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We report a detailed temperature dependent study of polarized piezoreflectance (PzR) for two GaInP epilayers that exhibit different degrees of ordering in the range between 25 and 500 K. The polarized PzR measurements showed anisotropic character along the [110] and [1 0] directions for the ordered sample. The PzR spectra were fitted using the first derivative of a Lorentzian line shape functional form. The direct band-to-band, crystal-field splitting to band and spin-orbit splitting to band transition energies which are denoted as E 0 , E 0 + c and E 0 + s respectively, at various temperatures, were accurately determined. The temperature dependences of these near band-edge critical point transition energies were analysed by the Varshni expression and an expression containing the Bose-Einstein occupation factor for phonons. The parameters that describe the temperature variation of the transition energies are evaluated and discussed.

Au-doped rhenium disulfide (Au-ReS 2 ) layer crystals were grown by chemical vapour method with iodine as the transporting agent. The thin samples were investigated by polarization-dependent transmittance ( T ) and photoreflectance (PR) at 20 K with the optical polarization along and perpendicular to the crystal b -axis. The Au-ReS 2 showed band gap anisotropy with respect to the b -axis. By comparing with the undoped sample, the indirect band gap deduced from the polarization-dependent T spectra shows a more pronounced red shift than the band edge excitonic transitions determined from polarization-dependent PR spectra.

Several samples of Zn–Sb alloys were prepared by doping with different amount of In (0, 5, 10 and 15 at%) using cold-pressing and sintering techniques. The X-ray powder diffraction patterns reveal that In-doped samples are complex phase mixture, which consist of β-Zn4Sb3 as the main phase and ZnSb, InSb and In as secondary phases. The thermoelectric properties for these alloys were studied by means of thermal and carrier transport measurement in the temperature range between 40 and 700 K. For all studied samples, the Seebeck coefficients were found to be positive in whole the temperature range under investigation. It suggests that the hole-type carriers dominate the thermoelectric transport. Temperature dependence of electric resistivity, Seebeck coefficient and thermal conductivity for these alloys were analyzed. From the experimental analysis, the indium doping in Zn–Sb alloys leads to an increase in Seebeck coefficient and electric resistivity and leads to a decrease in thermal conductivity of β-Zn4Sb3-based materials. The dimensionless figure of merit, ZT for these alloys were evaluated and discussed. It was found that the sample with 10 at% indium doping exhibits a larger figure of merit in the 40–700 K temperature range.

A GaAs/InGaAs quantum-well structure was grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The quantum well was graded from 25% to 15% indium (from the bottom to the top of the channel). Hall measurements were made to characterize the concentration and mobility of the two-dimensional electron gas (2DEG). The temperature-dependent photoluminescence (PL) and photoreflectance (PR) spectra of the structure of interest were obtained. Various intersuband features were observed in the PR spectra. Furthermore, a 1.5 μm gate-length high-electron mobility transistor (HEMT), fabricated on these layers, had an extrinsic transconductance of 127 mS/mm. The optical and electrical characteristics were determined simultaneously.

The electronic structures of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ single crystals are investigated using a first-principles density-of-states (DOS) calculation, ultraviolet photoelectron spectroscopy (UPS), and electrolyte electroreflectance (EER). The total and partial DOS were calculated by the full-potential linearized-augmented-plane-wave method. From the calculations, the main contribution near the band edge of $\mathrm{Re}{X}_{2}$ $(X=\mathrm{S},\mathrm{S}\mathrm{e})$ is determined to be dominated by the nonbonding Re d orbitals. The valence-band DOS is experimentally verified by the UPS measurements. EER measurements were performed in the energy range of 1.3\char21{}6 eV. The EER spectra exhibit sharp derivativelike structures in the vicinity of the band-edge excitonic transitions as well as higher-lying interband transitions. Transition energies are determined accurately. From the experimental and the theoretically calculated results, probable energy-band structures of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ are constructed.

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.

Single crystals of rhenium-doped MoS 2 have been grown by chemical vapor transport method using bromine as a transporting agent. Single crystalline platelets up to 10×10 mm 2 surface area and 2 mm in thickness were obtained. By analyzing the X-ray diffraction profile, the structure of the single crystals shows rhombohedral symmetry. Temperature-dependent electrical resistivity and Hall coefficient were measured between 20 and 300 K using the van der Pauw technique. The hall coefficient measurement indicates that all the samples are n-type in nature. The optical absorption measurements reveal that rhenium-doped MoS 2 is an indirect semiconductor. The energy gap shows a red shift by increasing the doping concentrations.

The temperature dependent electrical transport properties of n-type ReS2 and ReSe2 were investigated over an extended range from 30 to 630 K. The activation energies for the impurity carriers in the temperature range of 250–350 K for n-type ReS2 and ReSe2 were determined to be 178 and 103 meV, respectively. From the high temperature conductivity measurement, the thermal band gaps for the intrinsic carriers were evaluated to be 1.19 eV for ReS2 and 0.98 eV for ReSe2. We have also reported for the first time the electrical anisotropy of the ReS2 and ReSe2 single crystals in the van der Waals plane by measuring the resistivity along the different crystal orientations.

