Correlation between structural and electrical properties of InN thin films prepared by molecular beam epitaxy

The strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy are reported. The proposed growth model emphasizes the dominant role of the coalescence process in the formation of a dislocation network in 2H-InN. Edge type threading dislocations and dislocations of mixed character have been found to be the dominating defects in wurtzite InN layers. It is demonstrated that these dislocations are active suppliers of electrons and an exponential decay of their density with the thickness implies a corresponding decay in the carrier density. Room temperature mobility in excess of 1500 cm² V ⁻¹ s⁻¹ was obtained for 800 nm thick InN layers with dislocation densities of 3×10⁹ cm⁻².     

Compact 120 TW Ti:sapphire laser system with a high gain final amplifier

A 120 TW/36 fs laser system based on Ti:sapphire chirped-pulse amplification (CPA) has been successfully established in our lab. The final four pass Ti:sapphire amplifier pumped by an energetic single-shot Nd:YAG—Nd:glass laser was designed and optimized. With 24 J/8 ns pump energy at 532 nm, 300 mJ/220 ps chirped pulse was amplified to 5.98 J in this amplifier, and a total saturated gain of 20 was achieved. The focused intensity of compressed beam could reach to 10²⁰ W/cm² with the M² of 2.0.     

Picosecond optical parametric amplification of stimulated Raman as high peak-power source and ultra-sensitive preamplifier

We report the characteristics of the amplified stimulated Raman scattering (SRS) pulses generated in liquid benzene by a picosecond (ps) β-barium borate (BBO) optical parametric amplifier (OPA). When the OPA system was used as an energy amplifier for SRS pulses, with a pump energy of 2.4 mJ at 355 nm for the OPA, the maximum output energy of the amplified SRS was about 0.73 mJ for the signal and 0.18 mJ for the idler, the energy conversion efficiency was 30.4% from the pump beam to the amplified third order Stokes component at 635.1 nm. The total efficiency would be as high as 37.9% if the output of the idler is also included. The corresponding spectral line width of the amplified Raman pulse was 11.8 cm⁻¹ with a pulse width of 10.9 ps and a peak power of 67 MW. The OPA system was also used as an amplifier for very weak Raman signal, the slope gain factor of this amplifier was found to be as high as 4.1 × 10⁷ and the energy detection limit was as low as 14.8 aJ per pulse, or 48 photons at 635.1 nm, in particular. Such a detection limit corresponds to approximately 0.5 photons per pulse if the time-gate of the OPA is reduced to 150 fs and it is about the same as or even better than a recently report on the 0.75 photons detection limit for a 150-fs OPA of coherent signal at 800 nm.     

Temperature dependent photoluminescence from ZnO/MgZnO multiple quantum wells grown by pulsed laser deposition

We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range 1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of 3.35–3.68 eV with decreasing thickness in agreement with the calculated values.     

Combustion of nano-aluminum and liquid water

An experimental investigation on the combustion behavior of nano-aluminum (nAl) and liquid water has been conducted. In particular, linear and mass-burning rates of quasi-homogeneous mixtures of nAl and liquid water as a function of pressure, mixture composition, particle size, and oxide layer thickness were measured. This study is the first reported self-deflagration on nAl and liquid water without the use of any additional gelling agent. Steady-state burning rates were obtained at room temperature (25 °C) using a windowed vessel for a pressure range of 0.1–4.2 MPa in an argon atmosphere, particle diameters of 38–130 nm, and overall mixture equivalence ratios () from 0.5 to 1.25. At the highest pressure studied, the linear burning rate was found to be 8.6 ± 0.4 cm/s, corresponding to a mass-burning rate per unit area of 6.1 g/cm² s. The pressure exponent at room temperature was 0.47, which was independent of the overall mixture equivalence ratio for all of the cases considered. The mass-burning rate per unit area increased from 1.0 to 5.8 g/cm² s for an equivalence ratio range of 0.5–1.25. It varied inversely to particle diameter, increasing by 157% when the particle diameter was decreased from 130 to 50 nm at  = 1.0.     

Nucleation, growth and morphology of Co thin films on Ag(1 1 0)

Scanning tunneling microscopy (STM) experiments reveal that Co growth on Ag(1 1 0), at coverages of Co < 1 ML and low substrate temperatures (150 K), involves a concomitant insertion of Co into the top Ag layer and exchange of Ag out onto the surface. At 300 K, coverages of Co > 1 ML gives rise to a 3D nanocluster growth on the surface, with the clusters covered by Ag. Depending slightly on coverage, the clusters have a typical diameter of 3 nm and a height of 0.4 nm. Upon annealing to 500 K, major changes are observed in the morphology of the surface. STM and AES show that there is a reduction of the number of Co islands on the surface, partly due to subsurface Co cluster migration and partly due to sintering into larger clusters.     

