Effect Of Intrinsic Defects Research Articles

Effects of intrinsic defects on the electronic structure and magnetic properties of CoFe2O4: A first-principles study

The cobalt ferrite (CoFe2O4) with spinel structure has achieved a great interest as a very important magnetic material which has covered a wide range of applications. The formation condition and energy of possible intrinsic point defects have been investigated by the first-principles calculations, and the effects of the intrinsic point defects on the electronic and magnetic properties of CoFe2O4 have been analyzed. It is found that the growth conditions have a great effect on the formation energy of intrinsic point defects, and each point defect with its fully ionized state is the most stable for the intrinsic point defects with various charge states. In an oxygen rich environment, the cation vacancies are easy to form shallow acceptors，which is conducive to the strength of the p-type conductivity. While in the metal rich environment, the oxygen vacancies tend to form donors which lead to the n-type conductivity. There exists extra levels in the band gap when point defects are present, resulting in a reduction of the band gap. The net magnetic moment depends highly on the defects.

First-principles calculations of electronic and optical properties of aluminum-doped β-Ga2O3 with intrinsic defects

In this manuscript, the effects of intrinsic defects on the electronic and optical properties of aluminum-doped β-Ga2O3 are investigated with first-principles calculations. Four types of defect complexes have been considered: AlGa2O3VO (Al-doped β-Ga2O3 with O vacancy), AlGa2O3VGa (Al-doped β-Ga2O3 with Ga vacancy), AlGa2O3Gai (Al-doped β-Ga2O3 with Ga interstitial) and AlGa2O3Oi (Al-doped β-Ga2O3 with O interstitial). The calculation results show that the incorporation of Al into β-Ga2O3 leads to the tendency of forming O interstitial defects. And the bandgap of AlGa2O3 is 4.975eV, which is a little larger than that of intrinsic β-Ga2O3. When O vacancies exist, a defect energy level is introduced to the forbidden band as a deep donor level, while no defective energy levels occur in the forbidden band with O interstitials. After Al-doped, a slightly blue-shift appears in the intrinsic absorption edge, and an additional absorption peak occurs with O vacancy located in 3.69eV.

A first-principles study on the origin of magnetism induced by intrinsic defects in monolayer SnS2

The effect of intrinsic defects on the structural, electronic and magnetic properties of monolayer SnS2 has been studied using density functional theory. Among the possible intrinsic defects, S vacancy is the most energetically favorable intrinsic defect under the Sn-rich condition, while S-on-Sn anti-site defect is most likely to form under the S-rich condition. Besides, S-on-Sn anti-site defect can realize p-type semiconductor behavior. Additionally, Sn vacancy, Sn-on-S anti-site and S adsorption on the top of S atom from the upper triple layer defects can induce magnetism, which originates mainly from the nearest-neighbor S atoms close to the Sn vacancy, Sn substitutional atom and S adatom, respectively. Moreover, the room temperature ferromagnetism is possible to be realized in monolayer SnS2 with S adsorption on the top of S atom from the upper triple layer defects. These predictions give a new path to explore monolayer SnS2-based magnetic nanomaterials.

Effects of intrinsic defects on effective work function for Ni/HfO2 interfaces

The effective work functions and formation energies for Ni/HfO2 interfaces with and without defects, including interfacial intrinsic atom substitution and atom vacancy in interfacial layer were studied by first-principles methods based on density functional theory (DFT). The calculated results of the formation energies indicate that the interfaces with O–Ni combining bonds in the interfacial region are more energetically favorable and a small amount O vacancy is comparatively easy to form in O–Ni interface, especially under O-rich situation. Moreover, the results of our calculations also reveal that, (1) the effective work functions strongly depend on the type of interface, interface roughness and atom substitution content in the interface region; (2) for Hf–Ni interfaces, two calculated effective work functions without and with Ni substitutions in whole interfacial Hf layer are good for nMOS and pMOS effective work function (EWF) engineering, respectively; (3) the EWFs are sensitive to Hf vacancy rather than Ni vacancy in interfacial layer for Hf–Ni interfaces; (4) oxygen vacancies can result in a decrease of effective work function for O–Ni interfaces. Additionally, we establish an expected theoretical relationship that variations of the EWFs are in proportion to that of interface dipole density. Finally, ionic valence state and occupied state are used to qualitatively analyze and explain the effects of interfacial defects on the EWF in metal-oxide interfaces. Our work suggests that controlling interfacial intrinsic atom substitution and interface roughness are attractive and promising ways for modulating the effective work function of Ni/HfO2 interfaces.

