Carbon-related Defects Research Articles

Monotonic variation in carbon-related defects with Fermi level in different conductive types of GaN

This study examined the photoluminescence (PL) of samples of GaN and found that the intensity of the peak of luminescence related to carbon impurities in p-GaN changed with the doping concentration of Mg. However, the results of a secondary ion mass spectrometry test showed that the concentration of carbon impurities did not change correspondingly. Moreover, we observed changes in the relative strength of the peak related to carbon impurities in the PL spectra of a series of samples of n-type conductive GaN. This suggests a connection between the behavior of carbon-related defects and the conductivity of GaN. The results show that the variation in carbon-related defects was monotonic. As the Fermi level approached the conductive band, carbon-related defects that generated higher-energy photonics became more dominant in the PL spectra in the series from p-type to n-type GaN.

Band gap measurements of monolayer h-BN and insights into carbon-related point defects

Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is (6.8 ± 0.2) eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of (0.7 ± 0.2) eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level.

Luminescence properties of hexagonal boron nitride powders probed by deep UV photoluminescence spectroscopy.

Deep ultraviolet (UV) photoluminescence (PL) spectroscopy was employed to study the luminescence properties of hexagonal boron nitride (h-BN) crystal powders after annealing the samples at different temperatures in the range of 100–900 °C for 1 h in ambient air. The PL spectrum from the h-BN powder samples annealed around 700 °C showed strong luminescence intensity at 5.49 eV along with enhanced phonon-assisted band-edge emission at 5.90 eV. Additionally, it revealed sharp atomic-like emission lines in UV region at 4.10, 4.12, 4.14, and 4.16 eV with line widths less than 1 nm from the annealed samples which were not present in the unannealed samples. Power- and temperature-dependent PL measurements of the sharp atomic-like emission lines exhibited robust nature of the energy peak positions. Based on the theoretical reports, the sharp emission lines could be carbon-related defects.

Interfacial electrical and chemical properties of deposited SiO2 layers in lateral implanted 4H-SiC MOSFETs subjected to different nitridations

In this paper, SiO2 layers deposited on 4H-SiC and subjected to different post deposition annealing (PDA) in NO and N2O were studied to identify the key factors influencing the channel mobility and threshold voltage stability in lateral implanted 4H-SiC MOSFETs. In particular, PDA in NO gave a higher channel mobility (55 cm2V−1s−1) than PDA in N2O (20 cm2V−1s−1), and the subthreshold behavior of the devices confirmed a lower total amount of interface states for the NO case. On the other hand, cyclic gate bias stress measurements allowed to separate the contributions of interface states (Nit) both on the upper and bottom parts of the 4H-SiC band gap and near interface oxide traps (NIOTs) in the two oxides. In particular, it was found that NO annealing reduced the total density of charges trapped at the interface states down to 3 × 1011 cm−2 and those trapped inside the oxide down to 1 × 1011 cm−2. Electron energy loss spectroscopy demonstrated that the reduction of these traps in the NO annealed sample is due to the lower amounts of sub-stoichiometric silicon oxide (~1nm) and carbon-related defects (<1nm) at the interface, respectively. This correlation represents a further step in the comprehension of the physics of the SiO2/4H-SiC interface explaining the mobility and threshold voltage behavior of 4H-SiC MOSFETs. Furthermore, the experimental results indicate that an accurate control of SiC re-oxidation during post-deposition annealing in MOSFET technology is a key factor to improve the mobility and threshold voltage stability.

Atomistic Mechanism of 4 H - SiC/SiO2 Interface Carrier-Trapping Effects on Breakdown-Voltage Degradation in Power Devices

