Detailed explanation: Ultra-high brightness 4-element red LED characteristics

Light-emitting diodes ( LEDs ) can convert electricity into direct light, so they are classified as semiconductor solid-state components.

LEDs fully utilize the advantages of not using color filters, being able to directly extract light of a specific color, and low-voltage driving . They are now widely used as display light sources for various electronic devices and are applied in a variety of fields.

In recent years, the luminous efficiency of LEDs has been improved, and with the practical application of blue and green LEDs, as shown in Figure 1, LEDs have become the main light source for traffic lights, rear combination lamps, backlight modules for LCDs , and various displays and lighting, and their application range continues to expand.

Next, this article will introduce the method of improving the luminous efficiency of the AlGaInP 4-element red light-emitting diode, which can reduce the light loss of the light-emitting diode substrate, has a metal reflective film layer, high brightness, and a luminous efficiency of 48 lm/W, which is 4 times that of traditional structure light-emitting diodes, as well as the electrical, optical , and other characteristics of the metal reflective film light-emitting diode (MR-LED).

Development History

In addition to AlGaAs epitaxial wafers, AlGaInP epitaxial wafers have been commercialized for traditional red light emitting diodes.

If electrodes are made on the surface of AlGaInP epitaxial wafers and then cut into dies, light emitting diode chips can be made. However, traditional structured light emitting diodes are affected by the bottom substrate, and the light absorption loss is very large. It is generally believed that the luminous efficiency of 12 lm/W is the maximum limit of red light emitting diodes.

In view of this, the researchers set up a metal reflector (MR) inside the LED component and developed a new structure of red light emitting diodes, achieving a luminous efficiency of 48 lm/W, which is 4 times higher than the traditional structure.

Methods for improving the luminous efficiency of metal reflective film LEDs

As shown in Figure 2(a), conventional light-emitting diode light sources use the energy obtained by the recombination of electrons injected into the luminescent material (luminescent layer) of semiconductor solid-state components and positive holes to generate light. The electrical-to-light conversion efficiency can reach more than 70% for low-defect AlGaInP crystals, and the improvement in material properties can be considered quite sufficient.

As shown in Figure 2(a), the light generated by the chip is transmitted inside the semiconductor and then taken out to the outside of the component through the surface of the LED component. The light extraction efficiency of a simple red LED structure is only about 10%. In order to effectively improve the luminous efficiency of red LEDs, it is necessary to improve the structure design and process of the LED to increase the surface transmittance and interface reflectivity.

Red light emitting diodes are made on a GaAs single crystal substrate, using a lattice-integrated 3-element mixed crystal AlGaAs or AlGaInP 4-element mixed crystal light-emitting layer, using the GaAs single crystal substrate as a light-emitting component and the bottom supporting substrate. Since GaAs has the property of absorbing red light, it is also called the Absorbing Substrate Type (AS Type: Absorbing Substrate Type).

As shown in Figure 3(a), when the AS Type substrate was first developed, the light-emitting layer was made on top of the GaAs substrate. Due to the component surface of this structure, the reflected light and the light toward the substrate side are all absorbed by the substrate, so only a low electrical-to-light conversion efficiency of 8 lm/W can be achieved.

Although the main purpose of developing a substrate is to improve the luminous efficiency, as shown in Figure 3(b), the substrate type is basically a semiconductor multilayer reflector (DBR: Distributed Bragg Reflector) inserted type. This structure uses a semiconductor multilayer reflector to reflect light toward the substrate side, achieving an electrical light conversion efficiency of 12 lm/W.

However, the semiconductor multilayer reflector (DBR) has a structural defect that it is difficult to reflect light in an oblique direction. Therefore, the light from the LED radiated in all directions will not be transmitted to the substrate side, and the structure is greatly limited.

In order to improve the luminous efficiency of red light emitting diodes, researchers conducted in-depth research on structures that can completely reflect light incident obliquely to the substrate, and developed a metal reflective film light emitting diode (MR-LED) as shown in Figure 3(c). The reflective structure uses a metal film and has high reflective properties for light incident obliquely in addition to the vertical direction.

