Irradiated 4H-SiC Research Articles

Positron annihilation spectroscopy guided by two-component density functional theory calculations distinguishes irradiation-induced vacancy type in 4H-SiC

Based on two-component density functional theory integrated with the projector augmented-wave basis and incorporating both calculated and experimental data from Positron Annihilation Spectroscopy (PAS), this study introduces a novel method for identifying and analyzing specific types of vacancies when multiple types of vacancies are coexisting. This method was then tested on 4H-SiC irradiated by 300 keV C4+ ion beams. By calculating charge density to analyze positron annihilation lifetime spectroscopy and calculating wave functions to analyze slow positron-beam Doppler broadening spectroscopy, for the first time, silicon monovacancies (VSi) and carbon monovacancies (VC) in irradiated 4H-SiC were quantitatively detected separately, allowing them to be distinguished with high accuracy. In addition, a decreasing trend in the relative percentage of VC with increasing irradiation dose, consistent with that expected when irradiating with carbon ions, was also observed, illustrating both the effectiveness and potential of this method for broader applications in material defect analysis. This study not only addresses the challenges of identifying multiple coexisting vacancy types using PAS but also extends the applicability and depth of PAS in fields such as nuclear energy, aerospace, and semiconductors.

A DLTS analysis of alpha particle irradiated commercial 4H-SiC Schottky barrier diodes

4H-SiC Schottky barrier diodes (SBDs) were exposed to 5.4 MeV alpha particles with fluences of 2.55× 1011 cm−2, 5.11 × 1011 cm−2 and 7.67 × 1011 cm−2, respectively. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) was used to determine the structure and cross-sectional elemental composition of the device, while current–voltage and capacitance–voltage profiling were used to determine the primary electrical device-characteristics before and after irradiation. EDS revealed the presence of a <1 μm Ti layer, covered by 5 μm Al layer, in intimate contact with the SiC. Deep level transient spectroscopy (DLTS), performed in the temperature range 15–310 K, revealed one dominant peak around 50 K (Ec - 0.07 eV) in the unirradiated samples. This peak showed asymmetry suggesting that it may consist of more than one defect. Notably, Z1/2, the carbon vacancy-related (Vc) defect commonly observed in as-grown n-type 4H-SiC, was not detected in the unirradiated reference sample. After irradiation, a broad peak emerged around 280 K (at 80 Hz), most likely Z1/2, having a shoulder around 180 K, was detected. Increasing the fluence resulted in a corresponding decrease in the concentration of the electron trap observed around 50 K (Ec - 0.07 eV), while the concentration increases for the defect detected around 280 K. Notably, the concentration of Z1/2 was found to be strongly fluence dependent and linked to what we believe is a related to a silicon vacancy transition, labelled S1/2 in literature. Laplace DLTS confirmed that the peak observed around 50 K is composed of multiple defects.

Interaction of Stacking Faults with point/extended defects in Fe-He irradiated 6H-SiC

The study explored the microstructure evolution of 6H-SiC that underwent sequential iron and helium ion irradiation with energies of 2.5 MeV and 500 keV, respectively, at room temperature, followed by annealing at 1500°C for two hours. Following irradiation, the entire damaged layer underwent amorphization. However, during subsequent annealing, epitaxial recrystallization took place, resulting in the formation of defected polycrystalline 6H-SiC characterized by the presence of Fe-rich clusters, cavities, and stacking faults. Fe-rich cavities were found to predominantly form at the edges of the stacking faults, as revealed by XTEM. The interaction of microstructural defects is further investigated via first-principles calculations. The periphery of the stacking faults has been identified as the primary location for the emergence of vacancy clusters, serving as favorable sites for the accumulation of point defects, including Fe atoms. This behavior can be attributed to the combined effects of mechanical and electronic energy relaxation mechanisms. Mechanically, the presence of stacking faults allows for the release of elastic energy that had been stored at the boundary. Electronically, the energy relaxation arises from the saturation of C- and Si-dangling bonds. Both of these processes contribute to the observed behavior, highlighting the intricate interplay between mechanical and electronic factors in the system. The low point defect migration energy barriers in the vicinity of the stacking faults promise high recombination, which can limit cavity growth and enhance radiation resistance. The study not only offers valuable insights into the mechanism of cavity/stacking faults interaction, contributing to a better understanding of radiation damage in 6H-SiC but also demonstrates that 6H-SiC material containing stacking faults could serve as a viable alternative to 3C-SiC for nuclear application.

