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Register a new userSelf-assembled nano-columns in Bi2Se3 grown by molecular beam epitaxy
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Layered van der Waals (vdW) materials grown by physical vapor deposition techniques are generally assumed to have a weak interaction with the substrate during growth. This leads to films with relatively small domains that are usually triangular and a terraced morphology. In this paper, we demonstrate that Bi2Se3, a prototypical vdW material, will form a nano-column morphology when grown on GaAs(001) substrates. This morphology is explained by a relatively strong film/substrate interaction, long adatom diffusion lengths, and a high reactive selenium flux. This discovery paves the way toward growth of self-assembled vdW structures even in the absence of strain.
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We report the growth of self-assembled Bi2Se3 quantum dots (QDs) by molecular beam epitaxy on GaAs substrates using the droplet epitaxy technique. The QD formation occurs after anneal of Bismuth droplets under Selenium flux. Characterization by atomic force microscopy, scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy and X-ray reflectance spectroscopy is presented. The quantum dots are crystalline, with hexagonal shape, and have average dimensions of 12 nm height (12 quintuple layers) and 46 nm width, and a density of $8.5 cdot 10^9 cm^{-2}$. This droplet growth technique provides a means to produce topological insulator QDs in a reproducible and controllable way, providing convenient access to a promising quantum material with singular spin properties.
High-index Bi2Se3(221) film has been grown on In2Se3-buffered GaAs(001), in which a much retarded strain relaxation dynamics is recorded. The slow strain-relaxation process of in epitaxial Bi2Se3(221) can be attributed to the layered structure of Bi2Se3 crystal, where the epifilm grown along [221] is like a pile of weakly-coupled quintuple layer slabs stacked side-by-side on substrate. Finally, we have revealed the strong chemical bonding at the interface of Bi2Se3 and In2Se3 by plotting differential charge contour calculated by first-principle method. This study points to the feasibility of achieving strained TIs for manipulating the properties of topological systems.
The accurate control of the crystal phase in III-V semiconductor nanowires (NWs) is an important milestone for device applications. In this work, we present a method to select and maintain the wurtzite (WZ) crystal phase in self-assisted NWs. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the zinc-blende (ZB) or WZ phase is the V/III flux ratio. The latter can be monitored by changing the As flux, and drives the system toward a stationary regime when the wetting angle of the Ga droplet falls in a target interval, typically in the 90{deg} - 125{deg} range for the WZ phase growth. The analysis of the in situ RHEED evolution, high-resolution scanning transmission electron microscopy (HRSTEM), dark field transmission electron microscopy (DF-TEM), and photoluminescence (PL) data all confirm the control of an extended few micrometers long pure WZ segment obtained by MBE growth of self-assisted GaAs NWs with a V/III flux ratio of 4.0.
Quantum anomalous Hall (QAH) effect is a quantum Hall effect that occurs without the need of external magnetic field. A system composed of multiple parallel QAH layers is an effective high Chern number QAH insulator and the key to the applications of the dissipationless chiral edge channels in low energy consumption electronics. Such a QAH multilayer can also be engineered into other exotic topological phases such as a magnetic Weyl semimetal with only one pair of Weyl points. This work reports the first experimental realization of QAH multilayers in the superlattices composed of magnetically doped (Bi,Sb)$_2$Te$_3$ topological insulator and CdSe normal insulator layers grown by molecular beam epitaxy. The obtained multilayer samples show quantized Hall resistance $h/Ne$$^2$, where $h$ is the Plancks constant, $e$ is the elementary charge and $N$ is the number of the magnetic topological insulator layers, resembling a high Chern number QAH insulator.
SrxBi2Se3 is a candidate topological superconductor but its superconductivity requires the intercalation of Sr by into the van-der-Waals gaps of Bi2Se3. We report the synthesis of SrxBi2Se3 thin films by molecular beam epitaxy, and we characterize their structural, vibrational and electrical properties. X-ray diffraction and Raman spectroscopy show evidence of substitutional Sr alloying into the structure, while transport measurements allow us to correlate the increasing Sr content with an increased n-type doping, but do not reveal superconductivity down to 1.5K. Our results suggest that Sr predominantly occupies sites within a quintuple layer, simultaneously substituting for Bi and as an interstitial. Our results motivate future density functional studies to further investigate the energetics of Sr substitution into Bi2Se3.
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