侧向外延生长GaN的结构特性

研究了在刻有图形的GaN"衬底”上用HVPE方法侧向外延生长(ELO)GaN的结构特性.在SiO2衬底上侧向外延生长GaN已经实现,并得到了平面的ELO GaN薄膜.采用扫描电子显微镜、透射电微镜和原子力显微镜技术研究了这种ELO GaN材料的结构和表面形貌.原子力显微镜图像表明:在ELO范围中的4μm2面积上不存在明显的阶状形貌.透射电子显微镜的观测表明在ELO范围内位错密度很低.在接合的界面上没观察到有空隙存在.但观测到晶格的弯曲高达3.3°,这被归因为由GaN层下的"籽层”和接合界面处的水平倾料和猝灭所产生的螺旋位错的积聚.     

侧向外延法生长的高质量GaN及其外延缺陷的观察

在有条状SiO2图形的GaN“模板”上,侧向外延方法生长了高质量的GaN。荧光显微镜的结果表明在SiO2掩膜区有成核过程发生。原因可能是SiO2的质量不高,为GaN的生长提供了一些成核中心。在GaN层的厚度达到4．5μm后,侧向的融合开始发生。侧向生长的速度与垂直生长速度几乎相同。在所有的SiO：掩膜上方都形成了空洞。样品在240℃熔融的KOH中腐蚀13min。在SiO2掩膜区生长的GaN,其腐蚀坑密度(相当于穿透位错密度)减少到几乎为零。而在窗口区生长的GaN,腐蚀坑密度仍然很高,达到10^8cm^-2量级。同时,我们发现具有不同窗口尺寸的样品在SiO2掩膜区上侧向生长的CaN的晶体质量基本相同,与窗口区的宽度几乎无关。室温光荧光结果表明侧向外延法生长的CaN中的晶格失配应力已被部分释放。     

SiC衬底上外延GaN:Mg材料特性研究

利用扫描电子显微镜、拉曼散射光谱和光致发光谱对在SiC衬底上采用MOCVD异质外延的GaN:Mg薄膜特性进行研究发现：除了一部分Mg原子替代Ga原子呈现受主性外，大部分Mg原子以间隙原子状态（Mg_i）存在，并且在缺陷或者位错处大量聚集引起薄膜张力应力减小，薄膜在降温过程中由于应力不均匀会在部分区域内出现大量的裂纹；Mg的掺杂会加剧GaN无序化程度，致使薄膜质量变差；而室温下的PL谱测量表明蓝带发光由DAP（深施主—浅受主）复合引起，其中D为Mg_GaV_{N 关键词： GaN∶Mg 异质外延 扫描电子显微镜 拉曼散射 光致发光谱}     

使用SiNx原位淀积方法生长的GaN外延膜中的应力研究

采用低压MOCVD系统,在生长过程中使用SiNx原位淀积的方法产生纳米掩模,并在纳米掩模上进行选区生长和侧向外延制备了GaN外延薄膜.使用拉曼光谱和光荧光的手段对GaN外延膜中的残余应力进行了研究.研究发现,用SiNx原位淀积出纳米掩模后,GaN生长将由二维向三维转变,直到完全合并为止.利用拉曼光谱和光荧光谱分别研究了薄膜中的残余应力,两者符合得很好;这种方法生长出的GaN薄膜的应力分布较传统的侧向外延更加均匀;并且从中发现随着生长过程中SiNx原位淀积时间的增加,生长在其上的GaN外延膜中的残余应力减小.这是因为,随着SiNx原位淀积时间的增加,SiNx纳米掩模的覆盖度也增大.因此侧向外延区的比例增大,残余应力随之减小.     

