Abstract
In optical receiver systems where the system noise is circuit-limited, avalanche photodiodes (APDs) offer a benefit compared to conventional photodiodes as their internal gain mechanism can lead to an improved system signal-to-noise ratio. The internal gain in APDs comes from the process of impact ionization in which high energy carriers collide with bound electrons within a crystalline material, liberating them to move freely within the lattice. However, this amplification process itself creates noise as it is stochastic. Therefore, the benefit of an APD is limited to the point at which its noise contribution begins to dominate the system.
Applications in the mid-wave infrared (MWIR) are particularly attractive for APDs as high sensitivity is often required to detect the low signal levels present in applications like space imaging or surveillance. In the MWIR, the dark current contribution to noise is particularly problematic as the thermal generation of carriers is high in the narrow bandgap materials required to absorb MWIR light. However, this problem can be relatively easily solved as the devices can be cooled to reduce the thermal generation component of the dark current, albeit at the cost of a physically larger receiver. The second main contributor to noise in the MWIR is the excess noise factor. In an optical receiver, the excess noise factor will fundamentally limit its sensitivity. Therefore, it is crucial to make the excess noise factor as small as possible. For my thesis, I have focused on reducing the detector’s excess noise factor to improve receiver sensitivity.
In collaboration with the University of Texas at Austin, I have investigated several devices that fall into three regimes of excess noise: conventional single-carrier ionization (k ~ 0), heterojunction-dominated multiplication (excess noise factor ~ 1), and an intermediate between the two. I first started by examining four important considerations when measuring the excess noise of low k-factor materials, like AlInAsSb presented in this thesis. I then designed, fabricated, and characterized a Al0.05InAsSb-based separate absorption, charge, and multiplication (SACM) APD for operation out to 3.5 µm. The device achieves a maximum gain of around 850 and has a unity-gain external quantum efficiency of 24% at 3 µm. It is the first AlInAsSb-based APD capable of efficiently absorbing 3 µm light. The SACM APD has a k-factor of 0.04, meaning it falls in the single-carrier ionization regime of excess noise (regime 1).
In tandem, I experimentally measured the excess noise factor of 2- and 3-step staircase APDs at an operating frequency of 70 kHz. To do so, I first needed to develop a new noise measurement setup capable of operating in the kHz range. I also needed to develop a new measurement methodology for measuring the noise for the staircase APDs, as the one we typically use to characterize conventional APDs did not apply. At a gain of ~4 and ~7.5 for 2- and 3-step devices, respectively, their excess noise factors are near unity as theoretically predicted by Capasso and Teich.
To continue the investigation, I proposed a hybrid device combining the MWIR absorption of an SACM APD with the near-unity excess noise of a staircase APD. The device was designed jointly between our group at UVA and our collaborators at UT Austin. Growth was done at UT Austin. This hybrid “SACMcase” APD has a near unity excess noise factor at a gain of ~4, and it can absorb light out to 3 µm. At a gain of 4, the SACMcase has an excess noise factor nearly three times lower than that of commercial InGaAs/InP SACM APDs.
Finally, I fabricated and characterized a cascaded multiplier structure, combining a single-step staircase multiplier with a bulk conventional multiplier. The idea of this design is to achieve a larger gain than a single-step staircase APD (gain of 2) while maintaining the near-unity excess noise. This device was designed and grown by my collaborators at UT Austin. The resulting device had an excess noise factor of ~1.3 at a gain of ~6. Compared to 2- and 3-step staircase APDs, this device has a dark current density nearly four times lower.
