P-type Conditions Research Articles

Stability and migration of charged oxygen interstitials in ThO2 and CeO2

Density functional theory calculations have been carried out to study the stability and migration of charged oxygen interstitials in ThO2 and CeO2. The calculations demonstrate that the oxygen interstitial is likely to lose electrons under p-type conditions and gain electrons under n-type conditions. Neutral Osplit0 and singly positive Osplit+ O–O〈110〉 split interstitials, and doubly negative octahedral (Oocta.2-) oxygen interstitial are found to be the lowest-energy configurations within a certain Fermi energy range. In both oxides, the Osplit0 is the most mobile, and the migration energies of the split oxygen interstitials in ThO2 are lower than in CeO2, indicating higher oxygen interstitial mobility in ThO2 than in CeO2.

Vacancy defects in indium oxide: An ab-initio study

First-principles density functional theory is employed to study the electrical behavior of oxygen and indium vacancies in indium oxide (In 2O 3). The oxygen vacancy is found to be a double donor. The indium vacancy is a triple acceptor, which >can be a compensation center in n-type In 2O 3, leading to n-type carrier reduction. However, its high formation energy under p-type conditions makes it unlikely to be a source of p-type carriers by itself.

Study of Native Defects and Transition-Metal (Mn, Fe, Co, and Ni) Doping in a Zinc-Blende CdS Photocatalyst by DFT and Hybrid DFT Calculations

We use first-principles DFT and hybrid DFT calculations to investigate the formation of native defects and transition-metal (Mn, Fe, Co, and Ni) doping in zinc-blende CdS and their effect on the electronic structures. Our results reveal that Cd vacancies, S vacancies, interstitial Cd, and interstitial S are dominant native defects, in good agreement with experimental results. Except interstitial S, other native defects do not contribute to visible light absorption. Transition-metal dopants tend to substitute a lattice Cd atom under S-rich conditions and occupy a tetrahedral interstitial site under p-type and Cd-rich conditions. The doping becomes difficult with increasing the atomic number of the transition metal. Co doping does not contribute to visible light absorption. Mn, Fe, and Ni doping, especially interstitial Ni doping, is able to narrow the band gap, and thus, these transition metals are good dopant candidates to tailor the visible light absorption property of nanosized CdS photocatalysts.

First-Principles Calculations of Electronic Structure and Solution Energies of Mn-Doped BaTiO3

Doping with 3d transition metals, particularly Mn, is thought to play an important role in determining the reliability of dielectrics used in multi-layer ceramic capacitors (MLCCs). However, a detailed examination of the electronic structure, solution energies and compensation mechanisms of these systems is lacking. In this paper, the quantitative analysis of the substitution of Mn in perovskite-type BaTiO3 using first-principles calculations in combination with chemical thermodynamics is reported. The solution energies of dopants with vacancy and n-type and p-type charge compensations have been systematically calculated. Substitution onto the two crystallographically different cation sites in cubic BaTiO3 under four different thermodynamic conditions with different chemical potentials is also examined. Mn is found to be stable on Ti sites under all conditions examined, although its charge state varies. In the oxidizing limit, Mn substitutes for Ti as a Mn4+ ion, but in the reducing limit, Mn substitutes for Ti as a Mn2+ ion compensated by the formation of an O vacancy. Depending on the Fermi level of the system, the valence state of Mn varies from Mn4+ under p-type conditions, to Mn2+ under n-type conditions. Mn3+ is not found to be stable. These results agree well with the experimentally determined site preferences and valence states of Mn, and help to further elucidate the features of Mn-doped BaTiO3 at the atomic level.

Ab initio study of La-doped BaSnO3 proton conductor

The hydration properties of Lanthanum-doped barium stannate have been studied using density functional calculations. The interaction energy between elementary defects (proton-dopant and oxygen vacancy-dopant) have been calculated in charged states corresponding to p-type conditions. Surprisingly, the most attractive energies are not found for first-neighbor relative positions. The geometric distortions and the electronic densities of state in the different configurations involved are presented. With these data, two simple models are used to estimate the energy of the hydration reaction.

Valency configuration of transition metal impurities in ZnO

We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM = Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn1-xTMxO, the localized TM2+ configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy eF close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with eF close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.

