As the luminous efficiency of LEDs continues to increase, LEDs are undoubtedly one of the most important sources of light in recent years. On the one hand, by virtue of its light, thin, short and small characteristics, on the other hand, by its package type of drop resistance, shock resistance and special luminous shape, LEDs do give a very different light source choice for the general public. However, it has been seen that the development of LEDs includes heat dissipation problems and the use of special light-emitting patterns of light-emitting diodes. How to overcome and test the research and development capabilities of various manufacturers.
LEDs have a big feature, that is, they have low-current, low-voltage driving power-saving characteristics, and such characteristics are attracting everyone's attention at the same time as the world's energy shortage and the promotion of green environmental protection concepts in various countries. At present, in addition to the development of new energy sources, governments in various countries have also invested considerable efforts in improving the efficiency of existing electrical equipment and environmental protection research. While research and development on how to reduce industrial electricity consumption, the current consumption of household appliances with a penetration rate of about 80% has gradually received attention.
In terms of lighting, according to the results estimated by the Energy Commission, if the fluorescent lamps (66-75 lm/W) with higher luminous efficiency are used instead of the traditional 60W incandescent bulbs, the lighting time is 3,500 hours per year. Calculated, the annual electricity savings can be about 689 million kWh (about 88,600 kW).
Fluorescent lamps have the advantages of high luminous efficiency and low manufacturing cost, but because the lamps of fluorescent lamps contain mercury, the materials used for encapsulating fluorescent lamps are mainly glass that absorbs ultraviolet rays, and glass. Fragile properties combined with the difficulty of recycling mercury waste can cause serious environmental pollution. Therefore, the EU has already stipulated that these mercury-containing products will be banned in 2007. Therefore, the development of new lighting sources has become the target of development by governments. LEDs (light emitting diodes), which we usually call LEDs, are At present, the focus of development in lighting in various countries.
LED light emitting principle
The so-called light-emitting diode is basically a conventional pn diode, but its main function is not used for rectification, but the electron hole of the junction portion is promoted by the current passing through the pn junction after the positive bias is applied. Combine and shine.
The wavelength of the light emitted by the LED depends on the wavelength of the semiconductor material used by the diode, and also on the mixing ratio between the different materials. Figure 2 shows the relationship between the energy band, lattice constant and emission wavelength of each luminescent material. It can be seen that the current red, yellow and green light are mainly InGaAlP materials, while the blue and green light are mainly InGaN materials.
LED process technology
For semiconductor light-emitting diodes, lattice matching is a major issue, because for most III-V semiconductors, there is no just-suitable substrate that can carry the epitaxial layer above, and the growing Lei The crystal lattice size of the crystal layer must match the lattice of the substrate so as not to cause lattice defects due to stress factors, so that the photons emitted by the component are absorbed by the defects, and the luminous efficiency of the components is greatly reduced. The earliest III-V semiconductor heteroepithelar (heteroepitaxy) uses GaAs as the substrate and grows the epitaxial layer of GaAlAs on it. Because the lattices of these two materials are very similar, the epitaxial layer and the substrate The stress between the two is extremely small, so there is not much trouble in the development process.
However, the epitaxial crystals that have been developed in the future, such as GaAs1-xPx, grow on the GaAs substrate, or the GaAsxP1-x grows on the GaP substrate. Therefore, in optoelectronic materials, the ratio of binary, ternary or even quaternary materials is often adjusted, so that one can match the lattice structure of the substrate by the ratio of different atoms of different sizes, or because of the adjustment of the semiconductor. The size of the energy gap, and adjust the wavelength of the light-emitting component, the only way to adjust the epitaxial parameters is also more complicated, it can be seen that the epitaxial technology can be called the core of the semiconductor light-emitting component technology.
At the same time as the epitaxial method is improved, the structure of the epitaxial crystal is continuously improved. The earliest structure is of course the traditional p-?n junction LED, but its luminous efficiency can not be significantly improved, so the use of a single heterojunction (Single? Heterojunction, SH) structure began to be used in the epitaxy In the process of the process, the efficiency of minority carrier injection in the diode can be improved, and thus the luminous efficiency is significantly improved. Later, a double Heterojunction (DH) structure was developed. The energy gap between the two sides of the structure is higher than that of the middle, so that the bilateral carriers can be injected into the intermediate layer very effectively and the carriers are completely It is trapped in this range and produces very high photoelectric conversion efficiency. The latest method is of course to use a quantum structure in the epitaxial layer. When the thickness of the intermediate layer of the double heterojunction structure is gradually reduced to a few 10 angstroms (A), the electron or the hole produces a quantum effect, which can greatly enhance the photoelectricity. The effect of the conversion.
