Author: Ze Peng, Xie Mingxun / Epistar
For the current red LEDs , it is not impossible to have better extraction efficiency. This is due to the special design of the penetrating and reflective layers. The product has been online in the two products of Epistar. Development is complete.
Many of the LEDs based on AlGaInP are currently used to provide red light sources for traffic signals and automotive brake lights. However, this type of device can be more successfully applied to commercial products if the cost per unit of lumens is lower. For example, a projector, a backlight of a liquid crystal television, and a color temperature modulation device.
The method of reducing the cost per unit lumen can use new techniques to improve the efficiency of LDE, such as improving the growth conditions or the way the device is processed, and increasing the internal quantum efficiency of the device to the theoretical limit, thus improving the luminescence extraction efficiency of the LED device.
There are already many technologies for development research towards this goal, but none are suitable. A distributed Bragg reflector (DBR) is added to the LED to reduce the absorption of the source on the GaAs substrate, but the efficiency of the reflected source at oblique incident angles is relatively low due to significant optical losses. The improved way can replace the substrate with a penetrable substrate, such as sapphire or GaP, but it still has drawbacks because these methods do not release an effective heat transfer power coefficient and still require the maximum drive current. And lumen output values. However, the surface pattern still increases the light output, but it is not easy to control the boundary and etch range if using conventional chemical etching techniques.
The heat conduction problem has recently been used in a new way to energize and thermally conduct the substrate by switching the epitaxial layer. However, even with this advanced approach, optical efficiency is only 50 lm/W for many commercial 620 nm wavelength LEDs. This means that the production of high-brightness LEDs comes from a combination of different technologies, so that the customer's expectations for efficiency cannot be met.
In any case, in Taiwan's Jingyuan Optoelectronics Co., Ltd., we have a new series of AlGaInP LEDs that have not yet been published, which can produce much greater efficiency than current technologies. For these products, we named them P and A series (although they were originally named Phoenix and Aquarius LEDs respectively). The main feature is that the excitation efficiency of light is at least 50%, all thanks to the additional surface pattern of the multilayer film structure and the different levels of reflection. More importantly, we can use the original equipment to manufacture, and the quantity can be fully loaded.
We call the multilayer film structure of this patent "Lambert Penetrating and Reflecting Film Layers" because they follow the cosine scattering law of Johann Heinrich Lambert. The reflection or divergence of these structures has a great intensity in the direction perpendicular to the surface and the weakest in the most oblique angle (see the definition in Figure 1)
Figure 1: (a) The Lambertian penetration surface has the highest penetration rate perpendicular to the surface (light transmission intensity is proportional to cos[θ], θ represents the angle from the substrate plane to the vertical position), this figure The length of the arrow indicates the intensity of the light source. (b) The Lambertian reflecting surface produces the same intensity distribution, and the crystal photoelectric has confirmed the transmission characteristics (c) and reflector of Lambertian itself. (d) The angle θ of the incident beam is 0°, 30° and 60°, and the observed angle is between 5° and 80°.
Our P-series and A-series LED features Lambertian's emission and reflection on the top and bottom of the device, respectively. Most of the light emitted by the emitter goes straight ahead, and only a very small portion of the light is returned to the device, which may be absorbed by the quantum. At the same time, the reflector will return most of the light source to the substrate through a specific angle to avoid multiple reflections inside the chip.
We made our P-series chips created by Lambertian reflectors on the GaP surface, which is the top layer of AlGaInP in the GaAs-oriented epitaxial layer (Fig. 2a). This wafer is first bonded to silicon before GaAs is removed. Next we etch away the n-type plating to form a Lambertian reflector and clearly define the gold p-type in contact with the back side of the silicon substrate. The problem with wafers is as follows. Before the devices are bonded, the wafers are first cut into individual chips.
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