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The strong light-matter interaction in transition metal dichalcogenide (TMD) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS2 and MoSe2 MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then, we cover them with exfoliated hBN to finally obtain an encapsulated sample: exfoliated hBN/TMD ML/MBE hBN. We observe improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO2 substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge-free substrate for fabricating TMD-based heterostructures on a larger scale. [ABSTRACT FROM AUTHOR]

Hydrogen (H) plays an important role in determining the reliability and performance of HfO2- and Al2O3-based high-k dielectric electronic devices. In order to understand H behavior, deuterium (D), an isotope of H, was introduced into the poly-Si cap of Al2O3/HfxSi1-xO2/SiO2 high-k stacks by ion implantation. Atom probe tomography was used to image the D distribution in samples annealed under different conditions. The results clearly demonstrated that the D atoms were trapped at the interface of poly-Si and Al2O3 after annealing at 900 K for 10 min. Thus, it is possible that Al2O3 blocks the H atoms at the surface, preventing them from diffusing into the high-k dielectrics during the H2 annealing process in current fabrication technology. The current work also exhibits an example of investigating H behavior in semiconductors by atom probe tomography. [ABSTRACT FROM AUTHOR]

Noncontact temperature measurements based on Raman scattering were performed on 4H-SiC with hexagonal defects. These measurements show that the four-phonon process makes a greater contribution to the E2(TO) mode than to the E1(TO) mode. The longer lifetimes of E2(TO) and E1(TO) phonons in hexagonal defects demonstrate that there are fewer possible decay channels than in the defect free zone. The absence of electronic Raman peaks in the hexagonal defects suggests that hexagonal defects seriously limit the uniformity of the nitrogen distribution. The intensity of electronic Raman spectra is related to the density of neutral nitrogen atoms. [ABSTRACT FROM AUTHOR]

The electronic states of manganese in p-type GaN are investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra d-shell transition 4T2(F)-4T1(F) of Mn4+ ions. PLE spectrum of the Mn4+ [4T2(F)-4T1(F)] luminescence reveals intracenter excitation processes via the excited states of Mn4+ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra-d-shell 4T1(P)-4T1(F) and 4A2(F)-4T1(F) transitions of Mn4+, respectively. In addition to the intrashell excitation processes, a broad PLE band involving charge-transfer transition of the Mn4+/3+ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn4+/3+ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using ab initio calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in p-type GaN:Mn when the Fermi energy is lower than 1.11 eV above the valence band maximum. [ABSTRACT FROM AUTHOR]

The optical properties of Mn-Mg-codoped epitaxial GaN were studied. Addition of Mg acceptors quenches the weak manganese-related photoluminescence (Pl) band at 1.3 eV in GaN:Mn and a series of sharp PL peaks are observed at 1 eV in codoped epilayers. The change in PL spectra indicates that the Mg addition stabilizes the Mn4+ charge state by decreasing the Fermi level. The 1 eV PL peaks are tentatively attributed to intracenter transitions involving Mn4+ ions. Spin-allowed 3d-shell 4T2-4T1 transitions and their phonon replicas are involved. The relative intensities of the sharp peaks are strongly dependent on the excitation wavelength, indicating that the optically active Mn4+ centers involved in the separate peaks are different. The temperature dependence of the PL spectrum suggests the presence of at least three distinct Mn4+ complex centers. [ABSTRACT FROM AUTHOR]

Discusses the generation of a phosphorous-rich structure on the indium phosphide surface during metalorganic vapor-phase epitaxy. Adsorption of phosphorous dimers, alkyl groups and hydrogen atoms onto a layer of P atoms; Production of c and p domains of the adsorbed dimers.

We have developed a kinetic model for arsine decomposition on GaAs(001) that accounts for the effect of the surface reconstructions on the rates of As4 and As2 desorption. The main assumption of the model is that As2 desorbs more slowly from Ga sites than from As sites. The model accurately simulates the temperature-programmed desorption of arsenic from GaAs(001). In addition, it reveals how the As coverage and the As2 and As4 production rates depend on the conditions employed during GaAs metalorganic vapor-phase epitaxy. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Demonstrates the epitaxial growth of neodymium gallate thin films on (110) lanthanum aluminum oxygen[sub 3] substrate by metalorganic chemical vapor deposition. Use of neodymium (Nd) and gallium (Ga) beta-diketonate precursors; Details of the annealing of amorphous Nd-Ga-O precursor film in oxygen atmosphere; Results of cross-sectional electron microscopy.

