Role of native defects in wide-band-gap semiconductors

Emerging computational and machine learning methodologies for proton-conducting oxides: materials discovery and fundamental understanding

This review presents computational and machine learning methodologies developed during a 5-year research project on proton-conducting oxides. The main goal was to develop methodologies that could assist in materials discovery or provide new insights into complex proton-conducting oxides. Through these methodologies, three new proton-conducting oxides, including both perovskite and non-perovskites, have been discovered. In terms of gaining insights, octahedral tilt/distortions and oxygen affinity are found to play a critical role in determining proton diffusivities and conductivities in doped barium zirconates. Replica exchange Monte Carlo approach has enabled to reveal realistic defect configurations, hydration behavior, and their temperature dependence in oxides. Our approach 'Materials discovery through interpretation', which integrates new insights or tendencies obtained from computations and experiments to sequential explorations of materials, has also identified perovskites that exhibit proton conductivity exceeding 0.01 S/cm and high chemical stability at 300   ∘ C.

Hydrogen Impurities in ZnO: Shallow Donors in ZnO Semiconductors and Active Sites for Hydrogenation of Carbon Species

ZnO, as a low-cost yet significant semiconductor, has been widely used in solar energy conversion and optoelectronic devices. In addition, Cu/ZnO-based catalysts can convert syngas (H₂, CO, and CO₂) into methanol. However, the main concern about the intrinsic connection between the physical and chemical properties and the structure of ZnO still remains. In this work, efforts are made to decipher the physical and chemical information encoded into the structure. Through using NMR-IR techniques, we, for the first time, report a new ZnO model with three H⁺ cations incorporated into one Zn vacancy. ¹H magic-angle spinning NMR and IR spectra demonstrate that Ga³⁺ cations are introduced into the Zn vacancies of the ZnO lattice, which replace the H⁺ cation, and thus further confirm the feasibility of our proposed model. The exchange between the H⁺ cation in Zn vacancies and the D₂ gas phase shows that ZnO can activate H₂ because of the quantized three H⁺ cations in the defect site.

Boron-oxygen complex yields n-type surface layer in semiconducting diamond

Diamond is a wide-bandgap semiconductor possessing exceptional physical and chemical properties with the potential to miniaturize high-power electronics. Whereas boron-doped diamond (BDD) is a well-known p-type semiconductor, fabrication of practical diamond-based electronic devices awaits development of an effective n-type dopant with satisfactory electrical properties. Here we report the synthesis of n-type diamond, containing boron (B) and oxygen (O) complex defects. We obtain high carrier concentration (∼0.778 × 10²¹ cm^-3) several orders of magnitude greater than previously obtained with sulfur or phosphorous, accompanied by high electrical conductivity. In high-pressure high-temperature (HPHT) boron-doped diamond single crystal we formed a boron-rich layer ∼1-1.5 μm thick in the {111} surface containing up to 1.4 atomic % B. We show that under certain HPHT conditions the boron dopants combine with oxygen defects to form B-O complexes that can be tuned by controlling the experimental parameters for diamond crystallization, thus giving rise to n-type conduction. First-principles calculations indicate that B₃O and B₄O complexes with low formation energies exhibit shallow donor levels, elucidating the mechanism of the n-type semiconducting behavior.

Redistribution of native defects and photoconductivity in ZnO under pressure

Control and design of native defects in semiconductors are extremely important for industrial applications. Here, we investigated the effect of external hydrostatic pressure on the redistribution of native defects and their impact on structural phase transitions and photoconductivity in ZnO. We investigated morphologically distinct rod- (ZnO-R) and flower-like (ZnO-F) ZnO microstructures where the latter contains several native defects namely, oxygen vacancies, zinc interstitials and oxygen interstitials. Synchrotron X-ray diffraction reveals pressure-induced irreversible phase transformation of ZnO-F with the emergence of a hexagonal metallic Zn phase due to enhanced diffusion of interstitial Zn during decompression. In contrast, ZnO-R undergoes a reversible structural phase transition displaying a large hysteresis during decompression. We evidenced that the pressure-induced strain and inhomogeneous distribution of defects play crucial roles at structural phase transition. Raman spectroscopy and emission studies further confirm that the recovered ZnO-R appears less defective than ZnO-F. It resulted in lower photocurrent gain and slower photoresponse during time-dependent transient photoresponse with the synergistic application of pressure and illumination (ultra-violet). While successive pressure treatments improved the photoconductivity in ZnO-R, ZnO-F failed to recover even its ambient photoresponse. Pressure-induced redistribution of native defects and the optoelectronic response in ZnO might provide new opportunities in promising semiconductors.

