Donor Defect States Research Articles

Low-Defect-Density Monolayer MoS2 Wafer by Oxygen-Assisted Growth-Repair Strategy.

Atomic chalcogen vacancy is the most commonly observed defect category in two dimensional (2D) transition-metal dichalcogenides, which can be detrimental to the intrinsic properties and device performance. Here a low-defect density, high-uniform, wafer-scale single crystal epitaxial technology by in situ oxygen-incorporated "growth-repair" strategy is reported. For the first time, the oxygen-repairing efficiency on MoS2 monolayers at atomic scale is quantitatively evaluated. The sulfur defect density is greatly reduced from (2.71 ± 0.65) × 1013 down to (4.28 ± 0.27) × 1012 cm-2, which is one order of magnitude lower than reported as-grown MoS2. Such prominent defect deduction is owing to the kinetically more favorable configuration of oxygen substitution and an increase in sulfur vacancy formation energy around oxygen-incorporated sites by the first-principle calculations. Furthermore, the sulfur vacancies induced donor defect states is largely eliminated confirmed by quenched defect-related emission. The devices exhibit improved carrier mobility by more than three times up to 65.2 cm2 V-1 s-1 and lower Schottky barrier height reduced by half (less than 20 meV), originating from the suppressed Fermi-level pinning effect from disorder-induced gap state. The work provides aneffective route toward engineering the intrinsic defect density and electronic states through modulating synthesis kinetics of 2D materials.

Healing Donor Defect States in CVD-Grown MoS2 Field-Effect Transistors Using Oxygen Plasma with a Channel-Protecting Barrier.

Molybdenum disulfide (MoS2 ), a metal dichalcogenide, is a promising channel material for highly integrated scalable transistors. However, intrinsic donor defect states, such as sulfur vacancies (Vs ), can degrade the channel properties and lead to undesired n-doping. A method for healing the donor defect states in monolayer MoS2 is proposed using oxygen plasma, with an aluminum oxide (Al2 O3 ) barrier layer that protects the MoS2 channel from damage by plasma treatment. Successful healing of donor defect states in MoS2 by oxygen atoms, even in the presence of an Al2 O3 barrier layer, is confirmed by X-ray photoelectron spectroscopy, photoluminescence, and Raman spectroscopy. Despite the decrease in 2D sheet carrier concentration (Δn2D = -3.82×1012 cm-2 ), the proposed approach increases the on-current and mobility by 18% and 44% under optimal conditions, respectively. Metal-insulator transition occurs at electron concentrations of 5.7×1012 cm-2 and reflects improved channel quality. Finally, the activation energy (Ea ) reduces at all the gate voltages (VG ) owing to a decrease in Vs , which act as a localized state after the oxygen plasma treatment. This study demonstrates the feasibility of plasma-assisted healing of defects in 2D materials and electrical property enhancement and paves the way for the development of next-generation electronic devices.

A Class of Auxiliary Passivators for Polymer Dielectrics

AbstractHigh‐electrical‐strength polymer dielectrics are essential for advanced devices with high power and/or high integration densities and film capacitors with high energy‐storage densities. Key factors affecting the polymer dielectric electrical strength are deep‐level defect states, which lead to electron and hole accumulation. Numerous deep‐level defect states lead to charge accumulation in the polymer dielectric during operation, contributing to local electric field distortion and resulting in flashover or breakdown. In this work, first‐principles calculations and experiments reveal that VH (i.e., H vacancies) in the polymer dielectric molecular chain can create defect states deep in the bandgap. RCl (R = Li, Na, K) can be used for passivating the deep‐level polymer dielectric defect states. In addition, the passivation mechanisms are analyzed. The RCl cations can passivate deep‐level defect states into shallow‐level acceptor defect states because the RCl dipole moment regulates the deep‐level defect state energy. The RCl anions can passivate deep‐level defect states into shallow‐level donor defect states by forming a stable covalent bond between carbon and lone‐pair electrons. This work supports the design of high‐electrical‐strength polymer dielectrics.

