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We have used femtosecond time-resolved optical techniques to study fundamental materials issues in III-nitride semiconductors relevant to the realization of high quality ultraviolet photodetectors. Intensity dependent pump-probe reflectivity and transmission measurements have been employed in the investigation of carrier dynamics in AlGaN alloys with Al content ranging from ~0.15 to 0.4. For the Al0.15Ga0.85N sample, the intensity dependence of the (Delta) R decay suggests that at high intensity the shallow traps are saturated and ultrafast nonradiative recombination dominates the carrier dynamics. For the Al0.25Ga.75N and Al0.4Ga0.6N samples (Delta) R decays faster with decreasing intensity and changes sign. Moreover, the decays are faster for a given in tensity in the higher Al content sample. This behavior suggests that in these cases the dynamics are governed by trapping at localized states that become deeper and more numerous as the Al content increases. Within this context the sign change in (Delta) R in A;0.4Ga0.6N may be indicative of the onset of photoinduced absorption associated with the excitation of carriers from the localized states to the bands, which has also been observed in time-resolved transmission measurements. This localization may be associated with alloy fluctuations that broaden the absorption edge of the material and degrade the long-wavelength performance of photodetectors. In addition, time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurzite GaN. The velocity overshoot is observed at fields well below the field at which the calculated peak steady-state velocity occurs, and it increases with electric field up to ~320 kV/cm, at which field a peak velocity of 7.25x107 cm/s is attained within the first 200 fs after photoexcitation. These results are consistent with theoretical Monte Carlo calculations incorporating a GaN full-zone band structure, which show that because of band nonparabolicity in the (Gamma) valley the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (>325kV/cm). This behavior may have important implications for avalanche photodiodes, for which electrons are promoted to higher lying bands for participating in the avalanche process.
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