Yu-Hua Kao

Cadmium telluride quantum-dot-doped glass by the sol-gel technique

CdTe quantum dot (QD)-doped sodium borosilicate (NBS) glasses with particle sizes ranging from 2.4 nm to 8.5 nm were synthesized by the sol-gel technique. Two sol-gel methods were developed. Method (1) involves the immersion of CdO-doped gels in Na2Te/methanol (MeOH) solution for 1 to 4 days at 60 degrees Celsius, followed by heat-treatment at 540 degree Celsius or 570 degrees Celsius for 6 to 12 hours in an inert atmosphere. In method 2, CdTe QDs were directly formed at various temperatures (520 degrees Celsius to 570 degrees Celsius) under a reducing atmosphere. The synthesis of CdTe QDs in the NBS glass was accomplished using cadmium nitrate [Cd(NO3)2] and telluric acid (H6TeO6) as the starting materials. The number density and particle size distribution of the CdTe nanocrystallites depend on the heat- treatment conditions for the reduction of Te6+ to Te2- ions and their subsequent ions react with Cd2+ ions to form CdTe. The absorption edges of these glasses were blue-shifted due to quantum confinement depending on the concentration of the initial salt and the heat- treatment conditions.

Improving CdS quantum-dot materials by the sol-gel method

In this paper we discuss our approaches and results for the preparation of improved semiconductor quantum dot materials by the sol-gel method. By taking advantage of the low- temperature sol-gel route, the nanostructure and optical properties of CdS quantum dots were greatly improved for device applications. Pore-free sodium borosilicate (NBS) glasses and organically modified silicates (ORMOSILs) were used as matrices for the CdS nanocrystallites. Results from both degenerate four-wave mixing and pump-probe techniques indicated large third-order nonlinear responses of the order of 10-6 esu from CdS-doped NBS glasses. By using potassium ion exchange, the first CdS-doped channel waveguides were fabricated in NBS glasses. Propagation of femtosecond laser pulses through the waveguide was investigated. Pulse breakup effect that may be the result of soliton formation was observed in these waveguides. Polydimethylsiloxane (PDMS) was uses as the organic component to modify the silica gel. CdS-doped ORMOSILs derived from the PDMS- TEOS system exhibits improved mechanical property and film-forming ability compared to purely inorganic gels. Both CdS-doped PDMS-TEOS ORMOSILs and NBS glasses were much more resistant to photodarkening than the glasses made by melting. The use of the bifunctional ligand 3-aminopropyltriethoxysilane (APTES) significantly reduced the average particle size and narrowed the size distribution of CdS quantum dots in silica gels and densified NBS glasses.

Role of vanadium in organometallic vapor phase epitaxy grown GaAs

The electrical and photoluminescent properties of vanadium incorporated into GaAs epitaxial layers from a VO(OC2H5)3 source during organometallic vapor phase epitaxy were examined. The vanadium concentration in the GaAs was controllably varied from 1016 to 1018 atomsu2009cm−3. Deep level transient spectroscopy showed the presence of an electron trap at Ec−0.15 eV which increased in concentration with vanadium content of the epitaxial layers. A maximum value of 8×1015 cm−3 for this trap was obtained. There were no midgap electron traps associated with vanadium. In intentionally Si‐doped epitaxial layers, co‐doping with vanadium was observed to have no effect in reducing the carrier density when the Si concentration was ≥4×1016 cm−3. The net carrier concentration profiles resulting from 29Si implantation into GaAs containing 1018 cm−3 of total V had sharper tails than for similar implantation into undoped material, indicating the presence of less than 1016 cm−3 V‐related acceptors. Photoluminescent spectra exhi...

Optical waveguide based on CdS quantum-dot-doped sodium borosilicate glass fabricated by the sol-gel technique

Semiconductor CdS quantum dot-doped sodium borosilicate glasses were successfully fabricated by the sol-gel technique. The dot size and concentration of CdS are controlled by chemistry and processing conditions. Optical absorption spectra were measured for these quantum dot-doped glasses, and absorption shoulder due to the exciton effect was observed. Nonlinear optical properties of high third-order susceptibility, (chi) (3), and room-temperature optical gain were observed. Potassium ion exchange was performed on the glass, and channel waveguides were successfully fabricated in the CdS-doped glass. The waveguide was tested with single mode laser pulses, and single mode output was demonstrated. Spectral broadening of the output pulses was observed.

