Wen-Ray Chen

Synthesis of the Gold Nanocubes by Electrochemical Technique

This investigation demonstrates the rapid synthesis of a large quantity of uniform-sized gold nanocubes by an electrochemical method, using a surfactant solution and acetone. A redshift is observed in ultraviolet-visible absorption spectra as the shape of gold nanoparticles changes from spherical to cubic. The selected area electron diffraction patterns reveal that the gold nanocubes are single crystalline with lattice constant a = 4.068 A. The nanocube edge is about 30 nm long. The gold nanocubes are truncated structures, as revealed by high-resolution transmission electron microscopy analysis. The effect of acetone addition on the shape of particles is also discussed.

High responsivity of GaN p - i - n photodiode by using low-temperature interlayer

Gallium nitride p-i-n ultraviolet photodiodes with low-temperature (LT)-GaN interlayer have been fabricated. It was found that the dark current of photodiode with LT-GaN interlayer is as small as 143pA at 5V reverse bias. It was also found that the responsivity of the photodiode with LT-GaN interlayer can be enhanced at a small electric field (∼0.4MV∕cm) due to the carrier multiplication effect. The UV photocurrent gain of 13 and large ionization coefficient (α=3.1×105cm−1) were also observed in the detector with LT-GaN interlayer. Furthermore, we can achieve a large peak responsivity of 2.27A∕W from the photodiode with LT-GaN interlayer.

Strain-Compensated GaAsN/InGaAs Superlattice Structure Solar Cells

In this work, we investigate the metal organic chemical vapor deposition (MOCVD) growth of GaAsN/InGaAs strain-compensated superlattice cells in view of their application in solar cells. The compressive strain in InGaAs layers is matched by the tensile strain in GaAsN layers, overcoming the lattice-mismatch limitation. GaAsN/InGaAs strained layer superlattice cells, lattice-matched to GaAs, are proposed to extend the long-wavelength absorption of the bottom cell in a cascade solar cell structure. A strain-compensated superlattice solar cell with 0.6 µm GaAsN/InGaAs incorporated in the intrinsic region of p–i–n GaAs cells was fabricated. Compared with the fabricated InGaAs and InGaNAs cells, it was found that the GaAsN/InGaAs superlattice cell can lower the band gap energy and extend the absorption the most, followed by the InGaAs cell. In addition, the efficiency of the GaAsN/InGaAs superlattice cell was 4.3%, which is comparable to that of the InGaNAs cell. The GaAsN/InGaAs strain-compensated superlattice structure shows many characteristics required to make it a candidate for the next-generation multijunction solar cells, which means this design can be used as the third junction of future-generation ultrahigh-efficiency three- and four-junction devices.

Surface plasma resonant effect of gold nanoparticles on the photoelectrodes of dye-sensitized solar cells

In this study, we prepared different shapes of gold nanoparticles by seed-mediated growth method and applied them on the photoelectrodes of dye-sensitized solar cells (DSSCs) to study the surface plasma resonant (SPR) effect of gold nanoparticles on the photoelectrodes of dye-sensitized solar cells. The analyses of field emission scanning electron microscopy show that the average diameter of the spherical gold nanoparticles is 45 nm, the average length and width of the short gold nanorods were 55 and 22 nm, respectively, and the average length and width of the long gold nanorods were 55 and 14 nm, respectively. The aspect ratio of the short and long gold nanorods was about 2.5 and 4, respectively. The results of ultraviolet–visible absorption spectra show that the absorption wavelength is about 540 nm for spherical gold nanoparticles, and the absorption of the gold nanorods reveals two peaks. One is about 510 to 520 nm, and the other is about 670 and 710 nm for the short and long gold nanorods, respectively. The best conversion efficiency of the dye-sensitized solar cells with spherical gold nanoparticles and short and long gold nanorods added in is 6.77%, 7.08%, and 7.29%, respectively, and is higher than that of the cells without gold nanoparticles, which is 6.21%. This result indicates that the effect of gold nanoparticles on the photoelectrodes can increase the conductivity and reduce the recombination of charges in the photoelectrodes, resulting in the increase of conversion efficiency for DSSCs. In addition, the long gold nanorods have stronger SPR effect than the spherical gold nanoparticles and short gold nanorods at long wavelength. This may be the reason for the higher conversion efficiency of DSSCs with long gold nanorods than those of the cells with spherical gold nanoparticles and short gold nanorods.

