Strain modulation of electronic and optical properties of monolayer MoSi₂N₄

Highlights

• The electronic and optical properties of monolayer MoSi₂N₄ under in-plane strain are studied.

• Biaxial compressive strain can transform the energy band from a direct bandgap to an indirect bandgap.

• In-plane strain can modulate the hole effective mass.

• MoSi₂N₄ has an excellent light absorption performance in the ultraviolet region.

Abstract

The strain engineering is an important approaches to modulate the electronic and optical properties of materials. Recently, a new two dimensional material monolayer MoSi₂N₄ has been successfully synthesized with excellent electronic performance. The electronic properties of monolayer MoSi₂N₄ without and with in-plane strain are systematically investigated by first-principles calculation. The calculation results reveal monolayer MoSi₂N₄ is an indirect bandgap semiconductor with bandgap 1.74 eV (PBE) and 2.31 eV (HSE06), but it can be transformed to a direct bandgap under the certain in-plane strain, such as 3% and 4% biaxial compressive strain. In-plane strain can effectively modulate the band structure, bandgap, and the carrier effective mass. Furthermore, the optical properties of unstrained and transformed to direct bandgap situations are calculated. The light absorption capacity of monolayer MoSi₂N₄ is stronger in the ultraviolet band, and can be changed when it is transformed to a direct bandgap. The electrical and optical properties can be modulated by strain engineering, making it is a promising candidate of strain-modulated optoelectronic devices.

Introduction

In recent years, two-dimensional (2D) materials have become a new research hotspot. Compared with traditional bulk materials, monolayer or few layers 2D materials often have high carrier mobility and flexibility characteristics. Therefore, it has a brighter future in flexible electronic devices [1,2]. At present, researchers have conducted in-depth research on 2D materials including graphene [[3], [4], [5]], transition metal dichalcogenides (TMDs) [[6], [7], [8], [9]], black phosphorus (BP) [10,11] etc. Among them, graphene has ultra-high carrier mobility (10,000 cm²V⁻¹s⁻¹) [12] and excellent thermal conductivity properties [13], but the zero-bandgap feature limits its application. TMDs have moderate bandgap (1–2eV) and strong interlayer coupling, but the carrier mobility is not as high as other 2D materials [14,15]. BP is difficult to apply on a large scale due to its environmental instability [16]. More 2D materials with application value are still to be discovered and studied. Very recently, a new 2D material MoSi₂N₄ has been successfully synthesized by chemical vapor deposition (CVD) [17], and band structure is similar to MoS₂. According to the previous studies, MoSi₂N₄ is an indirect bandgap semiconductor with high stability [18]. The experimentally measured bandgap value of monolayer MoSi₂N₄ is 1.94 eV, and its carrier mobilities of electrons and holes are 270 cm²V⁻¹s⁻¹ and 1200 cm²V⁻¹s⁻¹, respectively, which are four to six times higher than the carrier mobility of monolayer MoS₂. Besides, MoSi₂N₄ has a good piezoelectric coefficient of 1.15 pm/V [19]. 2D semiconductor carrier transport capability, which can be modulated by coupling the strain-induced piezoelectric charge with potential of semiconductor properties, which dramatically modulates the photoresponse performance of flexible 2D materials‐based optoelectronics [2]. Thus, it is predicted that MoSi₂N₄ will have excellent application prospects in the field of electronics and optoelectronic devices. Nevertheless, the indirect bandgap of MoSi₂N₄ means that phonons need to be involved to provide momentum when electrons transition from the top of valence band to the bottom of conduction band, which is detrimental to the optoelectronic performance.

Strain is a prominent channel to improve the semiconductor performance [20]. Under external strain, the crystal lattice will be tensile or compressed, which can affect the interaction between atoms, and then change band structure and electronic properties. Strain can also be implemented as a tunable function of optical properties in 2D materials i.e., strain conditions that produce different optical leap behaviors. There have been many studies on the strain modulation of 2D materials. For example, applying biaxial strain on phosphorene will induce a transition from direct bandgap to indirect bandgap [21]. Volker Sorger et al. demonstrated a photodetection technique base on strain engineering of molybdenum telluride covered with silicon photonic waveguides, which a new photodetector operates effectively at 1550 nm communication wavelengths with a response rate of 0.5 AW^-1 [22]. In addition, it was found experimentally that tensile strain reduced the contact energy barrier between MoS₂ and gold, where the change in contact potential barrier was attributed to the strain-induced increase in the electron affinity of MoS₂ monolayer, and the strain-induced modulation of the potential barrier was also shown to affect the photoresponse behavior in MoS₂ flexible photodetectors through energy band bending [23].

In our work, the effect of in-plane strain engineering on the band structure, effective mass and optical properties of monolayer MoSi₂N₄ has been systematically investigated using the first-principles calculation. We find that the band structure transforms to direct bandgap at the biaxial strain of −3% and −4%, and the hole effective mass in this state decreases, which leads to a presumed increase in hole mobility and an improvement in electrical properties. The optical properties of MoSi₂N₄ are also modulated in this state. Our results indicated that MoSi₂N₄ is a promising material for strain-engineered photodetectors.