H. P. Hsu

Composition dependent lattice dynamics in MoSxSe(2–x) alloys

We report on room temperature, polarization-resolved Raman scattering measurements on layered crystals of the series MoSxSe(2–x) (0u2009≤u2009xu2009≤u20092) grown by chemical vapor transport technique. The results reveal two distinct sets of features related to the E2g1 and A1g modes of pure members of series. As composition x changes, the in-plane E2g1 mode shows two-mode behavior, whereas the out-of-plane A1g mode presents more complex evolution. The MoSe2-like branch reveals the splitting associated with the altering arrangement of S and Se atoms around Mo and the resulting changes in the dipole moment of the molecule. The X-ray diffraction measurements confirm that the samples are single-phase materials of 2H-type structure over the entire range of the sulfide composition x, while the scanning transmission electron microscopy imaging reveals a random arrangement of the S and Se atoms. Modified random-element-isodisplacement model is adopted to predict the behavior of the individual modes in the alloys. The model succ...

Above-room-temperature photoluminescence from a strain-compensated Ge/Si0.15Ge0.85 multiple-quantum-well structure

Photoluminescence (PL) of a strain-compensated Ge/Si0.15Ge0.85 multiple-quantum-well (MQW) structure was studied above room temperature, in the range of 300–440u2009K. Both direct and indirect radiative recombination PL features were observed. The relative intensity of direct to indirect recombination markedly increases with the increase of temperature. The enhancement of PL from direct recombination above RT has been attributed to the thermal excitation of carriers from L-type to Γ-type confined states. This extends the potential applicability of Ge/SiGe MQW as light emitters on a Si-based platform and is favorable for applications in metal-oxide-semiconductor integrated circuits which normally operate above RT.

Piezoreflectance study of near band edge excitonic-transitions of mixed-layered crystal Mo(SxSe1-x)2 solid solutions

The temperature dependence of the spectral features in the vicinity of the direct band edge of mixed-crystals Mo(SxSe1-x)2 solid solutions is measured in the temperature range of 25–295u2009K by using piezoreflectance (PzR). The near band-edge excitonic transition energies of Mo(SxSe1-x)2 solid solutions were determined accurately from a detailed line-shape fit of the PzR spectra. The near band-edge excitonic transition energies were found to vary smoothly with the increase of S content x, indicating that the natures of the direct band edges of Mo(SxSe1-x)2 solid solutions are similar. The temperature dependences of near band edge transition energies were analyzed using Bose-Einstein expressions in the temperature range from 25 to 295u2009K. The parameters that described the temperature variation of the energies and broadening function of the excitonic transitions were evaluated and discussed.

Temperature dependent surface photovoltage spectra of type I GaAs1−xSbx/GaAs multiple quantum well structures

We present temperature dependent surface photovoltage spectra of GaAs1−xSbx/GaAs multiple quantum well structures. Our previous studies [Sitarek et al., J. Appl. Phys. 105, 123523 (2009)] have identified all features present in the surface photovoltage spectra and showed weak type-I band alignment in the investigated GaAsSb/GaAs system. By analyzing the changes in the relative intensity of features near the energy of fundamental transitions caused by the temperature variations, we are able to determine the energy difference between electronic states localized in GaAsSb quantum well and conduction band edge in the GaAs barrier. In addition, the Bose-Einstein parameters that describe the temperature dependences of 1hh-1e transitions are evaluated and discussed.

Temperature dependence of energies and broadening parameters of near band edge interband excitonic transitions in wurtzite ZnCdMgSe

An optical characterization of two wurtzite ZnCdMgSe crystalline alloys grown by the modified high-pressure Bridgman method was carried out by temperature-dependent photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 10–300 K and photoreflectance (PR) measurements between 300–400 K. Low temperature PL spectra of the investigated samples consisted of the excitonic line, the “edge-emission” due to radiative recombination of shallow donor-acceptor pairs and a broad band related to recombination through deep level defects. Three excitonic features, A, B, and C, in the vicinity of band edge were observed in the CER and PR spectra. The peak positions of band edge excitonic features in the PL spectra were shifted slightly toward lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase in the CER-PL shift with the increasing of Mg content in the investigated crystals was explained by the rising of ...

Doping with Nb enhances the photoresponsivity of WSe2 thin sheets

In this study, we used chemical vapor transport to grow undoped and niobium (Nb)-doped tungsten diselenide (WSe2) thin sheets and then investigated the structural and photoelectronic characteristics of both samples. X-ray photoelectron spectroscopy confirmed the presence of Nb atoms in the Nb-doped WSe2 sample. Hall effect measurements of the transport behavior of the carriers in the undoped and Nb-doped WSe2 indicated that the Nb-doped WSe2 was of p-type and had a higher carrier concentration and lower mobility than the undoped WSe2. The current density–voltage characteristics indicated that doping with Nb led to a decrease in resistance. Photoconductivity measurements revealed that the responsivity of the Nb-doped WSe2 was an order of magnitude greater than that of the undoped WSe2. Thus, doping Nb atoms into WSe2 not only provides effective carriers but also enhances the photoresponsivity significantly. Accordingly, doping WSe2 with Nb atoms would appear to be useful for the fabrication of highly sensi...

Photoreflectance study of direct-gap interband transitions in Ge/SiGe multiple quantum wells with Ge-rich barriers

Photoreflectance (PR) was used to study the direct-gap interband transitions in strain-compensated Ge/SiGe multiple quantum well (MQW) structures with Ge-rich SiGe barriers grown by low-energy plasma-enhanced chemical vapor deposition. The PR spectra revealed a wide range of possible optical transitions in the MQW structure. Detailed lineshape fits to the PR spectra and comparison to a theoretical calculation based on the envelope-function approximation with conduction band-offset and stain compensation factor as adjust parameters led to the identification of various interband transitions. The results demonstrated that PR is a powerful technique for nondestructive optical characterization of Ge/SiGe MQW structures.

Contactless Electroreflectance and Photoluminescence Study of the Sb Surfactant Effects on InGaAsN Multiple Quantum Wells

The InGaAsN/GaAs multiple quantum wells structure grown on GaAs substrate with Sb surfactant treatment by metalorganic vapor-phase epitaxy has been characterized as a function of temperature in the temperature range between 15 and 300 K by the techniques of contactless electroreflectance (CER) and photoluminescence (PL). The interband transition energies are determined via a lineshape fit to the CER spectra and exhibit a blue-shift with the incorporation of Sb. A careful analysis of the CER and PL spectra has led to the identification of various excitonic transitions. The parameters that describe the temperature dependence of the interband transition energies are evaluated. The optical behavior results from the Sb surfactant treatment are presented and discussed.