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Abstract
A new method for characterizing lithium niobate +/-Z ferroelectric polarization domains using Auger electron spectroscopy (AES) is presented. The domains of periodically poled samples are found to be differentiable using the peak amplitude of the Auger oxygen KLL transition, with -Z domains having a larger peak-amplitude as compared to +Z domains. The peak amplitude separation between domains is found to be dependent on the primary beam current, necessitating a balance between the insulating samples charging under the primary beam and achieving sufficient signal to noise in amplitude separation. AES amplitude-based domain characterization is demonstrated for fields of view (FOV) ranging from 1 ${\mathrm{\mu}}$m to 78 ${\mathrm{\mu}}$m. Domain spatial resolution of 91 nm is demonstrated at 1 ${\mathrm{\mu}}$m FOV.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ferroelectric materials, with their built-in electric polarization fields, have found numerous applications in optics due to their wavelength conversion capabilities [1–7]. One material of particular interest is periodically poled lithium niobate (PPLN), for its numerous nonlinear optical applications [2,8–11]. Periodically switching the direction of the ferroelectric polarization enables quasi-phase matching (QPM) with optical beams polarized along the largest element of the nonlinear tensor, allowing high nonlinear conversion efficiency [12–16]. One limitation to PPLN is that the most efficient first-order QPM processes often require sub-micron poling periods [3,8,9,17]. A new non-destructive domain characterization method with sub-micron resolution that complements existing methods would be useful for the study and understanding of sub-micron ferroelectric domains [3].
There are several established methods for ferroelectric domain characterization, though they each have their individual limitations. Perhaps the most common method is visualizing the domains after selective etching in HF acid [18,19], which has the effect of only etching the -Z domain surface [20–26], however this method is destructive for surface applications. Piezoresponse force microscopy (PFM) is a scanning probe microscopy technique that images domains based on the phase of the sample’s piezoelectric response to an AC voltage applied to the probe [27,28]. The method has nano-scale resolution, but measuring domain wall width is limited by probe tip size [29,30], and the origin of the image contrast is complicated [31–35], as is data interpretation [36–39]. Scanning electron microscopy (SEM) is a useful technique for visualizing domain boundaries. However, due to the complexity of pyroelectric and ferroelectric lithium niobate (LN), +/-Z domain identification is often ambiguous due to variations in domain shading with changes in beam exposure and beam parameters [40–42]. Two optical methods, Raman spectroscopy (RS) and Cherenkov second harmonic generation (CSHG) microscopy, have been demonstrated. While these methods are advantageous for their ability to probe the volume of LN crystals [43–47], the imaging resolution for both methods is limited to $\approx$ 500 nm by the probing laser’s diffraction limit [46,48–50].
Ferroelectric domain characterization in magnesium doped lithium niobate (MgLN) using the oxygen KLL (O KLL) peak energy shifts in the Auger electron spectroscopy (AES) spectra was recently demonstrated [51]. However, insufficient peak energy separation at smaller fields of view (FOV) limited the spatial resolution to a few microns. In the present work, a new Auger electron spectroscopy method for ferroelectric domain characterization in lithium niobate species is reported, which uses peak amplitude separation of the oxygen KLL peak. We demonstrate non-destructive surface domain characterization of PPLN with unambiguous determination of +$/$-Z domains with sub-micron spatial resolution.
2. Experiment
Wafers of un-doped, congruent PPLN were obtained from G&H with a 15 ${\mathrm{\mu}}$m poling period and were diced into square 10 mm chips. The PHI 710 Auger Nanoprobe loading process is described in a previous work [51]. Due to the insulating property of LN, the sample is tilted to 75$^{\circ }$ relative to the electron beam axis in order to limit sample charging by increasing the portion of the beam that is reflected off the sample surface and decreasing embedded charge [52,53]. The sample is raised up until the region of interest is brought into the focal point of the Auger electron detector, a cylindrical mirror analyzer (CMA), and the SEM image is focused with the primary beam set to 5 kV. The AES energy range is chosen to include the oxygen KLL transition (O KLL, nominally at 531 eV [54]), the most prominent AES elemental peak on LN samples. A fitting routine in MATLAB is applied to each AES spectrum in order to determine the peak energy and amplitude of the O KLL peak. The fit function is a Gaussian with a linear background and a non-zero y-intercept. The sloped background accounts for the contribution due to inelastically scattered Auger electrons at energies below the peak that are not present at high energies.
