Engineering Carrier Effective Masses in Ultrathin Quantum Wells of ${\mathrm{IrO}}_{2}$

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of ${IrO}_{2}$

Jason K. Kawasaki, Choong H. Kim, Jocienne N. Nelson, Sophie Crisp, Christian J. Zollner, Eric Biegenwald, John T. Heron, Craig J. Fennie, Darrell G. Schlom, and Kyle M. Shen

Phys. Rev. Lett. 121, 176802 – Published 25 October 2018

Abstract

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide ${IrO}_{2}$ . Using a combination of in situ angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile ${IrO}_{2}$ have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of ${IrO}_{2}$ in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure.

Received 26 February 2018
Revised 9 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.176802

Physics Subject Headings (PhySH)

Electronic structure

2-dimensional systems Iridates Quantum wells Thin films

Angle-resolved photoemission spectroscopy Density functional theory Molecular beam epitaxy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jason K. Kawasaki^1,2,3,4,*, Choong H. Kim^5,6, Jocienne N. Nelson², Sophie Crisp², Christian J. Zollner⁷, Eric Biegenwald³, John T. Heron³, Craig J. Fennie⁷, Darrell G. Schlom^1,3, and Kyle M. Shen^1,2,†

¹Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA
²Laboratory for Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA
³Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
⁴Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706, USA
⁵Center for Correlated Electron Systems, Institute for Basic Science, Seoul, Korea
⁶Department of Physics and Astronomy, Seoul National University, Seoul, Korea
⁷Department of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA

^*Present address: Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706, USA.
^†kmshen@cornell.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 121, Iss. 17 — 26 October 2018

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Structural and transport characteristics of ultrathin ${IrO}_{2}$ thin films grown on (110) ${TiO}_{2}$ . (a) X-ray diffraction $θ - 2 θ$ scans (Cu $K α$ ). (b) Atomic force microscopy topograph of a 4 monolayer (ML) thick film, with step heights corresponding to 1 ML. (c) Low-energy electron diffraction ( $h ν = 150 eV$ ) pattern of the 4 ML film showing a sharp ( $1 \times 1$ ) pattern and indicating a well atomically ordered surface. (d) Resistance versus temperature measured along [001], showing retained metallicity for all films. (e) Effective 2D carrier density (defined as inverse of the Hall coefficient $1 / R_{H} q$ , where $q =$ fundamental charge) versus temperature, showing hole dominated transport.
Reuse & Permissions
Figure 2
ARPES measurements of metallic quantum well states. (a) Cuts along $k_{x} ∥ [1 \bar{1} 0]$ through (0,0) for films 50–4 ML thick (measured using He $I α$ ). The left side of each plot is the raw data and the right side is the second derivative ( $- (\partial^{2} I / \partial E^{2})$ , i.e., the curvature) of the raw data. Symbols denote EDC and MDC fits. Solid and dotted curves for 50 ML are the bulk $GGA + SO$ calculation [including zone folding at the (110) surface 19], curves for 9 ML and thinner are empirical guides to the eye. (b) Three-dimensional $GGA + SO$ Fermi surface (upper) and ARPES measured slice at constant $k_{z}$ through Fermi surface (lower) in the bulklike limit (50 ML). The constant $k_{z} = 0.76 π / d_{110}$ slice is denoted by the gray plane through the three-dimensional Fermi surface, as determined from fitting to a free-electronlike model of final states [19]. (c) Energy dispersion curves (measured using He $II α$ ) through (0,0) tracking the evolution of quantum well states. (d) Measured energy of the subband bottom versus film thickness (circles) and fit to the phase accumulation model (curves). Fits to the $n = 2$ subband for the 3 and 4 ML thick films (black data points) are extrapolated from EDC fits in the region near $k_{/ /} = 0$ . (e) Schematic Fermi surface in the 2D limit (upper) and measured Fermi surface for the 4 thick ML film (lower).
Reuse & Permissions
Figure 3
Subband mass enhancement of ${IrO}_{2}$ extracted from ARPES (solid symbols) and comparison with DFT slab calculations (open symbols) and discrete $k_{z}$ sampling of the bulk dispersion (dotted line). The color coding by subband index is the same as used in Fig. 2.
Reuse & Permissions
Figure 4
Origin of mass enhancement. (a) ARPES dispersions for the 7.5 ML thick film (red circles) and comparison with a $GGA + SO$ slab calculation for 8 ML thick ${IrO}_{2}$ (left, black symbols) and discrete $k_{z}$ sampling of the $GGA + SO$ bulk calculation (right, blue curves). (b) Schematic of how quantized subbands ( $E$ versus $k_{/ /}$ ) arise from discrete $k_{z}$ sampling of the bulk dispersion $E (k_{/ /}, k_{z})$ . Sampling of $k_{z}$ is in increments of $π / (N d)$ , where $N$ is the number of monolayers and $d = d_{110}$ is the monolayer spacing.
Reuse & Permissions

Physical Review Letters

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO2

Jason K. Kawasaki, Choong H. Kim, Jocienne N. Nelson, Sophie Crisp, Christian J. Zollner, Eric Biegenwald, John T. Heron, Craig J. Fennie, Darrell G. Schlom, and Kyle M. Shen

Phys. Rev. Lett. 121, 176802 – Published 25 October 2018

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of ${IrO}_{2}$