Semi-insulating Silicon Carbide Research Articles

Enhanced Crystallinity of SrTiO3 Films on a Silicon Carbide Substrate: Structural and Microwave Characterization

Thin films of strontium titanate, which reveal high structure quality and tunable properties prospective for microwave applications at room temperature, were grown on a semi-insulating silicon carbide substrate using magnetron sputtering for the first time. The films’ growth mechanisms were studied using medium-energy ion scattering, and the films’ structures were investigated using X-ray diffraction. The electrical characteristics of planar capacitors based on strontium titanate films were measured at a frequency of 2 GHz using a high-precision resonance technique. It is shown that the tendency to improve the crystalline structure of strontium titanate film with an increase in the substrate temperature is most pronounced for films deposited at elevated working gas pressure under low supersaturation conditions. Planar capacitors formed on the basis of oriented SrTiO3 films on silicon carbide showed tunability n = 36%, with a loss tangent of 0.008–0.009 at a level of slow relaxation of capacitance, which is significantly lower than the data published currently regarding planar tunable ferroelectric elements. This is the first successful attempt to realize a planar SrTiO3 capacitor on a silicon carbide substrate, which exhibits a commutation quality factor more than 2500 at microwaves.

Monolayer MoS[formula omitted] CVD-growth on SiC substrates assisted with KCl

This study examines the growth of molybdenum disulfide (MoS2) on semi-insulating silicon carbide (SI-6H-SiC) substrates using chemical vapor deposition (CVD). Raman and photoluminescence measurements were utilized to characterize pristine and grown with a potassium chloride (KCl) nucleation promoter MoS2. The Raman spectra showed a frequency difference (Δω) of 18.7 cm−1 between the A1g and E2g modes for pristine MoS2, confirming its monolayer nature, while KCl-modified MoS2 exhibited a Δω of 20.4 cm−1. Pristine MoS2 demonstrated compressive strain due to lattice mismatch, with values up to −1.4%, and a low dependency of charge doping shift on Δω. Conversely, KCl-modified MoS2 flakes showed smaller strain, up to −0.4%, and a significant dependency of charge doping shift on Δω. Electrical characterization of 2-Terminal devices indicated a ratio of the high-resistance state to low-resistance state close to 1 for pristine MoS2 and 1.75 for KCl-modified MoS2. Low-temperature photoluminescence measurements suggest that the observed behavior is due to the interaction of MoS2 with KCl.

Fabrication of silicon carbide color center nanoparticles by femtosecond laser ablation in liquid

Combined with the fabrication of low-dimensional materials, certain silicon carbide color centers can become excellent platforms for nano-optics, and be applied to quantum and biomedical fields. In this work, color center nanoparticles were prepared based on high purity semi-insulating silicon carbide substrate using liquid-assisted femtosecond laser machining. Effects of multi-pulse array ablation and line scanning fabrication were investigated, and parameters such as repetition frequency and pulse number were also optimized. We employed photoluminescence spectroscopy, field emission scanning electron microscopy and transient fluorescence spectroscopy to characterize optical properties and micromorphology of nanoparticles. The broadband photoluminescence within the range of 850–950 nm should be attributed to silicon vacancy color centers, and V1/V1’ zero-phonon lines were confirmed at low temperature. It is noted that the femtosecond laser annealing is critical for the luminescence enhancement of color center nanoparticles. The evolution of particles and cavitation bubbles during processing was elucidated. After optimization, silicon vacancy color center nanoparticles were prepared by multi-pulse (107) array ablation and 200 kHz pulse repetition frequency, with an average diameter of approximately 15 nm and an average density reaching 273.94 counts/μm2. The sample can be preserved stably in the form of nanoparticle solutions. Processing methods and corresponding conclusions of this study would be applicable to the femtosecond laser fabrication and micromorphology control of color center nanoparticles of various hard-brittle semiconductors.

