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.