CdTe thin film solar cells: Interrelation of nucleation, structure, and performance

The performance of CdTe solar cells as prototype of thin film solar cells strongly depends on film morphology. The needs for high solar cell performance using thin film materials will be addressed covering nucleation and growth control of thin film materials. In order to understand the basic growth mechanisms and their impact on cell performance, we have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS) using the integrated ultra-high vacuum system DAISY-SOL. CdTe thin films were deposited on TCO/CdS substrates (transparent conductive oxide) held at 270 °C to 560 °C. The properties of the films were determined before and after CdCl₂ treatment using X-ray diffraction and electron microscopy. In addition, solar cells were prepared to find correlations between material properties and cell efficiency. At low sample temperature the films tend to form compact layers with preferred (111) orientation which is lost at elevated temperatures above 450 °C. For CdS layers without (0001) texture there is in addition a low temperature regime (350 °C) with (111) texture loss. After activation treatment the (111) texture is lost for all deposited layers leading to strong recrystallisation of the grains. But the texture still depends on the previous growth history. The loss of (111) texture is evidently needed for higher performance. A clear correlation between cell efficiency and the texture of the CdTe film is observed.     

A review of crystallographic textures in chemical vapor-deposited diamond films

Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond.     

A review of crystallographic textures in chemical vapor-deposited diamond films

Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the〈100〉direction to that along the 〈111〉direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond.     

Influence of thickness on the crystallography and surface topography of TiN nano-films deposited by reactive DC and pulsed magnetron sputtering

TiN films of 50 nm and 500 nm thickness were deposited on M2 tool steel substrates by reactive closed field unbalanced magnetron sputtering operating in direct current (DC) and pulsed magnetron sputtering (PMS) modes. Parameters of the crystallographic structure and surface roughness and their evolution during the films growth were analyzed via X-ray diffraction and atomic force microscopy. The obtained results show that all the analyzed films have polycrystalline and mono-phase (TiN) structures. In the 50 nm films, the in plane crystallographic texture that formed was 100% {111}. During film growth a weakening of the preferred crystallographic orientation and a decrease of the concentration of lattice imperfections occurred. Both processes are more pronounced in the film deposited by PMS compared to that deposited by DC sputtering. Film growth is accompanied by increasing of surface smoothness. Pulsing the target power led to a decrease of the mean surface roughness of both the 50 nm and 500 nm films.     

The effect of texture and internal stresses on the perpendicular anisotropy of vacuum evaporated ferromagnetic films

The effect of crystallographic texture and macrostresses on perpendicular anisotropy was studied. The effect of texture is shown to be considerable for materials with high magnetocrystalline anisotropy (Co, Fe). Tensile stresses can both increase and reduce perpendicular anisotropy (in materials with positive and negative magnetostriction respectively). In the films of Fe in the presence of texture [100] contribution of stresses to perpendicular anisotropy may be positive, zero or negative depending on the degree of the texture perfection.     

Texture and secondary extinction measurements in Al\Ti stratified films by X-ray diffraction

In the present work the effects of secondary extinction has been studied using standard X-ray diffractometer and texture diffractometer techniques. For this purpose the texture of Al\Ti stratified films have been measured and evaluated. It is shown that these films have a very sharp single component texture. Therefore the evaluation and the interpretation of the measurement data is only possible when secondary extinction correction is carried out. The effect of secondary extinction is evaluated on the 111 pole figures of the stratified films with different layer pair numbers. The volume fraction of the main texture component is calculated from pole figures and it is shown that the grains of the films are practically completely oriented with 111 crystallographic direction parallel to the film surface normal. Moreover in the case of pole density measurements the secondary extinction values themselves are simultaneously calculated.     

Effects of sputtering pressure and nitrogen concentration on the preferred orientation of AIN thin films

Aluminium-nitride films were prepared on glass substrates by reactive radio frequency (r.f.) magnetron sputtering in argon/nitrogen gas mixtures containing 25 ~ 75 1/2; nitrogen at substrate temperatures below 150C. It is important to control the crystallographic orientation and the surface morphology of the films with the deposition parameters for surface-acoustic-wave (SAW) devices. The change of crystallographic orientation with the sputtering pressure and the nitrogen concentration was calculated from the texture coefficient of the (0002) plane based on X-ray diffraction (XRD) patterns. It was found that a change of the c-axis from a parallel to a normal orientation, with respect to the substrate surface, occurred with a decrease in the sputtering pressure and an increase in the nitrogen concentration. From observations of the cross-section and the surface morphology, aluminium-nitride films exhibited a columnar structure and the grain size at the film surface increased an increase in the sputtering pressure and with a decrease in the nitrogen concentration.     

