Yung-Chen Cheng

Dependence of composition fluctuation on indium content in InGaN/GaN multiple quantum wells

The information on the variations of indium composition, aggregation size, and quantum-well width is crucially important for understanding the optical properties and, hence, fabricating efficient light-emitting devices. Our results showed that spinodal decomposition could occur in InGaN/GaN multiple quantum wells with indium content in the range of 15%–25% (grown with metal–organic chemical-vapor deposition). A lower nominal indium content led to a better confinement of indium-rich clusters within InGaN quantum wells. The InGaN/GaN interfaces became more diffusive, and indium-rich aggregates extended into GaN barriers with increasing indium content. It was also observed that indium-rich precipitates with diameter ranging from 5 to 12 nm preferred aggregating near V-shaped defects.

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.

Nanostructures and carrier localization behaviors of green-luminescence InGaN/GaN quantum-well structures of various silicon-doping conditions

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.

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.

Excitation power dynamics of photoluminescence in InGaN/GaN quantum wells with enhanced carrier localization

Excitation-power dynamics of near-band-edge photoluminescence (PL) peak position in InxGa1−xN∕GaN multiple quantum wells (x∼0.15) was analyzed as a function of well width. The analysis was based on energy reference provided by photoreflectance (PR) spectra. The difference in spectral position of the PR feature and low-excitation PL band (the Stokes Shift) revealed carrier localization energy, which exhibited a remarkable sensitivity to the well width, increasing from 75meV in 2nm wells to about 250meV in 4nm wells. Meanwhile collating of the PR data with the flat-band model for the optical transition energy in quantum wells rendered a relatively weak (0.5MV∕cm) built-in piezoelectric field. The blueshift of the PL peak position with increasing photoexcitation power density was shown to be in qualitative agreement with the model of filling of the band-tail states with some contribution from screening of built-in field in the thickest (4nm) wells. Increased incident photon energy resulted in an additional b...

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.

Low-Threshold Stimulated Emission in ZnO Thin Films Grown by Atomic Layer Deposition

In this study, high-quality ZnO thin films were grown on sapphire substrates by atomic layer deposition (ALD), followed by high-temperature postdeposition annealing. A thin Al2O3 layer was subsequently deposited by ALD on the ZnO surface to reduce detrimental surface states. Photoluminescence measurements conducted in a backscattering configuration at room temperature show that the ZnO film exhibited stimulated emission with a low threshold intensity of 35.1 kW/ cm2. This may be attributed to the high-quality ZnO film and Al2O3 surface passivation layer grown by ALD, as well as the Al doping effect caused by the thermal diffusion of Al from the sapphire into the ZnO. Results show that ZnO films grown by the ALD technique are applicable to next-generation short-wavelength photonic devices.

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...