高頻調變砷化鎵/砷化鋁鎵系列近紅外光發光二極體之研究

Study of High-Frequency Modulation of GaAs/AlGaAs Near-Infrared Light-Emitting Diodes

陳尚甫(Shang-Fu Chen)

指導教授 ： 吳孟奇 ； 何充隆

國立清華大學/電機資訊學院/電子工程研究所/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00061

摘要

在本論文中，我們製作以原子層沉積的氧化鎵鋅薄膜作為砷化鎵/砷化鋁鎵高頻調變近紅外光發光二極體元件的電流散布層，並探討其對元件特性的影響。其中氧化鎵鋅薄膜呈現約 3.3 x 10-4 Ω-cm 的電阻率及橫向高片電阻7.8 Ω/□；與p型重摻雜砷化鎵之間存在低的特徵接觸電阻值約 1.7 × 10-5 Ω-cm2。為了得知元件中電阻、電容的限制如何影響元件頻寬的特性，我們針對不同發光孔徑的元件進行電流-電壓 (I-V) 及電容-電壓 (C-V) 特性的量測。而在近紅外光發光二極體的元件特性方面，對於有著59、84、109、134、159及184 µm發光孔徑及直徑80 µm焊接墊的元件，在注入電流20 mA的情況下，隨著發光孔徑的增加所呈現的順向偏壓為1.6至1.7 V；串聯電阻值則介於5.3至6.1 歐姆 (Ω)。在- 5 V偏壓下，對於不同發光孔徑面積的元件，總電容 (CT) 分別為11.6、14.4、17.9、21.8及31.8 pF。由於總電容與元件接面的面積成正比，因此由不同發光孔徑面積的元件電容特性，可取得直徑80 µm的焊接墊其焊接墊電容（Cp）值約為8.8 pF。
另外，在注入電流50 mA的情況下，隨著元件發光孔徑的增加顯示其光輸出功率分別為4.6、5.4、5.7、5.8、6.2及6.3 mW。而元件的峰值發光波長約為757 nm，在固定電流25 mA的情況下。 在元件製程設計方面，藉由環型金屬電極的設計及低電阻率的氧化鎵鋅電流散佈層的運用，使得注入到局限區域內的電流密度增加，以致元件頻率響應在50 mA下，隨發光孔徑的增加， 3dB 截止頻寬分別達107.8、90.6、74.6、66.8、48.3與45.4 MHz。
另外，也藉由少數載子生命期的觀點去驗證氧化鎵鋅薄膜在砷化鎵/砷化鋁鎵近紅外光發光二極體電流散布的效果。文中主要也分析了元件發光區域面積，對於少數載子生命期的影響，並進一步求得在定電流下，發光二極體達到3dB頻寬為100 MHz時，所需的元件發光區域面積大小。就我們所製作的近紅外光發光二極體而言，在注入電流分別為 10、20、30、40和50 mA的情況下，達到3dB頻寬為100 MHz所需的元件發光區域面積依序為1.4 x 10-6 、5.1 x 10-6、1.7 x 10-5、3.4 x 10-5及 5.1 x 10-5 cm2。然而，在不同直徑的元件發光區域下，相同的電流密度有相同少數載子生命期，其少數載子生命期與電流密度之間存在著開根號成反比的關係，而與元件發光區域的直徑無關。這也代表著氧化鎵鋅薄膜在近紅外光發光二極體中有良好的電流散布特性，而顯現出元件有較低的接觸電阻和順向導通電壓。

關鍵字

高頻調變 ； 砷化鎵/砷化鋁鎵 ； 近紅外光發光二極體 ； 氧化鎵鋅 ； 原子層沉積 ； 特徵接觸電阻 ； 環形電極 ； 少數載子生命期 ； 3-dB 頻寬

並列摘要

In this dissertation, we investigate the fabrication and characterization of high-frequency modulation of GaAs/AlGaAs near-infrared (NIR) light-emitting diodes (LEDs) by using gallium-doped zinc oxide (GZO) prepared by atomic layer deposition (ALD) as the current-spreading layer. The GZO film reveals a low resistivity of ~ 3.3 x 10-4 Ω-cm, a high sheet resistance of 7.8 Ω/□ in the lateral direction. For the GZO contacts to p+-type GaAs, the minimum specific contact resistance of 1.7 x 10-5 Ω-cm2 is obtained. In order to obtain the device RC-limited bandwidth, we performed the current-voltage (I-V) and capacitance-voltage (C-V) measurements on devices with different aperture diameters. The LEDs with the aperture diameter of 59, 84, 109, 134, 159, and 184 μm, and the same bonding pad diameter of 80 μm have a forward voltage of 1.6-1.7 V and the series resistance of 5.3-6.1 Ω at 20 mA with decreasing the aperture diameter. The total capacitance (CT) in the LEDs with different aperture areas at the reverse bias of 5 V is about 11.6, 14.4, 17.9, 21.8, 26.5, and 31.8 pF, respectively. Since the total capacitance is proportional to the area of junction, the pad capacitance (Cp) with aperture diameter of 80 µm is calculated as about 8.8 pF.
And the LED reveals a light output power of 4.6, 5.4, 5.7, 5.8, 6.2, and 6.3 mW at 50 mA with increasing the aperture diameter, respectively. The measured peak wavelength (λpeak) of the device biased at 25 mA is about 757 nm. By the design of a ring-shaped electrode overlapping with GZO film, the 3-dB frequency (f3dB) for the NIR LED biased at 50 mA decreases with aperture diameter, and achieves a maximum of 107.8, 90.6, 74.6, 66.8, 48.3, and 45.4 MHz owing to the increase of injected current density into the confined region.
However, we evaluate the current spreading of GZO on NIR LEDs from the aspect of minority carrier lifetime. We also analyze the effects of aperture area of the NIR LEDs on minority carrier lifetime and further investigate the required injection current to achieve the 3-dB frequency bandwidth of 100 MHz. As to our fabricated NIR LEDs, the aperture area required to achieve the 3-dB frequency bandwidth of 100 MHz at a driving current of 10, 20, 30, 40, and 50 mA is 1.4 x 10-6, 5.1 x 10-6, 1.7 x 10-5, 3.4 x 10-5, and 5.1 x 10-5 cm2, respectively. At the same injection current density, the LEDs with different aperture areas always exhibit the same minority carrier lifetime, which is inversely proportional to the square root of current density, but not depend on the aperture diameter. It means that the GZO layer plays a good current spreading in the lateral direction from the ohmic contact of NIR LEDs, which shows to the lower contact resistance and lower forward voltage.

並列關鍵字

high-frequency modulation ； GaAs/AlGaAs ； near-infrared light-emitting diodes ； gallium-doped zinc oxide ； atomic layer deposition ； specific contact resistance ； ring-shaped electrode ； minority carrier lifetime ； 3-dB frequency bandwidth

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