The high controllability of molecular-beam epitaxy (MBE) for the growth of nanostructures relies on the use of a monitoring technique, which is reflection-high  energy electron diffraction (RHEED) in a typical MBE system. Even sub-monolayer  control of the thickness of quantum well structures is achievable using the RHEED  oscillation. However, as the semiconductor nanostructures are diversified into  lower-dimensional structures including quantum wires and dots, more sophisticated  monitoring techniques has become required for full characterization of the  nanostructures beyond the thickness of two-dimensional layered structures. In this work, we present in situ X-ray diffraction techniques enabling growth monitoring of a wide variety of low-dimensional structures. Our experiments were performed using a molecular-beam epitaxy (MBE) chamber integrated with an X-ray difftactometer at a synchrotron beamline, 11XU, at SPring-8 [1].     Among all the semiconductor nanostructures, quantum wells are playing the most important roles in technological applications today. Recently, considerable efforts have been devoted to the growth of relaxed layers with a small dislocation density in the aim of strained channel field effect transistors and multi-junction solar cells. In situ X-ray diffraction is suitable for monitoring the development of dislocations and residual strains because of its high angular resolution. Results of high-speed three-dimensional X-ray reciprocal space mapping during the growth of InGaAs on GaAs(001) will be presented [2,3].    Because of their extremely anisotropic shape, quantum wires are interesting from  the viewpoints of crystal growth phenomena as well as the quantum effects in electronic structures. A remarkable phenomenon observed in III-V semiconductor nanowires is polytypism. The evolution of the crystal structure of semiconductor nanowires during growth has been investigated by in situ X-ray diffraction [4].     Finally, the growth of quantum dots, which are the ultimate quantum structure,  has been investigated using in situ X-ray diffraction at different growth temperatures. The evolution of lateral and vertical size of self-assembled InAs/GaAs(001) quantum dots was determined during growth. It was found that there was a good correlation between the structural and optical properties determined by in situ X-ray diffraction and post-growth photoluminescence spectroscopy, respectively [5].

[1] M. Takahasi, Y. Yoneda, H. Inoue, N. Yamamoto and J. Mizuki, Jpn. J. Appl. Phys. 41, 6247 (2002).

[2] H. Suzuki, T. Sasaki, A. Sai, Y. Ohshita, I. Kamiya, M. Yamaguchi, M. Takahasi and S. Fujikawa, Appl. Phys. Lett. 97, 041906 (2010).

[3] W. Hu, H. Suzuki, T. Sasaki, M. Kozu and M. Takahasi, J. Appl. Cryst. 45, 1046  (2012).

[4] W. Hu, M. Takahasi, M. Kozu and Y. Nakata, J. Phys.: Conf. Ser. 425, 202010 (2013).

[5] M. Takahasi and T. Kaizu, J. Crystal Growth 311, 1761 (2009).

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