Properties Of Native Point Defects Research Articles

The role of native point defects and donor impurities in the electrical properties of ZnSb2O4: a hybrid density-functional study.

The ceramic material zinc antimony oxide ZnSb2O4 has promising electrical and magnetic properties, making it suitable for various applications such as electrochemical and energy storage. However, the effects of point defects and impurities on its electrical properties have never been revealed. Here, we employ hybrid density-functional calculations to investigate the energetics and electronic properties of native point defects and donor impurities in ZnSb2O4. The energetically favorable configurations of native point defects under selected growth conditions (O-rich and O-poor) are identified based on the calculated formation energies. The study finds no shallow donor and shallow acceptor defects with low formation energies. Still, the oxygen vacancy (VO) has the lowest formation energy among the donor-type defects under O-rich and O-poor conditions. However, it acts as a very deep acceptor, making it unlikely to provide free electron carriers to the conduction band. Moreover, electron carriers are likely to be compensated by the formation of zinc vacancies (VZn) and the zinc substituted for antimony (ZnSb), which behave as dominant acceptors. Our analysis of the charge neutrality conditions estimates that the Fermi level of undoped ZnSb2O4 would be pinned in a range that is 2.60 eV to 3.12 eV above the VBM for O-rich to O-poor growth conditions, respectively, suggesting that this material is semi-insulating. The possibility of enhancing free electron carriers by doping with Al, Ga, In, and F impurities is also investigated. Our results, however, indicate that high n-type conductivity is hindered by self-compensation in which the impurities additionally act as electron killers. Our results suggest that other impurity candidates and approaches may need to be considered to efficiently dope this material into n-type. Overall, this work paves the way for point defect engineering in this class of ternary oxides.

Comparative studies of interatomic potentials for modeling point defects in wurtzite GaN

In this paper, a new version of the Stillinger–Weber (SW) potential for wurtzite GaN is presented, by which we systematically explore the structural and thermodynamical properties of native point defects and their complexes. In parallel, the semi-empirical Modified Embedded-Atom Method (MEAM) potential is selected for comparison. The SW and MEAM potentials are assessed by the reproduction of the fundamental properties of wurtzite GaN and by the ability to describe the inversion domain boundaries and the wurtzite–rocksalt phase transition. Then the structural search of native point defects and their complexes in GaN is implemented using both SW and MEAM potentials with the benchmark of Density Functional Theory (DFT) calculations. Besides vacancies and antisites, four N and five Ga interstitials are confirmed by refining the DFT calculations, among which two N split interstitials N+−N⟨21̄1̄0⟩ and N+−Ga⟨011̄0⟩, and two Ga split interstitials, Ga+−Ga⟨011̄0⟩−g and Ga+−N⟨011̄0⟩, are observed for the first time. The SW potential correctly predicts the octahedral occupation GaOct to be the most stable Ga interstitial, while the MEAM potential predicts the ground state of the N+−N⟨011̄0⟩ split interstitial (N+−N⟨011̄0⟩−g) as the most stable N interstitial. However, neither of the two potentials could simultaneously generate the most stable configurations of N and Ga interstitials. The investigations of point defect complexes reveal that N octahedral Frenkel [FrenkelOct(N)] and paired antisite (NGaGaN) defects are unstable and get converted into VN⊕N+−N⟨011̄0⟩−g configurations with different separations between VN and N+−N⟨011̄0⟩−g point defects based on the DFT calculations. The formation energies calculated by the DFT and SW potential demonstrate that Schottky, Ga octahedral Frenkel [FrenkelOct(Ga)], and VN⊕N+−N⟨011̄0⟩−g point defect complexes are energetically feasible and that they should not dissociate into two isolated point defects. In contrast, the MEAM potential predicts the dissociation to be exothermic for Schottky and VN⊕N+−N⟨011̄0⟩−g. Overall, the structural features concerned with N–N or Ga–Ga bonds relaxed by the SW potential are more consistent with DFT calculations than the MEAM counterpart.

Electronic and thermodynamic properties of native point defects in V2O5: a first-principles study.

