Influence of the substrate nature on the properties of ZnO thin films

Zinc metallic films, deposited onto different substrates, were submitted to a thermal oxidation process, in air, in order to obtain ZnO thin films. X-ray diffraction patterns revealed that as-obtained ZnO thin films were polycrystalline and have a wurtzite (hexagonal) structure. The temperature dependences of the electrical conductivity during some heating/cooling cycles were studied and the conduction mechanism was interpreted in terms of Seto model. The sensitivity of ZnO thin films, at five gases, was investigated and it was established that ethanol is the test gas that produces the most significant changes in the electrical resistance of all the studied films. Some correlations between the oxidation temperature and the substrate nature and the parameters which characterize the structural and electrical properties of ZnO thin films have been established.     

Properties of polysilicon films annealed by a rapid thermal annealing process

This paper describes the electrical and mechanical properties of polysilicon films annealed by a rapid thermal annealing (RTA) process. The results show that doped polysilicon films annealed by RTA process have conductivity below 100 ω sq⁻¹. This is sufficient for application for integrated micromechanical structures. Also, the stress non-uniformity, as well as the average stress in polysilicon films annealed by RTA, is acceptable for manufacturing flat cantilever beams. The average stress of 2 μm polysilicon films is compressive and typically less than 200 MPa.     

Nanocrystalline ZnO thin films: optoelectronic and gas sensing properties

Nanocrystalline Zinc oxide thin films have been deposited by sol–gel spin coating technique and then have been analyzed before and after a suitable thermal annealing in order to test their applications in various reducing and oxidizing gases. ZnO thin films were highly sensitive and selective for NH₃ gas. The spectrophotometric and conductivity measurements have been performed in order to determine the optical and electrical properties of zinc oxide thin films. The structure and the morphology of such material have been investigated by high resolution electron microscopy and small area electron diffraction. The average particle size is in 60–70 nm.     

Surface morphological, structural, electrical and optical properties of annealed vanadyl tetra tert-butyl 2, 3 naphthalocyanine thin films

Vanadyl Tetra Tert-Butyl 2, 3 Naphthalocyanine (VTTBNc) thin films have been grown at room temperature by physical vapor deposition technique. The article describes the role of air and vacuum annealing on VTTBNc thin film surface morphology, structure, electrical conductivity and optical absorbance on the basis of respective measurements like atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray Diffractogram (XRD), DC electrical conductivity with integrated electrodes and UV-visible absorption spectra.     

Characterization of proteins and fibroblasts on thin inorganic films

The ability of biomaterial surfaces to regulate cell behavior requires control over surface chemistry and material microstructure. One of the goals in the development of silicon-based biomedical devices such as biosensors or drug delivery systems is improved biocompatibility which may be achieved through the deposition or adsorption of thin films. In this study, films of single crystal silicon, stoichiometric and low stress silicon nitride, doped and undoped polysilicon, as well as Arg-Gly-Asp (RGD) peptide adsorbed surfaces characterized in terms of protein adsorption or cellular adhesion for a period of four days. Protein adsorption studies using fibrinogen and albumin, two proteins implicated in cellular adhesion and surface activity, reveal that low stress silicon nitride surfaces have a 223%±2.50% greater protein adsorption compared to undoped polysilicon surfaces, followed by silicon nitride, unmodified silicon, and doped polysilicon surfaces, respectively. The thickness of the adsorbed albumin and fibrinogen layer on various thin films was measured by ellipsometry and compared to contact angle measurements. The greatest cellular adhesion was observed on undoped polysilicon, followed by unmodified (control) silicon, low stress silicon nitride, silicon nitride, and doped polysilicon surfaces. Cellular binding supports the differential protein adsorption found on modified and unmodified silicon surfaces. Understanding the biological response to thin films will allow us to design more appropriate interfaces for implantable diagnostic and therapeutic silicon-based microdevices.     

Thermal Conductivity Measurements of Thin Amorphous Silicon Films by Scanning Thermal Microscopy

Thermal conductivity measurements of thin amorphous silicon films performed with a micro-thermistance mounted on an atomic force microscope are presented. A specific thermal model is implemented, and an identification procedure is proposed to extract the film contribution from the apparent thermal conductivity. Results show agreement with the literature regarding interface resistance data, but lower thermal conductivity values are obtained.     

