β-Ga2O3 can be used not only for power components, but also for LED chips, various sensor elements, and image sensors, and has a wide range of applications. Among them, LED chip substrates using GaN-based semiconductors are the most promising applications. In particular, it is worth mentioning that β-Ga2O3 is suitable for high-power LEDs that require large drive currents.
GaN-based LED chips are widely used for LEDs with shorter light wavelengths such as blue, violet, and ultraviolet. Among them, the blue LED chip is an important basic component of the white LED. GaN-based blue LED chips are now manufactured on sapphire substrates.
Compared with a sapphire substrate, the transmittance of ultraviolet light and visible light is 80% and the resistivity of the β-Ga2O3 substrate is about 0.005 Ωcm, which has good conductivity.
The higher the transmittance, the easier it is to extract the light emitted from the light emitting layer of the LED chip to the outside, which is expected to increase the light output power and the light emission efficiency. Moreover, since the conductivity is high, it is also possible to adopt a vertical structure in which the anode and the cathode are formed on the surface and the back surface of the LED chip, respectively. In contrast, the sapphire substrate has an insulating property, so the lateral structure of the anode and the cathode is horizontally arranged.
Compared with the lateral structure, the vertical structure can not only reduce the element resistance and thermal resistance, but also make the current distribution uniform. Since the smaller the element resistance and thermal resistance, the smaller the amount of heat generated by the LED chip, the higher the drive current is.
The vertical structure tends to make the current distribution uniform, so even if a large current flows, the LED chip is not easily damaged. In addition, the current evenly flows through the LED chip, which also reduces uneven light emission. Therefore, the optical output power per unit area of ​​the β-Ga2O3 substrate can be estimated to be more than 10 times higher than that of a conventional sapphire substrate product using a lateral structure.
SiC substrates can also achieve vertical structures, but their cost is high. With β-Ga2O3, it is expected that the substrate can be manufactured at a lower cost.
SiC substrates also have problems with element characteristics. The blue light absorption characteristics and resistance of the SiC substrate are in a trade-off relationship. Suppression of blue light absorption increases the resistance. Therefore, there is a limit to the reduction of the element resistance.
Optical output power is five times that of commercially available products Although a GaN-based LED chip using a β-Ga2O3 substrate is currently under development, certain results have been obtained. For example, the research team of the National Institute of Information and Communication Research (NICT) in Japan has prototyped a 300-μm-square LED element with a wavelength of 450 nm. In this device, an n-type GaN layer, an active layer of a multiple quantum well structure of InGaN/GaN, and a p-type GaN layer (FIG. A-1) were stacked on an n-type Ga 2 O 3 substrate via a buffer layer by MOCVD. An n-type electrode of Ti/Au is formed on the substrate side, and an Ag-type p-type electrode is formed on the other side.
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The prototype had a light output of 170mW at a drive current of 1200mA (Figure A-2). Compared with a commercially available 300 μm square lateral blue LED chip, light output power of 5 times or more can be achieved. In addition, by improving the light emitting layer and the light extraction structure, it is also expected to increase the light output power by a factor of two.
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In addition, NICT's research team also produced a beta-Ga2O3 substrate LED chip with reduced component resistance. The chip size is 300μm square and the operating voltage is only 3.3V when the drive current is 200mA (Figure A-3). The size of the transverse structure of the commercial product in the drive current of 200mA, the operating voltage up to 4.7V. Since the operating voltage is low, the amount of heat generated when driving with a large current can be reduced.
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The thermal resistance drops below 1/10. In addition, the heat resistance of the prototype LED chip is very low. By encapsulating the p-layer side of the LED chip downward, thermal resistance can be suppressed (Figure A-1). When AuSn is used as the bonding metal of the die bonding portion, and the LED chip size is 1 mm square, it is estimated that the thermal resistance of the active layer to the bonding metal is less than 0.1 DEG C/W, which is only 1 of the horizontal structure of the same size. /10 to 1/100.
Moreover, the current distribution of the trial LED chips is also uniform. To investigate the current distribution, the team examined the in-plane temperature distribution inside the 1mm square LED chip. The results show that even if the element temperature increases by an average of 70°C, the maximum in-plane temperature difference is only 7°C.
