Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate.

G. F. Nataf,M. Guennou,J. Kreisel,C. Mathieu,P. Grysan,C. L. Rountree,N. Barrett,D. Martinotti

doi:10.1038/srep33098

Abstract

The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM – electrons reflected) to Low Energy Electron Microscopy (LEEM – electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field.

Highlights

The understanding of domain structures, domain walls, currently attracts a significant attention in the field of-ferroic materials
The quantitative difference in surface potential between under and over-focusing far from the domain walls is due to the fact that we image charged regions and as discussed with respect to Fig. 5 the intensity changes as a function of focussing and the complementary error function returns a different value for the Mirror Electron Microscopy (MEM)-Low Energy Electron Microscopy (LEEM) transition
We used Mirror and Low Energy Electron Microscopy to carry out a microscopic characterization of the domains and domain walls at the surface of magnesium-doped lithium niobate (Mg):LNO

Summary

Sample Description and Experimental Methods

The sample was a 10 × 10 mm2 5% Mg-doped single crystal of lithium niobate (0001) supplied by HC Photonics Corp. The sample was at U0 +SV, where the Start Voltage (SV) defines the incident electron energy with respect to the sample surface. At low start voltages (SV → 0 V), the incident electrons are reflected by the potential above the surface, i.e. Mirror Electron Microscopy (MEM). At higher voltages (typically, 1 V < SV < 10 V) they penetrate the sample and backscattering occurs, i.e. Low Energy Electron Microscopy (LEEM). Since the band gap of lithium niobate is large (3.8 eV31), electrons penetrating the sample can induce charging, distorting the LEEM images. Piezo-response Force Microscopy (PFM) images were obtained using an Innova (Bruker) microscope, after the LEEM experiments. Silver paste on the rear of the sample provides a back electrode

Results and Discussion

Pup Pdown

Conclusions

Additional Information