Temperature-Dependent Thermal Conductivity of Undoped Polycrystalline Silicon Layers

Polycrystalline silicon is used in microelectronic and microelectromechanical devices for which thermal design is important. This work measures the in-plane thermal conductivities of free-standing undoped polycrystalline layers between 20 and 300 K. The layers have a thickness of 1 m, and the measurements are performed using steady-state Joule heating and electrical-resistance thermometry in patterned aluminum microbridges. The layer thermal conductivities are found to depend strongly on the details of the deposition process through the grain size distribution, which is investigated using atomic force microscopy and transmission electron microscopy. The room-temperature thermal conductivity of as-grown polycrystalline silicon is found to be 13.8 W·m^-1·K^-1and that of amorphous recrystallized polycrystalline silicon is 22 W·m^-1·K^-1, which is almost an order of magnitude less than that of single-crystal silicon. The maximum thermal conductivities of both samples occur at higher temperatures than in pure single-crystalline silicon layers of the same thickness. The data are interpreted using the approximate solution to the Boltzmann transport equation in the relaxation time approximation together with Matthiessen's rule. These measurements contribute to the understanding of the relative importance of phonon scattering on grain and layer boundaries in polysilicon films and provide data relevant for the design of micromachined structures.     

Nitrogen effect on spin-coated ZnO-based p–n homojunctions: structural,optical and electrical characteristics

In this work, ZnO:Al–N/ZnO:Al and ZnO:Ag–N/ZnO:Al homojunctions were deposited by means of spin coating method using precursors obtained by sol gel chemistry. The optical, structural and electrical properties of spin coated undoped and M-doped ZnO thin films (M?=?Al, Ag–N and Al–N) using ammonium hydroxide as a nitrogen source are reported. The films showed the wurtzite type structure with a c-axis (002) preferential orientation. The films showed a surface morphology consisting of wrinkles, which were constituted of nanocrystals in the range of ～?20 nm. The thin films were highly transparent in the visible region of the electromagnetic spectrum. The optical band gap of the films was close to 3.30 eV. Hall Effect measurements indicated that undoped and Al doped ZnO thin films showed an n-type conductivity, whereas ZnO:Al–N and ZnO:Ag–N thin films exhibited p-type conductivity, probably related to the formation of dual acceptor complexes related to nitrogen. Two types of p–n homojunctions (ZnO:Al–N/ZnO:Al and ZnO:Ag–N/ZnO:Al) were fabricated by means of sol–gel spin-coating method. In both cases, a rectifying behavior was observed, as revealed by current–voltage measurements.     

Experimental determination of the thermal conductivity of thin films

Two techniques have been developed to determine experimentally the thermal conductivity of thin solid films of thickness 500 Å or more at low and high temperatures. The first technique is a steady state and is suitable for measurements above room temperature. The method enables the thermal conductivity of eight film specimens to be measured simultaneously. The second technique is a transient one (an adaptation of Ioffe's method for bulk materials) and is suitable for measurements in the temperature range 100–260 K. The two techniques have been used to make measurements of thin films of copper and various crystalline and amorphous semiconductors. The values of the thermal conductivity for thick copper films by both techniques agree quite well with the bulk values.     

Piezoelectric micromachined ultrasonic transducers based on PZT thin films

This paper describes fabrication and characterization results of piezoelectric micromachined ultrasonic transducers (pMUTs) based on 2-microm-thick Pb(Zr0.53Ti0.47O3) (PZT) thin films. The applied structures are circular plates held at four bridges, thus partially unclamped. A simple analytical model for the fully clamped structure is used as a reference to optimize design parameters such as thickness relations and electrodes, and to provide approximate predictions for coupling coefficients related to previously determined thin film properties. The best coupling coefficient was achieved with a 270-microm plate and amounted to kappa2 = 5.3%. This value compares well with the calculated value based on measured small signal dielectric (epsilon = 1050) and piezoelectric (e3l,f = 15 Cm(-2)) properties of the PZT thin film at 100 kV/cm dc bias. The resonances show relatively large Q-factors, which can be partially explained by the small diameters as compared to the sound wavelength in air and in the test liquid (Fluorinert 77). A transmit-receive experiment with two quasi-identical pMUTs was performed showing significant signal transmission up to a distance of 20 cm in air and 2 cm in the test liquid.     

