Charged Defect States Research Articles

Silicene on Monolayer PtSe2: From Strong to Weak Binding via NH3 Intercalation.

We study the properties of silicene on monolayer PtSe2 by first-principles calculations and demonstrate a much stronger interlayer interaction than previously reported for silicene on other semiconducting substrates. This fact opens the possibility of a direct growth. A band gap of 165 meV results from inversion symmetry breaking and large spin-splittings in the valence and conduction bands from proximity to monolayer PtSe2 and its strong spin-orbit coupling. It is also shown that the interlayer interaction can be effectively reduced by intercalating NH3 molecules between silicene and monolayer PtSe2 without inducing charge transfer or defect states near the Fermi energy. A small NH3 diffusion barrier makes intercalation a viable experimental approach to control the interlayer interaction.

Distribution and Charge State of Iron Impurities in Intentionally Contaminated Lead Halide Perovskites

Impurity contamination in thin-film solar cells remains an uncertain risk due to the little-known impact of impurities on recombination. Building upon previous work, in which we intentionally contaminated lead halide perovskite (LHP) solar cells with iron, we further examine the distribution and charge state of iron-induced defects in LHP films using synchrotron-based X-ray techniques. X-ray absorption measurements suggest that iron-rich regions, which form among iron feedstock concentrations that exceed 100 ppm, most closely resemble the chemistry of Fe2O3. Iron distributed within the bulk may form a mix of Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , the latter of which is not expected to be recombination active, potentially allowing LHPs to incorporate more iron than traditional semiconductors. X-ray beam induced current measurements show little correlation between the presence of iron-rich regions and charge collection, which further suggests low recombination activity at these sites. These results further elucidate the recombination behavior caused by iron incorporation in LHP films, revealing insight into how inhomogeneous incorporation of impurities may mitigate photovoltaic performance degradation.

Enhancement of polaron-hopping-based a.c. conduction in semiconducting STS (Se-Te-Sn) glass by silver incorporation.

In the present work, we have synthesized novel Se78-xAgxTe20Sn2 (0 ≤ x ≤ 6) semiconducting glasses for improving thermally activated a.c. conduction in STS glass (x = 0). The results of the experimental analysis of thermally activated a.c. conduction have been reported in terms of frequency and temperature of Se78-xAgxTe20Sn2 (0 ≤ x ≤ 6) glasses. The frequency dependent experimental data of a.c. conductivity σac show that σac follows the Jonscher universal power law. Variation of a.c. conductivity with temperature obeys the Arrhenius relation. Further analysis confirms that the density of defect states can be determined on the basis of a correlated barrier hopping model using the frequency/temperature dependence of a.c. conductivity σac (T,ω). The analysis of composition dependence of various electric parameters like potential barrier height Wm, hopping distances Rmin and Rω, dielectric constant (ε') and the density of defect states N shows that hopping conduction is improved after silver incorporation into the parent glass. This is explained in terms of a dipolar model by considering the dipoles of Ag+ ions with negatively charged defect states.

Carrier Trapping by Vacancy‐Type Defects in Mg‐Implanted GaN Studied Using Monoenergetic Positron Beams

Vacancy‐type defects in Mg‐implanted GaN are probed using monoenergetic positron beams. Mg+ ions are implanted to provide a 500‐nm‐deep box profile with Mg concentrations, [Mg], of 1 × 1017–1 × 1019 cm−3 at room temperature. In the as‐implanted samples, the major defect species is a complex of a Ga vacancy (VGa) and a nitrogen vacancy (VN). After annealing above 1000 °C, the major defect species is changed to vacancy clusters due to vacancy agglomeration. This agglomeration is suppressed, and the agglomeration onset temperature is decreased with a decreasing [Mg]. For samples with [Mg] ≥ 1 × 1018 cm−3, the trapping rate of positrons by vacancy‐type defects decrease after annealing above 1100–1200 °C. This decreases is attributed to the change in the defect charge states from neutral to positive due to a downward shift of the Fermi level. The carrier trapping/detrapping properties of the vacancy‐type defects and their time dependences are also revealed.

