Polycrystalline Thin Film Materials Research Articles

Nanocrystalline polymorphic oxide perovskite-based high-κ low-leakage thin film materials

We report on thin-film nanocrystalline polymorphic BaTiO3 for enabling high dielectric and low leakage response. BaTiO3-Al2O3 bi-layer thin films based on nanocrystalline BaTiO3 containing the tetragonal and hexagonal polymorphs were deposited by atomic layer deposition. Polymorphic BaTiO3 film combined with a ≈ 3.5-nm thick Al2O3 layer located between the BaTiO3 film and top electrode exhibits dielectric constant of 108 or 130 and leakage currents 2.2 × 10−8 A/mm2 or 1.3 × 10−7 A/mm2 at 1 MV/cm, respectively. Where x-ray photoemission analysis of barrier heights for the metal-BaTiO3-Al2O3-metal structure point to using the polymorphic BaTiO3 interspersed between Al2O3 layers in tri-layered dielectric thin film capacitors, the outstanding combination of high dielectric constant and low leakage current for the polymorphic BaTiO3 - Al2O3 thin film stacks is superior to common high-κ polycrystalline thin film materials.

Correlated TKD/EDS - TEM - APT analysis on selected interfaces of CoSi2 thin films

Correlative analysis is a powerful way to relate crystallographic and chemical information to the properties of materials. In this work, a procedure is proposed to select and analyze interfaces of polycrystalline thin film materials through correlative transmission Kikuchi diffraction/energy dispersive X-ray spectroscopy (TKD/EDS), transmission electron microscopy (TEM) and atom probe tomography (APT). TKD provides information on the crystallographic orientation. The EDS analysis performed together with TKD in the scanning electron microscope (SEM) makes chemical information available allowing phases of similar crystal structure, but with a different composition to be distinguished. The information of TKD/EDS can be correlated to successive TEM and APT analysis on selected interfaces for structural and chemical analysis at the atomic scale. An interface of an epitaxial orientated grain of a polycrystalline CoSi2 thin film on (111)Si is selected and analyzed. The selected interface has a twin character and shows facets of different orientation and area. Site-specific segregation of Ge to junctions of the facets is evidenced. The correlation between local strain from misfit (defects) at the interface and segregation is discussed.

Thin-film solar cells exceeding 22% solar cell efficiency: An overview on CdTe-, Cu(In,Ga)Se2-, and perovskite-based materials

Already, several technologies of polycrystalline thin-film photovoltaic materials have achieved certified record small-cell power conversion efficiencies exceeding 22%. They are CdTe, Cu(In,Ga)(S,Se)2 (CIGS), and metal halide perovskite (PSC), each named after the light-absorbing semiconductor material. Thin-film solar cells and modules require very little active material due to their very high absorption coefficient. Efficient production methods with low materials waste, moderate temperatures, attractive cost structures, and favorable energy payback times will play a strong role in market development as thin-film technologies reach full maturity, including mass production and the standardization of production machineries. In fact, the first two technologies have already been developed up to the industrial scale with a market share of several GW. In this review article, we outline similarities and differences between these high-efficiency thin-film technologies from both the materials and the industrial point of view. We address the materials characteristics and device concepts for each technology, including a description of recent developments that have led to very high efficiency achievements. We provide an overview of the CIGS industry players and their current status. The newcomer PSC has demonstrated its potential in the laboratory, and initial efforts in industrial production are underway. A large number of laboratories are experimenting through a wide range of options in order to optimize not only the efficiency but also stability, environmental aspects, and manufacturability of PSC. Its high efficiency and its high bandgap make PSC particularly attractive for tandem applications. An overview of all these topics is included here along with a list of materials configurations.