Atomic-scale revealing defects in ion irradiated 4H-SiC

Here we identified atomic-scale defects in ion irradiated 4H-SiC, by integrated differential phase contrast technique equipped on aberration-corrected transmission electron microscopy and Raman scattering spectrometer. In 21 MeV Ni+ ions irradiated 4H-SiC, in which electronic energy loss is dominant almost at the whole penetration depth, Si- and C-related vacancies and interstitials are successfully imaged. In 900 keV Si+ ions irradiated 4H-SiC, however, the electronic energy loss plays the key role near the surface, and the nuclear and electronic energy loss are comparable at the depth of the peak damaged zone. As a result, 900 keV Si+ ions produce more point defects, atomic-scale in-plane and out-of-plane lattice distortions, and plenty of dislocations at the depth of damage peak. Meantime, these imaged point defects may provide direct detailed information for the vibration modes of defects in the Raman spectrum of the damaged 4H-SiC. This study illustrates the compelling intrinsic nature of point defects induced by energetic ions, providing important fundamental information for nuclear and electronic applications.

Photoluminescence evidence for silicon Frenkel defects in electron irradiated 4H SiC

The nature of defects in 4H SiC was studied by means of low temperature photoluminescence before and after energy-controlled electron irradiation. Analysis of experimental data from irradiation at energies above and below the Si displacement energy together with subsequent annealing leads to the conclusion that Si Frenkel defects have been detected experimentally in this material. Reasons why these are formed in some cases rather than carbon vacancy carbon antisite pairs are explored.

Research on the surface damage of Si+ and H+ co-implanted 6H-SiC before and after annealing

6 MeV Si ions with fluences of 2 × 1016 ions/cm2 and 400 keV H ions with fluences of 5 × 1016 ions/cm2 implantation experiments were performed for single-crystal 6H-SiC samples. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the evolutions of the surface damage and internal microstructure of these irradiated 6H-SiC samples before and after annealing. Bumps were observed on the surface of the Si+ ions irradiated samples after annealing. The internal damage caused by irradiation recovered well after annealing, and the peak damage zone disappeared after the 1200 °C annealing process. Circular bubbles occurred on the surface of the H+ ions irradiated samples, which peeled off after annealing. The morphology and distribution of bubbles and spalling on the Si+-H+ dual ion beam irradiated sample surface were more irregular as the more severe damage induced by the pre-implanted Si ions hindered the aggregation and migration of hydrogen atoms.

Monitoring of the recovery of ion-damaged 4H-SiC with in situ synchrotron X-ray diffraction as a tool for strain-engineering

In situ thermal annealing (673-1273 K) during X-ray diffraction synchrotron measurements was performed to monitor the strain level as a proxy to follow the recovery of 300 keV Ar ion irradiated 4H-SiC single crystals. Results show that, when exposed to Ar ions at a fluence of 6.7 $$\times$$ 10 $$^{14}$$ ions/cm $$^2$$ (0.7 dpa), the material suffers a maximum strain of 12 $$\%$$ that reduces to 2 $$\%$$ after the final anneal at 1273 K. In the same time, the disorder derived from the XRD data also demonstrates a thermal recovery of the crystalline structure. Hence, this work presents ion irradiation as a means to induce specific crystalline order and depth-controlled strain states within a few 100 s of nm window in 4H-SiC.

Distinguishing the EH1 and S1 defects in n-type 4H-SiC by Laplace DLTS

We report on the low-energy electron and fast neutron irradiated 4H-SiC studied by deep-level transient spectroscopy (DLTS) and Laplace DLTS. Irradiations introduced two defects, E c −0.4 eV and E c −0.7 eV. They were previously assigned to carbon interstitial (Ci) labeled as EH1/3 and silicon-vacancy (V Si) labeled as S1/2, for the low-energy electron and fast neutron irradiation, respectively. This work demonstrates how Laplace DLTS can be used as a useful tool for distinguishing the EH1 and S1 defects. We show that EH1 consists of a single emission line arising from the Ci(h), while S1 has two emission lines arising from the V Si(h) and V Si(k) lattice sites.