The epitaxial technology proposed here is mainly for the GaAs series in which the emission wavelength of the III-V material is concentrated in the red and yellow wavelength bands. This series of LEDs developed earlier and achieved better results earlier. However, if you want to obtain a full-color semiconductor light source, you must develop semiconductor light-emitting diodes in the blue and green wavelengths. In addition, GaN series light-emitting diodes have made significant progress in recent years.
GaN process troubles are successfully overcome
The materials used in blue and green light-emitting diodes are mainly ZnSe and GaN in the early stage. Because ZnSe has reliability problems, it allows GaN to have more room for development. However, the early research on GaN has not been able to make significant progress, mainly because it has not been able to find a substrate that matches the lattice constant of GaN, resulting in an excessively high defect density in the epitaxial crystal, and thus the luminous efficiency cannot always be improved.
Another reason why GaN cannot be broken is that the P-GaN portion of the module is not easily grown, and the doping of P-GaN is too low, and the mobility of the hole is also low. Until 1983, S. Yoshida and others in Japan first used high-temperature aluminum nitride (AlN) as a buffer layer on a sapphire substrate, and then the grown GaN obtained better crystallization. After that, I. Akasaki of Nagoya University and others used MOCVD to grow an AlN buffer layer at a low temperature (600 ° C) to obtain a mirror-like GaN above the high temperature growth. In 1991, Nichia Co. researcher S. Nakamura used the non-crystalline buffer layer of low-temperature grown GaN to grow specular-like GaN at high temperature. At this time, the problem of the epitaxial portion has been obtained. A major breakthrough.
On the other hand, in 1989, Professor Akasaki Yong used electron beam irradiation of magnesium (Mg)-doped P-GaN to obtain P-type GaN. After that, Nakamura Shuji of Nichia Corporation directly used thermal annealing at 700 °C to complete P. The production of GaN has finally broken through two major problems that have plagued the development of GaN.
In 1993, Nichia used the above two studies to successfully develop a GaN blue light-emitting diode that emits one candle (Candela), which has a lifetime of tens of thousands of hours. Later, green LEDs, blue and green diodes have been developed.
Manufacturers are committed to improving LED efficiency
The luminous efficiency of a light-emitting diode is generally referred to as the external quantum efficiency of the component, which is the product of the internal quantum efficiency of the component and the extraction efficiency of the component. The internal quantum efficiency of the component is actually the electro-optical conversion efficiency of the component itself, which is mainly related to the characteristics of the component itself, such as the energy band of the component material, defects, impurities, and the epitaxial composition and structure of the component. The extraction efficiency of the component refers to the number of photons that can be measured outside the component after the photons generated inside the component are absorbed, refracted, and reflected by the component itself. Therefore, the factors related to the extraction efficiency include the absorption of the component material itself, the geometry of the component, the refractive index difference of the component and the packaging material, and the scattering characteristics of the component structure.
The product of the above two efficiencies is the illuminating effect of the entire component, that is, the external quantum efficiency of the component. Early component development focused on improving its internal quantum efficiency. The method is mainly to improve the quality of epitaxial crystals and change the structure of epitaxial crystals, so that the electrical energy is not easily converted into thermal energy, thereby indirectly improving the luminous efficiency of LEDs, and about 70% can be obtained. Theoretical internal quantum efficiency. However, such internal quantum efficiency is almost close to the theoretical limit. Under such conditions, it is impossible to increase the total light quantity of the component by the internal quantum efficiency of the lifting component, that is, the external quantum efficiency is 2 to 3 times higher than the current one. Lifting the removal efficiency of components has become an important issue. The current methods for improving the efficiency of component removal can be divided into the following directions:
Change in grain appearance - TIP structure
Conventional LED dies are fabricated to a standard rectangular appearance. Because the difference between the refractive index of the general semiconductor material and the encapsulating epoxy resin is large, and the critical angle of the total reflection of the interface is small, and the four sections of the rectangle are parallel to each other, the probability that the photon leaves the semiconductor at the interface becomes smaller, so that the photon can only be Internal total reflection until it is absorbed, turning the light into a hot form, resulting in a less luminous effect. Therefore, changing the shape of the LED is a method for effectively improving the luminous efficiency. The TIP (Truncated? Inverted? Pyramid) type grain structure developed by HP, the four sections will no longer be parallel to each other, and the light can be effectively extracted, and the external quantum efficiency is greatly increased to 55%. With an efficiency of up to 100 lumens per watt, it is the first LED to achieve this goal.