Effects of intrinsic defects and extrinsic doping on the electronic and photocatalytic properties of Ta3N5

The improvement of n-type conductivity is an effective strategy to enhance the photocatalytic activity of Ta3N5.

Impurity induced crystallinity and optical emissions in ZnO nanorod arrays

We report the growth of ZnO nanocrystallites doped with impurities such as B, N and S by green chemistry route using ultrasound. The effect of intrinsic defects and impurity doping on the structural and optical properties of ZnO nanostructures has been studied and discussed. Characterization studies carried out using x-ray diffraction (XRD) reveal the change in lattice parameters and crystallinity of ZnO in the presence of dopant. This has been explained on the basis of the dopant substitution at regular anion and interstitial sites. Study on surface morphology by field emission scanning electron microscopy (FESEM) shows a change from particle-like structure to aligned nanorods nucleated at definite sites. Elemental analysis such as x-ray photon electron spectroscopy (XPS) has been carried out to ascertain the dopant configuration in ZnO. This has been corroborated by the results obtained from FTIR and Raman studies. UV–vis light absorption and PL studies show an expansion of the band gap which has been explained on the basis of Moss–Burstein shift in the electronic band gap of ZnO by impurity incorporation. The optical emissions corresponding to excitonic transition and defect centres present in the band gap of ZnO is found to shift towards lower/higher wavelength sides. New PL bands observed have been assigned to the transitions related to the impurity states present in the band gap of ZnO along with intrinsic defects.

Effects of Intrinsic Defects and Substrate Transfer Doping on the Electronic Structure of Sb2Te3 Thin Films on Graphene

We study how the electronic structure of the Sb2Te3 thin films on graphene changes with intrinsic defects and substrate transfer doping using first-principles calculations. We find that for freestanding Sb2Te3 thin films without defects band gap decreases with increasing film thickness, and that the behavior of three-dimensional topological insulators does not appear up to seven quintuple layers. The orbital degeneracy is broken by defects and substrate transfer doping, leading to the Rashba effect and a reduced band gap. By decreasing the defect ratio of Sb vacancies (VSb) as the primary source of p-type conductivity and increasing that of Te-on-Sb antisites (TeSb) as the primary source of n-type conductivity, coupled with a transfer doping using a graphene substrate, the energy band of Sb2Te3 thin films can be tuned from the p-type to the n-type. Differences in the local defects of Sb2Te3 can result in variations in the work function and intrinsic defects have a stronger effect on the work function than...

Effects of intrinsic defects on methanthiol monolayers on Cu(111): A density functional theory study

Density functional theory calculations were used to examine the effects of intrinsic surface defects of Cu(111) on the adsorption of methylthiol (CH3SH). The examination covers both the initial non-dissociative adsorption and the subsequent dissociation reaction pathways to form intermediate and final reaction products. By comparing the most probable adsorption structures likely formed after the adsorption of CH3SH on Cu(111) with and without the presence of adatoms (Cu(ad)) and vacancies, this computational work offers new insights about the geometry and thermodynamic stability of these structures. Particularly, it reveals a new type of surface complexes having two CH3S bonding to one Cu(ad) (referred therein as CH3S-Cu(ad)-CH3S). In addition, this work also yields new reaction dynamics results on transition states and activation barriers. The results reveal that the presence of Cu(ad) indeed significantly changes the kinetics of adsorption and dissociation of CH3SH on Cu(111). The most kinetically favorable reaction pathway turns out to be that involving the formation of a special surface complex formed by one Cu(ad) plus two CH3S fragments from the dissociation of CH3SH, with the two S atoms located at the bridge sites of Cu(111). Finally, this work also gives simulated scanning tunneling microscopic images for the most important adsorption species in the course of the transition from CH3SH∕Cu(111) to CH3S∕Cu(111), which may stimulate future experimental studies of self-assembled monolayers on practical metal substrates such as thiols on copper.

Effect of intrinsic defects on electronic structure of bilayer graphene: First-principles calculations

We carry out the first-principles calculations to investigate the electronic properties of bilayer graphene. The ultrasoft pseudopotential density functional methods within the local density approximation (LDA) are applied. The band structures and density of states of bilayer graphene are investigated. Intrinsic defects, including Stone–Wales (SW) defect and atom vacancy, are considered, and proper theoretical analysis about its electronic properties is given. Finally, we arrive at the point that Stone–Wales defect can generate a small energy gap in bilayer graphene, and the vacancy defect can bring in a magnetic moment of bilayer graphene. The results may be valuable for the application of bilayer graphene in electronic devices.

