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Nanoscale Measurement of Laser-Induced Temperature Rise and Field Evaporation Effects in CdTe and GaN David R. Diercks, and Brian P Gorman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02126 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 22, 2015

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The Journal of Physical Chemistry

Nanoscale Measurement of Laser-Induced Temperature Rise and Field Evaporation Effects in CdTe and GaN David R. Diercks* and Brian P. Gorman Metallurgical and Materials Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States

ABSTRACT The measurement of laser induced temperature change and its influence on the field evaporation behavior in nanoscale CdTe and GaN specimens was assessed through systematic studies using laser pulsed atom probe tomography. For CdTe, the laser was shown to induce a linear thermal response in the material. Using the determined relationships, a phase map of the field evaporation behavior was created. This shows that at high base temperatures, high laser energies, or low fields, significant Cd sublimation occurs, leading to apparently Terich measured compositions. In contrast, the highest fields result in simultaneous evaporation of multiple Te species, leading to apparently Cd-rich measured compositions. For GaN, increasing laser energy reduced the applied bias necessary for a given detection rate, whereas base temperature changes produced no significant effect on the evaporation behavior - indicative of a largely athermal evaporation mechanism. Similarly, the laser energy and bias affected the measured compositions, whereas the base temperature did not.

Additionally, the field 1

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evaporation behavior in GaN appears unusual in that there is a strong crystallographic dependence resulting in a non-uniform field being maintained across the apex of the specimen. These methods are useful beyond atom probe analyses for elucidating information about bonding and optoelectronic properties.

Introduction Nanoscale compound semiconductor structures are being used or investigated for use in a number of applications including displays, transistors, light emitting diodes, detectors, and photovoltaics.1-5

For these applications it is valuable to know how external forces such as

applied field, photonic excitation, and temperature affect the optoelectronic properties on the nanoscale. In particular, these can dramatically affect the electronic band structure in ways that are not observed for bulk materials. One possible effect is a change in laser absorption and the resulting thermal energy transferred to the specimen.6 Atom probe tomography (APT), while more commonly used for determining the precise three-dimensional composition and structure of materials, can also be used for assessing these nanoscale interactions. APT has the ability to obtain tens of parts-per-million composition sensitivity along with subangstrom spatial resolution.7,8 While initial implementations of APT were performed via voltage pulsing, the relatively widespread commercial use of a laser for pulsing has now expanded the materials applications of this technique. Laser-pulsed APT analysis has now been demonstrated for a number of compound semiconductor materials for which voltage-pulsed analysis is, at the very least, quite challenging.9-16 This allows for an unprecedented level of characterization of these materials, expanding the experimentally achievable understanding of them. Starting with Müller17 and Gomer,18 it was recognized that field desorption of species under most conditions is a thermally activated process. This was demonstrated by the fact that the 2 ACS Paragon Plus Environment

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electric field strength required for lattice field evaporation decreased as the temperature of the specimen was increased. Kellogg similarly established that laser pulsing of metals can produce a thermal response great enough to overcome the activation barrier.19 Moreover, for materials that have large electronic band gaps in unbiased conditions, lasers are still thought to induce a thermal response when the specimens are under sufficient bias. 6,20-23 Therefore, experimental measurement of the influence of temperature along with the influence of the laser provides important understanding of not only the field desorption processes, but also the underlying interactions and bonding within the material. This has been previously explored by APT for some elemental materials, but not for compound semiconductors.

One major challenge is the

difficulty in acquiring voltage-pulsed data from these materials.

This means that methods

previously used by Kellogg on metals for determining the effective temperature induced by the laser pulse19 are not possible. Those relied on comparing the electric field strength reduction for voltage pulsed evaporation as a function of specimen temperature to the laser pulsed evaporation at a fixed specimen temperature.

Thus, a series of measurements of the voltage-pulsed

evaporation field reduction as a function of specimen temperature provided a calibration curve against which the detection of field evaporation at a given laser power setting could be compared to determine the corresponding temperature. Vurpillot et al.24 used a pump-probe method to estimate the temperature at the specimen tip in the nanoseconds following laser illumination. However, this method still requires a voltage-pulsed calibration curve and uses a voltage pulse as the probe. And more recently Vurpillot et al.25 looked at the variation in evaporation rate of tungsten as a function of laser energy for two different temperatures and extracted a linear relationship between the laser energy and temperature. Methods based on modeling laser– semiconductor interactions in the presence of a high dc electric field have been used to estimate

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the temperature rise in semiconductors based on laser energy absorption by holes and energy transfer to the lattice.26 This, however, requires prior knowledge of the laser absorption and carrier properties of the materials at the analysis conditions, which may not be readily available for many specimens of interest. Non-metallic compounds have an additional complication in that previous analyses have demonstrated that these materials have a dependence of the apparent stoichiometry on the analysis conditions used during the data collection.16,27-31 The effect of laser energy on the apparent compositions has been reported multiple times; whereas the effect of the base temperature on these results has been significantly less explored.

