Electrically benign behavior of grain boundaries in polycrystalline CuInSe2 films

The classic grain-boundary (GB) model concludes that GBs in polycrystalline semiconductors create deep levels that are extremely harmful to optoelectronic applications. However, our first-principles density-functional theory calculations reveal that, surprisingly, GBs in CuInSe2 (CIS) do not follow the classic GB model: GBs in CIS do not create deep levels due to the large atomic relaxation in GB regions. Thus, unlike the classic GB model, GBs in CIS are electrically benign, which explains the long-standing puzzling fact that polycrystalline CIS solar cells with remarkable efficiency can be achieved without deliberate GB passivation. This benign electrical character of GBs in CIS is confirmed by our scanning Kelvin probe microscopy measurements on Cu(In,Ga)Se2 chalcopyrite films.     

Investigation of local electronic transport and surface potential distribution of Cu(In,Ga)Se2 thin-films

Local current mapping and surface potential distributions on polycrystalline Cu(In,Ga)Se₂ (CIGS) films are investigated by conductive atomic force microscopy and Kelvin probe force microscopy. The two kinds of samples fabricated by co-evaporation had extremely different conversion efficiencies of 10% and 0.2% for stoichiometric and Cu- and Se-deficient compositions, respectively. We examined the microscopic reasons for the differences in the local electrical properties. Current mapping and current–voltage behaviors were measured at intragrain regions (IGs) and grain boundaries (GBs). Electronic transport between a Pt scanning probe and the CIGS layer is explained by the Schottky conduction mechanism. The surface potential distribution shows an intriguing relation with topological variation, inferring that a local built-in potential is possibly formed on positively charged GBs. The surface potential is about 100 mV, which shows energy band bending near GBs in the films. Exciton separation near GBs is explained by the bending of the conduction and valence bands, which is sensitive to compositional and structural inhomogeneities.     

Effect of Cu deficiency on the optical properties and electronic structure of CuIn1−xGaxSe2

Spectroscopic ellipsometry measurements of CuInSe₂ (CIS) and CuIn_1−xGa_xSe₂ (CIGS) over a range of Cu compositions reveal that there are important differences in electronic and optical properties between α-phase CIS/CIGS and Cu-poor CIS/CIGS. We find a reduction in the imaginary part of the dielectric function ?₂ in the spectral region, 1-3 eV. This reduction can be explained in terms of the Cu-3d density of states. An increase in band gap is found for Cu-poor CIS and CIGS due to the reduction in repulsive interaction between Cu-3d and Se-4p states. We also characterize the dielectric functions of polycrystalline thin-film α-phase CuIn_1−xGa_xSe₂ (x=0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn_1−xGa_xSe₂. The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.     

Shear-strain induced structural relaxation of Cu Σ3 [110](112) symmetric tilt grain boundary: The role of foreign atoms and temperature

Grain boundaries (GBs) relaxation is a promising and effective strategy to improving GB stability or stabilizing nanocrystalline metals. However, previous studies mainly focused on nanocrystalline pure metals and GB behaviors therein, without considering the role of foreign atoms such as impurity or alloying atoms in GB relaxation. In this work, the shear-strain induced structural relaxation of pure Cu Σ3 [110](112) symmetric tilt GBs (STGBs), and the effects of foreign elements (Fe and Ni) and temperature on the GB relaxation were investigated in detail by molecular dynamics method. The results show that shear strain can trigger the structural relaxation of pure, Fe- and Ni-containing Cu GBs by the emission of Shockley partial dislocations from Cu GBs. Both Fe and Ni have impediment effects on the shear-strain induced GB relaxation, though the content of Fe or Ni atom (0.00165 at.%) is quite low in the GB model. The temperature cannot trigger GB relaxation independently within the considered temperature range, but play a positive role in the shear-strain induced structural relaxations of pure, Fe- and Ni-containing Cu Σ3 [110](112) STGBs. Our work might gain new insights into the mechanically induced GB relaxation in nanocrystalline copper and could be beneficial for improving the stability of Cu GBs.     

Preferential penetration path of gallium into grain boundary in practical aluminium alloy

The preferential penetration of a liquid metal along grain boundaries (GBs) in polycrystalline metals is a well-known phenomenon. Gallium-decorated GB networks in rolled aluminium alloys have been visualized three-dimensionally using the high-resolution synchrotron radiation computer tomography (SRCT). The distribution of gallium concentration along GBs was measured using X-ray absorption. Statistical correction for blurring was performed to raise accuracy of the measurement, and then compared with orientation mapping by the SEM/EBSP method on the surface of a tomographic specimen. The pancake-like grain microstructure formed by a rolling process causes the anisotropy of penetration direction. Although the gallium penetrated into high-angle GBs, all of the high-angle GBs are not necessarily decorated by the gallium. The reason for this may be explained by considering geometrically possible penetration paths that seem to be dependent on local grain arrangement and GB structure through each path. The dependence of the gallium concentration on the rotation axis of misorientation has been found along the high-angle GBs. Especially, GBs with a specific misorientation (〈221〉 as a misorientation axis) showed high gallium concentration.     