Improving Defect-Based Quantum Emitters in Silicon Carbide via Inorganic Passivation.

Defect-based color centers in wide-bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite-vacancy pairs (CAV centers) in 4H-SiC, which serve as single-photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect-based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers.

Spatial charge inhomogeneity and defect states in topological Dirac semimetal thin films of Na3Bi.

Topological Dirac semimetals (TDSs) are three-dimensional analogs of graphene, with carriers behaving like massless Dirac fermions in three dimensions. In graphene, substrate disorder drives fluctuations in Fermi energy, necessitating construction of heterostructures of graphene and hexagonal boron nitride (h-BN) to minimize the fluctuations. Three-dimensional TDSs obviate the substrate and should show reduced EF fluctuations due to better metallic screening and higher dielectric constants. We map the potential fluctuations in TDS Na3Bi using a scanning tunneling microscope. The rms potential fluctuations are significantly smaller than the thermal energy room temperature (ΔEF,rms = 4 to 6 meV = 40 to 70 K) and comparable to the highest-quality graphene on h-BN. Surface Na vacancies produce a novel resonance close to the Dirac point with surprisingly large spatial extent and provide a unique way to tune the surface density of states in a TDS thin-film material. Sparse defect clusters show bound states whose occupation may be changed by applying a bias to the scanning tunneling microscope tip, offering an opportunity to study a quantum dot connected to a TDS reservoir.

Optical switching of defect charge states in 4H-SiC

We demonstrate optically induced switching between bright and dark charged divacancy defects in 4H-SiC. Photoluminescence excitation and time-resolved photoluminescence measurements reveal the excitation conditions for such charge conversion. For an energy below 1.3 eV (above 950 nm), the PL is suppressed by more than two orders of magnitude. The PL is recovered in the presence of a higher energy repump laser with a time-averaged intensity less than 0.1% that of the excitation field. Under a repump of 2.33 eV (532 nm), the PL increases rapidly, with a time constant 30 μs. By contrast, when the repump is switched off, the PL decreases first within 100–200 μs, followed by a much slower decay of a few seconds. We attribute these effect to the conversion between two different charge states. Under an excitation at energy levels below 1.3 eV, VSiVC0 are converted into a dark charge state. A repump laser with an energy above 1.3 eV can excite this charged state and recover the bright neutral state. This optically induced charge switching can lead to charge-state fluctuations but can be exploited for long-term data storage or nuclear-spin-based quantum memory.

Charge state of vacancy defects in Eu-doped GaN

Eu ions have been doped into GaN in order to achieve red luminescence under current injection, where coupling between the Eu ions and intrinsic defects such as vacancies are known to play an important role. However, the charge state of the vacancies and the impact it would have on the optical and magnetic properties of the Eu ions have not been explored. Through a combination of first-principle calculations and experimental results, the influence of the charge state of the defect environment surrounding the Eu ions has been investigated. We have identified two Eu centers that are related through the charge state of a local vacancy defect. These two centers were found to exhibit a mutual metastability, such that each center can be excited in one configuration and emit as the other. This metastability was found to be dependent on temperature and the wavelength of the excitation laser. Furthermore, one of these centers was found to have an effective magnetic $g$ factor that is substantially larger than what is expected for an isolated $\mathrm{E}{\mathrm{u}}^{3+}$ ion and is explained by a change in the charge state of the defect environment around the Eu. This prediction could also offer a new explanation for the saturation magnetization previously observed in GaN : Eu and other GaN:RE systems.

