Strain relaxation in SiGe layer during wet oxidation process

The strain relaxation in SiGe layer on silicon substrate during wet oxidation at 1000 °C was investigated. It was proposed that the competition between Ge accumulation and diffusion led to different strain-relaxation behaviors. At the very beginning, Ge atoms at the oxidizing interface were quickly accumulated due to the high oxidation rate resulting in the additional nucleation of misfit dislocations (therefore a lot of threading dislocations) to relieve stress after the thickness of the Ge condensed layer was larger than the critical value. And then, when the Ge accumulation rate was less than the diffusion rate, Ge content started to decrease from a maximum value and the strain in the SiGe layer was mainly relieved through surface roughing and the degree of strain relaxation reached a maximum. When the samples were further oxidized, Ge accumulation could be neglected because of the self-limiting oxidation and the Ge diffusion dominated the consequent processes. As a result, Ge content at the interface was reduced, with the contribution of the strain relaxation in SiO₂ viscously, leading to the decrease of degree of strain relaxation in the SiGe layers slowly.     

Effect of oxidizing environments on the diffusion-segregation boron distribution in the thermal silicon dioxide-silicon system

The diffusion-segregation boron distribution in the silicon dioxide-silicon system upon oxidation in different environments is studied by secondary-ion mass spectrometry and numerical simulation. The coefficient of boron segregation at the SiO₂/Si interface and the enhancement of boron diffusion in silicon as functions of the type of oxidizing environment (dry oxygen, wet oxygen, and the presence of hydrochloric acid vapor), the orientation of the silicon surface, and the temperature of oxidizing annealing are obtained. A qualitative model is proposed based on the assumption that the segregation mass transfer of boron through the SiO₂/Si interface is associated with the generation of nonequilibrium intrinsic interstitials.     

磷通过热生长二氧化硅层在硅中的扩散

用四探针测量薄层电导方法及阳极氧化去层技术,测定了磷在硅中扩散的具体分布,在恒表面浓度下,它们偏离余误差函数分布。如认为这是由于扩散系数是杂质浓度的函数,实验得到了当杂质浓度大于10¹⁹原子/厘米³时,扩散系数随杂质浓度增加而增大的强烈依赖关系。用同样方法测定了磷通过二氧化硅层后在硅中扩散的具体分布,研究了这些杂质分布的特性,实验表明,不同厚度的氧化层在1300℃高温下仍具有掩蔽效应,在完全掩蔽失效时间附近,杂质分布的共同特点是表面浓度较低(～10¹⁷原子/厘米³)、结较浅(～1微米)。对不同厚度的氧化层,经过足够的时间后,硅中表面浓度不受氧化层厚度的影响,而只由扩散源的蒸气压决定。磷通过氧化层后扩散的具体分布情况还与扩散源的性质、条件等密切相关。扩散过程中观察到的氧化层厚度增长有可能影响表面附近杂质的具体分布情况。     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

X-ray spectral analysis of the interface of a thin Al<Subscript>2</Subscript>O<Subscript>3</Subscript> film prepared on silicon by atomic layer deposition

The extent and phase chemical composition of the interface forming under atomic layer deposition (ALD) of a 6-nm-thick Al₂O₃ film on the surface of crystalline silicon (c-Si) has been studied by depthresolved, ultrasoft x-ray emission spectroscopy. ALD is shown to produce a layer of mixed Al₂O₃ and SiO₂ oxides about 6–8 nm thick, in which silicon dioxide is present even on the sample surface and its concentration increases as one approaches the interface with the substrate. It is assumed that such a complex structure of the layer is the result of interdiffusion of oxygen into the layer and of silicon from the substrate to the surface over grain boundaries of polycrystalline Al₂O₃, followed by silicon oxidation. Neither the formation of clusters of metallic aluminum near the boundary with c-Si nor aluminum diffusion into the substrate was revealed. It was established that ALD-deposited Al₂O₃ layers with a thickness up to 60 nm have similar structure.     

Plasma treatment studies of MIS devices

Silicon nitride films have emerged as the possible future dielectrics for ultra large scale integration (ULSI). Because the interface state density of silicon nitride/silicon interface in metal insulator semiconductor (MIS) configuration is more than an order of magnitude larger than that of silicon dioxide/silicon interface, plasma treatment studies on silicon nitride films have been undertaken for the possible improvement. Accordingly, silicon nitride films of various composition have been prepared by plasma enhanced chemical vapor deposition (PECVD) system using silane(SiH₄) and ammonia(NH₃) with nitrogen(N₂) as the diluent and MIS devices have been fabricated with as well as without plasma treated silicon nitride as the insulator. A considerable improvement in the silicon nitride/silicon interface is observed on ammonia plasma treatment while nitrous oxide(N₂O) plasma treatment studies have resulted in the establishment of a novel plasma oxidation process.     

