Carrier transport in undoped and doped films of amorphous arsenic triselenide

  • Alan Sharp

    Student thesis: Doctoral Thesis


    The work reported in this thesis was carried out at the Physics Department of Dundee College of Technology between October 1978 and October 1981. The aim of the work was to examine the nature of the changes in electrical properties of amorphous chalcogenide films fabricated using different techniques, and to study the modifications to these properties produced by the introduction of impurities.

    In vitreous films of arsenic triselenide, both the dark d.c. conductivity and hole drift mobility have been studied as a function of temperature and applied electric field. Measurements of the d.c. conductivity have been extended to lower temperatures than in previous studies (down to 165K as compared with 280K). A full discussion of the interpretation of these results in terms of a simple model with trap levels approximately 0.3eV, 0.4eV and 0.6eV above the valence band mobility edge is presented. A new analysis is developed from the concept of a thermally activated capture process. This analysis yields a value for the capture coefficient_y of traps 0.6eV above the mobility edge of 1.1 + 0.5 x 10-7 cm /s at 300K, which agrees very well with other estimates from transient photoconductivity measurements in the literature.

    Similar measurements on evaporated arsenic triselenide are reported. These results are discussed in terms of a model similar to that proposed for the vitreous material. The densities of localised states in the mobility gap calculated on the basis of this model are one to two orders of magnitude greater than those calculated for vitreous films. The temperature dependence of the current pulse shape in evaporated films was observed to be less marked than in vitreous films.

    Measurements of the d.c. conductivity and drift mobility in arsenic triselenide prepared by r.f. sputtering are reported for the first time. The behaviour of these films differed significantly from that observed for the other films. The time-of-flight current transients decayed extremely quickly making measurements difficult. Electron transits were observed in arsenic triselenide for the first time.

    Films doped by co-sputtering Ni and Cu dopants were also studied. Additions of less than 2 at% produced an increase in the room temperature conductivity of 4 orders of magnitude for less than a 10% change in optical band gap. A shift in the Fermi level in heavily Ni doped films of at least 0.3eV is inferred.
    Date of AwardJun 1983
    Original languageEnglish
    Awarding Institution
    • Council for National Academic Awards

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