Si_3N_4/Si表面Si生长过程的扫描隧道显微镜研究

利用原位扫描隧道显微镜和低能电子衍射分析了Si的纳米颗粒在Si3N4 /Si(111)和Si3N4 /Si(10 0 )表面生长过程的结构演变 .在生长早期T为 35 0— 10 75K范围内 ,Si在两种衬底表面上都形成高密度的三维纳米团簇 ,这些团簇的大小均在几个纳米范围内 ,并且在高温退火时保持相当稳定的形状而不相互融合 .当生长继续时 ,Si的晶体小面开始显现 .在晶态的Si3N4 (0 0 0 1) /Si(111)表面 ,Si的 (111)小面生长比其他方向优先 ,生长方向与衬底Si(111)方向一致 .最后在大范围内形成以 (111)为主的晶面 .相反 ,在非晶的Si3N4 表面 ,即Si3N4 /Si(10 0 ) ,Si晶体的生长呈现完全随机的方向性 ,低指数面如 (111)和 (10 0 )面共存 ,但它们并不占据主导地位 ,大部分暴露的小面是高指数面如 (113)面 .对表面生长过程进行了探讨并给出了合理的物理解释     

Si3N4在h-AlN上的晶体化与AlN/Si3N4纳米多层膜的超硬效应

采用反应磁控溅射法制备了一系列具有不同Si₃N₄层厚度的AlN/Si₃N₄纳米多层膜,利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.研究了Si₃N₄层在AlN/Si₃N₄纳米多层膜中的晶化现象及其对多层膜生长结构与力学性能的影响.结果表明,在六方纤锌矿结构的晶体AlN调制层的模板作用下,通常溅射条件下以非晶态存在的Si₃N₄层在其厚度小于约1nm时被强制晶化为结构与AlN相同的赝形晶体,AlN/Si₃N₄纳米多层膜形成共格外延生长的结构,相应地,多层膜产生硬度升高的超硬效应.Si₃N₄随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.分析认为,AlN/Si₃N₄纳米多层膜超硬效应的产生与多层膜共格外延生长所形成的拉压交变应力场导致的两调制层模量差的增大有关. 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 赝晶体 超硬效应     

AlN/Si3N4纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN, Si₃N₄薄膜和不同调制周期的AlN/Si₃N₄纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si₃N₄层的晶体结构和多层膜的硬度依赖于Si₃N₄层的厚度.当AlN层厚度为4.0nm、 Si₃N₄层厚度 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 应力场 超硬效应     

Sr/Si界面沉积SrTiO3初始生长阶段的扫描遂道显微术研究

本文工作利用脉冲激光沉积术(PLD)和超高真空扫描隧道显微术(UHV-STM),研究了在Sr/Si(001)-(2×1)衬底表面上真空室温沉积几个单层SrTiO₃薄膜的初始生长过程.经660 ℃退火处理后,Sr/Si衬底表面上形成了纳米岛状结构.经分析,这些纳米小岛为C49-TiSi₂和 C54-TiSi₂.实验结果表明,在没有氧气的情况下退火,Sr/Si界面无法有效阻止SrTiO₃薄膜与Si衬底之间的相互作用. 关键词： 脉冲激光沉积术(PLD) 扫描隧道显微镜(STM) 3')" href="#">SrTiO₃ 2')" href="#">C54-TiSi₂     

改善Si3N4俘获层过擦现象的第一性原理研究

采用第一性原理方法对如何改善电荷俘获存储器的过擦现象进行了研究. 过擦是由于氮空位中Si原子对电荷的局域能力弱导致, 因此, 在Si₃N₄超胞中分别建立了以C, N, O替换氮空位中的Si原子的缺陷结构作为本文的研究模型. 分别计算了擦写之后体系的巴德电荷分布、相互作用能、态密度, 借以分析替位原子对过擦的影响. 巴德电荷分布的计算结果表明, Si₃N₄在O替位128号Si后的过擦现象被明显改善; C替位128号Si也可以改善过擦, 但由于C替位对电荷的局域作用变弱, 不利于电荷的存储实现; N替位128号Si则不能改善过擦; 而在162和196号Si位置, 三种原子的替换均无法改善过擦现象. 相互作用能的研究表明, 在128号Si位置, 三种原子都能够和氮空位形成团簇, 在体系中稳定存在. 特别地, O替位Si后, 体系中两缺陷的相互吸引作用最弱, 从而写入的电荷能够短暂的打破O团簇的稳定性, 实现电荷重构, 将电荷局域在O团簇周围. 此外, 态密度的分析结果表明O在128号Si位置能够在Si₃N₄禁带中引入深能级缺陷, 深能级局域电荷的能力强. 以上分析证明, O替位可以很好的改善Si₃N₄中的过擦现象. 本文的研究结果为电荷俘获存储器改善过擦提供了一种方法, 对提高器件的电荷保持特性和优化存储窗口具有指导意义.     

