Thin films of V–VI compound semiconductors (Bi2Se3, Bi2Te3 and Sb2Te3) have been synthesized recently as three-dimensional topological insulators (TIs). Although these materials have been used as thermoelectric materials for many years, for future studies and applications of the topological surface states, a major bottleneck remains the lack of high-quality bulk materials that have very few defects and the Fermi level can be moved to inside the bulk bandgap. In this paper, we review the use of molecular beam epitaxy (MBE) technique to achieve high-quality TI materials. Furthermore, the use of layered growth in MBE affords us the fabrication of heterostructures, such as quantum wells and superlattices. Thus, it may further enable additional studies and applications, similar to those of conventional semiconductor heterostructures but with the novel properties of TI. We explore the growth mechanism, providing a detail discussion on the growth parameters of thin-film synthesis by MBE. Then we discuss more complex cases, such as functional doping, heterostructures and superlattices. Potential new properties in such quantum structures are discussed. Finally, we give an outlook on this material system for both fundamental studies and applications. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssr.201307003

Review of 3D topological insulator thin‐film growth by molecular beam epitaxy and potential applications

Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems that are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by molecular beam epitaxy on graphene. Reflection high-energy electron diffraction images exhibited sharp streaky features indicative of a high-quality GaSe layer produced via a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW heterointerface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting to an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of the few layers of GaSe are located at the Γ point at a bindin...

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van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties.

Two-dimensional few-layered materials exhibit unique and exotic electronic properties when compared to their bulk counterparts. Here, the authors report the role of dimensionality on the electronic properties of a van der Waals GaSe/graphene heterostructure. The strong thickness dependence of the GaSe band gap is demonstrated experimentally using scanning tunneling spectroscopy. The GaSe band gap decreases as the number of layers increases due to quantum confinement effects. The particular GaSe `Mexican hat' band structure is further explored by angle-resolved photoemission spectroscopy and by theoretical calculations. Interestingly, an electron transfer from graphene to GaSe has also been detected. It results in $n$-type doped thin GaSe films, opposite to the $p$-type character GaSe bulk.

Tunable quasiparticle band gap in few-layer GaSe/graphene van der Waals heterostructures

Intermetallic coumpound formation at metal–metal interfaces was observed for thin film couples of Al and transition metals such as Cr, Ti, Hf, Zr, Co, Ta, Pd, and Pt. The Al–Cr system was investigated in some detail using a conductance method and nuclear backscattering for reaction‐rate measurements. The compounds CrAl7 and Cr2Al11 were identified and their growth was found to be diffusion controlled. The temperature dependence of the rate constant for CrAl7 obeyed an Arrhenius plot from 300° to 450 °C with an activation energy of 1.91±0.1 eV. The Al‐rich phases of the remaining transition metals were investigated; the compounds either exhibited a planar interface with parabolic growth kinetics or an irregular interface with nonparabolic growth. An empirical relationship involving the melting point and stability of the compounds was proposed to explain the different growth kinetics.

KINETICS OF COMPOUND FORMATION IN THIN FILM COUPLES OF Al AND TRANSITION METALS.

Constitutive flow behavior and hot workability of the powder metallurgy processed 20 vol.%SiC(P)/2024Al composite were investigated using hot compression tests. The modified Arrhenius-type constitutive equations were presented with the values of material constants in consideration as a function of strain. Dynamic material model (DMM) and modified DMM were used to construct the power dissipation efficiency maps, and Ziegler's instability criterion and Gegel's stability criterion were used to build instability maps. The presence of finer SiC(P) and more boundaries resulting from smaller 2024Al powders shifted the dynamic recrystallization domain of the 2024Al matrix to higher strain rate and lower temperature ranges and decreased the peak value of power dissipation efficiency. Large instable regions were found in the form of flow localization and cavitations located at the matrix/SiC(P) interfaces and within the SiC(P) clusters. By comparison, the Gegel's stability criterion was more sensitive to the instability zones than the Ziegler's instability criterion for this material. (c) 2010 Elsevier B.V. All rights reserved.

Constitutive flow behavior and hot workability of powder metallurgy processed 20 vol.%SiCP/2024Al composite

yield (;30%) is as low as that of state-of-the-art bulk material, and the interface is atomically abrupt. The initial film deposits are epitaxial islands, and subsequent growth is in the Frank‐van der Merwe mode. With the islands already relaxed at the nucleation stage and coalescing to essentially uniform coverage with the first monolayer of deposition, GaSe on Si~111! provides an example of van der Waals epitaxy. However, it is difficult to understand how epitaxy ~crystallographic alignment with the substrate! can occur in such a case, where the film is incommensurate starting from the initial nuclei. A mechanism for alignment of the islands is proposed: they are aligned with the silicon substrate through the influence of dangling bonds at their perimeter, being ‘‘quasicommensurate ’’ by virtue of their small lateral size. Although discommensurate regions are created as the islands grow laterally, there is simply no change in their orientation. © 1997 American Institute of Physics. @S0021-8979~97!05011-1# The hexagonal, layered semiconductor, GaSe, has been shown to grow on Si~111! by molecular beam epitaxy ~MBE! in a remarkable way: the film is crystallographically aligned with the substrate ~and remains so after the accumulation of 1 mm! and this epitaxy occurs in spite of the fact that, starting from sub-monolayer coverage, the film is misfitting and relaxed. 1

