, M/Mikulla and W. Kowalsky, Appl. Phys. Lett 62, p.129 1993!.