Dominant Native Defects Research Articles

Defects and oxygen impurities in ferroelectric wurtzite Al1−xScxN alloys

III-nitrides and related alloys are widely used for optoelectronics and as acoustic resonators. Ferroelectric wurtzite nitrides are of particular interest because of their potential for direct integration with Si and wide bandgap semiconductors and unique polarization switching characteristics; such interest has taken off since the first report of ferroelectric Al1−xScxN alloys. However, the coercive fields needed to switch polarization are on the order of MV/cm, which are 1–2 orders of magnitude larger than oxide perovskite ferroelectrics. Atomic-scale point defects are known to impact the dielectric properties, including breakdown fields and leakage currents, as well as ferroelectric switching. However, very little is known about the native defects and impurities in Al1−xScxN and their effect on the dielectric and ferroelectric properties. In this study, we use first-principles calculations to determine the formation energetics of native defects and unintentional oxygen incorporation and their effects on the polarization switching barriers in Al1−xScxN alloys. We find that nitrogen vacancies are the dominant native defects, and unintentional oxygen incorporation on the nitrogen site is present in high concentrations. They introduce multiple mid-gap states that can lead to premature dielectric breakdown and increased temperature-activated leakage currents in ferroelectrics. We also find that nitrogen vacancy and substitutional oxygen reduce the switching barrier in Al1−xScxN at low Sc compositions. The effect is minimal or even negative (increases barrier) at higher Sc compositions. Unintentional defects are generally considered to adversely affect ferroelectric properties, but our findings reveal that controlled introduction of point defects by tuning synthesis conditions can instead benefit polarization switching in ferroelectric Al1−xScxN at certain compositions.

Remarkable Stability and Optoelectronic Properties of an All-Inorganic CsSn0.5Ge0.5I3 Perovskite Solar Cell

Sn-Ge mixed perovskites have been proposed as promising lead-free candidates in the photovoltaics (PV) field. In this work, we discovered a stable P1 phase Sn-Ge mixed structure (CsSn0.5Ge0.5I3) with an appropriate band gap value of 1.19 eV, which manifests its unique structural stability and physics properties. The thermodynamic stability of this mixed structure under different growth conditions and all possible native defects are depicted in detail. The formation energies and dominant native point defects indicate that P1 phase CsSn0.5Ge0.5I3 exhibits unipolar self-doping behavior (p-type conductivity) and good defect tolerance while the growth condition changes. In addition, the calculation of light absorption confirmed that the P1 phase has a higher light absorption coefficient than that of MAPbI3 in the visible light range, showing excellent light absorption. Our work not only provides theoretical guidance for unraveling the unusual structural stability of Sn-Ge mixed perovskites, but also offers a useful scheme to modulate the stability and optoelectronic properties of Ge-based perovskites through alloy engineering.

Universal machine learning framework for defect predictions in zinc blende semiconductors

SummaryWe develop a framework powered by machine learning (ML) and high-throughput density functional theory (DFT) computations for the prediction and screening of functional impurities in groups IV, III–V, and II–VI zinc blende semiconductors. Elements spanning the length and breadth of the periodic table are considered as impurity atoms at the cation, anion, or interstitial sites in supercells of 34 candidate semiconductors, leading to a chemical space of approximately 12,000 points, 10% of which are used to generate a DFT dataset of charge dependent defect formation energies. Descriptors based on tabulated elemental properties, defect coordination environment, and relevant semiconductor properties are used to train ML regression models for the DFT computed neutral state formation energies and charge transition levels of impurities. Optimized kernel ridge, Gaussian process, random forest, and neural network regression models are applied to screen impurities with lower formation energy than dominant native defects in all compounds.

Native Atomic Defects Manipulation for Enhancing the Electronic Transport Properties of Epitaxial SnTe Films.

