O-rich Growth Conditions Research Articles

A comprehensive study of defects in gallium oxide by density functional theory

Ultrawide bandgap gallium oxide (Ga2O3) is a promising material for power semiconductor devices and deep ultraviolet (UV) solar-blind photodetectors. Understanding the properties of point defects in Ga2O3 is necessary to realize better-performing devices. A comprehensive study based on density functional theory (DFT), using the generalized gradient approximation (GGA): Perdew-Burke-Ernzerhof (PBE) exchange–correlation functional, of point defects in corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 is presented. The point defects include vacancies, interstitials, antisites, and extrinsic impurities in various phases of Ga2O3. Defect formation energies, charge transition energy levels, and defect concentrations variation with temperature are listed and presented under both gallium-rich (Ga-rich) and oxygen-rich (O-rich) growth conditions. The formation energy diagrams predict that the Ga2O3 phases favor the formation of Ga and O vacancies and the incorporation of extrinsic impurities. The calculations also show that the charge transition levels are deep inside the bandgap regardless of the Ga2O3 phase and growth environment. The impacts of temperature on the intrinsic point defects are analyzed by probing the vibrational modes of β-Ga2O3 thin films grown in a metal–organic chemical vapor deposition (MOCVD) system at 700 °C temperature and annealed at 1100 °C in an O-rich environment.

Effect of hydrogen on magnetic properties in MgO studied by first-principles calculations and experiments

We investigated the effects of both intrinsic defects and hydrogen atom impurities on the magnetic properties of MgO samples. MgO in its pure defect-free state is known to be a nonmagnetic semiconductor. We employed density-functional theory and the Heyd–Scuseria–Ernzerhof (HSE) density functional. The calculated formation energy and total magnetic moment indicated that uncharged {mathrm{V}}_{mathrm{Mg}}^{0} and singly charged {mathrm{V}}_{mathrm{Mg}}^{-1} magnesium vacancies are more stable than oxygen vacancies (VO) under O-rich growth conditions and introduce a magnetic moment to MgO. The calculated density of states (DOS) results demonstrated that magnetic moments of VMg result from spin polarization of an unpaired electron of the partially occupied valence band, which is dominated by O 2p orbitals. Based on our calculations, VMg is the origin of magnetism and ferromagnetism in MgO. In contrast, the magnetic moment of the magnetic VMg-MgO crystal is suppressed by hydrogen (H) atoms, and unpaired electrons are donated to the unpaired electronic states of VMg when the defect complex Hi-VMg is formed. This suggests that H causes a reduction in magnetization of the ferromagnetic MgO. We then performed experimental studies to verify the DFT predictions by subjecting the MgO sample to a thermal treatment that creates Mg vacancies in the structure and intentionally doping the MgO sample with hydrogen atoms. We found good agreement between the DFT results and the experimental data. Our findings suggest that the ferromagnetism and diamagnetism of MgO can be controlled by heat treatment and hydrogen doping, which may find applications in magnetic sensing and switching under different environmental conditions.

Correlation-assisted Peierls gap in divalent metal ion (Zn+2, Cd+2, Mg+2, Ca+2, Sr+2 and Ba+2) doped M1-VO2: First principle analysis

This work investigates energetic, electronic structures and optical properties of (M+2 = Zn, Cd, Mg, Ca, Sr, and Ba) doped M1-VO2 systems using density functional theory (DFT) with DFT + U extension to describe the strong V d-d orbital correlations. The results show that M+2 doping ions increase the internal stress, which weakens the stability of insulating states. The calculated formation energies indicate that this incorporation is possible at high concentrations and more favorites under O-rich growth conditions. In addition, analysis of the electronic structure reveals that the decrease of Eg1 values at 12.5% of Zn+2 doping is not only due to the excess electrons generating by oxygen vacancies as reported by experimental results. Multiple degrees of freedom contribute to the gap change in both case of AEMs or TM doping, where the strength of the covalent bonds reflects which; electronic correlation, structural distortion and orbital filling are the dominants factors. Furthermore, the energy upshift of the π∗ band due to the decrease of V–O apical bond lengths leads to the gap widening between O2p and V3dπ* orbitals and the weakly reduction of transition energy.

Structural Properties of Thin ZnO Films Deposited by ALD under O-Rich and Zn-Rich Growth Conditions and Their Relationship with Electrical Parameters.

