Growth,electrical conductivity and microindentation studies of CuInS2 single crystals

Summary Single crystals of copper indium disulfide (CuInS₂) have been grown by chemical-vapour transport technique (CVT) using iodine as the transporting agent. The obtained phase was checked by X-ray diffractometry and the presence of copper, indium and sulfur in the grown crystals was confirmed by Energy-Dispersive Spectrum Analysis (EDSA). The mechanical properties of the grown crystals were studied using microindentation analysis. Optical-transmission measurements were done to determine the energy gap of the grown crystals. The four-probe technique was used to measure the electrical properties of the grown crystals. The as-grown crystals were found to bep-type conducting and they were converted ton-type by suitable annealing treatment. The electrical parameters of bothn- andp-type crystals were measured.     

Electron and hole doping in NiO

We have investigated hole doped (by lithium) and electron-doped (by nickel metal) NiO with photoemission (PES), inverse photoemission (IPES) and low and high energy electron energy loss spectroscopy (EELS). Both types of doping create empty states approximately in the middle of the charge transfer gap of undoped NiO.     

Electron and hole injection in metal-oxide-nitride-oxide-silicon structures

The kinetics of electron and hole accumulation in metal-oxide-nitride-oxide-semiconductor structures is studied. Experimental data are compared with a theoretical model that takes into account tunnel injection, electron and hole capture by traps in amorphous silicon nitride SiN_x, and trap ionization. Agreement between experimental and calculated data is obtained for the bandgap width E _g = 8.0 eV of amorphous SiO₂, which corresponds to the barrier for holes Φ_h = 3.8 eV at the Si/SiO₂ interface. The tunneling effective masses for holes in SiO₂ and SiN_x are estimated at m _h ^* ≈ (0.4–0.5)m ₀. The parameters of electron and hole traps in SiN_x are determined within the phonon-coupled trap model: the optical energy W _opt = 2.6 eV and the thermal energy W _T = 1.3 eV.     

Electron and hole traps in N-GaAs crystals

Deep traps were measured and their electronic and optical properties were determined by junction capacitance techniques in n-GaAs crystals grown by different methods. Four electron-traps and four hole-traps were detected. An electron trap was not observed in LPE wafers. A hole trap at 0.45 eV above the top of the valence band was detected in all wafers measured here.     

Synthesis and Characterization of CuInS2 Nanoparticles

Optics and Spectroscopy - CuInS2 nanoparticles coated with ZnS are synthesized. The luminescence and absorption spectra of nanoparticles are measured and the dependences of the luminescence spectra...     

Electron and hole effective masses in self-assembled quantum dots

Electron and hole effective masses in self-assembled InAs/GaAs quantum dots are determined by fitting the energy levels calculated by a single-band model to those obtained by a more sophisticated tight-binding method. For the dots of various shapes and dimensions, the electron effective-mass is found to be much larger than that in the bulk and become anisotropic in the dots of large aspect ratio while the hole effective-mass becomes almost isotropic in the dots of small aspect ratio. For flat InAs/GaAs quantum dots, the most appropriate value for the electron and hole effective-mass is believed to be the electron effective-mass in bulk GaAs and the vertical heavy-hole effective-mass in bulk InAs, respectively.     

Structural and galvanomagnetic properties of CuInS2 films

Thin copper-indium-disulphide films were prepared by thermal evaporation technique. X-ray diffraction analysis of the compound used for evaporation showed a tetragonal polycrystalline structure. Differential Thermal Analysis (DTA) of this compound showed two exothermic peaks at 585 and 632 °C. Thin films with thicknesses of 0.14 and 0.27 nm have a deposition rate 10 nm/min, while those with thicknesses of 0.54 and 0.56 nm have a deposition rate 48 nm/min. The obtained films have polycrystalline structure as shown from the electron diffraction study. A growth process was detected in the films by transmission electron microscopy as the film thickness increases. The surface topography was revealed by scanning electron microscopy. The variations of Hall mobility and carrier concentration with magnetic induction were studied. The resistivity-temperature relationship was investigated, from which the activation energies before and after annealing were found to be 0.2, 0.3 and 0.055 eV, respectively.     