Si基Si3N4/SiO2双层膜驻极体的电荷储存与输运

利用等温表面电位衰减及热刺激放电(thermally stimulated discharge,TSD)方法探讨了恒栅压电晕充电经常压化学气相沉积(APCVD)的Si基Si3N4和热生长SiO₂双层薄膜驻极体电荷的存储特性.结果表明:在常温环境中,300℃高温下,以及95%相对湿度时的60℃条件下,所有试样表现出极好的电荷储存稳定性.对于负电晕充电试样,其电荷输运受慢再捕获效应(slow retrapping effect)控制;用热离子发射模型来描述了正电晕充电Si₃N₄/SiO₂驻极体的正电荷输运特性. 关键词： 驻极体 薄膜 电荷储存 热离子发射     

a-Si3N4纳米粒子的激光法制备及能级结构研究

给出激光化学汽相沉积法制备a-Si₃N₄纳米粒子的原理和经验公式,在特定工艺参数下获得平均粒径为6.5nm的优质a-Si₃N₄纳米粒子,用紫外光谱研究其能级结构,发现与小粒子有关的峰状光谱结构和能带分裂现象,描述了a-Si₃N₄纳米粒子的物理结构图象,确认硅错键≡Si—Si≡,硅悬挂键≡Si₃⁰在富硅a-Si₃ 关键词：     

Sr/Si(100)表面TiSi2纳米岛的扫描隧道显微镜研究

为了解半导体衬底与氧化物之间存在的相互作用,以及量子尺寸效应对不同再构体的影响,制备了1—2个原子层厚的TiSi₂/Si(100)纳米岛,并使用扫描隧道显微镜(STM)表征手段详细地研究了TiSi₂ /Si(100)纳米岛的电子和几何特性. 结果发现:这些纳米岛表面显示出明显的金属性;其空态STM图像具有典型的偏压依赖性:在高偏压下STM 图像由三聚物形成的单胞构成,并在低偏压下STM 图像显示为密堆积的图案,这些不同的图案反映出不同能量位的态密度有明显差异. 关键词： 2纳米岛')" href="#">TiSi₂纳米岛 Sr/Si(100)表面 扫描隧道显微镜     

(Ca3N2)n(n=1—4)团簇结构与性质的密度泛函理论研究

用密度泛函理论(DFT)的杂化密度泛函B3LYP方法在6-31G^*基组水平上对(Ca₃N₂)_n(n=1—4)团簇各种可能的构型进行几何结构优化,预测了各团簇的最稳定结构.并对最稳定结构的振动特性、成键特性、电荷特性和稳定性等进行了理论分析.结果表明,(Ca₃N₂)_n(n=1—4)团簇最稳定构型中N原子为3—5配位,Ca—N键长为0.231—0.251nm,Ca—Ca键长为0.295—0.358nm;N原子的自然电荷在-1.553e—-2.241e之间,Ca原子的自然电荷在1.035e—1.445e之间,Ca和N原子间相互作用呈现较强的离子性,Ca₃N₂和(Ca₃N₂)₃团簇有相对较高的动力学稳定性. 关键词： 3N₂)_n(n=1—4)团簇')" href="#">(Ca₃N₂)_n(n=1—4)团簇 密度泛函理论 结构与性质     

离子注入Zn的Si(001)基片热氧化制备纳米ZnO团簇及其生长行为研究

采用离子注入技术将Zn离子注入Si（001）基片,并在大气环境下加热氧化制备了ZnO纳米团簇.利用电子探针、薄膜X射线衍射仪、原子力显微镜和透射电子显微镜,对注入和热氧化后的薄膜成分、表面形貌和微观结构进行表征,探讨了热氧化温度以及注入剂量对纳米ZnO团簇的成核过程及生长行为的影响.结果表明,Zn离子注入到Si基片表面后形成了Zn纳米团簇,热氧化过程中Zn离子向表面扩散,在表面SiO₂非晶层和Si基片多晶区的界面处形成纳米团簇.热氧化温度是影响ZnO纳米团簇结晶质量的一个重要参数.随着热氧化温度的升高,金属Zn的衍射峰强度逐渐变弱并消失,而ZnO的（101）衍射峰强度逐渐增强.当热氧化温度高于800 ℃以后,ZnO与SiO₂之间开始发生化学反应形成Zn₂SiO₄. 关键词： ZnO纳米团簇 离子注入 微观结构 形貌分析     

Formation of CrSi2 nanoislands on Si(111)7 × 7 and epitaxial growth of silicon overlayers in Si(111)/CrSi2 nanocrystallites/Si heterostructures

Low-energy electron diffraction and differential reflectance spectroscopy are used to study the self-formation of chromium disilicide (CrSi₂) nanoislands on a Si(111) surface. The semiconductor properties of the islands show up even early in chromium deposition at a substrate temperature of 500°C, and the two-dimensional growth changes to the three-dimensional one when the thickness of the chromium layer exceeds 0.06 nm. The maximal density of the islands and their sizes are determined. The MBE growth of silicon over the CrSi₂ nanoislands is investigated, an optimal growth temperature is determined, and 50-nm-thick atomically smooth silicon films are obtained. Ultraviolet photoelectron spectroscopy combined with the ion etching of the specimens with embedded nanocrystallites demonstrates the formation of the valence band, indicating the crystalline structure of the CrSi₂. Multilayer epitaxial structures with embedded CrSi₂ nanocrystallites are grown.     

