Au-free recessed Ohmic contacts to AlGaN/GaN high electron mobility transistor: Study of etch chemistry and metal scheme

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.