All-optical switching application of germano-silicate optical fiber incorporated with Ag nanocrystals

银纳米晶体掺杂的高非线性石英光纤的全光转换应用

香蕉型分子的真空气相沉积薄膜研究

非手性的香蕉型分子因为独特的形状和极性自组装性能，能形成具有手性结构和特殊光电性能（如铁电性、反铁电性，二次谐波产生效应）的液晶，是近年来有机光电领域中一类非常令人感兴趣的材料。本论文以三环弯核分子间一二（对辛烷氧基苯乙烯基）苯［m-bis（4-p-octoxysryrenyl）benzene，m-OSB］为模型化合物，探索香蕉型分子真空气相沉积薄膜的生长行为。首先，采用示差扫描量热法（DSC）、偏光显微镜、X一射线衍射、透射电子显微镜以及分子模拟等手段表征了m一OSB的体相结构。m-OSB存在两个晶相：相I（T＜66℃）和相11（66oC＜T＜157℃），157oC以上为各向同性态。确定相I为正交晶系，PZI2121空间群，晶胞参数为a＝7．43A，b：6．34A，c＝72．07A，α=β=γ=90°；分子模拟结果显示m一OSB排列成反铁电的层状结构，分子的长轴方向与c轴平行。相n的结构与相I非常相似。这些表征结果为薄膜形态结构的解释提供了依据。然后，采用真空蒸镀的方法制备m-OSB的单层和多层薄膜，用原子力显微镜、透射电子显微镜以及X-射线薄膜反射等手段研究了薄膜形态、结构和薄膜对温度的稳定性。由于香蕉型分子的特殊形状，m-OSB的气相沉积薄膜表现出了特殊的形貌、生长机理和相行为。在单层薄膜生长中，二维岛在40℃以下为圆形或椭圆形的滴状岛，在60℃以上为枝晶状岛，该生长形貌随基底温度的演变与传统的扩散受限凝聚（DLA）理论所预言的结果相反，AFM观察发现滴状岛是从最初沉积的无序膜分解得来的，类似"去润湿"现象；枝晶状岛是从无序膜中成核再通过分子的扩散生长形成的；单层薄膜无论是滴状岛还是枝晶状岛都不稳定，在室温下放置一段时间后均转化为双层的晶体；在多层膜的生长中，观察到了m-OSB形成了高有序、大尺寸的准外延膜。这些在有机气相沉积薄膜生长中都是第一次报道，论文对所观察到的生长现象和相变现象给予了合理的解释。通过对m-OSB气相沉积薄膜的成核和生长过程的系统研究，获得了薄膜形态结构与分子形状和制备参数的关系，对进一步理解有机分子气相沉积薄膜的生长规律和制备高有序的连续薄膜具有重要意义。

High-Saturation-Power and High-Speed Ge-on-SOI p-i-n Photodetectors

Ge-on-silicon-on-insulator p-i-n photodetectors were fabricated using an ultralow-temperature Ge buffer by ultrahigh-vacuum chemical vapor deposition. For a detector of 70-mu m diameter, the 1-dB small-signal compression power was about 110.5 mW. The 3-dB bandwidth at 3-V reverse bias was 13.4 GHz.

Room-Temperature Continuous-Wave Operation of InGaN-Based Blue-Violet Laser Diodes with a Lifetime of 15.6 Hours

We report our recent progress of investigations on InGaN-based blue-violet laser diodes (LDs). The room-temperature (RT) cw operation lifetime of LDs has extended to longer than 15.6 h. The LD structure was grown on a c-plane free-standing (FS) GaN substrate by metal organic chemical vapor deposition (MOCVD). The typical threshold current and voltage of LD under RT cw operation are 78 mA and 6.8 V, respectively. The experimental analysis of degradation of LD performances suggests that after aging treatment, the increase of series resistance and threshold current can be mainly attributed to the deterioration of p-type ohmic contact and the decrease of internal quantum efficiency of multiple quantum well (MQW), respectively.

