Electrical properties and uniformity of two dimensional electron gases grown on cleaned SiGe virtual substrates

Abstract

The low temperature electrical properties of modulation-doped two dimensional electron gases (2DEGs) in the SiGe system were studied. The effects on the electrical properties of removing the substrate from the growth chamber after the growth of the virtual substrate, chemically cleaning the virtual substrate, and then growing the modulation-doped structure on a thin SiGe buffer were investigated. The results demonstrate that the carrier density and mobility decrease as the regrowth interface is moved closer to the 2DEG. The uniformity of the regrown wafers was also investigated. A monotonic increase in carrier density and a decrease in mobility were observed towards the edge of the wafers. Appropriate mechanisms will be discussed.

High mobility electron gases in Si/Si0.77Ge0.23quantum wells at 1.7 K

“…Those structures listed in table 1 have uniform supply layer doping. The effect of non-uniform doping which increases towards the Si cap (consistent with SIMS measurements [3][4][5][6]) has also been investigated, but was not found to have a significant influence on the sheet density.…”

“…Figure 1 shows the calculated X2-valley band edge for the Si/Si 0.77 Ge 0.23 structure described in section 1, using the geometry and doping parameters described in [3][4][5][6]. The inset figure of the quantum well eigenstates shows that at 1.7 K, the Fermi level does not extend beyond the ground state subband.…”

“…The quantum wells modelled are based on recently reported experimental structures [3][4][5][6]. These consist of a 9 nm Si quantum well put down on relaxed Si 0.77 Ge 0.23 , a 17 nm undoped Si 0.77 Ge 0.23 spacer layer, an 87 nm supply layer doped with As donors ∼5 × 10 24 m −3 , followed by a 4 nm Si cap.…”

“…A temperature gradient across the wafers during growth resulted in variation of the well and spacer layer thickness across the wafer which might explain the different sheet densities. Electron micrographs revealed contrast above the SiGe spacer at the onset of As doping and in the SiGe buffer at the regrowth interface due to the formation of Si rich layers which might trap electrons [5]. Variation of the unintentional doping in the spacer is also likely to modify the sheet charge.…”