The design of super LEDs from the perspective of diamond heat dissipation and light emission

Taiwan's Diamond Technology Center (DTC) replaced the two insulating layers of epoxy resin on printed circuit boards with diamond-like carbon (DLC) coatings, which can significantly extend the life of current LED lighting products (such as street lights). DTC has also developed Diamond Islands Wafer (DIW) as a substrate for producing super LEDs . Super LEDs can emit extremely strong ultraviolet light (UV), and its intensity does not decrease due to high temperatures, but becomes brighter. Super LED semiconductors, including diamond, cubic boron nitride (cBN) and aluminum nitride (AlN), have extremely wide energy gaps and can even be made into solid-state UV lasers, whose light energy density will be far greater than existing gas (such as Eximer) lasers.

Light Revolution

本世纪开始就啟动了光的革命， 已经用了百餘年的白炽灯(Incandescent Lamp)及萤光灯(Fluorescent Lamp)即将走入歷史。就在未来的几年，LED 发光将成照明(如路灯)及显示(如电视)的主流。2009 年全球LED 的总产值约76 亿美元，日本为产值第一的国家，台湾则为产量最大的地区(图1)。

Figure 1: LED production is rising year by year

Taiwan is the manufacturing kingdom of semiconductors and also has the largest number of MOCVD crystal growth machines. Taiwan has taken advantage of this wave of the optical revolution and has become the main producer of the largest number of LED chips. However, just like the expensive royalties that were paid for CDs in the past and DRAMs today, LED patents are also subject to foreign companies (Figure 2). The "smile curve" makes the profits generated by foreign companies' patents far greater than the value of Taiwan's hard work in producing LEDs.

Figure 2: The cross-licensing of global LED patents has marginalized Taiwanese manufacturers

Diamond Technology

Although Taiwan's semiconductor manufacturing technology is controlled by others, its diamond products, which are the most advanced semiconductor materials, are the best in the world. In 1999, the Diamond Technology Center (DTC), a joint venture between Kinik Company and Dr. Song Jianmin, launched the world's first diamond array (DiaGrid) diamond disc, which has long become a standard product for "Chemical Mechanical Planarization" (CMP) in the production of integrated circuits (Figure 3). DTC's patents have also forced 3M to retreat, and have been authorized to major diamond product companies in the United States, Japan and China.

Figure 3: Standard products for "chemical mechanical planarization" that are essential for the early production of integrated circuits

Taiwan's Diamond Technology Center (DTC) replaced the two insulating layers of epoxy resin on printed circuit boards with diamond-like carbon (DLC) coatings, which can significantly extend the life of current LED lighting products (such as street lights). DTC has also developed Diamond Islands Wafer (DIW) as a substrate for producing super LEDs. Super LEDs can emit extremely strong ultraviolet light (UV), and its intensity does not decrease due to high temperatures, but becomes brighter. Super LED semiconductors, including diamond, cubic boron nitride (cBN) and aluminum nitride (AlN), have extremely wide energy gaps and can even be made into solid-state UV lasers, whose light energy density will be far greater than existing gas (such as Eximer) lasers.

DTC has developed a series of diamond coatings, including CVD and PVD (Figure 4).

Figure 4: Technical diagram of DTC coated with diamond-like carbon (DLC)

DTC's diamond technology can enhance the vision of LED design. DTC has corresponding diamond products for the upstream, midstream and downstream of LED, which can make Taiwan's future LED products unique in the world with diamond value (Figure 5).

Figure 5: Illustration of a design example of a super LED developed by DTC

Diamond Circuit Board

LED 的亮度会随温度的升高而降低，而其寿命更会急据缩短。目前LED 的下游散热片多以铝片制成，其上的铜制电路乃以绝热的环氧树脂(Epoxy)隔开绝缘。环氧树脂的热传导係数(0.5 W/mK)比铝(275 W/mK)低数百倍，LED 芯片产生的热乃久聚难散。DTC 以比铜热传导係数(400W/mK)更高的DLC(500 W/mK)绝缘铜导线，因此可以达到显着的冷却效果(图6～图14)。DLC 披覆的印刷电路板(Print Circuit Board，PCB)，已经供应给台湾多家的LED 制造厂家，更将和中国的海安晶钻公司合作量产。

Figure 6: Thermal diffusivity measured by laser flash (ASTM E-1461DIN) shows that DLC is much higher than copper foil

