4H-SiC Epitaxial Layers Research Articles

TEM Investigation on High Dose Al Implanted 4H-SiC Epitaxial Layer

Within this work, the effect of high dose Al ion implantation on 4H-SiC epitaxial layer is displayed. Through TEM investigation it is demonstrated that the implanted surface is suitable as seed for subsequent epitaxial regrowth generating a crystal free of extended defects. In order to assess the defects within the projected range of the ion implanted area, High Angle Annular Dark Field STEM (HAADF-STEM) analyses were performed demonstrating the atomic arrangement of the lattice in correspondence of the dislocation loop and the deviation of the crystallographic planes of 4H-SiC, driven by stress relaxation, that determine the staircase configuration of the implant pattern. Further emphasis is given to the detailed analysis of the precipitates atomic structure, whose preferential localization is ascertained. Using Energy-Dispersive X-ray spectroscopy (EDS) analysis, the precipitate is finally established as Al crystal with an FCC structure.

Optimizing the chemical vapor deposition process of 4H–SiC epitaxial layer growth with machine-learning-assisted multiphysics simulations

This work addresses a novel technique for selecting the best process parameters for the 4H–SiC epitaxial layer in a horizontal hot-wall chemical vapor reactor using a transient multi-physical (thermal-fluid-chemical) simulation model and combined with a machine-learning model. An experiment was performed to validate the feasibility of the numerical model. Secondly, a single-factor analysis was conducted to investigate the effects of process parameters, including the deposition temperature, inlet-flow volume, rotational speed of the susceptor, and cavity pressure, on the quality of the 4H–SiC epitaxial layer. Finally, a machine learning algorithm, the ant colony optimization-back propagation neural network (ACO–BPNN), was employed to develop the input/output model and optimize process parameters for obtaining a high-quality epitaxial layer and reducing the optimization cycle and costs. Notably, the optimized process was validated by real experiments, where the error between calculation and experiment is 4.03 % for deposition rate and 0.49 % for coefficient of variation, respectively. The results highlight the model as reliable and lay the foundation for the CVD growth of the 4H–SiC epitaxial layer.

Observation of broad triangular Frank-type stacking faults and characterization of stacking faults with emission wavelengths below 430 nm in 4H–SiC epitaxial layers

Frank-type stacking faults in 4H–SiC epitaxial layers were investigated using room-temperature photoluminescence mapping and high-angle annular dark-field high-resolution scanning transmission electron microscopy. Remarkably, the intrinsic Frank-type stacking fault (5,2) and intrinsic multilayer Frank-type stacking fault (4,2), which have been reported to have elongated triangular shapes to date, were found to have broader triangular shape, which was typical shape of Shockley-type stacking faults. The shaping of stacking faults could be explained based on the directions of the partial dislocations bounding the stacking faults. Formation models for the intrinsic (5,2) and (4,2) stacking faults were proposed. In addition, we observed Frank-type stacking faults (3,2,2,3) and (3,3) and Shockley-type stacking faults (3,3,3,2,3,2) and (3,2,3,3,3,2), which were not reported before. All of these stacking faults were composed with the stacking numbers 2 and 3 in the Zhdanov notation and showed very close characteristic photoluminescence emission wavelengths of 427–428 nm. Formation models for the stacking faults of (3,2,2,3), (3,3), (3,3,3,2,3,2), and (3,2,3,3,3,2) were proposed, also. Based on the proposed formation models of (4,2) and (3,3) stacking faults, formation models for the Frank-type stacking faults (4,1) and (3,2) were discussed. Finally, we pointed out that the (4,1) Frank-type stacking fault should be the intrinsic not the extrinsic fault, and the (3,2) Frank-type stacking fault is the extrinsic fault.

Dual configuration of shallow acceptor levels in 4H-SiC

Acceptor dopants in 4H-SiC exhibit energy levels that are located deeper in the band gap than the thermal energy at room temperature (RT), resulting in incomplete ionization at RT. Therefore, a comprehensive understanding of the defect energetics and how the impurities are introduced into the material is imperative. Herein, we study impurity related defect levels in 4H-SiC epitaxial layers (epi-layers) grown by chemical vapor deposition (CVD) under various conditions using minority carrier transient spectroscopy (MCTS). We find two trap levels assigned to boron impurities, B and D, which are introduced to varying degrees depending on the growth conditions. A second acceptor level that was labeled X in the literature and attributed to impurity related defects is also observed. Importantly, both the B and X levels exhibit fine structure revealed by MCTS measurements. We attribute the fine structure to acceptor impurities at hexagonal and pseudo-cubic lattice sites in 4H-SiC, and tentatively assign the X peak to Al based on experimental findings and density functional theory calculations.

