Shou-Yi Kuo

Effects of RF power on the structural, optical and electrical properties of Al-doped zinc oxide films

Abstract In this study, we discussed the effects of growth parameters on the structural and optical properties of Al-doped zinc oxide (AZO) deposited at room temperature by radio-frequency magnetron sputtering. The AZO films have been characterized in detail using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, Hall-effect measurement system and UV–visible spectrophotometer. It was found that the morphological, structural, electrical and optical properties of AZO films are greatly dependent on sputtering power. Collision between sputter species and surface morphology play important roles in optoelectrical properties of AZO films. According to our experimental results, the AZO films can be used in versatile devices to meet various requirements.

Optical properties of a -plane InGaN/GaN multiple quantum wells on r -plane sapphire substrates with different indium compositions

A-plane InxGa1−xN/GaN (x=0.09, 0.14, 0.24, and 0.3) multiple-quantum-wells (MQWs) samples, with a well width of about 4.5 nm, were achieved by utilizing r-plane sapphire substrates. Optical quality was investigated by means of photoluminescence (PL), cathodoluminescence, and time resolved PL measurements (TRPL). Two distinguishable emission peaks were examined from the low temperature PL spectra, where the high- and low-energy peaks were ascribed to quantum wells and localized states, respectively. Due to an increase in the localized energy states and absence of quantum confined Stark effect, the quantum efficiency was increased with increasing indium composition up to 24%. As the indium composition reached 30%, however, pronounced deterioration in luminescence efficiency was observed. The phenomenon could be attributed to the high defect densities in the MQWs resulted from the increased accumulation of strain between the InGaN well and GaN barrier. This argument was verified from the much shorter carrier...

High extraction efficiency GaN-based light-emitting diodes on embedded SiO2 nanorod array and nanoscale patterned sapphire substrate

In this paper, GaN-based LEDs with a nanoscale patterned sapphire substrate (NPSS) and a SiO2 photonic quasicrystal (PQC) structure on an n-GaN layer using nanoimprint lithography are fabricated and investigated. The light output power of LED with a NPSS and a SiO2 PQC structure on an n-GaN layer was 48% greater than that of conventional LED. Strong enhancement in output power is attributed to better epitaxial quality and higher reflectance resulted from NPSS and PQC structures. Transmission electron microscopy images reveal that threading dislocations are blocked or bended in the vicinities of NPSS layer. These results provide promising potential to increase output power for commercial light emitting devices.

Dependence of luminescence efficiency on dopant concentration and sintering temperature in the erbium-doped Ba0.7Sr0.3TiO3 thin films

We found the dependence of luminescence efficiency on Er3+ concentration and sintering temperature in the Er-doped Ba0.7Sr0.3TiO3 (BST) thin films is governed by crystallinity and ion–ion interaction. X-ray diffraction and Raman studies of the sol-gel prepared samples show that the BST polycrystalline phase occurred when the sintering temperature reaches 700 °C, whereas, it becomes worse for temperature above 700 °C resulting from phase separation and the Er3+ concentration exceeding 3 mol % due to charge compensation mechanism. The observed green emission reaches maximum at sintering temperature 700 °C and 3 mol % Er3+ ions concentration. We also showed the Er dopant does not affect the dielectric property of BST thin films in C–V measurement and the Ba0.7Sr0.3TiO3 films doped with Er3+ ions may have potential use for electroluminescence devices.

Effect of the Ca content on the electronic structure of Pb1−xCaxTiO3 perovskites

This study performs O K- and Ti L3,2-edge x-ray absorption near-edge structure (XANES) measurements and first-principles pseudopotential calculations for the electronic structures of ABO3-type Pb1−xCaxTiO3 (x=0–1) perovskites. The features in the O K-edge XANES spectra are found to be contributed primarily by hybridization between O 2p and Ti 3d, Pb 6p, and Ca 3d orbitals. The O K-edge XANES spectra reveal that partial substitution of A cations, Pb, by Ca not only decreases O 2p–Pb 6p but also O 2p–Ti 3d hybridization. The Ti L3,2-edge measurements find that the off-center displacement of Ti, and hence, ferroelectricity persist up to a Ca concentration between 0.3 and 0.4.