Modulating the growth conditions: Si as an acceptor in (110) GaAs for high mobility p-type heterostructures

We implement metallic layers of Si-doped (110) GaAs as modulation doping in high mobility p-type heterostructures, changing to p-growth conditions for the doping layer alone. The strongly autocompensated doping is characterized in bulk samples first, identifying the metal-insulator transition density and confirming classic hopping conduction in the insulating regime. To overcome the poor morphology inherent to Si p-type (110) growth, heterostructures are then fabricated with only the modulation-doping layer grown under p-type conditions. Such heterostructures show a hole mobility of μ=1.75×105cm2∕Vs at density p=2.4×1011cm−2. We identify the zero-field spin-splitting characteristic of p-type heterostructures, but observe a remarkably isotropic mobility and a persistent photoconductivity unusual for p heterojunctions grown on other facets. This modulated growth technique is particularly relevant for p-type cleaved-edge overgrowth and for III-V growth chambers, where Si is the only dopant.

Energetics of native point defects in cubic silicon carbide

In this work we present a detailed investigation of native point defects energetics in cubic SiC, using state-of-the-art first principles computational method. We find that, the carbon vacancy is the dominant defect in p-type SiC, regardless the growth conditions. Silicon and carbon antisites are the most common defects in n-type material in Si-rich and C-rich conditions respectively. Interstitial defects and silicon vacancy are less favorite from the energetic point of view. The silicon vacancy tends to transform into a carbon vacancy-antisite complex and the carbon interstitial atom prefers to pair to a carbon antisite. The dumbbell structure is the lowest-energy configuration for the isolated carbon interstitial defect, and the tetrahedral interstitial silicon is a stable structure in p-type and intrinsic conditions, while in n-type material the dumbbell configuration is the stable one. Our results suggest that, in samples grown in Si-rich stoichiometric conditions, native defects are a source of n-doping and of compositional unbalance of nominally intrinsic SiC, in accord with experimental evidence.

Bi incorporation in GaN andAlxGa1−xNalloys

Bismuth has been recently suggested as a possible surfactant in the growth of GaN samples in place of As and Sb. In this work we use a density-functional first-principles technique to investigate the solubility and electrical activity of Bi impurities in GaN and AlGaN, two key parameters for surfactants that are so far unknown for bismuth. We find that the Bi solubility in GaN and AlGaN is very low. In n-type and moderate p-type conditions Bi behaves like a donor or an isoelectronic impurity substituting Ga at the cation site, while in extreme p-type conditions a multiple-donor ${\mathrm{Bi}}_{\mathrm{N}}$ defect is formed that hinders efficient p-type doping. In AlGaN, Al and Bi substitutional defects show a tendency to pair, giving origin to an Al-Bi complex, whose electronic properties are similar to those of an isolated ${\mathrm{Bi}}_{\mathrm{Ga}}$ impurity. The results of our investigations suggest that the use of Bi as surfactant is incompatible with an efficient p-type doping. Finally we examine the features of the defects extrinsic levels, identifying the current data on Bi-doped GaN luminescence as a conduction-band--to--donor recombination process for the ${\mathrm{Bi}}_{\mathrm{Ga}}$ defect.

Energetics of native defects in ZnO

We have investigated the formation energies and electronic structure of native defects in ZnO by a first-principles plane-wave pseudopotential method. When p-type conditions are assumed, the formation energies of donor-type defects can be quite low. The effect of self-compensation by the donor-type defects should be significant in p-type doping. Under n-type conditions, the oxygen vacancy exhibits the lowest formation energy among the donor-type defects. The electronic structure, however, implies that only the zinc interstitial or the zinc antisite can explain the n-type conductivity of undoped ZnO.

Stability of native defects in hexagonal and cubic boron nitride

We investigate, through first-principles calculations, the stability and electronic structure of self-interstitials and vacancies in both hexagonal (graphite-like) and cubic boron nitride. We find that the self-interstitials ${\mathrm{N}}_{i}$ and ${\mathrm{B}}_{i}$ in hexagonal boron nitride $(h\ensuremath{-}\mathrm{BN})$ have low formation energies, comparable to those of the vacancies ${V}_{\mathrm{N}}$ and ${V}_{\mathrm{B}}.$ For instance, we find that ${\mathrm{N}}_{i}$ is the most stable defect in $h\ensuremath{-}\mathrm{BN}$ under N-rich and p-type conditions followed by the nitrogen vacancy. This is consistent with experimental findings of large concentrations of nitrogen interstitials and vacancies, and of the trapping of nitrogen in the hexagonal phase of BN thin films grown by ion-bombardment assisted deposition techniques. In contrast, in cubic boron nitride $(c\ensuremath{-}\mathrm{BN})$ the self-interstitials have high formation energies as compared to those of the vacancies. As a consequence, the formation of vacancy-interstitial pairs in kickout processes would typically require much more energy in $c\ensuremath{-}\mathrm{BN}$ than in $h\ensuremath{-}\mathrm{BN}.$ This suggests that a possible role of the ion bombardment in favoring the growth of $c\ensuremath{-}\mathrm{BN}$ films is to generate a much larger amount of defects in the hexagonal phase than in the cubic phase.