3. Results
The SEM image in Fig. 1(a) shows the approximate location of ten sequential AES survey areas on the PPLN sample. With an FOV of 78 ${\mathrm{\mu}}$m, survey areas are spaced half of a poling period apart, such that adjacent scan areas alternate between +/-Z domains. Light/dark blue colored survey areas are in -Z domains and red/orange colored survey areas are in +Z domains. The +/-Z domains were confirmed independently using HF etching on a nominally identical die diced from the same wafer. The boxes are numbered according to their sequential scan order and are placed in such a configuration that subsequent survey areas are farther away from each other in order to help mitigate charging effects. The ten AES spectra shown in Fig. 1(b) are color-coded so that each spectrum can be matched with the survey area in which the spectrum was taken, as indicated by the coloring of the boxes in the SEM image in Fig. 1(a). Figure 1(b) demonstrates that the -Z domains (blue colors) and +Z domains (red/orange colors) are distinguishable by their separation in both peak energy and peak amplitude: a comparable shift is observed in the Auger electron peak energy and the Auger electron count rate (amplitude). This is consistent with the previously published results of peak energy separation at 100 ${\mathrm{\mu}}$m FOV on MgLN [51] although at slightly smaller size scale. The domain differentiation demonstrated in Fig. 1 is non-destructive and did not involve HF etching as was done in previous work [51]. It is important to note that the domain orientation in the SEM image is ambiguous, but can be determined when performed in conjunction with the differentiated AES spectra.
Fig. 1. (a) 78 ${\mathrm{\mu}}$m FOV SEM image of PPLN with associated AES survey areas, numbered and color-coded to spectra in (b). (b) Auger O KLL transition spectra from PPLN. Colored spectra correspond to the 10 colored survey areas in the SEM image in (a). Light/dark blue curves correspond to -Z domains and red/orange curves correspond to +Z domains. The peak of each spectrum, as determined by the fitting routine, is plotted with a circle/triangle for -/+Z domains, respectively. 78 ${\mathrm{\mu}}$m FOV was chosen such that 10 evenly spaced survey areas alternate +/-Z domains.
The Auger O KLL transition on LN samples was observed to drift by tens of eV across an Auger session containing many surveys, likely due to charging issues [51]. Because of this, relative measurements of peak energy separation were made for each survey, rather than using absolute energy values. This is accomplished by subtracting each individual survey’s mean peak energy from the peak energy of each survey area (in this case 10, 5 from each +/-Z domain), such that peak energies can be visualized more clearly as a deviation from the survey mean. The same is done for peak amplitude, though the variations in the mean amplitudes were relatively constant within each set of surveys at a given primary beam current. In Figs. 2(a) and 2(b), the deviation from the mean for peak energy and amplitude, respectively, are plotted for 31 surveys at different primary beam currents. The survey areas in survey numbers 5–27 correspond to the 10 color-coded AES survey areas in the 78 ${\mathrm{\mu}}$m FOV SEM image shown in Fig. 1(a). As a control group, the first and last sets of surveys (surveys 1–4 and 28–31) were taken by moving the sample such that all ten different survey areas on the crystal were on a -Z domain. These control surveys are denoted ‘All -Z’ in Figs. 2(a) and 2(b). In the ‘All -Z’ surveys blue/red separation is not observed, indicating that the separation seen between +/- Z domains is not a function of the procedure used to perform the surveys. The different background colors used in Figs. 2(a) and 2(b) denote changes in the primary beam current, as indicated in the second row of labels at the top of the figures. The spectra shown in Fig. 1(b) correspond to survey #9 in Fig. 2. As previously noted, the light/dark blue circles are in -Z domain and the red/orange triangles are in +Z domain, in order to better see the separation of the plotted peak energy and amplitude values. In the deviation from the mean peak energy plot in Fig. 2(a), -$/$+Z domains (blue$/$red colors) are separated at 10 nA, and mostly separated at 5 nA, however at lower primary beam currents, the domains are not well differentiated by their measured peak energies. In contrast, in the deviation from the mean peak amplitude plot in Fig. 2(b), -$/$+Z domains (blue$/$red colors) are separated at all primary beam currents. There is a strong correlation between peak amplitude separation and primary beam current (though too high beam current causes problematic sample charging). This demonstrates that peak amplitude separation is a more pronounced method for domain characterization at this FOV over a wider range of beam currents compared to the peak energy separation method [51].