Er-doped sol-gel glasses for integrated optics

Erbium-doped glasses are of great interest for optical fiber telecommunications at 1.5 micrometers . The sol-gel process offers many advantages for synthesizing materials for integrated optical devices. Some of these advantages include high-purity, low-temperature synthesis, and excellent control and flexibility over composition and design. In this paper, we report the first Er-doped sol-gel waveguides. We examined several approaches for alleviating the OH- quenching problem associated with conventional sol-gel processes. We prepared the first ion exchangeable Er-doped glasses which can be fabricated into integrated optical devices. The concept of molecular docking of Er as a coordinate complex in gel matrices to improve the luminescence properties has been applied. We also exploited the concept of using the evanescence field to couple energy into an Er-doped sol-gel layer on top of a glass optical device. New progresses in modifying the properties of gel matrices for integrated optical device applications are presented.

Preparation of narrow size distribution CdS quantum dots in sodium borosilicate glass by the sol-gel process

Successful fabrication of nonlinear optical devices such as nonlinear waveguides and switches requires high nonlinearity of semiconductor quantum dot nanocomposites and their ability to form waveguides. Nano-sized CdS quantum dots with narrow size distribution can be fabricated via the sol-gel process by using a bifunctional ligand which anchors the Cd2+ ions homogeneously to the matrix. In this investigation, 3- aminopropyltriethoxysilane, H2N(CH2)3Si(OCCH5)3, has been used to synthesize CdS quantum dots with small diameters and narrow size distributions in sodium borosilicate gels and fully dense glasses. This matrix composition allows waveguide fabrication by ion exchange of sodium with potassium and has the advantage of low- temperature densification to form optical quality glasses. In 8 nominal wt% CdS-doped 5Na2O-30B2O3-65SiO2 glass, the dots have an average size of 2.8 nm, a maximum in the distribution in the range of 2.0 - 2.5 nm, and a standard deviation of 0.9.

CdS quantum dot materials by the sol-gel method

Colored-oxide glass filters containing CdS quantum dots were found to exhibit third-order optical nonlinearity in 1983. However, the dot sizes were somewhat large and the size distributions were wide. The sol-gel method proved to be a better alternative for CdS quantum dot materials. This paper describes the advantages of the sol-gel method, and how it has been exploited to prepare two types of CdS quantum dot materials. These are fully dense sodium borosilicate glasses and organically-modified silicates, Ormosils. Both materials give high values of third-order nonlinearity and excellent optical transparency.

Sol-gel process for glass integrated optics

The sol-gel process allows the synthesis of a wide variety of amorphous as well as crystalline materials which can exhibit useful passive or active optical properties. The process offers many advantages, such as low-temperature synthesis, excellent control and flexibility over composition and design. For applications in future integrated optics, the sol-gel process is flexible in making various kinds of optical components which either have been successfully made or have potential in their realization. This paper examines a number of the state-of-the-art optical components fabricable by the sol-gel process for glass integrated optics. Major examples furnished are in glass substrates and waveguides, third-order nonlinear materials, lasers and optical amplifiers, optical fibers, and gradient-index lenses. The benefits as well as limitations by using the sol-gel approach will be critically presented.

Electrical and Optical Properties of Vanadium in Omvpe-Grown GaAs

The electrical and photoluminescent properties of vanadium incorporated into GaAs epitaxial layers from a VO(OC 2 H 5 ) 3 source during organometallic vapor phase epitaxy were examined. The vanadium concentration in the GaAs was controllably varied from 10 16 to 10 18 atoms cm −3 . Deep level transient spectroscopy showed the presence of an electron trap at E c – 0.15 eV which increased in concentration with vanadium content of the epitaxial layers. A maximum value of 8 × 10 15 cm −3 for this trap was obtained. There were no midgap electron traps associated with vanadium. In intentionally Si-doped epitaxial layers, co-doping with vanadium was observed to have no effect in reducing the carrier density when the Si concentration was > 4 × 10 16 cm −3 . The net carrier concentration profiles resulting from 29 si implantation into GaAs containing 10 18 cm −3 of total V had sharper tails than for similar implantation into undoped material, indicating the presence of less than 10 16 cm −3 V-related acceptors. Photoluminescent spectra exhibited the characteristic V +3 intracenter emission at 0.65∼0.75 eV. No other deep level photoluminescence was detected. For a V concentration of 10 16 cm −3 only 2.5 × 10 13 cm −3 was electrically active. Over the entire V concentration investigated this impurity was predominantly (≥99%) inactive.