Native Point Defects Research Articles

Identification of Ge vacancies as electronic defects in methyl- and hydrogen-terminated germanane

We use a combination of optical and electrostatic surface science techniques to measure electronically active native defects in multilayer GeCH3 and GeH, two-dimensional (2D) functionalized materials. Chemical processing techniques coupled with density functional theory enable us to identify the specific physical nature of both native point defects and synthesis-related impurities which can limit the optical and charge transport properties of these materials. Direct comparison of optical measurements with calculated electronic levels provides identification of these localized, deep level gap states and confirms partial H-passivation of dangling bonds, revealing synthesis and processing methods needed to control specific defects and optimize these 2D materials for emergent solid state-electronics.

Effects of Al2O3 gate insulator on the instability of amorphous indium-gallium zinc oxide thin film transistors

This study analyzes bottom-gated amorphous indium-gallium zinc oxide (a-IGZO) thin film transistors (TFTs) with different gate insulators of Si3N4 and Si3N4/Al2O3. The aim is to investigate the effect of Al2O3 on the instability of IGZO TFTs with respect to native point defects. Although the DC properties of Si3N4/Al2O3 devices are inferior to those of Si3N4 devices, the bias stress-induced hump is not observed in Si3N4/Al2O3 devices. From calculations of the oxygen exchange kinetics between the insulator and IGZO, it appears that more oxygen atoms diffuse from Al2O3 to the IGZO than from the Si3N4. This oxygen diffusion is confirmed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Auger electron spectroscopy depth-profiling analysis. In addition, more oxygen interstitials with fewer zinc interstitials are detected at the IGZO interface with Al2O3. From the results, it can be concluded that the distribution of native defects in Al2O3 devices reduces the instability of IGZO TFTs.

Effect of high temperature, high pressure annealing on GaN drift layers for vertical power devices

In this work, we evaluate the effect of the novel symmetric multicycle rapid thermal annealing (SMRTA) process on both Mg ion implanted and non-implanted thick unintentionally doped GaN drift layers for vertical power devices. The typical p-type behavior and restoration of implant damage are observed in implanted samples, but on non-implanted samples a reduction in background carrier concentration and associated reduction in leakage current and increase in breakdown voltage is observed. This indicates that the capping/annealing process itself is not detrimental to the crystal, and the annihilation of native point defects in the process has beneficial effects for device structures.

Origin of fast-response photocurrent in ZnO thin film

We study the origin of fast-response photocurrent in ZnO system grown by dc-unbalanced magnetron sputtering. Current-voltage (I-V) measurements show that ZnO thin film is sensitive to UV–visible wavelength and the response time is about 5 ms. This typical sensitivity is related to the contribution of native point defects in electrical transport, as indicated by a space-charge-limited current regime. I-V curves in vacuum, Ar, and O2 environments reveal that oxygen vacancies (VO) play an important role in adsorption and desorption processes. Using absorption and temperature-dependent photoluminescence spectroscopy, the presence of VO is confirmed and dominates even at low temperature. The strong contribution of VO promotes low excitonic absorption, resulting in low photocurrent and fast-response detector. We propose that fast-response ZnO photodetector is not only originated from Schottky barrier but also from VO-related defect states. This result is important to improve the functionalities of ZnO for optoelectronic applications.

Native point defects of semiconducting layered Bi2O2Se

Bi2O2Se is an emerging semiconducting, air-stable layered material (Nat. Nanotechnol. 2017, 12, 530; Nano Lett. 2017, 17, 3021), potentially exceeding MoS2 and phosphorene in electron mobility and rivalling typical Van der Waals stacked layered materials in the next-generation high-speed and low-power electronics. Holding the promise of functional versatility, it is arousing rapidly growing interest from various disciplines, including optoelectronics, thermoelectronics and piezoelectronics. In this work, we comprehensively study the electrical properties of the native point defects in Bi2O2Se, as an essential step toward understanding the fundamentals of this material. The defect landscapes dependent on both Fermi energy and the chemical potentials of atomic constituents are investigated. Along with the bulk defect analysis, a complementary inspection of the surface properties, within the simple context of charge neutrality level model, elucidates the observed n-type characteristics of Bi2O2Se based FETs. This work provides important guide to engineer the defects of Bi2O2Se for desired properties, which is key to the successful application of this emerging layered material27.

Detection of local vibrational modes induced by intrinsic defects in undoped BaSi2 light absorber layers using Raman spectroscopy

We fabricate BaSi2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi2. Vibration analysis based on first-principles calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560 cm−1, respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios RBa/RSi from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at RBa/RSi = 2.2. Furthermore, the position of Raman peaks at approximately 490 cm−1 shifts to a lower wavenumber, depending on RBa/RSi and thereby the density of point defects.