Si3N4Si量子点电子结构的理论研究

采用经表面优化的对称球形团簇作Ｓｉ₃Ｎ₄，Ｓｉ晶态量子点的模型，利用紧束缚近似和ｒｅｃｕｒｓｉｏｎ方法研究了它们的电子结构，给出了导带底和价带顶位置随量子点尺寸的变化。得到了３２８原子Ｓｉ₃Ｎ₄量子点、３２３原子Ｓｉ量子点的中心原子局域态密度及平均态密度，并讨论了态密度和光谱结构的关系，中心原子局域态密度能较好地描述量子点的光谱，这一点得到了实验结果的证实。 关键词：     

Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

Time-resolved observation of CVD-growth of silicon on Si(111) with STM

Scanning tunneling microscopy is used to study the epitaxial growth of silicon on Si(111)-(7×7) by Chemical Vapour Deposition (CVD) of disilane (Si₂H₆) at elevated substrate temperatures directly during the growth process. Different kinetic processes, as island nucleation, growth and coarsening and step flow have directly been imaged as a function of temperature and Si₂H₆ flow. On a substrate with a low defect concentration several growth modes depending on the flux and the total coverage are distinguished: the formation of multi-level islands as a transient mode leaving the substrate partially uncovered up to 20 bilayers, a transition to layer-by-layer growth when the multi-level islands initially formed coalesce and the formation of three-dimensional islands with tetrahedral shape at higher growth rates which are only metastable due to the presence of hydrogen at the surface. The equilibrium shape of two-dimensional islands is a hexagon whereas the kinetically influenced shape during growth is triangular.     

Interfacial reaction barriers during thin-film solid-state reactions: The crystallographic origin of kinetic barriers at the NiS2/Si(111) interface

Epitaxial NiSi₂ islands have been grown on Si(111) substrates by the direct reaction of nickel vapour with the silicon substrate in ultra-high vacuum at 400° C. Growth kinetics was shown to depend on the orientation of the islands: A-oriented islands grow about ten times faster than B-oriented ones, with the ratio of the advance rates of the main growth fronts even reaching 30. Applying plan-view transmission electron microscopy and high-resolution electron microscopy of cross sections, a corresponding difference was found in the structure of the NiSi₂/Si(111) growth front: Steps at the B-oriented growth front were of three or six interplanar (111) spacings in height, whereas at the A-oriented growth front step-like defects of less than one interplanar (111) spacing in height were observed. These observations are explained by an atomic-scale model of the solid-state reaction, which involves the diffusion of nickel to the interfaces and the nucleation and subsequent lateral propagation of interfacial steps. The difference in the reaction kinetics originates from the presence of kinetic reaction barriers at the NiSi₂/Si(111) growth fronts, the barrier at the B-front being higher owing to the lower formation rate of steps of triple atomic height than that of steps of lower height at the A-NiSi₂/Si(111) growth front.     

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

Reactive epitaxy of cobalt disilicide on Si(100)

The growth of cobalt disilicide on the Si(100) surface by reactive epitaxy at T=350°C was studied within the 10–40 ML cobalt coverage range. A new method of mapping the atomic structure of the surface layer by inelastically scattered medium-energy electrons was employed. The films thus formed were shown to consist of CoSi₂(221) grains of four azimuthal orientations turned by 90° with respect to one another. This domain structure originates from substrate surface faceting by (111) planes, a process occurring during silicide formation. B-oriented CoSi₂(111) layers grow epitaxially on (111) facets.     

Epitaxial growth of fully relaxed Si0.75Ge0.25 on SOI substrate

A fully relaxed Si_0.75Ge_0.25 film with low dislocation densities is fabricated by epitaxial growth on SOI substrate without depositing graded buffers. The relaxation mechanism of the SiGe layer directly grown on SOI substrate is also analyzed. For SiGe grown on SOI with low Ge content, the strain is redistributed between SiGe and the top Si of SOI substrate, and the strain residing in SiGe layer can be fully relaxed by the formation and expansion of dislocation half-loops near the SiGe/Si interface. The surface morphology and crystal quality of all samples are analyzed by optical microscopy and transmission electron microscopy (TEM), respectively. Compared to the Si_0.75Ge_0.25 layer epitaxially grown on graded buffer, the Si_0.75Ge_0.25 directly grown on SOI substrate appears good surface morphology and perfect crystal quality.     

Influence of a predeposited Si1−x Gex layer on the growth of self-assembled SiGe/Si(001) islands

The growth of self-assembled Ge(Si) islands on a strained Si_1?xGe_x layer (0% < x < 20%) is studied. The size and the surface density of islands are found to increase with Ge content in the Si_1?xGe_x layer. The increased surface density is related to augmentation of the surface roughness after deposition of the SiGe layer. The enlargement of islands is accounted for by the decrease of the wetting layer in thickness due to the additional elastic energy accumulated in the SiGe layer and to enhanced Si diffusion from the Si_1?xGe_x layer into the islands. The increase in the fraction of the surface occupied by islands leads to a greater order in the island arrangement.