Optical properties and electronic transitions of SnO2 thin films by reflection electron energy loss spectroscopy

Optical properties of SnO₂ thin films in the 4–60 eV energy range are determined by reflection electron energy loss spectroscopy. Bulk and surface electron loss functions, real and imaginary parts of the dielectric function, refraction index, extinction and absorption coefficients are obtained from the analysis of the electron energy loss spectra. Electronic transitions are identified through the interpretation of the optical data. The samples (250–500 nm thick) were produced by ion beam-induced chemical vapor deposition. It is found that the compacity of the SnO₂ thin films affects their optical properties and therefore the relative intensity of the observed electronic transitions. The advantages of this method to determine optical properties of thin films are discussed. Inelastic mean free paths (6.2, 17 and 41 Å for electrons traveling in SnO₂ with kinetic energies of 300, 800 and 2000 eV, respectively) are obtained from the corresponding inelastic electron scattering cross-sections.     

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.     

Interdiffusion in vacuum-deposited dielectric thin films

The observation of interdiffusion imperfection in vacuum-deposited optical thin films is reported. We measured the diffusion lengths of interdiffusion in dielectric thin films of typical combinations of coating materials (TiO₂–SiO₂ and TiO₂–MgF₂) by analyzing the atomic concentration data along the depth obtained with X-ray photoelectron spectroscopy technique. The resultant diffusion lengths in the deposited layers of dielectric coating materials are not small, hence the effects of interdiffusion should be taken into account in the design of optical filters.     

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.     

Optical characterization of amorphous Ga50Se45S5 films

The transmission spectra of thermally evaporated Ga₅₀Se₄₅S₅ films were measured over the wavelength range 300–900 nm. A simple method, suggested by Swanepoel, was used for the determination of the optical constants and thickness of the films. Increasing the thickness of the film beyond 450 nm does not affect the optical constants. The dependence of the absorption coefficient on the photon energy (hν) at the edge of the absorption band is well described by the relation hν=β(hν−E_opt)² with an optical gap equals 2.4 eV. A good fit of the experimental points with Tauc relation indicates that non-direct transition is the most probable mechanism responsible for the photon absorption inside the investigated film.     

Crystal orientation dependence of the in-plane dielectric properties for Ba(Sn0.15Ti0.85)O3 thin films

Barium tin titanate Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films with (1 0 0), (1 1 0) and (1 1 1) orientation were grown on (1 0 0), (1 1 0) and (1 1 1) LaAlO₃ (LAO) single-crystal substrates through sol–gel process, respectively. The in-plane dielectric properties of the films were measured on interdigital capacitor (IDC). Films with the (1 1 1) orientation had larger relative dielectric constant and larger tunability against the dc bias electric field than (1 0 0)- and (1 1 0)-oriented films. This difference in dielectric properties in these three kinds of oriented BTS films may be attributed due to change in the direction and magnitude of electric polarization in orientation engineered BTS films. This work clearly reveals the dielectric properties of BTS films exhibited a strong sensitivity to crystal orientation.     

高压退火对0.65PMN-0.35PT薄膜结构、形貌及电学性能的影响

利用射频磁控溅射技术在LaNiO₃/SiO₂/Si(100)基底上制备了厚 度约为250 nm的0.65PMN-0.35PT(PMN-PT)薄膜. 研究高压氧氛围退火方式对PMN-PT薄膜晶体结构、形貌以及电学性能的影响. 经过XRD测试发现,在高压氧气氛围中, 温度为400℃下退火后的PMN-PT薄膜具有纯的钙钛矿相结构, 具有完全的(100)择优取向, 且衍射峰尖锐, 表明经过高压退火后的薄膜结晶极为充分. SEM表面形貌测试结果显示, 经高压退火处理的PMN-PT薄膜表面呈现出棒状或泡状的形貌. 铁电性能测试表明: 氧气氛围压强4 MPa, 退火时间4h的PMN-PT薄膜样品具有较好的铁电性能, 其剩余极化强度P_r达到10.544 μC/cm², 且电滞回线形状较好, 但漏电流较大, 这可能是由于其微结构所导致.同时介电测试发现: PMN-PT薄膜样品具有极好的介电性能, 其在1 kHz下测试的介电常数ε_r达到913, 介电损耗tgδ 较小, 仅为0.065. 关键词： 射频磁控溅射 高压退火 0.65PMN-0.35PT 介电     

