Luminescent materials for white light LEDs

Phosphorous materials for white light LEDs

1. Phosphors
can convert various incident energies into photons for output. Materials with this function are called fluorescent. The incident forms include electron beams, radiation (α, γ...) rays, etc. Generally,
the photons emitted are visible light, which can be directly felt by the human eye. Materials with this function are called phosphors. There are about 30 types of materials that can be used in the UV light band for phosphors; the matrix of the materials includes halides, sulfides, oxides, etc.

2. Phosphorous Theory
In order to produce phosphors with high luminous brightness (efficiency), it is necessary to study the substances that can emit light in the crystals. This part is called the luminescence center. There are many different materials for the luminescence center. The most common one is made of yellowish metal ions diffused in the matrix. In order to make the luminescence center in the matrix work, sometimes an activator needs to be added. Even in order to make the activator more effective, other co-activators need to be added. For example, in the zinc silicate matrix, when the Mn activator is doped, it will be excited by ultraviolet rays or electron beams, and a bright green color will be stimulated. In general phosphors, due to the different materials of the matrix and the activator, the light waves emitted will also be different, especially the activator has the greatest influence. To be a substance that acts as an active agent, a specific atomic structure is required, in which the electrons themselves have energy. When the external energy excites the electrons in the ground state to the excited state, the excited electrons become unstable
free electrons. When the electrons are in a high energy level, they are active free electrons that will release energy. In the process of jumping from the high energy level to the ground state, energy will be released. At this time, the light waves released are photons, but the energy released will vary depending on the energy of the photons themselves, and the distance of the transition will make the light waves emitted different. The difference in light waves will also make the energy different, and the photons released at this time are a phenomenon of natural excitation; this can be proved by the transition mechanism theory of energy level spectrum splitting in modern physics, as shown in (Figure 4).

Figure 4: II-VI Group: Cu, Al energy transition diagram.
The phosphors used in LED epitaxy chips have the characteristics of absorption spectrum. LED epitaxy chips will emit multi-wavelength light waves, and the phosphor coating will absorb the light waves emitted by the LED first and then convert them into white light through the material. The luminous color of the phosphor is determined by the width of the forbidden band and the energy released when the electron-hole pairs are generated. The latter changes with the different active ions.
Doping different materials in II-VI group materials will produce different luminous light sources. These phosphors are excited by high-energy photons and emit light. At the same time, it is proved that the matrix crystal is covered on the active agent metal ions, and the active agent is turned into an idealized inorganic crystal in the luminescence mechanism.

Figure 5: Epitaxial growth, testing, and packaging process flow chart

3. II-VI Compound Semiconductor
Since II-VI compound semiconductors have strong ionic bonds, materials with a larger forbidden band Eg than those of IV or III-V can be used as photoconductive components in the visible light region, while materials with a smaller forbidden band can be used as infrared light detectors. On the other hand, when a III-group dopant is substituted at the II position of a II-VI compound semiconductor, or a VII-group dopant is substituted at the VI position, it can become a donor. On the contrary, when a I-group dopant is substituted at the II position, or a V-group dopant is substituted at the VI position, it can become an acceptor. N-type and P-type semiconductors can be made. Therefore, it is not easy to control the production of N-type and P-type II-VI semiconductors by impurity doping. Therefore, is II-VI semiconductor N-type or P-type? It is determined by the composition of its constituent elements and which carrier is easy to move.

IV. II-VI Compound Phosphorous Manufacture
1. II-VI Materials: The fluorescent properties of II-VI and the concentration of the active agent used
are less than those of other phosphors. Usually, the concentration of the active agent used in phosphors is several moles, while the concentration of II-VI is
0.01 mole. It is extremely sensitive to the purity of impurities, especially transition metal materials, which
will react at a level of 0.1ppm. Therefore, the crystallization of the matrix is ​​very simple, and a very high-purity
phosphor is required.
2. Manufacturing Process
○1 Mixing (mixing of activator and flux): For the refined 1 mole II-VI phosphor, the amount of flux is 0.1 mole and the heavy metal activator is added at 10-3 to 10-5 mole, and mixed and stirred.
○2 Drying: Put it in a quartz crucible and heat it to dry it. (Remove moisture)
○3 Firing: After drying, place the II-VI group phosphor in an electric furnace and heat it until it reaches about 900℃-1000℃, keep the temperature constant for about 60 minutes, and then cool it down.
○4 Cleaning: After cooling, clean it with dilute acid, then with water, and then dry it.
○5 Crystal dispersion: Grind it with a mortar, fineness: 300mesh, and separate it with a sieve.
○6 Surface treatment: Clean it with dilute acid, then with water, and then with steam water.
○7 Drying: Heat and dry it again. ○8 Sieve separation: Use a sieve to separate it again.
○9 Forming the phosphor: Stir the phosphor and PVA-Cr into a slurry.