Low-temperature deposition of stoichiometric HfO2 on silicon: Analysis and quantification of the HfO2/Si interface from electrical and XPS measurements

An understanding of the exact structural makeup of dielectric interface is crucial for development of novel gate materials. In this paper a study of the HfO₂/Si interface created by the low-temperature deposition ultrathin stoichiometric HfO₂ on Si substrates by reactive sputtering is presented. Analysis, quantification and calculation of layer thickness of an HfO₂/Hf-Si-O_x/SiO₂ gate stack dielectrics have been performed, using X-ray photoelectron spectroscopy (XPS) depth profile method, angle resolved XPS and interface modeling by XPS data processing software. The results obtained were found to be in good agreement with the high frequency capacitance-voltage (C-V) measurements. The results suggest a development of a complex three layer dielectric stack, including hafnium dioxide layer, a narrow interface of hafnium silicate and broad region of oxygen diffusion into silicon wafer. The diffusion of oxygen was found particularly detrimental to the electrical properties of the stack, as this oxygen concentration gradient leads to the formation of suboxides of silicon with a lower permittivity, κ.     

A model for phosphorus segregation at the silicon-silicon dioxide interface

A new model for phosphorus segregation at the Si-SiO₂ interface is derived and verified by experimental data. The model considers for the first time, a third phase, the interface layer itself, in addition to the Si and SiO₂ phases, and the dynamics of the three-phase system is described in terms of rate equations. In particular, the phosphorus compound formation in the interface layer (phosphorus pile-up), which renders the dopant electrically inactive to a large extent, is described as a competition of the dopant in silicon and in silicon dioxide in filling and depleting a constant density of interface traps. Our model allows an unambiguous correlation of the dopant concentration on both sides of the interface with the integral dose of the interface phosphorus pile-up. Experimental data for different phosphorus concentrations, different temperatures, and different oxidation ambients, including inert anneals, are fitted by a single curve.     

Relation between oxidation microstructure and the maximum energy product loss of a Sm2Co17 magnet oxidized at 500 ℃

The oxidation microstructure and maximum energy product (BH)_max loss of a Sm(Co_0.76, Fe_0.1, Cu_0.1, Zr_0.04)₇ magnet oxidized at 500 ℃ were systematically investigated. Three different oxidation regions were formed in the oxidized magnet: a continuous external oxide scale, an internal reaction layer, and a diffusion zone. Both room-temperature and high-temperature (BH)_max losses exhibited the same parabolic increase with oxidation time. An oxygen diffusion model was proposed to simulate the dependence of (BH)_max loss on oxidation time. It is found that the external oxide scale has little effect on the (BH)_max loss, and both the internal reaction layer and diffusion zone result in the (BH)_max loss. Moreover, the diffusion zone leads to more (BH)_max loss than the internal reaction layer. The values of the oxidation rate constant k for internal reaction layer and oxygen diffusion coefficient D for diffusion zone were obtained, which are about 1.91 × 10^-10 cm²/s and 6.54 × 10^-11 cm²/s, respectively.     

纳米尺度下Si/Ge界面应力释放机制的分子动力学研究

采用分子动力学方法研究了纳米尺度下硅(Si)基锗(Ge)结构的Si/Ge界面应力分布特征,以及点缺陷层在应力释放过程中的作用机制.结果表明:在纳米尺度下, Si/Ge界面应力分布曲线与Ge尺寸密切相关,界面应力下降速度与Ge尺寸存在近似的线性递减关系;同时,在Si/Ge界面处增加一个富含空位缺陷的缓冲层,可显著改变Si/Ge界面应力分布,在此基础上对比分析了点缺陷在纯Ge结构内部引起应力变化与缺陷密度的关系,缺陷层的引入和缺陷密度的增加可加速界面应力的释放.参考对Si/Ge界面结构的研究结果,可在Si基纯Ge薄膜生长过程中引入缺陷层,并对其结构进行设计,降低界面应力水平,进而降低界面处产生位错缺陷的概率,提高Si基Ge薄膜质量,这一思想在研究报道的Si基Ge膜低温缓冲层生长方法中初步得到了证实.     

