Yi-Yin Chung

Impact of localized states on the recombination dynamics in InGaN/GaN quantum well structures

Multiple-component decays of photoluminescence (PL) in InGaN/GaN quantum wells have been widely reported. However, their physical interpretations have not been well discussed yet. Based on wavelength-dependent and temperature-varying time-resolved PL measurements, the mechanism of carrier transport among different levels of localized states (spatially distributed) in such an indium aggregated structure was proposed for interpreting the early-stage fast decay, delayed slow rise, and extended slow decay of PL intensity. Three samples of the same quantum well geometry but different nominal indium contents, and hence different degrees of indium aggregation and carrier localization, were compared. The process of carrier transport was enhanced with a certain amount of thermal energy for overcoming potential barriers between spatially distributed potential minimums. In samples of higher indium contents, more complicated carrier localization potential structures led to enhanced carrier transport activities. Free ...

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.

Multiple-component photoluminescence decay caused by carrier transport in InGaN/GaN multiple quantum wells with indium aggregation structures

Based on wavelength-dependent and temperature-varying time-resolved photoluminescence (PL) measurements, the mechanism of carrier transport among different levels of localized states (spatially distributed) in an InGaN/GaN quantum well structure was proposed for interpreting the early-stage fast decay, delayed slow rise, and extended slow decay of PL intensity. The process of carrier transport was enhanced with a certain amount of thermal energy for overcoming potential barriers between spatially distributed potential minimums. With carrier supply in the carrier transport process, the extended PL decay time at wavelengths corresponding to deeply localized states can be as large as 80 ns.

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.

Strong green luminescence in quaternary InAlGaN thin films

We observed strong green luminescence (around 2.37 eV) in two InAlGaN thin film samples, which were originally prepared for applications in the UV range. Based on the observation of the InN peaks and the extended distributions, corresponding to InGaN compounds, in the x-ray diffraction patterns, such green luminescence was attributed to the formation of indium-rich clusters, which formed localized states, in the quaternary films of low indium content (2%). Carrier trapping of such localized states, particularly when carriers received thermal energy such that they could overcome certain potential barriers around the clusters, resulted in strong green luminescence. The observed optical behaviors, including the blueshift phenomenon of photoluminescence spectral peak position in varying temperature, showed that the green luminescence was quite different from the previously reported yellow luminescence, which resulted from defects.

Two-component photoluminescence decay in InGaN/GaN multiple quantum well structures

Two-component decay of time-resolved photoluminescence (TRPL) intensity in three InGaN/GaN multiple quantum well samples were observed. The first-decay component was attributed to exciton relaxation of free-carrier and localized states; the second-decay one was dominated by the relaxation of localized excitons. The second-decay lifetime was related to the extent of carrier localization or indium aggregation and phase separation. The lifetime of free-carrier states was connected with the defect density. Based on the temperature-dependent data of PL and stimulated emission (SE), the localization energies of the three samples were calibrated to show the consistent trend with the second-decay lifetime and previous material analyses.

Microstructure studies of InGaN/GaN multiple quantum wells

Material microanalyses of InGaN/GaN quantum well structures revealed better confinement of indium-aggregated clusters within wells in samples of lower indium contents and indium-rich precipitates aggregating near V-shape defects.

Carrier dynamics in InGaN/GaN multiple quantum well structures

We report the fast and slow decay lifetimes of multi- component photoluminescence (PL) intensity decays in the time-resolved photoluminescence measurements at the room temperature and a low temperature. The fast decay component was essentially due to carrier dynamics, that is, carrier flow between strongly localized and weakly localized states. Such a carrier relaxation process results in extremely long PL decay time for strongly localized states at the low temperature. At room temperature, because of thermal energy and hence carrier escape form strongly localized states, effective lifetimes becomes stronger.

Optical characteristics of InGaN/GaN quantum well structures with embedded quantum dots

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.

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.