P-type SnTe-based compounds have attracted extensive attention because of their high thermoelectric performance. Previous studies have made tremendous efforts to investigate native atomic defects in SnTe-based compounds, but there has been no direct experimental evidence so far. On the basis of MBE, STM, ARPES, DFT calculations, and transport measurements, this work directly visualizes the dominant native atomic defects and clarifies an alternative optimization mechanism of electronic transport properties via defect engineering in epitaxially grown SnTe (111) films. Our findings prove that positively charged Sn vacancies (VSn) and negatively charged Sn interstitials (Sni) are the leading native atomic defects that dominate electronic transport in SnTe, in contrast to previous studies that only considered VSn. Increasing the substrate temperature (Tsub) and decreasing the Te/Sn flux ratio during film growth reduces the density of VSn while increasing the density of Sni. A high Tsub results in a low hole density and high carrier mobility in SnTe films. The SnTe film grown at Tsub = 593 K and Te/Sn = 2/1 achieves its highest power factor of 1.73 mW m-1 K-2 at 673 K, which is attributed to the optimized hole density of 2.27 × 1020 cm-3 and the increased carrier mobility of 85.6 cm2 V-1 s-1. Our experimental studies on the manipulation of native atomic defects can contribute to an increased understanding of the electronic transport properties of SnTe-based compounds.

Depth-resolved cathodoluminescence and surface photovoltage spectroscopies of gallium vacancies in β-Ga2O3 with neutron irradiation and forming gas anneals

The gallium vacancy is one of the dominant native point defects in β-Ga2O3, one that, together with its complexes, can have a major effect on free carrier densities and transport in this wide bandgap semiconductor. We used a combination of depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to identify the optical and energy-level properties of these defects as well as how their defect densities and spatial distributions vary with neutron irradiation and temperature-dependent-forming gas anneals. These studies reveal optical signatures that align closely with theoretical energy-level predictions. Likewise, our optical techniques reveal variations in these defect densities that are consistent with hydrogen passivation of gallium vacancies as a function of temperature and depth from the free Ga2O3 surface. These techniques can help guide the understanding and control of dominant native point defects in Ga2O3.

Intrinsic and complex defect engineering of quasi-one-dimensional ribbons Sb2S3 for photovoltaics performance

${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ has attracted great attention recently as a prospective solar cell absorber material. In this work, intrinsic defects, dopants, and their complexes in ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ are systematically investigated by using hybrid functional theory. ${\mathrm{V}}_{\mathrm{Sb}}$ and ${\mathrm{V}}_{\mathrm{S}}$ are dominant native defects and pin the Fermi level near the midgap, which is consistent with the high resistivity observed experimentally. Both ${\mathrm{V}}_{\mathrm{Sb}}$ and ${\mathrm{V}}_{\mathrm{S}}$ introduce deep levels inside the band gap, which can trap free carriers. Our calculated deep transition levels of ${\mathrm{V}}_{\mathrm{Sb}}$ and ${\mathrm{Sb}}_{\mathrm{S}}$ are consistent well with the results of the deep-level transient spectroscopy measurement. We further study dopants (including Cu, Ti, Zn, Br, and Cl) in ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ and find that Zn and Br/Cl are shallow acceptors and donors, respectively, which may be used to control the carrier and trap densities in ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$. In addition, the defect complexes, i.e., $\mathrm{Cu}{(\mathrm{Zn})}_{\mathrm{Sb}}+{\mathrm{V}}_{\mathrm{S}}$ and $\mathrm{Cl}{(\mathrm{Br})}_{\mathrm{S}}+{\mathrm{V}}_{\mathrm{Sb}}$ are also investigated. The interaction between the donor and acceptor defects makes the defect levels of complexes shallower and less detrimental to carrier transport.

Hybrid‐Functional Study of Native Point Defects and Ti/Fe Impurities in α‐Al2O3

First‐principles calculations with the most accurate hybrid functional are used to investigate the effect of native point defects and Ti/Fe impurities on electrical and optical properties of α‐Al2O3. For dominant native point defects, the Al vacancy (VAl), O vacancy (VO), and Al interstitial (Ali) defects are considered, and it is found that only VAl defect provides the absorption line in the visible light region. Regarding Ti‐doped or Fe‐doped sapphire, it is found that their colors should be related to intra‐atomic d–d transitions. For the Ti–Fe pair, it is found that the ground‐state configuration is TiIII–FeIII, and its binding energy is very high, indicating that this pair can exist in high concentration. The calculated optical absorption energy associated with Ti–Fe pair is ≈1.92 and 1.84, which is appropriate for sapphire to be blue. It is, therefore, suggested that the intervalence charge transfer (IVCT) mechanism is the cause of blue color in blue sapphire.