Fig. 2. (a) Peak energies (deviation from the mean) of all 10 survey areas, for 31 separate AES surveys with 78 ${\mathrm{\mu}}$m FOV and different primary beam currents (indicated by labels at top of graph and changing background color). (b) Peak amplitudes (deviation from the mean) for the 31 surveys with 78 ${\mathrm{\mu}}$m FOV and different primary beam currents. Ordinate axis values are in kilocounts/second. In surveys 5–27, the ten numbered survey areas correspond to alternating -/+Z domains as shown in the SEM image in Fig. 1(a), while in the control group (surveys 1–4 and 28–31, denoted ‘All -Z’) all ten survey areas were placed within a region of the crystal with only -Z domain.
Consequently, the ability of peak amplitude separation to characterize +/-Z domains at smaller FOV is investigated. In Fig. 3(a–c) the SEM images with AES survey areas at 9 ${\mathrm{\mu}}$m, 3 ${\mathrm{\mu}}$m, and 1 ${\mathrm{\mu}}$m FOV are shown, respectively. For these smaller FOVs, the five survey areas on the left hand side of the image are all located in -Z domain, while the survey areas on the right are in +Z domain, and the color-coding has been adjusted accordingly so that light/dark blue circles (red/orange triangles) denote data derived from -Z (+Z) domains. Figure 3(d) shows the deviation from the mean peak amplitude for sets of surveys with different FOVs and primary beam currents, as indicated along the top of the plot and by the changing background colors. The initial 8 and final 12 surveys are a control group, performed with all ten survey areas located in a region of the crystal with only -Z domain, as described above. They are labeled ‘All -Z’ in Fig. 3(d). There are a few instances where area 9 (cyan color), which is located closest to the domain wall on the -Z side, crosses over or nearly crosses into the +Z domains’ amplitudes. We speculate this is due to sample charging causing the electron beam location to drift slightly during scanning such that area 9 is sometimes straddling the domain wall. Furthermore, at 1 ${\mathrm{\mu}}$m FOV and 1nA primary beam current, peak amplitude separation is poor, also thought to be due to image drift. The level of peak amplitude separation is well correlated with primary beam current, however the use of large primary beam current must be balanced against increased sample charging and the associated image drift, which limit spatial resolution. Furthermore, peak amplitude separation between domains is not strongly dependent on FOV, as was observed with peak energy separation [51]. The amplitude differentiation at 1 ${\mathrm{\mu}}$m FOV at 5nA and 2nA in Fig. 3(c) (surveys 27–31, and 43–47) is especially interesting as it demonstrates 91nm spatial resolution between the central survey areas 9 and 2, center to center.
Fig. 3. (a–c) SEM images of 9 ${\mathrm{\mu}}$m, 3 ${\mathrm{\mu}}$m, and 1 ${\mathrm{\mu}}$m FOVs at 5 nA primary beam current, with associated AES survey areas. At 1 ${\mathrm{\mu}}$m FOV, the separation between two adjacent survey areas is 91 nm, center to center. (d) Peak amplitude (deviation from the mean) for several sets of surveys for different FOVs and primary beam currents (as indicated at the top of the figure). The surveys labelled “All -Z” were performed with the survey areas in a different part of the crystal, with all areas in -Z domain.
