Pd/p-GaN欧姆接触退化机理

Degradation Mechanism of Pd/p-GaN Ohmic Contacts

近几年,Ⅲ-Ⅴ族半导体GaN由于其宽直接带隙,在高温、高功率器件方面得到了广泛研究。但是,目前GaN器件的性能依然受到了p型欧姆接触性能不良的限制,在长期使用过程或高温环境中激光器等器件性能退化严重。因此,获得性能优异的p-GaN接触仍然是一个巨大的挑战。虽然Pd基的金属体系已然在p-GaN获得了欧姆接触,但是Pd与GaN接触之后的微观结构及其高温特性尚不为人知。本文针对常用于p型GaN接触的第一层金属Pd材料,讨论了Pd/p-GaN接触界面的特性和退化机制。通过四探针测试仪、X射线光电子能谱(XPS)和原子力显微镜(AFM)实验测试和分析对比,发现Pd/p-GaN界面受到氧气和温度影响的退化过程。高温退火在界面处促成Ga-Pd合金相生成利于形成良好的接触,但是在有氧参与的情况下,金属的氧化反应超越其他因素成为主导,致使界面和性能发生明显的退化。温度越高退化越严重,甚至表面形貌状态完全改变,由平滑的原子台阶形貌转化呈现出树枝状晶粒状态。因此,保持Pd与p-GaN界面清洁、控制界面的氧成分不仅是形成合金态获得良好接触的关键,而且也关系着器件的长期稳定和可靠,是防止器件性能衰减和退化要害所在。

GaN-based wide bandgap semiconductors and devices have been quickly emerging as a new frontier and cutting-edge technique.It is necessary to fabricate Ohmic contact for GaN device application.Pd is one of candidate metals for fabricating metal/p-GaN Ohmic contacts.Under the identical preparation conditions,different thickness Pd films were sputtered on p-GaN epilayers in sputtering chamber directly connected with MOCVD growth system.After the metal deposition,the samples were then annealed at 300℃and 600℃in ambient atmosphere of N2∶O2=4∶1 or under vacuum conditions for 90 s.They were measured and characterized by four-probe Van de Pauw,X-ray photoelectron spectroscopy(XPS),and atomic force microscopy(AFM).It is found that the electrical properties and surface morphologies of samples were dependent on annealing ambient atmosphere and heat treatment temperature conditions.When the samples were annealed at 600℃under high vacuum conditions,Ga can be diffused out from Pd/p-GaN interface.It would be helpful to make Ga and Pd reactive to become alloy as well as to produce Ga vacancy for Ohmic contact.While the samples were annealed in N2∶O2=4∶1 environment,however,oxidation of metal Pd and Ga becomes a dominant factor resulting in degradation of the Ohmic contacts.The higher the annealing temperature in oxygen environment,the worse the electrical properties.Surface morphologies of the annealed samples were completely changed from a smooth atomic step morphology to a dendritic polycrystal grain state if oxygen was participated into the interface during the annealing process.Therefore,avoiding oxygen incorporation at Pd/p-GaN interface shall be a key role in forming alloy and Ohmic contact,which favors the stability and reliability of device performance.