Size Effect in Germanium Nanostructures Fabricated by Pulsed Laser Deposition
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ABSTRACT We have fabricated Ge nanostructures buried in A1N and AI 20 3 matrices grown on Si(1 11) and sapphire substrates by pulsed laser deposition. Our approach involved threedimensional island growth of low band-gap material followed by a layer of wide bandgap material. The nanodots were uniformly distributed in between alternating layers of AIN or A120 3. It was observed that these nanodots exhibit crystalline structure when grown at 300-500 °C. The average size of Ge islands was determined to be '-5-15 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 upto 295 cm' which is caused by quantum confinement of phonons in the Ge-dots. The photoluminescence of the Ge dots (size '-15nm) was blue shifted by --0.266 eV from the bulk Ge value of 0.73 eV at 77 K, resulting in a distinct peak at -1.0 eV. The spectral positions of both EI and E2 transitions in the absorption spectra at room temperature and 77K shift toward higher energy as the Ge dot size decreases. The interpretation of these behaviors in terms of quantum confinement is discussed in this work, and the importance
of pulsed laser deposition in fabricating novel nanostructures is emphasized INTRODUCTION The quantum confinement effects in semiconductor systems with reduced dimensions have attracted considerable attention[I-3]. Their optical and electronic properties are of special interest as they can be modified by altering their structure. This makes quantum-confined semiconductors very promising for photoelectronic device applications. Indirect bandgap semiconductors such as Si and Ge have very poor luminescent efficiency since the band center transition is optically forbidden. By making the crystallite size of these semiconductors smaller than the exciton Bohr radius, the resulting quantum confinement produces[4 ]a sharp increase in the oscillator strength and shifts the luminescence to higher energies. The effective Bohr radius aa for excitons in bulk Ge is calculated to be about 24 nm[5], which is much larger than that of bulk Si (4.9 nm)[6]. These electronic conditions lead to an expectation that it is much easier to change the electronic structure around the band gap of Ge.
163 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
EXPERIMENT In this work, we report a novel technique[7] to fabricate multilayers of Ge nanostructures buried in aluminum nitride (AIN) and aluminum oxide (A120 3) matrices on Si(l 11) and sapphire substrates. Our approach involved three-dimensional growth of Ge islands on AIN or A1203 substrate, the size of Ge islands (dots) is controlled by the amount of Ge and substrate temperature. AIN is a wide bandgap material (Ec- 6.2 eV) with dc dielectric constant -9.14[8]. The pulsed laser deposition (PLD) technique[9], used in the present work, is known for growing high quality structures at lower substrate temperatures. Moreover, using pulsed laser deposited Ge/A1N heterostructures, we can minimize
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