Room Temperature Ohmic contact on n-type GaN using plasma treatment
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Internet Journal Nitride Semiconductor Research
Room Temperature Ohmic contact on n-type GaN using plasma treatment Ho Won Jang1, Jong Kyu Kim1, Chang Min Jeon1 and Jong-Lam Lee1 1Pohang
University of Science and Technology,
(Received Monday, January 10, 2000; accepted Monday, April 9, 2001)
Surface pretreatment using Cl2 plasma was applied to n-type GaN and Ti/Al ohmic contacts with resistivity of ~ 10-6 Ω cm2 , realized without annealing. Using synchrotron radiation photoemission spectroscopy, it was observed that the Fermi level moved by 0.5 eV toward the conduction band edge and the atomic ratio of Ga/N was increased by the treatment. This suggests that a number of N vacancies were produced at the treated surface and the Fermi level was pinned at the energy level of N vacancies near the conduction band. The N vacancies acting as donors for electrons produced a number of electrons, resulting in the near surface region to be in the degenerate state. Both the shift of Fermi level and the production of electrons at the treated surface lead to the reduction in contact resistivity through the decrease of the effective Schottky barrier for conduction of electrons.
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Introduction
Ohmic contacts on GaN with a low contact resistivity attract continuously great interest in improving electrical and optical performance of various GaN-based devices such as optical devices and high temperature/ high power electric devices [1] [2] [3] [4] [5] [6] [7]. For n-type GaN, ohmic contacts with contact resistivity as low as 8 × 10-6 Ω cm2 were achieved using Ti/Al metallization after annealing at 900 °C for 30 s [8]. Due to the formation of a TiN layer associated with the interaction of Ti with GaN, a high concentration of N vacancies, VN, could be created near the interface, causing the GaN to be heavily doped n-type. This Ti/Al metal scheme has been used as the standard ohmic contact on n-type GaN [9] [10]. In the fabrication of a device, high temperature annealing degrades the device performance. The development of nonalloyed contacts could prevent the device from such thermal degradation during the fabrication process. In addition, adopting the nonalloyed contact in the fabrication process of the device could lead to improvement in process freedom. The pretreatment of the surface prior to metal deposition plays a key role in reducing the contact resistivity on GaN [11]. A number of Ga vacancies, VGa, acting as an acceptor for electrons, were found at the surface region of p-type GaN when the surface of GaN were treated using aqua regia solution [12]. Thus, the Fermi level position shifts to an
energy level of acceptor defects, resulting in a decease of contact resistivity from the decrease of the Schottky barrier height for hole transport. This suggests that room temperature ohmic contacts could be achieved through production of a donor-type defect, VN using proper surface treatment [13]. It has been shown that reactive ion etching n-GaN surfaces improves the contact resistance [9] [13]. However, there is no reported work
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