A density-functional theory investigation on disorption of O2 on Sn(111) and its comparison with initial oxidation on the X(111) (X=Si,Ge, Sn,Pb) surfaces

The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn(111) 2×2 surface. The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn(111) induced by oxygen adsorption are studied. Based on this, the adsorption behaviours of O₂ on X(111) (X=Si, Ge, Sn, Pb) surfaces are analysed, and the most stable adsorption channels of O₂ on X(111) (X=Si, Ge, Sn, Pb) are identified. The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared. The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Structural,optical,and electrical properties of Cu-doped ZrO_2 films prepared by magnetron co-sputtering

Copper(Cu)-doped ZrO_2(CZO) films with different Cu content(0 at.%～ 8.07 at.%) are successfully deposited on Si(100) substrates by direct current(DC) and radio frequency(RF) magnetron co-sputtering. The influences of Cu content on structural, morphological, optical and electrical properties of CZO films are discussed in detail. The CZO films exhibit ZrO_2 monocline(ˉ111) preferred orientation, which indicates that Cu atoms are doped in ZrO_2 host lattice. The crystallite size estimated form x-ray diffraction(XRD) increases by Cu doping, which accords with the result observed from the scanning electron microscope(SEM). The electrical resistivity decreases from 2.63 ?.cm to 1.48 ?·cm with Cu doping content increasing, which indicates that the conductivity of CZO film is improved. However, the visible light transmittances decrease slightly by Cu doping and the optical band gap values decrease from 4.64 eV to 4.48 eV for CZO films.     

The comparison between CdS thin films grown on Si(111) substrate and quartz substrate by femtosecond pulsed laser deposition

Two kinds of cadmium sulfate (CdS) thin films have been grown at 600 °C onto Si(111) and quartz substrates using femtosecond pulsed laser deposition (PLD). The influence of substrates on the structural and optical properties of the CdS thin films grown by femtosecond pulsed laser deposition have been studied. The CdS thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman spectroscopy. Although CdS thin films deposited both on Si(111) and quartz substrates were polycrystalline and hexagonal as shown by the XRD , SEM and AFM results, the crystalline quality and optical properties were found to be different. The size of the grains for the CdS thin film grown on Si(111) substrate were observed to be larger than that of the CdS thin film grown on quartz substrate, and there is more microcrystalline perpendicularity of c-axis for the film deposited on the quartz substrate than that for the films deposited on the Si substrate. In addition, in the PL spectra, the excitonic peak is more intense and resolved for CdS film deposited on quartz than that for the CdS film deposited on Si(111) substrate. The LO and TO Raman peaks in the CdS films grown on Si(111) substrate and quartz substrate are different, which is due to higher stress and bigger grain size in the CdS film grown on Si(111) substrate, than that of the CdS film grown on the amorphous quartz substrate. All this suggests that the substrates have a significant effect on the structural and optical properties of thin CdS films. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 78.67.-n; 42.62.-b     

Infrared spectroscopic ellipsometry of (Pb, La)(Zr, Ti)O3 thin films on platinized silicon

Lead lanthanum zirconate titanate (PLZT) thin films with different La concentrations (x), whose composition is x/40/60, have been grown directly on Pt/Ti/SiO₂/Si (100) substrates by a modified sol-gel method. X-ray diffraction analysis shows that the PLZT thin films are polycrystalline. The infrared optical properties of the PLZT thin films have been investigated using the infrared spectroscopic ellipsometry in the spectral range of 2.5–12.5 μm. By fitting the measured ellipsometric spectra (Ψ and Δ) with a three-phase model (air/PLZT/Pt), and a derived classical dispersion relation for the thin films, the optical constants and thicknesses of the thin films have been obtained. The refractive index of the PLZT thin films decreases with increasing La concentrations, however, the extinction coefficient increases with increasing La concentrations except for the PLZT(4/40/60) thin films. The values of the effective static charge calculated for the PLZT thin films, which state that PLZT belongs to a mixed ionic-covalent compound, decrease with increasing La concentrations. Moreover, the refractive index at the wavelength of 2.5 μm changes linearly with the effective static charge. The plot of the calculated infrared optical absorption versus wavelength for the Ni/PLZT/Pt multilayer structures with various La concentrations is given and indicates that the PLZT thin films are excellent candidates for ferroelectric infrared detectors and focal plane arrays.     

