Ion Implanted Er and Tb in SiO 2 for Electroluminescence in MOS Diodes
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Ion Implanted Er and Tb in SiO2 for Electroluminescence in MOS Diodes Ch. Buchal, S. Coffa1, S. Wang, and R. Carius Institut für Schicht- und Iontechnik (ISI-IT), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany 1 CNR-IMETEM, Strada le Primosole 50, 95121Catania, Italy
ABSTRACT Efficient infra-red and visible electroluminescence(EL) has been obtained from implanted rare earth ions in the SiO2 of a silicon-metal-oxide-semiconductor (MOS) diode structure at room temperature. The rare earth ions are excited by the direct impact of hot electrons tunneling through the oxide at electric fields larger than 6 MV/cm. The internal quantum efficiencies of Er and Tb implanted MOS diodes are estimated to be 10 % and 3 %, respectively. The high quantum efficiency is due to the high impact excitation cross-section of more than 10- 15cm2. These observations on MOS structures are an experimental proof for efficient light generation by hot electron impact.
INTRODUCTION The intra 4f-transitions of erbium emit light at 1.5 µm, which is an attractive wavelength for telecommunication. Therefore a lot of work has been carried out towards the development of light emitters from Si or Si based materials in combination with Er. Successful devices have been fabricated by doping p-n-junctions in silicon with erbium [1]. It has demonstrated that “hot" carriers are efficient to stimulate the 1.5 µm luminescence of Er [2-4]. Impact excitation relies on one kind of carrier only and permits the use of a relatively simple device structure [4-6]. In order to sustain a carrier distribution with electrons of high kinetic energy, a wide bandgap material is favoured. At lower bandgap, it is the onset of the avalanche process which limits the carrier energy [7]. The emission efficiency also appears to increase with the bandgap value [8], and electroluminescence has been reported from various wide bandgap materials [9-11]. Recently, efficient LEDs have been demonstrated, based on the wide bandgap semiconductor GaN [12]. In order to probe the upper limit of this concept, we select SiO2 as host for rare earth ions. SiO2 has a bandgap of 9 eV. The Er and Tb doped MOS diodes have been fabricated as shown in fig. 1a. When electrons tunnel through the MOS oxide of the doped MOS diodes, the electrons will impact the rare earth ions as illustrated in Fig. 1b. Since the average electron energy in the oxide can reach values as high as 5eV [5], their energy is even sufficient to excite visible luminescence.
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Figure 1. (a) The schematic structure of Er or Tb implanted MOS diode, (b)Energy band diagram to illustrate the impact excitation process.
EXPERIMENTAL The MOS-type light emitters were fabricated in our clean-room as follows: 1. Dry thermal oxidation of a p-doped Si(100) wafer to a total oxide thickness of 200 nm. 2. Fabrication of circular windows in the SiO2 by HF-etching through a PMMA lithographic mask. The window diameters were 0.2, 0.5 and 1 mm. 3. Growth of a very high quality MOS-type SiO2 of 50 nm thick
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