Effects of accumulated strain on the surface and optical properties of stacked 1.3 mu m InAs/GaAs quantum dot structures

We report the effects of accumulated strain by stacking on the surface and optical properties of stacked 1.3 mu m InAs/GaAs quantum dot (QD) structures grown by MOCVD. It is found that the surface of the stacked QD structures becomes more and more undulated with stacking, due to the increased strain in the stacked QD structures with stacking. The photoluminescence intensity from the QD structures first increases as the stacking number increases from 1 to 3 and then dramatically decreases as it further increases, implying a significant increase in the density of crystal defects in the stacked QD structures due to the accumulated strain. Furthermore, we demonstrate that the strain can be reduced by simply introducing annealing steps just after growing the GaAs spacers during the deposition of the stacked QD structures, leading to significant improvement in the surface and optical properties of the structures. (C) 2007 Elsevier B.V. All rights reserved.

A photonic wire-based directional coupler based on SOI

A photonic wire-based directional coupler based on SOI was fabricated by e-beam lithography (EBL) and the inductively coupled plasma (ICP) etching method. The size of the sub-micron waveguide is 0.34 mu m x 0.34 mu m, and the length in the coupling region and the separation between the two parallel waveguides are 410 and 0.8 mu m, respectively. The measurement results are in good agreement with the results simulated by 3D finite-difference time-domain method. The transmission power from two output ports changed reciprocally with about 23 nm wavelength spacing between the coupled and direct ports. The extinction ratio of the device was between 5 and 10 dB, and the insertion loss of the device in the wavelength range 1520-1610 nm was between 22 and 24 dB, which included an about 18.4 +/- 0.4 dB coupling loss between the taper fibers and the polished sides of the device. (c) 2008 Elsevier B.V. All rights reserved.

Effect of indium-doped interlayer on the strain relief in GaN films grown on Si(111)

Crack-free GaN films have been achieved by inserting an Indoped low-temperature (LT) AlGaN interlayer grown on silicon by metalorganic chemical vapor deposition. The relationship between lattice constants c and a obtained by X-ray diffraction analysis shows that indium doping interlayer can reduce the stress in GaN layers. The stress in GaN decreases with increasing trimethylindium (TMIn) during interlayer growth. Moreover, for a smaller TMIn flow, the stress in GaN decreases dramatically when In acts as a surfactant to improve the crystallinity of the AlGaN interlayer, and for a larger TMIn flow, the stress will increase again. The decreased stress leads to smoother surfaces and fewer cracks for GaN layers by using an In-doped interlayer than by using an undoped interlayer. In doping has been found to enhance the lateral growth and reduce the growth rate of the c face. It can explain the strain relief and cracks reduction in GaN films. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Recent progresses of Si-based photonics in chinese main land

Si-based photonic materials and devices, including SiGe/Si quantum structures, SOI and InGaAs bonded on Si, PL of Si nanocrystals, SOI photonic crystal filter, Si based RCE (Resonant Cavity Enhanced) photodiodes, SOI TO (thermai-optical) switch matrix were investigated in Institute of Serniconductors, Chinese Academy of Sciences. The main results in recent years are presented in the paper. The mechanism of PL from Si NCs embedded in SiO2 matrix was studied, a greater contribution of the interface state recombination (PL peak in 850 similar to 900 nm) is associated with larger Si NCs and higher interface state density. Ge dots with density of order of 10(11) cm(-2) were obtained by UHV/CVD growth and 193 nm excimer laser annealing. SOI photonic crystal filter with resonant wavelength of 1598 nm and Q factor of 1140 was designed and made. Si based hybrid InGaAs RCE PD with eta of 34.4% and FWHM of 27 nut were achieved by MOCVD growth and bonding technology between InGaAs epitaxial and Si wafers. A 16x16 SOI optical switch matrix were designed and made. A new current driving circuit was used to improve the response speed of a 4x4 SOI rearrangeable nonblocking TO switch matrix, rising and failing time is 970 and 750 ns, respectively.

