High Concentrations of Erbium in Crystal Silicon by Thermal or Ion-Beam-Induced Epitaxy of Erbium-Implanted Amorphous Si
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HIGH CONCENTRATIONS OF ERBIUM IN CRYSTAL SILICON BY THERMAL OR ION-BEAM-INDUCED EPITAXY OF ERBIUM-IMPLANTED AMORPHOUS SILICON J. S. Custer, A. Polman, E. Snoeks, and G. N. van den Hoven FOM Institute for Atomic and Molecular Physics Kruislaan 407, 1098 SJ Amsterdam, the Netherlands ABSTRACT Solid phase epitaxy and ion-beam-induced epitaxial crystallization of Er-doped amorphous Si are used to incorporate high concentrations of Er in crystal Si. During solid phase epitaxy, substantial segregation and trapping of Er is observed, with maximum Er concentrations trapped in single 3 crystal Si of up to 2 x 1020 /cm . Ion-beam-induced regrowth results in very little segregation, 3 with Er concentrations of more than 5 x 1020 /cm achievable. Photoluminescence from the incorporated Er is observed. INTRODUCTION There is substantial technological interest in achieving efficient light emission from crystal Si (c-Si). Unfortunately, because of its indirect band gap, Si exhibits very inefficient band-to-band luminescence. Ennen et al. have pointed out the potential of rare-earth ions as optical dopants in semiconductors, including Si [1,2]. Rare-earth ions in the correct charge state exhibit luminescent intra-4f transitions, which are shielded from the surroundings by filled outer electron shells. This produces a nearly host-independent luminescent transition. Erbium is of particular interest because Er 3+ has a transition from the first excited state to the ground state at a wavelength of 1.5 pm, which is important in optical communication technology. There are three major steps to actually use Er-doped Si. First, high concentrations of Er have to be incorporated in Si. Second, the Er must be made optically active, that is have the 3+ charge state. Finally, the Er ions must be efficiently pumped by electrical excitation. In this paper, we will concentrate on the first step, incorporating Er in c-Si.
Because
the solubility limit of Er in Si is not known, it is not clear a priori what concentrations can be easily achieved. By analogy to the transition metals, though, it is likely that the solubility 3 of Er is relatively small (- 101" - 10'a Er/cm ). This concentration is too low to make useful optical devices [3]. We demonstrate here that by using non-equilibrium crystal growth, at least 3 5 X 1020 Er/cm (1 at.%) can be incorporated in c-Si. This is done by recrystallizing an Er-doped amorphous Si (a-Si) layer on c-Si, using either thermal solid phase epitaxy (SPE) [4] or ion-beaminduced epitaxial crystallization (IBIEC) [5]. Both methods are capable of trapping impurities in c-Si above their solid solubility limits [6-8]. Thermal SPE results in significant segregation and 3 trapping of Er [9,10], with trapped concentrations of up to 2 x 1020 Er/cm achievable. IBIEC at 320*C leads to little segregation and nearly complete trapping of Er at concentrations of at least 5 x 1020 Er/cm 3 . Both types of samples exhibit photoluminescence (PL) characteristic of Er, and the PL intensity can be optimized by post-growth anneals. EXPERIM
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