The InGaN/GaN:Si multiple quantum well (MQW) samples used in this study were grown by low-pressure low-pressure metal-organic vapour phase epitaxy (LP-MOVPE) on high-pressure grown bulk GaN mono-crystalline substrates. Two InGaN/GaN samples were investigated. One of them was grown at 780°C (sample A) . In the case of the second (sample B), the GaN barriers were grown at 900°C, but InGaN at 730°C.

For both samples the indium composition in the QWs reaches the level of  about 20at%. The local indium composition was measured through strain measurements by digital processing from the lattice fringes' images of Transmission Electron Microscopy. We compare the  analysis performed  for two zone axis [11-10] and [11-20] with the use of  axial and off-axis illumination.  During TEM investigations the formation of  “false indium  clusters “ of the size of 2-4 nm  was observed for both samples just after 2 minutes of LaB₆ electron beam illumination at 200kV. Figures (a) and (b)  show the cross-section of sample B.

These images have been taken in the first seconds of illumination and they clearly show the inhomogeneous thickness of the QWs. These fluctuations have a length of 53 nm and the QW thickness changes from 2.5 nm  to 0.5 nm .The maximum composition reaches 23% of indium, but can drop below 5% in the “valleys”. The abrupt 1 nm high step  of the width QW is shown in figure (c). Sample A, which  barriers and wells were grown at 730°C, shows a perfect lateral homogeneity with 3 nm thick QWs  and an indium composition of 19.5at%. The QW with the mentioned modulations for sample A,  can be explained by the desorption of indium during the 30s rise of the temperature necessary for the growth of the GaN barriers.

The work was partly supported by the EU project ITN RAINBOW 213238-2 and Polish Scientific Network  project No.  71/E-64/BWSN-0108/2008.

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