Ying-Chi Huang

Crystal structure and band-edge transitions of ReS2-xSex layered compounds

Abstract Single crystals of the ReS 2− x Se x solid solution series have been grown by the chemical vapor transport method using Br 2 as a transport agent. From analyzing the X-ray patterns, the series crystals are determined to be single-phase and crystallized in the triclinic layered structure. The lattice parameters of the compound series are evaluated and discussed. The optical absorption edge was measured on basal-plane at room temperature. The results reveal that ReS 2− x Se x are indirect semiconductors and their energy gaps are determined. The band gap energy varies smoothly with the Se composition x , indicating that the nature of band edges are similar for the end members ReS 2 and ReSe 2 , and the compounds of intermediate compositions.

A bifunctional copolymer additive to utilize photoenergy transfer and to improve hole mobility for organic ternary bulk-heterojunction solar cell.

To realize the high efficiency organic photovoltaics (OPVs), two critical requirements have to be fulfilled: (1) increasing the photon energy absorption range of the active layer, and (2) improving charge separation and transport in the active layer. This study reports the utilization of THC8, a novel fluorescence-based polymer containing propeller-shaped di-triarylamine and fluorene moieties in the active layer consisting of poly-3-hexylthiophene and [6,6]-phenyl-C61-butyric acid methyl ester to form a ternary bulk heterojunction. The results showed that the high absorbance and strong fluorescence of THC8 at 420 and 510 nm, respectively, broadened the spectral absorption of the OPV, possibly through Förster resonance energy transfer. In addition, the morphology of the device active layer was improved with the addition of a suitable amount of THC8. Consequently, the charge transport property of the active layer was improved. The best power conversion efficiency (PCE) of the device with THC8 was 3.88%, a 25% increase compared to the PCE of a pristine OPV.

Improving the efficiency of an organic solar cell by a polymer additive to optimize the charge carriers mobility

We investigate the effect of a high hole mobility triarylamine-based conjugated polymer on a bulk hetero-junction organic solar cell. We employed a polymer blend consisting of poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and poly(N-(4 -(9,9-dioctyl-fluoren-2-yl)phenyl)-N,N′,N′-triphenyl-l,4-phenylenediamine) (PFLAM) as active materials. The hole mobility of PFLAM is ∼10−3 cm2 V−1 s−1, which is similar to the electron mobility of PCBM. Addition of PFLAM improves the hole mobility of the photovoltaic cell augmenting the charge balance of the system. The overall efficiency gain for such a device is 34%.

Room-temperature photoreflectance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with varied quantum well compositional profiles

Using room-temperature photoreflectance (PR) and photoluminescence (PL) we have characterized four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures with varied quantum well compositional profiles. Several features from the InGaAs modulation doped quantum well portion of the samples have been observed in addition to signals from the AlGaAs, GaAs, and GaAs/AlGaAs superlattice (SL) buffer layer. The PR spectra from the InGaAs quantum well channel can be accounted for by a line shape function which is the first derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the concentration of two-dimensional electron gas in addition to the energies of the intersubband transitions. The lowest lying intersubband transition has been confirmed by a comparison of the PR and PL spectra. From the difference of intersubband transition energies, the surface segregation effect...

Room-temperature phototransmittance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with varied quantum well compositional profiles

We have studied the effects of the two-dimensional electron gas (2DEG) and indium surface segregation in four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures using room-temperature phototransmittance (PT) and photoluminescence (PL) measurements. The PT spectra from the InGaAs modulation-doped quantum well channel can be accounted for by a lineshape function which is the first-derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the concentration of 2DEG in addition to the energies of the intersubband transitions. The lowest-lying intersubband transition has been confirmed by a comparison of the PT and PL spectra. From the difference of intersubband transition energies, the surface segregation effects of indium atoms are demonstrated.

Contactless electroreflectance study of strained Zn0.79Cd0.21Se/ZnSe double quantum wells

We have studied various excitonic transitions of strained Zn0.79Cd0.21Se/ZnSe double quantum wells, grown by molecular beam epitaxy on (100) GaAs substrates, using contactless electroreflectance (CER) at 15 and 300 K. A number of intersub-band transitions in the CER spectra from the sample have been observed. An analysis of the CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The conduction-band offset Qc is used as an adjustable parameter to study the band offset in the strained Zn0.79Cd0.21Se/ZnSe system. The value of Qc is determined to be 0.67±0.03.

Near-band-edge optical properties of molecular beam epitaxy grown ZnSe epilayers on GaAs by modulation spectroscopy

A study of near-band-edge optical properties of ZnSe epilayers grown on GaAs substrates using various modulation techniques is presented. We compare the contactless electroreflectance (CER) and piezoreflectance spectra to ascertain that our ZnSe epilayers of 1.2 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. The defect-related transitions near the ZnSe/GaAs interface are also compared by identifying the photoreflectance and other spectra. In addition, in order to observe the temperature-dependent energy splitting and strains, we present a detailed investigation of the heavy-hole and light-hole related transition energies as a function of temperature in the 15–200 K range by identifying the excitonic signatures in the CER spectra. We have also calculated the energy splitting between heavy-hole and light-hole valence bands by utilizing the temperature-dependent elastic constants for ZnSe and the thermal expansion coefficients for ZnSe and GaAs. Both the experimental result and ...

THE STRUCTURAL AND OPTICAL PROPERTIES OF HIGH QUALITY ZNTE GROWN ON GAAS USING ZNSE/ZNTE STRAINED SUPERLATTICES BUFFER LAYER

This work studied the structural and optical properties of ZnTe epilayers grown on GaAs substrates with ZnSe/ZnTe strained superlattices buffer layers. Material properties were characterized using cross-sectional transmission electron microscopy, photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR). PL spectra clearly distinguished the strong free exciton peaks, weak donor-acceptor pair, Y lines, and oxygen-bound exciton peaks, indicating the high quality of the films. In addition, the CER and PzR spectra were compared to ascertain that ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. An attempt was also made to identify the origins of the near-band-edge transitions of ZnTe epilayer in the CER and PzR spectra by comparing these spectra with PL spectra and the second harmonic frequency CER. By doing so, the interference below the band gap of ZnTe could be effectively eliminated. Moreover, the energy splitting between heav...

Temperature dependence of the band-edge exciton of a epilayer on GaAs

We have measured the temperature dependence of the spectral features near the direct bandgap of a film, grown by molecular beam epitaxy on a (001) GaAs substrate, in the temperature range between 15 and 300 K using contactless electroreflectance. The parameters that describe the temperature variations of the energy and broadening function of the band-edge exciton have been evaluated.

Contactless electroreflectance and piezoreflectance studies of temperature-dependent strain in ZnTe/GaAs heterostructures with ZnSe/ZnTe superlattice buffer layers

The temperature-dependent optical properties of ZnTe epilayers grown on GaAs substrates by molecular beam epitaxy with and without ZnSe/ZnTe strained-layer superlattice (SLS) buffer layers have been studied using contactless electroreflectance (CER) and piezoreflectance (PzR). Our ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain according to the results shown in CER and PzR spectra. Furthermore, the strain induced energy splitting between heavy- and light-hole valence bands in the ZnTe epilayer can be reduced by using the ZnSe/ZnTe SLS buffer layers. We have also justified the temperature-dependent energy splitting between heavy- and light-hole valence bands for ZnTe through theoretical calculations. Discrepancy between experiments and calculations indicates that the residual mismatch-induced strain as well as the thermally induced strain during cooling must be taken into account at the same time.