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substrate

Material name

Substrates are divided into drawing substrates and chemical substrates. Drawing substrate refers to filling the whole layout with pictures or text to make it Shading 。 The most common chemical substrates are nitrides Substrate material, etc.

Chinese name: substrate
Classification: Drawing substrate, chemical substrate

Materials: Sapphire, silicon, silicon carbide
Other materials: GaAS、AlN、ZnO

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four Material evaluation

Drawing substrate

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Fill the entire layout with pictures or text Shading 。

Material Introduction

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Basic Introduction

nitrides Of substrate material research and development GaN based semiconductor with increased font reset broadband gap at short wavelength light-emitting diode 、 Laser and UV detector , and high temperature Microelectronic device It shows a broad application prospect; For environmental protection, it is also suitable for environmental protection material system. Semiconductor lighting Industrial development Each of the main stages shown in the classification can form a distinctive industrial chain. Countries all over the world have now invested a lot of human, financial and material resources to expect to make breakthroughs in GaN based high-power devices and occupy the commanding heights in this field. The paper "Application prospect of nitride substrate materials and semiconductor lighting" introduces part of the application prospect of nitride substrate materials and semiconductor lighting.

GaN, AlN, InN and their alloys are GaN series materials as new materials. To evaluate the substrate materials, it is important to consider the factors of the substrate materials and find a more suitable substrate for the development of GaN based technology. The evaluation of substrate materials should take into account the Lattice matching , substrate and epitaxial film Coefficient of thermal expansion Matching, substrate and epitaxial film chemical stability Matching, difficulty of material preparation and cost. InN's Epitaxial substrate Materials are widely used now. Self-supporting Homoepitaxy The development of substrate Independent intellectual property rights Nitride of semiconductor laser High power High brightness Semiconductor lighting LED and high power Microwave device Waiting is very important. The paper "Evaluation factors, research and development of nitride substrate materials" introduces some contents of evaluation factors, research and development of nitride substrate materials.

application prospect

GaN is direct band gap Material, its light Transition probability One higher than the indirect band gap Order of magnitude 。 Therefore, the wide band gap GaN based semiconductors can be used in short wavelength light-emitting diodes, lasers and UV detector , and high-temperature microelectronic devices show broad application prospects; For environmental protection, it is also suitable for environmental protection material system.

In 1994, Japan's Nicha Company made a breakthrough in GaN/Al2O3. In 1995, GaN devices were commercialized for the first time. In 1998, GaN based light-emitting diode LED market The scale is US $500 million. In 2000, market size Expand to US $1.3 billion. According to the estimation of authoritative experts, GaN based LEDs and the Al2O3 substrate they use are international market The market growth period will reach 50 years. GaN based LED and its Al2O3 substrate have unique and excellent physical and chemical properties, and have a long-term durability 。 It is expected that the market size of GaN based devices will expand to US $3 billion in 2005, and the market size of Al2O3 substrates used in GaN based devices will expand to US $500 million.

Semiconductor lighting The classification of industrial development shows several major stages, each of which can form a distinctive industrial chain:

(1) Phase I

The first stage (special lighting era, before 2005), including: instrument and meter indication; Gold display, indoor and outdoor advertising; traffic lights 、 Signal lamp 、 beautiful Lights Automobile lamp ； Indoor permanent light suspended ceiling Light, color changing light Lawn lamp ； Urban landscape Beautifying architectural contour lights, bridges expressway , tunnel guiding street lights, etc.

(2) Phase II Phase II (lighting era, 2005~2010), including CD DVD 、H-DVD optical storage ； Laser gold display; Entertainment Bar code , printing, image recording; Medical laser; Open up new fields of fixed lighting, derive new lighting industries, lay the foundation for general lighting applications, etc. (3) The third stage The third stage (the era of general lighting, after 2010), including the application of the above two stages, has also fully entered the general lighting market, accounting for 30-50% market share 。

Up to now (in the first stage, the era of special lighting), medium and low power blue Light emitting diode (LED) , green LED White LED , blue and purple LED, etc. have achieved mass production and entered the commercial market. High power blue light-emitting diode (LED)、 laser diode (LD) and full band InN GaN will lead to new and greater business opportunities, such as, optical storage 、 Optical communication Etc. Achieve high power Blue LED (LED), laser diode (LD) and full band InN GaN are practical and commercialized, which requires appropriate substrate materials. Therefore, with the development of GaN materials and devices, it is necessary to find a substrate material matching GaN to further improve the quality of epitaxial films.

