多层薄膜瞬态热弹反射光栅实验及其理论分析和有限元数值计算

多层薄膜/基片材料的热学性质受各层薄膜及基片的共同影响,因此同时评估各层材料的热学性质是非常重要的。激光诱导瞬态热栅技术是一种研究材料表面及亚表面光声光热现象有效的方法,因此可用于研究多层材料的热学性质。本文利用瞬态热弹反射光栅方法研究多层薄膜/基片结构材料的热扩散率,首先对Al/ZnO/Si基片结构的多层材料进行激光诱导瞬态热栅的光衍射检测实验,然后通过对该瞬态热栅的光衍射强度信号进行理论分析及有限元数值模拟,得到Al/ZnO/Si各层热扩散率对其表面温度场分布的影响,进而计算铝膜和氧化锌膜热扩散率对瞬态热栅引起衍射光信号的贡献。最后将理论分析与实验结果进行多参数拟合,在基片的参量已知的情况下,可同时得到铝膜和氧化锌膜的热扩散率。     

Thermal and optical properties of Cd2SnO4 thin films using photoacoustic spectroscopy

Cadmium stannate (Cd₂SnO₄) thin films were prepared by the RF magnetron sputtering technique on glass substrates with substrate temperatures of room temperature (RT), 100°C, 200°C and 300°C. Photoacoustic analyses were made to obtain the thermal diffusivity and the optical bandgap values of the Cd₂SnO₄ thin films. The change in thermal diffusivity of the films with the substrate temperature was analyzed. The optical bandgap values obtained from the photoacoustic spectroscopy were compared with the values obtained from the optical transmittance spectra. X-ray photoelectron spectroscopic (XPS) studies confirm the formation of stoichiometric films. Surface morphological studies by atomic force microscopy (AFM) revealed the crystalline nature of the films deposited at 100°C.     

Thermal characterization of film-on-substrate system by photothermal method and genetic algorithm

For a film-on-substrate system, in the case of thin films with lower thermal diffusivities compared to substrates, the phase characterization of the photothermal signals is analyzed. Moreover, the numerical estimations of multiparameter are performed, which show the feasibility of simultaneous determination of the thermal diffusivities of the film and the substrate, as well as the thermal interface resistance of the film/substrate. Because the thermal diffusivity of the thin-film and the thermal interface resistance may be highly correlated, a genetic algorithm is used as an estimation method for the determination of the thermal properties of thin films with low thermal diffusivity.     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.     

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.Currently supported by the LLE fellowship     

Determination of Thermal Conductivity of Liquids by a Multi-Point Laser Pump Method

As an advanced optical method, a multi-point pump method is presented for measurements of thermo-physical properties of liquids. Meanwhile, based on the laser-induced thermal grating method, a new theory model is presented and used to analyse the thermal effects caused by the multi-point laser pump, by which the thermal conductivity of liquids can be obtained. The results of some typical liquids, such as water, ethanol and acetone, are presented and are consistent with those of acknowledged values, demonstrating that the multi-point method is simple and useful for characterizing thermal properties of liquids.     

Thermal characterization of TiC<Subscript>x</Subscript>O<Subscript>y</Subscript> thin films

Thermal wave characterization of thin films used in industrial applications can be a useful tool, not just to get information on the films' thermal properties, but to get information on structural-physical parameters, e.g. crystalline structure and surface roughness, and on the film deposition conditions, since the thermal film properties are directly related to the structural-physical parameters and to the deposition conditions. Different sets of TiC_XO_Y thin films, deposited by reactive magnetron sputtering on steel, have been prepared, changing only one deposition parameter at a time. Here, the effect of the oxygen flow on the thermal film properties is studied. The thermal waves have been measured by modulated IR radiometry, and the phase lag data have been interpreted using an Extremum method by which the thermal coating parameters are directly related to the values and modulation frequencies of the relative extrema of the inverse calibrated thermal wave phases. Structural/morphological characterization has been done using X-ray diffraction (XRD) and atomic force microscopy (AFM). The characterization of the films also includes thickness, hardness, and electric resistivity measurements. The results obtained so far indicate strong correlations between the thermal diffusivity and conductivity, on the one hand, and the oxygen flow on the other hand.     