As described above, an LED chip using a [beta]-Ga2O3 substrate is very suitable for high-current applications. In the use of such substrates for LED products, NICT's research team is aiming to introduce products in fiscal year 2012 and promote development in the practical direction.
GaN-based LED chips are widely used for LEDs with shorter light wavelengths such as blue, violet, and ultraviolet. Among them, the blue LED chip is an important basic component of the white LED. GaN-based blue LED chips are now manufactured on sapphire substrates.
Compared with a sapphire substrate, the transmittance of ultraviolet light and visible light is 80% and the resistivity of the β-Ga2O3 substrate is about 0.005 Ωcm, which has good conductivity.
The higher the transmittance, the easier it is to extract the light emitted from the light emitting layer of the LED chip to the outside, which is expected to increase the light output power and the light emission efficiency. Moreover, since the conductivity is high, it is also possible to adopt a vertical structure in which the anode and the cathode are formed on the surface and the back surface of the LED chip, respectively. In contrast, the sapphire substrate has an insulating property, so the lateral structure of the anode and the cathode is horizontally arranged.
Compared with the lateral structure, the vertical structure can not only reduce the element resistance and thermal resistance, but also make the current distribution uniform. Since the smaller the element resistance and thermal resistance, the smaller the amount of heat generated by the LED chip, the higher the drive current is.
The vertical structure tends to make the current distribution uniform, so even if a large current flows, the LED chip is not easily damaged. In addition, the current evenly flows through the LED chip, which also reduces uneven light emission. Therefore, the optical output power per unit area of ​​the β-Ga2O3 substrate can be estimated to be more than 10 times higher than that of a conventional sapphire substrate product using a lateral structure.
SiC substrates can also achieve vertical structures, but their cost is high. With β-Ga2O3, it is expected that the substrate can be manufactured at a lower cost.
SiC substrates also have problems with element characteristics. The blue light absorption characteristics and resistance of the SiC substrate are in a trade-off relationship. Suppression of blue light absorption increases the resistance. Therefore, there is a limit to the reduction of the element resistance.
Optical output power is five times that of commercially available products Although a GaN-based LED chip using a β-Ga2O3 substrate is currently under development, certain results have been obtained. For example, the research team of the National Institute of Information and Communication Research (NICT) in Japan has prototyped a 300-μm-square LED element with a wavelength of 450 nm. In this device, an n-type GaN layer, an active layer of a multiple quantum well structure of InGaN/GaN, and a p-type GaN layer (FIG. A-1) were stacked on an n-type Ga 2 O 3 substrate via a buffer layer by MOCVD. An n-type electrode of Ti/Au is formed on the substrate side, and an Ag-type p-type electrode is formed on the other side.
The prototype had a light output of 170mW at a drive current of 1200mA (Figure A-2). Compared with a commercially available 300 μm square lateral blue LED chip, light output power of 5 times or more can be achieved. In addition, by improving the light emitting layer and the light extraction structure, it is also expected to increase the light output power by a factor of two.
In addition, NICT's research team also produced a beta-Ga2O3 substrate LED chip with reduced component resistance. The chip size is 300μm square and the operating voltage is only 3.3V when the drive current is 200mA (Figure A-3). The size of the transverse structure of the commercial product in the drive current of 200mA, the operating voltage up to 4.7V. Since the operating voltage is low, the amount of heat generated when driving with a large current can be reduced.
The thermal resistance drops below 1/10. In addition, the heat resistance of the prototype LED chip is very low. By encapsulating the p-layer side of the LED chip downward, thermal resistance can be suppressed (Figure A-1). When AuSn is used as the bonding metal of the die bonding portion, and the LED chip size is 1 mm square, it is estimated that the thermal resistance of the active layer to the bonding metal is less than 0.1 DEG C/W, which is only 1 of the horizontal structure of the same size. /10 to 1/100.
Moreover, the current distribution of the trial LED chips is also uniform. To investigate the current distribution, the team examined the in-plane temperature distribution inside the 1mm square LED chip. The results show that even if the element temperature increases by an average of 70°C, the maximum in-plane temperature difference is only 7°C.
As described above, an LED chip using a [beta]-Ga2O3 substrate is very suitable for high-current applications. In the use of such substrates for LED products, NICT's research team is aiming to introduce products in fiscal year 2012 and promote development in the practical direction.
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