The $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_{2}$ interface in the termination area is a crucial component in limiting high-temperature reverse-bias (HTRB) reliability for $\mathrm{Si}\mathrm{C}$-based high-voltage devices. However, the atomic structure and carrier-trapping behavior of the $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_{2}$ interface defects therein and the underlying physical mechanisms of breakdown-voltage (${V}_{\mathrm{BD}}$) variation are still largely unclear. Here, the $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_{2}$ interface defects of 4H-$\mathrm{Si}\mathrm{C}$ gate turn-off (GTO) thyristors before and after HTRB stress are investigated by transient capacitance measurements and density-functional-theory (DFT) calculations. It is found that the bias stress at 4.4 kV enlarges the interface state density at ${E}_{C}\ensuremath{-}0.60$ eV to ${E}_{C}\ensuremath{-}1.33$ eV by electron capturing. As a result, the negative interface charge is generated. As high-resolution transmission electron microscopy reveals the presence of excess carbon near the $\mathrm{Si}\mathrm{C}$ surface, DFT calculations are focused on carbon-related interface defects to clarify the atomic and electronic structures of the $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_{2}$ interface trap and assign them to negatively charged excess split-interstitial carbon at the interface. Furthermore, technical computer-aided-design simulation further proves that the negatively charged $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_{2}$ interface defect is the main cause for the observed ${V}_{\mathrm{BD}}$ degradation after the HTRB test, which leads to a strong electric field crowding effect. These results not only provide deep physical insights underlying ${V}_{\mathrm{BD}}$ degradation in HTRB-stressed high-voltage devices, but are also of significant importance in the optimizations of device structure and oxidation technology for $\mathrm{Si}\mathrm{C}/{\mathrm{Si}\mathrm{O}}_{2}$ interfaces in high-voltage $\mathrm{Si}\mathrm{C}$ devices.

Reaction of NO molecule at 4H-SiC/SiO2 interface: an ab initio study for the effect of NO annealing after dry oxidation

The reaction of the NO molecule at the 4H-SiC/SiO2 interface after dry oxidation is theoretically investigated on the basis of ab initio calculations. On the Si-face, the reaction of the NO molecule results in the dissociation of the C–C single bond and the formation of Si4–N bonds with the CO2 molecule. In contrast, the C=C double bond changes into the C–C bond with the formation of Si3–N bonds and CO2 desorption on the C-face. This C–C bond on the C-face can be removed by the incorporation of an additional NO molecule. For the resultant interface structures, no localized electronic states are generated around the energy gap of 4H-SiC, suggesting that the NO annealing process is crucial to reduce the carbon-related defects at the 4H-SiC/SiO2 interface. Furthermore, the reaction on the NO molecule at the interface with CO desorption after dry oxidation is discussed in order to clarify the behavior of the NO molecule during the annealing process at 4H-SiC/SiO2 interfaces.

Defects and Passivation Mechanism of the Suboxide Layers at SiO₂/4H-SiC (0001) Interface: A First-Principles Calculation

The origination of poor quality remains debating at the as-grown SiO2/4H-SiC (0001) interface during the thermal oxidation process. A low electron density layer (SiO $_{x \text {(0.3} \,{ ) is observed at the Si-terminated SiO2/4H-SiC (0001) interface in experiment, different from the previous reports on carbon-related defects. In this article, the SiO2/4H-SiC (0001) interface modeled with an interfacial SiO $_{x {(0.3} \, { suboxide layer (~1 nm) is systematically studied by first-principles calculations. According to the calculated electronic structures, the Si–Si antibonding structures in the SiO x layer are the dominating defects that cause obvious gap states, while the oxygen and silicon vacancy are not. The energy positions of the defect states are located at ~0.72 eV above the valence band maximum (VBM), which indicates the Si–Si defect is the potential influence factor for the p-channel Silicon carbide (SiC)-MOSFET. We further study several passivation schemes by introducing nitrogen, phosphorus, boron, and aluminum at the interface to analyze the passivation mechanism. According to the calculated results of the passivation models, nitrogen passivation is an effective method, fully removing the gap states from Si–Si defects by forming Si3N, while boron passivation works for vacancy defect at the suboxide layers. The theoretical results prove the dominated defect structures at the interfacial SiO x layer and suggest that a combination passivation strategy (nitrogen and boron) may be an effective method to further improve the SiO2/4H-SiC interface.

Single artificial atoms in silicon emitting at telecom wavelengths

Given its unrivaled potential of integration and scalability, silicon is likely to become a key platform for large-scale quantum technologies. Individual electron-encoded artificial atoms either formed by impurities or quantum dots have emerged as a promising solution for silicon-based integrated quantum circuits. However, single qubits featuring an optical interface needed for large-distance exchange of information have not yet been isolated in such a prevailing semiconductor. Here we show the isolation of single optically-active point defects in a commercial silicon-on-insulator wafer implanted with carbon atoms. These artificial atoms exhibit a bright, linearly polarized single-photon emission at telecom wavelengths suitable for long-distance propagation in optical fibers. Our results demonstrate that despite its small bandgap (~ 1.1 eV) a priori unfavorable towards such observation, silicon can accommodate point defects optically isolable at single scale, like in wide-bandgap semiconductors. This work opens numerous perspectives for silicon-based quantum technologies, from integrated quantum photonics to quantum communications and metrology.

