We fabricated a high-efficiency infrared light emitting diode (LED) via dressed-photon-phonon (DPP) assisted annealing of a p-n homojunctioned bulk Si crystal. The center wavelength in the electroluminescence (EL) spectrum of this LED was determined by the wavelength of a CW laser used in the DPP-assisted annealing. We have proposed a novel method of controlling the EL spectral shape by additionally using a pulsed light source in order to control the number of phonons for the DPP-assisted annealing. In this method, the Si crystal is irradiated with a pair of pulses having an arrival time difference between them. The number of coherent phonons created is increased (reduced) by tuning (detuning) this time difference. A Si-LED was subjected to DPP-assisted annealing using a 1.3？μm ( ？eV) CW laser and a mode-locked pulsed laser with a pulse width of 17？fs. When the number of phonons was increased, the EL emission spectrum broadened toward the high-energy side by 200？meV or more. The broadening towards the low-energy side was reduced to 120？meV. 1. Introduction Direct transition type semiconductors are mainly used in semiconductor light emitting diodes (LEDs) [1, 2]. The reason for this is that the probability of electric dipole transitions, in other words, the radiative recombination probability, is high. Also, the emission wavelength is determined by the bandgap energy, , of the material used. Therefore, for example, InGaAsP epitaxially grown on an InP substrate is mainly used as the active layer for near-infrared LEDs with emission wavelengths of 1.00–1.70？μm (0.73–1.24？eV), which includes the optical fiber communication wavelength band. Shortcomings with this approach are that InP is highly toxic [3], and In is a rare resource. Silicon (Si), on the other hand, is a semiconductor having low toxicity and no concerns about depletion of resources; however, its emission efficiency is low since it is an indirect transition type semiconductor. Therefore, Si is usually not suitable as a material for use in LEDs. Nevertheless, there is a great demand for the use of Si in light emitting devices, and there has been extensive research into improving its emission efficiency. For example, there has been research into making Si emit light in the visible region by utilizing the quantum size effect of Si and by using porous Si [4], a Si/SiO2 superlattice structure [5, 6], and Si nanoprecipitates in SiO2 [7], as well as research into making Si emit light in the near-infrared region by doping it with light-emitting materials, such as erbium (Er)-doped Si [8] and
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