Electrical Performance of Ta-Based Ohmic Contacts on Undoped AlGaN/AlN/GaN Heterostructures

Undoped AlGaN/AIN/GaN Heterostructures Yunju Sun and L. F. Eastman 426 Phillips Hall, Cornell University, Ithaca NY 14853 E-mail: _ du Phone: 607-255-1431/607-342-0651 Fax: 607-255-4742 In order to improve the performance of the HFETs, the electrical characteristics of ohmic contacts formed on top of the heterostructures are extremely important in order to achieve a high transconductance and a high saturation current. Previously, thermally stable low resistance with SiCl4 plasma treatment was achieved using Ti/Al/Mo/Au multilayer ohmic contacts on n-GaN [1]. Its thickness and annealing condition were later optimized based on the electrical performance and edge acuity on Undoped AlGaN/GaN HEMTs structure without any plasma surface treatment [2]. The optimized thickness and annealing condition were Ti/Al/Mo/Au (15 nm/90 nm/40 nm/50 nm) and 800 for less than 30 sec respectively. In order to enhance the sheet charge confinement and improve electron mobility at heterojunction interface, a 10-15 A AlN interbarrier was inserted between AlGaN and GaN [3] HEMTs structure. From the experimental results, it was more difficult to get low contact resistance on Undoped AlGaN/GaN heterostructure with AlN interbarrier based on the same optimized Ti/Al/Mo/Au ohmic metal stack. In this report, in order to reduce the contact resistance, same material structure ( Undoped Al.27Ga.73N 230A/AlN 15A/GaN) was used, and Ta/Ti/Al/Mo/Au as a total of five metal layers with Ti/Al/Mo/Au fixed at 15 nm/90 nm/40 nm/50 nm and with Ta at a thickness of 30, 50, and 75 A respectively were evaporated on it. The electrical performance was also compared with the one using Ta/Ti/Al/Ti/Au metal layers on the same material structure. As shown in Fig. 1, with fixed Ti/Al/Mo/Au overlayers, a transfer resistance as low as 0.105 ohm-mm was obtained with Ta at a thickness of 75 A annealed at a cycle of 700 for 1 min followed by 800 for 20 sec. The transfer resistance increased as the Ta layer thickness decreased from 75 A to 30 A. Since comparing with the ohmic metal stack using Ti (4.33 eV) as a bottom layer, Ta has a lower work function (4.25 eV), it can be beneficial for the formation of low contact resistance. However, there could be a fluctuation of barrier height before the inner metal reaches a thickness that the metallic character of the film has been well established [4], which can be related to the dependence of contact resistance on the thickness of inner thin metal film. In Fig. 1, sheet resistance underneath the metal was also measured by "end-resistance" method, and the lowest value is 16.24 ohm/square for the one using 75 A Ta. Fig. 2 and Fig. 3 show the total resistance as a function ofTLM metal spacing with good linear fit (0.9997-0.9999) and I-V characteristics of ohmic contacts (9.5 ptm spacing) after annealing at 700 for 1 min followed by 800 for 20 sec respectively for Ti/Al/Mo/Au and Ti/Al/Ti/Au metal stack with an insertion of Ta as a bottom layer. As we can see, under same annealing condition, the contact resistance is a lot higher using Ti/Al/Ti/Au as overlayers. Higher annealing temperature above 800 can be required to reduce the resistance. The rootmean-square surface roughness after annealing was measured by AFM, as shown in Fig. 4. Ta/Ti/Al/Ti/Au contact has the highest value of surface roughness comparing with those of Ta/Ti/Al/Mo/Au contacts. As shown in Fig.5, the annealed Ta 75A/Ti/Al/Mo/Au contacts also had good edge acuity. The creeping of metal is less than 65 nm.