Concentration Of Native Point Defects Research Articles

Experimental and thermodynamic modeling of Al2O3 corundum and TiO2 rutile structures forming on Ti-6Al-4V powder during oxidation

The high affinity of Ti-6Al-4 V (Ti64) powder to oxygen impacts the quality of the powder during additive manufacturing. The oxide scale consists of a double layer with α-Al2O3 in the outer part and TiO2 in the inner one. Experimental and thermodynamic results are used to further understand the formation of these two oxide layers. Inert marker revealed the anionic growth of the duplex oxide scale. The thermodynamic results highlighted that interstitial aluminum enters and diffuses fast within TiO2 (rutile) to form Al2O3 as the surface layer. Moreover, α-Al2O3 is considered as a non-stoichiometric oxide with a very small excess of oxygen as dumbbells at high partial pressures of oxygen. A remarkable effect of the impurities on the concentration of native point defects is observed in this phase. Finally, a reactional mechanism with elementary steps for the oxidation of Ti64 powder is proposed.

Influence of oxygen-rich and zinc-rich conditions on donor and acceptor states and conductivity mechanism of ZnO films grown by ALD—Experimental studies

Understanding the origin of the strong difference of electrical parameters between as grown and annealed undoped ZnO films prepared at a temperature range of 100–200 °C by thermal atomic layer deposition is essential for their future applications. In this paper, we show that the conductivity drop by up to 4 orders of magnitude as effect of post-growth annealing is accompanied by multiple simultaneous effects like a two orders of magnitude decrease of hydrogen impurity content, a strong width reduction of the luminescence peaks, and an increase of crystallite sizes influencing the carrier scattering. We show that a level of structural and optical improvement as well as the final electrical parameters of annealed films strongly depend on the previously employed growth temperature, which is related to transition from oxygen- to zinc-rich conditions influencing a type and concentration of native point defects. The growth temperature does not only influence the bandgap energy but also the binding energies of existing donors and the relative ratio between the number of donors and acceptors; hence, it determines the final electrical characteristics of the films. This means that electrical properties of undoped ZnO-atomic layer deposition films can be tuned by native defects engineering.

Donor and acceptor characteristics of native point defects in GaN

The semiconducting behaviour and optoelectronic response of gallium nitride is governed by point defect processes, which, despite many years of research, remain poorly understood. The key difficulty in the description of the dominant charged defects is determining a consistent position of the corresponding defect levels, which is difficult to derive using standard supercell calculations. In a complementary approach, we take advantage of the embedded cluster methodology that provides direct access to a common zero of the electrostatic potential for all point defects in all charge states. Charged defects polarise a host dielectric material with long-range forces that strongly affect the outcome of defect simulations; to account for the polarisation, we couple embedding with the hybrid quantum mechanical/molecular mechanical approach and investigate the structure, formation and ionisation energies, and equilibrium concentrations of native point defects in wurtzite GaN at a chemically accurate hybrid-density-functional-theory level. N vacancies are the most thermodynamically favourable native defects in GaN, which contribute to the n-type character of as-grown GaN but are not the main source, a result that is consistent with experiment. Our calculations show no native point defects can form thermodynamically stable acceptor states. GaN can be easily doped n-type, but, in equilibrium conditions at moderate temperatures acceptor dopants will be compensated by N vacancies and no significant hole concentrations will be observed, indicating non-equilibrium processes must dominate in p-type GaN. We identify spectroscopic signatures of native defects in the infrared, visible and ultraviolet luminescence ranges and complementary spectroscopies. Crucially, we calculate the effective-mass-like-state levels associated with electrons and holes bound in diffuse orbitals. These levels may be accessible in competition with more strongly-localised states in luminescence processes and allow the attribution of the observed 3.46 and 3.27 eV UV peaks in a broad range of GaN samples to the presence of N vacancies.

Characterization of Graphite/ZnO Schottky Barriers Formed on Polar and Nonpolar ZnO Surfaces

The authors demonstrate that the electrical properties of Schottky junctions fabricated by the deposition of colloidal graphite strongly depend on the crystallographic orientation of the ZnO substrate. The current‐voltage, capacitance‐voltage, and impedance measurements indicate that near‐ideal Schottky junctions form on c‐plane, while on a‐ and m‐plane the junctions are laterally inhomogeneous. This behavior is assigned to higher concentration of native point defects in the near‐surface region of nonpolar ZnO substrates. The authors further present an extended equivalent circuit model, which corresponds to actual structure of the junctions, and sheds light on their electrical transport properties.

