High-purity Semi-insulating 4H-SiC 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.

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.

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

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

Charge transport in semi insulating bulk 4H-Silicon carbide: Effect of metallization and wafer homogeneity

We investigated the effect of metallization and spatial homogeneity on the charge collection of sensors prepared from high purity semi-insulating 4H-SiC bulk wafer. We used Au, Ni, Cr or graphene electrical contacts subsequently prepared on the same chip. We also tested sensors prepared from different positions on the wafer. Laser-induced transient current technique (L-TCT) and alpha spectroscopy were used to characterize the charge transport in the sensors. It was found that different sensors polarize at DC bias in the time interval of 0.2–270 s. We observed significant deviation of current transients measured on the sensor with a graphene contact from the sensors with metal contacts, which is attributed to the surface plasmon formed on the graphene-SiC interface. We demonstrated that the sensor polarization is independent of the metallization, whereas it strongly depends on the location on the wafer from which the sensor was cut. This testifies to a significant spatial inhomogeneity of the wafer. We also tested the electrical current relaxation after step-DC biasing as a simple method for the characterization of the sensor polarization. We found that the relaxation profile has extremes in the similar positions as the L-TCT profiles of collected charge. We also showed that using pulse biased L-TCT we are able to choose suitable sensors with slow polarization and to design optimal pulsing conditions including necessary depolarization times for the alpha spectroscopy measurement and L-TCT techniques to achieve long-term stable charge collection. In case of 241Am alpha spectroscopy charge collection efficiency 5–24 % was observed. We framed up model depicting all observed phenomena assuming the space charge formation induced by a blocking cathode in an n-type material and related lifetime reduction. We evaluated the space charge formed in the sensors after 104 s biasing, which ranged from 1.7 × 1014 cm−3 to 7 × 1014 cm−3 in respective sensors.

Effect of hydrogen on the unintentional doping of 4H silicon carbide

High-purity semi-insulating (HPSI) 4H silicon carbide (4H-SiC) single crystals are critical semiconductor materials for fabricating GaN-based high-frequency devices. One of the major challenges for the growth of HPSI 4H-SiC single crystals is the unintentional doping of nitrogen (N) and boron (B). The addition of hydrogen has been supposed to mitigate unintentional doping. However, the underlying mechanism has not been well understood. In this work, the role of hydrogen in the growth of HPSI 4H-SiC single crystals is investigated by first-principles formation-energy calculations. We find that the addition of hydrogen significantly mitigates N doping while hardly affecting B doping. Once hydrogen is added, hydrogen may adsorb at the growing surface of 4H-SiC, leading to surface passivation. Since N can react with hydrogen to form stable NH3 (g), the chemical potential of N is reduced, so that the formation energy of N in 4H-SiC increases. Hence, the critical partial pressure of nitrogen required for the growth of HPSI 4H-SiC single crystals increases by two orders of magnitude. Moreover, we reveal that the adjustment of relative B and N doping concentrations has a substantial impact on the Fermi energy of HPSI 4H-SiC. When the doping concentration of N is higher than that of B, N interacts with carbon vacancies (VC) to pin the Fermi energy at Z1/2. When the doping concentration of B is higher than that of N, the Fermi energy is pinned at EH6/7. This explains that the resistivity of unintentionally doped HPSI 4H-SiC may vary.

Photoexcited carrier dynamics in semi-insulating 4H-SiC by Raman spectroscopy

The photoexcited carrier dynamics of high-purity (HPSI) and vanadium-doped semi-insulating (VDSI) 4H-SiC irradiated by lasers with different wavelengths and powers were investigated. Raman spectra were measured at room temperature and the photoexcited carrier concentrations were extracted from the Raman line shape analysis of longitudinal optical phonon–plasmon coupled mode. It was found that the longitudinal optical (LO) peaks of HPSI and VDSI did not shift with laser power variations, due to a low concentration of photoexcited carriers, when a 532- nm laser was used. However, when a 355- nm laser was adapted, the relationship between the photoexcited carrier concentrations and the laser power was found to be nonlinear because of the dominance of trap-assisted Auger (TAA) recombination. The coefficient of TAA recombination was laser power–dependent. The proposed carrier dynamic model deepens the understanding of the physical mechanism of semi-insulating SiC irradiated by nanosecond laser and provides an insight into the interpretation of experimental phenomena related to laser energy in optoelectronic devices.

