X-ray reflectivity and atomic force microscopy studies of MOCVD grown AlxGa1-xN/GaN superlattice structures

The grazing incidence X-ray reflectivity (GIXR) technique and atomic force microscopy (AFM) were exploited to obtain an accurate evaluation of the surfaces and interfaces for metalorganic chemical vapor deposition grown Al<,x>Ga<,1-x>N/GaN superlattice structures. The X-ray diffraction results have been combined with reflectivity data to evaluate the layer thickness and Al mole fraction in the AIGaN layer. The presence of a smooth interface is responsible for the observation of intensity oscillation in GIXR, which is well correlated to step flow observation in AFM images of the surface. The structure with a low Al mole fraction (x = 0.25) and thin well width has a rather smooth surface for the R<,rms> of AFM data value is 0.45 nm.     

MOCVD AlxGa1-xN/GaN超晶格结构的X射线反射谱与原子力显微研究

采用掠入射X射线反射谱技术与原子力显微技术对属有机化合物化学气相淀积生长的AlxGa1-xN/GaN超晶格结构的表面和界面进行了精确表征。结合高分辨率X射线衍射谱与反射谱数据分析获得外延层各层厚度与AlGaN层的Al摩尔组分。掠入射x射线反射谱的显著强度振荡与原子力显微镜所观察到的台阶流动形貌表明了平整的界面和表面的存在。研究发现,低Al组分（x=0.25）且阱宽小的样品界面与表面粗糙度最小,通过原子力显微技术得到的表面粗糙度均方根偏差为0.45nm。     

Si间隔层对Al(1wt.%Si)/Zr极紫外多层膜热稳定的作用研究

为了提升Al/Zr多层膜的热稳定性,采用直流磁控溅射方法制备了18个带有不同厚度Si间隔层的Al(1 wt.%Si)/Zr多层膜,并将这些样品分别进行了不同温度(100~500 ℃)的真空退火,退火时间为1 h.利用X射线掠入射反射(GIXR)和X射线衍射(XRD)的方法来研究Si间隔层对Al/Zr多层膜热稳定性的作用.GIXR测量结果表明:随着Si间隔层厚度的增大,Al膜层的粗糙度减小,而Zr膜层的粗糙度增大;XRD测量结果表明:Al和Zr膜层粗糙度的变化是由于退火后膜层中晶粒尺寸不同造成的.相比于没有Si间隔层的Al/Zr多层膜,引入厚度为0.6 nm的Si间隔层可以有效提升Al/Zr多层膜的热稳定性.     

Stability and interface abruptness of InxGa1−xN/InyGa1−yN multiple quantum well structures grown by OMVPE

The abruptness of hetero-interfaces in InGaN multiple quantum well structures is shown to degrade when a high temperature growth follows growth of the multiple quantum well (MQW) region, as is generally required for the growth of full device structures. We have analyzed MQW samples both with and without high temperature GaN “cap” layers, using x-ray diffraction (XRD), grazing incidence x-ray reflection (GIXR), and photoluminescence. While all of these techniques indicate a degradation of the MQW structure when it is followed by growth at high temperature, GIXR is shown to be especially sensitive to changes of heterointerface abruptness. GIXR measurements indicate that the heterojunctions are less abrupt in samples that have high temperature cap layers, as compared to samples with no cap layer. Furthermore, the degree of roughening is found to increase with the duration of growth of the high temperature cap layer. The degradation of the heterointerfaces is also accompanied by a reduction in the intensity of satellite peaks in the x-ray diffraction spectrum.     

Investigation of nanometer-scale films using low angle X-ray reflectivity analysis in IPOE

The X-ray low angle reflectivity measurement is used to investigate single and bilayer films to determine the parameters of nanometer-scale structures,three effectual methods are presented by using X-ray reflectivity analysis to provide an accurate estimation of the nanometer film structures. The parameters of tungsten (W) single layer, such as the material density, interface roughness and deposition rate, were obtained easily and speedily. The base metal layer was introduced to measure the profiles of single low Z material film. A 0.3 nm chromium (Cr) film was also studied by low angle reflectivity analysis.     

