Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule, and the Staebler-Wronski effect

Enthalpy-entropy compensation in the slow Arrhenius process

The Meyer-Neldel compensation law, observed in a wide variety of chemical reactions and other thermally activated processes, provides a proportionality between the entropic and the enthalpic components of an energy barrier. By analyzing 31 different polymer systems, we show that such an intriguing behavior is encountered also in the slow Arrhenius process, a recently discovered microscopic relaxation mode, responsible for several equilibration mechanisms both in the liquid and the glassy state. We interpret this behavior in terms of the multiexcitation entropy model, indicating that overcoming large energy barriers can require a high number of low-energy local excitations, providing a multiphonon relaxation process.

Phenomenological Model for Defect Interactions in Irradiated Functional Materials

The ability to tailor the performance of functional materials, such as semiconductors, via careful manipulation of defects has led to extraordinary advances in microelectronics. Functional metal oxides are no exception – protonic-defect-conducting oxides find use in solid oxide fuel cells (SOFCs) and oxygen-deficient high-temperature superconductors are poised for power transmission and magnetic imaging applications. Similarly, the advantageous functional responses in ferroelectric materials that make them attractive for use in microelectromechanical systems (MEMS), logic elements, and environmental energy harvesting, are derived from interactions of defects with other defects (such as domain walls) and with the lattice. Chemical doping has traditionally been employed to study the effects of defects in functional materials, but complications arising from compositional heterogeneity often make interpretation of results difficult. Alternatively, irradiation is a versatile means of evaluating defect interactions while avoiding the complexities of doping. Here, a generalized phenomenological model is developed to quantify defect interactions and compare material performance in functional oxides as a function of radiation dose. The model is demonstrated with historical data from literature on ferroelectrics, and expanded to functional materials for SOFCs, mixed ionic-electronic conductors (MIECs), He-ion implantation, and superconductors. Experimental data is used to study microstructural effects on defect interactions in ferroelectrics.

Eco-friendly and facilely prepared silica modified amorphous titania (TiO2–SiO2) electrocatalyst for the O2 and H2 evolution reactions

A silica modified amorphous titania (TiO₂–SiO₂) electrocatalyst was prepared by a simple, cheap, and scalable preparation procedure. The catalyst is active in the oxygen and hydrogen evolution reactions, leading to a promising bifunctional electrocatalyst.

25th anniversary article: microstructure dependent bias stability of organic transistors

Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge-trapping sites: the semiconductor, the dielectric, and the semiconductor-dielectric interface. The charge-trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias-stress-induced charge trapping in OFETs with the aim of minimizing operational instability.

Operational stability of organic field-effect transistors

Organic field-effect transistors (OFETs) are considered in technological applications for which low cost or mechanical flexibility are crucial factors. The environmental stability of the organic semiconductors used in OFETs has improved to a level that is now sufficient for commercialization. However, serious problems remain with the stability of OFETs under operation. The causes for this have remained elusive for many years. Surface potentiometry together with theoretical modeling provide new insights into the mechanisms limiting the operational stability. These indicate that redox reactions involving water are involved in an exchange of mobile charges in the semiconductor with protons in the gate dielectric. This mechanism elucidates the established key role of water and leads in a natural way to a universal "stress function", describing the stretched exponential-like time dependence ubiquitously observed. Further study is needed to determine the generality of the mechanism and the role of other mechanisms.

Stability of polymer solar cells

Organic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll-to-roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

X-RAY Photoemission Spectroscopic Study of Light-Induced Structural Changes in Amorphous Silicon

Light- and annealing-induced structural changes in undoped hydrogenated amorphous silicon (a-Si:H), pure amorphous silicon (a-Si) and crystalline silicon (c-Si) have been investigated in detail by X-ray photoemission spectroscopy (XPS). Both the Si2s and Si2p peaks in a-Si:H films were found to shift simultaneously to lower binding energies by the same amount with illumination time, and nearly reach saturation at about 0.06 eV after one hour of light-soaking at the intensity used. In contrast to the metastable changes in electronic properties [Staebler-Wronski effect (SWE)], the light-induced shifts in both peaks are unstable even at room temperature and can be reversed by annealing with a lower activation energy than that for the SWE. The absence of metastable XPS changes in pure a-Si and c-Si suggests that hydrogen is actively involved in the light-induced structural changes. Furthermore, visible light exposure produces XPS changes in a-Si:H less effectively than X-ray irradiation, despite its much higher incident intensity, indicating a high-energy photon irradiation effect. Our present results suggest that essentially the whole Si network structure is affected by light-soaking or X-ray irradiation, and becomes more stable after repeated irradiation-annealing training. These structural changes may be an independent metastable phenomenon or a precursor process of the SWE.

