Two-dimensional semiconducting SnP2Se6 with giant second-harmonic-generation for monolithic on-chip electronic-photonic integration

Two-dimensional (2D) layered semiconductors with nonlinear optical (NLO) properties hold great promise to address the growing demand of multifunction integration in electronic-photonic integrated circuits (EPICs). However, electronic-photonic co-design with 2D NLO semiconductors for on-chip telecommunication is limited by their essential shortcomings in terms of unsatisfactory optoelectronic properties, odd-even layer-dependent NLO activity and low NLO susceptibility in telecom band. Here we report the synthesis of 2D SnP2Se6, a van der Waals NLO semiconductor exhibiting strong odd-even layer-independent second harmonic generation (SHG) activity at 1550 nm and pronounced photosensitivity under visible light. The combination of 2D SnP2Se6 with a SiN photonic platform enables the chip-level multifunction integration for EPICs. The hybrid device not only features efficient on-chip SHG process for optical modulation, but also allows the telecom-band photodetection relying on the upconversion of wavelength from 1560 to 780 nm. Our finding offers alternative opportunities for the collaborative design of EPICs.


Experimental setup for the SCCVT and materials characterization
The precursors including Sn powder, P powder and Se powder were sealed into a quartz tube with vacuum pressure less than 10 mPa, together with fluorophlogopite mica as the growth substate. A microreactor with confined space is formed by stacking freshly cleaved mica vertically, as shown in Supplementary Figure 1 The high-resolution X-ray photoelectron spectroscopy (XPS) spectrum was conducted on the as-grown samples on mica substate. As shown in Supplementary   Figure 3, we can see that the oxidation state of Se 2was clearly identified by the two characteristic peaks of Se 3d5/2 (~ 54.8 eV) and Se 3d3/2 (~ 55.9 eV), while the two peaks at 487.5 and 495.9 eV can be attributed to the Sn 3d5/2 and Sn 3d3/2 satellite, respectively. In addition, another two specific peaks were also observed, including P 2p3/2 and P 2p1/2 at 132.6 and 134.8 eV. The results demonstrate the synthesis and ideal atomic stoichiometry of the SnP2Se6 nanosheets.  High-angle annular dark field scanning transmission electron microscope (HAADF-STEM) was used to characterize the SnP2Se6 nanosheets. Elemental energy-dispersive X-ray spectroscopy (EDS) measurements give the density of the elements (Supplementary Figure 5). The sample utilized for TEM characterization has a thickness of about 14 nm. After eliminating the influence of Cu from TEM grid, we can see that element including Sn, P and Se are clearly detected, together with perfect stoichiometric ratio around 1:2:6 for SnP2Se6. EDS mapping was also performed, from which we can see that all the elements are distributed uniformly across the sample, indicating the high crystalline quality of SnP2Se6.

DFT calculations of band structure
First-principles DFT calculations were carried out to predict the electric structure of

Polarization-dependent SHG measurement of SnP 2 Se 6
We carried out polarized SHG measurements to investigate the NLO properties of SnP2Se6. A femtosecond (fs) laser with a central wavelength of 1550 nm was linearly polarized and focused on the samples. The SHG intensity of samples were collected by rotating the sample with a step of 15° from 0° to 360°. The incident laser was initially polarized along the armchair axis of SnP2Se6.As shown in Supplementary Figure 7, the intensity changes periodically with the change of angle, and it can be fitted using the following formular 1 : where I is the SHG intensity at angle θ, and I0 is the maximum SHG intensity.

SHG measurement of MoTe 2 and susceptibility calculation
Semiconducting TMDC monolayers are expected to exhibit strong SHG response.
However, most of them deliver relatively low susceptibility at telecom wavelength (1310−1550 nm). Among of them, a highest susceptibility (χ (2) ~2.5×10 -9 mV -1 ) was demonstrated for MoTe2 monolayer, thus it is utilized as the counterpart for comparison.
where Pω and P2ω represent the excitation laser power and SHG power, respectively; is the thickness of the sample; ε0 and c are dielectric constant and the speed of light in vacuum, respectively; A is the area of incident laser spot; nω and n2ω denote the linear refractive indices of the sample at the fundamental and SH frequencies, respectively.
As P2ω of the SnP2Se6 sample cannot be measured directly, the second-order nonlinear susceptibility of SnP2Se6 ( ) can be measured by comparing with MoTe2 in identical test conditions.
According to previous report, the second-order nonlinear susceptibility of MoTe2 ( ) at 1560 nm excitation wavelength is about 2.5×10 -9 m V -1 , we can estimate is approach 1.32×10 -9 m V -1 for SnP2Se6.
Particularly, the value of χ (2) relies on accurate knowledge of several experimental parameters, such as frequency and duration of the excitation pulse, the shape and size of the focused fundamental spot at the sample, and the relation between the measured spectral counts and the actual SH power. Hence, the values should be viewed as an order of magnitude estimate for references 3 .

