Synthesis and optical properties of europium-doped ZnS: Long-lasting phosphorescence from aligned nanowires

Quasi-aligned Eu2+-doped wurtzite ZnS nanowires on Au-coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [0110] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu2+ ions form an isoelectronic acceptor level and yield a high density of bound excitions, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co-dopant Cl- ions can serve not only as donors, producing a donor-acceptor pair transition with the Eu2+ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu2+, and yield long-lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal-field strength around Eu2+. As a result, not only have an enhanced Eu2+ -4f(6)5d(1)-4f(7) intra-ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu2+ related emissions are shifted to shorter wavelengths.

Growth of aligned single-walled carbon nanotubes under ac electric fields through floating catalyst chemical vapour deposition

Through floating catalyst chemical vapour deposition(CVD) method, well-aligned isolated single-walled carbon nanotubes (SWCNTs) and their bundles were deposited on the metal electrodes patterned on the SiO2/Si surface under ac electric fields at relatively low temperature(280 degrees C). It was indicated that SWCNTs were effectively aligned under ac electric fields after they had just grown in the furnace. The time for a SWCNT to be aligned in the electric field and the effect of gas flow were estimated. Polarized Raman scattering was performed to characterize the aligned structure of SWCNTs. This method would be very useful for the controlled fabrication and preparation of SWCNTs in practical applications.

Quasi-aligned ZnO nanotubes grown on Si substrates

Quasi-aligned ZnO nanotubes have been grown on silicon substrates by metalorganic chemical vapor deposition without using any catalyst. Two kinds of ZnO nanotubular structures were found: Nanotubes with single walls and nanotubes with double walls. The nanotubes were grown along the [001] direction. Room-temperature photoluminescence measurements of the ZnO nanotubes indicate strong ultraviolet emission and weak green emission. A new growth mode for these ZnO nanotubes is proposed, which can be used to prepare other nanotubular structures. (c) 2005 American Institute of Physics.

On the formation of well-aligned ZnO nanowall networks by catalyst-free thermal evaporation method

Two-dimensional ZnO nanowall networks were grown on ZnO-coated silicon by thermal evaporation at low temperature without catalysts or additives. All of the results from scanning electronic spectroscope, X-ray diffraction and Raman scattering confirmed that the ZnO nanowalls were vertically aligned and c-axis oriented. The room-temperature photoluminescence spectra showed a dominated UV peak at 378 nm, and a much suppressed orange emission centered at similar to 590 nm. This demonstrates fairly good crystal quality and optical properties of the product. A possible three-step, zinc vapor-controlled process was proposed to explain the growth of well-aligned ZnO nanowall networks. The pre-coated ZnO template layer plays a key role during the synthesis process, which guides the growth direction of the synthesized products. (C) 2007 Elsevier B.V. All rights reserved.

Combined structure of ZnO vertical well-aligned nanorods and net-like structures on AIN/sapphire

ZnO vertical well-aligned nanorods were grown on A1N/sapphire by using metal-organic chemical vapor deposition. We first observed the ZnO net-like structures under the nanorods. The different strain was determined in these two layers by using double crystal X-ray diffraction, Raman spectra, which revealed that the nanorods were relaxed and the net-like structures were strained. The optical properties of two layers were measured by using the cathodoluminescence and photo luminescence and the shift of UV peaks was observed. Moreover, the growth mechanism of the ZnO nanorods and the net-like structures is discussed. (c) 2007 Elsevier B.V. All rights reserved.

Structural, photoluminescence, and field emission properties of vertically well-aligned ZnO nanorod arrays

The self-assembled growth of vertically well-aligned ZnO nanorod arrays with uniform length and diameter on Si substrate has been demonstrated via thermal evaporation and vapor-phase transport. The structural, photoluminescence (PL), and field emission properties of the as-prepared nanorod arrays were investigated. The PL spectrum at 10 K shows a strong and sharp near-band gap emission (NBE) peak ( full width at half-maximum (FWHM) = 4.7 meV) and a weak neglectable deep-level emission (DL) peak (I-NBE/I-DL= 220), which implies its good crystallinity and high optical quality. The room-temperature NBE peak was deduced to the composition of free exciton and its first-order replicas emissions by temperature-dependent PL spectra. The field emission measurements indicate that, with a vacuum gap of 400 Am, the turn-on field and threshold field is as low as 2.3 and 4.2 V/mu m. The field enhancement factor beta and vacuum gap d follows a universal equation.

