4H-SiC Epitaxial Layers Research Articles

Characterization of 4H-SiC Epitaxial Layers and High-Resistivity Bulk Crystals for Radiation Detectors

Defect and electrical characterization of bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates is reported. Optical microscopy, electron beam induced current (EBIC) imaging, current-voltage ( $I$ – $V$ ) measurements, thermally stimulated current (TSC) spectroscopy (94 K–620 K), Hall effect, and van der Pauw measurements have been conducted for characterization and defect correlation studies. Both epitaxial layers exhibited relatively shallow levels related to Al, B, $L$ - and D-centers. Deep level centers in the n-type epitaxial layer peaked at ${\sim} 400$ K ( $E_{a} \sim 1.1$ eV), and ${\sim} 470$ K were correlated with $IL_{2}$ defect and 1.1 eV center in high-purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ${\sim} 290$ K ( $E_{a} = 0.82\hbox{--}0.87$ eV) that was attributed to $IL_{1}$ and HK2 centers, and at ${\sim} 525$ K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band-gap. Results of EBIC and optical microscopy showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layer. The $I$ – $V$ characteristics of the devices on SI epitaxial layer exhibited steps corresponding to the ultimate trap filling of deep centers. The high-temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep-level electron trap centers. The responsivity of the n-type epitaxial 4H-SiC detector in the soft X-ray energy range is reported for the first time.

Highly sensitive x-ray detectors in the low-energy range on n-type 4H-SiC epitaxial layers

Schottky diodes on n-type 4H-SiC epitaxial layers have been fabricated for low-energy x-ray detection. The detectors were highly sensitive to soft x-rays and showed improved response compared to the commercial SiC UV photodiodes. Current-voltage characteristics at 475 K showed low leakage current revealing the possibility of high temperature operation. The high quality of the epi-layer was confirmed by x-ray diffraction and chemical etching. Thermally stimulated current measurements performed at 94–550 K revealed low density of deep levels which may cause charge trapping. No charge trapping on detectors’ responsivity in the low x-ray energy was found.

Defects annealing in 4H–SiC epitaxial layer probed by low temperature photoluminescence

The thermal evolution of defects induced in 4H–SiC by multiple implantation of C ions was investigated by Low Temperature Photoluminescence in the temperature range 450–1000K. The photoluminescence spectra show sharp luminescent lines (alphabet lines) in the wavelength range 426–440nm upon irradiation and thermal treatment at 450K induces the appearance of a new line at 427nm (DI centre). The trend shown by the luminescence lines as a function of the temperature is quite complex. The alphabet lines intensity increases up to 850K, whereas at higher temperature decreases with an activation energy of 2.0eV, suggesting that the defect, responsible for these lines, is the Si-vacancy. The luminescence yield of DI centre is always increasing as a function of the temperature, with a higher slope from 750K, suggesting a correlation to the reconfiguration and to the annealing of point defects.

Study of Defects Generated by Standard- and Plasma-Implantation of Nitrogen Atoms in 4H-SiC Epitaxial Layers

A comparison is made between two kinds of Nitrogen implantations for the formation of thin n+p junctions in p-type Silicon Carbide (SiC) epitaxial layers. The standard beam ion implantations and PULSIONTM processes were performed at two distinct energies (700 eV and 7 keV) and the subsequent annealing was held at 1600°C in a resistive furnace specifically adapted to SiC material. Positron Annihilation Spectroscopy (PAS) and unpolarized infrared reflectivity (IR) measurements were carried out before and after the annealing, respecively. Despite the presence of deep vacancy clusters near the as-implanted sample surfaces, no extended defects were detected after the annealing. Plasma implanted samples prove to contain a lower point defect concentration than beam implanted samples. The concentration of these defects (resulting from plasma process) is higher in the plane parallel to the optical axis, which denotes an energy spreading alongside the dopant distribution.

