The authors study the effect of etch chemistry and metallization scheme on recessed Au-free Ohmic contacts to AlGaN/GaN heterostructures on silicon. The effect of variation in the recess etch chemistry on the uniformity of Ohmic contact resistance has been studied using two different etch chemistries (BCl3/O2 and BCl3/Cl2). Experiments to determine the optimum recess etch depth for obtaining a low value of contact resistance have been carried out, and it is shown that near-complete etching of the AlGaN barrier layer before metallization leads to the lowest value of contact resistance. Furthermore, two metal schemes, namely, Ti/Al and Ti/Al/Ti/W, are investigated, and it is found that the Ti/W cap layer on Ti/Al leads to low contact resistance with a smooth contact surface morphology. The effect of maintaining unequal mesa and contact pad widths on the extracted values of contact resistance and sheet resistance using the linear transfer length method (LTLM) has been studied. This is important as LTLM structures are used as monitors for process control during various steps of fabrication. It is shown that the extracted contact resistance and sheet resistance values are reliable when the mesa width is equal to the contact pad width. Finally, a possible mechanism for carrier transport in the Ohmic contacts formed using this process has been discussed, based on temperature dependent electrical characterization, and the field emission mechanism is found to be the dominant mechanism of carrier transport. A low Ohmic contact resistance of 0.56 Ω mm, which is one of the lowest reported values for identical metal schemes, and good contact surface morphology has been obtained with moderate post-metal annealing conditions of 600°C.

中文翻译：

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与 AlGaN/GaN 高电子迁移率晶体管的无金凹式欧姆接触：蚀刻化学和金属方案的研究

作者研究了蚀刻化学和金属化方案对硅上 AlGaN/GaN 异质结构的凹陷无 Au 欧姆接触的影响。已经使用两种不同的蚀刻化学物质（BCl3/O2 和 BCl3/Cl2）研究了凹槽蚀刻化学物质的变化对欧姆接触电阻均匀性的影响。已经进行了确定最佳凹槽蚀刻深度以获得低接触电阻值的实验，结果表明，在金属化之前几乎完全蚀刻 AlGaN 阻挡层会导致接触电阻值最低。此外，研究了两种金属方案，即 Ti/Al 和 Ti/Al/Ti/W，发现 Ti/Al 上的 Ti/W 帽层导致低接触电阻和光滑的接触表面形态。已经研究了使用线性转移长度方法 (LTLM) 保持不等台面和接触焊盘宽度对接触电阻和薄层电阻的提取值的影响。这很重要，因为 LTLM 结构在制造的各个步骤中用作过程控制的监视器。结果表明，当台面宽度等于接触焊盘宽度时，提取的接触电阻和薄层电阻值是可靠的。最后，基于温度相关的电特性，讨论了使用该过程形成的欧姆接触中载流子传输的可能机制，并且发现场发射机制是载流子传输的主要机制。0.56 Ω mm 的低欧姆接触电阻，这是相同金属方案的最低报告值之一，