Fundamental Resolution of Difficulties in the Theory of Charged Point Defects in Semiconductors

A defect's formation energy is a key theoretical quantity that allows the calculation of equilibrium defect concentrations in solids and aids in the identification of defects that control the properties of materials and device performance, efficiency, and reliability. The theory of formation energies is rigorous only for neutral defects, but the Coulomb potentials of charged defects require additional ad hoc numerical procedures. Here we invoke statistical mechanics to derive a revised theory of charged-defect formation energies, which eliminates the need for ad hoc numerical procedures. Calculations become straightforward and transparent. We present calculations demonstrating the significance of the revised theory for defect formation energies and thermodynamic transition levels.

Importance of doping, dopant distribution, and defects on electronic band structure alteration of metal oxide nanoparticles: Implications for reactive oxygen species

Metal oxide nanoparticles (MONPs) are considered to have the potency to generate reactive oxygen species (ROS), one of the key mechanisms underlying nanotoxicity. However, the nanotoxicology literature demonstrates a lack of consensus on the dominant toxicity mechanism(s) for a particular MONP. Moreover, recent literature has studied the correlation between band structure of pristine MONPs to their ability to introduce ROS and thus has downplayed the ROS-mediated toxicological relevance of a number of such materials. On the other hand, material science can control the band structure of these materials to engineer their electronic and optical properties and thereby is constantly modulating the pristine electronic structure. Since band structure is the fundamental material property that controls ROS-producing ability, band tuning via introduction of dopants and defects needs careful consideration in toxicity assessments. This commentary critically evaluates the existing material science and nanotoxicity literature and identifies the gap in our understanding of the role of important crystal structure features (i.e., dopants and defects) on MONPs' electronic structure alteration as well as their ROS-generation capability. Furthermore, this commentary provides suggestions on characterization techniques to evaluate dopants and defects on the crystal structure and identifies research needs for advanced theoretical predictions of their electronic band structures and ROS-generation abilities. Correlation of electronic band structure and ROS will not only aid in better mechanistic assessment of nanotoxicity but will be impactful in designing and developing ROS-based applications ranging from water disinfection to next-generation antibiotics and even cancer therapeutics.

Ab initio study on the stability of N-doped ZnO under high pressure

We perform first-principles density functional theory calculations to examine the stability of nitrogen-doped wurtzite ZnO under pressure.

Manifestation of intrinsic defects in the band structures of quaternary chalcogenide Ag2In2SiSe6 and Ag2In2GeSe6 crystals

Complex studies on the band structures of novel Ag₂In₂SiSe₆ and Ag₂In₂GeSe₆ crystals were performed.

Theory of Point Defects and Complexes in GaN

We have studied the electronic and energetic properties of native defects, impurities and complexes in GaN applying state-of-the-art first-principles calculations. An analysis of the numerical results gives direct insight into defect concentrations and impurity solubility with respect to growth parameters (temperature, chemical potentials) and into the mechanisms limiting the doping levels in GaN. We show how compensation and passivation by native defects or impurities, solubility issues, and incorporation of dopants on other sites influence the acceptor doping levels. The role of hydrogen in enhancing the p-type doping is explained in detail. We also discuss the mechanisms responsible for the experimentally observed limitation of the free-carrier concentration in p-type GaN.

Native Defects in Wurtzite GaN And AlN

The results of an extensive theoretical study of native defects in GaN and of vacancies in AlN are presented. We have considered cation and anion vacancies, antisites, and intersti-tials. The computations were carried out using quantum molecular dynamics, in supercells containing 72 atoms. Due to the wide gap of nitrides, the formation energies of defects depend strongly on the position of the Fermi level. The N vacancy in GaN introduces a shallow donor level that may be responsible for the n-type character of as-grown GaN.Other defects introduce deep states in the gap, with strongly localized wave functions.

