Thinning of Cascaded Multiplication Layers in Avalanche Photodiodes for High-Speed and High-Power-Tolerant Performance

The thinning of the multiplication (M) layers in high-speed avalanche photodiodes (APDs) is an effective way to boost the gain-bandwidth product (GBP) and reduce excess noise. However, such downscaling usually comes at the price of a huge leakage current (>1 μA) induced by direct tunneling through the final thin M-layer (<100 nm) and degradation of the RC-limited bandwidth due to the decrease in the thickness of the depletion layer. In this work, we demonstrate how a p-side up top-illuminated APD structure with a thick InP collector layer buried below the thin and cascaded In<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</sub>Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.48</sub>As based multiplication layer can effectively relax the trade-offs among the dark current, RC-limited bandwidth, multiplication gain, and avalanche delay time. This advanced device structure is then applied and the performance of APDs with different thin M-layer thicknesses (<50 nm) explored. Under 1.55 μm wavelength excitation, a device fabricated with a large active window (mesa) diameter of 10 (20) μm and an optimized M-layer thickness exhibits a dark current as low as ∼0.4 μA and a high responsivity (2.8 A/W; gain = 9.3) at 0.9 V<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub>. This device also exhibits excellent dynamic performance, including a wide optical-to-electrical bandwidth (44 GHz at 0.84 A/W), an extremely large GBP of 1.03 THz, and a high saturation current (12 mA), which corresponds to a large millimeter-wave (MMW) output power (∼0 dBm) at 45 GHz. The exceptional speed performance coupled with the wide dynamic range and simple top-illuminated structure opens up new possibilities to further enhance the sensitivity of 50 G passive optical networks (PONs).