图 7：披覆DLC 的PCB 在加电20 分鐘后不同LED 的表面温度差异＜ 1℃。未披覆DLC 者温差可达3℃(350 mA)或9℃(1000 mA)

Figure 8: The cross-section of DLC heat dissipation shows that the temperature gradient of the LED is significantly reduced, and the heat flow quickly flows from the chip to the edge

Figure 9: DLC PCB (DTC product) for high power (＞ 5W) LED

Figure 10: DLC coating can dramatically reduce the thermal resistance of aluminum plate

Figure 11: The thermal resistance of the DLC PCB is not only minimal, but also does not increase with the increase of LED power.

Figure 12: DLC-coated PCB can improve the brightness of red, green and blue LEDs

Figure 13: Heat dissipation effect of LED street lights made of DLC-coated PCB

Figure 14: The rapid heat dissipation of DLC-coated PCB can effectively slow down the attenuation of LED brightness

[page]

Usually, materials with fast heat conduction (such as metal) have a very low rate of heat radiation (< 1%), while materials with high heat radiation (such as plastic) have very low thermal conductivity (< 1 W/mK). DLC can combine the best of both worlds, and can conduct heat and radiate heat at high speed. In fact, DLC can radiate heat to air molecules at room temperature in far infrared (such as 10 μm wavelength) like a black body. If the above-mentioned DLC PCB is coated with a layer of DLC on the exposed surface (such as the back), the heat of the LED can be continuously radiated to the air, just like wearing the outer coat of atomic fans.

LED middle heat dissipation layer

The LED chip can also be soldered to the silicon wafer support (submount) by flip chip method, and then bonded to the PCB. Since the thermal conductivity of silicon (150 W/mK) is lower than that of aluminum (250W/mK), the heat generated by high-power LEDs will be blocked by the silicon wafer. The silicon wafer coated with DLC can easily become a heat-transmitting (HeaThruTM) interface, reducing the junction temperature of the LED (Figure 15-16) (Reference 1).

Figure 15: LED silicon wafer submount coated with DLC cools more and faster than SiO2. The higher the current, the more significant the cooling effect of DLC.

Figure 16: DLC coated Si Submount epitaxial wafer (6 inches) (DTC product) can be directly soldered to the LED flip chip epitaxial wafer

DTC will also work with Crystal Diamond to develop a support for Boron Doped Diamond (BDD). BDD is formed by chemical vapor deposition (CVD) using a DC arc. Since BDD is conductive, it can directly become the electrode of the LED, so that the LED can be reduced in area and form vertical stacks. Vertical LED chips have higher luminous efficiency than traditional lateral current chips.

Diamond Film Heat Dissipation

Although DLC has good heat dissipation effect, the thermal conductivity of polycrystalline diamond film (1200 W/mK) can be doubled, making its cooling effect on GaN chips more prominent (Figure 17).

Figure 17: Appearance of CVD polycrystalline diamond film epitaxial wafer (Zhongsha catalog) and its effect on cooling GaN

If the LED needs to dissipate heat quickly with silicon crystals covered with polycrystalline diamond film, the diamond epitaxial wafer can be first welded to the LED epitaxial wafer, and then the sapphire substrate can be peeled off to make a diamond film-bonded LED chip (Figure 18).

Figure 18: Schematic diagram of LED manufacturing process with diamond substrate

Polycrystalline diamond film has a remarkable effect in cooling LEDs, and its ability to suppress hot spots becomes stronger as the LED current increases (Figure 19). Therefore, the brightness of LEDs based on diamond film can be greatly improved.

Figure 19: Effective cooling of polycrystalline diamond film can significantly improve the brightness of LEDs

单晶钻石衬底的GaN

在高温(e.g.1200℃)下单晶钻石可藉AlN 过渡而长出GaN 磊晶。由于单晶钻石的热传导率可比多晶倍增，单晶钻石底GaN 的散热效果会比前述的多晶钻石膜更加明显(图20)。

Figure 20: Luminescence effect of epitaxial GaN on single crystal diamond (Ib)

[page]

Aluminum nitride LED

materials include so-called superhard materials, including diamond and cubic boron nitride (cBN). The lattice rigidity of superhard materials is so large that the transmission of phonons is super fast, so superhard materials become ultrasonic materials. Since sound can vibrate and transmit energy quickly, superhard materials are also superthermal conductive materials that can quickly remove heat energy.

