Charge Transition Levels Research Articles

The Fermi energy as common parameter to describe charge compensation mechanisms: A path to Fermi level engineering of oxide electroceramics

Chemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.

Deep Levels and Electron Paramagnetic Resonance Parameters of Substitutional Nitrogen in Silicon from First Principles.

Nitrogen is commonly implanted in silicon to suppress the diffusion of self-interstitials and the formation of voids through the creation of nitrogen-vacancy complexes and nitrogen-nitrogen pairs. Yet, identifying a specific N-related defect via spectroscopic means has proven to be non-trivial. Activation energies obtained from deep-level transient spectroscopy are often assigned to a subset of possible defects that include non-equivalent atomic structures, such as the substitutional nitrogen and the nitrogen-vacancy complex. Paramagnetic N-related defects were the object of several electron paramagnetic spectroscopy investigations which assigned the so-called SL5 signal to the presence of substitutional nitrogen (NSi). Nevertheless, its behaviour at finite temperatures has been imprecisely linked to the metastability of the NSi center. In this work, we build upon the robust identification of the SL5 signature and we establish a theoretical picture of the substitutional nitrogen. Through an understanding of its symmetry-breaking mechanism, we provide a model of its fundamental physical properties (e.g., its energy landscape) based on ab initio calculations. Moreover by including more refined density functional theory-based approaches, we calculate EPR parameters (↔g and ↔A tensors), elucidating the debate on the metastability of NSi. Finally, by computing thermodynamic charge transition levels within the GW method, we present reference values for the donor and acceptor levels of NSi.

P-Type Impurities in 4H-SiC Calculated Using Density Functional Theory

We have investigated the p-dopant potential of 14 different impurities (Be, B, F, Mg, Al,Ca, Sc, Cu, Zn, Ga, In, Ba, Pt, and Tl) within 4H-SiC via Density Functional Theory (DFT) calcu-lations using a hybrid density functional. We analyse the incorporation energies of impurity atomson Si and C sites as well as the character of lattice distortion induced by impurities. The calculatedthermal ionization energies confirm that Al and Ga on the Si site are the best candidates for p-dopingof 4H-SiC. Although we find some correlation of incorporation energies with atomic radii of impuri-ties, the difference in chemical interaction with neighbouring atoms and strong lattice distortions playimportant roles in determining the impurity incorporation energies and charge transition levels. Wefind Al to still be the best and most industrially viable p-dopant for 4H-SiC.

Stability of doped and undoped ScNbO4 compound: Insight from first principle calculations

The quest for potential applications for a newly synthesized material is pragmatic and essential for building new devices. Doping it with rare-earth ions is one way to tune and approve its properties. We present a density functional calculation on the pristine and doped ScNbO4 compounds in this paper. In this context, we used the charged point defect procedure within the projector augmented wave method. The result of the phonon dispersion plot shows that ScNbO4 is dynamically stable in the monoclinic wolframite-type structure. According to the electronic calculations, ScNbO4 has a wide band gap, which favors a variety of UV and other optical applications, including the luminescence effect. To analyze the capability of the title compound as a host of doped rare-earth ions, a number of ions were evaluated by checking the site and growth conditions. Furthermore, the application of the Freysoldt methodology showed that the charge transition level in the doped ScNbO4 material has a stable valence band maximum under rich and poor Yb growing conditions. The analysis of the result shows that the p-type structure is more favorable to growth, paving the way for LED applications.

Defects in WS2 monolayer calculated with a nonlocal functional: any difference from GGA?

Density functional theory (DFT) with generalised gradient approximation (GGA) functionals is commonly used to predict defect properties in 2D transition metal dichalcogenides (TMDs). Since GGA functionals often underestimate band gaps of semiconductors and incorrectly describe the character of electron localisation in defects and their level positions within the band gap, it is important to assess the accuracy of these predictions. To this end, we used the non-local density functional Perdew—Burke—Ernzerhof (PBE)0-TC-LRC to calculate the properties of a wide range of intrinsic defects in monolayer WS2. The properties, such as geometry, in-gap states, charge transition levels, electronic structure and the electron/hole localisation of the lowest formation energy defects are discussed in detail. They are broadly similar to those predicted by the GGA PBE functional, but exhibit numerous quantitative differences caused by the degree of electron and hole localisation in charged states. For some anti-site defects, more significant differences are seen, with both changes in defect geometries (differences of up to 0.5 Å) as well as defect level positions within the band gap of WS2. This work provides an insight into the performance of functionals chosen for future DFT calculations of TMDs with respect to the desired defect properties.