Energetics of halogen impurities in thorium dioxide

Defect energies for halogen impurity atoms (Cl, Br and I) in thoria are calculated using the generalized gradient approximation and projector augmented plane wave potentials under the framework of density functional theory. The energy to place a halogen atom at a pre-existing lattice site is the incorporation energy. Seven sites are considered: octahedral interstitial, O vacancy, Th vacancy, Th-O di-vacancy cluster (DV) and the three O-Th-O tri-vacancy cluster (NTV) configurations. For point defects and vacancy clusters, neutral and all possible defect charge states up to full formal charge are considered. The most favourable incorporation site for Cl is the singly charged positive oxygen vacancy while for Br and I it is the NTV1 cluster. By considering the energy to form the defect sites, solution energies are generated. These show that in both ThO2-x and ThO2 the most favourable solution equilibrium site for halides is the single positively charged oxygen vacancy (although in ThO2, I demonstrates the same solubility in the NTV1 and DV clusters). Solution energies are much lower in ThO2-x than in ThO2 indicating that stoichiometry is a significant factor in determining solubility. In ThO2, all three halogens are highly insoluble and in ThO2-x Br and I remain insoluble. Although ½Cl2 is soluble in ThO2-x alternative phases such as ZrCl4 exist which are of lower energy.

Electron capture by vacancy-type defects in carbon-doped GaN studied using monoenergetic positron beams

Vacancy-type defects in GaN with different carbon concentrations ([C]=2×1016−1×1018cm−3) were probed using monoenergetic positron beams. 1.5-μm-thick GaN layers were grown on Si substrates by metalorganic vapor phase epitaxy. Measurements of Doppler broadening spectra and positron lifetimes indicated that the major defect species in the GaN layers was a Ga vacancy coupled with nitrogen vacancies. The positron trapping rate of the defects decreased with increasing [C], which was attributed to the downward shift of the Fermi level position due to C acceptor and the resultant change in the defect charge state (neutral to positive). Under illumination, the defect charge state was changed from positive to neutral, which was attributed to the trapping of the excited electrons by the defects (V++e−→V0). The effect of illumination was suppressed for the GaN layer with [C]≥1018cm−3, which was ascribed to the excellent electron trapping property of C in GaN.

Electronic properties of B and Al doped graphane: A hybrid density functional study

Using a hybrid density functional theory approach parametrized by Heyd, Scuseria and Ernzerhof (HSE06 hybrid functional), we study the energetics, structural and electronic properties of a graphane monolayer substitutionally doped with the B (BCH) and Al (AlCH) atoms. The BCH defect can be integrated within a graphane monolayer at a relative low formation energy, without major structural distortions and symmetry breaking. The AlCH defect relaxes outward of the monolayer and breaks the symmetry. The density of states plots indicate that BCH doped graphane monolayer is a wide band gap semiconductor, whereas the AlCH defect introduces the spin dependent mid gap states at the vicinity of the Fermi level, revealing a metallic character with the pronounced magnetic features. We further examine the response of the Al dependent spin states on the multiple charge states doping. We find that the defect formation energy, structural and electronic properties can be altered via charge state modulation. The +1 charge doping opens an energy band gap of 1.75eV. This value corresponds to the wavelength in the visible spectrum, suggesting an ideal material for solar cell absorbers. Our study fine tunes the graphane band gap through the foreign atom doping as well as via defect charge state modulation.

Defect induced charge trapping in C-doped α-Al2O3

The charged defect states of C-doped α-Al2O3 are investigated systematically with the density functional theory to study their thermodynamic stability and possible effects on the crystal structure and electrical conductivity. Our results reveal that under reducing (Al-rich) synthesis conditions, the most stable defect configuration is CO−2 with a deep (-1|-2) thermodynamic transition level. As a result, C defects are expected to act as double acceptors by introducing appreciable deep electron traps in the host band gap of α-Al2O3. These results are consistent with the proposal that a large number of F+-centers are formed as charge compensators to CO−2 ions in α-Al2O3.

Accurate formation energies of charged defects in solids: A systematic approach

Defects on surfaces of semiconductors have a strong effect on their reactivity and catalytic properties. The concentration of different charge states of defects is determined by their formation energies. First-principles calculations are an important tool for computing defect formation energies and for studying the microscopic environment of the defect. The main problem associated with the widely used supercell method in these calculations is the error in the electrostatic energy, which is especially pronounced in calculations that involve surface slabs and two-dimensional materials. We present an internally consistent approach for calculating defect formation energies in inhomogeneous and anisotropic dielectric environments and demonstrate its applicability to the cases of the positively charged Cl vacancy on the NaCl (100) surface and the negatively charged S vacancy in monolayer ${\mathrm{MoS}}_{2}$.

