CN Defects Research Articles

Enhancing oxygen evolution reaction via hydrogen plasma treatment: Unveiling the functionality of CN defects and the role of Fe in NiFe Prussian blue analogs

AbstractThe rational design of electronic and vacancy structures is crucial to regulating and enhancing electrocatalytic water splitting. However, creating novel vacancies and precisely controlling the number of vacancies in existing materials systems pose significant challenges. Herein, a novel approach to optimize the concentration of the CN vacancy (VCN) in the NiFe Prussian blue analog (PBA) nanocubes is designed by incorporating the H2 or O2 plasma treatment. The relationship between the VCN and catalysis is analyzed, and results show that a moderate concentration of VCN (6.5%) can enormously enhance oxygen evolution reaction (OER) activity of NiFe PBA. However, an excessive amount of VCN disrupts the crystal structure and hinders the transportation of charge carriers, consequently leading to inferior OER. Furthermore, the VCN significantly activates the activity of Fe sites, inducing preferential adsorption of OH− on Fe sites, followed by adsorption on Ni sites, thereby optimizing the reaction pathway and significantly promoting OER performance. In addition, VCN also suppresses Fe leaching, giving the catalyst excellent durability. This study reveals the feasibility of creating unconventional defects in nanomaterials and precisely controlling the number of vacancies for diverse catalytic and energy applications.

Theoretical investigation of electronic structure and thermoelectric properties of CN point defects in GaN

GaN bulk with CN point defects has been theoretically calculated using Density Functional Theory and Boltzmann transport theory. Pure GaN and GaN with two CN of the nearest neighbor, the second-nearest neighbor, and the third-nearest neighbor were carried out to investigate the energy band, density of states, and thermoelectric properties. Compared to pure GaN, it was found that impurity levels appear, which lead to the moving down of the Fermi level and then the moving up of top valence bands in GaN with CN defects. Combining with total density of states (TDOS) and partial density of states (PDOS), we found C(p) makes the main contribution to the moving down of the Fermi level. Then, the highest value of figure-of-merit (ZT) was calculated to be 2.66 of GaN with two CN of the nearest neighbor, while it is only 0.94 of pure GaN at 900 K for p-type. ZT of pure GaN and GaN with CN defects are almost the same at room temperature. In short, impurity levels that appeared in GaN in the energy band were related to CN point defects, which are useful for the improvement of thermoelectric properties on GaN at high temperatures. The research is useful for the application of GaN-based semiconductors.

A Combination of Ion Implantation and High‐Temperature Annealing: The Origin of the 265 nm Absorption in AlN

The commonly observed absorption around 265 nm in AlN is hampering the outcoupling efficiency of light‐emitting diodes (LEDs) emitting in the UV‐C regime. Carbon impurities in the nitrogen sublattice (CN) of AlN are believed to be the origin of this absorption. A specially tailored experiment using a combination of ion implantation of boron, carbon, and neon with subsequent high‐temperature annealing allows to separate the influence of intrinsic point defects and carbon impurities regarding this absorption. Herein, the presented results reveal the relevance of the intrinsic nitrogen‐vacancy defect VN. This is in contradiction to the established explanation based on CN defects as the defect causing the 265 nm absorption and will be crucial for further UV‐LED improvement. Finally, in this article, a new interpretation of the 265 nm absorption is introduced, which is corroborated by density functional theory (DFT) results from the past decade, which are reviewed and discussed based on the new findings.

Stability of the CNHi Complex and the Blue Luminescence Band in GaN

The dissociation of the CNHi complex in GaN is studied in detail using photoluminescence measurements and first‐principles calculations. The blue luminescence (BL2) band with a maximum at 3.0 eV is caused by electron transitions from an excited state located at 0.02 eV below the conduction band to the ground state of the CNHi donor with the 0/+ level 0.15 eV above the valence band. The dissociation releases hydrogen atom, and the remaining CN defect with the −/0 state at 0.92 eV above the valence band is responsible for the yellow band (YL1) with a maximum at about 2.2 eV. The dissociation of the CNHi complex can be caused by the photoinduced defect reaction mechanism under UV illumination at low temperature (≈20 K), leading to the bleaching of the BL2 band and simultaneous rise in the YL1 band. The bleaching is reversible. Alternatively, the complex dissociates after annealing at temperatures above 600 °C. The activation energy of this process (3–4 eV, depending on the annealing geometry) corresponds to the removal of hydrogen from the sample and not to the dissociation of the complex itself.

