Growth and Chemical Thermodynamics Analysis of SiC Film on Si Substrate by Heating Polystyrene/Silica Bilayer Method

采用改进的PS/OCS/Si(111)叠层热解法,在石英高温管式炉中,常压流通Ar气氛下,于1300 o C制备出了无层错空洞缺陷的晶态SiC薄膜．采用红外、X射线衍射和扫描电镜等分析方法对薄膜样品进行了表征．并研究了这种方法制备SiC薄膜的化学热力学过程．通过对反应平衡常数和吉布斯自由能的计算,初步确定了生成SiC的主反应的发生顺序以及反应体系的平衡状态,并论证了流通的Ar气氛对于反应的持续进行是必须的．该方法制备的薄膜为SiO2/SiC/Si(111)结构,这种结构对于半导体MOS器件的应用是非常有利的．该SiO2层形成反应后期降温阶段,并且可以通过RCA清洗工艺(标准半导体清洗工艺)彻底清除．目前还没有发现用这种方法生长SiC薄膜的报道．     

SiC polytypes process affected by Ge predeposition on Si(111) substrates

Structural and optical measurements were performed on silicon carbide (SiC) samples containing several polytypes. The SiC samples investigated were grown on (111) Si substrates by solid source molecular beam epitaxy (SSMBE). Several quantities of Ge were predeposited before the growth procedure. The influence of Ge on the SiC polytypes formation was studied by X-Ray, FTIR and μ-Raman characterizations methods. The spectra of the samples with less than one Ge monolayer exhibit a mixture of 2H, 15R and 3C–SiC polytypes. This mixture is due to the mismatch between the heterostructure layers. We propose that the Ge predeposition in the heterostructure can be used to stabilize and unify the polytypes formation.     

相变区硅薄膜拉曼和红外光谱分析

采用等离子体增强化学气相沉积(PECVD)技术制备了一系列不同氢稀释率下的硅薄膜,采用拉曼散射光谱和傅里叶红外光谱技术研究了非晶/微晶相变区硅薄膜的微观结构变化,将次晶结构(paracrystalline structure)引入到非晶/微晶相变区硅薄膜结构中,提出了次晶粒体积分数(f_p),用来表征硅薄膜中程有序程度。结果表明,氢稀释率的提高导致硅薄膜经历了从非晶硅到微晶硅的相变过程,在相变区靠近非晶相的一侧,硅薄膜表现出氢含量高、结构致密和中程有序度高等特性,氢在薄膜的生长中主要起到表面钝化作用。在相变区靠近微晶相的一侧,硅薄膜具有氢含量低、晶化率高和界面体积分数小等特性,揭示了氢的刻蚀作用主控了薄膜生长过程。采用扫描电子显微镜对样品薄膜的表面形貌进行分析,验证了拉曼散射光谱和傅里叶红外光谱的分析结果。非晶/微晶相变区尤其是相变区边缘硅薄膜结构特性优良,在太阳能电池应用中适合用作硅基薄膜电池本征层。     

Growth mechanism and morphology of Ge on Pb covered Si(111) surfaces

We study the mechanism and surface morphology in epitaxial growth of Ge on Pb covered Si(111) using a scanning tunneling microscope (STM). We find that Ge adatoms have a very large diffusion length at room temperature. The growth is close to perfect layer-by-layer for the first two bilayers. Surface roughness increases gradually with the film thickness, but no 3D islands are found at room temperature. For growth at 200°C, 3D Ge islands appear after completion of the second bilayer. At room temperature, we believe, the Pb layer enhances surface diffusion and the descending-step motion of Ge adatoms, but the ascending-step motion is hindered and thus 3D island growth is suppressed.     

SiC formation on Si(100) via C60 precursors

The interaction between C₆₀ molecules and the Si(100) surface and the preparation of silicon-carbide thin films by thermal reaction of C₆₀ molecules with the Si(100) surface have been investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, reflection high-energy electron diffraction and atomic force microscopy measurements. The effects of annealing temperature and C₆₀ coverage on the SiC formation will be discussed. It is found that the C₆₀ molecules bond covalently with silicon, and the number of bonds increase upon increasing the annealing temperature. Annealing at T≥830°C entails the formation of stoichiometric silicon carbide clusters that coalesce to form a continuous SiC layer when the C₆₀ coverage is greater than one monolayer. Deep pits acting as silicon diffusion channels are present with dimensions that increase with the amounts of C₆₀.

The interaction of C₆₀ with the SiC surface was also investigated. It is found that a similar covalent interaction is present in the two systems C₆₀/Si and C₆₀/SiC.     

