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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

Germanium quantum wells on silicon

Germanium quantum well (QW) structures are based on commercially available silicon wafers, but the QW cannot be grown directly on silicon. A feature of the silicon-germanium material system is strain. While both silicon and germanium form the same diamond-like crystal structure, the lattice parameter of germanium is 4.2% larger than that of silicon. We want the germanium to grow in a uniform 2-dimensional layer – “Frank–van der Merwe” growth – but actually after the first few monolayers the germanium would form of 3-dimensional structures – “Stranski–Krastanov growth” – because of the strain. So to grow a strained Ge QW on silicon we need a SiGe “buffer” which relaxes the strain as it grows. LEPECVD in Como can grow buffers graded from pure Si up to the final SiGe content of 70%, whereas the other groups use CVD to first deposit a thick Ge layer and then grade back down to 80%. 

Prerequisite knowledge

  • Semiconductors 
  • Epitaxy 
    • Hooke’s law, stress and strain

Main takeaways

  • Ge QWs cannot be grown directly on Si. 
  • By LEPECVD we can grow forward-graded buffers but these are very thick. 
  • These would take too long to grow by CVD, so other groups deposit a thick Ge layer and then use reverse-grading to reach an alloy suitable for a strained Ge QW. 

Futher thinking

True or False: The roughness of the virtual substrate is critical because if the peak-valley roughness range reaches values similar to the QW thickness, the QW may be physically isolated into detached regions.

Further reading

The LEPECVD growth system is reviewed in G. Isella et al., Low-energy plasma-enhanced chemical vapor deposition for strained Si and Ge heterostructures and devices. Solid State Electron. 48 (8) 1317--1323 (2004) http://dx.doi.org/10.1016/j.sse.2004.01.013

Reverse grading is presented in V. A. Shah et al., Reverse graded relaxed buffers for high Ge content SiGe virtual substrates. Appl. Phys. Lett. 93 (19) 192103 (2008) https://doi.org/10.1063/1.3023068