Determination of cesium distributions in oxides of MOS structures by photoinjection studies

The photoinjection technique of Berglund and Powell has been used for the determination of cesium profiles in the oxide films of MOS-structures. Measurements of photocurrent as a function of the applied voltage provide information on the distribution of charges in the vicinity (10–200 Å) of both the interfaces of the insulating layer. The charge profiles to be measured were introduced into SiO₂ by ion implantation. Cesium ions, which cause positive surface charge and show great stability under temperature-bias stress, have been implanted before and after oxidation. In the first case, cesium ions were implanted into the silicon surface, which was subsequently oxidized. After oxidation, the cesium ions were found mainly at both the interfaces and only a few of them in bulk SiO₂. In the second case, cesium ions were implanted into SiO₂ at an energy of 5 keV. The cesium distribution measured by the photoinjection technique agrees within 5 Å with that predicted by implantation theory.