Carrier trapping and recombination at carbon defects in bulk GaN crystals grown by HVPE

We investigate the impact of carbon defects on carrier trapping and recombination processes in high-quality GaN crystals using transient absorption spectroscopy (TAS). The different wavelengths for probing carrier dynamics enable discrimination of electron- and hole-capture processes by carbon acceptors (CN), and two charge states (−1 and 0) of the CN are demonstrated from TAS by tuning the pump fluence. The hole-capture coefficients for the CN− and CN0 states are at least ten times larger than that for Fe centers. On the other hand, electrons can also recombine efficiently with holes at the 0/+ level of the CN defect. The resulting carrier trapping on carbon in GaN would mediate carrier recombination and significantly reduce the efficiency of optoelectronic devices even for a low defect concentration (∼1015 cm−3).

Defect Engineering on Boron Nitride for O2 Activation and Subsequent Oxidative Desulfurization.

The rational design of highly active hexagonal boron nitride (h-BN) catalysts at the atomic level is urgent for aerobic reactions. Herein, a doping impurity atom strategy is adopted to increase its catalytic activities. A series of doping systems involving O, C impurities and B, N antisites are constructed and their catalytic activities for molecular O2 have been studied by density functional theory (DFT) calculations. It is demonstrated that O2 is highly activated on ON and BN defects, and moderately activated on CB and CN defects, however, it is not stable on NB and OB defects. The subsequent application in oxidative desulfurization (ODS) reactions proves the ON and C-doped (CB , CN ) systems to be good choice for sulfocompounds oxidization, especially for dibenzothiophene (DBT). While the BN antisite is not suitable for such aerobic reaction due to the extremely stable B-O* -B species formed during the oxidation process.

High Breakdown Voltage and Low Buffer Trapping in Superlattice GaN-on-Silicon Heterostructures for High Voltage Applications.

The aim of this work is to demonstrate high breakdown voltage and low buffer trapping in superlattice GaN-on-Silicon heterostructures for high voltage applications. To this aim, we compared two structures, one based on a step-graded (SG) buffer (reference structure), and another based on a superlattice (SL). In particular, we show that: (i) the use of an SL allows us to push the vertical breakdown voltage above 1500 V on a 5 µm stack, with a simultaneous decrease in vertical leakage current, as compared to the reference GaN-based epi-structure using a thicker buffer thickness. This is ascribed to the better strain relaxation, as confirmed by X-Ray Diffraction data, and to a lower clustering of dislocations, as confirmed by Defect Selective Etching and Cathodoluminescence mappings. (ii) SL-based samples have significantly lower buffer trapping, as confirmed by substrate ramp measurements. (iii) Backgating transient analysis indicated that traps are located below the two-dimensional electron gas, and are related to CN defects. (iv) The signature of these traps is significantly reduced on devices with SL. This can be explained by the lower vertical leakage (filling of acceptors via electron injection) or by the slightly lower incorporation of C in the SL buffer, due to the slower growth process. SL-based buffers therefore represent a viable solution for the fabrication of high voltage GaN transistors on silicon substrate, and for the simultaneous reduction of trapping processes.

Impact of growth conditions on intrinsic carbon doping in GaN layers and its effect on blue and yellow luminescence

In this study, the GaN films were deposited in the multilayered structures under different growth conditions. SIMS analysis showed that intrinsic carbon incorporation around two orders can be controlled effectively by varying the growth parameters. Defects analysis by cross-sectional cathodoluminescence (X-CL) is demonstrated as a useful analytical technique for unambiguously understanding the effect of incorporation of the intrinsic carbon concentration on the yellow luminescence (YL) and blue luminescence (BL) band intensities. It was inferred from the experimental data that the observed BL and YL intensities are due to the electronic transitions through the isolated CN and CNON complex, respectively. This finding is in contrast to the theoretically published literature that two charged states of CN are responsible for both BL and YL which sought that the BL emission to be always smaller in comparison to YL emission. Our experimental data, for all different growth conditions showed the prominent BL in comparison to YL except for the growth at sufficiently lower temperature which is considered to be more conducive for higher oxygen incorporation into the stoichiometry. This increases the probability of CNON formation which is manifested in the enhancement of YL intensity over BL as the formation energy of CNON complex is lower than the CN defect. Thus, X-CL analysis method can be key to address the challenges of understanding the impurity character in multi-thin layered GaN structures.

