The Nature of Localized States and the Effect of Doping in Amorphous Semiconductors

H. Fritzsche

《Chinese Journal of Physics》 15卷2期 (1977/07) Pp. 73-91

並列摘要

Recent experiments revealed that the chemistry of electrically active centers is quite different in two classes of amorphous semiconductors, the tetrahedrally bonded materials (Si, Ge,...) and the chalcogenide glasses (Se, As2Se3, Ge-As-Se,...). Doping of amorphous Si and Ge has been achieved by glow-discharge decomposition of silane (or germane) to which controlled amounts of phosphine, arsine or diborane are added. The room temperature conductivity can be changed in a controlled manner by 10 orders of magnitude toward n-type and p-type conduction. These doped materials open up interesting new fields of application: Schottky barrier, p-n, and p-i-n junctions as well as field effect transistors have already been made. The preparation techniques as well as the electrical, optical, and magnetic properties (ESR) will be reviewed. It is found that hydrogen, incorporated at levels between 15-25 atomic percent during glow discharge preparation, plays an important role in reducing the high density of gap states normally found in evaporated or sputtered amorphous Si or Ge films.Doping of chalcogenide glasses may be possible after understanding the very different defect chemistry in these materials. The majority of experimental results can be explained by supposing that some of the normally two-fold coordinated chalcogen atoms, C2, are present in positively charged three-fold coordination, The particular notation is abbreviated, and negative one-fold coordination, The particular notation is abbreviated. These so-called valence alternation centers pin the Fermi level near the gap center by means of the reaction The equation is abbreviated. This reaction is exothermic by the amount of the negative effective correlation energy proposed by Anderson, Street and Mott. Group V elements which normally are three-fold coordinated, P3, may also form valence alternation centers The equation is abbreviated. The equilibrium valence alternation chemistry governs the doping characteristics of chalcogenide glasses in the presence of other charged impurity centers.