Semi-insulating 4H-SiC Research Articles

Influence of crystal orientation and incident plane on n-type 4H-SiC wafer slicing by using picosecond laser

Laser slicing has been considered as the most efficient technique for slicing SiC wafers with the advantages of small kerf width (loss) and crack, low roughness on cleaved surface, high quality and efficiency, etc. Here, laser slicing of n-type 4H-SiC was demonstrated by using a homemade 1064 nm picosecond laser combined with mechanical stretch stripping. The influence of laser processing along [11 2¯ 0] and [1 1¯ 00] crystal orientations on Si-face and C-face of n-type 4H-SiC, specifically on the internal laser ablation lines and cracks, the peeling tensile strength, and the surface roughness of the peeled surfaces was investigated. The semi-insulating 4H-SiC wafer laser slicing was conducted for comparison. Under the same laser conditions, laser slicing of semi-insulating 4H-SiC was easier than that of n-type. The laser modification quality along [1 1¯ 00] orientation was better than that along [11 2¯ 0] orientation for both the semi-insulating and n-type 4H-SiC and the reasons were explained. The results indicated that the optimal laser slicing scheme detailed laser scanning along [1 1¯ 00] orientation and incidence from the C-face. Finally, a 6-inch, 420.36 µm-thick n-type 4H-SiC wafer was successfully sliced.

4H-SiC ultraviolet photodetector array with vertical MSM configuration

Abstract The increasing demand for ultraviolet (UV) imaging in extreme conditions, such as high temperatures and strong radiation, has spurred advancements in UV photodetector arrays. Traditional metal-semiconductor- metal (MSM) 4H-SiC UV photodetector array, with their planar structure and M×N electrode connections, face challenges in circuit design. Our research utilizes a vertical design for building the photodetector array, reducing the connections to just M+N, thereby simplifying the circuit design and signal processing. Utilizing semi-insulating 4H-SiC wafer and TiN electrodes, we developed an 8×8 vertical MSM photodetector array. Tested under a 365 nm light source at 10.5 mW/cm2 and a 5V bias, the array demonstrated low dark currents, high contrasts under illumination, and a 100% operational yield. With average photo current and dark currents of 1.56×10-8 A and 2.94×10-13 A, respectively, the average photo-to-dark current ratio (PDCR) exceeded 5×104 Our design effectively minimized sneak path currents and achieved a low crosstalk rate of 0.46%, enabling the capture of clear, high-contrast images. This marks a significant advancement in the application of MSM 4H-SiC UV photodetectors for imaging in extreme conditions.

Demonstration of SiC n-channel MOSFETs fabricated on a high-purity semi-insulating substrate and investigation of the short-channel effects

N-channel MOSFETs fabricated on a high-purity semi-insulating (HPSI) 4H-SiC substrate were demonstrated. The fabricated MOSFETs exhibited normally-off transistor operation and the peak field effect mobility (μ FE,peak) was 30 cm2 V−1 s−1, which was lower than that of the p−-body MOSFETs (N A = 2 × 1015 cm−3). The critical channel length (L crit) was 1.48 μm for the HPSI MOSFETs, which was shorter than that for the p−-body MOSFETs. In the HPSI MOSFETs, electrons trapped by the compensating defects in the HPSI substrate increase as the Fermi level moves up, which may be the main cause for the resulting low μ FE,peak and short L crit.

In-Line Charaterization of HPSI SiC Wafers Using High Resolution Surface Photovoltage Spectroscopy (HR-SPS)

High purity semi-insulating (HPSI) 4H-SiC wafers from different vendors have been studied by surface photovoltage spectroscopy (SPV). It is demonstrated that the surface photovoltage signal height can be used to discriminate between non-compensated and compensated material, and that the SPV signal is also proportional to the bulk resistivity, at least for non-compensated 4H-SiC material.

Demonstration of SiC-on-Insulator Substrate with Smart Cut™ Technology for Photonic Applications

Silicon-carbide-on-insulator (SiCOI) is a promising platform for photonic integrated circuits. However, the development of this new photonic platform is hindered by the lack of high-quality commercial SiC-on-insulator substrates. In this study, we present a demonstration of the transfer of a single crystalline semi-insulating 4H-SiC thin film on a SiO2 insulated substrate at 150 mm wafer scale using the Smart Cut™ technology. We describe the development of SiCOI substrates and their characterization at each key step of the process. In particular, we provide a detailed study of bow compensation related to the implanted SiC donor substrate. The quality of the transferred SiC layer was investigated as a function of the final annealing temperature applied. The optical indices of the bulk SiC were measured using spectroscopic ellipsometry, and an advanced model has been used to take into account the strong birefringence of the silicon carbide film. Finally, simulations were conducted to design a preliminary set of basic and advanced photonic devices.