Impact of sidewall etching on the dynamic performance of GaN-on-Si E-mode transistors

The aim of this paper is to investigate the role of the etching of the sidewalls of p-GaN on the dynamic performance of normally-off GaN HEMTs with p-type gate. We analyze two wafers having identical epitaxy but with different recipes for the sidewall etching, referred to as “Etch A” (non-optimized) and “Etch B” (optimized). We demonstrate the following relevant results: (i) the devices with non-optimized etching (Etch A), when submitted to positive gate bias, show a negative threshold voltage shift and a decrease in Ron, which are ascribed to hole injection under the gate and/or in the access regions; (ii) transient characterization indicates the existence of two trap states, with activation energies of 0.84 eV (CN defects) and 0.30 eV. The latter (with time-constants in the ms range) is indicative of the hole de-trapping process, possibly related to trap states in the AlGaN barrier or at the passivation/AlGaN interface; (iii) by optimizing the p-GaN sidewall etching (for the same epitaxy) it is possible to completely eliminate the threshold voltage shift. This indicates that hole injection mostly takes place along the sidewalls.

Point defect reduction in MOCVD (Al)GaN by chemical potential control and a comprehensive model of C incorporation in GaN

A theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters is presented. It enables systematic point defect reduction by chemical potential control in metalorganic chemical vapor deposition (MOCVD) of III-nitrides. Experimental corroboration is provided by a case study of C incorporation in GaN. The theoretical model is shown to be successful in providing quantitative predictions of CN defect incorporation in GaN as a function of growth parameters and provides valuable insights into boundary phases and other impurity chemical reactions. The metal supersaturation is found to be the primary factor in determining the chemical potential of III/N and consequently incorporation or formation of point defects which involves exchange of III or N atoms with the reservoir. The framework is general and may be extended to other defect systems in (Al)GaN. The utility of equilibrium formalism typically employed in density functional theory in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth is confirmed. Furthermore, the proposed theoretical framework may be used to determine optimal growth conditions to achieve minimum compensation within any given constraints such as growth rate, crystal quality, and other practical system limitations.

Magnetic moment changed by interlayer charge transfer in vertical graphene/C-doped hexagonal boron nitride heterostructure

Using density functional calculations, we study atomic and electronic structures of monolayer hexagonal boron nitride (h-BN) and vertical graphene/h-BN heterostructures with substitutional carbon defects. For monolayer h-BN, a carbon atom substituted for a boron or nitrogen atom causes spin polarization. In the presence of an adjacent graphene sheet, however, the spin magnetic moment disappears owing to interlayer charge transfer for a CB defect. Interestingly, two carbon defects in a hexagon of monolayer h-BN do not induce magnetic properties. In contrast, two CB defects (two CN defects) can have spin polarization in the vertical graphene/h-BN heterostructures because of interlayer charge transfer.

High voltage trapping effects in GaN-based metal–insulator–semiconductor transistors

This paper presents an analysis of the high voltage trapping processes that take place in high-electron mobility transistors based on GaN, with a metal–insulator–semiconductor (MIS) structure. The study is based on combined pulsed and transient measurements, carried out with trapping voltages in the range from 50 to 500 V. The results indicate that: (i) dynamic Ron is maximum for trapping voltages between 200 and 300 V, and decreases for higher voltage levels; (ii) Ron-transient measurements reveal the presence of a dominant trap with activation energy Ea1 = 0.93 eV and of a second trap with activation energy equal to Ea2 = 0.61 eV; (iii) the deep level transient spectroscopy (DLTS) signal associated to trap Ea1 is completely suppressed for high trapping voltages (VDS = 500 V). The results are interpreted by considering that the trap Ea1 is located in the buffer, and originates from CN defects. The exposure to high drain voltages may favor the depletion of such traps, due to a field-assisted de-trapping process or to the presence of vertical leakage paths.