We present magneto-photoluminescence measurements in a hybrid two-dimensional semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in the MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results are interpreted in terms of a spin-dependent charge transfer between the MoSe2 monolayer and the nickel film. The build-up of circular polarization is observed up to 120 K, mainly limited by the trion emission that vanishes with temperature. [ABSTRACT FROM AUTHOR]

We have characterized the (2×4) and (4×2) reconstructions of GaAs and InAs (001) that are present in a metalorganic vapor-phase epitaxy (MOVPE) reactor. Scanning tunneling micrographs show that these surfaces are terminated with arsenic and gallium (or indium) dimers. The (2×4) dimer row exhibits a mottled appearance, which is ascribed to the adsorption of alkyl groups on some of the sites. On the (4×2), <10% of the surface is covered with small (2×4) islands. These results show that, in the MOVPE environment, the GaAs and InAs surface structures are nearly the same as those found in ultrahigh vacuum molecular beam epitaxy. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Surfaces of GaAs (001) were prepared by metalorganic vapor-phase epitaxy and characterized by scanning tunneling microscopy, x-ray photoelectron spectroscopy, infrared spectroscopy, and low-energy electron diffraction. Upon removal from the reactor, the gallium arsenide surface exhibits a (1×2) reconstruction, which is a disordered variant of the c(4×4). The disorder arises from the presence of adsorbed alkyl groups. Heating the sample to 350 °C desorbs the hydrocarbons and produces a well-ordered c(4×4) structure. A model is proposed for the alkyl-terminated (1×2) reconstruction. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure at the atomic scale and optical properties of GaN-based green light-emitting diodes (LEDs) before and after in situ degradation were investigated by spherical aberration corrected scanning transmission electron microscopy and temperature-dependent micro-photoluminescence. Indium (In) interstitial atoms existed in the degraded sample, due to the small-bond-energy In atoms deviating from their normal lattice position, caused by the relaxation of the InGaN well. Both the peak wavelengths of the original and degraded green LEDs had similar temperature-dependent behaviors, due to the localization states in the InGaN well. These wavelengths indicate that the degradation had little influence on the localization states. However, the emission peak of the degraded green LED redshifted by 1.6 nm at 300 K, and the integrated intensity decreased by 36.8%, compared to the peak and intensity of the original sample, respectively. Based on first-principles calculations, the calculated bandgap for the relaxation of the InGaN well was small. Therefore, the wavelength redshifted, and the luminous efficiency of the green LED decreased after degradation. These features are attributed to a decreased bandgap due to the relaxation of the InGaN well; increased defect density, resulting from In interstitial atoms; and an increase in the InGaN well thickness. [ABSTRACT FROM AUTHOR]

A simple scheme to produce strong magnetic fields due to cold electron flow in an open-ended coil heated by high power laser pulses is proposed. It differs from previous generation of magnetic fields driven by fast electron current in a capacitor-coil target [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. The fields in our experiments are measured by B-dot detectors and protonradiography, respectively. A 205 T strong magnetic field at the center of the coil target is generated in the free space at I?² of 6.85 × 1014 W cm-2 µm², where I is the laser intensity, and ? is the laser wavelength. The magnetic field strength is proportional to I?². Compared with the capacitor-coil target, the generation mechanism of the magnetic field is straightforward and the coil is easy to be fabricated. [ABSTRACT FROM AUTHOR]

Carbon tetrachloride dissociatively adsorbs on the Ga-rich (4×2) reconstruction of GaAs (001) at 200 °C. Upon heating to 440 °C, the chlorine desorbs as GaCl, which etches the surface. Scanning tunneling micrographs reveal that this reaction transforms the (4×2) into a Ga-rich (3×2) structure that is interlaced with As-rich (2×4) phases. The (3×2) is well ordered, while the (2×4) phases exhibit a high degree of disorder. This work establishes the surface reaction pathway for carbon doping of GaAs with CCl[sub 4].© 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) thin film composed of Tb nanocrystals with uniform orientations is fabricated directly from Tb65Fe25Al10 alloy melt upon quenching. The Tb nanocrystals with vertical height of 15–30 nm and lateral width of 10–20 nm are assembled in the nanocrystalline array on an amorphous substrate. The formation mechanism for the aligned Tb nanocrystals is discussed. The single-step formation method may provide a new and flexible alternative to fabricate nanostructured films or arrays used for submicron devices. [ABSTRACT FROM AUTHOR]