Preparation and characterisation of certain II-VI, I-III-VI2 semiconductor thin films and transparent conducting oxides

There is an increasing demand for renewable energies due to the limited availability of fossil and nuclear fuels and due to growing environmental problems. Photovoltaic (PV) energy conversion has the potential to contribute significantly to the electrical energy generation in the future. Currently, the cost for photovoltaic systems is one of the main obstacles preventing production and application on a large scale. The photovoltaic research is now focused on the development of materials that will allow mass production without compromising on the conversion efficiencies. Among important selection criteria of PV material and in particular for thin films, are a suitable band gap, high absorption coefficient and reproducible deposition processes capable of large-volume and low cost production. The chalcopyrite semiconductor thin films such as Copper indium selenide and Copper indium sulphide are the materials that are being intensively investigated for lowering the cost of solar cells. Conversion efficiencies of 19 % have been reported for laboratory scale solar cell based on CuInSe2 and its alloys. The main objective of this thesis work is to optimise the growth conditions of materials suitable for the fabrication of solar cell, employing cost effective techniques. A typical heterojunction thin film solar cell consists of an absorber layer, buffer layer and transparent conducting contacts. The most appropriate techniques have been used for depositing these different layers, viz; chemical bath deposition for the window layer, flash evaporation and two-stage process for the absorber layer, and RF magnetron sputtering for the transparent conducting layer. Low cost experimental setups were fabricated for selenisation and sulphurisation experiments, and the magnetron gun for the RF sputtering was indigenously fabricated. The films thus grown were characterised using different tools. A powder X-ray diffractometer was used to analyse the crystalline nature of the films. The energy dispersive X-ray analysis (EDX) and scanning electron microscopy i (SEM) were used for evaluating the composition and morphology of the films. Optical properties were investigated using the UV-Vis-NIR spectrophotometer by recording the transmission/absorption spectra. The electrical properties were studied using the two probe and four probe electrical measurements. Nature of conductivity of the films was determined by thermoprobe and thermopower measurements. The deposition conditions and the process parameters were optimised based on these characterisations.

Defect analysis of semiconductor thin films for photovoltaic applications using photo-luminescence and photo-conductivity

The present thesis can be divided into three areas:1) the fabrication of a low temperature photo-luminescence and photoconductivity measuring unit 2) photo-luminescence in the chalcopyrite CulnSez and CulnS2 system for defect and composition analysis and 3) photo-luminescence and photo-conductivity of In:JS3. This thesis shows that photo-luminescence is one of most essential semiconductor characterization tool for a scientific group working on photovoltaics. Tools which can be robust, non-destructive, requiring minimal sample preparation for analysis and most informative of the device applications are sought after by industries and this thesis is towards establishing photo-luminescence as "THE" tool for semiconductor characterization. The possible application of photo-luminescence as a tool for compositional and quality analysis of semiconductor thin films has been worked upon by this thesis. Photo-conductivity complement photo-luminescence and together they provide all the information required for the fabrication of an opto-electronic device.

Non-Destructive Evaluation of Ion-Implanted Semiconductor Thin Films Using Photothermal Deflections Spectroscopy

Materials and equipment which fail to achieve the design requirements or projected life due to undetected defects may require expensive repair or early replacement. Such defects may also be the cause of unsafe conditions or catastrophic unexpected failure, and will lead to loss of revenue due to plant shutdown. Non-Destructive Evaluation (NDE) / Non Destructive Testing (NDT) is used for the examination of materials and components without changing or destroying their usefulness. NDT can be applied to each stage of a system’s construction, to monitor the integrity of the system or structure throughout its life.

Photothermal beam deflection for non-destructive evaluation of semiconductor thin films

