Updates of Iowa State University S. Dumpala, S. Broderick and K. Rajan Sep – 18, 2013

Summary Refinements in Environmental chamber set up for in-situ gas reactions Oxidation studies of Al using new set up of E-cell Preliminary analysis of silicon oxidation results in comparison with ReaxFF simulations (Adri’s papers) Further analysis of silicon oxidation growth mechanisms and sub oxide species at different temperatures Study of Si tips from Maryland using APT

E-Cell APT Results Bulk Alumina PhaseBulk Al Phase Aluminum Oxidation  3D atomic scale interfacial analysis (structural and chemical)  Stoichiometry of different phases observed  Inter-diffusional characteristics of elements

Reduced Contamination with In-situ E-Cell Oxidation In-situ E-cell Oxidation Numerous additional peaks representing contamination that were detected in ex-situ oxidation were absent in in-situ oxidation results. Ex-situ Oxidation

Increased Mass Resolution with In-situ E-Cell Oxidation Lower mass resolution with longer tails of the peaks were seen in ex-situ In-situ E-cell Oxidation Ex-situ Oxidation

APT of Si Tips from Maryland : Anode Voltage: 250 V of Argon exposure 50 monolayers150 monolayers Bare Si Tip Laser APT – 1nj (laser power) Higher Ar content in beam exposed tips compared to bare Si tip

Mass Spectra 1.Small peak of Ar 2.Check other condition tips (Different beam currents) 3.Different deposition thickness of monolayers 4.FIM studies 50 monolayers 150 monolayers Bare Si Tip

Oxidation of Silicon ReasFF Simulations APT- Experimental Hyper thermal oxidation (atomic and molecular oxygen beam source) Dynamic study Smaller time scales (3pc) Monolayer detection Plasma oxidation (ambient oxygen) Static study (post deposition study) Longer time scales (minutes) Sub nano scale detection

Interfacial Diffusion – Interfacial Width 383 K548 K

383 K548 K Diffusion Profiles – Sub Oxides

Interface region – Between bulk Si, and bulk silica (region II from proxigrams) The total number of sub oxide species increase with increase temperature, observed similar trend in APT results Relative amounts/ratio of Si +1 (Si2O), Si +2 (SiO) at two different temperatures agrees with simulations Silica layer – 1. Surface, 2. Bulk (Si +4 components in interface) Average Concentration (at%) Analysis of Interfacial Sub Oxides – Comparison of Simulations with APT Atom Probe (Interfacial Region) ReaxFF Simulation

For 8 ML, the number of Si 4+ components is much higher at low temperatures than at high temperatures, indicating that the initial growth of the silica (SiO2) layer occurs much faster at low than at high temperature After 32 ML, number of Si 4+ components is almost same at all temperatures indicating that the silica layer now grows faster at higher temperatures, but its nucleation started later APT results also indicate the presence of almost same number of Si 4+ (SiO2) components at both the temperatures Average Concentration (at%) Analysis of Si+4 (SiO2) Oxides – Comparison of Simulations with APT Atom Probe (Silica Region) ReaxFF Simulation

Growth Mechanism - Low Temperature APT K Stage I - Growth of sub oxides and an incipient silica layer growth Stage II -1) Continued growth of sub oxides and also growth of silica observed, 2) Inward growth rate of sub oxides drops (high activation energy) (mainly Si2O) due to fast conversion of Si +1 Si +2 Si +4 Stage III, IV - Growth of sub oxides and silica slowed down ReaxFF K

Stage I - Growth of sub oxides and an incipient silica layer growth Stage II -1) Continued growth of sub oxides and also growth of silica observed, 2) Inward growth rate of sub oxides still continues does not drop as in lower temperature case Stage III, IV – 1) Continued growth of sub oxides (interstitial neutral oxygen atoms surmount the activation energy barrier at threshold T of 500 K), 2) Interface (consisting of sub oxides) is thicker than low T and 3) Inward growth of silica slows down APT K ReaxFF K Growth Mechanism - High Temperature

Effect of Temperature on Growth Mechanism APT K APT K ReaxFF

Future Study of thermal oxidation (silicon) case Analysis of bonding information in different regions (bulk, interface) from APT data that could offer complimentary information to the bond length and bong angle analysis by ReaxFF.