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The van der Waals epitaxial growth of GaSe on Si(111)

Several 265‐nm‐thick metastable pseudomorphic Ge0.12Si0.88 films grown on a Si(100) substrate by molecular‐beam epitaxy were implanted at room temperature with 100 keV phosphorus ions to a dose of 1.5×1015/cm2. The implantation amorphizes the top portion (∼190 nm) of the GeSi layer and leaves the rest of the film single crystalline. Implanted and nonimplanted samples were subsequently annealed simultaneously in vacuum for 30 min from 400 to 800 °C. The implanted samples undergo layer‐by‐layer solid‐phase‐epitaxial regrowth during annealing at or above 500 °C. The regrown GeSi layer is relaxed with a high density of threading dislocations (∼1010–1011/cm2). The nonamorphized portion of the layer remains fully strained when annealed between 400 and 600 °C. At or above 700 °C misfit dislocations are observed at the Si/Ge0.12Si0.88 interface. After 800 °C annealing the strain in the whole epilayer is fully relaxed. The strain relaxation is facilitated by the implantation. The presence of phosphorus in GeSi rai...

Solid‐phase epitaxial regrowth and dopant activation of P‐implanted metastable pseudomorphic Ge0.12Si0.88 on Si(100)

A dynamic Monte Carlo simulation (MCS) program was developed to discribe the processes of interfacial modification of materials by ion beam mixing and applied to the bilayer and multilayer Al/Pd system. For the bilayer system, the MCS results for the dependence of the mixing rate on the film thickness of top layer (Al) show that the optimum film thickness for ion beam mixing corresponding to the mean damage depth in the Al layer. The dynamic MCS results show that the number of moved Pd atoms is larger than that of Al for small Al overlayer thickness due to collisional nature. In the case of multilayer system, the mixing rate depends on the structure of the multilayered system, and differs from the bilayered system. The mixing rate has a maximum value at the second interface with no dependence on structure. The maximum mixing rate for the multilayer system is larger than that of the bilayer system by a factor of 2. The energy deposition due to the nuclear collisions at the interface and the total number of intermixed atoms across the interface are found to play an important role for these interfacial mixing characteristics.

Dynamic Monte Carlo simulation for cascade interfacial mixing

Atomic transport in the radiation enhanced diffusion (RED) region has been studied from the shifts of a marker layer in ion beam mixed Pd Co and Pd Au bilayers 80 keV Ar+ with a dose of 15 × 1016 ions/cm2 were irradiated into the bilayers at temperature region from 90 K to 700 K In the Pd Co system, the atomic flux of Pd (JPd) transported across the interface is nearly same with JCo in the thermal spike region, while JPd is always larger than JCo in the RED regio However, in the Pd Au system, JPd is nearly same with JAu in both of the thermal spike and RED regions We have developed a model to describe the atomic transport in the RED region, which predicts that the atom with small cohesive energy has more mobility than that with large cohesive energy

Cohesive energy effects on the atomic transport induced by ion beam mixing

Evaporated Al/Cr bilayer thin films were irradiated by 80 keV Ar+ at doses in the range from 1 × 1015 to 2 × 1016 Ar+/cm2 at room temperature in order to investigate the Ar+ induced interfacial mixing behavior and the phase formation and transition by Ar+ bombardment. Ion bombardment induces intermixing across the Al/Cr interface and mixing variance increases with increasing ion dose. Cascade and thermal spike models are found to be not adequate for the ion beam mixing mechanism at room temperature in this system. The Al13Cr2 phase is formed as an initial phase by ion beam mixing and then transforms into the Al11Cr2 or Al4Cr phases at subsequent ion bombardment. This result is discussed in terms of the enhanced atomic mobility and the thermodynamical driving force by introducing the concept of an effective heat of formation.

J.H. Song

Papers

The van der Waals epitaxial growth of GaSe on Si(111)

Solid‐phase epitaxial regrowth and dopant activation of P‐implanted metastable pseudomorphic Ge0.12Si0.88 on Si(100)

Dynamic Monte Carlo simulation for cascade interfacial mixing

Cohesive energy effects on the atomic transport induced by ion beam mixing

Ar + induced interfacial mixing and phase formation in the Al/Cr system