The formation of native point defects in semiconductors and their behaviors play a crucial role in material properties. Although the native defects of V2O5 include vacancies, self-interstitials, and antisites, only oxygen vacancies have been extensively explored. In this work, we carried out first-principles calculations to systematically study the properties of possible native defects in V2O5. The electronic structure and the formation energy of each defect were calculated using the DFT+U method. Defect concentrations were estimated using a statistical model with a constraint of charge neutrality. We found that the vanadyl vacancy is a shallow acceptor that could supply holes to the system. However, the intrinsic p-type doping in V2O5 hardly occurred because the vanadyl vacancy could be readily compensated by the more stable donor, i.e., the oxygen vacancy and oxygen interstitial, instead of holes. The oxygen vacancy is the most dominant defect under oxygen-deficient conditions. However, under extreme O-rich conditions, a deep donor of oxygen interstitial becomes the major defect species. The dominant oxygen vacancy under synthesized conditions plays an important role in determining the electronic conductivity of V2O5. It induces the formation of compensating electron polarons. The polarons are trapped at V centers close to the vacancy site with the effective escaping barriers of around 0.6 eV. Such barriers are higher than that of the isolated polaron hopping (0.2 eV). The estimated polaron mobilities obtained from kinetic Monte Carlo simulations confirmed that oxygen vacancies act as polaron-trapping sites, which diminishes the polaron mobility by 4 orders of magnitude. Nevertheless, when the sample is synthesized at elevated temperatures, a number of thermally activated polarons in samples are quite high due to the high concentrations of oxygen vacancies. These polarons can contribute as charge carriers of intrinsic n-type semiconducting V2O5.

First-principles theoretical analysis and electron energy loss spectroscopy of vacancy defects in bulk and nonpolar (101¯) surface of GaN

We determine atomic structure, electronic structure, formation energies, magnetic properties of native point defects, such as gallium (Ga) and nitrogen (N) vacancies, in bulk and at the nonpolar (101¯0) surface of wurtzite gallium nitride (w-GaN) using first-principles density functional theory (DFT) based calculations. In bulk and at the (101¯0) surface of GaN, N vacancies are significantly more stable than Ga vacancies under both Ga-rich and N-rich conditions. We show that within DFT-local density approximated N vacancies form spontaneously at the (101¯0) surface of GaN when doped to raise the Fermi level up to ≈1.0 eV above valence band maximum (VBM) while with valence band edge correction it is 1.79 eV above VBM. We provide experimental evidence for occurrence of N vacancies with electron energy loss spectroscopy measurements, which further hints the N vacancies at surface to the source of auto-doping which may explain high electrical conductivity of GaN nanowall network grown with molecular beam epitaxy.

Hybrid functional study of native point defects and impurities in ZnGeN2

Using hybrid density functional theory, we investigate the properties of native point defects and hydrogen and oxygen impurities in ZnGeN2, a wide-band-gap semiconductor that is promising for applications in electronic and optoelectronic devices. We find that cation antisites have the lowest formation energies amongst all of the native point defects for a wide range of chemical potential conditions. However, native point defects cannot act as sources of doping. Unintentional n-type conductivity in ZnGeN2 must be attributed to impurities: substitutional oxygen on a nitrogen site and interstitial hydrogen act as donors.

Self-diffusion in single crystalline silicon nanowires

Self-diffusion experiments in single crystalline isotopically controlled silicon nanowires with diameters of 70 and 400 nm at 850 and 1000 °C are reported. The isotope structures were first epitaxially grown on top of silicon substrate wafers. Nanowires were subsequently fabricated using a nanosphere lithography process in combination with inductively coupled plasma dry reactive ion etching. Three-dimensional profiling of the nanosized structure before and after diffusion annealing was performed by means of atom probe tomography (APT). Self-diffusion profiles obtained from APT analyses are accurately described by Fick's law for self-diffusion. Data obtained for silicon self-diffusion in nanowires are equal to the results reported for bulk silicon crystals, i.e., finite size effects and high surface-to-volume ratios do not significantly affect silicon self-diffusion. This shows that the properties of native point defects determined from self-diffusion in bulk crystals also hold for nanosized silicon structures with diameters down to 70 nm.