Effect of Intrinsic Defects in High-Purity Semi-Insulating 4H-SiC on Reverse Current–Voltage Characteristics of Schottky Barrier Diodes

Schottky barrier diodes are fabricated using high-purity semi-insulating 4H-SiC. Under certain measurement conditions, the reverse current–voltage (IR–V) characteristics of the diodes exhibit a peak, with the diodes appearing to behave as negative resistance diodes. To investigate the effect of intrinsic defects in the 4H-SiC on the IR–V characteristics, the transient reverse currents of the diodes are measured. Without any assumptions regarding defects, a graphical peak analysis method, discharge current transient spectroscopy (DCTS), which uses the isothermal transient reverse current, can determine the densities and emission rates of defects. From simulation of the IR–V characteristics using the densities and emission rates of intrinsic defects determined by DCTS, the effect of the intrinsic defects in high-purity semi-insulating 4H-SiC on the IR–V characteristics of its Schottky barrier diodes is elucidated. It is found that DCTS is suitable for determining the densities and emission rates of electrically active defects in semi-insulating semiconductors.

Electronic structure of BN nanotubes with intrinsic defects N B and B N and isoelectronic substitutional impurities P N, As N, Sb N, In B, Ga B, and Al B

The effect of intrinsic defects and isoelectronic substitutional impurities on the electronic structure of boron-nitride (BN) nanotubes is investigated using a linearized augmented cylindrical wave method and the local density functional and muffin-tin approximations for the electron potential. In this method, the electronic spectrum of a system is governed by a free movement of electrons in the interatomic space between cylindrical barriers and by a scattering of electrons from the atomic centers. Nanotubes with extended defects of substitution N B of a boron atom by a nitrogen atom and, vice versa, nitrogen by boron B N with one defect per one, two, and three unit cells are considered. It is shown that the presence of such defects significantly affects the band structure of the BN nanotubes. A defect band π(B, N) is formed in the optical gap, which reduces the width of the gap. The presence of impurities also affects the valence band: the widths of s, sp, and p π bands change and the gap between s and sp bands is partially filled. A partial substitution of the N by P atoms leads to a decrease in the energy gap, to a separation of the Ds(P) band from the high-energy region of the s(B, N) band, as well as to the formation of the impurity Dπ(P) and Dπ*(P) bands, which form the valence-band top and conduction-band bottom in the doped system. The influence of partial substitution of N atoms by the As atom on the electronic structure of BN nanotubes is qualitatively similar to the case of phosphorus, but the optical gap becomes smaller. The optical gap of the BN tubule is virtually closed due to the effect of one Sb atom impurity per translational unit cell, in contrast to the weak indium-induced perturbation of the band structure of the BN nanotube. Introduction of the one In, Ga or Al atom per three unit cells of the (5, 5) BN nanotube results in 0.6 eV increase of the optical gap. The above effects can be detected by optical and photoelectron spectroscopy methods, as well as by measuring electrical properties of the pure and doped BN nanotubes. They can be used to design electronic devices based on BN nanotubes.

Ion-driven deuterium retention in tungsten

The ion-driven retention of deuterium in polycrystalline tungsten (PCW) is studied experimentally and theoretically as a function of temperature, incident ion energy, and ion fluence. Deuterium retention was investigated by thermodesorption spectroscopy and ion beam analysis. The peculiarities of deuterium behavior in PCW such as (i) ion-induced defect formation at low-energy implantation and (ii) higher D retention of low-energy ions (60–200eV) compared to high-energy ions (3keV) at high fluences are considered. The effect of intrinsic defects (dislocations, vacancies, grain boundaries) and ion-induced defects (vacancies, dislocations, deuterium clusters) is discussed for polycrystalline tungsten.

Theoretical and experimental studies on oxygen vacancy in p-type ZnO

Zinc oxide (ZnO) is one of the most important transparent conducting oxides (TCOs) and has attracted much attention due to its unique properties and possible applications in UV light emitting diodes and laser diodes. The realization of high quality reproducible p-type ZnO is proven difficult because of the strong self-compensation effect of intrinsic defects. In this study, we explicitly investigated one of the native defects, oxygen vacancy, by means of theoretical and experimental studies. It was predicted that oxygen vacancy is a deep donor in ZnO. We suggest that the oxygen vacancy should be suppressed in p-type ZnO. In the experimental study on P-doped ZnO, we further explore the role of oxygen partial pressure in the fabrication of p-type ZnO by using RF sputtering method. We also found that the conduction type for P-doped ZnO thin films can be controlled by adjusting the oxygen partial pressure. In this study, we demonstrated that p-type conduction can be stabilized in a narrow oxygen partial pressure window. This research may contribute to fabricating the high quality p-type ZnO.