Ellipsometry-based conductivity extraction in case of phosphorus doped polysilicon

In this work, we investigated a new method for thin films electrical conductivity extraction based on spectroscopic ellipsometry. This has been enabled through the correlation between the films conductivity and their ellipsometric properties. Indeed, it has been demonstrated that numerous ellipsometric fitting-based approaches can provide, in an indirect way, the electrical characteristics of thin films. The study was focused on electrical conductivity, but doping level or carriers’ mobility can also be extrapolated from ellipsometric measurements. Among various possibilities leading to electrical properties extraction, we can cite the extremal values of Ψ and Δ ellipsometric angles, their associated wavelengths, the mean square error and the maximal and minimal reflectivities ratio. Otherwise, the correlation between extrinsic conductivity and ellipsometric parameters evolution has been confirmed in case of low doping levels with particular behavior after annealing. This contactless method has been successfully applied to polycrystalline silicon films deposited on oxidized, p-type monocrystalline substrates, by low pressure chemical vapor deposition technique, and lightly or heavily phosphorus doped by diffusion. The feasibility of the method has been proven in this case, but also in other cases like implanted polysilicon layers or silicon-on-insulator (not included here).     

The As-prepared surfaces of C-axis-oriented Nd1Ba2Cu3Oy thin films characterized using scanning probe microscopes

Recently, superconducting Nd₁Ba₂Cu₃O_y (Ndl23) thin films with high superconducting transition temperature (T _c) have been successfully fabricated at our institute employing the standard laser ablation method. In this paper, we report the results of surface characterization of the Nd123 thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) and an atomic force microscope (AFM) system operated in air. Clear spiral pattern is observed on the surfaces of Nd123 thin films by STM and AFM, suggesting that films are formed by two-dimensional island growth mode. Contour plots of the spirals show that the step heights of the spirals are not always the integer or half-integer number of thec-axis parameter of the structure. This implies that the surface natural termination layer of the films may not be unique. This result is supported byI-V STS measurements. The surface morphology of the Nd123 thin films is compared with that of thec-axis-oriented Y₁Ba₂Cu₃S_y thin films. Surface atomic images of the as-prepared Nd123 thin films are obtained employing both STM and AFM. STS measurements show that most of the surfaces are semiconductive. The results of STS measurements together with the fact that we are able to see the surface atomic images using scanning probe microscopes suggest that exposure to air does not cause serious degradation to the as-prepared surfaces of Nd123 thin films.     

The As-prepared surfaces of C-axis-oriented Nd1Ba2Cu3Oy thin films characterized using scanning probe microscopes

Recently, superconducting Nd₁Ba₂Cu₃O_y (Ndl23) thin films with high superconducting transition temperature (T _c) have been successfully fabricated at our institute employing the standard laser ablation method. In this paper, we report the results of surface characterization of the Nd123 thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) and an atomic force microscope (AFM) system operated in air. Clear spiral pattern is observed on the surfaces of Nd123 thin films by STM and AFM, suggesting that films are formed by two-dimensional island growth mode. Contour plots of the spirals show that the step heights of the spirals are not always the integer or half-integer number of thec-axis parameter of the structure. This implies that the surface natural termination layer of the films may not be unique. This result is supported byI-V STS measurements. The surface morphology of the Nd123 thin films is compared with that of thec-axis-oriented Y₁Ba₂Cu₃S_y thin films. Surface atomic images of the as-prepared Nd123 thin films are obtained employing both STM and AFM. STS measurements show that most of the surfaces are semiconductive. The results of STS measurements together with the fact that we are able to see the surface atomic images using scanning probe microscopes suggest that exposure to air does not cause serious degradation to the as-prepared surfaces of Nd123 thin films.     

Conductivity and nanoscale morphology of thin films prepared from indolo[2,3-a]carbazole and 11,12-dioctylindolo[2,3-a]carbazole

A systematic study on the thin film morphology of heteroacenic derivatives such as indolo[2,3-a]carbazole and 11,12-dioctylindolo[2,3-a]carbazole has been carried out using scanning force microscopy and optical microscopy techniques. The investigation has comprised the preparation of a series of thin films by combining different solvents (dimethylformamide and tetrahydrofurane), substrates (glass, gold, silicon and aluminium) and deposition techniques (spin coating and thermal evaporation) we found a wide variety of self-assembled structures. In addition, conductivity measurements have been performed, showing a very large spread in conductivity values. Some of the samples give quite high conductivity while others conduct very poorly, even in the case of samples prepared with essentially the same conditions. These results indicate that morphology is very critical for the final conductivity of the thin films. In addition we conclude that for some of the films prepared the homogeneous thin film approximation must be revised otherwise obtained conductivity measurements lead to significant error.     

