Single-photon avalanche diode (SPAD) based sensors and systems enable a variety of applications in biomedical, automotive, consumer, and security domains. While several established standard technologies, which can facilitate the design of SPAD-based systems are already in existence, challenges remain for the development of deep sub-micron monolithic integration of circuits and SPADs. In this work, we present SPADs along with pixel circuits in a standard GF 55 nm BCDL process. Two different designs demonstrate the flexibility allowed by the technology for a variety of applications. Both shallow and deep junction SPADs present excellent noise performance of less than 1 cps/μm2 at 3 V excess bias. An integrated passive-quench active-recharge (PQAR) circuit is used in conjunction with the SPADs, which enables a dead time of less than 2 ns, easily allowing for high dynamic range applications that require < 100 Mcps such as quantum communication and information technologies. The deep and shallow junction SPADs demonstrate an afterpulsing probability of < 0.5 % and < 2 % at 3V excess bias, respectively. The dead time is adjustable through analog control of the active-recharge circuit, allowing for afterpulsing reduction to below 0.1 % while maintaining Mcps operation. The shallow junction, which has a breakdown voltage of about 18 V and a peak sensitivity at 430 nm is particularly interesting for applications requiring low supply voltage, whereas the deep SPAD, which demonstrates < 4 % photon detection probability (PDP) at 940 nm, can be implemented in LiDAR sensors that require enhanced sensitivity at near-infrared (NIR) wavelengths. The measured timing jitter of both SPADs is < 50 ps FWHM at 3 V excess bias and 780 nm.
Single-photon avalanche diodes (SPADs) are direct photon-to-digital detectors that enable scalable arrays with Poisson-limited signal-to-noise ratio and picosecond timing resolution. However, SPAD detectors require a guard-ring structure to prevent lateral edge breakdown. The guard ring, in addition to pixel electronics, reduces the sensitive area within the pixel, often below 50%. We present the simulation, design and characterization of microlens structures to increase the effective fill factor and SPAD photon detection efficiency. The main challenges in designing microlenses for SPADs are a relatively large SPAD pitch and a low native fill factor, requiring high microlens efficiency over a wide angular distribution of light. In addition, we addressed the requirements of several designs in the same technology, featuring native fill factors which range from 10.5% to 28%, by carrying out the microlens fabrication at wafer reticle level. The fabrication process starts with creating a photoresist microlens master, used to fabricate a mould for microlens imprints. After dispensing a UV curable hybrid polymer on top of the SPAD array, the mould is used to imprint the microlens array shape, and then cured with UV exposure. By using microlenses, we were able to increase the initial fill factor to more than 84% effective fill factor for a 28.5 μm pixel pitch. We also explore the influence of the passivation layer on the SPAD photon detection efficiency.
It is numerically shown that Al/Sb free InGaAs unipolar barrier detectors with superior performance compared to the conventional heterojunction detectors can be constructed. Compositionally graded layers provide the transition between the high bandgap InGaAs barrier and the lattice matched InGaAs absorber layers. In addition, the delta doped layers remove the valence band offset in order to block only majority carriers and allow unimpeded flow of minority carriers. More than one order of magnitude reduction in the dark current is observed while photocurrent remains nearly unchanged. Proposed barrier structure utilized in this study is not limited to short wave infrared (SWIR) and can be applied to a variety of materials operating in various infrared regions.
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