Intrinsic defects in gallium sulfide monolayer: a first-principles study

The electronic and magnetic properties of native point defects, including vacancies (VGa and VS), antisites (GaS and SGa) and interstitials (Gai and Si) in monolayer and bulk GaS, were systemically studied using the density functional theory method.

Theoretical Study of Optical Properties of Native Point Defects in α-Al2O3

Using first-principles calculations based on density functional theory with generalized gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation functional, we studied the native point defects in α-Al2O3. We found that oxygen vacancy (VO), Al vacancy (VAl), and Al interstitial (Ali) are the dominant defects in α-Al2O3 under both Al- and O-rich growth conditions. Because the optical properties of α-Al2O3 are important for many device applications, the optical excitation levels due to these native point defects were studied. Our results revealed that the absorption and emission bands at ∼3.0 and 6.0 eV should be associated with VO (F-center) in good agreement with previous works. Moreover, the observed emission peak at ∼3.8 eV is very close to the calculated emission energy of Ali. The absorption and emission due to other native point defects were also reported.

Properties of Point Defects in Silicon: New Results after a Long-Time Debate

The contributions of vacancies and self-interstitials to silicon (Si) self-diffusion are a matter of debate since many years. These native defects are involved in dopant diffusion and the formation of defect clusters and thus influence many processes that take place during Si single crystal growth and the fabrication of silicon based electronic devices. Considering their relevance it is remarkable that present data about the properties of native point defects in Si are still limited and controversy. This work reports recent results on the properties of native point defects in silicon deduced from self-diffusion experiments below 850°C. The temperature dependence of silicon self-diffusion is accurately described by contributions due to vacancies and self-interstitials assuming temperature dependent vacancy properties. The concept of vacancies whose thermodynamic properties change with temperature solves the inconsistency between self-and dopant diffusion in Si but further experiments are required to verify this concept and to prove its relevance for other material systems.

Contributions of vacancies and self-interstitials to self-diffusion in silicon under thermal equilibrium and nonequilibrium conditions

Since many years, the contribution of vacancies ($V$) and self-interstitials ($I$) to silicon (Si) self-diffusion is a matter of debate. Native defects and their interaction among themselves and with foreign atoms influence the processes taking place during device fabrication, starting with the growth of Si single crystals and ending with doping of nanosized electronic devices. Considering this relevance, it is remarkable that present data about the properties of native point defects in Si are still limited and controversial. This work reports experiments on self-diffusion in Si for temperatures between 650${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C and 960${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C to verify recent results of Shimizu et al. [Phys. Rev. Lett. 98, 095901 (2007)] that give rise to inconsistencies in $V$-mediated self- and dopant diffusion. Two different structures of isotopically controlled epitaxial layers of Si are used for the diffusion study. One structure consisting of 20 bilayers of ${}^{29}$Si/${}^{28}$Si was grown by molecular beam epitaxy (MBE). The other structure with a ${}^{28}$Si layer sandwiched between natural Si was grown by means of chemical vapor deposition. Self-diffusion in (${}^{29}$Si/${}^{28}$Si)${}_{20}$ multilayers (ML) was analyzed by means of secondary ion mass spectrometry (SIMS) and neutron reflectometry, whereas self-diffusion in ${}^{\mathrm{nat}}$Si/${}^{28}$Si/${}^{\mathrm{nat}}$Si sandwich (SW) structures was measured with SIMS only. Analysis of the experimental profiles reveals an enhanced self-diffusion in ML compared to SW structures. The enhanced diffusion is ascribed to the dissolution of $V$- and $I$-related defect clusters grown-in during MBE. On the other hand, self-diffusion in the SW structures accurately confirms the data of Shimizu et al. that are considered to represent data for thermal equilibrium conditions. The temperature dependence of self-diffusion is described by $V$- and $I$-mediated contributions with temperature-dependent thermodynamic properties of $V$. This interpretation can solve the inconsistency between self- and dopant diffusion in Si, but further experiments are required to verify this concept.