Local current–voltage behaviors of preferentially and randomly textured Cu(In,Ga)Se2 thin films investigated by conductive atomic force microscopy

Electrical transport properties on polycrystalline Cu(In,Ga)Se₂ (CIGS) (Ga/(In+Ga) ≈35%) thin films were examined by conductive atomic force microscopy. The CIGS thin films with a (112) preferential or random texture were deposited on Mo-coated glass substrates. Triangular pyramidal grain growths were observed in the CIGS thin films preferentially textured to the (112) planes. Current maps of the CIGS surface were acquired with a zero or non-zero external voltage bias. The contrast of the images on the grain boundaries and intragrains displayed the conduction path in the materials. Local current–voltage measurements were performed to evaluate the charge conduction properties of the CIGS thin films.     

Strain mapping in nanocrystalline grains simulated by phase field crystal model

In recent years, the phase field crystal (PFC) model has been confirmed as a good candidate to describe grain boundary (GB) structures and their nearby atomic arrangement. To further understand the mechanical behaviours of nanocrystalline materials, strain fields near GBs need to be quantitatively characterized. Using the strain mapping technique of geometric phase approach (GPA), we have conducted strain mapping across the GBs in nanocrystalline grains simulated by the PFC model. The results demonstrate that the application of GPA in strain mapping of low and high angles GBs as well as polycrystalline grains simulated by the PFC model is very successful. The results also show that the strain field around the dislocation in a very low angle GB is quantitatively consistent with the anisotropic elastic theory of dislocations. Moreover, the difference between low angle GBs and high angle GBs is revealed by the strain analysis in terms of the strain contour shape and the structural GB width.     

Effect of hydrostatic pressure on the migration of tilt grain boundaries in Sn- and Al-bicrystals

Abstract

The use of hydrostatic pressure as an intensive parameter for investigation of grain boundaries (GBs) migration in bicrystals is potentially important in gaining additional information on the mechanism of GB migration. This is because an analysis of the pressure dependence of the GB mobility yields a new activation parameter of the migration process, namely the activation volume V* which quantitatively is the difference between the volumes of the system in the ground and activated states. Only a few experiments to study the pressure dependence of GB mobility are known [1,2]. These experiments were made on polycrystalline materials. They provide the average data for all GBs in polycrystal and do not permit the determination of the connection between GB structure and value of activation volume V* of GB migration. In studying the mobility of single boundaries of a given type, there is the possibility of connecting the activation volume with the GB structural peculiarities, in particular, of determining the misorientation dependence of the activation volume, that is, of determining V* for GBs having various degrees of ordering.     

A non-selenization technology by co-sputtering deposition for solar cell applications

This work presents a novel method to form polycrystalline Cu(In(1-x)Ga(x))Se(2) (CIGS) thin film by co-sputtering of In─Se and Cu─Ga alloy targets without an additional selenization process. An attempt was also made to thoroughly elucidate the surface morphology, crystalline phases, physical properties, and chemical properties of the CIGS films by using material analysis methods. Experimental results indicate that CIGS thin films featured densely packed grains and chalcopyrite phase peaks of (112), (220), (204), (312), and (116). Raman spectroscopy analysis revealed chalcopyrite CIGS phase with Raman shift at 175 cm(-1), while no signal at 258 cm(-1) indicated the exclusion of Cu(2-x)Se phase. Hall effect measurements confirmed the polycrystalline Cu(In,Ga)Se₂ thin film to be of p type semiconductor with a film resistivity and mobility of 2.19×10(2) Ω cm and 88 cm(2)/V s, respectively.     

A study of the determination of grain boundary diffusivity and energy through the thermally grown oxide ridges on a Fe-22Cr alloy surface

Abstract

The grain boundaries (GBs) present in polycrystalline materials are important with respect to materials behaviour and properties. During the transient stage of oxidation, the higher GB diffusivity results in heterogeneous oxidation structures in the form of oxide ridges that emerge along the alloy GBs. In an attempt to delve into the more fundamental aspects of the GBs, such as GB energy, the size of the oxide ridges was quantitatively measured by atomic force microscopy on the post oxidation surface of a Fe-22 wt % Cr alloy after an oxidation exposure at 800 °C in dry air. The GB diffusivity was calculated utilising the ridge size data and the relationship between the GB diffusivity and the GB characteristics was determined. Furthermore, the GB energy was calculated from the GB diffusivity data, also to make comparison with the data available in the literature. The absolute value of the calculated GB energy was quite close to the values reported in the literature. However, compared to the extremely low temperature (0 K) data-set from the literature, the data-set obtained from this study showed much less spread. The smaller variation range may be attributed to the higher temperature condition (1073 K) in this study.     