Superhard materials have extremely high band gaps. They and AlN are semiconductors for super LEDs and can be made into ultra-high power ultraviolet LEDs. To upgrade LEDs, old ones must be replaced with new ones, and luminescent materials with wider band gaps must be used. GaN is the mainstream of current LED chips. GaN and AlN are isostructural compounds, and they can form mixed crystals of solid solutions. AlN has a wider band gap (6.2 eV) and can emit deep ultraviolet light (wavelength of about 210 nm) through electroluminescence (EL) (Table 1).

Table 1: Comparison of semiconductor characteristics of LEDs

In the GaN lattice, Al gradually replaces Ga to transition to AlN. During this process, the light emission of the LED will change from blue to purple and finally enter the ultraviolet region (Figure 21).

Figure 21: Substituting Al in the GaN lattice can shorten the wavelength of LED light to deep UV.

The lattices of AlN and SiC are similar, and AlN/SiC LEDs have been produced in prototype form (Figures 22 and 23), but are currently difficult to produce due to the lack of appropriate epitaxial wafers. However, the Diamond Islands Wafer (DIW) described below can solve this manufacturing problem.

Figure 22: AlN LED structure and its electroluminescence waveform

Figure 23: AlN can excite deep ultraviolet light with a wavelength of 210 nm

cBN LED

Aluminum nitride and wurtzite boron nitride (wBN) are also heterogeneous phases, while sphalerite boron nitride, namely cubic boron nitride (cBN), is an allotrope with wBN. cBN is a higher-frequency luminescent body that can electro-excite ultraviolet light of ultrashort wavelength (about 200 nm) (Figures 24-25).

Figure 24: Design of cBN LED and carrier concentration of Mg-dyne cBN

Figure 25: IV curve of cBN chromatin

The allotrope of cBN, hBN, is a two-dimensional (planar) semiconductor. Its energy gap is 5.97 eV, and its electroluminescence wavelength is 215 nm (Figure 26). When the atoms of hBN are substituted with C, an N-type semiconductor is produced, while when they are substituted with Be, a P-type semiconductor is produced.

Figure 26: The luminescence waveform of hBN, with a peak wavelength of 215 nm

AlN on Diamond

There is no epitaxial wafer for commercial production of single crystal AlN, but it can be grown from silicon epitaxial wafer to GaN epitaxial, and then transition to AlN epitaxial through Al/Ga substitution. However, the atomic spacing of silicon is much larger than that of GaN, so the generated GaN lattice defects are very high (such as 109/cm2), which reduces the internal quantum effect of the LED. However, depositing a thin layer (such as 10 nm) of amorphous InN on silicon can greatly reduce the stress of GaN epitaxial, which helps to reduce its lattice defect density. Another method is to use cubic TiN (hexagonal ZnO can also be grown on Sapphire substrate as an intermediate layer) as an intermediate layer. The lattices of TiN, ZnO and GaN are quite matched (Mismatch ＜2%), and this intermediate layer can effectively reduce the defect density of GaN lattice. In addition, using graphene as an intermediate layer is also an effective method. The graphene layer is a two-dimensional lattice that can be deformed in the third dimension, so that it can flexibly adjust the degree of tightness of the matching other semiconductor lattices (Figure 27) (References 9-10).

Figure 27: Atomic distances in a tetrahedral bonded semiconductor

除了以硅晶为基材配合中间层长出GaN 磊晶之外，钻石的(111)表面也可在高温(1200℃)长出AlN 的磊晶(文献11)。除此之外，钻石在真空裡加热至1200℃使其縐褶的(111)面扯平成磊晶的石墨层(Graphene)后就可在1100℃或更低的温度长出GaN 的磊晶。还有一个更有效的方法就是以原子层沉积(Atomic Layer Deposition，ALD)法通入甲烷(Methane，CH4)及硅烷(Silane，SiH4)并渐进增加C/Si 的比率。每次进气后都以电浆解离的氢气或氟气移除非晶格的沉积。这样就可以使钻石的表面长出数奈米的SiC。由于SiC 和AlN 的晶格匹配，以SiC 为中间层可长出完美的AlN 磊晶。

wBN and diamond have similar lattices. Using the (111) surface of boron doped diamond (BDD) as the substrate, wBN epitaxial growth can be achieved (Figure 28). wBN and AlN are heterogeneous phases and can form a solid solution (B, Al)N. Using this eutectic as a transition layer, AlN single crystals can also be grown on diamond.

