Ohmic contacts to Si-implanted and un-implanted n-type GaN
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We report on ohmic contacts to Si-implanted and un-implanted n-type GaN on sapphire. A ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. Metal layers of Al and Ti/Al were investigated. On un-implanted GaN, post metalization annealing was performed in an RTA for 30 seconds in N 2 at temperatures of 700, 800, and 900'C, A minimum specific contact resistance (re) of 1.4x10-5 f2-cm 2 was measured for Ti/Al at an annealing temperature of 800'C. Although these values are reasonably low, variations of 95% in specific contact resistance were measured within a 500 P.m distance on the wafer. These results are most likely caused by the presence of compensating hydrogen. Specific contact resistance variation was reduced from 95% to 10% by annealing at 900'C prior to metalization. On Si-implanted GaN, un-annealed ohmic contacts were formed with Ti/Al metalization. The implant activation anneal of 1120'C generates nitrogen vacancies that leave the surface heavily n-type, which makes un-annealed ohmic contacts with low contact resistivity possible. INTRODUCTION
Obtaining stable, low resistance ohmic contacts is essential for the fabrication of most practical semiconductor devices. For GaN and other highly ionic semiconductors, evidence suggests that contact barrier heights depend directly on the difference between the work function of the metal and the electron affinity of the semiconductor [1]. Consequently, of the common
metals used in semiconductor processing, we expect Al and Ti to form ohmic contacts to n-type GaN fairly easily due to their relatively small work functions [2,3]. The quality of ohmic contacts, however, will be greatly influenced by the condition of the semiconductor surface prior to metalization as well as heat treatment of the material after the contacts have been formed. In this paper we present the results of our investigations of the formation of ohmic contacts to Siimplanted and un-implanted n-type GaN. Surface preparation, pre-metalization and postmetalization treatments are discussed. EXPERMENT Annealed Ohmic Contacts to Undoped GaN Metal was deposited by evaporation at 5x10 7-10-6 Torr. Ring shaped contacts were then formed using a photolithography and liftoff process. The ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. For a large ring to gap-spacing ratio, the ring contact geometry reduces to the standard transmission line model (TLM) structure [4]. For practical ring radii (200 lim) and spacings (5-45 jtm), though, small, geometrical correction factors are necessary to compensate for the difference between the TLM and ring layouts. 855 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
Our studies indicate that, while Al forms an ohmic contact to undoped n-type GaN, the specific contact resistance for this metalization is very high. This is probably due to the presence of an oxide layer on the GaN surface which reacts with the Al to form insulat
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