Quantum Electron Transport through Functional Interfaces · 2015. 4. 14. · Quantum Electron...

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Quantum Electron Transport through Functional Interfaces Zhongchang Wang 1 , Susumu Tsukimoto 1 , Mitsuhiro Saito 1 , Yuichi Ikuhara 1,2 1 WPI, Advanced Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan 2 Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan Quantum electron transport through metal-semiconductor interfaces is of strong current interest because of the miniaturization of devices both in the semiconductor technology as well as in the emerging oxide-based electronics. However, obtaining an atomistic understanding of the impact of buried interface structures on the quantum transport remains challenging because interpretation of the atomic-resolution interface images is not always straightforward and the acquired structures need further to be bridged to properties on a quantum level [1]. This can be difficult to obtain by experiment alone but is likely with additional theoretical calculations. Here, combining the nonequilibrium Green’s function technique with density functional theory (DFT), we present quantum transport properties of several technologically important interfaces and demonstrate that origin underlying the interface-related issues can be understood and manipulated at the atomic level. One interface issue currently limiting the wide-gap semiconductor processing is the trial-and-error designing of contact materials for SiC. SiC has been investigated extensively because its physical properties are desirable for next-generation high-power and high-temperature electronics [2]. However, most of its intriguing inherent properties rely critically on and are often limited by the ohmic contact to SiC. Although it has been found that deposition of TiAl-based metals can yield significantly low contact resistance and that the formation of ohmic contact can be ascribed to an interface between SiC and Ti 3 SiC 2 generated after annealing [3], an understanding of how the interface reduces Schottky barrier has not yet been well developed. Using advanced transmission electron microscopy, we attributed qualitatively the ohmic contact formation to an epitaxial, coherent, and atomically ordered interface [4]. Qualitatively, our theoretical calculations predict that this interface can trap an atomic layer of carbon (Fig. 1) and hence enable lowered Schottky barrier and enhanced quantum transport (inset of Fig. 1). Another interface of interest is the Pt-SrTiO 3 -Pt heterojunction because the physical properties of SrTiO 3 can be tuned effectively via intentional electron doping so as to meet many applications. However, little is known about how these extra electrons distribute, especially along the conductance channel (i.e., Ti-O-Ti network), which matters because the degree of electron localization is critical for quantum transport. Interestingly, we find that the intrinsically closed channel in SrTiO 3 opens after doping substitutional atoms of higher valency (e.g., La or Nb in Fig. 2) for Sr or Ti, resulting in enhancement in electron transmission at Fermi level and drastic increase in current with bias [5]. This switch of channel, which could be promising for novel device concepts, is suggested to be due to the redistribution of states on the orbitals of channel atoms. In addition to the oxide SrTiO 3 , we are also investigating electron transport through MgO grain boundary because it is suggested that this boundary can confine electrons within the empty space inside its dislocation cores [6,7]. This unusual electron-trapping behavior should be crucial for addressing a wide range of physical problems that may © 2010 Japan Fine Ceramics Center AMTC Letters Vol. 2 (2010) 18

Transcript of Quantum Electron Transport through Functional Interfaces · 2015. 4. 14. · Quantum Electron...

Page 1: Quantum Electron Transport through Functional Interfaces · 2015. 4. 14. · Quantum Electron Transport through Functional Interfaces Zhongchang Wang 1, Susumu Tsukimoto1, Mitsuhiro

Quantum Electron Transport through Functional Interfaces

Zhongchang Wang1, Susumu Tsukimoto1, Mitsuhiro Saito1, Yuichi Ikuhara1,2

1WPI, Advanced Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan 2Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan

Quantum electron transport through metal-semiconductor interfaces is of strong current interest because of the miniaturization of devices both in the semiconductor technology as well as in the emerging oxide-based electronics. However, obtaining an atomistic understanding of the impact of buried interface structures on the quantum transport remains challenging because interpretation of the atomic-resolution interface images is not always straightforward and the acquired structures need further to be bridged to properties on a quantum level [1]. This can be difficult to obtain by experiment alone but is likely with additional theoretical calculations. Here, combining the nonequilibrium Green’s function technique with density functional theory (DFT), we present quantum transport properties of several technologically important interfaces and demonstrate that origin underlying the interface-related issues can be understood and manipulated at the atomic level.

