Separation of Optical Thin Film and Substrate Absorption by Obliquely-crossed Photothermal Deflection: Theory

Abstract

An explicit three-dimensional calculation of c.w. modulated photothermal deflection (PTD) with obliquely crossed geometry, including optical thin film and substrate absorption, has been presented. Theoretical results show that the obliquely crossed PTD technique is feasible to separate the thin film and substrate absorption, and to measure low thin film absorption under high substrate absorption by appropriately adjusting the cross-point position of the excitation and probe beams. Measurement of the thin film absorption is not affected by the thermal properties of the substrate. The obliquely crossed PTD also provides a method for real-time monitoring of thin film generation.     

Photodiodes based on fullerene semiconductor

Fullerene thin films have been deposited by thermal evaporation on glass substrates at room temperature. A comprehensive optical characterization was performed, including low-level optical absorption measured by photothermal deflection spectroscopy. The optical absorption spectrum reveals a direct bandgap of 2.3 eV and absorption bands at 2.8 and 3.6 eV, which are related to the creation of charge-transfer excitons. Various photodiodes on indium-tin-oxide coated glass substrates were also fabricated, using different metallic contacts in order to compare their respective electrical characteristics. The influence of a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) buffer layer between the indium-tin-oxide electrode and the fullerene semiconductor is also demonstrated. These results are discussed in terms of the workfunction for each electrode. Finally, the behaviour of the external quantum efficiency is analyzed for the whole wavelength spectrum.     

Study on the Spatial Resolution of Imaging Technique for Absorption Loss Measurement of Optical Coatings

The uniformity of optical coatings becomes more and more important as large diameter optical devices are widely used. Absorption loss in optical components, particularly in optical coatings, is a limiting factor in high-power laser applications. This article analyzes the main factors, which affect the spatial resolution of three techniques for surface absorption loss measurement, including the photothermal deflection technique, the surface thermal lens technique, and the photothermal detuning technique. The influence of the size of the heating and probe beam on the photothermal detuning technique is studied in detail. Experiments are conducted to study the photothermal signal of the photothermal detuning technique for absorption measurement of the optical coating point by point. The results show that the main factors, which affect the spatial resolution of imaging measurements for absorption loss of coatings, are the heating beam size and the step accuracy of the sample translation stage. The heating and probe beam sizes has a significant impact on the application of the photothermal detuning technique. Experimental result shows that the photothermal detuning technique can be used for imaging of absorption loss measurements of optical coatings. The results provide theoretical and experimental supports for further application of the photothermal detuning technique.     

Optically transparent carbon nanowire thin film

Thin films of amorphous carbon nanowire (a-CNW) have been fabricated from crossed multi-walled carbon nanotube (MWCNT) thin film. The fabrication was done by means of ion beam irradiation on various substrates. It is found that the a-CNW thin films show electrical conduction behaviour, and electrical conductivity varies after annealing. In addition, the transmission spectra in the visible range reveal that the film has above 90% optical transmission. It can be ascribed to the fact that the decreased crystallinity of MWCNTs by ion beam irradiation has caused the incremental increase of optical transmission. We also report on a method for cutting or destroying a-CNWs using low-energy focused electron beam from a scanning electron microscope.     

Deposition of SiO2 films with high laser damage thresholds by ion-assisted electron-beam evaporation

SiO(2) thin films ( approximately 100 nm thick) with transmittivity and a laser damage threshold nearly equal to those of bulk material are deposited on silica substrates by the technique of ion-assisted electron-beam evaporation. The influence of film packing density on the laser damage threshold is investigated by the technique of photoacoustic probe beam deflection. It is shown that films with lower packing density may have a higher laser damage threshold and as a consequence better heat dissipation.     

Morphology, structural and optical characterization of electron beam evaporated bromoaluminium phthalocyanine (BrAlPc) thin films

Bromoaluminium phthalocyanine (BrAlPc) thin films have been deposited onto pre-cleaned glass substrates by electron beam evaporation technique. Thin films have been characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and optical absorption (UV–Vis) spectra. XRD studies of BrAlPc thin film deposited at room temperature shows the indication of the α-phase. FESEM images have shown that the most of particles are spherical in shape with an average size about 26–34 nm. Using UV–Visible spectra over the wavelength range 300–800 nm, the optical band gap, absorption coefficient and extinction coefficient of BrAlPc thin films are evaluated. The optical absorption measurements of thin films show that the absorption mechanism is due to direct transition.     

