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In-segregation induced anomalous behavior of band gap and its pressure coefficient in InAlN and InGaN. Theory and Experiment
|Izabela Gorczyca , Tadeusz Suski|
Polish Academy of Sciences, Institute of High Pressure Physics (UNIPRESS), Sokolowska 29/37, Warszawa 01-142, Poland
It is believed that indium cations play a specific role in In containing nitride compounds. It was proved that In-addition, even in a small amount to nitride compounds leads to an enhancement of light emission intensity in LEDs and LDs and many works were devoted to explain this phenomenon. The most popular model dealing with this subject was proposed by Chichibu and Nakamura. It relates an enhancement of light emission with In-segregation effect. There exists a variety of results that confirm effects of fluctuations in In distribution in nitride alloys. Transmission electron microscopy, cathodoluminescence, and microphotoluminescence demonstrate wide spread of the In-segregation scale, from few nanometers to micrometers. In spite of a very intensive studies, the issue of the spatial and energetic scale of In-fluctuations is still not well understood.
In this work we are interested in InAlN and InGaN alloys. We present results of ab-initio calculations showing a strong modification of the band gap, EG and its pressure coefficient, dEG/dp, as a function of In-content and In-segregation rate (in nanometer scale). The unusual decrease in both the band gaps and their pressure coefficients with In content is observed in the case of clustered In arrangements. We compare theoretical results with the experimental data on EG and dEG/dp variations with In content.
An explanation of the observed phenomena is proposed on the basis of density of states and bond lengths analysis. We found that the unusual behavior of EG and dEG/dp as functions of In concentration comes from the In-induced changes of the states at the valence band top. It relates to the increase of the valence band width due to (hybridization) admixture of In p and d states into the uppermost N-originating valence states. The detailed analysis of the lattice relaxation shows that strong interaction of In and N- states is related to the bonds between In and N-atoms, being shorter in the clustered InAlN and InGaN alloys than in InN itself.
Presentation: Invited oral at E-MRS Fall Meeting 2009, Symposium A, by Izabela Gorczyca
See On-line Journal of E-MRS Fall Meeting 2009
Submitted: 2009-05-04 13:08 Revised: 2009-06-07 00:48