Structure,phase composition,and nanohardness of vacuum-annealed multilayer fullerite/aluminum films

The influence of the number of layers and thermal annealing on the structure, elemental and phase compositions, and nanohardness of multilayer fullerite/aluminum films has been studied by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray microanalysis, and nanoindentation. The results demonstrate that sequential growth of five aluminum layers and four fullerite layers, each 50 nm in thickness, on oxidized single-crystal silicon substrates leads to the formation of textured films, which retain 111 texture after vacuum annealing at 620 K (τ = 5 h). In the case of the growth of bilayer films of greater thickness, C₆₀(200 nm)/Al(300 nm), the fullerite and aluminum have a polycrystalline structure with no growth texture. Thermal annealing of the bilayer films leads to the formation of a new phase, Al_xC₆₀. The materials studied here possess enhanced nanohardness compared to pure aluminum and fullerite films.     

Influence of morphologic texture on stress analysis by X-ray and neutron diffraction in single-phase metallic materials

In this work, a study on the influence of morphologic texture on the residual stress determination by diffraction in metallic materials with cubic and hexagonal symmetry is proposed. To this end, elastic self-consistent model has been developed to properly take into account the morphologic texture. Extreme crystallites morphologies (sphere, disc and fibre) were studied, and coupled with the crystallographic texture to reflect the combined effect of morphologic and crystallographic texture in elasticity. In the case of morphologic texture, a stronger influence than the crystallographic texture on the estimated residual stresses (several tens of MPa difference) was observed. We propose a methodology through a scale transition model to take into account the influence of these different morphologies in the stress analysis by diffraction methods. The main purpose of this work was to make the best choice for lattice planes (hkl) used for residual or internal stress analysis, in elasticity, depending on the morphologic (and crystallographic) texture of the polycrystal, especially when the usual X-ray Elasticity Constants (XECs) are used instead of the stress factors.     

Barrier layer,geometry and alloying effects on the microstructure and texture of electroplated copper thin films and damascene lines

The effect of two different barrier materials (sputtered Ta and TaN) and their layering schemes on the microstructure of the sputtered copper seed and electroplated Cu films and interconnects was investigated using electron backscatter diffraction and X-ray diffraction. The films had a predominantly (111) texture and the microstructural change with the change of barrier layer material was minimal. The sidewall constraint effect on the microstructure development in narrow damascene trenches on Ta barrier was also examined by analyzing 0.5 μm deep lines over a range of different line widths (0.2–5.0 μm). The constraint due to the sidewall produces an [011] orientation along the line length. The in-plane texture strengthens with decreasing line width. The crystallographic texture was a function of line width with the intermediate line widths (near 1 μm) having the weakest texture and highest fraction of twin boundaries compared to the other line widths. The effect of alloying on the microstructure and texture was examined in similar damascene lines with a Cu–1% Ag seed layer before electroplating with pure Cu. The Ag addition decreased the average grain size without affecting the texture.     

Lattice Expansion by Adding Oxygen to Control Crystallographic Orientations in Chemically Ordered L10 FePt Films

We fabricated L1₀ FePt thin films by sputtering in reactive oxygen on polycrystalline glass substrates, and we investigated the magnetic properties and crystallographic orientations of the films. Oxygen addition during the FePt deposition promoted heteoroepitaxial growth by decreasing the lattice misfit with the Ag underlayer. In an oxygen/argon ratio of 1.5-3.0 vol.%, the in-plane lattice parameter of the FePt films expanded, and the lattice misfit with the Ag underlayer decreased from 6.3 to 3.9% in the as-deposited state, as determined by grazing incidence X-ray diffraction (GIXRD). Annealing at 700degC for 1 min produced a heteroepitaxially grown L1₀ (001) texture with a large out-of-plane coercivity of 8.8 kOe and a nucleation field of kOe. Transmission electron microscopy showed that average grain size in the as-deposited films was about 4-5 nm and was in the range of 10-15 nm in the annealed films, indicating that there was some grain growth.     

