Traditional applications for 2-micron photodetectors have been largely dominated by passive remote sensing where detectors having bandwidth of even one megahertz are deemed sufficient. The onus in such applications is to achieve low dark current through active cooling. The advent of high-power, 2-micron-wavelength lasers has made coherent LiDARs viable for active sensing applications. Such a system needs photodetectors that can handle high local oscillator optical power and have large bandwidth. Through a combination of high coherent gain and small integration time, a large signal-to-noise ratio can be achieved. Operation at high optical power levels reduces the significance of photodiodes' dark current. As a result, uncooled operation at room temperature is feasible, simplifying the overall instrument design.
Lattice-mismatched, uncooled, 2.2-μm wavelength cutoff, InGaAs photodiodes and balanced photoreceivers with band-width up to 25 GHz were developed. The responsivity at 2.05μm is 1.2 A/W, and the 1-dB compression, optical current handling of these photodiodes is 10 mA at 7V reverse bias. Such high-current-handling capacity allows these photodiodes to operate with a higher DC local oscillator (LO) power, allowing more coherent gain and shot-noise-limited operation. The impulse response of these devices shows rise time/fall time of ~15 ps, and full width half maximum of ~20 ps. These highspeed detectors can find utility in several 2-micron-wavelength applications including pulsed LiDARs, microwave photonics, and next-generation telecommunication links based on photonic bandgap fibers.
The epitaxial layer structure of the p-i-n photodiode is shown in the figure. The device was grown on a (100) oriented n-doped InP substrate. The growth sequence starts with an n-doped graded buffer layer of InAsyP1-y. The buffer layer is lattice-matched to InAs0.33P0.67 and InP at the top and bottom, respectively. The buffer layer contains compositionally abrupt interfaces that minimize the propagation of lattice defects into the active layers grown subsequently. As a result, the dark current and reliability of the photodiode are improved. An intrinsic In0.72Ga0.28As absorption layer is grown on top of the buffer layer. Finally, an InAs0.33P0.67 contact layer is grown to provide the anode for the photodiodes. It is noteworthy that the lattice constant of the epitaxial layers increases along the growth direction. The resulting compressive strain prevents the epitaxial material from cracking. The photodiodes were fabricated using standard planar processing steps.