Identifying the ground state structures of point defects in solids

Point defects are a universal feature of crystals. Their identification is addressed by combining experimental measurements with theoretical models. The standard modelling approach is, however, prone to missing the ground state atomic configurations associated with energy-lowering reconstructions from the idealised crystallographic environment. Missed ground states compromise the accuracy of calculated properties. To address this issue, we report an approach to navigate the defect configurational landscape using targeted bond distortions and rattling. Application of our workflow to eight materials (CdTe, GaAs, Sb2S3, Sb2Se3, CeO2, In2O3, ZnO, anatase-TiO2) reveals symmetry breaking in each host crystal that is not found via conventional local minimisation techniques. The point defect distortions are classified by the associated physico-chemical factors. We demonstrate the impact of these defect distortions on derived properties, including formation energies, concentrations and charge transition levels. Our work presents a step forward for quantitative modelling of imperfect solids.

Electrically stimulated optical spectroscopy of interface defects in wide-bandgap field-effect transistors

Wide-bandgap semiconductors such as silicon carbide, gallium nitride, and diamond are inherently suitable for high power electronics for example in renewable energy applications and electric vehicles. Despite the high interest, the theoretical limit regarding device performance has not yet been reached for these materials. This is often due to charge trapping in defects at the semiconductor-insulator interface. Here we report a one-to-one correlation between electrically stimulated photon emission and the threshold voltage shift obtained from a fully processed commercial 4H-SiC metal-oxide-semiconductor field-effect power transistor. Based on this observation, we demonstrate that the emission spectrum contains valuable information on the energetic position of the charge transition levels of the responsible interface defects. We etch back the transistor from the reverse side in order to obtain optical access to the interface and record the emitted light. Our method opens up point defect characterization in fully processed transistors after device passivation and processing. This will lead to better understanding and improved processes and techniques, which will ultimately push the performance of these devices closer to the theoretical limit.

Photoluminescence from Be‐Doped GaN Grown by Metal‐Organic Chemical Vapor Deposition

A systematic photoluminescence study of Be‐doped GaN grown by metal‐organic chemical vapor deposition is presented. All Be‐doped samples show the ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV and the yellow luminescence (YLBe) band with a maximum at ≈2.15 eV in GaN:Be having various concentrations of Be. The UVLBe band is attributed to the shallow state of the BeGa acceptor with a delocalized hole. The YLBe band is caused by a Be‐related defect, possibly the polaronic state of the BeGa acceptor with the charge transition level at 0.3 eV above the valence band. This broad band exhibits unusual properties. In particular, it always shows two steps in its thermal quenching. The second step at T ≈ 200 K is attributed to the emission of holes from the 0.3 eV level to the conduction band. The origin of the first step remains unexplained.

Structure, electronic properties, and energetics of oxygen vacancies in varying concentrations of SixGe1−xO2

SiGe alloys, widely used in various technological applications, are typically interfaced with a thermally grown oxide layer that is composed of ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{O}}_{2}$, a composite material which is also used for technological applications in its own right. Point defects in this oxide layer influence the electronic and structural properties, which can detrimentally affect the desired application. In this paper, we use ab initio calculations to investigate the canonical oxygen vacancy in systems of varying compositions of ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{O}}_{2}$. We find that the electronic structures and geometries of the vacancies remain qualitatively similar to their well-known analogs in ${\mathrm{SiO}}_{2}$ and ${\mathrm{GeO}}_{2}$ regardless of the composition and similar to previous results in the literature on Ge-doped ${\mathrm{SiO}}_{2}$. They show a wide distribution of formation energies and one-electron levels across the various concentrations of ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{O}}_{2}$. However, our results show that the factor defining their quantitative behavior is not the concentration, rather it is the chemistry of the atoms around the vacancy, each combination of which has its own distribution of properties. The resulting charge transition levels similarly cover a wide range of the band gap. These results aid the understanding of reliability issues in technological applications.

