I. Introduction
At present, AlGaInP quaternary light-emitting diodes generally use a GaAs substrate. Since the forbidden band width of the GaAs substrate is narrower than that of AlGaInP, the photons emitted downward by the active region will be absorbed, so that the luminous efficiency is greatly reduced. To avoid substrate absorption, a layer of distributed Bragg reflection layer (DBR) is typically added between the substrate and the active layer to reflect light incident on the substrate, reducing GaAs absorption. Since the DBR reflective layer can only effectively reflect light in a small angle (usually qDBR<20°) in the normal direction, most of the other light far away from the normal direction is absorbed by the GaAs substrate, so the effect of improving the light effect is limited. .
In order to improve the luminous efficiency, studies have been started on other substrates instead of GaAs absorbing substrates. One of the methods is to replace the GaAs substrate (TS) with a GaP substrate transparent to visible light, that is, bonding the epitaxial layer structure with a thick GaP window layer on the GaP substrate by bonding technique, and etching away the GaAs liner. At the bottom, the luminous efficiency can be more than doubled, and the transparent characteristics of GaP increase the luminous area. However, this process has the disadvantages of low yield, complicated equipment, and high manufacturing cost. In recent years, Taiwan has begun research on the fabrication of flip-chip AlGaInP red light chips. Because the process is suitable for mass production and the manufacturing cost is low, it has attracted wide interest.
Second, vertical chip structure and process
This paper introduces the AlGaInP red light vertical structure ultra-high brightness LED chip manufacturing method. First, MOCVD epitaxy is performed, and a material having a high thermal conductivity of Si, SiC, or metal is used as a substrate, and an epitaxial layer of the LED is bonded thereon to form a chip. The structure is: firstly, the Au layer is evaporated on the surface of the high thermal conductivity material as a mirror surface and a bonding layer, and the LED epitaxial layer is adhered to the high thermal conductivity substrate material by heat and pressure, and then selectively etched. The method is to peel off the original GaAs substrate, and then form an LED chip with a high thermal conductivity material as a substrate by evaporation, etching, surface roughening and the like. Since the Au film has a very high reflectance for red light and yellow light, and can reflect light of all incident angles (q a 90°), the light extraction efficiency can be improved by nearly three times. Reflectivity calculation and test results of DBR and metal reflective layer (a) Normal reflection spectrum calculated by DBR structure (q=0°) (b) Metal reflection layer reflection spectrum (q=0°~90°)
Third, LED performance test results
Table 1: Comparison of 12mil red LED chip parameters for different substrates with the same active area structure (measured under I=20mA condition)
Table 1 shows the photoelectric performance test results of 12mil dies and different substrate red LED chips. It can be seen from the table that the 20 mA test current, although the mirror substrate LED chip voltage slightly increased from 1.9V to 1.92V (Figure 3), but its brightness increased by 3 times, the main reason is the mirror lining The bottom itself has a high reflectivity and can reflect the light incident at various angles. In addition, the surface roughening effectively reduces the multiple reflection, refraction and absorption of light inside the material, providing more opportunities for the photon to exit; and the mirror substrate LED The increase in voltage is mainly due to the increase in surface roughness of the chip after roughening, which affects the transmission of current, and the bonding layer also increases the voltage.
Fig. 3 Different substrate red LEDs volt-ampere characteristic curve The wafer pattern of 12 mil different substrate LED chips is tested under 20 mA. As shown in Fig. 4, the brightness and wavelength distribution of the mirror substrate red LED are very uniform. As shown in Figure 5, at 20 mA, the 12 mil mirror red LED with a center wavelength of 623 nm has a luminous efficacy of 50 lm/W, and the luminous efficacy is about 2.5 times that of a conventional substrate LED.
Figure 4 Different substrate LED brightness, wavelength wafer map (a) ordinary substrate LED brightness wafer map (b) common substrate LED wavelength wafer map (c) mirror substrate LED brightness wafer map (d) mirror lining Bottom LED wavelength wafer map
Fourth, the conclusion:
The high-intensity AlGaInP red LED chip developed by the project has greatly improved the brightness of the LED chip, and the high thermal conductivity of the substrate material has high mechanical strength and high thermal conductivity, which can greatly improve the high temperature characteristics of the product. To improve product reliability, there are advantages that other types of LEDs can't match in high-power applications. In addition, since the mirror structure does not require a thick GaP window layer and a DBR layer, the epitaxial material consumption can be greatly reduced, and the epitaxial cost is lower than that of a conventional GaAs substrate. The successful development and large-scale production of high thermal conductivity mirror-substrate high-brightness LED is an effective way to obtain low cost and high stability and play an important role in high-power applications.
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