Carbon as a source for yellow luminescence in GaN: Isolated CN defect or its complexes

We study three carbon defects in GaN, isolated CN and its two complexes with donors CN–ON, and CN–SiGa, as a cause of the yellow luminescence using accurate hybrid density functional calculation, which includes the semi-core Ga 3d electrons as valence electrons and uses a larger 300-atom supercell. We show that the isolated CN defect yields good agreement with experiment on the photoluminescence (PL) peak position, zero-phonon line, and thermodynamic defect transition level. We find that the defect state of the complexes that is involved in the PL process is the same as that of the CN defect. The role of the positively charged donors (ON or SiGa) next to CN is to blue-shift the PL peak. Therefore, the complexes cannot be responsible for the same PL peak as isolated CN. Our detailed balance analysis further suggests that under thermal equilibrium at typical growth temperature, the concentration of isolated CN defect is orders of magnitude higher than the defect complexes, which is a result of the small binding energy in these complexes.

Magnetic properties of 3d transition metals and nitrogen functionalized armchair graphene nanoribbon

First-principles density functional theory (DFT) calculations were performed to study the stability and magnetic properties of TM–CxNy (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn; x/y = 0, 1 and 2 and x + y = 2) edge defects in armchair graphene nanoribbon (9-AGNR). The results showed that the edge functionalization of 9-AGNR by TM–N2 defects is energetically more favorable compared to the edge functionalization by TM–C2 and TM–CN defects. Furthermore, we found that edge TM–CxNy defects in 9-AGNR support high and localized magnetic moments. The magnetic moment of TMs in TM–CxNy edge defects increases with an increase in the number of unpaired electrons in the 3d orbitals, and the calculated magnetic moment is largest for an edge Mn–N2 defect. The analysis of magnetization of TMs reveals that the magnetic moment mainly comes from the electrons in the 3d orbitals and the contribution of 4s electrons to the total magnetic moment is very small. Our calculations also show that the magnetic moment of TMs in edge TM–N2 defects in AGNR is independent of edge TM–N2 defect concentrations and AGNR width in the present study. The interedge magnetic coupling between the edge TM–N2 defects is explored by investigating the competing ferromagnetic and antiferromagnetic spin structures. We found that the interedge magnetic coupling depends on the type of TMs in edge TM–N2 defects and TM–N2 edge defect concentrations.

Impact of carbon on trap states in n-type GaN grown by metalorganic chemical vapor deposition

The effect of excess C incorporation on the deep level spectrum of n-type GaN grown by metalorganic chemical vapor deposition was investigated. Low-pressure (LP) growth conditions were used to intentionally incorporate excess C compared to atmospheric pressure (AP) growth conditions. GaN samples with high C content are found to be highly resistive, and samples codoped with C and Si are heavily compensated. From a comparison of deep level optical spectroscopy and deep level transient spectroscopy measurements of the LP-grown codoped GaN:C:Si sample with the AP-grown unintentionally doped GaN, two deep levels at Ec−Et=1.35 and 3.28 eV are observed to have a direct relation to excess C incorporation. Comparing these activation energies to previous theoretical studies strongly suggests that the levels may be associated with a C interstitial and CN defect, respectively. These results suggest that C forms not only a shallow acceptor level but also a deep acceptor level in GaN, and these levels contribute to the compensation of the free carriers in n-type GaN:C.

The crooked neck dwarf muscular dysgenesis in fowl is due to a selective alteration of the somitic myogenic cell line.

Muscular dysgenesis in trunk and limb regions of the crooked neck dwarf (cn/cn) fowl is characterized by a complete disorganization of the muscles, starting at 7.5 days of incubation and resulting, at the end of the incubation period, in a profound muscular atrophy. It has previously been attributed to progressively extending defects of the myotubes. In this paper, embryonic cn/cn head and neck muscles were subjected to histological and ultrastructural analysis. The mononucleated myoblasts of the skeletal muscles are not diseased. Pathology is only expressed in the multinucleated cells, mainly by impaired sarcomerogenesis and distension of the sarcoplasmic reticulum. In the non-skeletal (cardiac or smooth) muscles, the connective tissue scaffolding and the ultrastructural features are similar to those of normal muscles at the same age. The present report confirms that the cn defect is confined to the skeletal muscle cells. All of them belong to the same lineage, which is contained in the somitic mesoderm, whether the latter becomes segmented or not during embryogenesis.