Formation of high quality gallium nitride thin films on Ga-diffused Si(1 1 1) substrate

High quality gallium nitride thin films have been successfully grown on the Ga-diffused Si(1 1 1) substrates through ammoniating Ga₂O₃ thin films deposited by r.f. magnetron sputtering. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), atomic force microscope (AFM) and photoluminescence (PL) were used to characterize the synthesized samples. The analyses reveal that the formed films are high quality polycrystalline hexagonal gallium nitride. The as-formed GaN films show a flat surface topography with RMS roughness varied from 29 to 48 Å. The strong near-band-edge-emission peak around 368 nm was observed at room temperature. This is a novel method to fabricate GaN thin films based on the direct reaction between Ga₂O₃ and NH₃ on the Ga-diffused Si(1 1 1) substrates.     

反应前驱物中n(S)∶n(Cd)对CdS薄膜结构及发光特性的影响

采用化学水浴以CdCl2·H2O、CS( NH2)2、NH4Cl、NH3·H2O和去离子水作为反应前驱物制备CdS纳米晶薄膜.采用扫描电镜( SEM)、X射线衍射(XRD)、透射光谱和稳态荧光光谱,研究了反应前驱物中不同的n(S)∶n(Cd)对所制备的CdS薄膜的形貌、结构和光学性能的影响.结果表明:反应前驱物中n(S...     

Optical Analysis of Nanocrystalline ZnO Films Coated on Porous Silicon by Radio Frequency (RF) Magnetron Sputtering

Zinc oxide thin films have been deposited onto porous silicon (PSi) substrates at high growth rates by radio frequency (RF) sputtering using a ZnO target. The advantages of the porous Si template are economical and it provides a rigid structural material. Porous silicon is applied as an intermediate layer between silicon and ZnO films and it contributed a large area composed of an array of voids. The nanoporous silicon samples were adapted by photo electrochemical (PEC) etching technique on n-type silicon wafer with (111) and (100) orientation. Micro-Raman and photoluminescence (PL) spectroscopy are powerful and non-destructive optical tools to study vibrational and optical properties of ZnO nanostructures. Both the Raman and PL measurements were also operated at room temperature. Micro-Raman results showed that the A₁(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at around 522 and 530 cm^–1, re- spectively. PL spectra peaks are distinctly apparent at 366 and 368 cm^–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The peak luminescence energy in nanocrystalline ZnO on porous silicon is blue-shifted with regard to that in bulk ZnO (381 nm). The Raman and PL spectra pointed to oxygen vacancies or Zn interstitials which are responsible for the green emission in the nanocrystalline ZnO.     

High pyroelectric Ba0.65Sr0.35TiO3 thin films with Ba0.65Sr0.35RuO3 seeding-layer for monolithic ferroelectric bolometer

Ba_0.65Sr_0.35TiO₃ (BST) thin films were deposited by RF sputtering with a very thin Ba_0.65Sr_0.35RuO₃ (BSR) seeding-layer on Pt/Ti/SiO₂/Si substrate. The crystallization of BST thin films and the surface morphology of BSR seeding-layer were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD patterns show that the BSR seeding-layer affected the orientation of BST thin film, which is highly a-axis textured. It was also found that the BSR seeding-layer had a marked influence on the dielectric properties of BST thin films. Comparing with BST thin films directly deposited on Pt electrode, the dielectric relaxation can be suppressed and dielectric constant increased due to a possible reduction of interface oxygen vacancies at BST/BSR interface. Moreover, J–V measurement indicates that the leakage current density of BST thin films on BSR seeding-layer were greatly reduced compared with that of BST thin films directly on Pt electrodes. The pyroelectric coefficient of BST thin films with BSR seeding-layer is 7.57 × 10⁻⁷ C cm⁻² K⁻¹ at 6 V/μm at room temperature (RT). Our results reveal that high pyroelectric property of BST thin film could be achievable using BSR seeding-layer as a special buffer.     

