Dong Sing Wuu

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E2(high) and A1(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 103) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.

Pulsed laser deposition of ITO/AZO transparent contact layers for GaN LED applications

In this study, indium-tin oxide (ITO)/Al-doped zinc oxide (AZO) composite films were fabricated by pulsed laser deposition and used as transparent contact layers (TCLs) in GaN-based blue light emitting diodes (LEDs). The ITO/AZO TCLs were composed of the thin ITO (50 nm) films and AZO films with various thicknesses from 200 to 1000 nm. Conventional LED with ITO (200 nm) TCL prepared by E-beam evaporation was fabricated and characterized for comparison. From the transmittance spectra, the ITO/AZO films exhibited high transparency above 90% at wavelength of 465 nm. The sheet resistance of ITO/AZO TCL decreased as the AZO thickness increased, which could be attributed to the increase in a carrier concentration, leading to a decrease in the forward bias of LED. The LEDs with ITO/AZO composite TCLs showed better light extraction as compared to LED with ITO TCL in compliance with simulation. When an injection current of 20 mA was applied, the output power for LEDs fabricated with ITO/AZO TCLs had 45%, 63%, and 71% enhancement as compared with those fabricated using ITO (200 nm) TCL for the AZO thicknesses of 200, 460, and 1000 nm, respectively.

Growth evolution of SixNy on the GaN underlayer and its effects on GaN-on-Si (111) heteroepitaxial quality

The GaN epilayers were grown on Si(111) substrates via combining the techniques of AlN buffer, the graded AlGaN structure and the SixNy interlayer by metalorganic chemical vapor deposition. The SixNy interlayers with various growth times of 0–60 s were introduced into the growth of GaN epilayers. To thoroughly realize the growth evolution of SixNy, measurements of atomic force microscopy, field emission scanning electron microscopy and nano-Auger electron spectroscopy were performed. From the measurement by transmission electron microscopy, it can be proven that nanocrystalline SixNy is preferentially located at the dislocation cores and pits during the growth process. For the fabrication of GaN/graded AlGaN/AlN/Si, the full width at half maximum of the X-ray diffraction rocking curve at the GaN(102) plane was reduced effectively from 965 to 771 arcsec by inserting SixNy into the GaN epilayer, which resulted from the bending and annihilation of dislocations.

Growth and Characterization of Epitaxial ZnO Nanowall Networks Using Metal Organic Chemical Vapor Deposition

ZnO nanowall networks with a honeycomblike pattern on GaN/sapphire substrates were deposited by metalorganic chemical vapor deposition (MOCVD) without using any metal catalysts. The effects of growth temperature and VI/II ratio on the surface morphology and optical properties of ZnO nanowall networks were investigated by scanning electron microscopy (SEM) and photoluminescence (PL) analysis. The SEM image obtained shows a prominent nanowall-network structure when the growth temperature is higher than 550 ?C. The wall width and network size of ZnO nanowall-network structures grown by MOCVD were found to change depending on DEZn flow rate. The surface morphology of ZnO structures was observed at different time intervals from 10 to 40 min to investigate the formation mechanism of ZnO nanowall networks. The room-temperature PL measurement of ZnO nanostructures grown on GaN/sapphire substrates shows high-intensity ultraviolet peaks at 385 nm without any ?green peak?. The PL spectrum suggests that the quantum confinement effects are caused by the nanostructure of ZnO.

Defect annihilation mechanism of AlN buffer structures with alternating high and low V/III ratios grown by MOCVD

A novel structure was built by alternating high and low V/III ratios to improve the quality of the AlN layer at a lower temperature (1100 °C) as compared with conventional growth temperatures (≥1300 °C). This novel structure was applied to fabricate the stacking structure and superlattice layers. We designed four different AlN epitaxial structures to verify the effect of these new structures on the crystallinity. The dislocation density decreased by about one order of magnitude from 7.8 × 109 cm−2 to 7.4 × 108 cm−2 after inserting the superlattice in the middle AlN layer. The full width at half maximum values of AlN (0002) and (10−12) also were improved. Interestingly, we found that the growth mechanism and dislocation behavior were greatly dependent on the inserting sites of the superlattice by transmission electron microscopy. As the superlattice was grown first, the growth of the AlN layer was dominated by an island mode forming a column-like structure with twist boundaries. Most of the threading dislocations were decreased by bending and combining with each other along the grain boundary in the columnar structure. We believed that this novel structure could help achieve the growth of an AlN epilayer with optimal quality at a low temperature under the consideration of prolonging the lifetime of the heating system.

