Research Highlights:
Development of LEEM system to enable growth on complex samples and collaboration with MBE laboratories:
The group is highly interested on nucleation processes on complex samples with multiple layers that can lead to strain patterns on the sample surface. A new preparation chamber has been added to the system, enabling the study of complex samples grown externally. The external samples are provided with an As capping layer, that is removed under UHV conditions in the new preparation chamber. A line-of-sight-mass spectroscopy system has been installed to monitor the desorption of the protecting layer. We are now able to study epitaxial processes carried out on samples grown externally.
A BandIt system and an extra source, specially design to be operated upside down, provide more flexibility and accurate temperature measurements. A miniaturized flux meter installed on a sample holder has been designed, and fabricated, to monitor fluxes before growth and increase reproducibility of growth conditions. Collaborations with epitaxial laboratories and industrial partners have been established and further funding has already been secured to continue developing the system and apply it to spintronics (evolution of electronics using the electron spin).
A publication on the development of the system is expected to be submitted in the next 6-12 months to Review of Modern Instruments or similar journal.
Development of novel technique to enable visualization of phase diagram, and phase boundary dynamics:
The reconstruction of the surface prior to epitaxy has been shown to have a strong effect on the layers or nanostructures grown on top [(Ohtake, Mano, Hagiwara, & Nakamura, 2014)]. We have developed a new method that enables the visualization of the surface phase diagram of the material in real space, as well as observation of the dynamics of phase transformations in complex system such as GaAs.
This technique has been used to study the phase diagram of GaAs. During a droplet epitaxy process, Gallium diffuses from the droplet creating a gradient of Gallium content across the surface of the material. This new technique allows us to visualize the different surface reconstructions that occur at each Ga content, and their stability.
A publication on the development of this technique is expected to be submitted in the next 3 months to Ultramicroscopy journal.
Observation of phase metastability on GaAs surfaces:
Gallium Arsenide epitaxy has a major significance for current optoelectronics and photonics research, e.g. Molecular Beam Epitaxy has been shown to enable growth of GaAs based quantum dot lasers on Silicon [S. Chen, Nat. Photon. 10, 307-311 (2016)]. GaAs surface shows a number of different surface structures depending on its composition As-rich (4x4) and (2x4) to Ga rich (8x2), (6x6) and (4x6) [A. Ohtake, Surf. Sci. Rep. 63, 295-327 (2008)]. The transition between phase transitions is understood to be 1st order and shows phase coexistence through an extended temperature range [J.B. Hannon et al. Phys. Rev. Lett. 86, 4871-4874 (2001)].
In-situ characterization during MBE is carried out using RHEED which averages the structure of the surface over the width of the crystal. Therefore, it is impossible to observe whether small regions with different structure coexist. Our LEEM results reveal the existence of small regions with a metastable surface reconstruction, which could affect nucleation processes during epitaxy. A theoretical model reproducing the formation of this phase has been produced.
A publication describing these processes and their implications will be submitted in the next 6 months. We aim at submitting the article to Physical Review Letters.
Observation of nucleation dynamics during droplet epitaxy and nucleation of nanostructures:
We have studied the nucleation of nanostructures through Stranski-Krastanov growth of InAs quantum dots on GaAs and using droplet epitaxy [K. Watanabe, Jap. J. App. Phys. 39 L79-L81]. Our LEEM system enables us to visualize the different surface phases coexisting during growth. A detailed study of the evolution of the surface phases, their interaction and significance for the growth has been produced. We are aiming to submit a publication to Nanoletters (or ACS Nano) in the next 6 months.
