Photoelectronic Properties of a-Si:Se x and a-Si:Te x Thermally Evaporated Films
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PHOTOELECTRONIC PROPERTIES OF a-Si:Sex AND a-Si:Tex THERMALLY EVAPORATED FILMS. F.G. Wakim, Electrical Engineering Dept., Lowell Univ., Lowell, MA 01854 ABSTRACT The presence of selenium (Se) or Telurium (Te) in amorphous silicon films seems to improve the quality of these films by reducing the number of unsatisfied or dangling bonds and changing the optical gap and thermal activation energy. The thermal activation energies of a-Si:Sex and a-Si:Tex were always larger than that of a-Si films. The measured optical gap can be tailored to some value between 0.8 eV and 1.8 eV depending on the use of Se or Te and on the relative concentrations. The presence of Se or Te in a-Si films seems to have the same effect as that of hydrogen in a-Si films prepared by glow discharge of silane and used in solar cells. INTRODUCTION In glasses containing selenium (Se) or Telurium (Te) the short range order is preserved as in crystals where each selenium atom is bonded to two neighbors (1). This could change with little added energy to one positively charged triply coordinated atom and another negatively charged singly coordinated atom which is known as valence alteration pair (2). This makes it possible for Se or Te to bond to one, two, or three atoms, and satisfy dangling bonds in neighboring atoms. Amorphous silicon (a-Si) films are known to have microvoids (3) that are generated during the preparation of the films where many dangling bonds are present. The presence of Se or Te in a-Si could satisfy many dangling bonds at the microvoids. The presence of Se or Te in a-Si films has the same effect on its resistivity and optical gap as that of hydrogen. It has been found (3, 4) that the presence of hydrogen in a-Si which were prepared by glow discharge have a low density of defect states in the mobility gap. The absence of defects makes it possible to dope a-Si and make p-n junctions, which can be used as solar cells. It is assumed that hydrogen bonds covalently to silicon and satisfy dangling bonds at vacancies and at surfaces of microvoids (4) thus reducing the number of defects. Due to the similarity in the bonding situation of Te or Se with Si and that of hydrogen it seems likely that p-n junctions can be obtained in a-Si containing Se or Te. EXPERIMENTAL Different methods were used in preparing the films, but the most successful was one in which high purity silicon was evaporated using an electron gun (e-gun) evaporation source in an ultrahigh vacuum system. Se or Te were evaporated simultaneously with silicon in another boat which was placed several inches away from the e-gun source. The starting pressure in the system was 10- 9 Torr which increased to 10-7 Torr during evaporation.
Mat. Res. Soc. Symp. Proc. Vol. 70. ' 1986 Materials Research Society
344
The films were deposited on different substrates for different types of measurements such as electrical conductivity, optical absorption and for transmission or scanning electron microscope studies. The resulting a-Si, a-Si:Se and a-Se:Tex films ranged in thickness from 0.1 t
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