Investigations of Nanoscale Defects in Crystalline and Powder ZnSe Doped With Fe for Laser Application

The theoretical and experimental analysis of the ternary system Zn‐Se‐Fe for obtaining new fundamental information on phase using X‐ray analysis and inductively coupled plasma mass‐spectrometry is carried out. New experimental data of isothermal annealing in the Zn‐Se‐Fe system at the temperatures 730, 814, 1073 K by X‐ray studies confirms the reliability of theoretical isothermal sections. The information on Fe solubility and concentrations of native point defects of atomic scale at bi‐ and monovariant conditions at 1073 and 1273 K have been carried out. Taking into account the new data on phase equilibria, it is possible to fabricate active ZnSe:Fe2+ elements to create an effective laser operating at the temperature of 300 K with 1.52 Joule emission energy with efficiency of ≈50% relative to the absorbed energy.

Comprehensive insights into point defect and defect cluster formation in CuInSe2

The concentration of native point defects in CuInSe2 powder material as a function of stoichiometry has been experimentally determined by neutron powder diffraction. A correlation between the Cu/In ratio and the density of VCu as well as InCu has been established and their concentrations are quantified. It is demonstrated, that assuming the spontaneous formation of defect pairs, the density of native point defects is reduced significantly by an order of magnitude. The functionality of a solar device, assuming same conditions like in the analyzed material, may be explained by a neutralization due to the formation of electrically inactive defect complexes.

Fermi level stabilization energy in cadmium oxide

We have studied the effects of high concentrations of native point defects on the electrical and optical properties of CdO. The defects were introduced by irradiation with high energy He+, Ne+, Ar+, and C+ ions. Increasing the irradiation damage with particles heavier than He+ increases the electron concentration until a saturation level of 5×1020 cm−3 is reached. In contrast, due to the ionic character and hence strong dynamic annealing of CdO, irradiation with much lighter He+ stabilizes the electron concentration at a much lower level of 1.7×1020 cm−3. A large shift of the optical absorption edge with increasing electron concentration in irradiated samples is explained by the Burstein–Moss shift corrected for electron-electron and electron-ion interactions. The saturation of the electron concentration and the optical absorption edge energy are consistent with a defect induced stabilization of the Fermi energy at 1 eV above the conduction band edge. The result is in a good agreement with previously determined Fermi level pinning energies on CdO surfaces. The results indicate that CdO shares many similarities with InN, as both materials exhibit extremely large electron affinities and an unprecedented propensity for n-type conductivity.

First‐principles study on the concentrations of native point defects in high‐dielectric‐constant binary oxide materials

AbstractThe intrinsic concentrations of point defects in high‐k binary oxide materials of HfO2, ZrO2, Y2O3 and La2O3 are evaluated on the basis of first‐principles calculations. Oxygen defects are found to dominate over a wide range of the oxygen chemical potential. Neutral oxygen vacancies are likely to be responsible for electron trapping in the investigated materials. In HfO2 and ZrO2, oxygen Frenkel pairs are likely to form. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the Fermi level pinning in as-grown GaInNAs(Sb)/GaAs quantum wells with indium content of 8%–32%

Modified van Hoof structures containing GaInNAs(Sb) quantum wells (QWs) with indium content varying from 8% to 32% have been investigated using contactless electroreflectance (CER) spectroscopy. In CER spectra, both the QW transitions and GaAs-related Franz–Keldysh oscillations (FKOs) have been clearly observed. The band gap discontinuity at the GaInNAs(Sb)/GaAs interface has been determined by analyzing the QW transitions. The built-in electric field in the GaAs cap layer has been extracted from the FKO periodicity. The Fermi level position in the GaInNAs(Sb) QW has been determined through knowledge of the electric field in the GaAs cap layer and band gap discontinuity in the GaInNAs(Sb)/GaAs QW. It has been found that the Fermi level is pinned for all samples at the same energy, ∼4.7 eV below the vacuum level. The Fermi level is located very close to the Fermi level stabilization energy, ∼4.9 eV below the vacuum level. A high concentration of native point defects in the as-grown material is the reason for the Fermi level pinning at this energy.