Estimating complete band diagrams of non-ideal heterointerfaces by combining ellipsometry and photoemission spectroscopy

In this work, we show that spectroscopic ellipsometry can be combined with photoemission spectroscopy to obtain complete interface band diagrams of non-ideal semiconductor heterointerfaces, such as interfaces between thin-film polycrystalline materials. The non-destructive ellipsometry measurement probes the near-interface bandgap of the two semiconductors (including the buried semiconductor) after the interface has formed. This is important in the non-ideal case where chemical processes during interface growth modify the electronic properties of the two separated surfaces. Knowledge of near-interface bandgaps improves accuracy in conduction band offset measurements of non-ideal interfaces, and it sheds light on their device physics. Both of those positive outcomes are demonstrated in the Cu2ZnSnS4/CdS interface used here as a case study, where the bandgap of both materials decreases by up to 200 meV from the bulk to the near-interface region. This finding reveals a preferential electron-hole recombination channel near the interface, and it yields corrected values for the interfacial conduction band offset.

Post deposition surface treatments to enhance the quality of polycrystalline CdTe thin films for photovoltaic applications

Cadmium telluride (CdTe) is one of the topmost thin film polycrystalline materials used in the photovoltaics (PV) industry today and post deposition surface treatment has been a major step used in the production process for improving the photovoltaic quality of the CdTe material. In the present study, several post deposition surface treatment processes including CdCl2 treatment were carried out on CdTe material and the properties of the materials were then analyzed with the intension of gaining an understanding of the effect of the post deposition process on the material properties and identifying better post deposition treatment processes that can be used to improve the PV quality of the material. In this study, CdTe thin films were potentiostatically electrodeposited using the typical three electrode electrolytic cell consisted of a saturated calomel reference electrode and a high purity graphite counter electrode. 3CdSO4.8H2O and TeO2 were used as the cadmium and the tellurium precursors respectively and CdTe layers were electrodeposited on fluorine doped tin oxide (FTO) glass substrates and glass/FTO/CdS surfaces at pre-identified growth conditions namely; cathodic deposition potential of 650 mV, pH of 2.3 and temperature of 65 °C. Subsequently, deposited samples were annealed in air with CdCl2 and thereafter, comparable samples of glass/FTO/CdTe and glass/FTO/CdS/CdTe were subjected to surface etching with diluted HCl (DH), Br2-CH3OH (BM), HNO3-H3PO4 (NP), K2Cr2O7-H2SO4 (DS) and K2Cr2O7-CH3OH (DM). Surface treated samples were then characterized for their electrical, optical, elemental, morphological and structural properties using photo-electrochemical cell measurements, optical absorption spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy and X-ray diffraction spectroscopy respectively. The study reveals that, post deposition surface treatments with BM and NP etchings enhance the material qualities of polycrystalline CdTe layers to be used for fabrication of PV devices.

Mesoscale Functional Imaging of Materials for Photovoltaics

Further global adoption of photovoltaic energy conversion technologies is contingent on sustained progress toward widespread grid-parity. For that, solar cell materials composed of microscale grains and nanoscale boundaries show the highest potential due to their large theoretical efficiency and low-cost fabrication methods. Here we outline the current challenges facing hybrid perovskites and prevalent thin-film polycrystalline materials for photovoltaics. We offer our perspective on how mesoscale functional imaging can enable a complete understanding of the physical and chemical processes restricting their performance, completing the materials science structure–properties–processing–performance tetrahedron. First, we present the key characteristics of hybrid perovskites, CdTe, CIGS, CZTS, and pc-GaAs, emphasizing their main limitations. Second, we discuss how novel imaging methods based on electron and scanning probe microscopies can be realized to provide quantitative information about the relevant para...

