Improvement of Au‐Free, Ti/Al/W Ohmic Contact on AlGaN/GaN Heterostructure Featuring a Thin‐Ti Layer and Low Temperature Annealing

Abstract

In this study, a Ti/Al/W ohmic contact and the factors that determine the optimal annealing temperature are investigated to obtain Au-free highelectron mobility transistor (HEMT) with a low thermal budget. The results show that a thin-Ti layer of 2.7 nm with a low specific contact resistance of 2.54 Â 10 À6 Ω Á cm 2 (0.358 Ω mm) is realized in 10 min at a low annealing temperature of 500 C. It is found that a thinner Ti layer leads to a lower annealing temperature because Al needs to diffuse through the Ti layer and make contact with AlGaN to obtain linear I-V characteristics. In addition, Ti is essential in removing the natural oxide film on AlGaN surface, and the minimum thickness of the Ti layer is 2 nm. The Au-free AlGaN/GaN HEMT on a silicon wafer shows comparable characteristics with the conventional Au-containing HEMT, which leads to low cost manufacturing of HEMTs in existing silicon-device fabrication lines.

Au‐Free Ohmic Contacts and Their Impact on Sub‐Contact Charge Carrier Concentration in AlGaN/GaN Heterostructures

“…The lowest contact resistance was achieved for the last sample annealed at T RTA ¼ 900 C with R c ¼ 0.74 Ω mm. This is in the expected range for non-recessed Ti/Al-based Au-free contact metallization schemes, which have resistances between 1.1 and 0.4 Ω mm after annealing at 500⋯900 C. [4,[21][22][23][24][25] If using a pre-deposition recess etching step of the sub-contact area, contact resistances between 0.6 and 0.2 Ω mm can be achieved after annealing at 600⋯550 C. [26][27][28][29] The RMS surface roughness for this sample was as low as 4.6 nm (measured over a length of 1500 μm, using high-resolution profilometry). The similar shape of Figure 5 and 7 in the region between T RTA ¼ 650⋯850 C seems to indicate that the total resistance R total of the test device is dominated by the contact resistance R c .…”

A Comprehensive Review of Recent Progress on GaN High Electron Mobility Transistors: Devices, Fabrication and Reliability

“…The low annealing temperatures and the Au-free metal stacks would be beneficial to the device electrical and reliability performance [83] and are also compatible with foundry CMOS processes which is plausible for transferring to large scale production.…”

“…Recently, Yoshida and Egawa [83] reported Ti/Al/W Ohmic contacts on AlGaN/GaN. The contact resistance was about 0.358 Ω•mm with a low annealing temperature of 500 • C. They found that thinner Ti layer can lead to a lower annealing temperature, because Al needs to diffuse through the Ti layer in order to make Ohmic contact with AlGaN.…”

Current Trends in the Development of Normally-OFF GaN-on-Si Power Transistors and Power Modules: A Review

“…57 Various metal stack use different metal types and process schemes. [58][59][60][61] Other complex contact and field plate structures are also currently under intense investigation. 62 The gate metal stack is typically p-type GaN and Ni/Au.…”

“…65 Gold-free contact fabrication is another recent research trend which allows GaN device processes compatible with the CMOS processes to promote further integrations. 59,66,67 As the dimensions of the devices are reduced to the nm scale, they are now highly dependent on the atomic and molecular behavior of the material, which is very sensitive to the process parameters. Therefore, the research and development of advanced process techniques and precision controls are crucial for the development of high-performance GaN power transistors.…”