Correlation of Size and Photoluminescence for Ge Nanocrystals in SiO 2 Matrices
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ABSTRACT Synthesis and size-dependent photoluminescence has been performed for Ge nanocrystals in Si0 2 matrices with average diameters between 2 nm and 9 nm, formed by room temperature
ion implantation into Si0 2 followed by precipitation during vacuum thermal anneals. Nanocrystal size distributions obtained from electron microscopy data were used in conjunction with a quantum-confined exciton recombination model[l] to generate calculated photoluminescence spectra, which were compared with experimental spectra.
INTRODUCTION Recently there has been considerable interest in luminescent group IV semiconductor nanocrystals inspired by a potential for fabricating integrated optoelectronic devices and optical amplifiers on Si substrates. A large fraction of this work has been devoted to luminescence mechanisms in porous silicon, for which high quantum efficiency photoluminescence (PL) has been reported[2]. It has been widely suggested that this PL is due to the "quantum size effect" at nanometer-scale features of group IV semiconductor nanocrystals. Theoretical work has suggested that sufficiently small crystals, of the order of a few nanometers, would exhibit a direct bandgap behavior, allowing efficient radiative recombination of electrons and holes[l, 3, 4]. Another related and promising approach for Si-based optoelectronic devices may be luminescent group IV nanocrystals embedded in an Si0 2 matrix. Silicon dioxide has been extensively characterized, is more mechanically robust and is relatively insensitive to the ambient environment, unlike porous silicon. Luminescent Si and Ge nanocrystals in Si0 2 have been fabricated by several groups using rf co-sputtering or ion implantation, and subsequent thermal annealing to induce precipitation[6, 7, 8, 9]. However, systematic comparisons between observed PL spectra and PL spectra calculated for measured crystal size distributions have not been performed. In the present work, Ge-implanted samples annealed at different temperatures were systematically characterized using transmission electron microscopy (TEM), and the size distributions obtained were used to assess an excitonic recombination model of PL[1].
EXPERIMENTS AND RESULTS Thermally-grown silicon dioxide films 100 nm thick were implanted with 70 keV Ge ions at a dose of 2 x 1016 /cm 2 (about 5 at. % concentration), the energy being chosen to place the peak of the concentration profile approximately in the center of the film. These samples were subsequently annealed for 40 min at 600 TC, 800 TC, 1000 TC and 1200 *C. Plan-view TEM was performed to obtain crystal size distributions and ascertain crystal quality and structure. Both bright-field and high-resolution images were acquired. Photoluminescence spectra were measured at room-temperature, excited by 50 mW of 457 nm radiation from an Ar ion laser 181 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
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Figure 1: Bright-field transmission electron micrographs of Ge nanocrystals annealed for 40 min at (a) 600 0C, (b) 800 0 C
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