Evidence of Formation of Superdense Nonmagnetic Cobalt

Because of the presence of 3d transition metals in the Earth’s core, magnetism of these materials in their dense phases has been a topic of great interest. Theory predicts a dense face-centred-cubic phase of cobalt, which would be nonmagnetic. However, this dense nonmagnetic cobalt has not yet been observed. Recent investigations in thin film polycrystalline materials have shown the formation of compressive stress, which can increase the density of materials. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.2–1.4 times the normal density of Co. This has been revealed by X-ray reflectometry experiments, and corroborated by polarized neutron reflectometry (PNR) experiments. Transmission electron microscopy provides further evidence. The magnetic depth profile, obtained by PNR, shows that the superdense Co layers near the top of the film and at the film-substrate interface are nonmagnetic. The major part of the Co film has the usual density and magnetic moment. These results indicate the possibility of existence of nonmagnetic Co in the earth’s core under high pressure.

Development of an in situ temperature stage for synchrotron X-ray spectromicroscopy.

In situ characterization of micro- and nanoscale defects in polycrystalline thin-film materials is required to elucidate the physics governing defect formation and evolution during photovoltaic device fabrication and operation. X-ray fluorescence spectromicroscopy is particularly well-suited to study defects in compound semiconductors, as it has a large information depth appropriate to study thick and complex materials, is sensitive to trace amounts of atomic species, and provides quantitative elemental information, non-destructively. Current in situ methods using this technique typically require extensive sample preparation. In this work, we design and build an in situ temperature stage to study defect kinetics in thin-film solar cells under actual processing conditions, requiring minimal sample preparation. Careful selection of construction materials also enables controlled non-oxidizing atmospheres inside the sample chamber such as H2Se and H2S. Temperature ramp rates of up to 300 °C/min are achieved, with a maximum sample temperature of 600 °C. As a case study, we use the stage for synchrotron X-ray fluorescence spectromicroscopy of CuIn(x)Ga(1-x)Se2 (CIGS) thin-films and demonstrate predictable sample thermal drift for temperatures 25-400 °C, allowing features on the order of the resolution of the measurement technique (125 nm) to be tracked while heating. The stage enables previously unattainable in situ studies of nanoscale defect kinetics under industrially relevant processing conditions, allowing a deeper understanding of the relationship between material processing parameters, materials properties, and device performance.

Growth and characterization of Cu2ZnSn(SxSe1−x)4 alloys grown by the melting method

I2–II–IV–VI4 quaternary Cu2ZnSn(SxSe1−x)4 (CZTSSe) alloys were successfully grown by the melting growth method. The powder X-ray diffraction (XRD) pattern of the CZTSSe alloys showed preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure. In Raman spectra, the A1 mode peaks expected for CZTS and CZTSe were observed in all samples, and no secondary phases were observed. The CZTSSe alloys in this study were slightly Cu-poor, Zn-rich and VI-rich, similar to compositions of polycrystalline thin-film materials that have achieved high photovoltaic conversion efficiencies. Single phase and homogeneous CZTSSe alloys were obtained.

Ferroelastic Domains and Anisotropy in Lead Free Piezoelectrics

Our research investigates the correlations between domain texture and microstructural features, including crystallographic texture in bulk and thin film polycrystalline materials to understand the development of piezoelectric and other anisotropic properties in a number of rapidly evolving lead free piezoelectric materials. We investigate approaches to understanding polarization distributions by starting from polarization measurements. In addition, 2D and 3D microstructural simulations are carried out in all types of ferroelectrics to rationalize and then engineer their equilibrium and kinetic response. This paper discusses recent findings associated with bulk piezoelectricity, phase stability, and ferroelastic and ferroelectric domain motion for materials such as Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) and Bi0.5Na0.5TiO3 (BNT). Conventional and synchrotron-based x-ray diffraction, electron and optical microscopy and piezoelectric characterization techniques are employed to assess texture, both as a function of poling and temperature. The coupling between microstructure and the inherent directional biases fundamental to piezoelectric and ferroelectric performance enable consideration of orientation and anisotropy in systems with unique constraints.

