Undoped Strained Ge Quantum Well with Ultrahigh Mobility of Two Million

We develop a method to fabricate an undoped Ge quantum well (QW) under a 32 nm relaxed Si_0.2Ge_0.8 shallow barrier. The bottom barrier contains Si_0.2Ge_0.8 (650 °C) and Si_0.1Ge_0.9 (800 °C) such that variation of Ge content forms a sharp interface that can suppress the threading dislocation density (TDD) penetrating into the undoped Ge quantum well. The SiGe barrier introduces enough in-plane parallel strain (ε_∥ strain -0.41%) in the Ge quantum well. The heterostructure field-effect transistors with a shallow buried channel obtain an ultrahigh two-dimensional hole gas (2DHG) mobility over 2 × 10⁶ cm²/(V s) and a very low percolation density of (5.689 ± 0.062) × 10¹⁰ cm^-2. The fractional indication is also observed at high density and high magnetic fields. This strained germanium as a noise mitigation material provides a platform for integration of quantum computation with a long coherence time and fast all-electrical manipulation.