  Liquid xenon time projection chambers are promising detectors to search for
neutrinoless double beta decay (0$\nu \beta \beta$), due to their response
uniformity, monolithic sensitive volume, scalability to large target masses,
and suitability for extremely low background operations. The nEXO collaboration
has designed a tonne-scale time projection chamber that aims to search for
0$\nu \beta \beta$ of \ce{^{136}Xe} with projected half-life sensitivity of
$1.35\times 10^{28}$~yr. To reach this sensitivity, the design goal for nEXO is
$\leq$1\% energy resolution at the decay $Q$-value ($2458.07\pm 0.31$~keV).
Reaching this resolution requires the efficient collection of both the
ionization and scintillation produced in the detector. The nEXO design employs
Silicon Photo-Multipliers (SiPMs) to detect the vacuum ultra-violet, 175 nm
scintillation light of liquid xenon. This paper reports on the characterization
of the newest vacuum ultra-violet sensitive Fondazione Bruno Kessler VUVHD3
SiPMs specifically designed for nEXO, as well as new measurements on new test
samples of previously characterised Hamamatsu VUV4 Multi Pixel Photon Counters
(MPPCs). Various SiPM and MPPC parameters, such as dark noise, gain, direct
crosstalk, correlated avalanches and photon detection efficiency were measured
as a function of the applied over voltage and wavelength at liquid xenon
temperature (163~K). The results from this study are used to provide updated
estimates of the achievable energy resolution at the decay $Q$-value for the
nEXO design.