Morphological and electrical properties of SrTiO_3/TiO_2/SrTiO_3 sandwich structures prepared by plasma sputtering

SrTiO_3(STO) and TiO_2 are insulating materials with large dielectric constants and opposite signs of the quadratic coefficient of voltage(α). Insertion of a TiO_2 thin film between STO layers increases the linearity of the capacitance in response to an applied voltage, to meet the increasing demand of large-capacitance-density dynamic random access memory capacitors. Both STO and TiO_2 suffer from the problem of high leakage current owing to their almost equivalent and low bandgap energies. To overcome this, the thickness of the thin TiO_2 film sandwiched between the STO films was varied. A magnetron sputtering system equipped with radio frequency and direct current power supply was employed for depositing the thin films. Ti N was deposited as the top and bottom metal electrodes to form a metal–insulator metal(MIM) structure,which exhibited a very large linear capacitance density of 21 fF/um~2 that decreased by increasing the thickness of the TiO_2 film. The leakage current decreased with an increase in the thickness of TiO_2, and for a 27-nm-thick film, the measured leakage current was 2.0 × 10~(-10) A. X-ray diffraction and Raman spectroscopy revealed that Ti N, STO, and TiO_2 films are crystalline and TiO_2 has a dominant anatese phase structure.     

溴代酞菁铜衍生物薄膜的折射率和吸收特性

通过旋涂法在单晶硅片上制备了一系列含有不同数目溴取代基的酞菁铜衍生物：一溴代－四烷氧基酞菁铜（MBTAPcCu），二溴代－四烷氧基酞菁铜（DBTAPcCu），四溴代－四烷氧基酞菁岣（TBTAPcCu）和无溴代－四烷氧基酞菁铜（NBTAPcCu）薄膜，利用同步旋转起偏－检偏器（RAP）型的宽谱扫描全自动椭圆偏振光谱仪测定了上述薄膜的椭偏光谱，获得了该系列薄膜的复折射率、复介电常数和吸收系数，研究了溴取代基对酞菁铜衍生物薄膜光学性质的影响。     

Structural and optical studies of thermally evaporated CoPc thin films

The structure of the thermally evaporated cobalt phthalocyanine (CoPc) thin film in the β-form is investigated, and shows a single strong peak indicating preferential orientation in the (1 0 0) direction. Some structural parameters such as crystallite grain size, dislocation density and the number of crystallites per unit surface area are determined.The spectral parameters are determined by applying the electronic orbital transitions.But the optical parameters are deduced using band-model consideration for thin films of Pc.The spectral and optical parameters have also been investigated by using the spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm.The absorption spectra recorded in the UV–VIS region show two absorption bands of phthalocyanine (Pc) molecule, namely the Soret band (B) and the Q-band. The Q-band shows its characteristic splitting (Davydov splitting) with ΔQ=0.23 eV.Some of the important spectral parameters, namely optical absorption coefficient (α), molar extinction coefficient (_molar), oscillator strength (f), electric dipole strength (q²) and absorption half bandwidth (Δλ) of the principle optical transitions have been evaluated.The fundamental and the onset indirect energy gaps could be estimated as 2.90 + or − 0.05 and 1.51 eV, respectively.The refractive index showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. From analysis of dispersion curves, the dielectric constants, the dispersion parameters and the molar polarizability were obtained.All the above parameters were obtained for films as deposited and as annealed. No remarkable annealing effect on many parameters was observed.     