Effects of disk rotation rate on the growth of ZnO films by low-pressure metal-organic chemical vapor deposition

ZnO films were grown at low pressure in a vertical metal-organic vapor deposition (MOCVD) reactor with a rotating disk. The structural and morphological properties of the ZnO films grown at different disk rotation rate (DRR) were investigated. The growth rate increases with the increase of DRR. The ZnO film grown at the DRR of 450 revolutions per minute (rpm) has the lowest X-ray rocking curve full width at half maximum and shows the best crystalline quality and morphology. In addition, the crystalline quality and morphology are improved as the DRR increased but both are degraded when the DRR is higher than 450 rpm. These results can help improve in understanding the rotation effects on the ZnO films grown by MOCVD. (C) 2007 Elsevier B.V. All rights reserved.

Optical response of high-level bandgap in one-dimensional photonic crystal applying in-plane integration

A new broadband filter, based on the high level bandgap in 1-D photonic crystals (PCs) of the form Si vertical bar air vertical bar Si vertical bar air vertical bar Si vertical bar air vertical bar Si vertical bar air vertical bar Si vertical bar air vertical bar Si is designed by the plane wave expansion method (PWEM) and the transfer matrix method (TMM) and fabricated by lithography. The optical response of this filter to normal-incident and oblique-incident light proves that utilizing the high-level bandgaps of PCs is an efficient method to lower the difficulties of fabricating PCs, increase the etching depth of semiconductor materials, and reduce the coupling loss at the interface between optical fibers and the PC device. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

Hydrogen sensors based on AlGaN/AIN/GaN Schottky diodes

Pt/AlGaN/AIN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300K is obtained at a fixed forward current after switching from N-2 to 10%H-2+N-2. The sensor responses under different concentrations from 50ppm H-2 to 10%H-2+N-2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N-2 and air atmosphere at 300 and 373K are compared. Oxygen in air accelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity of the sensor are calculated.

Electroluminescence from Ge on Si substrate at room temperature

A Ge/Si heterojunction light emitting diode with a p(+)-Ge/i-Ge/N+-Si structure was fabricated using the ultrahigh vacuum chemical vapor deposition technology on N+-Si substrate. The device had a good I-V rectifying behavior. Under forward bias voltage ranging from 1.1 to 2.5 V, electroluminescence around 1565 nm was observed at room temperature. The mechanism of the light emission is discussed by the radiative lifetime and the scattering rate. The results indicate that germanium is a potential candidate for silicon-based light source material. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3216577]

The field emission properties of nonpolar a-plane n-type GaN films grown on nano-patterned sapphire substrates

Si-doped nonpolar a-plane GaN films were grown on nanopatterned sapphire substrates by a low-pressure metal organic chemical vapor deposition (MOCVD) system. The structure, morphology and field emission properties of the sample were studied by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and field emission measurement. The XRD analysis shows that the sample is a nonpolar a-plane (11 (2) over bar0) GaN film. The field emission measurement shows that the nonpolar GaN films exhibit excellent field emission properties with a threshold emission field of as low as 10 V/mu m at a current density of 0.63 mu A/cm(2), and a high field emission current density of 74 mA/cm(2) at an applied field of 24 V/mu m. Moreover, the Fowler-Nordheirn plot of the sample fits a near linear relation. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mn-AlInN: a new diluted magnetic semiconductor

Mn ions have been incorporated into MOCVD grown Al1-x In (x) N/GaN thin films by ion implantation to achieve the room temperature ferromagnetism in the samples. Magnetic characterizations revealed the presence of two ferromagnetic transitions one has Curie points at similar to 260 K and the other above room temperature. In-diffusion of indium caused by the Mn implantation leads to the partition of AlInN epilayer into two diluted magnetic semiconductor sub-layers depending on the Mn concentration. The Curie temperature of 260 K is assigned to the layer having lower concentration, whereas T (c) above room temperature is assumed to be associated to the layer having higher Mn concentration.