Correlation of grain boundary extra free volume with vacancy and solute segregation at grain boundaries: a case study for Al

Grain boundary extra free volume (GB EFV) can be considered as fundamental microstructural parameter for polycrystalline or nano-crystalline materials. Here, we present a systematic first principles study on a group of representative symmetric tilt grain boundaries of Al with various EFVs subjected to vacancy formation and Mg segregation. All grain boundaries were constructed using the coincident site lattice (CSL) and the structural unit (SU) models. It was found that the SU model is superior to the CSL in describing FCC-Al GBs, the same as we previously revealed for BCC-Fe. The predicted relation between GB misorientation angle and EFV, and the predicted EFV criteria for a stable GB, both agree with available experimental observations. Vacancy formation and Mg segregation show stronger preference to those GBs with high EFV values, due to the resultant high levels of atomic disorder. These findings not only provide a new, atomistic perspective on the significance of EFV, but also suggest a viable means of predicting GB properties based on direct experimental characterisation of GB EFVs.     

On the Origin and Energy of Triple Junction Defects Due to the Finite Length of Grain Boundaries

King [1] established that due to the discrete nature of their dislocation structure, finite length grain boundaries (GBs) in polycrystalline materials possess discrete values of misorientation angle. For a GB with a length that is not a multiple of the GB period, this leads to the formation of specific disclinations at their junctions with neighboring GBs, which compensate the difference between the misorientations of finite and infinite boundaries. In the present paper the origin of these compensating disclinations within GB triple junctions is elucidated and their strength is calculated using the disclination-structural unit model. It is shown that for a GB with length of about 10 nm the junction disclinations can have a strength value not more than 1°, in contrast to King's calculations that indicate much larger values. Elastic energies of triple junctions due to compensating disclinations are calculated for both equilibrium and non-equilibrium structures of a finite length GB, which differ by the position of the grain boundary dislocation network with respect to the junctions. The calculations show that triple junction energies are comparable to dislocation energies, and that compensating disclinations can play a significant role in the properties of nanocrystalline metals with grain sizes less than about 10 nm.     

Relationship between Electrical Activity and Grain Boundary Structural Configuration in Polycrystalline Silicon

Temperature dependent electron beam induced current (EBIC) technique has been applied to investigate the electrical activities of grain boundaries (GBs) in polycrystalline silicon. The GB character, misorientation and orientation of GB plane, were analyzed using a FE-SEM/EBSP/OIM system prior to the EBIC measurements. The EBIC contrasts were found to depend on GB character; low GBs showed weak contrasts compared with general GBs at any temperatures, and also demonstrated to vary at GB irregularities such as boundary steps. These results indicate that electrical properties depend on the orientation of the GB plane as well as the misorientation. On the other hand, there existed less differences in temperature dependence of EBIC contrast irrespective of GB characters. The EBIC contrast decreased with increasing temperature, showed a minimum around 250 K, then increased again with further increasing temperature. The resulting temperature dependence of EBIC contrast probably comes from the combination of two types of recombination processes of carriers. One is related to a shallow level associated with an inherent GB structure, though the exact energy levels also would probably depend on GB structures, and the other to a deep level associated with impurities segregated at GBs, which acts as recombination center.     

Grain Boundary Channel Formation During Interdiffusion in Alloys

We present results of an experimental study of a new phenomenon accompanying grain boundary (GB) interdiffusion: the hole channel formation along GBs. The objects for study were plates of a homogeneous Cu-5 at.% Sn alloy, which were annealed at 800°C in purified hydrogen. Porous zones were found with GB hole channels perpendicular to the surface and practically equidistant from one another. The porous zone propagation and the average pore size growth at early stages of annealing obey a parabolic law. The observed processes are caused by nucleation and growth of the Cu₃Sn phase at the free surface. The new phase works as diffusion pump pumping out Sn atoms from the alloy towards the growing compound layer. The GB channel formation has been described as a relaxation process accompanying GB interdiffusion of Sn and Cu atoms with unequal partial diffusion coefficients (D _Sn>D _Cu). Excessive vacancies appearing at the GBs due to the inequality between D _Sn and D _Cu are absorbed by bulk and GB sinks, and tensile stresses appear near the GBs stimulating hole channels or groove formation.     

