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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....

We compared the temperature dependent spectral variations of the amplified spontaneous emission (ASE) between InGaN/GaN quantum well samples of no doping, well doping, and barrier doping of silicon. The comparisons were particularly made between two series of samples with a low and a high indium content. The results show that a multi-peak ASE spectral feature and a low energy stimulated emission peak, existing at the photoluminescence shoulder, could be observed only in the high-indium-content, barrier-doped sample. Such results are supposed to originate from the formation of quantum dots of various sizes, concentrations, and shapes under the condition of barrier doping in the sample. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We compared the optical and material properties of two high-indium InGaN thin layers between the as-grown and post-growth thermally annealed conditions. In one of the samples, the major part of photoluminescence spectrum was shifted from the original yellow band into the blue range upon thermal annealing. Cathodo-luminescence (CL) spectra showed that the spectral shift occurred essentially in the photons emitted from a shallow layer of the InGaN film. The deeper layer in the as-grown film contributed blue emission because it had been thermally annealed during the growth of the shallow layer. The spectral shift was mainly attributed to the shrinkage of cluster size through spinodal decomposition upon thermal annealing. The attribution was supported by the observations in the CL, X-ray diffraction (XRD), time-resolved photoluminescence (TRPL) and, high–resolution transmission electron microscopy (HRTEM) results. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Material and optical analyses of three InGaN/GaN quantum well (QW) samples with different silicon doping conditions were conducted. Quantum dot (QD) structures were observed in samples of silicon doping either in barriers or wells. Optical characterization indicated that quantum-confined Stark effect (QCSE) has been tremendously reduced, particularly in the barrier-doped sample. However, contrary to the interpretation in the past, the major mechanism for QCSE reduction is due to strain relaxation, instead of carrier screening, in conjunction with the formation of QD structures. Such a conclusion is supported by the smaller change of Stokes shift upon silicon doping in wells. In the well-doped sample, besides strain relaxation, enhanced carrier localization might represent another important mechanism for photon emission improvement.

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.

Size and distribution of indium-rich quantum dots (QDs) are important parameters for improving photon emission efficiency of InGaN/GaN quantum well (QW) structures. Our results showed that post-growth thermal annealing of such a sample with temperature ranging from 800°C to 900°C led to a better confinement of indium-rich clusters near InGaN QW layers. Transmission electron microcopy (TEM) and energy filter TEM results manifested that the sizes of indium-rich QDs were reduced with increasing annealing temperature. Also, the size homogeneity was improved. Quasi-regular arrays of indium-rich QDs embedded in InGaN QWs were observed in the sample of 900°C annealing. X-ray diffraction also showed the enhancement of InN relative intensity. Photoluminescence measurements revealed blue shifts of photon emission spectral peak, indicating stronger quantum confinement after thermal annealing. However, such a process of regular QD formation disappeared when annealing temperature was increased to 950°C. In this situation, coarsening of indium-rich clusters occurred and their distribution became irregular.

Temperature-dependent femtosecond pump–probe experiments are performed to explore the ultrafast carrier-relaxation processes in an InGaN∕GaN quantum-well sample, in which nanometer-scale cluster structures have been identified. Combined with the time-resolved photoluminescence results, we can identify three stages of carrier relaxation. The fast-decay time, ranging from several hundred femtoseconds to 1 picosecond, 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 picoseconds, 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 t...

The results of photoluminescence (PL), detection-energy-dependent photoluminescence excitation (DEDPLE), excitation-energy-dependent photoluminescence (EEDPL), and strain state analysis (SSA) of three InGaN/GaN quantum-well (QW) samples with silicon doping in the well, barrier and an undoped structure are compared. The SSA images show strongly clustering nanostructures in the barrier-doped sample and relatively weaker composition fluctuations in the undoped and well-doped samples. Differences in silicon doping between the samples give rise to the differences in DEDPLE and EEDPL spectra, as a result of the differences in carrier localization. In addition, the PL results provide us clues for speculating that the S-shaped PL peak position behavior is dominated by the quantum-confined Stark effect in an undoped InGaN/GaN QW structure.

Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content (19%) was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence (PL) measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.

