Spark Plasma Sintering of Undoped SnO_2 Ceramics

Various parameters in spark plasma sintering（SPS）,such as sintering temperature,holding time,heating rate,and pressure,were adopted to investigate their effects on the densification of pure SnO2 power.The obtained experimental data show that the SPS process enhances densification.The high-density undoped SnO2 ceramics （96.6% of theoretical） was obtained at much lower temperature （1000℃）,within a much shorter time,compared to the conventional sintering process.The high-density undoped SnO2 ceramics （96.6% of theoretical） were obtained by SPS,under the condition of temperature：1000℃,pressure：40MPa,heating-rate：200℃/min,and holding time：3min

Temperature Effects on Gas Sensing Properties of Electrodeposited Chlorine Doped and Undoped n-Type Cuprous Oxide Thin Films

As one of the most widely used domestic fuels, the detection of possible leakages of Liquefied Petroleum (LP) gas from production plants, from cylinders during their storage, transport and usage is of utmost importance. This article discusses a study of the response of undoped and chlorine doped electrodeposited n-type Cuprous Oxide (Cu2O) films to of LP gas. Undoped n-type Cu2O films were fabricated in an electrolyte bath containing a solution of sodium acetate and cupric acetate whereas n-type chlorine doped Cu2O thin films were prepared by adding a 0.02 M cuprous chloride (CuCl2) into an electrolyte solution containing lactic acid, cupric sulfate and sodium hydroxide. The n-type conductivity of the deposited films was determined using spectral response measurements. The structural and morphological properties of the fabricated films were monitored using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Due to doping, the overall conductivity of the chlorine doped n-type Cu2O films increased by several orders of magnitude. The temperature dependent gas responses of both the undoped and chlorine doped n-type Cu2O thin films to the LP gas was monitored by measuring the electrical resistance (R), and using the contact probe method at a constant gas flow rate of 0.005 ml/s. Upon exposure to gases, both doped and undoped films showed a good response to the gas by increasing/decreasing the electrical resistance by ΔR. The undoped n-type Cu2O thin films showed a negative response (ΔR 2O thin films initially showed a positive response (ΔR > 0) to the LP gas which then reversed its sign to give a negative response which peaked at 52°C. The positive response shown by the chlorine doped Cu2O films vanished completely at 42°C.

Vanadium Defects Formation Mechanism in Undoped GaN Grown on Silicon

V defects in GaN layer grown on Si (111) using metalorganic chemical vapor deposition (MOCVD) were investigated by atomic force microscopy (AFM), plan-view transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometer (EDS). Dislocations are the origination of V defects. Stress field around dislocations induce the concentration of C atoms, furthermore, slow growth rate on those {10-11} planes are suggested as being responsible for the initiation of V defects. The formation mechanism of V defects was discussed.

Superconductivity in Undoped CaFe_2As_2 Single Crystals

Single crystals of undoped CaFe2As2 are grown by an FeAs self-flux method, and the crystals are quenched in ice-water rapidly after high-temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature-dependent x-ray diffraction measurements. Superconductivity below 25 K is observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-ll superconductor. For comparison, we systematically characterize the properties of the furnace-cooled, quenched, and post-annealed single crystals, and find strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe2As2 single crystals.

Investigation of EL2 Defect in 10 MeV Electron Irradiated Undoped Semi－insulating LEC GaAs

The induces defects and especially the EL2 defect in 10 MeV electron irradiated undoped semi-insulating(SI) LEC GaAs samples were investigated by using optical transient current spectroscopy (OTCS)technique.The results indicate that the density of EL2 defect of irradiated GaAs decreases and the density of EL6 defect in-creases at lower fluence levels. At higher fluences, we observe an increase in density of the EL2 level, howev-er,the density of the EL6 is decreased. It is suggested that on lower fluences, 10 MeV electron irradiation causes the dissociation of the EL2 defect, and may be used to decrease the main donor level EL2 in SI-GaAs.

