One-step growth of ZnO from film to vertically well-aligned nanorods and the morphology-dependent Raman scattering

We observed a transition from film to vertically well-aligned nanorods for ZnO grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. A growth mechanism was proposed to explain such a transition. Vertically well-aligned homogeneous nanorods with average diameters of similar to 30, 45, 60, and 70 nm were grown with the c-axis orientation. Raman scattering showed that the E-2 (high) mode shifted to high frequency with the decrease of nanorod diameters, which revealed the dependence of nanorod diameters on the stress state. This dependence suggests a stress-driven diameter-controlled mechanism for ZnO nanorod arrays grown on sapphire (0001) substrates. (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.

Improvement of the radiation hardness of SIMOX buried layers using nitrogen implantation

An investigation of hardening the buried oxides (BOX) in separation by implanted oxygen (SIMOX) silicon-on-insulator (SOI) wafers to total-dose irradiation has been made by implanting nitrogen into the BOX layers with a constant dose at different implantation energies. The total-dose radiation hardness of the BOX layers is characterized by the high frequency capacitance-voltage (C-V) technique. The experimental results show that the implantation of nitrogen into the BOX layers can increase the BOX hardness to total-dose irradiation. Particularly, the implantation energy of nitrogen ions plays an important role in improving the radiation hardness of the BOX layers. The optimized implantation energy being used for a nitrogen dose, the hardness of BOX can be considerably improved. In addition, the C-V results show that there are differences between the BOX capacitances due to the different nitrogen implantation energies.

Transition films from amporphous to microcrystalline silicon and solar cells

A series of hydrogenated silicon films near the threshold of crystallinity was prepared by very high frequency plasmaenhanced chemical vapor deposition (VHF-PECVD)from a mixture of SiH4 diluted in H-2. The effect of hydrogen dilution ratios R = [H-2]/[SiH4] on the microstructure of the films was investigated. The photoelectronic properties and stability of the films were studied as a function of crystalline fraction. The results show that the diphasic films gain both the fine photoelectric properties like a-Si: H and high stability like mu w-Si:H. By using the diphasic silicon films as the intrinsic layer, p-i-n junction solar cells were prepared. Current-voltage (J-V) characteristics and stability of the solar cells were measured under an AM1.5 solar simulator. We observed a light-induced increase of 5.2% in the open-circuit voltage (V-oc) and a light-induced degradation of similar to 2.9% inefficiency.

Electrical and optical coupling in an electroabsorption modulator integrated with a DFB laser

Electrical and optical coupling in an electroabsorption (EA) modulator integrated with a distributed feedback (DFB) laser have been investigated. The integrated device is treated as a three-port optoelectronic device with two electrical ports and one optical output port. The scattering parameters of this three-port device have been measured in the designed experiment. The measured results indicate that there exists the electrical coupling between the DFB laser and EA modulator of the integrated light source whenever the current applied to the laser section is below or above the threshold current, and the optical coupling will have stronger influence on the frequency responses than the electrical coupling when the bias current is above the threshold. A small-signal equivalent circuit model for the integrated device is established considering both the electrical and internal optical coupling. Experiments show that the equivalent circuit model is reasonable and the determined element values are correct. Based on the measurement and modeling, the influences of the electrical and optical coupling on the high-frequency responses are investigated and the effective measure to eliminate the additional modulation in the DFB laser are discussed.

Weak antilocalization and beating pattern in an InGaAs/InAlAs quantum well

We have observed the weak antilocalization (WAL) and beating SdH oscillation through magnetotransport measurements performed on a heavily delta-doped In0.52Al0.48As/In0.53Ga0.47As/In0.5Al0.48As single quantum well in an applied magnetic field up to 13 T and a temperature at 1.5 K. Both effects are caused by the strong Rashba spin-orbit (SO) coupling due to high structure inversion asymmetry (SIA). The Rashba SO coupling constant alpha and zerotield spin splitting Delta(0) are estimated and the obtained values are consistent from different analysis for this sample. (c) 2007 Elsevier Ltd. All rights reserved.

