| Summary: | 碩士 === 國立臺灣科技大學 === 電子工程系 === 105 === IV-VI compound semiconductors tin monosulfide and tin monoselenide have been grown by chemical vapor transport (CVT) method using I2 as a transport agent. Detailed characterization of the materials were carried out by using energy-dispersive X-ray spectroscopy (EDS) , X-ray diffraction (XRD) , high-resolution transmission electron microscopy (HRTEM) , Raman scattering techniques. The SnX material crystal structure is orthorhombic. Lattice constants of SnS are a = 4.2582Å, b = 3.9860Å, c = 11.4258Å and those of SnSe are a = 4.4499Å, b =4.1569Å, c = 11.6091Å, respectively.
According to Raman result, the vibration modes of SnS and SnSe show selection rule for the linearly polarized lights along a and along b axis. The SnX (X=S, Se) shows in-plane anisotropy on the c plane. From the results of transmittance and thermoreflectance, the band gaps and direct interband transitions also show polarization dependency with the linearly polarized lights along a and along b axis. In polarized thermoreflectance (PTR) experiment of SnS, it has a direct band-gap transition at 1.20 eV and an interband transition at 1.616 eV when E//b. For E⊥b, the 1.616 eV transition disappears, the direct-gap transition feature becomes narrow and its energy position shifts to 1.15 eV. Owing to the results of transmittance and thermoreflectance are comparable near band edge, we can infer that SnS is a direct band gap semiconductor. For the PTR result of SnSe, the direct band gap transition is at 0.99 eV and two transitions are at 1.258 eV and 1.374 eV when E//b. For the E⊥b condition, two transitions of 1.258 eV and 1.374 eV disappear, while direct-gap transition become narrow and shifts to 0.94 eV. The SnSe is also a direct band gap semiconductor.
For electrical measurements, the conductivity of SnX was determined to be 2.76×10-1 (-cm)-1 for SnS and 1.944×10-1 (-cm)-1 for SnSe, respectively. The surface photovoltage (SPV) measurements of SnS and SnSe also show in-plane anisotropy of the photovoltaic response spectra with E//b and E⊥b conditions. The SnX (X=S, Se) also reveal thermos-electric voltage generated from the samples by hot-probe measurements. From the polarity of thermoelectric voltage, the SnS and SnSe are p-type semiconductors. According to the optical and electrical measurement results, the SnX (X=S, Se) can have both photovoltaic and thermoelectric capability, and which possess the potential for application in the future optoelectronics and energy devices.
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