Effect of band gap on the red luminescence capability of Pr-doped InGaN layers grown by molecular beam epitaxy

The effect of In concentration on the red photoluminescence (PL) from trivalent Pr rare-earth (RE) ions in InGaN layers is investigated. A series of Pr-doped InxGa1-xN thin layers with 0 <= x <= 0.15 is grown by radio-frequency plasma-assisted molecular beam epitaxy on GaN/sapphire templates. Photoexcitation with a He-Cd laser results in characteristic red emission at 652 nm, corresponding to the P-3(0)-F-3(2) transition of Pr3+. The Pr PL is the highest for samples grown under slightly N-rich conditions, but the "yellow luminescence" caused by Ga vacancies is also present. The intensity of the characteristic Pr PL in InxGa1-xN:Pr decreases by one order of magnitude when increasing the In content from x=0 to 0.15, even though the matching of the band gap to the Pr3+ energy states is better. Low-temperature PL measurements show that the de-excitation energy backtransfer process from excited Pr3+ ions is more severe for In-richer samples, although the amount of backtransfer cannot be the sole reason for the significant drop of the Pr3+ emission with the band gap reduction. The dissociation of bound excitons from Pr-related traps as well as the presence of In-related exciton localization centers are also likely to decrease the excitation of the Pr3+ ions. In view of ongoing efforts to search for efficient RE-doped III-V emitters, the results indicate that the formation/dissociation rate of RE-bound excitons is one of the key factors that determine the RE emission yield. (C) 2008 American Institute of Physics.