GaN 外延生长方法及生长模式

[{"attributes":{"color":"var(--weui-LINK)"},"insert":"半导体工程师"},{"insert":" "},{"attributes":{"color":"var(--weui-FG-2)"},"insert":"2023-06-10 08:16"},{"insert":" "},{"attributes":{"color":"var(--weui-FG-2)"},"insert":"发表于北京"},{"insert":"\n01\n"},{"attributes":{"bold":true},"insert":"GaN 异质衬底外延生长方法"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"insert":"由于GaN在高温生长时N的离解压很高，很难得到大尺寸的GaN单晶材料，因此，为了实现低成本、高效、高功率的GaN HEMTs器件，研究人员经过几十年的不断研究，并不断尝试利用不同的外延生长方法在Si衬底上实现高质量的外延生长GaN基材料。GaN材料的生长是在高温下，通过TMGa分解出的Ga与NH3的化学反应实现的，生长GaN需要一定的生长温度，且需要一定的NH3分压。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"align":"center"},"insert":"\n"},{"insert":"当前GaN的外延生长方法有：氢化物外延生长法（HVPE）]、分子束外延（MBE）和金属有机化学气相沉积法（MOCVD），其特点如下表2-1所示。"},{"attributes":{"align":"justify"},"insert":"\n\n\n"},{"attributes":{"bold":true},"insert":"1.1 金属有机物气相沉积法（MOCVD）"},{"attributes":{"align":"justify","header":2},"insert":"\n\n"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"MOCVD（金属有机物气相沉积法）是在气相外延生长的基础上发展起来的一种新型气相外延生长技术。在采用MOCVD法制备GaN单晶的传统工艺中，通常以三甲基镓作为镓源，氨气作为氮源，以Si作为衬底，并用氢气和氮气的混合气体作为载气，将反应物载入反应腔内，加热到一定温度下使其发生反应，能够在衬底上生成GaN的分子团，在衬底表面上吸附、成核、生长，最终形成一层GaN单晶薄膜。采用MOCVD法制备的产量大，生长周期短，适合用于大批量生产，但生长完毕后需要进行退火处理，最后得到的薄膜可能会存在裂纹，会影响产品的质量。"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"align":"justify","header":2},"insert":"\n"},{"attributes":{"bold":true},"insert":"1.2 分子束外延法（MBE）"},{"attributes":{"align":"justify","header":2},"insert":"\n\n"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"用MBE法（分子束外延法）制备GaN与MOCVD法类似，主要的区别在于镓源的不同。MBE法的镓源通常采用Ga的分子束，NH"},{"attributes":{"script":"sub"},"insert":"3"},{"insert":"作为氮源，制备方法与MOCVD法相似，也是在衬底表面反应生成GaN。用该方法可以在较低的温度下实现GaN的生长，一般为700 ℃左右。较低的温度可以有效减少反应设备中NH"},{"attributes":{"script":"sub"},"insert":"3"},{"insert":"的挥发程度，但低温使得分子束与NH"},{"attributes":{"script":"sub"},"insert":"3"},{"insert":"的反应速率减小。较小的反应速率可以在制备过程中对生成GaN 膜的厚度进行精确控制，有利于对该工艺中的生长机理进行深入研究，但对于外延层较厚的膜来说反应时间会比较长，在生产中发挥的效率欠佳，因此该方法只能用于一次制备少量的GaN薄膜，尚不能用于大规模生产。"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"align":"justify","header":2},"insert":"\n"},{"attributes":{"bold":true},"insert":"1.3 氢化物气相外延法（HVPE）"},{"attributes":{"align":"justify","header":2},"insert":"\n"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"insert":"HVPE（氢化物气相外延法）与上述两种方法的区别还是在于镓源，此方法通常以镓的氯化物GaCl"},{"attributes":{"script":"sub"},"insert":"3"},{"insert":"为镓源，NH"},{"attributes":{"script":"sub"},"insert":"3"},{"insert":"为氮源，在衬底上以1000 ℃左右的温度生长出GaN晶体。用此方法生成的GaN晶体质量比较好，且在较高的温度下生长速度快，但高温反应对生产设备，生产成本和技术要求都比较高。"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"采用以上传统方法制备GaN薄膜，对其质量好坏的主要影响因素是Si与薄膜晶格的相配程度。欲制备无缺陷的薄膜，首先要满足两者之间尽量小的晶格失配度；其次，两者的线膨胀系数也要相近。"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"insert":"表1-1 GaN外延生长方法的优缺点"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"color":"black"},"insert":"制备方法"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"外延生长过程"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"优点"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"缺点"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"氢化物气相外延法"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"在金属镓上流过HCl,形成GaCl蒸汽，当他流到衬底上，与氨气反应，沉积形成GaN。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"①生长速度快"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"②可以比较精确地控制膜厚"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"①高温反应对生产设备、生产成本和技术要求都比较高。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"金属有机物气相沉积法"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"气体或者固体分子在高温下热裂解生成团簇，通过载气扩散到基片上，在催化剂的作用下排列、反应、生长、沉积。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"①适合于工业化生产"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"②GaN晶体质量好"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"①过程比较复杂"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"②反应速率影响因素多"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"③温度高，原材料消耗大"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"分子束外延法"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"在真空中亿原子束或分子束溅落到衬底上，并在衬底上按一定的结构有序排列，形成晶体薄膜。"