Universal behavior of the pressure coefficient of the light absorption and emission in InGaN structures
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Universal behavior of the pressure coefficient of the light absorption and emission in InGaN structures. 33HUOLQ76XVNL3:L QLHZVNL,*RUF]\FD6àHSNRZVNL
High Pressure Research Center, “Unipress”, ul Sokolowska 29/37, 01-142 Warsaw, Poland M. Hansen, S.P. DenBaars University of California, Santa Barbara, CA 93106 B. Damilano, N. Grandjean and J. Massie Centre de Recherche sur l'Hetero-Epitaxie et ses Applications, Valbonne, France ABSTRACT We have studied an influence of pressure on the emission and absorption spectra measured from various types of InGaN structures such as epilayers, quantum wells and quantum dots. While the known pressure coefficients of GaN and InN bandgaps are in the range 40-25 meV/GPa, the experimental observation for the light emission shift with pressure for InGaN alloys is dramatically different. With the increasing In content and thus decreasing emission energy the observed pressure coefficients become very small eventually reaching zero or even slightly negative values! We have observed a much weaker trend for the decrease of the pressure coefficient for the absorption edges of InGaN. First principle calculations of InGaN band structure and its modification with a pressure are not able to explain the huge effect observed in the emission experiment but are in a good agreement with the results obtained in optical absorption measurements. We discuss here the possible mechanisms which can account for extremely low pressure coefficient of the light emission and the discrepancy between sensitivity light emission and absorption on applied pressure in InGaN alloys.
INTRODUCTION The mechanism of the radiative transitions in InGaN structures has been intensively studied for the last 10 years. This remarkable material is characterized by an extremely high radiative efficiency accompanied by a large Stokes shift of the emission spectra with respect to light absorption and big nonlinear bowing parameter of its energy gap [1]. Since Chichibu et al. [2] has put forward the hypothesis of the existence of In rich quantum dot like regions in InGaN, corresponding carrier localization was frequently called as a reason explaining peculiar behavior of this material. Application of high-pressure is enabling tool to distinguish between the transitions involving extended band states or localized states because they should have very different pressure coefficients. The localized states have pressure coefficients much lower than that corresponding to the extended states (band gap transitions) [3] . In 1995 we measured the influence of pressure on the emission from the commercially available blue and green Nichia LEDs [4] revealing that both electroluminescence and photoluminescence peaks react surprisingly weak to this external perturbation. Weak, means here that the pressure induced shift of the photoluminescence was up to four time smaller than that predicted for the energy gap of this materials. At this time it was tempting to associate this observation with some peculiar properties of InGaN layers
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