常用间隔层材料软X射线多层膜的设计

为了研究常用间隔层材料的软X射线多层膜的材料设计,利用Lawrence-Livermore国家实验室提供的软X射线多层膜设计程序进行模拟计算,得到了软X射线波段内的具有高反射率的膜系。结果表明,Mo在相当大的波段范围内具有良好的光学特性;除了在12.44nm是首选膜系外(除4.36nm外),其它波长都有较高的反射率;U作为吸收层材料,在相当大的波段范围内,与其它间隔层材料配对的多层膜也具有较高的反射率。这对多层膜膜系的材料选择有一定的指导意义。     

Effects of alloying elements on cobalt silicide formation

Alloying elements can substantially affect the formation of cobalt silicide. A comprehensive study of phase formation was performed on 23 Co alloys with alloying element concentrations ranging from 1 at.% up to 20 at.%. Using in-situ characterization techniques in which x-ray diffraction (XRD) and elastic-light scattering are monitored simultaneously, we follow the formation of the silicide phases and the associated variation in surface roughness in real time during rapid thermal annealing. For pure Co silicide, we detect the formation of all stable silicide phases (Co₂Si, CoSi, and CoSi₂) as well as abnormal grain growth in the Co film and thermal degradation of the silicide layer at high temperatures. The effect of the various additives on phase formation was determined. The roughness of the interface was also measured using grazing incidence x-ray reflectivity (GIXR). We show that by selecting an alloy with a specific composition, we can change the phase-formation temperatures and modify the final CoSi₂ film texture and roughness.     

Investigation of the degradation of smooth SiGe epitaxial layer on Si substrate

The degradation of smooth SiGe epitaxial layer was investigated by transmission electron microscopy (TEM), X-ray reflectivity (XRR) and atomic force microscopy (AFM). It was shown from AFM results that the crosshatch was formed with increasing annealing temperature, which indicated the degradation of smooth surface. The surface degradation was caused by the internal dislocations, which were observed by plan-view TEM (PTEM) and cross-sectional TEM (XTEM). From XTEM, the sharp interface between SiGe top layer and Si substrate was broadened and there were a lot of 60° dislocations formed in SiGe top layer, which resulted in the crosshatch on the surface. The crosshatch was also verified by PTEM.     

SiC/Mg multilayer reflective mirror for He-Ⅱ radiation at 30.4 nm and its thermal stability

In applications of solar physics, extreme ultraviolet imaging of solar corona by selecting the HeⅡ (λ = 30.4 nm) emission line requires high reflectivity multilayer mirrors. Some material combinations were studied to design the mirrors working at a wavelength of 30.4 nm, including SiC/Mg, B4C/Mg, C/Mg, C/Al, Mo/ Si, B4C/Si, SiC/Si, C/Si, and Sc/Si. Based on optimization of the largest reflectivity and the narrowest width for the multilayer mirror, a SiC/Mg material combination was selected as the mirror and fabricated by a magnetron sputtering system. The layer thicknesses of the SiC/Mg multilayer were measured by an X-ray diffractometer. Reflectivities were then measured on beamline U27 at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. At a wavelength of 30.4 nm, the measured reflectivity is as high as 38.0%. Furthermore, a series of annealing experiments were performed to investigate the thermal stability of the SiC/Mg multilayer.     

Porous silicon as an intermediate buffer layer for GaN growth on (100) Si

We report preliminary results on the growth of GaN on (100) Si substrate using porous silicon (PS) as an intermediate buffer layer. The growth was in situ monitored by laser beam reflectivity. Analysis of the evolution of the reflectivity signal indicates a change from relatively flat surface to rough one as the growth temperature (T_g) is increased. At a temperature of about 1050°C, the growth rate is very low and the reflected signal intensity is constant. When the growth temperature is varied, no drastic change of the porosity of the intermediate layer was detected. Scanning electron microscope (SEM) observations of the GaN/SP/Si structure revealed a good surface coverage at 500°C. When T_g increases, the structure morphology changes to columnar like structure at 600°C, and well-developed little crystallites with no preferential orientation appear at 800°C. These observations agree well with the X-ray diffraction (XRD) analysis. A preferential hexagonal growth is obtained at low growth temperature, while cubic phase begin to appear at elevated temperatures.     