Effect of Misfit Strain on Pyroelectric Properties of (111) Oriented Pb(Zr1-xTix)O3 Thin Films

Based on the Landau-Dovenshire theory, the thermodynamic potential of (111) oriented films is constructed to investigate the pyroelectric properties of Pb(Zr_1-xTi_x)O₃ thin films. Due to the presence of nonlinear coupling terms resulted from the (111) epitaxy with substrates, the effects of misfit strain and electric field on the phase transitions at room temperature are more complex than that of (001) and (110) oriented films. Pb(Zr_0.5Ti_0.5)O₃ thin films with the Ti composition around the morphotropic phase boundary (MPB) have the giant dielectric and pyroelectric properties. Moreover, Pb(Zr_1-xTi_x)O₃ thin films grown on the tensile substrates which induce the triclinic γ phase have the larger dielectric and pyroelectric properties than that on the compressive substrates. Therefore, the physical properties of (111) oriented Pb(Zr_1-xTi_x)O₃ thin films can be adjusted by choosing the suitable substrates.     

过渡金属与F共掺杂ZnO薄膜结构及磁、光特性

采用溶胶-凝胶法在玻璃衬底上制备了过渡金属元素与F共掺杂Zn_0.98-xTM_xF_0.02O (TM_x=Cu_0.02, Ni_0.01, Mn_0.05, Fe_0.02, Co_0.05)薄膜, 进而利用X射线衍射仪、扫描电子显微镜、紫外-可见透过谱、光致发光及振动样品磁强计等研究了薄膜的表面形貌、微结构、禁带宽度及光致发光(PL)和室温磁学特性. 研究表明: 掺杂离子都以替位的方式进入了ZnO晶格, 掺杂不会破坏ZnO的纤锌矿结构. 其中Zn_0.93Co_0.05F_0.02O薄膜样品的颗粒尺寸最大, 薄膜的结晶度最好且c轴择优取向明显; Zn_0.93Mn_0.05F_0.02O薄膜样品的颗粒尺寸最小, 薄膜结晶度最差且无明显的c轴择优取; Cu, Ni, Fe与F共掺杂样品的颗粒尺寸大小几乎相同. TM掺杂样品均表现出很高的透过率, 同时掺杂后的薄膜样品的禁带宽度都有不同程度的红移. PL谱观察到Zn_0.98-xTM_xF_0.02O薄膜的发射峰主要由较强的紫外发射峰和较弱的蓝光发射峰组成. Zn_0.93Mn_0.05F_0.02O薄膜样品的紫外发光峰最弱, 蓝光发射最强, 饱和磁化强度最大; 与之相反的是Zn_0.96Cu_0.02F_0.02O薄膜, 其紫外发光峰最强, 蓝光发射最弱, 饱和磁化强度最小. 结合微结构和光学性质对Zn_0.98-xTM_xF_0.02O薄膜的磁学性质进行了讨论.     

(Ga,Mn)As光调制反射光谱

室温下我们研究了稀磁半导体(Ga,Mn)As的光调制反射(PR)光谱,观测到来自样品的Franz-Keldysh振荡(FKO)信号。随着Mn原子浓度的增加,PR线形展宽,但是临界点E₀和E₀+Δ₀没有明显的移动。根据FKO振荡数据,计算得到样品表面电场强度随Mn原子掺杂浓度的增加而增强。测量到与Mn原子掺杂相关的杂质带,其能量位置离GaAs价带边~100 meV。根据样品的表面电场强度和表面耗尽层模型,估算样品的空穴浓度为~10¹⁷cm^-3,较低的空穴浓度可能与样品具有较低的居里温度有关,或测量的PR信号来自于样品中外延层的部分耗尽区域。     