In-plane anisotropic optical and electrical properties of layered ReS2 crystals are reported. The optical anisotropic effects for E || b polarization and E b polarization were studied through polarization-dependent piezoreflectance and optical absorption measurements. The resistivity perpendicular to the b-axis was shown to be several times larger than that along the b-axis.

Polarization-dependent absorption measurements of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ single crystals have been carried out in the temperature range between 25 and 500 K. A significant shift towards lower energies has been observed in the transmittance spectra of $\mathbf{E}\ensuremath{\parallel}\mathbf{b}$ polarization with respect to those corresponding to $\mathbf{E}\ensuremath{\perp}\mathbf{b}$ polarization. Analysis reveals that the absorption edges of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ are indirect allowed transitions. The parameters that describe the temperature dependence of the absorption edges with different polarizations in the van der Waals plane are evaluated. The results indicate that the electron-phonon coupling constants for $\mathbf{E}\ensuremath{\parallel}\mathbf{b}$ polarization are considerably larger than those of $\mathbf{E}\ensuremath{\perp}\mathbf{b}$ polarization.

We have measured the temperature dependence of the spectral features in the vicinity of direct band-edge excitonic transitions of single crystals from 25 to 300 K using piezoreflectance (PzR). From a detailed lineshape fit of the PzR spectra, the energies and broadening parameters of the A and B excitons have been determined accurately. The origin of these excitonic transitions is discussed. The transition energies and their splittings vary smoothly with the tungsten composition x, indicating that the natures of the direct band edges are similar for the compounds. In addition, the parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed.

Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces where the layers are electronically and vibrationally coupled. Isolated monolayers show changes in electronic structure and lattice vibration energies, including a transition from indirect to direct bandgap. Here we present a new member of the family, rhenium disulphide (ReS2), where such variation is absent and bulk behaves as electronically and vibrationally decoupled monolayers stacked together. From bulk to monolayers, ReS2 remains direct bandgap and its Raman spectrum shows no dependence on the number of layers. Interlayer decoupling is further demonstrated by the insensitivity of the optical absorption and Raman spectrum to interlayer distance modulated by hydrostatic pressure. Theoretical calculations attribute the decoupling to Peierls distortion of the 1T structure of ReS2, which prevents ordered stacking and minimizes the interlayer overlap of wavefunctions. Such vanishing interlayer coupling enables probing of two-dimensional-like systems without the need for monolayers.

Recently, rhenium disulfide (${\mathrm{ReS}}_{2}$) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ${\mathrm{ReS}}_{2}$ crystals. Unlike the well-known members of transition metal dichalcogenides (TMDs), ${\mathrm{ReS}}_{2}$ crystallizes in a stable distorted-$1T$ structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ${\mathrm{ReS}}_{2}$. Experimentally, irradiation with 3-MeV He${}^{+2}$ ions was used to break the strong covalent bonds in ${\mathrm{ReS}}_{2}$ flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic $\ensuremath{\alpha}$ particle irradiation. In order to understand the energetics of possible vacancies in ${\mathrm{ReS}}_{2}$ we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1--3${\ensuremath{\mu}}_{\mathrm{B}}$. Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.

ReS2 and ReSe2 are diamagnetic indirect semiconductors and belong to the family of transition-metal dichalcogenides crystallizing in the distorted octahedral layer structure of triclinic symmetry. We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps Egd in the temperature range betwee 25 and 526 K using piczoreflectance (PzR). The polarization dependence of the PzR spectra provides conclusive evidence that the features are associated with the interband excitonic transitions from different origins. From a detailed lineshape fit to the PzR spectra, we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated and discussed.

Polarization dependent piezoreflectance study of the band-edge excitons of ReS2 as a function of temperature in the range of 25 K ≤ T ≤ 525 K is reported. The parameters that describe the temperature variation of the transition energies and boradening function of the excitons have been evaluated and discussed.

We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps ${\mathrm{E}}_{\mathrm{g}}^{\mathrm{d}}$ of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ in the temperature range between 25 and 450 K using piezoreflectance (PzR). From a detailed line-shape fit to the PzR spectra we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated.

The optical absorption of synthetic ReS2 and ReSe2 single crystals is reported over a temperature range from 25 to 300 K. Analysis reveals that the absorption edges of ReS2 and ReSe2 are indirect allowed transitions. The indirect band gaps at various temperatures are determined and their temperature dependence is analyzed by the Varshni equation [Physica 34, 149 (1967)] and an empirical expression proposed by O’Donnel and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of energy gaps of these two materials are evaluated and discussed.