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.

Modified divacancies in 4H-SiC

Divacancies near or at lattice defects in SiC, the PL5–PL7 photoluminescence centers, are known to have more favorable optical and spin properties for applications in quantum technology compared to the usual divacancies. These centers were previously predicted to be divacancies near stacking faults. Using electron paramagnetic resonance, we observe PL5, PL6, and four other divacancy-like centers, labeled PLa–PLd, in electron-irradiated high-purity semi-insulating (HPSI) 4H-SiC. From the observed fine-structure D-tensors, we show that these centers including PL6, which has so far been believed to be an axial center, all have C1h symmetry. Among these, PLa, PLc, and PLd are basal divacancies and PL5 and PL6 are slightly deviated from axial symmetry, while PLb is different from others with the principal Dzz axis of the D-tensor aligning at ∼34° off the c-axis. We show that these modified divacancies are only detected in one type of HPSI materials but not in commercial n- and p-type substrates or n-type pure epitaxial layers irradiated by electrons regardless of surface treatments which are known to create stacking faults.

Carrier Trapping Effects on Forward Characteristics of SiC p-i-n Diodes Fabricated on High-Purity Semi-Insulating Substrates

Carrier trapping effects of high-concentration deep levels in high-purity semi-insulating (HPSI) silicon carbide (SiC) substrates on device characteristics were investigated. Vertical p-i-n diodes were fabricated on HPSI 4H-SiC substrates with ion implantation processes. The fabricated diodes showed good rectification. The experimental forward current&#x2013;voltage (<inline-formula> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula>&#x2013;<inline-formula> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula>) characteristics were analyzed by using drift-diffusion (DD) simulation that considers deep levels both as recombination centers and carrier traps. The DD simulation revealed that high-concentration carrier traps flatten the potential distribution in the i-type region and, therefore, increase the recombination current and ideality factor in the low-bias region. The simulated <inline-formula> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula>&#x2013;<inline-formula> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> characteristics assuming a high trap density of <inline-formula> <tex-math notation="LaTeX">$5 \times {10}^{16} \mathrm { \text {cm}^{-{3}}}$ </tex-math></inline-formula> showed good agreement with experimental ones in the temperature range of 295&#x2013;600 K.

Space charge formation in the high purity semi-insulating bulk 4H–silicon carbide

We investigated the carrier transport and space charge formation in the high purity semi-insulating 4H-SiC bulk single crystal. Using the Laser–induced Transient Current Technique we observed anomalous short current waveform oscillations that are caused by very short carrier lifetime ≤ 1 ns. We conclude from a detailed inspection of current waveform shape completed by the analysis of collected charge that the electron mobility decreases in an increasing electric field. The saturation value of electron drift velocity 8.7 × 106 cm2s-1 was evaluated. The Hall effect measurement revealed the n-type electrical conductivity and dominant donor energy of ED = 0.85 eV. We also observed the blocking character of prepared Au contacts, which induce the electron depletion and positive space charge formation in biased sample. The continuous increase of the space charge in a wide time interval of 10-1 s – 104 s, attested to testified on a presence of a dispersed impurity band localized below the Fermi energy. Based on these findings, we proposed a theoretical model describing the observed phenomena very well.

Output Characteristics of Side-Illuminated Photoconductive Semiconductor Switch Based on High Purity Semi-Insulating 4H-SiC

Photoconductive semiconductor switch (PCSS) allowing side illumination was fabricated on high purity semi-insulating (HPSI) 4H-SiC. A 532-nm pulsed laser with variable optical energy was used to trigger the PCSS. The performance of the PCSS was characterized under the two different load conditions, 50-Ω load and 0.05-Ω, with a current viewing resistor (CVR). The PCSS exhibited significantly different output characteristics for the two different loads. The equivalent resistance of the PCSS with the 50-Ω load, which was calculated from the output voltage and current, was inversely proportional to the optical energy, but the one with a 0.05-Ω load exhibited saturation behavior with the optical energy. While the times at peak output with the 50-Ω load were similar at various optical energies, the times at peak output with the 0.05-Ω load were dependent on the optical energy. Output current oscillation was also observed after the PCSS was turned off in the case of 0.05-Ω load condition. The different output characteristics for the different load resistances were analyzed using the transient response of the equivalent circuits. The PCSS exhibited a minimum on-state resistance of 0.27 Ω with the optical energy of 8 mJ and a maximum output current of 657 A at the bias voltage of 4.8 kV. The operating voltage of the PCSS was limited by surface flashover, which caused an additional output pulse following the first output pulse.