钽掺杂二氧化钛薄膜的光电性能研究

采用脉冲激光沉积法 (PLD), 以石英玻璃为衬底制备了钽掺杂TiO₂薄膜并研究了薄膜样品的光电性质. 沉积氧气分气压从0.3 Pa变化到0.7 Pa时薄膜样品的帯隙变化范围是3.26 eV到3.49 eV. 通过测量电阻率随温度的变化关系确定了薄膜内部的主要导电机理. 在150 K到210 K温度范围内, 热激发导电机理是主要的导电机理; 而在10 K到150 K范围内; 电导率随温度的变化复合Mott的多级变程跳跃模型 (VRH); 在210 K到300 K范围内, 电阻率和exp(b/T)^1/2呈正比关系. 关键词： 2')" href="#">Ta掺杂TiO₂ 脉冲激光沉积法 薄膜 导电机理     

Deposition and characterization of carbon nitride films from hexamethylenetetramine/N2 by microwave plasma-enhanced chemical vapor deposition

Carbon nitride thin films were deposited on Si(1 0 0) substrate by microwave plasma-enhanced chemical vapor deposition (PECVD). Hexamethylenetetramine (HMTA) was used as carbon and nitrogen source while N₂ gas was used as both nitrogen source and carrier gas. The sp³-bonded C---N structure in HMTA was considered significantly in the precursor selection. X-ray diffraction analysis indicated that the film was a mixture of crystalline - and β-C₃N₄ as well as graphitic-C₃N₄ and β-Si₃N₄ which were not easily distinguished. Raman spectroscopy also suggested the existence of - and β-C₃N₄ in the films. X-ray photoelectron spectroscopy study indicated the presence of sp²- and sp³-bonded C---N structures in the films while sp³C---N bonding structure predominated to the sp² C---N bonding structure in the bulk composition of the films. N was also found to be bound to Si atoms in the films. The product was, therefore, described as CN_x:Si, where x depends on the film depth, with some evidences of crystalline C₃N₄ formation.     

MgO(111)衬底MgB2 超薄膜的制备和性质研究

利用混合物理化学气相沉积法(HPCVD)在MgO(111)衬底上制备了干净的MgB₂超导超薄膜. 在背景气体压强, 载气氢气流量以及沉积时间一定的情况下, 改变B₂H₆的流量, 制备得到不同厚度系列的MgB₂超导薄膜样品, 并测量了其超导转变温度 T_c, 临界电流密度J_c等临界参量. 该系列超导薄膜沿c轴外延生长, 表面具有良好的连接性, 且有很高的超导转变温度T_c(0) ≈ 35-38 K和很小的剩余电阻率ρ(42 K) ≈ 1.8-20.3 μΩ·cm^-1. 随着膜厚的减小而减小, 临界温度变低, 而剩余电阻率变大. 其中20 nm的样品在零磁场, 5K时的临界电流密度J_c ≈ 2.3×10⁷ A/cm². 表明了利用HPCVD在MgO(111)衬底上制备的MgB₂超薄膜有很好的性能, 预示了其在超导电子器件中广阔的应用前景. 关键词： MgO(111)衬底 2超薄膜')" href="#">MgB₂超薄膜 混合物理化学气相沉积     