There has been some

speculation on the causes of these deviations from the known compositions.

The current

prevailing theories are the loss of some species due to detector dead time,32 as neutral particles,2730,32

and the uncorrelated evaporation of some species.27,30,33

There is not much direct

experimental evidence for any of these scenarios, though recent work on GaN demonstrated a correlation in the laser energy, apparent stoichiometry, and the measured background, suggesting that an apparent gallium deficit at low laser energy (high field) is due to uncorrelated evaporation whereas a nitrogen deficit at high laser energy (low field) is due to loss of neutral nitrogen species.30 Similar results were seen for oxygen content in ceria.28 Furthermore, there may be different methods of loss of neutral species during analysis, as both thermal27,28,30 and direct electronic excitation 29,34 methods have been postulated. Regardless of mode of species loss, this apparent composition dependence can be particularly important when trying to extract quantitative information from the data as the usefulness of the technique for analyzing materials of unknown composition is diminished if the correct stoichiometry is not measured. Additionally, such fundamental analyses can provide greater


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understanding of temperature, photonic, and field induced effects on nanoscale materials. Therefore, this work (1) presents a method for quantifying the effective temperature generated in a material by the pulsed laser, (2) relates the analysis conditions to the measured stoichiometry, and (3) uses those analyses to provide additional insight into the laser absorption and field evaporation processes. Experimental Methods Systematic studies of the relationships of laser energy, base temperature, applied bias, and measured stoichiometry for the compound semiconducting materials CdTe and GaN were performed using a Cameca LEAP 4000X Si APT instrument. This instrument has a 355nm laser (~ 3µm spot diameter at 4σ) and a straight flight path which was set to 90 mm. The data collection was done for a fixed detection rate at multiple laser energies while maintaining the base temperature constant, then for multiple base temperatures while the laser energy was kept constant. All other user-controlled variables were kept constant throughout. Additionally, all of the analyses for a given material were performed consecutively on the same specimen. This avoided the complicating effects of specimen-to-specimen geometric variations.

The APT

analyses were preceded and followed by transmission electron microscopy (TEM) imaging of the specimens using a Philips CM200 instrument and compatible hardware35 to measure and account for changes in tip radii in order to normalize the fields to those of the initial radii. Cameca’s IVAS 3.6.6 software was used for the reconstructions. Before beginning the series of APT analyses for a given material, an initial alignment of the specimen to the electrode and stabilization of the evaporation was performed during which ~ 300,000 ions were detected. While these were factored into the determination of the evolution of the tip radii, these data were not included in the other results.



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The positioning of the specimen relative to the electrode can have a large effect on the field generated at the specimen apex, therefore the analyses for the different laser energies were performed consecutively without moving the specimen. The adjustment of the base temperature does introduce movement of the specimen relative to the electrode due to the thermal expansion of the specimen mount and stage components. To compensate for this, the laser position was used as a reference point to which the tip of the specimen was returned after the temperature stabilized. While this is a source of uncertainty in the measurements at the different base temperatures, this method of realignment appeared to produce consistent positioning to within a couple of microns. The evolution of the radius for each specimen was taken from the TEM images. By overlaying the images from before and after analysis as shown in Figure 1, the depth and volume of the material removed during a series of runs was calculated. While in APT analysis not all of the removed volume typically makes it through the electrode aperture to the detector, it was assumed that the same fraction of the removed volume was collected during each analysis on a given specimen. For these calculations, the tip shape was approximated as a frustum, which has a volume given by:

= ℎ + +

(1)

where h is the height, r is the initial radius, and R is the final radius. Since all of these values were known from the TEM images, the calculation of the overall volume removed was straightforward. Then the volume removed during each run (Vn) was taken as the total volume (VT) multiplied by the number of ions detected during that run (in) divided by the total number of ions detected from all of the runs (iT):

=

(2)


