Overlayer stress effects on defect formation in Si and Ge

Point-defect formation energies in bulk crystalline materials such as Si and Ge are material specific quantities defined for the case of formation at a free surface, but in many cases of technological interest, point defects are formed at the interface between the crystalline substrate and a strained material overlayer. Here the energy cost of generating a bulk point defect at the overlayer/substrate interface is modified by the stress interaction during defect formation, leading to an effective supersaturation or undersaturation in the bulk, relative to the ‘equilibrium’ concentration expected for the case of a free surface. This in turn impacts on diffusion, defect formation and activation of dopant impurities in the substrate. We present current experimental evidence for this phenomenon, based on studies of B diffusion under tensile-strained nitride layers, and discuss the likely implications for dopant activation in Si and Ge.     

Understanding the High Voltage Behavior of LiNiO2 Through the Electrochemical Properties of the Surface Layer

Nickel-rich layered oxides are adopted as electrode materials for EV's. They suffer from a capacity loss when the cells are charged above 4.15 V versus Li/Li⁺. Doping and coating can lead to significant improvement in cycling. However, the mechanisms involved at high voltage are not clear. This work is focused on LiNiO₂ to overcome the effect of M cations. Galvanostatic intermittent titration technique (GITT) and in situ X-ray diffraction (XRD) experiments are performed at very low rates in various voltage ranges (3.8–4.3 V,). On the “4.2–4.3 V” plateau the R2 phase is transformed simultaneously in R3, R3 with H4 stacking faults and H4. As the charge proceeds above 4.17 V cell polarization increases, hindering Li deintercalation. In discharge, such polarization decreases immediately. Upon cycling, the polarization increases at each charge above 4.17 V. In discharge, the capacity and dQ/dV features below 4.1 V remain constant and unaffected, suggesting that the bulk of the material do not undergo significant structural defect. This study shows that the change in polarization results from the electrochemical behavior of the grain surface having very low conductivity above 4.17 V and high conductivity below this threshold. This new approach can explain the behavior observed with dopants like tungsten.     

Atomic structure of point defects in compound semiconductor surfaces

The present work reviews the progress in the determination and understanding of the atomic structure of point defects and dopant atoms exposed in and below cleavage surfaces of III–V semiconductors during recent years. By critically evaluating the present experimental data and theoretical concepts, we discuss the methods of identification of the types of defects and dopant atoms, the determination of defect energy levels, electrical charge states, as well as lattice relaxation, and the deduction of the physical mechanisms governing the interactions between different defects and/or dopant atoms, the formation of defect complexes, the compensation of dopant atoms, the pinning of the Fermi-level, and the stability of defects. Finally, the methodology to extract the concentrations and types of bulk defects and the physics governing bulk defects is examined.     

Metrology of semiconductor device structures by cross-sectional AFM

Atomic force microscopy (AFM) in air has been used to study various III–V semiconductor heterostructures. Topography of the (110) cleaved cross-sections has been examined where oxidation processes modify the surface and allow the structures to be investigated. It is shown that surface height differences of as little as 0.4 Å are sufficient to distinguish between layers, and that quantum wells of as little as 7 nm width are detectable.     

Experimental evidence and physical models of fatigue crack initiation

Results of the study of the early fatigue damage in a number of model and structural crystalline materials using modern experimental techniques are presented. The dislocation structure of the persistent slip bands and the evolution of the surface relief resulting in the formation of persistent slip markings during cyclic loading are documented. The dislocation mechanisms leading to production of point defects in cyclic loading are described and point defect production and annihilation rates are derived. The kinetics of point defect migration is characterized. The physically based models of the surface relief formation describing the formation of extrusions and intrusions are presented. The models are confronted with experimental evidence. It is concluded that intrusions representing sharp surface crack-like defects play the principal role in the initiation of fatigue cracks.     

Assessing charge carrier trapping in silicon nanowires using picosecond conductivity measurements

Free-standing semiconductor nanowires on bulk substrates are increasingly being explored as building blocks for novel optoelectronic devices such as tandem solar cells. Although carrier transport properties, such as mobility and trap densities, are essential for such applications, it has remained challenging to quantify these properties. Here, we report on a method that permits the direct, contact-free quantification of nanowire carrier diffusivity and trap densities in thin (～25 nm wide) silicon nanowires-without any additional processing steps such as transfer of wires onto a substrate. The approach relies on the very different terahertz (THz) conductivity response of photoinjected carriers within the silicon nanowires from those in the silicon substrate. This allows quantifying both the picosecond dynamics and the efficiency of charge carrier transport from the silicon nanowires into the silicon substrate. Varying the excitation density allows for quantification of nanowire trap densities: for sufficiently low excitation fluences the diffusion process stalls because the majority of charge carriers become trapped at nanowire surface defects. Using a model that includes these effects, we determine both the diffusion constant and the nanowire trap density. The trap density is found to be orders of magnitude larger than the charge carrier density that would be generated by AM1.5 sunlight.     