Powder samples of the IrTe system were prepared from the elements and characterized by an X-ray powder diffraction technique. Polycrystalline powder with pyrite phase was then utilized as the starting material for the subsequent crystal growth. Single crystals of Ir 3 Te 8 with mirror-like surfaces are grown in a tellurium flux by a slow cooling technique. The stoichiometry of the selected crystals is discussed. Optical reflectivity and electrical transport measurements have revealed a metallic behavior of the crystals. Magnetic susceptibility measurement indicates that Ir 3 Te 8 is a diamagnetic material.

The α-phase Bi(2)O(3) (α-Bi(2)O(3)) is a crucial and potential visiblelight photocatalyst material needless of intentional doping on accommodating band gap. The understanding on fundamental optical property of α-Bi(2)O(3) is important for its extended applications. In this study, bismuth oxide nanowires with diameters from tens to hundreds nm have been grown by vapor transport method driven with vapor-liquid-solid mechanism on Si substrate. High-resolution transmission electron microscopy and Raman measurement confirm α phase of monoclinic structure for the as-grown nanowires. The axial direction for the as-grown nanowires was along122. The band-edge structure of α-Bi(2)O(3) has been probed experimentally by thermoreflectance (TR) spectroscopy. The direct band gap was determined accurately to be 2.91 eV at 300 K. Temperaturedependent TR measurements of 30-300 K were carried out to evaluate temperature-energy shift and line-width broadening effect for the band edge of α-Bi(2)O(3) thin-film nanowires. Photoluminescence (PL) experiments at 30 and 300 K were carried out to identify band-edge emission as well as defect luminescence for the α-Bi(2)O(3) nanowires. On the basis of experimental analyses of TR and PL, optical characteristics of direct band edge of α-Bi(2)O(3) nanowires have thus been realized.

The surface formation oxide assists of visible to ultraviolet photoelectric conversion in α-In2Se3 hexagonal microplates has been explored. Hexagonal α-In2Se3 microplates with the sizes of 10s to 100s of micrometers were synthesized and prepared by the chemical vapor transport method using ICl3 as a transport agent. Many vacancies and surface imperfection states have been found in the bulk and on the surface of the microplate because of the intrinsic defect nature of α-In2Se3. To discover physical and chemical properties and finding technological uses of α-In2Se3, several experiments including transmission electron miscopy (TEM), X-ray photoelectron spectroscopy (XPS), surface photovoltage (SPV), photoluminescence (PL), surface photoresponse (SPR), photoconductivity (PC), and thermoreflectance (TR) measurements have been carried out. Experimental results of TEM, XPS, SPV, PL, and SPR measurements show that a surface oxidation layer α-In2Se3-3xO3x (0 ≤ x ≤ 1) has formed on the crystal face of α-In2Se3 in environmental air with the inner layer content close to In2Se3 but the outermost layer content approaching In2O3. The near band edge transitions of α-In2Se3 microplates have been probed experimentally by TR and PC measurements. The direct band gap of α-In2Se3 has been determined to be 1.453 eV. The SPV result shows a maximum quantum efficiency of the surface oxide α-In2Se3-3xO3x (0 ≤ x ≤ 1) that presents a peak photoresponse near 2.18 eV. The analyses of SPV, SPR, PL, TR, and PC measurements revealed that the surface oxide layer facilitates the conversion of the ultraviolet to the visible range while the native defects (Se and In vacancies) sustain photoconductivity in the near-infrared region. On the basis of the experimental results a wide-energy-range photodetector that combines PC- and SPR-mode operations for α-In2Se3 microplate has been made. The testing results show a well-behaved function of photoelectric conversion in the near-infrared to ultraviolet region via the auxiliary forming of surface oxide on the crystalline face of the α-In2Se3 microplates.

2D black phosphorus (BP) and rhenium dichalcogenides (ReX2 , X = S, Se) possess intrinsic in-plane anisotropic physical properties arising from their low crystal lattice symmetry, which has inspired their novel applications in electronics, photonics, and optoelectronics. Different from BP with poor environmental stability, ReX2 has low-symmetry distorted 1T structures with excellent stability. In ReX2 , the electronic structure is weakly dependent on layer numbers, which restricts their property tunability and device applications. Here, the properties are tuned, such as optical bandgap, Raman anisotropy, and electrical transport, by alloying 2D ReS2 and ReSe2 . Photoluminescence emission energy of ReS2(1-x) Se2x monolayers (x from 0 to 1 with a step of 0.1) can be continuously tuned ranging from 1.62 to 1.31 eV. Polarization behavior of Raman modes, such as ReS2 -like peak at 212 cm-1 , shifts as the composition changes. Anisotropic electrical property is maintained in ReS2(1-x) Se2x with high electron mobility along b-axis for all compositions of ReS2(1-x) Se2x .

