Achieving high n-dopant concentrations in germanium and abrupt vertical dopant profiles is challenging with traditional implantation because of fast-dopant diffusion during annealing. Now researchers from the University of New South Wales in Australia have demonstrated that it is possible to stack multiple, narrow and closely spaced layers of phosphorus atoms in germanium crystal, achieving high dopant concentrations while maintaining atomic level control of the interfaces in the crystal.

Reporting their work in the journal Nanotechnology, the team has developed a doping process based on repetition of atomically controlled doping cycles (see image). Each doping cycle comprises three steps: i) the adsorption of phosphine molecules in ultra-high vacuum onto a near defect-free and atomically flat Ge(001) surface; ii) a thermal anneal to drive the incorporation of P atoms into the crystal matrix; and iii) a Ge overgrowth by molecular beam epitaxy to embed the incorporated P dopants in a high quality crystal without altering their atomically sharp vertical distribution.

The results of a combined scanning tunnelling microscopy, secondary ion mass spectroscopy and electrical characterization study showed the process to be reproducible and robust. The processing technique allows near identical and abrupt dopant profiles to be embedded in the crystal, with an atomically flat surface recovered at each doping step.

Large window for growth

In delta doping of semiconductors, a trade-off is usually required to achieve abrupt dopant profiles at the expenses of the crystal growth quality. But in germanium, due to the relatively low melting point (936 °C), there is a large window for high-quality growth at relatively low temperatures (400–550 °C). This ultimately allows the growth of high-quality layers (roughness 1 Angstrom) without compromising on dopant redistributions, with dopants moving vertically less than 4 atomic planes throughout the whole fabrication process.

Upon decreasing the spacing between doped layers, currently set to 9 nm, the team intends to increase the electrically active donor concentrations in germanium well beyond 1020cm–3. Furthermore, having recently demonstrated planar atomic-scale devices in germanium based on lateral STM patterning of single delta-doped layers (more info here ), the results presented in the journal Nanotechnology represent a key milestone towards extending this promising nanofabrication technique to atomically precise 3D architectures.

For a timeline of the work, click here.