Accurate first-principles detailed-balance determination of auger recombination and impact ionization rates in semiconductors

The technologically important prediction of Auger recombination lifetimes in semiconductors is addressed by means of a fully first-principles formalism, based on precise energy bands and wave functions provided by the full-potential linearized augmented plane wave code. The minority carrier Auger lifetime is determined by two related approaches: (i) a direct evaluation within Fermi's golden rule, and (ii) an indirect evaluation, based on a detailed balance formulation combining Auger recombination and its inverse process, impact ionization, in a unified framework. Lifetimes determined with the direct and indirect methods show excellent consistency between them (i) for n-doped GaAs and (ii) with measured values for GaAs and InGaAs. This indicates the computational formalism as a new sensitive tool for use in materials performance optimization.     

The unique carrier mobility of Janus MoSSe/GaN heterostructures

Heterostructure is an effective approach in modulating the physical and chemical behavior of materials. Here, the first-principles calculations were carried out to explore the structural, electronic, and carrier mobility properties of Janus MoSSe/GaN heterostructures. This heterostructure exhibits a superior high carrier mobility of 281.28 cm²·V⁻¹·s⁻¹ for electron carrier and 3951.2 cm²·V⁻¹·s⁻¹ for hole carrier. Particularly, the magnitude of the carrier mobility can be further tuned by Janus structure and stacking modes of the heterostructure. It is revealed that the equivalent mass and elastic moduli strongly affect the carrier mobility of the heterostructure, while the deformation potential contributes to the different carrier mobility for electron and hole of the heterostructure. These results suggest that the Janus MoSSe/GaN heterostructures have many potential applications for the unique carrier mobility.     

Theoretical insight into the carrier mobility anisotropy of hole transport material Spiro-OMeTAD

All-solid-state organic-inorganic halide perovskite solar cells (PSCs) have attracted wide attention due to the rapid progress of power conversion efficiency in recent years. Hole transport material (HTM) in PSCs plays the role of extracting and transporting photo-excited holes. Anisotropy of carrier mobility is one important property for semiconductors, however, which still remains unclear for the dominant HTM spiro-OMeTAD used in PSCs. Based on Density Functional Theory (DFT) and Marcus theory, we for the first time conducted investigations on the anisotropy of carrier mobility along representative crystal planes of spiro-OMeTAD by recombination energy λ and electronic coupling integral V. Results indicate that the holes and electrons show transport orientations consistency parallel to the (010), (101) and (111) crystal planes while inconsistency was found parallel to (100), (110), (011) and (001) crystal planes (with an angle ranged from 40° to 70° between the hole and electron transport directions). Our work embodies the theoretical significance of controllable and oriented fabrication of HTM in PSCs.     

Magnetic properties of Co-doped SnO2 at different carrier concentrations

Magnetic properties of Co-doped wide-gap semiconductor SnO₂ were studied theoretically by using the PPLCAO first-principles computational scheme. Since the carrier plays an important role on magnetic properties about diluted magnetic semiconductors (DMS) materials, we discuss the origin of magnetic moments and the magnetic ordering mechanism with different carrier concentration in Co-doped SnO₂ based on calculated spin density distribution. It is found that, the RKKY interaction is dominated in the magnetic coupling in Co-doped SnO₂.     

Determination of electric transport properties in the pre- and post-breakdown regime ofp-germanium

Conductivity and Hall-effect measurements were performed on single-crystallinep-doped germanium, electrically driven into low-temperature avalanche breakdown via impurity impact ionization. The electric transport properties were determined as a function of the applied electric field in the pre- and post-breakdown regime. The characteristic field dependence of the carrier density, mobility, and drift velocity was found to be reflected in smooth variations of the integral current flow. The breakdown mechanism was demonstrated to involve a mobility which sensitively depends upon the density of the mobile charge carriers. Our experimental findings are qualitatively explained by simple model approaches developed from established breakdown theories.     

等价阴-阳离子共掺杂调节ZnO的能带结构及其光催化活性

采用等价阴-阳离子共掺的方法调节ZnO的能带结构,提高其光催化分解水制氢的效率。计算结果表明：等价阴-阳离子共掺不仅减小了ZnO的带隙,使其在可见光区域的光吸收增强,而且能有效抑制电子-空穴的复合,提高载流子迁移率。（Cd+Te）共掺杂的ZnO是较理想的水分解的光催化剂,因为它具有合适的带隙、较少的电子-空穴复合中心、增强的可见光区域的光吸收和与水氧化还原势相匹配的带边位置。等价阴-阳离子共掺的方法也可运用到其它宽带隙的半导体中以提高光催化活性。     

Monolayered semiconducting GeAsSe and SnSbTe with ultrahigh hole mobility

High carrier mobility and a direct semiconducting band gap are two key properties of materials for electronic device applications. Using first-principles calculations, we predict two types of two-dimensional semiconductors, ultrathin GeAsSe and SnSbTe nanosheets, with desirable electronic and optical properties. Both GeAsSe and SnSbTe sheets are energetically favorable, with formation energies of −0.19 and −0.09 eV/atom, respectively, and have excellent dynamical and thermal stability, as determined by phonon dispersion calculations and Born–Oppenheimer molecular dynamics simulations. The relatively weak interlayer binding energies suggest that these monolayer sheets can be easily exfoliated from the bulk crystals. Importantly, monolayer GeAsSe and SnSbTe possess direct band gaps (2.56 and 1.96 eV, respectively) and superior hole mobility (~20 000 cm²·V⁻¹·s⁻¹), and both exhibit notable absorption in the visible region. A comparison of the band edge positions with the redox potentials of water reveals that layered GeAsSe and SnSbTe are potential photocatalysts for water splitting. These exceptional properties make layered GeAsSe and SnSbTe promising candidates for use in future high-speed electronic and optoelectronic devices.     