One interface issue currently limiting the wide-gap semiconductor processing is the trial-and-error designing of contact materials for SiC. SiC has been investigated extensively because its physical properties are desirable for next-generation high-power and high-temperature electronics [2]. However, most of its intriguing inherent properties rely critically on and are often limited by the ohmic contact to SiC. Although it has been found that deposition of TiAl-based metals can yield significantly low contact resistance and that the formation of ohmic contact can be ascribed to an interface between SiC and Ti3SiC2 generated after annealing [3], an understanding of how the interface reduces Schottky barrier has not yet been well developed. Using advanced transmission electron microscopy, we attributed qualitatively the ohmic contact formation to an epitaxial, coherent, and atomically ordered interface [4]. Qualitatively, our theoretical calculations predict that this interface can trap an atomic layer of carbon (Fig. 1) and hence enable lowered Schottky barrier and enhanced quantum transport (inset of Fig. 1).

Another interface of interest is the Pt-SrTiO3-Pt heterojunction because the physical properties of SrTiO3 can be tuned effectively via intentional electron doping so as to meet many applications. However, little is known about how these extra electrons distribute, especially along the conductance channel (i.e., Ti-O-Ti network), which matters because the degree of electron localization is critical for quantum transport. Interestingly, we find that the intrinsically closed channel in SrTiO3 opens after doping substitutional atoms of higher valency (e.g., La or Nb in Fig. 2) for Sr or Ti, resulting in enhancement in electron transmission at Fermi level and drastic increase in current with bias [5]. This switch of channel, which could be promising for novel device concepts, is suggested to be due to the redistribution of states on the orbitals of channel atoms. In addition to the oxide SrTiO3, we are also investigating electron transport through MgO grain boundary because it is suggested that this boundary can confine electrons within the empty space inside its dislocation cores [6,7]. This unusual electron-trapping behavior should be crucial for addressing a wide range of physical problems that may

© 2010 Japan Fine Ceramics Center

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influence device functionality. The results concerning transport through the MgO grain boundary sandwiched between two Au electrodes will be presented in the poster. References [1] Z. Wang et al., Adv. Mater. 21 (2009) 4966. [2] Z. Wang et al., Phys. Rev. B 79 (2009) 045318. [3] S. Tsukimoto et al., J. Electron. Mater. 33 (2004) 460. [4] Z. Wang et al., Phys. Rev. B 80 (2009) 245303. [5] Z. Wang et al., Appl. Phys. Lett. 94 (2009) 252103. [6] K. P. McKenna et al., Nature Mater. 7 (2008) 859. [7] Z. Wang et al., Appl. Phys. Lett. 95 (2009) 184101. FIG. 1. Z-contrast image of the 4H-SiC/Ti3SiC2 interface in the Ohmic-contact sample. The overlay shows the most stable interface model obtained from the density functional theory (DFT), where the one layer of carbon trapped at interface is highlighted. The right inset presents the transmission spectra at 0 V for the interfaces with and without interfacial C. Of the two cases, the interface with C shows a larger transmission coefficient at Fermi level (EF), implying that, compared to the interface without C, electrons permeate the interface in an easier way. FIG. 2. Charge density plot in the energy window [EF-0.5eV, EF] for (a) non-doped system and the systems doped with (b) La, (c) Ca, (d) Ba, (e) Nb, and (f) Hf. Note that the La has a higher valency than Sr and the Nb than Ti. The brown filled balls represent Ti atoms, while the blue ones O atoms, regardless of their sizes. The conductive channels open only in the doping cases of La and Nb.

Interface

DFT-Aided HAADF-STEM Image

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E-EF (eV)

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