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Van der Waals heterostructures open up vast possibilities for applications in optoelectronics, especially since it was recognized that the optical properties of transition-metal dichalcogenides (TMDC) can be enhanced by adjacent hBN layers. However, although many micrometer-sized structures have been fabricated, the bottleneck for applications remains the lack of large-area structures with electrically tunable photoluminescence emission. In this study, we demonstrate the electrical charge carrier tuning for large-area epitaxial MoSe 2 grown directly on epitaxial hBN. The structure is produced in a multistep procedure involving Metalorganic Vapor Phase Epitaxy (MOVPE) growth of large-area hBN, a wet transfer of hBN onto a SiO 2 /Si substrate, and the subsequent Molecular Beam Epitaxy (MBE) growth of monolayer MoSe 2 . The electrically induced change of the carrier concentration is deduced from the evolution of well-resolved charged and neutral exciton intensities. Our findings show that it is feasible to grow large-area, electrically addressable, high-optical-quality van der Waals heterostructures.

Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from specific symmetry operation that connects the spin sublattices. In this report, we show with ab initio calculations that semiconductive MnSe exhibits altermagnetic spin-splitting in the wurtzite phase as well as a critical temperature well above room temperature. It is the first material from such a space group identified to possess altermagnetic properties. Furthermore, we demonstrate experimentally through structural characterization techniques that it is possible to obtain thin films of both the intriguing wurtzite phase of MnSe and more common rock-salt MnSe using molecular beam epitaxy on GaAs substrates. The choice of buffer layers plays a crucial role in determining the resulting phase and consequently extends the array of materials available for the physics of altermagnetism.

Distributed Bragg Reflectors (DBR) are well-established photonic structures that are used in many photonic applications. However, most of the DBRs are based on different materials or require post-process etching which can hinder integration with other components in the final photonic structure. Here, we demonstrate the fabrication of DBR structures consisting only of undoped boron nitride (BN) layers with high refractive index contrast by using metal-organic chemical vapor deposition (MOCVD). This has been achieved in a single process, without the need for any post-process etching. The difference in the refractive index of the component BN layers stems from different degrees of porosity of the individual BN layers, which is a direct result of a different growth temperature. The fabricated DBR structures consist of 15.5 pairs of BN layers and exhibit a reflectance of 87 ± 1% at the maximum. The wavelength of maximum reflectance can be tuned from 500 nm up to the infrared region (IR), by simply adjusting the growth periods of subsequent BN layers. We also demonstrate that the fabricated structures can be used to create an optical microcavity. The fabricated DBRs are very promising candidates for future applications, for example in combination with single-photon emitters in h-BN, which could allow the building of a cavity-based all-BN single-photon source., (Creative Commons Attribution license.)

Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers although only for materials of sufficient crystalline quality and so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow the growth of large-scale, high-quality TMD monolayers but also allow the growth to be performed directly on large-scale epitaxial hBN. In this work, we address this problem and demonstrate that MoSe 2 of high optical quality can be directly grown on epitaxial hBN on an entire 2 in. wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by metal organic vapor phase epitaxy (MOVPE) and as a second step MoSe 2 is deposited on top by molecular beam epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole 2 in. wafer with only ±0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC [formula] surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 Å from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC [formula] are loosely bound to the SiC substrate. Moreover, the structural observations reveal a certain degree of disorder between the graphene planes, which manifests itself in a rotation of the layers and in an increase in the interplanar spacing between certain carbon layers from 3.35 Å, which is characteristic for graphite, up to 3.7 Å. Graphene grown on 8° off-axis SiC [formula] substrates covers the steps of SiC and as a result disorder seems to be not as pronounced as it is on the on-axis SiC [formula] substrate. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and micro-Raman investigations of epitaxial graphene on 4H-SiC on-axis and 4° off-axis are presented. The STM images show that there is superimposed on 1×1 graphene pattern the carbon nanomesh of honeycomb 6×6 structure with the lattice vector of 17.5 Å. The TEM results give evidence that the first carbon layer is separated by 2 Å from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 Å. It is also found in TEM that the graphene layers cover atomic steps, present on 4° off-axis SiC(0001) surface, indicating a carpetlike growth mode. However, a bending of graphene planes on atomic steps of SiC apparently leads to generation of stress which leads to creation of edge dislocations in the graphene layers. [ABSTRACT FROM AUTHOR]

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1-xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed. [ABSTRACT FROM AUTHOR]

Monolayer transition-metal dichalcogenides (TMDs) manifest exceptional optical properties related to narrow excitonic resonances. However, these properties have been so far explored only for structures produced by techniques inducing considerable large-scale inhomogeneity. In contrast, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of the interesting optical effects. Here, we report for the first time on the MBE-grown TMD exhibiting narrow and resolved spectral lines of neutral and charged exciton. Moreover, our material exhibits unprecedented high homogeneity of optical properties, with variation of the exciton energy as small as ±0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth is presented for MoSe 2 and includes the use of atomically flat hexagonal boron nitride substrate. This recipe opens a possibility of producing TMD heterostructures with optical quality, dimensions, and homogeneity required for optoelectronic applications.

Grating metamaterials were fabricated with electron beam lithography on CdTe/CdMgTe modulation doped structures with two non-interacting quantum wells. Two types of samples were studied: with etched gratings and with gratings formed by deposition of Au stripes. The polarization properties at THz frequencies of the gratings were determined at room temperature. It was shown that Au gratings formed a linear polarizer, while etched gratings did not polarize THz radiation. Transmission of circularly polarized THz radiation at low temperatures through a sample with no grating showed a strongly circularly polarized cyclotron resonance transition. Transmission of this radiation through a sample with an etched grating showed a magnetoplasmon transition that was almost perfectly linearly polarized. We concluded that magnetoplasmons in metamaterials with etched gratings are linearly polarized excitations, possibly with a small contribution of a circular component. This work opens the possibility of the detailed study of the polarization of magnetoplasmons, which has not been explored in the past.

The Raman scattering has been studied in heterostructures composed of a thin MoS2 flake and a 1-1.5 nm layer of thermally evaporated gold (Au). There have been Au nanoislands detected in the heterostructure. It has been found that their surface density and the average size depend on the MoS2 thickness. The Raman scattering spectrum in the heterostructure with a few monolayer MoS2 only weakly depends on the excitation (resonant vs. non-resonant) mode. The overall Raman spectrum corresponds to the total density of phonon states, which is characteristic for disordered systems. The disorder in the MoS2 layer is related to the mechanical strain induced in the MoS2 layer by the Au nanoislands. The strain results in the localization of phonon modes, which leads to the relaxation of the momentum conservation rule in the scattering process. The relaxation allows phonons from the whole MoS2 Brillouin zone to interact with electronic excitations. Our results show that the Au nanoislands resulted from thermal evaporation of a thin metal layer introduce substantial disorder into the crystalline structure of the thin MoS2 layers. [ABSTRACT FROM AUTHOR]