OBSERVATION OF STRAIN-RELAXATION-INDUCED SIZE EFFECTS IN P-TYPE SI SIGE RESONANT-TUNNELING DIODES/

We have studied the effect of strain relaxation in small Si/SiGe reson ant-tunneling diodes (RTD's) on the tunneling of holes through these s tructures. We have used RTD's mesa-etched into dots and wires, the lat eral dimensions ranging from 10 mu m down to 230 nm. In the dots we fi nd a very strong shift of the light-hole (LH) resonance in the tunneli ng spectrum as the dot diameter decreases below 1 mu m, while the posi tion of the heavy-hole (HH) resonance is constant. In the wires, on th e contrary, this size effect in the tunneling is completely absent: bo th peak positions are constant. This behavior, including the surprisin g insensitivity of the tunneling spectrum to the wire width, arises fr om 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 I-II-I and LH barrier heights, and the HH-LH splitti ng in the quantum well. The combined effect of these quantities on the peak voltages agrees qualitatively with the experimental data, when w e assume that in the dots the relaxation is biaxial, while in the wire s it is predominantly uniaxial. The interpretation is also consistent with magnetotunneling-spectroscopy data, which reflect the in-plane an isotropy of the LH quantum-well subband. We find for all dot diameters a fourfold rotational symmetry of the shift of the LH resonance and f or the wires a remarkable transition from a fourfold to a pronounced t wofold rotational symmetry of this shift as the wire width decreases b elow 900 nm. This transition is interpreted as evidence for the strong influence of uniaxial relaxation on the in-plane dispersion.