Frank Partial Dislocation in 4H-SiC Epitaxial Layer by MSE Method

We have been trying to improve a quality of crystal, using the metastable solvent epitaxy (MSE) method, one of the solution methods. In MSE, a Frank-type fault is formed by conversion of a threading screw dislocation (TSD) in the substrate. To study the status of the growth, we performed plane-viewed TEM observation. Analysis of Burgers vectors in the TEM image showed Frank PDs (Partial Dislocations) which do not include a components and Frank PDs which include a components. The total Burgers vectors of Frank-type fault including a components are represented as b=a/3+c, which indicates some TSDs in the substrate also include a components.

Electron Beam Induced Current Observation of Dislocations in 4H-SiC Introduced by Mechanical Polishing

Electron beam induced current (EBIC) observations have been carried out to investigate the influence of mechanical polishing (MP) direction on the dislocations formation at the Si-face c(0001) of 4H-SiC epitaxial layers. Two opposite MP directions (defined by polish pad moving direction) have been compared, which are [11-20] off-cut directions along step-up and step-down, respectively. It has been found that high density of dislocations have been formed along the polish paths for the 8o off samples with polishing pad moved in step-up direction. By contrast, step-down polishing samples have shown no significant dislocation increase although shallow polish scratches were observed. Similar experiments have also been carried out for 4o off samples, showing step-up MPs introduced more dislocations than step-down ones. The results are discussed in terms of forces along the slip plane [11-20](0001) effectively exerted by the abrasive particles on the steps.

Tunneling Atomic Force Microscopy Studies on Surface Growth Pits Due to Dislocations in 4H-SiC Epitaxial Layers

The morphological and electrical properties of surface growth pits caused by dislocations in 4H-SiC epitaxial layers were characterized using tunneling atomic force microscopy. The characteristic distribution of the tip current between the metal-coated atomic force microscopy tip and the SiC was observed within a large surface growth pit caused by a threading screw dislocation. The current was highly localized inside the pit and occurred only on the inclined surface in the up-step direction near the pit bottom. This paper discusses the causes and possible mechanisms of the observed tip current distribution inside surface growth pits.

Ultraviolet Photoluminescence Imaging of Stacking Fault Contraction in 4H-SiC Epitaxial Layers

Shockley stacking fault (SSF) contraction in 4H-SiC was investigated, in-situ, under varying temperature and ultraviolet (UV) intensity. Contraction of single SSFs at room temperature was observed for the first time under low power UV excitation of 0.04 W/cm2. At temperatures above 150 °C, complete SSF contraction occurred for UV power at 0.2 W/cm2. In contrast to expansion, SSF contraction occurred in discrete jumps between pinning sites along existing C-core partials. Luminescence from the pinning sites suggest they may be local concentrations of point defects. Additionally, a change in the line direction of the Si-core partials by ~25o off the direction was observed.

A Theoretical and Experimental Comparison of 4H- and 6H-SiC MSM UV Photodetectors

This paper reports on fabrication and modeling of 4H- and 6H-SiC metal-semiconductor-metal (MSM) photodetectors (PDs). MSM PDs have been fabricated on 4H-SiC and 6H-SiC epitaxial layers, and their performance analyzed by MEDICI simulation and measurements. The simulations were also used to optimize the sensitivity by varying the width and spacing of the interdigitated electrodes. The fabricated PDs with 2 µm wide metal electrodes and 3 µm spacing between the electrodes exhibited, under UV illumination, a peak current to dark current ratio of 105 and 104 in 4H-SiC and 6H-SiC, respectively. The measured spectral responsivity of 6H-SiC PDs was higher compared to that of 4H-SiC PDs, with a cutoff at 407 nm compared to 384 nm in 4H-SiC PDs. Also the peak responsivity occurred at a shorter wavelength in 6H material. A high rejection ratio between the photocurrent and dark current was found in both cases. These experimental results were in agreement with simulation.

The Effect of Growth Conditions on Carrier Lifetime in N-Type 4H-SiC Epitaxial Layers

Carrier lifetime has been studied as a function of C/Si ratio and growth rate during epitaxial growth of n-type 4H-SiC using horizontal hot-wall CVD. Effort has been put on keeping all growth parameters constant with the exception of the parameter that is intended to vary. The carrier lifetime is found to decrease with increasing growth rate and the highest carrier lifetime is found for a C/Si ratio of 1. The surface roughness was correlated with epitaxial growth conditions with AFM analysis.

