Observation of strain-relaxation-induced size effects inp-type Si/SiGe resonant-tunneling diodes

Abstract

We have studied the effect of strain relaxation in small Si/SiGe resonant-tunneling diodes (RTD's) on the tunneling of holes through these structures. We have used RTD's mesa-etched into dots and wires, the lateral dimensions ranging from 10 \ensuremath{\mu}m down to 230 nm. In the dots we find a very strong shift of the light-hole (LH) resonance in the tunneling spectrum as the dot diameter decreases below 1 \ensuremath{\mu}m, while the position of the heavy-hole (HH) resonance is constant. In the wires, on the contrary, this size effect in the tunneling is completely absent: both peak positions are constant. This behavior, including the surprising insensitivity of the tunneling spectrum to the wire width, arises from a substantial degree of strain relaxation in the SiGe layers of the devices. This interpretation is supported by the strain dependencies we derive for the HH and LH barrier heights, and the HH-LH splitting in the quantum well. The combined effect of these quantities on the peak voltages agrees qualitatively with the experimental data, when we assume that in the dots the relaxation is biaxial, while in the wires it is predominantly uniaxial. The interpretation is also consistent with magnetotunneling-spectroscopy data, which reflect the in-plane anisotropy of the LH quantum-well subband. We find for all dot diameters a fourfold rotational symmetry of the shift of the LH resonance and for the wires a remarkable transition from a fourfold to a pronounced twofold rotational symmetry of this shift as the wire width decreases below 900 nm. This transition is interpreted as evidence for the strong influence of uniaxial relaxation on the in-plane dispersion.