The structural, optical, and electrical properties of ZnO are intimately intertwined. In the present work, the structural and transport properties of 100 nm thick polycrystalline ZnO films obtained by atomic layer deposition (ALD) at a growth temperature (Tg) of 100–300 °C were investigated. The electrical properties of the films showed a dependence on the substrate (a-Al2O3 or Si (100)) and a high sensitivity to Tg, related to the deviation of the film stoichiometry as demonstrated by the RT-Hall effect. The average crystallite size increased from 20–30 nm for as grown samples to 80–100 nm after rapid thermal annealing, which affects carrier scattering. The ZnO layers deposited on silicon showed lower strain and dislocation density than on sapphire at the same Tg. The calculated half crystallite size (D/2) was higher than the Debye length (LD) for all as grown and annealed ZnO films, except for annealed ZnO/Si films grown within the ALD window (100–200 °C), indicating different homogeneity of charge carrier distribution for annealed ZnO/Si and ZnO/a-Al2O3 layers. For as grown films the hydrogen impurity concentration detected via secondary ion mass spectrometry (SIMS) was 1021 cm−3 and was decreased by two orders of magnitude after annealing, accompanied by a decrease in Urbach energy in the ZnO/a-Al2O3 layers.

Mechanical properties of M-Zr (M=Cr, Al, Mn) co-doped ceria: A first-principles study

Zr-doped ceria has been extensively been used in three-way catalyst industry due to the excellent physical and chemical properties. Co-doping technology is always employed to improve the stability of the materials by hindering the phase segregation into CeO2 and ZrO2 individual phases. In this paper, we have intensively studied the stability of M-Zr co-doping systems (M= Cr, Al and Mn) under different growth conditions and under the applied stress. The results show that the O-rich growth condition is very beneficial to the formation of these compounds. Compared with Cr and Mn atoms, Al atoms produce a different type of lattice distortions, which all can improve the stability of the whole system. Among these candidates, Al-Zr co-doping can be most easily formed. Furthermore, our researches demonstrate that under a small strain range, co-doping might increase the mechanical properties of the systems by the solid solution strengthening.

A three-order-parameter bistable magnetoelectric multiferroic metal

Using first-principles calculations we predict that the layered-perovskite metal Bi5Mn5O17 is a ferromagnet, ferroelectric, and ferrotoroid which may realize the long sought-after goal of a room-temperature ferromagnetic single-phase multiferroic with large, strongly coupled, primary-order polarization and magnetization. Bi5Mn5O17 has two nearly energy-degenerate ground states with mutually orthogonal vector order parameters (polarization, magnetization, ferrotoroidicity), which can be rotated globally by switching between ground states. Giant cross-coupling magnetoelectric and magnetotoroidic effects, as well as optical non-reciprocity, are thus expected. Importantly, Bi5Mn5O17 should be thermodynamically stable in O-rich growth conditions, and hence experimentally accessible.

Energetics and electronic structure of native point defects in α-Ga2O3

We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects (Ga and O vacancies, interstitials, and a Ga vacancy-O vacancy pair) in corundum structured α-Ga2O3. Either under a Ga- or O-rich growth condition, the negatively-charged Ga vacancy and the positively-charged Ga interstitial on a site surrounded by six O atoms are dominant when the Fermi level approaches the conduction and valence band edges, respectively. These defects would compensate carrier electrons and holes, respectively. Ga-rich conditions relatively suppress the formation of the Ga vacancy and, therefore, are suited for extrinsic n-type doping of α-Ga2O3.

First-principles calculation of the structure and electronic properties of Fe-substituted Bi2Ti2O7

We performed first-principles calculations to investigate the formation energy, geometry structure, and electronic property of Fe-doped Bi2Ti2O7 systems with different Fe doping content. The calculated formation energies indicate that the substitutional configurations of Fe-doping Bi2Ti2O7 are easy to obtain under O-rich growth condition, but their thermodynamic stability decreases with the increase of Fe content. The calculated spin-resolved density of states and band structures indicate that the introduction of Fe into Bi2Ti2O7 brings high spin polarization. The spin-down impurity levels in FexBi2−xTi2O7 and spin-up impurity levels in FexBi2Ti2−xO7 systems locate in the bottom of conduction band and narrow the band gap significantly, thus leading to the absorption of visible light. Interestingly, the impurity states in FexBi2−xTi2O7 are the efficient separation center of photogenerated electron and hole, and less affected by Fe doping content, in comparison, the levels of impurity band in FexBi2Ti2−xO7 systems are largely effected by the Fe doping content, and high Fe doping content is the key factor to improve the separating rate of photogenerated electron and hole.

