STUDIES OF SEMICONDUCTOR CLUSTERS IN SUPERSONIC BEAMS

Cold semiconductor clusters of Si, Ge, and GaAs have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The neutral clusters were characterized by F$\sb 2$ (7.89 eV) and ArF (6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. A semiconductor cluster ion apparatus has been developed for characterizing ionic cluster beams and mass selecting a particular ion for subsequent studies with various photolysis methods such as photodetachment, photofragmentation, and photoelectron spectroscopy. Ga$\sb{\rm x}$As$\sb{\rm y}$ clusters with x + y $>$ 10 thus generated have compositions ranging from Ga$\sb{\rm x+y}$ through Ga$\sb{\rm x}$As$\sb{\rm y}$ to As$\sb{\rm x+y}$ which closely follow a statistical binomial distribution. In the smaller clusters, strong variations were observed from this binomial distribution as a result of kinetic effects in the cluster formation process. An even/odd alternation in the ionization potential (IP) and the electron affinity (EA) of GaAs clusters was observed, which only depended upon the total number of atoms in cluster. GaAs clusters with an even total number of atoms had higher IP but lower EA than their neighboring odd ones. This suggests that the even clusters have fully paired singlet ground states with no dangling bonds. Electron affinities of Si, Ge, and GaAs clusters have been roughly determined as a function of cluster size by detecting the photoelectron intensity dependence upon the probing laser fluence. In general, the EA of cluster increases monotonically with increasing cluster size extrapolating towards the EA of the corresponding bulk semiconductor material. The fragmentation patterns of Si and Ge negative ion clusters observed were remarkably similar, implying that they have almost the same structures. In contrast with GaAs and metal clusters, Si and Ge anions fragmented by fission into mainly 5-10 atom size range, and the six and ten atom anions were the most favorite daughters. Photofragmentation processes were found to be competitive with photodetachment. For Si and Ge, one-photon detachment and one-photon fragmentation occurred simultaneously in certain laser wavelength ranges. Detachment became dominant at high photon energies