氮化鎵系列發光元件上P型接觸電極之特性研究Characteristics; of p-type Contact on GaN-Based Light Emitting Devices

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题名:	氮化鎵系列發光元件上P型接觸電極之特性研究Characteristics;of p-type Contact on GaN-Based Light Emitting Devices
作者:	敦俊儒;Chun-Ju Tun
贡献者:	光電科學研究所
关键词:	透明電極;氧化鋅;雷射二極體;發光二極體;氮化鎵;transparent contact layer;TCL;LD;LED;AZO;ZnO;GaN
日期:	2005-10-01
上传时间:	2009-09-22 10:28:03 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本論文所使用的氮化物材料是以有機金屬化學氣相沈積系統成長的。主要針對氮化鎵系列藍紫光雷射、發光二極體上的p型接觸電極之特性進行研究。在藍紫光雷射方面，為了降低接觸電阻，矽摻雜之In0.23Ga0.77N/GaN短週期超晶格接觸層被成長在InGaN/GaN多層量子井雷射二極體上以取代p型氮化鎵作為接觸層。藉由加入In0.23Ga0.77N/GaN短週期超晶格可得到具有較低順向偏壓、較小的串聯電阻與較低的臨界電流的雷射二極體。另一方面，在脈衝操作下，具有短週期超晶格的雷射二極體也有較長的雷射開啟時間。當脈衝寬度由300ns增加至2μs時，以Pt為接觸電極的雷射二極體的雷射開啟時間為以鎳/金作為接觸電極的三倍。因此，具有短週期超晶格接觸層的雷射二極體能顯著的降低接觸電阻並增加元件的熱穩定性。在發光二極體方面，為了增加電流散佈效應並提升光粹取率，具有高穿透率、低電阻率的接觸電極是必須的。鋁摻雜氧化鋅(AZO)是其中一種具有潛力的材料。在本論文中，AZO的電阻率在經過熱處理後可降低至4.1×10-4 Ω-cm，同時也可觀察到穿透光譜在熱處理後有藍移的趨勢。穿透光譜的藍移可能是由於Burstein-Moss效應造成的。當熱處理溫度為700?C時可以達到最高的光學能隙3.69eV。此外在波長範圍為380nm~700nm時，AZO也具有超過85%的高穿透率。然而AZO直接應用於p型氮化鎵上無法形成歐母接觸，但可藉由在AZO與p型氮化鎵之間加入一層鎳金屬(Ni)或氧化鎳(NiOx)來達成歐母接觸。未經熱處理的Ni/AZO薄膜的穿透率在可見光波段只有70%，在經過800?C熱處理後可提升至85%以上。而所有的NiOx/AZO薄膜在可見光波段都具有高於85%的穿透率。Ni/AZO和NiOx/AZO在p型氮化鎵上的接觸電阻分別為1.1 ± 0.3 ×10-2和1.6 ± 0.8 ×10-2 Ω-cm2。由光電子能譜量測可知，經過800?C 熱處理之Ni/AZO 與 NiOx/AZO具有較佳的電性，可能是由於介面顯著的反應造成的我們將800 oC熱處理的AZO，Ni/AZO和NiOx/AZO薄膜應用在氮化鎵系列高功率發光二極體上作為透明的電流散佈層，元件的尺寸為1×1mm2。AZO薄膜在p型氮化鎵上無法形成歐姆接觸，以Ni/AZO薄膜為透明電極的元件在注入電流為400mA時光輸出會達到飽和，但是以NiOx/AZO薄膜為透明電極的元件至注入電流為500mA時都尚未達到飽和。這可能是由於Ni/AZO薄膜的電阻率較NiOx/AZO薄膜高造成的。而Ni/AZO薄膜電阻率的升高可能是由於鎳原子擴散進入AZO中造成的。由光電子能譜縱深分佈曲線可看出先將鎳金屬進行氧化處理可以有效防止鎳原子擴散進入AZO中。相較於以鎳/金作為透明電極的元件，以Ni/AZO與NiOx/AZO為透明電極的元件的光輸出，在注入電流為350mA時，分別提升38.2%與60.6%。 In this dissertation, characteristics of p-type contact on GaN-based laser diodes and light emitting diodes are investigated. Si-doped n+-In0.23Ga0.77N/GaN short-period super-period superlattice (SPS) tunneling contact layer were used on InGaN/GaN multiple-quantum-well laser diode (LD) structures which were grown by metal-organic chemical vapor deposition (MOCVD). The SPS was used to instead of the p-type GaN as a top contact layer. The lower forward voltages, smaller series resistances, and lower threshold current of the LD with SPS contact layer were achieved. On the other hand, the LD with an SPS layer exhibited a longer lasing duration than that of LD without an SPS layer under pulse operation. When the widths of input pulse were lengthened from 300 ns to 2 s, the lasing duration of the LD with Pt contact was three times longer than that of the LD with Ni/Au contact. Therefore, one would like to conclude that nitride-based LDs with an SPS tunneling contact layer will significantly reduce the contact resistance and increase the thermal stability of the device reliability. In order to enhance the current spreading effect and light extraction efficiency, a high transparency and low resistivity contact layer is needed. Al-doped ZnO is one of the potential materials. The lowest resistivity of 4.1×10-4 Ω-cm for AZO films can be achieved by post-deposition annealing. A blue shift of transmission spectra can be observed after thermal annealing. This could be due to the Burstein-Moss effect. The highest optical band-gap of 3.69eV can be achieved when the annealing temperature is 700?C. Besides, all the AZO films exhibited high transmittance (> 85%) in the wavelength range from 380nm to 700nm. Al-doped ZnO(AZO) Ni/AZO and NiOx/AZO films were deposited on p-type GaN films followed by thermal annealing to form Ohmic contacts. The AZO/p-GaN contacts showed a non-Ohmic electrical characteristic. However, the electrical characteristic could be markedly improved by the insertion of Ni or NiOx between AZO and p-GaN. For the 1x1mm2 UV LEDs with Ni/AZO contacts, the light output approached to saturation point when the injection current was about 400mA. However, the saturation point was as high as 500mA for the LEDs with NiOx/AZO contacts. This could be due to the fact that the resistivity of Ni/AZO films was higher than that of NiOx/AZO films leading to a severe current crowding effect. The increased resistivity of the Ni/AZO films could be attributable to the interdiffusion between Ni and AZO films. Comparing to the LEDs with Ni/Au Ohmic contacts, the light output intensity of the LEDs with Ni/AZO and NiOx/AZO contacts were increased by 38.2% and 60.6% at 350mA, respectively.
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