Effect of High-Temperature Annealing on Yellow and Blue Luminescence of Undoped GaN

The effect of high-temperature annealing on the yellow and blue luminescence of the undoped GaN is investi- gated by photoluminescenee （PL） and x-ray photoelectron spectroscopy （XPS）. It is found that the band-edge emission in the GaN apparently increases, and the yellow luminescence （YL） and blue luminescence （BL） bands dramatically decrease after annealing at 700℃. At the annealing temperature higher than 900℃, the YL and BL intensities show an enhancement for the nitrogen annealed GaN. This fact should be attributed to the increment of the Ga and N vacancies in the GaN decomposition. However, the integrated PL intensity of the oxygen an- nealed GaN decreases at the temperature ranging from 900℃ to 1000℃. This results from the capture of many photo-generated holes by high-density surface states. XPS characterization confirms that the high-density surface states mainly originate from the incorporation of oxygen atoms into GaN at the high annealing temperature, and even induces the 0.34eV increment of the upward band bending for the oxygen annealed GaN at 1000℃.

Hyperfine splitting and ferromagnetism in CdS:Mn nanoparticles for optoelectronic device applications

Manganese(Mn)doped cadmium sulphide(Cd S)nanoparticles were synthesized using a chemical method.It was possible to decrease Cd S:Mn particle size by increasing Mn concentration.Investigation techniques such as ultraviolet-visible(UV-Vis)absorption spectroscopy and photoluminescence(PL)spectroscopy were used to determine optical properties of Cd S:Mn nanoparticles.Size quantization effect was observed in UV-Vis absorption spectra.Quantum efficiency for luminescence or the internal magnetic field strength was increased by doping Cd S nanoparticles with Mn element.Orange emission was observed at wavelength~630 nm due to ^(4)T_1→^(6)A_1 transition.Isolated Mn~(2+)ions arranged in tetrahedral coordination are mainly responsible for luminescence.Luminescence quenching and the effect of Mn doping on hyperfine interactions in the case of Cd S nanoparticles were also discussed.The corresponding weight percentage of Mn element actually incorporated in doping process was determined by atomic absorption spectroscopy(AAS).Crystallinity was checked and the average size of nanoparticles was estimated using the X-ray diffraction(XRD)technique.Cd S:Mn nanoparticles show ferromagnetism at room temperature.Transmission electron microscopy(TEM)images show spherical clusters of various sizes and selected area electron diffraction(SAED)patterns show the polycrystalline nature of the clusters.The electronic states of diluted magnetic semiconductors(DMS)ofⅡ-Ⅵgroup Cd S nanoparticles give them great potential for applications due to quantum confinement.In this study,experimental results and discussions on these aspects have been given.

Raman Study of Defects in SI－GaAs and Se－doped Epitaxial Layer Irradiated by 10 MeV Electrons

Raman scattering measurements on Se-doped GaAs epitaxial layers and semi-insulating (SI) GaAs irradi-ated by 10 Mev electrons have been investigated. Several defect-related features were observed. We suggestthat the 220 cm ￣-1mode is attribute to As_1 which is associated, at least in part, with EL2 and EL12 defects.For Sedoped samples, the Raman peaks at 205 and 258 cm￣-1 may be due to vibrational modes in small clus-ters of arsenic, and the 77 and 185 ￣-1modes are probably associated with disorder-activated first order Ra-man scattering.Irradiated results show that the small clusters of arsenic and disorder state are increased with in-creasing irradiation fluences. Other Raman peaks will also be discussed in this paper.

Annealing Effects on the Structural, Optical, and UV Photoresponse Properties of ZnO Nanostructures Prepared by RF-Magnetron Sputtering at Different Deposition Temperatures

Undoped ZnO nanostructures were deposited on SiO2/Si substrates via radio 11：equency magnetron sputtenng at different deposition temperatures （room temperature, 200, 300, and 400 ℃）. The prepared samples were annealed at 500 ℃ for 2 h under an N2 flow. The structural, surface morphological, optical, and photoresponse characteristics of ZnO nanostructures as deposited and after annealing were then investigated. The energy bandgaps of all samples after annealing （3.22-3.28 eV） decreased compared with those of the as-deposited specimens. The barrier height increased when the deposition temperature increased and reached 0.77 eV at 400 ℃ after annealing with a leakage current of 0.17 gA at a 5 V bias. The UV photodetector device which was deposited at the optimal temperature of 300 ℃, has 12.51 × 10^3% pho- tosensitivity, 2.259 pA dark current, 0.508 s response time, and 0.466 s recovery time. The dark current significantly decreased for all samples after annealing. The proposed UV photodetectors exhibit high performance, high photosensi- tivity, shorter response and recovery times, and excellent stability at lower bias voltages of 5 and 2 V.