Fabrication and Electrical Characteristics of Ti/Al Ohmic Contact to Si + Implanted GaN
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0892-FF14-03.1
FABRICATION AND ELECTRICAL CHARACTERISTICS OF Ti/Al OHMIC CONTACT TO Si+ IMPLANTED GaN Nobuyuki ITO*, Akira SUZUKI*, Mitsunori KAWAMURA*, Kazuki NOMOTO*, Takeshi KASAI**, Tomoyoshi MISHIMA***, Taroh INADA*, Tohru NAKAMURA*, Masataka SATOH* *Dept. of EECE and Research Center of Ion Beam Technology, Hosei University, Koganei, Tokyo 184-8584, Japan, [email protected] **Chemitronics, Musashimurayama, Tokyo 208-0023, Japan ***Hitachi Cable, Tsuchiura, Ibaraki 300-0026, Japan
ABSTRACT Ti/Al ohmic contact with an extremely low specific contact resistance has been formed by the deposition of Ti and Al films on Si+ implanted GaN. The ohmic contact formed by annealing at 600 oC of Ti film with a thickness of 50 nm and Al film with a thickness of 200 nm indicates the specific contact resistance of 2 x 10-6 ohm-cm2 for Si+ implanted GaN with a dose of 5 x 1013 cm-2. As Si ion dose increases to 5 x 1014 /cm2, the specific contact resistance is reduced to 2 x 10-8 ohm-cm2. It is revealed that the selective doping at high impurity concentration in the surface region by Si+ implantation is useful to reduce the contact resistance for Ti/Al contact to GaN.
INTRODUCTION The wide-band gap semiconductor GaN is of great interest as a candidate material for high-power and high-frequency electron devices because of high saturation electron velocity and high break-down field strength [1,2]. The ohmic contact with low contact resistance is strongly desired to reduce the power loss of GaN device through the decrease of series resistance in devices. To reduce the contact resistance, n-type epitaxial layer with high doping level is often grown on the active layer because the contact resistance is reciprocal to the doping concentration [3,4]. It has been reported that the specific contact resistance of 8.9 x10-8 ohm-cm2 is obtained by the deposition of Ti/Al/Ni/Au films on n-type GaN with a doping concentration of 4 x 1017 /cm3 and subsequently annealing at 900 oC for 30 sec in N2 gas flow [4]. However, in order to apply such ohmic contact structure to field effect transistor, the recess structure must be fabricated by etching the top contact layer over active layer. Ion implantation is a useful technique for doping of impurity into the selected area of GaN. Some workers reported that Si impruty implanted into GaN is highly activated as donor by annealing at temperatures above 1000 oC in N2 gas flow [5-8]. The sheet carrier concentration of 1.8 x 1015 /cm2 has been obtained for Si+ dose of 5 x 1015 /cm2 [6]. It is expected that the heavy doping in the surface region by Si+ implantation is applicable to fabricate a good ohmic contact to n-type GaN with a low contact resistance. There are, however, several reports about the formation and electrical properties of the ohmic contact to heavily Si+ implanted GaN [9-11] In this study, we investigated the Si+ implantation effect on the contact resistance and the surface structure of Ti/Al ohmic contact on GaN.
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Figure 1: Distribution of implanted Si in GaN, calcul
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