Metal Nanoparticle Layers

Metal (Mo,Ti,V)/a-Si ML systems (also called composite films) have been prepared by magnetron sputtering of both materials or by an rf glow discharge deposition of a-Si and thermal evaporation of metals [180, 186]. Discontinuous metal sublayers were formed by interrupting the growth process before the stadium of coalescence at a nominal layer thickness of around 1 nm. The thickness of a-Si layers was 100 nm. Scanning force microscopy measurements of surface topography revealed metal clusters with diameters of the a-Si:H surface roughness (2-8 nm). These clusters were arranged in chains up to several hundreds of nanometers in length. Cross-section TEM micrographs showed island-type metal layers having a typical thickness of 10 nm. It has been assumed that metal clustering is supported by the existing valleys of the a-Si:H surface structures. As in MLs with continuous layer, an material mixing has been observed at the interface. It is interesting to mention the observed asymmetry of the mixing. The mixed interface region from the growth of the metal on Si has significantly greater thickness (~10 nm) than that (a few nanometers) from the growth of the Si on the metal.

Nanoparticle layers of Ga, with relative low-size dispersion (<20%) and regular shape of a truncated sphere, have been produced by thermal evaporation of Ga on 5 nm thick SiOx films. Metal particles were formed in the liquid state by a self-organization process [189, 190], which is related to the partial wetting character of Ga with respect to the insulator [190]. The melting point of Ga in nanoparticle layers is considerably lowered with respect to the bulk materials and it is liquid at room temperature [191].

Self-organized growth has also been observed on deposition of cobalt on alumina [192-195]. Multilayers were

Table 2. Chalcogenide-based regular multilayers.

Multilayer

Preparation technique

Crystallization

a-Se/a-Se100-, Te,

(7.5 <

, < 30)

thermal evaporation [109, 110]; laser beam evaporation [82-85, 93]

laser beam crystallization [109, 110]

a-Se/CdSe

thermal evaporation [76, 77, 88, 99, 100, 105, 140]

furnace annealing [105, 140]; laser beam crystallization [100]

a-Se85Te15/CdSe

thermal evaporation [24, 27, 30, 76, 78]

a-Se/As2S3

thermal evaporation

laser beam crystallization [106]

a-Se/As2Se3

thermal evaporation

furnace annealing [72]

a-Te/As6Se94

thermal evaporation

furnace annealing [79], interdiffusion

As2S3/Se,Te100-,

(10 < ,

< 50)

thermal evaporation

laser beam processing [80], interdiffusion

As2S3/AsSe

thermal evaporation

laser beam processing [81], interdiffusion

nc-CdSe/a-SiO,

thermal evaporation

furnace annealing [93-96, 117, 201]

nc-CdSe/a-GeS2

thermal evaporation

furnace annealing [97, 98, 201], interdiffusion

nc-CdSe/ZnSe

thermal evaporation

furnace annealing [97, 98, 151], interdiffusion

prepared by sequential rf sputtering of Co and A!2O3 at room substrate temperature. It has been shown [193, 194] that nearly spherical crystalline Co clusters with diameters ranging from 0.8 nm to 4.5 nm were formed when nominal deposited Co thickness increases from 0.2 to 1 nm. The A!2O3 layer thickness was several nanometers. The metal nanocluster formation has been discussed in terms of a large difference of surface energies of the transition (and noble) metals and insulators [196]. Additionly, cross-section TEM of Co(0.7 nm)/Al2O3(3 nm) MLs has shown [195] an ordered structure (seen in a given range of thicknesses), which consists of Co clusters with a diameter of 3 nm, whose vertical arrangement from plane to the plane is not random but shows an additional self-organization related to topology reasons [195]. When Co is deposited on Al2O3, clusters are formed that display the in-plane order. The next Al2O3 layer is assumed to wet perfectly this Co island-type layer being undulated. During the next Co deposition, cluster formation takes place preferentially in the hollows of the wavy Al2O3 surface. The periodic repetition of this phenomenon is considered to be the origin of the self-organized Co growth observed. The last result illustrates that in contrast to a codeposition method, the multilayer approach allows preparation of composite structures in which the size and spacing of the clusters can be controlled directly by varying the thickness of the layers. If this fabrication method can be extended to ferromagnetic alloys, it might open a new route for the fabrication of advanced magnetic recording media.

As previously discussed, at semiconductor/semiconductor interface, significant material intermixing may take place which can strongly effect ML properties. Similarly, in a metal/oxide structure, one should pay attention to possible reactions at the interface. X-ray photoelectron spec-troscopy has been used to probe metal (Au, Ag, Mg, Mg-Ag alloy)/AlO3 interface structures. It has been found [197] that there is no chemical reaction at (Au, Ag)/AlO3 interfaces, while there is a significant reaction/diffusion at (Mg, Mg-Ag alloy)/AlO3 ones. The reaction involves formation of Mg and metallic Al and then followed by metallic Mg diffusion to the interface.

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