Bidimensional RF optoelectronic devices based on PtSe2 – BIRDS

The project contains all the necessary steps for the realization of optoelectronic devices from material synthesis to the technology demonstrator of functional devices, including nano-fabrication and optical and electrical characterizations. Each technological building block uses original and advanced techniques : (i) The PtSe2/PdSe2 materials, which are recently studied TMDCs exhibiting an adjustable bandgaps between 1.2/1.3 eV (monolayer) and 0/0,15 eV (multilayers); molecular beam epitaxy (MBE) and ion implantation which can provide layer by layer growth, thus enabling the control of thickness from mono to few layer material, (ii) The fabrication of van der Waals heterostructures allowing encapsulation of TMDCs between two layers of graphene, (iii) Device characterization over the 0.5–10 µm band in the optical domain and up to 67 GHz in the electrical domain, (iv) finally planar and vertical architectures making use of the 2D nature of the material.

Results include the synthesis of PtSe2 & PdSe2 and the demonstration of devices based on PdSe2.

PtSe2 synthesis has been first studied with an original method: Ion implantation. This method allowed us to implant in a sapphire substrate the equivalent of 5 PtSe2 monolayers at a depth of around 20 nm. The substrates are then annealed to induce PtSe2 crystallization. The films exhibit Raman spectra very similar to exfoliated PtSe2 spectra. Cross section images obtained by transmission electron microscopy show crystallites with random orientation and with various layer numbers.
PtSe2 growth has also been studied by molecular beam epitaxy on sapphire (0001). After optimization of growth parameters, we have obtained PtSe2 monolayers parallel to the substrate as shown by the TEM cross sections.
Raman spectra exhibit the PtSe2 peaks Eg and A1g with a full width at half maximum of 3.5 cm-1, i.e. a value smaller than published values.
PdSe2 nanoflakes have also been obtained by mechanical exfoliation. MOSFETS transistors based on PdSe2 have been integrated in 50 Ohms coplanar waveguides. The active part of the transistor includes a gold back gate, a 30-40 nm thick hexagonal Boron Nitride layer and a PdSe2 film. Transistors the first with thin (= 20 nm) and thick (= 90 nm) films have been studied. The inversion-depletion-accumulation modes have been observed with a depletion depth of 15 nm. An electron mobility of 110 cm2/V.s has been obtained at room temperature.
PdSe2 bandgap has been determined by transport measurements (Eg=0.15eV for PdSe2 thicknesses above 30nm, indirect bandgap). High electric field transport has shown that carrier mobility is almost intrinsic and is limited by optical phonons (thin films). It also highlighted the thermal instability for thick films due to the large number of thermally activated carriers. Finally, the first photodetection experiments have been performed and result from a bolometric mechanism.

MOSFETs based on Métal/h-BN/PdSe2 with a 10-µm channel length present a cutoff frequency in the GHz range with a product gain-bandwidth that compete with III-V devices.

PtSe2 layers in sapphire obtained by ion implantation are promising as saturable absorbers for mode locked lasers.
PtSe2 monolayers obtained by molecular beam epitaxy present a high crystalline quality and the BIRDS consortium currently studies high frequency optoelectronic devices based on these films.

A first paper has been published in «2D Materials«.

The intrinsic characteristics (doping, gap, mobility) of 2D materials can be largely modified by controlling their thickness and their environment (electric field, dielectric environment). These control possibilities have no equivalence in classical massive semiconductors and provide opportunities to develop new electronic, optical and optoelectronic devices.

The goal of the BIRDS project is to exploit the remarkable properties exhibited by the new 2D semiconductors, the transition metal dichalcogenides (TMDCs), for microwave optoelectronics, with, as first demonstrators, a 1.55 µm optoelectronic mixer and a large bandwidth (1-10 µm) infrared photodetector. The optoelectronic mixers that mix microwave signals with optical signals (after detection) will lead to a new generation of analog-digital convertors for the future telecommunication infrastructures (5G). An in-depth study of the large bandwidth photodetection will allow us to better understand the excellent photoresponsivity (4-6 A/W between 1 and 10 µm) obtained recently and to exploit these properties in an optimized device.

The scientific goal of BIRDS is to extend spectral performances of TMDC-based optical detectors beyond the natural limit of 1µm fixed by the usual bandgaps in these materials (MoS2, MoSe2, WS2, WSe2, etc..). Reaching longer wavelengths in the infrared (1.55 µm and also 2-10 µm) is more difficult but will lead to promising applications. Two strategies will be studied and combined: bandgap engineering by using an adjustable material between the semi-metal and the semiconductor regime depending on thickness and the field effect in p-n or p-i-n planar junction and in a vertical Graphene/PtSe2/Graphene heterostructure. For the p-i-n device dedicated to scientific studies, the junctions will be controlled with local back gates. The devices will be inserted in microwave waveguides for ultrawide band characterization of optoelectronic properties.

The project contains all the necessary steps for the realization of optoelectronic devices from material growth to the technology demonstrator of functional devices including nano-fabrication and optical and electrical characterizations. Each technological building block uses original and advanced techniques : (i) The PtSe2 material, which is a recently studied TMDC exhibiting an adjustable bandgap between 0,5 (for 5 layers) and 1,2 - 1,4 eV (monolayer); molecular beam epitaxy (MBE) which can provide layer by layer growth, thus enabling the control of thickness from mono to few layer material, (ii) The local back gate technology recently demonstrated on graphene and the fabrication of van der Waals heterostructures allowing encapsulation of TMDCs between two layers of graphene, (iii) Device characterization over the 0,5 – 10 µm band in the optical domain and up to 67 GHz in the electrical domain, (iv) finally varied architectures making use of the 2D nature of the material (planar and vertical junctions). The BIRDS project complements existing ANR projects which, to the best of our knowledge, are dedicated to the optoelectronic studies using TMDC operating only in the visible range or studying the electroluminescence with polarized spin in black phosphorus.

The S/T goals of BIRDS are (i) the MBE controlled synthesis of mono and few layer PtSe2 on 2 inches substrate, (ii) a planar photodiode p-i-n to study the generation mechanism of a photocurrent, (iii) a planar p-n wide-band photodiode on the 1-10 µm range and (iv) a trilayer vertical photodiode for 30 GHz optoelectronic mixing.