Ultrafast carrier dynamics in nano-clustered InGaN

Abstract

We perform fs degenerate pump-probe experiments on an InGaN/GaN quantum-well sample and an InGaN thin film of 800 nm in thickness, in which nm-scale cluster structures have been identified. In the InGaN/GaN quantum-well sample, we can identify three stages of carrier relaxation. The fast-decay time, ranging from several hundred fs to one ps, corresponds to the process reaching a local quasi-equilibrium condition, in which carriers reach a thermal distribution within one or a few nearby indium-rich clusters. The slow-decay time, ranging from tens to a couple hundred ps, corresponds to the process reaching a global quasi-equilibrium condition, in which carriers reach a thermal distribution among different clusters of various potential minima. In this stage, the mechanism of carrier transport over barriers between clusters dominates the relaxation process. Finally, carrier recombination dominates the relaxation process with the carrier lifetime in the range of a few ns. In the InGaN thin film sample, we can identify the variation of the space-averaged density of state with energy level in this sample. The carrier dynamics is controlled by the shift of effective bandgap and hence the behavior of band filling, which are determined by the combined effect of bandgap renormalization and phonon effect (bandgap shrinkage with increasing temperature). Two-photon absorption and free-carrier absorption can be observed when the corresponding density of state is low and hence the band-filling effect is weak. The variation of the space-averaged density of state with energy level can be due to the existence of indium-composition-fluctuation nanostructures, which is caused by the spinodal decomposition process.Keywords: pump-probe spectroscopy, ultrafast carrier dynamics, InGaN thin film, InGaN/GaN quantum-well INTRODUCTIONBecause of the large lattice mismatch between GaN and InN, two important phenomena in InGaN compounds have been widely observed, including indium composition fluctuation (or clustering) [1-4] and strain-induced piezoelectric field near an interface of a hetero-structure [5][6][7][8][9]. The former leads to carrier (exciton) localization and hence radiative efficiency improvement. The latter results in quantum-confined Stark effect (QCSE) and hence carrier separation in a carrier confined structure. Both phenomena are important in interpreting the photon emission mechanisms in such a compound. In particular, these two phenomena interplay in an InGaN/GaN quantum well (QW) structure, leading to complicated optical behaviors. Normally, when the hetero-structure induced strain is relaxed through a certain process, spinodal decomposition can be easily induced such that indium composition fluctuations and indium-rich clusters can be formed in such a sample [10,11]. On the other hand, if the strain is preserved, the solid miscibility between InN and GaN can be increased such that relatively more uniform InGaN ternary compounds can exist. In this situation, higher QW quality can be obtained.With the effects of carrier lo...