학술논문
AlGaInP계 수직형 적색 발광 다이오드의 광추출효율을 향상하기 위한 연구 / The Study for Increasing Light Extraction Efficiency of AlGaInP-based Vertical Red Light Emitting Diode
Document Type
Dissertation/ Thesis
Author
Source
Subject
Language
Korean
Abstract
본 논문에서는 AlGaInP 기반 수직형 적색 LED의 광추출효율을 개선할 수 있는 다양한 방법은 제시하였다. AlGaInP 기반 적색 LED는 화합물 반도체로서 대기와의 큰 굴절률 차이에 의해 빛이 외부로 빠져 나올 수 있는 각이 매우 좁다. 또한 GaAs 기판의 빛 흡수 문제로 인하여 LED 내부에서 발생되는 상당량의 빛이 흡수된다. Si 전도성 기판을 이용한 Wafer bonding process LED는 매우 효율적인 구조를 갖고 있다. 이 수직형 적색 LED의 장점은 GaAs 흡수기판을 제거하고 전도성 금속 기판을 이용하기 때문에 빛 흡수 문제를 해결하고 GaAs 기판 LED 보다 고전류에서 빛의 포화상태가 발생되지 않는 장점이 있다. 하지만 LED 내부의 빛을 더 효율적으로 빼내기 위한 노력은 계속 지속되고 있다.이번 논문에서는 이와 같은 문제를 해결하기 위하여 수직형 적색 LED의 반사막 효율을 개선하고, 간단한 방법의 n-표면 roughening을 형성 하였으며, ITO current spreading 층을 통해 n-ohmic 금속의 최적화된 pattern 모형을 제시 하였다. ITO를 이용한 ODR 반사막은 기존의 Ag 반사막 보다 42%의 높은 휘도증가를 보였으며 n-표면 roughening 형성을 통해 9000 nm의 etching 깊이를 갖는 조건에서 roughening이 형성되지 않은 수직형 LED 보다 130 %의 높은 휘도증가를 보였다. 또한 n-ohmic 금속의 metal pattern을 변경 하여 최적화된 패턴모형을 제시하였으며 상위 금속 ITO의 최적화된 current spreading 두께를 보고 하였다. 하지만 본 연구의 LED 소자들은 높은 광학적 특성에 비하여 전기적 특성은 기존의 수직형 LED보다 높은 구동 전압을 보였다. 이것은 ITO층 자체의 높은 저항으로 인하여 LED의 series 저항이 증가된 것으로 판단된다. 따라서 ITO의 저항을 더 낮출 수 있는 증착 조건에서 실험이 진행된다면 LED의 전기적 특성과 광학적 특성을 동시에 만족할 수 있는 고효율 발광다이오드 소자를 얻을 수 있을 것으로 기대된다.
In this study, we report on more efficient formations for increasing the light extraction efficiency of AlGaInP based vetical red LEDs. The quaternary AlGaInP material system, grown by metal organic chemical vapor deposition, has proven to be the best choice in many application such as interior and exterior automotive lighting, traffic lights, full color displays, and all kinds of indoor and outdoor signs. The major issue in quaternary AlGaInP LEDs design is the poor light extraction out of the chip due to the large difference of the refractive index between the GaP window (n=3.3) layer and air (n=1). Furthermore, the AlGaInP LEDs are grown on GaAs substrates that are opaque for the emitting wavelengths of this quaternary material. Hence, most of the light radiated from the active region is trapped in the device and is eventually absorbed by the GaAs absorbing substrate. In order to obtain high performance AlGaInP-based LEDs, it is significantly important to enhance the light extraction efficiency.In this study, we fabricated AlGaInP-based vertical LEDs by wafer bonding technique with Si substrate in order to increase the light extraction efficiency of LEDs. First, AlGaInP LED employing a conductive omni-directional reflector (ODR) using ITO and IZO films is presented. The omni-directional reflector consisted of p-type GaP, a transparent conducting oxide layer, and an Ag layer. The transmittance of the ITO and IZO films were found 96% and 95% after annealing process of 350 ℃ at the wavelength of 620 nm, respectively. The forward voltages of the LEDs with TCO/Ag at 20 mA were slightly higher than that of the LEDs with Ag due to the high resistivity of the TCO. The brightness of the AlGaInP LEDs with ODRs using ITO and IZO was increased by 42% and 38% compared with that of the LED with Ag reflector.Secondly , we have investigated the effect of n-side roughness on the light extraction efficiency of the vertical AlGaInP-based light emitting diodes (LEDs). chemical wet etching technique was produced by using a roughened surface with triangle-like morphology. A commonly used H₃PO₄-based solution was applied for chemical wet etching. The light extraction of AlGaInP LED was related to the n-side roughed surface morphology. The morphology of roughed surface is analyzed by the atomic force microscope (AFM). As a result, the roughed surface AlGaInP LED has a root-mean-square (RMS) roughness of 44 nm. The brightness shows 41% increase after roughening n-side surface, as compared to the ordinary flat surface LED. Also, the optimized condition of roughened surfaces was obtained for 900 nm of etching depth. It was shown that the luminous intensity of the AlGaInP-based LEDs with the n-side roughness was two times higher than that of the LEDs