利用蓝宝石衬底表面纹理化提高LED的发光效率

Abstract—A high light-extraction effificiency was demonstrated in the flflip-chip light-emitting diode (FCLED) with a textured sapphire substrate. The bottom side of a sapphire substrate was patterned using a dry etching process to increase the light-extraction effificiency. Light output power measurements indicated that the scattering of photons emitted in the active layer was considerably enhanced at the textured sapphire substrate resulting in an increase in the probability of escaping from the FCLED. The lightoutput power of the FCLED was increased by 40.2% for a 0.4- m deep FCLED with a periodic distance of 13- m mesh-type texture on the bottom side of the sapphire substrate.

THE PAST few years have brought continuing and rapid development in GaN-based light-emitting diodes (LEDs). GaN-based LEDs are currently being used in a variety of display and lighting applications, including traffific signals, full-color displays, automotive lighting, and general room lighting [1], [2]. It has been shown that the flflip-chip LED (FCLED) confifiguration is very effective in enhancing light extraction effificiency and can be used in high-power and high-effificiency LEDs [3], [7]. In an FCLED, light is extracted through the transparent sapphire substrate by reflflecting light on the p-contact metal layers [4]–[7]. However, the light extraction effificiencies of top-emitting and FCLEDs are still low due to the trapping of light inside the LEDs. Surface texturing methods have recently been introduced, in an attempt to reduce the loss of photons due to the internal reflflection at the interface of the top layer of p-type GaN and air. The probability of photons fifinding the escape cone can be increased due to the textured p-type surface. Even though a photon does not escape initially, the redirected photon can exist in the escape cone angle and subsequently be emitted from the top side of the LED to the air. The increase in light-extraction effificiency by surface texturing is attributed to angular randomization by photon recycling.

In this letter, the bottom side surface of a sapphire substrate in an FCLED was textured, in an attempt to improve the probability of photon escape through the textured sapphire substrate. To study the effect of surface textures on the light extraction ef- fificiency of FCLEDs, we examined the depth (0.2–0.4 m) and shape of textures (line-type and the mesh-type) on the bottom side of the sapphire. The textures on the bottom side surface of sapphire substrate were produced by a dry etching process. The light-output power of an FCLED with a textured sapphire surface was greatly improved compared to that of a conventional FCLED with a nontextured sapphire surface.

Fig1

Fig. 2 shows the optical microscope images of the textured sapphire surface of an FCLED with line shape and mesh shapes. It has been reported that the sapphire can be etched by a chlorine-based ICP [15], [16]. Sapphire is chemically very stable and the ion bombardment effect in ICP is important for breaking the strong chemical bonding of Al–O in the sapphire structure. In this letter, mixtures of Cl /Ar gases were used for the ICP etching of the sapphire substrate. It is believed that the Ar ions bombard the sapphire surface to remove oxygen from Al–O and the remaining Al can be removed by Cl through the formation of volatile AlCl [15], [16]. To confifirm the effect of the surface area of the bottom side of the sapphire on light extraction effifi- ciency, we etched the bottom side of a sapphire using etch masks with various shapes and depths.

FCLED with a mesh-textured sapphire of 0.4- m depth was increased by 40.2% at an injection current of 20 mA compared to a conventional FCLED. These results indicate that the surface texturing of sapphire substrate reduces the internal light reflflection and increases the light extraction effificiency. In other words, the photon probability to fifind the escape cone can increase due to the textured surface. Even though a photon does not escape at fifirst trial, the redirected photon can exist in an escape cone angle at second trial and emit from the sapphire to air. This mechanism was reported as angular randomization by photon recycling [8]. It is likely that optimization of the patterns of textures and etch depth will lead to a further increase in the light extraction effifi- ciency of FCLEDs. The distance from the active region to the Ag reflflector is also an important factor to extract more photons through the escape cone in the FCLED chip [4]. Therefore, we need to further optimize the distance from the active region to the reflflector to increase the light-extraction effificiency.