4H-SiC Epitaxial Layers Research Articles

Identification of dislocations in 4H-SiC epitaxial layers and substrates using photoluminescence imaging

Dislocations in n- and p-type substrates as well as in epitaxial layers (epilayers) were clearly identified using a photoluminescence (PL) imaging technique. Dislocations in epilayers show large/small bright spots or lines in infrared PL images, which correspond to threading screw/edge dislocations (TSDs/TEDs) or basal plane dislocations (BPDs), respectively. In contrast, dislocations in substrates exhibit large/small dark spots or dark lines in infrared PL images, corresponding to TSDs/TEDs or BPDs, respectively. These different features (bright/dark contrast) of dislocations may originate from the different densities of point defects or impurities.

A Comparison of the Microwave Photoconductivity Decay and Open-Circuit Voltage Decay Lifetime Measurement Techniques for Lifetime-Enhanced 4H-SiC Epilayers

This work compares the optical microwave photoconductivity decay (μPCD) and electrical open-circuit voltage decay (OCVD) techniques for measuring the ambipolar carrier lifetime in 4H-silicon carbide (4H-SiC) epitaxial layers. Lifetime measurements were carried out by fabricating P+/intrinsic/N+ (PiN) diodes on 100-μm-thick, 1 × 1014 cm−3 to 4.5 × 1014 cm−3 doped N-type 4 H-SiC epilayers, and measuring the lifetime optically using μPCD prior to metallization, then electrically using OCVD after contact deposition. Both as-grown epilayers as well as epilayers with improved lifetime (via thermal oxidation) were measured using both techniques. The observed ambipolar lifetime was improved from 1.4 μs on an unenhanced wafer to 4 μs on a wafer enhanced through the oxidation process as measured by μPCD. Little difference was observed between the μPCD and OCVD measurements on the unenhanced wafer; the ambipolar lifetime on the enhanced wafer measured by OCVD was approximately 5.5 μs, or 1.5 μs higher than the μPCD measurement. Continuous evaluation of the OCVD transient waveform was necessary due to the high lifetime in the enhanced wafer; shunt resistances included to discharge the P+/N junction capacitance were found to damp the OCVD response and yield low values for the measured lifetime. Simulation of the μPCD measurement including various surface recombination conditions yielded a good match to experimentally observed μPCD measurements for high values of the surface recombination velocity. The OCVD lifetime measurement technique is expected to yield measured lifetime values closer to the physical value due to its independence from surface conditions, provided that the experimental conditions are appropriately chosen.

On-axis 4H SiC Epitaxial Layers for High Power Applications

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Low Energy X-Ray and $\gamma$-Ray Detectors Fabricated on n-Type 4H-SiC Epitaxial Layer

Schottky barrier diode (SBD) radiation detectors have been fabricated on n-type 4H-SiC epitaxial layers and evaluated for low energy x- and γ-rays detection. The detectors were found to be highly sensitive to soft x-rays in the 50 eV to few keV range and showed 2.1 % energy resolution for 59.6 keV gamma rays. The response to soft x-rays for these detectors was significantly higher than that of commercial off-the-shelf (COTS) SiC UV photodiodes. The devices have been characterized by current-voltage (I-V) measurements in the 94-700 K range, thermally stimulated current (TSC) spectroscopy, x-ray diffraction (XRD) rocking curve measurements, and defect delineating chemical etching. I-V characteristics of the detectors at 500 K showed low leakage current ( nA at 200 V) revealing a possibility of high temperature operation. The XRD rocking curve measurements revealed high quality of the epitaxial layer exhibiting a full width at half maximum (FWHM) of the rocking curve ~3.6 arc sec. TSC studies in a wide range of temperature (94-550 K) revealed presence of relatively shallow levels ( ~0.25 eV) in the epi bulk with a density ~7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> related to Al and B impurities and deeper levels located near the metal-semiconductor interface.

High resolution alpha particle detection using 4H–SiC epitaxial layers: Fabrication, characterization, and noise analysis

In this article we report the fabrication and characterization of large area, room-temperature operable and very high resolution Schottky barrier detectors for alpha particles using 20μm thick n-type 4H–SiC epitaxial layers. Schottky barriers were fabricated by depositing circular nickel contacts of ~11mm2 area on the 4H–SiC epitaxial layers. Room temperature current–voltage measurements revealed very high Schottky barrier height of 1.6eV and extremely low leakage current of 3.5pA at an operating reverse bias of −90V. We also report an energy resolution of 0.29%, which is the best resolution obtained so far for uncollimated 5.48MeV alpha particles in 4H–SiC epitaxial detectors with such a large area. Very low micropipe density (<1cm−2) and low effective doping concentration (2.4×1014cm−3) in the epilayer helped to achieve a high resolution even with the large detector area and a broad source. A diffusion length of ~18.6μm for holes has been determined in these detectors following a calculation based on a drift-diffusion model. A noise analysis in terms of equivalent noise charge revealed that the white series noise due to the detector capacitance has substantial effect on their spectroscopic performance.