Charge collection efficiency study on neutron-irradiated planar silicon carbide diodes via UV-TCT

The material properties of silicon-carbide (SiC) make it a promising candidate for application as a particle detector at high beam rates. In comparison to silicon (Si), the increase in charge carrier saturation velocity and breakdown voltage allows for high intrinsic time resolution while mitigating pile-ups. A larger bandgap and higher atomic displacement threshold energy suppresses dark current and potentially improves radiation hardness, respectively. In addition to the lower susceptibility to temperature variations, it allows the operation of irradiated devices at room temperature and daylight illumination. Although already known for several decades, recent developments in industrial power electronics made SiC more accessible as a potential particle detector. Repeatability, large amplitudes and timing possibilities of signal pulses produced by the ultraviolet transient current technique (UV-TCT) allow for precise performance comparison of differing samples, while efficiently monitoring the influence of alternating external conditions. We present measurement results on the performance of neutron irradiated 4HSiC p-on-n planar diodes using such a setup. Dark current levels remain in the nA range for all fluences (5×1014neq/cm2−1×1016neq/cm2), while charge collection efficiency decreases with irradiation fluence, partially compensated when operating samples at reverse voltage conditions far above full depletion.

Threshold voltage instability and hysteresis in gamma-rays irradiated 4H-SiC junction field effect transistors

High dose irradiation effects of gamma-rays up to 17 MGy (H2O) on 4H-SiC junction field effect transistors (JFETs) were investigated. Due to the irradiation, gradual positive threshold voltage (Vth) shift as high as 0.5 V and continuous decrease in transconductance gm were observed. In addition, Vth instability and hysteresis appeared for the irradiated JFETs when the gate voltage (VG) sweep direction, sweep interval, i.e., averaged sweeping rate, sweep range, and delay time were changed. Increase of VG interval attributed to positive Vth shift for both forward and reverse directions, whereas narrowing of sweep range and increase of delay time resulting in a more noticeable negative shift of Vth for the reverse direction. Such Vth hysteresis indicates that capture and release of carriers predominantly took place via hole traps formed around the gate region due to high dose gamma-ray irradiation.

100 Mev Ag and 25 Kev He Induced Defects in 4H-SiC

We demonstrate the formation of radiative defects, known as color centres, in 100 MeV Ag and 25 keV He irradiated semi-insulating 4H-SiC using low-temperature (~77 K) photoluminescence (PL). In PL, the luminescence resulting from radiative recombination of photo-excited carriers (electrons and holes) is studied. In our work, a 266 nm deep UV excitation source is utilized for performing PL measurements. In the range, 850-1150 nm, the photoluminescent peaks ascribed to silicon vacancies (V Si) and unknown defect-related bands (UD3 and UD4) are observed after both ion irradiations. It has also been observed that till an optimum irradiation fluence there is an enhancement of photoluminescence from the ion irradiation-induced defects but further increase in irradiation fluence results in accumulation of photo-absorbing defects, providing pathways for non-radiative processes. In addition, ion irradiation has been shown to suppress the undesirable luminescence in the range 300-600 nm caused by the intrinsic defects in as-grown 4H-SiC.

IR photothermal and spectroscopic analysis of proton-irradiated 4H-SiC

The effect of the proton irradiation dose on thermal transport anisotropy, free carrier density, and defect formation in 4H-SiC is studied by thermal wave scattering and infrared spectroscopy. Thermal waves are generated by infrared laser pulses, and the in-plane and cross-plane thermal diffusion length are measured using the deflection of a laser probe beam due to the mirage effect. The effect of proton irradiation dose on in-plane and cross-plane thermal diffusivity is measured as a function of depth. Proton irradiation is shown to cause significant damage primarily in the direction perpendicular to the sample surface. Irradiation-induced free carriers contributing to heat transport in the sample plane are revealed by Kramers-Kronig analysis of the infrared reflectivity spectra of the studied samples. Measurements of thermal diffusion lengths in the irradiated samples are converted to depth profiles of defect density. It is shown that the highly damaged zone in irradiated 4H-SiC thickens and approaches the proton reaching depth with increasing the irradiation dose. Thermal wave scattering complemented by infrared spectroscopy is proposed as an effective approach for the directional analysis of irradiation-induced changes in physical and structural properties of materials.