Non-destructive testing (NDT) is the use of non-invasive techniques to determine the integrity of a material, component, or structure. Engineers and scientists use NDT in a variety of applications, including medical imaging, materials analysis, and process control.Photothermal beam deflection technique is one of the most promising NDT technologies. Tremendous R&D effort has been made for improving the efficiency and simplicity of this technique. It is a popular technique because it can probe surfaces irrespective of the size of the sample and its surroundings. This technique has been used to characterize several semiconductor materials, because of its non-destructive and non-contact evaluation strategy. Its application further extends to analysis of wide variety of materials. Instrumentation of a NDT technique is very crucial for any material analysis. Chapter two explores the various excitation sources, source modulation techniques, detection and signal processing schemes currently practised. The features of the experimental arrangement including the steps for alignment, automation, data acquisition and data analysis are explained giving due importance to details.Theoretical studies form the backbone of photothermal techniques. The outcome of a theoretical work is the foundation of an application.The reliability of the theoretical model developed and used is proven from the studies done on crystalline.The technique is applied for analysis of transport properties such as thermal diffusivity, mobility, surface recombination velocity and minority carrier life time of the material and thermal imaging of solar cell absorber layer materials like CuInS2, CuInSe2 and SnS thin films.analysis of In2S3 thin films, which are used as buffer layer material in solar cells. The various influences of film composition, chlorine and silver incorporation in this material is brought out from the measurement of transport properties and analysis of sub band gap levels.The application of photothermal deflection technique for characterization of solar cells is a relatively new area that requires considerable attention.The application of photothermal deflection technique for characterization of solar cells is a relatively new area that requires considerable attention. Chapter six thus elucidates the theoretical aspects of application of photothermal techniques for solar cell analysis. The experimental design and method for determination of solar cell efficiency, optimum load resistance and series resistance with results from the analysis of CuInS2/In2S3 based solar cell forms the skeleton of this chapter.

Photoluminescence studies of spray pyrolytically grown nanostructured tin oxide semiconductor thin films on glass substrates

SnO2 nanocrystalline thin films were deposited on glass substrates by the spray pyrolysis technique in air atmosphere at 375, 400, 425, 450 and 500 ◦C substrate temperatures. The obtained films were characterized by using XRD. The room temperature photoluminescence (PL) spectra of these films have near band edge (NBE) and deep level emission under the excitation of 325 nm radiation. NBE PL peak intensity decreased consistently with temperatures for samples prepared at 400, 450 and 500 ◦C, while a sudden reduction in intensity is observed for the sample prepared at 425 ◦C. A similar effect was observed for the optical transmittance spectra. These effects can be explained on the basis of the change in population of oxygen vacancies as indicated by the change in a values

Few-layer, large-area, 2D covalent organic framework semiconductor thin films.

In this work, we synthesize large-area thin films of a conjugated, imine-based, two-dimensional covalent organic framework at the solution/air interface. Thicknesses between ∼2-200 nm are achieved. Films can be transferred to any desired substrate by lifting from underneath, enabling their use as the semiconducting active layer in field-effect transistors.

Investigations On The Preparation And Properties Of Reactively Evaporated Lead Sulphide And Tin Telluride Thin Films

The work reported in this thesis is the preparation, and the structural, electrical and optical properties of reactively evaporated lead sulphide and tin telluride thin films. The three temperature method had been used for the preparation of these semiconductor thin films. In this preparation technique constituent elements are evaporated from separate sources with the substrate kept at a particular temperature. when one of the constituent element is a gas near room temperature, the method is often called reactive evaporation. It has been found for many materials that a stoichiometric interval exists with a limited range of flux and substrate temperature. Usually this technique is used for the preparation of thin films of high melting point compounds or of materials which decompose during evaporation. Tin telluride and lead sulphide are neither high melting point materials nor do they decompose on melting. But even than reactive evaporation offers the possibility of changing the ratios of the flux of the constituent elements within a wide range and studying its effect on the properties of the films

Nanogravimetric study of lead underpotential deposition on selenium thin films as a semiconductor alloy formation procedure

An electrochemical quartz crystal microbalance Au electrode modified with a Se thin film was used to investigate the electrochemical behavior of lead ad-atoms using underpotential deposition (UPD) conditions. A specific quasi-reversible process was observed during the reduction of Pb2+ on Se thin films in perchloric acid media. The charge density of Pb ad-atoms on Se thin film (46.86 mu C cm(-2)) suggests a recovery of 0.1 monolayers, which is in good agreement with EQCM data. The Se thin film can be successfully alloyed with Pb atoms that are deposited by chronoamperometry using time intervals large enough to allow for diffusion toward the inner Se phase. Linear sweep voltammetry combined with EQCM in perchloric acid was used to characterize the amount of Pb absorbed in the Se thin film. These findings offer a new strategy for alloy formation in semiconductor films using UPD as an effective tool to quantify the exact amount of the incorporated metal.