ZnSe-based mixed-color LEDs

II-VI mixed-color light-emitting diodes (LEDs) were prepared by molecular beam epitaxy. At 20 K, it was found that we could achieve a near white light emission from sample 1O2B quantum well with a Commission Internationale de Enluminure index of x=0.4 and y=0.45. By carefully designing the device structure, we should be able to achieve a phosphor free white light LED by using the combinations of ZnCdSe and ZnCdSeTe wells.

Effect of TiO2 nanotubes with TiCl4 treatment on the photoelectrode of dye-sensitized solar cells

In this study, we used the electrochemical anodization to prepare TiO2 nanotube arrays and applied them on the photoelectrode of dye-sensitized solar cells. In the field emission scanning electron microscopy analysis, the lengths of TiO2 nanotube arrays prepared by electrochemical anodization can be obtained with approximately 10 to 30 μm. After titanium tetrachloride (TiCl4) treatment, the walls of TiO2 nanotubes were coated with TiO2 nanoparticles. XRD patterns showed that the oxygen-annealed TiO2 nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO2 nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. After TiCl4 treatment, the efficiency of TiO2 nanotube photoelectrode for dye-sensitized solar cell can be improved up to 6.58%. In the analysis of electrochemical impedance spectroscopy, the value of Rk (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO2 nanotubes were treated with TiCl4. These results indicate that TiO2 nanotubes treated with TiCl4 can increase the surface area of TiO2 nanotubes, resulting in the increase of dye adsorption and have great help for the increase of the conversion efficiency of DSSCs.

Low Dark Current GaN p-i-n Photodetectors With a Low-Temperature AlN Interlayer

GaN p-i-n ultraviolet (UV) photodetectors (PDs) with a low-temperature (LT)-AlN interlayer were proposed and fabricated. It was found that the dark current of such detectors is as small as 28pA even at a high reverse bias of 40 V. Although the high potential barrier at the AlN-GaN interface would slightly reduce the responsivity of PD under low reverse biases, the high UV-to-visible rejection ratio of the PD with an LT-AlN interlayer could be achieved under high reverse biases due to its very low dark current. The rejection ratio of the PD with the LT-AlN interlayer is as large as 735 at the reverse bias of 40 V.

Enhanced Luminescence Efficiency of InGaN/GaN Multiple Quantum Wells by a Strain Relief Layer and Proper Si Doping

The effects of a strain relief layer (SRL) employed in the InGaN/GaN light-emitting diodes (LEDs) was demonstrated. The wavelength shift was reduced to as small as 2.5 nm by inserting a SRL between n-GaN and InGaN/GaN multiple quantum wells (MQWs). For the improvement of optical properties, a proper Si-doped layer was simultaneously added in the last several barriers of In0.08Ga0.92N/GaN SRL. It can be found that the output power was increased more than 25% as the Si doping level was increased up to 5 times in the last three barriers of SRL at an injection current of 20 mA. Furthermore, the forward voltages at 20 mA were almost the same for all LEDs with different doping levels and positions.

Improvement in Linearity of Novel InGaAsN-Based High Electron Mobility Transistors

We have fabricated InGaAsN-based high electron mobility transistors (HEMTs) using InGaAsN as the channel layer. An extremely large gate-voltage swing (GVS) up to 4.2 V can be achieved by utilizing the large conduction band offset between the GaAs spacer layer and the InGaAsN channel layer. However, the poor channel mobility and current density as a result of nitrogen-induced electrically active defects limit the transconductance (gm) performance. Attempts using various annealing temperatures have demonstrated that better device characteristics can be obtained via rapid thermal annealing at 700 °C. In this study, we investigate the effect of nitrogen-induced traps on the basis of Hall measurements and device characterizations of HEMTs. The improvement in GVS in the annealed samples is also discussed. Despite the relatively poor gain, InGaAsN HEMTs with excellent linearity performance after proper thermal annealing are expected to be compatible for novel InGaAsN-based optoelectronics integral circuits (OEICs).

P-down ZnSTeSe/ZnSe/GaAs heterostructure photodiodes

High-quality quaternary ZnSTeSe epitaxial layers with uniform carrier concentration of 1/spl times/10/sup 17/ cm/sup -3/ were successfully grown on p-GaAs substrates by molecular beam epitaxy. P-down ZnSTeSe/ZnSe/GaAs heterostructure photodiodes were also fabricated. It was found that the maximum quantum efficiency of the fabricated ZnSTeSe photodiodes was around 75% with a large spectral width of 500 nm.