Figure 28: Low temperature sputtering on the surface of diamond film can generate (B, Al)N intermediate layer and AlN polycrystal

Boron doped diamond (BDD) is a super P-type semiconductor with the highest carrier concentration and the highest hole mobility (see Table 1). Silicon-containing AlN is an excellent N-type semiconductor because its electron dissociation energy (0.25 eV) is lower than that of magnesium-containing P-type AlN (0.63 eV). This heterojunction can be made into LEDs that emit blue light and UV light at the same time (Figure 29).

Figure 29: The EL spectrum of AlN/Diamond(100) includes blue light at 2.7 eV and UV light at 4.8 eV.

[page]

Diamond Semiconductor

钻石半导体是超速计算机CPU 芯片的梦幻材料。它可在强电场、高频率、大电流及热温度下运作。钻石半导体也可制成多种极端的光电组件，例如「死光级」(Death Light)的雷射二极管(Laser Diode)。钻石半导体可以染质(Dopant)渗杂使其带电或(N-Type 或负极)或缺电(P-Type 或正极)。P-N的结合可使其成为晶体管(Transistor)、LED 或太阳电池。由于钻石的碳原子极小，染质只能使用很小的原子才能塞进钻石的紧密晶格。其中最容易取代碳原子的为比碳少一个电子的硼原子及比碳多一个电子的氮原子。以象棋比喻元素的週期表，碳为正中的国王，硼及氮为过与不及的左右护法。

Boron-doped silicon is a P-Type semiconductor, and boron-doped diamond is also a hole source. Phosphorus-doped silicon is an N-Type semiconductor, but phosphorus atoms are very large, and only a few atoms can be squeezed into diamond to become an electron source with high resistance. Therefore, the current of PN combination is very small, making it difficult to exert the excellent properties of diamond semiconductor. If nitriding is performed in diamond, although the concentration of valence electrons can be greatly increased, the nitrogen atoms bind the electrons too tightly, and the valence electrons cannot dissociate, which greatly increases the resistance and makes the current smaller (Figure 30).

图 30：钻石晶格内的电子能阶图。鋰可提供电子流动的通路

氮原子把额外的电子卡住，这个电子会推挤旁边的碳原子使其离开而造成空位(Vacancy)。若能使鋰(Li)原子扩散进入钻石的晶格，这些原子会填补氮原子旁边的空位，因此会形成氮鋰(LiN)的原子对(Atomic Pair)。鋰原子为金属，通常不能佔据钻石的晶格，但LiN 可取代两个碳原子。氮原子虽把电子绑死，但鋰原子提供一个低位能阶可使电子流动，因此可解决钻石半导体没有电子源的问题，这是钻石半导体的新思维(图31)。

图31：LiN 的电子染化轨道可把束缚的氮电子经鋰离子释放

Diamond LED

The development of diamond LED has been completed by the Japanese, and its design and excitation spectrum are shown in Figure 32.

Figure 32: Design and spectrum of diamond LED. The P type has B as the dye, while the N type has phosphorus.

Diamond UV LED has the advantages of high energy and super brightness. Even if the current density exceeds 2000A/cm2, its luminous efficiency is still not saturated. This current density is many times that of the quasi-UV (400 nm) LED made of AlGaN. In addition, the higher the temperature of the diamond LED, the greater the brightness (Figure 33). This is exactly the opposite of the heat tendency of traditional LEDs. Therefore, diamond LEDs can emit super-strong ultraviolet light at extremely high power. If the temperature is as high as 400℃, the tightly bound valence electrons of nitrogen can also be freed to become electron sources, so diamonds containing nitrogen can form N-type semiconductors, solving the problem of lattice defects caused by phosphorus mentioned above.

Figure 33: The relationship between current and intensity of diamond LED (left). The right figure shows the characteristics of the light emission increasing with temperature (℃) at 50 mA (right)

Diamond Island Epitaxial Wafer

The above-mentioned super LEDs lack suitable epitaxial wafers for production. Therefore, DTC has developed Diamond Island Epitaxial Wafer (DIW) for future production of high-power UV LEDs.

