Er-Doped Porous Silicon Led For Integrated Optoelectronics
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Recently, room-temperature luminescence from Si-based materials doped by Er has been demonstrated in: c-Si co-implanted by oxygen (Si:O:Er) [2]; semi-insulating polycrystalline Si (SIPOS) [3]; amorphous Si (a-Si) [4]; and porous Si (PSi), prepared by electrochemical etching of c-Si. In the case of PSi, erbium has been incorporated through ion implantation [5] or electroplating [6]. It has been suggested that the recombination of spatially confined electron-hole pairs in Si-nanograins, which is responsible for the visible luminescence in PSi [7, 8], promotes efficient energy transfer and excitation of Er ions incorporated in Si nanoclusters [5,6]. The alternative model suggests that the Er'÷ luminescence is mediated by photocarriers in the amorphous Si:O:H or Si:O-matrix produced by a thermal oxidation of PSi [9]. Recently, photo-(PL) and electroluminescence (EL) has been reported in partially oxidized PSi, but no evidence of the charge transfer mechanism related to Si nanocrystals has been found [10]. The purpose of this work is to study light-emitting properties of Er+ doped Si-rich silicon oxide SRSO(Er) and to develop a prototype of a SRSO(Er)-based light-emitting device (LED). EXPERIMENTAL PROCEDURE In this work the samples were produced by anodic etching of boron-doped c-Si wafers with a resistivity p = 1 - 0.05 Q cm in an HF-ethanol solution (1:1) under a current density J = 2-20 mA/cm2. The thickness of the PSi layers varies from 0.5 to 5 gim with porosities from 145 Mat. Res. Soc. Symp. Proc. Vol. 486 © 1998 Materials Research Society
40% to 60%. After a rinse in ethanol, the samples were immersed in an Er(NO 3)3/ethanol solution and negatively biased relative to the Pt electrode (according to the procedure developed in Ref. 6). The volume concentration of Er-ions incorporated in the PSi layer, which is given by a total charge passed during the electroplating, was estimated to be < 10'9 cm 3 . Different post treatments were used on PSi(Er÷) samples prior to Er+ electromigration step. We found that the RCA cleaning significantly (- 5 times) increases the Er+ related PL efficiency. Our PSi(Er÷) samples were rinsed -15 minutes in deionized H20, then boiled in a 1:3:15 (H20 2:HCl:H20) solution for 25 minutes, rinsed again in H 20 and dried at 150'C over 24 hours. All samples were subject to 2-steps thermal post-treatment: oxidation (annealing in air or ~10% 02 diluted by N 2 at temperatures 400-700'C) and densification (annealing at 1 100'C in N 2 or Ar). RESULTS The thermal activation and partial oxidation are known to be responsible for an increase of the Er-related PL efficiency in PSi:Er+ (see for example, Ref. [2, 3]). However, thermal oxidation of PSi can not be compared with conventional thermal oxidation of crystalline Si. A fragile PSi structure can be totally converted to porous glass at a temperature close to 9000 C, especially in the presence of water vapors. At the same time, the role of oxygen in the activation of the Er ions in Si is important. Low temperature oxidation at 600-700°C is not able t
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