Anisotropic defect distribution in He+-irradiated 4H-SiC: Effect of stress on defect distribution

Irradiation-induced anisotropic swelling in hexagonal α-SiC is known to degrade the mechanical properties of SiC; however, the associated physical mechanism and microstructural process remain insufficiently understood. In this study, an anisotropic swelling condition where the surface normal direction was allowed to freely expand with constraint in the lateral direction was introduced in 4H-SiC using selected-area He+ irradiation, and the internal defect distribution was investigated using transmission electron microscopy (TEM) and advanced scanning TEM. The defect distribution was compared to that in non-selected-area He+-irradiated 4H-SiC and electron-irradiated TEM-foil 4H-SiC. An anisotropic defect distribution was observed in the selected-area He+-ion-irradiated 4H-SiC, with interstitial defects preferentially redistributed in the surface normal direction ([0004]) and negative volume defects (such as vacancies and/or carbon antisite defects) dominantly located in the lateral directions ([112¯0] and [101¯0]). This anisotropy of the defect distribution was substantially lower in the non-selected-area He+-irradiated and electron-irradiated samples. The stress condition in the three samples was also measured and analyzed. In the selected-area He+-irradiated 4H-SiC, compressive stress was introduced in the lateral directions (([101¯0] and [112¯0])), with little stress introduced in the surface normal direction ([0004]); this stress condition was introduced at the beginning of ion irradiation. The compressive stress likely inhibits the formation of interstitial defects in the lateral directions, enhancing the anisotropy of the defect distribution in SiC.

Anisotropic Defect Distribution in He+-Irradiated 4h-Sic: Effect of Stress on Defect Distribution

Irradiation-induced anisotropic swelling in hexagonal α-SiC is known to degrade the mechanical properties of SiC; however, the associated physical mechanism and microstructural process remain insufficiently understood. In this study, an anisotropic swelling condition where the surface normal direction was allowed to freely expand with constraint in the lateral direction was introduced in 4H-SiC using selected-area He + irradiation, and the internal defect distribution was investigated using transmission electron microscopy (TEM) and advanced scanning TEM. The defect distribution was compared to that in non-selected-area He + -irradiated 4H-SiC and electron-irradiated TEM-foil 4H-SiC. An anisotropic defect distribution was observed in the selected-area He + -ion-irradiated 4H-SiC, with interstitial defects preferentially redistributed in the surface normal direction ([0004]) and negative volume defects (such as vacancies and/or carbon antisite defects) dominantly located in the lateral directions ([ ] and [ ]). This anisotropy of the defect distribution was substantially lower in the non-selected-area He + -irradiated and electron-irradiated samples. The stress condition in the three samples was also measured and analyzed. In the selected-area He + -irradiated 4H-SiC, compressive stress was introduced in the lateral directions (([ ] and [ ])), with little stress introduced in the surface normal direction ([0004]); this stress condition was introduced at the beginning of ion irradiation. The compressive stress likely inhibits the formation of interstitial defects in the lateral directions, enhancing the anisotropy of the defect distribution in SiC.

Optical confocal spectroscopy of SiC and AlN interfaces using Raman scattering and Optically Detected Magnetic Resonance

The thesis contains the investigation of transition layer between AlN and SiC and the investigation of AlN Raman peaks evolution with distance till interface. Moreover, ODMR spatial research of nonuniformly irradiated 4H-SiC was performed together with PL spatial spectroscopy.

Non-destructive evaluation of the strain distribution in selected-area He+ ion irradiated 4H-SiC

Residual strain in silicon carbide (SiC) greatly affects its physical and chemical properties and thus the performance of SiC-based devices. Herein, the detailed strain distribution in selected-area He+ ion-irradiated 4H-SiC was evaluated using the non-destructive techniques of electron backscattering diffraction and confocal Raman microscopy (CRM). In addition to the strain introduced in the irradiated area, excessive strain induced by irradiation-induced swelling also extended into the surrounding substrate. Furthermore, great compressive strain was concentrated around the interface between the irradiated and unirradiated areas. In the strain-introduced substrate, an A1(LO)/A1(LOPC) peak variation was detected by CRM, suggesting a variation of the carrier density.

