Semi-insulating 4H-SiC Substrates Research Articles

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.

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.

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.

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.

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.

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.

Effective channel mobility in phosphorus-treated 4H-SiC (0001) metal-oxide-semiconductor field-effect transistors with various p-body doping concentrations

Phosphorus treatment, which can substantially reduce the interface state density (D it), was used to investigate the impact of D it on effective channel mobility (μ eff) of 4H-SiC (0001) metal-oxide-semiconductor field-effect transistors (MOSFETs). A high μ eff of 126 cm2 V−1 s−1, which exceeds the reported phonon-limited mobility of 83 cm2 V−1 s−1 determined from Hall mobility of nitridation-treated MOSFETs, at a high effective normal field of 0.57 MV cm−1 was obtained in MOSFETs fabricated on a high-purity semi-insulating 4H-SiC substrate at room temperature. This high mobility may be caused by the difference of the density of electrons trapped at the interface states.

Epitaxial growth of TiN on (0001) semi-insulating 4H-SiC substrate by reactive sputtering

Growth of epitaxial TiN film on a semi-insulating single crystal (0001) SiC substrate has been achieved by DC magnetron reactive sputtering deposition. The effects of growth temperature and N2 flow ratio in Ar on film quality and growth rate were investigated. X-ray diffraction (XRD) characterization shows that all the deposited TiN films at 550 and 700 °C are heteroepitaxially grown on SiC with the epitaxial relationship between TiN and 4H-SiC in (111)TiN//(0001)SiC and [11¯0]TiN//[112¯0]4H-SiC. Increasing the N2 flow ratio from 4 to 8% results in improvement of the film quality and a decrease in the growth rate. The width of the X-ray rocking curve of TiN (111) reflection can reach a value as small as 249 arcsec and the resistivity of TiN film can be 21 μΩ·cm for deposition at 700 °C with 8% N2 flow ratio. Measurements by atomic force microscopy show that the surface roughness of the TiN film can be 1.1 nm. X-ray photoelectron spectroscopy reveals that TiN composition is nearly in stoichiometry for all the grown TiN films. Examinations by cross-sectional transmission electron microscopy show that TiN is directly grown on SiC and verify the epitaxial relationship from XRD.

Исследование сильно легированных эпитаксиальных пленок n-3C-SiC, выращенных методом сублимации на основе полуизолирующих подложек 6H-SiC

Heavily doped 3C-SiC films based on semi-insulating 6H-SiC substrates were obtained by sublimation epitaxy. The structural perfection of the obtained samples was monitored by X-ray diffractometry. The measurements of the photoluminescence and Hall effect spectra have confirmed the rather high perfection of the obtained epitaxial layers.

Study of heavily doped n-3C-SiC epitaxial films grown on 6H-SiC semi-insulating substraes by sublimation method

Heavily doped 3C-SiC films based on semi-insulating 6H-SiC substrates were obtained by sublimation epitaxy. The structural perfection of the obtained samples was monitored by X-ray diffractometry. The measurements of the photoluminescence and Hall effect spectra have confirmed the rather high perfection of the obtained epitaxial layers. Keywords: SiC, heteroepitaxy, cubic silicon carbide, X-ray diffractometry, Hall effect, photoluminescence.

Fabrication and characterization of AlGaN/GaN HEMTs with high power gain and efficiency at 8 GHz

State-of-the-art AlGaN/GaN high electron mobility structures were grown on semi-insulating 4H-SiC substrates by MOCVD and X-band microwave power high electron mobility transistors were fabricated and characterized. Hall mobility of 2291.1 cm2/(V·s) and two-dimensional electron gas density of 9.954 × 1012 cm–2 were achieved at 300 K. The HEMT devices with a 0.45-μm gate length exhibited maximum drain current density as high as 1039.6 mA/mm and peak extrinsic transconductance of 229.7 mS/mm. The f T of 30.89 GHz and f max of 38.71 GHz were measured on the device. Load-pull measurements were performed and analyzed under (–3.5, 28) V, (–3.5, 34) V and (–3.5, 40) V gate/drain direct current bias in class-AB, respectively. The uncooled device showed high linear power gain of 17.04 dB and high power-added efficiency of 50.56% at 8 GHz when drain biased at (–3.5, 28) V. In addition, when drain biased at (–3.5, 40) V, the device exhibited a saturation output power density up to 6.21 W/mm at 8 GHz, with a power gain of 11.94 dB and a power-added efficiency of 39.56%. Furthermore, the low f max/f T ratio and the variation of the power sweep of the device at 8 GHz with drain bias voltage were analyzed.