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Figure 1. Overlay of the TEM images of a CdTe atom probe specimen before and after APT analysis showing the device structure and indicating the volume of material removed. Implicit in this calculation is an assumption of constant detection rate of ions. While the detection rate parameter was held constant during the analyses, any significant change in the rate during the analyses, such as from undetected loss of species under one set of conditions but not under another set of conditions, would introduce some error in the values. Once the volumes Vn for each of the runs were known, the initial and final radii were calculated from equation 1, in the order of the analyses. That is, the final radius of one run was used as the initial radius of the subsequent run. For the APT analysis, the applied bias and radius are related to each other as described in the field equation for a needle-shaped tip: =

(3)

where F is the field, V is the applied bias, kf is the field factor which takes into account specimen geometry and local environment, and Rc is the radius of curvature of the specimen. Therefore, normalizing the applied bias at each analysis condition by the final radius after that analysis allowed for a comparison of the relative field required to maintain the set detection rate for each set of conditions. Additionally, a fixed detection rate does not necessarily result in a constant emission rate since the field-of-view of the specimen changes with specimen radius.



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Therefore, calibration curves of the bias vs. measured flux were used to compensate for this effect. More details concerning these calculations are provided in the Supporting Information. For each specimen the result was a relative field vs. laser energy curve and a relative field vs. base temperature curve. Since these relationships both have the relative field in common, the laser energy and the temperature could thus be related to each other mathematically. Therefore, laser energy vs. temperature curves can be produced. The measured compositions for each specimen as a function of laser energy and base temperature were also explored. The individual details specific to the analysis of each material are presented within their subsections in the results. Results and Discussion CdTe A CdTe specimen was taken from a device that was fabricated as described elsewhere.36 After growth, this device was subject to rapid thermal processing at 300 °C for 30 seconds followed by 320 °C for 30 additional seconds. The APT specimen was prepared via a focused ion beam liftout method in an FEI Co. Helios 600i focused ion beam – scanning electron microscope.37 TEM imaging and diffraction confirmed that the specimen was a single CdTe grain, so there was no variation in crystallography during the analysis. APT analyses were performed at base temperatures from 23 – 120 K and laser energies from 0.4 – 10 pJ. The pulse frequency was 500 kHz, the detection rate was set at 1 event per 200 pulses (0.5%), and each run was stopped after 2 million ions were detected. Figure 2a shows the relative field vs. laser energy curve at a constant base temperature of 23.9K. Figure 2b shows the relative field vs. base temperature curve at a constant laser energy of 6.1 pJ. These plots are both fit well by linear functions. For the CdTe analyses, a relative field of 1 was defined as the field


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where, extrapolating the data, the set detection rate would be maintained in the absence of thermal or laser contributions, as will be discussed in more detail later. If it is assumed that there are no temperature-dependent effects (e.g. that the change in base temperature does not significantly affect the laser absorption), then, by setting the relative field as a function of laser energy equal to the relative field as a function of base temperature, the linear relationships in Figures 2a and 2b produce the calibration between the laser energy and the resulting average tip temperature during the pulse as shown in Figure 2c. This methodology is similar in some ways to a couple of previously used techniques to estimate the temperature of metallic tips.19,25 From this laser energy vs. temperature curve, CdTe shows a linear fit over the range of temperatures and laser energies analyzed.

This suggests that the laser pulsing is primarily resulting in

thermally assisted field evaporation for CdTe, similar to metallic materials. This is consistent with what has been previously observed for laser interactions with CdTe38 and is not surprising given the ~1.6 eV band gap of CdTe at these temperatures39 and the 3.5 eV laser used for the pulsing.



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Figure 2. CdTe (a) Relative field versus laser energy at a constant base temperature of 23.9 K. (b) Relative field versus base temperature at a constant laser energy of 6.1 pJ. The error bars indicate the maximum and minimum relative fields during each analysis. The average variation in relative field during each analysis was 0.03. (c) Using the linear relationships in (a) and (b), the average additional temperature of the specimen tip induced by the laser pulse as a function of the laser energy.

Furthermore, the two linear relations in Figures 2a and 2b were then used to define the plane governing the relationship between relative field, laser energy, and base temperature, where relative field (F) = 1 was defined as the point where the laser energy (L) = 0 pJ and the base temperature (T) = 0 K:


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+ 29.3 + 492 = 492

(4)

Setting F and L = 0 in equation 4 suggests the temperature at which the set evaporation rate would be attained absent an applied field and laser pulsing. In this case it is 492 K. Previous research at vacuum levels a couple orders of magnitude greater than what was used in this analysis indicated a significant amount of Cd sublimation from CdTe starting at temperatures around 500 K.

Therefore, the observed trend appears consistent with previous research.

