Monitoring Structural and Electrical Properties of PtSe₂ Films for High Frequency Optoelectronics

PtSe₂ exhibits high carrier mobility and infrared absorption which are particularly suitable for high frequency optoelectronics at the 1.55 µm telecom wavelength. We have studied PtSe₂ films grown by molecular beam epitaxy (MBE) to demonstrate high-frequency photodetectors and mixers. Optoelectronic mixers can mix high-frequency optical and electrical signals, enabling innovative applications such as radio-over-fiber communication systems, wideband radio frequency transceivers for radar systems and satellite payload. We first measured the optical absorption at 1.55 µm and the conductivity of PtSe₂ films as a function of the number of monolayers (ML) and focused our studies on thick (12-16 MLs) semimetallic films that exhibit both good IR absorption and high conductivity. These are key parameters for demonstrating highly efficient optoelectronic mixers. To this end, we have carried out in-depth studies on the growth of PtSe₂ by MBE. To evaluate the crystalline quality of PtSe₂ films, most groups measure the full width at half maximum (FWHM) of the in-plane Eg Raman peak. Eg FWHM ≤ 5 cm^-1 was then an indicator of high crystalline quality. Monitoring not only E_g FWHM but also the FWHM of the out-of plane A1_g peak, we have optimized the MBE growth of large-scale PtSe₂ films on sapphire substrates. By performing a post-growth annealing, we demonstrated that the E_g and A_1g FWHM values reach the exceptionally low values of 3.6 cm^-1 and 3.5 cm^-1 respectively, as well as a high conductivity of 1.6 mS. Based on these studies, we propose a new figure of merit showing that both Eg and A1g FWHM values are required to evaluate the crystalline quality of PtSe₂ films. We show that the electrical conductivity of our films can be efficiently predicted using the FWHM of the A1g peak. Furthermore, using X-ray diffraction and advanced transmission electron microscopy techniques, we have identified the correlations between the crystalline structure and the electrical conductivity of the PtSe₂ films.
A14ML PtSe₂ film of high crystalline quality was grown on a 2-inch sapphire wafer. We then fabricated PtSe₂ based coplanar waveguides to demonstrate photodetectors at 1.55 µm with a record bandwidth of 60 GHz and the first PtSe₂-based optoelectronic mixer with 30GHz-bandwidth.