High Purity Se Research Articles

Multi-layered nanostructure Bi2Se3 grown by chemical vapor deposition in selenium-rich atmosphere

High quality multi-layered nanostructured bismuth selenide (Bi2Se3) with an asymmetric, elongated hexagonal morphology were obtained in a selenium-rich atmosphere by chemical vapor deposition (CVD) using high purity Se and Bi2Se3 powder as source materials. The measurement results reveal that Au particles only serve as the initial stage nucleation of the Bi2Se3 nanostructures at a low temperature. The Se powder in the source materials is helpful to ensure the stoichiometry and decrease the selenium vacancies, and causes the wide Bi2Se3 nanoribbons to grow along the lateral direction.

Sound velocity in liquid and glassy selenium

The speed of longitudinal sound waves at 7 and 22MHz has been measured in liquid, supercooled, and amorphous selenium, including the region around the glass transition temperature, Tg, near 35°C. In amorphous selenium the speed of shear waves at 7MHz was also measured. The experiments were performed with high purity Se (99.9999%) hermetically sealed in an evacuated silica ampoule. Four temperature regions with strongly different relaxation times can be distinguished between room temperature and the melting point: (1) a glassy state below Tg, which is stable on the time scale of the experiments, (2) a glassy state above Tg, which is metastable on the time scale of the experiments, (3) a region where homogeneous crystal nucleation occurs, and (4) a supercooled liquid, which is stable on the time scale of the experiments. Each region is marked by a change in the slope of the temperature dependence of the sound velocity. Near the glass transition temperature the velocities of longitudinal and transverse sound exhibit hysteresis with a step-like drop on heating and a more continuous rise on cooling. The step-like anomaly in sound velocity may be a general property of the glass transition.

Differential Scanning Calorimetry and X-Ray Photoelectron Spectroscopy Studies on Se100-xInx System

Chalocogenide glasses in the Se100-xInxsystem, in the composition range 0 ≤ x ≤ 29 (x in at%), were prepared from high purity Se and In. Differential scanning calorimetry results clearly indicate that the thermal stability of the glasses decreases upon increasing x. The values of the glass transition temperatures (Tg) were found to be composition independent which is interpreted by envisaging the formation of In2Se3 microclusters that do not contribute to the Tg values of the glasses and that these values remain to be essentially determined by that of the Sen chains of the Se glassy matrix. The X-ray photoelectron spectroscopy results lend further support to the formation of In2Se3 microclusters.

Crystallization of vacuum-evaporated Se studied by near infrared microscope

The crystallization of vacuum-evaporated Se can be greatly affected by impurities present either in the Se source or on the substrate surface [1-3]. The presence of oxygen (in the form of SeO2) in the Se source was found to enhance Se crystallization [1], due to formation of new nucleation sites on the substrate surface. In contrast, the addition of C1, As, and Te alloys in the Se source was observed to inhibit Se crystallization, due to deactivation of chain growth on the crystal surface [2, 3]. Crystallites were also observed in amorphous Se films after ageing in air, even at room temperature and without illumination (the activation energy for the crystallization of Se is about 1 eV) [4-7]. Along with reorganization of the Se molecules [6, 7], the adsorption of oxygen and water impurities on the Se surface is believed to play an important role. We have reported [8] that the crystallization of vacuum-evaporated high purity Se on glass, ITO (indium tin oxide), and PC (polycarbonate) substrates can be greatly enhanced and the structure of the crystallites modified if the substrates have been soaked in water prior to Se deposition. Crystallization was further increased by heating the Se films in air, the rate of crystal growth being faster than that in the vacuum chamber. These observations lead to the conclusion that water impurities, initially absorbed in the substrate during soaking, can 'enhance crystallization of Se by forming additional nucleation sites on the substrate surface. To further investigate this effect, and in particular, to effectively eliminate water molecules from the substrates, we examine the influence of several parameters affecting water content in the substrates on the crystallization of Se. These parameters include water soaking temperature and heating and pumping substrates in the vacuum chamber. Glas, ITO and PC substrates [8] were cleaned using reagent grade isopropanol and dried with a jet of dry pre-purified nitrogen. Some of the substrates were soaked in de-ionized water for 1 min and again

Reversible chemical modification of the electrical behavior of a-Se

a-Se films prepared from either high purity (99.999%) source material which has undergone hydrazine reduction or from high purity Se powder surface treated with hydrazine exhibit enhanced electron and diminished hole mobility lifetime (μτ) products relative to source material. The effect of this chemical treatment is stable enough to survive repeated distillation of the selenium. A history of hydrazine reduction or surface treatment, however, has no effect on hole or electron drift mobility. The effect of hydrazine exposure can be completely reversed by oxidation of selenium oxide with nitric acid followed by reprecipitation of selenium with SO 2. The effect of hydrazine on the photoelectronic behavior of a-Se is restricted to changes in the near midgap density of states and may involve, at least in part, chemical modification of pre-existing native defects.

Growth of undoped, high purity, high resistivity ZnSe layers by molecular beam epitaxy

The growth of undoped, high purity ZnSe layers by molecular beam epitaxy with extremely high purity Se source materials refined by sublimation is described. The resistivity of these layers is greater than 104 Ω cm. In low-temperature photoluminescence spectra, only free-exciton emission line becomes dominant and all the bound exciton emission lines are extinguished, which is characteristic of very high purity ZnSe. Detailed behavior of electrical and optical properties of normally undoped layers is remarkably dependent on the purification cycles of Se materials used as source materials. The results indicate that the high resistivity of undoped, high purity ZnSe layers is due to a reduction of residual donor impurities.

The effects of impurities upon photoluminescence and optically induced paramagnetic states in chalcogenide glasses

Photoluminescence (PL) and optically induced electron spin resonance (ESR) have been studied in As 2Se 3 glasses doped with Cu, Tl, I, Ag, In, and K and in B-doped As 2S 3 glass. In all cases there is no significant change in PL efficiency or intensity of induced ESR until dopant concentrations exceed ∼1 at.%. These results are in marked contrast the strong dependence of transient hole transport upon dopant concentration in the same glasses. These parallel but contrasting observations are discussed in terms of the predicted effects of dopants on defects in chalcogenide glasses postulated by Mott on the basis of the charged defect model of Mott, Davis and Street (MDS). PL and ESR studies of the AsSe glass system have revealed that oxygen contamination can severely quench PL efficiency and ESR intensity for As concentrations <1 at.%. The PL efficiency of high purity Se glass is shown to be approximately a factor of 10 higher than previously reported for nominally pure Se. In such high purity glassy Se, the addition of As dopants produces essentially no change in the PL until the As concentration exceeds ∼0.1 at.%. These results are consistent with the existence of twodistinctly different PL bands, one centered at 0.8 eV and characteristic of some defect in pure Se, and one centered at ∼0.9 eV which is produced by the introduction of As atoms. Within the framework of the MDS model these results appear to indicate a negatively charged defect (D −) as the PL center in pure Se.