Acceptor Action of Alkali Metals in II—VI Compounds as Detected by Electron Spin Resonance Techniques

Publisher Website
Google Scholar
Abstract
Experiments were performed to decide whether the introduction of alkali metals into II—VI compounds gives rise to the appearance of acceptor levels. In comparison with single crystals, powder samples are in most cases purer and it is much easier to dope powders rapidly and reproducibly. For that reason polycrystalline materials were used in this investigation. In order to obtain a quick and decisive answer to the above‐mentioned question we used electron spin resonance techniques to study the influence of alkali metals on the Fermi level position in these samples. This requires at least one indicator, the presence of which can readily be detected by the employed techniques. For that purpose Fe was chosen since the valence change of divalent Fe to trivalent Fe, caused by the introduction of lower‐lying acceptors, is easily detected at 77°K due to the appearance of the electron spin resonance signal of the trivalent Fe. The use of Fe as an indicator has the additional advantage of setting an upper limit to the depth of the acceptors introduced by the alkali metals, since the position of the Fe level is quite accurately known. If no acceptors were introduced the Fe was found to be introduced in the divalent form under the preparation conditions chosen and no resonance of trivalent Fe was detected. The simultaneous introduction of the alkali metals Li or Na in ZnS, CdS or ZnSe caused the appearance of resonances due to trivalent Fe. These results unequivocally show that these alkali metals introduce acceptor levels into the lattices investigated. The upper limit to the acceptor depth (optically, with respect to the valence band) are 1.4 eV for cubic ZnS, 1.3 eV for hexagonal CdS, and 1.1 eV for cubic ZnSe. The increase in the amount of Fe that is converted to the trivalent form by the introduction of the alkali metals on going from ZnS to CdS or ZnSe is discussed and explained in terms of the decreasing energy needed to ionize the electron from Fe centers into the conduction band.