Role Of Intrinsic Point Defects Research Articles

Role of Point Defects and Ion Intercalation in Two-Dimensional Multilayer Transition Metal Dichalcogenide Memristors.

Two-dimensional materials, in particular transition metal dichalcogenides (TMDs), have attracted a nascent interest in the implementation of memristive architectures. In addition to being functionally similar to synapses, their nanoscale footprint promises to achieve the high density of a biological neural network in the context of neuromorphic computing. However, in order to advance from the current exploratory phase and reach reliable and sound memristive performances, an understanding of the underlying physical mechanisms in TMD memristors seems imperative. Despite the distinctive transport medium inherent to multilayer TMDs, the memristance is routinely attributed to defects or metal atoms present in the system, with their precise contribution remaining elusive. Specifically, the role of intrinsic point defects in the formation of conductive channels, although shown for monolayer TMDs, is not conclusively studied for multilayer samples. In this work, using density functional theory (DFT) and nonequilibrium Green's function (NEGF) formalism, a systematic study is carried out to analyze the impact that defects and metal atoms produce on the out-of-plane conductivity of multilayer TMDs. MoS2, a representative of the 2H structural configuration, and PtS2, a representative of the 1T structure, the most common crystal arrangements among TMDs, are used for this analysis. It is found that the intrinsic sulfur vacancies, which are the dominant defects present in both TMDs, appear to be insufficient in causing resistive switching on the application of an external bias. The claim that the intrinsic point defects on their own can realize a valence change memory-type device by providing a controllable conductive channel through the van der Waals structure seems, according to our study, improbable. The presence of metallic atoms is demonstrated to be essential to trigger the memristive mechanism, emphasizing the proper choice of a metal electrode as being critical in the fabrication and optimization of memristors using TMDs.

Role of intrinsic point defects on electrocatalytic activity of a metal-free two-dimensional Polyaramid for enhanced hydrogen evolution reaction

Despite the current research aims at replacing expensive metals like Platinum with non-noble metals for producing hydrogen via hydrogen evolution reaction (HER), practical challenges like oxidation continue to be a significant concern. Our study aims at providing the metal-free alternative to such noble Pt-based catalysts. Taking care of the fact that the defects are inevitable during the material's growth process, we propose thermodynamically and thermally stable two-dimensional (2D) Polyaramid (2DPA) with possible intrinsic point defects as HER active materials with overpotentials comparable to that of Pt catalysts using density functional theory (DFT). Creating single carbon (VC), single nitrogen (VN), and single oxygen (VO) vacancies helps reducing its wide band gap upon creation of several defect states, thereby improving conductivity. VO-2DPA exhibits lowest ΔGH* value of −0.04 eV which is closest to thermoneutrality condition, i.e., ΔGH*≈0 while VN-2DPA is not found suitable for HER. The calculated low formation energies for generating point-vacancies under defect-poor growth conditions and the minimal bond length fluctuations within, serve as strong validation for the feasibility of their synthesis. Furthermore, underlying reaction mechanisms are also investigated for the HER process favouring the Heyrovsky reaction path over Tafel as the former requires lesser activation energy: 0.53 eV v. 0.86 eV for VC-2DPA and 0.34 eV v. 1.01 eV for VO-2DPA.

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Non-steady state transport of charge carriers. An approach based on invariant embedding method

In this work, we report on a model that describes the microscopic electrical transport as a transmission problem using the invariant embedding technique. Analytical expressions for the transport coefficients under non-steady-state conditions are derived allowing us to calculate carrier concentration and time-dependent conductivity. Employing measurable magnitudes, our theoretical results allow us to determine defect concentrations, carrier generation rates, cross sections of recombination, and capture by traps. This model can be employed to study the conduction processes of semiconductors and test their band and defect structure. In particular, time-dependent photoconductivity measurements of a ZnO microwire have been well fitted using our model indicating a relevant role of intrinsic point defects in this material.