Electronic structure and relative stability of the coherent and semi-coherent HfO2/III-V interfaces

III-V semiconductors are prominent alternatives to silicon in metal oxide semiconductor devices. Hafnium dioxide (HfO2) is a promising oxide with a high dielectric constant to replace silicon dioxide (SiO2). The potentiality of the oxide/III-V semiconductor interfaces is diminished due to high density of defects leading to the Fermi level pinning. The character of the harmful defects has been intensively debated. It is very important to understand thermodynamics and atomic structures of the interfaces to interpret experiments and design methods to reduce the defect density. Various realistic gap defect state free models for the HfO2/III-V(100) interfaces are presented. Relative energies of several coherent and semi-coherent oxide/III-V semiconductor interfaces are determined for the first time. The coherent and semi-coherent interfaces represent the main interface types, based on the Ga-O bridges and As (P) dimers, respectively. show that interface energy depends sensitively on the type and position of the defects and the atomic structure of the interface. Various coherent interfaces are stable and have band gaps free of defect states in spite of the interfacial structural defects. The semi-coherent interfaces include harmful As dimers and As dangling bonds. If kinetics contributes via the layer by layer oxide growth, the semi-coherent interfaces are formed under the experimentally relevant O-rich growth conditions. This is explained by the basic interfacial structural motifs and the electron counting rule (ECR). An oxidized (3 × 1) substrate has previously been used to decrease interface defect gap state density. A scenario, which explains why the oxidized substrate leads to a relatively small interface defect density, is presented.

Formation of VZn-NO acceptors with the assistance of tellurium in nitrogen-doped ZnO films

We report the formation of VZn-NO complex shallow acceptors in ZnO films. Through tellurium and nitrogen co-doping under O-rich growth condition, a TeZn-NO complex can be stably formed. A proper O-rich post-annealing process can make tellurium out of the films, and finally results in the formation of VZn-NO complex. This process has been evidenced by a set of characterization methods. Utilizing this approach, hole majority has been realized for a large windows of nitrogen concentration. Therefore, this study may help to advance the investigation on the p-type bottleneck of ZnO material.

A Density Functional Theory Study of Codoping Characteristics of Sulfur with Alkaline Earth in Delafossite CuAlO2**Supported by the National Natural Science Foundation of China under Grant Nos. 11347199, 51402244, and 11547311, the Specialized Research Fund for Doctoral Program of Higher Education of China under Grant No. 20130184120028, the Fundamental Research Fund for the Central Universities, China under Grant Nos. 2682014CX084,

The structural, electronic properties and formation energies of sulfur and alkaline earth codoped delafossite CuAlO2 have been investigated using the first-principles density functional theory calculations. Our results reveal that the volume of codoping systems increases with the increasing atomic radius of metal atoms. The formation energies under different growth conditions have been calculated, showing that the codoping systems are formed easily under O-rich growth conditions. Electronic band structures and density of states have been obtained. The decreased bandgaps, enhanced covalence and appearance of electron acceptors after codoping are all good for p-type conductivity.

First-principles study on the doping effects of Al in α-MnO2

In this paper, first-principles calculations have been implemented to study the structural relaxation, formation energies and electronic structure of Al doped α-MnO2. Both Al insertion and Al substitution reactions in the α-MnO2 were considered. Calculated formation energies indicate that Mn atom is easier to be displaced by Al atom under the O-rich growth condition compared with Al insertion reaction. Besides, it can be found that Al doping can afford acceptor impurity level which can accommodate electrons, thus contributing to the improvement of conductivity of α-MnO2. The conductivity of α-MnO2 is gradually improved with the increasing doping concentration of Al, and Al0.0417Mn0.9583O2 exhibits the best conductivity. Lastly, the electronic structure of Al0.0417Mn0.9583O2 was further investigated by analysis of total charge density and Bader charge. It is clear that Al doping can afford more electrons for α-MnO2, which also contributes to improvement of its conductivity.

The Electronic Structures and Optical Properties of Alkaline-Earth Metals Doped Anatase TiO2: A Comparative Study of Screened Hybrid Functional and Generalized Gradient Approximation.

