Development of AlGaInP red vertical structure ultra high brightness LED chip

Introductionsince

At present, AlGaInP four series of light emitting diode generally use GaAs substrate, GaAs substrate whose band gap width is narrower than the AlGaInP, produced by the active region down the emission of a photon will be absorbed, so that the luminous efficiency is greatly reduced. In order to avoid substrate absorption, a layer of distributed Prague reflector (DBR) is added between the substrate and the active layer to reflect the light to the substrate and reduce the absorption of GaAs. Due to the DBR reflection layer only to the normal direction of small angle (usually less than 20 degrees qDBR) can effectively reflect light, away from other normal incident light are absorbed most of the GaAs substrate, thus enhance light efficiency limited effect.

In order to improve the luminous efficiency, people began to replace the GaAs substrate with other substrates. One way is to use the visible light transparent substrate GaP to replace the GaAs substrate (TS), which uses bonding techniques with adhesive layer thickness GaP window layer on the GaP substrate, and etching the GaAs substrate, the luminous efficiency can be increased more than two times, at the same time the characteristics of transparent GaP luminous area increased. However, the process has the disadvantages of low qualification rate, complex equipment and high manufacturing cost. In recent years, the research on the fabrication of flip chip AlGaInP red chip has been carried out in Taiwan, which is widely used because of its high volume and low cost.

Two, vertical chip structure and process

In this paper, we introduce the method of making AlGaInP red light vertical structure super high brightness LED chip. First of all, MOCVD epitaxy, and then high thermal conductivity of Si, SiC, metal and other materials as a substrate, the LED epitaxial layer is bonded on it and made into chips. The structure is as follows:

In the production process of high thermal conductivity material on the surface of deposited Au layer as a mirror and the adhesive layer, and by heating and pressurizing the LED epitaxial layer and the thermal conductivity of the substrate material stuck together, and then choose the original method of corrosion corrosion of GaAs substrate off after deposition, etching, surface roughening etc. the process is made of high thermal conductivity material for LED chip substrate. The reflectivity of Au films is very high to red, light yellow, and can reflect all incident angles of light (q? 90 degrees), which can be used to improve the efficiency of light close to 3 times.

Reflectivity calculation and test results Figure 2 DBR and metal reflective layer (a) DBR structure calculation method to the reflection spectrum of (q=0 degrees) (b) (q=0 metal reflective layer reflection spectrum of ~90 degrees)

The test results of three LED, performance

Table 1: LED (I = 20mA) measurements for different active substrate structures with different substrate 12mil

Table 1 the test results of the optical properties of the 12mil size core and the different substrate red LED chips. You can see from the table 20mA test current, although the mirror substrate LED chip voltage slightly improved, from 1.9V to 1.92V (Figure 3), but its brightness increased by 3 times, the main reason is that since the height of the mirror substrate reflectivity and reflection angles of incident light, the surface roughening is weakened effectively the light in the material internal reflection, refraction and absorption, to provide more opportunities for photon emitted; and the mirror substrate LED voltage increase is mainly due to the coarsening after chip surface roughness increases, the impact of the current transmission at the same time, the adhesive layer also make voltage increased.

Figure 3 voltammetric characteristics of different substrate red LED curve

20mA under the test 12mil different substrate LED chip wafer figure as shown in Figure 4, mirror substrate red LED brightness, wavelength distribution is very uniform. As shown in Figure 5, 20mA current, the central wavelength of 12mil mirror substrate LED red light efficiency of up to 50lm/W, light effect is about 2.5 times more than the ordinary substrate LED.

Figure 4 different substrate LED brightness and wavelength of wafer map (a) common substrate brightness of LED wafer map (b) common substrate LED wavelength wafer map (c) mirror substrate LED brightness wafer map (d) mirror substrate LED wavelength wafer map

Figure 5 12mil red LED light curves of different substrates

Four. Conclusion:

The vertical structure of the metal reflector project research and development of high brightness AlGaInP light LED chip greatly improves the brightness of the LED chip, and the thermal conductivity of the substrate material with high mechanical strength, high thermal conductivity, high temperature characteristics can greatly improve the product, improve the reliability of products in high power applications there are other types of LED can not be compared the advantage of. In addition, due to the need of the thick GaP window layer and the DBR layer, the epitaxial material consumption can be greatly reduced, and the epitaxial cost is lower than that of the conventional GaAs substrate. The successful development and large-scale production of high brightness specular substrate LED is an effective way to obtain low cost and high stability, which can play an important role in high power applications.