To investigate the repeatability of the measurement, Fig. 4 shows a second set of AES data taken using a different die diced from the same PPLN wafer as the sample shown in Fig. 3. The two samples have nominally identical poling patterns. As with the previous sample, as shown in Fig. 4(a–c), in the SEM images at 9 ${\mathrm{\mu}}$m, 3 ${\mathrm{\mu}}$m, and 1 ${\mathrm{\mu}}$m FOVs, the five survey areas on the left hand side of the image are in -Z domain and the five surveys on the right are in +Z domain. In Fig. 4(d), the initial 12 and final 17 surveys are placed with all ten survey areas within the bulk -Z domain as a control group. In the control surveys, no differentiation is observed, whereas in surveys with both +/-Z domains there is good red/blue separation and thus good domain characterization based on the peak amplitude differentiation method. In a few instances at 10 nA, the +Z area closest to the domain wall (area 2, orange triangle) are higher than the other red/orange points, likely due to image drift caused by surface charging. At 1 nA beam current, there are a few instances of imperfect red/blue separation, likely due to insufficient signal to noise in the measured peak amplitude. It should be noted that on this sample, domain differentiation was possible for 1 nA surveys at 1 ${\mathrm{\mu}}$m FOV, whereas on the previous sample (used in Fig. 3), separation at 1 nA, 1 ${\mathrm{\mu}}$m FOV was poor, likely due to image drift. While domain differentiation using the amplitude separation method was possible on both samples, there were slight differences in the optimum configuration between the two nominally identical samples. Identifying and controlling the causes of these inconsistencies could lead to better domain characterization and finer spatial resolution. For underlying values for Figs. 1–4 see Data Files 1-4 in Dataset 1 [57].
Fig. 4. A second data set from a separate sample nominally identical to the one shown in Fig. 3. (a–c) SEM images of 9 ${\mathrm{\mu}}$m, 3 ${\mathrm{\mu}}$m, and 1 ${\mathrm{\mu}}$m FOVs, respectively, at 1 nA primary beam current and associated AES survey areas. (d) Peak amplitude (deviation from the mean) for a number of surveys at 9 ${\mathrm{\mu}}$m, 3 ${\mathrm{\mu}}$m, and 1 ${\mathrm{\mu}}$m FOVs and 10 nA and 1 nA primary beam currents. The surveys labeled “All -Z” were performed with the survey areas in a different part of the crystal, with all areas in -Z domain.
4. Discussion
The underlying physical reason for the peak amplitude separation between +/-Z domains is not well understood. In our previous work, the peak energy separation between domains was explained by the difference in potential due to polarization surface charges in each domain, such that Auger electrons escaping from +Z domains would be slowed more than those from -Z domains and thus their detected peak energy would be lower [51]. While the peak energy separation is observed to be FOV dependent, the peak amplitude separation is observed to be constant for different FOVs with the same primary beam current. Whether the potential difference between domains also causes the separation in Auger electron count rates (peak amplitude) is unclear.
One hypothesis is that the amplitude separation between domains originates from a change in the amplitude of the background and not the Auger O KLL peak itself. This hypothesis is discounted, since spectra from different domains are not separated by amplitude outside the region of the peak. Another hypothesis is that the amplitude separation is a result of the unique geometry of the experiment, in which the sample is tilted to 75$^{\circ }$ with respect to the axis of the electron beam and CMA detector. The CMA detector has an angular cone of acceptance of 42+/-6$^{\circ }$ and thus it is likely that the chip’s high tilt angle shadows some portion of the cone of acceptance. Perhaps slight variations between the domains’ surface potential vary this clipping. Consequently, the impact of incidence angle on count rate was investigated using a conducting aluminum sample. An external bias voltage is applied to the aluminum sample, ranging from 0 V to 12 V. As the applied bias is increased, a decrease in amplitude (Auger electron count rate) was observed (this investigation is described in greater detail below). This experiment was repeated at several lower incidence angles, including 0$^{\circ }$ (normal incidence), and although a slight shift in the amplitude was observed, qualitatively similar results were obtained for all incidence angles tested. Thus, although this effect could not be tested with an LN sample, these results suggest that the incidence angle to the sample does not have a significant impact on the AES signal amplitude for surfaces of different potential.
Another hypothesis is that the negative polarization charge of the -Z domain may slow the incident primary electrons more than the +Z domain, causing a higher output signal because the lower energy electrons are more likely to interact near the surface of the sample, 1–5 nm deep where Auger electrons are able to escape [55] and be detected. Experimentally, when the beam voltage (energy) of the primary electron beam is decreased from 5 kV to 3 kV with the same primary beam current on PPLN samples, the Auger signal increased. This may be evidence that lower energy primary electrons have a higher probability of interacting in the region where most detected Auger electrons are produced.
