Electrical Properties of Quantum Dots

IRG #1

Electrical Properties of Quantum Dots Grown on Ultra-Thin SOI

Figure 1 EFM images of four mesas of ultra-thin SOI with Ge overgrowth. Charge has been deposited on the upper right mesa (first, over an hour before the image was acquired) and the lower left mesa (second, only minutes before this image was acquired). Note the very different charge distributions on the two time scales.

IRG1 has discovered two novel epitaxial growth modes of Ge on ultra-thin silicon-on insulator (SOI) (Nature, in press). One of these modes results in the growth of concentric rings of self-assembled quantum dots at the edge of mesas patterned into the ultra-thin SOI. TEM images of the rings of quantum dots indicate that the ultra-thin silicon layer has been consumed during the growth of the dots. This suggests that the electrical resistance between quantum dots near the edge of the mesas will be very high.

IRG1 has investigated the electrical properties of these mesas of ultra-thin silicon (10 nm) and self-assembled quantum dots (1.5 nm total Ge deposition). We deposit charge in the center of a mesa via contact electrification with a conducting AFM tip. Subsequent imaging with electric force microscopy (EFM) allows us to follow the charge migration across the mesa. We find three types of behavior on three different time scales.

Figure 2 EFM image of the same region as Fig.1, acquired 20 minutes later.

On short time scales (Fig. 1, lower left mesa), the deposited charge runs towards the edge of the mesa, stopping at the rings of quantum dots, which present a barrier to charge flow. This spreading of charge is easily understood as a classic electrostatic problem: the charge is attempting to minimize electrostatic energy by maximizing the distance between charges.

Surprisingly, on medium time scales (~10 minutes), the charge redistributes itself uniformly over the entire central region of the mesa (Fig. 1 upper right, and Fig. 2 both charged mesas). This uniform charge distribution is far from the lowest-energy electrostatic configuration, and it may be due to the occupation of electronic traps in the center of the mesa that have long time constants. The specific mechanism involved is the subject of ongoing investigation.

Figure 3 AFM (blue) and EFM (green) images of an edge of a mesa acquired 20 min. after charge deposition by contact electrification. Note that the charge has leaked only partway into the concentric rings of quantum dots at the edge of the mesa.

On long time scales (~1 hour), the charge slowly seeps to the actual physical edge of the mesa. Figure 3 shows EFM and AFM cross-sections, acquired at an intermediate time when the charge has leaked partway into the concentric rings of quantum dots that line the edge of the mesa.

These electrical properties are the consequence of novel epitaxial growth, and the electrically isolated quantum dots we demonstrate may prove practically significant as elements in patterned-gate floating-gate memory.