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.

Native defects in monolayer GaS and GaSe: Electrical properties and thermodynamic stability

Structural, electronic and thermodynamic properties of native defects in GaS and GaSe monolayers are investigated by means of accurate ab-initio calculations. Based on their charge transition levels we assess the influence of the studied defects on the electrical properties of the monolayers. Specifically, we show that native defects do not behave as shallow dopants and their presence cannot account for the experimentally observed intrinsic doping. In addition, we predict that native defects are efficient compensation and recombination centers. Besides pointing out their detrimental nature, we also calculate the corresponding finite temperature formation energies and provide a window of growth conditions able to reduce the concentration of all relevant native defects.

Experimental and theoretical studies of native deep-level defects in transition metal dichalcogenides

Transition metal dichalcogenides (TMDs), especially in two-dimensional (2D) form, exhibit many properties desirable for device applications. However, device performance can be hindered by the presence of defects. Here, we combine state of the art experimental and computational approaches to determine formation energies and charge transition levels of defects in bulk and 2D MX2 (M = Mo or W; X = S, Se, or Te). We perform deep level transient spectroscopy (DLTS) measurements of bulk TMDs. Simultaneously, we calculate formation energies and defect levels of all native point defects, which enable identification of levels observed in DLTS and extend our calculations to vacancies in 2D TMDs, for which DLTS is challenging. We find that reduction of dimensionality of TMDs to 2D has a significant impact on defect properties. This finding may explain differences in optical properties of 2D TMDs synthesized with different methods and lays foundation for future developments of more efficient TMD-based devices.

Properties of Oxygen Vacancy and Hydrogen Interstitial Defects in Strontium Titanate: DFT + Ud,p Calculations

This work presents extensive theoretical studies focused on the mixed ion-electron transport in cubic strontium titanate (STO). A new approach to the description of this difficult system was developed within the framework of linear-scaling Kohn–Sham density functional theory, as realized in the ONETEP program. The description we present is free of any empirical parameters and relies on the Hubbard U and Hund’s J corrections applied to both Ti and O atoms. The proposed methodology was validated by considering perfect STO. Its calculated properties were found to be in close agreement with experiments and calculations at higher levels of theory. The validated approach was subsequently employed to study the oxygen vacancy (VO) and the hydrogen interstitial (IH), using very large supercells (625 ± 1 atoms). The relaxed configurations of defects were obtained through fastidious energy minimization and later analyzed from a number of perspectives. The calculated defect formation energies and charge transition levels (CTLs) were found to be in close agreement with the experiment. With the exception of the charge-neutral VO, all considered defects were found to introduce shallow states, located down to 0.2 eV below the conduction band. Our calculations revealed a large 1 eV difference in the thermodynamic and optical CTLs of the neutral VO, explaining the inconsistencies observed─till now─between conduction and spectroscopic measurements. The influence of defects on the bonding characteristics and the crystalline structure of STO was quantified, revealing that both VO and IH defects lead to a significant polarization and strong tilting of the TiO6 octahedra.

Ab initio investigations in amorphous silicon dioxide: Proposing a multi-state defect model for electron and hole capture

Experiments as well as theoretical calculations indicate that point defects in the amorphous SiO2 layer of electronic devices as well as in optical fibers are responsible for numerous stability and reliability phenomena, including noise, hysteresis, bias temperature instabilities and decreasing transmission efficiency. In addition to the well-known oxygen vacancy, hydrogen related defects such as the hydrogen bridge and the hydroxyl-E′ center have gained a considerable amount of attention in the recent past, as they are suspected to negatively influence the device performance by capturing charge carriers from e.g. both Si and SiC substrates in field effect transistors. Here, we present a thorough ab initio study of these oxide defects and develop a unified description of electron and hole capture processes in a single multi-state model, supported by a comprehensive analysis of various defect parameters like relaxation energies, charge transition levels, (meta-)stability and transition barriers. We show that a single oxide defect can have two different trap levels for both electron and hole capturing processes, which might be the cause of anomalous device degradation phenomena. Based on its low formation energy compared to other defect types, we find that the hydroxyl-E′ center is the most promising defect candidate to explain charge capture processes in Si/SiO2 systems. Furthermore, we argue that the reduced effect of positive bias temperature instability (PBTI) observed in MOS devices compared to its negative counterpart (NBTI) can be explained by the locations of the hydroxyl-E′ centers charge transition levels.

