Optical and structural studies of Ge nanocrystals embedded in AlN matrix fabricated by pulsed laser deposition

We have fabricated Ge nanostructures buried in a matrix of AlN grown on Si(111) by pulsed laser deposition at the substrate temperature of 500 degrees C. The characterization of these structures was performed using high-resolution transmission electron microscopy (HRTEM), photoluminescence, and Raman spectroscopy. The HRTEM results show that the Ge islands are single crystal with a pyramidal shape. The average size of Ge islands was determined to be similar to 15 nm, which could be varied by controlling laser deposition and substrate parameters. The Raman spectrum showed a peak of the Ge-Ge vibrational mode downward shifted up to 295 cm(-1) which is caused by quantum confinement of phonons in the Ge dots. The photoluminescence of the Ge dots (size similar to 15 nm) was blueshifted by similar to 0.266 eV from the bulk Ge value of 0.73 eV at 77 K, resulting in a distinct peak at similar to 1.0 eV. The transmission measurements carried out on different samples having Ge dot sizes of 7, 8, and 13 nm deposited on sapphire substrate showed the above band edge transitions of Ge, which were also blueshifted in accordance with the quantum confinement effect. The importance of pulsed laser deposition in fabricating novel nanostructures is emphasized. (C) 1999 American Institute of Physics. [S0003-6951(99)03735-3].