In addition fundamental research And Long term plan Considering that scientific and technological development increasingly needs to combine materials of different systems, namely Heterojunction Material Science. The application of covariant substrate can limit the lattice and thermal mismatch defects to the substrate, and lay the foundation for the development of new material systems. Several preparation technologies of covariant substrates have been proposed, such as self supporting substrates Bonding Twisted bonding, repositioned lattice Transition layer , and SOI And VTE substrate technology. It is expected that in the next 10 to 20 years, the preparation technology of large size, covariant substrates will achieve a breakthrough and be widely used in large mismatch Heterojunction material Growth and its associated light electronic device manufacture.

Countries all over the world have now invested a lot of human, financial and material resources in the hope of making a breakthrough in GaN based high-power devices and occupying the commanding height in this field.

Evaluation factors of nitride substrate materials and research and development of GaN, AlN, InN and their alloys are GaN series materials as new materials. To evaluate the substrate material, it is an important goal to comprehensively consider its factors and find a more suitable substrate for the development of GaN based technology.

Research and Development of InN Epitaxial Substrate Materials

InN epitaxial substrate materials are now widely used, including: InN; α－Al2O3(0001)； 6H-SiC； MgAl2O4(111)； LiAlO2 and LiGaO2; MgO ； Si； GaAs (111), etc.

III-V compounds, such as GaN, AlN and InN, have two crystal forms: one is Cubic system Of sphalerite Structure, and the other is Hexagonal system Of Wurtzite Structure. Focusing on blue light radiation Wurtzite structure Of Gallium nitride 、 Aluminium nitride 、 inn And mainly gallium nitride, aluminum nitride and indium nitride Solid solution 。 The forbidden band of these materials is Direct transition Type, so there is a high quantum efficiency 。 The solid solution produced by gallium nitride, aluminum nitride and indium nitride in different components and proportions Band gap width It can vary from 2.2eV to 6.2eV. In this way, using these solid solutions to make light-emitting devices is Photoelectric integrated material And the direction of device development.

(1) InN and GaN

because Heteroepitaxy Nitride films usually bring a lot of defects, which damage the performance of devices. Same as GaN, if it can be carried out on InN Homoepitaxy Growth can greatly reduce defects, so the performance of devices will have a huge leap.

Self-supporting Homoepitaxy GaN, AlN and AlGaN substrates are the most likely substrates for practical applications.

(2) Sapphire (α - Al2O3) and 6H SiC

α - Al2O3 single crystal, namely sapphire boule 。 (0001) sapphire is the most commonly used epitaxial substrate material of InN at present. The matching direction is: InN (001)//α - Al2O3 (001), InN [110]//α - Al2O3 [100] [11,12]. Because the substrate surface becomes AlON during nitridation before film growth, InN rotates 30 ° around the hexagonal lattice structure of α - Al2O3 (0001) substrate, so it loses Matching degree This is slightly less than the original 29%. Although the mismatch ratio between (0001) sapphire and InN lattice is as high as 25%, due to its hexagonal symmetry, the melting point is 2050 ℃, the highest working temperature Up to 1900 ℃, with good high temperature stability and mechanical mechanical property In addition, due to more research, more mature production technology and low price, it is still the most widely used substrate material.

6H SiC is only second to sapphire in its extensive application as a substrate material. Compared with sapphire, the lattice matching of 6H SiC and InN epitaxial films is improved. In addition, 6H SiC has blue light emission characteristics, and is a low resistance material, which can be used to make electrodes. This makes it possible for devices to fully test the epitaxial film before packaging, thus enhancing the competitiveness of 6H SiC as a substrate material. And because the layered structure of 6H SiC is easy to cleavage High quality Cleavage plane , which will greatly simplify the structure of the device; But at the same time, because of its Layered structure There are often steps on the surface of the substrate that introduce a large number of defects into the epitaxial film.