Light-induced changes in diphasic nanocrystalline silicon films and solar cells

A series of diphasic nanocrystalline silicon films and solar cells was prepared using different hydrogen dilution ratios of silane by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). It was observed that after light soaking the open circuit voltage (V-oc) of the diphasic solar cells increased, while that of amorphous silicon solar cells decreased. Raman scattering spectroscopy was performed on the series of diphasic silicon films before and after light soaking. It was found that after light soaking the nanostruclures in the diphasic nanocrystalline silicon films were changed. Both the grain sizes and grain volume fraction reduced, while the grain boundary components increased. These results provide experimental evidence for the conjecture that the light-induced increase in V-oc of the diphasic nanocrystalline solar cells might be induced by the changes in the nanostructure of the intrinsic layer. (c) 2006 Elsevier B.V. All rights reserved.

Microstructure characterization of transition films from amorphous to nanocrocrystalline silicon

Hydrogenated silicon (Si:H) films near the threshold of crystallinity were prepared by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a wide range of hydrogen dilution R-H = [H-2]/[SiH4] values of 2-100. The effects of H dilution R-H on the structural properties of the films were investigated using micro-Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. The obtained Raman spectra show that the H dilution leads to improvements in the short-range order and the medium-range order of the amorphous network and then to the morphological transition from amorphous to crystalline states. The onset of this transition locates between R-H = 30 and 40 in our case, and with further increasing R-H from 40 to 100, the nanocrystalline volume fraction increases from similar to23% to 43%, and correspondingly the crystallite size enlarges from similar to2.8 to 4.4 nm. The FTIR spectra exhibit that with R-H increasing, the relative intensities of both the SiH stretching mode component at 2100 cm(-1) and wagging mode component at 620 cm(-1) increase in the same manner. We assert that these variations in IR spectra should be associated with the formation of paracrystalline structures in the low H dilution films and nanocrystalline structures in the high H dilution films. (C) 2003 Elsevier Science B.V. All rights reserved.

Heteroepitaxial growth and annealing of gamma-Al2O3 thin films on silicon

The gamma-Al2O3 films were grown on Si (100) substrates using the sources of TMA (Al (CH3)(3)) and O-2 by very low-pressure chemical vapor deposition (VLP-CVD). It has been found that the gamma-Al2O3 film has a mirror-like surface and the RMS was about 2.5nm. And the orientation relationship was gamma-Al2O3(100)/Si(100). The thickness uniformity of gamma-Al2O3 films for 2-inch epi-wafer was less than 5%. The X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) results show that the crystalline quality of the film was improved after the film was annealed at 1000degreesC in O-2 atmosphere. The high-frequency C-V and leakage current of Al/gamma-Al2O3/Si capacitor were also measured to verify the annealing effect of the film. The results show that the dielectric constant increased from 4 to 7 and the breakdown voltage for 65-nm-thick gamma-Al2O3 film on silicon increases from 17V to 53V.

Studies of 6H-SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H-SiC were systematically studied. The main works are summarized as follows:The homoepitaxial growth on the commercially available single-crystal 6H-SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p(+)n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K.Using thermal evaporation, Ti/6H-SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V.n-Type 6H-SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of polysilicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to gamma rays. (C) 2002 Published by Elsevier Science B.V.

Structural properties and Raman measurement of AlN films grown on Si (111) by NH3-GSMBE

Epitaxial growth of AlN has been performed by molecular beam epitaxy (MBE) with ammonia. The structural properties of materials were studied by cross-sectional transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). XRD and TEM diffraction pattern confirm the AlN is single crystalline 2H-polytype with the epitaxial relationship of (0001)AlNparallel to(111)Si, [11 (2) over bar0](AlN)parallel to[110](Si), [10 (1) over bar0](AlN)parallel to[11 (2) over bar](Si). Micro-Raman scattering measurement shows that the E-2 (high) and A(1) (LO) phonon mode shift 9 cm(-1) toward the low frequency, which shows the existence of large tensile strain in the AlN films. Furthermore, the appearance of forbidden A, (TO) mode and its anomalous shift toward high frequency was found and explained. (C) 2002 Elsevier Science B.V. All rights reserved.