Structure and role of carbon-related defects in yttrium aluminum garnet

This paper presents ab-initio calculations of composition and energy structure of carbon-related defects in carbon-doped yttrium aluminum garnet, Y3Al5O12. Defect formation energies are considered for isolated point defects and their combinations in case of O-poor atmosphere, which is inherent to crystal growth under Ar + CO gas conditions, and O-rich atmosphere used at post-growth annealing. The point CAl and complex carbon-related defects, such as Ci-VAl, and CAl-VO with lower formation energies should play a role of electron acceptors that compete with oxygen vacancies for electron capture. These results are discussed in conjunction with specific properties of carbon-doped garnets, in particular, no F-type center formation in annealed crystals, even under high irradiation doses.

Improved electrical properties of NO-nitrided SiC/SiO2 interface after electron irradiation**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0100601) and the National Natural Science Foundation of China (Grant Nos. 61674169 and 61974159).

Effective improvement in electrical properties of NO passivated SiC/SiO2 interface after being irradiated by electrons is demonstrated. The density of interface traps after being irradiated by 100-kGy electrons decreases by about one order of magnitude, specifically, from 3×1012 cm−2⋅eV−1 to 4×1011 cm−2⋅eV−1 at 0.2 eV below the conduction band of 4H-SiC without any degradation of electric breakdown field. Particularly, the results of x-ray photoelectron spectroscopy measurement show that the C–N bonds are generated near the interface after electron irradiation, indicating that the carbon-related defects are further reduced.

Electrically detected magnetic resonance study on interface defects at nitrided Si-face, a-face, and m-face 4H-SiC/SiO2 interfaces

We investigated interface defects formed on a-face and m-face 4H-SiC/SiO2 interfaces after interface nitridation by nitric oxide (NO) post-oxidation annealing (POA). Using electrically detected magnetic-resonance spectroscopy, we observed interface defects on these faces. The a- and m-face interface defects were found to be similar to a carbon-related interface defect (the PbC center) observed on the standard Si-face, but their amounts were significantly lower than those of the Si-face after the same NO POA. Such a reduction was correlated with a drastic increase in the field-effect mobility (80–90 cm2 V−1 s−1) of the a- and m-face metal-oxide-semiconductor field-effect transistors after NO POA. We also found that over-nitridation caused the formation of two types of nitrogen-related defects on the Si-face. These nitrogen-related defects resemble the K center (Si dangling-bond center) observed in Si3N4.

Electron-spin-resonance and electrically detected-magnetic-resonance characterization on PbC center in various 4H-SiC(0001)/SiO2 interfaces

We characterized an intrinsic interface defect, called the “PbC center,” formed at 4H-SiC(0001)/SiO2 interfaces by means of electron-spin-resonance (ESR) and electrically detected-magnetic-resonance (EDMR) spectroscopies. The formation of the PbC center was observed with a spin density of 3–4 × 1012 cm−2 after standard thermal oxidation. This center could be effectively removed by the NO post-oxidation-anneal (POA) process or ultra-high-temperature oxidation and could be passivated by H atoms via the H2 POA process. There was a clear correlation between the PbC center and field-effect mobility (μFE) of 4H-SiC(0001) metal–oxide–semiconductor field effect transistors (MOSFETs). The PbC center decreased μFE because this center acts as electron traps, reducing the free-carrier density in the inversion channel of 4H-SiC(0001) MOSFET. We also examined the counter doping effect of NO POA by introducing 15N impurities; however, the counter doping of 15N donors was not detectable by ESR (much lower than 2 × 1011 cm−2). Highly sensitive EDMR measurements revealed that the PbC center has two isotropic hyperfine (HF) interactions at 1.3 and 6.8 mT and suggested that its main 13C HF interaction should be larger than 14 mT. Based on the present experimental data, the origin of the PbC center was ascribed as a carbon-related interface defect that forms a C–H bond after hydrogen passivation. This feature is similar to that of the porous-PbC centers (carbon dangling-bond centers) found in porous-SiC/SiO2 systems. However, their HF signatures indicated that the PbC center at 4H-SiC(0001)/SiO2 interfaces and the porous-PbC centers in porous-SiC should be different centers associated with different wave functions.

