Light emitting device with a stopper layer structure

#This invention relates to light emitting devices, and more particularly to improvements in electroluminescent device structures relating to reducing hot electron related effects, such as charging effects, and improving charge to breakdown and reliability of TCO electrode structures, with particular application for light emitters for solid state lighting.

#The generation of light from electroluminescent solid state light emitting devices (EL devices), as described in this application, is based on applying energy from an electric field to a light emitting structure including an active region or emissive layer. The emissive layer may comprise a wide band gap semiconductor or dielectric e.g. silicon nitride, silicon dioxide, or GaN, which may include luminescent centres, such as nanocrystals and/or rare earth species. It is important to deliver a minimum and controlled amount of energy to luminescent centres in an active light emitting layer in the device. If the energy of incident electrons is too low there will be no light emission possible. On the other hand, if the electrons possess too much energy there will be light emission but excess energy will be carried away in the form of heat, which reduces efficiency. Furthermore, hot electrons can be responsible for damage to the host matrix, result in charging, and ultimately contribute to breakdown and failure of the device under bias.

#For example, as described in pending United States Patent Publication No. 2008/0093608 filed Dec. 12, 2006, entitled “Engineered structure for solid state light emitters”, a device is described comprising a light emitting structure having a plurality of emissive layers, i.e. optically active layers comprising luminescent centres, that are separated by drift layers. The light emitting structure is disposed between electrodes for applying an electric field to the light emitting structure. One electrode, usually a top electrode, comprises a transparent conductive oxide (TCO), typically a layer of indium tin oxide (ITO) or other suitable transparent conductive material, which not only provides for electron injection, but also allows light to be extracted from the EL device. Electrons are accelerated and gain energy from an applied electric field as they traverse each drift layer, and energy is released as light from luminescent centres in the active layer by impact ionisation or impact excitation. The drift layers may alternatively be referred to as drift regions, buffer layers or acceleration layers.

#FIG. 1 shows an emission spectrum of a device having a TCO electrode comprising ITO, observed to be failing while under bias;

#FIG. 2 shows a schematic representation of an electroluminescent device comprising a multilayer emitter structure;

#FIG. 2A shows electron energy as a function of electron travel;

#A simplified schematic of elements of an electroluminescent device structure, similar to that described in copending United States Patent Publication No. 2008/0093608, are shown in FIG. 2. The light emitting device 10 comprises an electroluminescent emitter structure 20 disposed between electrode layers 21 and 25 for a applying an electric field to the emitter structure 20 (EL structure) for exciting luminescent centres. The EL structure 20 comprises a multilayer stack of at least one layer pair, and typically a plurality of layer pairs 22 each comprising an emissive (i.e. active) layer 12 and a corresponding drift (i.e. buffer) layer 13 adjacent the respective active layer 12, formed on a substrate 11. The emissive layers 12 comprise a concentration of luminescent centres, for example rare earth species and/or semiconductor nanoparticles, within a host matrix of a wideband gap semiconductor material or dielectric, for emitting light at a desired wavelength or range of wavelengths. The drift layers 13 comprises a wide band gap semiconductor or a dielectric material, and are of a specific thickness related to an excitation energy e of the emissive layers 12, so that electrons traversing a drift layer in an applied electric field gain sufficient energy from the electric field to excite the respective emissive layer 12 to emit light at the appropriate wavelength. Preferably, the kinetic energy in electron volts gained by an electron passing through the drift region is E×d where E is the electric field across a drift layer in V/cm and d is the thickness of each drift layer 13 is matched to a desired excitation energy e of the respective emissive layer 12 (i.e. E×d≧e). As described in the above mentioned patent application, the emitter structure 20 may comprise a plurality of layer pairs, in which all active layers 12 may emit light of the same wavelength. Alternatively, individual active layers 12 or groups or sets of active layers 12 may each emit light at a plurality of different wavelengths, which are combined to provide visible light having a desired spectrum, or CRI. The composition and thickness of each active layer 12 and each respective drift layer 13 is selected accordingly (although for simplicity in the schematics shown in FIG. 2 and other figures, the layers may be shown schematically with similar thicknesses).

#Referring to FIGS. 2 and 2A, the source of hot electrons can be explained. Analysis of device performance indicated that only a small percentage (˜1%) of incident electrons were being used effectively and transferring their energy to the optical centres (i.e. luminescent centres) in the emissive layers. In a device having an N+ substrate and operated with a DC bias as illustrated in FIG. 2, electrons gain energy from the applied electric field in the first drift layer 13a as they travel from the N+ substrate 11 towards the top TCO electrode 21. As electrons pass through the next emissive layer 12a, a small percentage of electrons will interact with optical centres and cause light emission, but other electrons will miss optical centres in this layer 12a and therefore lose very little energy, and as they enter a second drift layer 13b, they will continue to gain additional energy from the electric field. This results in a cascade of energy gain as electrons pass through subsequent drift layers in a multilayer structure as shown schematically in FIG. 2A. Hot electrons will quickly acquire enough energy to damage the drift or emissive regions leading to the observed charging and ultimately failure of the device under bias. Some electrons may have gained energies in excess of 3 eV on reaching the TCO electrode layer 21. As mentioned above, hot electrons may lead to deleterious effects such as charging, or breakdown within layers of emitter structure, or breakdown and decomposition of the TCO layer. In order to overcome the issue of damage and disassociation of the TCO from hot electrons, some means of cooling the incident electrons and reducing the electric field is required.

#A light emitting device structure 100 comprising a stopper layer structure, according to embodiments of the present invention, may be embodied as a device structure such as that shown schematically in FIG. 3. The light emitting device structure 100 comprises a substrate 11, which is typically N-type or P-type silicon, or other suitable conductive layer on a substrate of arbitrary conductivity to which electrical connections may be made. Thick field oxide (FOX) regions 28 are formed on the substrate 11 to define a device well region 27. The electroluminescent emitter layer structure (EL structure) 20 is deposited thereon, and a layer of a transparent conductive oxide (TCO layer) 21 formed thereon. Top contacts 23 to the TCO layer 21 are provide over the field oxide regions 28, that is a barrier layer 22 is provided on top of the TCO layer 21 and contacts 23 are defined thereon in a region over the field oxide regions. Contacts 23 are placed on top of the field oxide regions 28 so as not to block light emission from the emitter structure 20. Also, the FOX regions 28 provide a barrier to vertical current flow and confine current flow to the device well region 27. Thus the optically active region of the emitter structure 20 is confined within the device well region 27 between the FOX regions 28.