Development of p-i-n radiation detectors based on semi-insulating 4H-SiC substrate via dual-face ion implantation

To explore the potential of semi-insulating (SI) SiC substrate for radiation detector applications, p-i-n diodes with thick depletion layers are fabricated by using a dual-face ion implantation technique on SI 4H-SiC substrate. The Poole-Frenkel (P-F) emission is found responsible for the reverse leakage current transport of the p-i-n diode at elevated temperatures. The derived emission barrier height is likely induced by the residual boron-related shallow-level states, which are located ∼ 0.3 eV above the top of the valence band. Detection tests of α-particles are performed on each side of the p-i-n detector to determine the location of the depleted region, which reveals weak p-type conduction of the SI 4H-SiC substrate after high temperature post-implantation annealing. The energy resolution of the detector operating in photovoltaic mode is ∼ 4.3%, which would degrade at higher biases. The saturated charge collection efficiency (CCE) of ∼ 99% for detection of 5.48 MeV α-particles is obtained as reverse bias exceeds 80 V.

Graphene on SiC Substrate as Biosensor: Theoretical Background, Preparation, and Characterization.

This work is devoted to the development and optimization of the parameters of graphene-based sensors. The graphene films used in the present study were grown on semi-insulating 6H-SiC substrates by thermal decomposition of SiC at the temperature of ~1700 °C. The results of measurements by Auger and Raman spectroscopies confirmed the presence of single-layer graphene on the silicon carbide surface. Model approach to the theory of adsorption on epitaxial graphene is presented. It is demonstrated that the Green-function method in conjunction with the simple substrate models permit one to obtain analytical results for the charge transfer between adsorbed molecules and substrate. The sensor structure was formed on the graphene film by laser. Initially, a simpler gas sensor was made. The sensors developed in this study demonstrated sensitivity to the NO2 concentration at the level of 1–0.01 ppb. The results obtained in the course of development and the results of testing of the graphene-based sensor for detection of protein molecules are also presented. The biosensor was fabricated by the technology previously developed for the gas sensor. The working capacity of the biosensor was tested with an immunochemical system constituted by fluorescein and monoclonal antibodies (mAbs) binding this dye.

Growth of thin AlN nucleation layer and its impact on GaN-on-SiC heteroepitaxy

Abstract AlN nucleation layer (NL) with thickness of 35 nm was grown on semi-insulating 4H–SiC substrate by metal organic chemical vapor deposition (MOCVD). Focused on the evolutions of the surface microstructural morphology and crystalline quality, the growth mechanism of the thin NL was studied by series of growth procedures. As interval supply of ammonia (NH3) and alternative supply of Trimethylaluminum (TMA)/NH3 are applied to NL growth, complete coalescence of AlN nucleate islands with a smooth surface and low density of defects is obtained. In addition, the impact of the thin NL on the subsequent GaN buffer layer (BL) was also discussed. Overgrown on the well-coalesced NL, surface roughening and three-dimensional mode get predominant at the initial GaN growth, which facilitates high crystalline quality of GaN BL with low wafer bow.

Analyzing the Effects of Aluminum-Doped ZnO and Ag Layers for the Transparent Electrode Vertical PCSS

To figure out the effects of aluminum-doped ZnO (AZO) and Ag layers for the transparent electrode vertical (TEV) photoconductive semiconductor switch (PCSS) reported in our previous works, several different structures of high power, new vertical, extrinsic-triggered PCSS were designed and their performances are presented. All the PCSSs were fabricated on the vanadium compensated semi-insulating (VCSI) 4H-SiC substrates. The ON-state performance of all device versions was characterized by a test circuit without load resistor. Among all structures, the version with AZO transparent window, AZO subcontact layer, and silver mirror reflector has been testified to be the most reasonable structure, and shows the best results as well. Its minimum ON-state resistance is 6.11 Ω at the optical power density of 2.62 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The factor for the best ON-state performance of this structure is attributed to the improvement of utilization efficiency of laser energy, and good contact between the AZO and SiC substrate.

Enhanced Performance of a Visible Light Detector Made with Quasi-Free-Standing Graphene on SiC.