Non-destructive mapping of electrical properties of semi-insulating compound semiconductor wafers using terahertz time-domain spectroscopy

With an increasing demand for THz electronics applications, it is essential to obtain the electrical properties of semiconductor materials used in their fabrication. This study presents an approach to estimate the electrical properties of semi-insulating compound semiconductors using terahertz time-domain spectroscopy (THz-TDS). By employing THz-TDS in transmission mode, the resistivity and carrier concentration of semi-insulating Silicon Carbide (SiC) and Indium Phosphide (InP) wafers are non-destructively measured. The study utilizes the simplified Drude model and the Nelder-Mead algorithm to estimate the electrical properties from the THz-TDS transmission (S21) measurement data. The resistivity values of the SiC and InP samples, (1.42 ± 0.15) × 105 Ω cm and (3.0 ± 0.1) × 107 Ω cm, respectively, are in accordance with the manufacturer specifications. Moreover, the feasibility of non-destructive mapping of the electrical properties of the wafers is demonstrated, offering a novel tomographic inspection approach for online monitoring. This technique holds significant promise for enhancing production yields in the semiconductor industry.

30 GHz ferroelectric phase shifter on silicon carbide

Thin ferroelectric barium-strontium titanate (BST) films of high structure quality have been grown on semi-insulating silicon carbide substrates. The crystal structure and elemental composition of the obtained films were studied by the X-ray diffraction and Medium Energy Ions Scattering. Planar capacitors based on these BST films reveal the combination of high tunability and low losses at microwaves. 30 GHz ferroelectric phase shifter demonstrating a figure of merit of 22 deg/dB with a phase shift of Δϕ = 220 deg is implemented based on BST films on silicon carbide substrate.

Fabrication of Ohmic Contact on N-Type SiC by Laser Annealed Process: A Review

In recent years, because of stringent needs in the fabrication of silicon carbide (SiC) power devices, laser annealing has been introduced to achieve local ohmic contact. In this paper, the laser annealing research for the ohmic contact process of SiC power devices is reviewed, which is mainly divided into four aspects: laser process mechanism, ohmic contact electrode materials, and substrate materials. The effect of laser parameters on ohmic contact and the annealing process on SiC diode devices is also reviewed. Progress of other substrate materials, namely 6H-SiC and semi-insulating 4H-SiC-based devices with laser annealed ohmic contacts, is also briefly discussed, in which formation of semi-insulating SiC ohmic contacts is derived from laser irradiation at the interface to produce 3C-SiC. Some experiment results have been shown in the passage, such as XRD, SEM, TEM, etc. In the review, it points out that the direction of application and development of the laser annealing process for improving the ohmic contact of SiC power devices is highly encouraging.

Nearly Ideal Breakdown Voltage Observed in Lateral p-i-n Diodes Fabricated on a SiC High-Purity Semi-Insulating Substrate

Lateral p-i-n diodes with small i-region width (less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5 ~\mu \text{m}$ </tex-math></inline-formula> ) were fabricated by direct ion implantation into a high-purity semi-insulating silicon carbide (SiC) substrate. The breakdown voltage of the diodes increased with increasing the i-region width and a lateral effective breakdown electric field of 1.7–1.9 MV/cm was obtained. The experimental breakdown voltage well agreed with the technology computer-aided design (TCAD) simulation, indicating that deep levels contributing the semi-insulating property do not seriously deteriorate the breakdown voltage in case of the small i-region width.

Terahertz monolithic integrated narrow-band filter based on the silicon carbide substrate

This paper presented a narrow-band terahertz integrated cavity filter based on the silicon carbide (SiC) substrate. By introducing two metalized via-holes in a rectangular integrated cavity, an integrated coupling cavity was built, on the one hand, to operate as a coupling structure; on the other hand, to control the direct coupling between input and output feeding lines, further to control the frequency of the transmission zero. This novel approach was thoroughly investigated with attention paid to the position of the via-holes. Based on this approach, a terahertz narrow-band filter was realized on the semi-insulating SiC substrate, demonstrated good filtering performance, especially the stopband rejection characteristic, and established the groundwork for the production of the terahertz monolithic integrated circuit.

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.