Fabrication and performance of SnP 2 Se 6 field effect transistor
To investigate the electrical properties, field-effect transistors were fabricated after transferring SnP2Se6 nanosheets onto SiO2/Si substrate with the assistance of PMMA.
Ti/Au (10/50 nm) electrodes were patterned with standard photolithography process followed by electron beam evaporation. Supplementary Figure 10a and 10b where L, W, and Cg represent the FET channel length, width, and gate dielectric capacitance, respectively. The value of electron mobility is estimated to be 6.5 cm 2 V -1 s -1 at room temperature.
In addition, we have estimated the contact resistance of the FET device. The total resistance of a single field-effect transistor includes two parts: channel resistance and contact resistance (R=Rchannel+2Rcontact). We have constructed field effect transistors based on 16-nm-thick SnP2Se6 film with different channel lengths ranging from 2 to 8.7 μm (Supplementary Figure 10e), and each device has the same contact electrode to ensure the consistency of contact resistance. Supplementary Figure 10f  Thickness-dependent field-effect mobility and ON-OFF ratio were evaluated based on the data from more than forty devices. By extracting the data from the linear region of transfer characteristics, we can obtain an electron mobility of 15 cm 2 V −1 s −1 for a 20-nm-thick sample. Due to the enhanced gate electrostatic control in thin nanosheets, the ON/OFF ratio increases sharply from ∼10 2 to ∼10 5 as the nanosheet thickness decreases from 40 to 5.6 nm. On the contrary, the carrier mobility exhibits the opposite trend, ranging from 0.2 cm 2 V -1 s -1 to 10 cm 2 V -1 s -1 (Supplementary Figure 11). The interface scattering, which often occurs in FETs based on 2D ultrathin semiconductors, can be well utilized to explain the thickness-dependent characteristics 4,5 .
Supplementary Figure 11| Distributions of measured on/off current ratios and field-effect mobilities obtained from 30 typical SnP 2 Se 6 FETs with different thicknesses at room temperature As shown in Supplementary Figure 12, we measured the transfer curves of a 10nm-thick SnP2Se6 field effect transistor with temperature ranging from 7 to 300 K. As the temperature decreases from 300 to 200 K, the measured mobility would increase slightly from 3 to 4.5 cm 2 V -1 s -1 . With the further decreasing of temperature, the mobility would drop gradually, and a value of 0.6 cm 2 V -1 s -1 was estimated.
Supplementary Figure 12| a, Transfer curves of the SnP 2 Se 6 FET (10 nm-thick) at different temperature ranging from 7 to 300 K. b, Temperature dependent field effect mobility and on/off radio obtained from a.

Optoelectronic properties of SnP 2 Se 6 photodetector
For the wavelength-dependent photodetection measurement, a broadband laserdriven light with excitation wavelength ranging from 300 to 900 nm was utilized. The power intensity of light was calibrated and fixed at 2 mW cm -2 for the measurement.
Supplementary Figure 13 plots the I-V curves of 10-nm-thick phototransistor as exposed to different wavelengths of light, and the maximum photocurrent can be achieved with 700 nm illumination.

Numerical simulations of the SHG process in SiN microring resonator
The microring resonator is fabricated on 300 nm-thick SiN slab with a diameter of 35 μm, and the coupling length is designed to be 7. To reduce the transmission loss, the TM2 guiding mode in microring is finally converted to fundamental TM0 mode in pump through a mode converter, and the simulated conversion efficiency can reach up to 96% (Supplementary Figure 17).
Supplementary Figure 17| The mode conversion from TM 2 in microring to TM 0 in sub-waveguide through a mode converter.

Optical measurement of device based on SnP 2 Se 6 /SiN hybrid structure
The testing device set-up consists of a tunable laser (Santec TSL-710)

Investigation of SHG conversion efficiency with the thickness of the transferred SnP 2 Se 6 on the SiN microring resonator
The SHG efficiency for a lossless waveguide without pump depletion is given by the following expression 6,7 : The Q value in the micro ring determines the overall length of propagation L, Δ 2 represents the wave vector mismatch, and represents nonlinear overlap factor, which can be defined as: In this formular, θ is the angle formed by the guided-mode wave vector and the armchair direction of the SnP2Se6 crystal. However, since our device is fabricated based on a micro ring, this term can be eliminated by integral. Considering this, the equation (7) can be reduced to: harmonics for the situation of fixed propagation length and waveguide width, which is the primary cause for the difference in SHG conversion efficiency. Here, the waveguide width was set to be 1.26 m and the bending radius of the micro ring to 25 m.
According to Supplementary Figure 21, we can conclude that the number of material layers providing the optimal SHG conversion efficiency is around 64 in this device configuration. It's important to note that this value is highly relative to the fixed device.
Once the size parameter of the micro ring changes, the optimal conversion layer of the material will also change.
Supplementary Figure 21| The variation of SHG conversion efficiency with the thickness of the transferred SnP 2 Se 6 on the micro ring device, as measured by logarithmic scale.
To further prove the above conclusion, we have supplemented the experimental data of SHG efficiency in micro ring structures covered with different thicknesses of SnP2Se6 (Supplementary Figure 22). In summary, our device has preliminarily verified the feasibility of integration of frequency doubling conversion and photoelectric detection, but the specific performance indexes need to be further improved. Waveguide structure, Q value of resonant ring, sample thickness and other factors will have a great impact on the performance of the device, which will be the focus of our research in the future.