Nanostructure in the p-layer and its impacts on amorphous silicon solar cells

The open circuit voltage (V-oc) of n-i-p type hydrogenated amorphous silicon (a-Si:H) solar cells has been examined by means of experimental and numerical modeling. The i- and p-layer limitations on V-oc are separated and the emphasis is to identify the impact of different kinds of p-layers. Hydrogenated protocrystalline, nanocrystalline and microcrystalline silicon p-layers were prepared and characterized using Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), optical transmittance and activation energy of dark-conductivity. The n-i-p a-Si:H solar cells incorporated with these p-layers were comparatively investigated, which demonstrated a wide variation of V-oc from 1.042 V to 0.369 V, under identical i- and n-layer conditions. It is found that the nanocrystalline silicon (nc-Si:H) p-layer with a certain nanocrystalline volume fraction leads to a higher V-oc. The optimum p-layer material for n-i-p type a-Si:H solar cells is not found at the onset of the transition between the amorphous to mixed phases, nor is it associated with a microcrystalline material with a large grain size and a high volume fraction of crystalline phase. (c) 2006 Elsevier B.V. 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.

Research on multi-degree-of-freedom neurons with weighted graphs

In this paper, we redefine the sample points set in the feature space from the point of view of weighted graph and propose a new covering model - Multi-Degree-of-Freedorn Neurons (MDFN). Base on this model, we describe a geometric learning algorithm with 3-degree-of-freedom neurons. It identifies the sample points secs topological character in the feature space, which is different from the traditional "separation" method. Experiment results demonstrates the general superiority of this algorithm over the traditional PCA+NN algorithm in terms of efficiency and accuracy.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

1.55-mu m resonant cavity enhanced photodiode based on MBE grown Ge quantum dots

Self-assembly Ge quantum dots (QD) on Si and Si/Ge mutli-quantum-wells (MQW) are grown by MBE. The island size and island density was investigated by atomics force microscopy. Ten-layer and twenty-layer MQW were selected for photodiode device fabrication. In photoluminescence (PL), a broad peak around 1.55-mu m wavelength was observed with higher peak intensity for the 10-layer MQW which had less defects than the 20-layer sample. Resonant cavity enhanced (RCE) photodiodes were fabricated by bonding on a SOI wafer. Selected responsivity at 1.55 mu m was successfully demonstrated. (c) 2005 Elsevier B.V. All rights reserved.

A novel fast lock-in phase-locked loop frequency synthesizer with direct frequency presetting circuit

This paper proposes a novel, fast lock-in, phase-locked loop (PLL) frequency synthesizer. The synthesizer includes a novel mixed-signal voltage-controlled oscillator (VCO) with a direct frequency presetting circuit. The frequency presetting circuit can greatly speed up the lock-in process by accurately the presetting oscillation frequency of the VCO. We fully integrated the synthesizer in standard 0.35 mu m, 3.3 V complementary metal-oxide-semiconductors (CMOS) process. The entire chip area is only 0.4 mm(2). The measured results demonstrate that the synthesizer can speed up the lock-in process significantly and the lock-in time is less than 10 mu s over the entire oscillation frequency range. The measured phase noise of the synthesizer is -85 dBc/Hz at 10 kHz offset. The synthesizer avoids the tradeoff between the lock-in speed and the phase noise/spurs. The synthesizer monitors the chip temperature and automatically compensates for the variation in frequency with temperature.

Growth of ZnO single crystal by chemical vapor transport method

ZnO crystals were grown by CVT method in closed quartz tube under seeded condition. Carbon was used as a transport agent to enhance the chemical transport of ZnO in the growth process. ZnO single crystals were grown by using GaN/sapphire and GaN/Si wafer as seeds. The property and crystal quality of the ZnO single crystals was studied by photoluminescence spectroscopy and X-ray diffraction technique.

Study on surface morphology of GaN growth by MOCVD on GaN/Si(111) template

The surface morphology of GaN grown by MOCVD on GaN/Si template was studied. Rough morphology and deep pinhole defects on some surface areas of the samples were observed and studied. The formation of rough morphology is possibly related to Ga-Si alloy produced due to poor thermal stability of template at high temperature. The deep pinhole defects generated are deep down to the surface of MBE-grown GaN/Si template. The stress originated from the large thermal expansion coefficient difference between GaN and Si may be related to the formation of the pinhole defects. The surface morphology of the GaN can be improved by optimizing the GaN/Si template and decreasing the growth temperature.

