High Crystalline Quality PtSe₂ Films for High Frequency Optoelectronics

Transition metal dichalcogenides (TMDs) are very attractive two-dimensional materials for optoelectronic applications, due to their various and tunable bandgaps, to the strong light-matter interaction per 2D layer and to the possibility to transfer and integrate these 2D materials on CMOS or photonic platforms.<br/>While the most well-known TMDs (MoS₂, WSe₂,…) exhibit a bandgap in the visible range, PtSe₂ is an original and remarkable TMD particularly adapted for optoelectronics in the infrared (IR) domain. It exhibits a bandgap varying from 1.2eV (monolayer) to 0.2eV (bilayer) and becomes semi-metallic from few layers to bulk. Thus, PtSe₂ absorption can be easily tuned to absorb IR light by varying its thickness from two to multilayers.<br/>PtSe₂ is air stable and particularly adapted for high-frequency optoelectronics with a very high theoretical (4000 cm²/V.s) and a high experimental (e.g. ~ 200 cm²/V.s) carrier mobility at room temperature (Note that larger mobilities are reported in the literature which remain however difficult to measure due to the difficulty to estimate actual carrier concentration in semimetal multi-layered PtSe₂).<br/>Due to its high intrinsic qualities, PtSe₂ is studied to fabricate high frequency photodetectors operating at the 1.55 µm telecom wavelength. Based on transferred thin PtSe₂ films grown by CVD, 17 GHz bandwidth photodetectors have been fabricated on a SiO₂/Si substrate and 35 GHz bandwidth waveguide photodetectors have been realized on a silicon nitride platform.<br/>High frequency optoelectronics requires both efficient photodetection and high-speed transport and thus materials exhibiting high crystalline quality.<br/>PtSe₂ has been synthesized by mechanical exfoliation, CVD, thermally assisted conversion and molecular beam epitaxy (MBE). To evaluate PtSe₂ crystalline quality, most groups have studied the full width at half maximum (FWHM) of the in-plane Eg Raman peak and shown that thinner is the Eg peak, higher is the crystalline quality and the electrical conductivity.<br/>Here, we have studied the growth of high quality PtSe₂ by MBE on sapphire, which is a low cost, insulating substrate and have fabricated high frequency optoelectronic devices on 2 inches sapphire substrates. We show 8-nm thick PtSe₂ films exhibiting average FWHM Eg values as low as 3.7 cm^-1and electrical conductivities as high as 1.5 mS. Integrated in a coplanar wave guide, we demonstrated 1.55 µm photodetectors with a 60 GHz bandwidth and optoelectronic mixers exhibiting a flat response between 1 and 30 GHz. Optoelectronic mixing is a key technical enabler for future broadband and multifunctional RF systems such as radio-over-fiber communication systems, radar systems, and satellite payloads.<br/>To further increase PtSe₂ crystalline quality, we are studying two methods: step-guided epitaxy and van der Waals epitaxy. Step-guided epitaxy on sapphire consists in growing on dedicated vicinal surfaces to favor the oriented nucleation of the crystals at the step edges (this method has been demonstrated by other groups for MoS₂ and WS₂ on sapphire). We will also show first results targeting van Der Waals (vdW) epitaxy of PtSe₂ on h-BN.