Healing of donor defect states in monolayer molybdenum disulfide using oxygen-incorporated chemical vapour deposition

Two-dimensional molybdenum disulfide (MoS2) is a semiconductor that could be used to build scaled transistors and other advanced electronic and optoelectronic devices. However, the material typically exhibits strong n-type doping, low photoluminescence quantum yields and high contact resistance with metals, behaviour that is often attributed to the presence of donor states induced by sulfur vacancies. Here we show that oxygen-incorporated chemical vapour deposition can be used to passivate sulfur vacancies and suppress the formation of donor states in monolayer MoS2. First-principles calculations and spectroscopy measurements are used to reveal the formation of molybdenum–oxygen bonding at the sulfur vacancy sites and the absence of donor states in oxygen-incorporated MoS2. Compared with MoS2 fabricated via chemical vapour deposition without oxygen, oxygen-incorporated MoS2 exhibits enhanced photoluminescence, higher work function and improved contact resistance with a lower Schottky barrier (less than 40 meV) at the metal/MoS2 interface. Sulfur vacancies in monolayer molybdenum disulfide can be passivated using an oxygen-incorporated chemical vapour deposition technique, which results in less n-type doping, enhanced photoluminescence and decreased contact resistance compared with growth without oxygen.

Electron energy loss spectroscopy and first-principles study of GaN via Zn doping

The electronic structure of GaN and GaN:Zn was investigated by electron energy loss spectroscopy and first-principles calculations. In the low-loss spectrum, the interband transitions are assigned to the observed energy loss peaks. After Zn doping, impurity levels are introduced to the density of states and hybrid orbitals of N 2p and Zn 3d are formed around the Fermi level. In the nitrogen K-edge, an additional peak was observed due to the formation of donor defect states. A core-hole effect is believed to be significant for simulation of the N K-edge for both GaN and GaN:Zn.

The point defects induced ferromagnetism in ZnO semiconductor by terbium doping via co-precipitation method

In this work, the undoped and terbium (Tb) doped ZnO nanoparticles were synthesized by a simple chemical co-precipitation method. X-ray diffraction patterns of the undoped and Tb doped ZnO exhibit the wurtzite hexagonal crystal structure, here the changing of crystallite size and unit cell volume is noticed with doping level. The anticipated elements in the synthesized samples are confirmed using energy dispersive X-rays analysis and X-ray photoelectron spectroscopy. The hexagonal wurtzite crystal structure of the synthesized materials is also confirmed from the µ-Raman study. From photoluminescence (PL) spectra, near band edge emission (NBE) peak of ZnO and defects related peaks intensities are increased by Tb doping. The red shift (NBE) observed in Tb doped ZnO samples as compared to undoped ZnO. The PL and micro-Raman results are strongly confirming the co-existing of oxygen vacancies (VO) and zinc interstitials (Zni) defect sates in the Tb doped ZnO samples. So, the presence of these donor defect states (VO and Zni) is main reason for the strong ferromagnetism in 0.09 mol% Tb doped ZnO.

Effect of inversion layer at iron pyrite surface on photovoltaic device

Iron pyrite has great potential as a thin-film solar cell material because it has high optical absorption, low cost, and is earth-abundant. However, previously reported iron pyrite solar cells showed poor photovoltaic characteristics. Here, we have numerically simulated its photovoltaic characteristics and band structures by utilizing a two-dimensional (2D) device simulator, ATLAS, to evaluate the effects of an inversion layer at the surface and a high density of deep donor defect states in the bulk. We found that previous device structures did not consider the inversion layer at the surface region of iron pyrite, which made it difficult to obtain the conversion efficiency. Therefore, we remodeled the device structure and suggested that removing the inversion layer and reducing the density of deep donor defect states would lead to a high conversion efficiency of iron pyrite solar cells.

Ionization of high-density deep donor defect states explains the low photovoltage of iron pyrite single crystals.

Iron pyrite (FeS2) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress, the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (VOC). Here we report a comprehensive investigation on {100}-faceted n-type iron pyrite single crystals to understand its puzzling low VOC. We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals. These insights were used to develop a circuit model analysis for the electrochemical impedance spectroscopy that allowed a complete characterization of the bulk and surface defect states and the construction of a detailed energy band diagram for iron pyrite crystals. A holistic evaluation revealed that the high-density of intrinsic surface states cannot satisfactorily explain the low photovoltage; instead, the ionization of high-density bulk deep donor states, likely resulting from bulk sulfur vacancies, creates a nonconstant charge distribution and a very narrow surface space charge region that limits the total barrier height, thus satisfactorily explaining the limited photovoltage and poor photoconversion efficiency of iron pyrite single crystals. These findings lead to suggestions to improve single crystal pyrite and nanocrystalline or polycrystalline pyrite films for successful solar applications.

