銦錫氧化物電極與表面結構改進新穎氮化鎵發光元件

Abstract

最近氮化鎵發光元件由於其多用途的應用和市場需求的迅速的發展吸引了眾人的目光，並使得相繼投入研究。自從氮化鎵發光二極體在西元一九九三年問世以來，高亮度氮化鎵發光二極體已成功地應用在行動電話鍵盤的發光模組、液晶顯示器背光源、照相機閃光燈和高色彩飽和度的戶外顯示器上。而這些發光元件更被視為將改變人們的生活模式並且舒緩嚴重的能源危機，不過依照目前元件的發光效率，氮化鎵發光二極體仍然比不上傳統的光源系統，因此氮化鎵發光二極體若要應用在固態照明上並且取代傳統的光源系統，許多限制了光輸出轉換效率的技術，譬如磊晶結構品質、P型半導體的歐姆電極、出光效率、以及散熱問題等等必須得到長足的改善。 在這篇論文當中，提出了幾種方法來改進氮化鎵發光元件其出光效率，包含使用具有較低吸收係數的導電電極來取代原有的金屬電極以及元件表面結構上的修改。在論文第一部分，採用銦錫氧化物來取代傳統的鎳金電極，並分析其發光元件的特性，銦錫氧化物為一具有高光穿透性的導電物質，不過由於銦錫氧化物與P型氮化鎵半導體的功函數差異甚大，因此使得銦錫氧化物在P型氮化鎵半導體上呈現蕭基接點的特性，因此在二者間插入一層薄的P型氮化鎵銦磊晶層，以降低其蕭基位能障，形成近似甌姆特性的接點。經由 XPS, XRD及 SIMS分析的結果，其介面形成機制主要是由於鎵原子的擴散並和氧化物的氧原子形成鍵結，造成鎵原子空缺，使得局部載子濃度的提高，進而提升內建電場強度和載子穿遂此介面的機率，因而降低此界面接觸電阻，形成近似歐姆接點。在分析此歐姆介面的主要電流傳導機制時，量測環境溫度對介面接觸電阻係數的影響，發現此介面電流的主要傳導機制和介面的合金條件相關，在不同的合金溫度處理下，當處理溫度由400oC提高成600oC時，此介面的主要傳導電流機制為熱場發射傳導傾向為熱離子發射傳導。 雖然銦錫氧化物無法形成較鎳金電極良好之甌姆接點於P型氮化鎵上，但在相當於元件正常工作條件27 A-cm-2的電流密度流過銦錫氧化物和P型氮化鎵的介面時，其接觸電阻係數約2.6 x 10-2 ohm-cm2 ，雖然仍未盡理想，但是已經足夠應用在二極體上，而不至於產生過多的串接電阻，進而損耗過多的能量。使用銦錫氧化物為電極的氮化鎵發光二極體其整體特性表現如下，當20 mA的電流注入時，順向電壓約為3.43 V，雖然比傳統上使用鎳金金屬電極的發光二極體高了約0.2 V， 但是外部量子效率和能量轉換效率卻分別提升了46% 和36%，這效率上的提升主要是減少了半透明金屬電極的吸收。 至於壽命試驗，經過500oC退火處理的銦錫氧化物氮化鎵發光二極體，表現了類似傳統上使用鎳金金屬層的氮化鎵發光二極體的可靠度行為。因此，藉由中間層P型氮化鎵銦磊晶層的加入，使得銦錫氧化物能夠應用在高亮度、高可靠度的氮化鎵發光元件上。 在製作發光元件時，採用平台式的結構，正負電極位於絕緣基板的同一面，所以元件的操作電流為橫向傳導。然而在這個結構下，橫向傳導電流可能導致正負電極附近電流過度擁擠，此效應將會影響元件的可靠度。因此，妥善的處理橫向傳導電流，避免元件在操作時產生局部過熱的現象是不可或免的。由於銦錫氧化物的導電特性遠不如金屬，所以當應用在氮化鎵發光元件時，必須考慮到透明電極厚度對元件特性的影響。當20 mA的電流注入時，60奈米、180奈米和300奈米厚銦錫氧化物薄膜電極的氮化鎵發光元件的順向電壓分別為3.45、3.42及3.32 V，而輸出光功率則幾乎沒有太大的分別，但是所對應的光輸出轉換效率則和順向電壓及串接電阻成反向關係。除此之外，從元件操作電流密度分佈的模擬，60奈米厚銦錫氧化物薄膜電極氮化鎵發光元件面臨嚴重電流散佈不均的問題，此問題將會導致電流擁擠效應且產生局部過熱的現象，在實驗中此元件在經過1008小時可靠度測試後，光輸出功率衰減了48%且仍在持續劣化中，而非呈現一穩定的光輸出；相對之下，300奈米厚銦錫氧化物薄膜電極氮化鎵發光元件在模擬中顯出均勻的電流散佈，且在經過1008小時可靠度測試後，光輸出功率呈現一穩定輸出且僅衰減27%，因此妥善的處理氮化鎵發光元件橫向傳導電流是必需的，尤其對於以低導電係數氧化物導體為電極材料的發光元件，顯得更為重要。 在論文的第二部份，提出兩種表面結構的改良以增進發光元件的出光效率。首先，製作以銦錫氧化物為電極的微尺寸結構，提出一自我對準網狀發光二極體，此新元件的軸向光強度較傳統結構提昇了至少10%，且並未對操作電壓及反向電流造成影響，同時輸出光有效地集中於正向，使得正向光強度在整個元件光輸出功率的比例遠高於傳統結構元件，此外經由改變網狀結構的尺寸及形狀，其外部量子效率的峰值也提升了5%。由於軸向光的集中性和外部量子效率的提升，使得此結構有助於在表面黏著型封裝和低功率消耗發光元件上的應用。其次，提出一簡單而不必增加製程步驟的表面結構來增加氮化鎵發光二極體的出光效率，已知在乾蝕刻製程中，不同的蝕刻條件可以造成被蝕刻表面呈現平坦的、六角狀孔洞、和奈米柱狀等等不同的型態，在此調整蝕刻條件造成被蝕刻面呈現六角孔洞型態，並應用在氮化鎵發光二極體的結構上。具有六角孔洞型態和平整型態的發光元件，不論是順向或者反向偏壓操作，皆呈現相似的電流電壓關係，表示蝕刻條件的變異並不會造成N型電極歐姆介面的破壞及被蝕刻側壁的損壞，導致順向偏壓及反向電流的增加，進而影響元件的電流電壓特性。在元件光輸出特性表現上，具有六角孔洞型態的氮化鎵發光二極體在直流電源20 mA操作下，正面亮度及整體光輸出功率分別較平整型態的氮化鎵發光二極體提高了27%及13%，這效率上的提升主要是六角孔洞型態的表面破壞了空氣-氮化鎵半導體-藍寶石的波導結構，使得部分原先因全反射現象而侷限在此波導結構的光子，透過六角孔洞而傳導入空氣中，因而增加了光強度和輸出功率。

GaN-based light-emitting devices have recently attracted much attention for their versatile applications and the rapid growth of market demand. Nowadays, the high-brightness GaN-based LEDs have already successfully applied in the handset keypad, LCD backlighting, camera flash light and full-color outdoor display since their commercial introduction in 1993. These devices are expected to change our life style and will save human beings from serious energy crisis. However, the light output efficiency is still insufficient as compared to that of a conventional light source. In