(658c) Use of Deuterium and Ammonia to Achieve a Clean Bandgap in Silicon Nanocrystals

Silicon (Si) nanocrystals (NCs) less than 5 nm in diameter exhibit size-dependent light emission and such materials can potentially be used to fabricate low cost, light emitting devices. However, the ratio of surface atoms to the total number of atoms in nanostructured materials is very large and surface effects play an important role in determining the optoelectronic properties. Dangling bonds and surface reconstructions create defect states in the bandgap, which can result in nonradiative recombination of excitons. Defect states can also be introduced by surface passivants such as oxygen, sulfur or nitrogen when present in the bridged or double bond configurations. This talk addresses, in detail, the effect of the various ND_x and deuteride species on the photoluminescence (PL) emitted from Si NCs subjected to thermal doses of ND₃, and to hot wire doses of D₂ and ND₃. We show the manner of filling the various ND_x and deuteride species plays an important role in determining the optoelectronic properties of Si NCs.

Si NCs less than 5 nm in diameter are grown on SiO₂ surfaces using hot wire chemical vapor deposition in an ultrahigh vacuum chamber. The dangling bonds and the reconstructed bonds at the NC surface are passivated and transformed with D and ND_x by using deuterium and/or deuterated ammonia (ND₃), which are predissociated over a hot tungsten filament prior to adsorption, or by adsorbing ND₃ directly on the Si surface. Temperature programmed desorption and X-ray photoelectron spectroscopy are used to follow the formation of silicon deuterides and ND_x species, and PL emitted, for an excitation wavelength of 405 nm, follows the corresponding optical properties. At low hot wire ND₃ doses PL emission is observed at 1000 nm corresponding to reconstructed surface bonds capped by predominantly monodeuteride and Si-ND₂ species. As the hot wire ND₃ dose is increased, di- and trideuteride species form and intense PL is observed around 800 nm that does not shift with NC size and is associated with defect levels resulting from ND_x insertion into the strained Si-Si bonds forming Si₂=ND. The PL intensity at 800 nm increases as the ND₃ dose is increased and the intensity increase is correlated to increasing concentrations of deuterides. At extremely high ND₃ doses, PL intensity decreases due to amorphization of the NC surface. Si NCs subjected to dissociative (thermal) exposures of ammonia followed by exposures to atomic deuterium exhibit size dependent PL and this can be attributed to the prevention of Si₂=ND species formation. A clean bandgap and maximum PL intensity result when the Si NC is fully relaxed (i.e., unreconstructed) and the surface dangling bonds are fully passivated.