Author: Vladimir Odnoblyudo
Red, orange and yellow dilute magnetic nitride LEDs have higher power and lower temperature sensitivity. Comparing the traditional red AlGaInP LED with the new InGaNP LED, it is found that the latter has a brighter illumination and is suitable for a large-screen color display backlight unit.
After participating in the competition in the general lighting market, the performance of blue and white LEDs has been greatly enhanced, which is distinct from the red, amber and yellow tubes used in large color screens, traffic lights and architectural lighting. In contrast, the latter performance is slow.
However, a new company, Quanlight, which has just been separated from the University of California, San Diego (UCSD), has developed a completely different chip manufacturing process that overcomes these obstacles and greatly improves the performance of LEDs. By replacing the conventional AlGaInP material with the new dilute magnetic nitride material InGaPN, it is possible to produce a high-brightness LED that is not sensitive to temperature changes. This will be useful for high quality images that require a steady red output for applications such as large screen color displays.
The core of this key technology comes from the findings of the University of California, San Diego (UCSD). The UCSD team led by charles Tu has made some progress on InGaPN before the establishment of Quanlight. This material contains approximately 1% nitrogen and appears to be another ideal material for red tubes. Once combined, InGaPN and GaP can produce a larger heterojunction than the AlInGaP-based LED. Calculations show that it produces LEDs with higher brightness due to greater current handling capability. In addition, although this material system is still immature, little known about it and difficult to grow, InGaPN has had some exciting preliminary results.
Conventional red LEDs have been used in automotive brake lights. When turning to InGaPN-based devices, the brightness of the lights is increased and the number of components required is reduced. Source: BMW
Researchers at UCSD managed to design an effective prototype LED that achieved several improvements in device performance as expected, including a reduction in the drift of the emission wavelength with temperature. Although these devices are sufficient for the initial study, MBE equipment can contaminate the material, which limits the brightness of the LED. Therefore, further research and development work needs to be transferred to commercial production plants, and the processes and equipment used therein are based on the MOCVD platform.
The commercialization of academic research for this purpose we created Quanlight and formed a research and development team. After raising a total of 4 million US dollars from private investors, we developed the epitaxial film through Bandwidth Semiconductor foundry service in early August 2006. Currently, we have made good progress and manufactured and sold according to the plan at the end of last year. Red LED epitaxial wafers are given to chip manufacturers. Now we plan to extend the range of epitaxial wafers to cover the orange and yellow bands from 585nm to 660nm. In addition, we are also beginning to license proprietary process technologies or to partner with other companies.
InGaPN LEDs have three main advantages over AlGaInP LEDs: lower manufacturing cost, better color temperature stability, and higher brightness at high currents.
Lower manufacturing costs are derived from a more streamlined manufacturing process that uses the same production equipment as traditional red LEDs. Conventional AlInGaP-based light-emitting tubes are grown on GaAs substrates. In order to increase the output power, the epitaxial wafers are usually placed on a transparent GaP substrate or mirror support. Our process dilute magnetic nitride can be directly grown on a GaP substrate, which reduces the two process steps of "epitaxial layer removal and pressure bonding" required for conventional fabrication; thereby reducing material costs.
In our devices, there is a small lattice mismatch between the GaP substrate and the InGaPN material, which means that the epitaxial wafer is pseudo-strained, but it allows for the integration of enough quantum wells in the LED to achieve high power. Output. The structure thus produced is similar in quality to the AlInGaP material grown on a GaAs substrate, but has the disadvantage that a GaP substrate prepared by vertical gradient condensation (VGF) is not commercially available. VGF is a manufacturing process capable of growing very low dislocation density pear crystals.
The substrate we are using is grown in a liquid-sealed straight-through (LEC) process, which is suitable for mass production of LEDs. The resulting device is commercially competitive in terms of brightness and reliability. At the same time, we are also working with PVA TePla to develop new technologies to prepare GaP pear crystals using the VGF method. We hope that this adventure will be successful because the VGF method is well known in the industry and has long been used to grow substrates of other materials. Although it is impossible to predict how profitable it will be after switching to VGF materials, we expect it to increase the lifetime and power output of InGaPN-based LEDs. We have begun to initially measure the LEDs produced on this platform and hope to achieve preliminary results in a short period of time.
The intrinsic properties of InGaNP ensure that the device's peak wavelength drifts with temperature will be less than that of AlInGaP devices. For applications that require a stable light source, such as a color display, the light emitting device is more attractive. At the UCSB laboratory, we found that the red LED made by the MBE method has improved color stability, but it is also effective for LEDs made by MOCVD. The LED was heated from 25 ° C to 125 ° C by an external heating method, and the peak emission wavelength at different temperatures was recorded. The test results are shown in Figure 1. The peak emission wavelength of the LED has only changed by 3 nm over the entire temperature range. Compared to the AlInGaP chip produced by a leading red LED manufacturer, the 3 nm offset is only the fifth of the latter. one.
