Papers by Author: Ching Hwa Ho

Abstract: Single crystals of undoped and Au-doped MoS₂ were grown by the chemical vapor transport method using iodine as a transporting agent. The doping effect of the Au was characterized by temperature-dependent piezoreflectance (PzR) spectroscopy measurements in broad range of temperatures from 25 to 300 K. The temperature dependent PzR spectra revealed prominent features in the vicinity of direct-band-edge excitons for both investigated samples. The energies and broadening parameters of the A and B excitons were determined via a detailed line shape fit of the PzR spectra. We observed that Au doping strongly reduces the splitting between A and B excitons as well as it causes the redshift of observed transitions in relation to the undoped MoS₂. The origin of observed excitons was discussed.

Abstract: Single crystals of Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) solid solutions have deen grown by chemical vapor transport technique using ICl₃ as a transport agent. Analyzing the X-ray diffraction patterns reveal that the as-grown CZTSSe solid solutions are crystallized in kesterite structure and the lattice parameters are determined. The S contents of the obtained crystals are estimated by Vegard’s law. The composition dependent band gaps of CZTSSe solid solutions are studied by electrolyte electroreflectance (EER) techniques. The band gaps of CZTSSe are evaluated by a lineshape fit of the EER spectra and are found to increase almost linearly with the increase of S content.

Abstract: Single crystals of Pb_1-xCd_xSe compounds with x = 0, 0.01, 0.03, 0.05, 0.07 and 0.1 were grown by vertical Bridgman method. The crystalline phase and stochiometry for these crystals were investigated by X-ray diffraction, SEM and electron-probe microanalysis (EPMA). The thermoelectric behaviors for the Pb_1-xCd_xSe crystals were studied by means of thermal and carrier transport measurements in the temperature range between 50K and 400K. X-ray diffraction and SEM analysis confirmed that as-grown Pb_1-xCd_xSe crystals are simgle phase. The experimental results showed that the PbTe sample is p-type semiconductor but Pb_1-xCd_xSe samples with x = 0.01, 0.03, 0.05, 0.07 and 0.1 are n-type semiconductors. Temperature dependences of resistivity, Seebeck coefficient, and thermal conductivity for the various compositions of Pb_1-xCd_xSe were analyzed. The dimensionless thermoelectric figure of merit ZT for these compounds was evaluated and discussed. It was found that Pb_0.95Cd_0.05Se exhibits the best thermoelectric performance. The maximum figure of merit (ZT) of Pb_0.95Cd_0.05Se is about 0.47 at 290 K.

Abstract: We have demonstrated structural and electronic properties of a series solar energy crystals Cu(Al_xIn_1-x)S₂ (0<=x<=1) by using measurement techniques of X-ray diffraction, polarized thermoreflectance (PTR), and X-ray photoelectron spectroscopy (XPS). Single crystals of Cu(Al_xIn_1-x)S₂ (0<=x<=1) (0 and E ^ polarizations. The PTR spectra clearly showed that the energy value of D increases with the increase of Al content x in the Cu(Al_xIn_1-x)S₂ (0<=x<=1) series due to the enhanced strain in the lattice. The composition-dependent crystal-field-splitting energies can be evaluated and determined to be D(x)= (10±2)+( 139±5)×x meV. Based on the experimental analyses, the crystal structure and valence-band structure of the Cu(Al_xIn_1-x)S₂ (0<=x<=1) (0<=x<=1) series are thus realized.

Abstract: A systematic optical characterization of a series of Mo1-xWxS2 (0 ≤ x ≤ 1) layered mixed crystals grown by chemical vapour transport method were carried out by using piezoreflectance (PzR) and Raman scattering measurements. From a detailed lineshape fit of the PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband transitions were determined accurately. The transition energies and their splittings vary smoothly with the tungsten composition x indicating that the nature of the band structure is similar for the Mo1-xWxS2 series compounds. The peaks of the two dominant first-order Raman-active modes, and , and several second-order bands are observed in the range of 250-450 cm-1. The peaks corresponding to mode show a one-mode type behavior, while the peaks of mode demonstrate two-mode type behavior for the entire series of Mo1-xWxS2 crystals. These behaviors were discussed on the basis of the lattice vibrational properties of 2H-MoS2 and 2H-WS2 compounds.

Abstract: In this study, Bi2Te3 was selected as a matrix material and an immiscible semiconductor material CdTe with a wider energy band gap was precipitated to form microstructures in it. For this purpose, a series of two-component composites (Bi2Te3)1-x/(CdTe)x, with x = 0.01, 0.05, 0.1, 0.15, 0.2 were prepared by melting, high speed rocking and quenching technique. The composition and microstructure of these two-component composites were examined by X-ray and SEM. Thermoelectric properties including Seebeck coefficient, electric resistivity, and thermal conductivity were measured from 30 K to 400 K. The compositional and temperature dependence of the microstructure and thermoelectric properties of (Bi2Te3)1-x/(CdTe)x composites were discussed. It is found that (Bi2Te3)0.99/(CdTe)0.01 exhibits a better thermoelectric performance than that of Bi2Te3 at elevated temperatures. The figure of merit (ZT) of (Bi2Te3)0.99/(CdTe)0.01 is about 0.64 at 400 K.

Abstract: Electronic structure of solar-energy related crystals of CuInS2 and CuAlS2 has been characterized using thermoreflectance (TR) measurement in the energy range between 1.25 and 6 eV. The TR measurements were carried out at room (~300 K, RT) and low (~30 K, LT) temperatures. A lot of interband transition features including band-edge excitons and higher-lying interband transitions were simultaneously detected in the low-temperature TR spectra of CuInS2 and CuAlS2. The energies of band-edge excitonic transitions at LT (RT) were analysed and determined to be =1.545 (1.535) and =1.554 eV (1.545 eV) for CuInS2, and =3.514 (3.486), =3.549 (3.522), and =3.666 eV (3.64 eV) for CuAlS2, respectively. The band-edge transitions of the and excitons are originated from the sulfur pp transitions in CuInS2 and CuAlS2 separated by crystal-field splitting. Several high-lying interband transitions were detected in the TR spectra of CuInS2 and CuAlS2 at LT and RT. Transition origins for the high-lying interband transitions are evaluated. The dependence of electronic band structure in between the CuInS2 and CuAlS2 is analysed and discussed.