First-principles Band Structures Calculation of Tin-phthalocyanine

SnPc(Tin-phthalocyanine)因在无机/有机二极管等光电结构器件中表现出了很多有趣的特性而备受关注.为了更深地理解载流子的传输特性,利用密度泛函理论,采用广义梯度近似(DFT-GGA),关联函数选择BLYP计算了SnPc的能带结构.从点波函数、能带带宽以及带隙分析了载流子的传输行为. 从前线轨道的带宽以及电子和空穴的有效质量,可以看到电子的传输要比空穴的传输容易两倍左右.而且,当研究费米能级附近的能带时,发现未占有带的带隙总体上要小于占有带的带隙,这表明在考虑声子参与的情况下,电子在带间的跳跃要比空穴容易得多.以上的事实说明SnPc是一种电子传输占主导的材料.     

Insulator-to-metal transition in selenium-hyperdoped silicon: observation and origin

Hyperdoping has emerged as a promising method for designing semiconductors with unique optical and electronic properties, although such properties currently lack a clear microscopic explanation. Combining computational and experimental evidence, we probe the origin of sub-band-gap optical absorption and metallicity in Se-hyperdoped Si. We show that sub-band-gap absorption arises from direct defect-to-conduction-band transitions rather than free carrier absorption. Density functional theory predicts the Se-induced insulator-to-metal transition arises from merging of defect and conduction bands, at a concentration in excellent agreement with experiment. Quantum Monte Carlo calculations confirm the critical concentration, demonstrate that correlation is important to describing the transition accurately, and suggest that it is a classic impurity-driven Mott transition.     

Time-resolved studies of electron and hole spin dynamics in modulation-doped GaAs/AlGaAs quantum wells

Spin dynamics in semiconductors have gained much interest in the past years due to the emerging field of semiconductor spintronics. This review is focussed on the observation and control of electron and hole spin dynamics in modulation-doped heterostructures based on the GaAs/AlGaAs material system. Modulation doping allows for the creation of two-dimensional electron and hole systems with high carrier mobility. By confining carriers to a two-dimensional sheet, the spin–orbit interaction is modified significantly. In addition to this, it can be further modified by changing the symmetry of the system, for example by externally applied or built-in electric fields along the growth direction. Our recent experimental results on spin dynamics in two dimensions are reviewed and discussed in connection with theoretical considerations. A brief overview of the current research challenges in this field is given.     

sp3sTight-binding parameters for transport simulations in compound semiconductors

A genetic algorithm approach is used to fit orbital interaction energies of sp3s* tight-binding models for the nine binary compound semiconductors consistent of Ga, Al, In and As, P, Sb at room temperature. The new parameters are optimized to reproduce the bandstructure relevant to carrier transport in the lowest conduction band and the highest three valence bands. The accuracy of the other bands is sacrificed for the better reproduction of the effective masses in the bands of interest. Relevant band edges are reproduced to within a few meV and the effective masses deviate from the experimental values typically by less than 10%.     

Trap-assisted recombination in disordered organic semiconductors

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)μ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.     

Crystallization in two-component Coulomb systems

The analysis of Coulomb crystallization is extended from one-component to two-component plasmas. Critical parameters for the existence of Coulomb crystals are derived for both classical and quantum crystals. In the latter case, a critical mass ratio of the two charged components is found, which is of the order of 80. Thus, holes in semiconductors with sufficiently flat valence bands are predicted to spontaneously order into a regular lattice. Such hole crystals are intimately related to ion Coulomb crystals in white dwarf and neutron stars as well as to ion crystals produced in the laboratory. A unified phase diagram of two-component Coulomb crystals is presented and is verified by first-principles computer simulations.     

Electromagnetically induced transparency and theoretical slow light in semiconductor multiple quantum wells

We report an experimental demonstration of electromagnetically induced transparency in the transient optical response in a cascade-type three-level system of GaAs/AlGaAs multiple quantum wells, and analyze the cascade-type three-level schemes with density matrix and Maxwell equations to then obtain the phase shift, group velocity, and group-velocity dispersion. The calculated group velocity is ??6.87×10⁴?m/s and the corresponding pulse delay is ??7?ps. Finally, we provide a convenient basis for investigating many-body effects in semiconductors.     

Electronic structure of a complex with charge transfer and carrier energy bands in thin films of semiconductors containing carbazole

The electronic structures of hole and electron carrier energy bands and complexes with charge transfer (CCT) in films of semiconductors containing carbazole and sensitized with TNP were determined by the methods of photoelectron and electron spectroscopy as well as by studying the wavelength dependence of the quantum yield of photogeneration at temperatures of 295 and 80 K. The hole carrier bands are determined by the top three filled electronic levels of the carbazole nuclei, while the electron carrier bands are determined by the bottom free electron level of TNP. The first three excited states of CCT are determined, respectively, by electronic transitions from the first three top filled electron levels of the carbazole nuclei to the bottom free electronic level of TNP. Photogeneration of current carriers is observed with excitation of CCT in the region of the second and third electronic transitions.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 7, pp. 47–51, July, 1989.     

Local solutions to high-frequency 2D scattering problems

We consider the solution of high-frequency scattering problems in two dimensions, modeled by an integral equation on the boundary of a smooth scattering object. We devise a numerical method to obtain solutions on only parts of the boundary with little computational effort. The method incorporates asymptotic properties of the solution and can therefore attain particularly good results for high frequencies. We show that the integral equation in this approach reduces to an ordinary differential equation.