Carbon passivation in solid-state light emitters

#The invention relates to solid-state lighting devices, and in particular to carbon doping for passivating semiconductor nano-particles in a wide bandgap semiconductor or dielectric matrix used as an active light-emitting layer in sold state lighting devices.

#The next generation of solid-state lighting is seeking to provide advances in brightness, efficiency, color, purity, packaging, scalability, reliability and reduced costs. The creation of light emitting devices from silicon based materials, upon which the modern electronic industry is built, has been the subject of intensive research and development around the world. The main obstacle has been the indirect energy gap of bulk silicon, which limits the efficiency to an extremely low level. However, one particular technology, based on silicon nano-particles, e.g. nanocrystals, formed through various techniques, has been able to overcome this difficulty.

#Prior art light emitting devices, such as those disclosed in United States Patent Publication Nos. 2004/149,353, entitled: “Doped Semiconductor Powder and Preparation Thereof”, published Aug. 5, 2004 in the name of Hill; 2004/151461, entitled: “Broadband optical pump source for optical amplifiers, planar optical amplifiers, planar optical circuits and planar optical lasers fabricated using group IV semiconductor nanocrystals”, published Aug. 5, 2004 in the name of Hill; 2004/214,362, entitled: “Doped semiconductor nanocrystal layers and preparation thereof”, published Oct. 28, 2004 in the name of Hill et al; and 2004/252,738, entitled: “Light emitting diodes and planar optical lasers using IV semiconductor nanocrystals”, published Dec. 16, 2004 in the name of Hill, which are incorporated herein by reference, have demonstrated that using silicon-rich silicon oxide (SRSO), which consists of silicon nano-particles embedded in a silicon dioxide (SiO2 or glass) matrix, reduces many of the problems associated with bulk silicon, and when doped with erbium, or other rare earth material, can exhibit efficient room temperature rare earth luminescence because of the high efficiency of the energy transfer process from excited nanocrystals to rare earth ions. Accordingly, the SRSO provides an alternative to thin film electroluminescent material. The silicon nano-particles act as classical sensitizer atoms that absorb incident photons or electrons and then transfer the energy to the rare earth ions, which then fluoresce in the infrared or visible wavelength ranges with several advantages compared to the direct fluorescence of the rare earth. First, the absorption cross-section of the silicon nano-particles is larger than that of the rare earth ions by more than three orders of magnitude. Second, as excitation occurs via an Auger-type interaction or via a Forster transfer process between carriers in the silicon nano-particles and rare earth ions, incident photons need not be in resonance with one of the narrow absorption bands of the rare earth. Unfortunately, existing approaches to developing such silicon nano-particle materials have only been successful at producing very low concentrations of the rare earth element, which is not sufficient for many practical applications.

#The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:

#FIG. 1 is a cross-sectional view of an electroluminescent solid-state device in accordance with an embodiment of the present invention;

#FIG. 2 is a cross-sectional view of a super-lattice semiconductor structure in accordance with the device of FIG. 1;

#With reference to FIG. 1, an embodiment of the present invention provides an electroluminescent solid-state device 1, which incorporates a conductive substrate 11, such as an N-type or a P-type silicon wafer with a light emitting multi-layer film structure 20, including one or more active layers with semiconductor nano-particles or other materials with luminescent centers, deposited thereon. The film structure 20 could be deposited by one of many appropriate methods, such as plasma enhanced chemical vapor deposition (PECVD), molecular beam epitaxy, pulsed laser deposition, sputtering, and sol-gel processes. A top optically-transparent, current-injection layer 21, e.g. indium tin oxide (ITO), is mounted on the film structure 20, which, along with a back electrical contact 5, enables AC or DC power to be applied thereto. Preferably, the current-injection layer 21 has a thickness of from 150 to 500 nm. Preferably, the chemical composition and the thickness of the current injection layer 21 are such that the semiconductor structure has a resistivity of less than 70 ohm-cm. A buffer electrical contact 22, e.g. TiN, is positioned between the front current-injection layer 21 and a top electrical contact 23, e.g. Al. The buffer contact 22 provides an ohmic contact point between the front current-injection layer 21 and the top electrical contact 23, while the top electrical contact 23 provides a suitable surface for wire bonding contact. Other suitable materials for the current injection electrodes 21 and buffer electrical contact 22 might alternatively be employed. A back reflector 24 can be provided between the film structure 20 and the substrate 11 to reflect light that is internally emitted towards the substrate 11 back towards the emitting surface, i.e. the ITO layer 21.

#The substrate 11, on which the film structure 20 is formed, is selected so that it is capable of withstanding high temperatures in the order of 1000° C. or more. Examples of suitable substrates include silicon wafers or poly silicon layers, either of which can be n-doped or p-doped, e.g. with 1×1020 to 5×1021 of dopants per cm3, fused silica, zinc oxide layers, quartz, sapphire silicon carbide, or metal substrates. Some of the above substrates can optionally have a thermally grown oxide layer, which oxide layer can be of up to about 2000 nm in thickness, a thickness of 1 to 20 nm being preferred. Some of the above substrates can optionally have a deposited electrically conducting layer, which can have a thickness of between 50 and 2000 nm, but preferably between 100 and 500 nm. The thickness of the substrate is not critical, as long as thermal and mechanical stability is retained.

#The multi-layer film structure 20 can be comprised of a single or of multiple active layers, each layer having an independently selected composition and thickness, e.g. semiconductor (group IV, such as Si, Ge, Sn and Pb) nano-particles in a host matrix, e.g. a dielectric or a wide bandgap semiconductor (group IV, such as Si, Ge, Sn and Pb) oxide or nitride with or without rare earth doping elements and with carbon doping, as will hereinafter described. By using active layers having different compositions, a multi-color structure can be prepared. For example, combining erbium, thulium and europium doped semiconductor nanocrystal layers in a single structure provides a structure that can fluoresce at green (terbium), blue (cerium), and red (europium) or colour combinations thereof. The layers can be either stacked or constructed side by side as separately controllable circuit elements.