Defects study in zinc blende ZnS utilizing optimized hybrid functional

Due to its wide band gap and widespread availability in single crystals as well as thin film, zinc sulfide (ZnS) has been applied in many areas. Unfortunately, experimental studies alone are not always sufficient to understand the electronic properties of semiconductors, especially their intrinsic point defects. It is thus necessary to perform theoretical calculations along with the characterization experiments to elucidate their defect characteristics. Density functional theory (DFT) with (semi)local exchange correlation is not often appropriate for describing defects in wide band gap semiconductors, while hybrid functional that do not enforce generalized Koopmans theorem (gKT) are not entirely reliable for predicting the properties of point defects in semiconductors. In this study, we have tuned exact-exchange parameters of hybrid functional to not only reproduce the experimental band gap but also satisfy the gKT for zinc blende (ZB) ZnS. With the new optimized hybrid functional, we study the intrinsic defects in ZB ZnS. Using this optimized hybrid functional we study the intrinsic defects of ZB ZnS. The results show that intrinsic defects have deep charge-transition levels. Among these defects, the zinc vacancy (Znvac) is energetically preferable in a n-type undoped sample either in Zn-rich or S-rich conditions. The calculated emission energy by the combination of an electron from the conduction band minimum and a trapped hole at Znvac2- is 3.17 eV. This calculation is the first theoretical result that explains the origin of the deep-blue emission in ZB ZnS.

Role of doping and defect quenching in antiferroelectric NaNbO3 from first principles

The antiferroelectric ${\mathrm{NaNbO}}_{3}$ is a candidate material for application in high-energy density dielectric capacitors. Various doping strategies have been used for installing the desired narrow double P-E loop behavior in this lead-free material. However, to unravel the influence of doping on the double P-E loops, we first need to understand the influence of doping on the defect equilibrium. In this study, we therefore calculate the formation energies, charge transition levels, and doping behavior of single defects and defect complexes in orthorhombic ${\mathrm{NaNbO}}_{3}$ doped with Ca, Sn, and Sr by means of density functional theory (DFT). The results show that substitutions with dopants have formation energies as low as Na and O vacancies. Additionally, all defect complexes show negative binding energy, which means they are stable and very likely to form. Lastly, we show how defect quenching influences the Fermi level, which reveals how the synthesis conditions can tailor the final defect distribution.

Light Absorption and Emission by Defects in Doped Nickel Oxide

Nickel oxide is a versatile p‐type semiconducting oxide with many applications in optoelectronic devices, but high doping concentrations are often required to achieve necessary electrical conductivity. In contrast to many other transparent oxide semiconductors, even moderate doping levels in NiO can lead to significant optical absorption in the visible spectral range, limiting the application range of the material. This correlation has been reported extensively in the literature, but its origin has been unknown until now. This work combines experimental data on optical properties from a variety of NiO samples with results from hybrid density functional theory calculations. It shows that strong electron–phonon interaction leads to a significant blueshift (0.6–1 eV) of electronic transitions from the valence band maximum to defect states by light absorption with respect to the thermodynamic charge transition levels. This essentially renders NiO a narrow‐gap semiconductor by defect band formation already at moderate doping levels, with strong light absorption for photon energies of approximately 1 eV. The calculations are also shown to be fully consistent with experimental data on defect‐related light emission in NiO.