To further explore the possible effects of surface potential on Auger O KLL peak amplitude, AES was performed on a conducting aluminum sample. The Al sample was tilted to 75$^{\circ }$ and the sample’s potential was varied with an external applied bias voltage. The results are shown in Fig. 5(a), with applied voltages listed along the top. The O KLL peak energy of the aluminum sample shifts an amount equal to the applied voltage (within +/-0.08 V). The stability of the peak energy at a given voltage on aluminum supports the idea that the peak energy is unstable on LN due to its insulating properties. Similar data to that in Fig. 5(a) was obtained for the peak amplitudes. The average amplitudes from 40 spectra (4 surveys of 10 different survey areas) at each voltage setting were averaged and plotted in Fig. 5(b). Due to issues with a varying background amplitude, the shift in amplitude (the red lines in Fig. 5(b)) was measured by subtracting the amplitudes measured with an applied voltage from the mean 0V amplitude measured just before and after (the black lines in Fig. 5(b)). The general trend of the amplitude increasing with increasing number of surveys may be due the AES chamber vacuum improving over time. As seen in Fig. 5(b), increasing (decreasing) the applied bias on the sample decreases (increases) the peak amplitude, by 0.2% per volt on average. In one reference, photoemission electron microscopy (PEEM) measurements of the +/-Z domains work functions found 6.2 eV and 4.6 eV, respectively, a separation of just 1.6 eV [56]. Applying this work function shift to our results would predict a shift of 0.3% in amplitude between the +/-Z domains. While the sign of the shift is consistent, the predicted value is approximately an order of magnitude lower than our calculated shift of 3.6% between +/-Z domains on PPLN. This may be due to material differences between aluminum and LN. These two experiments show that increasing beam voltage and applying external voltage bias both lower the Auger signal, which qualitatively support the hypothesis. However, a better understanding is needed to explain the quantitative differences, and whether -Z domains’ higher Auger signal is due to primary beam electrons losing more energy when incident upon the sample. While the physical mechanism underlying the amplitude shifts between the +/-Z domains is not fully understood, the empirical ability of the AES peak amplitude method to characterize PPLN domains was confirmed using the established HF etch method.
Fig. 5. AES of the O KLL transition on an aluminum sample tilted at 75$^{\circ }$ with an external applied bias (denoted along top of plot). (a) Peak energy values. A horizontal line is drawn at the mean energy for all surveys with no applied voltage (507.2 eV) as well as at offset energies corresponding to the applied voltage values [12, -3 -6 -9 -12]eV above and below. (b) Mean peak amplitudes for all ten spectra in all four surveys at each voltage setting. Black horizontal lines show the average amplitude of adjacent measurements with no applied voltage; these values are used as a baseline to determine the amplitude shift (red lines) for each measurement with an applied voltage. Nearest-neighbor baseline averages were used in this way in order to account for the overall upward drift in mean peak amplitude. See Data File 5 in Dataset 1 for underlying values [57].
5. Summary
A new method for using Auger electron spectroscopy to characterize ferroelectric domains in periodically poled lithium niobate is demonstrated. It is found that in addition to a relative peak energy shift of the Auger O KLL transition between +/- Z domains (as demonstrated previously [51]), +/-Z domains can be reliably differentiated using the peak amplitude separation of AES spectra. Advantages of this method include that it is non-destructive, it allows unambiguous determination of the +/-Z domains, and it has been demonstrated over a range of spatial resolutions spanning nearly two orders of magnitude, down to 91 nm. This is an improvement of two orders of magnitude over the spatial resolution achieved with peak AES energy separation in previous work on MgLN [51]. Further investigation is needed to understand the origins of AES peak amplitude shifts and to demonstrate the applicability of AES peak amplitudes as a non-destructive method for imaging ferroelectric domains with fine resolution over large fields of view.
Funding
National Science Foundation (1710128).
Acknowledgments
The authors gratefully acknowledge G&H for supplying the crystals, and Nathaniel Rieders and Christopher Ebbers for technical assistance and useful discussions. This work was performed in part at the Montana Nanotechnology Facility (MONT), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation under grant number ECCS-1542210. This material is based upon work supported by the National Science Foundation under grant number 1710128.
Disclosures
The authors declare no conflicts of interest.
Data availability
Supplementary data associated with this article can be found online Dataset 1, Ref. [57].
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