Fe-Cr-Al合金氧化膜形成机理电子理论研究

通过自编软件建立了Fe-Cr-Al合金表面、氧化膜/基体界面模型,采用递归法计算了合金元素在Fe-Cr-Al合金表面、氧化膜/基体界面的环境敏感镶嵌能、亲和能、结合能、态密度等电子结构参数.从电子层次系统研究了Fe-Cr-Al合金氧化膜的形成机理、稀土元素和杂质硫对氧化膜形成过程及黏附性的影响机理.研究表明Fe-Cr-Al合金中Al的偏聚驱动力远大于Y,Cr.氧化初期氧从合金表面向合金内部扩散,合金内部Al向合金表面扩散,使合金形成富铝、氧表面层;氧与Al间的亲和力较大（亲和能低）,氧原子容易与Al结合生成Al₂O₃保护膜;合金中加入Y后,Y在合金表面偏聚,抑制Al向合金表面扩散,氧化膜的横向生长得到有效控制,从而避免氧化膜皱褶形貌的发生,提高氧化膜的黏附性;合金内部的S通过扩散汇集在基体/氧化膜界面,S使界面区原子的总能增高,总态密度降低,减小了界面的稳定性,进而削弱氧化膜与合金基体的结合力. 关键词： 电子结构 高温氧化 Fe-Cr-Al合金     

Dipole formation and electrical properties of high-k/SiO2 interface according to the density of SiO2 interfacial layer

The effect of SiO₂ buffer layers with various atomic densities on the interface dipole of high-k/SiO₂ is confirmed. An ultrathin SiO₂ layer is formed on Si using the plasma-enhanced chemical vapor deposition (PECVD), H₂O₂ oxidation and nitric acid oxidation (NAOS). The atomic density ratio between the SiO₂ layer with various methods and the high-k is calculated respectively. As the oxygen density of the SiO₂ increased, the amount of the dipole and the flatband voltage (V_FB) shift decreased. Furthermore, leakage current density of the H₂O₂ (0.9 × 10⁻² A/cm²) due to the formation of low-density SiO₂, decreases by approximately six orders of magnitude when SiO₂ buffer layer is inserted using the NAOS (5.13 × 10^-8 A/cm²). Consequently, it is demonstrated that the dipoles that affects the V_FB shift is formed by the diffusion of oxygen ions between the high-k and SiO₂ interface, which has a significant effect of the MOS capacitor.     

Low temperature diffusion of oxygen in titanium and titanium oxide films

Work in our laboratory and elsewhere indicates that the thermal ignition characteristics of Ti-based pyrotechnics are controlled by diffusion of oxygen from a surface coating into the bulk of the metal. Diffusion of oxygen in Ti at temperatures greater than 700 K has previously been modeled using Fick's law. No work has been reported at lower temperature, and the results from previous high temperature oxidation studies are both inconsistent and insufficient to define diffusion behavior for the temperature range 400 to 800 K. We have conducted Ti oxidation experiments in oxygen ambients of 3 to 700 Torr, at temperatures from 400 to 800 K, for periods from 1 to 100 h. Oxygen concentration profiles were determined by Auger analysis combined with sputter depth profiling. Calibration of the Auger atomic concentrations were confirmed by Rutherford backscattering spectroscopic measurements. Results show growth of four distinct oxide regions which are consistent with the titanium-oxygen phase diagram. Growth of the oxide regions is independent of the oxygen partial pressure and appears to be diffusion controlled. Fick's law with a constant diffusion coefficient was assumed, and an Arrhenius expression for the oxygen diffusion coefficient in each region was evaluated from the experimental data. Activation energies for oxygen diffusion range from about 9 to 43 kcal/mol, and indicate that the thermal ignition mechanism is controlled by growth of a TiO₂ region adjacent to the gas-solid interface.     

The thermal oxidation of silicon the special case of the growth of very thin films

The thermal oxidation of silicon is generally modelled by Deal and Grove's theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO₂ interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.     

Radiation response of strained Si---SiO2 layered system

In the presented paper radiation effects in silicon-silicon dioxide systems in the presence of additional tensile and compressive stresses were investigated. These stresses were produced by placing additional layers of SiO₂ or Si₃N₄ on the back side of oxidized silicon wafers.The results obtained indicate that relaxation takes place in the oxide during irradiation and leads to the decrease of the compressive stress in the oxide film. Relaxation of strained bonds causes defect generation and the increase of the oxide charge density. This increase is larger for oxides with higher level of compressive stress. No significant changes of the density of surface states were observed since the additional coatings did not affect the strain gradient in the SiO₂.     

