Figure 1

EFM topography images taken with laser (a) off and (b) on. Both images were taken in constant frequency shift mode $\left(\Delta f=-31.66\text{Hz}\right)$ with $V_s=2\text{V}$ at 4.5 K. Scale $\text{bars}=200\text{nm}$. The EFM image with the laser on (b) shows noise (streaks) along the fast scan (horizontal) direction. This noise is characterized by measuring $\Delta f$ over the wetting layer [center circle in (a)] and the quantum dot [lower circle in (a)] at a constant height for 10 s. The PSD spectra $\left(S_{\Delta f}/\Delta f^2\right)$ of the $\Delta f$ shows increased low-frequency $\left(<100\text{Hz}\right)$ noise with laser irradiation only (c) over the wetting layer and not (d) over the quantum dot. This noise follows a $1/f^2$ dependence which indicates generation-recombination noise.Reuse & Permissions

Figure 2

(Color online) PSD spectra measured at locations indicated by squares with the same color (and letter) shown in the EFM topography image of an InAs quantum dot (inset). Leaving the quantum dot results in the appearance of generation-recombination noise as marked by the approximate $-2$ slope of the PSD. The data was taken with $V_s=2\text{V}$ at 77 K.Reuse & Permissions

Figure 3

The $\Delta f$ measured over the wetting layer at 4.5 K with $V_s=-1\text{V}$ and under laser irradiation. Over this time scale the $\Delta f$ resembles a multileveled random telegraph signal.Reuse & Permissions

Figure 4

(a) $\Delta f\text{−}{V}_s$ curves taken with (gray line) and without (black line) laser irradiation. The laser irradiation causes a negative shift in CPD (parabola apex) and an increased curvature of the $\Delta f\text{−}{V}_s$ curve. Both observations are a result of a change in surface charge due to the increase in minority carriers which are generated by the light and travel to the surface via electric field, reducing the band bending of the space-charge region near the surface. This effect, which occurs over the wetting layer, is illustrated by the schematic band diagram (b). The effect of the laser on the quantum dot is depicted in (c) where generated charge pairs are captured into the quantum dot and recombine so that an overall charge separation does not occur. Time traces of the $\Delta f$ for 10 s (d) for $V_s=-5\text{V}$ and (e) for $V_s=+3\text{V}$ are shown. The two time traces under light irradiation (gray line) in (d) and (e) were taken in the conditions indicated by the closed circles (a). More fluctuations are observed at $V_s=+3\text{V}$ than at $V_s=-5\text{V}$ under light irradiation because the applied electric field is complementing rather than opposing the built-in electric field caused by the initial band bending.Reuse & Permissions