Power semiconductor devices are key components in power conversion systems. Silicon carbide (SiC) has received increasing attention as a wide-bandgap semiconductor suitable for high-voltage and low-loss power devices. Through recent progress in the crystal growth and process technology of SiC, the production of medium-voltage (600?1700 V) SiC Schottky barrier diodes (SBDs) and power metal?oxide?semiconductor field-effect transistors (MOSFETs) has started. However, basic understanding of the material properties, defect electronics, and the reliability of SiC devices is still poor. In this review paper, the features and present status of SiC power devices are briefly described. Then, several important aspects of the material science and device physics of SiC, such as impurity doping, extended and point defects, and the impact of such defects on device performance and reliability, are reviewed. Fundamental issues regarding SiC SBDs and power MOSFETs are also discussed.

https://iopscience.iop.org/article/10.7567/JJAP.54.040103/pdf

Material science and device physics in SiC technology for high-voltage power devices

We present a comprehensive review of the properties of the epitaxial 4H silicon carbide polytype (4H–SiC). Particular emphasis is placed on those aspects of this material related to room, high-temperature and harsh environment ionizing radiation detector operation. A review of the characterization methods and electrical contacting issues and how these are related to detector performance is presented. The most recent data on charge transport parameters across the Schottky barrier and how these are related to radiation spectrometer performance are presented. Experimental results on pixel detectors having equivalent noise energies of 144 eV FWHM (7.8 electrons rms) and 196 eV FWHM at +27 °C and +100 °C, respectively, are reported. Results of studying the radiation resistance of 4H–SiC are analysed. The data on the ionization energies, capture cross section, deep-level centre concentrations and their plausible structures formed in SiC as a result of irradiation with various particles are reviewed. The emphasis is placed on the study of the 1 MeV neutron irradiation, since these thermal particles seem to play the main role in the detector degradation. An accurate electrical characterization of the induced deep-level centres by means of PICTS technique has allowed one to identify which play the main role in the detector degradation.

https://iopscience.iop.org/article/10.1088/0957-0233/19/10/102001/pdf

Silicon carbide and its use as a radiation detector material

The authors report a significant reduction in deep level defects and improvement of carrier lifetime in 4H-SiC material after carrying out carbon or silicon ion implantation into the shallow surface layer of 250nm and subsequent annealing at 1600°C or higher temperature. Reduction of Z1∕2 and EH6∕7 traps from 3×1013cm−3 to below the detection limit (5×1011cm−3) was observed by deep level transient spectroscopy in the material 4μm underneath the implanted layer. Minority carrier lifetime almost doubled in the implanted samples compared to the unimplanted samples. The authors propose that the implanted layer acts as a source of carbon interstitials which indiffuse during annealing and accelerate annealing out of grown-in defects in the layer underneath the implanted region.

Reduction of traps and improvement of carrier lifetime in 4H-SiC epilayers by ion implantation

A semi-insulating bulk single crystal of silicon carbide is disclosed that has a resistivity of at least 5000 Ω-cm at room temperature and a concentration of deep level trapping elements that is below the amounts that will affect the resistivity of the crystal, preferably below detectable levels. A method of forming the crystal is also disclosed, along with some resulting devices that take advantage of the microwave frequency capabilities of devices formed using substrates according to the invention.

Semi-insulating silicon carbide without vanadium domination

We report results from two-dimensional Raman spectroscopy studies of large-area epitaxial graphene grown on SiC. Our work reveals unexpectedly large variation in Raman peak position across the sample resulting from inhomogeneity in the strain of the graphene film, which we show to be correlated with physical topography by coupling Raman spectroscopy with atomic force microscopy. We report that essentially strain free graphene is possible even for epitaxial graphene.

Raman Topography and Strain Uniformity of Large-Area Epitaxial Graphene

The growth of high-purity, semi-insulating (HPSI) 4H-SiC crystals has been achieved using the seeded-sublimation growth technique. These semi-insulating (SI) crystals (2-inch diameter) were produced without the intentional introduction of elemental deep-level dopants, such as vanadium, and wafers cut from these crystals possess room-temperature resistivities greater than 109 Ωcm. Based upon temperature-dependent resistivity measurements, the SI behavior is characterized by several activation energies ranging from 0.9-1.5 eV. Secondary ion mass spectroscopy (SIMS) and electron paramagnetic resonance (EPR) data suggest that the SI behavior originates from deep levels associated with intrinsic point defects. Typical micropipe densities for wafers were between 30 cm-2 and 150 cm-2. The room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/mK for 4H-SiC, making these wafers suitable for high-power microwave applications.

