COULOMB STAIRCASE IN A SI/GE STRUCTURE

The transverse differential conductance of a p+-Si0.7Ge0.3/Si/p+-Ge/Si/p+-Si0.7Ge0.3 epitaxial structure, associated with hole tunnelling into isolated ultrasmall (about 10 nm) three-dimensional relaxed Ge islands, has been studied. The Ge islands are located in the plane between the two potential barriers formed by the Si layers 8 and 12 nm thick. Periodic conductance oscillations with a voltage period of approximately e/C (C being the capacitance of a Ge particle) have been observed. This result has been explained on the basis of a 'Coulomb staircase' effect which consists of the quantization of the electron (hole) charge on the ultrasmall particles caused by a Coulomb blockade of the current by the captured charges.