β-In2S3 for photovoltaic devices: investigation of the native point defects with ab initio first-principle calculations

Herein, we report a theoretical investigation based on DFT calculations devoted to the nature of charge carriers in the β-In2S3 material used as buffer in chalcopyrite thin-film solar cells. Our simulations led to unambiguous results concerning the incapability for this material to be a p-type semiconductor. Furthermore, it is demonstrated that the insertion of indium into a Td interstitial site seems to be the driving force leading to a natural n-type conductivity. Our calculations took into account different atmospheres in order to be directly comparable with experimental data. In particular, the competitive InS structure was considered. It is shown that the S-poor condition strongly lesser the defect formation energy and should be privileged to achieve high free electron concentrations for potential applications.

Native point defects and impurities in hexagonal boron nitride

Hexagonal boron nitride ($h$-BN) is a technologically important electronic and dielectric material. Recently, research into this material has intensified due to the discovery of bright single-photon-emitting color centers in the form of point defects, with potential applications in advanced quantum technologies. Despite this technological promise, the defect physics of this material is still largely unexplored. Here, the authors have performed detailed first-principles investigations of the defect properties of $h$-BN, finding that the defect physics of this material is dictated by impurities, in particular carbon, oxygen, and hydrogen. They show how the defect properties of this material can be selectively engineered by growth or processing conditions. These insights will be fundamental to controlled generation of single-photon emitters and for general defect engineering of this material.

Optical signatures of deep level defects in Ga2O3

We used depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to measure the effects of near-surface plasma processing and neutron irradiation on native point defects in β-Ga2O3. The near-surface sensitivity and depth resolution of these optical techniques enabled us to identify spectral changes associated with removing or creating these defects, leading to identification of one oxygen vacancy-related and two gallium vacancy-related energy levels in the β-Ga2O3 bandgap. The combined near-surface detection and processing of Ga2O3 suggests an avenue for identifying the physical nature and reducing the density of native point defects in this and other semiconductors.

Energetics of native defects, solute partitioning, and interfacial energy of Q precipitate in Al-Cu-Mg-Si alloys

The compound Al3Cu2Mg9Si7, is known as the Q-phase and forms as a thermodynamically stable precipitate during aging in the quaternary Al-Cu-Mg-Si system. We perform atomic-scale density functional theory (DFT) calculations of defect properties, solute partitioning, and interfacial stability of the Al3Cu2Mg9Si7 (Q) precipitate. We find: (i) simple native point defect (i.e., vacancies and anti-sites) thermodynamics can partially explain the experimentally observed off-stoichiometry, such as the observed variation of compositions, Al3+δCu2Mg9-δSi7 (Mg-deficient and Al-rich) in experiment. (ii) Calculated solute-partitioning energies of common solutes allow us to define general rules for site-preference in the Q-phase in terms of electronic structure and atomic radius. To validate our DFT predictions, we perform atom-probe tomography (APT) experiments for six-different elements (Zn, Ni, Mn, Ti, V, and Zr). The results show that the partitioning behavior of solutes Ni, Zn, and Mn are consistent with DFT predictions, but the transition elements (Ti, V, and Zr), which are anomalously slow diffusers in Al, partition to the Q-phase in constrast to DFT partitioning energies. (iii) For the low energy interface (112¯0)Q//(510)Al observed in needle shaped Q-precipitate, we survey various terminations and orientations and derive a low-energy interfacial structure. We find this low-energy interfacial model has Cu atoms nearest to the interface, which is in agreement with previous literature on Cu interfacial segregation at the Q′//α-Al interface. The computed interfacial energy (0.52 J/m2) and the corresponding structure will be useful input to future multi-scale modeling of microstructural evolution.

Reinventing a p-type doping process for stable ZnO light emitting devices

A tough challenge for zinc oxide (ZnO) as the ultraviolet optoelectronics materials is realizing the stable and reliable p-type conductivity. Self-compensation, coming from native donor-type point defects, is a big obstacle. In this work, we introduce a dynamic N doping process with molecular beam epitaxy, which is accomplished by a Zn, N-shutter periodic switch (a certain time shift between them for independent optimization of surface conditions). During the epitaxy, N adatoms are incorporated under the condition of (2 × 2) + Zn vacancies reconstruction on a Zn-polar surface, at which oxygen vacancies (VO), the dominating compensating donors, are suppressed. With the p-ZnO with sufficient holes surviving, N concentration ~1 × 1019 cm−3, is employed in a p-i-n light emitting devices. Significant ultraviolet emission of electroluminescence spectra without broad green band (related to VO) at room-temperature are demonstrated. The devices work incessantly without intentional cooling for over 300 h at a luminous intensity reduction of one order of magnitude under the driving of a 10 mA continuous current, which are the demonstration for p-ZnO stability and reliability.

