Silicon nanopore array structure using porous anodic alumina

A free-standing, bidirectionally permeable and ultra-thin (500-1000 nm) porous anodic alumina membrane was fabricated using a two-step aluminium anodization process, which was then placed on top of a silicon film as an etching mask. The pattern was transferred to silicon using dry-etching technology, and the silicon nanopore array structure was formed. The factors which afflct the pattern transfer process are discussed. Observation of the nanopatterned sample under a scanning electron microscope shows that the structure obtained by this method is made up of uniform and highly ordered holes, which attains to 125 nm depth. The photoluminescence spectrum from the nanopatterned sample,the surface of which has been thermal-oxidized, shows that the the luminesce is evidently enhanced, the mechanism of which is based on the normally weak TO phonon assisted bandgap light-emission process, and the physical reasons that underlic the enhancement have been analyzed. The PL results do show an attractive optical characteristic, which provides a promising pathway to achieve efficient light emission from silicon.

Electronic structure and exciton states in the freestanding ZnO nanorods

The electronic structure and exciton states of cylindrical ZnO nanorods with radius from 2 to 6 nm are investigated based on the framework of the effective-mass theory. Using the adiabatic approximation, the exciton binding energies taking account of the dielectric mismatch are solved exactly when the total angular momentum of the exciton states L = 0 and L = +/- 1. We find that the exciton binding energies can be enhanced greatly by the dielectric mismatch and the calculated results are almost consistent with the experimental data. Meanwhile, we obtain the optical transition rule when the small spin-obit splitting Delta(so) of ZnO is neglected. Furthermore, the radiative lifetime and linear optical susceptibilities chi(w) of the exciton states are calculated theoretically. The theoretical results are consistent with the experimental data very well. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3125456]

Design and Fabrication of AlGaN-Based Resonant-Cavity-Enhanced p-i-n UV PDs

AlGaN-based resonant-cavity-enhanced (RCE) p-i-n photodetectors (PDs) for operating at the wavelength of 330 nm were designed and fabricated. A 20.5-pair AlN/Al0.3Ga0.7N distributed Bragg reflector (DBR) was used as the back mirror and a 3-pair AlN/Al0.3Ga0.7N DBR as the front one. In the cavity is a p-GaN/i-GaN/n-Al0.3Ga0.7N structure. The optical absorption of the RCE PD structure is at most 59.8% deduced from reflectance measurement. Selectively enhanced by the cavity effect, a response peak of 0.128 A/W at 330 nm with a half-peak breadth of 5.5 nm was obtained under zero bias. The peak wavelength shifted 15 nm with the incident angle of light increasing from 0 degrees to 60 degrees.

High responsivity resonant-cavity-enhanced InGaAs/GaAs quantum-dot photodetector for wavelength of similar to 1 mu m at room temperature

The characteristics of a resonant cavity-enhanced InGaAs/GaAs quantum-dot n-i-n photodiode with only a bottom distributed Bragg reflector used as the cavity mirror, are reported. To suppress the dark current, an AlAs layer is inserted into the device structure as the blocking layer. It turns out that the structure still possesses the resonant coupling nature, and makes Rabi splitting discernible in the photoluminescence spectra. The measured responsivity spectrum of the photocurrent shows a peak at lambda = 1030 nm, and increases rapidly as the bias voltage increases. A peak responsivity of 0.75 A/W, or equivalently an external quantum efficiency of 90.3%, is obtained at V-bias = -1.4 V.

Design of Narrow-Gap TiO2: A Passivated Codoping Approach for Enhanced Photoelectrochemical Activity

To improve the photoelectrochemical activity of TiO2 for hydrogen production through water splitting, the band edges of TiO2 should be tailored to match with visible light absorption and the hydrogen or oxygen production levels. By analyzing the band structure of TiO2 and the chemical potentials of the dopants, we propose that the band edges of TiO2 can be modified by passivated codopants such as (Mo+C) to shift the valence band edge up significantly, while leaving the conduction band edge almost unchanged, thus satisfying the stringent requirements. The design principle for the band-edge modification should be applicable to other wide-band-gap semiconductors.

