Electronic transport properties of thin films of amorphous silicon
The work reported in this thesis was carried out at the Physics Department of Dundee College of Technology between May 1980 and October 1983. The aims of the work were; (i) to develop a computer-based system for the capture and analyses of fast transient waveforms, with particular emphasis on the "Time of Flight" experiment, (ii) to commission an r.f. sputter-deposition system capable of producing thin films of amorphous Silicon, and to evaluate the electronic transport properties of the material prepared over a wide range of deposition conditions, and (iii) to investigate the electronic transport properties of thin films of co-sputtered Silicon-Aluminium and Silicon-Antimony, and to evaluate the effectiveness of Aluminium and Antimony as electronic dopants. The apparatus constructed consists of an electron-gun capable of firing a pulsed electron beam, in the energy range 4-15 keV, at the top metallic contact of a semiconductor specimen. A Datalab DL905 transient recorder was used to capture and digitise the characteristic waveforms, these waveforms being sent to the College's Dec-System-20 mainframe computer for analyses, via a NASCOM microcomputer. Computer programs were developed for the control of the experimental equipment and for the capture and analyses of the transient waveforms. Dark d.c. conductivity, electron drift mobility, transient and steady state photoconductivity and optical absorption experiments were carried out on thin films of amorphous Silicon. These measurements reveal wide variations in the electronic transport properties of specimens of amorphous Silicon prepared under seemingly identical deposition conditions. The experimental results are interpreted in terms of localised conduction band tail states extending to approximately 0.2 eV below the conduction band mobility edge, and a discrete set of localised states situated at 0.4 eV below the conduction band mobility edge. It is suggested that the extent of the tail states and the density of the discrete states are critically dependent upon the conditions of film deposition and upon the level of impurity atoms within the material. Silicon-Aluminium films were found to contain crystalline Aluminium is lands embedded with in an amorphous lattice. Silicon-Antimony films appeared homogeneous to are solution of 50 Angstroms. The addition of Aluminium or Antimony to the amorphous Silicon was seen to increase the room temperature conductivity and decrease the conductivity activation energy and the optical band gap. It is suggested that neither Aluminium or Antimony act as effective substitutional dopants for the particular films of amorphous Silicon prepared in this laboratory .
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