High-voltage AlGaN/GaN heterojunction lateral photoconductive semiconductor switch based on semi-insulating 4H-SiC substrate

A novel high-power AlGaN/GaN heterojunction lateral photoconductive semiconductor switch (PCSS) based on the SiC substrate is proposed, which achieves high dark-state resistance characteristics by groove etching. Under the action of biased electric field and incident laser, a high concentration of two-dimensional electron gas is formed at the heterojunction interface. The photo-generated free carriers transport along the heterojunction interface, which improves the utilization efficiency of photo-generated carriers. Moreover, the simulation of the current density distribution of the PCSS provides theoretical support for this phenomenon. Compared with the conventional GaN PCSS, the power capacity and conduction characteristics are further improved. The test results show that the output current of the device increases significantly after the introduction of the AlGaN/GaN heterojunction. At the biased voltage of ∼34 kV, the maximum output current of the AlGaN/GaN PCSS reaches 205 A.

Properties of Z1 and Z2 Deep-Level Defects in n-Type Epitaxial and High-Purity Semi-Insulating 4H-SiC

Properties of Z1 and Z2 Deep-Level Defects in n-Type Epitaxial and High-Purity Semi-Insulating 4H-SiC

Numerical investigation of laser doping parameters for semi-insulating 4H-SiC substrate

Semi-insulating (SI) 4H-polytype of silicon carbide (SiC) is a highly desirable wide bandgap semiconductor material for various applications in challenging environments owing to its exceptional characteristics such as high melting point, remarkable thermal conductivity, strong breakdown field, and excellent resistance to oxidation. This study investigates the critical laser processing parameters to operate a pulsed UV 355 nm laser to dope high-purity (HP) SI 4H-SiC substrates with boron. The doping process parameters are examined and simulated for this UV laser doping system using a liquid precursor of boron. Boron atoms create a dopant energy level of 0.3eV in the doped HP 4H-SiC substrates. Diffusion of boron atoms into 4H-SiC substrates modifies the hole density at 0.3eV energy level, and causing a variation in the dynamic refraction index, and absorption index. Consequently, the optical properties of boron doped samples, namely, transmittance, reflectance, and absorbance, can be modified. The current simulation reported in this study explains the motivation of UV optical doping strategy to dope SiC substrates. A beam homogenizer was used to control the laser spot used to generate doping process. The advantage of the beam homogenizer is demonstrated by producing flat-top beams with uniform intensity over a certain area defined by the focusing lens choice. A simple theoretical model is used to select the laser processing parameters for doping SiC substrates. These modeled parameters are used to determine the efficient laser processing parameters for our doping experiments.

Defect-engineered graphene-on-silicon-carbide platform for magnetic field sensing at greatly elevated temperatures

High-temperature electrical properties of p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) and high-purity on-axis 4H-SiC(0001) originate from the double-carrier system of spontaneous-polarization-induced holes in graphene and thermally-activated electrons in the substrate. In this study, we pre-epitaxially modify SiC by implanting hydrogen (H+) and helium (He+) ions with energies ranging from 20 keV to 50 keV to reconstruct its post-epitaxial defect structure and suppress the thermally-developed electron channel. Through a combination of dark current measurements and High-Resolution Photo-Induced Transient Spectroscopy between 300 K and 700 K, we monitor the impact of ion bombardment on the transport properties of SiC and reveal activation energies of the individual deep-level defects. We find that the ion implantation has a negligible effect on 6H-SiC. Yet in 4H-SiC, it shifts the Fermi level from ∼600 meV to ∼800 meV below the minimum of the conduction band and reduces the electron concentration by two orders of magnitude. Specifically, it eliminates deep electron traps related to silicon vacancies in the charge state (2-/-) occupying the h and k sites of the 4H-SiC lattice. Finally, we directly implement the protocol of deep-level defect engineering in the technology of amorphous-aluminum-oxide-passivated Hall effect sensors and introduce a mature sensory platform with record-linear current-mode sensitivity of approximately 80 V/AT with -0.03-%/K stability in a broad temperature range between 300 K and 770 K, and likely far beyond 770 K.

Spectroscopic properties of close-to-perfect-monolayer quasi-free-standing epitaxial graphene on 6H[sbnd]SiC(0001)

In this report, we present transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on 15-mm × 15-mm semi-insulating vanadium-compensated on-axis 6HSiC(0001), characterized in that its room-temperature direct-current Hall-effect-derived hole mobility μp= 5019 cm2/Vs, and its statistical number of layers (N), as indicated by the relative intensity of the SiC-related Raman-active longitudinal optical A1 mode at 964 cm−1, equals N= 1.05. The distribution of the ellipsometric angle Ψ measured at an angle of incidence of 50° and λ= 490 nm points out to N= 0.97. The close-to-unity value of N implies that the material under study is a close-to-perfect quasi-free-standing monolayer, which is further confirmed by High-Resolution Transmission Electron Microscopy. Therefore, its spectroscopic properties, which include the SiH peak at 2131 cm−1, the histograms of Ψ and Δ, and the Raman G and 2D band positions, widths, and the 2D-to-G band intensity ratios, constitute a valuable reference for this class of materials.

