Ferroelectric-tuned van der Waals heterojunction with band alignment evolution

Van der Waals integration with abundant two-dimensional materials provides a broad basis for assembling functional devices. In a specific van der Waals heterojunction, the band alignment engineering is crucial and feasible to realize high performance and multifunctionality. Here, we design a ferroelectric-tuned van der Waals heterojunction device structure by integrating a GeSe/MoS2 VHJ and poly (vinylidene fluoride-trifluoroethylene)-based ferroelectric polymer. An ultrahigh electric field derived from the ferroelectric polarization can effectively modulate the band alignment of the GeSe/MoS2 heterojunction. Band alignment transition of the heterojunction from type II to type I is demonstrated. The combination of anisotropic GeSe with MoS2 realizes a high-performance polarization-sensitive photodetector exhibiting low dark current of approximately 1.5 pA, quick response of 14 μs, and high detectivity of 4.7 × 1012 Jones. Dichroism ratios are also enhanced by ferroelectric polarization in a broad spectrum from visible to near-infrared. The ferroelectric-tuned GeSe/MoS2 van der Waals heterojunction has great potential for multifunctional detection applications in sophisticated light information sensing. More profoundly, the ferroelectric-tuned van der Waals heterojunction structure provides a valid band-engineering approach to creating versatile devices.

The bandgap modulation effect of P(VDF-TrFE) on MoS2 has been demonstrated in our previous work. 11,12 We can verify the bandgap reduction by measure the cutoff wavelength of the photoresponse. The bandgap of bulk GeSe is approximately 1.1−1.2 eV, which makes its intrinsic response with the cutoff wavelength of 1100 nm. With the modulation of P(VDF-TrFE), however, the detection range of GeSe is broadened to 1550 nm as shown in Supplementary Fig. 5.
To estimate the magnitude of bandgap reduction by the Stark effect, we calculate the electronic structure of bulk GeSe under different external electrical fields. Firstprinciple calculations are performed in VASP code within the projector-augmented plane-wave method. [13][14][15] The general gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) is adopted to describe the exchange-correlation potential. 16 To overcome the problem of bandgap underestimation in PBE functionals, the HSE06 hybrid functional [17][18][19] is used to calculate the band structures of layered GeSe. Energy cutoff of 450 eV is employed for the plane-wave basis. In calculations of few-layer GeSe, a vacuum larger than 15 Å is used to eliminate the interaction between adjacent images. The first Brillouin zone is sampled with an (8 × 8 × 1) Monkhorst-Pack grid for relaxation of layered GeSe. 20 All the structures are fully relaxed with a force tolerance of 0.01 eV/Å. From the electronic structures calculated by PBE method as shown in Supplementary Fig. 6, the bandgap of GeSe shows a strong dependence on its thickness, which is attributed to quantum confinement effect. With increasing thickness of GeSe, quantum confinement effect becomes less significant, resulting in decreasing bandgap of GeSe. The bandgap of GeSe is calculated to be 0.94 eV, which is close to the reported results. 10 HSE06 calculations obtain larger bandgap of 1.6 eV. The remnant polarization electric field intensity is calculated to be 1.26 × 10 9 V/m, which may induce a 0.3 eV bandgap reduction in GeSe according to the calculated value.

P(VDF-TrFE).
We explored the evolution in the electronic properties of GeSe with the applied top gate voltage (Vtg). Conductance measurements were performed in dual-gated GeSe transistors with a BN back gate and P(VDF-TrFE) top gate as shown in Supplementary   Fig. 8 The impact of the remnant polarization field on the GeSe was also investigated.
The density of thermally generated intrinsic carriers (ni) reveals the bandgap of a semiconductor. Their relationship is given by ni ∝ exp(-Eg/2kT), where Eg is the bandgap of GeSe and k is the Boltzmann constant. Meanwhile, the intrinsic carrier density can be obtained by the minimal conductance at the charge neutrality state. 22 They follow the equation of m = qnih. The temperature-dependency of conductance at Pup and Pdown states are plotted in Supplementary Fig. 8c and 8d. At high temperatures, it is difficult to find the minimum conductance, but the data shows an increasing trend of conductance with increasing temperature. More importantly, the conductance at Pdown state is always larger than that of Pup state. Consequently, the density of intrinsic carriers at Pdown state is higher, indicating a smaller Eg of GeSe.

Supplementary Note 3: Analysis of band alignment of GeSe/MoS2 VHJ.
1. To better understand the transport properties of the GeSe/MoS2 heterojunction, we analyzed the electrical properties of MoS2 and GeSe, respectively. To determine its band alignment, we calculated the mobility, carrier concentration, and the Fermi level positions of these two materials based on the measured data as shown in Supplementary Fig. 2. The specific methods are as follows.
where g2D is the density of states, kB is the Boltzmann constant, T is the temperature, EF-EC and EF-EV are the separations from Fermi level to conduction band minima (CBM) and valence band maxima (VBM), which we need to obtain by calculation.
The Boltzmann constant kB = 1.38  10 -23 J/K, temperature T = 300 K, the calculated carrier concentration (n, p), and density of states g2D are taken into equation where Is is the saturation current, kB is the Boltzmann constant, V is the applied voltage, T is the temperature, η is the ideality factor, and e is the electron charge. it is reasonable to conclude that the downward polarization electric field can penetrate the GeSe hundreds of nanometers thick and modulate the interfacial band alignment.