Effect of intrinsic defects on the electronic structure of BN nanotubes

The effect of intrinsic defects on the electronic structure of boron-nitrogen nanotubes (5, 5) and (9, 0) is investigated by the method of linearized associated cylindrical waves. Nanotubes with extended defects of substitution N B of a boron atom by a nitrogen atom and, vice versa, nitrogen by boron B N with an impurity concentration of 1.5 to 5% are considered. It is shown that the presence of such defects significantly affects the band structure of boron-nitrogen nanotubes. A defect band D π(B, N) is formed in the bandgap, which sharply reduces the width of the gap. The presence of impurities also affects the valence band: the widths of s, sp, and p π bands change and the gap between s and sp bands is partially filled. These effects may be detected experimentally by, e.g., optical and photoelectron spectroscopy.

Charge transfer luminescence in Yb3+-containing compounds

Abstract The characteristics of charge transfer transition of Yb3+ in selected compounds and complexes are reviewed. Systematic comparison of the optical and luminescence spectra as well as the decay kinetics of Yb-doped aluminium perovskites and garnets is provided. Effect of intrinsic defects and possible occurrence of Yb2+ is discussed and possible application areas are briefly mentioned.

A computational study of oxygen diffusion in olivine

Atomistic modelling techniques are used to study the rate-determining steps that limit diffusion of oxygen in forsterite. The activation energies for diffu- sion parallel to all three crystallographic axes by the vacancy and interstitial mechanisms are calculated. The activation energy for extrinsic vacancy diffusion is predicted to be isotropic with a barrier height of 119 kJ mol )1 . Conversely, in the interstitial case it is found to be anisotropic, with extrinsic activation ener- gies that range between 94 and 178 kJ mol )1 . The effect of intrinsic defects and two typical impurities, iron and hydrogen, upon diffusion is also considered. We find that the migration energy is slightly higher in iron-rich fayalite compared with forsterite and that the presence of hydrogen defects will not affect the diffusion mech- anism. These observations lead us to reinterpret existing experimental results on oxygen diffusion in natural olivine. We suggest that at low oxygen partial pressure the mechanism observed is a vacancy mechanism, while at high oxygen partial pressure the mechanism is interstitial. We believe that this change in mechanism is mediated by iron redox reactions. Taking this process into account, we derive activation energies in excellent agreement with those found experimentally in natural samples of olivine. The anisotropy of activation barriers and hence the change in diffusion rates with tempera- ture could be used to distinguish between the two mechanisms in future experimental work.

Effect of intrinsic defects on the electron mobility of gallium arsenide grown by molecular beam epitaxy and metal organic chemical vapor deposition

Temperature dependent electron mobility measurements are reported for lightly doped n-type gallium arsenide (GaAs) grown by metal organic chemical vapor deposition (MOCVD GaAs). Using the Brooks–Herring model, the charge state of the impurity scattering centers is deduced to be 1. The same measurements are reported for irradiated MOCVD GaAs, and it is deduced that the charge state of the defects introduced is also 1. These observations are different from previously reported mobility measurements on irradiated gallium arsenide grown by molecular beam epitaxy (MBE GaAs), in which it was deduced that multiply charged scattering centers are introduced by the irradiation. Photoluminescence and deep level transient spectroscopy are used to identify the defects present in the as-grown and in irradiated GaAs. Through the observation of the antisites, it is concluded that the MOCVD GaAs was grown under arsenic rich conditions and the MBE GaAs under gallium rich conditions. Moreover, the gallium vacancy defect (VGa) is observed in irradiated MOCVD GaAs, but not in irradiated MBE samples. The mobility measurements on the MOCVD samples imply that the VGa charge state defect is −1. It is speculated that the arsenic vacancy defect is introduced in irradiated MBE GaAs and its charge state is −2.

Materials Characterization of Nondestructive Readout Nonvolatile Memory Devices

AbstractPrototype ferroelectric thin film, nonvolatile memory, nondestructive readout (NDRO), semiconductor devices have been fabricated. The “1” and “0” logic states of these prototype devices are in principle determined by the modulation of the conductivity of a semiconductor film channel by the polarization state of the underlying ferroelectric thin film layer. Programmed resistance ratios of the two logic states of 5:1 are demonstrated. While the best performance to date has been achieved for devices that have a 40 nm ln2O3 film covering a 300 nm thick PZT 20/80 layer, we also develop criteria for selecting semiconductor films that will improve performance for this NDRO device design. Among the other semiconductor films that are characterized with respect to this criteria are boron doped Ge, ZnO and aluminum doped ZnO. It is demonstrated that by appropriate donor doping of ZnO films the effects of intrinsic defects are masked and that process temperatures can be extended by 300×C.