However, HP's TIPLED is only suitable for quaternary red light-emitting diodes that are easy to process. It is quite difficult for GaN series light-emitting diodes with extremely high hardness sapphire substrates. At the beginning of 2001, Cree used the same structural concept (Fig. 4), taking advantage of its substrate as SiC. It also successfully made GaN/SiC LEDs with beveled LEDs and greatly increased the external quantum efficiency to 32%. However, SiC substrates are much more expensive than Sapphire, so there is no further progress in this technology.
Surface roughness technique
By roughening the internal and external geometry of the component, the total reflection of the light inside the component is destroyed, and the efficiency of the component is improved. 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.
Chip bonding technology (waferbonding)
Because the light generated by the LED is subjected to multiple total reflections, most of it is absorbed by the semiconductor material itself and the packaging material. Therefore, when GaAs which absorbs light is used as the substrate of the AlGaInP? LED, the absorption loss inside the light-emitting diode is greatly changed, and the light extraction efficiency of the module is greatly reduced. In order to reduce the absorption of light emitted by the substrate from the substrate, HP first proposed a bonding technique for the transparent substrate. The so-called transparent substrate bonding technology mainly applies the pressure of the LED die in a high temperature environment, and pastes the transparent GaP substrate, and then removes the GaAs, thereby increasing the light extraction rate by twice.
The above-mentioned chip-paste technology is currently mainly applied to quaternary LED components, but recently it has also begun to apply this technology to GaN-LEDs. Osram Opto Semiconductors also released a new research in February 2003 - ThinGaN, which can increase the light-emitting efficiency of blue LEDs to 75%, which is three times higher than the traditional.
Flip chip packaging technology (Flipchip)
For GaN series materials using sapphire substrates, since the P-pole and N-pole electrodes must be on the same side of the component, if the conventional packaging method is used, the upper part of the component is illuminated at most. The surface will lose a considerable amount of light due to the blocking of the electrodes. The so-called Flip? Chip structure is to reverse the traditional components, and make a reflective layer with a higher reflectivity above the p-type electrode, so that the light originally emitted from the upper part of the component is derived from other illumination angles of the component, and the sapphire The edge of the substrate is taken out (see Figure 5). Such a method can reduce the amount of light output on the electrode side by about twice as much as the conventional package. On the other hand, because the flip-chip structure can directly contact the heat dissipation structure in the package structure directly by the electrode or the bump, the heat dissipation effect of the component is greatly improved, and the light quantity of the component is further improved.
White light led to the stage focus
While the luminous efficiency of various LEDs has begun to increase dramatically, the possibility of applying high-brightness LEDs to lighting has become higher and higher. The consideration of such an application lies in how to develop a white light emitting diode. At present, there are mainly three methods for using white light to form white light, which are respectively described as follows:
Single crystal blue LED and yellow phosphor
After the successful development of the blue light-emitting semiconductor, Nichia Corporation developed a white light-emitting diode. In fact, Nichia's white light-emitting diodes do not directly emit white light from the semiconductor material itself, but instead use a blue light-emitting diode to excite the yellow-light YAG phosphor coated on it. The yellow light generated by the phosphor is excited and used. The excited blue light complements to produce white light. At present, Nichia's commercial products use 460nm InGaN blue semiconductor to excite YAG phosphors to produce 555nm yellow light, which has been fully commercialized, and several other manufacturers Lulumineds Lighting, which is also developing high-brightness LEDs. Cree and Toyota Synthetic (Toyoda Gosei) are constantly competing in the LED market . With the continuous improvement of the luminous efficiency of blue crystal grains and the gradual maturity of YAG phosphor synthesis technology, the white light emitting diodes encapsulated by blue and yellow phosphors are the more mature white light emitting diode technologies.