金属/介质薄膜中声子热辐射的空间和各向异性研究

基于理论、实验和仿真相结合的方式,着重研究了金属/介质(MD)薄膜中声子热辐射的空间特性和各向异性。声子是由于晶格振动产生的元激发,是物质的内在属性。尽管声子不易调控,但是声子与其他光学激发的耦合会产生奇异的光学现象。特别是红外到太赫兹范围内的光子与极性介质中的声子强耦合产生表面声子激元（SPhP）。SPhP具有强局域、低损耗等特点,与等离子体（plasmon polaritons）形成互补,使得深亚波长光学成为可能。为了进一步了解声子吸收的内在理论基础,首先通过黄昆方程和超晶格连续介电模型在理论上分析了声子吸收。实验上,主要以SiO₂声子作为研究对象,利用等离子体增强化学气相沉积(PECVD)方法,分别在Si/Al（150 nm）薄膜和Si衬底上制备出500 nm厚的SiO₂薄膜。基于傅里叶红外光谱仪(FTIR),在垂直入射下得到热辐射光谱,通过热辐射光谱分析,并结合由时域有限差分算法（finite-difference time-domain,FDTD）计算得出的仿真光谱图,对比了MD薄膜结构和非MD薄膜结构中声子的热辐射,发现MD薄膜结构更能够有利于声子和SPhP的激发。根据Berreman效应,纵光学波（LO）声子只在倾斜入射时产生。光谱线没有呈现洛伦兹线型,因此,虽然LO声子在垂直入射时测得的热辐射图中不辐射,但同样影响横光学波（TO）声子辐射谱的线型。另外,利用FTIR对金属（Si/Al）/介质（SiO₂薄膜）进行热辐射转角测试,对热辐射转角图分析证明,Si/Al/SiO₂薄膜中SiO₂声子遵循LST(lyddano-sachs-teller)关系,纵横声子成对出现,且两种声子的空间辐射特性不同。改变偏振,发现在S偏振和P偏振下,声子热辐射呈现不同的模式, 体现出声子的空间各向异性。并且,声子与光子耦合可以激发SPhP,反过来,SPhP可以增强声子的吸收。基于MD结构,能够激发并调控SPhP和声子辐射行为,为红外器件的实现奠定了基础。     

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.     

Characterization of high-k gate dielectric/silicon interfaces

We have shown that, for thermally evaporated Ta₂O₅ or ZrO₂ thin films on Si(1 0 0), O₂ annealing at 300–500 °C causes the formation of an interfacial silicon oxide layer as thin as 1–2 nm which can be interpreted in terms of their high permeability to oxygen. And we have demonstrated how useful the energy loss spectra of photoexcited electrons from core levels such as O 1s are to measure the energy bandgaps of very thin insulators. With the combination of measured bandgaps and valence band lineups determined for X-ray photoelectron spectroscopy valence band spectra, we have determined the energy band alignments of Ta₂O₅ and ZrO₂ with Si(1 0 0) before and after the O₂ annealing at 500 °C. In addition, we have demonstrated that total photoelectron yield spectroscopy provides us direct information to quantify the energy distributions of both the defect states in the high-k dielectrics and the dielectric/Si(1 0 0) interface states over nearly entire Si bandgap.     

Determination and analysis of the dispersive optical constants of the 5,5′,6,6′-tetraphenyl-2,2′-bi([1,3]dithiolo[4,5-b][1,4]dithiinylidene)–DDQ complex thin film

The synthesis and optical properties of the 5,5′,6,6′-tetraphenyl-2,2′-bi([1,3]dithiolo [4,5-b] [1,4]dithiinylidene)–2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) complex thin film were investigated by the optical characterization. The optical constants such as refractive index, extinction coefficient and absorption coefficient were determined using the transmittance T(λ) and reflectance R(λ) spectra and the refractive index dispersion was analyzed using single oscillator of Wemple–Didomenico model. The single oscillator energy E₀ and the dispersion energy E_d were calculated. The effect of temperature on refractive dispersion and optical band gap E_g is also discussed. As a result, the annealing temperatures have an important effect on refractive index of thin film.     

Synthesis and optical characterization of c-axis oriented GaN thin films on amorphous quartz glass via sol–gel process

c-Axis oriented GaN nanocrystalline thin films were fabricated by nitridation of three different thin films of -GaO(OH), -Ga₂O₃ or β-Ga₂O₃ obtained by sol–gel technique on amorphous quartz glass substrates. All these GaN thin films showed near band edge emission at 390 nm and yellow luminescence at 570 nm. The crystalline nature and c-axis orientation as well as luminescence properties of the GaN thin films increased by several times by using a buffer layer of GaN on the substrate.     

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.     

Growth behavior of high k LaAlO3 films on Si by metalorganic chemical vapor deposition for alternative gate dielectric application

LaAlO₃ (LAO) is explored in this work to replace SiO₂ as the gate dielectric material in metal–oxide–semiconductor field effect transistor. Amorphous LAO gate dielectric films were deposited on Si (0 0 1) substrates by low pressure metalorganic chemical vapor deposition using La(dpm)₃ and Al(acac)₃ sources. The effect of processing parameters such as deposition temperature and precursor vapor flux on growth, structure, morphology, and composition of LAO films has been investigated by various analytical methods deeply. The film growth mechanism on Si is reaction limiting instead of mass transport control. The reaction is thermally activated with activation energy of 37 kJ/mol. In the initial growth stage, Al element is deficient due to higher nucleation barrier on Si. The LAO films show a smooth surface and good thermal stability and remain amorphous up to a high temperature of 850 °C. The electrical properties of amorphous LAO ultrathin films on Si have also been evaluated, indicating LAO is suitable for high k gate dielectric applications.     

