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We compared the nano-structures of three samples of ZnO thin film on sapphire under different growth temperature conditions. Although disconnected domain structures (on the scale of 100 nm in size) were observed in the samples of high-temperature (450 °C) growth, their crystal quality is generally better than the one grown at a low temperature (200 °C), either near or away from the sapphire interface. Lattice misfits and threading dislocations were observed within a domain with the separation of around 8 nm. The sample grown at the low temperature showed a continuous structure through the ZnO layer although void-like structures might exist inside. However, its crystal quality is relatively poorer. Of the two samples with high-temperature growth, the one with initial low-temperature growth had a larger domain structure (around 150 nm in size) and relatively lower crystal quality. In particular, strong strains existed near the interface of this sample. The samples of high-temperature growth generally have higher photon emission efficiencies. Temperature-dependent integrated photoluminescence intensities of the high-temperature-growth samples show that the exciton trapping by either intrinsic donors or acceptors leads to a higher thermal quenching rate in comparison with free excitons.

We study the depth-dependent variation of optical property beyond the critical thickness in an InGaN thin film. In the sample, both free-carrier and localized-state recombination activities are observed. The emission peak corresponding to the localized states in photoluminescence (PL) measurement becomes more prominent with increasing sample depth, implying stronger clustering in the deeper layers. Although the PL spectral peak variation is weak, that of cathodoluminescence (CL), corresponding to the activities of the localized states, shows a clear red shift trend with depth. The red shift trend is attributed to the stronger clustering behavior and possibly stronger quantum-confined Stark effect in the nano-clusters, which is due to the residual strain beyond the critical thickness, in a deeper layer.

We compare the results of strain state analysis (SSA) and photoluminescence (PL) of six InGaN/GaN quantum well samples with un-doped, well-doped, and barrier-doped structures. Based on the SSA images, a strain relaxation model is proposed for describing the nanostructure differences between the three sets of sample of different doping conditions. In the barrier-doped samples, the hetero-structure-induced strains are fully relaxed such that spinodal decomposition is effectively induced. Therefore, strongly clustering nanostructures are observed. In the well-doped samples, strains are partially relaxed and the spinodal decomposition process can be slightly induced. Hence, weaker composition fluctuations are observed. Then, in the un-doped samples, the un-relaxed strains result in higher miscibility between InN and GaN, leading to the relatively more uniform composition distributions. Between the low- and high-indium samples, higher indium content leads to a stronger clustering behavior. The strain relaxations in the well-doped and barrier-doped samples result in their unclear S-shaped behaviors of PL spectral peaks. The enhanced carrier localization and reduced quantum-confined Stark effect in the barrier-doped samples are responsible for their significant increases of radiative efficiency.

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.

The microstructure variations upon post-growth thermal annealing at different temperatures of InGaN/GaN quantum well (QW) structures of different well widths are demonstrated. The high-resolution transmission electron microscopy images showed that with a small QW width (2 nm), thermal annealing tended to form quantum dots (QDs) of larger sizes (3–5 nm) to cover the designated QW layers. In this situation, strain might be well relaxed and quantum-confined Stark effect (QCSE) was reduced, resulting in a higher radiative efficiency. Also, with the formed QD structures, the expected stronger carrier localization (CL) effect might make a significant contribution to the enhanced radiative efficiency. On the other hand, with a large well width (4 nm), thermal annealing tended to make the QW boundaries clearer. In this situation, it seemed that either QCSE was enhanced or CL was reduced, resulting in degraded optical quality.

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. The calibrated-radiative lifetimes in both silicon-doped samples showed the consistent trend of the formation of zero-dimensional (0-D) structure upon silicon doping. In optical characterization, the barrier-doped sample showed a blueshift of the photoluminescence (PL) peak, enhancement of integrated PL intensity, reduction of Stokes shift (SS), decrease of carrier-activation energy, and shortening of PL decay time. Except the insignificant PL peak shift, the well-doped sample showed similar trends, although they are not as prominent as the barrier-doped sample. Such results mainly originated from the reduction of the quantum-confined Stark effect (QCSE) within the clusters. 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 result of smaller changes of optical behavior in the well-doped sample, in which carrier screening is expected to be more significant. In this sample, besides strain relaxation, enhanced carrier localization (CL) might represent another important mechanism for photon-emission improvement.

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.

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.

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.

We report the study results of an InGaN/GaN multiple quantum well structure with a nominal indium content of 25%. The high-resolution transmission electron microscopy and x-ray diffraction show clear indium aggregation and phase separation. Stimulated emission data always show two major peaks in spectrum. The long- (short-) wavelength peak is assigned to the recombination of localized state carriers (free carriers). At low temperatures or optical pump levels, the localized-state recombination dominates the stimulated emission; however, at high temperatures or pump levels, the free-carrier recombination becomes dominant. The peak position corresponding to localized states changes little in spectrum as temperature or pump level varies. This result is attributed to carrier overflow, strain relaxation, and carrier shielding in increasing temperature or carrier supply.