Si／SiNx／SiO2多层膜的光致发光

采用射频磁控溅射法,制备了具有强光致可见发光的纳米Si/SiNx/SiO2多层膜,利用傅立叶红外吸收(FTIR)谱,光致发光(PL)谱对其进行了研究。用260nm光激发得到的PL谱中观察到高强度的392nm(3.2eV)和670nm(1.9eV)光致发光峰,分析认为它们分别来自于缺陷态≡Si-到价带顶和从导带底到缺陷态≡Si-的辐射跃迁而产生的光致激发辐射复合发光。PL谱中只有370nm(3.4eV)处发光峰的峰位会受退火温度的影响,结合FTIR谱认为370nm发光与低价氧化物—SiOx(x<2.0)结合体有密不可分的关系。当SiO2层的厚度增大时,发光强度有所增强,800℃退火后出现最强发光,认为具有较大SiO2层厚度的Si/SiNx/SiO2结构多层膜更有利于退火后形成Si—N网络,能够得到更高效的光致发光。用量子限制-发光中心(QCLC)模型解释了可能的发光机制,并建立了发光的能隙态(EGS)模型。     

Growth of manganese silicide films by co-deposition of Mn and Si on Si(1 1 1): A spectroscopic and morphological investigation

MnSi is a ferromagnetic compound with a Curie temperature of 29 K. Recent theoretical studies predict that 2 ML of MnSi epitaxially grown on Si show a ferromagnetic metallic ground state with spin polarization of about 50%. This would allow the development of spintronic devices based on the injection of spin-polarized current from a ferromagnetic metal into a semiconductor.In this context the possibility of growing in situ MnSi on the Si(1 1 1) surface has been explored. Thermal reaction, crystalline structure and electronic properties of the grown films have been studied in situ by photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), and low energy electron diffraction (LEED). Depositing a thin film of Mn on Si(1 1 1) the formation of ordered islands (with dimensions dependent on the amount of deposited Mn) is driven by annealing at selected temperatures, as already observed. Our preliminary studies show that by simultaneously depositing Mn and Si in 1:1 stoichiometry on Si(1 1 1) a large improvement in the homogeneity of the MnSi films is achieved.     

Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

红外光谱法研究射频功率对微晶硅薄膜微结构的影响

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,在硅衬底上以不同的射频功率生长微晶硅(μc-Si:H)薄膜,利用傅里叶变换红外透射光谱技术对薄膜进行测试.通过对红外透射光谱的高斯拟合分析,结果表明薄膜中的氢含量和硅氢键合模式跟射频功率密切相关;当射频率从30W增加到110W时,薄膜中的氢含量先减少后慢慢增加,而结构因子逐渐增加后再减小,并且硅氢键合模式由以SiH为主转变为以SiH2为主.并讨论了这些参量随射频功率变化的机理.     

多孔氧化硅薄膜阴极射线谱的研究

采用近常压下等离子体增强化学气相沉积的方法制备得到多孔氧化硅薄膜,并在沉积区域加载偏压对薄膜形貌和性质进行调制。扫描电镜显示偏压条件下沉积的薄膜更加蓬松多孔,常温下阴极射线谱表明:增大偏压的占空比,CL谱中发光峰的位置并不改变,但谱线强度增大。发光峰的位置说明薄膜中的主要成分是Si-O-Si基团,且该种基团以非桥键的形式存在,薄膜中还存在少量的Si-H键,这与红外光谱测试结果一致。X射线衍射结果表明薄膜中的SiO2是非晶态的。     

一种新型硅基3C-SiC的生长方法及光谱学表征

采用LPCVD技术, 以CH4和H2混合气体为反应源气, 在n-Si（111）衬底上生长3C-SiC晶体薄膜。H2在反应过程中作为稀释气体和运输气体, CH4作为碳源, 硅源有衬底硅来提供。利用X射线衍射分析仪、场发射扫描电子显微镜、激光拉曼光谱和傅里叶变换红外光谱分别研究3C-SiC薄膜的晶相结构、表面形貌及其光谱性质。结果表明此生长方法可以成功的成长出3C-SiC薄膜。     

UHV-MOCVD growth of TiO2 on SiOx/Si(1 1 1): Interfacial properties reflected in the Si 2p photoemission spectra

Metal–organic chemical vapour deposition growth of titanium oxide on moderately pre-oxidised Si(1 1 1) using the titanium(IV) isopropoxide precursor has been studied for two different growth modes, reaction-limited growth at 300 °C and flux-limited growth at 500 °C. The interfacial properties have been characterized by monitoring synchrotron radiation excited Si 2p photoemission spectra. The cross-linking from oxidised Si to bulk Si after TTIP exposure has been found to be very similar to that of SiO_x/Si(1 1 1). However, the results show that the additional oxidation of Si most probably causes a corrugation of the SiO_x/Si interface. Those conclusions are valid for both growth modes. A model is introduced in which the amorphous interface region is described as (TiO₂)_x(SiO₂)_y where x and y changes linearly and continuously over the interface. The model quantifies how (TiO₂)_x(SiO₂)_y mixing changes the relative intensities of the signals from silicon oxide and silicon. The method can be generalised and used for the analyses of other metal-oxides on silicon.     

