Quantum Confinement of Above-Band-Gap Transitions in Ge Quantum Dots
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ABSTRACT A number of research efforts have been focused on self-assembled germanium quantum dots in which indirect optical transitions take place across the band gap. However, many questions regarding the confined electronic state transitions of Ge quantum dots still remain unanswered. In the present report, we have deposited ten alternating layers of crystalline Ge quantum dots embedded in an A120 3 or an AIN matrix on sapphire substrates by pulsed laser deposition. The average dot sizes (73 A to 260 A) were controlled by the laser energy density, deposition time and substrate temperature. The spectral positions of both the El and the E2 transitions in the absorption spectra at room temperature and 77 K shift toward higher energy (AE 1=I.19 eV, AE2 =0.57 eV) as the Ge dot size decreases (73 A). Structural analysis using transmission electron microscopy and atomic force microscopy and the interpretation of optical absorption measurements in terms of quantum confinement of carriers in both transitions are presented. INTRODUCTION Electrical and optical properties of semiconductors are strongly modified as their sizes shrink below several nanometers. In the indirect band gap semiconductors, such as porous Si or Ge quantum dots (QD's), quantum size effects can be used to greatly increase the luminescence efficiency. The potential of being able to fabricate integrated nanometer-scale optoelectronic devices upon the existing silicon technology infrastructure is foreseen. A great deal of research has been focused on the self-assembly of Ge quantum dots by various growth techniques, such as molecular beam epitaxyI, chemical vapor deposition 2, and pulsed-laser deposition (PLD) 3. Existing optical characterization results of Ge quantum dots are mostly devoted to the quantum confinement effects in the region around the fundamental band gap. However, many questions regarding the confined electronic state transitions of Ge quantum dots still remain unanswered. Photoluminescence has been used to verify the quantum confinement effect of Ge dots embedded in silicon4 and silicon dioxide 5 matrices. However, it has been pointed out that photoluminescence from localized surface states of GeOx can also be significant.6 Absorption measurements are not as sensitive to surface state effects. Absorption
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Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
measurements also permit examination of the band structure away from the zone center. Tognini et al. fabricated Ge dots embedded in an A120 3 matrix by evaporationcondensation and measured the absorption spectra7 ,8 and the ultrafast carrier dynamics. 9 They found that as the dot size decreased the E2 peak blue shifted and the E1 peak weakened but did not shift. Kwok et al.' 0 investigated Ge dots grown by molecular beam epitaxy by resonant Raman scattering. They reported that Ei excitons are only weakly confined inside the quantum dot, but that the interaction between the El exciton and the Si-Ge interface was almost as strong as with the Ge optical phonon insid
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