Reduction of structural defects in thick 4H-SiC epitaxial layers grown on 4° off-axis substrates

By carefully controlling the surface chemistry of the chemical vapor deposition process for silicon carbide (SiC), 100 μm thick epitaxial layers with excellent morphology were grown on 4° off-axis SiC substrates at growth rates exceeding 100 μm/h. In order to reduce the formation of step bunching and structural defects, mainly triangular defects, the effect of varying parameters such as growth temperature, C/Si ratio, Cl/Si ratio, Si/H2 ratio, and in situ pre-growth surface etching time are studied. It was found that an in-situ pre growth etch at growth temperature and pressure using 0.6% HCl in hydrogen for 12 min reduced the structural defects by etching preferentially on surface damages of the substrate surface. By then applying a slightly lower growth temperature of 1575 °C, a C/Si ratio of 0.8, and a Cl/Si ratio of 5, 100 μm thick, step-bunch free epitaxial layer with a minimum triangular defect density and excellent morphology could be grown, thus enabling SiC power device structures to be grown on 4° off axis SiC substrates.

Very low specific contact resistance measurements made on a highly p-type doped 4H-SiC layer selectively grown by vapor-liquid-solid transport

This work reports on the performances of ohmic contacts fabricated on highly p-type doped 4H-SiC epitaxial layer selectively grown by vapor-liquid-solid transport. Due to the very high doping level obtained, the contacts have an ohmic behavior even without any annealing process. Upon variation of annealing temperatures, it was shown that both 500 and 800 °C annealing temperature lead to a minimum value of the Specific Contact Resistance (SCR) down to 1.3×10−6 Ω⋅cm2. However, a large variation of the minimum SCR values has been observed (up to 4×10−4 Ω⋅cm2). Possible sources of this fluctuation have been also discussed in this paper.

Characterization of Bar-Shaped Stacking Faults in 4H-SiC Epitaxial Layers by High-Resolution Transmission Electron Microscopy

Stacking faults in 4H-SiC epitaxial layers were investigated by micro-photoluminescence (PL) mapping at room temperature and by transmission electron microscopy (TEM) observation. Stacking faults with a peak emission wavelength of 420 nm were identified by the PL measurements with a He–Cd laser of 325 nm excitation wavelength. The shape of the stacking faults was revealed to be bar-shape by the micro-PL intensity mapping at emission wavelength of 420 nm. The stacking sequences of the bar-shaped stacking faults were determined as (...2, 3, 2...), (...2, 3, 3, 3, 2...), and (...2, 3, 3, 3, 3, 3, 2...) in the Zhdanov's notation by high-resolution TEM.

Influence of Contact Metallisation on the High Temperature Characteristics of High-κ Dielectrics

In this work SiC-based MIS capacitors have been fabricated with different contact/high-k dielectric combinations and the temperature dependence of the characteristics have been examined in an N2 ambient at temperatures between 323K and 673K. The structures utilise either a Pt or Pd catalytic gate contact and a TiO2 or HfO2 high-k dielectric, all of which are grown on a thin SiO2 layer, thermally grown on the Si face of a 4H SiC epitaxial layer. The MIS capacitors have been studied in an N2 ambient between 323K and 673K and observations show that VFB reduces with increasing temperature. The majority of this variation is caused a reduction in the Dit influencing the structures electrical characteristics, due to a shift in the semiconductors bulk potential, which is due to the lower VTH of SiC-based MOSFETs at high temperatures.