Single-phase, near-stoichiometric zinc stannate (Zn2SnO4) nanowires were synthesized by a direct vapor transport process on c-Al2O3 substrates under optimized growth conditions. The optimal growth temperature for Zn2SnO4 nanowires is above 700 °C. Structural characterization indicates that the nanowires had the single crystal cubic spinel structure and diameters of ~90 nm and they grew in the [1 direction. Low-temperature photoluminescence (PL) shows a strong emission peak at 3.7 eV, attributed to the near-band-edge-emission of Zn2SnO4 nanowires. Temperature-dependent PL results are consistent with the Varshni equation, and the band gap is redshifted by ~170 meV as the temperature is increased from 11 to 300 K. The obtained direct gap of Zn2SnO4 nanowires is 3.546 eV at 300 K. A UV photodetector based on as-grown Zn2SnO4 nanowires was fabricated by a simple and cost-effective process. The Zn2SnO4 nanowires exhibited UV photoconductivity, and good selectivity and decent response to UV. The efficient fabrication method, high chemical and thermal stability of the Zn2SnO4 nanowire UV photodetector made it very suitable for use in harsh environments.

Multilayer InSe with a thickness above ~20 nm, is a direct semiconductor with a peak absorption wavelength approaching λ = 1000 nm, which is a promising candidate for solar-energy conversion and 2D optoelectronics devices. We present herein the experimental observations of thickness-dependent conductivity and photoconductive-responsivity spectrum in multilayer InSe as well as optically enhanced transconductance gain in the multilayer InSe metal-semiconductor-field-effect transistor (MESFET) illuminated by a halogen lamp. The voltage-current (V-I) measurement result shows multilayer InSe belongs to a p-type semiconductor, which can form a p-channel FET device. Thickness (t) dependent conductivity (σ) of multilayer InSe reveals about six-order variation from 5076 (Ω-cm)−1 (t = 5 nm) to 2.56 × 10−3 (Ω-cm)−1 (t = 184 μm, bulk) following a relationship of σ ∝ t −1.38. The highest conductivity in a thin InSe (e.g. t = 5 nm) is due to the increase of carrier density when the thickness is decreased. The photoresponsivity spectrum of a Ag-InSe-Ag multilayer photoconductor demonstrates a prominent peak absorption at 1.1 ~ 1.3 eV, matches well with the direct-free-exciton energy of the InSe. A multilayer p-InSe MESFET was tested by V-I measurement. The transconductance was measured and determined to be The gm value will enhance about three times when the MESFET was placed under the illumination of a tungsten halogen lamp of a lower power density ~0.5 mW cm−2. All the experimental results demonstrate multilayer InSe a promising 2D material available for microelectronics and optoelectronics applications.

Polarization dependence of band-edge excitonic transitions in Cu(Al(0.5)In(0.5))S(2) [denoted as Cu(AlIn)S(2)] has been characterized using polarized-thermoreflectance (PTR) measurements with E ||111and E perpendicular111polarizations in the temperature range between 30 and 320 K. The measurements were done on as-grown {112} surface of the chalcopyrite crystal. The polarization dependence of the band-edge transitions of Cu(AlIn)S(2) clearly showed that the E(A) exciton is present prominently with E ||111polarization while the E(B) exciton appears significantly only in the E perpendicular111polarized spectra. For the unpolarized spectra, both E(A) and E(B) features were combined. The E(A) feature is closely related to the E(0) transition, while the E(B) feature is that of E(0) + Delta(0) transition in the chalcopyrite. The crystal-field splitting energy of Delta(0) of Cu(AlIn)S(2) at the valence-band top is determined accurately by PTR experiments. Temperature dependences of transition energies of E(A) and E(B) transitions were analyzed. The band-edge excitons reveal an anomalous temperature-energy shift with increasing the temperatures from 30 to 320 K due to the variation of Cu d electrons' contribution to valence band that affected by the native defects inside Cu(AlIn)S(2). The PTR technique is more effective in studying the band-edge structure of the chalcopyrite crystal.

We present a practical experimental design for performing photoluminescence (PL) and photoreflectance (PR) measurements of semiconductors with only one PL spectroscopic system. The measurement setup is more cost efficient than typical PL-plus-PR systems. The design of the experimental setup of the PL-PR system is described in detail. Measurements of two actual device structures, a high-electron-mobility transistor (HEMT) and a double heterojunction-bipolar transistor (DHBT), are carried out by using this design. The experimental PL and PR spectra of the HEMT device, as well as polarized-photoreflectance (PPR) spectra of the DHBT structure, are analyzed in detailed and discussed. The experimental analyses demonstrate the well-behaved performance of this PL-PR design.

In this report the optical properties of ReS2 are characterized using polarized thermoreflectance (PTR) measurements in the temperature range between 25 and 300 K. Single crystals of ReS2 were grown by chemical vapor transport method using Br2 as a transport agent. Crystal morphologies of the as-grown rhenium disulfides were shown to possess two different kinds of the structural phases after crystallization. Observing in detail on the crystallized solids, the crystal phases can be essentially divided into two distinct types of normal triclinic layer and tetragonal structure. The PTR experiments were done with optical polarizations along and perpendicular to the crystals' b-axis for both layer and tetragonal crystals. From the experimental analyses of PTR measurements the occurrence of structural change in ReS2 is mostly probable caused by the atomic bonding deformation along -axis, which is parallel to the Re4 parallelogram consisted diamond chains. Temperature dependences of the band-edge transitions for the different structural phases of ReS2 are analyzed. The parameters that describe temperature variations of the transition energies and broadening parameters for both layered and tetragonal ReS2 are evaluated and discussed.