Impact of carbon injection in 4H-SiC on defect formation and minority carrier lifetime

Point defects in silicon carbide (SiC) can act as charge carrier traps and have a pronounced impact on material properties such as the mobility and carrier lifetime. Prominent among these traps is the carbon vacancy (VC) with a demonstrated detrimental effect on the minority carrier lifetime in 4H-SiC epitaxial layers. Hence, a variety of methods for VC removal have been proposed. Common to all is that they involve some sort of C-injection into the epi-layer that leads to annihilation of VC with C interstitials (Ci) via the reaction Ci＋ VC→0̸. However, many studies on injection of Ci for removal of VC do not take into account the potential effect of any additional defects that are formed as a result of excess C in the material, including electrically active defects that introduce energy levels in the SiC band gap. Herein, we study the formation and impact of carbon related defects that are introduced in 4H-SiC epi-layers by injection of excess carbon. C-injection is achieved by annealing 4H-SiC epi-layers covered by a graphitized photoresist known as a C-cap at 1250°C for different durations. The resulting appearance of defects in the samples is monitored using deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) measurements, which reveal the formation of both minority and majority carrier traps as a result of the C-injection. Intriguingly, the injected carbon is also found to interact with a perennially present impurity in 4H-SiC epi-layers and a potentially lifetime limiting defect, namely boron. Furthermore, we monitor the minority carrier lifetime as a function of C-injection time and depth from the surface using cross-sectional time-resolved photoluminescence (TRPL) measurements. Both the defect distributions and the minority carrier lifetime are found to depend strongly on the duration of the C-cap anneal, with a marked depth dependence being present for all samples studied. The moderate temperature C-cap annealing treatment is proposed as a method for enhancing the carrier lifetime in n-type 4H-SiC epi-layers for power device applications.

Defects in epitaxial 4H-SiC revealed by exciton recombination

In this work, we conducted an analysis of 4H-SiC epitaxial layer grown on two distinct 4H-SiC substrates (both 6 inches in diameter) using non-invasive techniques such as micro-Raman spectroscopy, steady-state absorption spectroscopy and time-resolved photoluminescence spectroscopy. We have shown that despite the doping homogeneity, confirmed by micro-Raman and steady-state absorption spectroscopy, the carrier lifetime, assessed by monitoring the excitonic band at 3.2 eV by time-resolved photoluminescence spectroscopy, depends on the position on the wafer. This variability is attributed to the presence of defects, such as impurities or point defects, which are not uniformly distributed on the epitaxial layer and that, in addition to extended defects, affect the charge carrier recombination. Additionally, it is found that interactions with the underlying substrate could contribute to these effects as evidenced in regions of the substrate characterized by differences of doping.

β -rays induced displacement damage on epitaxial 4H-SiC revealed by exciton recombination

One of the most interesting wide-bandgap semiconductor is 4H-SiC that has an indirect wide-bandgap of 3.3 eV. This material holds great potential to develop power devices that find applications in the field of high-voltage and high-temperature electronics and harsh environments. In this study, we employed complementary noninvasive characterization techniques, including micro-Raman, optical absorption, steady-state, and time-resolved photoluminescence spectroscopy, to investigate the characteristics of a 12 μm thick epitaxial layer of 4H-SiC grown on 4H-SiC. Furthermore, we explored the impact of ionizing radiation on this material, utilizing β-rays and two x-ray sources. The doses are in the range of 1–100 kGy for electrons with energy of 2.5 MeV, 16 kGy for the first x-ray source (an x-ray tube with a W target operating at an anode bias voltage of 28 kV), and 100 kGy for the second x-ray source (an x-ray tube with a W target operating at an anode bias voltage of 100 kV). When exposed to the electron beam, the excitonic band at 3.2 eV exhibits a reduction in its lifetime as the deposited dose increases. In particular, in samples characterized by a greater amount of native defects, both extended and point defects, this effect becomes evident at lower deposited doses. Conversely, in the samples subjected to x-ray irradiation, these effects are not observed. These findings indicate that electron beam irradiation triggers the formation of defects associated with atomic displacement. Ultimately, we have examined the impact of thermal treatments in air, ranging from 100 to 900 °C, to investigate the recovery characteristics of 4H-SiC.