Additionally, this result is consistent with CdTe molecular beam epitaxy growth literature where the best growth results are achieved at temperatures of 275 °C and lower with excess Cd flux.4043

Moreover, these laser energy and base temperature data were analyzed with regards to the apparent composition that is measured. As has been mentioned previously, these parameters have an effect on the relative detection of species in compounds. Understanding these effects is useful both in terms of determining analysis conditions which give the correct composition and for further insight into the field evaporation processes of multi-element materials. Since the CdTe phase has little tolerance for off-stoichiometry,44,45 the apparent compositional variations measured by APT cannot be explained by material variations and, instead, are a result of the instrumental analysis conditions. Figures 3a and 3b show the measured Cd composition as a function of the laser energy (at base temperature = 23.9 K) and base temperature (at laser energy = 6.1 pJ). Based on the relationship between laser energy and temperature that was developed above, the measured Cd concentration can further be shown as a function of overall temperature attained by the tip during the laser pulse (base temperature plus laser pulse temperature) as shown in Figure 3c. Over the whole temperature range, from 35 K to 300 K, there is a smooth transition from a slightly Cd-rich measured composition to one that is slightly



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Cd-poor, with a couple notable exceptions. All of the points that fall on the curve in Figure 3c had base temperatures of 52 K and below; the two outliers had base temperatures greater than 70 K. The overall temperatures for which the known composition was produced were ~ 135 – 175 K.

Figure 3. (a) Measured Cd composition versus laser energy at a constant base temperature of 23.9 K. (b) Measured Cd composition versus base temperature at a constant laser energy of 6.1 pJ. (c) Using the relationship from Figure 2(c), the measured Cd composition as a function of the overall temperature of the specimen tip during the laser pulse. The diamonds in (c) indicate analyses where the base temperature was greater than 70 K. The dashed lines in the figures indicate the expected Cd composition. The above observations can further be plotted in terms of the relative contributions of the three major variables – the base temperature, the laser-induced temperature, and relative field. Figure 12 ACS Paragon Plus Environment

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4 shows the measured composition as a function of the relative contributions of each of these. For this, the 100% reference points were set by the values determined from equation 4 by setting the other values equal to zero. For example, the reference laser energy was 16.8pJ, because that is the value for L when T and F = 0. In Figure 4 the colors of the dots indicate the experimental deviation of the measured compositions from the known composition. These appear to naturally segregate into regions of similar deviation from the 1:1 Cd:Te composition. Therefore, contour lines were drawn to indicate these groupings and trends. These contour lines represent the difference in the measured Cd and Te concentrations. For example, a measured composition of 49.5 at% Cd and 50.5 at% Te would fall along the 1 at% contour line.

Figure 4. The base temperature, laser, and field plane defined by equation 4 overlaid with the 13 ACS Paragon Plus Environment


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measured CdTe compositions under several experimental conditions within the plane. The colored dots and contour lines indicate the deviations from the expected composition: green – stoichiometric, yellow 1 – 3 at%, orange 3 - 10 at%, red > 10 at%. The dashed line indicates the approximate transition from Te-rich to Cd-rich. Displayed in this way, there are several noteworthy aspects that become apparent. Stoichiometric measurements were achieved at only the coldest base temperatures with a relative laser contribution of 30% of its defined maximum and field contribution of 65% of its defined maximum. On either side of this region, compositions apparently either Te-rich or Cd-rich were produced.

Te-rich measurements resulted from higher base temperatures and greater laser

energies; Cd-rich measurements resulted from high fields. Interestingly, this transition between Te-rich and Cd-rich also implies that there are conditions using this variable set where Cd and Te would be lost at the same rate and, therefore, would also reproduce the expected stoichiometry, though at a diminished detection rate. This is approximated by the double dashed line in Figure 4. Following on that, the reconstructions of other specimens run at low base temperatures with the other conditions within the region indicated as “stoichiometric” in Figure 4, had detection efficiencies of 50 – 56 %, where pre- and post-APT analysis TEM images were used as constraints on the reconstruction parameters. 28,30,46-48 This is within or nearly within the ranges that have previously been determined for this type of detector49-51 and indicates that at those conditions, there is not a substantial loss of either species beyond those caused by the limitations of the detector. So why is there more significant loss of Cd and Te at other conditions? As has been shown previously, at high vacuum levels, Cd sublimation occurs at relatively low temperatures.38 This is consistent with what is shown in Figure 3c – at higher temperatures a greater loss of Cd 14 ACS Paragon Plus Environment

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occurs. This is particularly the case for a higher base temperature (Figure 3b), which again is consistent with Cd sublimation, since at a higher base temperature the specimen experiences a greater thermal load throughout the analysis. An interesting aspect of this is that the Cd loss was not uniform, the loss was concentrated in one slightly off-center region of the detector where only 30 at% Cd was detected as shown in the Supporting Information.

