*3.1. Epitaxial 3C-SiC Films*

A cross-sectional SEM view of the wafer, which allows for accurate estimation of film thickness (n<sup>+</sup>-, p-SiC, Si device film, and buried oxide), is shown in Figure 2a. This figure highlights various layers and the approximate thickness of each layer on the wafer used for the fabrication. The two epitaxial 3C-SiC films were measured, and their combined thickness determined to be ~10 μm. The SOI Si device layer (~26 μm), as well as the thin (~2 μm) buried oxide layer are also visible in this figure. The epitaxial n<sup>+</sup> film in the center of the wafer was quite rough with a mean surface roughness of ~244 nm and smoother near the wafer edge with a mean surface roughness of ~21 nm. Figure 2b shows surface morphology of the smooth n<sup>+</sup> layer, which was taken using a DI AFM (Dimension 3100). Although rough in the wafer center (Figure 2c), the surface roughness was low enough for thick layers of photoresist to properly cover the entire surface for the subsequent fabrication steps. However, this roughness would be expected to impact device electrical performance, particularly p-n diode leakage current.

**Figure 2.** Analysis of epitaxial SiC results. (**a**) Cross-section SEM micrograph of the 3C-SiC epi films on SOI. (**b**) AFM image (tapping mode) of the 3C-SiC epiwafer specular region on the wafer periphery that shows typical 3C-SiC surface morphology (mean roughness of ~21 nm). (**c**) AFM image (tapping mode) of the rough surface of the same epiwafer (center) (mean roughness of ~244 nm). The devices were fabricated from the center of the wafer.

#### *3.2. Fabricated All-SiC Neural Probe*

Epitaxial growth of single crystalline 3C-SiC with di fferent types of doping enables realization of a nearly monolithic probe from homogeneous SiC material. The all-SiC probe is a Michigan-style, planar neural probe with 16 electrodes for recording and stimulating neurons. The connector tab

has 18 metallic pads (approximately 0.8 mm by 0.4 mm) with through holes to which a commercial Omnetics connector is bonded. Two extra pads provide connections for the return and reference electrode wires. The diameter of the electrode sites is ~15 μm and width of the traces is ~10 μm. Figure 3 shows the optical and SEM micrographs of a free-standing probe.

The probe's shank, which contains the traces and electrode sites, is shown in Figure 3b. This figure shows a scanning electron micrograph of the electrode sites, which have *a*-SiC windows on top to allow contact with the extracellular environment. The traces and electrode sites are mesas formed from the n<sup>+</sup> 3C-SiC film. There are no metallic components on the shank, which is a homogeneous structure consisting entirely of SiC. The pads, which are shown in Figure 3c, contain titanium and gold layers in order to provide ohmic connections to external electronic devices via the Omnetics connector. However, since the metallic pads are not in direct contact with brain tissue, the issues regarding delamination of metallic parts and compatibility with CNS tissue are not a concern.

**Figure 3.** Physical characterization of the completed neural probe. (**a**) Optical image of a freestanding all-SiC probe after release. (**b**) SEM image of the shank tip showing four of the electrode sites and a magnified image of a single electrode site (inset). (**c**) SEM image of some of the metal contact pads with through holes. The shank is 5.1 mm long and the length of the tapered portion is 2.4 mm. The tab is 6.64 mm wide and 2.3 mm long, excluding the semi-circular top portion. The surface roughness of the electrode sites is shown in Figure 2c.

#### *3.3. Electrical and Electrochemical Characterization*

The doping density of the top n<sup>+</sup> 3C-SiC film was determined by measuring the capacitance voltage profile of the Schottky contact at 1 MHz and ND-NA was estimated to be ~10<sup>19</sup> cm<sup>−</sup>3. A similar measurement was also performed on the p-type epitaxial film exposed after DRIE processing and NA- ND was estimated to be ~10<sup>16</sup> cm<sup>−</sup>3. These measurements indicate the feasibility of p-n junction formation between two epi films and high electrical conductivity of the top semi-metallic n<sup>+</sup> film that formed the traces and electrode sites. EIS was done to confirm this expectation.

As shown in Figure 4a, current-voltage (I-V) measurements on individual diode structures had a rectifying effect due to the diode formed between the <sup>n</sup>+- and p-type epitaxial films. In order to measure turn-on and breakdown voltages and the reverse leakage current, the I-V plot for four diodes on the same wafer was measured. The averaged turn-on voltage for these four diodes was determined to be ~1.4 V, with an average leakage current less than 8 μArms. In addition, Figure 4a also contains a current-voltage curve, obtained from measurements on one of the IDEs, showing isolation between adjacent traces.

CV curves for four test microelectrodes of the same surface area (491 μm2) in 7.4 pH PBS are shown in Figure 4b. The upper (+800 mV) and lower (−600 mV) boundaries for the potential were based on the electrochemical window for Pt in water. The shape of the hysteresis cycle showed that the anodic and cathodic currents were charge balanced, with no indication of faradaic current resulting from oxidation or reduction reactions between +800 mV and −600 mV. However, the phase behavior of the electrode-electrolyte interface (Figure 4d) only supports a capacitive-dominant mechanism at higher frequencies (e.g., −61.2 ± 3.7◦ at 1 kHz), while at lower frequencies the phase indicates a faradaic current (e.g., −30.3 ± 4.9◦ at 100 Hz), which contrasts with earlier results from 4H-SiC

microelectrodes [33]. The average anodic charge storage capacity (CSC) was 15.4 ± 1.46 mC/cm<sup>2</sup> (mean ± standard deviation) and the cathodic CSC was 15.2 ± 1.03 mC/cm2. The average anodic charge per phase was 75.4 ± 5.06 nC and the average cathodic charge per phase was 74.8 ± 5.06 nC.

Figure 4c,d show the EIS results for the same four test microelectrodes. As expected, the impedance magnitude was found to increase with decreasing frequency. At a frequency of 1 kHz, the impedance was 165 ± 14.7 kΩ (mean ± standard deviation). The electrode-electrolyte interface was determined to be predominately capacitive, as indicated by the negative phase angles for higher frequencies (i.e., >1 kHz).

**Figure 4.** All-SiC p-n diode and n-p-n junction characterization and electrochemistry for four test microelectrodes with an area of 491 μm2. (**a**) I-V measured from a p-n diode and a n-p-n junction between adjacent traces fabricated on the same wafer used for probe fabrication. (**b**) The cyclic voltammetry curves swept between +800 mV and −600 mV with a scan rate of 50 mV/s. (**c**) EIS Impedance (Z) magnitude (~165 kΩ @1kHz) and (**d**) impedance phase angles. The curve for each microelectrode (b-d) is the average of three replicates.