Determination of Young's modulus and Poisson's ratio of thin films by combining sin 2ψ X-ray diffraction and laser curvature methods

In this study, we have proposed a nondestructive method to simultaneously determine the Young's modulus ( E) and Poisson's ratio ( ν) of polycrystalline thin film materials. The method involved independent stress measurement by laser curvature technique and strain components determination by sin 2 ψ X-ray diffraction (XRD) method, and afterward, elasticity theory was employed to calculate E and ν. The proposed method was applied on two model specimens, TiN and ZrN thin films, using synchrotron X-ray and laboratory X-ray sources, respectively. The cos 2 αsin 2 ψ XRD method which measured the strain for diffraction planes at different location was performed on the same film, and the previously determined E and ν were used to calculate the stress. The residual stresses derived from cos 2 αsin 2 ψ method were close to the stresses from laser curvature measurements, which validated the measured values of E and ν. The depth profile of residual stress of the TiN thin film was assessed using cos 2 αsin 2 ψ method by appropriately adjusting the X-ray incident angle. In addition, the E value determined from nanoindentation (NIP) may depend on the indentation depth. Therefore, one should be cautious when employing the NIP-determined E in sin 2 ψ or cos 2 αsin 2 ψ methods to calculate the residual stress because the modulus may not always give correct stress value.

Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin‐film solar cells

This short communication reports on achieving 18·8% total-area conversion efficiency for a ZnO/CdS/Cu(In,Ga)Se2/Mo polycrystalline thin-film solar cell. We also report a 15%-efficient, Cd-free device fabricated via physical vapor deposition methods. The Cd-free cell includes no buffer layer, and it is fabricated by direct deposition of ZnO on the Cu(In,Ga)Se2 thin-film absorber. Both results have been measured at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m2, 25°C, ASTM E 892 Global). The 18·8% conversion efficiency represents a new record for such devices (Notable Exceptions) and makes the 20% performance level by thin-film polycrystalline materials much closer to reality. We allude to the enhancement in performance of such cells as compared to previous record cells, and we discuss possible and realistic routes to enhance the performance toward the 20% efficiency level. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.

Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin-film solar cells

This short communication reports on achieving 18·8% total-area conversion efficiency for a ZnO/CdS/Cu(In,Ga)Se2/Mo polycrystalline thin-film solar cell. We also report a 15%-efficient, Cd-free device fabricated via physical vapor deposition methods. The Cd-free cell includes no buffer layer, and it is fabricated by direct deposition of ZnO on the Cu(In,Ga)Se2 thin-film absorber. Both results have been measured at the National Renewable Energy Laboratory under standard reporting conditions (1000 W/m2, 25°C, ASTM E 892 Global). The 18·8% conversion efficiency represents a new record for such devices (Notable Exceptions) and makes the 20% performance level by thin-film polycrystalline materials much closer to reality. We allude to the enhancement in performance of such cells as compared to previous record cells, and we discuss possible and realistic routes to enhance the performance toward the 20% efficiency level. Published in 1999 by John Wiley & Sons, Ltd. This article is a US government work and is in the public domain in the United States.

Research opportunities in polycrystalline compound solar cells

Polycrystalline photovoltaic (PV) materials show substantial promise for achieving the U.S. Department of Energy PV costs and performance goals. Cadmium telluride (CdTe) and CuInSe2 (CIS) both have recently demonstrated device efficiencies in the 15 percent range with promise of achieving 20 percent efficiencies. Large area thin film CIS and CdTe modules in the 0.4 to 0.7 m2 size have also been fabricated with aperture efficiencies approaching ten percent. In spite of these results, polycrystalline thin film materials are relatively unexplored. Fundamental research opportunities in materials for thin film solar cells can be categorized under three major headings, from the most general to the most specific: areas with long-range potentialities, areas with a larger exploratory content, and areas with specific need. Each of these issues can play a vital role in the development of improved solar cells. The discussion of research opportunities in this paper starts with the more general opportunities and works its way to the most specific ones: (i) updating the basic investigation of defect properties and compensation in Group II-VI and related materials; exploring the basic materials science of the growth process for synthesis from layers of the elements, and exploring the potentials of bandgap engineering; (ii) search for new materials, interactions between defects and grain boundaries in polycrystalline materials, and exploration of the p-i-n structure for solar cells in a more general way; and (iii) doping of and contacts to p-CdTe, junction transport and effects of heat treatments on CdTe and CuInSe2, and development of Group II-VI ternaries.