The structure, morphology and Raman scattering study on Mn-implanted nonpolar a-plane GaN films

Dilute magnetic nonpolar GaN films with a Curie temperature above room temperature have been fabricated by implanting Mn ions into unintentionally doped nonpolar a-plane (1 1 (2) over bar 0) GaN films and a subsequent rapid thermal annealing (RTA) process. The impact of the implantation and RTA on the structure and morphology of the nonpolar GaN films is studied in this paper. The scanning electron microscopy analysis shows that the RTA process can effectively recover the implantation-indUced damage to the surface morphology of the sample. The X-ray diffraction and micro-Raman scattering spectroscopy analyses show that the RTA process can just partially recover the implantation-induced crystal deterioration. Therefore, the quality of the Mn-implanted nonpolar GaN films should be improved further for the application in spintronic devices. (C) 2009 Elsevier B.V. All rights reserved.

Curvature Correction of FWHM in the X-Ray Rocking Curve of Bent Heteroepitaxial Films

A method for accurate determination of the curvature radius of semiconductor thin films is proposed. The curvature-induced broadening of the x-ray rocking curve (XRC) of a heteroepitaxially grown layer can be determined if the dependence of the full width at half maximum (FWHM) of XRC is measured as a function of the width of incident x-ray beam. It is found that the curvature radii of two GaN films grown on a sapphire wafer are different when they are grown under similar MOCVD conditions but have different values of layer thickness. At the same time, the dislocation-induced broadening of XRC and thus the dislocation density of the epitaxial film can be well calculated after the curvature correction.

Suppression of indium droplet formation by adding CCl4 during metalorganic chemical vapor deposition growth of InN films

In this work, the influences of CCl4 on the metalorganic chemical vapor deposition (MOCVD) growth of InN were studied for the first time. It was found that the addition of CCl4 can effectively suppress the formation of metal indium (In) droplets during InN growth, which was ascribed to the etching effect of Cl to In. However, with increasing of CCl4 flow, the InN growth rate decreased but the lateral growth of InN islands was enhanced. This provides a possibility of promoting islands coalescence toward a smooth surface of the InN film by MOCVD. The influence of addition of CCl4 on the electrical properties was also investigated.

Annealing behaviors of long-wavelength InAs/GaAs quantum dots with different growth procedures by metalorganic chemical vapor deposition

The effects of annealing on the optical properties of InAs/GaAs quantum dots (QDs) grown under different conditions by metalorganic chemical vapor deposition (MOCVD) are studied. A lower QD growth rate leads to an earlier and faster decrease of QD photoluminescence (PL) intensity with increasing annealing temperature. which is proposed to be related to the increased QD two-dimensional (2D)-three-dimensional (3D) transition critical layer thickness at low QD growth rate. High-quality GaAs cap layers grown at high temperature and a low deposition rate are shown to decrease the blueshift of the QDs' emission wavelength significantly during in-situ I h annealing experiments, which is important for the fabrication of long-wavelength InAs/GaAs QD lasers by MOCVD technique. (C) 2009 Elsevier B.V. All rights reserved.

Quantum efficiency and temperature coefficients of GaInP/GaAs dual-junction solar cell

GaInP/GaAs dual-junction solar cell with a conversion efficiency of 25.2% has been fabricated using metalorganic chemical vapor deposition (MOCVD) technique. Quantum efficiencies of the solar cell were measured within a temperature range from 25 to 160A degrees C. The results indicate that the quantum efficiencies of the subcells increase slightly with the increasing temperature. And red-shift phenomena of absorption limit for all subcells are observed by increasing the cell's work temperature, which are consistent with the viewpoint of energy gap narrowing effect. The short-circuit current density temperature coefficients dJ (sc)/dT of GaInP subcell and GaAs subcell are determined to be 8.9 and 7.4 mu A/cm(2)/A degrees C from the quantum efficiency data, respectively. And the open-circuit cell voltage temperature coefficients dV (oc)/dT calculated based on a theoretical equation are -2.4 mV/A degrees C and -2.1 mV/A degrees C for GaInP subcell and GaAs subcell.