Kung-Jen Ma

Quasiregular quantum-dot-like structure formation with postgrowth thermal annealing of InGaN'GaN quantum wells

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.

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.

Carrier relaxation in InGaN∕GaN quantum wells with nanometer-scale cluster structures

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...

Effects of post-growth thermal annealing on the indium aggregated structures in InGaN/GaN quantum wells

Abstract 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.

Stimulated emission study of InGaN/GaN multiple quantum well structures

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.

Ultrafast carrier dynamics in an InGaN thin film

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.

Long-period fiber grating effects induced by double-sided loading

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.

Investigation of laser ablation of CVD diamond film

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.