The band structure of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ series solids was probed experimentally by thermoreflectance (TR) measurements in the energy range of $2--6.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The TR measurements were done with the layered samples put in air and operated at room temperature. The TR spectra of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ series layers exhibit derivative-like spectral features in the vicinity of band edge as well as higher-lying interband transitions. The energies and broadening parameters of the interband transition features of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ are analyzed by detailed line-shape fits to the TR spectra. Compositional dependences of the transition energies of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ are analyzed. The presence of higher-lying interband transitions as well as the analysis of line-width broadening of the direct band gap indicates that the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ series should not be amorphous but belong to the polycrystalline solids with medium-range order. Based on the experimental results of TR measurements together with referring to previous band-structure calculations of crystalline and glassy $\mathrm{Ge}{X}_{2}$ $(X=\mathrm{S},\mathrm{Se})$, a probable experimental band structure near the fundamental edge of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ is proposed.

We report on the extensive structural and optical studies of Re-doped molybdenum disulfide (MoS2) grown by the chemical vapor transport (CVT) method using Br2 as a transport agent. To evaluate the influence of Re on the structural properties of crystals, we have conducted X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments. For optical characterization, we carried out piezoreflectance (PzR) and electrolyte electroreflectance (EER) measurements. The Re dopant clearly caused a structural change in MoS2 from a two-layer hexagonal (2H) structure to a three-layer rhombohedral (3R) structure, which has been clearly verified and identified herein.

The polarization dependence of band-edge excitonic transitions in Ag(In0.5Al0.5)S2 has been characterized using polarized thermoreflectance (PTR). The polarization-sensitive behavior of the and band-edge excitons was detected and characterized by angle-dependent PTR measurements from θ = 0 [] to 90° [] with respect to the chalcopyrite crystal's needle axis. The polarized photoconductivity of Ag(In0.5Al0.5)S2 has also been characterized using an array of white light-emitting diodes with the polarization angles ranging from 0 to 360°. The experimental results demonstrate that Ag(In0.5Al0.5)S2 is a suitable material for the fabrication of polarization-sensitive photodetectors applied in the visible region.

Excitonic transitions of the ${\mathrm{ReS}}_{2}$ triclinic layer semiconductor are studied using polarized photoreflectance (PPR) spectroscopy with optical polarizations along and perpendicular to the b axis in the temperature range of 25--300 K. The low-temperature PPR spectra reveal a prominent and enlarged excitonic series positioned at the higher-energy side with respect to previously identified band-edge excitons ${E}_{1}^{\mathrm{ex}}$ and ${E}_{2}^{\mathrm{ex}}$ [Phys. Rev. B 55, 15 608 (1997)]. A detailed line-shape analysis and the photosensitive characteristic of the excitonic sequence provide conclusive evidence that the series and those of band-edge excitons ${E}_{1}^{\mathrm{ex}}$ and ${E}_{2}^{\mathrm{ex}}$ are interband excitonic transitions originated from different origins. The possible origin of the excitonic sequence originates from nonbonding Re $5d$ ${t}_{2g}$ ${(d}_{\mathrm{xy}}{,d}_{{x}^{2}\ensuremath{-}{y}^{2}})$ states to antibonding S $3p$ ${\ensuremath{\sigma}}^{*}$ states. The sequence is shown to correspond to the Rydberg series starting with a principal quantum number $n=2.$ The temperature dependence of transition energies of the exciton series is analyzed. The parameters that describe the temperature variations of the excitonic transitions in ${\mathrm{ReS}}_{2}$ layers are evaluated and discussed.

The electronic transport properties in individual niobium disulphide (NbS2) nanoflakes mechanically exfoliated from the bulk crystal with three rhombohedral (3R) structure grown by chemical vapor transport were investigated. It is found that the conductivity values of the single-crystalline nanoflakes are approximately two orders of magnitude lower than that of their bulk counterparts. Temperature-dependent conductivity measurements show that the 3R-NbS2 nanoflakes exhibit semiconducting transport behavior, which is also different from the metallic character in the bulk crystals. In addition, the noncontinuous conductivity variations were observed at the temperature below 180 K for both the nanoflakes and the bulks, which is attributed to the probable charge density wave transition. The photoconductivities in the semiconducting nanoflakes were also observed under the excitation at 532 nm wavelength. The probable mechanisms resulting in the different transport behaviors between the NbS2 nanostructure and bulk were discussed.

