Low Voltage Electron Emission from BaTiO3 Thin Films Treated in Hydrochloric Acid

Low voltage electron emission from ferroelectric thin films consisting of BaTiO3 is reported. The 1 μm thick BaTiO3 thin films were deposited by spin coating and were annealed to improve their crystallinity. Then surface treatments using hydrochloric acid have been performed and very thin gold top electrodes were deposited. The electron emission capability of these thin films has been tested using a parallel cathode-positively biased anode arrangement in a vacuum chamber at 10−5 Pa. The turn on electric field for electron emission was 23 V/μm and we obtained an emission charge of about 200 pC/cycle at an electric field of 100 V/μm. [DOI: 10.1380/ejssnt.2008.164]


I. INTRODUCTION
Electron emission from ferroelectric materials due to a large change in dielectric displacement has been studied for many years due to its potential application to vacuum electronic devices, like flat panel displays and bright electron guns [1,2].Ferroelectric electron emission (FEE) is an unconventional electron emission effect which does not need an externally applied extraction field, since the field emission of the electrons is achieved by the spontaneous polarization field of the ferroelectric material itself.Two mechanisms have been proposed to describe the emission of the electrons from the surface of the ferroelectric material: the fast switching of the spontaneous polarization is considered responsible for the so called "weak" FEE, observed for the first time in 1984 by Rosenman et al. [3], while the "strong" FEE is plasma-assisted electron emission [4].The polarization reversal can be induced by applying a rise of temperature, a mechanical pressure pulse, an intense laser pulse or a high-voltage pulse via surface electrodes.However the last of these methods seems to be most effective for electron emission since the switching process is very fast and the charge compensating this process inside the ferroelectric material cannot follow quickly enough the spontaneous polarization change.
Due to its very high dielectric constant, relatively low coercive voltage as well as high remanent polarization, BaTiO 3 is recommended as a good candidate for its use as a ferroelectric cathode.Electron emission during phase transition of BaTiO 3 ceramic samples under vacuum conditions was observed by Kortov and Mints [5] and from γ-irradiated samples of powdered BaTiO 3 by Hayakawa et al. [6].Optically stimulated electron emission during phase transition of BaTiO 3 single crystals grown by Remeika method was reported by Rosenman et al. [7,8].All these experiments were carried out using BaTiO 3 ceramics or single crystals and typical applied voltage of several kV was required to induce the polarization reversal and electron emission from samples in the mm thickness range.However, as far as we know, there are no reports on electric field induced electron emission from BaTiO 3 thin films.
In this paper our recent investigations of the electron emission from BaTiO 3 thin films prepared by sol-gel method are presented.

II. EXPERIMENTAL
For our experiments we used a 10 % BaTiO 3 precursor solution (BT08B26-4, Mitsubishi Materials Corporation, Japan) and we used the spin coating method to obtain thin films onto nichrome substrates.The nichrome substrates (17 × 17 mm 2 ) were obtained from a commercial sheet, nichrome 80, with a thickness of 0.12 mm.Prior to the film deposition, the substrates were cleaned in ultrasonic bath with acetone and ethanol, and then rinsed in de-ionized water and dried by N 2 gas.For thin film deposition a two steps spin coating method was used: 500 min −1 for 3 seconds followed by 3000 min −1 for 30 seconds.After spin coating, each layer was pyrolized for 5 minutes at 380 • C onto a hot plate for solvent evaporation, and subsequently heated at 750 • C for 3 minutes in a furnace for crystallization of the film.This coating and heating procedure was repeated 10 times to achieve the desired thickness of 1 µm.Finally the samples were annealed at 750 • C for 2 hours and were allowed to cool down to room temperature at a very low cooling rate.

III. RESULTS AND DISCUSSIONS
The crystallization of the as-deposited thin films was evaluated using X-ray diffraction (XRD) measurements.Figure 1 shows the X-ray diffraction patterns for a 1 µm BaTiO 3 thin film.These diffraction patterns show a pure crystalline BaTiO 3 with no amorphous phase.The intensity of the diffraction peaks indicates a good crystallization of the films and no preferred orientation could be identified.
For electron emission experiments we thermally evaporated a very thin gold top electrode and we used the nichrome substrate as a bottom electrode.The top electrodes were circles with the diameter of 10 mm and over the entire surface of the top electrode microholes are present in high density, exposing the surface of the ferroelectric material.These microholes were realized due to a very high deposition rate of the gold combined with the pretreatment of the thin films in concentrated hydrochloric acid (HCl).Figure 2 shows the atomic force microscopy (AFM) images of the as-deposited BaTiO 3 thin film compared with a thin film treated in HCl for 45 seconds.
The surface roughness of the thin films was drastically increased from 3.2 nm for as-deposited thin films to about 16 nm for thin films exposed to hydrochloric acid.As a result, the gold electrode deposited onto these thin films consisted of numerous microholes as we can easily see in the field emission scanning electron microscope (FE-SEM) images presented in Fig. 3.
After top electrode deposition, the electron emission experiments were carried out using the experimental arrangement presented in Fig. 4. Samples were placed in a vacuum chamber at a pressure of 10 −5 Pa and a sine wave voltage with different frequencies was applied to the bottom electrode while the top electrode was grounded.The emitted electrons are detected by an anode consisting of a phosphor-coated glass screen situated at 5 cm in front of the emitting surface.When electron emission occurs, the emission current is recorded on a digital oscilloscope aligned with the anode.By applying an acceleration voltage to the screen, the emission image can be displayed simultaneously.
Electron emission tests have been performed using both, as-deposited and HCl treated thin films.In the case of the as-deposited thin films, we observed the electron emission image onto the phosphor screen with a turn on electric field of 50 V/µm, but the emission current was very small and could not be measured.On the other hand, for BaTiO 3 thin films pretreated for 45 seconds in HCl, stable emission image could be obtained onto the phosphor screen at an applied electric field as low as 23 V/µm and a frequency of 50 Hz.
Furthermore, for these samples we could obtain the I-V characteristics presented in Fig. 5.As we can see, the electron emission occurs under a negative pulse bias indicating a pure ferroelectric electron emission due to the polarization switching.From these I-V characteristics we calculated the emitted charge of 200 pC/cycle at the maximum applied voltage of 100 V. To compare the emission intensity for the two kinds of samples used in our experiments, we measured the luminance of the phosphor screen during the electron emission and the results are presented in Fig. 6.
For the samples treated in hydrochloric acid the luminance has a linear increase with the applied voltage with a maximum value of 10 cd/cm 2 , while for the as-deposited samples, the luminance is very small and has just a very slightly variation.The mechanism of the improvement of the electron emission in the hydrochloric acid treated BaTiO 3 thin films is under investigation, but we think that a higher surface roughness led to an increase in the charge stored on the surface of the films.

IV. CONCLUSIONS
In conclusion we obtained very low voltage electron emission from BaTiO 3 thin films.This reduction of the applied voltage was obtained for thin films treated in hydrochloric acids prior to the top electrode deposition.The electron emission occurred under a negative pulse bias.An emitted charge of 200 pC/cycle was obtained at an applied electric field of 100 V/µm.