Native point defects in non-stoichiometric GaAs doped with beryllium

Native point defects in non-stoichiometric GaAs layers doped with Be were observed. Ga vacancies were identified by positron annihilation. As Ga antisite defects were investigated by near infrared absorption and magnetic circular dichroism of absorption, respectively. We found the properties of the As Ga antisites very similar to that of As Ga in bulk GaAs, i.e. they exhibit the same MCDA spectra and quenching behavior. The concentration of native point defects was not influenced by the Be doping. Therefore, the thermal stability of non-stoichiometric GaAs : Be can not be explained with a different incorporation of native defects.

Incorporation and thermal stability of defects in highly p-conductive non-stoichiometric GaAs : Be

Non-stoichiometric GaAs is known to contain a high concentration of native point defects. The dominant defect in the epilayers is the arsenic antisite (As Ga), a deep double donor, which is incorporated at low growth temperatures (commonly between 200°C and 300°C) in molecular beam epitaxy (MBE). Consequently, p-doping of non-stoichiometric GaAs is difficult because large concentrations of acceptors are compensated by the As Ga. Recently, we found that despite this compensation effect we can achieve p-conductive GaAs : Be with almost one order of magnitude higher Be-doping than previously obtained in MBE grown GaAs. The kinetics of dopant incorporation during MBE growth at these low growth temperatures seem to allow pushing the doping concentration further beyond thermal equilibrium. The epilayers, which are about 1 μm thick, are pseudomorphic with a lattice mismatch to the substrate of up to Δ c/ c=–0.4%. They remain free of structural defects such as dislocations and stacking faults. After annealing at 600 oC only the highest doped epilayers show a reduction in the Be concentration although the layers remain ultrahigh p-conductive. The increased incorporation of Be as well as its unusual stability in non-stoichiometric GaAs is likely influenced by the native defects in these layers, double positively charged As Ga defects and probably neutral gallium vacancies ( V Ga). This novel material which is solely achievable through low-temperature growth may significantly enhance III–V semiconductor applicability due to ultrahigh doping capability with increased thermal stability.

Nonequilibrium experiments on self-diffusion in silicon at low temperatures using isotopically enriched structures

Self-diffusion in silicon has been studied using epitaxially grown isotopically enriched structures under nonequilibrium concentrations of native point defects created by thermal oxidation and nitridation. Comparing identical anneals for phosphorus, antimony, and self-diffusion in Si, we obtain experimental evidence for a dual vacancy–interstitial mechanism of self-diffusion with the possibility of a small substitutional exchange component. We determine that in the temperature range 800–1100°C, the interstitial-mediated fraction of self-diffusion is confined between 0.50 and 0.62. The corresponding activation enthalpies are 4.68 and 4.86eV for the interstitial and vacancy mechanisms, respectively. Furthermore, both mechanisms exhibit large activation entropies. This constitutes direct experimental evidence of the remarkable similarity between the energetics of these native point defects in silicon.

Native point defect analysis in non-stoichiometric GaAs: an annealing study

The electronic properties of MBE-grown GaAs at temperatures well below 400°C are governed by its high concentration of native point defects. It is vital for device applications to understand and control the point defect concentrations in this non-stoichiometric III–V compound. In this paper, we present a detailed analysis of the changes of the point defect concentrations upon thermal annealing in both undoped and p-doped low-temperature-grown GaAs (LT-GaAs). The temperature-dependent concentration of residual arsenic antisites (AsGa) after annealing is shown. Also the annealing behavior of the gallium vacancies (VGa) is investigated. Their role in the As diffusion will be discussed. The thermal stabilization of AsGa will be demonstrated in LT-GaAs:Be for annealing temperatures as high as 700°C for 30min.

Experimental evidence of the self-compensation mechanism in CdS

Microscopic origin of full electrical compensation of donor doped CdS was analyzed with perturbed angular correlation and Hall-effect measurements. Single crystals were implanted with radioactive 111 In and stable 115 In ions. Total In concentration ranged from 10 16 to 10 20/cm 3. A strong correlation was observed between electrical self-compensation and the formation of (In Cd–V Cd) pairs (A centers) as a result of thermal annealings. It is shown that the presence of In donors during thermal treatment under the S pressure provokes spontaneous formation of (doubly) ionized cation vacancies [V Cd]. During cooling, these vacancies form pairs with In donors (A center), which compensate the rest of the donors, leading to highly resistive material. The experiments presented provide direct evidence for self-compensation: doped crystals spontaneously create just a matching concentration of native point defects needed to compensate foreign doping atoms electrically. This holds for over four orders of magnitude of In concentrations.

Erratum: Concentration of native point defects in Si single crystals at high temperatures

Erratum: Concentration of native point defects in Si single crystals at high temperatures

Concentration of native point defects in Si single crystals at high temperatures.