High-purity semi-insulating 4H-SiC grown by the seeded-sublimation method

The production of large diameter, high quality SiC substrates is essential to realize the full potential of this important semiconductor material. The current status of SiC bulk sublimation growth for the production of these substrates is reviewed from an industrial point of view. Specific efforts towards larger diameter high quality substrates have led to the production of 50 and 75 mm diameter 4H and 6H wafers and the demonstration of high quality 100-mm wafers. We present thermal conductivity data for material of different doping levels, relevant for device applications. In SiC, micropipes are the most harmful defects for SiC device production. By continuous optimization of the growth process we were able to steadily decrease the micropipe density over the past several years, down to densities as low as 1.1 cm −2 for an entire 50-mm wafer, indicating that micropipes may be totally eliminated in the next few years. In order to achieve this goal for SiC substrates of increasing diameter, a thorough understanding of the growth process is essential. We will summarize results of modeling the growth process and its experimental verification. The effect of micropipe densities and their characteristic lateral distribution in SiC wafers on achievable device yields will be discussed, using large area Schottky diodes as an example.

Progress in the industrial production of SiC substrates for semiconductor devices

High-purity, semi-insulating (HPSI) 4H-SiC crystals with diameters up to 75 mm have been grown by the seeded sublimation technique without the intentional introduction of elemental deep-level dopants, such as vanadium. Wafers cut from these crystals exhibit homogeneous activation energies near mid gap and thermally stable semi-insulating (SI) behavior (>109 ohm-cm) throughout device processing. Secondary ion mass spectroscopy, deep-level transient spectroscopy, optical admittance spectroscopy, and electron paramagnetic resonance data suggest that the SI behavior originates from several deep levels associated with intrinsic point defects. Micropipe densities in HPSI substrates have been demonstrated to be as low as 10 cm−2 in 2-in. substrates, and the room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/cm·K for 4H-SiC. Devices fabricated on these HPSI wafers do not exhibit any substrate related back-gating effects and have power densities as high as 5.2 W/mm with 63% power added efficiency.

High-purity semi-insulating 4H-SiC for microwave device applications

We present results of a thermal anneal process that increases the minority carrier lifetime in SiC substrates to in excess of 3μs, compared to the starting as-grown substrates with lifetimes typically in the <10ns range. Measurement of lifetimes was conducted using microwave-photoconductive decay. Electron beam induced current measurements exhibited minority carrier diffusion lengths of up to 65μm, confirming the enhanced carrier lifetime of the annealed substrate material. Additionally, positron annihilation spectroscopy and deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1∕Z2, may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. The operational performance of p-i-n diodes employing drift la...

Effects of annealing on carrier lifetime in 4H-SiC

The current status of SiC bulk growth is reviewed, while specific attention is given to the effect of defects in SiC substrates and epitaxial layers on device performance and yield. The progress in SiC wafer quality is reflected in the achievement of micropipe densities as low as 0.92 cm−2 for a 3-inch n-type 4H-SiC wafer, which provides the basis for a high yielding fabrication process
 of large area SiC power devices. Using a Murphy Probe Yield Analysis for the breakdown characteristics of 10 kV PiN diodes we have extracted an “effective” defect density for 4H-SiC material to be as low as 30 cm−2, providing valuable information to further isolate and address the specific material defects critical for device performance. We address the problematic degradation of the forward characteristics (V f -drift) of bipolar SiC PiN diodes [CITE]. The underlying mechanism due to stacking fault formation in the epitaxial layers and possible effects of device processing are investigated. An improved device design is demonstrated, which effectively stabilizes this V f -drift. We show the progression in the development of semi-insulating SiC grown by the sublimation technique from extrinsically
 doped material to high purity semi-insulating (HPSI) 4H-SiC bulk crystals of up to 100 mm diameter without resorting to the intentional introduction of elemental deep level dopants, such as vanadium. Uniform resistivities in 3-inch HPSI wafers greater than 3 × 1011 Ω-cm have been achieved. Secondary ion mass spectrometry, deep level transient spectroscopy and electron paramagnetic resonance data suggest that the semi-insulating behavior in HPSI material originates from deep levels associated with intrinsic point defects. MESFETs produced on HPSI wafers are free of backgating effects and have resulted in the best combination of power density and efficiency reported to date for SiC MESFETs of 5.2 W/mm and 63% power added efficiency (PAE) at 3.5 GHz.

C. H. Carter

Papers

High-purity semi-insulating 4H-SiC grown by the seeded-sublimation method

Progress in the industrial production of SiC substrates for semiconductor devices

High-purity semi-insulating 4H-SiC for microwave device applications

Effects of annealing on carrier lifetime in 4H-SiC

Defects in SiC substrates and epitaxial layers affecting semiconductor device performance