# A Near-Ultraviolet Photodetector Based on the TaC: Cu/4 H Silicon Carbide Heterostructure
## 3 Results and Discussions
### Electrical and Optical Properties  
The resistivity and electrical conductivity measurements of the TaC: Cu alloy were obtained based on the sample deposited on a glass substrate using a four-point probe system (Ossila T2001A3 (Ossila, Sheffield, UK)). To ensure accuracy, measurements were conducted at three different positions of the sample, and the average values were calculated. The TaC: Cu alloy exhibited a resistivity of 14.1 \(\upmu\Omega\cdot\)cm and a high electrical conductivity of 9.3 KS/m at room temperature. This notable conductivity is attributed to the enhanced charge carrier density introduced by copper doping. The dielectric function (\(\psi\)) of the TaC: Cu film was characterized using variable-angle spectroscopic ellipsometry (VASE) over a spectral range from 200 nm to 1800 nm. Measurements were conducted at three distinct incident angles (55\({}^{\circ}\), 65\({}^{\circ}\), and 75\({}^{\circ}\)), and the resulting data were analyzed using a general oscillator model. The excellent agreement between the experimental data and the model-generated results, as illustrated in Figure 4a, validates the accuracy and reliability of the measurements. Furthermore, spectroscopic ellipsometry (SE) was performed over the same wavelength range, with a step size of 2 nm,  
Figure 3: (**a**) EDX elemental composition of TaC: Cu on a glass substrate (**b**) The schematic structure of the fabricated device (**c**) FIB image of the cross-section of the fabricated device with 300 nm scale par. to determine the optical constants of the TaC: Cu alloy. The analysis assumed a single uniform layer and ignored the surface roughness. The refractive index and film thickness were subsequently extracted from the ellipsometry fitting. Figure 4b depicts the real (n) and imaginary (k) components of the refractive index. The real component attains a maximum value of 3.9 at a wavelength of 1700 nm, whereas the imaginary component peaks at 1.58 at 700 nm and exhibits a gradual decline with increasing wavelength. This behavior indicates that the TaC: Cu alloy demonstrates a broad spectral detection capability, spanning from the ultraviolet (UV) to the near-infrared (NIR) regions. The film thickness, determined via ellipsometry without accounting for surface roughness, was measured to be 127 nm. The absorption and transmission characteristics of the proposed device were analyzed through simulations utilizing the Finite-Difference Time-Domain (FDTD) method, carried out using commercial Lumerical software (2024R2.1). Figure 5a illustrates the simulated structure, while the simulation setup is depicted in Figure 5b. A plane wave source spanning the spectral range of 200-1800 nm was injected along the \(Z\)-axis, with perfectly matched layer (PML) boundary conditions applied in the \(Z\)-direction and periodic boundary conditions in the \(X\) and \(Y\) directions. Monitors recorded the transmission (T) and reflection (R) data, while absorption was calculated using the relation \(1-R-T\)[58]. The refractive index of the TaC: Cu alloy and SiC substrate were imported into the simulation based on experimental measurements and values obtained from the published literature [59]. In addition, the thickness of the TaC: Cu alloy was set to 127 nm, as determined by ellipsometry measurements. As shown in Figure 5c, the simulation revealed a pronounced absorption peak of approximately 77% at 259 nm, with absorption decreasing to 73% at 405 nm and gradually diminishing across the near-infrared (NIR) region. Despite the decline, the film exhibited measurable absorption extending beyond 1800 nm. The simulated transmittance behavior of the TaC: Cu alloy on SiC is shown in Figure 5d. This hybrid structure exhibited near-zero transmittance in the 200-600 nm range due to high absorption in this region. However, transmittance steadily increased as the wavelength extended into the NIR regions.