This application claims priority to EP Application No. EP 19177581.6 filed on May 31, 2019, the content of which is in corporated herein in its entirety.

The present invention generally relates to the field of manu facturing of light-emitting diode chips .

More particularly, the present invention relates to increasing the light generating efficiency of aluminum gallium indium phos phide LEDs .

It is well known that the light generating efficiency of alu minum gallium indium phosphide (AlGalnP) LEDs is limited due to the non-radiant recombination of electron-hole pairs at the LEDs' mesa edge. This problem is particularly salient for very small AlGalnP-LEDs such as those used for high resolution mon itors and screens. Indeed, as the size of the AlGalnP-LEDs decreases, their circumference to surface ratio increases, which in turn increases the relative proportion of non-radiant recom bination at the mesa edge.

One known approach to solving this problem is to diffuse Zinc into the LED's mesa edge. This Zink diffusion leads to so-called quantum well intermixing, meaning that the bandgap of the op tically active material in the LED's mesa edge is increased. This in turn means that less electron-hole pairs can reach the mesa edge. Accordingly, the electron-hole pairs are confined to the LED's center and can recombine optically to generate light.

However, the drawback of diffusing Zinc is an increase in the non-radiant effects in the LED's center. There is thus a need for a different method of increasing the light generating efficiency of small AlGalnP-LEDs .

According to the present disclosure, this problem is solved by a method of treating a semiconductor wafer comprising a set of Aluminum Gallium Indium Phosphide light emitting diodes or Al- GalnP-LEDs to increase the light generating efficiency of the AlGalnP-LEDs,

wherein each ALGalnP-LED includes a core active layer for light generation sandwiched between two outer layers, the core active layer having a central light generating area and a peripheral edge surrounding the central light generating area,

the method comprising the step of treating the peripheral edge of the core active layer of each AlGalnP-LED with a laser beam, thus increasing the minimum band gap in each peripheral edge to such an extent that, during later operation of the AlGalnP-LED, the electron-hole recombination is essentially confined to the central light generating area.

By treating the peripheral edges of the core active layers of the AlGalnP-LEDs with a laser beam, non-radiant electron-hole recombination at the LEDs' edges is effectively suppressed. This increases the LEDs' light generating efficiency.

According to one embodiment, the laser beam treatment may in volve scanning the wafer with the laser beam according to a predefined pattern.

The photon energy of the laser beam may be higher than the minimum band gap of the core active layer and lower than the band gaps of the two outer layers such that, during the laser beam treatment, the laser beam energy is primarily transferred to the core active layer's peripheral edge.

Each AlGalnP-LED may be a red light LED. The wavelength of the laser beam may in particular be in the range of 550 to 640 nm.

The laser beam may have a Gaussian shape/profile.

The laser beam may be generated by a pulsed laser.

Prior to the laser beam treatment, the wafer may be heated to a background temperature to reduce the power requirements of the laser beam treatment.

The laser beam power density may be between 0.1 and 100 mJ/mm2, and preferably between 1 and 10 mJ/ mm2.

After the laser beam treatment, the wafer may be etched, thus obtaining, for each AlGalnP-LED, a chip preform, wherein, pref erably, after the etching, the wafer is diced into individual AlGalnP-LED chips, e.g. by laser cutting.

The duration of the laser beam treatment may be between Is and lOmin and preferably between 10s and 2min.

The present disclosure will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a semiconductor wafer prior to being subjected to the method of the present disclo sure ;

Figure 2 is a cross-sectional view according to the arrows II of Figure 1 of one AlGalnP-LED, which is part of the wafer of Figure 1;

Figure 3 is a flow diagram showing the steps of obtaining in dividual AlGalnP-LED chips, starting from the wafer shown in Figure 1, and involving the process of the present disclosure; and

Figure 4 shows the effects of the method of the present disclo sure on the bandgap shape of the peripheral edge of the core active layer of an AlGalnP-LED.

With reference to Figure 1, there is shown a semiconductor wafer 10 comprising a set of aluminum gallium indium phosphide light- emitting diodes (AlGalnP-LEDs ) 12. In the example of Figure 1, there are twenty LEDs 12 arranged in the wafer 10. Each LED is a PN junction, which has been formed in the wafer 10 according to well-known methods.

Figure 2 is a cross section of one of the LEDs 12. Each LED 12 includes a core active layer 14 for light generation sandwiched between two outer layers 16 and 18. The core active layer 14 has a central light generating area 20. This area 20 is iden tified by the dotted region. The central light generating area 20 is surrounded by a peripheral edge 22.

