Tunable band gap of diamond twin boundaries by strain engineering

Abstract

Diamond thin films are promising wide band gap semiconductor materials for applications in electronic and microwave devices. Revealing the mechanism of how twin boundaries impact the band gap of diamond is of critical importance since they are the most common defects in polycrystalline diamond films. Here, nanocrystalline diamond films are synthesized by microwave plasma-enhanced chemical vapor deposition. The atomic and electronic structures of diamond lamellar and fivefold twins, and their evolution behaviors with the increase of axial tensile strain, are investigated by combining aberration-corrected transmission electron microscopy with first-principles calculations. There is no intrinsic stress concentration at the lamellar twin boundary, and its band gap equals to that of the bulk diamond ( i.e. , 5.3 eV). An intrinsic in-plane compressive stress field is formed and the band gap is increased evidently ( i.e. , 0.6 eV) in the center of fivefold twins. When applying an axial tensile strain up to 15%, the band gaps of the bulk diamond, lamellar and fivefold twins reduce significantly to 2.2 eV, 2.1 eV and 2.4 eV, respectively, which are mainly due to the decrease of p z orbital energy caused by the increase of axial bond length during the tensile process. Under the same axial tensile strain, the band gap of the fivefold twin is always larger than those of the lamellar twin and the bulk diamond due to the formation of in-plane five-membered carbon rings in the center. The findings of the tunable band gap by strain engineering will benefit for the innovation and design of advanced diamond functional devices. • Atomic and electronic structures of diamond lamellar and fivefold twins under tensile strains have been investigated. • The band gap of fivefold twins is evidently larger than that of the bulk diamond. • Under axial tensile strains, the band gap of bulk diamond, lamellar and fivefold twins reduces significantly. • The reduction of band gap is due to the decrease of p z orbital energy caused by the increase of axial bond length.