等离子体增强原子层沉积AlN外延单晶GaN研究

Epitaxy Single Crystal GaN on AlN Prepared by Plasma-enhanced Atomic Layer Deposition

氮化镓(GaN)作为第三代半导体材料,具有较大的禁带宽度,较高的击穿电场强度、电子迁移率、热导系数以及直接带隙等优异特性,被广泛应用于电子器件和光电子器件中。由于与衬底的失配问题,早期工艺制备GaN材料难以获得高质量单晶GaN薄膜。直到采用两步生长法,即先在衬底上低温生长氮化铝(AlN)成核层,再高温生长GaN,才极大地提高了GaN材料的质量。目前用于制备AlN成核层的方法有磁控溅射以及分子束外延等,为了进一步提高GaN晶体质量,本研究提出在两英寸c面蓝宝石衬底上使用等离子体增强原子层沉积(Plasma-enhanced Atomic Layer Deposition,PEALD)方法制备AlN成核层来外延GaN。相比于磁控溅射方法,PEALD方法制备AlN的晶体质量更好;相比于分子束外延方法,PEALD方法的工艺简单、成本低且产量大。沉积AlN的表征结果表明,AlN沉积速率为0.1 nm/cycle,并且AlN薄膜具有随其厚度变化而变化的岛状形貌。外延GaN表征结果表明,当沉积厚度为20.8 nm的AlN时,GaN外延层的表面最平整,均方根粗糙度为0.272 nm,同时具有最好的光学特性以及最低的位错密度。本研究提出了在PEALD制备的AlN上外延单晶GaN的新方法,沉积20.8 nm的AlN有利于外延高质量的GaN薄膜,可以用于制备高电子迁移率晶体管及发光二极管。

As the third generation semiconductor material,gallium nitride(GaN)is widely used in electronic devices and optoelectronic devices due to its excellent characteristics such as wide band gap,high breakdown field strength,high electron mobility,outstanding thermal conductivity,and direct band gap.However,it is difficult to obtain high quality single crystal GaN thin films due to the mismatch between GaN material and substrate in early phase of preparation.Until the two-step growth method is proposed,in which the nucleation layer of aluminum nitride(AlN)is firstly grown on the substrate at low temperature,and then GaN is grown at high temperature,the quality of GaN is greatly improved.Nowadays,AlN nucleation layers are fabricated via magnetron sputtering and molecular beam epitaxy,etc.To further improve the quality of GaN crystals,this study used plasma-enhanced atomic layer deposition(PEALD)method to prepare AlN nucleation layers for the epitaxial growth of GaN on a two-inch c-plane sapphire substrate.Compared with the magnetron sputtering method and molecular beam epitaxy method,the crystal quality of AlN prepared by PEALD method displays advantages of simple process,low cost and high yield.Measurements on deposited AlN films show that the deposition rate is 0.1 nm/cycle and the films have island-like structures varying with its thickness.Epitaxial GaN measurements show that GaN epitaxial layer can obtain the smoothest surface with a root mean square roughness of 0.272 nm,the best optical properties,and the lowest dislocation density when AlN is deposited with a thickness of 20.8 nm.In conclusion,a new method of epitaxial single crystal GaN on AlN prepared by PEALD has been built with optimal deposition at 20.8 nm of AlN to obtain high quality GaN thin films,it can be used to prepare high electron mobility transistors and light-emitting diodes.