| Quantization due to strain gradients in silicon-based semiconductor nanostructures has been investigated both theoretically and experimentally. When vertical quantum dots are fabricated from strained Si/SiGe heterostructures, the creation of the stress-free lateral surface can cause strain relaxation through sidewall expansion, leading to a spatially inhomogeneous strain field. This strain field can induce lateral quantum confinement, in addition to the usual vertical confinement by the heterostructures. We employed magnetotunneling spectroscopy to probe the inhomogeneous-strain-induced quantization in deep submicron strained p-Si/SiGe vertical quantum dots. We demonstrated that the inhomogeneous strain relaxation on the lateral surface creates a ring-like potential near the perimeter of the cylindrical dot, which can confine hole states exhibiting quantum ring characteristics. Most recently, in an ultrasmall strained cylindrical quantum dot, we obtained the energy spectrums of single-particle and two-particle quantum ring ground state in magnetic fields. A periodicity of &phis; 0 for single-particle quantum ring ground state and a periodicity of &phis; 0/2 for two-particle quantum ring ground state in magnetic field B are observed, and the halved periodicity is the signature for singlet-triplet transition in two-particle quantum ring ground state. We are the first to report this observation. Since strain relaxation is not only determined by the size of the dot, but also determined by the geometry of the dot. We also investigated strain relaxation in elliptical and figure-eight quantum dots. An understanding about the geometry effect on the strain relaxation in the quantum dot is achieved. |
