Hsiang-Chen Wang

Temperature-dependent exciton dynamics in a ZnO thin film

We measured the temperature-dependent photoluminescence (PL) decay time of the mixed system of free exciton (FX) and donor-bound exciton (DX0) in a ZnO thin film. From the measured cw PL spectra, we also calibrated the individual spectral widths and integrated PL intensities of FX and DX0 by fitting the spectra with four Lorentzian functions. The radiative lifetime of the mixed system was then calibrated based on the thermal quenching rate of the total integrated PL intensity of the system. With the system radiative lifetime data, FX radiative lifetimes were estimated by using a theoretical relation between the lifetime and the spectral width. From the results of FX radiative lifetime, we calibrated the DX0 radiative lifetimes by considering the emission intensity ratio between FX and DX0. The results support our model that the DX0 radiative behavior is similar to that of FX when the thermal energy is smaller than the donor binding energy.

Optimal lighting of RGB LEDs for oral cavity detection

In this paper the optimal lighting for oral cavity detection is proposed. The illuminants consist of several LEDs with different intensity ratios and peak wavelengths, which can enhance the color difference between normal and abnormal regions in the oral cavity. An algorithm combined with multi-spectral imaging (MSI) and color reproduction technique is applied to find the best enhancement of this difference. The colored LEDs of the optimal lighting, the Color Rendering Index (CRI) of the illuminants, and comparison with traditional illuminants are discussed. The calculations show that color enhancement ability in the oral cavity is not entirely a function of the higher CRI of some illuminants, as the narrowband illuminants (LEDs) produce an image with greater contrast than the broadband spectra and higher CRI of traditional illuminants in the reddish oral environment. Accordingly, an illuminant with specific intensity ratio of red, green, and blue LEDs is proposed, which has optimal color enhancement for oral cavity detection. Compared with the fluorescent lighting commonly in the use now, the color difference between normal and inflamed tissues can be improved from 21.5732 to 30.5532, a 42% increase, thus making medical diagnosis more efficient, so helping patients receive early treatment.

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.

Visual perception enhancement for detection of cancerous oral tissue by multi-spectral imaging

Color reproduction systems based on the multi-spectral imaging technique (MSI) for both directly estimating reflection spectra and direct visualization of oral tissues using various light sources are proposed. Images from three oral cancer patients were taken as the experimental samples, and spectral differences between pre-cancerous and normal oral mucosal tissues were calculated at three time points during 5-aminolevulinic acid photodynamic therapy (ALA-PDT) to analyze whether they were consistent with disease processes. To check the successful treatment of oral cancer with ALA-PDT, oral cavity images by swept source optical coherence tomography (SS-OCT) are demonstrated. This system can also reproduce images under different light sources. For pre-cancerous detection, the oral images after the second ALA-PDT are assigned as the target samples. By using RGB LEDs with various correlated color temperatures (CCTs) for color difference comparison, the light source with a CCT of about 4500 K was found to have the best ability to enhance the color difference between pre-cancerous and normal oral mucosal tissues in the oral cavity. Compared with the fluorescent lighting commonly used today, the color difference can be improved by 39.2% from 16.5270 to 23.0023. Hence, this light source and spectral analysis increase the efficiency of the medical diagnosis of oral cancer and aid patients in receiving early treatment.

Effect of catalyst morphology on the quality of CVD grown graphene

The strong interest in graphene has motivated large effort in the scalable production of high-quality material. The potential of chemical vapor deposition on Cu foil to produce such graphene is impeded by lacking understanding of the relation between catalyst properties and graphene performance. We here present a systematic analysis of the catalyst morphology and its effect on electrical properties of graphene. We find that nanometer sized particles increase the density of bilayer regions but have no significant effect on carrier transport. Long wavelength roughness (waviness), on the other hand, generates defective graphitic regions that deteriorate carrier mobility. These findings shed light on the graphene formation process on Cu substrates and open a route to improve graphene quality for electronics applications.

Crystallinity improvement of ZnO thin film on different buffer layers grown by MBE

The material and optical properties of ZnO thin film samples grown on different buffer layers on sapphire substrates through a two-step temperature variation growth by molecular beam epitaxy were investigated. The thin buffer layer between the ZnO layer and the sapphire substrate decreased the lattice mismatch to achieve higher quality ZnO thin film growth. A GaN buffer layer slightly increased the quality of the ZnO thin film, but the threading dislocations still stretched along the c-axis of the GaN layer. The use of MgO as the buffer layer decreased the surface roughness of the ZnO thin film by 58.8% due to the suppression of surface cracks through strain transfer of the sample. From deep level emission and rocking curve measurements it was found that the threading dislocations play a more important role than oxygen vacancies for high-quality ZnO thin film growth.

Enhanced visualization of oral cavity for early inflamed tissue detection

We describe a color image reconstruction method that enables both direct visualization and direct digital image acquisition from one oral tissue by using various light sources and color compensating filters. In this method, the image of the oral tissue with white light emitting diodes (LEDs) with blue color compensating filter has a larger color difference between the normal and inflamed tissues. The enhanced visualization comes from the white light color mixing between the red normal tissue and bluish white light from the LEDs. With our method, we evaluate the perceived tissue reflectance in each pixel of the image and color reproduction with different illuminated spectra. Our approach to enhancement of visually perceived color difference between normal and inflamed oral tissue involves optimization of illumination and observation conditions by allowing a significant optical contrast of illuminated spectrum to reach the observers eyes. In comparison with a conventional daylight LED flashlight, a LED with blue filter as the illuminant for oral cavity detection enhances the color difference between normal and inflamed tissues by 32%.

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...

Synthesis of CIGS thin film by solvothermal route

This study presents the synthesis of copper indium gallium (di)selenide (CIGS) films by a solvothermal method. Four factors in CIGS synthesis are considered: In/Ga ratios, hydrogen contents during thermal annealing, thermal annealing temperatures, and annealing times. Experimental results show that the optimal parameters for CIGS film synthesis are the following: proportion of Cu:In:Ga:Se = 1:0.7:0.3:2; hydrogen content during thermal annealing, 5%; thermal annealing temperature, 600 °C; and annealing time, 100 min. The largest crystal grain size of a CIGS film synthesized using these optimal parameters is about 100 nm. The crystal grain size is also found to be inversely proportional to sheet resistance. This relationship holds true because a smaller crystal indicates more grain boundaries and defects. Thus, an electron encounters more barriers in the transmission process, and electric conductivity decreases.

Synthesis and characterization of ZnO/ZnMgO multiple quantum wells by molecular beam epitaxy

The growth of single and multiple (three) ZnO/ZnMgO quantum well samples on sapphire substrates, through a two-step temperature variation growth of ZnO buffer layers by molecular beam epitaxy (MBE), were investigated. For single quantum well (QW) growth, the thicker first ZnMgO barrier layer about 220 nm on the high-temperature growth ZnO (HT-ZnO) buffer layer, accumulated larger compressive stress, to achieve higher quality ZnO/ZnMgO QW growth. In the temperature-dependent photoluminescence (PL) results, the obvious S-shape variation of emission peak positions presented the stronger exciton confinement ability of QW in the higher magnesium concentrations of ZnMgO barrier layer growth. Compared to the control sample, the quantum confinement resulted in blueshift PL peaks of QW samples at low temperature. The multiple quantum well (MQWs) structure increased the exciton confinement ability to enhance the light emission efficiency of the sample. The three ZnO/ZnMgO MQWs structures were found clearly by high-resolution transmission electron microscopy.