It is not easy for metal reflective film light emitting diodes (MR-LEDs) to have the characteristics of light-emitting layer, metal reflective film reflectivity and low electrical impedance at the same time, and it is impossible to make a low-defect light-emitting layer on the metal reflective film. Therefore, researchers have taken countermeasures through the design of component structure to achieve the problem of both reflectivity and low electrical impedance. The defect problem of the light-emitting layer is solved through substrate replacement technology, using a low-defect AlGaInP light-emitting layer of the same grade as that on a GaAs single crystal substrate.

The substrate replacement technology is shown in Figure 4. (1) First, prepare a low-defect AlGaInP epitaxial silicon wafer, (2) then adhere the light-emitting layer to the bottom supporting substrate, and (3) finally remove the GaAs substrate from the already bonded wafer, and a structure with a light-emitting layer can be formed on the bottom supporting substrate.

Initial characteristics

Regarding the characteristics of the metal reflective film type LED (MR Type LED) and the substrate type LED (AS Type LED), the appearance of both chips is 300×300μm prism.

Figure 5 is a photo of the actual light emission of these two types of LEDs. From the photo, it can be seen that the metal reflective film type LED emits brighter than the substrate type LED.

As shown in the comparison of initial characteristics of metal reflective film type light emitting diodes (MR Type LED) and substrate type light emitting diodes (AS Type LED) in Table 1, the light beam is 1.92 lm when the forward current is 20mA, and the luminous efficiency of 48 lm/W can be achieved. The luminous efficiency of metal reflective film type light emitting diodes (MR Type) is more than 4 times that of substrate type light emitting diodes (AS Type).

FIG6 shows the forward current-beam characteristics of the two red LEDs. As can be seen from the figure, the light intensity, i.e., the beam, increases linearly with respect to the current. Even under the influence of heat, the light output does not decrease.

FIG7 shows the forward current-forward voltage characteristics of the two red LEDs. As can be seen from the figure, although the forward voltage of the metal reflective film type light-emitting diode (MR Type) is higher than that of the substrate type light-emitting diode (AS Type), it can still achieve the practical requirement of a forward voltage below 2.2V (IF=20mA), proving that even with a metal reflective film structure, the series impedance can be sufficiently reduced.

Distribution within the wafer surface

When LEDs are used side by side for display purposes, uneven distribution of light intensity will cause uneven brightness.

In addition, constant voltage driving requires the same forward voltage, so researchers investigated the distribution characteristics of the light beam and forward voltage within the surface of a 3-inch metal reflective film type LED (MR Type) wafer.

The in-plane distribution characteristics of the light beam are shown in Figure 8. The in-plane average is 1.86 lm (σ=0.05 lm), and the in-plane distribution is within the range of ±10%, which proves that the new red light emitting diode can achieve the requirements of high output and high uniformity.

The forward voltage distribution is shown in Figure 9. The average in-plane voltage is 1.98 lm (σ=0.02V), and the in-plane distribution is within the range of ±5%, which proves that the new red light emitting diode can also achieve the requirements of low voltage and high uniformity.

According to the above-mentioned in-plane distribution characteristics, it is almost exactly the same as the wafer distribution of the AS Type light-emitting diode, which proves that the distribution characteristics will not be significantly changed by the process of the metal reflective film structure.

Continuous power-on characteristics

Compared with substrate-based LEDs (AS Type), metal reflective film type LEDs (MR Type) are more susceptible to heat and pressure loads when manufacturing substrate-mounted structures, which may have adverse effects on the reliability of the components. Reliability tests are required to confirm this. Figure 10 shows the light beam, forward voltage, and reverse voltage test results

of a one-week continuous power-on characteristic change test at room temperature with IF=50mA .

The test results show that the characteristics of the two new types of red light emitting diodes have not changed at all, proving that sufficient reliability can be achieved even if a metal reflective film structure is used.

Conclusion

The above introduction uses a metal reflective film (MR) structure and a new type of high luminous efficiency AlGaInP 4-element red light emitting diode.

In order to improve the light extraction efficiency, researchers have developed the structural design and manufacturing technology of metal reflective film type light emitting diode (MR-LED), achieving an ultra-high luminous efficiency of 48 lm/W.

LEDs with ultra-high luminous efficiency can be used for outdoor identification purposes where very distinct color mixing is required, such as large outdoor displays that require vivid displays, or car tail combination lights, etc.

In addition, LED backlight modules for large-scale LCD displays, which have been slow to become practical due to insufficient luminous efficiency, are also areas where the new red light-emitting diodes can fully demonstrate their characteristics.