EPR Study of the Nitrogen Containing Defect Center Created in Self-Assembled 6H SiC Nanostructure

Triplet center with spin state S = 1 is detected in the EPR spectrum of the self-assembled 6H SiC nanostructure obtained by non-equilibrium boron diffusion into the n-type 6H SiC epitaxial layer (EL) under conditions of the controlled injection of the silicon vacancies at the temperature of T = 900°C. From the analysis of the angular dependences of the EPR spectrum and the numerical diagonalization of the spin Hamiltonian, the value of the zero-field splitting constant D and g-factor are found to be D = 1140•10-4см-1 and gpar = 1.9700, gper = 1.9964. Based on the hyperfine (hf) structure of the defect originating from the hf interaction with one 14N nuclei, the large value of the zero-field splitting constant D and technological conditions of the boron diffusion into the n-type 6H SiC EL, the triplet center is tentatively assigned to the defect center consisting of nitrogen atom and silicon vacancy.

Determination of the Electrical Capture Process of the EH&lt;sub&gt;6&lt;/sub&gt;-Center in n-Type 4H-SiC

Deep Level Transient Spectroscopy (DLTS) investigations at different temperatures and with various filling pulse lengths were performed on n-type 4H-SiC epitaxial layers using Iridium Schottky contacts to determine the electrical capture process of the EH6-center. The temperature dependence of the electrical capture cross section σ ~ T-2.0 suggests a cascade capture process, which is not thermally activated. Together with earlier work by Zippelius et al. [4] this proves the acceptor-type of the EH6-center.

Step-Bunching Free and 30 μm-Thick SiC Epitaxial Layer Growth on 150 mm SiC Substrate

The production of 150 mm-diameter SiC epi-wafers is the key to the spread of SiC power devices. Besides, step-bunching free surface leads to high-performance devices. We have developed the production technology of the epitaxial growth with smooth surface morphology for 4º off Si-face 4H-SiC epitaxial layers on 150 mm diameter substrates. The various area observations of the surface by optical surface analyzer, confocal microscope and atomic force microscope revealed that there was no conventional step-bunching in whole wafer surface. While creating step-bunching free surface is more difficult for thicker epilayer growth, we have achieved step-bunching free surface for 30-μm thick epilayer on a 150 mm diameter substrate. The typical values of thickness uniformity of the 30μm-thick epilayer are 0.5% (σ/mean) and 1.7% (range/mean). A few interfacial dislocations (IDs) were detected for the 150 mm-diameter epi-wafer by reflection X-ray topography. We have succeeded in removal of IDs by the optimized growth condition.

Temperature Dependence of the Band-Edge Injection Electroluminescence of 4H-SiC pn Structure

We present the injection electroluminescence spectra in the temperature range 290-800 K of 4H-SiC pn structure, which was formed by implantation of Al+ ions in low-doped n-type conductivity 4H-SiC epitaxial layer. The dominant emission band of injection electroluminescence (IEL) spectrum at room temperature was observed in the blue-green region; as the temperature is raised, the blue-green band is quenched, while UV band (near band-edge) IEL become dominant. The peak parameters of UV band at room temperature are: hmax  3.17 eV, full width at half maximum w  90 meV. The UV peak shifted in the long-wave direction with increasing temperature; the hmax (T) dependence was linear with the slope of -2.3∙10-4 eV/K. Both the IEL intensity of the UV peak at hmax and band width w increased upon heating. The w(T) dependence was linear with the slope of 2.9∙10-4 eV/K; intensity increased with the activation energy of 100-150 meV. The UV IEL band can be considered more probable to the band-band recombination and edge IEL increasing with rising temperature can be explained by the nonequilibrium charge carriers lifetime increasing.

Uniformity and Morphology of 10 x 100mm 4° Off-Axis 4H-SiC Epitaxial Layers and their Effect on Device Performance

Homoepitaxial layers with very good thickness and doping uniformity were grown on 4 inch 4˚ off-axis substrates in a 10x100mm planetary reactor. Process optimizations resulted in reduction of the size of the triangular defects. Aggressive pre-etching of the substrate prior to growth resulted in further suppression of the triangular defect concentration from 3-5cm-2 to 0.5cm-2 using the same growth processes. Even imperfect areas of the substrate with scratches show suppressed nucleation of triangular defects. JBS diodes with triangular defects show increased leakage depending on the size of the defects. This effect is more pronounced at higher voltages.

Deep Levels in P-Type 4H-SiC Induced by Low-Energy Electron Irradiation

We have characterized deep levels in as-grown and electron irradiated p-type 4H-SiC epitaxial layers by the current deep-level transient spectroscopy (I-DLTS) method. A part of the samples were irradiated with electrons in order to introduce defects. As a result, we found that electron irradiation to p-type 4H-SiC created complex defects including carbon vacancy or interstitial. Moreover, we found that observed deep levels are different between before and after annealing, and thus annealing may change structures of defects.