Insights into the effect of susceptor rotational speed in CVD reactor on the quality of 4H-SiC epitaxial layer on homogeneous substrates

In this work, 4H-SiC homoepitaxial layers were grown on 4°off-axis substrates at different susceptor rotation speeds by using a hot-wall horizontal CVD reactor. The effect of different susceptor rotation speed on the quality of 4H-SiC epitaxial layers in terms of thickness, thickness uniformity, crystallinity, surface morphology and morphological defects was investigated via Fourier transform infrared spectroscopy (FTIR), high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM), confocal differential interference contrast microscopy (CDIC), ultra-violet photo-luminescence spectroscopy (UV-PL), scanning electron microscopy (SEM), and micro-Raman spectroscopy, respectively. A flow field simulation was performed to explain the impact of susceptor rotation speed on the film deposition. The FTIR results suggested that the susceptor rotation speed could be an important factor to adjust thickness uniformity and deposition rate. The XRD patterns showed that crystallinity was independent of the susceptor rotation speed. The surface morphology can be improved by changing the susceptor rotation speed. According to CDIC scans, the down-fall related defects were reduced through the increase in the susceptor rotation speed. The origin of down-fall related defects was interpreted by Raman spectroscopy and speculative models. To sum up, the susceptor rotation speed is a crucial factor in increasing growth rate and improving uniformity. Also, the faster susceptor rotation speed helps reduce the number of down-fall related defects in the hot-wall CVD reactor.

Nanoscale Infrared Spectroscopic Characterization of Extended Defects in 4H-Silicon Carbide.

Extended defects in wide-bandgap semiconductors have been widely investigated using techniques providing either spectroscopic or microscopic information. Nano-Fourier transform infrared spectroscopy (nano-FTIR) is a nondestructive characterization method combining FTIR with nanoscale spatial resolution (∼20 nm) and topographic information. Here, we demonstrate the capability of nano-FTIR for the characterization of extended defects in semiconductors by investigating an in-grown stacking fault (IGSF) present in a 4H-SiC epitaxial layer. We observe a local spectral shift of the mid-infrared near-field response, consistent with the identification of the defect stacking order as 3C-SiC (cubic) from comparative simulations based on the finite dipole model (FDM). This 3C-SiC IGSF contrasts with the more typical 8H-SiC IGSFs reported previously and is exemplary in showing that nanoscale spectroscopy with nano-FTIR can provide new insights into the properties of extended defects, the understanding of which is crucial for mitigating deleterious effects of such defects in alternative semiconductor materials and devices.

Optoelectronic and structural characterization of trapezoidal defects in 4H-SiC epilayers and the effect on MOSFET reliability

To this day, trapezoidal defects are found in clusters and high counts in wafers representing the industry standard in terms of material quality being produced. This study sheds light on the nature, origin, behavior, and impact of this defect on device yield and reliability. Trapezoidal defects in 4H-SiC epitaxial layers were investigated by photoluminescence (PL) imaging, scanning electron microscopy (SEM), cathodoluminescence spectrum imaging (CLSI), SEM electron beam induced current (EBIC) imaging, and by transmission electron microscopy (TEM) observation. The bar-shaped stacking faults were identified by the PL and CL measurements with a peak emission wavelength of 420 and 450 nm. An optoelectronic behavioral study based on the recombination enhanced dislocation glide mechanism revealed how expanding dislocations and stacking faults interact with each other. Combining the luminescence and microscopy results, the nature of the stacking faults was identified as being a combination of Shockley-type and Frank-type stacking faults. The TEM analysis showed that these defects originate from the substrate and the stacking sequences of some of the faults were determined as (…2, 4, 2…) and (…2, 3, 2…) in the Zhdanov's notation by high-resolution TEM. The origin of this defect is speculated based on our results and previous reports. The EBIC imaging showed that the high density of SFs in these towers is a strong site of carrier recombination, which presumably has an impact on the transfer characteristics of SiC devices. Furthermore, these defects have shown to impact metal oxide semiconductor field effect transistors electrical performance via an increase in the on-state resistance depending on the coverage percentage of the tower of defects in the active area of the device.