Optimization of III–V compound semiconductor heterostructures for distributed Bragg reflector applications in VCSELs

A selectively oxidized vertical cavity surface emitting laser (VCSEL) has been designed and fabricated for operation at a wavelength of 1.546 μm. The device structure was grown on an InP substrate using III–V quaternary semiconductor alloys for the two mirror stacks and unstrained multi-quantum wells for the active layer. A threshold current as low as 2.2 mA has been achieved. The influence of the intentional and growth-related compositional grading at the heterointerfaces on the mirror reflectivity and laser characteristics has been investigated, and key sensitivities to laser performance have been determined.     

Photoluminescence characterization of a counterpropagating twin photons structure: hot carrier effects in AlGaAs

Counterpropagating signal and idler twin photons can be produced via parametric fluorescence by a pump beam incident at a certain angle on top of a III–V semiconductor waveguide, as long as the refractive indices of the waveguide material for the two photons––determined by the thicknesses and alloy compositions of the layers––satisfy the necessary in plane phase matching condition. A specially designed AlGaAs/AlAs counterpropagating twin photons structure is characterized by temperature and pumping power density dependence photoluminescence spectroscopy. Under the present conditions of photoexcitation the spectra show all the typical characteristics of hot carrier luminescence. The relaxation of photoexcited carriers and formation of hot electron gas are analysed and discussed. From the dependence on pumping power density of the carrier temperature extracted from the slope of the high energy tail of the spectra, it is shown that the main relaxation path of the hot electron gas is via emission of LO-phonons at low temperatures.     

Defect‐Mediated Polarization Switching in Ferroelectrics and Related Materials: From Mesoscopic Mechanisms to Atomistic Control

The plethora of lattice and electronic behaviors in ferroelectric and multiferroic materials and heterostructures opens vistas into novel physical phenomena including magnetoelectric coupling and ferroelectric tunneling. The development of new classes of electronic, energy‐storage, and information‐technology devices depends critically on understanding and controlling field‐induced polarization switching. Polarization reversal is controlled by defects that determine activation energy, critical switching bias, and the selection between thermodynamically equivalent polarization states in multiaxial ferroelectrics. Understanding and controlling defect functionality in ferroelectric materials is as critical to the future of oxide electronics and solid‐state electrochemistry as defects in semiconductors are for semiconductor electronics. Here, recent advances in understanding the defect‐mediated switching mechanisms, enabled by recent advances in electron and scanning probe microscopy, are discussed. The synergy between local probes and structural methods offers a pathway to decipher deterministic polarization switching mechanisms on the level of a single atomically defined defect.     

Surface erosion and modification by energetic ions

Surface erosion and modification by energetic highly charged and cluster ions are important in the development of semiconductor devices, TeV accelerators, fission and fusion reactors, and in the development of extreme ultra-violet lithography devices. Gas cluster ion beam (GCIB) surface treatment can significantly mitigate the high-gradient electric vacuum breakdown of rf-cavities. GCIB can also mitigate Q-slope drop in superconducting Nb-cavities. Various mechanisms of the highly charged ion (HCI) energy transfer into the solid target, such as hollow atom formation, charge screening and neutralization, shock wave generation, and sputtering were analyzed. Surface erosions caused by GCIB, HCI bombardments, and by low energy He⁺ and H⁺ ions typical for fission and fusion devices were studied by using molecular dynamics simulation. A He bubble splashing mechanism of liquid Li containing was developed, and surface erosion was simulated. The mechanism of bubble explosion could significantly contribute to the surface erosion at high ion fluxes and explain the existing experimental results.     

Stoichiometry of III–V compounds

Effects of stoichiometry control on electrical, optical and crystallographic features in III–V compounds are shown. The application of the optimum vapor pressure during annealing and crystal growth is shown to minimize the deviation from stoichiometric composition. Temperature-dependence of the optimum vapor pressure is also obtained. In view of the defect formation mechanism, existence of the stable interstitial arsenic atoms (I_As) in GaAs is emphasized. The mechanism of stoichiometry control is discussed on the basis of the equality of chemical potentials and the change of saturating solubility in the liquidus phase as a function of the vapor pressure.     