without the n-side roughness.Finally, we proposed a new vertical red LED structure using the dot n-ohmic metal patterns and the top surface ITO current spreading layer to enhance the optical and electric characteristics. we have investigated the vertical red LEDs using the ITO current spreading layer as a function of ITO thickness, and the n-ohmic metal patterns are modified by size and area. The AlGaInP-based vertical LED using the ITO current spreading thickness of 150 nm and the smallest n-ohmic metal are shown the higher luminous intensity of 353 mcd at 20 mA. The luminous intensity shows 20% increase than the conventional device.
In this study, we report on more efficient formations for increasing the light extraction efficiency of AlGaInP based vetical red LEDs. The quaternary AlGaInP material system, grown by metal organic chemical vapor deposition, has proven to be the best choice in many application such as interior and exterior automotive lighting, traffic lights, full color displays, and all kinds of indoor and outdoor signs. The major issue in quaternary AlGaInP LEDs design is the poor light extraction out of the chip due to the large difference of the refractive index between the GaP window (n=3.3) layer and air (n=1). Furthermore, the AlGaInP LEDs are grown on GaAs substrates that are opaque for the emitting wavelengths of this quaternary material. Hence, most of the light radiated from the active region is trapped in the device and is eventually absorbed by the GaAs absorbing substrate. In order to obtain high performance AlGaInP-based LEDs, it is significantly important to enhance the light extraction efficiency.In this study, we fabricated AlGaInP-based vertical LEDs by wafer bonding technique with Si substrate in order to increase the light extraction efficiency of LEDs. First, AlGaInP LED employing a conductive omni-directional reflector (ODR) using ITO and IZO films is presented. The omni-directional reflector consisted of p-type GaP, a transparent conducting oxide layer, and an Ag layer. The transmittance of the ITO and IZO films were found 96% and 95% after annealing process of 350 ℃ at the wavelength of 620 nm, respectively. The forward voltages of the LEDs with TCO/Ag at 20 mA were slightly higher than that of the LEDs with Ag due to the high resistivity of the TCO. The brightness of the AlGaInP LEDs with ODRs using ITO and IZO was increased by 42% and 38% compared with that of the LED with Ag reflector.Secondly , we have investigated the effect of n-side roughness on the light extraction efficiency of the vertical AlGaInP-based light emitting diodes (LEDs). chemical wet etching technique was produced by using a roughened surface with triangle-like morphology. A commonly used H₃PO₄-based solution was applied for chemical wet etching. The light extraction of AlGaInP LED was related to the n-side roughed surface morphology. The morphology of roughed surface is analyzed by the atomic force microscope (AFM). As a result, the roughed surface AlGaInP LED has a root-mean-square (RMS) roughness of 44 nm. The brightness shows 41% increase after roughening n-side surface, as compared to the ordinary flat surface LED. Also, the optimized condition of roughened surfaces was obtained for 900 nm of etching depth. It was shown that the luminous intensity of the AlGaInP-based LEDs with the n-side roughness was two times higher than that of the LEDs without the n-side roughness.Finally, we proposed a new vertical red LED structure using the dot n-ohmic metal patterns and the top surface ITO current spreading layer to enhance the optical and electric characteristics. we have investigated the vertical red LEDs using the ITO current spreading layer as a function of ITO thickness, and the n-ohmic metal patterns are modified by size and area. The AlGaInP-based vertical LED using the ITO current spreading thickness of 150 nm and the smallest n-ohmic metal are shown the higher luminous intensity of 353 mcd at 20 mA. The luminous intensity shows 20% increase than the conventional device.