Investigation of internal electric fields in graphene/6H-SiC under illumination by the Pockels effect.

In this paper, we introduce a method for mapping profiles of internal electric fields in birefringent crystals based on the electro-optic Pockels effect and measuring phase differences of low-intensity polarized light. In the case of the studied 6H-SiC crystal with graphene electrodes, the experiment is significantly affected by birefringence at zero bias voltage applied to the crystal and a strong thermo-optical effect. We dealt with these phenomena by adding a Soleil-Babinet compensator and using considerations based on measurements of crystal heating under laser illumination. The method can be generalized and adapted to any Pockels crystal that can withstand sufficiently high voltages. We demonstrate the significant formation of space charge in semi-insulating 6H-SiC under illumination by above-bandgap light.

A Method to Simulate Extrinsic Light Excitation of Vanadium-Compensated 6H-SiC

Extrinsic light excitation has much lower absorption coefficient compared to intrinsic light excitation, which can better utilize the “bulk” of semiconductor rather than a thin surface as the depth of light absorption is much larger, making it suitable for higher power applications. However, commercial technology computer aided design (TCAD) software has not developed a model for extrinsic light excitation. Therefore, we construct a model of Vanadium-compensated semi-insulating (VCSI) 6H-SiC photoconductive semiconductor switch (PCSS) illuminated with sub-bandgap light, and realize the process of light absorption at V deep acceptor level in Silvaco TCAD simulation by modifying the electron emission rate. Then, we simulate the transient response of 6H-SiC triggered by a nanosecond light pulse and discuss the feasibility of this method.

Extraordinary permittivity characterization of 4H SiC at millimeter-wave frequencies

For millimeter-wave power applications, GaN high-electron mobility transistors (HEMTs) are often grown epitaxially on a high-purity semi-insulating c-axis 4H-SiC substrate. For these anisotropic hexagonal materials, the design and modeling of microstrip and coplanar interconnects require detailed knowledge of both the ordinary permittivity ε⊥ and the extraordinary permittivity εǁ perpendicular and parallel, respectively, to the c-axis. However, conventional dielectric characterization techniques make it difficult to measure εǁ alone or to separate εǁ from ε⊥. As a result, there is little data for εǁ, especially at millimeter-wave frequencies. This work demonstrates techniques for characterizing εǁ of 4H SiC using substrate-integrated waveguides (SIWs) or SIW resonators. The measured εǁ on seven SIWs and eleven resonators from 110 to 170 GHz is within ±1% of 10.2. Because the SIWs and resonators can be fabricated on the same SiC substrate together with HEMTs and other devices, they can be conveniently measured on-wafer for precise material-device correlation. Such permittivity characterization techniques can be extended to other frequencies, materials, and orientations.

High-Temperature Annealing of High Purity Semi-Insulating 4H-SiC and Its Effect on the Performance of a Photoconductive Semiconductor Switch

High-Temperature Annealing of High Purity Semi-Insulating 4H-SiC and Its Effect on the Performance of a Photoconductive Semiconductor Switch

Evaluation of Hysteresis Response in Achiral Edges of Graphene Nanoribbons on Semi-Insulating SiC

Hysteresis response of epitaxially grown graphene nanoribbons devices on semi-insulating 4H-SiC in the armchair and zigzag directions is evaluated and studied. The influence of the orientation of fabrication and dimensions of graphene nanoribbons on the hysteresis effect reveals the metallic and semiconducting nature graphene nanoribbons. The hysteresis response of armchair based graphene nanoribbon side gate and top gated devices implies the influence of gate field electric strength and the contribution of surface traps, adsorbents, and initial defects on graphene as the primary sources of hysteresis. Additionally, passivation with AlOx and top gate modulation decreased the hysteresis and improved the current-voltage characteristics.

Epitaxial growth of TiC on (0001) 4H-SiC substrate by reactive sputtering

Heteroepitaxial TiC films were grown on semi-insulating (0001) 4H-SiC substrates at high temperature by using direct current (DC) magnetron reactive sputtering. The effect of the CH4 flux ratio in Ar on the crystalline quality of the films was investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the deposited TiC films are heteroepitaxially grown on 4H-SiC substrates with the heteroepitaxial relationship between TiC and 4H-SiC in (111)TiC // (0001)4H-SiC and [11¯0]TiC // [112¯0]4H-SiC. The full width at half-maximum (FWHM) of X-ray rocking curve (XRC) of (111)TiC reflection can reach a minimum value of 0.039° and the resistivity of TiC film can be ∼ 80 μΩ⋅cm for deposition at 700 °C with 4% CH4 flow ratio. X-ray photoelectron spectroscopy (XPS) shows that the composition of TiC film is nearly in stoichiometry for 4% CH4 flow ratio. From the TEM results, it is observed that the TiC film is divided into a strained layer and a relaxed layer when the film thickness reaches a critical value over about 70 nm.

