Piezoelectric Properties of GaN Self-Organized Quantum Dots
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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 AIN layer, about 15 nm thick, was deposited followed by the growth of a 1.5 ýLm thick AIN 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 AIN at 700'C (Ref. 6, 7). Next, they were covered by AIN 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 o ripened» under vacuum or not before further covering with AIN. The o large» dots were ripened under vacuum during about one minute before further capping with AIN. They were typically 4.1 ± 0. 4 nm high (17 nm diameter). The unripened dots were capped with AIN with no growth interruption. They were 2.3 ± 0.2 nm high (8 nm diameter). These figures were extracted from the analysis of high resolution electron microscopy (HREM) pictures. As shown in Fig.1, analysis of the HREM pictures taken along two directions rotated by 300 allowed to conclude that the GaN quantum dots are coherently matched to the (0001) plane of AIN, then experiencing a strong biaxial stress. As a consequence of
Fig. 1 HREM images of the same GaN island taken along (a) the 1/3[2,- 1,-I1,0] and (b) the [1010] directions, differing from 30'
the biaxial stress and of the cristallographic symmetry, a strong piezoelectric field is expected to be present in the dots [3,41. This hypothesis was confirmed by measuring the photoluminescence of the large dots. The result, plotted in figure 2, reveals that the PL peak of the large dots is observed at 2.95 eV, i.e. 0.5 eV below the bulk GaN energy gap.
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Photon Energy (eV) Figure 2: Photoluminescence of • large •, GaN QDs Actually, it has been shown elsewhere that the photoluminescence emission energy of GaN QDs was dramatically dependent on the QD size [8], with the • small ,, QDs exhibiting a PL peak centered at 3.75 eV, nearly 0.3 eV higher than the GaN bandgap. Consistent with the HREM results, this striking QD size effect was attributed to the presence of a giant piezoelectric field in the QDs along the c-axis [8]. This hypothesis was supported by PL experiments as a function of excitation power which revealed an increasing blue shift for increasing power, as an evidence of the screening of the piezoelectric field for an increased density of photocreated carriers. Finally, theoretical calculations were performed, allowing to conclude that the GaN QDs matched to the A1N matrix experience a piezoelectric field as high as 5.5 MV/cm, consistent with the experimental data available on the piezoelectric field values in GaN and related alloys [9-11]. The strong localization effects resulting from the reduced size of the GaN QDs was further con
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