Negative‐U Properties of the Deep Level E3 in ZnO

The deep level E3 is a commonly observed defect in bulk crystals and thin films of ZnO independently of the method of growth. Its chemical origin is still not finally understood. In this paper, previous research on the E3 level is reviewed and recent experimental results acquired by optical deep level transient spectroscopic (DLTS) methods are presented for the temperature range from 4 to 350 K, for photon energies between 0.25 and 4 eV, and for emission rates ranging from 10−3 to 104 s−1. The capture cross section of the optical transition of E3‐trapped electrons into the conduction band is described. Additionally, a second optical transition occurs for photon energies higher than 1 eV, which accounts for the lowering of the E3 DLTS signal at low temperatures under illumination. Further, it is unambiguously shown that the E3 defect can bind up to two electrons and that it has the properties of a negative‐U center. Photoluminescence measurements reveal that the E3 defect is directly connected to a radiative recombination channel at 2.09 eV. Further, electron paramagnetic resonance (EPR) measurements were performed with optical excitation in dependence on the temperature and photon energy. Interestingly, the temperature‐dependent EPR signal of is similar to that of the photo‐ionization cross‐section of the E3 defect. These experimental findings are discussed in comparison with published EPR results and results obtained by first‐principle calculations of native point defects in ZnO.

Defect Characterization, Imaging, and Control in Wide-Bandgap Semiconductors and Devices

Wide-bandgap semiconductors are now leading the way to new physical phenomena and device applications at nanoscale dimensions. The impact of defects on the electronic properties of these materials increases as their size decreases, motivating new techniques to characterize and begin to control these electronic states. Leading these advances have been the semiconductors ZnO, GaN, and related materials. This paper highlights the importance of native point defects in these semiconductors and describes how a complement of spatially localized surface science and spectroscopy techniques in three dimensions can characterize, image, and begin to control these electronic states at the nanoscale. A combination of characterization techniques including depth-resolved cathodoluminescence spectroscopy, surface photovoltage spectroscopy, and hyperspectral imaging can describe the nature and distribution of defects at interfaces at both bulk and nanoscale surfaces, their metal interfaces, and inside nanostructures themselves. These features as well as temperature and mechanical strain inside wide-bandgap device structures at the nanoscale can be measured even while these devices are operating. These advanced capabilities enable several new directions for describing defects at the nanoscale, showing how they contribute to device degradation, and guiding growth processes to control them.

Theoretical study of native point defects in strained-layer superlattice systems

We developed a theoretical approach that employs first-principles Hamiltonians, tight-binding Hamiltonians, and Green's function techniques to obtain energy levels arising from native point defects (NPDs) in InAs-GaSb and InAs-InAs1-xSbx strained layer superlattice (SLS) systems. In InAs and GaSb regions, we considered four types of NPDs—anion vacancy, cation vacancy, anion anti-site, and cation anti-site—as well as isoelectronic substitution at anion sites (Sb at the As site and As at the Sb site). Additionally, we considered three types of defects—the cation at the second anion site, the second anion at the cation site, and second anion vacancy—in the InAs1-xSbx alloy region of the SLS. For a selected few designs, we studied NPDs both in the bulk region and near the interfaces of the SLS. We have considered 12 designs of InAs-GaSb systems and two designs of InAs-InAs0.7Sb0.3 systems lattice-matched to the GaSb substrate. The calculated defect levels not only agreed well with available measurements, but also revealed the connection between mid-gap levels and specific NPDs. We further calculated defect formation energies both in compounds and in all superlattices considered above. Since the absolute value of defect formation energy depends considerably on growth conditions, we evaluated the formation energies in SLS with respect to their value in the corresponding bulk or alloy. The calculated defect formation energies, together with defect energy level results, allow us to identify a few promising SLS designs for high-performing photodetectors.

First principle study of native point defects in (ZnO)n nanoclusters (n = 34, 60)

First principle investigations of native point defects in (ZnO)n nanoclusters (n = 34, 60) based on density-functional theory within the general gradient approximation plus Hubbard U, for different types of isomers, were performed. For each type of defect, an optimization (relaxation) of structures geometry was performed and the basic properties of the band structure were investigated. The values for formation energy, HOMO–LUMO gap, as well as the partial density of states for each cluster were investigated to establish the influence of the defects on the electronic properties of the (ZnO)n nanoclusters. It was determined that the most favorable defects of the clusters structure were Zn and O vacancies.

