Semiconductor Nanoparticle Layers

Preparation Semiconductor nanocrystals (quantum dots) of ZnCdSe has been grown by molecular beam epitaxy on a ZnSe crystalline surface which was intentionally roughened [198]. High-resolution transmission electron microscopy studies have shown that the vertical size of nanoparticles was significantly smaller than their in-plane size and, also, the size distribution of those dots was rather wide.

Another technique has been developed [97, 188, 199201] for fabrication of CdSe nanoparticle layers buried in amorphous thin film matrices. Consecutive deposition of ultra-thin films of thermally evaporated CdSe on a relatively rough surface of the matrix material (SiOx, GeS2, ZnSe) has been applied. When developing this technique, it has been taken into account that during the nonepitax-ial growth of thin films, the top surface of these films is rough with amplitude of this roughness varying with the deposition techniques. In case of thermal evaporation of materials, this amplitude is ~d1/2 (d-film thickness) [202]

and, hence, the thicker the thermally evaporated layer, the greater its surface roughness. Multilayer of consecutively deposited SiOx (GeS2, ZnSe) and CdSe layers were fabricated in which the SiOx thickness dSiO was 20 times greater than that of CdSe layers dCdSe. One can expect that CdSe layers will be discontinuous rather than continuous. A schematic diagram of such a ML structure is shown in Figure 7. Cross-section high-resolution electron microscopy measurements have revealed [97, 199, 200] that, indeed, CdSe does not form a continuous layer. Instead, nearly spherical CdSe nanoparticles partly isolated, and partly in contact among each other, are disposed in a composite SiOx-CdSe "sublayer" of up to 10 nm in thickness (Fig. 8). It should be pointed out that nanoparticles are not grown in the surface valleys but their spatial distribution follows the morphology of the surface underneath. The great width of the CdSe nanoparticle spatial distribution also indicates that they should not be formed only in the surface valleys. The results indicate that the surface roughness favors the self-organized formation of CdSe nanoparticle layers rather than the growth of continuous thin films.

High-density Si nanoparticles have been successfully grown on Si3N4 and SiO2 thin film surfaces by hot-wire CVD using disilane, in which Si atoms were generated on a heated tungsten filament [203]. A highest density of 1.1 x 1012 cm-2 has been achieved with corresponding average nanoparticle size of about 5 nm. Sandwich-structured thin films of silicon nanoparticles embedded in Al2O3 matrices have been prepared by pulsed laser beam evaporation [203]. It has been found that the sharpness of the Si/Al2O3 interface depend strongly on annealing conditions.

The described approaches of production of nanoparticle layers buried in a thin film amorphous matrix makes possible the reproducible fabrication of nanocrystals for shorter times and at considerably lower annealing temperatures than those usually used for the nanocrystal growth in

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