Investigation on the strain relaxation of InGaN layer and its effects on the InGaN structural and optical properties

The evolution of strain and structural properties of thick epitaxial InGaN layers grown on GaN with different thicknesses are investigated. It is found that, with increase in InGaN thickness, plastic relaxation via misfit dislocation generation becomes a more important strain relaxation mechanism. Accompanied with the relaxation of compressive strain, the In composition of InGaN layer increases and induces an apparent red-shift of the cathodoluminescence peak of the InGaN layer. On the other hand, the plastic relaxation process results in a high defect density, which degrades the structural and optical properties of InGaN layers. A transition layer region with both strain and In composition gradients is found to exist in the 450-nm-thick InGaN layer.

Abnormal photoabsorption in high resistance GaN epilayer

Unintentionally doped GaN epilayers are grown by the metalorganic chemical vapor deposition (MOCVD). Photovoltaic (PV) spectroscopy shows that there appears an abnormal photoabsorption in some undoped GaN films with high resistance. The peak energy of the absorption spectrum is smaller than the intrinsic energy band gap of GaN. This phenomenon may be related to exciton absorption. Then metal-semiconductor-metal (MSM) Schottky photodetectors are fabricated on these high resistance epilayers. The photo spectrum responses are different when the light individually irradiates each of the two electrodes with the photodetector which are differently biased. When the excitation light irradiates around the reverse biased Schottky junction, the responsivity is almost one order of magnitude larger than that around the forward biased junction. Furthermore, when the excitation light irradiates the reverse biased Schottky junction, the peak energy of the spectrum has a prominent red-shift compared with the peak energy of the spectrum measured with the excitation light irradiating the forward biased Schottky junction. The shift value is about 28 meV, and it is found to be insensitive to temperature. According to the analyses of the distribution of the electric field within the MSM device and the different dependences of the response on the electric field intensity between the free carriers and excitons, a reliable explanation for the different response among various areas is proposed.

EXCIMER-LASER DOPING OF SPIN-ON DOPANT IN SILICON

Solid films containing phosphorus impurities were formed on p-type silicon wafer surface by traditional spin-on of commercially available dopants. The doping process is accomplished by irradiating the sample with a 308 nm XeCl pulsed excimer laser. Shallow junctions with a high concentration of doped impurities were obtained. The measured impurity profile was ''box-like'', and is very suitable for use in VLSI devices. The characteristics of the doping profile against laser fluence (energy density) and number of laser pulses were studied. From these results, it is found that the sheet resistance decreases with the laser fluence above a certain threshold, but it saturates as the energy density is further increased. The junction depth increases with the number of pulses and the laser energy density. The results suggest that this simple spin-on dopant pre-deposition technique can be used to obtain a well controlled doping profile similar to the technique using chemical vapor in pulsed laser doping process.

INVESTIGATION OF GAAS/SI MATERIAL BY X-RAY DOUBLE-CRYSTAL DIFFRACTION

GaAs epilayer films on Si substrates grown by molecular-beam epitaxy were investigated by the x-ray double-crystal diffraction method. The rocking curves were recorded for different diffraction vectors of samples. The results show that the unit-cell volumes of GaAs epilayers are smaller than that of the GaAs bulk material. The strained-layer superlattice buffer layer can improve the quality of the film, especially in the surface lamella. The parameter W' = W(expt)/(square-root \gamma-h\/gamma-0/sin 2-theta-B) is introduced to describe the quality of different depths of epilayers. As the x-ray incident angle is increased, W' also increases, that is, the quality of the film deteriorates with increasing penetration distance of the x-ray beam. Therefore, W' can be considered as a parameter that describes the degree of perfection of the epilayer along the depth below the surface. The cross-section transmission electron microscopy observations agree with the results of x-ray double-crystal diffraction.

PHOTOLUMINESCENCE AND SURFACE PHOTOVOLTAIC SPECTRA OF STRAINED INP ON GAAS

A high energy shift of the band-band recombination has been observed in the photoluminescence (PL) spectra of the strained InP epilayer on GaAs by metalorganic chemical vapor deposit. The strain determined by PL peak is in good agreement with calculated thermal strain. The surface photovoltalic spectra gives the information about energy gap, lattice mismatching, and composition of heteroepilayers, diffusion length, surface, and interface recombination velocity of minority carriers of heteroepitaxy layers.