The measured

composition of the rest of the specimen was ~ 45 at% Cd. This did not correspond with the incident laser direction and suggests a crystallographic or specimen shape dependency for the preferential loss of Cd. There were relatively few multiples (~4%) detected under these analysis conditions with Cd+ species (the main Cd peak) also being part of multiple hit events for ~4% of its counts. Ion correlation (Saxey) plots32 did not indicate the generation of neutral Cd species through dissociation of molecular ions. While the background totals did increase slightly at higher base temperatures, the increase was significantly less than the deficit seen in the Cd concentration. Therefore, this loss does not appear to be due to detector dead time during multiple hits or a significant amount of uncorrelated evaporation. This leaves loss of Cd neutral species from a mechanism other than dissociation of molecular ions or diffusion of Te up to the specimen apex as possible explanations. Unlike the localized Cd deficiency at high base temperatures, the apparent Te deficit occurred uniformly. Te loss occurred at conditions of low laser energy and correspondingly high fields. These conditions produced greater than 50% of the detected events as multiple hits. Counting just the ions that were part of multiple hit events, the composition was 54.8 at% Cd. For other analysis conditions which produced fewer than 5% of the events as multiple hits, the composition of ions involved in the multiple hit events was 51.7 at% Cd.

If that same

composition ratio were applied to the multiples for the high-field, high-multiples analyses, then



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the overall compositions for those analyses would be stoichiometric. Therefore, Te species appear more likely to have counts missed as part of a multiple hit event where they are incident on the detector close enough in space and time that some of these species are not being counted, as has been shown for boron in silicon.52 Even including the analyses with the large apparent deficiencies in Cd and Te, the relative field vs. overall temperature (base temperature plus laser pulse temperature) displays a linear fit as shown in Figure 5. This suggests that the field reduction is thermally driven similar to what has been observed for metals.19,25 It is worth noting that this relationship between field reduction and temperature seen here (0.21/100 K) is greater than what was seen in metallic tips, which would be related to differences in the evaporation field for CdTe versus those for W, Mo, and Rh.

Figure 5. Relative field versus overall specimen apex temperature (base temperature plus laser pulse temperature) for CdTe. The diamonds indicate analyses where the base temperature was greater than 70 K. The error bars indicate the maximum and minimum relative fields during each analysis. The average variation in relative field during each analysis was less than 0.03. Charge state ratios (CSRs) of species have been used as a proxy for measuring temperatures in specimens19,53 and also provide a means of maintaining the same surface field from specimen to specimen and instrument to instrument. In this case, the CSR for Cd+/Cd++ (using all of the 16 ACS Paragon Plus Environment

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isotopes), as shown in Figure 6, also provides a means of reproducing the known composition. This mirrors the data shown in Figure 3c, including the outliers which were run at higher base temperatures, which suggests that the Cd CSR is also a good proxy for the laser induced temperature, except for at higher base temperatures where Cd is likely subliming. The CSR of Te is not as reliable in these respects, which stems from the significant amount of Te2++, which causes an interfering overlap with all of the Te+ peaks.

Figure 6. Measured Cd composition versus the Cd+ / Cd++ ratio using all Cd isotopes. The diamonds indicate analyses where the base temperature was greater than 70 K. The dashed line in the figure indicates the expected Cd composition. GaN A GaN nanowire fabricated as described elsewhere54 (batch D062) was mounted for APT analysis using a previously described method.55 Of note, this mounting method did not involve the use of a focused ion beam (FIB) during any of the steps, and therefore the wire’s composition was not affected by FIB-implanted Ga. Also, this analysis was performed on only the 10 µm undoped GaN segment of the wire. APT analyses were performed at base temperatures from 23 – 115 K and laser energies from 0.002 – 0.5 pJ. The pulse frequency was 500 kHz, the detection



