Electronic Properties Of Semiconductors Research Articles

Simulation of Structural Evolution Using Time-Dependent Density-Functional Based Tight-Binding Method.

A faster and more efficient quantum mechanical simulation method for application to complicated issues of real systems beyond model cases has long been sought after. The density-functional based tight-binding (DFTB) method has successfully explained the atomistic and electronic properties of semiconductors, surfaces, and nanostructures. In addition, the time-dependent formalism implemented in DFTB showed high efficiency in terms of computational cost. In this study, we demonstrated the structural and electronic evolution of small molecules induced by a laser pulse using the time-dependent DFTB (TD-DFTB) method. We identified the critical fluence of the input laser for structural dissociations in carbon chains and fullerenes, which related to the structural stability. The excitation energies of several molecules calculated by TD-DFTB agreed with the experimental values.

Light soaking in metal halide perovskites studied via steady-state microwave conductivity

The light-soaking effect is the observation that under constant illumination the measured power conversion efficiency of certain solar cells changes as a function of time. The theory of the light-soaking in metal halide perovskites is at present incomplete. In this report, we employ steady-state microwave conductivity, a contactless probe of electronic properties of semiconductors, to study the light-soaking effect in metal halide perovskites. By illuminating isolated thin films of two mixed-cation perovskites with AM1.5 solar illumination, we observe a continual increase in photoconductance over a period of many (>12) hours. We can fit the experimentally observed changes in photoconductance to a stretched exponential function, in an analogous manner to bias-stressed thin-film transistors. The information provided in this report should help the community better understand one of the most perplexing open problems in the field of perovskite solar cells and, ultimately, lead to more robust and predictable devices.

Persistent photoconductivity at LaVO3–SrTiO3 interface

Light illumination is widely used to tune the electronic properties of semiconductors. We have studied the effect of temperature and wavelength of light on the photoconductivity and persistent photocurrent (PPC) on an insulating (3 unit cells (u.c.) of LaVO3 (LVO) on SrTiO3 (STO)) and a conducting (5 u.c. of LVO on STO) oxide interface. Photo induced insulator to metal transition has been observed for the insulating interface. For 405 nm wavelength light illumination, the conducting interface shows PPC at all temperatures, however for 532 nm wavelength light, it shows PPC state only above 220K. For insulating sample, PPC has been observed only for 405 nm light illumination above ~150K. A model band structure has been presented to understand the experimental observations.

First-principles study of electronic and diffusion properties of intrinsic defects in 4H-SiC

As a wide bandgap semiconductor, SiC holds great importance for high temperature and high power devices. It is known that the intrinsic defects play key roles in determining the overall electronic properties of semiconductors; however, a comprehensive understanding of the intrinsic defect properties in the prototype 4H-SiC is still lacking. In this study, we have systematically investigated the electronic properties and kinetic behaviors of intrinsic point defects and defect complexes in 4H-SiC using advanced hybrid functional calculations. Our results show that all the point defects in 4H-SiC have relatively high formation energies, i.e., low defect concentrations even at high growth temperatures. Interestingly, it is found that the migration barriers are very high for vacancies (>3 eV) but relatively low for interstitial defects (∼1 eV) in SiC. Meanwhile, the diffusion energy barriers of defects strongly depend on their charge states due to the charge-state-dependent local environments. Furthermore, we find that VSi in SiC, a key defect for quantum spin manipulation, is unstable compared to the spin-unpolarized VC–CSi complex in terms of the total energy (under p-type conditions). Fortunately, the transformation barrier from VSi to VC–CSi is as high as 4 eV, which indicates that VSi could be stable at room (or not very high) temperature.

Impedance Spectroscopy Revisited

AbstractImpedance spectroscopy has been widely used in characterizing the electronic properties of semiconductors. Its application in the perovskite photovoltaic research is a natural extension of the well‐established methodology in the new semiconducting material. However, it turns out that the published results are sometimes very strange, difficult to interpret, or the interpretations contradict each other. It is now clear that most of these troubles stem from the dominating influence of ionic contributions. Herein, some of the published data are reviewed and the possible problems are identified, which can be reconciled by taking into account the explicit contribution of the ionic behavior.