Internal modified structure of silicon carbide prepared by ultrafast laser for wafer slicing

Silicon Carbide (SiC) is an important substrate material for electronic components due to its excellent properties. Compared with wire sawing technology, laser slicing technology is a promising method that can achieve the slicing of SiC wafers without kerf-loss. In this paper, the modified layer for SiC slicing was obtained by ultrafast laser processing inside semi-insulating SiC wafers. We studied the effect of pulse duration on the internal modification process and discussed the formation mechanism of the modified layer according to morphology and composition. The effect of laser processing parameters on the morphology of the modified layer was systematically studied as well as tensile tests were carried out on samples with different morphologies of the modified layer, which revealed the effect of the modified layer on the peeling. The results show that the increase in pulse duration favors the formation of internal modified structures, which are mainly composed of amorphous Si, amorphous C, amorphous SiC and SiC crystals. The change of process parameters will affect the type of structure and the formation of cracks in the modified layer. Only the sample with a single-crack layer was exfoliated successfully, proving that the crack morphology has an essential influence on the peeling.

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.

Lateral spreads of ion-implanted Al and P atoms in silicon carbide

Lateral spreads of Al and P atoms implanted (10 ∼ 700 keV) into a high-purity semi-insulating 4H silicon carbide (SiC) substrate have been experimentally investigated. Scanning capacitance microscopy observation revealed that lateral distributions are highly dependent on the implantation energies of ions. Asymmetric lateral distributions were observed along the direction, which may be attributed to the 4° off-angle of the substrate. The lateral spreads are determined to be 0.33 μm for Al atoms, 0.38 ± 0.02 μm for P atoms along the direction, and 0.28 μm/0.38 μm for Al atoms, 0.25 μm/0.40 μm for P atoms along the direction (upstream/downstream side of step flow). Comparison between the lateral spreads of Al and P atoms is discussed with Al and P stopping powers and ranges in SiC.

Development of a World Class Silicon Carbide Substrate Manufacturing Capability

Silicon carbide (SiC) semiconductor substrates provide the foundation for revolutionary improvements in the cost, size, weight and performance of a broad range of military and commercial radio frequency (RF) and power switching devices. Due to the lack of a viable, native gallium nitride (GaN) substrate, semi-insulating (SI) SiC substrates are the substrate of choice for high power AlGaN/GaN High Electron Mobility Transistors (HEMTs) due to their near lattice-match to GaN, superior thermal conductivity and commercial availability. GaN has emerged as the technology of choice for RF power because of its superior output power capability compared to other semiconductors. Similarly, semi-conducting (N+) SiC substrates are required for fabrication of high voltage Schottky Diodes and Metal Oxide Semiconductor Field Effect Transistor (MOSFET) power switching devices. Critical to this realization is the availability of affordable, high quality, large diameter SI and N+ SiC substrates for production of GaN and SiC power semiconductors. SiC is unique in that bulk single crystals cannot be grown via traditional melt-based manufacturing processes such as Czochralski. Rather, a high temperature sublimation process is required. In the late 1980’s, pioneering physical vapor transport research taking place at North Carolina State University ultimately led to the formation of Cree Research and subsequently the wide bandgap semiconductor industry. U.S. Department of Defense investment in wide bandgap semiconductor development has easily exceeded $1B spawning the creation of an entirely new industry. During the early 1990’s, SiC physical vapor transport growth development was fraught with perceived insurmountable technical challenges associated with micropipes, polytype conversion, diameter expansion and crystalline defects. Despite these monumental crystal growth technology hurdles, SiC substrates are now manufactured at a cost and quality never thought possible. This article highlights more than 20 years of Air Force Research Laboratory (AFRL) sponsored development with II-VI aimed at positioning itself as a merchant, world-class manufacturer of SiC substrates.

Normally-off 400 °C Operation of n- and p-JFETs With a Side-Gate Structure Fabricated by Ion Implantation Into a High-Purity Semi-Insulating SiC Substrate

We demonstrate normally-off 400 °C operation of n-channel and p-channel junction field-effect transistors (JFETs) fabricated by an ion implantation into a common high-purity semi-insulating silicon carbide (SiC) substrate. The side-gate structure proposed in this letter has good controllability of threshold voltage. The present results assure the potential of JFET-based complementary logic integrated circuits (ICs) operational at high temperature.