Single crystal UVLED+RGB phosphor
Although the white light-emitting diode package technology using the blue crystal grain with the yellow YAG phosphor is a relatively mature technology, the white light-emitting diode packaged by such a method has several serious problems that cannot be solved. The first is the problem of uniformity, because the blue crystal grains that excite the yellow phosphor actually participate in the color matching of white light, so the shift of the wavelength of the blue crystal light, the change of the intensity, and the change of the thickness of the phosphor coating all affect the white light. Evenness. The most commonly seen example is a white light-emitting diode that is sealed in this way. The central part looks bluer (or whiter) and the area next to it looks yellower (the phosphor coating is thicker), each The color of white light-emitting diodes is different.
On the other hand, Nichia, which develops this technology, owns most of the patents related to the blue-ray process technology and the yellow-light YAG-based phosphor-related white light-emitting diodes, while the Japanese company adopts a policy of changing the oligopolistic market. For manufacturers who use blue crystal grains to match white phosphors to produce white light-emitting diodes, it is hard to say. The white light-emitting diode technology using blue light crystals with yellow fluorescent powder has more problems such as high white color temperature and low color rendering. Therefore, the development of a better-performing technology without patent problems is a major issue for all LED manufacturers.
UV? LEDs with three-color (R, G, B) phosphors provide another direction of research and development. The method mainly uses the UV LED which does not actually participate in the white light to excite the red, green and blue phosphors, and the three-color light emitted by the three-color phosphor is white light. Such a method is not particularly sensitive to the white light dispensed because the UV LED does not actually participate in the color matching of the white light, so the fluctuation of the wavelength and intensity of the UV LED is not particularly sensitive. White light with acceptable color temperature and color rendering properties can be prepared by selecting and proportioning phosphors of various colors. In terms of patents, the development of UV LED+RGB phosphors still has considerable room for development. However, although such a technique has various advantages, it still has considerable technical difficulties. These difficulties include the choice of the wavelength of the ultraviolet light of the phosphor (the excitation wavelength of the best conversion efficiency of the phosphor), the difficulty and the resistance of the UV? The development of UV packaging materials, etc., is to be resolved by each R&D unit.
Polycrystalline RGBLED
The crystal grains of red, blue and green colors are directly packaged together, and white light emitting diodes can be formed by directly combining white colors of red, green and blue colors. The method of directly packaging a white light diode into a white light diode is the first method for making white light, and the advantage is that it does not need to be converted by a phosphor, and the white color is directly formed by the three color crystal grains, except that the fluorescence can be avoided. In addition to the loss of powder conversion to obtain better luminous efficiency, it is also possible to achieve a full-color color-changing effect (variable color temperature) by separately controlling the light intensity of the three-color light-emitting diode, and by selecting the grain wavelength and intensity Better color rendering is obtained.
However, the disadvantage is that it is difficult to mix light, and the user can easily observe a variety of different colors in front of the light source, and see a colored shadow behind each of the masks. In addition, since the three crystal grains used are all heat sources, the heat dissipation problem is three times that of other package types, and the difficulty in use thereof is increased. At present, a white light emitting diode of a polymorphic RGB? LED package type can obtain an efficiency of about 25 to 30 lm/W. Mainly used in outdoor display billboards, outdoor landscape lights, color-changing wall washers, etc., where the heat dissipation problem is less serious. On the other hand, if the design of the electronic circuit is controlled, the polycrystalline RGB LED package type LED can be used as one of the main light sources to replace the backlight in the LCD backlight module currently using CCFL.
White LEDs must pass the heat test
Although with the gradual improvement of the luminous efficiency of white light-emitting diodes, the possibility of applying white light-emitting diodes to illumination is increasing, but it is obvious that a single white light-emitting diode has a low driving power supply voltage regulator. Therefore, in the current package type, it is unlikely that a single white light emitting diode can be used to achieve the lumens required for illumination.
