标题: 铟锡氧化物电极与表面结构改进新颖氮化镓发光元件
Novel GaN-based Light-emitting Devices with Indium Tin Oxide Contacts and Surface Structure Modifications
作者: 朱俊宜
Jiunn-Yi Chu
张国明
Kow-Ming Chang
电子研究所
关键字: 氮化镓;发光二极体;铟锡氧化物;GaN;LED;ITO
公开日期: 2006
摘要: 最近氮化镓发光元件由于其多用途的应用和市场需求的迅速的发展吸引了众人的目光,并使得相继投入研究。自从氮化镓发光二极体在西元一九九三年问世以来,高亮度氮化镓发光二极体已成功地应用在行动电话键盘的发光模组、液晶显示器背光源、照相机闪光灯和高色彩饱和度的户外显示器上。而这些发光元件更被视为将改变人们的生活模式并且舒缓严重的能源危机,不过依照目前元件的发光效率,氮化镓发光二极体仍然比不上传统的光源系统,因此氮化镓发光二极体若要应用在固态照明上并且取代传统的光源系统,许多限制了光输出转换效率的技术,譬如磊晶结构品质、P型半导体的欧姆电极、出光效率、以及散热问题等等必须得到长足的改善。
在这篇论文当中,提出了几种方法来改进氮化镓发光元件其出光效率,包含使用具有较低吸收系数的导电电极来取代原有的金属电极以及元件表面结构上的修改。在论文第一部分,采用铟锡氧化物来取代传统的镍金电极,并分析其发光元件的特性,铟锡氧化物为一具有高光穿透性的导电物质,不过由于铟锡氧化物与P型氮化镓半导体的功函数差异甚大,因此使得铟锡氧化物在P型氮化镓半导体上呈现萧基接点的特性,因此在二者间插入一层薄的P型氮化镓铟磊晶层,以降低其萧基位能障,形成近似瓯姆特性的接点。经由 XPS, XRD及 SIMS分析的结果,其介面形成机制主要是由于镓原子的扩散并和氧化物的氧原子形成键结,造成镓原子空缺,使得局部载子浓度的提高,进而提升内建电场强度和载子穿遂此介面的机率,因而降低此界面接触电阻,形成近似欧姆接点。在分析此欧姆介面的主要电流传导机制时,量测环境温度对介面接触电阻系数的影响,发现此介面电流的主要传导机制和介面的合金条件相关,在不同的合金温度处理下,当处理温度由400oC提高成600oC时,此介面的主要传导电流机制为热场发射传导倾向为热离子发射传导。
虽然铟锡氧化物无法形成较镍金电极良好之瓯姆接点于P型氮化镓上,但在相当于元件正常工作条件27 A-cm-2的电流密度流过铟锡氧化物和P型氮化镓的介面时,其接触电阻系数约2.6 x 10-2 ohm-cm2 ,虽然仍未尽理想,但是已经足够应用在二极体上,而不至于产生过多的串接电阻,进而损耗过多的能量。使用铟锡氧化物为电极的氮化镓发光二极体其整体特性表现如下,当20 mA的电流注入时,顺向电压约为3.43 V,虽然比传统上使用镍金金属电极的发光二极体高了约0.2 V, 但是外部量子效率和能量转换效率却分别提升了46% 和36%,这效率上的提升主要是减少了半透明金属电极的吸收。 至于寿命试验,经过500oC退火处理的铟锡氧化物氮化镓发光二极体,表现了类似传统上使用镍金金属层的氮化镓发光二极体的可靠度行为。因此,藉由中间层P型氮化镓铟磊晶层的加入,使得铟锡氧化物能够应用在高亮度、高可靠度的氮化镓发光元件上。
在制作发光元件时,采用平台式的结构,正负电极位于绝缘基板的同一面,所以元件的操作电流为横向传导。然而在这个结构下,横向传导电流可能导致正负电极附近电流过度拥挤,此效应将会影响元件的可靠度。因此,妥善的处理横向传导电流,避免元件在操作时产生局部过热的现象是不可或免的。由于铟锡氧化物的导电特性远不如金属,所以当应用在氮化镓发光元件时,必须考虑到透明电极厚度对元件特性的影响。当20 mA的电流注入时,60奈米、180奈米和300奈米厚铟锡氧化物薄膜电极的氮化镓发光元件的顺向电压分别为3.45、3.42及3.32 V,而输出光功率则几乎没有太大的分别,但是所对应的光输出转换效率则和顺向电压及串接电阻成反向关系。除此之外,从元件操作电流密度分布的模拟,60奈米厚铟锡氧化物薄膜电极氮化镓发光元件面临严重电流散布不均的问题,此问题将会导致电流拥挤效应且产生局部过热的现象,在实验中此元件在经过1008小时可靠度测试后,光输出功率衰减了48%且仍在持续劣化中,而非呈现一稳定的光输出;相对之下,300奈米厚铟锡氧化物薄膜电极氮化镓发光元件在模拟中显出均匀的电流散布,且在经过1008小时可靠度测试后,光输出功率呈现一稳定输出且仅衰减27%,因此妥善的处理氮化镓发光元件横向传导电流是必需的,尤其对于以低导电系数氧化物导体为电极材料的发光元件,显得更为重要。
在论文的第二部份,提出两种表面结构的改良以增进发光元件的出光效率。首先,制作以铟锡氧化物为电极的微尺寸结构,提出一自我对准网状发光二极体,此新元件的轴向光强度较传统结构提升了至少10%,且并未对操作电压及反向电流造成影响,同时输出光有效地集中于正向,使得正向光强度在整个元件光输出功率的比例远高于传统结构元件,此外经由改变网状结构的尺寸及形状,其外部量子效率的峰值也提升了5%。由于轴向光的集中性和外部量子效率的提升,使得此结构有助于在表面黏着型封装和低功率消耗发光元件上的应用。其次,提出一简单而不必增加制程步骤的表面结构来增加氮化镓发光二极体的出光效率,已知在干蚀刻制程中,不同的蚀刻条件可以造成被蚀刻表面呈现平坦的、六角状孔洞、和奈米柱状等等不同的型态,在此调整蚀刻条件造成被蚀刻面呈现六角孔洞型态,并应用在氮化镓发光二极体的结构上。具有六角孔洞型态和平整型态的发光元件,不论是顺向或者反向偏压操作,皆呈现相似的电流电压关系,表示蚀刻条件的变异并不会造成N型电极欧姆介面的破坏及被蚀刻侧壁的损坏,导致顺向偏压及反向电流的增加,进而影响元件的电流电压特性。在元件光输出特性表现上,具有六角孔洞型态的氮化镓发光二极体在直流电源20 mA操作下,正面亮度及整体光输出功率分别较平整型态的氮化镓发光二极体提高了27%及13%,这效率上的提升主要是六角孔洞型态的表面破坏了空气-氮化镓半导体-蓝宝石的波导结构,使得部分原先因全反射现象而局限在此波导结构的光子,透过六角孔洞而传导入空气中,因而增加了光强度和输出功率。
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.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT009011801
http://hdl.handle.net/11536/80525
显示于类别:Thesis


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