Vacancies and dopants in two-dimensional tin monoxide: An ab initio study

Layered tin monoxide (SnO) offers an exciting two-dimensional (2D) semiconducting system with great technological potential for next-generation electronics and photocatalytic applications. Using a combination of first-principles simulations and strain field analysis, this study investigates the structural dynamics of oxygen (O) vacancies in monolayer SnO and their functionalization by complementary lightweight dopants, namely C, Si, N, P, S, F, Cl, H and H2. Our results show that O vacancies are the dominant native defect under Sn-rich growth conditions with active diffusion characteristics that are comparable to that of graphene vacancies. Depending on the choice of substitutional species and its concentration within the material, significant opportunities are revealed in the doped-SnO system for facilitating n/p-type tendencies, work function reduction, and metallization of the monolayer. N and F dopants are found to demonstrate superior mechanical compatibility with the host lattice, with F being especially likely to take part in substitution and lead to degenerately doped phases with high open-air stability. The findings reported here suggest that post-growth filling of O vacancies in Sn-rich conditions presents a viable channel for doping 2D tin monoxide, opening up new avenues in harnessing defect-engineered SnO nanostructures for emergent technologies.

Polaron formation, native defects, and electronic conduction in metal tungstates

Iron tungstate (FeWO$_4$) and manganese tungstate (MnWO$_4$) belong to a family of wolframite-type materials that has applications in various areas, including supercapacitors, batteries, and multiferroics. A detailed understanding of bulk properties and defect physics in these transition-metal tungstates has been lacking, however, impeding possible improvement of their functional properties. Here, we report a first-principles study of FeWO$_4$ and MnWO$_4$ using screened hybrid density-functional calculations. We find that in both compounds the electronic structure near the band edges are predominantly the highly localized transition-metal $d$ states, which allows for the formation of both hole polarons at the Fe (Mn) sites and electron polarons at the W sites. The dominant native point defects in FeWO$_4$ (MnWO$_4$) under realistic synthesis conditions are, however, the hole polarons at the Fe (Mn) sites and negatively charged Fe (Mn) vacancies. The presence of low-energy and highly mobile polarons provides explanation for the good p-type conductivity observed in experiments and the ability of the materials to store energy via a pseudocapacitive mechanism.

Investigation on native defects of α-MgAgSb and its effects on thermoelectric properties using first principles calculations

α-MgAgSb is a promising thermoelectric materials having good performance at medium temperature. Native defects in α-MgAgSb are frequently reported experimentally and are tightly involved in the thermoelectric properties of α-MgAgSb. In this paper, all possible native defects in α-MgAgSb are calculated as well as detailed results are given and discussed. The concentrations of several dominant native defects, for example, VAg and AgSb, could reach up to 10−4 cm−3 at 540 K. Furthermore, the electronic structure and transport properties of α-MgAgSb with dominant native defects are investigated. Results show that the introduction of AgMg and VAg contributes to a much lower inertial mass and slight decrease in Seebeck coefficient. The lattice thermal conductivity is greatly reduced with the introduction of native defects. For α-MgAgSb with VAg, the peak ZT could reach up to 1.84 at 420 K. Our calculation demonstrates that defect engineering is an effective strategy to enhance thermoelectric performance of the materials.

Heavy and light hole transport in nominally undoped GaSb substrates

In this work, we report results of a study of electronic transport in nominally undoped p-type GaSb wafers typically employed as substrate material for the epitaxial growth of InAs/GaInSb type-II superlattices. Magnetic field dependent Hall-effect measurements and high-resolution mobility spectrum analysis clearly indicate p-type conductivity due to carriers in both the heavy and light hole bands. The extracted hole concentrations indicate a thermal activation energy of 17.8 meV for the dominant native acceptor-like defects. A temperature-independent effective mass ratio of 9.0 ± 0.8 was determined from the ratio of measured heavy and light hole concentrations. Over the 56 K–300 K temperature range, the light hole mobility was found to be 4.7 ± 0.7 times higher than the heavy hole mobility. The measured room temperature mobilities for the light and heavy holes were 2550 cm2/Vs and 520 cm2/Vs, respectively.

