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Germanium Quantum Technology

01 /What can CMOS technology do for quantum computing? 02 /Semiconductor devices and scaling with industrial approach 03 /Semiconductor foundry facilities: the good and the bad for qubits 04 /IMEC's latest qubit developments 05 /Summary and outlook 06 /Semiconductors for spin qubits 07 /Germanium quantum wells on silicon 08 /Germanium quantum wells on silicon-germanium 09 /Growth methods 10 /Characterization techniques 11 /Electrical characterization of germanium quantum wells 12 /Fabrication of H-FET 13 /Semiconductor qubits: vision and challenges 14 /Quantum materials 15 /Quantum dot qubits and their operation 16 /Types of quantum dot qubits 17 /Qubit Readout 18 /Qubit Control 19 /Qubit Coherence and Fidelity 20 /Multi-qubit control and quantum algorithms 21 /Quantum links and scaling 22 /Physics of holes 23 /Hole spin qubits 24 /Introduction to electronics for quantum computing 25 /Room temperature electronics 26 /Cryogenic qubit chip carriers 27 /Contribution to IGNITE 28 /Quantum Control Stack for Spin Qubits 29 /Auto-tuning a quantum computer 30 /Tuning and operation of arrays 31 /Finding operation points example 32 /What is auto-tuning? 33 /Neural network tuning 34 /Navigating charge stability diagrams 35 /Experimental implementation 36 /Towards universal quantum algorithms 37 /Classical error correction 38 /Quantum error correction 39 /Progress and challenges 40 /Introduction to quantum algorithms 41 /The first algorithms 42 /Quantum annealing 43 /Quantum Machine Learning

Growth methods

The earliest Ge QWs were grown by molecular beam epitaxy (MBE), which uses solid sources. The growth rate is generally quite low but is independent of substrate temperature. The system needs to be opened to refill the sources, making thick layers impractical. 

Chemical vapor deposition (CVD) uses gases such as silane (SiH4) and germane (GeH4), and sometimes hydrogen as a carrier gas. Reduced-pressure CVD is now standard technology for SiGe growth in research and industry. Growth is thermally activated by the hot substrate so can be 1-2 nanometres per second at high temperatures during buffer growth, but much slower at the lower temperatures required for strained layers. The material is of very high quality. 

In LEPECVD, CVD growth is enhanced with a low-energy plasma, so the growth can be fast and independent from substrate temperature. Well-defined QWs with sharp interfaces are grown at reduced plasma density and gas flow.

Main takeaways

  • MBE is a useful and flexible too for research but is impractical for the growth of large quantities of SiGe. 
  • Reduced-pressure CVD has become standard for growing high quality SiGe both in research and industry. 
  • LEPECVD is in use at the Politecnico di Milano in Como for the growth of thick SiGe layers for research.  

Further thinking

True or False: The substrate must be kept away from the plasma in the LEPECVD growth chamber so that ions do not damage the crystalline structure of the deposited layers.

Solution

Further reading

For MBE of Ge QWs, Y. H. Xie et al., Very high mobility two-dimensional hole gas in Si/GexSi1-x/Ge structures grown by molecular beam epitaxy. Appl. Phys. Lett. 63 (16) 2263--2264 (1993) https://aip.scitation.org/doi/10.1063/1.110547

For reduced-pressure CVD, Y. Bogumilowicz et al. High-temperature growth of very high germanium content SiGe virtual substrates. J. Cryst. Growth 290 (2) 523--531 (2006) http://dx.doi.org/10.1016/j.jcrysgro.2006.02.019

For LEPECVD, Hans von Känel et al., Very high hole mobilities in modulation-doped Ge quantum wells grown by low-energy plasma enhanced chemical vapor deposition. Appl. Phys. Lett. 80 (16) 2922--2924 (2002) https://aip.scitation.org/doi/10.1063/1.1470691