INFLUENCES OF ASYMMETRICALLY DISTRIBUTED DEFECT STATES AT REAR c-Si/a-Si:H INTERFACE ON PERFORMANCES OF SILICON HETERO-JUNCTION SOLAR CELLS

The performances of silicon double hetero-junction solar cells are investigated by the numerical simulator "Automat For Simulation of Hetero-structures (AFORS-HET)". Asymmetrical distribution of acceptor and donor defect states at the rear hetero-junction interface is introduced in both a-Si:H(n)/c-Si(p)/a-Si:H(p+) and a-Si:H(p)/c-Si(n)/a-Si:H(n+) structures in our study. The influences of total defect density and rear emitter parameters (doping concentration and band discontinuity to silicon substrate) on performances of the solar cells are analyzed. The simulated results indicate that the npp + cells with acceptor dominated defect states at the rear interface can obtain better performances, and the same for the pnn + cells with donor dominated defect states. Moreover, the npp + cells are more sensitive to the total interface defect density and its distribution mode due to the unfavorable band offset between the absorber and the rear emitter. But when total defect density is small, the npp + cells show more sensitivity to change of the doping concentration at the rear emitter than the pnn + cells, and conversely, the pnn + cells show more sensitivity to the distribution mode of defect states at the rear emitter than the npp + cells. Due to weaker inherent back diffusion ability of minority carrier, the pnn + cells with moderate band offset (0.25 ~ 0.37 eV ) at the rear hetero-junction are more sensitive to the incentive effect of localized charges at the interface formed by asymmetrically distributed defect states, and the interface with donor dominated defect states is a better option for the pnn + cells when the total defect density is relatively small.

First principles study of the ternary complex model of EL2 defect in GaAs saturable absorber

First principles calculations are performed for the perfect GaAs crystal, the double Ga vacancies (VGa)₂, and the ternary complex defect (AsGaVAsVGa), using the state-of-the-art computational method with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to correct the band gap and account for a proper description of the interaction between defects states and bulk states. Three shallow acceptor defect levels are found due to the creation of (VGa)₂ with nearest-neighbor As dangling bonds. However, for GaAs with the ternary complex defects (AsGaVAsVGa), the As antisite AsGa and the VAs'S nearest-neighbor Ga dangling bonds provoke several donor defect states. The lowest donor defect state locates at 0.85 eV below the bottom of conduction band, which is very close to the experimental observation of the EL2 defect level. In addition, structual evolution from (VGa)₂ defect to the ternary defect complex (AsGaVAsVGa) is simulated by ab initio molecular dynamic (MD) calculation at different temperatures. The MD results demonstrate that the ternary complex defect (AsGaVAsVGa) can be converted from the double Ga vacancies (VGa)₂ at room temperature, and it can exist stably at higher temperature. The present work is helpful to unravel the microstructure and the forming mechanism of the EL2 defect, to find out methods to improve the performance of the GaAs saturable absorber by changing the growth conditions of GaAs crystal.

An Extended Unified Schottky-Poole-Frenkel Theory to Explain the Current-Voltage Characteristics of Thin Film Metal-Insulator-Metal Capacitors with Examples for Various High-k Dielectric Materials

Historically, there is a controversy regarding the current-voltage (I-V) characteristics of thin film MIM (metal-insulator-metal) capacitors, which is quite frequently modeled by either the Schottky model or the Poole-Frenkel model. In this paper, the author points out that the two models actually can be unified. The physics underlying this model involves a non-uniform distribution of deep donor defect states such that a very large quantity of defect states exist at the two interface of the MIM capacitor while the density of defect states in the insulator bulk is relatively low, resulting in an M/n-i-n/M structure. This unified Schottky-Poole-Frenkel model can be further extended to include other effects like space charge limited current and tunneling. The effect of trap limited space charge limited current is also discussed. Examples of the application of this theory will be provided for MIM capacitors based on various high-k dielectric materials like tantalum oxide, titanium oxide, zirconium oxide and aluminum oxide.

Influence of the broken symmetry of defect state distribution at the a-Si:H/c-Si interface on the performance of hetero-junction solar cells

Taking into account the fact that the distribution of defect states at the interface does not have strictly symmetrical shape, we present a simulation study of a-Si:H(n)/c-Si(p) and a-Si:H(p)/c-Si(n) structures with regard to the defect states at the interface, band offsets and doping concentration of the emitter. The presented results suggest for a-Si:H(n)/c-Si(p) solar cells a strong influence of the introduced broken symmetry between acceptor and donor defect states on the open-circuit voltage, whereas the a-Si:H(p)/c-Si(n) structure benefits from inherent favorable band alignment and remains unaffected.

