Ohmic Contact Properties Depending on AlGaN Layer Thickness for AlGaN/GaN High Electron Mobility Transistor Structures
Authors: Takei Yusuke , Okamoto Mari , Saito Wataru , Tsutsui Kazuo , Kakushima Kuniyuki , Wakabayashi Hitoshi , Kataoka Yoshinori , Iwai Hiroshi 
Published: February 2020

Mo/Al/Ti or TiN/TiSi2ohmic contacts were fabricated on AlGaN/GaN HEMT structures having various thicknesses of AlGaN layers. Optimum thicknesses depending on annealing temperature were found, which should be correlated with the contact formation mechanism.
AlGaN/GaN HEMTs (high electron mobility transistors) have drawn lots of attention for high frequency and power applications owing to high mobility of 2DEG. One of the important issues of the devices is formation of ohmic contacts with low contact resistance. Since contact metal layers are usually placed on the AlGaN layer under which two-dimensional electron gas (2DEG) is induced, current passes through the AlGaN layer, a barrier layer for electrons, should be formed. Various models of the contact formation mechanism, such as the intrusion of metal into 2DEG [1] and formation of n-type region due to N vacancies [2], have been discussed. However, comprehensive understanding for the mechanism is still insufficient so far.
We expect that optimum AlGaN thickness is present to obtain low contact resistance because of a trade-off between increase in the resistance by increasing the thickness due to thicker barrier and that by decreasing the thickness due to depletion of 2DEG. In this work, we revealed the prospect experimentally, and discussed mechanism based on the phenomenon.
The wafers used in this study consist of 30-nm-thick undoped AlGaN with GaN layer epitaxially grown on Si (111) substrates. The top AlGaN layers were thinned by the cyclic etching in which oxidation in O3 ambient and removal by hydrofluoric acid were carried out, so that samples with various AlGaN thicknesses were prepared. Contact electrodes, Mo/Al/Ti (35/60/15nm) or TiN/TiSi2 (45/20nm) [3], were formed by sputtering followed by subsequent annealing (400-1100 oC for 1min. in N2) on the samples, as shown in Fig. 1. Contact resistances were evaluated by the TLM method.
A typical characteristic of the contact resistance vs. AlGaN thickness for the case of relatively low annealing temperature (650oC) is shown in Fig.2. The optimum AlGaN thickness to get the lowest resistance, 11.6 nm in this case, was clearly observed. On the other hand, in the case of higher annealing temperature (950oC or 1100oC), the monotonous decrease in contact resistance with the thickness was observed as shown in Fig.3. The increase in contact resistance in the region thinner than 11.6 nm is speculated to be due to depletion of 2DEG, which is common to the both low and high temperature cases. While, the increase in contact resistance for thicker layers at low temperature annealing, as shown in Fig.2, is probably attributed to high resistivity in the AlGaN layer. However, in the case of high temperature annealing, as shown in Fig.3, some mechanism of the metal intrusion and/or the conversion to n-type to overcome the thick barrier layer should work effectively.
In conclusion, dependences of contact resistance on thickness of the AlGaN layer were observed, and they indicated existence of the optimum thickness. Discussion of these properties correlated with mechanism of ohmic contact formation will be useful for further understanding of the contact technology for AlGaN/GaN HEMTs.
References
[1] L. Wang, et al., Appl. Phys. Lett., 87, 141915 (2005).
[2] B. Daele, et al., Appl. Phys. Lett., 89, 201908 (2006).
[3] K. Tsuneishi et al., ECS Transaction, 50, 477-480 (2013).