The Dopant Local Effect on the Stability of an Oxygen Vacancy and the Reliability of a Conductive Filament in Rutile Titanium Dioxide

Metal doping has long been recognized as a general strategy to modify electronic properties of semiconductors. The purpose here is to understand the behavior of oxygen vacancy when existing together with a metal dopant in rutile TiO2 through first‐principles calculation. For the coexistence of an oxygen vacancy and an Ag/Hf dopant, the dopant local effect determines that the formation energy of the oxygen vacancy depends on distances from it to the Ag/Hf dopant. Increased formation energy of an oxygen vacancy adjacent to an Ag dopant is attributed to the fact that the holes produced because of lower valence of Ag than Ti are filled by electrons left by the oxygen vacancy. The same valence of Hf as Ti results in that the electrostatic repulsive force pushes the Hf dopant outward from nearby oxygen vacancy, which lowers the formation energy of the oxygen vacancy. Furthermore, oxygen vacancies coexisting with an Ag/Hf dopant are of interest for investigating the effect of metal doping on the reliability of conductive filaments. The Hf dopant adjacent to the oxygen vacancies greatly enhances the strength of a conductive filament. The built‐in electric fields from localized electrons are considered as the seed action of metal doping in resistive switching devices.

Semiconductor properties of passive film and corrosion resistance of 2205 duplex stainless steel joint welded by laser hybrid welding

Purpose There are obvious differences in corrosion resistance of different 2205 welding joints with different ratios of austenite and ferrite, from the top to the bottom, the austenite content decreased gradually while the ferrite increased. In each region of welded joint, the pitting resistance number of ferrite is higher than that of austenite; pitting corrosion is more likely to occur in austenite phase first on the top region of the weld and in the secondary phase precipitates on the other regions of the weld. The fluctuation of the ratio of austenite and ferrite has a great influence on performance of passive film in 3.5 per cent NaCl solution. Design/methodology/approach To study the corrosion behavior of welded joint, the samples were obtained by laser hybrid welding. Pitting corrosion was studied in different area of welded joint. The Mott–Schottky curves of welded joints were measured to study the passive film on the different welded joint area. Findings Due to the difference of heat input and the limit of filler depth of the wire, the microstructure of duplex stainless steel laser welding joint has obvious difference in the thickness direction. In addition, there will be harmful secondary phase (such as chromium nitride and σphase) precipitates in the lower part of the joint. For the welded joint, the corrosion resistance decreases with the increase in the difference of the microstructure. Pitting corrosion usually takes the two phases as the nucleation point and grows up. The surface of 2205 duplex stainless steel laser hybrid welding joint cannot form a complete passive film in 3.5 per cent NaCl solution, and the more the ratios of austenite and ferrite deviate from equilibrium position (50:50), the worse the performance of passive film is. Originality/value In this paper, the authors attempt to establish the correlation between the semiconductor electronic properties of passive film and the difference of microstructures and the component in a joint welded by laser hybrid welding. The effect of passive film on the corrosion resistance of the weld was further investigated.

Scanning Photoelectron Spectroscopy of Tapered Cross Sections of Perovskite Solar Cells: Element Distribution, Band Alignment, Space Charge Layers and Photo-Voltage Distributions

Photoelectron spectroscopy is used for the characterization of chemical and electronic properties of semiconductor interfaces. As the information depth is extremely low, inner interfaces of device stacks are analyzed by depositing sequentially the respective contact material onto the base material in vacuum chambers integrated with the PES system. Thus elemental distribution including ionic state of the materials in contact as well as doping and electronic potential distribution induced by interface charge transfer can be monitored. For devices with non planar architecture or deposition processes not compatible with vacuum as both are given for perovskite solar cells, this procedure is not applicable. In this paper we demonstrate the photoelectron spectroscopic analysis of a complete working perovskite solar cell ranging from crystalline TiO2 on FTO front contact over perovskite intermixed in meso-TiO2 and the perovskite layer to Spiro-MeoTAD and Au back contact by preparing a tapered cross section of the cell stack. A wet chemically prepared mixed FAPbI3 MAPbBr3 solar cell of 18% efficiency is analyzed. As the analyzed spot size is in the range of the layer thicknesses a small angle tapered edge is prepared by polishing in order to drastically increase the local resolution across the cell stack. The tapered cross section is measured in the dark and under open circuit operando conditions. Using core level photoelectron spectroscopy the elemental distributions which forms in the wet chemical deposition process of the precursor solution are derived from the measured core level intensities. In addition, the electronic contact potentials are mapped, which allows conclusions on the band diagram of the full stack. We find that the perovskite absorber is n-type and a strong band bending in the p-type spiro-MeOTAD hole transport contact layer. Shifts of the core level emission binding energies under in situ illumination indicate that the cell photo-potential is induced to a large part at this back contact. We conclude that the diffusion voltage of the cells is not formed across the TiO2-perovskite front contact but mostly across the perovskite/SpiroMeOTAD/Au back contact. This example demonstrates that the wealth of information that can be obtained on full devices applying the method of scanning photoelectron spectroscopy on device cross section using a tapered edge approach.

