En-Chiang Lin

Cluster size and composition variations in yellow and red light-emitting InGaN thin films upon thermal annealing

We study thermal annealing effects on the size and composition variations of indium-aggregated clusters in two InGaN thin films with photoluminescence (PL) in the yellow and red ranges. The methods of investigation include optical measurement, nanoscale material analysis, and theoretical calculation. Such a study is important for determining the relation between the band gap and the average indium content of InGaN. In one of the samples, the major part of the PL spectrum is shifted from the yellow band into the blue range upon thermal annealing. In the other sample, after thermal annealing, a broad spectrum covering the whole visible range is observed. Cathodo-luminescence (CL) spectra show that the spectral changes occur essentially in the photons emitted from the shallow layers of the InGaN films. Photon emission spectra from the deeper layers are essentially unaffected by thermal annealing. The spectral changes upon thermal annealing are mainly attributed to the general trend of cluster size reduction....

Quantum-well-width dependencies of postgrowth thermal annealing effects of InGaN/GaN quantum wells

Optical measurements of temperature-dependent photoluminescence (PL) spectral peak, integrated PL intensity and PL decay time, and microstructure analyses with high-resolution transmission electron microscopy showed the strong dependencies of thermal annealing effects on quantum well (QW) width in InGaN/GaN QW structures. With different QW widths, different levels of strain energy were built. Upon thermal annealing, energy relaxation resulted in the reshaping of quantum dots and hence the changes of optical properties. Thermal annealing at 800 °C of a narrow QW width (2 nm) structure led to regularly distributed quantum dots (QDs) and improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) did not show QD formation. In this situation, even higher local strains around QWs were speculated. Also, degraded optical quality was observed.

Thermal annealing effects on an InGaN film with an average indium mole fraction of 0.31

We compared the optical and material properties of an InGaN thin film with an average indium content at 0.31 between as-grown and postgrowth thermally annealed conditions. The major part of the photoluminescence spectrum was shifted from the original yellow band into the blue range upon thermal annealing. Cathodoluminescence (CL) spectra showed that the spectral shift occurred essentially in a shallow layer of the InGaN film. The deeper layer in the as-grown sample contributed blue emission because it had been thermally annealed during the growth of the shallow layer. The spectral change was attributed to the general trends of cluster size reduction and possibly quantum-confined Stark effect relaxation upon thermal annealing. The attribution was supported by the observations in the CL, x-ray diffraction, and high-resolution transmission electron microscopy results.

Quantum dot structures and their optical properties of a high-indium InGaN film

Yellow luminescence from an InGaN film of high indium content shifted into blue emission upon thermal annealing. The shift was attributed to the quantum dot-like cluster size reduction through spinodal decomposition at thermal annealing.

Femtosecond pump-probe studies on carrier dynamics in InGaN/GaN quantum wells with indium aggregated quantum dot structures

Temperature-dependent pump-probe measurements were conducted for observing the process of carrier relaxation into localized states of quantum dots, which were formed through indium aggregation in InGaN/GaN quantum well structures of various parameters.

Dependencies of optical and material properties on nominal indium content and well width in InGaN/GaN quantum well structures

Optical properties and material microstructures of InGaN/GaN quantum wells structures with various nominal indium contents, quantum well widths, and different thermal annealing conditions were compared to show the effects of indium aggregations and strains.

Quantum dot formation with silicon doping in InGaN/GaN quantum well structures and its implications in radiative mechanisms

Optical properties and material microstructures of three InGaN/GaN quantum well (QW) samples with various silicon-doping concentrations in barriers were measured. From the high-resolution transmission electron microscopy images, quantum dots (QDs) of a few nm in size were observed in silicon-doped samples. The regularities of QDs in size, shape and distribution increased with doping concentration up to 5 x 1018 cm-3. Such observations implied that the reduction of quantum-confined Stark effect in such a sample was due to the relaxation of strain energy in QDs with silicon doping, besides the carrier screen effect. In other words, the microstructures were crucially changed with silicon doping in barriers. Also, the carrier localization effect was actually enhanced although potential fluctuation indeed became less randomly distributed. The calibrated radiative lifetimes in both silicon-doped samples showed the consistent trend of the formation of 0-D structure upon silicon doping.

Indium aggregated quantum dot structures in InGaN compounds

We will summarize the optical characteristics and microstructures of the indium-aggregated quantum dots in InGaN compounds, including intended InGaN/GaN quantum well structures and InGaN epi-layers. In quantum well structures, the dependencies of optical properties and material structures on well width and nominal indium content are to be discussed. Also, the effects of silicon doping will be reported. Furthermore, the effects of post-growth thermal annealing on the formation of quantum dot-like structures and their variations of optical properties will be presented. In InGaN epi-layers, we have studied the optical and material properties of such films with high indium contents. From the images of cathodo-luminescence and high-resolution transmission electron microscopy, quantum dot structures could be observed. After thermal annealing, the original yellow emission became blue in color in one of the samples. In the other sample, the original red emission became the combination of red, yellow and blue photons, leading to white in color.