CEA-Leti, a French technology research institute of the CEA, and Inac, reports a breakthrough toward large-scale fabrication of quantum bits, or qubits, the elementary bricks of future quantum processors. Researchers demonstrated on a 300 mm pre-industrial platform a new level of isotopic purification in a film deposited by chemical vapor deposition (CVD). This enables creating qubits in thin layers of silicon using a very high purity silicon isotope, 28Si, which produces a crystalline quality comparable to thin films usually made of natural silicon.
Qubits, the building blocks of quantum information, can be made in a broad variety of material systems, but when it comes to the crucial issue of large-scale integration, the range of possible choices narrows significantly. Silicon spin qubits have a small size and are compatible with CMOS technology, and offer advantages for large-scale integration compared to other types of qubits.
The introduction of isotopically purified 28Si has led to significant enhancement of the spin coherence time since the first qubits that relied on electron spins were reported in 2012. The longer spin coherence lasts, the better the fidelity of the quantum operations.
Quantum effects are essential to understanding how basic silicon micro-components work, but the most interesting quantum effects, such as superposition and entanglement, are not used in circuits. The CEA-Leti and Inac results showed that these effects can be implemented in CMOS transistors operated at low temperature.
CEA-Leti and Inac previously reported preliminary steps for demonstrating a qubit in a process utilizing a natural silicon-on-insulator (SOI) 300 mm CMOS platform. The qubit is an electrically controlled spin carried by a single hole in an SOI transistor. In a paper published in npj Quantum Information, CEA-Leti and Inac reported that an electron spin in an SOI transistor can also be manipulated by pure electrical signals, which enable fast and scalable spin qubits.
To help leverage nuclear spin free silicon in the CMOS platform, a silicon precursor was supplied by Air Liquide, using an isotopically purified silane of very high isotopic purity with a 29Si isotope content of less than 0.00250 percent, prepared by the Institute of Chemistry of High-Purity Substances at the Russian Academy of Sciences. The 29Si isotope is present at 4.67 percent in natural silicon and is the only stable isotope of silicon that carries a nuclear spin limiting the qubit coherence time.
A secondary ion mass spectrometry (SIMS) analysis conducted on the CVD-grown layer using this purified silane precursor showed 29Si concentration less than 0.006 percent, and 30Si less than 0.002 percent, while 28Si concentration was more than 99.992 percent. These unprecedented levels of isotopic purification for a CVD-grown epilayer on 300 mm substrates are associated with surfaces that are smooth at the atomic scale, as verified by atomic force microscopy (AFM), haze and X-ray reflectometry measurements.