The third advantage of InGaPN LEDs is that they illuminate brighter at high temperatures. They are derived from an excellent band structure that enhances the carrier confinement effect in the active region. InGaPN LEDs with GaP barriers produce a large band offset, typically 2-3 times the offset of the AlGaInP quantum well and the AlGaInP barrier band.
By comparing the output power of the Quanlight LED and the conventional red LED in the range of 25 ° C to 125 ° C, the results verify that the former is more powerful at higher temperatures (Figure 2).
The Quanlight device's transmit power at 125 ° C is 48% at 25 ° C, but the reference LED is only 25% of its initial value. When we completed the research and development of the device process, we hope that our materials can produce a device as bright as the AlInGaP chip at Room Temperature, and the brightness of the AlInGaP chip is about 2 times higher than that at 150 °C. The performance of InGaNP LEDs is improved at high temperatures, a feature that is more attractive for red and yellow traffic lights. Their minimum melting standards in the United States are 25 ° C and 74 ° C.
The enhanced carrier confinement effect in the active region is also beneficial for current handling, and the test device in the development phase has produced current densities of up to 9 A/mm2 (Figure 3). These tests were performed on the epitaxial wafer, not on each of the stripped chips. Therefore, it can be reasonably predicted that the effective parameters of the newly generated product will be lower. Nevertheless, we can predict that the saturation current density of InGaPN LEDs is 2-3 times that of AlInGaP based products.
The benefits of moving from a traditional red-light chip to an InGaPN device are threefold: higher current handling capability, which is helpful for LED packages and application engineers; the use of smaller devices at larger drive currents and the ability to emit light of the same brightness; or Fewer large size LEDs can be used in high power arrays. These methods can reduce the LED size and reduce the overall cost. Whether it is a smaller number of LEDs or the same number of LEDs, the total cost will be cheaper because of the small LED size.
Now we are testing the reliability of this red LED, which will be tested for 5,000 hours in the device under development; we also plan to compare the performance of LEDs made from substrates grown by LEC and VGF.
Ready to start
We have transferred the growth of the device to the MOCVD platform and optimized the epitaxial layer design. An increase in light output was observed from our device. In relation to the intrinsic properties of dilute magnetic nitrides, we can achieve these advantages without compromising the color and thermal stability of the illuminator.
Although many people in our field may think that dilute magnetic nitride is an incomprehensible material, it does not play its due role in the telecommunications sector. We have reason to believe that this device will be a commercial success. The epitaxial layer of a dilute magnetic nitride communication laser requires a high concentration of indium, which increases the stress of the material and reduces the lifetime and reliability of the device. However, the red, orange, and yellow InGaPN LEDs contain very little indium, which does not cause this problem.
We have gained a wealth of experience in the preparation of dilute magnetic nitrides, which has earned us a strong competitive edge compared to other companies that may be preparing to develop products based on this material. Although the epitaxial wafers were grown at Bandwidth Semiconductor, we still have the process knowledge and intellectual property in our hands, and the technical team is involved in all development and growth processes on site. This team led the development of this material.
When we launched more combinatorial products such as high-power red, brown and yellow epitaxial LEDs covering 585nm to 660nm, we have entered a fast-growing high-brightness market with a target of $500 million. The performance advantages of our products will be suitable for applications that require high power and stable color output. The temperature-induced wavelength drift of the light-emitting chip is reduced, which benefits the LCD TV backlight unit, the projector's light engine, outdoor display and other red-green-blue color mixing applications, that is, the control mechanism of the display device is simplified. At the same time, this new high-intensity red LED is also suitable for lighting in transportation, hazardous areas, theaters and buildings.
Applications such as traffic lights and automotive brake lights use AlInGaP LEDs to reduce energy and cost. For this type of high-power applications, InGaPN LEDs enable engineers to design more low-cost luminaires, as mentioned earlier: at higher Use smaller chips at the current or use fewer large LEDs in the array. Because Quanlight LEDs work efficiently at high temperatures, you need a housing that is more compact or that enhances heat dissipation.
In low-power applications where strict output specifications are lacking, such as Christmas tree lighting, this is the only inferior place for InGaPN LEDs. Currently, Quanlight is not aggressively pursuing this low-power application market because low-cost low-power AlInGaP chips are widely available. It is.
About the Author
Vladimir Odnoblyudov is the CTO of Quanlight, a Ph.D. at the University of California, San Diego, specializing in InGaPN LED research. He also developed 1.3-inch dilute magnetic nitride lasers at the Ioffe Institute of Physics and Technology in St. Petersburg for five years.
Neil Senturia is the CEO of Quanlight, a technology-born company with 25 years of experience in creating and managing startups.
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