Stability of Amorphous Silicon Thin Film Transistors

Dangling bond defects are created during positive bias stress of amorphous silicon thin film transistors and there is an energy barrier between 0.9 and 1 eV for this process. We have studied how this energy barrier depends on the material parameters of the amorphous silicon, namely hydrogen content, hydrogen bonding, Urbach energy and intrinsic, deposition induced stress. We observe no dependence on the hydrogen content or hydrogen bonding type, but we do observe a clear dependence on the Urbach energy and the intrinsic stress. These measurements support a localized model for defect creation involving Si-Si bond breaking and the switching of a neighboring H atom to stabilize the broken bond. These results suggest that stable amorphous silicon TFTs can be obtained at low deposition temperatures by control of the deposition-induced, intrinsic stress.

Understanding Phase Change Memory Reliability and Scaling by Physical Models of the Amorphous Chalcogenide Phase

Phase change memory (PCM) devices are based on the electrically-induced change of phase within an active chalcogenide material. PCM features large resistance window, fast threshold/phase switching and high endurance, thus motivating a broad interest as potential Flash replacement and/or nonvolatile storage class memory. Despite the relatively mature progress of research and technology, there is still a wide debate about the ultimate scaling perspective for PCMs. Structural relaxation, crystallization and noise affecting the amorphous chalcogenide phase need to be addressed by accurate physical models for a realistic scaling projection. This work discusses the scaling of PCM devices in terms of the conduction mechanisms and structural stability of the amorphous chalcogenide phase. Resistance window narrowing, current fluctuations, resistance drift and crystallization in the amorphous phase will be explained by a unified model for thermal excitation of the structure by many-phonon phenomena. The downscaling of the reset current, needed to reduce the cell area in memory arrays, and thermal disturb between adjacent cells during reset will be finally addressed to assess the scaling capability of high-density PCM crossbar architectures.

Formation of color centers in a soda-lime silicate glass by excimer laser irradiation

We have investigated defect generation in soda-lime silicate and iron-doped soda-lime silicate glasses by excimer laser irradiation in order to apply coloration due to radiation-induced defects as a coloring technique for practical glass products. The laser irradiation generated various kinds of defects, i.e., non-bridging oxygen hole centers (NBOHCs), E' centers, and trapped electron centers, as does x-ray and γ-ray irradiation. The amounts of generated NBOHCs, monitored by the absorption intensity, increased at first with the irradiation time for both the ArF and XeF lasers, and eventually became saturated. The saturated values for the ArF laser irradiation were almost the same regardless of the laser intensity, whereas those for the XeF laser irradiation were dependent on the intensity; a higher intensity generated a larger amount of NBOHCs. From the comparison of the energies of the photon and the absorption edge of the soda-lime silicate glasses, the defect generation reactions were expected to be one-photon and two-photon processes for the ArF and XeF lasers, respectively. In order to explain the defect generation behavior, we used a simple kinetic model in which the NBOHCs are reversibly generated and annihilated through the photo-reaction. The model includes a stretched exponential function, which is often observed for reactions occurring in amorphous materials. The dependences of the amounts of the generated NBOHCs on the irradiation time and intensity of the laser pulses derived from the model were consistent with the experimental results.

Microscopic description of light induced defects in amorphous silicon solar cells

Using a combination of quantum and classical computational approaches, we model the electronic structure in amorphous silicon in order to gain an understanding of the microscopic atomic configurations responsible for light-induced degradation of solar cells. We demonstrate that regions of strained silicon bonds could be as important as dangling bonds for creating traps for charge carriers. Further, our results show that defects are preferentially formed when a region in the amorphous silicon contains both a hole and a light-induced excitation. These results are consistent with the puzzling dependencies on temperature, time, and pressure observed experimentally.

Metastability of amorphous silicon from silicon network rebonding

We propose a network rebonding model for light-induced metastability in amorphous silicon, involving bonding rearrangements of silicon and hydrogen atoms. Nonradiative recombination breaks weak silicon bonds and generates dangling bond-floating bond pairs, with very low activation energies. The transient floating bonds annihilate, generating local hydrogen motion. Charged defects are also found. Support for these processes is found with tight-binding molecular dynamics simulations. The model accounts for major experimental features of the Staebler-Wronski effect including electron-spin resonance data, the t(1/3) kinetics of defect formation, two types of metastable dangling bonds, and hysteretic annealing.