Intrinsic energy conversions for photon-generation in piezo-phototronic materials: A case study on alkaline niobates

The creations of photons in response to mechanical stimulus in a crystal that has noncentral symmetry are the great fundamental physics responsible for numbers of important technologies. The underlying mechanism and complete theory for a precise explanation of the mechanical-photonic energy conversion phenomena is vital important. We take commercial piezoelectric LiNbO3 matrics as the example to interpret the detail mechanisms of energy conversions for the photon-generation through a native point defects study. It was found the Frenkel and Schottky type complex pairs as well as the antisite pair defects acting as energy harvesting and migration centers, which are very easy to form and active. It does to be the extra deep electron or hole traps levels near the valence or conduction band edge, respectively. That is the substantial energy reduction via a spontaneous equilibrium transformation from the complementarily charged individuals into agglomerated complexes. Such energy gain for both two processes turns to be independent to the variations of synthesis chemical potentials. In addition, the complex defects actually form independent to the variations of the chemical potentials. This leads to a coupling and exchange effect by them to continuously collect and transport host charges along the path via localized states to the deep recombination levels. The initiating energy barrier is small which ambient thermal stimulation or quantum tunneling can accomplish. The native sensitizers such as VNb2O5, VLiNbO3, NbLi are also the energy conversion centers to non-radiative resonant energy transfer onto the activator center at the Oi to transfer the energy into photon emissions. A generalized energy conversion mechanism has been unraveled in this work. This gives a solid theoretical reference for developing the mechanical-photonic energy conversion materials.

Theoretical and Experimental Investigations on Effects of Native Point Defects and Nitrogen Doping on the Optical Band Structure of Spinel ZnGa2O4

Impurity states in semiconductors are so important for optical related properties that understanding the role of native and imported defects is essential to design highly active semiconductors. Here, the structural and electronic properties of ZnGa2O4 with native atomic substitution, oxygen vacancies, and imported N doping are first investigated by first-principles calculations. It is demonstrated that native atomic substitution is energetically unfavorable and the most stable existence forms for N doping and O vacancies are N1– and O2+ states in most cases under various chemical environments. The band structures and density of states reveal that the photochemical property is significantly enhanced only by 2Ns doping with a great increase of valence band maximum relative to the Fermi level, whereas single-N atom doping or import of O vacancies or simultaneous import of N doping and O vacancies just generates impurity states in the band gaps. Moreover, experimental characterizations including X-ray photoel...

Effects of Ni doping and native point defects on magnetism of ZnO first-principles study

The magnetism sources and magnetic mechanism of Ni-doped ZnO remain unknown to date. In this study, the plane-wave ultrasoft pseudopotential technology of generalized gradient approximation +U based on spin-density-functional theory was used to investigate the magnetism sources and magnetic mechanism of Ni-doped ZnO. Results showed that the total magnetic moment of the ZnO system with Ni doping and O vacancy was similar to that of Ni-doped ZnO. Compared with the ZnO system with Ni doping and O vacancy, the doping system with Ni doping and Zn vacancy exhibited a larger magnetic moment, smaller formation energy, and better stability, and it was easier to dope. The magnetism of the Zn14NiO16 system mainly resulted from the hybrid coupling electron exchange effect among the O-2p (nearest to the Zn vacancy and Ni atom), Ni-3d, and Zn-4s orbits. This effect was based on the hole carriers in the complex as the medium. In the ZnO system with Ni doping and Zn vacancy, the magnetic moment slightly decreased with increasing doping amount. Results showed that the system with Ni doping and Zn vacancy was ferromagnetic. In addition, the Curie temperature of the doping system was at room temperature.

Resistive switching in individual ZnO nanorods: delineating the ionic current by photo-stimulation

Resistive switching in nanostructured metal oxide semiconductors has been broadly understood to originate from the dynamics of its native point defects. Experimental results of switching observed in individual n-ZnO nanorods grown on a p-type polymer is presented along with an empirical model describing the underlying defect dynamics necessary to observe bi-polar switching. Selective photo excitation of electrons into the defect states delineates the incidence and role of an ionic current in the switching behavior. The understanding further extends to the observance of a negative differential resistance regime that is often coincident in such systems. The analysis not only unifies the underlying physics of the two phenomena but also offers further confidence in the proposed mechanism. We conclude by demonstrating that the effective memresistance of such devices is a strong function of the operating bias and identify parameters that optimize switching performance.

Microkinetic Model for Oxygen Interstitial Injection from the ZnO(0001) Surface into the Bulk

Semiconductor surfaces provide efficient pathways for injecting native point defects into the underlying bulk. The present work constructs a quantitative microkinetic model for the injection of oxygen interstitial atoms from the polar Zn-terminated ZnO(0001) surface into the bulk. Rate constants for defect interaction with the surface and in the bulk were determined by a global optimization procedure of simulations fitted to self-diffusion profiles from isotopic gas–solid exchange experiments. Key activation barriers are 2.0 eV for injection, 0.62 eV for hopping diffusion, and 1.6 eV for lattice exchange. The injection barrier does not differ greatly from that for nonpolar TiO2(110), but the coverage of injectable oxygen increases with temperature, in contrast to the behavior of TiO2 and gas adsorption in general.