A base-emitter self-aligned multi-finger Si1-xGex/Si power heterojunction bipolar transistor

With a crystal orientation dependent on the etch rate of Si in KOH-based solution, a base-emitter self-aligned large-area multi-linger configuration power SiGe heterojunction bipolar transistor (HBT) device (with an emitter area of about 880 mu m(2)) is fabricated with 2 mu m double-mesa technology. The maximum dc current gain is 226.1. The collector-emitter junction breakdown voltage BVCEO is 10 V and the collector-base junction breakdown voltage BVCBO is 16 V with collector doping concentration of 1 x 10(17) cm(-3) and thickness of 400 nm. The device exhibited a maximum oscillation frequency f(max) of 35.5 GHz and a cut-off frequency f(T) of 24.9 GHz at a dc bias point of I-C = 70 mA and the voltage between collector and emitter is V-CE = 3 V. Load pull measurements in class-A operation of the SiGe HBT are performed at 1.9 GHz with input power ranging from 0 dBm to 21 dBm. A maximum output power of 29.9 dBm (about 977 mW) is obtained at an input power of 18.5 dBm with a gain of 11.47 dB. Compared to a non-self-aligned SiGe HBT with the same heterostructure and process, f(max) and f(T) are improved by about 83.9% and 38.3%, respectively.

Novel folding 8x8 silicon-based optical matrix switch with tapered waveguides and self-aligned corner mirrors

Novel folding 8 x 8 matrix switches based on silicon on insulator were demonstrated. In the design, single-mode rib waveguides and multimode interferences are connected by optimized tapered waveguides to reduce the mode coupling loss between the two types of waveguides. The self-aligned method was applied to the key integrated turning mirrors for perfect positions and low loss of them. A mixed etching process including inductively coupled plasma and chemical etching was employed to etch waveguides and mirrors, respectively. The compact size of the device is only 20 x 3.2 mm(2). The switch element with high switching speed and low power consumption is presented in the matrix. The average insertion loss of the matrix is about -21 dB, and the excess loss of one mirror is measured of -1.4 dB. The worst crosstalk is larger than 21 dB. Experimental results illuminate that some of the main characteristics of optical matrix switches are. developed in the modified design, which is in accord with theoretic analyses.

Synthesis and photoluminescence properties of vertically aligned ZnO nanorod-nanowall junction arrays on a ZnO-coated silicon substrate

Arrays of vertically well-aligned ZnO nanorod-nanowall junctions have been synthesized on an undoped ZnO-coated silicon substrate by a carbothermal reduction and vapour phase transport method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the nanostructures are well-oriented with the c-axis perpendicular to the substrate. The room temperature photoluminescence (PL) spectrum of the as-prepared ZnO nanostructure reveals a dominant near-band-edge (NBE) emission peak and a weak deep level (DL) emission, which demonstrates its good optical properties. Temperature-dependent PL spectra show that both the intensity of NBE and DL emissions increased with decreasing temperature. The NBE emission at 3.27 eV is identified to originate from the radiative free exciton recombination. The possible growth mechanism of ZnO nanorod-nanowall junctions is also proposed.

Effect of substrate temperature on the growth and photoluminescence properties of vertically aligned ZnO nanostructures

Vertically well-aligned ZnO nanoridge, nanorod, nanorod-nanowall junction, and nanotip arrays have been successfully synthesized on Si (100) substrates using a pulsed laser deposition prepared ZnO film as seed layer by thermal evaporation method. Experimental results illustrated that the growth of different morphologies of ZnO nanostructures was strongly dependent upon substrate temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies showed that the ZnO nanostructures were single crystals with a wurtzite structure. Compared with those of the other nanostructures, the photoluminescence (PL) spectrum of nanorod-nanowall junctions showed the largest intensity ratio of ultraviolet (UV) to yellow-green emission and the smallest full-width at half-maximum (FWHM) of the UV peak, reflecting the high optical quality and nearly defect free of crystal structure. The vertical alignment of the nanowire array on the substrate is attributed to the epitaxial growth of the nanostructures from the ZnO buffer layer. The growth mechanism was also discussed in detail. (c) 2006 Elsevier B.V. All rights reserved.

