Electrical Characterization of Ge Nanocrystals in Oxide Matrix
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Electrical Characterization of Ge Nanocrystals in Oxide Matrix Ivana Capan1, Maja Buljan1, Tea Misic-Radic1, Branko Pivac1, Nikola Radic1, Joerg Grenzer2, Vaclav Holy3, S. Levichev4, and Sigrid Bernstorff5 1
Ruñer Bošković Institute, 10000 Zagreb, Croatia Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany 3 Charles University in Prague, 12116 Prague, Czech Republic 4 University of Minho, 4710-057 Braga, Portugal 5 Sincrotrone Trieste, 34012 Basovizza, Italy 2
ABSTRACT We report on electrical properties in [(Ge+SiO2)/SiO2]×2 films deposited by magnetron sputtering on a periodically corrugated-rippled substrate and annealed in vacuum and forming gas. The rippled substrate caused a self-ordered growth of Ge quantum dots, while annealing in different environments enabled us to separate charge trapping in quantum dots from the trapping at the dot-matrix and matrix-substrate interfaces. We show that the charge trapping occurs mainly in Ge quantum dots in the films annealed in the forming gas, while Si–SiO2 interface trapping is dominant for the vacuum annealed films.
INTRODUCTION Germanium quantum dots QDs embedded in widebandgap dielectrics like amorphous silica or alumina matrices are widely investigated in the past decade due to its unique properties including very strong quantum confinement, electroluminescence, and photoluminescence, and possibility of storing of electric charge. Memory devices based on Ge QDs are very promising due to a long charge retention time in the Ge QDs, faster writing/erasing times, and low operating voltages [1,2] Therefore, numerous investigations have been performed on this system, where the influence of different properties on charge trapping and retention were investigated [1–5]. However, the main mechanism of the charge storage is still not completely clear. The charge trapping may occur: i) at substrate–matrix interface states, ii) at different traps inside the matrix, iii) at QDs confined states, and iv at QD–matrix interface states [6]. EXPERIMENT Here we present a study of electrical properties of Ge+SiO2/SiO2 films deposited on a periodically corrugated rippled Si substrate. The rippled substrate morphology combined with properly chosen deposition parameters caused the growth of spatially correlated Ge QDs with uniform sizes in amorphous SiO2 matrix. The films were annealed in vacuum and some of them were additionally annealed in a hydrogen-rich atmosphere forming gas. Hydrogen is very effective in the passivation of dangling bonds at the interfaces. This fact enables us to compare the charge storage in the systems with different centers of charge trapping.
The main part of the device prepared for electrical investigation are two bilayers consisting of Ge QDs inside a SiO2 matrix separated by a 5 nm SiO2 layer. The whole film is caped by a 20 nm thick SiO2 layer acting as a gate oxide. The film is grown by magnetron sputtering on a rippled, n-type, CZ Si (100) substrate at temperature of 500 °C. The structure of the films is investigated by transmission electron mic
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