Chemical bath deposition of indium sulphide thin films: preparation and characterization

Indium sulphide (In₂S₃) thin films have been successfully deposited on different substrates under varying deposition conditions using chemical bath deposition technique. The deposition mechanism of In₂S₃ thin films from thioacetamide deposition bath has been proposed. Films have been characterized with respect to their crystalline structure, composition, optical and electrical properties by means of X-ray diffraction, TEM, EDAX, optical absorption, TRMC (time resolved microwave conductivity) and RBS. Films on glass substrates were amorphous and on FTO (flourine doped tin oxide coated) glass substrates were polycrystalline ( phase). The optical band gap of In₂S₃ thin film was estimated to be 2.75 eV. The as-deposited films were photoactive as evidenced by TRMC studies. The presence of oxygen in the film was detected by RBS analysis.     

Thermal Conductivity Measurement of Submicron-Thick Films Deposited on Substrates by Modified ac Calorimetry (Laser-Heating Ångstrom Method)

Technological development, especially in microelectronics, necessitates the development of new and improved methods for measuring the thermal properties of materials, especially in the form of ultrathin films. Previously, modified ac calorimetry (laser-heating Ångstrom method) using a scanning laser as the energy source was developed and shown to provide accurate values of thermal diffusivity and derived thermal conductivity for a broad range of materials in the form of free-standing thin sheets or films, wires including fiber bundles, and some films on substrates. This paper describes further applications of the modified ac-calorimetry technique for measurements of the thermal conductivity of thin films deposited on substrates. It was used to measure successfully the thermal conductivities of 1000- to 3000-Å-thick aluminum nitride films, aluminum oxide films, etc., which were deposited on a glass substrate. It was also shown to be suitable for developmental measurements on submicron-thick chromatic films deposited on a PET substrate, which are photothermal recording layers, used in the media of CD-R drives of computer systems.     

Trace gas detection by laser intracavity photothermal spectroscopy

A novel laser intracavity photothermal detector is described. In this scheme, sample absorption of the pump laser power takes place within the cavity of a probe He-Ne laser causing modulation in the gain and in turn the output power. Comparison of this intracavity detector with two other photothermal techniques, namely, phase fluctuation optical heterodyne spectroscopy and thermal beam deflection, is made in terms of practicality and sensitivity. For in situ measurements, sensitivity of 0.5 x 10(-7) cm(-1) for a probe length of 3 cm has been achieved.     

光热偏转薄膜热导测试系统及其应用

根据光热偏转光谱（ＰＴＤＳ）原理，研制成功光热偏转法薄膜热导测试系统，对大量试样进行了测量，结果同文献参考值符合较好，测量重复精度优于５０％，为材料科学有关热性质的研究，特别是有关光热偏转薄膜热导率的研究提供了有力的检测手段，为研制高质量金刚石薄膜提供了依据。     

Effect of electron bombardment on the properties of ZnO thin films

Zinc oxide thin films deposited on quartz substrates were subjected to electron beam irradiation. The effect of electron bombardment on the structural, optical and luminescent properties of the film has been investigated. The electron bombardment leads to better crystallinity, enhanced UV emission and a red shift of 0.025 eV in the band gap of ZnO thin films. The changes in the material property are ascribed to the surface annealing effect of electron beam irradiation. However, the annealing effects induced by electron beam are found to be distinct compared to the high temperature (800 °C) thermal annealing.     

Intrinsic optical and structural properties of SrS thin films

SrS thin films were deposited by electron beam evaporation on heated silica substrates. The optical properties of the layers – complex refractive index and optical band gap –were derived from optical transmission spectra, measured by means of UV-VIS-NIR spectrophotometry. The influence of post-deposition annealing by rapid thermal processing (RTP) was studied. X-ray powder diffraction (XRD) was used to study the film crystal structure and preferential orientation.     

Measurements of thermal diffusivity of boron-silicon film-on-glass structure using phase detection method of photothermal deflection spectroscopy

The phase detection method of photothermal deflection spectroscopy in the transverse configuration was used to measure the overall thermal diffusivity of silicon-boron (Si-B) alloy film on Corning 7059 glass substrate. Results were attained by observing the phase of deflection of the probe beam when it scanned above the film surface relative to the pump beam. Measurements were repeated for different modulation frequencies of the pump beam. Furthermore, both bouncing and skimming configurations were used. The effect of varying the distance between the probe beam and film surface was investigated.     

Quantitative Thermal Microscopy Measurement with Thermal Probe Driven by dc+ac Current

Quantitative thermal measurements with spatial resolution allowing the examination of objects of submicron dimensions are still a challenging task. The quantity of methods providing spatial resolution better than 100 nm is very limited. One of them is scanning thermal microscopy (SThM). This method is a variant of atomic force microscopy which uses a probe equipped with a temperature sensor near the apex. Depending on the sensor current, either the temperature or the thermal conductivity distribution at the sample surface can be measured. However, like all microscopy methods, the SThM gives only qualitative information. Quantitative measuring methods using SThM equipment are still under development. In this paper, a method based on simultaneous registration of the static and the dynamic electrical resistances of the probe driven by the sum of dc and ac currents, and examples of its applications are described. Special attention is paid to the investigation of thin films deposited on thick substrates. The influence of substrate thermal properties on the measured signal and its dependence on thin film thermal conductivity and film thickness are analyzed. It is shown that in the case where layer thicknesses are comparable or smaller than the probe–sample contact diameter, a correction procedure is required to obtain actual thermal conductivity of the layer. Experimental results obtained for thin SiO\(_{\mathrm {2}}\) and BaTiO\(_{\mathrm {3 }}\)layers with thicknesses in the range from 11 nm to 100 nm are correctly confirmed with this approach.     