Increasing the equilibrium solubility of dopants in semiconductor multilayers and alloys

We have theoretically studied the possibility to control the equilibrium solubility of dopants in semiconductor alloys, by strategic tuning of the alloy concentration. From the modeled cases of C(0) in Si(x)Ge(1-x), Zn(-) and Cd(-) in Ga(x)In(1-x)P it is seen that under certain conditions the dopant solubility can be orders of magnitude higher in an alloy or multilayer than in either of the elements of the alloy. This is found to be due to the solubility's strong dependence on the lattice constant for size mismatched dopants. The equilibrium doping concentration in alloys or multilayers could therefore be increased significantly. More specifically, Zn- in a Ga(x)In(1-x)P multilayer is found to have a maximum solubility for x = 0.9, which is 5 orders of magnitude larger than that of pure InP.

Dissolution and re-crystallization processes in multiphase silicon stabilized tricalcium phosphate

Ultrasonically accelerated dissolution of multiphase silicon stabilized tricalcium phosphate powders in water or Earle's balanced salt solution transforms the powders into needle-like calcium deficient apatite crystals with the c-axis (001) oriented along the needle. Ion exchange with the solution occurs primarily in the first hours of immersion. The transformation is driven by an interaction between the crystal surface and adsorbed water leading to the growth of crystallites which have the most stable surface configuration. First principles calculations of the surface energies of various hydroxyapatite surfaces with and without adsorbed water shows that depending on the ion concentrations in the fluid that determine the chemical potential of tricalcium phosphate, either Ca-rich (010) or stoichiometric (001) layers are the dominant surfaces. The higher the chemical potential, the more elongated in the (001) direction the crystallites become to minimize the total surface energy. The loss of a calcium Ca(2+) compensated by the addition of two H(+) is strongly favoured energetically on the (001) and Ca-rich (010) surfaces. A high concentration of excess Si at grain boundaries may be partly responsible for the rapid transformation of multiphase Si-TCP.

Interfacial segregation and electro-diffusion of dopants in superlattices

A first-principles theory of interfacial segregation of dopants and defects in heterostructures is developed and applied to GAN/A1N superlattices. The results indicate that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude, depending on its chemical identity and charge state, and that these cannot be obtained from calculations for bulk constituents alone. In addition, the presence of an internal electric field in polar heterostructures induces electro-migration and accumulation of hydrogen at the appropriate interfaces.

Interfacial electronic traps at ZnSe/GaAs heterostructures studied by photomodulation Raman scattering

Photomodulation Raman scattering spectroscopy has been employed to study free charge trapping mechanisms at ZnSe-GaAs(001) heterostructure interfaces. This technique reveals that the interfacial region contains predominantly hole traps. Time dependent measurements of the photomodulated Raman scattering intensity show that interfacial charge-trap lifetime is ≈30 s for both electrons and holes.

Shallow donor state due to nitrogen-hydrogen complex in diamond

Based on an ab initio calculation, we propose a possible shallowing of a nitrogen (N) donor in diamond, in contrast to the traditional thinking that it is very deep. A complex defect of N and hydrogen (H), N-H-N, should be much shallower than an isolated N donor. A qualitative scenario for formation of the N-H-N defects is presented. The existence of this complex is strongly suggested by a recent discovery of a new muonium center in N-rich diamond.

Control of doping by impurity Cchemical potentials: predictions for p-type ZnO

Theoretical work has so far focused on the role of host-element chemical potentials in determining defect formation energies that control doping levels in semiconductors. Here, we report on our analysis of the role of the dopant-impurity chemical potential, which depends on the source gas. We present first-principles total-energy calculations that demonstrate a wide variation in the possible effective chemical potential of N. We account in detail for the recent puzzling observations of doping ZnO using N2 and N2O and predict that the use of dilute NO or NO2 gas would resolve the long-standing problem of achieving p-type ZnO.