Effects of carbon related defects on opto-electronic properties of β-Ga2O3: The first principle calculation

Calculations using the revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) reveal the opto-electronic effects of carbon in β-Ga2O3. Stable charged states with carbon dopant are proved by phonon calculations which can result in the deep levels in the band gap. Stronger distortion can happen around oxygen sites substituted by carbon than those at gallium sites leading to interactions changing. Temperature and oxygen pressure can have huge effect on the formation of substitutions and the electronic conductivity. As partial oxygen pressure increase, the substitution concentration increases, and decrease with temperature increasing. Electronic conductivity of substitution of carbon at oxygen sites can get enhanced hugely, and it increases with temperature increasing. Besides electronic properties, carbon substitution can also have great impact on optical properties. Red-infrared PL emissions can be induced with substitutions at gallium sites, and yellow and blue emissions can be achieved by substitutions with oxygen in different charged states. The work can effectively predict the impact of carbon substitution in β-Ga2O3 and be useful for improvement of device designs and optimizations.

Carbon pair defects in aluminum nitride

AlN bulk single crystals grown by the physical vapor transport method may be beneficially applied as substrates for deep ultraviolet light emitting devices or as a basic material for piezoelectric resonators operating at high temperatures. Identification of point defects which deteriorate the optical, electrical, and electromechanical properties of AlN crystals for such applications is the subject of the present work. Using Raman spectroscopy, two local vibrational modes (LVMs) were discovered at wave numbers of 1189 cm−1 and 1148 cm−1. By analyzing an AlN crystal intentionally enriched with the carbon isotope 13C, it is unambiguously shown that the two LVMs originate from two different, but in each case carbon-related defects. Furthermore, it is evidenced that the defect underlying the LVM at 1189 cm−1 contains exactly two carbon atoms. The tricarbon defect-related LVM reported earlier in an infrared absorption study is found to be Raman active at 1772 cm−1. The Raman scattering intensity of all three LVMs strongly depends on the photon energy of the exciting light what is interpreted as a resonance Raman effect. This allows linking the identified defects with their contribution to the strong, carbon-related ultraviolet absorption around 4.7 eV and proves that these defects introduce optically and electrically active deep levels in the bandgap of AlN.

Low resistive InGaN film grown by metalorganic chemical vapor deposition

Indium gallium nitride (InGaN) samples were grown on sapphire substrate with low temperature GaN buffer by metalorganic chemical vapor deposition (MOCVD) under varying growth conditions, such as temperature, pressure and ammonia flow. Although high indium composition is considered as indispensable parameter for sheet carrier concentration (ns) and low sheet resistance (Rs), our outcomes disclose that higher indium composition in InGaN film doesn't always have high ns and low Rs. Acceptor nature of carbon related defects, a major trapping sites for electrons, plays a crucial role to limit the carrier concentrations in the InGaN specially grown at low pressure, low temperature and low ammonia flow. Furthermore, study of metal contacts deposited on the grown samples by transmission line method (TLM) shows that carrier concentration is the most important factor to obtain low contact resistance. The measured contact resistance for the sample with 4.5× 1016 cm2 carrier concentration is 0.13 Ωmm which is among the lowest contact resistance for GaN based materials grown by MOCVD. This proves that the use of InGaN in source and drain contact region can surpassingly reduce contact resistance and significantly improve device performance of AlGaN/GaN device.

Proximity gettering technique using CH3O multielement molecular ion implantation for the reduction of the white spot defect density in CMOS image sensor

We have characterized CH3O ion-implanted epitaxial wafers in device fabrication processes. We confirmed that the CH3O ion-implanted wafers can reduce the density of white spot defects in CMOS image sensors. Evaluation of the CH3O ion-implanted region before and after device fabrication processes which included heat treatment and ion implantation, showed that two types of defects, namely stacking faults and carbon-related defects, exist in the CH3O ion implantation region getter the metallic impurities during device fabrication processes. In particular, it was clarified that stacking fault defects existing in the CH3O ion-implanted region are powerful gettering sinks for oxygen. Thus, it was considered that two types of gettering sink formed by CH3O ion implantation contribute to the reduction in the density of white spot defects. We believe that these characteristics can contribute to the improvement of fabrication processes for advanced CMOS image sensors.

