Polarity issues affecting III-V nitride semiconductors are reviewed with respect to their determination and control. A set of conditions crucial to the polarity control of GaN is provided for each of the following growth techniques; molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and hydride vapor phase epitaxy (HVPE). Although GaN films might have been deposited by identical growth methods using the same buffer layer technologies, there is often a conflict between the resulting polarities achieved by different research groups. In this paper, we present the implications of the conditions used in each of the processes used for two-step metalorganic chemical vapor deposition (MOCVD), demonstrating systematic control of the polarity of GaN films on sapphire substrates. The potential for confusion in polarity control will be explained, taking into account the implications clarified in our studies. The correlation between the polarity and the growth conditions will be discussed in order to provide a mechanism for the determination and control of the crystal polarity during the growth of GaN films.

A nonequilibrium kinetic model is used for predicting the time evolution of the Li atom concentrations (ground and excited states) in the plasma produced by excimer laser ablation of a LiNbO3 target. The model predicts a very high ionization degree (∼0.97) that agrees well with the one obtained experimentally (∼1). These results together with the prediction of high (and close to local thermodynamic equilibrium) population densities of the electronically excited Li upper energy levels provide an indirect support for an electronic rather than thermal ablation mechanism of Li atoms.