Surface Defects Enhanced Visible Light Photocatalytic H2 Production for Zn‐Cd‐S Solid Solution

In order to investigate the defect effect on photocatalytic performance of the visible light photocatalyst, Zn‐Cd‐S solid solution with surface defects is prepared in the hydrazine hydrate. X‐ray photoelectron spectra and photoluminescence results confirm the existence of defects, such as sulfur vacancies, interstitial metal, and Zn and Cd in the low valence state on the top surface of solid solutions. The surface defects can be effectively removed by treating with sulfur vapor. The solid solution with surface defect exhibits a narrower band gap, wider light absorption range, and better photocatalytic perfomance. The optimized solid solution with defects exhibits 571 μmol h^?1 for 50 mg photocatalyst without loading Pt as cocatalyst under visible light irradiation, which is fourfold better than that of sulfur vapor treated samples. The wavelength dependence of photocatalytic activity discloses that the enhancement happens at each wavelength within the whole absorption range. The theoretical calculation shows that the surface defects induce the conduction band minimum and valence band maximum shift downward and upward, respectively. This constructs a type I junction between bulk and surface of solid solution, which promotes the migration of photogenerated charges toward the surface of nanostructure and leads to enhanced photocatalytic activity. Thus a new method to construct highly efficient visible light photocatalysts is opened.     

Ultra high vacuum-based in situ characterization of compound semiconductor surfaces by a contactless capacitance–voltage technique

A ultrahigh vacuum contactless capacitance–voltage technique is described as a powerful in situ surface characterization of growth and processing steps for III–V device fabrication. The technique was applied to characterization of MBE-grown surfaces and to optimization of the silicon interface control layer-based surface passivation.     

Tunable SiO<Subscript>2</Subscript>/Si-based nanostructures

We model the formation of a nanoscale potential well with quantum wires on the semiconductor surface near the SiO₂/Si interface owing to a special charge distribution in the oxide. We consider an SiO₂/Si structure in the form of a cylindrical substrate covered with a coaxial oxide layer. The charge distribution in the oxide is taken to have the form of charged circular rings of finite thickness, coaxial with the cylindrical substrate. The parameters of the quantum wires are analyzed in relation to the charge distribution and density. Reducing the separation between two charged rings decreases the width of the quantum wires produced on the semiconductor surface and increases their depth.     

First-principles study of the phosphorous-vacancy pair in silicon

Using first-principles simulations, we investigate the structure and the energetics of the phosphorous-vacancy pair in silicon. Symmetry-unrestricted optimization of the atomic positrons gives an inward relaxation combined with a pairing component for both neutral and negatively charged defects. We find a small outward relaxation in the charge state transition from the negative to the neutral phosphorous-vacancy pair. We calculate the defect formation energies and the binding energy of the phosphorous atom to the silicon vacancy, and the heat of solution of phosphorous to silicon. The activation ] energy for phosphorous diffusion is discussed.     

Probing and Manipulating the Interfacial Defects of InGaAs Dual‐Layer Metal Oxides at the Atomic Scale

The interface between III–V and metal‐oxide‐semiconductor materials plays a central role in the operation of high‐speed electronic devices, such as transistors and light‐emitting diodes. The high‐speed property gives the light‐emitting diodes a high response speed and low dark current, and they are widely used in communications, infrared remote sensing, optical detection, and other fields. The rational design of high‐performance devices requires a detailed understanding of the electronic structure at this interface; however, this understanding remains a challenge, given the complex nature of surface interactions and the dynamic relationship between the morphology evolution and electronic structures. Herein, in situ transmission electron microscopy is used to probe and manipulate the structural and electrical properties of ZrO₂ films on Al₂O₃ and InGaAs substrate at the atomic scale. Interfacial defects resulting from the spillover of the oxygen‐atom conduction‐band wavefunctions are resolved. This study unearths the fundamental defect‐driven interfacial electric structure of III–V semiconductor materials and paves the way to future high‐speed and high‐reliability devices.     

Simulation study on discrete charge effects of SiNW biosensors according to bound target position using a 3D TCAD simulator

We introduce a simulation method for the biosensor environment which treats the semiconductor and the electrolyte region together, using the well-established semiconductor 3D TCAD simulator tool. Using this simulation method, we conduct electrostatic simulations of SiNW biosensors with a more realistic target charge model where the target is described as a charged cube, randomly located across the nanowire surface, and analyze the Coulomb effect on the SiNW FET according to the position and distribution of the target charges. The simulation results show the considerable variation in the SiNW current according to the bound target positions, and also the dependence of conductance modulation on the polarity of target charges. This simulation method and the results can be utilized for analysis of the properties and behavior of the biosensor device, such as the sensing limit or the sensing resolution.     

A parallel code for a self-consistent charge density functional based tight binding method: Total energy calculations for extended systems

We present a parallel version of a selfconsistent-charge density-functional based tight-binding (SCC-DFTB) method for total energy calculations and geometry optimizations of clusters and periodic structures. On single processor machines the SCC-DFTB method has been successfully applied to systems up to several hundred atoms with an accuracy comparable to sophisticated selfconsistent field density-functional theory (SCF-DFT) methods. The new parallel code allows to treat systems which are larger by an order of magnitude in reasonable time. The freely available ScaLAPACK and PBLAS libraries are used for linear algebra operations. We tested the scaling of our program for a realistic system (III–V semiconductor surface) with different sizes and give a short outlook on current applications.