Broad band spectroscopic ellipsometry for the characterization of photovoltaic materials

The availability of commercial spectroscopic ellipsometers (SE) has been restricted to the UV-visible range from 250–900 nm. Although this is useful for many applications, it must be extended to the near IR region (up to 1700 nm) for the study of the optical behavior of most photovoltaic materials. This paper discusses the development of a broad band (300–1700 nm) SE which has been used to measure the optical characteristics of various materials. Among these are the polycrystalline thin film materials, CuInSe2 and CdTe (for which single crystal samples have also been investigated), and materials for high efficiency cascade solar cells including InP, InGaAs and InGaAsP. Most of these data are not presently available over such a wide spectral range. Experimentally, a rotating polarizer-fixed analyzer ellipsometer with an a.c. detection system has been developed for accurate measurement of ψ and Δ, the relevant ellipsometric parameters, in the near IR. This approach has certain advantages over the rotating analyzer-fixed polarizer systems including reduced sensitivity to room light. The analytical methods include the use of a specially developed computer modeling program which gives ψ and Δ for a given set of values related to the film thickness (which may be finite or zero) and to the optical properties of the substrate.

High Tc Oxide Superconductors

This issue of the MRS BULLETIN is devoted to high Tc superconductivity. It is the sequel to a previous series of articles on the same subject which appeared in the MRS BULLETIN in January 1989. While the articles in the January 1989 issue emphasized the families of high Tc superconducting oxides known at that rime, as well as novel processing techniques and thin films, the papers in this issue focus on the physical properties of high Tc oxide superconductors.The quality of polycrystalline and single-crystal bulk and thin-film materials has improved to the point where researchers can now make reliable measurements of many physical properties representative of the intrinsic behavior of these materials. As a result, a broad spectrum of important issues such as the nature of the electronic structure, the type of superconducting electron pairing, the magnitude and temperature dependence of the superconducting energy gap, the behavior of fluxoids in the vortex state, etc., can be addressed meaningfully. Presently emerging is a consistent picture of the physical properties of the high Tc oxides, which will form the foundation to eventually developing an appropriate theory for the normal and superconducting states of these remarkable materials.

Measurement techniques in thin film polycrystalline materials and devices (solar cells)

A discussion of the relevance and applicability of basic bulk material and single-crystal semiconductor measurement techniques to thin film polycrystalline materials and devices is presented. The measurement techniques emphasized include resistivity and the Hall effect. Basic parametric relations for resistivity and the Hall effect coefficient are presented together with a discussion of the modifications due to film thickness, grain size, grain boundary conductivity and the presence of a substrate. The use of other electronic measurement techniques ( e.g. surface photovoltage, photoconductivity and capacitance-voltage methods) is reviewed. Automated materials characterization and analysis methods such as scanning electron microscopy, secondary ion mass spectrometry, Auger electron spectroscopy and X-ray crystallography are presented in their dual role of providing fundamental materials parameter values and of verifying results determined by other electronic methods. The technical literature is surveyed to determine the most widely applied thin film resistivity and Hall effect (mobility) analytical and experimental measurement methods. Widely used measurement techniques for other relevant parameters ( e.g. minority carrier lifetime, diffusion length etc.) are also identified, and an estimate of their efficacy in yielding results ( i.e. parameter values) for thin films consistent with overall device performance is made. The measurement approaches producing the most viable performance indicators are then analyzed for needed improvements, greater measurement reproducibility etc.