Tuning The Band Alignment Of GaSe/SnX₂（X=S,Se） 2D Van Der Waals Heterojunctions Via An Electric Field

Because of its ultrathin physical size and unique photoelectric performance,two-dimensional semiconductor not only possesses a considerable application prospect in the field of future communication,information and optoelectronics,but also is the star material leading the rapid development of integrated circuit technology in the post-Moore era.Van der Waals heterojunction strategy which combines different types of two-dimensional layered semiconductor materials by van der Waals forces can effectively adjust the properties of two-dimensional materials.This can not only combine the excellent properties of a single material,but also show extremely rich interface characteristics.In addition,due to their different band alignment（including type-Ⅰ,type-Ⅱ and type-Ⅲ）,two-dimensional van der Waals heterostructions have been considered excellent candidates for extending a variety of device applications to luminescent photovoltaics and field-effect transistors.The development of two-dimensional heterostructions can greatly promote the progress of integrated electronic device industry such as transistor,flexible device,detector and light-emitting diode which are the core of semiconductor heterostructures in the future.Therefore,how to better expand the diverse application fields of heterostructions,that is,to realize the conversion between different types of band alignment in a single heterostructure,is a key problem to be urgently solved in the future development of highly integrated electronic devices.In this paper,2D van der Waals heterostructions based on chalcogenides,including GaSe/SnS₂and GaSe/SnSe₂vdWHs,are systematically designed and studied.Meanwhile,based on density functional theory,the effect of vertical electric field on the band types and electronic properties of two-dimensional van der Waals heterostruction GaSe/SnS₂（SnSe₂）is investigated.The main results are as follows:In summary,our work demonstrates theoretically through DFT calculations that a vertical electric field can effectively,flexibly,and dynamically regulate the band structure and electronic properties of type-Ⅱ GaSe/Sn X₂（X=S,Se）vdWHs,making a good candidate material or photodetector and field-effect transistor applications.A positive electric field can control the band alignment from type-Ⅱ to type-Ⅰ with the electric field transformation values of about 0.39 V/（?）（0.12 V/（?））,while a negative electric field can transform from the type-Ⅱ to type-Ⅲ band alignments with the electric field values of about-0.18 V/（?）（-0.16 V/（?））in the GaSe/SnS₂（SnSe₂）Ga/Sn X₂（X=S,Se）vdWHs.We trace these surprising results to the conduction band and valence band edge position movements for the linear decrease of the GaSe,as well as the linear increase of the SnS₂（SnSe₂）with the applied electric field.For the electric field regulation,our designed GaSe/Sn X₂（X=S,Se）heterojunctions can complete the band-type regulation between type-Ⅰ,type-Ⅰ,and type-Ⅲ within a range of about 0.4V/（?）,or even 0.12 V/（?）,so GaSe-Sn X₂（X=S,Se）heterojunctions can be flexibly regulated.Moreover,type-Ⅰ vdWHs are suitable for LEDs,type-Ⅱ vdWHs for light capture in the potovoltaic field,and type-Ⅲ vdWHs for TFETs.In addition,since a heterojunction is inevitably affected by an applied electric field,the theoretical results of this study can be used to predict the response of a heterojunction in different electric fields.Therefore,the present work provides a direction for the development of tunable multiband alignment technology in 2D vdWHs and the application of multifunctional devices.