Piezoelectric Properties of GaN Self-Organized Quantum Dots
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T It is demonstrated that GaN quantum dots with the wurtzite structure grown by molecular beam epitaxy on AlN exhibit optical properties which, depending on the size of the dots, may be dominated by piezoelectric effects. In "large" quantum dots with an average height and diameter of 4.1 and 17 nm, respectively, the photoluminescence peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN bandgap, which is assigned to a piezoelectric field of 5.5 MV/cm present in the dots. The decay time of the photoluminescence was also measured. A comparison is carried out with theoretical calculation of the radiative lifetime. INTRODUCTION Despite the successful realization of blue LDs by Nakamura et al., serious problems are still to be overcome, related to the lack of adapted substrate. In particular, a reduction in the number of cristallographic defects is highly desirable in order to improve the lifetime and to reduce the threshold current of the LDs. This is partly achieved through the recent development of lateral overgrowth for GaN which leads to the reduction of the dislocation density by several orders of magnitude [1]. Alternately, the realization of devices with quantum dots (QDs) in the active layer appears promising, based on the theoretical prediction of a low threshold current and of a weak temperature dependence of the threshold current [2]. Furthermore, due to the reduced size of the QDs, they are expected to be virtually perfect, with most cristallographic defects out of the dots, which should result in a decrease in non radiative recombination. It is the aim of this article to adress this point in the case of selfassembled GaN QDs, with a particular attention paid to the peculiar properties resulting from the presence of a huge piezoelectric field which is one of the most fascinating aspect of nitrides [3,4]. 1 Downloaded from https://www.cambridge.org/core. IP address: 91.200.81.129, on 05 Jan 2019 at 06:19:15, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300003513
EXPERIMENT The samples were grown by molecular beam epitaxy (MBE) in a commercial MECA 2000 machine. The substrate was (0001) sapphire. After the nitridation step of the sapphire, a low temperature AlN layer, about 15 nm thick, was deposited followed by the growth of a 1.5 µm thick AlN buffer layer. The details of the growth procedure have been published elsewhere [5]. The GaN QDs were grown by depositing the equivalent of 3 monolayers of GaN on AlN at 700°C (Ref. 6, 7). Next, they were covered by AlN in order to smooth the surface again and the operation was repeated several times to obtain a superlattice of GaN QDs layers. The size of the dots was varied, depending on whether they were « ripened » under vacuum or not before further covering with AlN. The « large » dots were ripened under vacuum during about one minute before further capping with AlN. They were typically 4.1 ± 0. 4 nm high (17 nm diameter). The « small » unripened dots were capped with AlN with
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