Analysis on the development of vertical structure ultra-high brightness LED chips

1. Introduction

At present, AlGaInP quaternary light-emitting diodes generally use GaAs substrates. Since the bandgap width of GaAs substrates is narrower than that of AlGaInP, the photons emitted downward by the active region will be absorbed, which greatly reduces the luminous efficiency . In order to avoid substrate absorption, a distributed Bragg reflector (DBR) is usually added between the substrate and the active layer to reflect the light directed to the substrate and reduce the absorption of GaAs. Since the DBR reflector can only effectively reflect light within a small angle in the normal direction (usually qDBR < 20°), most of the other light far from the normal incident direction is absorbed by the GaAs substrate, so the effect of improving the light efficiency is limited.

In order to improve the luminous efficiency, people began to study other substrates to replace GaAs absorption substrates. One of the methods is to replace the GaAs substrate (TS) with a GaP substrate that is transparent to visible light, that is, to use bonding technology to bond the epitaxial layer structure with a thick GaP window layer to the GaP substrate, and etch the GaAs substrate. The luminous efficiency can be increased by more than one time, and the transparent characteristics of GaP greatly increase the luminous area. However, this process has the disadvantages of low pass rate, complex equipment and high manufacturing cost. In recent years, Taiwan has begun to study the production of flip-chip substrate AlGaInP red light chips. Because the process is suitable for mass production and has low manufacturing cost, it has aroused widespread interest.

2. Vertical chip structure and process

This article introduces the AlGaInP red light vertical structure ultra- high brightness LED chip manufacturing method carried out by our company. First, MOCVD epitaxy is carried out, and then high thermal conductivity Si, SiC , metal and other materials are used as substrates, and the LED epitaxial layer is bonded on it to make a chip. Its structure is: the process is to first evaporate the Au layer on the surface of the high thermal conductivity material as a reflective mirror and bonding layer, and then heat and pressurize the LED epitaxial layer and the high thermal conductivity substrate material together, and then use the selective etching method to corrode and peel off the original GaAs substrate, and then evaporate, etch, surface roughening and other processes to make an LED chip with a high thermal conductivity material as the substrate . Since the Au film has a very high reflectivity for red light and yellow light, and can reflect light at all incident angles (q a90°), the light output efficiency can be increased by nearly 3 times. Calculation and test results of reflectivity of DBR and metal reflective layer (a) Normal reflection spectrum calculated by DBR structure
(q=0°) (b) Reflection spectrum of metal reflective layer (q=0°~90°)

3. LED performance test results

Table 1: Comparison of parameters of 12mil red LED chips with the same active area structure and different substrates ( measured under I = 20mA )

Table 1 shows the optoelectronic performance test results of red LED chips with different substrates and 12mil die. From the table, we can see that under the 20mA test current, although the voltage of the mirror substrate LED chip has increased slightly from 1.9V to 1.92V (as shown in Figure 3), its brightness has increased by 3 times. The main reason is that the mirror substrate has high reflectivity and can reflect light incident at all angles. In addition, the surface roughening effectively reduces the multiple reflections, refractions and absorption of light inside the material, providing more emission opportunities for photons. The increase in the voltage of the mirror substrate LED is mainly due to the increase in the roughness of the chip surface after roughening, which affects the transmission of current. At the same time, the bonding layer also increases the voltage.

Figure 3 volt-ampere characteristic curve of red LED with different substrates. The wafer diagram of 12mil LED chips with different substrates tested at 20mA is shown in Figure 4. The brightness and wavelength distribution of the red LED with mirror substrate are very uniform. As shown in Figure 5, at 20mA, the luminous efficiency of the 12mil red LED with mirror substrate and central wavelength of 623nm can reach 50lm/W, which is about 2.5 times that of the LED with ordinary substrate.

Figure 4. Brightness and wavelength of LEDs on different substrates (a) Brightness of LEDs on ordinary substrates (b) Brightness of LEDs on ordinary substrates (c) Brightness of LEDs on mirror substrates (d) Brightness of LEDs on mirror substrates

IV. Conclusion:

The metal reflection vertical structure ultra-high brightness AlGaInP red light LED chip developed in this project has greatly improved the brightness of the LED chip. Moreover, due to the high mechanical strength and high thermal conductivity of the high thermal conductivity substrate material, it can greatly improve the high temperature characteristics of the product and improve the product reliability. It has unparalleled advantages over other types of LED in high power applications. In addition, since the mirror structure does not require a thicker GaP window layer and DBR layer, the consumption of epitaxial materials can be greatly reduced, and the epitaxy cost is lower than that of ordinary GaAs substrates. The successful development and large-scale production of high-brightness LEDs with high thermal conductivity mirror substrates is an effective way to achieve low cost, high stability, and can play an important role in high power applications.