The Test of a High-Power, Semi-Insulating, Linear-Mode, Vertical 6H-SiC PCSS

A high-power photoconductive semiconductor switch (PCSS) working in linear mode can be used for RF generation by modulating the illuminating light. Such a PCSS constructed of a semi-insulating 6H silicon carbide (6H-SiC) substrate and its characterization under a high electric field is presented. The PCSS is of a vertical structure, with one hollow/transparent electrode to let through the laser light and the other highly reflective to increase light absorption. First, to optimize the high-voltage withstanding, the factors affecting the field enhancement, such as substrate thickness, electrode profiles, and encapsulating materials, are discussed. Then, the PCSS is assembled and tested, and the 0.8-mm-thick 6H-SiC PCSS is able to work with electric fields up to 225 kV/cm and the power capacity up to 10.6 MW without failure when triggered by a Nd:YAG laser (532-nm, 17-ns full wavelength at half maximum). By varying the optical energy from 3 to 31.9 mJ, the minimum conducting resistance and the peak photocurrent of the PCSS are obtained. The PCSS fails with further increasing voltage, and the cause of the failure was identified to be a bulk breakdown. The result shows that the PCSS can work linearly and it is possible to be used in high-power compact RF generation.

Influence of the FIB parameters on the etching of planar nanosized multigraphene/SiC field emitters

The possibility of fabrication of planar field emission nanostructures based on multigrafene films on semiinsulating silicon carbide using focused ion beam (FIB) technology is considered in this paper. The effect of FIB-parameters on the etching of the structure was determined using a scanning probe microscope. Conductive probes were used for local studies of electrical properties of nanoscale structures. It was found that etching of planar field emission nanostructures at a current of 1 pA did not lead to a change in the depth of the treated area. An increase in current up to 10 pA was sufficient to initiate the etching process of a multigraphene film on the surface of silicon carbide.

Structural and microwave characterization of BaSrTiO3 thin films deposited on semi-insulating silicon carbide

Thin ferroelectric barium strontium titanate (BST) layers of high structure quality have been grown for the first time directly on semi-insulating silicon carbide substrates by RF magnetron sputtering of a ceramic target. The structural and microwave properties of the films were substantially improved by an intermediate annealing of the layers during the growing process. Prepared under this approach, BST films have a well-formed crystal structure, which has a positive effect on their electrical properties, specifically on nonlinearity and dielectric losses. Planar capacitors based on these BST films demonstrate the best combination of high tunability and low losses for BST/SiC structures at microwaves.

Study of the electric field strength in planar multigraphene/SiC field emission nanostructures with different arrangement of the electrode planes

The paper describes simulation of planar field emission nanostructures on the basis of graphene on a semiinsulating silicon carbide. The effect of arrangement of the electrode planes on the field strength and field gain is shown. The model is considered with a nanoscale interelectrode distance and a potential difference of 10 V.

High-Temperature Operation of n- and p-Channel JFETs Fabricated by Ion Implantation Into a High-Purity Semi-Insulating SiC Substrate

Silicon carbide (SiC) n-channel and p-channel junction field-effect transistors (JFETs) were fabricated by direct ion implantation into a high-purity semi-insulating 4H-SiC substrate toward complementary-JFET integrated circuit applications under a harsh environment. The fabricated n-JFET and p-JFET on a common substrate without an epitaxial layer show normal transistor operations up to 400 °C. Their electrical characteristics are comparable with theoretical estimations obtained from material parameters of n- and p-type SiC epitaxial layers. The present results assure that not only JFETs but also a variety of devices can be made by direct ion implantation while keeping material properties of epitaxially grown SiC.

The effect of adsorbates on the electrical stability of graphene studied by transient photocurrent spectroscopy

Adsorbate induced variations in the electrical conductivity of graphene layers with two different types of charge carriers are investigated by using the Transient Photocurrent Spectroscopy (TPS) measurement technique. In-vacuum TPS measurements taken for a duration of 5 ks revealed that the adsorption/desorption of atmospheric adsorbates leads to more than a 110% increment and a 45% decrement in the conductivity of epitaxial graphene (n-type) and chemical vapor deposition graphene (p-type) layers on semi-insulating silicon carbide (SiC) substrates, respectively. The graphene layers on SiC are encapsulated and passivated with a thin SiO2 film grown by the Pulsed Electron Deposition method. The measurements conducted for short periods and a few cycles showed that the encapsulation process completely suppresses the time dependent conductivity instability of graphene independent of its charge carrier type. The obtained results are used to construct an experimental model for identifying adsorbate related conductivity variations in graphene and also in other 2D materials with an inherently high surface-to-volume ratio.