Ultrafast carrier dynamics in InGaN quantum well channel based high electron mobility transistor structure

Abstract

Ultrafast pump probe measurements on InGaN quantum well (QW) channel based high electron mobility transistor (HEMT) structure enable a direct measurement of the transit/capture time of electrons from GaN buffer layer to InGaN channel layer. Carrier capture time of 75 ± 10 fs is measured and the same is found to be independent of excitation wavelength. However, the capture process is delayed by 160 ± 5 fs when carriers are excited in GaN buffer layer. This happens due to the inclusion of transit time of carriers in GaN layer. It is confirmed by varying the wavelength of excitation where a delay in carrier transit is seen at longer wavelength. It is also seen that the strength of a feature associated with the carriers excited in GaN layer and captured by InGaN QW channel layer mimics the absorption spectrum of GaN, which also confirms that the delay in capture process is caused by the carriers excited in GaN layer. It is found that the capture process can be delayed even up to 240 ± 5 fs when the pump wavelength is kept at 390 nm. It is explained by considering the absorption in a deeper part of GaN layer and via the shallow defect states. Further, it is found that the localization of carriers in InGaN QW channel layer occurs at the time scales of 85 ± 10 fs. The transit/capture/localization time parameters reported in this work provide a crucial set of information, which is essential for modulating the carrier dynamics in InGaN/GaN based HEMTs and other high speed optoelectronic devices.