Non-local Impact ionisation coefficients in Al0.8Ga0.2As and Ga0.8In0.2N0.05As0.94Sb0.01 for GaAs-based APDs and SPADs
Creators: name: Albeladi, Shadia
Publisher: name: Lancaster University
Year: 2024

description: This work attempts to develop AlGaAs/GaInNAsSb separate absorption and
 multiplication avalanche photodiodes (SAM APDs) and single photon
 avalanche diodes (SPAD). AlGaAs/GaInNAsSb is a promising structure, with
 significant potential benefits in detector and APD applications. The
 combination of the low-noise Al0.8Ga0.2As avalanche multiplication layer
 with the high detection capability of the lattice-matched GaInNAsSb, would
 introduce a new choice of high-speed and cheap GaAs-based APD for 1.3 and
 1.55 μm telecommunication applications. In this work, experimental
 measurements as well as a comparison to stochastic models were carried out
 to investigate the characterisation of different AlGaAs and GaInNAsSb
 materials and structures. 
 
 The use of GaInNAsSb, which is lattice-matched to GaAs, as an absorption
 layer in the SAM APD structure can exploit its narrow band gap energy to
 effectively detect wavelengths important to optical applications, such as
 1.55 μm. When designing SAM APDs, it is crucial to consider if the
 absorber and avalanche materials have comparable electron-to-hole
 ionization coefficient ratios. Opposing coefficient ratios of the absorber
 and multiplication layer would produce the worst case for APD noise and
 response time. An initial evaluation of the impact ionization coefficients
 and threshold energies of GaInNAsSb has been undertaken. Even though these
 coefficients have some degree of uncertainty, they can be used as a
 sufficient foundation for further research on this material. On the other
 hand, these coefficients can offer essential knowledge, such as the fact
 that beta is much greater than alpha, which would negatively affect noise
 in a poorly designed APD. In addition, the approximate fields where
 multiplication initiates and breaks down were identified. In the future,
 more work needs to be done to optimize these coefficients. 
 
 The large α/β ratio in bulk Al0.8Ga0.2As structure offers low excess noise
 and low dark currents. Also, the wide indirect band gap of Al0.8Ga0.2As
 enables fabrication of very thin multiplication widths without tunneling
 being a problem. Consequently, using Al0.8Ga0.2As in a SAM structure can
 take advantage of the thin multiplication layer, exhibit desirable
 characteristics suited to high-speed, low-noise avalanche photodiodes, and
 maximise the reduction in excess noise due to the non-local ionisation
 effects. In order to characterize thin APDs, the non-local electron and
 hole ionization coefficients and threshold energies using the hard dead
 space model were extracted in this work and compared with those of other
 materials. It is demonstrated that, despite the fact that previously
 investigated thin GaAs APDs can achieve a similar significance of dead
 space with the associated minimisation of excess noise, it is difficult to
 exploit this advantage due to the significant tunnelling in GaAs. On the
 other hand, it is possible to produce a thin Al0.8Ga0.2As APD with a dead
 space to ionisation path length ratio close to the fundamental limit,
 which leads to excellent low-noise multiplication without significant
 tunnelling. It is also found that in the case of the standard ideal p-i-n
 structure, these coefficients and threshold energies can effectively
 simulate multiplication in APDs as thin as 50 nm. This is substantially
 thinner than current state-of-the-art APDs. 
 
 The optimal charge sheet conditions for AlGaAs/GaInNAsSb SAM APDs can be
 determined by initially using GaAs as the absorber layer. Five samples
 were studied, and samples 3, 4, and 5 were found to be unsuitable for SAM
 APD applications as they failed to punch through. Sample 3 is suitable for
 SPAD applications operating at a higher voltage than the breakdown
 voltage. Sample 2 showed punch through into the absorber but kept the
 electric field below the required level for multiplication in the planned
 dilute nitride absorber layer. Therefore, the structure of sample 2 should
 be used as the nominal design for preparing to use a GaInNAsSb absorber
 layer. The AlGaAs/GaAs structure can be successfully simulated and
 redesigned based on correct coefficients to control the charge sheet
 conditions and achieve the desired punch-through voltage. 
 
 GaAs/AlGaAs SPADs with an extremely thin multiplication layer are found to
 exhibit a lower DCR than that in other common SPADs with relatively
 thicker multiplication layers, such as InGaAs/InP, InGaAs/InAlAs, and
 Ge/Si SPADs. On the other hand, these SPADs suffered from a higher
 afterpulsing probability than other SPADs. In the future, more work needs
 to be done to minimise the effect of afterpulsing, improving the SPAD’s
 performance. 
 
 Despite the fact that this work provides the foundation for this
 structure's essential understanding, a lot of work must be done
 before it can be developed into products which improve on state-of-the-art
 APDs and SPADs. Using the extracted coefficients and threshold energies of
 AlGaAs and GaInNAsSb with a more sophisticated model such as a randomly
 generated ionization path length (RPL) can help to maximise the potential
 of the materials in new APDs and SPADs simulating important
 characteristics such as excess noise, impulse response, and breakdown
 probability. There is also scope for more open-ended simulation to explore
 the theoretical limits of APDs with multiplication widths which are
 substantially thinner than previously employed.