Degenerate four-wave-mixing signals from dc- and ac-driven semiconductor superlattice

Within the one-dimensional tight-binding model;rnd chi-3 approximation, we have calculated four-wave-mixing (FWM) signals for a semiconductor superlattice in the presence of both static and high-frequency electric fields. When the exciton effect is negligible, the time-periodic field dynamically delocalizes the otherwise localized Wannier-Stark states, and accordingly quasienergy band structures are formed, and manifest in the FWM spectra as a series of equally separated continua. The width of each continuum is proportional to the joint width of the valence and conduction minibands and is independent of the Wannier-Stark index. The realistic homogeneous broadening blurs the continua into broad peaks, whose line shapes, far from the Lorentzian, vary with the delay time in the FWM spectra. The swinging range of the peaks is just the quasienergy bandwidth. The dynamical delocalization (DDL) also induces significant FWM signals well beyond the excitation energy window. When the Coulomb interaction is taken into account, the unequal spacing between the excitonic Wannier-Stark levels weakens the DDL effect, and the FWM spectrum is transformed into groups of discrete lines. Strikingly, the groups are evenly spaced by the ac field frequency, reflecting the characteristic of the quasienergy states. The homogeneous broadening again smears out the line structures, leading to the excitonic FWM spectra quite similar to those without the exciton effect. However, all these features predicted by the dynamical theory do not appear in a recent experiment [Phys. Rev. Lett. 79, 301 (1997)], in which, by using the static approximation the observed Wannier-Stark ladder with delay-time-dependent spacing in the FWM spectra is attributed to a temporally periodic dipole field, produced by the Bloch oscillation of electrons in real space. The contradiction between the dynamical theory and the experiments is discussed. In addition, our calculation indicates that the dynamical localization coherently enhances the time-integrated FWM signals. The feasibility of using such a technique to study the dynamical localization phenomena is shown. [S0163-1829(99)10607-6].

Simulation and fabrication of the SiC-based clamped-clamped filter

In this paper, the SiC-based clamped-clamped filter was designed and fabricated. The filter was composed of two clamped-clamped beam micromechanical resonators coupled by a spring coupling beam. Structural geometries, including the length and width of the resonator beam and coupling beam, were optimized by simulation for high frequency and high Q, under the material properties of SiC. The vibrating modes for the designed filter structure were analyzed by finite element analysis (FEA) method. For the optimized structure, the geometries of resonator beams and coupling beams, as well as the coupling position, the SiC-based clamped-clamped filter was fabricated by surface micromaching technology.

Recent progresses of SOI-based photonic devices - art. no. 60201R

SOI (Silicon on Insulator) based photonic devices, including stimulated emission from Si diode, RCE (Resonant Cavity Enhanced) photodiode with quantum structure, MOS (Metal Oxide Semiconductor) optical modulator with high frequency, SOI optical matrix switch and wavelength tunable filter are reviewed in the paper. The emphasis will be played on our recent results of SOI-based thermo-optic waveguide matrix switch with low insertion loss and fast response. A folding re-arrangeable non-blocking 4x4 matrix switch with total internal reflection (TIR) mirrors and a first blocking 16 x 16 matrix were fabricated on SOI wafer. The extinction ratio and the crosstalk are better. The insertion loss and the polarization dependent loss (PDL) at 1.55 mu m increase slightly with longer device length and more bend and intersecting waveguides. The insertion losses are expected to decrease 2-3 dB when anti-reflection films are added in the ends of the devices. The rise and fall times of the devices are 2.1 mu s and 2.3 mu s, respectively.

Design and Fabrication of Low-microwave loss Coplannar Waveguide and precise V groove on Silicon Substrate for Optoelectronic Packaging - art. no. 601938

Optoelectronic packaging has become a most important factor that influences the final performance and cost of the module. In this paper, low microwave loss coplanar waveguide(CPW) on high resistivity silicon(HRS) and precise V groove in silicon substrate were successfully fabricated. The microwave attenuation of the CPW made on HRS with the simple process is lower than 2 dB/cm in the frequency range of 0 similar to 26GHz, and V groove has the accuracy in micro level and smooth surface. These two techniques built a good foundation for high frequency packaging and passive coupling of the optoelectronic devices. Based on these two techniques, a simple high resistivity silicon substrate that integrated V groove and CPW for flip-chip packaging of lasers was completed. It set a good example for more complicate optoelectronic packaging.