(3) Magnesium aluminum spinel (MgAl2O4)

MgAl2O4 crystal , i.e Magnesium aluminate Crystal. MgAl2O4 crystal is High melting point (2130 ℃), high hardness (Mohs 8) Crystalline material , belongs to face centered cubic Crystal system ， Space group Fd3m, lattice constant 0.8085nm. MgAl2O4 crystal is excellent Acoustic medium Material, sound attenuation reduction in microwave section, microwave made of MgAl2O4 crystal delay line insertion loss Small. MgAl2O4 crystal and Si lattice Good matching performance Coefficient of expansion It is also similar to Si, so the deformation Small distortion and large-scale production Ultrahigh speed integrated circuit Speed ratio It is faster to make sapphire. In addition, MgAl2O4 crystal is also used as Superconductive material , has a very good effect. In recent years, there have been many studies on the epitaxial substrate materials of MgAl2O4 crystal for GaN. Because MgAl2O4 crystal has good lattice matching and thermal expansion The mismatch ratio between (111) face MgAl2O4 crystal and GaN lattice is 9% thermal stability and chemical stability , as well as good mechanical properties. MgAl2O4 crystal is currently one of the more suitable substrate materials for GaN. High quality GaN films have been successfully epitaxed on MgAl2O4 substrates and have been successfully developed Blue LED And LD. In addition, the most attractive aspect of MgAl2O4 substrate is that the laser cavity surface can be obtained by cleavage.

On the basis of previous studies, recent studies on MgAl2O4 crystal as the epitaxial substrate material for InN have also been reported in the literature. The matching direction between them is: InN (001)//MgAl2O4 (111), InN [110]//MgAl2O4 [100], InN rotates 30 ° around the square and hexagonal lattice structure of MgAl2O4 (111) substrate. The research shows that the mismatch ratio between (111) face MgAl2O4 crystal and InN lattice is 15%, and the lattice matching performance is much better than that of sapphire. The mismatch ratio between (0001) face sapphire and InN lattice is as high as 25%. Moreover, if the magnesium atom below the oxygen atom layer on the top layer occupies the effective coordination lattice position, as well as the oxygen lattice position, it is possible to further reduce the lattice mismatch ratio to 7%, which is much lower than that of sapphire. So MgAl2O4 crystal is very development potential The epitaxial substrate material of InN.

(4) LiAlO2 and LiGaO2

Previous research has focused on LiAl O2 And LiGaO2 are used as epitaxial substrate materials for GaN. The mismatch between LiAlO2 and LiGaO2 and GaN epitaxial films is quite small, which makes LiAlO2 and LiGaO2 become quite suitable epitaxial substrate materials for GaN. At the same time, as the epitaxial substrate material of GaN, LiGaO2 has its unique advantages: epitaxial growth After GaN, the LiGaO2 substrate can be corroded, leaving GaN epitaxial film, which will greatly facilitate the fabrication of devices. However, due to the Lithium ion Very active, under normal epitaxial growth conditions (for example, MOCVD The chemical atmosphere and growth temperature of single crystal As GaN epitaxial substrate materials, further research is needed. At present, LiAlO2 and LiGaO2 are rarely used as epitaxial substrate materials of InN.

（5）MgO

MgO crystal Cubic system , is a NaCl type structure, with a melting point of 2800 ℃. Because MgO crystal MOCVD The atmosphere is not stable enough, so it is seldom used, especially for InN films with higher melting point and growth temperature.

（6）GaAs

GaAs (111) is also the substrate material for growing InN thin films. When the nitriding temperature of the substrate is lower than 700 ℃, and the thickness of the grown InN film is less than 0.05 μ m, the InN film is of cubic structure. When the thickness of the grown InN film exceeds 0.2 μ m, the cubic structure disappears and all of them are transformed into hexagonal InN films. InN thin films in GaAs (111)

The nucleation mode on the substrate is very different from that on the α - Al2O3 (001) substrate White gemstone The columnar and fibrous structure of InN film grown on the substrate appears very flat on the surface.