Thermal oxidation temperature dependence of 4H-SiC MOS interface

The thermal oxidation temperature dependence of 4H-silicon carbide (SiC) is systematically investigated using X-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) measurements. When SiC is thermally oxidized, silicon oxycarbides (SiC_xO_y) are first grown and then silicon dioxide (SiO₂) is grown. It is identified by XPS that the SiO₂ films fall into two categories, called SiC-oxidized SiO₂ and Si-oxidized SiO₂ in this paper. The products depend on thermal oxidation temperature. The critical temperature is between 1200 and 1300 °C. The interface trap density (D_it) of the sample possessing Si-oxidized SiO₂, at thermal oxidation temperature of 1300 °C, is lower than SiC-oxidized SiO₂ at and below 1200 °C, suggesting that a decrease of the C component in SiO₂ film and SiO₂/SiC interface by higher oxidation temperature improves the metal-oxide-semiconductor (MOS) characteristics.     

Analysis of rapid dry oxidation of silicon

By analyzing the most recent models on rapid initial oxidation and the experimental data at low temperatures we prove unambiguously that neither enhanced nor retarded oxygen diffusion nor any kind of additional oxygen transport flux can account for anomalous initial regime of silicon dry oxidation.The rapid growth is mainly due to the enhanced oxygen solubility and partly to the enhancement of the reaction rate constantk _s. We argue that the reaction rate depends linearly on the oxygen solubility for low solubilities pertinent to dry oxidation but that it saturates at high solubilities characteristic for the wet oxidation.     

Oxidation study of silicon nanoparticle thin films on HOPG

Thin films of silicon nanoparticles (diameter 5-10 nm) were deposited on highly oriented pyrolytic graphite (HOPG) by low-pressure DC magnetron sputtering. The effect of different room-temperature oxidation techniques was investigated using XPS sputter-depth profiling. Both oxygen treatment during deposition (using an argon-oxygen mixture in the sputter gas) as well as post-deposition oxidation techniques (exposure to oxygen plasma beam, ambient air conditions) were studied. In all cases oxidation was found to involve the whole film down to the film/substrate interface, indicating a network of open pores. Depending on the type of oxidation between 15 and 25 at% of oxygen, mostly associated with low oxidation states of silicon, were detected in the interior of the film and attributed to oxidized surfaces of the individual silicon nanoparticles. The highest oxygen concentrations were found at the very film surface, reaching levels of 25-30% for films exposed to air or prepared by reactive magnetron sputtering. For the oxygen plasma-treated films even oxygen surface concentrations around 45% and fully oxidized silicon (i.e., SiO₂) were achieved. At the Si/HOPG interface formation of silicon carbide was observed due to intermixing induced by Ar-ion beam used for sputter-depth profiling.     

Theoretical study of sorption and diffusion of lithium atoms on the surface of crystalline silicon and inside it

The energy of the sorption and diffusion of lithium atoms on the reconstructed (4 × 2) (100) silicon surface in the process of their transport into near-surface layers, as well as inside crystalline silicon, at various lithium concentrations have been investigated within the density functional theory. It has been shown that single lithium atoms easily migrate on the (100) surface and gradually fill the surface states (T3 and L) located in channels between silicon dimers. The diffusion of lithium into near-surface silicon layers is hampered because of high potential barriers of the transition (1.22 eV). The dependences of the binding energy, potential barriers, and diffusion coefficient inside silicon on distances to the nearest lithium atoms have also been examined. It has been shown that an increase in the concentration of lithium to the Li_0.5Si composition significantly reduces the transition energy (from 0.90 to 0.36 eV) and strongly increases (by one to three orders of magnitude) the lithium diffusion rate.     

A kinetic Monte Carlo study of the initial stage of silicon oxidation: Basic mechanisms-induced partial ordering of the oxide interfacial layer

A kinetic Monte Carlo study of the early stage of silicon oxidation is presented. The model assembles the most recently published dedicated surface mechanisms: oxygen incorporations, migrations, charge transfer effects. Simulations of the thermal oxidation at typical manufacturing temperature and pressure conditions are discussed. As revealed recently through Density Functional Theory investigations, we observe hexagonal patterns that can be here extended over the surface giving rise to a new model system of the Si/SiO₂ interface as well as new associated specific defects. We show that our simulator is able to reproduce correctly the oxidation states of the silicon atoms which are specific of the Si/SiO₂ interface.