蓝光之路---蓝光二极管的制造为什么难？
鲍海飞2014-10-15
2014年，诺贝尔物理奖授予了发明高效率的蓝光二极管的中村修二，引起不少争议。但蓝光二极管的制造为什么这么难？究竟难在什么地方？大家更多关注的是谁获得了诺奖，而少有人关注问题的核心。从1998年中村修二在science上发表的综述文章可以回答这个问题。这个至少曾经难倒了多少英雄好汉，让多少人面对的无法逾越的难题，被他给解决了。该文中披露了制造半导体蓝光发光管道路上的三个重大问题，及其如何突破。
这篇综述的题目是：InGaN基蓝光二极管和激光二极管中结构缺陷的作用。【The Roles of Structural Imperfections in InGaN-Based Blue Light—Emitting Diodes and Laser Diodes，Shuji Nakamura，14 AUGUST 1998 VOL 281 SCIENCE www.sciencemag.org）】
摘要中似乎没有什么特别的强调，大意如下：
采用InGaN有源层取代GaN的有源层，从而获得了能够辐射琥珀色、绿色、蓝色以及紫外光颜色的高效率发光二极管。InGaN有源层中的铟(In)组份涨落导致的局域能态与高效率发光有关。虽然存在大量穿透位错(threading dislocations)，但是依然取得了每瓦5到30流明的蓝光和绿光的InGaN量子阱结构的发光二极管。穿透位错源于GaN和蓝宝石衬底之间，在蓝宝石衬底上外延生长GaN可以减少位错数，其后在有SiO2掩膜区上的GaN层上生长的InGaN多量子阱结构激光二极管寿命超过1万小时，位错使得激光二极管的阈值电流密度提高了。
（High efficiency light emitting diodes emitting amber, green,blue, and ultraviolet light have been obtained through the use of an InGaN active layer instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based emitting devices. The blue and green InGaN quantum-well structure light-emitting diodes with luminous efficiencies of 5 and 30 lumens per watt, respectively, can be made despite the large number of threading dislocations (13 108 to 1 3 1012 cm22 ). Epitaxially laterally overgrown GaN on sapphire reduces the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. InGaN multi-quantum-well structure laser diodes formed on the GaN layer above the SiO2 mask area can have a lifetime of more than 10,000 hours. Dislocations increase the threshold current density of the laser diodes.）
蓝光究竟有什么用处，为什么如此吸引人？在正文的前言中，大致描述如下：
亮度和耐用性使得发光二极管成为显示应用中的理想光源，半导体激光二极管在很多场合下，如从光通讯到CD光存储等方面得到了广泛的应用。但是，这些应用受到了限制，主要是没有能够有效发射蓝光的材料。在全色显示中，至少需要通常的红、绿、蓝三原色光，以合成出任意颜色的光；同时，也需要制作出一种发白光的器件，同白炽灯和荧光灯等相比，要消耗更低的能量，并更耐久好用；此外，短波长的光聚焦的斑点更小，这样，刻蚀在光盘上的信息量就越大。1996年上市的DVD（4.7G字节，CD 0.65G）采用的是红光AlInGaP激光器，如果用基于氮化物基N的III-V族紫光激光，那么对于CD来说，其存储密度将达到15G。紫光也有利于打印机和海底通讯等应用。III-V族氮化物具有直接带隙的特点，适合于发蓝光（直接带隙发光效率高，Si是间接带隙而发光效率低）。随组份变化，AlGaInN的带隙变化从2eV到6.2eV，利用该材料能够制造出从红光到紫外光的发射器件。
（The brightness and durability of light-emitting diodes (LEDs) make them ideal for displays, and semiconductor laser diodes (LDs) have been used in numerous device applications from optical communications systems to compact disk (CD) players. These applications have been limited, however, by the lack of materials that can emit blue light efficiently. Full-color displays, for example, require at least three primary colors, usually red, green, and blue, to produce any visible color. Such a combination is also needed to make a white light–emitting device that would be more durable and consume less power than conventional incandescent bulbs or fluorescent lamps. The shorter wavelength means that the light can be focused more sharply, which would increase the storage capacity of magnetic and optical disks. Digital versatile disks (DVDs), which came onto the market in 1996, rely on red aluminum indium gallium phosphide (AlInGaP) semiconductor lasers and have a data capacity of about 4.7 gigabytes (Gbytes), compared to 0.65 Gbytes for compact disks. By moving to violet wavelengths emitted by III-V nitride-based semiconductors, the capacity could be increased to 15 Gbytes. The violet III-V nitride-based LDs could also improve the performance of laser printers and undersea optical communications. Such III-V nitride-based semiconductors have a direct band gap that is suitable for blue light–emitting devices. The band gap energy of aluminum gallium indium nitride (AlGaInN) varies between 6.2 and 2.0 eV, depending on its composition, at room temperature (RT). Thus, by using these semiconductors, red- to ultraviolet (UV)-emitting devices can be fabricated.）