Thermal Conductivity Measurement of Submicron-Thick Aluminium Oxide Thin Films by a Transient Thermo-Reflectance Technique

Thermal conductivity of submicron-thick aluminium oxide thin films prepared by middle frequency magnetron sputtering is measured using a transient thermo-reflectance technique. A three-layer model based on transmission line theory and the genetic algorithm optimization method are employed to obtain the thermal conductivity of thin films and the interracial thermal resistance. The results show that the average thermal conductivity of 330- 1000nm aluminium oxide thin films is 3.3 Wm^-1K^-1 at room temperature. No significant thickness dependence is found. The uncertainty of the measurement is less than 10%.     

Preparation and Thermal Characterization of Diamond-Like Carbon Films

Diamond-like carbon （DLC） films are prepared on silicon substrates by microwave electron cyclotron resonance plasma enhanced chemical vapor deposition. Raman spectroscopy indicates that the films have an amorphous structure and typical characteristics. The topographies of the films are presented by AFM images. Effective thermal conductivities of the films are measured using a nanosecond pulsed photothermal reflectance method. The results show that thermal conductivity is dominated by the microstructure of the films.     

The effects of experimental parameters in the thermal diffusivity measurements of oriented polymer films using mirage effect: Computer simulation

Computer simulations have been carried out to study the effects of the experimental parameters when the mirage method has been applied to thermal diffusivity measurements of oriented polymer films. The parameters under study are the thermal diffusivity of the fluid surrounding the sample, the modulation frequency and the radius of the heating beam, the height and the radius of the probe beam, and the sample thickness and thermal diffusivity. Proposals for the optimum parameter values to maximize the measurement sensitivity for the sample diffusivity are made and the difficulties arising from the low diffusivity of the samples are described. It is also concluded that because the thermal properties of the fluid surrounding the sample have a strong contribution to the mirage signals, the signals do not include any simple feature corresponding to the sample diffusivity. Therefore it should be determined from the entire measurement data using regression methods.     

Thermal diffusivity measurement of thermally conducting films on insulating substrates

We report thermal diffusivity measurements of thin Niobium films on glass substrates, using OptoThermal Transient Emission Radiometry (OTTER). The result is a thermal diffusivity value ofD=(2.79±0.36)×10^–5 m²/s, which is within 17% of the accepted value for multicrystaline bulk niobium. The technique is remote sensing, non-destructive, and the measurements depend only on the thickness of the film and the thermal properties of the substrate.     

低温缓冲层对氧化锌薄膜质量的影响

利用金属有机化学气相沉积(MOCVD)法,在Si衬底上外延生长ZnO薄膜。为了改善氧化锌薄膜的质量,首先在Si衬底上生长低温ZnO缓冲层,然后再生长高质量的ZnO薄膜。通过XRD、SEM、光致发光(PL)光谱的实验研究,发现低温ZnO缓冲层可有效降低ZnO薄膜和Si衬底之间的晶格失配以及因热膨胀系数不同引起的晶格畸变。利用低温缓冲层生长的ZnO薄膜的(002)面衍射峰的强度要比直接在Si上生长的ZnO薄膜样品的高,并且衍射峰的半高宽也由0.21°减小到0.18°,同时有低温缓冲层的样品室温下的光致发光峰也有了明显的增高。这说明利用低温缓冲层生长的ZnO薄膜的结晶质量和光学性质都得到了明显改善。     

Thermal properties of 15-mol% gadolinia doped ceria thin films prepared by pulsed laser ablation

Thermal properties of 15-mol% gadolinia doped ceria thin films (Ce_0.85Gd_0.15 O_1.925) prepared by pulsed laser ablation on silicon substrates in the temperature range 473–973 K are presented. Thermal diffusivities and thermal conductivities were evaluated using photoacoustic spectroscopy. The influence of grain size on thermal properties of the films as a function of deposition temperature is studied. It is observed that the thermal diffusivity and the conductivity of these films decreases up to 873 K and then increases with substrate temperatures. The thermal properties obtained in these films are discussed on the basis of influence of grain size on phonon scattering.     