Ambipolarity of diluted hydrogen in wide-gap oxides revealed by muon study

Muon spin rotation has long been recognized as one of the few methods for experimentally accessing the electronic state of dilute hydrogen (H) in semiconductors and dielectrics, where muon behaves as a pseudo-H (designated by the elemental symbol Mu). Meanwhile, predictions on the electronic state of H in these materials by density functional theory (DFT) do not always agree with the observed states of Mu. Most notably, Mu frequently occurs in wide-gap oxides simultaneously in a neutral (Mu0) and a diamagnetic state (Mu+ or Mu−), which DFT calculations do not explain; they predict that H is stable only in a diamagnetic state with the polarity determined by the equilibrium charge-transition level (E+/−) vs the Fermi level. To address this issue, we developed a semi-quantitative model that allows a systematic understanding of the electronic states reported for Mu in the majority of oxides. Our model assumes that muons interact with self-induced excitons to produce relaxed-excited states corresponding to donor-like (MuD) and/or acceptor-like (MuA) states and that these states correspond to the non-equilibrium electronic level (E+/0 or E0/−) predicted by DFT calculations for H. The known experimental results are then explained by the relative position of E+/0 and E0/− in the host’s energy band structure. In addition, the model sheds new light on the polaron-like nature of the electronic states associated with shallow donor Mu complexes.

First-principles study of oxygen vacancy defects in orthorhombic Hf0.5Zr0.5O2/SiO2/Si gate stack

The gate defect of the ferroelectric HfO2-based Si field-effect transistor (Si FeFET) plays a dominant role in its reliability issue. The first-principles calculations are an effective method for the atomic-scale understanding of gate defects. However, the first-principles study on the defects of FeFET gate stacks, i.e., the metal/orthorhombic-Hf0.5Zr0.5O2/SiOx/Si structure, has not been reported so far. The key challenge is the construction of metal/orthorhombic-Hf0.5Zr0.5O2/SiOx/Si gate stack models. Here, the atomic structure and defect property of orthorhombic-Hf0.5Zr0.5O2/SiO2/Si gate stack are systematically studied by first-principles calculations. We use the Hf0.5Zr0.5O2(130) high-index crystal face as the orthorhombic ferroelectric layer and construct a robust atomic structure of the orthorhombic-Hf0.5Zr0.5O2/SiO2/Si gate stack without any gap states based on the electron counting rule. The calculated band offsets show that this gate structure is of type-I band alignment. Furthermore, the formation energies and charge transition levels (CTLs) of defects reveal that the oxygen vacancy defects are more favorable to form compared with other defects such as oxygen interstitial and Hf/Zr vacancy, and their CTLs are mainly localized near the Si conduction band minimum and valence band maximum, in agreement with the reported experimental results. The oxygen vacancy defects are responsible for charge trapping/de-trapping behavior in Si FeFET. This work provides an insight into gate defects and paves the way to carry out the first-principles study of ferroelectric HfO2-based Si FeFET.

Vacuum‐Referred Binding Energies of Bismuth and Lanthanide Levels in LiTaO3 Perovskite: Toward Designing Energy Storage Phosphor for Anti‐Counterfeiting, X‐Ray Imaging, and Mechanoluminescence

AbstractDiscovering UV‐light or X‐ray charged afterglow and storage phosphors with high charge carrier storage capacity remains challenging. Herein, a method is proposed by combining vacuum referred binding energy (VRBE) diagram construction and optimization of dopants’ concentration and compound synthesis. The refined chemical shift model, optical spectroscopy, and thermoluminescence will be combined to construct the VRBE diagram of LiTaO3 with the lanthanide and bismuth charge transition levels. Based on the constructed VRBE diagram of LiTaO3, Bi3+, and/or Ln3+ (Ln = Tb or Pr) doped LiTaO3 will be studied. By combining Bi3+ with Tb3+, Pr3+, or Bi3+ itself, Bi3+ emerges to act as a ≈0.62 eV deep electron trap, while Tb3+, Pr3+, or Bi3+ acts as about 1.5 eV deep hole capturing and recombination centres. The VRBE in the Bi2+ 2P1/2 ground state will be derived by thermoluminescence study. Proof‐of‐concept X‐ray imaging, compression force distribution monitoring, and color‐tailoring for anti‐counterfeiting will be demonstrated by using the developed Bi3+ and/or Ln3+ doped LiTaO3. This work promotes the understanding of trap level locations and on the trapping and release processes of charge carriers in Bi3+ and/or lanthanides doped inorganic compounds for rational design of new afterglow and storage phosphors.