Characterization of nanometer scale compositionally inhomogeneous AlGaN active regions on bulk AlN substrates

Abstract

Despite the recent progress in the development of III-Nitride semiconductor based ultraviolet light emitting diodes (UVLEDs), commercially available devices emitting at wavelengths shorter than 370 nm still suffer from poor wall plug efficiencies on the order of ~ 1%. One factor limiting the performance of these devices is the presence of a high density of dislocations that arise from the typical heteroepitaxial growth on c-plane sapphire substrates that result in non-radiative recombination. Previously we have reported on the growth of AlGaN active regions containing self-assembled nanometer scale compositional inhomogeneities (NCI-AlGaN) that demonstrate enhanced luminescence efficiency despite the presence of a large dislocation density [1]. This phenomenon is attributed to (1) the high density of NCI regions that improves the probability of carriers recombining radiatively within them rather than non-radiatively at a structural defect and (2) the subsequent concentration of carriers within the narrower band gap NCI regions that suppresses non-radiative recombination and enhances radiative efficiency due to the reduced radiative lifetime at high carrier density [2,3]. Nevertheless, time-resolved photoluminescence (TRPL) studies show that the reduction of dislocation density in these materials further improves the active region performance [4], a phenomenon that may be due to improved transport from the wider band gap matrix to the NCI and concomitant higher carrier concentration therein. Bulk AlN has significant advantages over conventional c-plane sapphire as a substrate for III-Nitride based UVLEDs including a low dislocation density (etch pit density <104 cm−2), a reduced lattice mismatch, a high thermal conductivity, and potentially a high transparency at wavelengths of interest. In this paper we report on the structural and optical characterization of NCI-AlGaN active regions grown on bulk AlN substrates. 800 nm-thick NCI AlGaN films having ~57% AlN by mole fraction were deposited directly on bulk AlN substrates by plasma-assisted molecular beam epitaxy. The films were characterized optically by temperature dependent cw-photoluminescence (cw-PL) and TRPL, and structurally by x-ray diffraction. Figure 1 shows the room temperature cw-PL of an NCI-AlGaN film deposited directly on a bulk AlN substrate, as well as that of a similar film deposited on a c-plane sapphire substrate using a 25 nm thick high temperature AlN buffer layer. The luminescence of both films is dominated by emission from the NCI-regions that is red shifted ~ 220 meV from the band gap of the alloy and peaks at 273 nm and 277 nm for the film on the AlN bulk substrate and film on the sapphire substrate, respectively. The intensity of the cw-PL for the film grown on the bulk AlN substrate is ~ 2x more intense than that observed for the film grown on sapphire, consistent with a reduction in dislocation density in this material due to a lower dislocation density in the AlN and better lattice match of the NCI layer to the AlN substrate. TRPL studies (Figure 2) of the NCI AlGaN film on AlN at low temperature, where non-radiative channels are frozen out, reveals a short radiative lifetime of ~330 ps which is faster than what we have previously observed for high quality multiple quantum well based active regions on bulk AlN or commercially available LEDs at similar emission wavelength [5], providing further evidence of the improvement of this material. Moreover, the extended decay at low temperature, which may be associated with a longer radiative lifetime related to carrier localization at NCI/matrix interfaces, is significantly suppressed compared to that in the quantum well samples discussed above or NCI AlGaN samples of higher dislocation density (not shown). X-ray rocking curve studies of NCI AlGaN films grown on bulk AlN exhibit a narrow on-axis