Influence of chemical reactions on the barrier height of metal semiconductor junctions

Abstract

In a recent paper on the electrical properties of metal surface barriers on the layer structures of GaS and GaSe [ 11 Kipperman and Van Leiden observed that the relation between the work function of the contact metal and the barrier height of the surface barrier is dependent on whether the heat of formation of the metal-sulphur compound considered is greater or smaller than that of GaS. For metals which do not form sulphur compounds with a heat of formation greater than that of GaS the barrier height varies linearly with the work function of contact metal. For metals with more stable sulphur compounds than GaS on the contrary, the barrier height is almost independent of the work function of contact metal and equals the value for Ga. The authors mentioned above assumed that this behaviour is due to a chemical interaction between the contact metal and the sulphur of Gas, leaving behind a thin Ga layer. In this paper we will prove that at relatively low temperatures, normally reached during evaporation of contact metals, such a reaction does occur. From barrier height studies reported in the literature[2-101 an indication is obtained that also in other metal semiconductor systems such a reaction occurs and influences the barrier height. In the present paper contact metals which can form compounds with a greater heat of formation than that of the semiconducting compound considered are called reactive. Other metals are called nonreactive. The distinction is made using heats of formation as given by Kubashewski et al. [ 1 I]. Small platelets of n-type GaS were obtained by the iodine transport process [ 121. Metal dots with a diameter of approximately 1 mm were evaporated onto the surface of these platelets from tunsten boats in a vacuum better than IO-’ torr. Al and Au being representatives of the reactive respectively non reactive metals were used as contact metals. In order that the amount of Ga formed by a reaction between GaS and contact metal should be large enough to make detection possible by other techniques than barrier height measurements, the samples were given a heat treatment. They were placed in a small ampoule of vitreous silica, which in turn was put in a bigger tube as is shown in Fig. 1. This tube was partially filled with GaS powder to prevent sublimation of the sample. After evacuation the system was heated in an electric furnace. After a heat treatment of 5 days at 300°C samples with Al contacts showed at microscopic examination some very small metal like droplets on the surface of the metal, giving an indication that a reaction had taken place. However too few reaction products had been formed to give evidence of the presence of Ga. In order to investigate whether free Ga had been formed, the heat treatment was repeated at 500°C for the same period of time. Microscopic examination revealed the same picture as seen before. However we were now able to show that the reaction products were liquid at room temperature by smearing together some of the droplets with a very thin needle. The photograph in Fig. 2 represents a part of the contact surface on which are seen some small droplets and a big one which shows a clear metallic lustre. It was observed in stirring a droplet with a needle that it solidified if the sample was cooled a few degrees centigrade. As