Temperature Dependence and Annealing Effects of Absorption Edges for Selenium Quantum Dots Formed By Ion Implantation in
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SELENIUM QUANTUM DOTS FORMED BY ION IMPLANTATION INSILICA GLASS A. Ueda*-', M. Wu*, R. Mu*, Y.-S. Tung*, D.O. Henderson*,", A. Meldrum**, R.A. Zuhr**, J.D. Budai**, C. W. White**, J.C. Keay***, L.C. Feldman*** *Chemical Physics Laboratory, Physics Department, Fisk University, Nashville, TN 37208, USA # e-mail: [email protected], ## corresponding author e-mail: [email protected] **Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37830, USA ***Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
ABSTRACT We have fabricated Se nanoparticles in silica substrates by ion implantation followed by thermal annealing up to 1000°C, and studied the Se nanoparticle formation by optical absorption spectroscopy, Rutherford backscattering spectrometry, X-ray diffraction, and transmission electron microscopy. The sample with the highest dose (1 x 10i7 ions/cm 2 ) showed the naonparticle formation during the ion implantation, while the lower dose samples (1 and 3 x 1016 ions/cm2) required thermal treatment to obtain nano-sized particles. The Se nanoparticles in silica were found to be amorphous. After thermal annealing, the particle sizes became larger than the exciton Bohr radius for bulk Se. Thus, the absorption edges for different doses approached the value of bulk after thermal annealing. The temperature dependent absorption spectra were also measured for this system in a temperature range from 15 to 300 K. INTRODUCTION Selenium is a semiconductor that has been extensively studied in the past because of the applications in photovoltaic cells, rectifiers, xerography, and so on. Solid selenium is known to occur as four major allotropes: an amorphous, trigonal, and two monoclinic phases [1]. In the solid and liquid states, selenium exists in two kinds of unit structures: rings and chains. The monoclinic allotropes are composed of eight-membered rings in two different stacking patterns, while the trigonal selenium is composed of helical chains. The glassy selenium consists of a mixture of rings and polymeric chains. For the trigonal phase, bulk selenium has the band gap of 1.7-2.2 eV, while monoclinic phase has the band gap of 2.4-2.6 eV. [2]. However, the values are varied in the literature and they are not yet well determined. It is also known that the temperature effect on the band gap of bulk selenium is relatively high ([dE/dT], = -14 x 10-4 eV/K), compared with the other semiconductors (-2-7 x 10-4 eV/K). Recently, quantum size effects on the optical properties on semiconductor nanocrystals have been observed. The third-order optical susceptibility for small semiconductor crystallites is found to be very large [3]. The nano-sized selenium quantum dots are expected to exhibit changes in electronic and vibrational characteristics due to the quantum size effects. The band gap is generally expected to be blue shifted from bulk value as the particle size decreases[4]. EXPERIMENTAL Selenium ions were implanted into fused silica substrates (Coming 7940) at an energy of 330
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