QTAIM method for accelerated prediction of band gaps in perovskites

Efficient prediction of electronic properties of semiconductors is a cornerstone in the rational design of materials for various technological applications, including optoelectronics, photovoltaics and catalysis. Topological analysis of electron density is a powerful tool to unravel the correlations between the composition of isostructural compounds and their electronic properties. Resorting to this approach in theory or in experiment requires an elaboration of descriptors connecting the composition with the electronic structure characteristics. In the current work, the application of chemical topology for prediction of band gaps is illustrated for the model systems of perovskite compounds. The correlations between the band gaps and the electron density at the bond critical points are established, enabling the construction of composition–property maps. The procedure applying PBE-based descriptors for evaluation of the band gaps, calculated with resources-demanding methods, such as hybrid functionals or GW, is outlined. Finally, it is demonstrated how topological indices can be used to predict the band gaps for yet unsynthesized materials on the basis of available experimental data for isostructural compounds.

Laser induced nano-patterning with atomic-scale thickness on an InAs/GaAs surface

Nano-patterned substrates fabricated by different techniques have been widely applied in high performance semiconductor devices. For the influence of surface morphology on semiconductor electronic properties and epitaxy growth, there is an increasing interest to realize atomic-scale surface modification by a simple and effective method. In this study, laser induced nano-patterning with atomic-scale thickness on an InAs/GaAs surface was studied in both atmosphere and vacuum environments. Periodic nano-grooves with an average depth of 1 to 2 atomic layers were fabricated by focused laser spot scanning in the air. The depth and width of nano-grooves increase with the laser energy intensity. The result of nano-patterning is attributed to atom desorption stimulated by photon-induced electronic excitation. To explore the situation in the vacuum, in situ two-beam laser interference was applied during InAs/GaAs sample MBE growth. Periodic InGaAs nano-islands were fabricated, which show significant influence on subsequent crystal growth. The formation of ordered InAs quantum dots agreeing with the atomic layer nano-pattern demonstrates the potential application of the technique in site-controlled quantum dot growth.

Promoting Charge Separation in Semiconductor Nanocrystal Superstructures for Enhanced Photocatalytic Activity

AbstractIncreasing the lifetime of photoexcited charge carriers in metal oxide semiconductor nanocrystals is essential for the promotion of their photocatalytic efficiency. In this context, different strategies are developed for tailoring the structural and electronic properties of semiconductors at the single nanocrystal level, mainly including band‐structure engineering, doping, catalyst‐support interaction tuning, and cocatalysts decoration. Recently, an alternative strategy for prolonging the lifetime of photoexcited charge carriers is discovered at the nanocrystal superstructure level. By assembling semiconductor nanocrystals into ordered superstructures, a new pathway is created for the spatial separation of charge carriers between neighboring nanocrystals within the superstructure network. The inter‐nanocrystal charge transfer in the superstructures enables the increased charge separation efficiency of photogenerated electron–hole pairs, prolongs their lifetime and, in turn, improves the photocatalytic activity. In this review, recent developments of nanocrystal‐superstructure‐based photocatalysts and their applications in catalyzing different reactions are summarized. Several perspectives in terms of challenges and future research in this area are highlighted.

(Invited) Surface Transfer Doping: A Novel Alternative to Classical Doping in Semiconductor Electronics

