Structure and electronic properties of step edges in the aluminum oxide film on NiAl(110)

Thin film aluminum oxide has been investigated at step edges of the NiAl(110) substrate by means of low temperature scanning tunneling microscopy and atomic force microscopy. A preference of the step edges for certain orientations and a restructuring of the substrate step edges during the oxidation were shown. The surface unit cell of the oxide film at the step edge is similarly extended as at the antiphase domain boundaries (APDBs), which are oxygen deficient defects with ${\text{F}}^{2+}$-like centers. The local electronic structure and the local work function at the step edges resemble that of the APDBs. Therefore, an oxide film structure at the step edge which is similar to the APDB as well as a carpetlike coverage of the substrate steps is concluded. This means there are ${\text{F}}^{2+}$-like centers at step edges.

Figure 1

(Color online) Directions of film and substrate of aluminum oxide on NiAl(110). The rectangular NiAl(110) surface unit cell $\left(0.29\times 0.41{\text{nm}}^2\right)$ is shown as gray solid box. The surface unit cells of the two domains (A and B) of the aluminum oxide film are rotated by $\pm 24°$ with respect to $\left[1-10\right]$. They measure 1.06 nm by 1.79 nm enclosing an angle of $88.7°$. The angular orientations of the APDB paths are pictured blue. All angles are plotted with respect to [0 0 1] (Ref. 26).

Figure 2

STM images, $50\times 50{\text{nm}}^2$. a) Three different terraces of the NiAl(110) surface. The bias voltage was set to $+1\text{V}$ and the tunneling current was regulated to 100 pA. b)-f) Typical images of thin film aluminum oxide. The APDBs (bright lines) and step edges of the substrate are clearly visible. Some APDBs are labeled (domain A or B, type I or II). Typical features of the film at the step edges are shown: b) The step edges are parallel orientated to the APDB. c) The orientation of the domain and the APDB are continued on the next terraces. d) The domain changes its orientation (A-B) at the step edges. e) APDB of type AII. f) APDB of type BI and BII. The bias voltage in pictures b-f) was set to $+3\text{V}$ and the tunneling current was regulated to 100 pA. The $\left[1-10\right]$ and [0 0 1] directions of the substrate indicated in figure a) are common to all images.

Figure 3

(Color online) Histogram of the orientation of the substrate step edges before oxide film preparation (a), with oxide film (b) and of the APDB (c). The angles are plotted with respect to the [0 0 1] direction of the substrate (see Fig. 1). Intervals with a size of $3°$ are summarized in one point. On the right hand side, STM images ($50\times 50{\text{nm}}^2$; $U_{\text{bias}}=3\text{V}$, and $I=100\text{pA}$) with a typical step edge or APDB are pictured.

Figure 4

(Color online) Picture of a step edge. a) STM picture, $12\times 13{\text{nm}}^2$, $U_{\text{bias}}=3\text{V}$, and $I=100\text{pA}$. b) AFM image of the step edge at the position shown in a), which is leveled to the upper terrace (i), the lower terrace (iii) and to the step edge (ii). The lattices of the surface unit cells on both terraces are pictured. A small gap between both lattices of 0.3 nm exists. The size of the image is $8\times 8{\text{nm}}^2$. The frequency shift was $-2.2\text{Hz}$ at a bias voltage of $-150\text{mV}$. c) The $z$ displacement of the tip along the dotted line in b).

Figure 5

(Color online) a) STM picture of the thin film aluminum oxide on NiAl(110) recorded with three different bias voltages ($-0.5$, $+2$, and $+4\text{V}$). The size is $12\times 20{\text{nm}}^2$ and the tunneling current is 100 pA. The step edge between two terraces is marked by a red circle, the APDB by a blue circle. b) Profiles of the $z$ position recorded at the dotted line in figure a) at 5 different bias voltages $\left(-0.5–+4\text{V}\right)$. The position of the APDB is marked blue, the step edge red. At bias voltages where the APDBs are clearly visible as rising in the STM picture (i.e., at $+2\text{V}$) there is also an additional rising at the step edge. Where the APDBs are hardly visible (i.e., at $-0.5\text{V}$), the step edges appears as simple steps. For a better visibility, the profiles are shifted vertically.

Figure 6

(Color online) STS of domain (black line), APDB (blue) and step edge (red). The $z$ displacement differentiated with respect to the bias voltage as a function of the bias voltage shows similar spectra at the APDB and at the step edges while the spectrum at the domain is significantly different.

Figure 7

(Color online) Contact potential spectroscopy. The determined contact potential at the domain (black), APDB (blue) and step edge (red) as well as the box plots of the statistical data are shown. The boxes indicate the range of the standard deviations and the whiskers the range of 5 to 95 % of the distribution. The small squares indicate the mean values. All measurements were performed with the same tip configuration at a frequency shift of $-1.1\text{Hz}$. The minimum of the frequency shift with this tip configuration was $-1.8\text{Hz}$.

Figure 8

(Color online) Simplified sketch of thin film aluminum oxide on NiAl(110) with one step edge of the substrate. Oxygen is plotted red, aluminum blue and nickel green. The oxide surface layer is marked with a yellow background, the oxide interface layer orange and the NiAl substrate gray. The $z$ values are taken from Ref. 3. The step height is 0.20 nm (Ref. 25).