Carrier localization, transportation and recombination in blue-emitting InGaN/GaN multiple quantum wells were analyzed using temperature-dependent photoluminescence spectroscopy, confocal laser scanning microscopy and time-resolved photoluminescence (TRPL). The temperature-dependent shift of PL intensity was fitted with Arrhenius equation and explained using two non-radiative channels, which are related with thermal activation of carriers from different confining potentials. The S-shaped shift of PL peak energy and inverse-S-shaped shift of PL full width at half maximum were explained with carrier localization and carrier transportation. The TRPL spectra taken at several different places from bright region to dark region in the confocal microscopic image showed that the fast decay life-time τ 1 increases with decreasing PL intensity, indicating a higher carrier transportation rate at bright region, while the slow decay life-time τ 2 decreases with decreasing PL intensity, indicating a higher probability of non-radiative recombination at dark region. 2 Previous research has been focused on studying the carrier localization effect in potential minimum using transmission electron microscopy, 3 nearfield scanning optical microscopy. 4 However, not too much work has been focused on the carrier transportation between different regions in the QW layer. In the present work, temperature-dependent photoluminescence (PL), confocal laser scanning microscopy (CLSM) and time-resolved photoluminescence (TRPL) are utilized to analyze the carrier localization and transportation behavior.
ExperimentalTwo InGaN/GaN MQWs samples were grown on c-plane sapphire substrate in a Veeco K465i GaN metal-organic chemical vapor deposition (MOCVD) reactor. Triethylgallium (TEGa) and trimethylindium (TMIn) were used as group III sources. Ammonia (NH 3 ) was used as group V source. Nitrogen was used as carrier gas. A 30-nm-thick GaN nucleation layer was grown first on the substrate, followed by a 0.5-μm-thick GaN buffer layer and a 2-μm-thick Si-doped n-type GaN layer. This preliminary structure serves as a GaN template for further growth. Analysis of the GaN template is as follow. The X-ray diffraction (XRD) ω(002) and ω(102) rocking curves scan indicated a TD density of approximately 4.4 × 10 8 cm −2 . The atomic force microscopy (AFM) surface morphology scan of the GaN template in a 5 × 5 μm 2 area exhibited a root mean square (RMS) roughness of 0.4 nm. Four-period MQWs were then grown on the GaN template, consisting of 2.7-nm-thick InGaN quantum well layers and 12.5 nm-thick GaN barrier layers, as was confirmed by XRD. Detailed growth conditions are described elsewhere. 5 The growth temperature was also tuned to achieve a target PL peak wavelength of ∼449 nm, corresponding to an indium composition of ∼14%. Figure 1 shows the system diagram of low-temperature PL system. A Verdi-G10-semiconductor-laser pumped laser system, generating 400 nm CW laser beam, was used as excitation source. The laser beam, after transmitting through an optical fib...