In response to this problem, the current main solutions can be roughly divided into two categories. One is to use a plurality of light-emitting diodes to form a light source module more conventionally, and the driving power required for each single light-emitting diode is generally The same is used (about 20-30 mA); the other method is the method used by several high-brightness LED manufacturers, that is, using the so-called large-grain process, where the size of the conventional die is no longer used. (0.3mm2), the die process is made to a larger size (0.6mm2 ~ 1mm2), and a high drive current is used to drive such a light-emitting component (typically 150-350 mA, currently up to 500 mA or more).
However, no matter which method is used, it will have to deal with extremely high heat in a very small LED package. If the components cannot dissipate these high heats, in addition to various packaging materials, the products will be reliable because of the difference in expansion coefficient between each other. The problem of degree, the luminous efficiency of the crystal grains will decrease significantly with the increase of temperature, and cause it to be significantly shortened. Therefore, how to dissipate the high heat in the assembly has become an important issue in the current LED packaging technology.
For the light-emitting diode, the most important thing is the luminous flux and light shape of the output. Therefore, one end of the light-emitting diode must not be shielded from light, and it needs to be coated with a high-transparency epoxy resin material. However, most of the current epoxy resins are non-thermally conductive materials. Therefore, for the current LED packaging technology, the main heat dissipation is by using a metal lead frame under the LED die to dissipate the components. The heat.
As far as the current trend is concerned, the choice of metal foot materials is mainly composed of materials with high thermal conductivity, such as aluminum, copper and even ceramic materials, but the thermal expansion coefficients between these materials and grains are very different, if they are directly Contact is likely to cause reliability problems due to stress generated between materials when the temperature rises, so an intermediate material having both a conductivity coefficient and a coefficient of expansion is generally used as a space between the materials. In view of the above concept, in 2003, Matsushita Electric made a plurality of light-emitting diodes on a multi-layer substrate module made of a metal material and a metal-based composite material to form a light source module, and the high-heat-conducting effect of the light source substrate was used to make the light source The output remains stable over long periods of use.
Also using the idea of ​​a high heat sink substrate, Lumileds applies it to large area die products. The Lumileds substrate is made of a copper material with a high conductivity, which is then connected to a special metal circuit board to take into account the circuit conduction and increase the heat transfer.
In addition to Lumileds, OsramOptoSemiconductors and Nichia Chemicals have introduced products with larger grains of more than 1W (Figures 8 and 9). From the high-brightness LED manufacturers who have introduced large-grain, high-power products, it seems that large-grain-related process and packaging technologies have gradually become the mainstream of high-brightness LEDs. However, the large-grain-related process and packaging technology not only enlarges the die area, but the related process and packaging technology still has a considerable threshold for traditional LED manufacturers, but if you want to push the LED to high-brightness illumination In the field, the development of related technologies is still a necessary process.
Solve the key problem LED future
With the gradual improvement of the luminous efficiency of LEDs in recent years, the possibility of using LEDs as illuminating sources is also increasing. However, while people only consider improving the luminous efficiency of LEDs, how to make full use of the characteristics of LEDs and the difficulties they may encounter when applying them to lighting is already the current goal of major lighting factories. The difficulties that have been seen so far include heat dissipation problems and the use of special light-emitting shapes of light-emitting diodes.
In terms of heat dissipation, although the light-emitting diode is called a cold light source, because of its current electro-optic efficiency, there is still considerable room for improvement, that is to say, there is still a considerable amount of electrical energy because it is not converted into light and causes excess heat energy, which is concentrated in the crystal. Granular size will cause serious heat dissipation problems. Therefore, good heat dissipation design and development of heat dissipation materials are the current focus.
In terms of the light-emitting shape of the light-emitting diode, the light-emitting diode has completely different light-emitting characteristics from the conventional light source. In addition to the extremely small size of the die itself, the different package types of the various light-emitting diodes are completely different. The illuminating light shape, so the design related to LED lighting applications will no longer be able to simply put a concentrating lens or mirror on the light source, but must undergo a more careful optical design. In this part, companies and R&D units have different directions, but in addition to developing technology, how to mass-produce these technologies is the key to whether solid-state light sources can become the mainstream of lighting sources in the future.
(Edit: Xiaotang)
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