Pinning Effect on Fermi Level in 4H-SiC Schottky Diode Caused by 40-MeV Si Ions

The changes in the electrical properties of 4H-SiC Schottky diodes caused by 40-MeV Si ions with different fluences are investigated. The irradiation fluences are selected as $1.0 \times 10^{9}$ , $8.9\times 10^{9}$ , and $1.5 \times 10^{10}$ cm−2, respectively. The diodes were characterized using deep-level transient spectroscopy (DLTS) and Keithley 4200-SCS semiconductor characterization system. The TRIM software is used to estimate the carbon distribution of vacancies in the irradiated 4H-SiC diodes by 40-MeV Si ions. As irradiation fluence increases, the ideality factors increase from 1.01 to 1.09, while the Schottky barrier height decreases from 1.16 to 1.09 eV. The free-carrier concentration $N_{d}$ decreases with the increasing fluence from $1.0 \times 10^{9}$ to $1.5 \times 10^{10}$ cm−2. The free-carrier removal rate is estimated to be $2.4 \times 10^{6}$ cm−1. Experimental and simulated results show that 40-MeV Si ions produce high concentrations of displacement defects ( $Z_{1}/Z_{2}$ center) in the 4H-SiC, leading to the Fermi level to be pinned at the deep levels.

Irradiation effect of primary knock-on atoms on conductivity compensation in N-type 4H-SiC Schottky diode under various irradiations

The electrical properties, defects and conductivity recombination mechanism of electron and ion irradiations were investigated in n-type 4H-SiC Schottky diodes. The incident particles were selected as 1 MeV electrons, 25 MeV C and 40 MeV Si ions, respectively. The defects in 4H-SiC Schottky diodes were characterized using deep level transient spectroscopy (DLTS). The primary knock-on atoms (PKAs) distribution in the irradiated 4H-SiC is calculated by SRIM code. According to the experimental results, there are significant differences between the defects produced by electrons and heavy ions. The complex peak combination can characterize the production rate of the cascades. The production rate of the cascades induced by heavy ions is much higher than that induced by electrons. Different conductivity compensation brought by different defects is the major reason for the significant difference in carrier removal rate.

Characterization of 167 MeV Xe ion irradiated n-type 4H-SiC

The nitrogen-doped, n-type 4H-SiC with 6 μm thick epitaxial layer was irradiated at the perpendicular incidence and room temperature by 167 MeV Xe+26 ions to a fluence of 5 × 1012 cm−2. The Monte Carlo simulation code, Stopping and Range of Ions in Matter (SRIM) was used to simulate the Xe26+ ions irradiated in SiC. The Rutherford backscattering spectrometry (RBS) analysis with 3.5 MeV He++ ions which have a projected range of ~9 μm in SiC (SRIM prediction) did not detect implanted Xe. Raman spectroscopy and imaging depth profiles clearly showed three distinct parts of the swift heavy ion (SHI)-irradiated 4H-SiC, i.e. 5.6 μm irradiated epitaxial layer, 6.9 μm damaged irradiated layer and non-irradiated 4H-SiC bulk. Raman spectroscopy showed new broad bands of crystalline SiSi and distorted SiC in the damaged irradiated layer. The new SiSi bands were attributed to the ordering in the SiSi homonuclear bonds without crystal amorphization. The scanning near-field optical microscopy (SNOM) results of the SHI-irradiated 4H-SiC sample displayed a very low signal counts (transmission) compared to the virgin sample, and this difference in the optical absorption could be correlated with the defects energy levels created within the bandgap of the sample upon irradiation.

Formation of Ohmic contacts on laser irradiated n-type 6H-SiC without thermal annealing

In this work, KrF excimer laser irradiation of n-type SiC is used to form Ohmic contacts at the interfaces between the irradiated SiC and various types of metals with different work functions without subsequent thermal annealing. Ohmic contacts are formed between laser-treated 6H-SiC and Ti at a laser fluence of 0.7 J/cm2. Moreover, in the fluence range of 0.7–1.3 J/cm2, Ohmic characteristics are also observed between irradiated 6H-SiC and Au, which is a representative inert metal. The laser-induced heavy doping effect reduces the thickness of the Schottky barrier between the metal and SiC, and the formation of graphene sheets on the irradiated SiC surface reduces the barrier height, resulting in the direct formation of Ohmic contacts. Our findings thus demonstrate the potential of this laser treatment method to achieve Ohmic contacts between n-type SiC and a broad range of metal electrodes without requiring high-temperature annealing.