Use of Vanadium Doping for Compensated and Semi-Insulating SiC Epitaxial Layers for SiC Device Applications

Vanadium doping from SiCl4 source during epitaxial growth with chlorinated C and Si precursors was investigated as a mean of achieving compensated and semi-insulating epitaxial 4H-SiC layers for device applications. Thin epilayers were grown at 1450°C with a growth rate of ~6 μm/h. Experiments at 1600°C resulted in the growth rates ranging from 60 to 90 µm/h producing epilayers with thickness above 30 µm. V concentrations up to about 1017cm-3 were found safe for achieving defect-free epilayer surface morphology, however certain degradation of the crystalline quality was detected by XRD at V concentrations as low as 3-5x1015 cm-3. Controllable compensation of nitrogen donors with V acceptors provided low-doped and semi-insulating epitaxial layers. Mesa isolated PiN diodes with V-acceptor-compensated n- epilayers used as drift regions showed qualitatively normal forward- and reverse-bias behavior.

Design, Fabrication, Characterization, and Evaluation of X-ray Detectors Based on n-type 4H-SiC Epitaxial Layer

The Schottky barrier diode (SBD) radiation detectors on n-type 4H-SiC epitaxial layer have been designed, fabricated, and evaluated for low energy x-rays detection. The detectors were found to be highly sensitive to x-ray flux in the 50 eV to few keV range and showed significantly improved response compared to the commercial of-the-shelf (COTS) SiC UV photodiodes. Thermally stimulated current (TSC) measurements performed in wide temperature range (94 - 550 K) revealed low density of deep level centers in the epitaxial layer. The high quality of the epitaxial layer was confirmed by XRD rocking curve measurements and defect delineating chemical etching. The detectors fabricated on 4H-SiC epitaxial layers exhibited low leakage current at 475 K (< 1 nA at 200 V) revealing great possibility of high temperature operation.

Silicon carbide detector for laser-generated plasma radiation

We present the performance of a Silicon Carbide (SiC) detector in the acquisition of the radiation emitted by laser generated plasmas. The detector has been employed in time of flight (TOF) configuration within an experiment performed at the Prague Asterix Laser System (PALS). The detector is a 5mm2 area 100nm thick circular NiSiC Schottky junction on a high purity 4HSiC epitaxial layer 115μm thick. Current signals from the detector with amplitudes up to 1.6A have been measured, achieving voltage signals over 80V on a 50Ω load resistance with excellent signal to noise ratios. Resolution of few nanoseconds has been experimentally demonstrated in TOF measurements. The detector has operated at 250V DC bias under extreme operating conditions with no observable performance degradation.

Surface photovoltage and Auger electron spectromicroscopy studies of HfO2/SiO2/4H-SiC and HfO2/Al2O3/4H-SiC structures

The electronic and chemical properties of the interface region in the structures obtained by the passivation of epitaxial n-type 4H-SiC layers with bilayers consisting of a 5nm-thick SiO2 or Al2O3 buffer film and high-κ HfO2 layer were investigated. The main aim was to estimate the influence of the passivation approach on the interface effective charge density (Qeff) from the surface photovoltage (SPV) method and, in addition to determine the in-depth element distribution in the interface region from the Auger electron spectroscopy (AES) combined with Ar+ ion profiling. The structure HfO2/SiO2/4H-SiC exhibited slightly superior electronic properties in terms of Qeff (in the range of −1011qcm−2).

Excess Carrier Lifetime in p-Type 4H-SiC Epilayers with and without Low-Energy Electron Irradiation

We measured excess carrier lifetimes in as-grown and low-energy electron-irradiated p-type 4H-SiC epitaxial layers by the microwave photoconductivity decay method. The carrier lifetime increases with the excitation density in the epilayers. This dependence suggests that the dominant recombination center in the epilayers has a larger capture cross section for electrons than for holes. The low-energy electron irradiation reduces the carrier lifetime in the epilayers, and 1000 °C annealing increases the carrier lifetime in the electron-irradiated samples.