&lt;i&gt;Ab Initio&lt;/i&gt; Study of MgTe Self-Interstitial (Mg&lt;sub&gt;i&lt;/sub&gt; and Te&lt;sub&gt;i&lt;/sub&gt;): A Wide Band Gap Semiconductor

We present results of defect formation energies and charge state thermodynamic transition levels of Mg and Te interstitials in MgTe wurzite structure. We use the generalized gradient approximation and local density approximation functionals in the framework of density functional theory for all calculations. The formation energies of the Mg and Te interstitials in MgTe for both the tetrahedral and hexagonal configurations were obtained. The Mg and Te interstitials in MgTe depending on the functional, introduced transition state levels that are either donor or acceptor within the band gap of the MgTe. The Te interstitial exhibit charge states controlled metastability, negative-U and DX centre properties. The Mg interstitial acts as deep or shallow donor and there is no evidence of acceptor levels found for the Mg interstitial.

Tailoring spin defects in diamond by lattice charging

Atomic-size spin defects in solids are unique quantum systems. Most applications require nanometre positioning accuracy, which is typically achieved by low-energy ion implantation. A drawback of this technique is the significant residual lattice damage, which degrades the performance of spins in quantum applications. Here we show that the charge state of implantation-induced defects drastically influences the formation of lattice defects during thermal annealing. Charging of vacancies at, for example, nitrogen implantation sites suppresses the formation of vacancy complexes, resulting in tenfold-improved spin coherence times and twofold-improved formation yield of nitrogen-vacancy centres in diamond. This is achieved by confining implantation defects into the space-charge layer of free carriers generated by a boron-doped diamond structure. By combining these results with numerical calculations, we arrive at a quantitative understanding of the formation and dynamics of the implanted spin defects. These results could improve engineering of quantum devices using solid-state systems.

(Invited) Toward GaN-Based Red LEDs: The Influence of Local and Extended Defect Environments on the Optical and Material Properties of GaN:Eu

The doping of Europium (Eu) into gallium nitride (GaN) has a promising future for solid state lighting applications.1 It has been shown that the Eu ions incorporate themselves into the GaN host in a variety of different defect environments.2 Locally, the defect environments around the Eu ions play a large role in the optical and magnetic properties of the ions themselves. Two of the main centers have been associated with the common vacancy defects in GaN, the gallium vacancy (VGa) and the nitrogen vacancy (VN).1 Once this association has been made, it becomes necessary to consider the charge state of the local vacancy defect. It now appears that one Eu center is actually a charged derivative of another center that was associated with a VGa. The two centers exhibit a temperature dependent meta-stability. Furthermore, the effective g-factor of the 7F2 multiplet for the “charged” center is far above the theoretical expectation for an isolated Eu ion,3 and more than two times larger than that of the other Eu centers. This is thought to be related to the delocalization of a captured carrier over the Eu-VGa complex. On the macroscale, the overall concentration of oxygen, which can be detrimental to device performance, was shown to influence the incorporation of the Eu ions.4 The oxygen was introduced into the samples due its presence in the standard Eu precursor Eu(DPM)3; therefore, a new liquid Eu source, which does not contain oxygen in its molecular structure, was developed. It was discovered that the removal of oxygen led to the broadening of the photoluminescence emission spectra, and the appearance of Eu-N surface precipitation. This was due to poor incorporation of the Eu ions onto Ga sites in the GaN, which was confirmed by Rutherford backscattering/channeling measurements. By intentionally reintroducing oxygen during growth, this issue was resolved. However, this issue can be subverted altogether by using a delta-doping growth structure, with alternating layers of GaN and GaN:Eu. The mechanism for this will be discussed. Moreover, it was recently found that the incorporation of Eu into the GaN matrix can drastically influence the material properties, even at dilute levels (< 1%). By using the delta-doping structure and lowering the growth temperature, the surface morphology, as well as the size and concentration of threading dislocations can be controlled. This control had a large influence on the optical and electrical properties of LEDs grown using these conditions. Not only was the output power per Eu layer thickness significantly enhanced, but an injection current of 20 mA was achieved under an applied voltage of <5V, which is the lowest value reported for this material.5 Lastly, the brightest LED had an output power of ~375µW, and an external quantum of efficiency of ~4.6%, which are the highest values reported for this system. [1] Y. Fujiwara and V. Dierolf, Jpn. J. Appl. Phys. 53, 05FA13 (2014). [2] N. Woodward et al., Opt. Mater. 33, 1050 (2011). [3] N. Woodward, et al., MRS Proceedings, 1342, mrss11-1342-v05-06 (2010). [4] B. Mitchell et al., Scientific Reports 6,18808 (2016). [5] A. Nishikawa et al., Appl. Phys. Lett. 97, 051113 (2010).