Atomic geometry and energetics of native defects in cubic boron nitride

We use first-principles calculations to investigate the electronic structure, atomic geometries and formation energies of point vacancies (VB , VN ), antisites (BN , NB ) and interstitials (Bi , Ni ) in cubic boron nitride (c-BN). We find that nitrogen and boron vacancies exhibit the lowest formation energies in nonstoichiometric c-BN for p-type and n-type conditions, respectively, showing intrinsic donor(VB ) and acceptor (VN ) characters. The equilibrium geometries show large outward breathing relaxations for both vacancies and for B N , with slight Jahn-Teller distortions from Td symmetry. For NB we nd an off-center distortion inducing a negative-U behavior in this center. For both interstitial centers we find stable confi- gurations in which an atomic pair occupies a nitrogen site in the lattice, N-N for Ni and N-B for Bi . These systems are stable for the different charge states investigated.

Atomic geometry and energetics of vacancies and antisites in cubic boron nitride

We use first-principles calculations to investigate the atomic geometries and formation energies of vacancies (VN,VB) and antisites (BN,NB) in cubic boron nitride. We find that VN and VB are the most stable defects in p-type and n-type conditions, respectively. They also exhibit intrinsic donor (VN) and acceptor (VB) characters, which makes them good candidates for compensation. The equilibrium geometries show large outward breathing relaxations for both vacancies and for BN, with a slight Jahn–Teller distortion from Td symmetry. For NB in neutral and negatives charge states, we find an off-center distortion, inducing a negative-U behavior.

Energetics of the As vacancy in GaAs: The stability of the 3+ charge state.

Total-energy calculations predict that the As vacancy in GaAs adopts a 3+ charge state under p-type conditions, and that the formation energy of ${\mathit{V}}_{\mathrm{As}}^{3+}$ is competitive with that of the gallium interstitial ${\mathrm{Ga}}_{\mathit{i}}^{3+}$. After a breathing-mode relaxation, which reduces the energy by more than 1.5 eV, the nearest-neighbor Ga atoms exhibit a nearly ${\mathrm{sp}}^{2}$ bonding configuration. On the basis of our results we propose that charged anion vacancies with ${\mathrm{sp}}^{2}$-bonded cation neighbors should be examined as possible mechanisms for the compensation of p-type doping in III-V and II-VI semiconductors, and as possible facilitators of acceptor diffusion and anion self-diffusion in these materials.

Atomic diffusion mediated by intrinsic point defects in GaAs and AlxGa1−xAs–GaAs superlattices

Point-defect-mediated atomic diffusion in GaAs and AlxGa1−xAs–GaAs superlattices is examined thermodynamically by focusing on activation enthalpy of diffusion. Through a review of available experimental results of impurity diffusion of Si, Zn, and Be, their diffusion phenomena are discussed by taking the characteristics of column-III-site-related point defect, such as Ga vacancy and arsenic-antisite, into consideration. It is suggested that Zn and Be diffusion should be mediated by As-antisite defects. On the other hand, Si diffusion is mediated by either Ga vacancy or As-antisite, depending on the growth method of materials and diffusion conditions. It is argued that As-antisite should be a mediator of diffusion in As-antisite-rich materials and with using As-rich diffusion source under p-type conditions. Cation self-diffusion or interdiffusion is also discussed in the same manner. Impurity-enhanced layer-disordering phenomena are examined by considering the reduction of defect energy of Ga vacancy and As antisite under n-type and p-type conditions, respectively. Beryllium enhanced and suppressed interdiffusion (cation self-diffusion) in MBE-grown AlxGa1−xAs–GaAs superlattices are interpreted in view of point-defect-mediated cation diffusion on the basis of the Fermi-energy dependence of point defect. In order to explain the phenomena, crystal-growth methods and surface-localized point defects which is responsible for Fermi-level stabilization are taken into consideration.