Alkaline-earth metallic dopant can improve the performance of anatase TiO2 in photocatalysis and solar cells. Aiming to understand doping mechanisms, the dopant formation energies, electronic structures, and optical properties for Be, Mg, Ca, Sr, and Ba doped anatase TiO2 are investigated by using density functional theory calculations with the HSE06 and PBE functionals. By combining our results with those of previous studies, the HSE06 functional provides a better description of electronic structures. The calculated formation energies indicate that the substitution of a lattice Ti with an AEM atom is energetically favorable under O-rich growth conditions. The electronic structures suggest that, AEM dopants shift the valence bands (VBs) to higher energy, and the dopant-state energies for the cases of Ca, Sr, and Ba are quite higher than Fermi levels, while the Be and Mg dopants result into the spin polarized gap states near the top of VBs. The components of VBs and dopant-states support that the AEM dopants are active in inter-band transitions with lower energy excitations. As to optical properties, Ca/Sr/Ba are more effective than Be/Mg to enhance absorbance in visible region, but the Be/Mg are superior to Ca/Sr/Ba for the absorbance improvement in near-IR region.

The effect of growth conditions, point defects and hydrogen on the electronic structure and properties of p-type (Al,N) codoped ZnO: A first principles study

The effects of point defects, hydrogen, and growth conditions on the electronic structure and properties of the (Al,N) codoped p-type ZnO have been investigated using the first principles method. The obtained results showed that the AlZn–NO–VZn complex is a shallow acceptor that can play an important role in achieving the p-type conductivity in the (Al,N) codoped ZnO films. Our results showed also that the electrical conductivity type in the (Al,N) codoped ZnO films strongly depends on the donor/acceptor concentrations ratio. The codoped ZnO films prepared under both Zn-rich and O-rich growth conditions with a donors/acceptors ratio of 1:2 have a p-type conductivity, while those prepared with a ratio of 1:1 cannot be p-type unless if they are prepared under O-rich conditions. The achieved p-type quality depends also on the used nitrogen doping source. To prepare p-type ZnO film of high quality using the (Al,N) codoping method, the use of NO or NO2 is recommended. The presence of donor defects such as oxygen vacancies and hydrogen will significantly affect the electronic properties of the (Al,N) codoped ZnO films, and if the concentration of these defects in the sample is high enough, the material can be easily converted to n-type.

A novel Pd3O9@α-Al2O3 catalyst under a hydroxylated effect: high activity in the CO oxidation reaction.

Considering the importance of palladium-based and doped metal-oxide catalysts in CO oxidation, we design a new Pd3O9@α-Al2O3 catalyst and simulate its efficiency under a hydroxylated effect. The structure, electronic structure and oxidation activity of the hydroxylated Pd3O9@α-Al2O3(0001) surface are investigated by density functional theory. Under the O-rich growth conditions, Pd preferentially replaces Al. The lowest formation energy of the Pd-doped α-Al2O3(0001) surface is 0.21 eV under conditions wherein the coverage of the Pd-doped α-Al2O3 is 0.75 on a pre-hydroxylated surface and the water coverage is 0.25, which leads to formation of a Pd3O9 cluster embedded in the Al2O3(0001) surface. The reaction mechanisms of CO oxidization have been elucidated first by CO adsorption and migration, second by O(v) formation with the first CO2 release, then by the first foreign O2 filling and CO co-adsorption, and finally by the second CO2 desorption and restoration of the hydroxylated Pd3O9@α-Al2O3(0001) surface. The rate-determining step is the formation of the first CO2 in the whole catalytic cycle. The results also indicate that the energy barrier for CO oxidization is obviously reduced compared to that of the undoped surface, which implies that the introduction of Pd can efficiently improve the oxidation reactivity of the α-Al2O3(0001) surface. Compared to the synthesized Ir1/FeO(x) (1.41 eV) and Pt1/FeO(x) (0.79 eV) catalysts, the reaction activation barrier of CO oxidation is lowered by 0.65 eV and 0.03 eV, respectively. Therefore, the Pd3O9@α-Al2O3 catalyst shows superior catalytic activity in CO oxidation. The present results enrich the understanding of the catalytic oxidation of CO by palladium-based catalysts and provide a clue for fabricating palladium-based catalysts with low cost and high activity.

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.

