G. Lucovsky

Defects in plasma-deposited a-Si: H

The relationships between hydrogen vibrational spectra, electron spin densities and refractive index are investigated for a range of plasma-deposited amorphous silicon-hydrogen alloys. Results are also presented on the morphology of thick films as shown by scanning electron microscopy. A model is proposed for the structural origin of defects in these alloys based on voids that grow perpendicular to the film surface and are associated with coupled SiH2 units.

A molecular model for the vibrational modes in chalcogenide glasses

Abstract We demonstrate the use of molecular models for calculating the optic mode frequencies in elemental and compound chalcogenide glasses. Our model gives good agreement with the optic mode frequencies reported for As 2 Se 3 , and for the polymeric fraction in liquid S and amorphous Se and Te. The model is not applicable to amorphous Si and Ge and III–V semiconductors.

Electronic and structural properties of plasma-deposited a-Si:O:H - The story of O2☆

Abstract Measurements of luminescence, optical absorption and infrared vibrational spectra are reported for thin films plasma-deposited from SiH4/O2/Ar mixtures. The addition of oxygen causes the optical absorption edge and luminescence peak position to shift monotonically to higher photon energy. Increases in the luminescence peak width and in the temperature of thermal quenching result in room temperature luminescence in the visible at high oxygen contents.

Hydrogen in amorphous semiconductors

Abstract Over the past 5 years, there has been growing interest in a class of amorphous semiconductors deposited in thin-film form in the presence of hydrogen. The interest has derived primarily from certain electronic properties, such as the ability to control the Fermi level by substitutional doping, that make the materials potential candidates for solar photovoltaic energy conversion and thin-film device applications. These same properties have also made the materials attractive “test-beds” for basic research into electronic processes and defect states in amorphous semiconductors.

Local structure and vibrational spectra of vAs2O3

Abstract We discuss the atomic displacements of the optically active vibrational modes of vitreous (v-)As2O3 through comparisons of the infrared (ir) and Raman response with the corresponding spectra of the two crystalline polymorphs, claudetite, a layer crystal similar to orpiment, and arsenolite, a molecular crystal based on the As4O6 molecule. We conclude from these comparisons that the structure of the glass is composed of AsO 3 2 pyramidal units that are corner connected to form a continuous random network. The character of the strong ir and polarized Raman modes in the vitreous form, suggests that the interconnection of these pyramidal units cannot be described by a random distribution of dihedral angles, but rather has peaks at angles characteristic of the different ordering in two-dimensional macromolecular layer basis of claudetite, and the As4O6 molecule. The comparisons are extended to v-As2S3 and vAs2Se3 where we conclude the dihedral angle distributions characteristic of an As4O6-like local geometry are less prevalent.

Intermediate range order in amorphous solids

Abstract Information about local atomic arrangements in amorphous solids is derived by a variety of techniques, X-ray RDFs, vibrational spectroscopy, NMR and EXAFSs; however, there is a scarcity of information about intermediate range order, in particular the distribution of dihedral angles. We show that this information can frequently be obtained from an analysis of polarized features in the Raman response.

Structural models for amorphous semiconductors and insulators

Abstract The atomic structure of an idealized amorphous semiconductor or insulator is formed by the spatial repetition of one or more basic molecular units in a way that cannot be related to any known crystalline structure, or indeed with the molecular units of any periodic array. It will be shown that information on the constitution of such molecular units, can, in many cases, be obtained directly from either X-ray diffraction, or Raman scattering and infra-red absorption studies. Unfortunately, the way in which these units are linked together to form the amorphous structure cannot as yet be deduced from experiment. The generation of information of this type has relied almost entirely on comparisons of the structural characteristics of hand or computer-built models with the corresponding experimental results. The extent to which unique structural assignments are produced by this approach will be discussed. Examples will include elemental amorphous semiconductors: Ge, Si. As, Se; compounds: As 2 Se 3 , GeSe 2 , SiO 2 ; and binary alloys: Ge 1− x Se x , As 1− x Se x . Then structures range from network glasses in which all atoms are part of one covalently bonded solid, to polymeric glasses in which intertwined molecular units of long covalently bonded atomic chains are interbonded by weaker van der Waals-like forces to form the solid.

Selenium, the Amorphous and Liquid States

There has been a long standing interest in the molecular structure of liquid (l- and amorphous (a-) selenium. Recently this has been extended to include the local atomic arrangements at intrinsic bonding defects in a-Se as well. This paper addresses these questions by reviewing past work in the context of recent experimental and theoretical studies. No attempt is made to be comprehensive in treating all of the relevant research reports and many important contributions will therefore appear solely as references in those papers which are specifically cited in this text.

Intimate valence alternation pairs in amorphous SiO2

Abstract The intrinsic bonding defects in a-SiO 2 are qualitatively similar to the VAPs in a chalcogenide glass; however, they do not exhibit a negative effective correlation energy. Relaxations at the C 1 − (1T) and C 3 + (3T) centers are related to the electronic structure. Finally, the bonding model provides an excellent description of the defects at the Si/SiO 2 interface.

Aspects of the Molecular Non-Molecular Transition in Se and Te

Se and Te undergo transitions to metallic structures with pressure. Anomalous variations in lattice parameters and frequencies with pressure are shown to be precursors of the transition.