Early LED component development focused on improving its internal quantum efficiency. The main method is to improve the quality of the chip and change the structure of the chip, so that the electrical energy is not easily converted into thermal energy, thereby indirectly improving the luminous efficiency of the LED, and the theory of about 70% can be obtained. Internal quantum efficiency.
With the development of epitaxial growth technology and multi-quantum well structure, the internal quantum efficiency of ultra-high brightness LEDs has been greatly improved. For example, the internal quantum efficiency of wavelength 625 nm AlGalnP-based ultra-high brightness LEDs can reach 100%. Close to the limit.
The photoelectric conversion efficiency of the semiconductor material itself is far higher than other illuminating light sources, so now increasing the external quantum efficiency of the chip is the key to improving the luminous efficiency. The main technical approaches and developments adopted at home and abroad are as follows:
1, the technology to change the shape of the chip
Krarnes et al. used a special tool to make the AlInGaP red LED tabletop into a flat-bottomed inverted pyramid chip, bonded to a transparent substrate, achieving an external quantum efficiency of more than 50%. The TIP structure reduces the transmission distance of light in the crystal, reduces the optical loss caused by internal reflection and absorption (active region absorption and free interceptor absorption, etc.), and greatly improves the chip characteristics, and the luminous efficiency reaches 100 lm/W (100 mA). , 610nm), the external quantum efficiency is up to 55% (650nm), while the face-down flip-chip structure makes the PN junction closer to the heat sink, improving the heat dissipation characteristics and improving the chip life. For conventional positive-load GaN-based LEDs, the chip can be etched 23 using wet etching. The sidewall tilt angle is used to increase the light extraction efficiency, and this method has now been mass-produced. The light emitted by the active layer of the LED is omnidirectional, and part of the light is emitted in the horizontal direction due to the relationship of refraction or reflection. This part of the light only increases the divergence of the light and the luminous efficiency of the element. Without much help, the light should be emitted more from the front. In GaN-based LEDs, according to Snell’s law <img alt=” teach you how to improve the luminous efficiency of LEDs”, that is, the critical angle of refraction is about 23 by the formula. Therefore, H3P04 and H2S04 mixed solution can be used and wet etching can be used to form a sidewall angle of about 23* with the vertical side at a certain temperature to change the light transmission direction and cause light to be emitted from the front side. The brightness is more concentrated and the brightness is improved. In this way, the total side reflection probability of the light can be increased, and the result is shown in FIG. This allows the brightness of the device to be more concentrated and the brightness to be improved.
“Teach you how to improve the luminous efficiency of LEDs”
2, flip chip technology
The GaN-based diode epitaxial wafer is generally grown on an insulating sapphire substrate, and the ohmic contact P electrode and N electrode can only be prepared on the same side of the epitaxial surface, and the light portion emitted from the front surface is absorbed by the contact electrode and shielded by the bonding wire. . The main cause of light absorption is that the conductivity of the P-type GaN layer is low. To meet the requirements of current spreading, the thickness of the translucent Ni-Au ohmic contact layer covering most of the surface of the epitaxial layer should be greater than 5-10 nm, but To minimize light absorption, the thickness of the Ni-Au ohmic contact layer must be very thin, so that a proper compromise should be given between the transmittance and the extended resistivity. The result of the compromise design must increase the power conversion. Limited. In 2001, Lumileds reported the application of flip-chip bonding technology on high-power AlnGaN-based chips, which avoids the influence of electrode pads and leads on light extraction efficiency, improves current diffusivity and heat dissipation, and the preparation of back-reflective film will be transmitted. The light below reflects back to the sapphire side of the light, further improving the light efficiency, external quantum efficiency of 21%, power conversion efficiency of 20% (20mA, 435nm), maximum power up to 400mW (drive current 1A, 435nm, chip size lmm* Lmm), its overall luminous efficiency increased by 1.6 times than the formal wear.
3. Growth distribution Bragg reflection layer (DBR) structure
A GaN-based chip with a DBR layer is grown by an epitaxial technique. The DBR is a layered structure in which two materials having different refractive indices are alternately grown periodically, and between the active layer and the substrate, the light that is incident on the substrate can be reflected back. The surface or the side can reduce the absorption of light by the substrate and improve the light extraction efficiency. However, since the DBR reflectivity decreases rapidly with the increase of the incident angle, the omnidirectional average still has a high optical loss, and the reflective film is not efficient.
4, surface roughening technology
The surface roughening mainly changes the direction of the light that satisfies the law of total reflection, and then passes through the interface without being totally reflected on the other surface or reflected back to the original surface, and can function as an anti-reflection. Such a method was first proposed by Nichia Chemical Co., Ltd. The roughening method basically forms a regular concave-convex shape on the geometry of the component, and the structure of the regular distribution is divided into two forms according to the position. One is to provide a concave-convex shape in the assembly, and the other is to make a regular concave-convex shape above the assembly and a reflective layer on the back of the assembly. Since the uneven shape can be provided at the interface of the GaN-based compound semiconductor layer by using a conventional process, the above-described first mode has high practicability. At present, if an ultraviolet component with a wavelength of 405 nm is used, an external quantum efficiency of 43% can be obtained, and the extraction efficiency is 60%, which is the highest external quantum efficiency and extraction efficiency in the world. In 1999, Fuji reported that an AlInGaN-based chip was bonded to a silicon substrate, and the substrate was removed by laser lift-off. The surface of the n-type GaN was photoetched by photoelectrochemical etching to form an ordered tapered shape to increase the luminescence intensity by 2.3 times.
5, photonic crystal technology
The shallow two-dimensional surface grating photonic crystal can avoid damage to the active region and introduce too much surface damage during the photonic crystal preparation process, which causes the internal quantum efficiency to decrease, and at the same time can exert the diffraction effect of the photonic crystal and change the incident angle of the light. The light extraction efficiency is increased by 1.7~2.7 times, and the fabrication process involves electron beam lithography or other etching processes.