Microcrystalline silicon films and tandem solar cells prepared by triode PECVD

This paper investigates the effects of the diphasic structure on the optoelectronic properties of hydrogenated microcrystalline silicon (mu c-Si:H) films prepared in a triode three-chamber plasma-enhanced chemical vapor deposition (PECVD) system. The influences of boron-compensation doping on the dark-and photo-conductivity of mu c-Si:H films are also described. A tandem solar cell with an entirely mu c-Si:H p-i-n bottom cell and an a-Si:H top cell has been prepared with an initial conversion efficiency of 8.91% (0.126 cm(2), AM1.5, 100 mW/cm(2)).

Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films(similar to 10 mu m) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Angstrom/s at the substrate temperature (T-s) of 1030 degrees C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn(+) junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm(2), 1 cm(2)).

Assessment of the structural properties of GaAs/Si epilayers using X-ray (004) and (220) reflections

We improved the method previously used to determine the lattice constants and misorientation of GaAs/Si by recording the patterns of X-ray (004) and (220) reflections. The (220) reflection was measured from the (110) cross section of a GaAs/Si epilayer. The structural properties of the GaAs/Si epilayers grown by metal-organic chemical-vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were investigated. The rotation angle of GaAs/Si epilayers grown by MOCVD using an a-Si buffer layer is very small and the lattice constants of these GaAs/Si epilayers agree quite well with elastic theory.

Characterization and improvement of GaAs layers grown on Si using an ultrathin a-Si film as a buffer layer

GaAs epilayers grown on Si by metalorganic chemical vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were characterized by deep-level transient spectroscopy (DLTS). Six electron traps with activation energies of 0.79, 0.67, 0.61, 0.55, 0.53 and 0.32 eV below the conduction band were determined by fitting the experimental spectra. Two of the levels, C (0.61 eV) and F (0.32 eV), were first detected in GaAs epilayers on Si and identified as the metastable defects M3 and M4, respectively. In order to improve the quality of GaAs/Si epilayers, another GaAs layer was grown on the GaAs/Si epilayers grown using MOCVD. The deep levels in this regrown GaAs epilayer were also studied using DLTS. Only the EL2 level was found in the regrown GaAs epilayers. These results show that the quality of the GaAs epilayer was greatly improved by applying this growth process.

Growth of GaAs on Si by using a thin Si film as buffer layer

We report a novel technique for growing high-quality GaAs on Si substrate. The process involves deposition of a thin amorphous Si film prior to the conventional two-step growth. The GaAs layers grown on Si by this technique using metalorganic chemical vapor deposition exhibit a better surface morphology and higher crystallinity as compared to the samples gown by conventional two-step method. The full width at half maximum (FWHM) of the x-ray (004) rocking curve for 2.2 mu m thick GaAs/Si epilayer grown by using this new method is 160arcsec. The FWHM of the photoluminescence spectrum main peak for this sample is 2.1 meV. These are among the best results reported so far. In addition, the mechanism of this new growth method was studied using high-resolution transmission electron microscopy.

PHOTOLUMINESCENCE SPECTRUM STUDY OF THE GAAS/SI EPILAYER GROWN BY USING A THIN AMORPHOUS SI FILM AS BUFFER LAYER

Recently, we reported successful growth of high-quality GaAs/Si epilayers by using a very thin amorphous Si film as buffer layer. In this paper, the impurity properties of this kind of GaAs/Si epilayers have been studied by using PL spectrum, SIMS and Hall measurement. Compared to a typical PL spectrum of the GaAs/Si epilayers grown by conventional two-step method, a new peak was observed in our PL spectrum at the energy of 1.462 eV, which is assigned to the band-to-silicon acceptor recombination. The SIMS analysis indicates that the silicon concentration in this kind of GaAs/Si epilayers is about 10(18) cm(-3). But its carrier concentration (about 4 x 10(17) cm(-3)) is lower than the silicon concentration. The lower carrier concentration in this kind of GaAs/Si epilayer can be interpreted both as the result of higher compensation and as the result of the formation of the donor-defect complex. We also found that the high-quality and low-Si-concentration GaAs/Si epilayers can be regrown by using this kind of GaAs/Si epilayer as substrate. The FWHM of the X-ray (004) rocking curve from this regrowth GaAs epilayer is 118 '', it is much less than that of the first growth GaAs epilayer (160 '') and other reports for the GaAs/Si epilayer grown by using conventional two-step method (similar to 200 '').