Hole-Capture Mechanisms in n-type GaN: Evidence for Additional Defects

To gain insight into the mechanisms by which acceptors capture holes in n-type GaN, researchers conducted pump fluence-dependent transient absorption (TA) measurements on the 03UID sample at a wavelength of 1050 nm. The calculated average photo-generated carrier concentrations (Dn) ranged from 5 x 10^15 cm^-3 at a pump fluence of 0.5 mJ/cm^2 to 2 x 10^16 cm^-3 at a pump fluence of 1.8 mJ/cm^2.

The researchers observed that the proportion of the fast recovery component, which arises from hole-trapping, decreased as the pump fluence increased. This indicates that the acceptors in GaN become saturated with photo-generated holes.

Initially, the researchers assumed that the only acceptor in n-type GaN was the negatively charged CN (CN-) and estimated its concentration (Ni) to be 4 x 10^15 cm^-3 using secondary ion mass spectrometry (SIMS). They then simulated the carrier dynamics under a pump fluence of 1.8 mJ/cm^2 using the single capture model with a hole-capture coefficient (Cp) of 2 x 10^-6 cm^3 s^-1.

However, the theoretical fitting (represented by the dashed line in Fig. 2) did not match the experimental result, except when Ni was increased to 8 x 10^15 cm^-3 (approximately twice the concentration of [C]). This suggests that there are other defects, either radiative or non-radiative, present in GaN with a concentration of approximately 4 x 10^15 cm^-3 and a similar hole-capture coefficient.