order to fulfill the requirements of applications to solid-state lighting, there are remained many technologies limiting the performance of devices to be improved such as crystal quality, p-type ohmic contact, emission extraction, thermal management, etc. In the dissertation, several approaches are utilized to improve light output efficiency of GaN-based LEDs including employing a lower absorptive current spreading layer and surface structure modifications. In Part 1, indium tin oxide (ITO) is employed to replace conventional Ni/Au contacts on p-GaN attributed to its high transparency characteristic. However, it is difficult to form an ohmic contact of ITO on p-GaN due to the large work function difference between ITO and p-GaN. Therefore, a thin p-type In0.1Ga0.9N layer is inserted as an intermediate layer to reduce the Schottky barrier height between ITO and p-GaN, because p-In0.1Ga0.9N is supposed to have a narrower band-gap than p-GaN. The transport mechanism of ITO ohmic contacts on p-GaN is characterized and investigated. Based on the variation of the contact resistivity with respect to the ambient temperature, the dominant transport mechanism of ITO/p-GaN interfaces varies with the post alloying temperature. The dominant transport mechanism has a tendency from thermionic-field emission to thermionic emission as rising alloyed temperature from 400oC to 600oC. From the X-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS) results, the out-diffusion of gallium atoms and the formation of Ga-O bonds would introduce the gallium vacancies and increase the net concentration of carriers beneath the contact, which would make the ITO/p-GaN contact reveal ohmic characteristics. Although ITO contacts does not reveal as good ohmic property as Ni/Au contacts, the contact resistivity is 2.6 x 10-2 ohm-cm2 at a current density of 27 A-cm-2 equivalent to that of 350 um-sized LEDs, and it is low enough for the application of LEDs. GaN-based LEDs with ITO contacts exhibit the forward voltage of 3.43 V at an injection current of 20 mA. The forward voltage is a little higher than the conventional LEDs by 0.2 V, but the external quantum efficiency and power conversion efficiency are raised by 46% and 36%, respectively. As for the life test, LEDs with ITO contacts annealed at 500oC exhibit a similar reliability as the LEDs with conventional Ni/Au contacts. Therefore, ITO contacts with a thin p-In0.1Ga0.9N intermediate can make GaN-based LED highly bright and reliable in practice. GaN-based LEDs are fabricated on insulating sapphire substrates, and mesa structures with lateral current conduction are utilized in the devices. However, the lateral current conduction could result in a severe current crowding phenomenon near either n-type or p-type electrode and thus impacts on the reliability of devices. Hence, it is necessary to handle the lateral current conduction to alleviate local hot spots formation as device operated. GaN-based LEDs with various quarter wavelength thicknesses