Large and accessible conductivity of charged domain walls in lithium niobate

Abstract

Ferroelectric domain walls are interfaces between areas of a material that exhibits different directions of spontaneous polarization. The properties of domain walls can be very different from those of the undisturbed material. Metallic-like conductivity of charged domain walls (CDWs) in nominally insulating ferroelectrics was predicted in 1973 and detected recently. This important effect is still in its infancy: The electric currents are still smaller than expected, the access to the conductivity at CDWs is hampered by contact barriers, and stability is low because of sophisticated domain structures or proximity of the Curie point. Here, we report on large, accessible, and stable conductivity at CDWs in lithium niobate (LN) crystals – a vital material for photonics. Our results mark a breakthrough: Increase of conductivity at CDWs by more than 13 orders of magnitude compared to that of the bulk, access to the effect via ohmic and diode-like contacts, and high stability for temperatures T ≤ 70 °C are demonstrated. A promising and now realistic prospect is to combine CDW functionalities with linear and nonlinear optical phenomena. Our findings allow new generations of adaptive-optical elements, of electrically controlled integrated-optical chips for quantum photonics, and of advanced LN-semiconductor hybrid optoelectronic devices.

Physics and applications of charged domain walls

“…38 Also, a correlation between the local concentration of charged dopants and the CDW conductance was established using the EELS TEM technique. 40 In 2017, record-breaking conductivity was reported for 180°C DWs in LiNbO 3 crystals, 6,76,99 while the earlier studies reported only transient currents 66 and d.c. conductivity under superbandgap illumination. 55 We highlight here the results from ref.…”

“…Here, the total resistance extracted from the I-V characteristic can be approximated by R = F/σ S where F ≈ 0.72•log(5d/a) is a dimensionless geometric factor accounting for a sharp convergence of stream lines of the current density from continuous bottom electrode to the tip. 6 Fortunately, it depends weakly on the ratio of the sample thickness d to the tip contact length a facilitating estimates of σ S and σ from experimental data.…”

“…55 We highlight here the results from ref. 6 . In this work, inclined (θ i ≈ 1°) positively charged walls (and also arrays of such walls) were engineered, using biased μm-sized tips.…”

“…106 Field-assisted poling of commercially available z-cut LiNbO 3 wafers combining tip and continuous electrodes leads to inclined H-H CDWs with θ i ⪅ 1°. 6,55,76 The origin of the inclinations is not quite clear; most probably, they are due to expansion of the field stream lines toward the continuous electrode. The inclination angle can be varied by doping with Mg and thermal treatments.…”

“…A conductivity wall/domain contrast up to 13 orders of magnitude has been recently reported. 6 A number of methods enabling CDW engineering have been developed, and first device prototypes exploiting CDWs appeared. 7,8 Various CDW issues were reviewed in refs [9][10][11] .…”