Nanostructured thin films

D. Nesheva, Z. Levi, Z. Aneva, I. Bineva

The work in the field was initiated by Prof. Elena Vateva more than 20 years ago with fabrication and characterization of amorphous multilayers based on selenium. Recently, the attention has been concentrated on formation and investigation of crystalline and amorphous semiconductor nanoparticles and nanocrystalline single layers.

Sample fabrication

1. Single layers, amorphous and amorphous-nanocrystalline multilayers

Thermal evaporation in vacuum is used to produce II-VI (CdSe, ZnSe, CdS) thin and ultra thin single layers. Two deposition approaches are applied: one-step deposition during which the substrates are fixed above the crucible and step-by-step deposition which involves the rotation of the substrates at a rate varying between 8 and 30 cycles/min as the substrates spend only 1/12 part of the cycle over the source. This approach allows fabrication of multilayers with smooth interfaces and well defined layer thickness.
       The step-by-step deposition approach is used for fabrication of high quality amorphous multilayers (Se/CdSe, Se/Se_100-xTe_x, x=5-30, CdSe/Se₈₅Te₁₅) as well as for preparation of amorphous-nanocrystalline multiple quantum wells from SiO_x/CdSe, GeS₂/CdSe, SiO_x/ZnSe.

     HRTEM cross section micrograph of a
    SiO_x(5 nm)/CdSe(5 nm)/SiO_x(5 nm)
     three layer structure.

2. Self-asembling of CdSe nanocrystals

Nanocrystals of CdSe embedded in amorphous SiO_x, GeS₂ and polycrystalline ZnSe thin film matrix (composite films) are successfully produced using a multilayer approach. Step-by-step physical vapor deposition of CdSe ultra thin films on the relative ‘rough’ surface of ‘one-step’ deposited layers from the matrix materials is applied. The matrix layer thickness is 20 times greater than the CdSe one. High-resolution electron microscopy proved the formation of CdSe nanocrystals with nearly spherical shape. The particles are partly isolated and partly in contact and their spatial distribution follows the surface morphology of the ‘matrix’ films. The average size of the nanoparticles is about two times greater than the nominal thickness of CdSe layer. The annealing of SiO_x- CdSe films at proper high temperatures allows formation of isolated CdSe nanocrystals homogeneously distributed in the SiO_x matrix.

     Cross-sectional view of the “2-nm” sample SiO_x upon annealing in air at 670 K for 80 min.
     The low magnification representation and the diffractogram point to CdSe nanoparticles
     of increased size and improved crystallinity with respect to the as-deposited sample.

3. Growth of Si nanoparticles in SiO_x matrix

SiO_x thin films with various oxygen contents (1.1?x?1.7) are produced by thermal evaporation of SiO in vacuum. The film composition is varied by varying the deposition rate and residual pressure under the vacuum chamber. Amorphous and crystalline Si nanoparticles are grown by furnace and rapid thermal annealing at various temperatures, for various times and in various ambients. Visible photoluminescence is observed at room temperature which, in the films with the same initial composition, is stronger in the films containing amorphous Si nanoparticles than in those having nanocrystals.

Photoluminescence spectra of SiO_x films with different oxygen content annealed
at 973 K for 60 min in argon (a), (b), and (c), upper spectra
or 1303 K for 60 min in nitrogen (a), (b), (c), and (d), lower spectra.
All samples have the same thickness of 0.2 mm.
All spectra were excited by the 488 nm of an Ar⁺ laser line and correspond to the same scale.

Ìetal-oxyde-silicon (MOS) structures containing layers with amorphous or crystalline silicon nanoparticles in a silicon oxide matrix are fabricated by sequential physical vapour deposition of SiO_x and RF sputtering of SiO₂ on p- or n-type crystalline silicon, followed by high temperature annealing in an inert gas ambient. Depending on the annealing temperature, 700^oC or 1000^oC, amorphous or crystalline silicon nanoparticles are formed in the SiO_x. High frequency C-V measurements demonstrate that these structures are suitable for production of nonvolatile memories.

Sample characterization

X-ray diffraction, high resolution electron microscopy and Raman scattering measurements are carried out (in collaboration with several European Institutes and Universities) in order to study the shape, structure, spatial and size distribution of CdSe and Si nanoparticles as well as to investigate material intermixing in both multilayers and composite films.
       Conductivity measurements in dark and under illumination with white and monochromatic light as well as thermally stimulated currents, optical transmission and photoluminescence measurements are performed in order to study carrier transport, recombination processes, defect state distribution and size-induced changes in the optical band gap of the CdSe and ZnSe well layers of multilayers and CdSe and Si nanoparticles.

Collaboration

• Institute of Physics, Carl von Ossietzky University Oldenburg, Oldenburg, Germany;

• Dundee University, Dundee, Scotland, UK;

• Institute of Physics, Center for Solid State Physics and New Materials, Belgrade, Serbia;

• National Polytechnical University of Athens, Greece;

• Max-Plank Institute of Microstructure Physics, Halle, Germany.