More than 1,000 metric tons of diamond single crystal abrasive grains are produced worldwide each year, and the price can be as low as $200/Kg (1Kg = 5000 carats), which is cheaper than silicon epitaxial wafers. Diamond abrasive grains are grown by catalyzing graphite growth with molten iron group metals (alloys of Fe, Co, and Ni) under ultra-high pressure (> 5GPa, 1 GPa = 10,000 atm). DTC is the world's leading developer of the technology to grow large (> 0.5 mm) diamond single crystals using arranged seed crystals, and can even grow single crystals with a hexahedral (cubic) shape (Figure 34). The cost of mass production is 10 diamond single crystals for every NT dollar (Reference 18).

Figure 34: DTC has developed technology to produce diamond cubic crystals, which can mass-produce diamond single crystals for LEDs.

The biggest feature of diamond LED is that it emits more amazing light when the temperature reaches 600℃. Conventional LEDs may burn out below 200℃. Diamond is a semiconductor that can input the highest power LED (inert gas protects it from oxidation). Since the LED chip is less than 0.5 mm, each single crystal can grow one LED. DTC's Diamond Islands Wafer (DIW) can be combined with the LED crystal growth production line (Figure 35).

Figure 35: Diamond Island epitaxial wafer design and entity

In the future, with dense cubic crystal arrangement, Diamond Island can cover 3/4 of the epitaxial wafer area (DTC products).

DIW 不仅可解决钻石外延片问题，更能以之长出cBN 及AlN 的磊晶。钻石与cBN 的晶格相同而cBN 可过渡至AlN，因此上述的超级LED 都可在未来纳入生产线。大量生产DIW LED 是台湾科技超越美、日、欧的机会。台湾也可藉此由半导体的硅晶岛(Silicon Island)升级到比金银岛更有价值的钻石岛(Diamond Island)。

Diamond Laser

Since Würtzite GaN is a hexagonal crystal, it will distort the lattice due to piezoelectric distortion, thus interfering with the uniformity of light emission. Although some people have tried to improve the light emission of LEDs by cutting out so-called non-polar GaN from extremely expensive GaN epitaxial wafers, the cost is sky-high, so it is not practical (Figure 36).

Figure 36: Cutting a GaN crystal yields non-polar planes (a or m)

Diamond is a cubic crystal system, and there is no polarity problem at all. The cBN or AlN epitaxial growth of the (100) crystal plane DIW is a cubic crystal system. Extending from the cubic AlN to GaN epitaxial growth will also be a cubic crystal system.

立方晶系的LED 以解理(Cleavage)面(100)为共振腔就能做出雷射二极管(Laser Diode，LD)，UV 的LD 为梦幻的死光材料，它可藉DIW 的实践制造生产。

Nitride phosphor

目前的白光LED 多以蓝光激发黄色的萤光粉(如YAG 或TAG)而互补组成白光。萤光粉的母体(Carrier)常为氧化物，而光源(Activator)的原子为稀土元素(如Ce、Eu)。由于氧化物为极性化合物(Ionic Compound)，它会吸收水份而逐渐潮解。更有甚者，光源的原子太大以致和母体结合的键能不强，在高温下大原子会扩散(Diffusion)及偏析(Segregation)使发出的光分散走样。除此之外，母体内光源原子的浓度不高，萤光粉吸收了自己发光的强度。

If the dream of super LED comes true, phosphor can be replaced by nitride. For example, (In, Ga)N can be used as both the matrix and the light source. By adjusting the ratio of In and Ga, any color of red, green, blue and their combination, including white light, can be obtained. This UV photoluminescence (PL) breaks away from the traditional thinking framework (paradigm) that phosphor must be powder and instead uses MOCVD to add multiple layers of light source epitaxy on the UVLED chip.

in conclusion

目前LED 的主流技术只是光革命的前奏曲，真正的突破为以超级LED 制成高功率的UV 光源。钻石岛外延片可为桥樑把高压的钻石合成技术和真空的气相合成方法结合起来而制造出超级LED，包括钻石LED、AlN/Diamond、cBN/Diamond LED、AlN/cBN LED 及其它待开发的次世代LED 产品。在这个起跑点上，台湾深入的钻石科技可领先制成全球首创的钻石岛外延片，这样就可以大规模生产超级LED。若台湾的LED 科技公司可以合作开发这项蛙跳技术，则台湾的LED 产业不仅可摆脱「微笑曲线」的魔咒，更可把台湾从硅晶岛升级到钻石岛。