of ITO films are fabricated and characterized. Chips with various ITO thick films show nearly coincident output power-current curves and exhibit an enhancement of 30% as compared with Ni/Au contacts. At a current of 20 mA, the forward voltage is around 3.45, 3.42, and 3.32 V for devices with 60, 180, and 300-nm-thick ITO contacts, respectively. Thus, the power efficiency of LEDs with thicker ITO contacts is higher than with thinner ITO contacts due to the less power consumption. Moreover, from the simulation of current density distribution in devices, the LEDs with 60nm-thick ITO contacts present a worse distribution and it is considered to cause a severe current crowding issue and introduce local hot spots as device operated. Consequently, LEDs with 60nm-thick ITO contacts suffered an output power degradation of 48% after 1008-hour stress. On the other hand, LEDs with 300nm-thick ITO contacts exhibits a stable output after 1008-hour stress with merely 27% decay. Therefore, it is very important to handle the lateral current conduction especially for devices with conductive oxide materials of low conductivity. In Part 2, two surface structure modifications were proposed to increase the light extraction coefficient. First, a feasible method for fabricating micro-LEDs with ITO contact is demonstrated. In comparison with the conventional structured LEDs, the self-aligned micro-net ones are a least 10% brighter in the normal direction and 25% higher in the ratio of luminescence to total output power without sacrifice of operating voltage and leakage current. Moreover, the peak value of external quantum efficiency can be increased by 5% by varying the dimensions and the density of the holes at low current driving. With higher normal luminescence and external quantum efficiency, LEDs with such a structure are quite useful in surface-mounting and low-power-consuming devices. Secondly, a simple way to increase extraction efficiency of GaN-based LEDs without taking any other extra processing step is presented. A mesa structure formed by dry etch is utilized in GaN-based LEDs, and the exposed n-GaN surface could reveal various morphologies, such as smooth surface, nano-rods or hexagonal cavities dependent on various etching conditions. LEDs with smooth morphology and hexagonal cavities on exposed n-GaN layers are fabricated and characterized. Both LEDs with various morphologies on n-GaN show very similar electrical properties. It means that the dry etching condition to reveal hexagonal cavities on n-GaN surface would neither do damage on the sidewalls of mesas nor deteriorate the n-type ohmic contacts. At 20-mA-current injection, the LEDs with hexagonal cavities on n-GaN exhibit higher normal luminescence and output power by 27% and 13% in comparison with LEDs with smooth surface. The enhancement is mainly attributed to that photons guided laterally through the air–GaN–sapphire structure are partially interfered and extracted into the air through the hexagonal cavities.