Influence of Growth Temperature on Carrier Lifetime in 4H-SiC Epilayers

Carrier lifetime and formation of defects have been investigated as a function of growth temperature in n-type 4H-SiC epitaxial layers, grown by horizontal hot-wall CVD. Emphasis has been put on having fixed conditions except for the growth temperature, hence growth rate, doping and epilayer thickness were constant in all epilayers independent of growth temperature. An increasing growth temperature gave higher Z1/2 concentrations along with decreasing carrier lifetime. A correlation between growth temperature and D1 defect was also observed.

Experimental determination of electron-hole pair creation energy in 4H-SiC epitaxial layer: An absolute calibration approach

Electron-hole pair creation energy (ε) has been determined from alpha spectroscopy using 4H-SiC epitaxial layer Schottky detectors and a pulser calibration technique. We report an experimentally obtained ε value of 7.28 eV in 4H-SiC. The obtained ε value and theoretical models were used to calculate a Fano factor of 0.128 for 5.48 MeV alpha particles. The contributions of different factors to the ultimate alpha peak broadening in pulse-height spectra were determined using the calculated ε value and Monte-Carlo simulations. The determined ε value was verified using a drift-diffusion model of variation of charge collection efficiency with applied bias.

Thick 4H-SiC Epitaxial Growth and Defect Reduction for Very High Voltage Bipolar Devices

The reduction of extended and point defects, including 8H in-grown stacking faults, basal plane dislocations (BPDs), and Z1/2 center, in 4H-SiC epitaxial layers was investigated. In the epitaxial growth process, a high density of 8H stacking faults was evaluated in the epilayer grown under a relatively high partial pressure of SiH4. By decreasing the SiH4 partial pressure or adding HCl to the conventional gas system (H2+SiH4+C3H8), the 8H stacking fault density was considerably reduced. By using 4° off-cut 4H-SiC substrates instead of an 8° off-cut one, an extremely low BPD density in the epilayer was achieved. No deep levels other than Z1/2 and EH6/7 centers were detected in the upper half of the bandgap for epilayers grown with the addition of HCl. The Z1/2 center concentration was reduced to <1 × 1012 cm−3 by tuning the growth parameters, and further reduced by the post-growth processes.

Fabrication of High-Resolution Nuclear Detectors Using 4H-SiC n-type Epitaxial Layers

ABSTRACTHigh resolution Schottky barrier detectors for alpha particles have been fabricated on 20 μm n-type 4H-SiC epitaxial layers. Schottky barrier contact structure was accomplished by deposition of 10 nm nickel on the Si face of the epilayers. The detectors were characterized for structural, electrical, and spectroscopic properties. Scanning electron microscopy and Nomarski optical microscopy revealed a micropipe density lower than 1 cm-2. The current-voltage (I-V) characteristics of the device exhibited very low leakage current of the order of 6.5 pA at an operating bias of 90 V. C-V measurements revealed a typical effective doping concentrations of 2.4 × 1014 cm-3 in these epilayers. The detectors were evaluated for alpha particles detection using a 241Am source. An energy resolution of ∼0.98% for 5.48 MeV alpha particles was observed. The separate contribution of charge carrier drift and diffusion to the total charge collection efficiency has been calculated in these detectors following a drift-diffusion model. Detailed electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system. Effect of shaping time, presence of source and bias on the ENC has been studied in details.

Schottky barrier detectors on 4H-SiC n-type epitaxial layer for alpha particles

Schottky barrier detectors have been fabricated on 50μm n-type 4H-SiC epitaxial layers grown on 360μm SiC substrates by depositing ∼10nm nickel contact. Current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out to investigate the Schottky barrier properties. The detectors were evaluated for alpha particle detection using a 241Am alpha source. An energy resolution of ∼2.7% was obtained with a reverse bias of 100V for 5.48MeV alpha particles. The measured charge collection efficiency (CCE) was seen to vary as a function of bias voltage following a minority carrier diffusion model. Using this model, a diffusion length of∼3.5μm for holes was numerically calculated from the CCE vs. bias voltage plot. Rise-time measurements of digitally recorded charge pulses for the 5.48MeV alpha particles showed a presence of two sets of events having different rise-times at a higher bias of 200V. A biparametric correlation scheme was successfully implemented for the first time to visualize the correlated pulse-height distribution of the events with different rise-times. Using the rise-time measurements and the biparametric plots, the observed variation of energy resolution with applied bias was explained.