真空紫外光学薄膜及薄膜材料

概述了真空紫外(VUV)波段光学薄膜及薄膜材料的研究进展,金属Al膜因到短至80nm还能提供较高的反射率而得到普遍关注,在高真空和30 nm/s左右沉积速率下沉积了保护层的金属Al膜在157 nm处反射率可达90%.介质氧化物薄膜机械应力小,环境稳定性比氟化物薄膜好,在190 nm以上波段应用较广泛,但在180 nm以下波段吸收大大增加而应由氟化物薄膜取代.氟化物薄膜带宽大、吸收系数小,沉积了致密SiO2保护层的氟化物高反膜,在中心波长180 nm处可得到接近99%的反射率,而且膜系的稳定性和抗激光损伤也大大提高.氟化物减反膜在157 nm处可得到0.1%以下的反射率;到目前为止氟化物薄膜最好的沉积工艺是电阻热蒸发.     

渐变反射率镜的设计与制备

按照高斯型渐变反射率镜(GRM)的参数要求,采用了中间层厚度渐变的方案对膜系和掩模板形状进行设计.根据薄膜的实际需求和具体的沉积设备,设计了掩模和掩模切换装置.在一次高真空环境下镀制了渐变反射率镜的所有膜系.采用直接测量的方法,测量了高斯型渐变反射率镜反射率的径向分布.测试结果表明,用这种技术制备的样晶,与设计要求基本一致.分析得出,掩模板形状与精度对镀制结果有影响.随着设计尺寸减小,掩模板对膜料分子的散射作用增强,使样品中心反射率小于设计要求,边缘出现旁瓣.提出了减小基片与掩模板之间的距离和提高膜厚监控的精度的改善方案.     

p型GaN上Pd/NiO/Al/Ni反射电极欧姆接触的热稳定性研究

研究了p型GaN上Pd/NiO/Al/Ni反射电极欧姆接触的比接触电阻率、热稳定性,以及光学反射率。与传统Pd/Al/Ni电极相比,Pd/NiO/Al/Ni电极的欧姆接触在氮气环境中经300℃下热处理10min后,仍保持低比接触电阻率(小于5×10-4Ω·cm2)和高反射率(大于80%@365nm)。研究获得的优化Pd/NiO层厚度为1nm/2nm,此时的Pd/NiO/Al/Ni反射电极既能形成良好的欧姆接触,拥有低比接触电阻率,又能减少对紫外光的吸收,保持高反射率。研究表明适当的NiO层厚度能够有效地防止热处理过程中上层Al金属向p-GaN表面层的渗入,对于制备高质量的Al基反射电极至关重要。     

引入GaN过渡层对Si（111）衬底上生长GaN外延的影响

本文研究了在Si(111)衬底上生长GaN外延层的方法。相比于直接在AlN缓冲层上生长GaN外延层,引入GaN过渡层显著地提高了外延层的晶体质量并降低了外延层的裂纹密度。使用X射线双晶衍射仪、光学显微镜以及在位监测曲线分析了GaN过渡层对外延层的晶体质量以及裂纹密度的影响。实验发现,直接在AlN缓冲层上生长外延层,晶体质量较差, X射线(0002)面半高宽最优值为0.686°,引入GaN过渡层后,通过调整生长条件,控制岛的长大与合并的过程,从而控制三维生长到二维生长过渡的过程,外延层的晶体质量明显提高, (0002)面半高宽降低为0.206°,并且裂纹明显减少。研究结果证明,通过生长合适厚度的GaN过渡层,可以得到高质量、无裂纹的GaN外延层。     

X-ray reflectivity of silicon on insulator wafers

The ability of X-ray reflectivity to analyse different silicon on insulator structures is underlined. The standard geometry with first reflection occurring at the surface gives information about the thickness, roughness, and density of the layers. Deeply buried interfaces, i.e. in between thick wafers, are analysed with a non-standard geometry (the first reflection occurs at the buried interface) and with a high-energy radiation. These two methods are, respectively, illustrated by the reflectivity measurements of (SiO₂/Si/SiO₂|bulk Si) and (bulk Si/thermal SiO₂|native SiO₂/bulk Si) bonded structures, and are explained in the framework of kinematic theory of X-ray reflectivity.     

