| dc.description.abstract | Over the last 20 years, high-speed avalanche photodiodes (APDs) have
played a vital role in the development of the Ethernet Passive Optical
Network (EPON) and 10 G-EPON. The commercially available
In0.52Al0.48As based 10 G APDs can usually provide an 8 dB higher
sensitivity than that of their p-i-n photodiode (PDs) counterparts. However,
in the next generation of 50 G-PON, the 3-dB bandwidth demonstrated by
APDs under moderate gain operation (~10) is insufficient to meet the
bandwidth requirements for 50 G operation (> 30 GHz), with less
pronounced benefits for the sensitivity of 50 G APD based receivers .
It has been demonstrated that the sensitivity at the receiver-end can be
improved by the incorporation of complex equalizer integrated circuits
(ICs) and high-power PDs, which are hybrid or monolithic integrated with
the semiconductor optical amplifier (SOA).
However, both the additional ICs and pre-amplified SOAs need large
extra bias currents which leads to a significant increase of overall power
consumption in the receiver-end. Moreover, the additional amplified
spontaneous emission (ASE) noise in the SOAs may result in marginal
improvement in sensitivity in the receiver-end.
ii
The thinning of the multiplication (M) layers in high-speed avalanche
photodiodes (APDs) is an effective way to boost up its gain-bandwidth
product (GBP) and reduce excess noise. However, such downscaling
usually comes with a price, a huge leakage current (> 1 μA) induced by
direct tunneling through the ultimate thin M-layer (< 100 nm) and
degradation of the RC-limited bandwidth due to the decrease in depletion
layer thickness.
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
In0.52Al0.48As based multiplication layer can fundamentally relax the tradeoffs among the dark current, RC-limited bandwidth, multiplication gain,
and avalanche delay time. Applying this advanced device structure, we
then explore the performance of APDs with different thin M-layer
thicknesses (< 50 nm). Under1.55 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 Vbr.
Moreover, this device 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.
iii
The excellent 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). | en_US |