The Optimizing Effect of Nitrogen Flow Ratio on the Homoepitaxial Growth of 4H-SiC Layers

In this study, a 4H-SiC homoepitaxial layer was grown on a 150 mm 4° off-axis substrate using a horizontal hot-wall CVD reactor. The research aimed to investigate the impact of varying the C/Si ratio and temperature while also changing the N2 flow rate and N2 flow ratio on the growth rate (thickness), doping, surface roughness, and uniformity of the large-size 4H-SiC epitaxial layer. The results indicate that the growth rate and thickness uniformity of the film increases with an increase in the C/Si ratio. Additionally, adjusting the N2 flow rate in a timely manner based on the change in the C/Si ratio is crucial to achieving the best epitaxial layer doping concentration and uniformity. The study found that, as the temperature increases, the film thickness and thickness uniformity also increase. The maximum thickness recorded was 6.2 μm, while the minimum thickness uniformity was 1.44% at 1570 °C. Additionally, the surface roughness reached its lowest point at 0.81 nm at 1570 °C. To compensate for the difference in thickness and doping concentration caused by temperature distribution and uneven airflow, the N2 flow ratio was altered. In particular, at a growth temperature of 1570 °C, a N2 flow ratio of 1.78 can improve the uniformity of doping by 4.12%.

Formation and propagation mechanism of complex stacking fault in 180 μm thick 4H-SiC epitaxial layers

The formation mechanism and propagation behavior of a complex stacking fault (SF) in thick silicon carbide epitaxial layers was investigated using depth-resolved section X-ray topography and ultraviolet photoluminescence (UVPL) imaging. The SF is initiated in the substrate by deflection of a threading screw dislocation onto two basal half planes creating a Frank fault. This fault propagates into the epitaxial layer and interacts with several threading mixed dislocations, which creates both Frank and Shockley SFs on the same basal plane. UVPL images using different spectral filters show various SF type conversions occurring during propagation of the complex SF.

Suppression of In-Grown SF Formation and BPD Propagation in 4H-Sic Epitaxial Layer by Sublimating Sub-Surface Damage before the Growth

It is known that basal plane dislocations (BPDs) and in-grown stacking faults (IGSFs) in the 4H-SiC epitaxial layer cause severe electrical degradation in SiC devices. The impact that sub-surface damage (SSD) on a production-grade 4H-SiC substrate with CMP-finished surface causes on both the BPD propagation and IGSF formation during epitaxial growth was investigated by Dynamic AGE-ing (DA). The substrates etched by DA sublimation etching to adjust the residual amount of SSD maintaining a smooth surface without macro step bunching were grown to observe BPD and IGSF density. The obtained results showed that these defect densities decreased exponentially with increasing etching depth. We demonstrated SSD introduced by mechanical processing led BPDs and IGSFs to extend or introduce to the epitaxial layer.

The Influence of Extended Defects in 4H-SiC Epitaxial Layers on Gate Oxide Performance and Reliability

For the ongoing commercialization of power devices based on 4H-SiC, increasing the yield and improving the reliability of these devices is becoming more and more important. In this investigation, gate oxide on 4H-SiC was examined by time-zero dielectric breakdown (TZDB) and constant current stress (CCS) time-dependent dielectric breakdown (TDDB) method in order to get insights into the influence of the epitaxial defects on the gate oxide performance and reliability. For that purpose, MOS capacitors with different gate oxides have been fabricated. Crystal defects in the epitaxial layers have been detected and mapped by ultraviolet photoluminescence (UVPL) and interference contrast (DIC) imaging. The results of the comparison of electrical data and surface mapping data indicate a negative influence on the leakage current behavior for some extended epitaxial defects. Results from TDDB measurement indicated numerous extrinsic defects, which can be traced back to gate oxide processing conditions and defect densities.

Structural investigation of triangular defects in 4H-SiC epitaxial layers as nucleation source for bar shaped stacking faults (BSSFs)

The formation of recombination-induced bar shaped stacking faults (BSSFs) during forward voltage operation of SiC devices, can lead to increased voltage drop and enhanced device degradation. In this study, a triangular epitaxial defect is identified as a nucleation source for the growth of BSSF in forward-biased 4H-SiC p-n diode test structures. We performed low and high voltage current emission microscopy measurements in order to detect the position of BSSFs in the active area of the device and in-depth structural analysis to locate their nucleation source. It was found that basal plane dislocations that converted into threading screw dislocations, close to the surface of the epitaxial layer and included in the triangular defect, act as nucleation source for the BSSFs. Those BSSFs expand from the top towards the bottom of the epitaxial layer, which is a newly reported expansion mechanism compared to the already reported BSSFs growing from the substrate/epitaxial layer interface towards the epitaxial layer surface.

