石墨烯过渡层对金属/SiC接触肖特基势垒调控的第一性原理研究

First principle study on modulating of Schottky barrier at metal/4H-SiC interface by graphene intercalation

由于SiC禁带宽度大,在金属/SiC接触界面难以形成较低的势垒,制备良好的欧姆接触是目前SiC器件研制中的关键技术难题,因此,研究如何降低金属/SiC接触界面的肖特基势垒高度(SBH)非常重要.本文基于密度泛函理论的第一性原理赝势平面波方法,结合平均静电势和局域态密度计算方法,研究了石墨烯作为过渡层对不同金属(Ag,Ti,Cu,Pd,Ni,Pt)/SiC接触的SBH的影响.计算结果表明,单层石墨烯可使金属/SiC接触的SBH降低;当石墨烯为2层时,SBH进一步降低且Ni,Ti接触体系的SBH呈现负值,说明接触界面形成了良好的欧姆接触;当石墨烯层数继续增加,SBH不再有明显变化.通过分析接触界面的差分电荷密度以及局域态密度,SBH降低的机理可能主要是石墨烯C原子饱和了SiC表面的悬挂键并降低了金属诱生能隙态对界面的影响,并且接触界面的石墨烯及其与金属相互作用形成的混合相具有较低的功函数.此外,SiC/石墨烯界面形成的电偶极层也可能有助于势垒降低.

In the production of SiC electronic devices,one of the main challenges is the fabrication of good Ohmic contacts due to the difficulty in finding the metals with low Schottky barriers of wide band gap SiC.Therefore,reducing the Schottky barrier height(SBH)at the metal/SiC interface is of great importance.In this paper,the effects of graphene intercalation on the SBH in different metals(Ag,Ti,Cu,Pd,Ni,Pt)/4 H-SiC interfaces are studied by combining the average electrostatic potential and local density of states calculation methods based on first-principles plane wave pseudopotential density functional theory.The calculation results show that single-layer graphene intercalation can reduce the SBH of metal/4 H-SiC contact.When the two layers of graphene are inserted,the SBH are further reduced.Especially,the contact between Ni and Ti exhibits negative SBH values,inferring that good Ohmic contacts are formed.When layers of graphene continue to increase,the SBH no longer changes obviously.By analyzing the differential charge density and the local density of states of the interface,the mechanism of SBH reduction may be that the dangling bonds on the SiC surface are saturated by the graphene C atoms and the influence of the metal-induced energy gap state at the interface is reduced,thereby reducing the interface state density.In addition,graphene and the corresponding new phases at the interface have low work functions.Moreover,an interfacial electric dipole layer may be formed at the SiC/graphene interface which also contributes to barrier reduction.