SiO2薄膜内部短程有序微结构研究

SiO₂薄膜是重要的低折射率材料之一, 针对离子束溅射(IBS)和电子束蒸发(EB)的SiO₂薄膜, 采用红外光谱反演技术获得在400–1500 cm^-1波数内的介电常数, 通过对介电能损函数的分析获得了两种薄膜在横向和纵向光学 振动模式下的振动频率和Si–O–Si键角.研究结果表明, 在EB SiO₂薄膜短程有序范围内, SiO₄的连接方式主要是类柯石英结构、3-平面折叠环和热液石英结构的SiO₄连接方式; 在IBS SiO₂薄膜短程有序范围内, SiO₄的连接方式复杂主要是类柯石英结构、3-平面折叠环、 4-平面折叠环结构和类热液石英结构. 关键词： 2薄膜')" href="#">SiO₂薄膜 离子束溅射 电子束蒸发 短程有序     

Dielectric and piezoelectric properties of(110) oriented Pb(Zr_(1-x)Ti_x)O_3 thin films

A phenomenological Landau–Devonshire theory is developed to investigate the ferroelectric, dielectric, and piezoelectric properties of(110) oriented Pb(Zr_(1-x)Ti_x)O_3(x = 0.4, 0.5, 0.6, and 0.7) thin films. At room temperature, the tetragonal a_1 phase, the orthorhombic a_2c phase, the triclinic γ_1 phase, and the triclinic γ_2 phase are stable. The appearance of the negative polarization component P_2 in the a_2c phase and the γ_1 phase is attributed to the nonlinear coupling terms in the thermodynamic potential. The γ phase of the Pb(Zr_(1-x)Ti_x)O_3 thin films has better dielectric and piezoelectric properties than the a_2c phase and the a_1 phase. The largest dielectric and piezoelectric coefficients are obtained in the Pb(Zr_(0.5)Ti_(0.5))O_3 thin film. The piezoelectric coefficient of 110–150 pm/V is obtained in the(110) oriented Pb(Zr_(0.5)Ti_(0.5))O_3 thin film, and the Pb(Zr_(0.3)Ti_(0.7))O_3 thin film has the remnant polarization and relative dielectric constant of 50 μC/cm~2 and 100, respectively,which are in agreement with the experimental measurements reported in the literature.     

脉冲激光沉积法制备高温压电薄膜0.20 BiInO$lt;sub$gt;3$lt;/sub$gt;-0.80PbTiO$lt;sub$gt;3$lt;/sub$gt;$lt;strong$gt;$lt;font color="#ff0000"$gt;$lt;font color="#ff0000"$gt;（已撤稿）$lt;/font$gt;$lt;/font$gt;$lt;/strong$gt;

采用脉冲激光沉积法制备了0.20BiInO₃-0.80PbTiO₃（20BI-PT）高温压电薄膜,并与0.15BiInO₃-0.85PbTiO₃（15BI-PT）样品进行了比较研究. X射线衍射谱显示,20BI-PT样品100峰出现了明显的劈裂,显示样品具有更高的四方对称性. FESEM图显示,20BI-PT样品中出现了部分111取向的三角形晶粒. 20BI-PT样品的铁电剩余极化（P_r）为～28 μC/cm²,矫顽场（E_c）为～120 kV/cm,相较15BI-PT样品,P_r略有增加,但同时E_c也有增加. 20BI-PT样品的横向压电系数（e_31,f）约为–4.7±0.6 C/m²,和15BI-PT相比几乎一样. 介电温度谱显示,20BI-PT 样品的居里温度比15BI-PT增加了约30 ℃,达590 ℃,且介电峰没有明显的频率依赖性. Rayleigh分析显示,20BI-PT样品中内在本征因素及可翻转畴对介电非线性的贡献和15BI-PT基本相同,但是外在因素的贡献没有15BI-PT的贡献大,这可能和20BI-PT样品中晶粒111相对取向率较高有关. 关键词： 薄膜 脉冲激光沉积 铁电 压电