High performance of 1.55 mu m DFB integrated with vertically tapered self-aligned spot-size converter

A new type of self-aligned spotsize converter (SSC) integrated 1.55 mum DFB lasers had been proposed in this article. The upper optical confinement layer and the butt-coupled tapered thickness waveguide were regrown simultaneously, which not only offered the separate optimization of the active region and the integrated SSC, but also reduced the difficulty of the butt-joint selective regrowth. The vertical and horizontal far field angles were 9degrees and 12degrees respectively, the 1- dB misalignment tolerance were both 3.6 and 3.4 mum. The directed coupling efficiency to tapered single mode fiber was 48%.

Selective-area MOCVD growth for distributed feedback lasers integrated with vertically tapered self-aligned waveguide

The growth pressure and mask width dependent thickness enhancement factors of selective-area MOCVD. grow th were investigated in this article. A, high enhancement of 5.8 was obtained at 130 mbar with the mask width of 70 mum. Mismatched InGaAsP (-0.5%) at the maskless region which could ensure the material at butt-joint region to be matched to InP was successively grown by controlling the composition and mismatch modulation in the selective-area growth. The upper optical confinement layer and the butt-coupled tapered thickness waveguide were regrown simultaneously in separated confined heterostructure 1.55 gm distributed feedback laser, which not only offered the separated optimization of the active region and the integrated spotsize converter, but also reduced the difficulty of the butt-joint selective regrowth. A narrow beam of 9degrees and 12degrees in the vertical and horizontal directions, a low threshold current of 6.5 mA was fabricated by using this technique. (C) 2003 Elsevier Science B.V. All rights reserved.

Room-temperature, ground-state lasing for red-emitting vertically aligned InAlAs/AlGaAs quantum dots grown on a GaAs(100) substrate

The effect of the growth temperature on the properties of InAlAs/AlGaAs quantum dots grown on GaAs(100) substrates is investigated. The optical efficiency and structural uniformity are improved by increasing the growth temperature from 530 to 560 degreesC. The improvements of InAlAs/AlGaAs quantum-dot characteristics could be explained by suppressing the incorporation of oxygen and the formation of group-III vacancies. Furthermore, edge-emitting laser diodes with six quantum-dot layers grown at 560 degreesC have been fabricated. Lasing occurs via the ground state at 725 nm, with a room-temperature threshold current density of 3.9 kA/cm(2), significantly better than previously reported values for this quantum-dot systems. (C) 2002 American Institute of Physics.

Structural, optical and intraband absorption properties of vertically aligned In0.32Ga0.68As/GaAs quantum dots superlattices

The self-organization growth of In0.32Ga0.68As/GaAs quantum dots (QDs) superlattices is investigated by molecular beam epitaxy. It is found that high growth temperature and low growth rate are favorable for the formation of perfect vertically aligned QDs superlattices. The aspect ratio (height versus diameter) of QD increases from 0.16 to 0.23 with increase number of bi-layer. We propose that this shape change play a significant role to improve the uniformity of QDs superlattices. Features in the variable temperature photoluminescence characteristics indicate the high uniformity of the QDs. Strong infrared absorption in the 8-12 mum was observed. Our results suggest the promising applications of QDs in normal sensitive infrared photodetectors. (C) 2001 Elsevier Science B.V. All rights reserved.

Blue-violet photoluminescence from large-scale highly aligned boron carbonitride nanofibers

We report on the strong blue-violet photoluminescence (PL) at room temperature from the large-scale highly aligned boron carbonitride (BCN) nanofibers synthesized by bias-assisted hot filament chemical vapor deposition. The photoluminescence peak wavelength shifts in the range of 470-390 nm by changing the chemical composition of the BCN nanofibers, which shows an interesting blue and violet-light-emitting material with adjustable optical properties. The mechanism for the shift of the PL peaks at room temperature is also discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)04427-2].