Engineering thin films of magnetic alloys and semiconductor oxides at the nanoscale

The thesis aims to exploit properties of thin films for applications such as spintronics, UV detection and gas sensing. Nanoscale thin films devices have myriad advantages and compatibility with Si-based integrated circuits processes. Two distinct classes of material systems are investigated, namely ferromagnetic thin films and semiconductor oxides. To aid the designing of devices, the surface properties of the thin films were investigated by using electron and photon characterization techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and energy-dispersive X-ray spectroscopy (EDS). These are complemented by nanometer resolved local proximal probes such as atomic force microscopy (AFM), magnetic force microscopy (MFM), electric force microscopy (EFM), and scanning tunneling microscopy to elucidate the interplay between stoichiometry, morphology, chemical states, crystallization, magnetism, optical transparency, and electronic properties. Specifically, I studied the effect of annealing on the surface stoichiometry of the CoFeB/Cu system by in-situ AES and discovered that magnetic nanoparticles with controllable areal density can be produced. This is a good alternative for producing nanoparticles using a maskless process. Additionally, I studied the behavior of magnetic domain walls of the low coercivity alloy CoFeB patterned nanowires. MFM measurement with the in-plane magnetic field showed that, compared to their permalloy counterparts, CoFeB nanowires require a much smaller magnetization switching field , making them promising for low-power-consumption domain wall motion based devices. With oxides, I studied CuO nanoparticles on SnO2 based UV photodetectors (PDs), and discovered that they promote the responsivity by facilitating charge transfer with the formed nanoheterojunctions. I also demonstrated UV PDs with spectrally tunable photoresponse with the bandgap engineered ZnMgO. The bandgap of the alloyed ZnMgO thin films was tailored by varying the Mg contents and AES was demonstrated as a surface scientific approach to assess the alloying of ZnMgO. With gas sensors, I discovered the rf-sputtered anatase-TiO2 thin films for a selective and sensitive NO2 detection at room temperature, under UV illumination. The implementation of UV enhances the responsivity, response and recovery rate of the TiO2 sensor towards NO2 significantly. Evident from the high resolution XPS and AFM studies, the surface contamination and morphology of the thin films degrade the gas sensing response. I also demonstrated that surface additive metal nanoparticles on thin films can improve the response and the selectivity of oxide based sensors. I employed nanometer-scale scanning probe microscopy to study a novel gas senor scheme consisting of gallium nitride (GaN) nanowires with functionalizing oxides layer. The results suggested that AFM together with EFM is capable of discriminating low-conductive materials at the nanoscale, providing a nondestructive method to quantitatively relate sensing response to the surface morphology.

Hall Effect and transient surface photovoltage (SPV) study of Cu3BiS3 thin films

Se presentan las propiedades eléctricas del compuesto Cu3BiS3 depositado por co-evaporación. Este es un nuevo compuesto que puede tener propiedades adecuadas para ser utilizado como capa absorbente en celdas solares. Las muestras fueron caracterizadas a través de medidas de efecto Hall y fotovoltaje superficial transiente (SPV). A través de medidas de efecto Hall se encontró que la concentración de portadores de carga n es del orden de 1016 cm-3 independiente de la relación de masas de Cu/Bi. También se encontró que la movilidad de este compuesto (μ del orden de 4 cm2V -1s-1) varía de acuerdo con los mecanismos de transporte que la gobiernan en dependencia con la temperatura. A partir de las medidas de SPV se encontró alta densidad de defectos superficiales, defectos que son pasivados al superponer una capa buffer sobre el compuesto Cu3BiS3.

Effect of Mn concentration and atomic structure on the magnetic properties of Ge thin films

This work reports on the magnetic properties of Ge(100-x)Mn(x) (x=0-24 at. %) films prepared by cosputtering a Ge+Mn target and submitted to cumulative thermal annealing treatments up to 500 degrees C. Both as-deposited and annealed films were investigated by means of compositional analysis, Raman scattering spectroscopy, magnetic force microscopy, superconducting quantum interference device magnetometry, and electrical resistivity measurements. All as-deposited films (either pure or containing Mn) exhibit an amorphous structure, which changes to crystalline as the annealing treatments are performed at increasing temperatures. In fact, the magnetic properties of the present Ge(100-x)Mn(x) films are very sensitive to the Mn content and whether their atomic structure is amorphous or crystalline. More specifically: whereas the amorphous Ge(100-x)Mn(x) films (with high x) present a characteristic spin glass behavior at low temperature; after crystallization, the films (with moderate Mn contents) are ferromagnetic at room temperature. Moreover, the magnetic behavior of the films scales with their Mn concentration and tends to be more pronounced after crystallization. Finally, the semiconducting behavior of the films, experienced by previous optical studies, was confirmed through electrical measurements, which also indicate the dependence of the resistivity with the atomic composition of the films. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3520661]

