A combination of red, green and blue light emitting diodes (LEDs) can produce white light. However, it has been noted that current white LEDs rarely use the above principle. “White” LEDs in production currently are typically modified blue LEDs i.e. Gallium Nitride (GaN)-based with Indium Gallium Nitride (InGaN)-active-layer LEDs that emit blue light having wavelengths between about 450 nm and 470 nm. Typically, the InGaN—GaN structure is covered with a yellowish phosphor coating comprising cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. Such a LED chip emits blue light, part of which is efficiently converted to a broad-spectrum centered at about 580 nm (yellow light emission) by the Ce3+:YAG material. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white light, the resulting shade typically referred to as “lunar white”. The pale yellow emission of the Ce3+::YAG material can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium, and can be further adjusted by substituting some, or all, of the aluminum in the YAG with gallium.
Due to the spectral characteristics of such LEDs, one problem that can arise is that the red and green colours of objects illuminated in the blue yellow light are not as vivid as in broad-spectrum light. Further, manufacturing variations and varying thickness in the phosphor coating typically cause LEDs to produce light with different colour temperatures, e.g. ranging from yellow to cold blue. As such, disadvantageously, the resulting LEDs are required to be sorted by their actual characteristics during manufacture.
As an alternative to the above, white LEDs can be made by coating near ultraviolet emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors with green emitting copper and aluminum doped zinc sulfide. This method is analogous to how typical fluorescent lamps work. However, one problem is that the ultraviolet light typically causes photo-degradation to the epoxy resin of the LEDs and other materials used in LED packaging. The photo-degradation typically leads to shorter lifetimes. Further, this method is typically less energy efficient than the blue LED with YAG:Ce phosphor coating type discussed above, since the Stokes shift of this method is larger, and thus, more energy is required to be converted to heat, even if light is produced under this method with better spectral characteristics and colour rendering.
In addition to the above, existing commercial white LEDs are typically expensive and typically utilize blue LEDs coated with a layer of phosphor which results in higher production costs. Furthermore, phosphor coating typically lowers the colour rendering index of the LEDs.
Therefore, there exists a need for a light emitting diode structure and a method of forming a light emitting diode structure that seek to address at least one of the above problems.