1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to an electroluminescence (EL) device with a silicon-rich oxide (SRO) film having a refractive index of about 1.72, to optimize photoluminescence (PL).
2. Description of the Related Art
The generation of light from semiconductor devices is possible, regardless of whether the semiconductor material forms a direct or indirect bandgap. High field reverse biased p-n junctions create large hot carrier populations that recombine with the release of photons. For silicon devices, the light generation efficiency is known to be poor and the photon energy is predominantly around 2 eV. The conversion of electrical energy to optical photonic energy is called electroluminescence (EL). Efficient EL devices have been made that can operate with small electrical signals, at room temperature. However, these devices are fabricated on materials that are typically not compatible with silicon, for example type III-V materials such as InGaN, AlGaAs, GaAsP, GaN, and GaP. An EL device built on one of these substrates can efficiently emit light in a narrow bandwidth within the visible region, depending on the specific material used. Additionally, type II-VI materials such as ZnSe have been used. Other type II-VI materials such as ZnS and ZnO are known to exhibit electroluminescence under ac bias conditions. These devices can be deposited onto silicon for use in light generating devices if special (non-conventional) CMOS processes are performed. Other classes of light emitting devices are organic light emitting diodes (OLEDs), nanocrystalline silicon (nc-Si), and polymer LEDs.
Silicon has conventionally been considered unsuitable for optoelectronic applications, due to the indirect nature of its energy band gap. Bulk silicon is indeed a highly inefficient light emitter. Among the different approaches developed to overcome this problem, quantum confinement in Si nanostructures and rare earth doping of crystalline silicon have received a great deal of attention. In particular, Si nanoclusters (nc) embedded in SiO2 have in recent years attracted the interest of the scientific community as a promising new material for the fabrication of a visible Si-based light source. Alternatively, Er-doped crystalline Si has been extensively studied to take advantage of the radiative intra-4f shell Er transition. Room-temperature operating devices with efficiencies of around 0.05% have been achieved. The device efficiency is very low and the process temperature is very high, normally over 1100° C.
However, these pioneering efforts in creating visible luminescence emanating from porous room-temperature silicon (Si), have spurred a tremendous amount of research into using nano-sized Si to develop a Si-based light source. One widely-used method of fabricating nanocluster Si (nc-Si) is to precipitate the nc-Si out of SiOx (where x<2), producing a film using chemical vapor deposition (CVD), radio frequency (RF)-sputtering, or Si implantation. This film is often called silicon-rich silicon oxide (SRSO) or silicon-rich oxide (SRO). Using the CVD or RF-sputtering processes, with a high-temperature annealing, a photoluminescence (PL) peak in the SRSO can typically be obtained in the wavelength range of 590 nanometers (nm) to 750 nm. However, these SRSO materials exhibit low quantum efficiency and have a stability problem, which reduces the PL intensity height over time, and limits their application to EL devices.
Er implantation, creating Er-doped nanocrystal Si, is also used in Si-based light sources. However, state-of-the-art implantation processes have not been able to distribute the dopant uniformly, which lowers the light emitting efficiency and increases costs. At the same time, there has been no interface engineering sufficient to support the use of such a dopant. Using the Si/SiO2 superlattice structure to control crystal size results in a slow, high-temperature deposition process that cannot simultaneously control both the Si particle size and the quality Si nanocrystal/SiO2 interface. The device efficiency is very low, which limits the device applications. In order to improve the device efficiency, a large interface area must be created between nanocrystal Si and SiO2.
A simple and efficient light-emitting device compatible with silicon, and powered by a dc voltage would be desirable in applications where photonic devices (light emitting and light detecting) are necessary. Efficient silicon substrate EL devices would enable a faster and more reliable means of signal coupling, as compared with conventional metallization processes. Further, for intra-chip connections on large system-on-chip type of devices, the routing of signals by optical means is also desirable. For inter-chip communications, waveguides or direct optical coupling between separate silicon pieces would enable packaging without electrical contacts between chips. For miniature displays, a method for generating small point sources of visible light would enable simple, inexpensive displays to be formed.
It would be advantageous if a Si-based EL device could be fabricated that emitted PL with a high intensity.
It would be advantageous if a simple, easy to measure characteristic of SRO films existed, that could be used as guide to fabricate a Si EL device with optimized PL intensity.