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rate was set at 1 event per 200 pulses (0.5%), and each run was stopped after 2 million ions were detected. Following the method described for CdTe in the previous section, the relative field vs. laser energy (at 23.5 K) and relative field vs. base temperature (at 0.006 pJ) curves were created as shown in Figures 7a and 7b. Additionally, another curve was produced at 0.05 pJ as shown in Figure 7b. This allowed for comparing the temperature dependent effects between two different laser energies. For the GaN analyses, the relative field was set based on the initial data collected at 0.002pJ and 23.5 K. When using the applied bias as a proxy for the field, it was observed that the normalized bias decreased slightly as base temperature was increased. However, it was also observed that the detected background increased substantially with increasing base temperature. Consequently, the CSR of Ga+/Ga++ was also examined as a means of estimating the field. From this, it was found that there was no significant change in CSR and, therefore, relative field, as base temperature was increased as shown in Figure 7b. This disparity between the applied bias method and CSR method suggests that the decrease in bias with increased base temperature was primarily, if not exclusively, due to an increased background signal, whereby the increase in background species caused the bias to decrease to maintain the set detection rate. Therefore, the CSRs were deemed a more reliable method of estimating the changes in field, so those were used for the analyses of the GaN specimen.


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Figure 7. GaN (a) Relative field versus laser energy at a constant base temperature of 23.5 K. Note that the laser energy is plotted on a log scale. (b) Relative field versus base temperature at constant laser energies of 0.006 pJ (diamonds) and 0.05 pJ (squares). The error bars indicate the maximum and minimum relative fields during each analysis. The average variation in relative field during each analysis was 0.02. Because the field does not display a significant temperature dependence in the base temperature range that was analyzed, it was not possible to produce a mathematical temperature versus laser energy relationship. On the other hand, the laser energy displayed a correlation to the field as shown in Figure 7a. This shows that while the laser is decreasing the required field, it does not appear to be doing so primarily via a thermally induced mechanism. Furthermore, the measured composition of the GaN nanowire showed a variation with analysis conditions, as has been shown previously.27,30 Graphs of the measured Ga composition as a function of laser energy and base temperature are displayed in Figures 8a and 8b. Strikingly, there is a strong variation with input laser energy, but little variation with base temperature. As previously mentioned, this was done for a series of base temperatures at two different laser energies – one in the apparent Ga-poor regime and one in the apparent Ga-rich regime. These indicate that while the laser was generating a response in the specimen as shown in Figure 7a and the laser energy was correlated with the measured composition, the variation in the measured composition of GaN cannot be strictly a thermal effect as had been previously postulated.30 Otherwise, adjusting the base temperature of the specimen would have produced shifts in the apparent composition.



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Figure 8. (a) Measured Ga composition versus laser energy. The diamonds are the results for the whole data set; the triangles are the results for just a 4nm diameter cylinder along the [0001] axis (b) Measured Ga composition versus base temperature at constant laser energies of 0.006 pJ and 0.05 pJ. The dashed lines in the figures indicate the expected Ga composition. But what is causing the evaporation of species if it is not substantially thermally driven? Photogenerated holes have been proposed as a possible mechanism of field evaporation in semiconductors and insulators26,329,34,56,57 resulting in either thermal or athermal contributions. It has been shown in MgO that trapping of holes at low-coordination oxygen sites can lead to nonthermal desorption of anionic species by laser excitation.58 Interestingly, these desorb as neutral species. A similar athermal process may be occurring in GaN. An additional possibility for the generation of neutral species is surface rearrangement leading to the desorption of N2 as has been shown in theoretical work for O2 from an MgO surface.59 During APT analysis, such desorbed neutral species may or may not be ionized depending on their local field. Dissociation of molecular ions can also result in undetected species, however, Saxey plots32 did not indicate a significant presence of these.A further clue in the nature of this interaction is the local compositional variations across the specimen. These did not show a correspondence with the incident laser direction, but did display the underlying crystallography. All of the analyses


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clearly reveal the [0001] axis of the GaN nanowire in the detector histogram. By placing a 4nm diameter cylindrical region of interest along the [0001] axis, the apparent compositions for just this close-packed direction were determined as shown in Figure 8a. For the whole range of laser energies this region produced compositions with a greater amount of N than those measured over the entire sample and the values remain relatively close to the expected stoichiometry over a much greater range of the laser energies. It can be seen that the deviation between the measured composition for the whole specimen and just the [0001] axis can become quite large. Thus, there appears to also be a crystallographic aspect to the measured variation. In the 2D charge state ratio images here (Supporting Information) and elsewhere,30,31 it is apparent that during analysis of c-axis GaN nanowires there is a non-uniform field across the tip, with the field being highest along the [0001]. Therefore, if neutral species are being desorbed from the specimen, those coming from this region are more likely to be ionized than species coming from elsewhere on the specimen. Since nitrogen has a higher ionization potential60 than Ga does,61 a higher field would result in an apparently greater concentration of nitrogen, consistent with what is observed in Figure 8a for the [0001] axis. The reason for the differences in field across the specimen likely stems from the local atomic coordination of the species. Athermal desorption of neutral oxygen in MgO was significant only for low-coordination sites; cleaved surfaces showed only thermal desorption.58 Away from the GaN [0001] axis, there are lower coordination directions along with significant curvature of the APT specimen, resulting in many corner and kink sites. Therefore, species in these locations are more easily desorbed. The result is a steady-state condition during analysis with a higher field around the [0001] axis and lower fields further away. With this differential field around the



