Blue Luminescence in Undoped and Zn-doped GaN

Abstract

ABSTRACTA broad band with a maximum at about 2.9 eV (blue band) is widely observed in the photoluminescence (PL) and cathodoluminescence (CL) spectra of unintentionally doped GaN grown by metalorganic chemical vapor deposition (MOCVD) or by hydride vapor phase epitaxy (HVPE). In some samples this band exhibits fine structure attributed to electron-phonon coupling, in others it appears featureless. Different defect origin and recombination mechanisms responsible for the blue band have been suggested in the past. The situation is complicated by the fact that bands similar in shape and position were observed also in Zn- and Mg-doped GaN, as well as in undoped GaN after dry or wet etching. We investigated PL in HVPE-grown undoped, Si- and Zn-doped GaN layers in wide temperature and excitation intensity ranges. We have found that the shape, temperature and excitation intensity dependencies of the blue band in undoped GaN are almost identical to those in GaN lightly doped with Zn. Moreover, in both undoped and Zn-doped samples we observed a distinctive set of peaks related to an exciton bound to the Zn acceptor. Although the exact structure of the Zn acceptor is still unknown, our experimental results unambiguously demonstrate that Zn impurity is responsible for the blue band in unintentionally doped GaN. The results of transient PL study are also consistent with the above attribution.