With reference to the left part of Figure 4, we will now describe the bandgap structure of the LED 12 shown in Figure 2. The bandgap diagram in Figure 4 shows the bandgap as a function of the depth T into the LED 12, see Figure 2. The upper outer layer 16 of the LED 12 is a P-doped layer. The bandgap shape of this P-type doped layer 16 is a P-ramp 24, followed by a P-setback 26. The bandgap of the P-setback 26 is denoted by BG1. The bandgap shape of the active layer 14 is, along the depth T, a series of quantum wells Q separated by barriers B. In the ex ample shown in Figure 4, active layer 14 has two quantum wells Q separated by one barrier B. The quantum wells Q define a bandgap BG2 , which is smaller than the bandgap BG1. The bandgap of the quantum wells Q is the minimum bandgap of the core active layer 14. The lower outer layer 18 has a bandgap shape compa rable to the one of the upper outer layer 16, with a setback 28, and ramp 30. However, the lower outer layer 18 is N-type doped .

Turning now to Figure 3, individual LED chips 38 with increased light generating efficiency are obtained from the wafer 10 as follows :

In a first step a) , a hatched zone Z of the wafer 10 is treated with a laser beam L, as depicted in the small process drawing located right to the flow diagram's first arrow 100. The laser beam treatment involves scanning wafer 10 according to a pre defined pattern. More precisely, the laser beam L scans the surface of the wafer 10, which is not taken up by the LEDs 12, and, on top of that, the peripheral edges 22 of the core active layers 14 of the LEDs 12. Different scanning patterns are pos sible, as long as the pattern involves the scanning of the peripheral edges 22 of the core active layers 14 of the LEDs 12.

The photon energy of the laser beam L is higher than the minimum bandgap BG2 of the core active layer 14 and lower than the bandgap BG1 of the two outer layers 16 and 18. Hence, during the laser beam treatment, the laser beam energy is primarily transferred to the core active layer' s peripheral edge 22.

The laser beam' s wavelength is chosen in particular such that only the quantum wells Q, the barriers Band the setbacks 26, 28 are optically stimulated, as illustrated by the arrows E in Figure 4. For example, the wavelength of the laser beam may be chosen anywhere within the range of 550 to 640 nm.

Furthermore, the shape or profile of the laser beam may in particular correspond to a Gaussian profile.

The laser beam may be a pulsed laser, such as a nano- pico- or femtosecond laser. The power density of the laser beam may be between 0.1 and 100 mJ per mm ² , and preferably between 1 and 10 mJ per mm ² . The overall duration of the wafer's laser beam treatment may be between 1 second and 10 minutes, and preferably between 10 seconds and 2 minutes.

The effect of the laser beam treatment on the bandgap structure is shown on the right hand side of Figure 4. It is apparent that the laser beam L, by locally heating the treated area, effec tively destroys the quantum wells Q. Hence, after the laser beam treatment, the peripheral edges 22 no longer have any core active layer 14. The laser beam treatment results in a mixing of the quantum well material with the barrier material (so- called quantum well intermixing), which increases the bandgap.

After the laser beam treatment 100, the semiconductor wafer 10 may be etched (so-called "Mesa etching"), thus obtaining for each LED 12, a chip preform 32. The etching step 102 is iden tified by the letter b) in Figure 3. The etched parts of the wafer 10 are highlighted by a crosshatch pattern Y. Each chip preform 32 has a central zone 34 and a peripheral boundary 36. The peripheral boundary 36 has been laser treated and thus lacks any core active layer 14. In contrast thereto, the central zone 34 still has a core active layer 14.

In order to obtain individual LED chips 38, wafer 10 is diced, e.g. by laser cutting, as shown in step 104 of Figure 3.

The laser treatment step 100 may optionally be preceded by a step of heating the semiconductor wafer 10 to a background temperature to reduce the power requirements of the laser beam treatment .

Thanks to the laser beam treatment of the present disclosure, during operation of the LED chips 38, the electron-hole recom bination is essentially confined to the central light generating area 20. This is because of the increased bandgap in the pe ripheral edges 22, which prevents electron-hole pairs from en tering the same . The laser treatment method of the present disclosure is espe cially useful for very small red LEDs, which are e.g. used as part of high resolution monitors and displays .

LIST OF REFERENCE SIGN

10 Semiconductor wafer

12 Light Emitting Diode (LED)

14 Core active layer

16 Outer layer

18 Outer layer

20 Central light generating area

22 Peripheral edge

24 P-ramp

26 P-setback

28 Setback

30 Ramp

32 Chip preform

34 Central zone

36 Peripheral boundary

38 LED-chip

100 Laser beam treatment step

102 Etching step

104 Dicing step

B Barrier

BG1 Bandgap

BG2 Bandgap

E Optical stimulation

L Laser beam

Q Quantum well

T Depth

Y Crosshatch pattern

Z Hatched zone