(446d) Hot Wire Chemical Vapor Deposition Kinetics For Germanium Nanoparticle Growth On Extended And Patterned Hafnia Surfaces

Proposed flash memory devices that use semiconductor nanocrystals to store charge are currently of interest due to their low operating power and potential for high device density. As nanotechnology devices become more prevalent, the controlled placement of nanoparticles within the device confines is important for device operation viability. We have previously reported the selective growth on Ge nanoparticles on hafnia at the bottom of 20 nm diameter features that are formed in a 12.5 nm-thick silica film. The growth is affected by the relative rates of Ge adatom accumulation on the hafnia surface and the removal of Ge adatoms at the silica surfaces that are at the perimeter of the hafnia regions. This paper explores the kinetics of Ge nanoparticle growth on hafnia. Mean-field nucleation growth kinetics descriptions are used to extract the critical cluster size, i*, and the activation energy for cluster formation. Studies on extended surfaces and studies on hafnia features that range in size from 0.20 to 100 µm are used to explore how surface diffusion affects the nucleation rates. The nanoparticles are grown on hafnia using hot wire chemical vapor deposition (HWCVD), which employs a heated (>1500 ºC) tungsten filament to crack GeH₄ and generate GeH_x radicals at a sufficiently high enough flux that parallel thermal CVD is effectively suppressed. This permits us to explore the effects of incident flux and growth temperature on the densities of Ge nanoparticles on hafnia surfaces.

Square boxes ranging in size from 0.20 to 100 µm were formed using a poly(methyl methacrylate) positive resist in conjunction with electron beam lithography. Reactive ion etching with CHF₃/O₂ is used to remove the 12.5 nm silicon dioxide film and expose the hafnia beneath the silicon dioxide. The research employs a silicon dioxide hard mask because our previous studies revealed temperatures at which Ge adatoms accumulate on hafnia and not silica; adatom accumulation is required for nanoparticle growth. After mask removal, Ge was selectively deposited only upon the hafnia with GeH₄ using HWCVD in the temperature range of 700-775 K and incident flux rate range of 0.1-4.2 ML/min. The temperature range assures Ge adatom desorption is insignificant from hafnia while adatom interactions with the silicon dioxide hard mask removes adatoms. Scanning electron microscopy is used to characterize the nanoparticles. Random arrays consisting of ~5 nm Ge nanoparticles were grown on hafnia with 3×10¹¹ cm^-2 densities that were independent of flux. The critical nuclei cluster size, i*, is estimated in the range 0-1 using mean-field nucleation theory from data obtained by having flux as the independent variable. This is consistent with the value of zero reported for molecular beam growth of Ge nanoparticles on silica. The activation energy for critical cluster formation is ~1.0 eV. HWCVD densities in confined micron sized regions is similar to deposition found on extended surfaces except at the highest examined temperature and lowest examined flux, 775 K and 0.1 ML/min, respectively. HWCVD in submicron features reveals how diffusion to the silica and removal at the perimeter competes with adatom arrival on the hafnia surface at all temperatures examined. By working at the temperature and flux extremes and in the smaller features, we are able to estimate the distances adatoms travel unimpeded before nucleation or addition to an existing particle as a function of temperature. Mass accumulation models lead to an activation energy of 0.43 eV for diffusion, again consistent with the diffusion activation energy for Si and Ge adatoms on the amorphous silica surface.