The excellent optoelectronic properties of graphene give it great potential for applications in optical detection. Among the graphenes obtained through many synthetic methods, epitaxial graphene obtained by thermal decomposition on silicon carbide has remarkable advantages for preparing photodetectors. In this research, epitaxial graphene has been successfully prepared on a silicon surface (0001) of semi-insulating 4H-SiC substrate with a size of 10 mm × 10 mm and epitaxial graphene has been converted to quasi-free-standing graphene by hydrogen passivation. Two metal-graphene-metal photodetectors were fabricated using the two types of graphenes above and the photo-absorption properties of detectors have been investigated under 650-nm laser illumination with different illumination powers. From a comparison of the performances between the two detectors, it was found that a photodetector fabricated with quasi-free-standing graphene shows enhanced performance under a light power of 0.018 mW. Responsivity and external quantum efficiency reach maxima of 5.11 A/W and 9.74%, respectively. This dramatic improvement is mainly due to the disappearance of the buffer layer in epitaxial graphene, providing a new method to achieve optimization of graphene-based opto-electrical devices.

High-voltage LDIMOSFETs on HPSI 4H-SiC substrate with dual field plates

High-voltage lateral double-implanted metal-oxide-semiconductor field-effect transistor on high-purity semi-insulating 4H-SiC substrate with dual field-plates on both gate and drain was experimentally demonstrated and analyzed by a simulation method. E-field crowding under gate and drain was successfully suppressed by the optimized dual field-plates so that high breakdown voltage of 1.6 kV was achieved at Ldrift = 20 μm. Also, we obtained a low Ron,sp (specific on-resistance) of 30 mΩ cm2 at Ldrift = 20 μm by shrinking the channel length and controlling doping concentration of a drift region.

SiC Vertical-Channel n- and p-JFETs Fully Fabricated by Ion Implantation

Silicon carbide (SiC) n-and p-channel junction field effect transistors (JFETs) with vertical channels were fabricated by direct ion implantation into a high-purity semi-insulating 4H-SiC substrate in order to further develop the path towards complementary JFET integrated circuits for applications in harsh environments. Compared with the conventional structure (lateral channel), the proposed structure is suitable for integration and inherently has a high transconductance owing to the double-gate configuration. The threshold voltage (Vth) can be controlled by mask design, while Vth in the conventional structure is solely determined by the ion implantation conditions. We demonstrate the transistor operation of the vertical-channel n-and p-channel JFETs fully fabricated by ion implantation.

High-Temperature Hall Effect Sensor Based on Epitaxial Graphene on High-Purity Semiinsulating 4H-SiC

In this report, we demonstrate a novel high-temperature Hall effect sensor that is based on quasi-free-standing monolayer graphene epitaxially grown on high-purity semiinsulating (SI) on-axis 4H-SiC(0001) substrate in a chemical vapor deposition process. To ensure statistical perspective, characteristics of 23 elements are determined as a function of temperature ranging from 300 to 770 K. Passivated with a 100-nm-thick atomic-layer-deposited aluminum oxide, the sensor offers current-mode sensitivity of 80 V/AT with thermal stability of -0.02%/K within the range between 300 and 573 K, and -0.06%/K between 573 and 770 K. The sensor's room-temperature output voltage is monitored in the magnetic field from -300 to +300 mT and its offset voltage at 0 T is assessed. Its high-temperature electrical properties are explained through a double-carrier transport involving spontaneous-polarization-induced holes in the graphene layer and thermally activated electrons emitted from a deep acceptor level related to silicon vacancy V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Si</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-/2-</sup> occupying the k site of the 4H-SiC lattice. The sensor is compared with a previously reported one on vanadium-compensated SI on-axis 6H-SiC(0001). The new sensor's applicability to magnetic field detection at high temperatures is verified.

Radiation Defects in Heterostructures 3C-SiC/4H-SiC

The effect of 8 MeV proton irradiation on n-3C-SiC epitaxial layers grown by sublimation on semi-insulating 4H-SiC substrates has been studied. Changes in sample parameters were recorded using the Hall-effect method and judged from photoluminescence spectra. It was found that the carrier removal rate (Vd) in 3C-SiC is ~100 cm−1, which is close to Vd in 4H-SiC. Compared with 4H and 6H silicon carbide, no significant increase in the intensity of the so-called defect-related photoluminescence was observed. An assumption is made that radiation-induced compensation processes in 3C-SiC are affected by structural defects (twin boundaries), which are always present in epitaxial cubic silicon carbide layers grown on substrates of the hexagonal polytypes.