Aluminum induced crystallization of strongly (111) oriented polycrystalline silicon thin film and nucleation analysis

A polycrystalline silicon thin film was fabricated on glass substrate by means of aluminum induced crystallization (AIC). Al and alpha-Si layers were deposited by magnetron sputtering respectively and annealed at 480A degrees C for 1 h to realize layer exchange. The polycrystalline silicon thin film was continuous and strongly (111) oriented. By analyzing the structure variation of the oxidation membrane and lattice mismatch between gamma-Al2O3 and Si, it was concluded that aluminum promoted the formation of (111) oriented silicon nucleus by controlling the orientation of gamma-Al2O3, which was formed at the early stage of annealing.

Homogeneous Epitaxial Growth of N,N '-di(n-butyl)quinacridone Thin Films on Ag(110)

The structural evolution of the ordered N-N' dibutyl-substituted quinacridone (QA4C) multilayers (3 MLs) has been monitored in situ and in real time at various substrate temperatures using low energy electron diffraction (LEED) during organic molecular beam epitaxy (MBE). Experimental results of LEED patterns clearly reveal that the structure of the multilayer strongly depends on the substrate temperature. Multilayer growth can be achieved at the substrate temperatures below 300 K, while at the higher temperatures we can only get one ordered monolayer of QA4C. Two kinds of structures, the commensurate and incommensurate one, often coexist in the QA4C multilayer. With a method of the two-step substrate temperatures, the incommensurate one can be suppressed, and the commensurate, on the other hand, more similar to the (001) plane of the QA4C bulk crystal, prevails with the layer of QA4C increasing to 3 MLs. The two structures in the multilayers are compressed slightly in comparison to the original ones in the first monolayer.

Rashba and Dresselhaus spin-orbit coupling in GaN-based heterostructures probed by the circular photogalvanic effect under uniaxial strain

The spin splitting in GaN-based heterostructures has been investigated by means of circular photogalvanic effect experiments under uniaxial strain. The ratios of Rashba and Dresselhaus spin-orbit coupling coefficients (R/D ratios) have been measured in AlxGa1-xN/GaN heterostructures with various Al compositions. It is found that the R/D ratio increases from 4.1 to 19.8 with the Al composition of the AlxGa1-xN barrier varied from 15% to 36%. The Dresselhaus coefficient of bulk GaN is experimentally obtained to be 0.4 eV angstrom(3). The results indicate that the spin splitting in GaN-based heterostructures can be modulated effectively by the polarization-induced electric fields.

Surface Emitting Distributed Feedback Quantum Cascade Laser around 8.3 mu m

We demonstrate surface emitting distributed feedback quantum cascade lasers emitting at wavelengths from 8.1 mu m at 90 K to 8.4 mu m at 210 K. The second-order metalized grating is carefully designed using a modified coupled-mode theory and fabricated by contact lithography. The devices show single mode behavior with a side mode suppression ratio above 18 dB at all working temperatures. At 90 K, the device emits an optical power of 101 mW from the surface and 199 mW from the edge. In addition, a double-lobe far-field pattern with a separation of 2.2 degrees is obtained in the direction along the waveguide.

High Characteristic Temperature InGaAsP/InP Tunnel Injection Multiple-Quantum-Well Lasers

We fabricate 1.5 mu m InGaAsP/InP tunnel injection multiple-quantum-well (TI-MQW) Fabry-Perot (F-P) ridge lasers. The laser heterostructures, including an inner cladding layer and an InP tunnel barrier layer, are grown by metal-organic chemical-vapor deposition (MOCVD). Characteristic temperature.. 0 of 160K at 20 degrees C is obtained for 500-mu m-long lasers. T-0 is measured as high as 88K in the temperature range of 15-75 degrees C. Cavity length dependence of T-0 is investigated.

The two- to three-dimensional growth transition of InAs/GaAs epitaxy layer studied by reflectance difference spectroscopy

In this work, we have adopted reflectance difference spectroscopy to study the evolution of InAs layer grown at different temperatures in GaAs matrix. Associated with the two- to three-dimensional growth transition of InAs layer, the transition energies and the in-plane optical anisotropy of InAs wetting layer exhibit abrupt changes. This provides a new way to decide the critical thickness h(c) for the growth transition. The obtained h(c)s are compared with those determined by atomic force microscope measurement, and discrepancy is found at high temperatures. The origin of the difference is clarified and the variations in hc with temperature are further discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3494043]

Intersubband absorption energy shifts in 3-level system for asymmetric quantum well terahertz emitters

Intersubband absorption energy shifts in 3-level system stemming from depolarization and excitonlike effects are investigated. Analytically, the expressions we derive present good explanations to the conventional 2-level results and bare potential transition energy results; and numerical results show that they are more exact than the previous studies to describe the 3-level system depolarization and excitonlike shift (DES) character especially for higher carrier density (more than 8 x 10(11) cm(-2)). One interesting detail we find is that the "large blue" DES becomes "slight redshift" in the low doping limit (less than 1.9 x 10(11) cm(-2)), which may be neglected by the previous studies of intersubband transitions. Temperature character of DES in the step well structure is also numerically studied. Finally the above are applied to calculate asymmetric step quantum well structures. The two main functional aspects of terahertz (THz) emitters are discussed and several basic optimizing conditions are considered. By adjusting the well geometry parameters and material composition systematically, some optimized structures which satisfy all of the six conditions are recommended in tables. These optimizations may provide useful references to the design of 3-level-based optically pumping THz emitters.

