Engineering the gain and bandwidth in avalanche photodetectors

Abstract

Avalanche and Single-Photon Avalanche photodetectors (APDs and SPADs) rely on the probability of photogenerated carriers to trigger a multiplication process. Photon penetration depth plays a vital role in this process. In silicon APDs, a significant fraction of the short visible wavelengths is absorbed close to the device surface that is typically highly doped to serve as a contact. Most of the photogenerated carriers in this region can be lost by recombination, get slowly transported by diffusion, or multiplied with high excess noise. On the other hand, the extended penetration depth of near-infrared wavelengths requires thick semiconductors for efficient absorption. This diminishes the speed of the devices due to the long transit time in the thick absorption layer that is required for detecting most of these photons. Here, we demonstrate that it is possible to drive photons to a critical depth in a semiconductor film to maximize their gain-bandwidth performance and increase the absorption efficiency. This approach to engineering the penetration depth for different wavelengths in silicon is enabled by integrating photon-trapping nanoholes on the device surface. The penetration depth of short wavelengths such as 450 nm is increased from 0.25 µm to more than 0.62 µm. On the other hand, for a long-wavelength like 850 nm, the penetration depth is reduced from 18.3 µm to only 2.3 µm, decreasing the device transit time considerably. Such capabilities allow increasing the gain in APDs by almost 400× at 450 nm and by almost 9× at 850 nm. This engineering of the penetration depth in APDs would enable device designs requiring higher gain-bandwidth in emerging technologies such as Fluorescence Lifetime Microscopy (FLIM), Time-of-Flight Positron Emission Tomography (TOF-PET), quantum communications systems, and 3D imaging systems.

Design and Fabrication of High-Efficiency, Low-Power, and Low-Leakage Si-Avalanche Photodiodes for Low-Light Sensing

“…The EQE of the device is compared against the literature at a fixed illumination wavelength of 850 nm. A proportionate reduction in the EQE from 39% to 22.5% is due to a significant reduction in the π-layer thickness from 2.0 to 0.8 μm. Finally, we have compared the multiplication gain ( M ) of the device.…”

“…In Figure d, we present the multiplication gain ( M ) of the APDs for 1 and 10 μW laser power for the 850 nm illumination wavelength. The multiplication gain is calculated at the unity gain voltage ( V M =1 = −1 V) and using M = ( I photo – I dark )/( I photo ( V M =1 ) – I dark ( V M =1 )) expression . The gain, M , increases rigorously with reduced laser power.…”

“…Thereafter, we show an ∼5× enhancement in the EQE at 850 nm with the introduction of PTMH. The PTMH works as a waveguide and allows lateral propagation by bending the light ,, almost 90°. This bending can be attributed to the diffraction of the incident light at the corners and edges of the PTMH as the feature size of the holes is comparable to that of the illumination wavelength .…”

“…Since the advent of impact-ionization-based avalanche photodiodes (APDs), − there has been an accelerated growth toward integrating high-speed optics on-board and on-chip. Despite significant development in the design optimization of Si-APD, the high operating voltage, low intrinsic EM wave absorption in Si, and high dark current remain challenging. − To enhance the detection efficiency, numerous methods such as metal-grating, , photon-trapping holes, ,, antireflection coating, and so on have been extensively explored.…”

On-chip hyperspectral detectors for fluorescence lifetime imaging

“…Here, 𝑉 𝑢𝑛𝑖𝑡𝑦 is the voltage at which there is no impact ionization, thereby gain is unity. We consider reverse bias voltage, V = -1V to be unity gain voltage as there is no significant change in the dark or photo current in its vicinity 30 .…”