那么，为什么蓝光二极管的制造很难？经历了将近三十年的时间才取得了今天突破性的成果，原来这里面隐藏着三个重大的涉及到材料生长的技术问题，当突破了这三个难题之后，问题便迎刃而解了。那么，在当时看来，究竟有哪三个技术突破呢？前言中继续讲道：
第一个突破是要生长出像镜面一样平坦的高质量GaN薄膜。采用AlN或GaN作为成核层，获得了高质量的镜面平坦的薄膜，该薄膜具有低的载流子浓度，高的空穴迁移率，以及强的荧光强度（提到了自己的研究工作和别人的研究工作）。之所以难于生长平坦的薄膜，主要是由于GaN和衬底之间存在较大的晶格失配和材料间较大的热涨系数差异，由此，在成膜后会引入残余应力，导致薄膜存在很多缺陷。实际上，对于衬底和待生长材料之间存在较大的晶格失配，需要在其间生长一层过渡层来解决，而中村修二是第一个采用GaN作为缓冲层的。
（The first breakthrough for III-V nitride-based semiconductors was the use of AlN (1, 2) or GaN (3, 4) nucleation layers for the GaN growth. By using these nucleation layers, it became possible to obtain high-quality GaN films with a mirrorlike flat surface, a low residual carrier concentration, high carrier mobilities, and a strong photoluminescence (PL) intensity. ）
第二个突破是终于制备出p-型的GaN，由此获得III-V族半导体发光的必要条件，同时阐述了为什么这么多年为什么一直没有做出来的原因。半导体发光管或激光管都需要一个pn结来完成光、电的泵浦或注入，实现电光的转换。对于半导体发光来说，来自于n区和p区的电子与空穴注入到有源区复合才能发光。相对来说，从一开始，n型的GaN很容易制备出来。但是，多年来，就是几乎得不到p型的GaN。因此，这成为妨碍III-V族实现发光或激光的一个主要原因。从70年代开始，很多人尝试着在GaN中掺杂Zn,Be,Mg,Cd等金属作为受主（空穴）杂质。但是，始终不清楚为什么就是没有制备出p型的GaN。其实，今天看来，当时是一直没有找到正确的工艺生长方法以及缺少对不同组元配比材料发光机理的认识等。
（The second big breakthrough for III-V nitride-based LEDs and LDs was that p-type GaN was obtained, and the reasons why p-type GaN had not been obtained were clarified. For the LEDs and LDs, a p-n junction is used to inject carriers (holes and electrons) into the active layers from p-type layer and n-type layer. Thus, control of both p-type and n-type conductivity is required to fabricate those devices. It was relatively easy to make n-type GaN from the beginning. However, it was virtually impossible to obtain p-type GaN films for many years (5, 6). The unavailability of p-type GaN films had prevented III-V nitrides from being used in light-emitting devices, such as blue LEDs and LDs. Since the 1970s, many people had tried to make p-type GaN by doping with Zn (7), Be (8), Mg (9), Cd (10), and similar metals as an acceptor impurity. However, unknown reasons prevented the formation of a low-resistivity, p-type GaN by doping. ）
1989年，Amano 等人利用有机化学气相外延获得了p型GaN薄膜，他们采用了掺杂了Mg作为受主杂质，并利用低能电子束辐射处理。在生长薄膜后，采用低能电子束辐射处理掺杂Mg的GaN薄膜，结果获得了低阻的p型GaN薄膜。这在1992年之前，也只有Amano 等人制备出了p型的GaN，而其中低能电子束辐射处理后薄膜的电阻变化机制还不清楚。这一年，中村修二等人利用氮气N2环境下退火，而非低能电子束辐射处理终于制作出了p-型GaN（提到了自己的研究工作）。利用N2退火的优点是经济、方便，处理薄膜的均匀性好，同时，最重要的是光辐射效率提高了（其中隐含着复杂的机理）。中村修二所生长的薄膜，其初始电阻率很高(106 ohmNaN)，而经过N2高温700°C处理之后变成了只有2 ohmNaN的p型GaN。随后，如果该薄膜在氨气NH3的条件下在400°C退火，那么，又得到了高阻的p型GaN。这是一个可重复的过程。其中，中村等人给出了其中的电阻变化的机制，涉及到NH3在高温下分解的过程，氢原子H在薄膜中有很强的扩散渗透能力，其中H与其中的金属发生了氢化过程，即在p型GaN中形成了中性复合物（或络合物）等过程，即空穴的补偿效应。即当薄膜在NH3环境下退火，NH3释放的H会渗透到薄膜内部并与薄膜中的金属反应，变成了中性复合物，将其中的空穴中和，从而使导电能力降低。而低温电子束辐射的结果与此类似。由此，一个二十多没有解开的谜团终于打开了。实际上，是中村找到了一套正确的薄膜生长工艺方法，包括薄膜的退火处理方式等，制备出了p型GaN。
