Comparison of multiple-quantum wells and quantum dots by below-bandgap photomodulated reflectivity

Large-area high-density patterns of quantum dots with a diameter of 200 nm have been prepared from a series of four multiple quantum well structures of different well width (40 A, 60 A, 80 A and 100 A) by electron lithography followed by ion beam etching. Below-bandgap photomodulated reflectivity spectra of the quantum dot samples and the parent heterostructures were then recorded at 10 K and the spectra were fitted to extract the linewidths and the energy positions of the excitonic transitions in each sample. The fitted results are compared with calculations of the transition energies in which the different strain states in the samples are taken into account. We show that the main effect of the nanofabrication process is a change in the strain state of the quantum dot samples compared with the parent heterostructures. The quantum dot pillars turn out to be freestanding, whereas the heterostructures are in a good approximation strained to the ZnTe lattice constant. The lateral size of the dots is such that extra confinement effects are not expected or observed.