193nm-ArF excimer laser irradiation of amorphous silicon (a-Si) thin film has great potential on the production of poly-crystalline silicon (poly-Si) thin film transistor liquid crystal displays (TFT-LCDs). The main reason for applying poly-Si thin films instead of a-Si thin films on fabricating high performance electronics devices is due to its higher electron mobility, which is strongly influenced by the film's surface morphology, grain size, microstructure and defect density. The specimens used in this study have a 100 nm-thick a-Si thin films deposited on glass substrate by plasma enhanced chemical vapor deposition (PECVD) technique. The effects of annealing parameters, such as shot number, repetition rate and the fluence, on the surface morphology and recrystallization behaviour of a-Si thin films were investigated. The results showed that the threshold fluence of partial melting of a-Si thin films at 1Hz and single shot was around 150 mJ/cm 2 . Further increasing the fluence and shot numbers leads to the formation of microvoids trapped inside poly-Si film due to the evolution of hydrogen gas during the laser annealing process. The surface morphology formed during the recrystallization of amorphous Si thin films is found depending upon the laser fluence and shot numbers. The super-lateral grown (SLG) poly-Si grain can be obtained at the fluence around 190 mJ/cm 2 .

Coupling with HE/sub 2/spl mu// modes resulted in a coupling coefficient variation period the same as that of micro bending. However, the coupling with HE/sub 1/spl mu// modes led to a period only one-half the micro bending period.

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.

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.

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.

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 10 18 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.

It is shown that post-growth thermal annealing of such a sample with temperature ranging from 800 to 900 °C led to a better confinement of indium rich clusters near InGaN quantum well layers. Transmission electron microcopy (TEM) and energy filter TEM results manifested that the sizes of indium-rich QDs were reduced with increasing annealing temperature. Also, the size homogeneity was improved. Quasi-regular arrays of indium-rich QDs embedded in InGaN quantum wells were observed in the sample of 900 °C annealing. X-ray diffraction also showed the enhancement of InN relative intensity. Photoluminescence measurements revealed blue shifts of photon emission spectral peak, indicating stronger quantum confinement after thermal annealing.

Periodical perturbations along an optical fiber can cause power coupling between the core mode and cladding modes for the applications of spectral filtering and derivative operations. Such perturbations can be generated through periodical loading on fiber. By applying loading onto fiber with two face-to-face, identical periodical groove structures, it was found that the long-period grating effects were dependent on the relative phase of the two periodical corrugations. Particularly, when the relative phase was zero (crest-to-crest), spectral filtering effects disappeared completely. The comparisons of such results between the cases of jacketed and un-jacketed fibers led to the conclusion that geometric deformation, instead of direct pressure-induced effect, was the dominating mechanism for generating spectral filtering functions in the double-sided loading configuration. The same conclusion can be applied to the single-sided loading device.

Summary form only given.Indium aggregation and phase separation due to the lattice mismatch between GaN and InN lead to the formation of localized states. It was widely accepted that the photoluminescence (PL) in InGaN samples and the output from a light-emitting diode came from the recombination of localized excitons. Several stimulated emission (SE) studies have led to the conclusion that their measured stimulated emission (SE) also originated from band-filled localized states. In this paper, we report a two-peak feature of SE from InGaN-GaN quantum well samples.

We compared the optical properties and material nanostructures between several InGaN∕GaN multiple quantum-well (QW) samples of different interfacial layers. In some of the samples, InN interfacial layers were inserted between the wells and barriers to improve the QW quality and hence the light-emission efficiency. Compared with a widely used barrier-doped QW structure, the insertions of the InN interfacial layers (silicon doped or undoped) do enhance the photon emission efficiencies. Of the two samples with InN interfacial layers, the one with intrinsic InN interfacial layers had the higher photoluminescence (PL) and electroluminescence (EL) efficiencies. Cluster structures are clearly observed in this sample, resulting in strong carrier localization. In this sample, we also observed a temperature-dependent S-shape variation in the PL spectral peak, a strong photoluminescence excitation (PLE) intensity, and a steep PL decay time variation beyond its peak as a function of temperature. On the other hand, bo...

Periodical perturbations along an optical fiber can cause power coupling between the core and cladding modes for the applications of spectral filtering and derivative operations. Such perturbations can be generated through periodical loading on fiber. By applying loading onto a fiber with two face-to-face, identical groove structures, it was found that the long-period grating effects were dependent on the relative phase of the two periodical corrugations. Particularly, when the relative phase was zero (crest to crest), spectral filtering effects disappeared completely. The comparisons of such results between the cases of jacketed and unjacketed fibers led to the conclusion that geometric deformation, instead of direct pressure-induced effects, was the dominating mechanism for generating spectral filtering functions in the double-sided loading configuration. The same conclusion can be applied to a single-sided loading device.