Large Enhancements in Optical and Piezoelectric Properties in Ferroelectric Zn_1‐xMg_xO Thin Films through Engineering Electronic and Ionic Anharmonicities

Abstract

Multifunctionality as a paradigm requires materials exhibiting multiple superior properties. Integrating second‐order optical nonlinearity and large bandgap with piezoelectricity can, for example, enable broadband, strain‐tunable photonics. Though very different phenomena at distinct frequencies, both second‐order optical nonlinearity and piezoelectricity are third‐rank polar tensors present only in acentric crystal structures. However, simultaneously enhancing both phenomena is highly challenging since it involves competing effects with tradeoffs. Recently, a large switchable ferroelectric polarization of ≈80 μC cm−2 was reported in Zn1‐xMgxO films. Here, ferroelectric Zn1‐xMgxO is demonstrated to be a platform that hosts simultaneously a 30% increase in the electronic bandgap, a 50% enhancement in the second harmonic generation (SHG) coefficients, and a near 200% improvement in the piezoelectric coefficients over pure ZnO. These enhancements are shown to be due to a 400% increase in the electronic anharmonicity and a ≈200% decrease in the ionic anharmonicity with Mg substitution. Precisely controllable periodic ferroelectric domain gratings are demonstrated down to 800 nm domain width, enabling ultraviolet quasi‐phase‐matched optical harmonic generation as well as domain‐engineered piezoelectric devices.

Strong nonlinear-polarization in ZnMgO epitaxial thin-films with Li incorporation

“…The highlighted results are as follows: (1) Moderate Li incorporation [Li/(Zn + Mg + Li) = 0.07] into Zn 0.67 Mg 0.33 O can counteract the reduction in χ 33 caused by the Mg incorporation (x = 0.33) by enhancing electronic polarization; (2) The Li occupation at octahedral interstitial sites of wurtzite ZnO effectively enhances the resistance to high-power laser by improving structural stability. The Li incorporated Zn 0.67 Mg 0.33 O [Li/(Zn + Mg + Li) = 0.07] thin-film shows a |χ 33 | of 36.1 pm V −1 under a peak power density (E p ) of 81.2 GW cm −2 , which is about 3.1 times larger than the value of the Zn 0.63 Mg 0.37 O without Li incorporation in the previously published literature [17]. This suggests the great potential of Zn 0.67 Mg 0.33 O thin-films with Li incorporation for applications to laser frequency conversion.…”

“…This indirectly proves that moderate Mg incorporation has positive influence on χ (2) of ZnO, because χ (2) is closely related to r c [16]. After that, Zu et al proposed that moderate Mg incorporation improved the χ (2) by increasing electronic anharmonicity and reducing ionic anharmonicity [17]. The demonstration of robust ferroelectricity in Zn 1−x Mg x O (x > 0.3) [18,19] motivates more interests to investigate periodic polarization reversal in the materials, which can effectively enhance conversion efficiency of the χ (2) process by realizing quasi-phase-matching.…”

“…The demonstration of robust ferroelectricity in Zn 1−x Mg x O (x > 0.3) [18,19] motivates more interests to investigate periodic polarization reversal in the materials, which can effectively enhance conversion efficiency of the χ (2) process by realizing quasi-phase-matching. However, it was found that Mg incorporation (x > 0.3) caused a sharp fall in χ (2) and the matrix element |χ 33 | of χ (2) tensor value of a Zn 0.63 Mg 0.37 O thin-film was 11.64 pm V −1 (χ 33 = 2d 33 [20]), which was smaller than that of ZnO thin-film (14.1 pm V −1 ) [17]. Therefore, more efforts are needed to improve χ (2) of ZnMgO thin-films with robust ferroelectricity.…”

Polarity Control of the Schottky Barrier in Wurtzite Ferroelectrics

“…This enhanced piezoelectric performance can be attributed to the alloying effect and introduction of much smaller Mg 2+ ions in the wurtzite lattice, 25 which have been hypothesized to lead to switchable spontaneous polarization. The improved electromechanical performance on magnesium substitution can also be attributed to an elastic softening of the wurtzite structure and flattening of double-well potential, as has been suggested using first- principles calculations in wurtzite-structured Sc-doped AlN alloys, 8,12 together with increasing fractions of c-axis-oriented nanocrystallites in the films. Dynamic Spectroscopic and Global Polarization Switching.…”

“…Given this, recent reports on the emergence of ferroelectricity and a large switchable polarization of more than 90 μC/cm 2 measured through macroscopic volume-average polarization switching transients in magnesium-substituted zinc oxide (Mg x Zn 1 –x O, x = 0.3–0.37), a simple metal oxide with a polar wurtzite crystal structure, have gained much attention. − This is because, unlike traditional complex perovskite oxides with multiple constituent elements, semiconducting zinc oxide is amenable to fabrication at relatively low temperatures by using different methods including low-cost solution-based processes. The magnesium-substituted zinc oxide thin films, therefore, have the potential to be integrated with CMOS processes and flow and present an attractive option beyond doped HfO 2 -based binary oxide ferroelectric thin films which has been under investigation with regard to metastable ferroelectric phase, wake-up effects, and advanced electronic device demonstrations.…”