Defect States In Thin Films Research Articles

ZnO gap states investigated using magnetic circular dichroism

We investigate gap states in a series of nonmagnetic ZnO films grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) using magnetic circular dichroism. This technique is shown to be sensitive enough to investigate defect states in thin films of disordered wide-gap semiconductors by differentiating between a loss of transmission due to scattering and absorption. The method is first applied to Zn- and O-terminated films grown by MBE. The O-terminated film shows a broad peak centred at ~2.4 eV which corresponds to the green PL signal, whereas the Zn-terminated film shows a magnetic circular dichroism (MCD) signal that increases slowly with energy. The method was then extended to PLD films. Oxygen vacancy states in both MBE and PLD-grown films were identified, and the introduction of low concentrations of Al was shown to quench the MCD signal at low energy by neutralising the paramagnetic oxygen vacancies. In PLD-grown ZnAlO we observed a negative MCD that increased in magnitude as the energy approached the band edge indicating a much more pronounced spin-split impurity and conduction band. These examples illustrate the versatility of the MCD technique in identifying gap states in a variety of different types of ZnO samples.

Effect of impurity (Te and Zn) incorporation in the density of defect states in thin films of Se 90In 10 and Se 75In 25 glassy alloys

In this paper we report the effect of Te and Zn incorporation on the density of defect states of two binary Se–In glassy systems. For this purpose, we have chosen here two well known Se 90In 10 and Se 75In 25 binary glassy alloys. Thin films of Se 90In 10, Se 75In 25, Se 75In 10Te 15 and Se 75In 10Zn 15 glassy alloys prepared by quenching method, were deposited on glass substrate using thermal evaporation technique. Current–voltage characteristics have been measured at various fixed temperatures in the thin films under study. Ohmic behavior was observed at low electric fields while at high electric fields current becomes superohmic. An analysis of the experimental data confirms the presence of space charge limited conduction in Se 90In 10, Se 75In 10Te 15 and Se 75In 10Zn 15 glassy alloys. It was found that the absence of space charge limited conduction in Se 75In 25 may be due to joule's heating at high fields. By applying the theory of space charge limited conduction, the density of defect states near Fermi level was calculated. The peculiar role of the additives (Te and Zn) in the pure binary Se 90In 10 and Se 75In 25 glassy alloys is also discussed in terms of electro-negativity difference between the elements involved.