Using Atom-Probe Tomography to Understand $\mathrm{Zn}\mathrm{O}\ensuremath{\mathbin:}\mathrm{Al}/{\mathrm{Si}\mathrm{O}}_{2}/\mathrm{Si}$ Schottky Diodes

Using Atom-Probe Tomography to Understand $Zn O ∶ Al / {Si O}_{2} / Si$ Schottky Diodes

R. Jaramillo, Amanda Youssef, Austin Akey, Frank Schoofs, Shriram Ramanathan, and Tonio Buonassisi

Phys. Rev. Applied 6, 034016 – Published 26 September 2016

Abstract

We use electronic transport and atom-probe tomography to study $Zn O ∶ Al / {Si O}_{2} / Si$ Schottky diodes on lightly doped $n$ - and $p$ -type Si. We vary the carrier concentration in the $ZnO ∶ Al$ films by 2 orders of magnitude, but the Schottky barrier height remains nearly constant. Atom-probe tomography shows that Al segregates to the interface, so that the $ZnO ∶ Al$ at the junction is likely to be metallic even when the bulk of the $ZnO ∶ Al$ film is semiconducting. We hypothesize that the observed Fermi-level pinning is connected to the insulator-metal transition in doped ZnO. This implies that tuning the band alignment at oxide/Si interfaces may be achieved by controlling the transition between localized and extended states in the oxide, thereby changing the orbital hybridization across the interface.

Received 13 November 2015

DOI:https://doi.org/10.1103/PhysRevApplied.6.034016

Physics Subject Headings (PhySH)

Thermionic emission

Diodes Interfaces

Sputtering Tomography

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

R. Jaramillo^1,*, Amanda Youssef², Austin Akey², Frank Schoofs³, Shriram Ramanathan^3,4, and Tonio Buonassisi²

¹Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
²Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
³School of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge, Massachusetts 02138, USA
⁴School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA

^*rjaramil@mit.edu

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Vol. 6, Iss. 3 — September 2016

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Figure 1
Measuring Schottky barrier heights. We show $J (V, T)$ data and activation energy plots for two AZO compositions, oxygen poor ( $R_{O_{2}} = - 16 %$ , left half) and oxygen rich ( $R_{O_{2}} = 27 %$ , right half). (a), (e) and (b), (f) Data for samples on $p$ -type and $n$ -type Si, respectively. The measurement $T$ is represented by the line color according to the color bar in (f). [(a) and (e), insets] The diode ideality factors for $p$ -type (purple) and $n$ -type (black) samples. (c), (g) Data at room $T$ measured over a wider bias range. (d), (h) The saturation current density ( $J_{S}$ ) shown as an activation energy plot. The slope of these lines yields the barrier height.
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Figure 2
AZO film properties and Schottky barrier heights on $n$ - and $p$ -type Si. (a) Resistivity (left axis, black curve) and conduction electron concentration (right axis, red curve) for AZO films as a function of relative oxygen content $R_{O_{2}}$ (see text). The yellow bar indicates the electron concentration corresponding to the insulator-metal transition in ZnO [31]. (b) Schottky barrier height for $AZO / {Si O}_{2} / Si$ diodes on both $n$ -type (black points) and $p$ -type (purple points) Si. The sum of barrier heights $Φ_{n} + Φ_{p}$ is shown in light blue.
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Figure 3
We compare the difference $(Φ_{p} - Φ_{n}) / 2$ found here to that reported in the literature for metals and ZnO, and to predictions of barrier heights based on charge neutrality level (CNL) and electron affinity rules of band alignment [5, 32, 33, 34, 35]. See Supplemental Material for details including the work function of ZnO and treatment of results in Ref. [35] [16].
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Figure 4
Al segregation at the $AZO / {Si O}_{2} / Si$ interface observed by APT. (Left) 2D contour plot of the Al concentration (at. %) for the oxygen-poor ( $R_{O_{2}} = - 24.5 %$ ) AZO layer on $n$ -type Si substrate. The 2D projection from the entire tip volume contour plots is generated by sampling the 3D grid in the $z$ direction with a distance between the sampling planes equal to 1 nm. (Right) 2D contour plots of the Al concentration for all six samples measured by APT. The dashed black line corresponds to the defined interface (5 at. % Si). 3D reconstruction artifacts such as curvature effects are caused by the difference in evaporation field needed to field evaporate the different elements present at the junction and the resulting change in the image compression factor. Also, notice the interface line being tilted, this is the result of sample preparation wherein the tip is deposited slightly offset from its vertical position. The measured Al concentrations are systematically inflated because the oxygen content cannot be accurately measured [16].
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Figure 5
Al concentration across the $AZO / {Si O}_{2} / Si$ interface, generated using a proxigram normal to an interface defined as 5 at. % Si. $n$ -type samples are shown with solid lines, $p$ -type samples are shown with dashed lines. Black, oxygen poor ( $R_{O_{2}} = - 24.5 %$ ). Orange, optimal ( $R_{O_{2}} = 0 %$ ). Blue, oxygen rich ( $R_{O_{2}} = 26.7 %$ ). (a) Al at. %. (b) Al concentration scaled by the Hall data measured on witness AZO films. The yellow bar indicates the free-electron concentration at the insulator-metal transition. The segregation of Al suggests that the concentration of ionized donors ${Al}_{Zn}^{\cdot}$ at the interface is always on the metallic side of the insulator-metal transition.
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Physical Review Applied

Using Atom-Probe Tomography to Understand ZnO∶Al/SiO2/Si Schottky Diodes

R. Jaramillo, Amanda Youssef, Austin Akey, Frank Schoofs, Shriram Ramanathan, and Tonio Buonassisi

Phys. Rev. Applied 6, 034016 – Published 26 September 2016

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Using Atom-Probe Tomography to Understand $Zn O ∶ Al / {Si O}_{2} / Si$ Schottky Diodes