Electroluminescence and Transmission Electron Microscopy Characterization of Reverse-Biased AlGaN/GaN Devices
Electroluminescence and Transmission Electron Microscopy Characterization of Reverse-Biased AlGaN/GaN Devices
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DOI：
10.1109/tdmr.2012.2221464
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发表时间：
2013-03-01
影响因子：
2
通讯作者：
Zanoni, Enrico
中科院分区：
文献类型：
--
作者：
Cullen, David A.;Smith, David J.;Zanoni, Enrico
反向偏压应力测试已应用于超过 50 个 AlGaN/GaN 高电子迁移率晶体管，这些晶体管采用相同的工艺制造，但具有不同的 AlN 摩尔分数和 AlGaN 势垒层厚度值，以及不同的衬底（SiC 和蓝宝石）。还比较了两组具有不同缺陷类型和密度的器件，这些缺陷类型和密度与不同的生长条件和成核层的选择有关。当进行栅极漏极（或栅极漏极和源极短路）反向偏置测试时，所有器件都呈现出相同的与时间相关的故障模式，包括栅极漏电流的显着增加。这种失效机制在阶跃应力实验期间，当超过某个负栅极电压或“临界电压”时突然发生，或者在恒压测试期间的某个时间（定义为“击穿时间”）突然发生。电致发光（EL）显微镜被系统地用于识别引起栅极反向电流增加的局部受损区域。这种电流的增加与 EL 强度的增加相关，并且只有当超过临界电压时，测试期间才会出现显着的 EL 发射。聚焦离子束铣削可在先前通过 EL 显微镜识别出的故障点处生成适合电子显微镜观察的横截面样品。在高缺陷率器件中，我们发现 V 缺陷与最初的高栅极漏电流相关，并且对应于未经处理的器件中已经存在的 EL 斑点。相反，事实证明，识别反向偏压测试引起的缺陷极其困难，只能发现从栅极边缘垂直延伸穿过 AlGaN/GaN 异质结的纳米尺寸裂纹或缺陷链。没有看到金属/半导体相互扩散或扩展缺陷区域的迹象。对 AlGaN 特性的弱依赖性、强工艺依赖性、时间依赖性以及通过 EL 和电子显微镜识别的局部损伤的特征表明，由工艺引起的栅极肖特基结弱点引发的多步失效机制，这增强了对预先存在的缺陷的电流注入。结果，产生或激活了更多缺陷，最终导致渗透导电路径和永久性损坏。器件栅极和沟道之间形成低阻抗路径，增加栅极漏电流并可能导致器件烧毁。
Reverse-bias stress testing has been applied to a large set of more than 50 AlGaN/GaN high electron mobility transistors, which were fabricated using the same process but with different values of the AlN mole fraction and the AlGaN barrier-layer thickness, as well as different substrates (SiC and sapphire). Two sets of devices having different defect types and densities, related to the different growth conditions and the choice of nucleation layer, were also compared. When subjected to gate-drain (or gate-to-drain and source short-circuited) reverse-bias testing, all devices presented the same time-dependent failure mode, consisting of a significant increase in the gate leakage current. This failure mechanism occurred abruptly during step-stress experiments when a certain negative gate voltage, or "critical voltage," was exceeded or, during constant voltage tests, at a certain time, defined as "time to breakdown." Electroluminescence (EL) microscopy was systematically used to identify localized damaged areas that induced an increase of gate reverse current. This current increase was correlated with the increase of EL intensity, and significant EL emission during tests occurred only when the critical voltage was exceeded. Focused-ion-beam milling produced cross-sectional samples suitable for electron microscopy observation at the sites of failure points previously identified by EL microscopy. In high-defectivity devices, V-defects were identified that were associated with initially high gate leakage current and corresponding to EL spots already present in untreated devices. Conversely, identification of defects induced by reverse-bias testing proved to be extremely difficult, and only nanometer-size cracks or defect chains, extending vertically from the gate edges through the AlGaN/GaN heterojunction, were found. No signs of metal/semiconductor interdiffusion or extended defective areas were visible. The weak dependence on AlGaN properties, the strong process dependence, the time dependence, and the features of the localized damage identified by EL and electron microscopy suggest a multistep failure mechanism initiated by a process-induced weakness of the gate Schottky junction, which enhances current injection into pre-existing defects. As a result, further defects are generated or activated, eventually resulting in a percolation conductive path and permanent damage. A low-impedance path between the device gate and the channel is formed, increasing gate leakage current and possibly resulting in device burnout.