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 NDx 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 NDx 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 NDx 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 NDx 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 NDx 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.
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