Ferroelectricity in Atomic Layer Deposited Wurtzite Zinc Magnesium Oxide Zn1- xMgxO

Abstract

Conformal deposition of wurtzite ferroelectrics, which is needed for their use in scaled nonvolatile memories, is challenging using current physical vapor deposition techniques. To overcome the conformality barrier, this work demonstrates ferroelectricity in wurtzite Zn_1-xMg_xO thin films prepared by plasma-enhanced atomic layer deposition, which is a non-line-of-sight deposition method. Films ranging in composition from x = 0.00 to x = 0.58 are predominantly wurtzite phase and exhibit a (0001)-texture. Increasing the magnesium content decreases the c/a ratio, increases the optical bandgap energy, increases the piezoelectric response, and enables polarization reversal. Clear polarization switching is demonstrated in 50 nm thick Zn_1-xMg_xO films by piezoresponse force microscopy in compositions containing x = 0.46 and x = 0.58.

Keywords: Atomic Layer Deposition; Ferroelectric; Thin Films; Wurtzite; Zinc Magnesium Oxide.

1

(a) Symmetric out-of-plane 2θ–ω X-ray diffraction patterns, (b) in-plane 2θ_χ X-ray diffraction patterns, (c) c/a lattice parameter ratio, and (d) optical bandgap energies across the Zn_1–x Mg _x O composition series for x = 0, 0.30, 0.42, 0.46, and 0.58.

2

(a) Bright-field TEM image of the Zn_0.42Mg_0.58O/Pt/Ti/SiO₂/Si stack with selected area diffraction pattern of the film (inset). (b) STEM-EDS elemental map indicating zinc and magnesium distribution throughout the thickness of the film, (c) STEM-HAADF atomic resolution image of the film structure, overlaid with dDPC showing the polar alignment of oxygen anions (red) and cations (green). (d) High-resolution transmission electron microscopy (HRTEM) image of the Zn_0.42Mg_0.58O film in the bulk region. The inset displays the corresponding fast Fourier transform (FFT).

3

Piezoresponse force microscopy images for Zn_1–x Mg _x O films with magnesium concentrations of (a) x = 0.00, (b) x = 0.30, (c) x = 0.42, (d) x = 0.46, and (e) x = 0.58.

4

Switching spectroscopy displaying off-field amplitude (green) and phase (black) of locally switched Zn_1–x Mg _x O for (a) x = 0.46 and (b) x = 0.58. The (c) piezoresponse force microscopy and (d) phase images of ferroelectric domain lithography performed by applying a DC bias to the cantilever to locally switch polarization within specified regions for the x = 0.58 film. Bias voltages of −16 and +16 V were applied to the cantilever, resulting in polarization switching over areas of 2.5 and 1 μm, respectively.