Understanding the Behavior of Native Point Defects in ZrC by First‐Principles Calculations

Phase stability and macroscopic performances of ZrC are closely related to the behavior of native point defects. In this study, structures and stabilities of native point defects in ZrC as well as diffusion of C‐related defects are investigated by first‐principles calculations. It is shown that the carbon vacancy (VC) and interstitials (Cis) are the dominant native point defects in ZrC. Six types of Ci configurations: two C‐trimers, one C‐tetrahedron, and three C‐dimers are identified with low defect formation energies. The VC has a high migration energy (4.39 eV) which suggests its low mobility in ZrC. The Cis have low diffusion energy barriers (from 0.26 to 1.29 eV) which lead to their high mobility. In addition, the impact of defects (VC, VZr, ZrC, and CZr) on bonding strengths of neighboring Zr–C bonds is discussed. Especially near VC, the 1NN (nearest neighboring) and 2NN Zr–C bonds are strengthened but 4NN Zr–C bonds are weakened. Interestingly, the 3NN Zr–C bonds are almost not affected by the presence of VC. These results may be closely related to the short‐range interactions and ordering of VC in nonstoichiometric ZrC.

Thermodynamic and quasi-chemical modeling of point defects in CdTe〈I〉

The carrier concentration and the densities of predominant point defects in CdTe〈I〉 crystals have been calculated as functions of two-step annealing parameters using quasi-chemical and thermodynamic modeling. We have identified the dominant native and impurity defect species, which govern the electrical properties of the material, and proposed a compensation model that takes into account not only native defects, ITe+ substitutional defects, and their complexes with native point defects, such as (VCd2−ITe+)−, and (VCd2−2ITe+)0, but also DX− centers. The equilibrium constants of formation of the VCd2−ITe+)−, defect complex and center have been evaluated.

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.

Capture kinetics at deep-level defects in MBE-grown CdTe layers

The results of deep-level transient spectroscopy (DLTS) investigations in n-type CdTe layers grown by the molecular-beam epitaxy (MBE) technique on lattice-mismatched GaAs substrates are described. Three electron traps and one hole trap, at rather low concentrations of the order of 1013 cm−3, have been revealed in the DLTS spectra measured under various bias conditions of Schottky diodes prepared on the as-grown CdTe layers. One of the electron traps has been attributed to electron states of dislocations on the ground of the logarithmic capture kinetics for capture of electrons into the trap states. The other three traps, displaying exponential capture kinetics, have been attributed to native point defects produced during the epitaxial growth of CdTe. The microscopic nature of the defects responsible for the traps is discussed taking into account recent results of first-principles calculations of the properties of dominant native defects in CdTe.

Optical and EPR study of point defects in CdSiP 2 crystals

Optical absorption and electron paramagnetic resonance (EPR) are used to identify and characterize point defects in large single crystals of cadmium silicon phosphide (CdSiP 2) grown by the horizontal gradient freeze method. The dominant native defects in these crystals are cadmium vacancies and phosphorus vacancies, and the primary unintentional impurities are Fe +, Fe 2+, Fe 4+, and Mn 2+ ions. Optical absorption in the region between 1.5 and 2.5 μm was studied at temperatures from 10 to 300 K. At lower temperatures, zero-phonon lines and phonon sidebands observed near 1.87 μm are assigned to Fe 2+ ions (similar features observed at 10 K near 1.08 μm are assigned to Fe 4+ ions). At higher temperatures, an unidentified absorption band has a peak near 1.95 μm and a vibronically broadened absorption band due to the Fe 2+ ions has a peak near 1.75 μm. EPR lines due to Fe + and Mn 2+ ions appear near g=2. Illumination at 5 K with 632.8 nm laser light produces changes in the intensity of the Fe + EPR spectrum and introduces a three-line EPR spectrum caused by hyperfine interactions with two phosphorus nuclei. This latter spectrum is assigned to singly ionized cadmium vacancies ( V cd - ). Optical and EPR results from two CdSiP 2 samples exhibiting different defect behaviors are compared.