Study of self-sensitization of wide-gap oxides to visible light by intrinsic defects: From Terenin to the present days

We present the results of the investigation of the role of intrinsic point defects in wide-bandgap oxides photo-sensitization. This is one of numerous directions of researches pioneered by academician A.N. Terenin in the Leningrad (St. Petersburg) State University and continued by his disciples. The step-by-step experimental investigations of photo-excitation of the electronic sub-system of wide-bandgap oxides and further relaxation channels are analyzed in situ in three phases: solid – adsorbate – gas. The spectroscopy of optical activation and the spectroscopy of diffuse reflection are used for identification of initial and final excited states of the system. Photoluminescence (PL) and thermo-stimulated luminescence (TSL) are used to study radiative channels. Thermo-programmed desorption (TPD), FTIR and ESR spectroscopies are used to study the precursors and adsorbed species in the excited system. The final steps – photo-adsorption/desorption and chemical reactions in gaseous phase are analyzed using the mass-spectrometry (MS). In situ UV (8.43eV) Photoelectron spectroscopy (UPS) is used to monitor the electro-physical parameters of the sample: Fermi level EF, band bending VS, density of occupied states (DOS) and dipole potential δ. Common and specific features of wide-bandgap oxides – insulators and semiconductors – are analyzed.

Mechanisms of dislocation generation in Si structures with strained layers: Intrinsic point defects in dislocation nucleation

The peculiarities of dislocation production in silicon compositions with elastically strained layers fabricated by the molecular-beam epitaxy technique (SiGe/Si heterostructures) and by direct bonding of Si(110)/Si(100) wafers are studied with transmission electron microscopy. The role of intrinsic point defects during the process of nucleation of misfit dislocations is explained. The surplus concentration of these defects in heterostructures was produced via low-temperature epitaxial growth of buffer layers or with ion implantation of elastically strained heterostructures. The model of “optimal” and “inverse” intrinsic point defects providing an explanation for the relaxation of misfit strains in heterostructures is proposed.

Photoluminescence and Photoluminescence Excitation Spectroscopy of Cu(In,Ga)Se2Thin Films

AbstractThe role of intrinsic point defects on radiative recombination in Cu(In,Ga)Se2thin films was investigated by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Experiments were performed on device-grade polycrystalline layers and single crystal thin films. PL transitions identified by others as indicating a shallow state with an ionization energy of ∼16 meV is proposed to be a transition into band tail states rather than a distinct shallow defect. The presence of deep levels contributing to radiative recombination does not necessarily preclude the material from producing a high efficiency device and may suggest the absence of dominant non-radiative recombination pathways. The band edge width as measured by PLE and the separation of this edge from defect states are suggested to be potentially effective indicators of the quality of a material. Luminescence that appears to be connected with the absence of Na in the growth process persists in high Ga alloy, Na containing materials, suggesting that Na may become ineffective in passivating or eliminating certain defects in high Ga material.

Misfit Dislocations in SiGe/Si Heterostructures: Nucleation - Propagation - Multiplication

We present experimental data on the effect of low-temperature buffer layers on the dislocation structure formation in SiGe/Si strained-layer heterostructures under thermal annealing. Specific subjects include mechanisms of misfit dislocation nucleation, propagation and multiplication as well as the role of intrinsic point defects in these processes. Samples with lowtemperature Si (400°C) and SiGe (250°C) buffer layers were grown by MBE. In general, the processes of MD generation occur similarly in the heterostructures studied independently of the alloy composition (Ge content: 0.15, 0.30) and kind of buffer layer. Intrinsic point defects related to the low-temperature epitaxial growth influence mainly the rate of misfit dislocation nucleation.

Misfit dislocation nucleation and multiplication in fully strained SiGe/Si heterostructures under thermal annealing

An evolution of dislocation structure formed in fully strained Si 1− x Ge x /Si(0 0 1) heterostructures during thermal annealing was studied. Heterostructures with Ge content x = 0.15 and 0.30 were grown by MBE on low-temperature Si(400 °C) and SiGe(250 °C) buffer layers. The main attention was devoted to the initial stages of strain relaxation and to the role of intrinsic point defects in misfit dislocation nucleation. A mechanism is proposed for the misfit dislocation nucleation at heterogeneous sources placed within SiGe epitaxial layer.

Self Diffusion in SiC: the Role of Intrinsic Point Defects

Self Diffusion in SiC: the Role of Intrinsic Point Defects