Fabrication of the uniform CdTe quantum dot array on GaAs substrate utilizing nanoporous alumina masks

Fabrication of quantum dot array (QDA) is attractive for applications in electronic and optoelectronic devices. The CdTe QDAs have potential applications in optoelectronic devices of visible range. One of the major challenges in fabricating QDAs is the uniformity and reproducibility in size and spatial distribution. The uniformity and reproducibility of QDs can be improved by using the nanoporous alumina mask. The geometry of porous alumina is schematically represented as a close-packed array of columnar hexagonal cells, each containing a central pore normal to the substrate. The well-ordered nanoporous alumina masks were able to obtain by two-step anodizing processes from aluminum in oxalic acid solutions at low temperature. The pore size, thickness, and density of nanoporous alumina mask can be controlled with the anodization voltage, time, and electrolyte. The CdTe QDAs on the GaAs substrate was grown by molecular beam epitaxy method using the porous alumina masks. The temperature of substrate and source (Cd, Te) was an important factor for the growth of CdTe QDs on GaAs substrate. The CdTe QDAs of 80 nm dot size was fabricated; using the porous alumina masks (300 nm thickness) of pore diameter (80 nm) and density (10¹⁰ /cm²).     

MBE growth of nitride-based photovoltaic intersubband detectors

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×10¹⁹ cm⁻³. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.     

Bragg grating writing through the polyimide coating of high NA optical fibres with femtosecond IR radiation

Bragg gratings are written with ultrafast 800 nm radiation and a phase mask through the polyimide polymer coatings of commercially available high NA fibres that are both unloaded and loaded with high pressure hydrogen gas. For polyimide coated fibres with very high germanium core concentrations, index modulations greater than 1 × 10⁻⁴ are induced. Stable core index modulations 60% of their original value were present after 115 h at 500 °C.     

Laser scribing of polycrystalline thin films

We have investigated the use of several different types of lasers for scribing of the polycrystalline materials used for thin-film solar cells: CdTe, CuInGaSe₂ (CIGS), ZnO, SnO₂, Mo, Al, and Au. The lasers included four different neodymium–yttrium–aluminum garnet (Nd:YAG) (both 1064 and 532 nm wavelengths), a Cu vapor (511/578 nm), an XeCl excimer (308 nm), and a KrF excimer (248 nm). Pulse durations ranged from 0.1 to 250 ns. We found that the fundamental and frequency-doubled wavelengths of the Nd:YAG systems work well for almost all of the above materials except for the transparent conductor ZnO. The diode-laser-pumped Nd:YAG was particularly convenient to use. For ZnO the uv wavelengths of the two excimer lasers produced good results. Pulse duration was found generally not to be critical except for the case of CIGS on Mo where longer pulse durations (≥250 ns) are advantageous. The frequently observed problem of ridge formation along the edges of scribe lines in the semiconductor films can be eliminated by control of intensity gradients at the film through adjustment of the focus conditions.     

Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn₂O₅ was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at 45 K. A broad peak below 6 K in the heat capacity measurements corresponds to the crossover transition of Tb³⁺ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn₂O₅ with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by using neutron wavelength of 4.8 Å in the temperature range of 1.8–250 K. Magnetic scattering was separated from nuclear coherent and spin incoherent scattering contributions. Long-range ordered magnetic peaks were observed below 39 K which is consistent with the heat capacity results. The drastic increasing intensities below 6 K indicate the ferromagnetic transition in Tb³⁺ orderings.     

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P₂O₅-doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ_n0.5 cm² V ⁻¹ s⁻¹ and a carrier concentration of n3×10¹⁷ cm⁻³ at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.     

From Zn microspheres to hollow ZnO microspheres: A simple route to the growth of large scale metallic Zn microspheres and hollow ZnO microspheres

Large scale metallic Zn microspheres and hollow ZnO microspheres are synthesized by thermal evaporation and vapor transport by heating a ZnO/graphite mixture at 1000 °C. Firstly, metallic Zn microspheres are fabricated with diameters in the range of 1–10 μm. The Zn microspheres are then annealed at 600 °C in air, which form hollow semiconducting ZnO microspheres. EDX and XRD spectra reveal that the oxidized material is indeed ZnO. Room temperature photoluminescence spectra of the oxidized material show a sharp peak at 380 nm and a wider broad peak centered at 490 nm. This growth mechanism is discussed and further investigated for other metallic and metal oxide microstructures.     

Hybrid system of iron porphyrin on aminosilanized c-Si

Adsorption of iron porphyrin (Fe^IIITPPS₄) Fe(III)meso-tetra(4-sulfonatophenyl) porphine on aminosilanized surface of crystalline Si (c-Si) was investigated. Formation of nanometric structures of Fe^IIITPPS₄ on c-Si, the surface of which has been modified by various silanization procedures, was studied. Aqueous, ethanol and acetone solutions of 3-aminopropyltrietoxysilane (APTES) were prepared for deposition on c-Si by spinning or immersion techniques. Smooth homogeneous APTES films of thickness 100–500 nm were produced by multiple spin coating procedure. Thin APTES films of thickness 2.5 nm were fabricated by dipping technique followed by washing procedure. Hybrid system of Fe^IIITPPS₄/APTES/Si was prepared from a drop of Fe^IIITPPS₄ aqueous solution put on aminosilanized Si surface or by dipping the Si wafer in Fe^IIITPPS₄ aqueous solution. Nanostructures of size 50–250 nm were formed along with large rings of Ø50–100 μm which have been observed at chemisorption of highly concentrated (1 mM) Fe^IIITPPS₄ aqueous solution. Spectroscopic ellipsometry was used to characterize the APTES layer and to investigate the aggregation state of Fe^IIITPPS₄. The studies provided allowed one to presume that covalent bonds were formed between amino-groups of APTES and functional groups of sulfonic acid in porphyrin leading to immobilization of Fe^IIITPPS₄ on Si substrate.     

