Wu-Yih Uen

Optical properties of the ZnSe1−xTex epilayers grown by molecular beam epitaxy

ZnSe1−xTex epilayers were grown on a GaAs (001) substrate with 0°, 3°, 10°, and 15° tilts toward [110] by molecular beam epitaxy. The energy gap was found to increase with the substrate tilt angle. In addition, a Te-bound exciton and an exciton bound to the Te cluster in the photoluminescence spectra have been identified. The threshold temperature for the observation of the Te-bound exciton in the photoluminescence spectrum of ZnSe1−xTex epilayers was found to increase with the Te concentration.

Characterizations of InN films on Si(111) substrate grown by metal-organic chemical vapor deposition with a predeposited In layer and a two-step growth method

In this study, a two-step growth method with a thin predeposited indium (In) layer is reported to grow high-quality indium nitride (InN) films on silicon (Si) substrates by metal-organic chemical vapor deposition. The surface morphologies of the InN films exhibited good surface quality as determined by scanning electron microscope and atomic force microscope. The wurtizite structure with lattice constants c=5.69A and a=3.44A of the InN films was observed by high-resolution transmission electron microscopy. A 540arcsec full width at half maximum of InN(0002) diffraction peak and a 0.756eV room-temperature interband transition energy of the InN films were also observed from x-ray diffraction and photoluminescence spectroscopy, respectively. A small temperature coefficient of ∼0.036meV∕K for the band gap of InN was determined. Such a small value could be explained by the small lattice mismatch and thermal expansion of InN on the Si substrate.

Characterizations of ZnTe bulks grown by temperature gradient solution growth

In this work, zinc telluride (ZnTe) bulk crystals have been grown for the first time by the temperature gradient solution growth (TGSG) technique. Hall effect and capacitance–voltage (C–V) measurements were used to determine the room temperature electrical properties, and low temperature photoluminescence (PL) measurements were used to analyze the optical properties. As grown undoped ZnTe shows p-type conductivity with the carrier concentration (1–2) × 1015 cm−3, mobility about 50 cm2/V s, resistivity about 80–90 Ω cm. Otherwise, heavily doped p-ZnTe can be achieved by phosphorus doping. The carrier concentration demonstrates a logarithmic increase with the doping amount. The Hall carrier concentration up to 8×1017 cm−3 (4.7×1018 cm−3 from C–V measurement), resistivity low to around 0.15 Ω cm was achieved with a doping amount of 4000 ppm ZnP2. Besides, the room temperature PL spectra exhibits a pure green luminescence of energy 2.259 eV (λ=549.5 nm). The peak intensifies with increasing ZnP2 doping amount and begins to saturate when the added amount is over 8000 ppm. The 9 K PL spectra also demonstrates that the excitonic zone is much more intense than for deep level band, indicating a good sample quality. It is plausible that using the TGSG technique can afford high-quality phosphorus-doped p-type ZnTe substrates for device applications.

High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers

In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-mean-square value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (101̅2) reflections in the double crystal X-ray rocking curve were ~330 and 450 respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <; 6× 108 cm-2. From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaN-based LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of ~110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.

The effect of a-GaAs/a-Si double buffer layers on GaAs-on-Si as determined by transmission electron microscopy

A high-quality GaAs epilayer was successfully grown on a Si substrate using a-GaAs/a-Si double buffers, which were crystallized by thermal cyclic annealing. Double crystal x-ray diffraction measurements showed that the full width at half maximum of the rocking curve for the GaAs epilayer was only 102 arcs. The effect of a-GaAs/a-Si double buffers was examined in detail by transmission electron microscopy. Transmission electron microscopy characterizations revealed obvious bending behaviour of dislocations in the a-GaAs/a-Si buffered epilayer. Most threading dislocations and planar defects were confined within the crystallized double buffers. The dislocation density was reduced to 2.6 × 106 cm−2 at a thickness of 1.0 µm in the first epitaxial layer and was further improved to under 1.3 × 106 cm−2 in the second epitaxial layer. Transmission electron microscopy observation and analysis revealed a defect reduction mechanism.