Side-Illuminated Photoconductive Semiconductor Switch Based on High Purity Semi-Insulating 4H-SiC

High purity semi-insulating (HPSI) 4H-silicon carbide (SiC) was used to fabricate lateral and vertical photoconductive semiconductor switches (PCSSs). The lateral PCSSs were illuminated from the frontside (fPCSS) or the backside (bPCSS). The side-illuminated vertical PCSS (vPCSS) was designed to increase the light-matter interaction volume. A 532-nm pulsed laser with adjustable energy was utilized to excite the PCSSs. The turn-on time was found to be highly dependent on the optical illumination energy, and the full-width at half-maximum of the PCSSs output waveforms was related to the peak output voltage. The output electrical pulse from the vPCSS exhibited a shorter turn-on time and a larger pulsewidth than the two types of lateral PCSSs. The vPCSS outperformed the fPCSS and bPCSS in terms of minimum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm{\scriptscriptstyle ON}$ </tex-math></inline-formula> -state resistance and output pulse amplitude under the same optical illumination energy. The vPCSS, which utilizes a large effective contact area to collect photogenerated carriers, also had higher photon absorption efficiency by arranging the optical path at a right angle to the carrier transport. The vPCSS exhibited a minimum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm{\scriptscriptstyle ON}$ </tex-math></inline-formula> -state resistance of 0.34 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> at optical illumination energy of 8 mJ.

Deep levels related to the carbon antisite–vacancy pair in 4H-SiC

Photo-induced current transient spectroscopy (PICTS) and electron paramagnetic resonance (EPR) are used to study irradiation-induced defects in high-purity semi-insulating (HPSI) 4H-SiC. Several deep levels with the ionization energy ranging from 0.1 to ∼1.1 eV have been observed in irradiated and annealed samples by PICTS. Among these, two deep levels, labeled E370 and E700 at ∼0.72 and ∼1.07 eV below the conduction band, respectively, are detected after high-temperature annealing. The appearance and disappearance of these two deep levels and the EPR signal of the positive C antisite–vacancy pair (CSiVC+) in the sample annealed at 1000 and 1200 °C, respectively, are well correlated. Based on data from PICTS and EPR and the energies predicted by previous calculations for different charge states of dominant intrinsic defects, the E370 and E700 levels are suggested to be related to the charge transition levels (0|–) and (+|0), respectively, of the C antisite–vacancy pair. The activation energy of Ea ∼ 1.1 eV in commercial HPSI 4H-SiC materials is, therefore, reassigned to be related to the single donor (+|0) level of CSiVC.

Charge state control of the silicon vacancy and divacancy in silicon carbide

Color centers in silicon carbide (SiC), such as the negative silicon vacancy (VSi−) and neutral divacancy (VSiVC0), have recently been shown to be promising quantum bits (qubits) for a variety of applications in quantum communications and sensing. Considerable effort has been spent on improving the performance of these optical spin qubits, and the instability of their charge state is an important issue to be solved. Using electron paramagnetic resonance to monitor the charge state of dominant intrinsic defects in n-type, high-purity semi-insulating and p-type 4H-SiC, we reveal carrier compensation processes and the windows of the Fermi level that allow us to obtain stable VSi− and VSiVC0 in equilibrium. We show that stable VSi− and VSiVC0 ensembles can be obtained in n-type (p-type) via controlling the concentration of the Si vacancy (the C vacancy and the C antisite–vacancy pairs). The charge-state control of single VSi− and VSiVC0 emitters is expected to be possible in pure p-type layers by controlling the concentration of the C vacancy. In ultrapure materials, optical repumping is required for charge state control of single emitters.

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.

The Effects of Illumination on Point Defects Detected in High Purity Semi-Insulating 4H-SiC

The electrical characterization of high-purity semi-insulating 4H-SiC is carried out by means of current deep level transient spectroscopy (I-DLTS). Measurements are performed by employing either an electrical or optical pulse (below/above bandgap). The study performed on as-grown material, either annealed or oxidized, reveals the presence of six levels with ionization energies in the 0.4-1.3 eV range.