（7）Si

single crystal Si， It is widely used semiconductor material 。 It is very interesting to use Si as the substrate material of InN, because it is possible to integrate InN based devices with Si devices. In addition, Si technology has been quite mature in the semiconductor industry. It can be imagined that if the device quality InN epitaxial film can be grown on the Si substrate, it will greatly simplify the manufacturing process of InN based devices and reduce the size of devices.

（8）ZrB2

ZrB2 is a new substrate for nitride epitaxy first proposed by Japanese scientists in 2001. ZrB2 and nitride lattice Match, and it has a matching Coefficient of thermal expansion And high conductivity 。 Mainly used Flux method And floating zone growth.

Self-supporting Homoepitaxy The development of substrate is important for the development of nitride with independent intellectual property rights semiconductor laser High power and brightness Semiconductor lighting LED and high power Microwave device Waiting is very important.

Material selection

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For production LED chip For example, the selection of substrate materials is the primary consideration. Which suitable substrate should be used depends on the equipment and LED Device requirements.

At present, there are generally three kinds of materials on the market that can be used as substrates:

· sapphire （ Al2O3 ）

· silicon (Si)

· silicon carbide （SiC）

Sapphire substrate Generally, GaN based materials And device Epitaxial layer It mainly grows on sapphire substrate. Sapphire substrate has many advantages: first, the Production technology Mature and good device quality; Secondly, sapphire has good stability and can be used in the process of high temperature growth; Finally, sapphire mechanical strength High, easy to handle and clean. Therefore, most processes generally use sapphire as the substrate. Made of sapphire substrate LED chip 。 There are also some problems with sapphire as the substrate for LED chips, such as Lattice mismatch and Thermal stress Mismatch, which will produce a lot of defects in the epitaxial layer, and make the subsequent device processing difficult. Sapphire is a kind of insulator , normal temperature resistivity More than 1011 Ω· cm , cannot be made in this case vertical structure Device; Generally, only n-type and p-type electrodes are fabricated on the upper surface of the epitaxial layer. Two electrodes are fabricated on the upper surface, which reduces the effective light-emitting area and increases photolithography and etching in device manufacturing Process , resulting in Material utilization rate Reduce and increase costs. Due to the difficulty of P-type GaN doping, the method of preparing metal transparent electrodes on p-type GaN is widely used to make the current diffuse to achieve the goal of uniform luminescence. However, the metal transparent electrode generally absorbs about 30%~40% of the light, and the GaN based material Chemical properties Stable, high mechanical strength, difficult to carry out Etching Therefore, better equipment is needed during etching, which will increase production costs 。 The hardness of sapphire is very high, which is second only to that of natural materials diamond However, in the manufacturing process of LED devices, it needs to be thinned and cut (from 400nm to about 100nm). The addition of equipment to complete thinning and cutting processes will increase a large investment. The thermal conductivity of sapphire is not very good (about 25W/(m · K) at 100 ℃). Therefore, when using LED devices, a large amount of heat will be transferred; Especially for high-power devices with large area, thermal conductivity is a very important consideration. In order to overcome the above difficulties, many people try to Photoelectric device It is directly grown on silicon substrate to improve thermal conductivity and conductivity.

There are some silicon substrates at present LED chip Silicon substrate is adopted. The chip electrode on the silicon substrate can adopt two contact modes, namely, L contact (horizontal contact) and V contact (vertical contact), hereinafter referred to as L electrode and V electrode. Through these two contact modes, the current inside the LED chip can flow horizontally or vertically. Because the current can flow longitudinally, the luminous area of the LED is increased, thereby improving the luminous efficiency of the LED. Because silicon is A good conductor of heat Therefore, the thermal conductivity of the device can be significantly improved, thereby extending the life of the device.

silicon carbide Substrate silicon carbide substrate CREE Specially using SiC material as the substrate) LED chip The electrode is L-shaped and the current flows longitudinally. The devices made with this substrate have excellent conductivity and thermal conductivity, which is conducive to making high-power devices with large area. Silicon carbide substrate LED chip 。 use Sapphire substrate LED chip with silicon carbide substrate Thermal conductivity of silicon carbide substrate ( thermal conductivity 490W/(m · K) is more than 10 times higher than the sapphire substrate. Sapphire itself is Hot bad conductor And the bottom needs to be used when making devices Silver glue Solid crystal The heat transfer performance of this silver adhesive is also very poor. use silicon carbide The chip electrode of the substrate is L-shaped, and the two electrodes are distributed on the surface and bottom of the device. The heat generated can be directly derived from the electrode; At the same time, this substrate does not require current diffusion layer Therefore, the light will not be absorbed by the material of the current diffusion layer, which improves the light output efficiency. But compared with sapphire substrate, silicon carbide Manufacturing cost It is relatively high, and the corresponding cost needs to be reduced to realize its commercialization.