Electronic properties of semiconductors can be controlled by introduction of appropriate dopants into the bulk lattice. In a recently discovered phenomenon, known as “surface transfer doping,” free electrons or holes can be generated in undoped semiconductor surfaces, without the introduction of foreign atoms, by controlled deposition of thin layer of chemical moieties having appropriate electronic structure. In this type of doping, the nature of electrical conductivity and the direction of electron transfer depend crucially on the relative positions of the Fermi level of semiconductor and the chemical potential of electrons in the surface chemical moieties. This doping phenomenon was first observed in undoped hydrogenated diamond, which showed a p-type surface conductivity when exposed to ambient air.1 In this case, surface conductivity develops spontaneously as a result of electron transfer from diamond to an adsorbed water film on the surface.2 , 3 , 4 Since then, this effect has been exploited for other systems, such as diamond/C60,5 graphene/C60. In addition, the process has been shown to affect electrical, optical and electrochemical properties of several technologically important materials such as single-walled carbon nanotubes,6 graphene,7 gallium nitride and zinc oxide.8 This presentation will give an overview of the recent progress in this field and its impact on the emerging technologies. Gi, R. S., et al., Jpn. J. Appl. Phys. (1995) 34, 5550Maier, F., et al., Phys. Rev. Lett. (2000) 85 (16), 3472Chakrapani, V., et al., Science (2007) 318 (5855), 1424Foord, J. S., et al., Diam. Relat. Mater. (2002) 11 (3-6), 856Strobel, P., et al., Nature (2004) 430 (6998), 439Chakrapani, V., et al., ECS Solid State Lett. (2013) 2 (11), M57Chen, W., et al., J. Am. Chem. Soc. (2007) 129 (34), 10418Chakrapani, V., et al., J. Am. Chem. Soc. (2008) 130 (39), 12944

Advances in vibrational spectroscopy provide new ways for characterizing semiconductor impurities

Improvements in computing push the boundaries of vibrational spectroscopy, allowing for observation of new electronic properties of semiconductors with light-mass element impurities.

Light-Induced Aggregation of Gold Nanoparticles and Photoswitching of Silicon Surface Potential

Hybrid nanomaterials having tunable properties that can be reversibly conducted by external stimuli, in particular light, are of great importance since they enable synergetic behavior between their components and enable the design of stimuli responsive smart materials and surfaces. Here we describe the formation of organic-inorganic hybrid nanoparticles that photochemically aggregate and their effect on the electronic properties of a semiconducting surface, as a function of external irradiation. The inorganic component consists of 3 nm gold nanoparticles while the organic component is a covalently attached, photochromic spiropyran derivative. Aggregation/deaggregation patterns in solution were obtained and analyzed by UV-Vis spectroscopy and transmission electron microscopy upon photoswitching. The assembly of spiropyran-modified gold nanoparticles on Si/SiO2 surface proved useful in photo-tuning the electronic properties of semiconductors measured by contact potential difference.

Synthesis of Unsymmetric Monosubstituted and Disubstituted Dinaphthothiophenes

Dinaphthothiophenes (DNTs) are a class of compounds with potential uses in organic semiconductors and the synthesis of unsymmetric catalysts. Symmetrical or asymmetrical addition of functional groups to the DNT structure may be desired for steric bulk in binaphthyl catalyst synthesis or tuning the electronic properties of semiconductors. Thus, versatility of functional group addition is a great asset in DNT synthesis. Until now, no versatile and concise methods for the synthesis of unsymmetrically substituted DNTs have been reported. Herein, we report three synthetic routes for the creation of three different classes of DNTs. Each route involves the successive addition of two functionalized styryl groups to a thiophene ring, followed by a photocyclization to form the desired asymmetric DNT. Various novel unsymmetrically monosubstituted and disubstituted dinaphtho[2,1‐b:1′,2′‐d]thiophenes, dinaphtho[1,2‐b:1′,2′‐d]thiophenes, and dinaphtho[1,2‐b:2′,1′‐d]thiophenes were synthesized from 2‐bromothiophene,2,4‐dibromothiophene, and 3,4‐dibromothiophene in three or four steps. These methods can be used to synthesize a wide variety of unsymmetrically functionalized DNTs.

Size Dependence of Doping by a Vacancy Formation Reaction in Copper Sulfide Nanocrystals

AbstractDoping of nanocrystals (NCs) is a key, yet underexplored, approach for tuning of the electronic properties of semiconductors. An important route for doping of NCs is by vacancy formation. The size and concentration dependence of doping was studied in copper(I) sulfide (Cu2S) NCs through a redox reaction with iodine molecules (I2), which formed vacancies accompanied by a localized surface plasmon response. X‐ray spectroscopy and diffraction reveal transformation from Cu2S to Cu‐depleted phases, along with CuI formation. Greater reaction efficiency was observed for larger NCs. This behavior is attributed to interplay of the vacancy formation energy, which decreases for smaller sized NCs, and the growth of CuI on the NC surface, which is favored on well‐defined facets of larger NCs. This doping process allows tuning of the plasmonic properties of a semiconductor across a wide range of plasmonic frequencies by varying the size of NCs and the concentration of iodine. Controlled vacancy doping of NCs may be used to tune and tailor semiconductors for use in optoelectronic applications.

Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface.