Performance comparison of p-side-up thin-film AlGaInP light emitting diodes with aluminum-doped zinc oxide and indium tin oxide transparent conductive layers

Transparent conductive layers (TCLs) deposited on a GaP window layer were used to fabricate high-brightness p-side-up thin-film AlGaInP light-emitting diodes (LEDs) by the twice wafer-transfer technique. Indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) were used as TCLs for comparison. The TCLs improved droop of external quantum efficiencies (EQE) of LEDs and junction temperature, which result in increasing the light output power and thermal stability of the LEDs. The droop efficiency of Ref-LED, ITO-LED and AZO-LED were 64%, 27% and 15%, respectively. The junction temperature of ITO-LED and AZO-LED reduced to 49.3 and 39.6 °C at an injection current of 700 mA compared with that (80.8 °C) of Ref-LED. The LEDs with AZO layers exhibited the most excellent LED performance. The emission wavelength shifts of LEDs without a TCL, with an ITO layer, and with an AZO layer were 17, 8, and 3 nm, respectively, when the injection current was increased from 20 to 1000 mA. The above results are promising for the development of AZO thin films to replace ITO thin films for AlGaInP LED applications.

Rapid-Thermal-Processed BaTiO3 Thin Films Deposited by Liquid-Source Misted Chemical Deposition

BaTiO3 thin films deposited on the RuO2(250 nm)/Ru(20 nm)/TiN(200 nm)/Ti(20 nm)/(100)Si substrates by liquid-source misted chemical deposition are reported. The rapid thermal processing (RTP) technique was used for post deposition annealing. It was found that the strain was released and grain size increased for BaTiO3 films treated at high RTP temperature or for long RTP time. The interface between BaTiO3 and the bottom electrode was still sharp for the RTP-treated sample at 950°C. The leakage current density decreases as the RTP temperature increases. It can be decreased to 2.09 nA/cm2 under a supply voltage of 1.5 V. The dielectric constant can be increased up to 250 measured at 100 kHz for the sample treated by RTP at 950°C. The improvements in the BaTiO3 characteristics are due to the fact that RTP can enhance the crystallinity, relax the strain, alleviate the impurities in the films and does not result in significant interdiffusion of the materials.

Defect formation mechanism and quality improvement of InAlN epilayers grown by metal–organic chemical vapor deposition

The effect of growth pressure on defect formation in InAlN epilayers grown on GaN/sapphire templates by metal–organic chemical vapor deposition was systematically investigated in this study. From X-ray diffraction measurements, it was found that a serious phase separation occurred in the InAlN epilayers grown at 1 × 104 Pa (100 mbar). The inhomogeneity of the In composition was observed at the beginning of the InAlN growth as examined by transmission electron microscopy. The initial In composition inhomogeneity close to the InAlN/GaN interface was confirmed to play an important role in the formation of V-shaped defects and the phase separation. When the growth pressure increased from 1 × 104 Pa (100 mbar) to 5 × 104 Pa (500 mbar), the phase separation diminished over 3 × 104 Pa (300 mbar), and the In content continuously increased from 6.0 to 25%. However, in spite of the fact that there was no phase separation in the InAlN layer grown at 3 × 104 Pa (300 mbar), the inhomogeneity of the In composition still existed near the surface instead of the InAlN/GaN interface. This was caused by the fact that the In adatoms preferred to accumulate at the V-shaped defects which were induced by the low surface mobility and parasitic reaction of Al adatoms. Two distinct formation mechanisms of the V-shaped defects at the low and high growth pressures were confirmed. To explore the effect of thickness on the epilayer quality, a series of InAlN samples (In = ~20%) with various thicknesses ranging from 5 to 125 nm were investigated. The InAlN epilayer with a thickness of 10 nm showed the optimum crystallinity and minimum surface roughness. A higher growth pressure (≥3 × 104 Pa (300 mbar)) and a thinner thickness (≤10 nm) favored the In composition homogeneity and suppressed the formation of V-shaped defects. Both key growth parameters were demonstrated in detail to achieve a high-quality InAlN epilayer for device applications.

Study of MgXZn1-XO Alloys (0<x<0.15) by X-Ray Absorption Spectroscopy

High-resolution K-edge x-ray absorption data are presented for Mg, Zn and O of Mg1-xZnxO films. A detailed analysis of the extended x-ray absorption fine structure by using the IFEFFIT program is given, and the Zn form chemical bonds with O are obtained. The x-ray absorption near-edge structure of Mg, Zn and O K-edge are investigated, and the electronic structures of Mg1-xZnxO with various compositions are studied.

Preparation of Cu2ZnSnS4 Thin Film by So-Gel Spin-Coated Deposition

In this work Cu2ZnSnS4 (CZTS) suitable for the absorption layer in solar cells was successfully prepared by sol-gel spin-coated deposition. CZTS precursors were prepared by using solutions of copper (II) chloride, zinc (II) chloride, tin (IV) chloride, and thiourea. The CZTS with texture surface structures, resulting from 3 times of stacks through the cycles of spin-coated and synthesized (at 320 °C) processes, is found to be merged well together, and the thickness of the CZTS reaches ~ 3 μm. The kesterite crystallinity of the CZTS designated from the x-ray diffraction of (112), (200), (312), and (322) planes of CZTS were obtained. The optical-energy gap of the CZTS is about 1.5 eV. The average optical-absorption coefficient of the CZTS is ~ 2.4 x 104 cm-1, and the high absorption band of the CZTS covers most of the solar irradiation spectrum. This makes the CZTS the most potential material for solar cells. The chemical composition Cu:Zn:Sn:S = 30:14:16:40 of the CZTS is obtained at a synthesized temperature of 320 °C.