（In 1989, Amano et al. (11) obtained p-type GaN films by using Mg-doping as an acceptor impurity and a post low-energy electron-beam irradiation (LEEBI) treatment by means of metal organic chemical vapor deposition (MOCVD) growth method. After the growth, LEEBI treatment was performed for Mg-doped GaN films to obtain a low-resistivity p-type GaN film. The LEEBI treatment was thought to displace Mg through the energy of the electron beam irradiation. In spite of this achievement in forming p-type GaN, only Amano et al. had succeeded in obtaining p-type GaN until 1992 because the mechanism of the LEEBI treatment was not understood exactly. In 1992, Nakamura et al.(12, 13) obtained p-type GaN films by thermal annealing of GaN films in a N2-ambient instead of the LEEBI treatment. Before thermal annealing, the resistivity of Mg-doped GaN films was 1.3X106 ohmNaN. After thermal annealing at temperatures above 700°C, the resistivity dropped to 2 ohmzcm (12). Low-resistivity p-type GaN films, which were obtained by N2-ambient thermal annealing, showed a resistivity as high as 1X106 ohmzcm after NH3-ambient thermal annealing at temperatures above 600°C (13).They proposed that atomic hydrogen produced by NH3 dissociation at temperatures above 400°C was related to the acceptor compensation mechanism (13). A hydrogenation process whereby acceptor-H neutral complexes were formed in p-type GaN films was proposed (13). The formation of acceptor-H neutral complexes causes acceptor compensation. This hydrogenation process has been accepted as the acceptor compensation mechanism of p-type III-V nitride-based semiconductor by many researchers (14–18). Theoretical calculations of this hydrogen passivation were made by Neugebauer and Van De Walle (18). Thus, in 1992, the 20-year mystery of p-type GaN was resolved.）
第三个突破是高质量的InGaN薄膜已经成功制备出来。在所有的III-V 氮基的从红光到紫外光的发光管和激光器中，InGaN是有源层的关键部分，在此结构层中注入的电子与空穴发生复合辐射光子。尽管它很重要，但是无论用光泵浦的方法还是电注入的方法，一直也没有人制备出室温下工作的、能够完成带-带辐射跃迁高质量的薄膜。1992年，中村修二利用MOCVD的双气流法，改变其中的In组份，从而成功制备出了高质量的具有带-带跃迁辐射的从绿光到紫外光的薄膜，最后，制备出了InGaN厚的多量子阱结构（单个量子阱厚度为2.5nm）并最终确认了其强辐射荧光谱。其中，在GaN中掺杂少量的In是实现强的带-带跃迁辐射的关键，这与深能级能态有关。这里，中村修二提出并实现了双气流沉积的方式是技术上的一个主要突破。
（The third big breakthrough was that high-quality InGaN films have become available. As mentioned above, an InGaN active layer is used for all of the III-V nitride-based LEDs and LDs to emit red to UV light. Thus, InGaN is the most important compound semiconductor among III-V nitride compounds because the InGaN active layer emits light by the recombination of the injected electrons and holes into the InGaN. In spite of its importance, no one had succeeded in obtaining high-quality InGaN films that could emit a strong band-to-band emission at RT by optical pumping or current injection (19–21). In 1992, Nakamura and Mukai (22) succeeded in growing high-quality InGaN films that emitted strong band-to-band emission from green to UV by changing the In content of InGaN with a two-flow MOCVD method. Finally, Nakamura et al. (23) grew a InGaN multi-quantum-well (MQW) structure and confirmed an enhanced strong PL intensity from quantized energy levels in a InGaN well layer with a thickness of 25 Å. Adding a small amount of In into the GaN is very important to obtain a strong band-to-band emission at RT. The reason is related to the presence of deep localized energy states (24–28).）
实际上，InGaN发光机制还是很复杂的，其中，其发光机理与材料复杂的结构和特性有关。InGaN和GaN中结构缺陷显著不同。前言中，下面一段描述了继他之后，其它研究人员都开始采用InGaN进行发光的研究等问题。
