Purest Germanium Crystals towards Single-Electron Devices: Technology and Research

• Applicant: Privatdozentin Dr. R. Radhakrishnan Sumathi
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 509113935

Germanium (Ge) has been demonstrated as the purest material among all the bulk semiconductor materials. Ultra-high purity Ge (U-HPGe) single crystals with net charge carrier concentration well below the intrinsic level has greatly stimulated interest both in basic physics and application of Ge during recent years. Especially high-purity combined with tailored very low-level doping and co-doping is very promising to investigate the physical properties of such unique single crystalline materials up to a single-electron limit of a dopant and to reveal new physical phenomena at the quantum limit. Synthesis and processing of materials to such a high purity is very challenging. This project aims to developing new processes and means to accomplish the required ultra-high purity in Ge with a net carrier concentration below 10^10 cm^-3. The two main processes, namely zone refining and Czochralski crystal growth will be enhanced and optimized towards attaining the world’s purest Ge crystals. The research also includes crystals with selected enriched or depleted isotopic contents. Various possible doping approaches like, gas-phase doping during crystal growth, diffusion doping, as well as neutron transmutation doping will be studied to achieve the targeted doping with ultra-low residual concentrations (10^12 – 10^13 cm^-3) of electrically active impurities. Probing the attractive fundamental properties of crystals co-doped with acceptors of different valences slightly above their residual level is also of fundamental interest. The main scientific questions that can be addressed are the effect of grown-in dislocations, point defect complexes, impurities and other structural defects on the dynamics of the charge carriers. Conventional characterization methods will be combined with new analytical techniques to characterize the defects and quantify the impurities well below the ppt (parts per trillion) level. The scientific outcome of this intended project will be acquiring a basic knowledge on the fundamental properties of highly pure, very lightly doped elemental semiconductor material, while at the same time a nationally inherited technology will be established for such a novel material. This work will pave the way for future research and development of research-grade materials for electronics, optoelectronics and quantum computers/communications. The objectives and tasks of the project have strong interrelated contents and require a tight cooperation of the partners, Leibniz-Institut für Kristallzüchtung (IKZ) and Humboldt-Universität zu Berlin (HUB) with their complementary expertise in crystal growth, characterization and as well as researching the fundamental properties of this new, not yet available material.

• DFG Programme: Research Grants