III-VI compounds In2S3 and In2Se3 are potential optical-absorption materials used for solar energy conversion and photon detection applications. The natural surface oxide and intrinsic chalcogen vacancies in In2X3 (X = S, Se) facilitate photoelectric conversion in near infrared (NIR) to ultraviolet (UV) region. In this work, In2S3 and In2Se3 crystals have been grown by chemical vapor transport method using ICl3 as a transport agent. The as-grown In2S3 presents β phase with a tetragonal lattice (β-In2S3) while In2Se3 reveals a hexagonal layer structure of α phase (α-In2Se3). The band-edge property of β-In2S3 and α-In2Se3 has been evaluated by transmittance and thermoreflectance spectroscopy. The direct band gaps have been determined to be Eg = 1.935 eV for β-In2S3, and Eg = 1.453 eV for α-In2Se3, respectively. The optoelectronic and photoelectric conversion properties of III-VI In2S3 and In2Se3 are examined by surface photovoltage, surface photoconductive response, photoluminescence, and photoconductivity measurements. The experimental evaluations show In2X3 a well-functional material in photoelectric conversion and photodetection from near IR to UV region with the auxiliary of intrinsic defects and surface formation oxides existed in the chalcogenides.

The band edge and higher-lying interband transitions of CuAl(Se0.5S0.5)2 compound have been characterized using thermoreflectance (TR) measurements at room temperature. Single crystals of CuAl(Se0.5S0.5)2 were grown by chemical vapor transport method using ICl3 as the transport agent. The TR measurement results showed three interband transitions denoted as E1, E2, and E3 detected near the band edge of CuAl(Se0.5S0.5)2 chalcopyrite compound. The lowest-energy transition is the main band-edge transition positioned at E1 = 2.96 eV, the second is E2 = 3.05 eV, and the other higher-energy transition is E3 = 3.202 eV, respectively. Polarized-thermoreflectance (PTR) measurements showed that the E1 and E2 transition features are observed only with the linearly polarized light along the e|| (needle axis) direction while the E3 transition largely appears with the electrical field of e perpendicular (⊥) to the needle axis. The E1 and E2 transitions may originate from the valence-band top while the E3 transition is closely related to the valence-band splitting in the crystal. To characterize the anisotropic properties of electronic structure of CuAl(Se0.5S0.5)2, PTR measurements were carried out over a wide energy range of 2.5-6 eV. The interband transitions belonging to E|| or E⊥ polarization are respectively identified. On the basis of experimental analysis, the electronic structure near the fundamental band edge of CuAl(Se0.5S0.5)2 has thus been determined.

We have grown γ-phase layered In2Se3 crystals with various thicknesses from hundreds of nm to hundreds of μm. The as-grown layers showed different crystal colors from light yellow to dark red owing to their different thicknesses. The direct band edge of γ-In2Se3 has been probed by transmittance and thermoreflectance measurements at room temperature. An amorphous effect mainly dominates the thickness-dependent band gap changes of the selenide.

We have detected diffuse X-ray emission within the planetary nebula shell of Abell 30, using a 21.8 ks ROSAT Position Sensitive Proportional Counter observation. The spectrum is very soft, with almost all source counts at energies below 0.4 keV. The extended emission is real because most of the detected counts are within the 0.2-0.4 keV band, which is not seriously plagued by the electronic ghost-image problem. The best-fit Raymond & Smith plasma emission models give plasma temperatures of (2-4.5) × 105 K. The X-ray emission could arise from the hot shocked stellar wind or the central H-poor nebula shocking the ambient H-rich material. Low plasma temperatures are expected in shocked stellar wind, if there is a strong heat conduction at the interface between the hot shocked wind and the surrounding cool nebular shell, or if the fast stellar wind is mass-loaded by the ejecta clumps. Low plasma temperatures are also expected behind the moderate shock of the H-poor shell advancing into the H-rich envelope. High-resolution X-ray images of A30 are needed to determine more accurately the relationship between the X-ray emission and the nebular shells.

The evaluation of band gap, exciton, and electronic structure of an energy compound is crucial and essential. We report direct optical evidence of band gap, band-edge exciton, and white-light photoelectric conversion for a solar-energy material In6S7 herein. The direct gap of In6S7 is determined to be 0.93 eV. The value is extremely suitable for absorption of full sunlight spectrum extension to near infrared region.

We explore the structural, electronic, photoluminescent, metallic, and semiconducting characteristics of 2D pure Zn-metal nanoplates. The 2D pure Zn-metal nanoplates are synthesized by the hot-filament metal-oxide vapor deposition (HFMOVD) technique. They have an average diameter and thickness of ∼520 and ∼144 nm, respectively. The results of the electronic and crystalline structure studies reveal the 2D nanoplates to be pure Zn hexagonal crystals, which can provide a wide-range photoluminescence from ultraviolet (UV) to red light emissions at room temperature. The measured valence-band and the calculated band-structure of the 2D pure Zn-metal nanoplates verify that the UV and blue light arise from the 3d–sp interband transitions, while the green, yellow, and red lights come from the valence-conduction interband transitions at a bandgap that is only present in the 2D nanoplates. Therefore, the 2D pure Zn-metal nanoplates possess not only metallic, but also semiconducting characteristics.