Interaction of Point Defects with Grain Boundaries in fcc Metals

Structures of several symmetrical tilt grain boundaries (GBs) with different tilt axes in Cu and Al and their interaction with vacancies and interstitials are studied using atomistic computer simulations with embedded-atom potentials. The lowest defect formation energy in a GB is found to correlate with the GB energy in both Cu and Al. Importantly, vacancies and self-interstitials in GBs have comparable formation energies, suggesting that both defects are equally important for GB diffusion and other properties. Vacancies in GBs can be either localized at certain sites or be delocalized over several sites. Some GB sites do not support a stable vacancy at all. Self-interstitial atoms can occupy relatively open interatomic positions, form split dumbbell configurations, or give rise to highly delocalized displacement zones. These structural forms of point defects have been observed across the whole set of twelve GBs in Cu and six GBs in Al studied in this paper as well as in our previous work [Interface Science 11, 131–148 (2003)]. It is suggested that these structural forms are general to all GBs in fcc metals. They can be explained by the existence of internal stresses and alternating tension and compression regions in the GB core.     

Effect of copassivation of Cl and Cu on CdTe grain boundaries

Using a first-principles method, we investigate the structural and electronic properties of grain boundaries (GBs) in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this Letter, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs.     

多晶石墨烯拉伸断裂行为的分子动力学模拟

采用分子动力学模拟方法研究了不同晶界对石墨烯拉伸力学特性及断裂行为的影响. 定义了表征晶界能量特性的新参量缺陷能, 并以此为基础分析了晶界结构的能量特性. 探讨了晶界对弹性模量和强度极限等的影响以及强度对晶界能量特性的依赖关系. 结果表明: 晶界能量特性可以间接反映晶界强度; 同时, 晶界中缺陷会使实际承载碳键数量小于名义承载碳键数, 从而在较大范围内影响弹性模量. 分析了不同晶界的断裂过程, 发现了裂纹扩展方向的强度依赖性: 低强度晶界主要是以碳键直接断裂为主要方式的沿晶断裂, 而高强度晶界通常是碳键直接断裂和Stone-Wales翻转过程交替进行下的穿晶断裂. 研究结果可为石墨烯器件的设计制造提供理论指导.     

Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method

Deformation behaviors of bicrystalline and nano-polycrystalline structures of various tilt angles and inclination angles in two dimensions are investigated in detail using a two-mode phase field crystal model.The interaction between grain boundary(GB)and dislocation is also examined in bicrystals and nano-polycrystals that both contain asymmetric and symmetric tilt GBs,with energy analysis being carried out to analyze these processes.During deformation simulations,we assume the volume of each simulation cell at every time step is coincident with that of the initial state just before deformation.Our simulation results show that the behaviors of symmetric and asymmetric GBs in bicrystals and nano-polycrystals differ from each other depending on tilt angle and inclination angle.A new dislocation emission mechanism of interest is observed in bicrystals which contain low angle symmetric tilt GBs.Low angle GB has a higher mobility relative to high angle GB in both bicrystalline and nano-polycrystalline structures,as does asymmetric GB to symmetric GB.The generation,motion,pileup and annihilation of dislocations,grain rotation and grain coalescence are observed,which is consistent with the simulation results obtained by molecular dynamics.These simulation results can provide strong guidelines for experimentation.     

Multi-timescale investigation of radiation damage near TiO2 rutile grain boundaries

To understand the interactions between defects and grain boundaries (GBs) in oxides, two atomistic modeling methods were used to examine the role of GBs in a model system, rutile TiO₂, in modifying radiation-induced defect production and annealing. Molecular dynamics was used to investigate defect production near a symmetric tilt GB at both 300?K and 1000?K. The damage production is found to be sensitive to the initial distance of the primary knock-on atom from the GB. We find three distinct regimes in which GBs have different effects. Similar to GBs in metals, the GB absorbs more interstitials than vacancies at certain distances while this behavior of biased loading of interstitials diminishes at other distances. Further, we obtain the statistics of both interstitial and vacancy clusters produced in collision cascades in terms of their compositions at two temperatures. Perfectly stoichiometric defect clusters represent a small fraction of the total clusters produced. Moreover, a significant reduction in the number of interstitial clusters at 1000?K compared to 300?K is thought to be a consequence of enhanced migration of interstitials towards the GB. Finally, the kinetic properties of certain defect clusters were investigated with temperature accelerated dynamics, without any a priori assumptions of migration mechanisms. Small interstitial clusters become mobile at high temperatures while small vacancy clusters do not. Multiple migration pathways exist and are typically complex and non-intuitive. We use this kinetic information to explain experimental observations and predict their long-time migration behavior near GBs.