Non-local Impact ionisation coefficients in Al0.8Ga0.2As and Ga0.8In0.2N0.05As0.94Sb0.01 for GaAs-based APDs and SPADs
Loading...
Date
2024-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Lancaster university
Abstract
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
iv
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.
Description
Keywords
single photon avalanche diodes (SPAD)