Spontaneous quantum dot formation in InGaN layers
The high radiative efficiency of nitride-based light-emitting diodes (LED) despite high dislocation densities in heteroepitaxially grown layers is often attributed to localization centers within InGaN layers. Excitons in InGaN have a Bohr radius of around 3 nm, which approximately sets the length scale on which fluctuating properties of an InGaN layer may lead to quantum-dot-like carrier confinement. As outlined at the beginning of this chapter, compositional fluctuations in InGaN are promoted by In segregation but may also be driven by phase separation. Fluctuations within InGaN layers on the length scale of a few nanometers are often observed by cross-sectional transmission electron microscopy. One has argued that the TEM investigation itself artificially introduces such fluctuations if exposure times are taken too long (>1 min.), but time series show that fluctuations are resolved even with short exposure times (<1 min.) [53, 54]. Figure 5.15 shows a statistical analysis of In fluctuation and thickness deviations within InGaN/GaN samples grown by MOVPE on Si(111) substrates . The analysis is based on cross-sectional TEM images taken at several locations across the sample. The average In composition and mean thickness of the InGaN layers were confirmed by X-ray diffraction measurements. As can be seen, both thickness and In concentration are widely scattered and the length scales are small enough to provide 3D localization.
Fig. 5.15. Statistical analysis based on TEM investigations on different sample areas. Compositional fluctuations and thickness deviations from the average values are well resolved. (Reprinted from , © 2004, with permission from Elsevier.)
Fig. 5.16. Resonantly excited time-resolved photoluminescence spectra taken at different excitation power densities. The steady spectral distribution of the signal directly proves 3D carrier localization. (Reprinted with permission from , © The Japan Society of Applied Physics.)
Complementary results from luminescence experiments exhibiting sharp emission lines with emission linewidths of the order of 500 peV confirm QD formation . From time-resolved PL spectra shown in Fig. 5.16 an average size of the localization centers of 3-4 nm and a very broad energy distribution are confirmed [56-58].
Significant impact on the localization centers by post-growth annealing and growth interruptions has been identified . Annealing of samples exhibiting pronounced fluctuations after growth at temperatures above 950° C is leading to the disappearance of In-rich clusters in InGaN/GaN-MQW layers . Growth interruptions in an ammonia atmosphere after quantum well growth promote In desorption out of the layers. For short growth interruptions of 5-30s, excess In which is initially incorporated into clusters within the quantum well layers is removed and the in-plane localization of charge carriers is reduced, leading to decreasing photoluminescence intensity . The magnitude of compositional fluctuations also increases with In content in the well layers . Especially for In concentration xIn = 0.3, quantum dot formation is evidenced by numerous means. Temperature-dependent photoluminescence measurements indicate a tendency towards enhanced in-plane localization with increasing well thickness from 1.2 to 3.9 nm. Corresponding activation energies for carrier delocalization are 27.1 meV, 31 meV, and 35 meV .