A silicon pn-diode was embedded into a microcavity composed of a buried metal silicide as bottom reflector and a Si=SiO 2 Bragg mirror as top reflector. Spectral narrowing and an increased intensity of the Si bandgap electroluminescence was observed.Introduction: The realisation of Si-based, electrically driven light emitters is a key requirement for the implementation of low-cost Si-based optoelectronics [1]. Realising Si-based light sources in a process technology compatible with mainstream microelectronics technology is one of the big challenges of semiconductor technology. Because of its indirect bandgap Si has a low radiative recombination rate, leading to efficiencies of the bandgap electroluminescence (EL) in the range of 10 À6 [2,3]. Recently, different approaches have led to an increase of the power efficiency of the bandgap EL by more than three orders of magnitude up to values of 0.1-1% [4-6]. These approaches are based on pn-diodes, where either the non-radiative lifetime is increased by using high-purity floatzone Si, combined with surface texturing to improve the outcoupling efficiency [5], or where specific defects introduced by ion implantation enhance the radiative recombination rate through carrier confinement effects [4,6]. However, the spectral width and temporal response of these devices still constrains their practical application. One possible route for a further enhancement of the efficiency of these devices is a photonic confinement of the emitting layer. III-V semiconductor based LEDs gained significantly in performance by incorporation into microcavities (MCs) [7]. Planar MCs enhance the brightness, efficiency and directionality of the emission from a high-index material and lead to more than an order of magnitude increase in the spectral power density [8]. An MC consists of a cavity with i  l MC =2n thickness (i integer, l MC resonance wavelength of the MC, n refractive index of the cavity material) embedded between two highly reflecting mirrors. For III-V LEDs molecular epitaxy growth allows the deposition of Bragg mirrors, which act simultaneously as electrical contacts to the active layer. This is a technological problem for Si-based devices, where the Bragg mirrors have to be fabricated from insulating dielectric materials thus inhibiting to electrically drive the active layer. Previously, electrically driven MCs were realised, where the active layer and the Bragg mirror consisted of porous Si [9,10]. These devices suffered from the low stability of porous Si under highvoltage current injection. Here we present a proof-of-principle of an electrically driven MC based on bulk Si as active layer.
Light emitting pn-diodes were fabricated on a 5.8 mm thick n-type Si device layer of a silicon-on-insulator (SOI) wafer using standard silicon technology and boron implantation. The thickness of the Si device layer was reduced to 1.3 mm, corresponding to a 4l-cavity for l ¼ 1150 nm light. Electroluminescence spectra of these low Q-factor microcavities are presented. Addition of Si/SiO 2 Bragg reflectors on the top and bottom of the device (3.5 and 5.5 pairs, respectively) is predicted to lead to spectral emission enhancement by $270. r
An electrically driven silicon light emitting diode with two distributed Bragg reflectors is reported. The active material is a Si pn-junction fabricated by boron ion implantation into an n-type silicon-on-insulator wafer. The cavity with a thickness of a few wavelengths is formed by amorphous Si/ SiO 2 multilayer stacks. A strong narrowing and enhancement of the electroluminescence at a resonant wavelength of = 1146 nm is observed with a quality factor of Q = 143 and a finesse of F = 11.
Recent progress on electrically driven silicon based light emitters is reviewed, with emphasis on our work on light emitting pn diodes (LED) and MOS devices doped with rare-earth elements. The LEDs were fabricated by high-dose boron implantation, producing nanoscale modifications in the material. The electroluminescence (EL) efficiency increases with temperature, reaching 0.1% (wall plug efficiency) at room temperature for optimized conditions. Such devices were integrated into a microcavity. In the MOS devices, the oxide was implanted with various rare-earth elements, resulting in strong EL in the visible (Tb) and ultraviolet (Gd). External quantum efficiencies in excess of 10% are reported. q
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