Enhanced charge storage characteristics of silicon nanocrystals fabricated by electron-beam coevaporation of Si and SiOx(x=1 or 2)

In this article, a simple and flexible electron-beam coevaporation (EBCE) technique has been reported of fabrication of the silicon nanocrystals (Si NCs) and their application to the nonvolatile memory. For EBCE, the Si and SiOx(x=1 or 2) were used as source materials. Transmission electron microscopy images and Raman spectra measurement verified the formation of the Si NCs. The average size and area density of the Si NCs can be adjusted by increasing the Si:O weight ratio in source material, which has a great impact on the crystalline volume fraction of the deposited film and on the charge storage characteristics of the Si NCs. A memory window as large as 6.6 V under +/- 8 V sweep voltage was observed for the metal-oxide-semiconductor capacitor structure with the embedded Si NCs.

Light extraction of GaN LEDs with 2-D photonic crystal structure

Ultraviolet photo-lithography is employed to introduce two-dimensional (2D) photonic crystal (PC) structure on the top surface of GaN-based light emitting diode (LED). PC patterns are transferred to 460-nm-thick transparent indium tin oxide (ITO) electrode by inductively coupled plasma (ICP) etching. Light intensity of PC-LED can be enhanced by 38% comparing with the one without PC structure. Rigorous coupled wave analysis method is performed to calculate the light transmission spectrum of PC slab. Simulation results indicate that total internal reflect angle which modulated by PC structure has been increased by 7 degrees, which means that the light extraction efficiency is enhanced outstandingly.

1.55 mu m GaInNAs resonant-cavity-enhanced photodetector grown on GaAs

We report the design, growth, fabrication, and characterization of a GaAs-based resonant-cavity-enhanced (RCE) GaInNAs photodetector operating at 1.55 mu m. The structure of the device was designed using a transfer-matrix method (TMM). By optimizing the molecular-beam epitaxy growth conditions, six GaInNAs quantum wells were used as the absorption layers. Twenty-five (25)- and 9-pair GaAs/AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. At 1.55 mu m, a quantum efficiency of 33% with a full width at half maximum of 10 nm was obtained. The dark current density was 3x10(-7) A/cm(2) at a bias of 0 V and 4.3x10(-5) A/cm(2) at a reverse bias of 5 V. The primary time response measurement shows that the device has a rise time of less than 800 ps. (c) 2005 American Institute of Physics.

Structure, stability and photoelectronic properties of transition films from amorphous to microcrystalline silicon

A series of hydrogenated silicon films near the threshold of crystallinity was prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) from a mixture of SiH4 diluted in H, The effect of hydrogen dilution ratios R-H = [H-2]/[SiH4] on microstructure of the films was investigated. Photoelectronic properties and stability of the films were studied as a function of crystalline fraction. The results show that more the crystalline volume fraction in the silicon films, the higher mobility life-time product (mu tau), better the stability and lower the photosensitivity. Those diphasic films contained 8%-31% crystalline volume fraction can gain both the fine photoelectronic properties and high stability. in the diphasic (contained 12% crystalline volume fraction) solar cell, we obtained a much lower light-induced degradation of similar to 2.9%, with a high initial efficiency of 10.01% and a stabilized efficiency of 9.72% (AM1.5, 100 mW/cm(2)). (c) 2005 Elsevier B.V. All rights reserved.

High-performance 1.55 mu m low-temperature-grown GaAs resonant-cavity-enhanced photodetector

A 1.55 mu m low-temperature-grown GaAs (LT-GaAs) photodetector with a resonant-cavityenhanced structure was designed and fabricated. A LT-GaAs layer grown at 200 degrees C was used as the absorption layer. Twenty- and fifteen-pair GaAs/AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. A responsivity of 7.1 mA/W with a full width at half maximum of 4 nm was obtained at 1.61 mu m. The dark current densities are 1.28x10(-7) A/cm(2) at the bias of 0 V and 3.5x10(-5) A/cm(2) at the reverse bias of 4.0 V. The transient response measurement showed that the photocarrier lifetime in LT-GaAs is 220 fs. (c) 2006 American Institute of Physics.

