Blue light-emitting diodes (LED's), utilizing InGaN-based multi-quantum well (MQW) active regions deposited by organometallic chemical vapor epitaxy (OMVPE), are one of the fundamental building-blocks for current solid-state lighting applications. Studies [1,2] have previously been conducted to explore the optical and physical properties of the active MQW's over a variety of different OMVPE growth conditions. However, the conclusions of these papers have often been contradictory, possibly due to a limited data set or lack of understanding of the fundamental fluid dynamics and gas-phase chemistry that occurs during the deposition process.Multi-quantum well structures grown over a range of pressures from typical low-pressure production processes at 200 Torr, up to near-atmospheric growth conditions at 700 Torr, have been investigated in this study. At all growth pressures, clear trends of gas-phase chemical reactions are observed for increased gas residence times (lower gas speeds from the injector flange and lower rotation rates) and increased V/III ratios (higher NH 3 flows).Confocal microscopy, excitation-dependent PL (PLE), and time-resolved photoluminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Confocal emission images show spatially-separated bright and dark regions. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting microscopic spatial localization of high indium content regions. As the growth pressure and gas residence times are reduced, a larger difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities can be obtained, indicating that higher optical quality material can be realized. Optimized MQW's grown at high pressure exhibit higher PLE slope intensities and IQE characteristics than lower pressure samples. Results on simple LED structures indicate that the improvement in MQW optical quality at high pressures translates to higher output power at a 110 A/cm 2 injection current density. 505

Confocal laser scanning microscopy and time-resolved photoluminescence (TRPL) spectroscopy were used to study blue-emitting InGaN/GaN multiple quantum wells. Spatial and spectral variations of photoluminescence (PL) were observed over submicron-scale regions. Spectral measurements showed that the bright regions have a higher PL intensity as well as smaller peak energy than the dark regions. Correlations among the bright region-dark region PL peak energy difference, the average PL intensity, the PL FWHM, the bright region PL intensity, and the extent of PL intensity fluctuation were observed. As the energy difference increased, the average PL intensity, the PL FWHM, and the bright region PL intensity increased, with a higher degree of areal PL intensity fluctuations. TRPL measurements and calculations showed that the effective PL lifetime at bright regions was longer than that at dark regions, and bright region lifetime increases as energy difference increases, possibly as a result of stronger confinement.

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...

Scanning confocal microscopy is used to study blueemitting Indium Gallium Nitride (InGaN)/Gallium Nitride (GaN) multi-quantum wells grown by metal-organic chemical vapor deposition under different growth conditions. Sub-micrometer scale spatial and spectral variation of photoluminescence (PL) has been observed. Spectrum measurement shows the PL peak in bright region is red-shifted comparing with that in dark region, and that the peak intensity of bright region is at least twice as strong as that of dark region. Images show defect luminescence features which are about 500 nm in diameter and have PL peak at around 550 nm. Experiments show that reducing In/Ga ratio, increasing growth pressure and increasing NH 3 flow rate can all increase the localization effect and result in the increase of sample average PL intensity. Moreover, average PL intensity increases with the increasing of bandgap difference and PL peak intensity difference between bright and dark regions in PL.