Dislocation-related leakage-current paths of 4H silicon carbide

Improving the quality of 4H silicon carbide (4H-SiC) epitaxial layers to reduce the leakage current of 4H-SiC based high-power devices is a long-standing issue in the development of 4H-SiC homoepitaxy. In this work, we compare the effect of different type of dislocations, and discriminate the effect of dislocation lines and dislocation-related pits on the leakage current of 4H-SiC by combining molten-KOH etching and the tunneling atomic force microscopy (TUNA) measurements. It is found that both the dislocation lines of threading dislocations (TDs) and the TD-related pits increase the reverse leakage current of 4H-SiC. The dislocation lines of TDs exert more significant effect on the reverse leakage current of 4H-SiC, which gives rise to the nonuniform distribution of reverse leakage current throughout the TD-related pits. Due to the different Burgers vectors of TDs, the effect of TDs on the reverse leakage current of 4H-SiC increases in the order to threading edge dislocation (TED), threading screw dislocation (TSD) and threading mixed dislocation (TMD). Basal plane dislocations (BPDs) are also found to slightly increase the reverse leakage current, with the leakage current mainly concentrated at the core of the BPD. Compared to the effect of TDs, the effect of BPDs on the reverse leakage current of 4H-SiC is negligible. Our work indicates that reducing the density of TDs, especially TMDs and TSDs, is key to improve the quality of 4H-SiC epitaxial layers and reduce the reverse leakage current of 4H-SiC based high -power devices.

Growth of 4H-SiC epitaxial layers at temperatures below 1500 °C using trichlorosilane (TCS)

TCS (SiHCl3) is widely used in the rapid growth of SiC. However, in the study of using TCS to grow 4H-SiC, the growth temperature is mainly concentrated in 1550–1650 °C, and the research on lower growth temperature is lacking. In this study, TCS was used as the silicon precursor and epitaxial layers were grown in the temperature range of 1350–1500 °C. The results show that the single crystal type 4H-SiC epitaxial layer with good quality can be obtained at the growth temperature of 1450 ℃.

Detection of fast neutrons using 4H-SiC radiation detectors

The particle detector based on a low concentration 4H-SiC epitaxial layer shows promising properties for the detection of various types of ionizing radiation. The wide bandgap energy of the 4H-SiC semiconductor material (3.23 eV at room temperature) allows the detector to operate reliably at room temperature and at elevated temperatures up to several hundred degrees Celsius. The 80 >m thick 4H-SiC epitaxial layer grown on a 350 >m 4H-SiC substrate was used for detector preparation. The active area of the detector was defined by a 3 mm Schottky contact. The current-voltage measurement shows a reverse current of less than 30 pA up to 1000 V. Capacitance-voltage measurements show that the epitaxial layer is completely depleted at bias voltages above 250 V. The detector has been tested with neutrons of energies from 370 keV up to 14.9 MeV. Neutrons were produced by three nuclear reactions p-T, d-D and d-T. The measured spectra clearly identified the elastic and inelastic scattering at silicon and carbon atoms of detector material.

Effects of defects in a 4H-SiC material on the breakdown behavior of a Schottky barrier diode

Substrate TSDs propagate into an epilayer and cause breakdown holes.

Fabrication and characterization of high-resolution 4H-SiC epitaxial radiation detectors for challenging reactor dosimetry environments

Reactor dosimetry environments require radiation detectors that are capable of operating at high temperatures in extremely high neutron and gamma-ray dose rates. Silicon carbide (SiC) is one of the most promising wide bandgap semiconductors (3.27 eV) for harsh environment applications due to its radiation hardness, high breakdown voltage, high electron saturation velocity, and high thermal conductivity. In this paper, we summarize the prospect of Schottky barrier radiation detectors, fabricated on highly crystalline low-defect detector-grade n-type 4H-SiC epitaxial layers with thickness ranging from 20 to 250 lm, for harsh environment applications. A comprehensive discussion on the characterization of the parameters that influence the energy resolution has been included. The usage of electrical and radiation spectroscopic measurements for characterizing the junction and rectification properties, minority carrier diffusion lengths, and energy resolution has been elaborated. Characterization of crucial factors that limit the energy resolution of the detectors such as charge trap centers using thermally stimulated transient techniques is summarized. Finally, the effect of neutron fluence on the performance of the 4H-SiC detectors is discussed.