Sequential coupling transport for the dark current of quantum dots-in-well infrared photodetectors

The dark current characteristics and temperature dependence for quantum dot infrared photodetectors have been investigated by comparing the dark current activation energies between two samples with identical structure of the dots-in-well in nanoscale but different microscale n-i-n environments. A sequential coupling transport mechanism for the dark current between the nanoscale and the microscale processes is proposed. The dark current is determined by the additive mode of two activation energies: E-a,E-micro from the built-in potential in the microscale and E-a,E-nano related to the thermally assisted tunneling in nanoscale. The activation energies E-a,E-micro and E-a,E-nano decrease exponentially and linearly with increasing applied electric field, respectively.

Electrical bistability and negative differential resistance in diodes based on silver nanoparticle-poly(N-vinylcarbazole) composites

An electrically bistable device has been fabricated using nanocomposite films consisting of silver nanoparticles and a semiconducting polymer by a simple spin-coating method. The current-voltage characteristics of the as-fabricated devices exhibit an obvious electrical bistability and negative differential resistance effect. The current ratio between the high-conducting state and low-conducting state can reach more than 103 at room temperature. The electrical bistability of the device is attributed to the electric-filed-induced charge transfer between the silver nanoparticles and the polymer, and the negative differential resistance behavior is related to the charge trapping in the silver nanoparticles. The results open up a simple approach to fabricate high quality electrically bistable devices by doping metal nanoparticles into polymer.

Interplay effects of temperature and injection power on photoluminescence of InAs/GaAs quantum dot with high and low areal density

In this report, we have investigated the temperature and injection power dependent photoluminescence in self-assembled InAs/GaAs quantum dots (QDs) systems with low and high areal density, respectively. It was found that, for the high-density samples, state filling effect and abnormal temperature dependence were interacting. In particular, the injection power-induced variations were most obvious at the temperature interval where carriers transfer from small quantum dots (SQDs) to large quantum dots (LQDs). Such interplay effects could be explained by carrier population of SQDs relative to LQDs, which could be fitted well using a thermal carrier rate equation model. On the other hand, for the low density sample, an abnormal broadening of full width at half maximum (FWHM) was observed at the 15-100 K interval. In addition, the FWHM also broadened with increasing injection power at the whole measured temperature interval. Such peculiarities of low density QDs could be attributed to the exciton dephasing processes, which is similar to the characteristic of a single quantum dot. The compared interplay effects of high-and low-density QDs reflect the difference between an interacting and isolated QDs system.

40 Gbits/s all-optical clock recovery for degraded signals using an amplified feedback laser

All-optical clock recovery for the return-to-zero modulation format is demonstrated experimentally at 40 Gbits/s by using an amplified feedback laser. A 40 GHz optical clock with a root-mean-square (rms) timing jitter of 130 fs and a carrier-to-noise ratio of 42 dB is obtained. Also, a 40 GHz optical clock with timing jitter of 137 fs is directly recovered from pseudo-non-return-to-zero signals degraded by polarization-mode dispersion (PMD). No preprocessing stage to enhance the clock tone is used. The rms timing jitter of the recovered clock is investigated for different values of input power and for varying amounts of waveform distortion due to PMD.

Broadly Tunable Grating-Coupled External Cavity Laser With Quantum-Dot Active Region

A broadly tunable and high-power grating-coupled external cavity laser with a tuning range of more than 200 nm and a similar to 200-mW maximum output power was realized, by utilizing a gain device with the chirped multiple quantum-dot (QD) active layers and bent waveguide structure. The chirped QD active medium, which consists of QD layers with InGaAs strain-reducing layers different in thickness, is beneficial to the broadening of the material gain spectrum. The bent waveguide structure and facet antireflection coating are both effective for the suppression of inner-cavity lasing under large injection current.

Blue-shift photoluminescence from porous InAlAs

A porous InAlAs structure was first obtained by electrochemical etching. Nano-pore arrays were formed when the In0.52Al0.48As membrane was anodized at constant voltages in an HF aqueous solution. These self-assembled structures showed evident blue-shift photoluminescence emissions. While a quantum size effect alone underestimates the blue-shift energy for a sample with a relatively large average pore wall thickness, a novel effect caused by the asymmetric etching is proposed to account for this phenomenon. The results inferred from the x-ray double crystal diffraction are in good agreement with the experimental data.

Strain effects on optical polarisation properties in (11(2)over-bar2) plane GaN films

We present the theoretical results of the electronic band structure of wurtzite GaN films under biaxial strains in the (11 (2) over bar2)-plane The calculations are performed by the kappa p perturbation theory approach through using the effective-mass Hamiltonian for an arbitrary direction The results show that the transition energies decrease with the biaxial strains changing from -0 5% to 0 5% For films of (11 (2) over bar2)-plane, the strains are expected to be anisotropic in the growth plane Such anisotropic strains give rise to valence band mixing which results in dramatic change in optical polarisation property The strain can also result in optical polarisation switching phenomena Finally, we discuss the applications of these properties to the (11 (2) over bar2) plane GaN based light emitting diode and lase diode

Effects of silicon incorporation on composition, structure and electric conductivity of cubic boron nitride thin films

We have achieved in-situ Si incorporation into cubic boron nitride (c-BN) thin films during ion beam assisted deposition. The effects of silicon incorporation on the composition, structure and electric conductivity of c-BN thin films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and electrical measurements. The results suggest that the content of the cubic phase remains stable on the whole with the incorporation of Si up to a concentration of 3.3 at.%, and the higher Si concentrations lead to a gradual change from c-BN to hexagonal boron nitride. It is found that the introduced Si atoms only replace B atoms and combine with N atoms to form Si-N bonds, and no evidence of the existence of Si-B bonds is observed. The resistance of the Si-doped c-BN films gradually decreases with increasing Si concentration, and the resistivity of the c-BN film with 3.3 at.% Si is lowered by two orders of magnitude as compared to undoped samples.