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specimen apex, it is not possible to measure a uniform composition across the specimen; a conclusion also reached by Mancini et al.31 Additional support for an athermal process can be found in estimating the temperature increase due to the laser pulse using an adiabatic approximation, ∆T = Iτβ/Cp, where ∆T is the temperature change, I is the laser intensity, τ is the laser pulse length, β is the absorption coefficient, and Cp is the heat capacity.62 Taking τ = 10 ps, β = 1.5 x 105 cm-1,63 Cp = 1 J/cm3·K,64 and I = 1 x 106 W/cm2 for 0.02pJ (which gave the expected GaN stoichiometry as shown in Figure 8a) gives a ∆T ≈ 2 K. Even with the absorption coefficient enhanced to be fully metal-like due to the high fields,6 this laser induced temperature change would be only 20K, which was shown by the measurements where the base temperature was changed to be insufficient to affect a significant change in the evaporation behavior. At relatively high laser energies (>1 pJ), previous research has shown evaporation displaying an apparent lack of nitrogen on the side of the specimen where the laser is incident.30 This seems likely to be a thermal response. Indeed, the adiabatic approximation indicates a change in temperature of hundreds of degrees for that range of laser energies. Also consistent with this, for MgO higher laser fluences were found to result in more thermal desorption and less athermal desorption.58 Similarly, field evaporation of Si below 110K has been observed to be athermal, while exhibiting a temperature dependence above 110 K.65 The CSR of Ga+/Ga++ can serve as a metric of not only the field, but also for the measured composition as shown in Figure 9. By comparing the CSR versus composition measured for the whole field-of-view with that from just the [0001] axis, it is seen that there is an inconsistency between the two. This indicates that there is a crystallographic dependence of the relationship between the field and measured composition. Therefore, there will be some variation for a


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particular Ga CSR in the measured composition for GaN depending on the analysis direction and the field-of-view. In this case, the ROI around the [0001] axis comprised just 0.3% of the analyzed volume, so its effect on the overall data was negligible.

Figure 9. Measured Ga composition versus the Ga+ / Ga++ ratio for the whole specimen and for just the [0001] axis. The dashed line in the figure indicates the expected Ga composition. For low laser energies, where the measured composition was Ga-deficient, there was both an increased background and a substantial number of multiple hits. The increased background may arise from the relatively high field leading to Ga ion evaporation uncorrelated with the laser pulse. Analysis of the multiple hit events indicate that Ga species were more likely to have counts missed as part of a multiple hit event. Neither one of these individually could account for all of the missing Ga, but in sum they were sufficient to make up that difference. Conclusions Consecutive APT analyses of the same CdTe specimen using different laser energies and base temperatures elucidated the relationship between laser energy and specimen temperature. It was found that there was a linear relationship between these variables indicating that the field evaporation occurs by a thermally assisted process. Further, the plane governing the relationship between field, laser energy, and base temperature was overlaid with the resulting measured



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compositions for a variety of analysis conditions.

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These indicated that stoichiometric

measurements were achieved at only the coldest base temperatures (< 50 K) with a laser plus base temperature contribution of 135 – 175 K and a field contribution of 65% of its defined maximum. At higher fields the composition appeared Cd-rich; at lower fields the composition appeared Te-rich. The apparent Cd-rich regions were due to preferential loss of Te in multiple hit events. The apparent Te-rich regions were due either to sublimation of Cd from regions where the field was insufficient for ionization or the diffusion of Te species to the specimen apex. Similar APT analyses of a GaN nanowire specimen indicated that increased laser energy reduced the applied bias necessary to maintain a set detection rate, but base temperatures from 25 K to 115 K had no significant effect on the field. Likewise, the laser energy affected the measured composition, whereas the base temperature did not. This was shown to be the case for conditions which gave a N-rich measured composition as well as for those that resulted in a Garich measurement.

