Multiferroics is a versatile material containing more than one ferroic order, including ferroelectricity (FE) and ferromagnetism (FM). The material has several applications in field-effect transistors, high-density memories, and spintronic devices. Unfortunately, two-dimensional (2D) single-phase multiferroics come with the challenges of the difference of d-orbital occupation of metal ions. The d-orbit of FM usually is partially filled to form metal. In contrast, FE forms an insulator due to the empty of fully d-orbital, and FM and FE orders are highly susceptible to temperature.
2D FE α-In2Se3 comes with exceptional photoelectric properties and non-volatile switching nature, such as excellent light absorption efficiency, thickness-dependent bandgap, and high stability in air. In nanodevices, α-In2Se3 comes into contact with metal electrodes to enhance interfacial carrier injection. Unfortunately, a Schottky barrier is produced at the metal-semiconductor interface due to the strong Fermi level pinning effect, which in effect plaques the transport performance of nanodevices.
Chemical doping and external electric field are among the several efforts that have been made in an attempt to modulate the Schottky barrier. Irrespective, it’s still challenging to implement these efforts in reality because of large electric field and space constraints. This is why it’s still challenging to choose a suitable 2D magnetic metal combined with α-In2Se3 to develop robust multiferroic devices and low resistance Ohmic contact.
Choosing appropriate magnetic metal electrodes to develop van der Waals heterojunctions appears to be a sure path to realizing electronic properties of multifunctional devices by employing ferroelectric polarization inversion. Incredibly, 2D FM metals, showing unique properties such as high conductivity, room-temperature ferromagnetism, low-temperature charge-density-wave order, have been successfully synthesized in the past few years. For FE, it can cause a large depolarization field in the ferroelectric domain due to the spontaneous polarization, which can be employed to regulate the carrier transport in ferroelectric-based devices.
First-principles show that transitions from insulator-to-half metal and ferroelectric-to-multiferroic at heterointerfaces can occur in 2D multiferroic heterostructures implying that polarization reversal can yield some novel and unintended alterations. The Schottky barrier height is an important parameter to distinguish between Schottky and Ohmic contacts. Fermi level pinning effect is majorly induced by the strong heterointerface interaction, where metal-induced gap states, interface dipole at the interface, and chemical disorder/defect induced gap states are the main influencing factors. Fortunately, recent studies have reported that the Fermi level pinning effect can be controlled due to the weak interface van der Waals interaction. Therefore, this implies that disorder/defect-induced gap states can be ignored in 2D can der Waals heterostructures because of no gangling bonds. As a result, controlling the Schottky barrier height and suppressing the Fermi level pinning effect are the keys to achieving Ohmic contact.
Researchers Huamin Hu (PhD candidate) and Professor Gang Ouyang from the Hunan Normal University, China, proposed a non-volatile electrical/light control transition from p-type to n-type Ohmic contact to improve the interface carrier transport in α-In2Se3-based van der Waals heterojunctions. They then employed density functional theory to explore the interface contact properties in α-In2Se3-based van der Waals heterojunctions. Their research work is published in the journal Applied Surface Science.
The authors investigated a series of α-In2Se3-based van der Waals multiferroic heterojunctions that the room temperature FM 1T-MX2 (M = Mn,V, Cr; X = Se, Te) were used as the electrodes in terms of density functional theory computations.
The authors observed that only the case of MnSe2 reserved its crystal structure and was affected by the polarization of α-In2Se3. For the case of MnSe2/α-In2Se3 junctions, the junction exhibited improved p-type Schottky contact when the α-In2Se3 was in α-P↑ polarization state. When α-In2Se3 reversed to the α-P↓ polarization state, the n-type Ohmic contact occurred due to charge transfer induced by polarization reversal, which led to the destruction of the depletion region.
Consequently, the observed conversion made Schottky barrier height disappear and overcame the Fermi level pinning effect to yield low resistance Ohmic contact. By analyzing further through spin density and kinetic pathway, the authors discovered the transition was intrinsic and retained its properties of magnetic and non-volatile switching nature.
Going by the exceptional photoelectric performance of α-In2Se3, the α-In2Se3-based heterojunction showed a significant switching from p-type Schottky to n-type Ohmic contacts. The findings of this study will help in the design of future nano-field effect transistors and high-density memories.
Huamin Hu and Gang Ouyang*. First-principles calculations of interface engineering for 2D α-In2Se3-based van der Waals multiferroic heterojunctions. Applied Surface Science, issue 545 (2021), 149024.