Photoluminescence of silicon carbonitride thin films: The interdependence of post-deposition annealing and growth temperature

Silicon carbonitride (SiCN) thin films have recently emerged as the promising class of multifunctional materials because of their unique properties inherited from binary submatrices of silicon carbide, silicon nitride, and carbonitride. The luminescence mechanism of such ternary materials is complex and is influenced by slight changes in the film composition, deposition temperature, and post-deposition annealing temperature. This paper reports the temperature dependence of luminescent centres in SiCN thin films grown using the electron cyclotron plasma enhanced chemical vapour deposition (ECR PECVD) at different deposition temperatures, which are subsequently subjected to a wide range of annealing temperatures. With the increase of the deposition temperature, the photoluminescence (PL) sub-band positioned at around 1.8 eV (650 nm) intensifies. Following the annealing, the changes of the PL spectra at higher annealing temperatures are dominated by the emission related to this PL sub-band. The white luminescence of SiCN thin films observed under excitation by a 325 nm laser is explained in terms of the radiative defects at localized carbide- and nitride-related states. We assign the changes of the intensity and shape of the PL-sub bands mainly to the evolution of carbon-related defects. The observed variation of the PL emission of different samples was linked with the film restructuring and original growth temperature. We discuss how the post-deposition thermal annealing influences the emissions associated with the carbon and nitride states differently in the samples grown at different deposition temperatures, which have different compositions.

Performance improvement in 4H-SiC(0001) p-channel metal-oxide-semiconductor field-effect transistors with a gate oxide grown at ultrahigh temperature

We fabricated 4H-SiC(0001) p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs) with gate oxide formed at 1600 °C under an oxygen partial pressure of 0.3 kPa. The fabricated pMOSFETs showed superior performance, suggesting suppression of carbon-related defects at the SiO2/SiC interface. A further performance improvement was achieved by subsequently employing forming gas annealing at 1200 °C. In particular, a peak field-effect mobility of up to 13 cm2V−1 s−1, 11% of the intrinsic hole mobility, was demonstrated. Moreover, the transistors had a stable threshold voltage up to 200 °C and high immunity against bias-temperature stress, suggesting improved reliability.

Process-dependent mechanical and optical properties of nanostructured silicon carbonitride thin films

Amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were grown using electron cyclotron resonance chemical vapour deposition using a mixture of methane, nitrogen, and silane as precursors. The origin of the variation of macroscopic properties such as hardness (H), elastic modulus (E), photoluminescence (PL), and the optical band gap was investigated through their correlation with the microscopic features of a-SiCN:H thin films as a function of the process parameters, including the deposition temperature and methane gas flow rate. From a microstructural perspective, the thin films were investigated using x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, elastic recoil detection, transmission electron microscopy, and x-ray diffraction. It is verified that an increase of the substrate temperature resulted in the substitution of hydrogen atoms mainly by carbon atoms, causing the density of the silicon carbide-related structures to increase in the amorphous structure of the a-SiCN:H thin films. Hardness and elastic modulus were found to increase with the deposition temperature and decreased with an increase of the methane gas flow during the deposition, resulting in higher carbon content in the films. The observed changes are ascribed to the reduced density of the weak hydrogen terminated bonds and the variation of the relative bond density of Si–C to Si–N bonds. In addition, the thin films were depth profiled using a slow positron beam to investigate the role of vacancies. The observed increase of the positronium formation with increasing deposition temperature was found to correlate with the variation of PL, where an enhancement of the visible emission originating from carbon-related defects was observed. A set of optimized process parameters to fabricate a-SiCN:H thin films with improved visible emission and hardness properties is suggested.

Relationship between band-offset, gate leakage current, and interface states density at SiO2/4H-SiC (000-1) interface

We investigated the relationship between the band-offset, the gate leakage current, and the interface states density on SiO2/4H-SiC (000-1) structures via hard x-ray photoelectron spectroscopy and electrical measurements. From the observed band-offset, we found that conduction band offset (ΔEc) depended on the oxidation procedure, but valence band-offset (ΔEv) did not. In addition, ΔEv was larger than ΔEc, indicating that electron tunneling was more predominant than hole tunneling. SiO2 prepared by a dry oxidation procedure exhibited the highest gate leakage current onset and the highest interface states density and the largest ΔEc, whereas a wet oxidation procedure produced the lowest gate leakage current onset, the lowest interface states density, and the smallest ΔEc. Oxygen annealing after wet oxidation effectively increased gate leakage current onset, which increased the interface states density and the ΔEc. These results were related to hydrogen atoms and carbon related defects at the SiO2/4H-SiC (000-1) interface.