Investigation of Near-IR Emission from Hydrogenated Nanocrystalline Silicon – The Oxygen Defect Band

AbstractHydrogenated nanocrystalline silicon (nc-Si:H), a mixture of nanometer sized crystallites and amorphous silicon tissue, demonstrates a photoluminescence band centered at ∼ 0.7 eV, which emerges in response to annealing at an onset temperature of ∼ 200–300 °C. This temperature range correlates well with hydrogen effusion spectroscopy studies, and evidence suggests thermal liberation of hydrogen from grain boundary regions allows oxidation of crystallite surfaces during annealing. We tentatively attribute the 0.7 eV PL in nc-Si:H to deep donor defect states related to oxygen precipitates, and argue for the possible involvement of dislocations inside of crystallites to accompany these precipitates.

Defect physics of the ordered defect compound CuIn 3Se 5

The activation energies of acceptor and donor defect states in the ordered defect compound CuIn 3Se 5 are calculated by using a simple model based on the effective-mass theory for single, double and triple point defect centers. It is found that the values of these energies for both shallow and moderately deep levels are in reasonable agreement with those determined from experimental data. From the analysis of the results, a tentative assignment of the origins of these centers has been made.

Defect physics of ternary chalcopyrite semiconductors

The activation energy of acceptor and donor defect states in chalcopyrite compounds are calculated by using model based in the effective-mass theory for the case of single-, double- and triple-point defect centers. It is found that the values of these energies thus calculated for shallow and moderate deep levels are in reasonable agreement with those obtained from experimental data. From the analysis of the results, most of these levels have been identified as due to the presence of cation and anion vacancies and interstitials, and cation–cation and cation–anion antisite disorders.

Polarized surface differential reflectivity and oxygen chemisorption on InP(110) surfaces

We present surface differential reflectivity (SDR) results on the polarization dependence of optical transitions and on the oxidation of InP(110) surfaces in the energy range between 2.0 and 4.0 eV. The surface dielectric function has been derived for the light electric vector along [110] and [001] directions. The data show two anisotropic peaks at 2.6 eV (excited with the light electric vector along the [110] direction) and 3.5 eV (excited with the light electric vector along the [001] direction) and an almost isotropic behaviour for the peak at 3.1 eV. Absorption kinetics is analyzed by following the reflectivity variation between 1 × 102 and 2 × 106 langmuirs of molecular oxygen at three selected spectral energies. Several well-defined steps of the oxidation process are clearly resolved and discussed in terms of disappearance of intrinsic surface states, creation of acceptor and donor defect states, and growth of In2O3.

Polarized surface differential reflectivity and oxygen chemisorption on InP(110) surfaces

Abstract We present surface differential reflectivity (SDR) results on the polarization dependence of optical transitions and on the oxidation of InP(110) surfaces in the energy range between 2.0 and 4.0 eV. The surface dielectric function has been derived for the light electric vector along [110] and [001] directions. The data show two anisotropic peaks at 2.6 eV (excited with the light electric vector along the [110] direction) and 3.5 eV (excited with the light electric vector along the [001] direction) and an almost isotropic behaviour for the peak at 3.1 eV. Absorption kinetics is analyzed by following the reflectivity variation between 1 × 10 2 and 2 × 10 6 langmuirs of molecular oxygen at three selected spectral energies. Several well-defined steps of the oxidation process are clearly resolved and discussed in terms of disappearance of intrinsic surface states, creation of acceptor and donor defect states, and growth of In 2 O 3 .

Clean and oxygen covered InP(110) surfaces differential reflectivity

We present surface differential reflectivity results on the polarization dependence of optical transitions and on the oxidation of InP(110) surfaces in the energy range between 2.0 and 4.0 eV. The surface dielectric function has been computed for light electric vector along [110] and [001] directions. Three well defined optical peaks have been detected at the following energies: 2.6 eV (light electric vector along [110] direction), 3.1 eV (unpolarized), and 3.5 eV (light electric vector along [001] direction). Absorption kinetics is analyzed by following the reflectivity variation between 1×102 and 2×106 L of molecular oxygen at three selected spectral energies. Several well defined steps of the oxidation process are clearly resolved and discussed in terms of disappearance of intrinsic surface states, creation of acceptor and donor defect states, growth of In2O3.

Oxygen chemisorption on cleaved InP(110) surfaces studied with surface differential reflectivity.

We present surface-differential-reflectivity results on the oxidation of InP(110) surfaces in the energy range between 2.0 and 4.0 eV. Absorption kinetics is analyzed by following the reflectivity variation between 1\ifmmode\times\else\texttimes\fi{}${10}^{2}$ and 2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ langmuirs of molecular oxygen at three selected spectral energies. Several well-defined steps of the oxidation process are clearly resolved and discussed in terms of the disappearance of intrinsic surface states, the creation of acceptor and donor defect states, and the growth of ${\mathrm{In}}_{2}$${\mathrm{O}}_{3}$.