Several estimative factors of image quality have been developed for approaching the human perception objectively<sup>1-3</sup>. We propose to take systematically distorted videos into the estimative factors and analyze the relationship between them. Several types of noise and noise weight were took into COSME standard video and verified the image quality estimative factors which were MSE (Mean Square Error), SSIM (Structural SIMilarity), CWSSIM (Complex Wavelet SSIM), PQR (Picture Quality Ratings) and DVQ (Digital Video Quality). The noise includes white noise, blur and luminance...etc. In the results, CWSSIM index has higher sensitivity at image structure and it could estimate the distorted videos which have the same noise type at the different levels. PQR is similar to CWSSIM, but the ratings of distribution were banded together; SSIM index divides the noise types into two groups and DVQ has linear relationship with MSE in the logarithmic scale.
Coupling of a InGaN/GaN multi-quantum well (MQW) and semitransparent metal layer is shown to result in dramatic
enhancement of spontaneous emission rate by the surface plasmon effect in the optical spectral range. A five-pairs
18.5nm InGaN/GaN MQW is positioned 175nm, form various thickness (t=5~50nm) silver layer. And periodic patterns
(p=0.25~0.8μm) are defined in the top semitransparent metal layer by e-beam lithography, which are grating structures
can be incorporated into the metal film to excite surface plasmon between the interference of the metal film and
semiconductor. We have experimentally measured photoluminescence intensity and peak position of spontaneous
emission of the fabricated structures and compared with the unprocessed samples, whilst still ensuring that most of the
emission takes place into the surface plasmon (SP) mode. And the implication of these results for extracting light by
reducing total internal reflection (TIR) from light emission diode is discussed.
In this study, ZnO:Al(AZO) Ni/AZO and NiO<sub>x</sub>/AZO films were deposited on p-type GaN films followed by thermal
annealing to form Ohmic contacts. After thermal annealing, the resistivities reduced from 5×10<sup>-3</sup> to 4.4×10<sup>-4</sup> Ω-cm,
2.6×10<sup>-3</sup>Ω-cm, and 1.1×10<sup>-3</sup>Ω-cm for AZO, Ni/AZO, and NiO<sub>x</sub>/AZO films, respectively. The Ohmic characteristic
could be highly improved after inserting Ni and NiO<sub>x</sub> between AZO and p-GaN. Both the Ni/AZO and NiO<sub>x</sub>/AZO
contacts exhibit Ohmic characteristic after annealed at 800°C in N<sub>2</sub> ambient. The light transmittance of Ni/AZO and
NiO<sub>x</sub>/AZO films were higher than 80% in the range of 380-700nm after the 800°C -annealing treatment. In addition,
we fabricated InGaN/GaN MQW LEDs with a dimension of 1×1mm<sup>2</sup> using the transparent Ni/AZO and NiO<sub>x</sub>/AZO
Ohmic contact as a current spreading layer for p-GaN in order to increase the light extrication efficiency. For the LED
with Ni/AZO contact, the light output approach to saturation when the injection current was about 400mA. But the
light output still doesn't approach to saturation when the injection current was 500mA for the LED with NiO<sub>x</sub>/AZO
contact. This may be due to that the resistivity of Ni/AZO was higher than that of NiO<sub>x</sub>/AZO and exhibit more heavy
current clouding effect. The increasing of resistivity may be due to the interdiffusion of Ni into AZO. Comparing to
GaN LED with Ni/Au ohmic contact, the light output intensity of LEDs with Ni/AZO and NiO<sub>x</sub>/AZO contacts was
increased by 41% and 60% at 350mA, respectively.
Red and green emissions are observed from P ion implanted ZnO. Red emission at ~680 nm (1.82 eV) is associated with the donor-acceptor pair (DAP) transition, where the corresponding donor and acceptor are interstitial zinc (Zn<sub>i</sub>) and interstitial oxygen (O<sub>i</sub>), respectively. Green emission at ~ 516 nm (2.40 eV) is associated with the transition between the conduction band and antisite oxygen (O<sub>Zn</sub>). Green emission at ~516nm (2.403 eV) was observed for ZnO annealed at 800 oC under ambient oxygen, whereas, it was not visible when it was annealed in ambient nitrogen. Hence, the green emission is most likely not related to oxygen vacancies on ZnO sample, which might be related to the cleanliness of ZnO surface, a detailed study is in progress. The observed micro-strain is larger for N ion implanted ZnO than that for P ion implanted ZnO. It is attributed to the larger straggle of N ion implanted ZnO than that of P ion implanted ZnO. Similar phenomenon is also observed in Be and Mg ion implanted GaN.
Vertical-cavity surface-emitting lasers with variant compressively strained InGaAlAs quantum wells have been investigated. The valence band structures, optical gain spectra, and threshold properties of InGaAlAs/AlGaAs quantum wells are compared and analyzed. The simulation results indicate that the characteristics of InGaAlAs quantum wells can be improved by increasing the amount of compressive strain in quantum well. Furthermore, the properties of VCSELs with these compressively strained InGaAlAs quantum wells are studied numerically. The results of numerical calculations show that the threshold current and maximum output power can be enhanced by using higher compressively strained InGaAlAs quantum well. However, when the compressive strain is larger than about 1.5%, further improvement of the laser performance becomes minimal. The effects of the position and aperture size of the oxide-confinement layers on the laser performance are also investigated. Variation of the oxide layer design is shown to affect the current distribution which makes the temperature in the active region different. It is the main reason for the power roll-off in the VCSEL devices.
High-performance, blue micro-size InGaN light emitting diodes (LEDs) with diameters of 3 to 20 μm have been fabricated. An ion implantation technique and a 12 micron electro-ridge were used to simplify fabrication processes. The 3 to 20μm LEDs that exhibited a large emission photon blue shift (87.5meV ~52.9meV) were observed in electro-luminescence (EL) spectra. Under an increased injection current, the quantum wells become populated with charge carriers, which screened the internal piezoelectric field and caused the energy blue shift of EL eventually. A high injection current caused a high junction temperature that narrowed the band gap (red shift). The size dependent energy shift is largely owing to the competition between the blue and the red shifts. At a bias voltage of 8.96V (which is 140% of the turn on voltage, 6.4V), the 10 μm device exhibited an injection current of 7.9mA. This value exceeds that in literature, i.e., 4mA at a bias voltage of 14V (which is 140% of the turn on voltage, 10V). This phenomenon may be owing to that the ion implantation and electro-ridge designs herein involved a lower series resistance. The external quantum efficiencies (E.Q.E.) of the micro size LEDs herein were all 0.4%~3.3%, which is better than the values reported in literature, which were ranged between 0.004% and 1.29% for an individual LED and an array LED, respectively. The E.Q.E. of the 15μm device at maximum injection current had the optimum value yet obtained for micro-size LEDs. The dependence of the blue shift and the E.Q.E. on the size warrants further study.
Homoepitaxial and heteroepitaxial ZnO films were grown by plasma-assisted molecular beam epitaxy (P-MBE). Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0001) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. There exists a residual strain in the heteroepitaxial ZnO, which is ε = -0.25%. Low-intensity excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. The quality of ZnO epilayers is better than that of ZnO substrate. However, under high-intensity excitation by a N2 laser, the emission due to exciton-exciton collisions dominates the PL spectrum from heteroepitaxial ZnO layer but is not observed from homoepitaxial ZnO layer.