(Color online) Index of refraction (left axis) and optical bandgap, Eg (right axis) vs the relative Ti thickness in the nine films presented in this study. Relative Ti thickness is defined as , where is estimated from the expected deposition rate of TiO2 alone. The values of amorphous Al2O3 and TiO2 are in close agreement with those reported in the literature. From this figure, it is evident that by adjusting the amount of Ti (Al) in the NLs, one can modulate the index of refraction and bandgap of the material.
(Color online) XPS of NLs [Al2O3 + 4TiO2] (I), [Al2O3 + TiO2] (II), and [4Al2O3 + TiO2] (III) after 10 s sputtering through the oxide layer. (a) Al 2p XPS results showing a nearly symmetric peak centered at ∼74.9 eV for all NLs. This peak can be fit with two lines at positions ∼75.1 and 74.6 eV, corresponding to fully oxidized Al2O3 and Al-O-Ti bonding, respectively; (b) Ti 2 p XPS results. This spectrum consists of three peaks at ∼457.8, ∼458.5, and ∼459.6 eV corresponding to Ti2O3, Ti-O-Al, and fully oxidized TiO2, respectively; (c) The O 1 s peak is centered at ∼531.5 eV and can be fit with four distinct components. Two predominant peaks at ∼531.0 and ∼531.5 eV are associated with TiO2 and Al2O3, respectively, and two less intense peaks at ∼532.4 and ∼533.6 eV correspond to hydroxyl groups and H2O.
(Color online) (a) Results of the CV measurements at 1 MHz for forward and reverse voltage sweeps from −5 V to + 10 V at 0.1 V resolution on 100 μm diameter capacitors. The arrows indicate the voltage sweep direction. The TiO2 film shows the largest value of accumulation capacitance ( ), while the pure Al2O3 has the smallest. The rest of the NLs show capacitance values in between those of Al2O3 and TiO2; (b) Measured dielectric constants shown in the bottom data set “Experimental” correspond to that of the NLs and SiO2 capacitors in series. Subtracting the contribution from the 2.0 nm SiO2 layer yields an increase in the dielectric as shown in the top data set of (b); (c) Current–voltage (I–V) characteristics under positive and negative biases for the Al2O3, TiO2, and NLs, on 100 μm diameter capacitors. The TiO2 film exhibits the largest amount of leakage current ∼10−5 A/cm2 at 1 V in forward bias, whereas the pure Al2O3 exhibits the lowest, ∼10−9 A/cm2 at 1 V. The NLs follow a trend of decreasing leakage current with increasing Al incorporation as indicated by the arrow.
(Color online) Z-contrast micrographs from the cross-section Si/TiO2/Al2O3 nanolaminated layers: (a) low-resolution micrograph showing the total NL thickness. Note the darker band at the interface with Si; (b) High-resolution micrograph showing the Si dumbbells with the amorphous layers on top of the substrate. The darker contrast suggests the presence of lower atomic number (Z) elements; (c) the higher magnification from the outline area in (b) showing the presence of the amorphous phase.
(Color online) (a) Cross-section micrograph using GIFF detector with indicated positions of the electron beam for the EELS studies; (b) EELS spectra showing the presence of O and Ti in position #1, (c) The same elements from position #2, and (d) from position #3. On the areas of the NL (like #3 and #4) the intensity of the Ti line is changing by about +/−10% counts from place to place; (e) higher energy spectra showing Si and Al presence in the black area #2; (f) The Al presence in area #3, #4. Similarly, as for Ti L3, the intensity of the Al Kα is alternating by about 10–15% from one area to another.
Parameters of the TiO2, Al2O3, and NLs included in this study. Thickness and index of refraction were determined by spectroscopic ellipsometry using a Cauchy model to fit the data. The optical bandgap was experimentally determined by a reflectance measurement. The dielectric constant was calculated from C-V plots using the capacitance of the oxides measured in the accumulation region. Errors in the measurements are within 5%.
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