Study on picosecond laser stealth dicing of 4H-SiC along [[formula omitted]] and [[formula omitted]] crystal orientations on Si-face and C-face

Stealth dicing of semi-insulating 4H-SiC along [112¯0] and [11¯00] crystal orientations on Si-face and C-face was conducted by using infrared picosecond laser in combination with three-point bending split. It was demonstrated that the laser ablation points inside 4H-SiC samples, the critical fracture load, and the dicing quality of 4H-SiC samples were closely related to the crystal orientation. The critical fracture load of 4H-SiC along [112¯0] orientation is smaller than that along [11¯00] orientation. The longitudinal lengths of the laser ablation points increase with the increase of laser incident distance. For the same laser-modified layer, as laser was incident from C-face and processed along [112¯0] orientation, the laser ablation points have larger longitudinal length. The reasons for the impact of crystal orientation on laser-modified points were analyzed. The fractured 4H-SiC samples have chipping width less than 3 μm and section roughness less than 500 nm. Moreover, the dicing quality along [112¯0] orientation is better than that along [11¯00] orientation. This research can provide technical support for precision dicing of SiC wafers in semiconductor industry.

Test of KW Class Photonic Microwave Generation Using Vanadium-Compensated 6H-SiC PCSS and Burst-Mode-Operation Pulse Laser

Photoconductive semiconductors operating in linear mode can be used for adaptive high-power microwave (HPM) generators by modulating incident light. This paper presents the design scheme and preliminary test results of a narrowband kW class adaptive photonic microwave generator by employing a wide-bandgap semi-insulating 6H-SiC photoconductive semiconductor and a burst-mode laser. The experimental scheme of the generator is described along with the circuit simulation of the radio frequency generator. The laser operated at a wavelength of 532 nm with a pulse width of 100 ns and a repetition rate of 100 Hz. The laser modulates the frequencies of the generated microwave, and preliminary tests are conducted in the frequency range of 0.8–1.2 GHz. The maximum output microwave power of the designed scheme reaches up to 2.6 kW when the bias voltage is 10 kV. The results confirm that this method can be used for high-power frequency-adjustable microwave signal generation. The microwave source system presented in this paper has continuously adjustable frequency and flexible waveform control.

Discrimination of dislocations in 4H-SiC by inclination angles of molten-alkali etched pits

Discrimination of dislocations is critical to the statistics of dislocation densities in 4H silicon carbide (4H-SiC), which are routinely used to evaluate the quality of 4H-SiC single crystals and homoepitaxial layers. In this work, we show that the inclination angles of the etch pits of molten-alkali etched 4H-SiC can be adopted to discriminate threading screw dislocations (TSDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) in 4H-SiC. In n-type 4H-SiC, the inclination angles of the etch pits of TSDs, TEDs and BPDs in molten-alkali etched 4H-SiC are in the ranges of 27°−35°, 8°−15° and 2°−4°, respectively. In semi-insulating 4H-SiC, the inclination angles of the etch pits of TSDs and TEDs are in the ranges of 31°−34° and 21°−24°, respectively. The inclination angles of dislocation-related etch pits are independent of the etching duration, which facilitates the discrimination and statistic of dislocations in 4H-SiC. More significantly, the inclination angle of a threading mixed dislocations (TMDs) is found to consist of characteristic angles of both TEDs and TSDs. This enables to distinguish TMDs from TSDs in 4H-SiC.

Effect of temperature on growth of epitaxial layer on semi-insulating 4H-SiC substrate

4H-SiC has excellent physical and chemical properties such as wide band gap and high electron mobility and can be used in microwave and radio frequency devices. Epitaxial layer on semi-insulating (SI) substrate of 4H-SiC is a key structure for the fabrication of radio frequency devices. However, two-dimensional growth easily occurs during the epitaxy process to form 3C-SiC. Therefore, it is necessary to study the crystal polytype change in the epitaxial layer. This paper studied the effect of temperature on the epitaxial layer during epitaxy on 4H-SiC SI substrates using trichlorosilane and ethylene as source gases. The Si-face and the C-face were respectively selected as the growth planes, and the temperature varied in the range of 1400℃–1600℃. The samples were characterized using optical microscopy, atomic force microscopy, and Raman spectroscopy. The surface topography obtained by epitaxial growth was uniform, and the surface roughness of the smooth region was less than 1.0 nm. 3C-SiC was obtained at low temperature, and 4H-SiC was obtained at higher temperature.