使用SiNx原位淀积方法生长的GaN外延膜中的应力研究

采用低压MOCVD系统，在生长过程中使用SiN_x原位淀积的方法产生纳米掩模，并 在纳米掩模上进行选区生长和侧向外延制备了GaN外延薄膜.使用拉曼光谱和光荧光的手段对 GaN外延膜中的残余应力进行了研究.研究发现，用SiN_x原位淀积出纳米掩模后 ，GaN生长将由二维向三维转变，直到完全合并为止.利用拉曼光谱和光荧光谱分别研究了薄 膜中的残余应力，两者符合得很好；这种方法生长出的GaN薄膜的应力分布较传统的侧向外 延更加均匀；并且从中发现随着生长过程中SiN_x原位淀积时间的增加，生长在 其上的GaN外延膜中的残余应力减小.这是因为，随着SiN_x原位淀积时间的增加 ，SiN_x纳米掩模的覆盖度也增大.因此侧向外延区的比例增大，残余应力随之减 小. 关键词： GaN x原位淀积')" href="#">SiN_x原位淀积 拉曼 光荧光 残余应力     

气相外延GaN的拉曼散射

测量了在蓝宝石衬底上气相外延生长GaN的拉曼散射谱.除观察到已被确认的两个E2,一个A1(TO)和一个E1(TO)声于振动以外,在734±3cm-1处观察到一个散射峰且从实验上确认其为GaN的纵向光学声子模E1(LO).而且发现其强度与外延层晶体质量密切相关.A1(TO)和高频E2散射峰相对强度变化显示不同生长条件引起的外延层质量的变化.     

柱状与孔状图形衬底对MOVPE生长GaN体材料及LED器件的影响

在柱状图形蓝宝石衬底（PSS-p）和孔状图形蓝宝石衬底（PSS-h）上外延了GaN体材料和LED结构并进行了详细对比和分析.X射线衍射仪（XRD）和原子力显微镜（AFM）测试结果表明,PSS-h上体材料的晶体质量和表面形貌都优于PSS-p上体材料的特性,通过断面扫面电子显微镜（SEM）照片看出PSS-h上GaN的侧向生长是导致这种差异的原因.另外,基于PSS-p和PSS-h上外延的LED材料制作而成的器件结果表明,其20?mA下光功率水平相比普通蓝宝石衬底（CSS）分别提高了46%和33%.通过变温光荧光 关键词： 蓝宝石图形衬底 氮化镓 发光二极管 侧向生长     

MOCVD生长Si衬底上HT-AlN缓冲层低生长压力对GaN薄膜的影响

采用金属有机化学气相沉积（MOCVD）技术在Si（111）衬底上外延GaN薄膜,对高温AlN（HT-AlN）缓冲层在小范围内低生长压力（6.7~16.6 kPa）条件下对GaN薄膜特性的影响进行了研究。研究结果表明GaN外延层的表面形貌、结构和光学性质对HT-AlN缓冲层的生长压力有很强的的依赖关系。增加HT-AlN缓冲层的生长压力,GaN薄膜的光学和形貌特性均有明显改善,当HT-AlN缓冲层的生长压力为13.3 kPa时,得到无裂纹的GaN薄膜,其（002）和（102）面的X射线衍射峰值半高宽分别为735 arcsec和778 arcsec,由拉曼光谱计算得到的张应力为0.437 GPa,原子力显微镜（AFM）观测到表面粗糙度为1.57 nm。     

GaN薄膜拉曼散射光谱的研究

利用拉曼散射光谱对在SiC衬底上采用MOCVD异质外延的未故意掺杂GaN薄膜特性进行研究发现E2模式向频率低的方向漂移表明在GaN薄膜中存在张力,由于SiC衬底不平整度增加引起更多位错的出现,从而引起拉曼谱中E2模式的加宽,因此通过选择平整度较好的衬底可以减小缺陷密度,提高薄膜的质量,此外A1(LO)模式的出现与强度可以用来表征未掺杂GaN的薄膜质量。     

淀积在不同小倾角蓝宝石衬底的n型GaN的研究

采用金属有机物化学淀积技术在不同倾角（0°—03°）的蓝宝石衬底上外延n型GaN.通过原子力显微镜观察到n型GaN均呈台阶流生长模式,02°和03°倾角衬底的n型GaN表面台阶朝向相同、分布均匀,明显地看到在0°倾角衬底的n型GaN表面由台阶重构直接导致的台阶朝向随机分布、疏密不匀的形貌.电子背散射分析表明,在0°倾角衬底的n型GaN外延层的应力随外延厚度增加而增加,而02°和03°倾角衬底的n型GaN外延层的应力没有明显的变化.电学和光学特性研究表明,02°和03°倾角衬底的n型GaN有较高的电子浓度和较低的黄光带与近带边强度之比. 关键词： 金属有机物化学淀积 氮化物 原子力显微镜 光致发光     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.