A multilayer structure consisting of 20 layers of InAs self-assembled quantum dots was grown by molecular beam epitaxy and observed by transmission electron microscopy. The positions of quantum dots in a quantum-dot layer were not correlated with those in the lower quantum-dot layer because of the thick (70 nm) GaAs spacer layer. Threading dislocations were observed, which originated from large strain-relaxed (incoherent) InAs islands in a quantum-dot layer. The dislocations were 30° dislocations in the [112] and [112] directions, and were generated by a misfit between large InAs islands and a GaAs overlayer.

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