Enhanced photoresponse from the ordered microstructure of naphthalocyanine-carbon nanotube composite film

Naphthalocyanine-sensitized multi-walled carbon nanotube (NaPc-MWNT) composites have been synthesized through the pi-stacking between naphthalocyanine (NaPc) and carbon nanotubes. The resultant nanocomposites were characterized with a scanning electron microscope (SEM), a transmission electron microscope (TEM), and by UV - vis absorption and photocurrent spectra. The long-range ordering was observed in the NaPc - MWNT composites by using a TEM. The enhancement in the absorption intensity and the broadening of the absorption wavelength observed in the composite films, which were due to the attachment of NaPc on the MWNT surface, is discussed based on the measured UV - vis absorption spectra. Furthermore, the photoconductivity of the poly( 3-hexylthiophene)(PAT6) - NaPc - MWNT composite film was found to increase remarkably in the visible region and broaden towards the red regions. These new phenomena were ascribed to the larger donor/acceptor (D/A) interface and the formation of a biconsecutive D/A network structure, as discussed in consideration of the photoinduced charge transfer between PAT6 and NaPc - MWNT.

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed. (c) 2005 Elsevier B.V. All rights reserved.

Structure and photoluminescence of InGaAs quantum dots formed on an InAlAs wetting layer

We have developed a new self-assembled quantum dot system where InGaAs dots are formed on an InAlAs wetting layer and embedded in the GaAs matrix. The structure is realized by special sample designation and demonstrated by low-temperature photoluminescence measurements. In contrast to the traditional InAs/GaAs quantum dots dominated by the ensemble effect, the temperature dependence of the photoluminescence of such a quantum dot structure behaves as decoupled quantum dots. This can be attributed to the enhanced potential confinement for the dots provided by a higher-energy barrier in the wetting layer.

High energy ion implantation enhanced intermixed quantum well structures and their absorption characteristics in passive optical waveguides

We have shown that high energy ion implantation enhanced intermixing (HE-IIEI) technology for quantum well (QW) structures is a powerful technique which can be used to blue shift the band gap energy of a QW structure and therefore decrease its band gap absorption. Room temperature (RT) photoluminescence (PL) and guided-wave transmission measurements have been employed to investigate the amount of blue shift of the band gap energy of an intermixed QW structure and the reduction of band gap absorption, Record large blue shifts in PL peaks of 132 nm for a 4-QW InGaAs/InGaAsP/InP structure have been demonstrated in the intermixed regions of the QW wafers, on whose non-intermixed regions, a shift as small as 5 nm is observed. This feature makes this technology very attractive for selective intermixing in selected areas of an MQW structure. The dramatical reduction in band gap absorption for the InP based MQW structure has been investigated experimentally. It is found that the intensity attenuation for the blue shifted structure is decreased by 242.8 dB/cm for the TE mode and 119 dB/cm for the TM mode with respect to the control samples. Electro-absorption characteristics have also been clearly observed in the intermixed structure. Current-Voltage characteristics were employed to investigate the degradation of the p-n junction in the intermixed region. We have achieved a successful fabrication and operation of Y-junction optical switches (JOS) based on MQW semiconductor optical amplifiers using HE-IIEI technology to fabricate the low loss passive waveguide. (C) 1997 Published by Elsevier Science B.V.

AlGaN/GaN/InGaN/GaN DH-HEMTs structure with an AlN interlayer grown by MOCVD

A novel AlGaN/GaN/GaN/GaN double heterojunction high electron mobility transistors (DH-HEMTS) structure with an AlN interlayer on sapphire substrate has been grown by MOCVD. The structure featured a 6-10 nm In0.1Ga0.9N layer inserted between the GaN channel and GaN buffer. And wer also inserted one ultrathin. AlN interlayer into the Al/GaN/GaN interface, which significantly enhanced the mobility of two-dimensional electron gas (2DEG) existed in the GaN channel. AFM result of this structure shows a good surface morphology and a low dislocation density, with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 mu m x 5 mu m. Temperature dependent Hall measurement was performed on this sample, and a mobility as high as 1950 cm(2)/Vs at room temperature (RT) was obtained. The sheet carrier density was 9.89 x10(12) cm(2), and average sheet resistance of 327 Omega/sq was achieved. The mobility obtained in this paper is about 50% higher than other results of similar structures which have been reported. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.