Morphology,microstructure, and residual stress in EBPVD erbia coatings

The electron-beam physical vapor deposition of erbium-oxide coatings onto sapphire wafers is investigated to evaluate processing effects on the residual stress state and microstructure. The erbium-oxide coatings are found to be in a compressive stress state. The crystallographic texture of the erbium-oxide coating is evaluated using X-ray diffraction along with an assessment of forming the cubic erbia phase as a function of substrate temperature. In addition to the cubic erbia phase, an orthorhombic phase is found at the lower deposition temperatures. A transition is found from a two-phase erbium-oxide coating to a single phase at deposition temperatures above 948 K. The variation in morphology with deposition temperature observed in fracture cross-sections is consistent with features of the classic zone growth models for vapor-deposited oxide coatings. For high-temperature applications, a deposition process temperature above 948 K is seen to produce a stoichiometric, fully dense, and equiaxed-polycrystalline coating of cubic erbia.     

Fabrication, structural and electrical characterization of lanthanum tungstate films by pulsed laser deposition

Films of lanthanum tungstate, 3 μm in thickness, were fabricated by means of pulsed laser deposition on a Pd foil. The films were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and their electrical conductivity was measured at temperatures between 400 and 800 °C in different gas atmospheres. The films' structure and electrical characteristics are close to what is reported in the literature for corresponding polycrystalline material. The films exhibit fairly high proton conductivity at elevated temperatures, which make them interesting for components in hydrogen-related technologies. Changes in microstructure and the crystallographic orientation observed at higher temperatures were accompanied by changes in the conductivity characteristics.     

Structure and microstructure of EB-PVD yttria thin films grown on Si (111) substrate

Structure and microstructure of yttria thin films grown by electron beam physical vapour deposition on a stationary Si (111) substrate at room temperature (RT), 500° and 700 °C, were investigated by the grazing-incidence X-ray diffraction and scanning electron microscopy, respectively. X-ray photoelectron spectroscopy provided information on the surface contamination from the atmosphere and the Y oxidation state. A strong effect of the deposition temperature and the vapour flux incidence angle was found. The film deposited at RT is polycrystalline with very fine grains of the body-centered cubic (bcc) crystallographic symmetry. An increase of deposition temperature results in a rapid growth of bcc grains with an improved crystalline structure. Moreover, the based-centered monoclinic phase appears for the deposition temperature of 700 °C. Preferred grain orientation (texture) with two main components, (400) and (622), was observed in the films deposited at 500 °C whereas no texture was found for 700 °C. The microstructure exhibits the columnar feather-like structure of different degrees of perfection which can be explained by the shadowing effects caused by an oblique vapour flux incidence angle. Surface morphology of the films is governed by a combination of the triangular and four-sided (square) columns. All films were found to be dense with a little porosity between the columns.     

Two-dimensional versus three-dimensional post-deposition grain growth in epitaxial oxide thin films: Influence of the substrate surface roughness

Epitaxial thin films made of nanosized yttria-stabilized zirconia islands deposited on (0001) sapphire substrates are synthesized by sol-gel dip-coating followed by a high-temperature post-deposition thermal annealing procedure. At high temperatures, a competitive growth process takes place that allows to obtain thin films made of atomically flat islands with an in-plane diameter typically ten times higher than the thickness or on the contrary inducing the formation of dome-shaped islands. Apart from having a different shape, these islands are also characterized by a different crystallographic orientation with respect to the substrates respectively (001) and (111). In this paper, we investigate the influence of the substrate surface roughness on this competitive grain growth process. The deposition on epi-polished substrates results in a two-dimensional (2D) island growth, whereas the deposition on rough substrates results in a three-dimensional (3D) growth of dome-shaped nanosized islands. The films have been characterized by atomic force microscopy and high-resolution X-ray diffraction using the reciprocal space mapping technique.     

Temperature dependent biaxial texture evolution in Ge films under oblique angle vapor deposition

The morphology and texture of Ge films grown under oblique angle vapor deposition on native oxide covered Si(001) substrates at temperatures ranging from 230 °C to 400 °C were studied using scanning electron microscopy, X-ray diffraction and X-ray pole figure techniques. A transition from polycrystalline to {001}<110> biaxial texture was observed within this temperature range. The Ge films grown at substrate temperatures < 375 °C were polycrystalline. At substrate temperatures of 375 °C and 400 °C, a mixture of polycrystalline and biaxial texture was observed. The 230 °C sample consisted of isolated nanorods, while all other films were continuous. The observed biaxial texture is proposed to be a result of the loss of the interface oxide layer, resulting in epitaxial deposition of Ge on the Si and a texture following that of the Si(001) substrates used. The rate of oxide loss was found to increase under oblique angle vapor deposition.     