Photoluminescence of ZnO nanostructures grown by the aqueous chemical growth technique

Zinc oxide nanostructured films were grown by the aqueous chemical growth technique using equimolar aqueous solutions of zinc nitrate and hexamethylenetetramine as precursors. Silicon(100) and glass substrates were placed in Pyrex glass bottles with polypropylene autoclavable screw caps containing the precursors described above, and heated at 95 C for several hours. X-ray diffraction 2θ/θ scans showed that the only crystallographic phase present was the hexagonal wurtzite structure. Scanning electron microscopy showed the formation of flowerlike ZnO nanostructures, consisting of hexagonal nanorods with a diameter of a few hundred nanometers. The photoluminescence spectra of the ZnO nanostructures were recorded at 18–295 K using a cw He–Cd laser (325 nm) and a pulsed laser (266 nm). The ZnO nanostructures exhibit an ultraviolet emission band centered at 3.192 eV in the vicinity of the band edge, which is attributed to the well-known excitonic transition in ZnO.     

Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode distributed Bragg reflector fiber laser

In this paper, a novel all-optical microwave generation technique based on a dual-wavelength single-longitudinal-mode (SLM) distributed Bragg reflector (DBR) fiber laser is proposed and demonstrated. By exploiting spatial hole burning (SHB) effect, this laser could provide stable dual-wavelength SLM operation with a wavelength separation of 0.088 nm corresponding to the microwave signal at 10.484 GHz with a 3 dB bandwidth of 28 kHz. By appropriately adjusting the pump power, dual-wavelength oscillation could be maintained at different temperatures. We have theoretically analyzed the mechanism for microwave generation of the proposed DBR laser, and the calculated microwave frequency is in good agreement with our experimental results. Furthermore, experimental observation shows both of the laser wavelengths and generated microwave signals have good stability at room temperature.     

Structure and properties of the one-dimensional cobalt oxide CaCo2O4

We have studied crystal structure and transport properties of the quasi one-dimensional cobalt oxide CaCo₂O₄. The CaCo₂O₄ phase crystallizes in calcium-ferrite type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. Large thermoelectric power (S  150 μV/K at 390 K) with metallic temperature dependence of S, moderate resistivity (ρ  2.9 × 10⁻¹ Ω cm at 390 K) with carrier localization at low temperature, and normal thermal conductivity (κ  6.3 W/Km at 390 K) were observed. The phonon mean-free path was calculated from the observed data, as a function of temperature. The long phonon mean-free path (l  24 Å at 300 K) implies that the thermal conductivity could be suppressed by impurity scattering of phonons with partial element substitution.     

Possible effects of a cosmological constant on black hole evolution

We explore possible effects of vacuum energy on the evolution of black holes. If the universe contains a cosmological constant, and if black holes can absorb energy from the vacuum, then black hole evaporation could be greatly suppressed. For the magnitude of the cosmological constant suggested by current observations, black holes larger than 4×10²⁴ g would accrete energy rather than evaporate. In this scenario, all stellar and supermassive black holes would grow with time until they reach a maximum mass scale of 6×10⁵⁵ g, comparable to the mass contained within the present day cosmological horizon.     

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was 200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu³ ions was not present. Transmission electron microscopy shows crystalline Eu₂O₃ formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp ⁵D_O–⁷F₂ transition of the Eu³⁺ ion at around 615 nm was observed.     

Using multiple quantum coherence to increase the N resolution in a three-dimensional TROSY HNCO experiment for accurate PRE and RDC measurements

We present a new version of the 3D TROSY HNCO pulse scheme, referred to as HR-TROSY HNCO, with comparable resolution in the ¹⁵N dimension to a 2D ¹H–¹⁵N HSQC experiment. In the conventional 3D TROSY HNCO, the constant time period (1/2J_NC  32 ms) severely limits the maximum resolution in the ¹⁵N dimension. In the HR-TROSY HNCO experiment presented here, both constant time periods (32 ms each) for coherence forward and backward transfer between ¹⁵N and ¹³C′ are utilized to double the ¹⁵N evolution time. This leads to a dramatic enhancement in peak separation along the ¹⁵N dimension, making the HR-TROSY HNCO an ideal pulse scheme for accurate paramagnetic relaxation enhancement and residual dipolar coupling measurements.