APT and RDF analysis of Ge(CoMn) Danny Perea, Jim Riley, and Lincoln Lauhon December 2008

FIB specimen fabrication for LEAP 10 µm K. Thompson et al. / Ultramicroscopy 107 (2007) 131–139 FIB courtesy of Praneet Adusumilli Thin film super lattice sample 1.Using FIB & micromanipulator, a sample wedge is cut from substrate 2.Sample wedge is then ‘welded’ onto Si micropost tip 3.Sample wedge is then cut to leave a small portion of the sample attached to the micropost tip Cut

FIB specimen fabrication for LEAP 1 µm K. Thompson et al. / Ultramicroscopy 107 (2007) 131–139 FIB courtesy of Praneet Adusumilli Thin film superlattice Si micropost 4.Sample is then sharpened into a needle-like shape by annular milling using the FIB. The tip radius is approx nm

Thin film Ge(CoMn) superlattice structure Approx. composition of codoped layers according to Tsui: –Ge: ~90% –Co: ~7% –Mn: ~3% Ge:CoMn Ge 12 nm 2.3 nm

Ge 2+ Ge + Mn 2+ Co 2+ Mn + Co + LEAP Reconstruction 43 x 43 x 71 nm 3 H H2OH2O Mass spectrum representing entire reconstructed volume Ge Mn Co Only a fraction of Ge is shown

1D composition profile 10 nm ~2nm thin slice taken parallel to growth direction Ge not shown Mn Co An enhanced Co & Mn conc. is observed at both ‘interfaces’ (top & bott) of each codoped layer. The Mn profile is relatively symmetric, while the Co profile shows more Co at the initial interface of each layer. This may be an artifact of the MBE growth procedure used. Is the Co asymmetry related to the exposure sequence? Growth Direction Analysis Direction

Isolation of single codoped layer for analysis 43 x 43 x 12 nm 3 Inhomogeneous distribution of Mn and Co can be seen Ge not shown Mn Co ~ 600K atoms (Co+Mn+Ge) Measured Comp. Mn: 3.1% Co: 6.6%

Co-Mn Clustering ~2 nm slice taken normal to analysis direction ~2 nm slice viewed from side 2D composition map of thin slice above it Observation of clusters may be evidence of spinodal decomposition

Radial Distribution Function (RDF) analysis = number i atoms in radial shell around k th x atom centered at r = total number of atoms in shell = total number x atoms in volume = overall concentration of i atoms in total sampled volume Adapted from C. Sudbrack et al, Phys. Rev. B 73, (2006) Initially we wanted to use RDF analysis to see whether we could determine dopant paring configurations.

An example: Co-Co RDF RDF analysis was applied to a single codoped layer (black curve) Values above (below) 1.00 represent a positive (negative) correlation which means that the average Co concentration at that radial distance is greater (less) than the Co concentration of the entire sampled volume. Values at 1.00 represent a random distribution. To determine whether the magnitude of the RDF is statistically significant, we ran an RDF analysis on a simulated random alloy of the same composition as was measured from the single codoped layer (red curve). Positional uncertainly was also included, which accounts for the positional uncertainly in the atom probe data. A detector efficiency of 50% was also accounted for. RDF Curve represents the average of 10 simulated random alloys r  1.5 nm may represent the average radius of the ‘cluster’ size Unable to determine dopant pairing configurations due to limited spatial resolution (~0.3 nm) Co atoms centered around Co atoms

Since most of the Co (Mn) is found within the Co & Mn rich clusters, the Co-Co & Co-Mn show very similar RDF curves. Mn atoms centered around Co atoms Co-centered RDF analysis

Since most of the Mn (Co) is found within the Co & Mn rich clusters, the Mn-Mn & Mn-Co show very similar RDF curves. Mn-centered RDF analysis

The shape of the Ge- RDFs may be due to the fact that a large amount of Ge lies inside and outside the CoMn rich clusters. We do not yet have a good explanation for the differences between Ge-Co and Ge-Mn. Ge-centered RDF analysis

Proxygram analysis 43 x 43 x 12 nm 3 10% Co+Mn isoconcentration surface(s) Ge: 15%; Co: 100%; Mn:100% The proxigram is a composition profile that extends normal from an isoconcentration surface (interface), such that positive distances extend inside Co & Mn rich clusters, while negative distances extend outside. Inside cluster Average composition of Mn and Co outside the clusters

Cluster interconnectedness Using isoconcentration surfaces, we are able to qualitatively study the clusters. 10% Co+Mn isoconcentration surface Ge not shown

Cluster interconnectedness 10% Co+Mn isoconcentration surface Ge not shown Using isoconcentration surfaces, we are able to qualitatively study the clusters.

Cluster interconnectedness 10% Co+Mn isoconcentration surface Ge not shown Using isoconcentration surfaces, we are able to qualitatively study the clusters.

Outstanding Questions What is the exposure sequence used to grow the superlattice structure? Can the growth procedure be used to explain the asymmetry in the 1D Co composition profile? How well does the average composition of Co and Mn within a single codoped layer measured by APT (Mn: 3.1%; Co: 6.6%) compare with what has previously been measured using other techniques? What quantitative tools can we apply to characterize the cluster size, orientation (directionality), connectedness?