二维X-AlN(X=C,Si,TC)半导体的可见光调控与反常热输运

Visible light modulation and anomalous thermal transport in two-dimensional X-AlN(X=C,Si,TC)semiconductor

在二维材料中,平面六方氮化铝(AlN)对开发电子器件至关重要.但宽带隙限制了其应用,为进一步突破性能瓶颈,本文采用结构搜索的方法找到一种新型孔状皱面的AlN构型,由于其特殊的孔状构型,可在孔中引入C, Si原子与碳三角环(TC)形成新型二维X-AlN (X=C, Si, TC)结构,从而提升其光学与热学性能.结果表明:1)在电子结构方面,由于X-pz电子的局域性,费米面附近产生的孤立能带将带隙值从4.12 eV (AlN)分别降至0.65 (C-AlN)和1.85 eV (Si-AlN),显著改善了AlN的宽带隙. TC-AlN由于碳三角环间C-p_(z)杂化形成离域π键,使能量降低,实现了间接带隙到直接带隙的转变. 2)热输运方面,与AlN, C/Si-AlN相比, TCAlN由于碳三角环间强共价键抑制了垂直面内的声子振动,极大地增强了热导率.此外,在X-AlN中施加双轴应变,热导率出现先上升后下降的异常变化趋势,这是源于随应变增强的N—N键带来的低非谐性与声子模软化降低群速度之间的竞争.本工作给出了调控二维AlN性能的新路径,为提高半导体电子、光学与热学性能提供有力指导.

Aluminum nitride(AlN)is of paramount importance in developing electronic devices because of excellent stability and thermal transport performance.However,lack of novel materials which can provide colorful physical and chemical properties seriously hinders further digging out application potential.In this work,we perform an evolutionary structural search based on first-principles calculation and verify the dynamic and thermal dynamic stability of porous buckled AlN and X-AlN(X=C,Si,TC)structural system,which constructs by introducing C,Si atoms and triangular carbon(TC)into the porous vacancy of AlN,by calculating phonon spectra and first-principles molecular dynamic simulations.Structural deformation becomes gradually serious with the increase of structural unit size and significantly influences structural,electronic,and thermal transport properties.Firstly,we point out that a flat energy band appears around the Fermi level in C-AlN and Si-AlN because of weak interatomic interaction between C/Si and the neighbor Al atoms.Unoccupied C-/Si-pz and Al-pz do not form bond and only a localized flat band near Fermi level arises,and thus the absorption peaks of structures are enhanced and the red shift occurs.Bonding state of bond from hybridized C-p_(z) orbitals in triangular carbon of TC-AlN lowers the energy of conduction band at K point in the first Brillouin zone and the corresponding antibonding state raises the band at G,therefore transition from indirect bandgap of AlN to direct bandgap of TC-AlN appears.Secondly,porous buckled AlN shows the lowest thermal conductivity due to asymmetric Al—N bonds around the porous vacancy and vertically stacked N—N bonds.Introduced C and Si atoms both reduce structural anharmonicity,while the former has a relatively small distortion,and so it has a higher thermal conductivity.Triangular carbon in TC-AlN hinders phonon scattering between FA mode and other phonon modes and has the weakest anharmonicity because of the strongest bond strength,and obtains the highest thermal transport performance.Finally,we unveil the physical mechanism of anomalous thermal conductivity in X-AlN system by modulating the biaxial tensile strain.Enhanced vertical N—N bonds dominate thermal transport due to its weaker anharmonicity with a slightly strain,and when tensile strain is above the 4%,soften phonon modes reduce phonon velocity and thus hinders the thermal transport process.Therefore,occurs the anomalous thermal transport behavior,i.e.thermal conductivity first rises and then drops with applied biaxial strain increasing.Our work paves the way for modulating twodimensional AlN performance and provides a new insight for designing promising novel two-dimensional semiconductors.