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We report on a new platform for separate absorption and multiplication avalanche photodiodes (SAM APDs), developed using 6.1A materials lattice matched to GaSb. Materials and quantum structures grown on GaSb substrates have become of great technological significance for infrared detectors, with absorber options covering the important SWIR, MWIR and LWIR wavebands. The new APD platform is able to exploit these well-established absorber materials, by linking them to a new avalanche multiplication medium, Al0.9Ga0.1As0.08Sb0.92 in a SAM architecture. This opens up the potential for linear and Geiger mode APDs operating across the infrared wavebands for applications such as environmental monitoring, gas sensing and active imaging. In particular, extension of the operating wavelength for active systems with single photon sensitivity, beyond the currently available 1.55µm, is compelling due to the commensurate reduction in solar background photon flux.
Recent characterisation has shown that Al0.9Ga0.1As0.08Sb0.92 exhibits desirable avalanche multiplication characteristics. Most notably tunnelling current is negligible due to the material’s large indirect bandgap, while the ionisation coefficients are found to exceed those of other III-V materials. In this work these characteristics are exploited to design and fabricate SAM APDs with breakdown voltages below 15V. This is significantly lower than alternative technologies such as AlInAs/InGaAs APDs, which typically breakdown in excess of 25V in order to limit the magnitude of deleterious tunnelling current. Such a low breakdown voltage is desirable because it greatly aids the design of ROICs enabling future 2D imaging APD and SPAD arrays.
In this work Al0.9Ga0.1As0.08Sb0.92/In0.22Ga0.78As0.19Sb0.81 SAM APDs are designed, fabricated and characterised. The detector’s spectral response covers the SWIR window, reaching 2.5µm. Grading between the absorption and multiplication layers is refined to achieve high quantum efficiencies both at room temperature and more challengingly at 77K. Operation below breakdown as a linear mode APD and above breakdown as a Geiger mode SPAD when cooled, are studied and reported. As a further proof of the new SAM APD platform, an Al0.9Ga0.1As0.08Sb0.92/InAs0.91Sb0.09 detectors is also realised, extending responsivity into the MWIR window.
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