Investigation of native defects and property of bulk ZnO single crystal grown by a closed chemical vapor transport method

Hall effect, Raman scattering, photoluminescence spectroscopy (PL), optical absorption (OA), mass spectroscopy, and X-ray diffraction have been used to study bulk ZnO single crystal grown by a closed chemical vapor transport method. The results indicate that shallow donor impurities (Ga and Al) are the dominant native defects responsible for n-type conduction of the ZnO single crystal. PL and OA results suggest that the as-grown and annealed ZnO samples with poor lattice perfection exhibit strong deep level green photoluminescence and weak ultraviolet luminescence. The deep level defect in as-grown ZnO is identified to be oxygen vacancy. After high-temperature annealing, the deep level photoluminescence is suppressed in ZnO crystal with good lattice perfection. In contrast, the photoluminescence is nearly unchanged or even enhanced in ZnO crystal with grain boundary or mosaic structure. This result indicates that a trapping effect of the defect exists at the grain boundary in ZnO single crystal.

Native and artificial radiation-induced defects in montmorillonite. An EPR study

A natural montmorillonite containing radiation-induced defects was studied with Electron Paramagnetic Resonance (EPR) spectroscopy (X- and Q-band). A first dominant native defect, namely native defect 1, is identified. It gives rise to an orthorhombic spectrum with gx= 2.004 ± 0.005 gy= 2.010 ± 0.003, gz= 2.065 ± 0.002. Simulation of the EPR spectrum at X- and Q-band reveals a second native defect with isotropic spectrum at g= 2.019 ± 0.005. Both are electron holes trapped on oxygen atoms of the structure. The native defect 1 is located on an oxygen-silicon bond or a non-bonding orbital parallel to the c* axis. These defects are annealed at 500°C and the half-life determined for native defect 1 is circa 3,000 years. Irradiations with beta rays produced two additional hole centers of lower stability and distinct EPR parameters. Artificial irradiations show that montmorillonite can be used as a dosimeter in a large dose range.

EPR and electrical studies of native point defects in ZnSiP2 semiconductors

We present the first detection of native defects in ZnSiP2. Similar to p-type ZnGeP2, the EPR spectra of the zinc vacancy VZn−, the phosphorus vacancy VP0, as well as of group IV anti-site SiZn+(GeZn+) could be proved. The influence of the group IV-ions on the bonding behavior for the different lattice sites is reflected in the differences of the EPR parameters in both lattices. In both materials, VP0 could be observed only by photo-excitation, whereas the silicon anti-site SiZn+ was detectable also in the dark contrary to GeZn+. This is connected with the shift of the Fermi level in the n-type ZnSiP2 samples. In disagreement with the recharching model developed for the dominant native defects in ZnGeP2 we could detect both SiZn+ and VZn− in the dark. This contradiction could be resolved by scanning surface potential microscopy measurements, which have shown that the Fermi level position laterally varies in these samples, realizing the paramagnetic states of both defects in different regions of the sample.

EPR studies of native and impurity-related defects in II–IV–V 2 semiconductors

Magnetic resonance studies of native defects in Zinc germanium diphosphide (ZnGeP 2) and their energy level positions in the band gap are reviewed and first results on defects in ZnSiP 2 are presented. The contribution of the dominant native defects to the optical absorption of ZnGeP 2 crystals is revealed by photo-EPR investigations in combination with the first ODMR experiments. The EPR results published for Mn and Fe in the II–IV–V 2 chalcopyrites are summarized. The observation of some new Mn centers and the first experimental detection of antiferromagnetic Mn–Mn coupled pairs in ZnGeP 2 are presented. In addition, new results concerning Mn-induced local changes of the free parameter x f of the chalcopyrite structure at the impurity site are discussed.