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"①生长温度低"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"②生长反应过程简单"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"③实时监控生长表面的结构、成分和膜厚，均匀性较好"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"①生长速率慢"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"②不能满足大规模商业化生产的要求"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"black"},"insert":"③采用等离子体辅助方式时，容易造成高能离子对于薄膜的损伤"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"insert":" "},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"经过分析了不同的GaN外延生长方法，虽然分子束外延技术可以在较低的温度下实现GaN的生长，其生长反应过程简单，可以实时监控生长表面的结构、成分和膜厚，生长温度低，均匀性较好，但是由于这种方法的生长速率较慢，可以精确地控制膜厚，不能满足大规模商业化生产的要求，而且当采用等离子体辅助方式时，容易造成高能离子对于薄膜的损伤。而金属有机化学气相沉积法的生长速率适中，可以比较精确地控制膜厚，特别适合于工业化生产GaN基外延材料，这种方法目前已经成为使用最多、外延生长材料和器件质量最高的方法。"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"align":"justify","header":2},"insert":"\n"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"bold":true},"insert":"02"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"bold":true},"insert":"异质外延生长的基本模式"},{"attributes":{"align":"justify"},"insert":"\n\n\n\n"},{"attributes":{"color":"black"},"insert":"一般来讲，异质外延有三种生长模式：Frank-van der Merwe 生长模式（层状生长模式）、Volmer-Weber生长模式（岛状生长模式）和Stranski-Krastanow生长模式（先层状生长再岛状生长）"},{"attributes":{"color":"black","script":"super"},"insert":"[30-32]"},{"attributes":{"color":"black"},"insert":"，这三种生长模式如图4-1所示。"},{"attributes":{"align":"justify"},"insert":"\n\n\n"},{"attributes":{"bold":true},"insert":"2.1 Frank-van der Merwe 生长模式"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"color":"black"},"insert":"层-层生长模式一般发生于晶格常数比较匹配，晶格失配较小，衬底与外延层之间的键能较高的两种异质材料之间。当外延层材料的的表面自由能σ"},{"attributes":{"color":"black","script":"sub"},"insert":"f"},{"attributes":{"color":"black"},"insert":"与界面能σ"},{"attributes":{"color":"black","script":"sub"},"insert":"i"},{"attributes":{"color":"black"},"insert":"之和远小于衬底材料的表面自由能σ"},{"attributes":{"color":"black","script":"sub"},"insert":"s"},{"attributes":{"color":"black"},"insert":"时，衬底材料将非常强烈地趋于完全覆盖衬底表面（即层-层生长模式），也就是外延层与衬底浸润，因为此生长模式会使整个体系的总表面自由能降低。于是沉积物质会先在衬底表面二维成核再扩展成层，然后在一层生长结束后再进行下一层的生长，如此按逐层生长的模式进行。"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"bold":true},"insert":"2.2 Volmer-Weber 生长模式"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"color":"black"},"insert":"当σ"},{"attributes":{"color":"black","script":"sub"},"insert":"s"},{"attributes":{"color":"black"},"insert":"<σ"},{"attributes":{"color":"black","script":"sub"},"insert":"f"},{"attributes":{"color":"black"},"insert":"+σ"},{"attributes":{"color":"black","script":"sub"},"insert":"i"},{"attributes":{"color":"black"},"insert":"时，外延层与衬底表面不能形成浸润层，为了使表面能降低以使外延层材料的表面面积最小化，外延层材料会在衬底表面形成许多三维小岛。随着外延层材料沉积的继续进行，这些众多的小岛逐渐长大形成柱状岛，并彼此汇聚，最终形成表面粗糙的薄膜。在岛状结构中会有释放应变产生的失配位错，岛与岛之间存在着小角度的取向差别，在彼此汇聚时会产生位错密度很高的边界层。"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"bold":true},"insert":"2.3 Stranski-Krastanow生长模式"},{"attributes":{"align":"justify"},"insert":"\n\n"},{"attributes":{"color":"black"},"insert":"当外延层材料的表面自由能σ"},{"attributes":{"color":"black","script":"sub"},"insert":"f"},{"attributes":{"color":"black"},"insert":"与界面能σ"},{"attributes":{"color":"black","script":"sub"},"insert":"i"},{"attributes":{"color":"black"},"insert":"之和略大于或者略小于衬底材料表面自由能σ"},{"attributes":{"color":"black","script":"sub"},"insert":"S"},{"attributes":{"color":"black"},"insert":"时，外延生长会大大依赖于衬底与外延层之间的晶格匹配情况。GaN在蓝宝石衬底上的异质外延生长就属于此种情况。一开始生长时外延层材料与衬底浸润，先形成几个原子层厚度的浸润层。随着沉积的进行，应变逐渐积累，最后会通过形成三维岛的形式来释放应力。由于应变能不是通过形成位错来释放的，所以小岛中不含有位错。"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"来源：半导体封装工程师之家"},{"attributes":{"align":"justify"},"insert":"\n"},{"attributes":{"color":"var(--weui-FG-0)"},"insert":"半导体工程师"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"半导体经验分享，半导体成果交流，半导体信息发布。半导体行业动态，半导体从业者职业规划，芯片工程师成长历程。"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"155篇原创内容"},{"attributes":{"align":"justify"},"insert":"\n"},{"insert":"\n"}]