Temperature-dependent photoluminescence of CuInS2 quantum dots

Temperature-dependent photoluminescence (PL) spectroscopy of CuInS₂ core and CuInS₂/ZnS core–shell quantum dots (QDs) was studied for understanding the influence of a ZnS shell on the PL mechanism. The PL quantum yield and lifetime of CuInS₂ core QDs were significantly enhanced after the QD surface was coated with the ZnS shell. The temperature dependences of the PL energy, linewidth, and intensity for the core and core–shell QDs were studied in the temperature range from 92 to 287 K. The temperature-dependent shifts of 98 meV and 35 meV for the PL energies of the QDs were much larger than those of the excitons in their bulk semiconductors. It was surprisingly found that the core and core–shell QDs exhibited a similar temperature dependence of the PL intensity. The PL in the CuInS₂/ZnS core–shell QDs was suggested to originate from recombination of many kinds of defect-related emission centers in the interior of the cores.     

Electron and hole spectra of silicon quantum dots

In the framework of perturbation theory, the first several one-particle energies and wave functions for electrons and holes (six for each) in spherical silicon quantum dots are obtained in the envelope function approximation (kp method). It is shown that the model of an isotropic dispersion relation with the mean reciprocal effective mass is applicable for the ground state of holes in the valence band. Anisotropy of the dispersion relation, which takes place for bulk semiconductors, becomes significant for the electron ground state in the conduction band as well as for all excited (both electron and hole) states.     

CuInS2纳米晶的制备和发光性质

以十二硫醇为溶剂,通过选择合适的金属源制备了各种尺寸的CuInS₂量子点。观察到随着粒子的尺寸减小,其吸收和发光光谱明显蓝移,存在明显的量子尺寸效应。通过在CuInS₂纳米晶表面包覆ZnS壳层,发现随着壳层厚度增加,其发光量子效率明显提高,最大达到了48%;继续增加壳层厚度,其发光量子效率反而降低。进一步测量它们的荧光寿命,发现包覆ZnS壳层后的CuInS₂纳米晶的荧光寿命明显增加,证实表面包覆明显减少其表面的无辐射复合中心,提高了其发光效率。进一步制备了CuInS₂/ZnS核壳量子点发光二极管,并对其电致发光性质进行了研究。     

Electrical properties of cadmium and zinc doped CuInS2

The electrical resistivity and mobility of Cd and Zn doped CuInS₂ single crystals grown by a Bridgman technique have been investigated. Crystals annealed in Cd or Zn vapor at high temperatures (～ 800°C) exhibit degenerate behaviour while those crystals annealed at more moderate temperatures (～650°C) show ionized impurity conduction with a shallow activation energy on the order of 0.004 eV. Resistivities as low as 0.15 Ω-cm and mobilities as high as 90 cm²/v-sec have been observed.     

Electron hole generation and propagation in an inhomogeneous collisionless plasma

The generation of "trains" of electron holes in phase space due to an external electrostatic disturbance is investigated by using a Vlasov-Ampere code with open boundary conditions. Electron holes are produced mostly during the initial phase of the wave-plasma interaction, with a given drift velocity which is maintained until they exit the integration box, even in the presence of plasma inhomogeneities. They present macroscopic features, a dipolar electrostatic field and an electron density perturbation, which can be exploited for diagnostic purposes. Their equilibrium is intrinsically kinetic, in that they are accompanied by a stationary hole in the electron distribution function.     

Infrared study of the lattice vibrations in CuInS2

Infrared reflectivity and absorption spectra of CuInS₂ are measured in the wave-number range from 180 to 700cm^–1for the polarization directions perpendicular and parallel to the tetragonalc axis of the crystals. The optical dispersion parameters of the fundamental lattice modes are determined for both polarization directions and compared with previously published data. All the structures observed in the absorption spectra in the wavenumber range beyond the fundamental lattice bands can be interpreted in terms of two-phonon combination modes due to zone-centre phonons.     

Radiative and photoelectric properties of CuInS2 single crystals

The photoluminescence and photocurrent spectra of CuInS₂ single crystals grown by the Bridgman method are studied at temperatures of 80 and 300 K. The photosensitivity spectrum is observed in the shortwave photoluminescence band. From the watt-ampere characteristics of photoconductivity, a linear mechanism of recombination of minority charge carriers is established for an illumination level of up to 100 mW/cm² in the temperature range of 80–300K.