Absolute infrared absorption measurements in optical coatings using mirage detection

Absolute measurements of the optical absorption at λ=10.6 μm of BaF₂ thin film grown on a ZnSe substrate and bare ZnSe substrate surface were performed by collinear photothermal deflection technique. Absorption values were obtained in an absolute manner by fitting experimental data to the theoretical expressions of beam deflection for two particular cases: when thermal wave extends far from the laser spot and the opposite, when the heated region does not stretch beyond the laser spot. The validity of the theoretical models were tested in the range of modulation frequencies from 20 Hz to 2700 Hz. This method was also applied to the analysis of fused silica and sapphire bulk samples in order to be used as reference absorbing media in the infrared spectral range. Optical absorption of BaF₂ thin film and ZnSe substrate surface were also deduced from the same theoretical approach using sapphire as a bulk reference medium. From both calculation methods an absorptance of 1200 ppm for BaF₂ thin film and of 425 ppm for ZnSe plate surface were measured.     

Thermal Conductivity of AlN and SiC Thin Films

The thermal conductivity of AlN and SiC thin films sputtered on silicon substrates is measured employing the 3ω method. The thickness of the AlN sample is varied in the range from 200 to 2000 nm to analyze the size effect. The SiC thin films are prepared at two different temperatures, 20 and 500°C, and the effect of deposition temperature on thermal conductivity is examined. The results reveal that the thermal conductivity of the thin films is significantly smaller than that of the same material in bulk form. The thermal conductivity of the AlN thin film is strongly dependent on the film thickness. For the case of SiC thin films, however, increased deposition temperature results in negligible change in the thermal conductivity as the temperature is below the critical temperature for crystallization. To explain the thermal conduction in the thin films, the thermal conductivity and microstructure are compared using x-ray diffraction patterns.     

不同衬底上低温生长的ZnO晶体薄膜的结构及光学性质比较

采用电子束反应蒸发方法,在单晶Si(001)及玻璃衬底上低温外延生长了沿c轴高度取向的单晶ZnO薄膜,并对沉积的ZnO晶体薄膜的结构和光学性质进行了分析比较。通过对ZnO薄膜的X射线衍射(XRD)分析及光致荧光激发谱(PLE)测量,研究了衬底材料结构特性、生长温度及反应气氛中充O     

Laser induced deflection technique for absolute thin film absorption measurement: optimized concepts and experimental results

Using experimental results and numerical simulations, two measuring concepts of the laser induced deflection (LID) technique are introduced and optimized for absolute thin film absorption measurements from deep ultraviolet to IR wavelengths. For transparent optical coatings, a particular probe beam deflection direction allows the absorption measurement with virtually no influence of the substrate absorption, yielding improved accuracy compared to the common techniques of separating bulk and coating absorption. For high-reflection coatings, where substrate absorption contributions are negligible, a different probe beam deflection is chosen to achieve a better signal-to-noise ratio. Various experimental results for the two different measurement concepts are presented.     

Characterization of optical, electrical and structural properties of silverphthalocyanine thin films

Silverphthalocyanine thin films are deposited on to glass substrates by thermal evaporation technique. Optical data have been obtained from both absorption and reflectivity spectra over the wavelength range 350–900 nm. The absorption coefficient α and extinction coefficient k are estimated from the spectrum. The mechanism of optical absorption follows the rule of direct transition. Using α and k, the refractive index and the dielectric constants are determined. Electrical conductivity studies are done at different substrate temperatures and using the Arrhenius plot the activation energy in the intrinsic region and impurity region is estimated. From the X-ray diffractograms of AgPc thin films subjected to heat treatments the variation of grain size is also studied. The scanning electron microscopy images are taken to study the surface morphology of the films. Silver phthalocyanine thin film is expected to find application in the fabrication of organic transistors and LED devices.     

Laser double-heterodyne integrated microscopy of magnetoelectric nanofilms

Aspects of photothermal characterization of magnetoelectric nanofilms are presented. The parameters of the study include photomodulated surface reflectance and displacement of various nanostructures. A subsequent application is developed for assessment of film thickness by the use of optical absorption coefficient. The method refers to nanofilms deposited on transparent substrates, where the laser power is absorbed selectively in the film. The analytical approach is based on ultrasensitive laser double-heterodyne probe featuring resolution of thickness up to fractions of nanometre at improved translational resolution compared to ellipsometry.