Se Molecules Research Articles

Molecular Beam Epitaxially Grown ZnSe(001) Surface Studied by the In Situ Observation of RHEED Intensity

The desorption of Zn and Se atoms or molecules on the (001) surface of a ZnSe/GaAs epitaxial layer were studied by means of reflection high-energy electron diffraction (RHEED). The temporal behavior of the specular intensity observed from two azimuths, [110] and [1̄10], was well understood by taking the surface condition into consideration; i.e., the surface terrace is elongated toward the [1̄10] direction and excessively adsorbed Se molecules exsist as well. We also confirmed that a higher-energy electron beam enhances the desorption of Se molecules and adatoms.

Ultraviolet irradiation effect on the MBE growth of ZnSe/GaAs observed by RHEED

Ultraviolet light irradiation effects on the MBE growth of ZnSe/GaAs(001) were investigated by means of RHEED observation. One of the prominent effects induced by the UV irradiation is to enhance the desorption of Se molecules. The other is that the decay of the RHEED oscillations becomes faster. These results imply the enhanced possibility of three-dimensional growth, which leads to the inferior surface morphology of the ZnSe/GaAs epilayers grown under photo-irradiation. Possible speculation on photo-MBE growth mechanisms is also given.

High-resolution transmission electron microscopy of inorganic materials in cellular and topological random systems.

The structure of amorphous Se films, in the topological random system, has been studied by the computer-simulation, electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM). As an example of HRTEM of the cellular random system, our recent investigation on Ba-ferrites is reviewed beforehand. In HRTEM images of spin-glass BaTi2-xSnxFe4O11 (x less than 0.6), magnetic clusters or clusters of FeO6-octahedra surrounded by TiO6-octahedra have been found. The structure of BaSn2Fe4O11 (x = 2) has been determined. The ordering of Sn (Ti) and Fe ions increases with increasing x, which interprets the change from the spin-glass state to nonspin-glass state at x = 0.6. It is shown that a cluster of polarized lattice ions is detectable in images of polar-glass BaxK2-xFexTi6-xO13 (x greater than or equal to 1.2). The structure models of amorphous Se films forming on a substrate have been constructed on a computer, and their radial distribution function (RDF) and HRTEM images have been calculated. Experimental RDF and HRTEM images have been obtained from vacuum-deposited amorphous Se films and are compared with the calculated ones. It is concluded that in the as-deposited films most of Se molecules may be composed of Se atoms as small as three and that by the electron-beam irradiation the molecules link to form spiral chains in amorphous state and then arrange to have the hexagonal crystal structure.

High Resolution Electron Microscopic Observation of Crystallization and Decomposition of Amorphous Selenium Films

The crystallization and decomposition process of amorphous Se films, which takes place by electron beam irradiation in an electron microscope, has been investigated by high resolution electron microscopy. A vacuum-deposited film 5 nm thick has the structure unit of Se molecules composed of as few as three or four atoms. It transforms to the hexagonal crystal, through an amorphous structure whose unit is long-chain molecules. An amorphous layer covering the crystallized layer has been observed in the film before the complete crystallization. It has been found that Se atoms, which are a segment of a chain, are removed from the crystal surface in the early stage of the decomposition

Optimal crystal growth condition of ZnSe grown by molecular beam epitaxy

The crystal quality of ZnSe grown on the (100) GaAs substrate by molecular beam epitaxy is characterized by ellipsometry and the Rutherford backscattering method. The growth rate and refractive index are measured as a function of the beam pressure ratio of Zn and Se molecules and the substrate temperature. It is discussed that the refractive index reflects the degree of packing of atoms in crystal films. The most close-packed ZnSe crystals are grown at the stronger Se beam pressure by several % than the Zn. The optimal crystal growth temperature around 270°C is confirmed by temperature dependence of growth rate and the refractive index. Substrate temperature dependence of the Rutherford backscattering spectra also shows that MBE ZnSe crystals of the best quality are grown near this temperature.

Monte Carlo Simulation of the Growth of ZnSe by MBE

ABSTRACTA Monte Carlo study of the growth of ZnSe by Molecular beam epitaxy is presented. The study is focused on the role of surface kinetic reactions on the structural quality of the epilayers. Two different models for the incorporation of Se molecules, one with a highly reactive physisorbed state and the other with a relatively nonreactive physisorbed state are employed for simulations. It is shown that the structural quality of the epilayers is very sensitive to the flux ratio if the physisorbed state is relatively nonreactive. It is also shown that if the physisorbed state is highly reactive, good quality epilayers are obtained over a wide range of flux ratio.

Thermal Crystallization of Amorphous Se Film

The crystallization of amorphous Se film was studied by measuring the rate at which Se molecules were thermally evaporated and sublimated from the film. It was found that the activation energy of the crystallization was equal to the average of the activation energies required for the evaporation and sublimation of Se molecules. Using this result, a model for the crystallization mechanism of amorphous Se is proposed.