Photoelectric properties of tin disulfide

In recent years, investigations of the crystal structure and electrical and optical properties of layered compounds of type MX2 ~where M is a metal and X is a chalcogen! have elicited considerable interest. Many of these compounds possess polytypes, which ensures that a whole range of crystal modifications, each having different properties, can exist for a single chemical compound. One such compound is the widegap semiconductor SnS2 . At this time, more than ten polytypes of this compound are known; depending on the structure of the polytype, the width of the bandgap varies over a wide range from 0.81 to 3.38 eV. The commonest polytype of SnS2 has a sequence (AgB)(AgB) of alternating layers, and is usually denoted by 2H according to its sulfur layers. However, since its unit cell contains only one molecule, it is the simplest structure this compound can have and, hence, should be denoted by 1H . The width of the forbidden band of this modification, according to optical measurements, is 2.18 eV. SnS2 crystals are rather strongly photosensitive, a fact of considerable interest since the photosensitivity is present in the visible region of the spectrum. However, very few papers have made it their task to study the photoconductivity of these crystals, and as far as we know no one has investigated the photo-EMF. In order to fill in this gap, we measured the photoconductivity and photo-EMF of SnS2 crystals doped during growth with Cu, Au, Zn, Cd, In, Ga, and P impurities. Our measurements were made in the constantcurrent regime, using a monochromator as a light source with a photon energy range from 1 to 4 eV. The contacts were made using silver paste. In order to measure the photoconductivity, they were deposited on one surface and screened from the light. The photo-EMF was measured on the opposite side of the sample as the immediate vicinity of the contact was illuminated by a fixed beam from the monochromator. Our results are shown in Fig. 1. It is clear from this data that a well-defined strong maximum is observed in the photoconductivity spectrum at an energy of about 2.3 eV, which was practically the same for all the samples, a weaker peak at an energy of 2.85 eV, which in some samples had the form of a shoulder, and a maximum in the neighborhood of 1.65 eV, which did not appear in all samples. The first feature is obviously connected with the edge of the fundamental absorption band and corresponds to an indirect optical transition to the absolute minimum of the conduction band. The peak at energy of 2.85 eV is probably associated with direct transitions to a higher-lying minimum, which agrees with