Thermo-optical switching in Si/Si1-xGexdistributed Bragg reflectors

The authors report thermo-optical switching in a 92 layer Si/Si _0.7Ge_0.3 distributed Bragg reflector (DBR) grown by molecular beam epitaxy. Depending on the layer periodicity, this structure exhibited a positive or negative reflectivity switching λ=1.06 μm, with a switch-on time of less than 20 ns and reflectivity contrast ratios greater than 50%     

45°角入射的13.1nm软X射线多层膜的研制

报道了对４５°入射角高反的１３．１ｎｍ软Ｘ射线多层膜反射镜的研制情况。利用在星光装置中进行的软Ｘ射线激光等离子体实验测量多层膜反射率的方法，获得了２６．２％的实测反射率，该反射率已达到理论反射率的７０％。     

Microwave Bistatic Reflectivity Dependence on the Moisture Content And Matric Potential of Bare Soil

Results are presented of an experimental program to determine the functional dependence of the microwave reflectivity of nonvegetated soil surfaces upon volumetric soil moisture and matric potential. A combination evaporation-drainage field experiment was conducted on a bare Captina silt loam with reflectivity, soil moisture content, and matric potential monitored for extended time periods. Results show that for a restricted pressure range (approximately -0.05 to -0.75 bar) there is excellent linear correlation between the log of bistatic reflectivity and both volumetric moisture content and matric potential. Layering effects due to steep moisture content (and matric potential) gradients in the profile are demonstrated to have two distinct and significant effects on the reflectivity response. At near saturation of rough surfaces a very thin dry surface layer appears to modify the effective roughness. This leads to a saturation of reflectivity at high moisture contents. As the surface proceeds to dry further, deeper layers produce coherent interference patterns in the reflectivity response, particularly at the higher frequencies.     

与GaAs基VCSEL同材料体系高对比度亚波长光栅的设计

设计了一种用于850 nm GaAs基VCSEL的高折射率对比度亚波长光栅(HCG),整体结构采用GaAs材料体系,包含光栅层及为缓解其应力问题而设计的应力缓冲层和以AlGaAs或AlAs氧化后形成的AlOx低折射率亚层。通过Rsoft软件对HCG的反射特性进行仿真研究,分析了不同光栅参数对反射谱的作用规律,重点探究了应力缓冲层和低折射率亚层对光栅特性的影响。设计了中心波长850 nm的TM模HCG,反射率大于99. 9%的带宽可达91 nm,与中心波长之比达到10. 7%,同时TE模的反射率不超过90%,显示出了良好的偏振选择性。该结构可以替代VCSEL中的P型分布式布拉格反射镜,提供高反射率、宽带宽,并改善由不同材料体系所导致的应力问题,提高器件稳定性。     

Toward two-dimensional self-organization of nanostructures using wafer bonding and nanopatterned silicon surfaces

The structure of ultrathin silicon layers obtained by molecular hydrophobic bonding is investigated. The twist and tilt angles between the two crystals are accurately controlled. The buried Si|Si interface is observed by transmission electron microscopy and by grazing incidence X-ray techniques. For low twist angle values (/spl psi/<5/spl deg/) plane view observations reveal well-defined dislocation networks. Cross-section observations give evidence that the dislocation networks are localized at the bonding interfacial plane with no threading dislocation. Grazing incidence small angle X-ray scattering measurements confirm the good quality of the bonding interface as well as the quality of the dislocation networks. Grazing incidence X-ray diffraction is also used and shows the long-range order of the periodic strain field in the silicon layer. It shows, especially, the interaction between the dislocations. X-ray reflectivity was employed and estimated that the interfacial thickness (i.e., thickness of the bonding) lower than 1 nm decreases when the twist angle increases. The nanopatterned surface is then investigated by scanning tunneling microscopy and X-ray methods. To validate these substrates for long-range order self-organization, the growth of Si and Ge quantum dots is finally achieved.