The concentration of dominant native defects in a float-zoned Si single crystal at high temperatures was directly determined from the difference between the macroscopic linear thermal expansion and the lattice-parameter thermal expansion. The concentration fraction of native defects in thermal equilibrium at 1300 K is (3.6\ifmmode\pm\else\textpm\fi{}2.7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$, or (1.8\ifmmode\pm\else\textpm\fi{}1.3)\ifmmode\times\else\texttimes\fi{}${10}^{16}$ atoms ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. It is found that vacancies are predominant over interstitial atoms in concentration.

Influence of dislocations on diffusion-induced nonequilibrium point defects in III-V compounds

Diffusion of elements migrating via a substitutional-interstitial mechanism in III-V compounds may induce nonequilibrium concentrations of native point defects. It has generally been assumed in the literature that, in the presence of dislocations, the point defects approach their thermal equilibrium concentrations. In contrast, it will be shown here that in III-V compounds the most favorable concentration a perturbed mobile species in one sublattice can reach corresponds to that of establishing a local equilibrium relation with another mobile species in the other sublattice if long-range transport of the defects to crystal surfaces is absent.

Use of precision density and lattice parameter measurements for study of point defects in single crystals of semiconductors: II. Determination of homogeneity regions of compounds, the Vegard law application

The method of determining semiconductor compound homogeneity regions by means of measuring the lattice parameter and density of heat treated samples, as well as the method of determining effective geometrical size and concentration of impurity atoms and native point defects in semiconductor crystals with the sphalerite lattice, have been described. Information on methods of studying the nature, formation mechanisms and concentrations of native point defects in the semiconductor AIIIBV compounds has been generalized.

Structural quality of Hg1-xCdxTe: Equilibrium point defects.

The Phillips--Van Vechten dielectric theory for estimating enthalpies of defect formation and defect concentrations is applied to CdTe and narrow-gap ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te (${\mathrm{Hg}}_{0.8}$${\mathrm{Cd}}_{0.2}$Te). It is found that favorable ionization levels in CdTe lead to large charged native defect densities (\ensuremath{\sim}${10}^{18}$/${\mathrm{cm}}^{3}$; \ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}${10}^{17}$/${\mathrm{cm}}^{3}$ net at 1365 K), which can condense into extended defects, such as dislocations and Te inclusions at lower temperatures. Intentional incorporation of small amounts of impurities or use of substitutional alloys, such as ${\mathrm{Cd}}_{0.96}$${\mathrm{Zn}}_{0.04}$Te, has often been suggested as a means of improving the quality of CdTe crystals, and addition of Zn or Mn has also been suggested as a means of reducing cation-vacancy concentrations in Hg-rich ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te. It is shown that equilibrium concentrations of native point defects are not reduced substantially by adding small amounts of other elements either to CdTe or to Hg-rich ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te, although macroscopic and nonequilibrium phenomena, such as formation of dislocations, may be significantly affected. Increasing metallicity upon addition of Hg to the alloy and the volatility of elemental Hg lead to preferential loss of Hg from ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te, which must be balanced, in equilibrium, by a larger proportion of Hg in the external phase. The observed strong p-type character of ${\mathrm{Hg}}_{0.8}$${\mathrm{Cd}}_{0.2}$Te, in contrast to the strongly compensated character of CdTe at high temperature, results both from this and from highly favorable cation-vacancy ionization levels.

Role of arsenic (As2, As) in controlling the quality of GaAs grown by MBE: Theoretical studies

We report results of theoretical investigations to understand the atomistic mechanisms responsible for controlling the surface roughness and concentration of native point defects in MBE grown GaAs. We have simulated growth of GaAs using both As2 and As as the sources of anions in the vapor phase using Monte Carlo techniques. In the case of As2 as a source, we have examined several possible reaction pathways for the dissociative chemisorption of As2 and their influence on the quality of the grown system (under anion overpressure). Different reaction pathways introduce different constraints for the incorporation of atoms from the vapors. These simulations indicate that lowering the constraints for As2 chemisorption decreases the defect nucleation site concentration but leads to a rougher growth front profile. We also report results of studies on GaAs growth using monatomic As as a source. The mechanisms responsible for As and As2 incorporation are expected to be very different and our model studies show important differences in the quality of the material grown. We have also examined the effect of varying the ratio of Ga to As vapor pressure on GaAs growth using monatomic As as a source and find that it has an important role in controlling the quality of the growth front profile.