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The InN quantum dots (QDs) grown on a GaN buffer layer have been widely studied for their potential applications, such as solar cells, infrared light-emitting diodes and biosensors etc.. However, the InN growth temperature was restricted due to the issue of low dissociation temperature. Leading In (indium) adatoms are less mobile in finding the energetically favored site, resulting in high density structure defects that generally exist in the conventional metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy growth of InN QDs. The flow rate modulation epitaxy (FME) technique was proposed to overcome the In adatoms migration issue and was proven to be a suitable method for preparing high optical quality InN QDs. Nevertheless, the low density of InN QDs grown by the FME technique is a drawback for most optoelectronic devices based on the QDs’ active layer which prefer high density QDs. In other words, the growth technique for high density QDs plays an important role. It was well known that the adatom kinetic processes, such as adsorption, desorption, and diffusion at surfaces, are considered to be key parameters controlling the dot density, material quality and surface morphology. Recently, the pulsed-mode (PM) MOCVD method was reported as a useful approach for preparing high density InN QDs on GaN buffer layers. However, most of these reports could only achieve one benefit, either density or optical quality. Therefore, in this study we proposed a new method to grow high density InN QDs while maintaining the optical quality. The two-step growth method consists of one growth cycle for step-1 and two growth cycles in step-2. The duration of one growth cycle is 50 sec, including a 10 s TMIn flow process, a 20 s NH3 process, and two 10 s purge processes intervened in between. The two-step growth included step-1 for high density InN nucleation at 575 oC, and then, the reactor temperature was raised to 650 oC in 5 min increments to decompose the poor material quality InN islands. The aim of the step-1 growth was only to create high density nucleation sites on the GaN buffer layer. The reactor temperature was then reduced to 600oC to enable the growth of InN QDs with high density and high optical property in step-2 of the process. The experimental result show that the high density 1.5×1010 cm− 2 truncated hexagonal pyramidal InN QDs can be grown by the two-step growth method. The InN nucleation, InN decomposition and In adatom diffusion in the step-1, annealing and the step-2 process, respectively, determined the density, shape and size distribution of the InN QDs. The bimodal size distribution of the two-step growth InN QDs can be attributed to the high mobility of the incoming adatoms or/and existing islands, which favors coalescence process. The double-peak feature in the photoluminescence (PL) is related to the ground-state emission from the InN QDs with bimodal size-distribution. The low residual carrier concentration (1.8×1018 cm− 3), large confinement energy (342 meV) and intense PL emission for 6.1 nm high InN QDs indicated that a two-step growth is suitable for QD device applications. |
