InGaAs for infrared photodetectors. Physics and technology

Contributed paperFrom Eq. (6), the quantum efficiency at the certain advalue is maximised whenR-2d= R ea . (7)1 2Figure 11 shows the maximum h max as a function of adfor various R 2 values. To attain the highest h, near unity R 2is desirable. For conventional detectors, without the cavityeffect (R 2 = 0), quantum efficiency higher than 90% can berealized only for a thick (> 2 µm) active layer (curve R 2 =0in Fig. 11). For RCE detector, when R 2 = 0.99, the h > 90%can be obtained for < 0.05-µm thick layer (a =10 4 cm –1 ).This fact is very helpful towards improving the high-speedperformance of detectors reducing the electronic transit time.Fig. 12. Optical field distribution in RCE detector as a function ofwavelength and position. Top and bottom mirrors are 5 and 15periods GaAs/AlAs. Cavity is optimised for the wavelength of0.9 µm (after Refs. 8 and 9).Fig. 10. Wavelength dependence of quantum efficiency for RCEdetectors (after Ref. 7).The spatial distribution of the optical field inside thecavity is a specific feature of the cavity. It arises from thestanding wave formed by the two propagating waves. Acalculated distribution of the optical field is presented inFig. 12. It follows that h, which is derived from the powerabsorbed in the active region, depends on a position of theactive region in the optical field. In order to obtain maximumvalue of the detector efficiency, the active regionshould be placed in a wave antinode position.The standing wave effect is conveniently included inthe calculation of as an effective absorption coefficient,i.e., a eff = SWE × a, where SWE is the standing wave coefficient.The SWE coefficient is an explicit function of thecavity properties [8,9].Earlier, we confined our consideration of h to a normalincidence. The changes of a detector quantum efficiencyversus angle incidence are presented in Fig. 13. The calcu-Fig. 11. Maximum quantum efficiency as a function of normalisedabsorption coefficient (after Ref. 7).Fig. 13. Changes of h for off-angle incidence at l = 1.0 µm andl = 1.5 µm and different finesses F = 10 and F = 20 (after Ref. 7).Opto-Electron. Rev., 12, no. 1, 2004 J. Kaniewski 143