Fanyan Chen

Laser molecular beam epitaxy and characterization of perovskite oxide thin films

More than 10 kinds of perovskite oxide thin films and their heterostructures have been successfully fabricated on SrTiO3(STO) (0 0 1) substrates by laser molecular beam epitaxy (laser MBE). Measurements of atomic force microscopy (AFM) and high-rt solution transmission electron microscopy (HRTEM) reveal that the surfaces and interfaces are Atom level smooth. The unit cell layers and the lattice structure are perfect. The enhancement of secund-harmonic generation (SHG) in BaTiO3/SrTiO3 9BTO/STO) superlattices and the thickness dependent structural characteristics of BaTiO3 (BTO) thin fillns were observed. The all-prrovskite oxide P-N junctions have been successfully fabricated and better I-V curves were observed

Synthesis and Electrochemical Properties of Two-Dimensional Hafnium Carbide

We demonstrate fabrication of a two-dimensional Hf-containing MXene, Hf3C2Tz, by selective etching of a layered parent Hf3[Al(Si)]4C6 compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm-3 for lithium and sodium ions batteries, respectively, at a current density of 200 mAg-1 after 200 cycles. Thus, Hf3C2Tz MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

Electronic characteristics of InAs/GaAs self-assembled quantum dots by deep level transient spectroscopy

A deep level transient spectroscopy technique has been used to determine the emission activation energies and capture barriers for electrons and holes in InAs self-assembled quantum dots embedded in GaAs. The ground electron and hole energies relative to their respective energy band edges of GaAs are 0.13 and 0.09 eV. Measurements show that the capture cross section of quantum dots is thermally activated. The capture barrier of quantum dots for electrons and holes are 0.30 and 0.26 eV, respectively. The results fit well with the results of photoluminescence spectroscopy measurements

Growth and characterization of SrMoO3 thin films

Highly conductive SrMoO3 thin films with good crystallinity and smooth surface were grown on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The effects of substrate temperature and oxygen pressure on the structure, surface morphology, and electrical properties were studied. In the range of substrate temperatures from 560 degreesC to 640 degreesC and oxygen pressure from 10 (-3) to 10 (-4) Pa studied in our experiments, high-quality SrMoO3 thin films were produced. Beyond the top temperature or oxygen pressure limit, SrMoO4 appears as an impurity phase. High-resolution transmission electron microscopy (HRTEM) study shows that the SrMoO3 film has high-quality crystallinity and an epitaxial nature. The root-mean-square surface roughness of the film deposited at 2.5 x 10 (-4) Pa is 3.9 Angstrom. The films exhibit metallic conduction, which results from the delocalized electrons from Mo. X-ray photoelectron spectroscopy (XPS) measurements characterized its core level spectra and the valence band

Td-MoTe2: A possible topological superconductor

We measured the magnetoresistivity properties of Td-MoTe2 single crystal under the magnetic field up to 33 T. By analyzing the Shubnikov–de Haas oscillations of the longitudinal resistance Δρxx, a linear dependence of the Landau index n on 1/B is obtained. The intercept of the Landau index plot is 0.47, which is between 3/8 and 1/2. This clearly reveals a nontrivial π Berrys phase, which is a distinguished feature of the surface state in Td-MoTe2 single crystal. Accompanied by the superconductivity observed at TC = 0.1 K, Td-MoTe2 may be a promising candidate of the topological superconductor and opens a door to study the relationship between the superconductivity and topological physics.

Influence of growth conditions on Al-Ga interdiffusion in low-temperature grown AlGaAs/GaAs multiple quantum wells

Low-temperature growth and subsequent rapid thermal anneal were used to intermix Al and Ga atoms in AlGaAs/GaAs multiple quantum wells (QWs). The intermixed samples were characterized by photoluminescence (PL) spectroscopy, and the observed blue shifts in PL energies are interpreted as the result of modification of the QW shape due to the enhanced Al-Ga interdiffusion in the samples. The enhancement of interdiffusion was found to be strongly dependent on the growth and annealing conditions. In addition, the saturation behavior of Al-Ga interdiffusion was also observed.

Fabrication of BaTiO 3 P-N Junctions by Laser Molecular Beam Epitaxy

A series of BaTiO 3 p-n junctions have been successfully fabricated including BaTi 1 m x Nb x O 3 /BaTi 1 m x In x O 3 BaTi 1 m x In x O 3 /SrTi 1 m x Nb x O 3 by Laser molecular beam epitaxy. Better I-V curves of p-n junctions have been obtained.

Photoluminescence of low-temperature AlGaAs/GaAs multiple quantum wells

The photoluminescence (PL) of low-temperature-grown AlGaAs/GaAs multiple quantum wells (LT-MQWs) has been investigated and compared with a normal-temperature-grown AlGaAs/GaAs MQW structure (NT-MQWs) implanted with protons. The as-grown LT-MQWs show much more inter!sive PL than the as-implanted NT-MQWs. Upon anneal, the PL from the LT-MQWs and the NT-MQWs exhibits different behaviors. For the NT-MQWs, the PL intensity increases monotonously with rising annealing temperature due to the decrease of implantation-induced defects. By contrast, the PL from the LT-MQWs is drastically quenched, and the PL intensity drops nearly three orders of magnitude after anneal at 600 degrees C. The annealing behavior of the PL from the LT-MQWs is attributed to the formation of As clusters that act as deep trap centers for the photoexcited carriers and cause the quenching of the PL intensity. After anneal, enhanced interface intermixing and roughening have been observed in the LT-MQWs.

Copper–SiC whiskers composites with interface optimized by Ti 3 SiC 2

In the present work, we propose Ti3SiC2 as the interlayer material to improve the interfacial bonding properties of SiC whiskers-reinforced copper matrix composites. For the first time, Ti3SiC2 coating was in situ fabricated on the surface of SiC whiskers in a molten salt bath. Ti3SiC2-coated SiC whiskers- and bare SiC whiskers-reinforced copper matrix composites were fabricated by spark plasma sintering. The influence of the Ti3SiC2 interlayer on the interfacial properties, mechanical properties, and thermal conductivity of the composites was investigated. The results show that the Ti3SiC2 interlayer can largely improve the interfacial bonding properties of the composites. Therefore, the composites reinforced with Ti3SiC2-coated SiC whiskers exhibit much higher tensile strength than the composites reinforced with bare SiC whiskers. The results also show that the Ti3SiC2 interlayer could decrease the thermal conductivity of the composites in certain extent.

Electronic characteristics of InAs self-assembled quantum dots

Deep-level transient spectroscopy and photoluminescence studies have been carried out on structures containing self-assembled InAs quantum dots formed in GaAs matrices. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the deep-level transient spectroscopy technique. From analysis of deep-level transient spectroscopy measurements it follows that the quantum dots have electron levels 130 meV below the bottom of the GaAs conduction band and heavy-hole levels at 90 meV above the top of the GaAs valence band. Combining with the photoluminescence results, the band structures of InAs and GaAs have been determined