1994年，中村修二等人实现了InGaN/AlGaN双异质结构的蓝光LED，随后的1995年，研制出来InGaN单量子阱蓝/绿二极管，随后，又在InGaN/GaN/AlGaN集的异质结构中研制出紫外光/琥珀色的LED。在中村修二之后，其它国家和实验室的研究小组也相继有了报道和研究。所有这些器件都采用了InGaN,作为有源层，而不是GaN，因为利用GaN有源层制造的器件其发光效率很低，其原因至今不是很清楚（当时）。但是，一个显著的现象是，在有源层InGaN中存在着密度相当高的穿透位错密度，这些位错密度从108 到1012 cm22，是来源于GaN和蓝宝石衬底之间的界面，其界面的存在着15%晶格失配。总之，在以InGaN为有源层的发光材料或器件中，其发光效率要远比其它组份结构的高很多。在通常的发光管中，如AlGaAs 和AlInGaP中，器件的发光性能是受到点缺陷和结构缺陷显著影响的。而在III-V族氮化物基的器件中，似乎并不受到这些结构缺陷的影响。这反映出在半导体凝聚态物理中，存在着很多尚十分不太清楚的结构复杂性（局域态、悬挂键、团簇等）和发光机制等。
（In 1994, Nakamura et al. developed blue InGaN/AlGaN double heterostructure LEDs (29) and then developed blue/green InGaN single quantumwell (SQW) structure LEDs in 1995 (30). Then, UV/amber LEDs (31, 32) and RT violet laser light emission in InGaN/GaN/AlGaN–based heterostructures under pulsed operations were achieved (33). Since Nakamura et al.’s report of pulsed operation, many groups have reported pulsed operation of the LDs using the same structure (34–40). The latest results showed that the lifetime can be as long as 1000 (41) and 10,000 hours (42) under RT continuous-wave (CW) operation. Also, highpower LDs were fabricated using epitaxially lateral overgrown GaN (ELOG) (43) and GaN substrates (44). All of these lightemitting devices use an InGaN active layer instead of a GaN active layer because it is difficult to fabricate a highly efficient light-emitting device using a GaN active layer, for reasons not yet understood. Also, the InGaN active layer in these LEDs and LDs include a large number of threading dislocations (TDs), from 1X108 to 1X1012 cm22, that originate from the interface between GaN and the sapphire substrate due to a large lattice mismatch of 15% (24, 45). The TDs are thought to form as a result of a complex set of interactions that include the interface energy, the nucleation density, and island coalescence (46). In spite of this large number of dislocations, the efficiency of the InGaN-based LEDs and LDs is much greater than that of the conventional III-V compound semiconductor (AlGaAs and AlInGaP)-based LEDs and LDs. In many conventional optoelectronic devices, the device performance has been limited by the control of both point defects and structural defects in these materials. However, these recent reports now suggest that III-V nitride–based devices are less sensitive to dislocations than conventional III-V semiconductors.）
前言很长，随后又描述了各研究组对穿透位错的起源、位错密度、有源层的发光机制、载流子的扩散等研究工作。但指出，无论怎样，只有掺杂In的GaN才具有高的发光效率。限于该文章的篇幅长度，只把前言中涉及到的三个主要技术突破做了简单的描述，余下的具体结构和特性只能留给感兴趣的读者自行发掘探幽了。
简单说，在实现III-V族半导体蓝光方面的研究中，解决的三个重大突破是：解决了界面晶格失配的问题，成功制造出了p型GaN，成功制备出InGaN蓝光等系列高发光效率的材料。其中，他首先采用了以GaN作为蓝宝石衬底上的缓冲生长层，后面两项具有决定性的突破技术都是由中村独立完成，提出并实施了垂直与水平的双气流法(two-flow，TF-MOCVD)的薄膜制备工艺新方法，利用该方法制备出高效率发光的InGaN结构层，提出了InGaN中H的作用及其影响发光效率的空穴补偿机制等。
如果说1992年以前的中村修二是个默默无闻的研究人员，那么在92年APL上发表文章之后，便一跃成为闪亮的明星了，到1998年在science上发表综述文章，中村修二在几年之间就成为一个重量级的人物了。至于其后移民大洋彼岸、以及与公司就专利之争的官司等，中村修二的研究道路上，有很多值得我们反思的东西。该工作包含了重大的技术突破，该技术解决了困扰业界多年来亟待解决的问题，在光存储、光通信、以及光处理等领域方面有重大应用。人们只看到诺奖光环闪亮无比，但诺奖的道路上是艰辛无比。
和大家一样，之所以对中村修二先生的工作感兴趣，是因为他获得了诺贝尔奖引起了我们的关注，是因为他在看似平凡的人生道路上在走出了一条属于自己的人生道路；同时，我也曾研究过一段时间的红外半导体发光管，已经生疏了很多年。故此，今天才有机会学习了一下这个‘新’东西。
精彩的人生，人生的精彩，在于你的眼界，在于你的智慧，在于你的性格，在于你的机遇，在于你的胆识。
走别人不曾走过的路，走别人不敢走的路，走别人走不过去的路，才能路转溪桥忽见，才能蓦然回首，才能看到一路不同的风景！
相关专题：2014年诺贝尔奖
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