Thermal induced changes of lead zirconium titanate films and their consequences for liquid crystal devices applications

Thin films of lead zirconate titanate (PZT) were obtained by a modified sol–gel route on float glass and indium tin oxide (ITO)-covered float glass substrates. Different thermal treatments were performed on the deposited films in the temperature range 100–700°C. Spectroscopic ellipsometry was used to investigate the optical properties of the deposited films, and the changing optical absorption spectra were interpreted in terms of the growth of two different crystal phases, pyrochlore and ferroelectric perovskite, as a function of annealing temperature. Moreover, a specific resonance at 1.9 nm was detected when thin PZT films are deposited on ITO substrates and was attributed to a particular charge distribution at the interface. Finally, the performance in rectifying the electro-optical response of asymmetric nematic liquid crystal cells was tested for some of the films undergoing different thermal treatments.     

Sol-gel制备的Yb3+,Er3+掺杂光致发光膜及其上转换光致发光性能

以钛酸正丁酯、异丙醇、硝酸镱和硝酸铒为起始原料,乙酰丙酮为络合剂,硝酸为催化剂,采用溶胶-凝胶法和旋转镀膜工艺制备出无掺杂和镱、铒共掺杂二氧化钛光致发光薄膜。利用接触角测定仪测定玻璃基底和溶剂之间的接触角,通过TG、XRD和FESEM分别对溶胶溶液或薄膜样品进行热分析、物相分析和特征形貌表征,研究了掺杂元素、不同煅烧温度对薄膜物相组成的影响。结果表明:异丙醇与玻璃基底的接触角最小,被选为最佳溶剂。当煅烧温度达到700 ℃且保温2 h时,可得到晶型较好的无掺杂和镱、铒共掺杂薄膜,其颗粒形貌为球形结构,平均尺寸不大于50 nm,薄膜表面均匀致密且较光滑。荧光分光光度计的测试结果表明,镱、铒共掺杂薄膜具有较强的光致发光性能。     

Characteristics of sculptured Cu thin films and their optical properties as a function of deposition rate

Sculptured copper thin films were deposited on glass substrates, using different deposition rates. The nano-structure and morphology of the films were obtained, using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Their optical properties were measured by spectrophotometry in the spectral range of 340-850 nm. The real and imaginary refractive indices, film thickness and fraction of metal inclusion in the film structure were obtained from optical fitting of the spectrophotometer data.     

Stable p-ZnO thin films by oxygen control using reverse spray dynamics

Semiconducting ZnO thin films were deposited on glass substrates by a modified CVD method using reverse spray of the precursor solutions. The films were characterized by X-ray diffraction (XRD) and Hall effect measurements at room temperature. The results of XRD analysis revealed polycrystalline nature of the grown films with different crystallographic orientations. The type of conductivity and the carrier concentration as determined from Hall effect measurements were dependent on the deposition temperature and annealing conditions. Oxygen control at 220 °C produced p-ZnO film with high hole mobility (193 cm²/V s). The electrical conductivity was correlated to the stoichiometry of the grown films.     

Theoretical and experimental study of photo-modulated reflectivity detections for transparent film/opaque substrate structures

A photo-modulated reflectivity method is used to evaluate thermal properties of bilayered structures composed of transparent films deposited on opaque substrates. In order to calculate the photo-modulated reflectivity variations of the bilayered structures induced by focused modulated laser irradiations, a 3-D theoretical model is presented. Due to the complexity of the 3-D theory, a finite element method is used to simulate the laser-induced temperature fields, thermoelastic displacement fields and dielectric constant variations, and finally the photo-modulated reflectivities. Then a quantitative investigation of the thermal properties of the transparent films deposited on opaque substrates can be performed by the experimental measurements and theoretical simulations. As examples, the thermal conductivities of ZnO and diamond films deposited on silicon substrates are evaluated and discussed. PACS 44.10.+i; 42.55.-f; 51.70.+f; 68.60.-p     

Observation of ultra-broadband terahertz emission from ZnTe films grown by metaloganic vapor epitaxy

ZnTe films were grown on (0001) sapphire substrates by the metalorganic vapor phase epitaxy (MOVPE) method. Single crystalline (111) ZnTe epitaxial layers were confirmed by x-ray diffraction, reflection high-energy electron diffraction, and cathodoluminescence measurements. Emission of THz radiation with a spectral distribution up to 40 THz was clearly observed from the ZnTe film with a thickness of 10 μm. The results show that MOVPE is a promising growth method for obtaining high-quality ZnTe epitaxial films on sapphire substrates, which paves the way for obtaining thinner ZnTe films to provide a flatter frequency response in THz device applications.