Electrodeposited Co-Pt thin films for magnetic hard disks

ew baths for Co-Pt electrodeposition have been developed and developed and ECD thin films (≤0.3μm) have been prepared and characterized structurally (XRD), morphologically (SEM), chemically (EDS) and magnetically (VSM); their improved corrosion, oxidation and wear resistance have been ascertained. Such alloys appear suitable candidates for magnetic storage systems, from all technological viewpoints. The originally formulated baths contain Co-NH₃-citrate complexes and Pt-p salt (Pt(NH₃)₂(NO₂)₂). Co-Pt thin films of fcc structure are deposited obtaining microcrystallites of definite composition. At Pt 30 at% we obtain fcc films with a=0.369 nm, H_C=80 kA m, and high squareness; increasing Co and decreasing Pt content in the bath it is possible to reduce the Pt content of the deposit, obtaining fcc structures containing two types of microcrystals with a = 0.3615 nm and a = 0.369 nm deposited simultaneously. NaH₂PO₂ additions to the bath have a stabilizing influence on the fcc structure of a = 0.3615 nm, Pt 20 at% and H_C as high as 200 kA/m, with hysteresis loops suitable for both longitudinal or perpendicular recording, depending on the thickness. We have prepared 2.5 in. hard disks for magnetic recording with ECD Co-Pt 20 at% with a polished and texturized ACD Ni-P underlayer. Pulse response, 1F & 2F frequency and frequency sweep response behaviour, as well as noise and overwrite characteristics have been measured for both our disks and high-standard sputtered Co-Cr-Ta production disks, showin improved D₅₀ for Co-Pt ECD disks. The signal-to-noise ratio could be improved by pulse electrodeposition and etching post-treatments.     

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.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Effects of rapid thermal annealing on crystallinity and Sn surface segregation of Ge_(1-x)Sn_x films on Si(100) and Si(111)

Germanium-tin films with rather high Sn content(28.04% and 29.61%) are deposited directly on Si(100) and Si(111)substrates by magnetron sputtering. The mechanism of the effect of rapid thermal annealing on the Sn surface segregation of Ge_(1-x)Sn_x films is investigated by x-ray photoelectron spectroscopy(XPS) and atomic force microscopy(AFM). The x-ray diffraction(XRD) is also performed to determine the crystallinities of the Ge_(1-x)Sn_x films. The experimental results indicate that root mean square(RMS) values of the annealed samples are comparatively small and have no noticeable changes for the as-grown sample when annealing temperature is below 400℃. The diameter of the Sn three-dimensional(3 D) island becomes larger than that of an as-grown sample when the annealing temperature is 700℃. In addition, the Sn surface composition decreases when annealing temperature ranges from 400℃ to 700℃. However, Sn bulk compositions in samples A and B are kept almost unchanged when the annealing temperature is below 600℃. The present investigation demonstrates that the crystallinity of Ge_(1-x)Sn_x/Si(111) has no obvious advantage over that of Ge_(1-x)Sn_x/Si(100) and the selection of Si(111) substrate is an effective method to improve the surface morphologies of Ge_(1-x)Sn_x films. We also find that more severe Sn surface segregation occurs in the Ge_(1-x)Sn_x/Si(111) sample during annealing than in the Ge_(1-x)Sn_x/Si(100) sample.     

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.     

Deposition of MgO films by pulsed mid-frequency magnetron sputtering

MgO films were deposited by pulsed mid-frequency magnetron sputtering from metallic targets in the mixture of Ar and O₂ gas. The surface morphology, crystalline structure, and optical properties were characterized by using atomic force microscopy (AFM), X-ray diffraction (XRD), and spectroscopic ellipsometry, respectively. The secondary electron emission coefficients of MgO films were measured by using a self-made apparatus in He gas. A pronounced hysteresis phenomenon of target voltage, current, and deposition rate with increasing and decreasing O₂ flow rate was observed. The structure of films deposited at a metallic mode changes from Mg phase to the mixed Mg and MgO phase, and the films have a very rough surface. All the films deposited at oxide mode have high transparency and smooth surface, and show (2 2 0) preferred orientation growth. The refractive index and extinction coefficient at a wavelength of 670 nm for MgO films deposited at oxide mode with a O₂ flow rate of 3 sccm are 1.698 and 1.16×10⁻⁴, respectively. The secondary emission coefficient at a E/p of 57.8 V/(cm Torr) for MgO films deposited at a O₂ flow rate of 3 sccm is 0.16, which is higher than that of MgO films deposited by e-beam evaporation.