Electrically isolated SiGe quantum dots

Abstract

A variation of electric force microscopy ͑EFM͒ is used to measure the electrical isolation of SiGe quantum dots ͑QDs͒. The SiGe QDs are grown on mesas of ultrathin silicon on insulator. Near the mesa edges, the thin silicon layer has been incorporated into the QDs, resulting in electrically isolated QDs. Away from the edges, the silicon layer is not incorporated and has a two-dimensional resistivity of less than 800 T⍀ per sq, resulting in relatively short RC times for charge flow on the mesa. The EFM technique we use here is a powerful probe of samples and devices with floating-gate geometries.

Three-dimensional stacking of self-assembled quantum dots in multilayer structures

“…In this case, as a result of the increasing Ge dot size in the superlattice stack, no complete planarization of the thin Si spacer layers is achieved and, as a result, the dot alignment switches to an oblique replication at a certain point of superlattice growth [55]. Other examples for non-vertical stackings possibly related to non-planarized spacer layer morphologies include self-assembled InP/GaInP quantum dot stacks [79] as well as InAs/InP [80] and InAs/InAlAs [45,81] quantum wire multilayers, in which oblique interlayer dot alignments with varying correlation angles as well as staggered wire stackings have been observed by TEM (see Fig. 2(d)).…”

Quantitative analysis of electric force microscopy: The role of sample geometry

“…As the SiGe domes form, they incorporate the surrounding ultra-thin silicon layer, electrically isolating themselves from their surroundings. 20 Figures 4͑a͒ and 4͑b͒ show tapping mode atomic force microscopy ͑AFM͒ and 2-EFM images of the edge of such a mesa. Because of the complex sample topography, our EFM images are acquired in "lift-mode" on a Digital Instruments Nanoscope IIIa with an Extender Electronics Module.…”

“…Sample morphology and growth specifics are described in detail elsewhere. 20 Briefly, a thin layer of silicon ͑10 nm͒ sitting on oxide is laterally patterned into 10 m ϫ 10 m mesas; Ge is then deposited at 700°C. The germanium diffuses to the edge of the mesa, where it forms SiGe domes.…”

“…Thus, ⌬v 2 is a measure of anything that influences the tipsample capacitance, such as the local dielectric constant and geometry. Although this type of VMEFM ͑2-EFM͒ has received relatively little attention, [17][18][19][20] it is ideal for investigating the interplay between geometry and local dielectric constant. Because 2-EFM does not require electrical contact to the sample surface-only a backgate is required-it is also particularly useful for samples with electrically isolated or floating elements, such as nanowires 21 or nanotubes 22 on oxide.…”

“…20 Off the mesa ͓situation ͑i͔͒ the silicon has been removed, exposing the silicon dioxide underneath. The predicted ratio of 2.5:1 for these two situations matches well with the experimentally measured ratio of 2.4:1, with no adjustable parameters.…”