Superconducting Ga-overdoped Ge layers capped with SiO${}_2$: Structural and transport investigations

Superconducting Ga-rich layers in Ge are fabricated by Ga implantation through a thin SiO${}_2$ cover layer. After annealing in a certain temperature window, Ga accumulation at the SiO${}_2$/Ge interface is observed. However, no Ga-containing crystalline phases are identified. Thus it is suggested that the volatile Ga is stabilized in an amorphous mixture of all elements available at the interface. Electrical transport measurements reveal $p$-type metallic conductivity and superconducting transition. The superconducting properties of the samples with high Ga concentration at the interface change dramatically with etching the amorphous surface layer. A critical temperature of 6 K is measured before, whereas after etching it drops below 1 K. Therefore, one can conclude that the superconducting transport is based on two different layers: a Ga-rich amorphous phase at the interface and a heavily Ga-doped Ge layer. Finally, the comparison of the transport properties of Ga-rich Ge with those of Si demonstrates distinct differences between the interface layers and even the deeper-lying doped regions.

Figure 1

tridyn simulation of the elemental depth profiles after implantation of 100 keV, 4 $\times$ 10${}^{16}$ cm${}^{-2}$ Ga in Ge covered with 30 nm SiO${}_2$. About 14 nm of this cover layer is removed due to sputtering. The SiO${}_2$/Ge interface after implantation is indicated by a vertical line. Long-ranging tails of recoil atoms are formed.

Figure 2

Surface structure investigated with SEM after implantation of (100) Ge covered with 13 nm SiO${}_2$ with a Ga fluence of 2 $\times$ 10${}^{16}$ cm${}^{-2}$. Randomly distributed, dendritelike structures having a diameter of about 500 nm occur on the surface. The upper inset shows the topography obtained by a detailed AFM measurement.

Figure 3

Results of the RBS/C analyses of samples implanted with a Ga fluence of (a) 2 $\times$ 10${}^{16}$ cm${}^{-2}$ (1.2 MeV He${}^{+}$) and (b) 4 $\times$ 10${}^{16}$ cm${}^{-2}$ (1.7 MeV He${}^{+}$) with depth scale for the implanted Ge layer. After implantation the layers are amorphous. Rapid thermal annealing leads to a single-crystalline layer structure. The inset shows the signal of some residual Si and O that is detected even after surface etching.

Figure 4

High-resolution TEM images of samples implanted with 4 $\times$ 10${}^{16}$ cm${}^{-2}$ and RTA processed at (a) 830 ${}^{\circ }$C as well as (e) 890 ${}^{\circ }$C before etching the SiO${}_2$ cover layer. (b) Homogeneous structure at the grain boundaries. (c) Crystalline Ge nanoclusters located in the SiO${}_2$. (d) Amorphous precipitates located at grain boundaries contain mainly SiO${}_2$. (f) The amorphous SiO${}_2$ clusters are also present after RTA at 890 ${}^{\circ }$C.

Figure 5

ToF-SIMS profiles of different elements for the samples implanted with 4 $\times$ 10${}^{16}$ cm${}^{-2}$ Ga. (a) Ga profiles with SiO${}_2$ obtained by ToF-SIMS. To evaluate the Ga concentration at the SiO${}_2$/Ge interface, additional AES measurements are shown in the inset. (b) Ge${}_2$ and SiO${}_3$ signal of the samples investigated by ToF-SIMS.

Figure 6

(a) Sheet resistance and (b) Hall fluence at a temperature of 10 K versus annealing temperature before and after surface etching for both implanted Ga fluences.

Figure 7

Temperature-dependent sheet resistance measurements before and after surface etching for implantation of (a) 4 $\times$ 10${}^{16}$ cm${}^{-2}$ and (b) 2 $\times$ 10${}^{16}$ cm${}^{-2}$.

Figure 8

Temperature dependence of the sheet resistance for the 830 ${}^{\circ }$C RTA-processed sample at different applied magnetic fields before surface etching.

Figure 9

Low-temperature resistance measurements surface etching. Implantation of 4 $\times$ 10${}^{16}$ cm${}^{-2}$ and RTA between 830 ${}^{\circ }$C and 870 ${}^{\circ }$C leads to an intrinsic superconducting transition below 0.5 K. The low-dose samples show a sharp transition because of the high Ga concentration in the layer.