Growth and characterization of semi-insulating GaN films grown by MOCVD

High resistivity unintentionally doped GaN films were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition. The surface morphology of the layer was measured by both atomic force microscopy and scanning electron microscopy. The results show that the films have mirror-like surface morphology with root mean square of 0.3 nm. The full width at half maximum of double crystal X-ray diffraction rocking curve for (0002) GaN is about 5.22 arc-min, indicative of high crystal quality. The resistivity of the GaN epilayers at room temperature and at 250 degrees C was measured to be approximate 10(9) and 10(6) Omega(.)cm respectively, by variable temperature Hall measurement. Deep level traps in the GaN epilayers were investigated by thermally stimulated current and resistivity measurements.

Growth of high quality semi-insulating InP single crystal by suppression of compensation defects

Deep level defects in as-grown and annealed SI-InP samples were investigated by thermally stimulated current spectroscopy. Correlations between electrical property, compensation ratio, thermal stability and deep defect concentration in SI-InP were revealed. An optimized crystal growth condition for high quality SI-InP was demonstrated based on the experimental results.

Temperature and power-density-dependent inter-shell energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling in InAs/GaAs quantum dot (QD) structures as a function of the excitation power density by using rnicro-photoluminescence spectroscopy at different temperatures. In addition to the emission bands of exciton recombination corresponding to the atom-like S, P and D, etc. shells of QDs, it was observed that some extra states V between the S and P shells, and D' between the P and D shells appear in the spectra with increasing number of excitons occupying the QDs at a certain temperature. The emergence of these inter-shell excitonic levels is power density and temperature dependent, which is an experimental demonstration of strong exciton-exciton exchange interaction, state hybridization, and coupling of a multi-exciton system in QDs. (c) 2006 Elsevier B.V. All rights reserved.

Research on nitrogen implantation energy dependence of the properties of SIMON materials

With different implantation energies, nitrogen ions were implanted into SIMOX wafers in our work. And then the wafers were subsequently annealed to form separated by implantation of oxygen and nitrogen (SIMON) wafers. Secondary ion mass spectroscopy (SIMS) was used to observe the distribution of nitrogen and oxygen in the wafers. The result of electron paramagnetic resonance (EPR) was suggested by the dandling bonds densities in the wafers changed with N ions implantation energies. SIMON-based SIS capacitors were made. The results of the C-V test confirmed that the energy of nitrogen implantation affects the properties of the wafers, and the optimum implantation energy was determined. (c) 2005 Elsevier B.V. All rights reserved.

Optical properties of inGaAs/GaAs quantum wells grown by Sb-assisted molecular beam epitaxy

It is found that both methods using either continuous Sb supply or pre-deposition of a very thin Sb layer are efficient for the Sb-assisted molecular beam epitaxy growth of highly strained InGaAs/GaAs quantum wells (QWs). The emission of QWs is extended to long wavelength close to 1.25 mu m with high luminescence efficiency at room temperature. The influence of rapid thermal annealing (RTA) on the photoluminescence intensity critically depends on the annealing temperature and duration for highly strained QWs. A relatively low RTA temperature of 700 degrees C with a short duration of 10 s is suggested for optimizing the annealing effect. (c) 2005 Elsevier B.V. All rights reserved.

Time-resolved photoluminescence spectra of self-assembled InAs/GaAs quantum dots

Two types of InAs self-assembled Quantum dots (QDs) were prepared by Molecular beam epitaxy. Atomic force microscopy (AFM) measurements showed that, compared to QDs grown on GaAs substrate, QDs grown on InGaAs layer has a significantly enhanced density. The short spacing (several nanometer) among QDs stimulates strong coupling and leads to a large red-shift of the 1.3 mu m photoluminescence (PL) peak. We study systematically the dependence of PL lifetime on the QDs size, density and temperature (1). We found that, below 50 K, the PL lifetime is insensitive to temperature, which is interpreted from the localization effects. As T increases, the PL lifetime increases, which can be explained from the competition between the carrier redistribution and thermal emission at higher temperature. The increase of carriers in QDs migrated from barriers and wetting layer (WL), and the redistribution of carriers among QDs enhance the PL lifetime as T increases. The thermal emission and non-radiative recombination have effects to reduce the PL lifetime at higher T. As a result, the radiative recombination lifetime is determined by the wave function overlapping of electrons and holes in QDs, and QDs with different densities have different PL lifetime dependence on the QDs size. (c) 2005 Elsevier B.V. All rights reserved.

Silicon-on-insulator based optical waveguide and integrated switch matrix

zhangdi于2010-03-29批量导入