Point defects in Cd(Zn)Te and TlBr: Theory

The effects of various crystal defects on the performances of CdTe, Cd1−xZnxTe (CZT), and TlBr for room-temperature high-energy radiation detection are examined using first-principles theoretical methods. The predictive, parameter-free, atomistic approaches used provide fundamental understanding of defect properties that are difficult to measure and also allow rapid screening of possibilities for material engineering, such as optimal doping and annealing conditions. Several recent examples from the author's work are reviewed, including: (i) accurate calculations of the thermodynamic and electronic properties of native point defects and point defect complexes in CdTe and CZT; (ii) the effects of Zn alloying on the native point defect properties in CZT; (iii) point defect diffusion and binding leading to Te clustering in Cd(Zn)Te; (iv) the profound effect of native point defects—principally vacancies—on the intrinsic material properties of TlBr, particularly its electronic and ionic conductivity; and (v) a study on doping TlBr to independently control the electronic and ionic conductivity.

Stability and mobility of native point defects in AlH 3

We investigate the properties of native point defects in AlH 3 using density functional theory (DFT). The results indicate that mass transport in AlH 3 is mediated by positively charged hydrogen vacancies ( V H + ), with an activation energy of 1.72 eV that is in good agreement with experimentally observed values for the dehydrogenation of AlH 3. An accurate description of the point-defect properties requires use of an advanced hybrid DFT/Hartree–Fock approach with the functional of Heyd, Scuseria, and Enzerhof (HSE), as revealed by a detailed comparison with results obtained using a conventional generalized gradient approximation functional. The HSE produces a more accurate band gap, and significant differences are found for the formation energies of the defects between HSE and PBE. We discuss the differences in terms of an alignment of the band structures and defect transition levels on an absolute energy scale.

Chemical diffusion in CdTe:Cl

High-temperature in situ galvanomagnetic measurements are reported in chlorine-doped CdTe, [Cl] ≈4 × 1018 cm−3 at temperatures T = 600–700 °C near Cd saturation. The chemical diffusion coefficient is determined by means of relaxation of electrical conductivity after a step-like change of ambient Cd pressure (PCd) and approximated well by a trial function . Both and equilibrium conductivity are calculated on the basis of a defect model in which donors ClTe are compensated by divalent acceptors Cd vacancies (VCd) and monovalent acceptor complexes (ClTeVCd). The properties of native point defects are consistent with undoped CdTe. It is shown that can be fit well assuming different diffusion coefficients for singly and doubly charged VCd.

Properties of native point defects in In1−xAlxN alloys

The electrical and optical properties of the In-rich InAlN alloys are strongly influenced by native point defects. Here the effects of the defects are studied using 2 MeV He+ irradiation to vary the defect concentration. Localized native defects in In1−xAlxN(x < 0.45) are predominantly donors, with energy levels located above the conduction band edge. Accordingly, the electron concentration increases and the optical absorption edge blue shifts with increasing irradiation fluence before saturating at high fluences. Saturation occurs when the Fermi level reaches the Fermi level stabilization energy, which is the average energy of localized native defects in semiconductors, at 4.9 eV below the vacuum level. The energy position of the native defects also explains the initial increase followed by the quenching of the photoluminescence (PL) intensity, as well as the blue shift in the PL peak, with increasing irradiation fluence.

Comment on “Self-Diffusion in Silicon: Similarity between the Properties of Native Point Defects”

A Comment on the Letter by Ant Ural, P. B. Griffin, and J. D. Plummer, Phys. Rev. Lett. 83, 3454 (1999). The authors of the Letter offer a Reply.

Self-Diffusion in Silicon: Similarity between the Properties of Native Point Defects

Self-diffusion measurements in silicon are extended to the range 800--900 \ifmmode^\circ\else\textdegree\fi{}C by monitoring ${}^{30}\mathrm{Si}$ diffusion in isotopically enriched structures. Comparing P, Sb, and self-diffusion under nonequilibrium conditions, we determine that the interstitial-mediated fraction of self-diffusion is confined between 0.50 and 0.62 in the temperature range of 800--1100 \ifmmode^\circ\else\textdegree\fi{}C. This allows activation enthalpies of 4.68 and 4.86 eV to be determined for the interstitial and vacancy mechanisms, respectively. Both mechanisms are also found to exhibit large activation entropies. This result is in contrast to values extracted from metal diffusion experiments.