First-principles study on transition metal-doped anatase TiO2

The electronic structures, formation energies, and band edge positions of anatase TiO2 doped with transition metals have been analyzed by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The model structures of transition metal-doped TiO2 were constructed by using the 24-atom 2 × 1 × 1 supercell of anatase TiO2 with one Ti atom replaced by a transition metal atom. The results indicate that most transition metal doping can narrow the band gap of TiO2, lead to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability. The theoretical calculations could provide meaningful guides to develop more active photocatalysts with visible light response.

Investigation on the electrical properties of anatase and rutile Nb-doped TiO2 by GGA(+U)

Crystal structure, electronic properties, and stability of anatase and rutile Nb-doped TiO2 (Nb:TiO2) compounds with different doping concentrations are studied by the combination of GGA and GGA+U methods within the density functional theory based first-principle calculation. And the main research work and contents are listed as follows: The anatase Nb:TiO2 appears as a degenerated semiconductor which behaves as an intrinsic metal. Its metallic property arises from Nb substitution into the Ti site, providing electrons to the conduction band. In contrast, the rutile Nb:TiO2 shows insulating behaviors. Ionization efficiency of Nb in anatase Nb:TiO2 is higher than that in rutile. We expect that anatase Nb:TiO2 is a potential material for transparent conducting oxide (TCO) while rutile Nb:TiO2 is not. The doped systems show different electronic characteristics, such as band structure, Fermi energy, and effective mass of carriers at different doping levels. In higher dopant concentration nNb, the ionization efficiency decreases slightly. Calculated defect-formation energy shows that Ti-rich material growth conditions are not in favor of the introduction of Nb while Nb can be easily doped in Nb:TiO2 under O-rich growth conditions. Nb dopant is difficult to be doped at higher doping level for both anatase and rutile Nb:TiO2.

Study on the p-type conductivities and Raman scattering properties of N+ ion-implanted O-rich ZnO thin films

The p-type N doped ZnO thin films are fabricated using radio-frequency magnetron sputtering technique in O-rich growth condition together with the direct N+ ion-implantation and annealing. The conductivities and Raman scattering properties of the samples are studied by Hall measurements and Raman spectra respectively. Hall measurements indicate that the optimal p-type ZnO film can be obtained when the sample is annealed at 600 ℃ for 120 min in N2 ambience, and its hole concentration is about 2.527×1017 cm-3. N+-implantation induces three additional vibrational modes in ZnO, which are located at 274.2, 506.7 and 640.4 cm-1 respectively. In the process of the annealing, by comparing the electrical properties and Raman speetra of the samples, we find that the competition between intrinsic donor defects and the activation of N acceptors plays a crucial role in the p-type formation of ZnO:N films during annealing.

New insight into the enhanced visible-light photocatalytic activities of B-, C- and B/C-doped anatase TiO2 by first-principles

The geometry structures, formation energies and electronic properties of the B-, C- and B/C-doped anatase TiO2 were investigated by the density functional theory (DFT) calculations of first-principles. The results indicated that the visible-light absorption and photocatalytic activities of the B-, C- and B/C-doped anatase TiO2 were not only influenced by the energy gaps (Eg) and the distributions of impurity states, but also affected by the locations of Fermi levels (EF) and the energies of the edges of band gaps (Ev for the top of valence bands and Ec for the bottom of conduction bands). However, the above four factors changed with the doped models of TiO2. The impurity states in the band gaps reduced the maximum energy gaps in the band gaps, which is responsible for the absorption of visible light. The Fermi levels at the bottom of conduction bands indicated the existence of Ti(3+) ions, which enhanced the separation rates of photogenerated electrons and holes. Further, the energies of the edges of band gaps, determining the dominant types of oxidants (O2(-), hole, ˙OH) in the photocatalytic degradation, were discussed. Moreover, the stability of the doped TiO2 depended on its growth conditions (O-rich or Ti-rich environment). The O-rich growth condition is beneficial to the substitutional B and C atoms to Ti atoms, while the Ti-rich growth condition is favorable to the other doped TiO2 including the most stable co-doped TiO2 with the interstitial B atom and the substitutional C atom to O atom. In addition, our results also showed that the B/C-doped TiO2 inherited the partial electronic properties of single-doped TiO2, but also exhibited many new electronic properties, implying that the electronic properties of co-doped systems are not a mechanical mixture of those of both single-doped systems.