Comparing XRD data for 225C and 300C growth of Si-Heusler.

1. Comparing XRD data for 225C and 300C growth of Si-Heusler. Some composition assumption for sample grown at 225C.

2. S230 grown at 300C Si Comp = 22.9 at.% S239 grown at 225C Si Comp = 27.8 at.%

3. Things I noticed: • (022) intensity higher for 225C growth at Heusler Comp • Large area of higher intensity and in-plane lattice matching (65-90)%Co • FWHM shows similar trend • No Significant difference in in-plane peak width for the two growth temperatures • (111) very wide at 225C growth • On the 300C sample, the strain is different for each reflection (the position traces do not collapse on top of each other) wereas they agree in the 225C sample • Comparing Zero Strain w/Si concentration suggests zero strain if Si was 25at% • Zero Strain doesn’t necessarily correspond with highest intensity of narrowest peak

4. Comparing Chemical Order The idea here is to measure the chemical ordering qualitatively. I’ve divided the integrated intensity of each superlattice peak by the fundamental to remove variations in structural disorder. F=(022)c=(014)h S1=(002)c=(102)h S2=(111)c=(011)h No significant changes in intensity within the “good” region (65-85)% Co No difference in S1/F for the two temperatures Significant difference in intensities for S2/F: the (111) peak is MUCH stronger relative to the fundamental peak for 300C growth

5. Comparing phi-scan FWHM FWHM vs Comp at Two Growth Temps Example of two components in s239 HeuslerStoichiometry Data Compare • Voigt could not fit well, so I found FWHM via max value and located max/2 in the data after background subraction • S239 had 2 components probably due to 2 distinct populations of grain sizes. I fit the wider as a polynomial background • Clearly, the sample grown at 225C has a narrower width at the Heusler composition

6. L-scans of (014) on s231 (Si~19at%)Grown at 150C and 1/3 as Thick as Others • A couple bad spots at ~90% Co • Zero strain @ ~84% Co • Position/Strain changes monotonically/linearly • Width strange

7. Some General Conclusions • Si-concentration is a bigger factor to determine strain than Co/Mn ratio • Higher Co concentration is better ordered than Heuslerstoichiometry regardless of growth temp according to intensities • Large region of composition gives good ordering according to peak widths • These samples are still too different to make a definitive statement on growth temperature • Different Si concentrations • Different layer ordering when grown • Different thicknesses (at least the 150C sample) • Improving the study: • Study Si-dependant samples • Grow all three samples in a immediately after each other rather than venting or composition recalibration to ensure const Si btwn samples

8. In-Plane Out-of-Plane S241: Ternary Sample Grown at 225C • L and K scans of the (014) reflection • Phi scans are difficult to fit – just calc FWHM from data • Data taken at corner of ternary sample nearest Co Apex • Along Diagonal boundary, Co~80% of metal concentration • Along base, Si~10% Phi-scan FWHM

9. Ternary General Conclusions • Brightest peaks correspond with narrowest width which occurs at the edge of the region corresponding to Co 80% (out of metal concentration). This is the spot of best crystal structure • Best crystal structure DOES NOT correspond to zero strain, OR Heusler Stoichiometry • Don’t know Si concentration yet

10. XRF Analysis of s238,39,41 Used to tweak first slide’s results and to correlate structural data

11. XRF analysis on binary samples: s238(150C), s239(225C) • Compared XRF thicknesses w/ AA monitor – Quartz crystal expected thickness during growth • Both samples came to significantly lower thicknesses • Since reduction ~ same for each element, composition was not effected significantly • This is NOT effected by Duke profilometer readings, since both are calibrated to the same calibration sample and same measured sample thickness • Fluctuation seems fairly large for Si and Co between the two samples. Mn held steady.

12. Binary Compositions • Despite fluctuations, the offset of the Mn in s238 seems to have compensated and produced similar Si-concentrations • Although Si is higher as expected, Co/Mn=2 pos changed and gives near zero strain (see slide 1) • NOTE: we have no structural data (RHEED or XRD) on s238.

13. XRF analysis on Ternary • Composition values obtained and used to correlate structural results between samples. • Quantitative thickness comparisons detailed in table at bottom right. • Total thickness comparison for s241 is better, giving 95% the expected value. • Co is nominally the same low value seen in other samples • Mn thickness matches expected here, giving large fluctuation from binary samples • Si thickness measurement was very low even though Liang had tried to increase the Si amount

14. Curiosity: Co Zero Line Misalignment • Co Zero line seen here and in previous Si-Heusler Ternary is not at 60 degrees. • The Equal percentage lines (prev. slide) are not parallel with this line and are actually closer to 60 degrees • While equal percentage lines could be off due to experimental/analytical errors, the boundary can not Co Zero Line

15. Possible causes of discrepancy of thicknesses from expected • Liang says this AA lamps were at the end of their lifetime (each of them) and could cause a systematic error in deposition • Could they cause reduction AND fluctuation? • Another possibility is the Quartz crystal may have been positioned improperly • would explain a systematic reduction in deposition, but good Mn agreement on s241 counts against this possibility • XRF fitting is still very much a black box • I tried fitting the data with different conditions but could find no conditions that would improve the values • Stefan says as long as fitting procedure same for Standard and Sample, fitting should be robust

16. Correlating Results (s230, s239, s241) • Interpolated s241missing data assuming planar geometry • Used XRF data to correlate Binary to Ternary sample • Right: Blue trace=s239 & Green trace=s230 • Green dot is Si-Heusler • Expected: • Pleateau of width and zero in-plane strain in large compositional region, including Heusler • Brightest spot & narrowest width (best ordering) not at Heusler Stoichiometry • Best ordering at high Co • Not Expected: • Heusler sees ~1/4% strain? • 25% Si does not intersect w/best ordering?

17. S239 (Binary) -> s241 (Ternary) • Both grown at 225C • Binary 590A; Ternary 370A • Layers: Binary = CoMnCoSi… Ternary = CoMnSi… • Ternary data sampled at Binary compositions to compare XRD results • Intensity traces remarkably similar (factor of ~½ comes from thickness difference) • L-Positions identical • In-Plane positions have strange discrepancies below 70% Co, otherwise agree • In general, width in ternary larger than in binary • Wider Out-of-Plane due to thickness difference but not much • Why is Ternary In-Plane wider? • Phi-direction continues the trend with larger width

18. S230 (Binary) -> s241 (Ternary) • Binary grown at 300C; Ternary 225C • Binary 540A; Ternary 370A • Layers: Binary = CoMnCoSi… Ternary = CoMnSi… • Intensity traces again similar This time the Binary is weaker • Intensity peak at ~68% in both data sets (shows up more if linear scale) • L-Positions again identical • In-Plane positions agree well with slight offset from 1 in the ternary (probably diffractometer calibration) • In general, width in ternary larger than in binary • Wider Out-of-Plane due to thickness difference but not much • Why is In-Plane wider? • Phi-direction continues the trend with larger width. Slightly more discrepancy

19. New General Conclusions • I would say this shows good reproduction of data and gives us an idea of our error • There could be a systematic error in composition causing the strain measurements to be off from the Heusler (1% is too far off to be from miscalibration of diffractometer) or there could be some Mn loss into the Ge substrate causing the Heusler comp to shift