Performance comparison of the three substrates LED chip Three commonly used substrate materials. See Table 1 for comprehensive performance comparison of these three substrate materials. Table 1 Performance comparison of three substrate materials In addition to the above three commonly used substrate materials, GaAS AlN 、 ZnO Such materials can also be used as substrates, usually according to the design needs.

Material evaluation

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1. Structure matching of substrate and epitaxial film: crystal structure Same or similar lattice constant Small mismatch, good crystallinity, low defect density;

2. Substrate and epitaxial film Coefficient of thermal expansion Matching: the matching of thermal expansion coefficient is very important. The difference between the thermal expansion coefficient of the epitaxial film and the substrate material is too large, which may not only reduce the quality of the epitaxial film, but also affect the quality of the device working process Medium, component damage caused by heating;

3. Substrate and epitaxial film chemical stability Matching: the substrate material should have good chemical stability epitaxial growth It is not easy to decompose and corrode in the temperature and atmosphere of chemical reaction The quality of epitaxial film is reduced;

4. The difficulty and cost of material preparation: In consideration of the needs of industrial development, the preparation of substrate materials should be simple and the cost should not be very high. The substrate size is generally not less than 2 inches.

Currently, there are many substrate materials for GaN based LEDs, but there are only two kinds of substrates that can be used for commercialization, namely sapphire and silicon carbide Substrate. Other substrates such as GaN, Si and ZnO are still in the research and development stage, which is still a long way from industrialization.

Gallium nitride ：

The most ideal substrate for GaN growth is GaN Single crystal , which can greatly improve the crystal quality of the epitaxial film and reduce dislocation density , improve device working life , improve luminous efficiency , improve device operation current density 。 However, it is very difficult to prepare GaN bulk single crystals, and there is no effective method so far.

zinc oxide ：

The reason why ZnO can be a candidate substrate for GaN epitaxy is that they have very striking similarities. both crystal structure Same, very small lattice recognition, Band gap width Proximity (energy band Discontinuity Small value, contact barrier Small). However, the fatal weakness of ZnO as GaN epitaxial substrate is that epitaxial growth It is easy to decompose and corrode in the temperature and atmosphere of. At present, ZnO semiconductor material It cannot be used to manufacture optoelectronic devices or high-temperature electronic devices, mainly because the material quality cannot reach the device level and the P-type doping problem has not been really solved, and the equipment suitable for the growth of ZnO based semiconductor materials has not been successfully developed.

sapphire:

The most common substrate for GaN growth is Al2O3. Its advantages are chemical stability Good, no absorption visible light The price is moderate and the manufacturing technology is relatively mature. Thermal conductivity Although the difference is in the device Small current There is no obvious lack of exposure in the work, but the power type device large current The problem under work is very prominent.

silicon carbide ：

SiC is only second to sapphire in its extensive application as a substrate material. At present, there is no third substrate for GaNLED Commercialization Production. SiC substrate has good chemical stability, good electrical conductivity, good thermal conductivity, and does not absorb visible light, but its disadvantages are also prominent. For example, the price is too high, the crystal quality is difficult to reach the level of Al2O3 and Si, and the mechanical processing performance is relatively poor. In addition, SiC substrate absorbs less than 380 nm ultraviolet , is not suitable for developing UV LED 。 Due to the beneficial conductivity and thermal conductivity of SiC substrate, it can better solve the heat dissipation problem of power type GaNLED devices Semiconductor lighting technology The field occupies an important position.

Compared with sapphire, the lattice matching of SiC and GaN epitaxial films is improved. In addition, SiC has blue emission characteristics and is a low resistance material, which can be fabricated

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