Influence of In doping on electro-optical properties of ZnO films

Thin metallic films of Zn and In/Zn were deposited onto glass substrates by thermal evaporation under vacuum. The metallic films were submitted to a thermal oxidation in air, at 623 K, for different oxidation times (30–90 min), in order to be oxidized. Structural and morphological analyses (X-ray diffraction, transmission electron microscopy and scanning electron microscopy) revealed that the obtained undoped and In-doped ZnO thin films possess a polycrystalline structure. Transmission spectra were recorded in spectral domain from 280 to 1400 nm. The influence of In doping and oxidation parameters as well, on the optical parameters (transmittance, optical bandgap, Urbach energy) were analysed. It was clearly evidenced that by In doping, the optical properties of ZnO films were improved. The temperature dependence of electrical conductivity was studied using surface-type cells with Ag electrodes. The obtained results indicate that In-doped ZnO films exhibit an enhancement of electrical conductivity with few orders of magnitude when compared with non-doped ones.     

Microwave conductivity of very thin graphene and metal films

The surface resistance of Ag, Au and A1 thin conducting films deposited on low loss dielectric substrates at microwave frequencies using TE011 mode single post-dielectric resonator (10-13.22 GHz) was measured to calculate their conductivity in relation to layers thickness. This method enabling measurements near metal-insulator percolation transition was also applied for epitaxial graphene deposited on semi-insulating SiC. Moreover, effective microwave conductivity has been determined for intentionally made aluminum island structure where the DC conductivity is equal to zero. Special attention was paid to films thickness measurements which is critical for accuracy of sheet resistance calculation. Conductivity of thin metal layers and very thin graphene was compared.     

Role of oxygen in enhancing N-type conductivity of CuInS2 thin films

Post-growth treatments in air atmosphere were performed on CuInS₂ films prepared by the single-source thermal evaporation method. Their effect on the structural, optical and electrical properties of the films was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical reflection and transmission and resistance measurements. The films were annealed from 100 to 350 °C in air. The stability of the observed N-type conductivity after annealing depends strongly on the annealing temperature. Indeed it is shown that for annealing temperatures above 200 °C the N-type conductivity is stable. The resistance of the N-CuInS₂ thin films correlates well with the corresponding annealing temperature. The samples after annealing have direct bandgap energies of 1.45-1.50 eV.     

Preparation and investigation of VOx thin films of n- and p-types

In this work we present first results on the synthesis of vanadium oxide semi-transparent conducting thin films of p- and n-types. The films were deposited by thermal evaporation method in vacuum, on: silicon, glass, sapphire, and gold substrates. Temperature of substrate during deposition was set around 250 °C. As deposited films were of a p-type of conductivity. Thermal annealing at 420 °C of as-deposited films in air at atmospheric pressure resulted in a change in the conductivity type.Optical, electrical and thermal properties of the deposited films were studied. The topography of the films was studied using AFM microscope. P-N structures were created using VO_x films on silicon and glass substrates. Electrical measurements had shown a non-linear behaviour of the samples.     

Influence of radio frequency power on structure and ionic conductivity of LiPON thin films

Lithium phosphorus oxynitride (LiPON) thin films as solid electrolytes were prepared by radio frequency magnetron sputtering of a Li₃PO₄ target in ambient nitrogen atmosphere. The influence of radio frequency (rf) power on the structure and the ionic conductivity of LiPON thin films has been investigated. The morphology, composition, structure and ionic conductivity of thin films were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and a.c. impedance measurement. It was found that ionic conductivity of LiPON thin films increases with N content in thin films. XPS measurements reveal that ionic conductivity also keeps relativity with the structure of thin films. Higher the N _t/N _d ratio, higher will be the ionic conductivity of LiPON thin films. And both of them can be improved by increasing rf power from 1·5 W/cm² to 5·5 W/cm².     

薄膜热导率测试方法研究进展

薄膜材料的热导率一般与其相应的块体材料有较大的差异.由于其厚度较小,对块体材料热导率的测试方法一般不适用于薄膜材料,因此近几十年来,研究工作者们发明了很多新的用于薄膜热导率的测试方法.综述了薄膜热导率的一些常用测试方法,着重介绍了薄膜热导率的最新测试方法的特点及其适用范围,并针对广泛使用的3ω法进行了详细介绍.