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Metal-insulator transitions in epitaxial rocksalt-structureCr1−x/2N1−xOx(001)
Physical Review B ( IF 3.2 ) Pub Date : 2020-12-01 , DOI: 10.1103/physrevb.102.235102
Mary E. McGahay , Sanjay V. Khare , Daniel Gall

Substitutional replacement of N with O in epitaxial CrN(001) layers is achieved by reactive sputter deposition in a mixed N2 and Ar+O2 atmosphere. O incorporation facilitates Cr vacancies, yielding a rocksalt-structure solid solution Cr1x/2N1xOx with a single compositional parameter x and a measured lattice constant that decreases from a=0.4175 to 0.4116 nm for x=0 to 0.59. First-principles calculations predict da/dx=+0.0200, −0.0018, and −0.0087 nm for CrN1xOx, Cr1x/3N1xOx, and Cr1x/2N1xOx, respectively, and confirm, in combination with ion-beam compositional analyses and x-ray diffraction results, a vacancy concentration of x/2 per cation site. The room-temperature resistivity decreases by over two orders of magnitudes from ρ=1.5×103 to 7.7×106Ωm for x=00.26. This is accompanied by a transition from a negative to a positive temperature coefficient of resistivity, an increase in the Hall mobility from μH=0.37 to 1.7cm2/Vs, an increase in the carrier concentration n=1.1×1020 to 4.9×1021cm3, and a decrease in the calculated band gap from 0.54 to 0 eV. However, ρ increases again to 6.9×105Ωm for x=0.34 and to >10Ωm for x0.59, with a corresponding drop in μH and the opening of a gap. The maximum in the measured carrier density and conductivity at x=0.26 is attributed to a maximum in the density of states near the Fermi level, in perfect agreement with calculations using supercells with randomly distributed O substitutions and Cr vacancies that predict a maximum at x=0.25±0.05. The measurements indicate insulator-to-metal and subsequent metal-to-insulator transitions at x=0.04±0.03 and 0.30±0.03, respectively. The transition to metallic conduction is attributed to substitutional O on anion sites acting as donors, while the measured n for x=0.010.26 quantitatively indicates charge compensation by cation vacancies that act as acceptors. The insulating properties at large x are likely caused by increased electron correlation effects.

中文翻译:

外延岩盐结构中的金属-绝缘体过渡Cr1-x / 2N1-xOx(001)

外延CrN(001)层中的O用N取代N是通过在混合状态下进行反应溅射沉积来实现的 ñ2氩气+Ø2大气层。O的掺入促进Cr的空位,产生岩石盐结构的固溶体1个-X/2ñ1个-XØX 具有单个成分参数 X 和测量的晶格常数从 一种=0.4175 到0.4116 nm X=0至0.59。第一性原理计算预测d一种/dX=+0.0200,-0.0018和-0.0087 nm ñ1个-XØX 1个-X/3ñ1个-XØX1个-X/2ñ1个-XØX并结合离子束组成分析和X射线衍射结果确定空位浓度为 X/2每个阳离子位点。室温下的电阻率相对于室温下降了两个数量级以上ρ=1.5×10-37.7×10-6Ω 对于 X=00.26。这伴随着电阻率从负温度系数到正温度系数的跃迁,霍尔迁移率从μH=0.371.7厘米2/VS,载流子浓度增加 ñ=1.1×10204.9×1021厘米-3,并将计算出的带隙从0.54降低到0 eV。然而,ρ 再次增加到 6.9×10-5Ω 对于 X=0.34>10Ω 对于 X0.59,相应的下降 μH和差距的扩大。测得的载流子密度和电导率中的最大值X=0.26 归因于费米能级附近的状态密度的最大值,这与使用具有随机分布的O取代和Cr空位的超级单元进行的计算完全吻合,而Cr空位预测了 X=0.25±0.05。测量结果表明绝缘子到金属以及随后的金属到绝缘子的转变在X=0.04±0.030.30±0.03, 分别。向金属导电的过渡归因于阴离子位点上作为供体的取代O,而ñ 对于 X=0.010.26定量表示通过充当受体的阳离子空位进行的电荷补偿。绝缘性能大X 可能是由于电子相关效应增加所致。
更新日期:2020-12-02
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