Temperature dependence of current conduction in semi-insulating 4H-SiC epitaxial layer

We have investigated temperature dependence (94 K–650 K) of current conduction in semi-insulating 4H-SiC epitaxial layer. The epitaxial layer was grown on highly doped n-type (0001) 4H-SiC substrate using chemical vapor deposition with dichlorosilane precursor. The current—voltage (I-V) characteristics exhibited steps at ∼1 V and ∼70 V that were attributed to the filling of deep level centers by injected electrons. Correlation of the I-V characteristics with the results of thermally stimulated current measurements showed that deep centers peaked at 242 K, 285 K, and 500 K, were responsible for the steps in the I-V characteristics. Slow processes of the injected carrier capture on traps resulted in the I-V characteristic with negative differential resistance.

Characterization of deep levels in n-type and semi-insulating 4H-SiC epitaxial layers by thermally stimulated current spectroscopy

We have investigated deep level centers in n-type and semi-insulating (SI) 4H-SiC epitaxial layers by thermally stimulated current (TSC) spectroscopy. The epitaxial layers were grown using chemical vapor deposition utilizing a dichlorosilane precursor. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D-centers. A deep level center with an activation energy of 1.1 eV, peaked at ∼400 K, was detected in the n-type epitaxial layer and correlated with the IL2 level and the 1.1 eV center in a high purity bulk SI 4H-SiC. The TSC spectra of the SI epitaxial layer was dominated by the peaks at 525–585 K that we attributed to intrinsic defects and their complexes with energy levels close to the middle of the bandgap. The TSC spectra of SI epitaxial layer exhibited peaks with different current polarity which is explained by thermoelectric effect and the built-in electric field reversal. The results of the transfer length method measurements of the SI epitaxial layer and the room temperature current-voltage (I-V) characteristics of both epitaxial layers are also reported.

Dependence of 4H-SiC Epitaxial Layer Quality on Growth Conditions with Wafer Size Corresponding to 150 mm

ABSTRACTHomoepitaxial growth on 4H-SiC Si-face substrates with sizes corresponding to 150 mm was carried out. The influence of growth conditions for uniformity and epitaxial defect density was investigated. A 150 mm size was realized by using two 76.2 mm wafers lined up in a radial direction. C/Si ratio is found to be a major parameter for controlling triangular defect density and the generation of step bunching. As a result, the surface morphology without bunched step structure and the triangular defect density with 0.5 cm−2 were obtained by decreasing C/Si ratio to 1.0 on the size corresponding to 150 mm. Under this condition, good carrier concentration and thickness uniformity of σ/mean =15.2 % and 1.7 % could be obtained.

Atomic layer deposited high-k Hf x Al(1−x)O as an alternative gate dielectric for 4H-SiC MIS based transistors

Hf x Al(1- x )O film grown by atomic layer deposition (ALD) on n-type 4H-SiC (0001) epitaxial layer has been studied. Measurements show that it has relatively high breakdown electric field of 16.4 MV/cm, high dielectric constant of 16.3 and low gate leakage current of 2.47×10-5 A/cm2 at E =5 MV/cm, which makes ALD Hf x Al(1- x )O a great potential candidate gate dielectric for 4H-SiC MIS based transistors.

Deep levels in iron doped n- and p-type 4H-SiC

Deep levels were detected in Fe-doped n- and p-type 4H-SiC using deep level transient spectroscopy (DLTS). One defect level (EC–0.39 eV) was detected in n-type material. DLTS spectra of p-type 4H-SiC show two dominant peaks (EV + 0.97 eV and EV + 1.46 eV). Secondary ion mass spectrometry measurements confirm the presence of Fe in both n- and p-type 4H-SiC epitaxial layers. The majority of the capture process for Fe1, Fe2, and Fe3 is multi-phonon emission assisted. These three detected peaks are suggested to be related to Fe.