Quantum oscillations in Pb/Si (111) heterostructure system

This paper summarizes our recent work on the study of quantum size effects (QSE) and novel physical properties of the Pb/Si (111) heterostructure. Two different types of samples were investigated. One is wedge-shaped Pb islands, and the other is atomically flat Pb thin films. With scanning tunneling microscopy (STM) manipulation, we observed an intriguing morphology dynamics of the islands that swings between two extreme energy states, like that in a classical pendulum. We show that the dynamics is a result of the competition between the QSE and the classical step free energy minimizing effect. For the second type of the samples, the QSE is studied in terms of thickness-dependent film stability, electronic structure and physical properties by using STM, angle-resolved photoemission spectroscopy (ARPES) and transport measurement. The results consistently reveal the formation of quantum well states (QWS) due to electron confinement in the films. This size effect could greatly modify the electronic structure near the Fermi level and lead to quantum oscillations in superconductivity, electron-phonon coupling and thermal expansion. The work unambiguously demonstrates the possibility of quantum engineering of physical properties of thin films by exploiting well-controlled and thickness-dependent QSE.     

Properties of the SiO2/SiC interface investigated by angle resolved studies of the Si 2p and Si 1s levels and the Si KLL Auger transitions

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL_2,3L_2,3 Auger transitions from SiO₂/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered √3×√3 reconstructed 4H-SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si⁺¹ and Si⁺⁴, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra.The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si₂O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å.The SiO₂ chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO₂ shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined.The relative final state relaxation energy ΔR(2p) is determined from the modified Auger parameter. This yields a value of ΔR(2p)=−1.7 eV and implies, for SiO₂/SiC, a “true” chemical shift in the Si 2p level of only ≈0.4 eV for oxide layers of up to 10 Å thick.     

Photoinduced non-linear optical diagnostic of SiNxOy/Si111 interfaces

Anisotropic (elliptically polarized) photoinduced second harmonic generation (PISHG) in SiN_xO_y/Si1 1 1 films was proposed for contact-less monitoring of specimens with different nitrogen to oxygen (N/O) ratios. As a source for the photoinducing light, we used a nitrogen Q-switched pulse laser at wavelengths of 315, 337 and 354 nm as well as doubled frequency YAG–Nd laser wavelength (λ=530 nm). The YAG : Nd pulse laser (λ=1.06 μm; W=30 MW; τ=10–50 ps) was used to measure the PISHG. All measurements were done in a reflected light regime. We found that the output PISHG signal was sensitive to the N/O ratio and the film thickness. Measurements of the PISHG versus pumping wavelengths, powers, incident angles as well as independent measurements of the DC-electric field induced second harmonic generation indicate the major role played in this process by axially symmetric photoexcited electron–phonon states. The SiN_xO_y films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and 30 nm and with an N/O ratio between 0 and 1 were obtained. Electrostatic potential distribution at the Si1 1 1–SiN_xO_y interfaces was calculated. Comparison of the experimentally obtained and quantum chemically calculated PISHG data are presented. High sensitivity of anisotropic PISHG to the N/O ratio and film thickness is revealed. The role of the electron–phonon interactions in the dependencies observed is discussed. We have shown that the PISHG method has higher sensitivity than the traditional extended X-ray absorption fine structure spectroscopic and linear optical method for films with the N/O ratio higher than 0.50.     

Manganese silicide single crystal and films deposited on Si(1 1 1): A comparative spectroscopic study

A comparative experimental study is presented of the electronic properties of MnSi films grown on Si(1 1 1) and of MnSi single crystals, using X-ray absorption spectroscopy (XAS), and core level and valence band photoemission spectroscopy (PES). No significant differences in the electronic structure of the two systems can be found.Absorption measurements on the Mn 2p threshold show a mixed valence ground state, where the multiplet structure is washed out by the hybridisation of the Mn 3d states with the Si sp states. These results are also confirmed by photoemission (PE) spectra from the valence band and the Mn 3s, 3p and 2p core levels.Strong attention has been paid to the effect of contamination. The occurrence of multiplet effects in the absorption spectra indicates unambiguously the localisation of the Mn 3d electrons in Mn-O bonds, which strongly influences the electronic properties of these systems.     

Optical second-harmonic investigations of the isothermal desorption of SiO from the Si(100) and Si(111) surfaces

The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10⁻¹ to 10⁻⁶ ML s⁻¹. From their temperature dependence between 780 and 1000 K, activation energies of E_A=3.4±0.2 eV and E_A=4.0±0.3 eV and pre-exponential factors of ν₀=10^16±1 s⁻¹ and ν₀=10^20±1 s⁻¹ for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer.     

Investigation of the formation of silicon nanocrystals by annealing of amorphous SiCx/c-Si structures

In this work, we study the changes in the optical properties of 300-nm-thick hydrogenated amorphous silicon carbide layers after an annealing process. Both intrinsic and phosphorus-doped amorphous silicon carbide layers (a-SiC_x:H) were deposited on silicon wafers by plasma enhanced chemical vapour deposition (PECVD) at 400 °C and annealed in a quartz furnace at 800 °C. The presence of randomly oriented silicon nanocrystals was confirmed by X-ray diffraction (XRD) measurements after the partial recrystallization process only in the doped layers. The presence or the absence of the nanocrystals clearly changes the Fourier transform infrared (FTIR) spectra. From the fitting of the experimental curves with the model of Lorentz oscillators, the refractive index and the extinction coefficient of the different layers were obtained.