Optical properties of indium nitride nanorods prepared by chemical-beam epitaxy

The optical properties of indium nitride nanorods grown by chemical-beam epitaxy are investigated by photoluminescence (PL) and Raman spectroscopy. The PL peaks show a blue shift from 0.69 to 0.79 eV, which is associated with a decrease in the size of the nanorods from 40 to 5 nm. Judging from the Raman spectra and transmission electron diffraction of these nanorods, it can be concluded that the quantum size effect is the most likely factor causing the PL shift, rather than the strain or Moss–Burstein effects.

Non-antireflective Scheme for Efficiency Enhancement of Cu(In,Ga)Se2 Nanotip Array Solar Cells

We present systematic works in characterization of CIGS nanotip arrays (CIGS NTRs). CIGS NTRs are obtained by a one-step ion-milling process by a direct-sputtering process of CIGS thin films (CIGS TF) without a postselenization process. At the surface of CIGS NTRs, a region extending to 100 nm in depth with a lower copper concentration compared to that of CIGS TF has been discovered. After KCN washing, removal of secondary phases can be achieved and a layer with abundant copper vacancy (V(Cu)) was left. Such compositional changes can be a benefit for a CIGS solar cell by promoting formation of Cd-occupied Cu sites (Cd(Cu)) at the CdS/CIGS interface and creates a type-inversion layer to enhance interface passivation and carrier extraction. The raised V(Cu) concentration and enhanced Cd diffusion in CIGS NTRs have been verified by energy dispersive spectrometry. Strengthened adhesion of Al:ZnO (AZO) thin film on CIGS NTRs capped with CdS has also been observed in SEM images and can explain the suppressed series resistance of the device with CIGS NTRs. Those improvements in electrical characteristics are the main factors for efficiency enhancement rather than antireflection.

Catalyst-free growth of indium nitride nanorods by chemical-beam epitaxy

We demonstrate the growth of indium nitride (InN) nanorods on sapphire by chemical-beam epitaxy without a catalyst. The nanorods are synthesized nearly unidirectionally along the ⟨001⟩ direction and the diameters varied in the range of 20–40nm with In∕N flow ratio. Single-crystalline wurtzite structure is verified by x-ray diffraction and transmission electron microscopy. Raman measurements show that these wurtzite InN nanorods have sharp peaks E2 (high) at 491cm−1 and A1 (LO) at 593cm−1.

Decreasing giant splitting of longitudinal and transverse optical phonons in PbxSr1-xTiO3 due to Pb covalency

In contrast to the increase of giant splitting of longitudinal optical (LO) and transverse optical (TO) phonons in polycrystalline BaxSr1−xTiO3 [Phys. Rev. B. 64, 224103-1 (2001)], a decreasing splitting of the softest A1(1TO) and the hardest A1(3LO) was observed in PbxSr1−xTiO3. A similar structure but anomalous discrepancies in lattice dynamics of ABO3 perovskite are related to the diverse electronic states of Pb2+ and Ba2+ ions. In this article, Pb–O interaction is proposed to be more hybridized than Ba–O, and thus the change of effective charge to the LO–TO splitting may be the dominant mechanism in PbxSr1−xTiO3, while unit-cell volume change is in the BaxSr1−xTiO3 systems.

Enhanced broadband and omnidirectional performance of Cu(In,Ga)Se2 solar cells with ZnO functional nanotree arrays

An effective approach is demonstrated for enhancing photoelectric conversion of Cu(In,Ga)Se2 (CIGS) solar cells with three-dimensional ZnO nanotree arrays. Under a simulated one-sun condition, cells with ZnO nanotree arrays enhance the short-circuit current density by 10.62%. The omnidirectional anti-reflection of CIGS solar cells with various ZnO nanostructures is also investigated. The solar-spectrum weighted reflectance is approximately less than 5% for incident angles of up to 60° and for the wavelengths primarily from 400 nm to 1000 nm. This enhancement in light harvesting is attributable to the gradual refractive index profile between the ZnO nanostructures and air.