A combinatorial study on the influence of Cu addition, film thickness and heat treatment on phase composition, texture and mechanical properties of Ti-Ni shape memory alloy thin films

A series of amorphous Ti-Ni-Cu based thin films with compositional spread and different thicknesses was produced at room temperature using a magnetron sputtering device and crystallized by means of different heat treatments. Phase composition and crystallographic orientation of the films were characterized using a Multi-Axial X-ray Diffraction approach and their mechanical properties were assessed using nanoindentation. The results show that the addition of Cu and thermal treatments can be used to significantly alter the phase composition, the texture and the microstructure of the films. Film thickness plays an important role in determining of the amount of martensitic phase at room temperature as well as the amount of Ti-rich and Ni-rich phases. In particular a thickness threshold between 350 nm and 800 nm was shown to be critical for the triggering of the texture in the austenitic phase.     

Influence of microstructure on the electrochemical performance of tin-doped indium oxide film electrodes

The effect of the microstructure of tin-doped indium oxide (ITO) films on their electrochemical performance was studied using three redox probes, tris(2,2'-bipyridyl ruthenium(II) chloride (Ru(bpy)3(2+/3+)), ferrocyanide (Fe(CN)6(4-/3-)), and ferrocenemethanol (FcCH2H(0/+)). ITO films were deposited using dc magnetron sputtering under a variety of conditions that resulted in films having different degrees of crystallinity, crystallographic texture, sheet resistance, surface roughness, and percent tin. It was found that the electron transfer for all three redox probes used in this study was more efficient at polycrystalline films than at amorphous ITO films. This effect is more pronounced at faster scan rates. The crystallographic texture of the ITO films, surface roughness, and a change in sheet resistance from 7.9 to 13.7 ohms/square did not have an effect on electron-transfer kinetics. ITO films deposited using a 1 wt % SnO2 target and having sheet resistance comparable to films deposited using a 10 wt % SnO2 target had dramatically different microstructure from the films with higher weight percent Sn and were shown to perform poorly when used as electrode materials. We believe that the dramatic differences in electron-transfer kinetics observed at the various ITO films can be attributed to either the different density of defect sites along the grain boundaries or defect sites caused by substitutional Sn in the film.     

Structure evolution of the biaxial alignment in sputter-deposited MgO and Cr

The growth of biaxially aligned layers, i.e. layers with both a preferential out-of-plane and an in-plane crystallographic orientation, on non-aligned metallic substrates is investigated. Unbalanced magnetron sputtering on an inclined substrate is used to deposit the layers.This method enables us to grow biaxially aligned layers for different classes of materials with different physical and chemical properties. The results for biaxially aligned MgO which is a cubic metal oxide (FCC rocksalt structure), and pure metallic chromium films (BCC) are presented.A comparison between biaxially aligned MgO and Cr concerning the microstructure and crystallographic texture is discussed. A correlation between the sputter deposition parameters on the biaxial alignment of both materials is observed. Both materials have a columnar V-shape structure with a faceted surface, corresponding to zone T of the well known structure zone model of Thornton. The MgO layers exhibit a [111] out-of-plane orientation, while Cr layers have an [100] preferential orientation. MgO as well as Cr show a strong in-plane alignment.     

Effect of substrate temperature on structural, morphological and optical properties of crystalline titanium dioxide films prepared by DC reactive magnetron sputtering

Titanium dioxide (TiO₂) thin films have been deposited with various substrate temperatures by dc reactive magnetron sputtering method onto glass substrate. The effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. Chemical composition of the films was investigated by X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) analysis of the films revealed that they have polycrystalline tetragonal structure with strong (101) texture. The surface morphological study revealed the crystalline nature of the films at higher substrate temperatures. The TiO₂ films show the main bands in the range 400–700 cm^?1, which are attributed to Ti–O stretching and Ti–O–Ti bridging. The transmittance spectra of the TiO₂ thin film measured with various substrate temperatures ranged from 75 to 90 % in the visible light region. The optical band gap values of the films are increasing from 3.44 to 4.0 eV at growth temperature from 100 to 400 °C. The structural and optical properties of the films improved with the increase in the deposition temperature.