Long-wavelength, high-speed avalanche photodiodes and APD arrays
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In optical receivers avalanche photodiodes (APDs) are frequently the photodetectors of choice due to high sensitivity afforded by their internal gain. APD noise and bandwidth are strongly dependent on the carrier impact ionization statistics in the APD gain region. Low noise comparable to silicon APDs was observed in Al0.9Ga0.1As APDs, due to larger carrier impact ionization coefficients ratio. A long-wavelength AlGaAs APD was therefore developed using a direct wafer-bonding technology to demonstrate long-wavelength absorption on GaAs substrates. For InP-based long-wavelength APDs, InAlAs lattice-matched to InP provides better noise and speed performances than InP due to larger ratio of carrier impact ionization coefficients. A thinner InAlAs layer provides even lower multiplication noise and a higher gain-bandwidth product than a bulk material due to pronounced carrier dead-space effect. Demonstrating high-speed, high-sensitivity APDs for fiber-optic communications is one of the primary tasks for this work. For 10Gb/s telecommunications, high carrier saturation velocities (>10^7cm/s) in In0.53Ga0.47As allow a thick absorbing layer (>=1µm) in APDs. In order to improve APD sensitivity, a thin InAlAs material was further incorporated to achieve high avalanche gains with less bandwidth penalty. A ~-29.5dBm optical receiver sensitivity was achieved using a 200-nm InAlAs APD. For the 40Gb/s applications, carrier saturation velocity sets an upper limit of absorbing layer thickness to less than a quarter micron. It is necessary to separate photon absorption from carrier transport to fundamentally circumvent speed constraint on APD’s responsivity. One solution is using a waveguide structure through which the incoming light is coupled into a thin absorbing layer in a direction normal to that carriers transport. Using this type of device structure, sensitivity becomes a function of device length rather than just the absorbing layer thickness. An InAlAs waveguide APD with both high responsivity (>0.62A/W) and broad bandwidth (>34GHz) was demonstrated, based on this scheme. In addition to telecommunications, long-wavelength InAlAs APDs, which provide broad wavelength coverage, high detection sensitivity, high gain uniformity, low dark currents at room temperature, and broad bandwidth, are also promising for the near-infrared three-dimensional imaging applications. A high-speed 40X40 InAlAs APD array was demonstrated for this application.