First-principles identification of charge-transition levels of native defects in BaF2

This paper reports on semilocal and hybrid density functional analysis of charge-transition levels of native defects in BaF2 structure. The transition level is defined as the Fermi level where two defect charge states have the same formation energy. The errors arising from the small supercell size effects have been relieved through extrapolating the formation energies to the limit of infinite supercell size. The level placement in the corrected band gap is achieved using a correction factor obtained from the difference between the valence band maxima in semilocal and hybrid calculations. The band gap size from hybrid calculation is validated using the full-potential, linearized augmented planewave method with the modified Becke–Johnson exchange potential. Our results are sufficiently accurate and, thus, significant for direct comparison with experiments.

Probing the effect of point defects on the leakage blocking capability of Al0.1Ga0.9N/Si structures using a monoenergetic positron beam

Vacancy-type defects in Al0.1Ga0.9N were probed using a monoenergetic positron beam. Al0.1Ga0.9N layers with different carbon doping concentrations ([C] = 5 × 1017−8 × 1019 cm−3) were grown on Si substrates by metalorganic vapor phase epitaxy. The major defect species in Al0.1Ga0.9N was determined to be a cation vacancy (or cation vacancies) coupled with nitrogen vacancies and/or with carbon atoms at nitrogen sites (CNs). The charge state of the vacancies was positive because of the electron transfer from the defects to CN-related acceptors. The defect charge state was changed from positive to neutral when the sample was illuminated with photon energy above 1.8 eV, and this energy range agreed with the yellow and blue luminescence. For the sample with high [C], the charge transition of the vacancies under illumination was found to be suppressed, which was attributed to the trapping of emitted electrons by CN-related acceptors. With increasing [C], the breakdown voltage under the reverse bias condition increased. This was explained by the trapping of the injected electrons by the positively charged vacancies and CN-related acceptors.

Dielectric relaxation in glassy Se 90 Cd 6 In 4

In the present Letter frequency (5 × 102 Hz–1 × 105 Hz) and temperature (308–333 K) dependence of dielectric parameter of Se90Cd6In4 glassy alloy has been examined. Conventional melt quench technique has been applied to prepare Se90Cd6In4 glassy alloy. It is observed that dielectric constant (e′) and dielectric loss factor (e″) change with frequency and temperature. The dielectric loss data has been utilised to determine the barrier height W m, which is in accordance with the Elliott's theory of hoping of charge carriers over potential barrier between charge defect states in case of chalcogenide glasses.

Interplay between strain, defect charge state, and functionality in complex oxides

We use first-principles calculations to investigate the interplay between strain and the charge state of point defect impurities in complex oxides. Our work is motivated by recent interest in using defects as active elements to provide novel functionality in coherent epitaxial films. Using oxygen vacancies as model point defects, and CaMnO3 and MnO as model materials, we calculate the changes in internal strain caused by changing the charge state of the vacancies, and conversely the effect of strain on charge-state stability. Our results show that the charge state is a degree of freedom that can be used to control the interaction of defects with strain and hence the concentration and location of defects in epitaxial films. We propose the use of field-effect gating to reversibly change the charge state of defects and hence the internal strain and corresponding strain-induced functionalities.