Fast Photoconductive Photodetectors Employing Iron-Diffusion into Epitaxial GaAs

Abstract

Iron in gallium arsenide forms a deep acceptor level at 0.46 ev above the valence band.l This energy level can be used to overcompensate nGaAs layers resulting in highresistivity ptype material. ^2,3 Although the level was identified as a hole trap,⁴ it is expected in p-type materials to function as a recombination center rather than a trap. Hence fast response without a long tail can be realized. The photodetectors on the iron-diffused layers exhibit short decays limited by the carrier lifetime rather than the carrier transit time: Voltage pulses with full widths at half-maximum (FWHMs) as short as 72 ps were observed in spite of the large spacing of 15 pµm between the fingers of interdigitated electrodes. An analysis revealed that the deconvolved FWHM is about 50 ps and the transit distance of electrons is less than 5 µm.⁵ Then an improvement in responsivity can be expected with a finer electrode pattern fitted to the transit distance. This approach is important in increasing the “reduced” sensitivity of lifetime-limited devices. Here we report on the characteristics of the devices with a finger spacing/width (L/W) combination of 7µµm/3µm in comparison with those of l5µm/5µm.