The electrical conductivity σ of amorphous Si 100−x Ti x alloys prepared by ion-beam sputtering has been measured over a temperature range between 4.2 and 290 K. The temperature dependence of σ is well expressed by a formula derived by the variable-range hopping theory with an exponent n ∼ 0.40 for x ≤ 11.5, whereas, for x ≥ 12.8, it is better expressed by a formula describing the conduction by electrons liberated from weakly localized states through inelastic electron-electron scattering. Thus, the metal-insulator transition takes place at 11.5 < x c < 12.8 in this system. The electronic structure of these alloys has been studied by measuring XPS and UPS spectra. Furthermore, DV-Xα calculation has been carried out for certain Si-Ti alloy clusters of diamond structure to simulate the electronic structure of the amorphous alloys. The calculated total and partial density of states well reproduces the features of the photoemission spectra. The calculation also provides information on the localization of electronic states in the amorphous alloys. §1. IntroductionThe structure, stability and electronic properties of semiconductor (Si, Ge)-transition-metal (TM) alloys have long been the subjects of a number of investigations. For understanding the characteristic properties of the alloys, many experimental and theoretical studies have been carried out, among which those revealing the nature of chemical bonding in TM silicides appear to be fundamentally most important. 1, 2) A stable crystalline silicide is readily formed in a stoichiometric composition at a contact interface between Si and a TM when the couple is annealed at an appropriate temperature, and it is known to provide a good ohmic contact between them. 3) In addition, the formation of a non-stoichiometric solid solution of Si and a TM is also important in the semiconductor technology. It is readily formed by coevaporation or cosputtering onto a substrate, although it usually takes a metastable amorphous structure. The atomic structure, 4) thermal stability 5) and electronic properties 6, 7) of amorphous Si-TM alloys have also been studied and their bonding characteristics have been revealed. Specifically, much attention has been payed to Ti-silicides in crystalline and amorphous forms in recent years [8][9][10][11] The objective of the present study is to make clear the electronic structure and electronic transport properties of amorphous Si-Ti alloys over a wide range of composition. Amorphous Si(or Ge)-TM alloys manifest the metal-insulator transition (MIT) at a critical concentration x c of the TM which usually falls between ∼10 and ∼15 at.%. This transition, called Anderson transition, has long been investigated for a variety of alloy systems and its mechanisms have been theoretically explored. 12-15) We have investigated the MIT effects in amorphous Si-Pd, 16) Si-Ni, 17) Ge-Ag 18) and GePd 19, 20) alloys, and related them with the bandstructure or bonding characteristics of the alloys as revealed by photoemission spectroscopy. The reason for choosing the...

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