β-In2S3 is a nontoxic window layer material usually used in a thin-film solar cell. Transition metal (TM)-incorporated In2S3 has been proposed to promote conversion efficiency in In2S3 because multi-photon absorption by an intermediate band (IB) would happen in the sulfide. In this paper, band-edge and photoelectric-conversion properties of Nb-substituted In2S3 have been probed by thermoreflectance (TR), photoconductivity (PC), and photo–voltage–current (Photo V–I) measurements. The crystals of niobium-incorporated In2S3 with different Nb contents of In1.99S3:Nb0.01, In1.995S3:Nb0.005, and undoped In2S3 were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent. X-ray diffraction measurements showed that the as-grown In2S3:Nb compounds are β-phase crystals with tetragonal structure. Lattice constants of the β-In2S3:Nb show shrinkage with the increase of the Nb content in the In2S3. Experimental TR spectra reveal four transition features including direct gap (E0), valence band to donor (EVS), valence band to IB (EIB), and acceptor to IB [Ed(IB)] transitions detected in the Nb-substituted In2S3 compounds. The band gap (E0) shows a reduction and the crystal color changes from fresh red, to red, to dark red with increasing Nb content in the undoped β-In2S3, β-In1.995S3:Nb0.005, and β-In1.99S3:Nb0.01. The PC and Photo V–I measurements verified that high photoelectric-conversion efficiency occurred in the β-In2S3 with higher niobium content. The intermediate band (IB) was formed by Nb substitution with indium in the β-In2S3 [i.e. detected by EIB≈1.52 eV and Ed(IB) ≈ 1.42 eV]. The IB state should mainly dominate the multi-photon absorption capacity and enhance the photoelectric-conversion yield in the indium sulfide.

Band edge property of whole series Cu(Al x In 1-x )S 2 (0≤x≤1) chalcopyrite crystals has been characterized using polarized-thermoreflectance (PTR) measurements in the temperature range between 30 and 320 K. The polarization dependency of band-edge transitions in the Cu(Al x In 1-x )S 2 (0≤x≤1) series clearly shows that E ∥ exciton (E o ) is only present in the E ∥ polarization while E ⊥ exciton (E o + Δ) appears merely in the E ⊥ polarized spectra. The energy separation between E ∥ and E ⊥ is due to the crystal field splitting energy (Δ) in the Cu(Al x In 1-x )S 2 crystalline series. The E o and E 0 +Δtransitions show an anomalous temperature-energy shift with respect to the increase of temperatures from 30 to 320 K. The anomalous temperature-energy shift reveals a maximum turn-over temperature T MTO appeared at different temperatures for each of the Cu(Al x In 1-x )S 2 (0 < x < 1) compounds. Both the crystal-field splitting energy (Δ) and maximum turn-over temperature (T MTO ) varied linearly with the Al composition x indicated that the variations of bonding and electronic structures of the Cu(Al x In 1-x )S 2 series are correlated with the change in lattice constants. The copper d electron's contribution should mainly dominate the band-edge nature in the Cu(Al x In 1-x )S 2 (0 < x < 1) chalcopyrite series.

The β-phase indium sulfide is a solar-energy absorption material with superior photoelectric conversion function applied in the near infrared to ultraviolet region due to a lot of vacant sites and surface states naturally existing in the crystals. In this study, the surface oxidation states of β-In2S3 single crystal have been characterized using surface photoconductive response (SPR) measurements in the temperature range between 30 and 300 K. In comparison with the traditional photoconductivity (PC) measurements, the SPR measurement characterizes a sample with an electric field perpendicular to the as-grown surface of β-In2S3, and this operation is similar to the general electric-field operation worked in a thin-film solar cell. There are a lot of defect, band-to-band, and surface-state transitions that can be detected in the temperature-dependent SPR spectra of 30–300 K. The defect transitions below band gap may come from the transitions of indium vacancies → sulfur vacancies in β-In2S3. The surface-state transitions are caused by higher-energy state transitions formed by β-In2S3−3x O3x . The intensities of some specific surface-state transitions are significantly enhanced in the SPR experiments as compared with those in the traditional PC measurements. The near-band-edge transitions below and above band gap of β-In2S3 are identified, and their temperature dependences of energy shift are analysed and discussed.

The solar-energy absorption material β-In2S3 was characterized using temperature-dependent photoconductivity (PC) measurements in the temperature range between 30 and 340 K in this study, and thermoreflectance (TR) and photoluminescence (PL) measurements were carried out to identify near-band-edge transitions in the β-In2S3 tetragonal crystal. The experimental analyses of PL, PC, and TR confirmed that β-In2S3 is a direct semiconductor with a band gap of 2.073 eV at 30 K and 1.935 eV at 300 K. The PL and PC spectra manifest some defect-related features in the β-In2S3 single crystal. Two defect emissions and two band-edge luminescences were simultaneously detected in the PL spectrum at 30 K, and the temperature-dependent PC spectra of β-In2S3 from 160 to 300 K reveal an additional defectlike or band-to-band feature with an energy located above the conduction band edge (EC). The temperature dependences of the PC transition features in the β-In2S3 defect crystal were analyzed. The origin and mechanism of all ...