Intrinsic p-type ZnO films fabricated by atmospheric pressure metal organic chemical vapor deposition

The structural, electrical, and optical properties of ZnO films fabricated by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) under various gas flow ratios of [H2O]/[DEZn] (VI/II ratio) ranging from 0.55 to 2.74 were systematically examined. Hall effect measurements exhibited an evident effect of the VI/II ratio on the conduction type of the intrinsic films. An n-type film was fabricated at the VI/II ratio=0.55; however, p-type ZnO films with the hole concentration of the order of 1017 cm−3 could be achieved at VI/II ratios higher than 1.0. In particular, the highest mobility of 91.6 cm2/V s and the lowest resistivity of 0.369 Ω cm have been achieved for the specimen fabricated at the VI/II ratio=1.10. Moreover, room-temperature photoluminescence (PL) measurements demonstrated an interstitial Zn (Zni) donor defect related emission at 2.9 eV for the n-type film, while a Zn vacancy (VZn) acceptor defect related one at 3.09 eV for the p-type films. The existence of material intrinsic ...

Enhancing the Emission Efficiency of In0.2Ga0.8N ∕ GaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates

Enhancing the Emission Efficiency of In0.2Ga0.8NOGaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates Zhen-Yu Li, Wu-Yih Uen, Ming-Hua Lo, Ching-Hua Chiu, Po-Chun Lin, Chih-Tsang Hung, Tien-Chang Lu, Hao-Chung Kuo, Shing-Chung Wang,* and Yen-Chin Huang Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Electronic Engineering, College of Electrical Engineering and Computer Science, Chung Yuan Christian University, Chung-Li 32023, Taiwan

Characterizations of Ga-doped ZnO films on Si (111) prepared by atmospheric pressure metal-organic chemical vapor deposition

Gallium-doped ZnO films were grown on p-Si(111) substrates by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as a n-type dopant gas. The structural, electrical, and optical properties of ZnO:Ga films obtained by varying the flow rate of TEG from 0.56to3.35μmol∕min were examined. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga doping plays a role in forming microstructures in ZnO films. A flat surface with a predominant orientation (101) was obtained for the ZnO:Ga film fabricated at a flow rate of TEG=2.79μmol∕min. This film also revealed a lowest resistivity of 4.54×10−4Ωcm, as measured using the van der Pauw method. Moreover, low temperature photoluminescence (PL) emission recorded at 12K demonstrated the Burstein Moss shift of PL line from 3.365to3.403eV and a line broadening from 100to165meV as the TEG flow rate varied from 0.56to2.79μmol∕min. This blueshift behavior o...

Damp-Heat induced performance degradation for InGaP/GaAs/Ge triple-junction solar cell

We performed accelerated tests on sealed and nonsealed InGaP/InGaAs/Ge triple-junction (TJ) solar cells in a complex high temperature and high humidity environment and investigated the electrical properties over time. The degradation of energy conversion efficiency in nonsealed cells was found to be more serious than that in sealed cells. The short-circuit current (ISC), open-circuit voltage (VOC), and fill factor (FF) of sealed cells changed very slightly, though the conversion efficiency decreased 3.6% over 500 h of exposure. This decrease of conversion efficiency was suggested to be due to the deterioration of silicone encapsulant. The ISCVOC, and FF of nonsealed cells decreased with increasing exposure time. By EL and SEM analysis, the root causes of degradation can be attributed to the damage and cracks near the edge of cells induced by the moisture ingress. It resulted in shunt paths that lead to a deterioration of the conversion efficiency of solar cell by increasing the leakage current, as well as decreasing open-circuit voltage and fill factor of nonsealed solar cells.

Luminescence mechanisms of silicon-rich nitride films fabricated by atmospheric pressure chemical vapor deposition in N2 and H2 atmospheres

This work examines possible luminescence mechanisms of silicon-rich nitride (SRN) films that were fabricated by atmospheric pressure chemical vapor deposition (APCVD). Under an ambient gas of either H2 or N2, two SRN films were deposited using the same precursors of Si and N. While photoluminescence (PL) measurements of both as-deposited specimens revealed an intense luminescence band (1.8–3.8 eV), which was observable by the naked eye, a detailed examination of the high energy band of the PL spectra over 2.8 eV yielded different results for those samples that were fabricated in different ambiences. To determine the reason for these differences, Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy were conducted, suggesting unique chemical bonds and elemental ratio of nitrogen to silicon in SRN films. Further analysis involving plan-view high-resolution transmission electron microscopic observations of SRN films demonstrated the embedding of Si quantum dots (Si QDs), but with some ...