Electric and Morphology Studies of Ohmic Contacts on AlGaN/GaN
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Electric and Morphology Studies of Ohmic Contacts on AlGaN/GaN V.Tilak, R.Dimitrov, M.Murphy, B.Green, J.Smart, W.J.Schaff, J.R.Shealy and L.F.Eastman School of Electrical Engineering, Cornell University, Ithaca, NY –14850 Abstract AlGaN/GaN is a promising system for high power electron devices. Quality of ohmic contacts is a critical parameter in determining the performance of the device. Although we have achieved a transfer resistance (Rc) of 0.35Ωmm and ρc of 9.5X10-7 Ωcm-2 the morphology and edge acuity of the contacts are poor. The standard ohmic contact recipes consist of a combination of Titanium and Aluminum with Nickel and/or Gold. This is annealed at 8000C9500C [1-5]. In this work we study ohmic contacts on unintentionally doped Al0.3Ga0.7N/GaN system. We look at ratios of Ti/Al from 0 to 2 to determine which is the optimum ratio in terms of surface morphology and electrical characteristics. From our studies we conclude that morphology of a Ti/Al contact is good over a ratio of 0.3 and the contact resistance is minimized at a Ti/Al of 0.6. The ohmic contacts are improved electrically if a layer of gold is added on top. The best electrical contacts however were obtained with a four layer recipe of Ti/Al/Ti/Au, which gave contact resistance (Rc) around 0.45Ωmm, but the morphology of the contacts was poor. Introduction The AlGaN/GaN semiconductor system has emerged as a candidate for high power and high temperature electronics [6]. Although the mobility is much less than that of GaAs, the saturation velocity and breakdown fields encountered are much higher in GaN. Also, higher sheet concentration of two dimensional electron gas are formed in unintentionally doped (UID) AlGaN/GaN system due to polarization effects, which increases the normalized current flowing in the device. These factors make them attractive candidates for high power microwave applications. An important factor in the performance of these HFETs is the quality of the ohmic contacts. It is desirable to have ohmic contacts with contact resistances < 1Ωmm, as this reduces the knee voltage and thereby improves the efficiency. As theses HFETs are high current structures, which are used in high power applications, it is good to minimize the contact resistance to decrease heating of the device. Another factor in the quality of the ohmic contact is its morphology and edge acuity. Because of the high temperatures needed to make ohmic contacts in this system, the morphology and edge acuity of the contacts are poor. Good edge acuity is critical for reducing the source gate spacing while keeping the source drain spacing constant in Hefts. This improves the breakdown voltage of the device, which is useful for high power applications. Ohmic contacts on n-type AlGaN/GaN usually are a combination of Titanium/Aluminum and/or Nickel, Gold [2-4]. Although there have been many reports of ohmic contacts with Ti/Al [1,5], there are few reports of comparative studies between different compositions of Ti/Al. In this work, we study various ratios of Titanium/Aluminum
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