Impact Ionisation in Wide Band-Gap III-V Quaternary Alloys

This work is concerned with the investigation of the characteristics of two III-V quaternary
alloy systems, (AlxGa1-x)0.52In0.48P and AlxGa1-xAs0.56Sb0.44. These materials are studied
primarily for their application as multiplication regions in SACM avalanche photodiodes for
the detection of SWIR wavelengths.
The impact ionisation coefficients of (AlxGa1-x)0.52In0.48P have been extracted from
multiplication and excess noise data for 7 PIN structures of different aluminium concentrations
across the full composition range from x = 0 to x = 1. These coefficients are compared with
those for the ternary alloy AlxGa1-xAs, which change similarly with variations in alloy
composition. Both alloys show a sharp reduction in hole-initiated impact ionisation at a similar
threshold of aluminium content around x = 0.65, without a similar change in electron-initiated
ionisation. This indicates that the advantages associated with a wide α/β ratio can be maintained
in these alloys with a significant reduction in aluminium content. Possible mechanisms for the
changes in the impact ionisation coefficients with alloy composition are discussed in terms of
the material band-structures. It is suggested that the suppression of hole impact ionisation may
be due to the Γ band-gap becoming sufficiently large that ionisation events scattering into the
Γ valley do not generally occur.
Al0.85Ga0.15As0.56Sb0.44 PIN and NIP structures of various thicknesses have been studied in
detail, with the objective of providing a comprehensive picture of the impact ionisation
characteristics of this alloy. Multiplication and excess noise have been measured under a range
of carrier injection conditions. Variations in excess noise factor and bandwidth between
structures of different thicknesses are discussed, and impact ionisation coefficients have been
extracted which agree with multiplication data for 10 different structures across a wide electric
field range. Excess noise in this alloy is found to decrease with increasing structure thickness,
and the measured noise characteristics are found not to agree with those predicted by local or
RPL models for the extracted ionisation coefficients.
Complementary 1500nm PIN and NIP structures of Al0.75Ga0.25As0.56Sb0.44 and
Al0.55Ga0.45As0.56Sb0.44 have been studied, in order to understand how the characteristics of the
AlxGa1-xAs0.56Sb0.44 system vary with alloy composition. Multiplication and excess noise data
have been measured under a range of injection conditions, and these are compared with the
reported data for the higher-aluminium compounds of the alloy system. Preliminary impact
ionisation coefficients have also been extracted for these alloys from the multiplication data,
and the variation of these coefficients with alloy composition is compared to that in
(AlxGa1-x)0.52In0.48P and AlxGa1-xAs. These results indicate that excess noise in the
AlxGa1-xAs0.56Sb0.44 alloy system increases as aluminium content decreases, but the measured
noise for each alloy is lower than that which would be expected given their respective α/β ratios.
Possible sources of error in excess noise measurement are also discussed. The mechanisms
considered include device series resistance, improper or insufficiently frequent setup
calibration, and inaccurate determination of primary photocurrent. Example data are given to
show how these factors may distort measured excess noise factor, particularly in the case of
low-noise alloys for which it is desirable to measure this parameter to a high degree of accuracy.