These suggest that the primary evaporation mechanism for GaN is an

athermal one, with a thermal process dominating at only the highest laser energies, where composition measurements are extremely Ga-rich. Further, the composition variations across the specimen for a given analysis condition were governed by the development of a non-uniform field around the apex of the specimen, where a higher field is present at the [0001] axis. The result is an apparently greater concentration of N along this direction.

Ga-deficient

measurements appear due to conditions which result in both a high number of uncorrelated Ga ion evaporation events and uncounted Ga ions due to multiple hit events. While the thermal, photonic, and field responses of nanoscale compound semiconductors, as determined here, have clear implications for atom probe analysis, they also reveal fundamental


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information that can be used for better understanding the bonding and optoelectronic properties on this scale. Supporting Information. mass spectra from CdTe analyses, reconstruction of a specimen that was measured as Cd-deficient, TEM images of a GaN nanowire before and after APT analysis, mass spectra from GaN analyses, a 2D histogram of the CSRs across a GaN specimen, calibration curves of the bias vs. measured flux, and discussion of the tip shape evolution and estimates of the associated errors for CdTe. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Address correspondence to [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS The atom probe instrument used in this work was supported under NSF-DMR award 1040456. The authors thank N. Sanford for helpful discussions regarding GaN. REFERENCES 1.

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Table of Contents Graphic


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Figure 1. Overlay of the TEM images of a CdTe atom probe specimen before and after APT analysis showing the device structure and indicating the volume of material removed. 82x37mm (300 x 300 DPI)


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Figure 2. CdTe (a) Relative field versus laser energy at a constant base temperature of 23.9 K. (b) Relative field versus base temperature at a constant laser energy of 6.1 pJ. The error bars indicate the maximum and minimum relative fields during each analysis. The average variation in relative field during each analysis was 0.03. (c) Using the linear relationships in (a) and (b), the average additional temperature of the specimen tip induced by the laser pulse as a function of the laser energy. 82x164mm (300 x 300 DPI)



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Figure 3. (a) Measured Cd composition versus laser energy at a constant base temperature of 23.9 K. (b) Measured Cd composition versus base temperature at a constant laser energy of 6.1 pJ. (c) Using the relationship from Figure 2(c), the measured Cd composition as a function of the overall temperature of the specimen tip during the laser pulse. The diamonds in (c) indicate analyses where the base temperature was greater than 70 K. The dashed lines in the figures indicate the expected Cd composition. 82x163mm (300 x 300 DPI)


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Figure 4. The base temperature, laser, and field plane defined by equation 4 overlaid with the measured CdTe compositions under several experimental conditions within the plane. The colored dots and contour lines indicate the deviations from the expected composition: green – stoichiometric, yellow 1 – 3 at%, orange 3 - 10 at%, red > 10 at%. The dashed line indicates the approximate transition from Te-rich to Cdrich. 177x145mm (300 x 300 DPI)



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Figure 5. Relative field versus overall specimen apex temperature (base temperature plus laser pulse temperature) for CdTe. The diamonds indicate analyses where the base temperature was greater than 70 K. The error bars indicate the maximum and minimum relative fields during each analysis. The average variation in relative field during each analysis was less than 0.03. 82x56mm (300 x 300 DPI)


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Figure 6. Measured Cd composition versus the Cd+ / Cd++ ratio using all Cd isotopes. The diamonds indicate analyses where the base temperature was greater than 70 K. The dashed line in the figure indicates the expected Cd composition. 82x54mm (300 x 300 DPI)



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Figure 7. GaN (a) Relative field versus laser energy at a constant base temperature of 23.5 K. Note that the laser energy is plotted on a log scale. (b) Relative field versus base temperature at constant laser energies of 0.006 pJ (diamonds) and 0.05 pJ (squares). The error bars indicate the maximum and minimum relative fields during each analysis. The average variation in relative field during each analysis was 0.02. 82x99mm (300 x 300 DPI)


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Figure 8. (a) Measured Ga composition versus laser energy. The diamonds are the results for the whole data set; the triangles are the results for just a 4nm diameter cylinder along the [0001] axis (b) Measured Ga composition versus base temperature at constant laser energies of 0.006 pJ and 0.05 pJ. The dashed lines in the figures indicate the expected Ga composition. 82x101mm (300 x 300 DPI)



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Figure 9. Measured Ga composition versus the Ga+ / Ga++ ratio for the whole specimen and for just the [0001] axis. The dashed line in the figure indicates the expected Ga composition. 82x51mm (300 x 300 DPI)


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Nanoscale Measurement of Laser-Induced ... - ACS Publications

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