Scanned gate microscopy of surface-acoustic-wave-induced current through a depleted one-dimensional GaAs channel

We present scanned gate microscopy images of the surface-acoustic-wave (SAW)-induced current through a depleted GaAs one-dimensional channel. The images show a crescent-shaped feature, which splits into two fragments when the tip bias is taken more negative. This is consistent with depopulation of the SAW minima when the electron energy is out of equilibrium with the source two-dimensional electron system, where the maximum longitudinal potential gradient, along the length of the channel, sets a limit on the SAW-induced current. Part of a circular feature located near the channel exit is seen in the same images, which is probably caused by a low-amplitude reflected SAW with transport induced by a turnstile mechanism. Image analysis provides several parameters that are related to the electrostatic landscape at the channel entrance and are critical for controlling electron capture by the SAW potential. The parameters are the maximum potential gradient, the length of the channel entrance, and the precise location of the maximum potential gradient.

Figure 1

(Color online) Illustration of experimental setup. SAWs are generated by applying a rf signal to the interdigitated transducer at the left end of the device. A negative bias applied to the central surface gates defines the SAW channel from the 2DES. Conductance and SAW-induced current measurements are made through the source and drain ohmic contacts.

Figure 2

(a) EFM image of the channel. The contour that has a channel width equal to the lithographic channel width is highlighted. The same contour is reproduced on subsequent images to locate the surface electrodes. (b) SGM image of conductance made with $V_{\text{gate}}=-0.6\text{V}$ and $V_{\text{tip}}=-2\text{V}$. The image identifies the location and size of microconstrictions. Both images were made at 400 mK in the absence of SAWs. Scale bars are $1\mu \text{m}$.

Figure 3

Experimental SGM images of SAW-induced current. Before each image was generated, $V_{\text{gate}}$ was adjusted so the background current was zero to remove the influence of the broad potential from the biased cantilever. Images (d)–(f) are enhanced versions of images (a)–(c), made by displaying only negative currents (charge flowing from right to left). All images were made at 400 mK in the presence of SAWs. Scale bars are $1\mu \text{m}$.

Figure 4

[(a)–(c)] Simulated SGM images made by calculating the maximum potential gradient along the length of the channel in the presence of a negatively biased tip, plotted as a function of tip position. A cross locates the point of maximum potential gradient, in the absence of the tip perturbation. Scale bars are $1\mu \text{m}$. [(d)–(f)] Simulated potential at the entrance of the channel with the tip located (d) outside the channel entrance and in the channel entrance with (e) small and (f) large negative tip bias. Arrows indicate the maximum potential gradient in each case.