Role of humidity in local anodic oxidation: A study of water condensation and electric field distribution

This paper deals with the analysis of the influence of humidity on the process of local anodic oxidation carried out by atomic force microscope (AFM) on GaAs (100) surfaces. Recent experiments have shown that the height and half width of oxide nanolines do not increase monotonously with relative humidity, but for lower relative humidities $\left(<50\%\right)$ the lines comparable in size to those prepared at 90% were obtained. However, their height and width along the lines revealed significant variations. To better understand these phenomena, the AFM force-distance spectroscopy measurements together with computer simulations of an electric-field distribution and water bridge formation between the tip and the substrate at different relative humidities were carried out. Our experiments on AFM force-distance spectroscopy have not proved an enhanced water condensation between the tip and the surface at lower humidities. However, the simulations of the electric field in the vicinity of the tip at the early stages of the oxidation process at low relative humidities showed an increase in the average intensity in the oxide layer promoting the diffusion of oxidizing species toward the substrate and, hence, the formation of oxide lines under these conditions. Finally, our simulations on water bridge variations along the tip track showed that at lower humidities there are higher relative standard deviations in the size of the water bridge while the tip is being moved along the surface. This indicates why the oxide lines showed a bigger variability in size.

Figure 1

Local anodic oxidation by AFM. A water bridge between the tip and the surface with the (a) meniscus detail and the values of the Kelvin radius (in nm) for (b) different relative humidity.

Figure 2

Force versus distance spectroscopy $\left(\text{RH}=91\%\right)$.

Figure 3

(Color online) Test arrays of oxide nanolines on GaAs(100) fabricated as a function of the tip-surface voltage and tip speed at the relative humidity (a) 40% and (b) 75%. The nanolines prepared at lower humidity show up a bigger variability in height and width.

Figure 4

Maximum capillary force between the tip and the surface during the sample retraction as a function of relative water humidity. In the inset, the force values averaged over the test sites and provided with the corresponding standard deviations.

Figure 5

The break-free length as a function of relative humidity corresponding to the capillary force experiments in Fig. 4

Figure 6

(Color online) Simulation of water meniscus in four different planes at one surface site.

Figure 7

(Color online) The width of the water bridge between the tip and the surface as a function of relative humidity; ratios of the standard deviation to the mean width (bottom right inset); development of an area given by the projection of a simulated water bridge into the surface during the tip movement from the left to the right for two different humidities (top left inset).

Figure 8

The equipotential lines in the vicinity of an AFM tip and the model sample consisting of a native GaAs oxide layer and GaAs substrate. Without the water bridge at the (a) relative humidity 0% and with the water bridge at the (b) relative humidity 70%. The thickness of the tip-sample gap, GaAs oxide layer, and GaAs substrate chosen for the simulations was 0.3 nm, 2.5 nm, and 10 nm, respectively.

Figure 9

The radial profiles of the magnitude of electric intensity in the GaAs oxide layer 0.25 nm deep below its surface for different relative humidities; the radial $\overline{x}$ component of electric-field intensity [inset (a)]; the average value of the magnitude of electric intensity in the oxide layer under the water bridge [inset (b)].