S.Y. Hu

The electrical and optical anisotropy of rhenium-doped WSe2single crystals

Single crystals of rhenium-doped WSe2 with large edge plane have been grown by the chemical vapour transport method using bromine as the transporting agent. From the x-ray diffraction patterns, both the doped and undoped crystals are found to crystallize in the 2H structure. The role of rhenium in affecting both the electrical and optical properties of the WSe2 crystals is examined. The thicker Re-doped WSe2 samples enable easier exploration of the electrical and optical anisotropies of the materials both parallel and perpendicular to the crystal c-axis. The samples are n-type in nature from Hall measurements and the conductivity anisotropy decreases drastically as a result of doping. Photovoltage measurements revealed that both undoped and Re-doped WSe2 are indirect semiconductors. The indirect band gap of the doped sample showed considerable red shift compared with the undoped sample and also exhibited anisotropy along and perpendicular to the c-axis. The anisotropy in the band gap is attributed to crystal anisotropy, which was also studied via the polarization-dependent electrolyte electroreflectance measurements taken along the van der Waals plane (; ) and the as-grown edge plane (, ; ). The strong dependence of excitonic transitions A and B on polarization compared with that of the undoped samples.

Observation of persistent photoconductivity in 2H‐MoSe2 layered semiconductors

We report the observation of persistent photoconductivity (PPC) effect in 2H‐MoSe2 layered semiconductors. The decay behavior of PPC can be well described by a stretch-exponential function. Experimental results indicate that the lattice relaxation of DX-like impurity is responsible for PPC in MoSe2. In addition, the small capture barrier created by lattice relaxation and the temperature-dependent resistance measurement exhibit that the magnitude of PPC effect in MoSe2 is small, consistent with the PPC behavior in indirect gap materials.

Raman spectra investigation of InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition

Raman analysis of the A1 (LO) mode of AlGaN-like phonons of the InAlGaN/GaN heterostructures in the composition range 1.38% ≦ In ≦ 2.73% and 8.01% ≦ Al ≦ 13.97% is presented. The line shape of A1 (LO) mode of AlGaN-like phonons was observed to exhibit a significant asymmetry and Raman linewidth toward the lower energy side. The spatial correlation model is discussed and is shown to account the line shape. The spatial correlation model calculations also indicate the lack of a long-range order in the higher Al to In ratio of InAlGaN/GaN alloys. These results were confirmed by x-ray diffraction and the correlation length L decreases as the increasing of Al to In ratio corresponding to the absence of the long-range order in the alloy. The Raman linewidth of the AlGaN-like A1 (LO) mode was found to exhibit a maximum at the higher Al to In ratio indicative of a random disordered alloy system.

Temperature-Dependent Excitonic Luminescence in ZnO Thin Film Grown by Metal Organic Chemical Vapor Deposition

In this study, the excitonic luminescence behaviors of ZnO thin films in the temperature range of 10–300 K were investigated. The photoluminescence (PL) spectrum exhibits the bound-exciton emission and the donor-acceptor recombination accompanying its multiphonon replicas at low temperatures. The observed features exhibit redshift with an increase in temperature, and the temperature dependence of the peak position was analyzed by the Varshni empirical expression. The study showed the dominant presence of bound excitonic transition below 160 K or free excitonic transition at higher temperatures for the observed PL spectrum. The free exciton emission can be observed up to room temperature. The Debye temperature of ZnO was evaluated by taking into account the elastic constants of ZnO and utilized as a parameter in the Varshni empirical expression giving an accurate description of the free exciton emission behavior up to room temperature.