Precursors’ order effect on the properties of sulfurized Cu2ZnSnS4 thin films

A dc magnetron sputtering-based method to grow high-quality Cu2ZnSnS4 (CZTS) thin films, to be used as an absorber layer in solar cells, is being developed. This method combines dc sputtering of metallic precursors with sulfurization in S vapour and with post-growth KCN treatment for removal of possible undesired Cu2−xS phases. In this work, we report the results of a study of the effects of changing the precursors’ deposition order on the final CZTS films’ morphological and structural properties. The effect of KCN treatment on the optical properties was also analysed through diffuse reflectance measurements. Morphological, compositional and structural analyses of the various stages of the growth have been performed using stylus profilometry, SEM/EDS analysis, XRD and Raman Spectroscopy. Diffuse reflectance studies have been done in order to estimate the band gap energy of the CZTS films. We tested two different deposition orders for the copper precursor, namely Mo/Zn/Cu/Sn and Mo/Zn/Sn/Cu. The stylus profilometry analysis shows high average surface roughness in the ranges 300–550 nm and 230–250 nm before and after KCN treatment, respectively. All XRD spectra show preferential growth orientation along (1 1 2) at 28.45◦. Raman spectroscopy shows main peaks at 338 cm−1 and 287 cm−1 which are attributed to Cu2ZnSnS4. These measurements also confirm the effectiveness of KCN treatment in removing Cu2−xS phases. From the analysis of the diffuse reflectance measurements the band gap energy for both precursors’ sequences is estimated to be close to 1.43 eV. The KCN-treated films show a better defined absorption edge; however, the band gap values are not significantly affected. Hot point probe measurements confirmed that CZTS had p-type semiconductor behaviour and C–V analysis was used to estimate the majority carrier density giving a value of 3.3 × 1018 cm−3.

A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Thin films of Cu2SnS3 and Cu3SnS4 were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu2SnS3 phase changes. For a temperature of 350 ◦C the films were composed of tetragonal (I -42m) Cu2SnS3. The films sulfurized at a maximum temperature of 400 ◦C presented a cubic (F-43m) Cu2SnS3 phase. On increasing the temperature up to 520 ◦C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu3SnS4 was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm−1 for tetragonal Cu2SnS3, 303 and 355 cm−1 for cubic Cu2SnS3, and 318, 348 and 295 cm−1 for the Cu3SnS4 phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 104 cm−1. The estimated band gap energy was 1.35 eV for tetragonal (I -42m) Cu2SnS3, 0.96 eV for cubic (F-43m) Cu2SnS3 and 1.60 eV for orthorhombic (Pmn21) Cu3SnS4. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4 are 4.59 × 10−2 cm, 1.26 × 10−2 cm, 7.40 × 10−4 cm, respectively.

Study of optical and structural properties of Cu2ZnSnS4 thin films

Cu2ZnSnS4 is a promising semiconductor to be used as absorber in thin film solar cells. In this work, we investigated optical and structural properties of Cu2ZnSnS4 thin films grown by sulphurization of metallic precursors deposited on soda lime glass substrates. The crystalline phases were studied by X-ray diffraction measurements showing the presence of only the Cu2ZnSnS4 phase. The studied films were copper poor and zinc rich as shown by inductively coupled plasma mass spectroscopy. Scanning electron microscopy revealed a good crystallinity and compactness. An absorption coefficient varying between 3 and 4×104cm−1 was measured in the energy range between 1.75 and 3.5 eV. The band gap energy was estimated in 1.51 eV. Photoluminescence spectroscopy showed an asymmetric broad band emission. The dependence of this emission on the excitation power and temperature was investigated and compared to the predictions of the donor-acceptor-type transitions and radiative recombinations in the model of potential fluctuations. Experimental evidence was found to ascribe the observed emission to radiative transitions involving tail states created by potential fluctuations.