Meicheng Li

High electron mobility and low sheet resistance in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel heterostructure

High electron mobility and low sheet resistance were achieved in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel (DC) heterostructure. Two-dimensional electron gas (2DEG) of the DC heterostructure was divided into the double channels and the room-temperature mobility was increased to 1430 cm2/V s by reducing the 2DEG density in each channel, compared with low electron mobility (1090 cm2/V s) for lattice-matched AlInN/AlN/GaN single-channel heterostructure. It was found that the 2DEG mobility was limited by thickness of the AlN interlayer between the double channels. After the structure optimization, the room temperature electron mobility of the DC heterostructure reached 1570 cm2/V s with sheet resistance of 222 Ω/◻.

Phase stability, oxygen nonstoichiometry and magnetic properties of BiFeO3−δ

The phase stability of polycrystalline BiFeO3−δ (BFO) was measured using coulometric titration in the technologically important temperature range for growth of epitaxial thin films, 680°C–800°C. BFO exhibits only a very narrow oxygen nonstoichiometry, i.e., δ<0.01. For the most nonstoichiometric sample obtainable at 800°C (containing a maximum Fe2+ fraction of 2mol%) the saturation magnetization was 3.1×10−4μB∕Fe, a factor of around 2 higher than a nominally fully oxygenated sample. Hence, the contribution of Fe2+ to the ferromagnetism is small. The decomposition products of BFO below the oxygen phase stability line are Bi25FeO40−x and γ-Fe2O3.

Highly Birefringent Two-Mode Photonic Crystal Fibers With Near-Zero Flattened Dispersion

Through introducing three sizes of holes in cladding, highly birefringent two-mode photonic crystal fibers (PCFs) are proposed. The full-vector finite-element method (FEM) is applied in the calculation of the transverse electric field vector distributions of the two modes, their effective indices, modal birefringence, chromatic dispersion, and confinement losses. The defect holes near the core region are deliberately designed to obtain high birefringence. Furthermore, the structural parameters are carefully selected to flatten chromatic dispersion curve and reduce confinement losses.

Distribution of dislocations in GaSb and InSb epilayers grown on GaAs (001) vicinal substrates

GaSb and InSb epilayers grown on GaAs (001) vicinal substrates misoriented toward (111) plane were studied using high resolution x-ray diffraction. The results show that GaSb and InSb epilayers take on positive crystallographic tilt, and the asymmetric distribution of 60 degrees misfit dislocations in {111} glide planes have an effect on the tilt. In addition, the vicinal substrate influences the distribution of the threading dislocations in {111} glide planes, and the density of dislocation in the (111) plane is higher than in the ((1) over bar(1) over bar1) plane. A model was proposed to interpret the distribution of full width at half maximum, which can help us understand the formation and glide process of the dislocations

Quenching mechanisms of the optical centers in Eu3+-doped nanophosphors under charge transfer excitation

As the size of Eu3+-doped phosphors decreases in nanoscale, the top of the valance band is elevated and the zero-phonon charge transfer (CT) energy is decreased; the rigidity of the lattice environment is decreased and the CT state (CTS) coordinate offset is enlarged. The decrease in zero-phonon CT energy and the enlargement of CTS coordinate offset mean the displacement of CTS in the configurational coordinate diagram (CCD). The CTS displacement in CCD results in the decrease in CTS feeding to the emitting D5 states and the increase in transition probability from the CTS to the F7 states, the excitation of the optical centers tends to be relaxed by sending phonons to the host lattice. Based on the quenching mechanisms of the optical centers progressively clarified in this paper, a feasible coating method is proposed to improve the luminescence efficiency for Eu3+-doped nanophosphors under CT excitation.

Single-polarization polarization maintaining optical fiber with large stress birefringence and high homogeneity

A type of high-birefringence polarization maintaining (PM) fiber was designed and fabricated using a modified chemical vapor deposition (MCVD) process. Different from conventional stress-applied PM fibers, the shape of the stress jacket is like a “capsule.” By study of the cross sectional component distribution and a calculation of stress birefringence using the finite element method, the structure and composition of capsule PM fiber were optimized and the hybrid MCVD process was developed. With advantages in homogeneity and temperature performance, capsule fiber has great potential in applications such as fiber gyroscopes, fiber hydrophones, and other optical fiber sensors.

Determination of interface structure and atomic arrangements for strained InAs/Ga1-xInxSb superlattices by high-resolution transmission electron microscopy

The lattice structure of the InAs/Ga1−xInxSb interface has been studied in cross-section by high-resolution transmission electron microscopy. The atomic arrangement at the plane of the interface is analyzed based on the image characteristics. Possible bonding configurations are discussed. The results suggest that interface formation is first driven by charge balance. The shift in the interplanar separations associated with this modulation may lead to distortions of the interfacial structure of Ga1−xInxAs-like. The morphological evolution at GaAs-like interface is accompanied by interface misfit dislocations and compositional fluctuations near the interface associated with segregation.The lattice structure of the InAs/Ga1−xInxSb interface has been studied in cross-section by high-resolution transmission electron microscopy. The atomic arrangement at the plane of the interface is analyzed based on the image characteristics. Possible bonding configurations are discussed. The results suggest that interface formation is first driven by charge balance. The shift in the interplanar separations associated with this modulation may lead to distortions of the interfacial structure of Ga1−xInxAs-like. The morphological evolution at GaAs-like interface is accompanied by interface misfit dislocations and compositional fluctuations near the interface associated with segregation.

High birefringence photonic crystal fiber design

A novel high birefringence photonic crystal fiber (PCF) was proposed, which consists of hexagonal inner ring and octagonal outer ring. A full vector finite element was applied to investigate the mode birefringence and chromatic dispersion. It has been demonstrated from the calculated results that high birefringence to the order of 0.001 can be achieved and ultra-flattened dispersion of -0.8~+0.2 ps/(km•nm) is obtained in 1.5 to 1.65μm wavelength range.

Investigation of GaSb epilayer grown on vicinal GaAs(001) substrate by high resolution x-ray diffraction

GaSb epilayers grown on GaAs(001) vicinal substrate misoriented towards (111) plane were studied using high-resolution x-ray diffraction (HRXRD). The results show that GaSb epilayers exhibit positive crystallographic tilt and the distribution of 60 degrees misfit dislocations (MDs) is imbalanced. The vicinal substrate also leads to the anisotropy of the mosaic structure, i.e. the lateral coherent lengths in [1 (1) over bar0] directions are larger than those in [110] directions. Furthermore, the full-width at half maximum (FWHM) of the off-axis peaks varies with the inclination angle, which is a result of different dislocation densities in the {111} glide planes.

X-ray determination of mosaic structure in variable thickness EuBa2Cu3O7-δ thin films

Employing the Srikant method of extrapolating the rocking curve widths of a series of diffraction peaks across a range of angles of inclination, the out-of-plane (mosaic tilt) and in-plane (mosaic twist) misorientation angles of a series of varying thickness EuBa2Cu3O7-δ thin films has been deduced. The twist angle is seen to increase as the film thickness decreases below 200 nm, from 0.2° for films approaching the bulk to 0.35° for the thinnest film (50 nm) investigated. The tilt angle follows a similar trend, increasing more strongly from 0.05° to 0.4°. Furthermore, it is shown that the tilt and twist misorientations in this class of materials are independent in nature, implying that limited information regarding the in-plane behaviour can be inferred from purely out-of-plane measurements. The reduction in the degree of misorientation in thicker films can be associated with a decreasing threading dislocation density, and correlates with the decrease in the critical current density of R123 thin films with increasing film thickness.