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 .

Diamond, having many advanced physical and mechanical properties, is one of the most important materials used in the mechanical, telecommunication and optoelectronic industry. However, high hardness value and extreme brittleness have made diamond extremely difficult to be machined by conventional mechanical grinding and polishing. In the present study, the microwave CVD method was employed to produce epitaxial diamond films on silicon single crystal. Laser ablation experiments were then conducted on the obtained diamond films. The underlying material removal mechanisms, microstructure of the machined surface and related machining conditions were also investigated. It was found that during the laser ablation, peaks of the diamond grains were removed mainly by the photo-thermal effects introduced by excimer laser. The diamond structures of the protruded diamond grains were transformed by the laser photonic energy into graphite, amorphous diamond and amorphous carbon which were removed by the subsequent laser shots. As the protruding peaks gradually removed from the surface the removal rate decreased. Surface roughness (Ra) was improved from above 1μm to around 0.1μm in few minutes time in this study. However, a scanning technique would be required if a large area was to be polished by laser and, as a consequence, it could be very time consuming.

Summary form only given. The formation of quantum-dot structures will induce strain energy in the quantum well layer, optical measurements of photoluminescence (PL) spectral peak and microstructure analyses with high-resolution transmission electron microscopy (HRTEM) showed the strong dependency on quantum well (QW) width in InGaN/GaN QW structures. Quantum-dot-like structures are formed because of the large lattice mismatch between InN and GaN, which will result in potential fluctuations and hence the effect of carrier localization (CL). Due to the strain energy, quantum dot structures will also be built during the growth of InGaN QW layers. Therefore, the designated QW width becomes a crucial factor in cluster formation and the consequent photon emission characteristics. In this study, we had done our first time to calculate the strain energy distribution around the QDs. Due to the higher strain energy built in the QW layer, the QDs structures are clearly observed on 4 nm well widths (w40) sample. High strain energy is expected in w40, due to the higher strain energy might be relaxed through the formation of thread dislocations or local lattice distortions in the QW layers, and they also led to quite poor photon emission efficiency. PL spectral peak variations with temperature of the three samples under various thermal annealing conditions. After thermal annealing, the S-shape variation of PL peak is maintained in each case. Thermal annealing results in blue shifts of PL peak in w20 and red shifts in w30 and w40. At 3 nm well widths, more gathering of indium structures appear in barriers and again affected the distribution of strain energy and the atomic-scale HRTEM images of sample. However, especially at 4 nm well widths, because the larger well width leads to larger strain energy, spinoidal decomposition becomes less effective. In this situation, the strain energy might be relaxed through the formation of more stacking faults or local lattice distortions. Stacking faults and local distortions were easily observed in atomic-scale HRTEM images of the sample. In conclusion, we have shown different level distribution of strain energy observed in each sample of different QW widths.

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.

We compared the results of optical characterization between five InGaN/GaN quantum well samples of different well widths. Temperature dependencies of photoluminescence (PL) spectral positions, integrated PL intensities, and PL intensity decay times at PL peaks of all the five samples showed three temperature ranges of different variation trends. The radiative efficiencies of the samples in the high temperature range had the same decay slope, which is supposed to be determined by the defect structures outside clusters. The radiative efficiencies in the medium temperature range varied among samples, indicating different defect structures in the regions between coupled clusters in different samples. Consistent results of temperature dependent variations between the integrated PL intensity and PL decay time among these samples provided clues for reasonable interpretations. Also, we showed the strong dependencies of thermal annealing effects on quantum well (QW) width in InGaN/GaN QW structures. Thermal annealing at 800 °C of a narrow QW width (2 nm) structure led to improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) resulted in degraded optical quality.

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 perform femtosecond degenerate pump-probe experiments on an InGaN thin film of 800 nm in thickness. The observed temperature-, pump-photon-energy-, and pump-intensity-dependent variations of ultrafast carrier dynamics manifest the variation of the space-averaged density of state with energy level in this sample. The carrier dynamics is controlled by the shift of effective band gap and hence the behavior of band filling, which are determined by the combined effect of band-gap renormalization and phonon effect (band-gap shrinkage with increasing temperature). Two-photon absorption and free-carrier absorption can be observed when the corresponding density of state is low and hence the band-filling effect is weak. The variation of the space-averaged density of state with energy level can be due to the existence of indium-composition-fluctuation nanostructures, which is caused by the spinodal decomposition process, in the sample.

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.