Fig. 6

The average (Ave.) grain size dimension \(\langle d \rangle \) in the top (blue) and bottom (red) Al layers for sample groups A–D. Sample groups A and B were both oxidized for \(\hbox {t}_{\mathrm{ox}} = 10\) min at pressures of \(\hbox {p}_{\mathrm{ox}} = 0.01\) and 0.1 mbar, respectively. Sample groups C and D were oxidized for \(\hbox {t}_{\mathrm{ox}} = 30\) min under the oxidation pressures of \(\hbox {p}_{\mathrm{ox}} = 0.01\) mbar and \(\hbox {p}_{\mathrm{ox}} = 0.1\) mbar, respectively. The data indicates that changing oxidation pressure did not have a prominent effect on the grain size in any of the Al layers. However, the grain size in the top Al layer (tall blue columns) is significantly affected by the oxidation time for formation of the \(\hbox {AlO}_{{\mathrm{x}}}\) barrier layer, where longer oxidation time results in smaller grains. However, the average grain size in the bottom layers (short red columns) did not show a significant change with oxidation time