Dopants modify the electronic properties of semiconductors, including their susceptibility to etching. In semiconductor nanowires doped during growth by the vapor-liquid-solid (VLS) process, it has been shown that nanofaceting of the liquid-solid growth interface influences strongly the radial distribution of dopants. Hence, the combination of facet-dependent doping and dopant selective etching provides a means to tune simultaneously the electronic properties and morphologies of nanowires. Using atom-probe tomography, we investigated the boron dopant distribution in Au catalyzed VLS grown silicon nanowires, which regularly kink between equivalent ⟨112⟩ directions. Segments alternate between radially uniform and nonuniform doping profiles, which we attribute to switching between a concave and convex faceted liquid-solid interface. Dopant selective etching was used to reveal and correlate the shape of the growth interface with the observed anisotropic doping.

Tuning the near-gap electronic structure of Cu2O by anion–cation co-doping for enhanced solar energy conversion

Doping is an effective strategy to tune the electronic properties of semiconductors, but some side effects caused by mono-doping degrade the specific performance of matrixes. As a model system to minimize photoproduced electron-hole pairs recombination by anion–cation co-doping, we investigate the electronic structures and optical properties of (Fe[Formula: see text]+[Formula: see text]N) co-doped Cu2O using the first-principles calculations. Compared to the case of mono-doping, the Fe[Formula: see text]N[Formula: see text] (a Fe (N) atom substituting a Cu (O) atom) co-doping reduces the energy cost of doping as a consequence of the charge compensation between the iron and nitrogen impurities, which eliminates the isolated levels (induced by mono-dopant) in the band gap. Interestingly, it is found that the contributions of different host atoms (Cu and O) away from anion (N) and cation (Fe) dopants to the variation of near band gap electronic structure of the co-doped Cu2O are different. Moreover, co-doping reduces the band gap and increases the visible-light absorption of Cu2O. Both band gap reduction and low recombination rate are critical elements for efficient light-to-current conversion in co-doped semiconductor photocatalysts. These findings raise the prospect of using co-doped Cu2O with specifically engineered electronic properties in a variety of solar applications.

Nanopatterning of Group V Elements for Tailoring the Electronic Properties of Semiconductors by Monolayer Doping.

Control of the electronic properties of semiconductors is primarily achieved through doping. While scaling down the device dimensions to the molecular regime presents an increasing number of difficulties, doping control at the nanoscale is still regarded as one of the major challenges of the electronic industry. Within this context, new techniques such as monolayer doping (MLD) represent a substantial improvement toward surface doping with atomic and specific doping dose control at the nanoscale. Our previous work has explained in detail the atomistic mechanism behind MLD by means of density-functional theory calculations (Chem. Mater. 2016, 28, 1975). Here, we address the key questions that will ultimately allow one to optimize the scalability of the MLD process. First, we show that dopant coverage control cannot be achieved by simultaneous reaction of several group V elements, but stepwise reactions make it possible. Second, using ab initio molecular dynamics, we investigate the thermal decomposition of the molecular precursors, together with the stability of the corresponding binary and ternary dopant oxides, prior to the dopant diffusion into the semiconductor surface. Finally, the effect of the coverage and type of dopant on the electronic properties of the semiconductor is also analyzed. Furthermore, the atomistic characterization of the MLD process raises unexpected questions regarding possible crystal damage effects by dopant exchange with the semiconductor ions or the final distribution of the doping impurities within the crystal structure. By combining all our results, optimization recipes to create ultrashallow doped junctions at the nanoscale are finally proposed.

Charge Transfer Reductive in situ Doping of Mesoporous TiO2 Photoelectrodes - Impact of Electrolyte Composition and Film Morphology.

Some material properties depend not only on synthesis and processing parameters, but may furthermore significantly change during operation. This is particularly true for high surface area materials. We used a combined electrochemical and spectroscopic approach to follow the changes of the photoelectrocatalytic activity and of the electronic semiconductor properties of mesoporous TiO2 films upon charge transfer reductive doping. Shallow donors (i.e. electron/proton pairs) were introduced into the semiconductor by the application of an external potential or, alternatively, by band gap excitation at open circuit conditions. In the latter case the effective open circuit doping potential depends critically on electrolyte composition (e.g. the presence of electron or hole acceptors). Transient charge accumulation (electrons and protons) in nanoparticle electrodes results in a photocurrent enhancement which is attributed to the deactivation of recombination centers. In nanotube electrodes the formation of a space charge layer results in an additional decrease of charge recombination at positive potentials. Doping is transient in nanoparticle films, but turns out to be stable for nanotube arrays.