Near band edge transitions of CuAlS2 chalcopyrite crystals have been characterized using temperature-dependent thermoreflectance (TR) spectroscopy in the temperature range between 30 and 340 K. A lot of interband transition features of EAn=2, EAn=3, EBn=2, EC, ED, EE, and EF were detected in the low-temperature TR spectrum of 3–6 eV at 30 K. Transition energies of the TR features of CuAlS2 are analyzed. Transition origins of the features are evaluated. Optical-electric conversion behavior of CuAlS2 was evaluated by photoconductivity (PC) measurement at 300 K. The PC spectrum reveals that the maximum photoresponse occurs near the energy positions of EAn=2, EAn=3, and EBn=2 features. The features are the fundamental band edge excitons which dominate photoelectric conversion behavior of CuAlS2 ultraviolet solar-energy material.

Single crystals of CuInS 2 were grown by a chemical vapor transport method using IC1 3 as a transport agent. The electronic structure of CuInS 2 was experimentally characterized by temperature-dependent thermoreflectance (TR) measurements in a wide energy range of 1.25-6 eV between 20 and 320 K. A pair of excitonic doublets of E ex1 1(d) and E ex2 1(d) near 3.4 eV were observed in the low temperature TR spectrum at 20 K. The temperature- energy shift of the E ex1 1(d) and E ex2 1(d) features identified the origin of the excitonic doublets. The E 1 excitons were confirmed to be interband transitions from the nonbonding Cu 3d state in the valence band (E v ) to the S 3p antibonding state at the conduction band (E C ) bottom. There were also a lot of derivative-like spectral features observed near the bandedge as well as high lying interband transitions of CuInS 2 . The origins of the critical-point transitions of CuInS 2 were evaluated. X-ray photoelectron spectroscopic (XPS) measurements were implemented to identify the band blocks below the top of the valence band. Based on the experimental analyses of TR and XPS, together with previous band-structure calculations of CuInS 2 , an experimental band structure that describes a fundamental band-edge structure of CuInS 2 was then constructed.

The band-edge property and built-in electric fields of two different Al(x)Ga(1-x)N/GaN(AlGaN/GaN) heterostructures (HSs) with and without an additional AlGaN inserted layer were studied by thermoreflectance (TR), photoluminescence (PL), and contactless electroreflectance (CER) techniques. The PL spectra characterize the band-edge luminescence property of GaN. Free exciton transitions of AlGaN and GaN were probed experimentally by TR. Prominent Franz-Keldysh oscillations (FKOs) of GaN and the opposite FKO phase of AlGaN were simultaneously detected by the additional AlGaN inserted sample with CER owing to the enhancement effect of built-in electric fields of GaN and AlGaN with the same polarity direction. Optoelectronics properties of the two HSs were characterized by the experimental analyses.

In this study we analyzed the structural and electronic properties of a new morphological form, one-dimensional (1D) Ta2O5 nanorod arrays, which were synthesized by hot filament metal vapor deposition. Field-emission scanning electron microscopy (FESEM) showed the 1D Ta2O5 nanorods to be arranged in a large-area high-density array about 50 nm wide and approximately 550 nm long. X-ray photoemission spectroscopy (XPS) revealed not only the electronic structures and chemical properties of the 1D Ta2O5 nanorods but also their stoichiometric Ta and O compositions. Photoluminescence (PL) spectra showed intensive green-light, yellow-light and red-light emissions at room temperature. These emissions simultaneously emerged from the trap levels of oxygen vacancies within the Ta2O5 bandgap. The emission results strongly indicate that the 1D Ta2O5 nanorods are good room-temperature visible-light emitters.

We have measured temperature-dependent photoreflectance (PR) and photoluminescence (PL) spectra of three symmetric linearly graded GaAs/GaInAs high electron mobility transistor (gHEMT) structures with opposite channel-band slopes in the temperature range between 30 and 300 K. Analyses of the PL and PR spectra indicate the existence of two-dimensional electron gas (2DEG) in the graded-channel well of the three gHEMTs in the temperature range of 30–300 K. The comparison of PL and PR spectra facilitates the identification of channel-well transitions in the gHEMTs with different channel-band slopes. Inter-subband transitions, Fermi-level energies, 2DEG and the built-in electric field of the three gHEMTs with dissimilar channel-band slopes are evaluated. The well widths for the gHEMTs of different channel-band slopes are estimated. The temperature dependences of the channel inter-subband transition energy and built-in electric field of the three gHEMTs are analyzed and discussed.