热退火对AlGaN/GaN异质结上碳化钨肖特基接触电学和结构特性的影响
Thermal Annealing Effect on Electrical and Structural Properties of Tungsten Carbide Schottky Contacts on AlGaN/GaN heterostructures
摘要：
碳化钨（WC）接触是一种新型的AlGaN/GAN异质结无金肖特基金属化材料。WC/AlGaN接触的电学和结构/成分特性的演变已经被监测为退火温度在400至800°C范围内的函数。从二极管的正向电流-电压特性中提取的WC/AlGaN界面处的肖特基势垒高度（$Φ$ B）从沉积态和400°C退火样品中的0.8 eV降低到800 °C退火后的0.56 eV。$Φ$ B的这种大幅降低伴随着反向偏置漏电流的相应增加。透射电子显微镜结合电子能量损失谱分析揭示了在沉积态和400°C退火样品中均存在均匀分布在WC层中的氧（O）。相反，在800 °C下退火之后，观察到在与AlGaN的界面处的2-3 nm薄W-O-C层中的氧积累，以及在膜内形成W2 C晶粒（通过X射线衍射分析确认）。这种界面W-O-C层的形成可能是800°C退火肖特基二极管中$Φ$ B降低和漏电流增加的主要原因，而WC膜内部O含量的降低可以解释金属层电阻率的降低。结果提供了一个评估的应用程序的WC肖特基接触AlGaN/GAN异质结的工艺条件。
英文摘要：
Tungsten carbide (WC) contacts have been investigated as a novel gold-free Schottky metallization for AlGaN/GaN heterostructures. The evolution of the electrical and structural/compositional properties of the WC/AlGaN contact has been monitored as a function of the annealing temperature in the range from 400 to 800°C. The Schottky barrier height ($Φ$B) at WC/AlGaN interface, extracted from the forward current-voltage characteristics of the diode, decreased from 0.8 eV in the as-deposited and 400°C annealed sample, to 0.56 eV after annealing at 800 °C. This large reduction of $Φ$B was accompanied by a corresponding increase of the reverse bias leakage current. Transmission electron microscopy coupled to electron energy loss spectroscopy analyses revealed the presence of oxygen (O) uniformly distributed in the WC layer, both in the as-deposited and 400°C annealed sample. Conversely, oxygen accumulation in a 2-3 nm thin W-O-C layer at the interface with AlGaN was observed after the annealing at 800 °C, as well as the formation of W2C grains within the film (confirmed by X-ray diffraction analyses). The formation of this interfacial W-O-C layer is plausibly the main origin of the decreased $Φ$B and the increased leakage current in the 800°C annealed Schottky diode, whereas the decreased O content inside the WC film can explain the reduced resistivity of the metal layer. The results provide an assessment of the processing conditions for the application of WC as Schottky contact for AlGaN/GaN heterostructures.
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