Effects of surface tunneling of two-dimensional hole gases in undoped Ge/GeSi heterostructures

Light effective hole mass in undoped Ge/SiGe quantum wells

“…The α values confirm that the mobility is limited by scattering from the dielectric/semiconductor interface, as previously observed in Si/SiGe and Ge/SiGe H-FETs. [10,24,29,30] Despite the close proximity to the dielectric interface, the shallower quantum well has a remarkable peak mobility of 1.64 × 10 5 cm 2 /Vs at p = 1.05 × 10 12 cm −2 , 2.4× larger than previous reports for quantum wells positioned at a similar distance from the surface. [24] At higher density the mobility starts to drop, possibly due to occupation of the second subband or to different scattering mechanisms becoming dominant.…”

“…[10,24,29,30] Despite the close proximity to the dielectric interface, the shallower quantum well has a remarkable peak mobility of 1.64 × 10 5 cm 2 /Vs at p = 1.05 × 10 12 cm −2 , 2.4× larger than previous reports for quantum wells positioned at a similar distance from the surface. [24] At higher density the mobility starts to drop, possibly due to occupation of the second subband or to different scattering mechanisms becoming dominant. The deeper quantum well (t = 44 nm) has a higher mobility of 2.6 × 10 5 cm 2 /Vs at a much lower density of 2.9 × 10 11 cm −2 , as expected due to the larger separation from the scattering impurities.…”

The germanium quantum information route

“…A high mobility of 5×10 5 cm 2 /Vs at T = 1.7 K was measured in Ge/SiGe heterostructures field effect transistors (H-FETs) 9 using an industry-compatible high-κ dielectric, setting new benchmarks for holes in shallow buried-channel transistors. Further improvements are expected by an engineered optimization of the stack and device fabrication parameters, such as strain in the quantum well strain, barrier thickness 45,154 , and dielectric deposition conditions.…”

“…The superior quality achieved in reverse-graded Ge/SiGe heterostructures enabled a plethora of quantum transport studies in modulation-doped etched Hall-bar devices 89,141,[155][156][157][158][159][160][161][162][163][164][165][166][167][168] and, more recently, in undoped H-FETs 45,90,140,154,[169][170][171] . Initial expectations from bandstructure considerations were confirmed and the knowledge-base of confined holes in planar Ge was advanced.…”

Low percolation density and charge noise with holes in germanium

“…The strained Ge QW is uniform, has a constant thickness of 16 nm, and is separated from the SiO x /Al 2 O 3 dielectric stack by a Si 0.2 Ge 0.8 barrier. We chose a Si 0.2 Ge 0.8 barrier thickness t = 55 nm to suppress surface tunneling from the strained Ge QW [30], whilst achieving a sharp confinement potential for quantum dots. We achieve smooth interfaces between the Ge QW and nearby Si 0.2 Ge 0.8 and between then Si 0.2 Ge 0.8 barrier and the dielectric, highlighting the high-quality of epitaxy and device processing.…”