## 5 Material Platforms for Integrated Photonics
\begin{table}
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**Material Platform** & **Si** & **Si\({}_{3}\)N\({}_{4}\)** & **InP** & **GaAs** &
\begin{tabular}{} **Lithium Niobate (LiNbO\({}_{3}\))** \\ \end{tabular} \\ \hline Material type & Semiconductor & Dielectric & Semiconductor & Semiconductor & EO crystal \\ \hline Refractive index & \textasci{} & \textasci{} & \textasci{} & \textasci{} \\ \hline Transparency range & 1.1 μm to 9 μm & 0.4 μm to 5 μm & 1 μm to 2.6 μm & 0.9 μm to 1.8 μm & 0.35 μm to 5 μm [118] \\ \hline Nonlinear coefficient & Low & Low & High & High & Very high [119] \\ \hline WG loss & Low & Low & Very low & Low & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 2: General comparison of different material platforms. [PERSON] et al. presented a novel van der Waals PN heterojunction PD, comprising p-type black phosphorous and n-type molybdenum telluride, seamlessly integrated into a Si\({}_{3}\)N\({}_{4}\) WG (Figure 5a) [141]. Figure 5b illustrates the energy band diagrams of the black phosphorous/molybdenum ditelluride van der Waals heterostructure in both non-equilibrium (top panel) and thermal equilibrium states (bottom panel). To accurately assess \begin{table}
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**Material Platform** & **Si** & **Si\({}_{3}\)N\({}_{4}\)** & **InP** & **GaAs** & **Lithium Niobate (LiNbO\({}_{3}\))** \\ \hline Fabrication method & CMOS-compatible [120] & CMOS-compatible [120] & Epitaxial growth & Epitaxial growth & Bulk crystal growth \\ \hline Applications & Optical interconnects, sensors [121; 122] & Filters, sensors, modulators sensors [122; 123; 124] & Lasers, detectors [125; 126] & Lasers, detectors [127; 128] & Modulators, switches [129; 130] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 4: Two-dimensional materials, synthesis, and applications. the intrinsic responsivity and internal quantum efficiency of the PD, it was essential to quantitatively extract the absolute light absorption by the black phosphorous/molybdenum ditelluride heterostructure in the waveguiding mode. This was achieved using a Mach-Zehnder interferometer (MZI) based on the Si\({}_{3}\)N\({}_{4}\) WG, depicted in Figure 5c. The intrinsic electric field within the PN heterojunction notably reduced the dark current and enhanced responsivity. When a 1 V bias was applied from the n-type molybdenum telluride to the p-type black phosphorous, the dark current remained below 7 nA, which was over two orders of magnitude lower than that of other WG-integrated black phosphorus PDs. This configuration achieved an intrinsic responsivity of up to 577 mA \(W^{-1}\). Notably, the van der Waals PN heterojunction could be further optimized through electrostatic doping, enhancing its rectification properties and boosting the responsivity to 709 mA \(W^{-1}\). Additionally, the PD demonstrated a response bandwidth of approximately 1.0 GHz and maintained consistent photodetection across a broad spectral range, from 1500 to 1630 nm, as verified experimentally [141]. The integration of this van der Waals PN heterojunction PD on a chip, featuring low dark current, high responsivity, and rapid response, offered significant potential for developing efficient on-chip PDs suitable for various photonic integrated circuits based on Si, LN, polymers, and other materials. Figure 5: **(a)** Graphical illustration of the WG-integrated van der Waals PN heterojunction PD, featuring stacked p-doped black phosphorous and n-doped molybdenum ditelluride layers, which are evanescently coupled with the Si\({}_{3}\)N\({}_{4}\) WG’s guiding mode [141], **(b) Top**: Band profiles of black phosphorous and molybdenum ditelluride in the non-equilibrium state.