Two-step surface treatment technique: Realization of nonalloyed low-resistance Ti/Al/Ti/Au ohmic contact to n-GaN

Abstract

A novel two-step surface treatment method has been developed to realize low resistance nonalloyed ohmic contact to n-type GaN doped with Si to 6×1017 cm−3. The removal of native oxide (oxides and hydroxides) formed on GaN surface is crucial for successful creation of nonalloyed low resistance ohmic contact. In the case of GaN, plasma etching of the material surface prior to metal deposition holds promise for developing such nonalloyed ohmic contacts. In this article, the effects of the postetch chemical treatment of the n-type GaN surface on the Ti based nonalloyed contact performance have been investigated. Contacts on samples without reactive ion etching (RIE) showed Schottky behavior. However, contacts on samples with 15 s of RIE using Cl2 showed ohmic behavior. The contact resistivity of this contact reached to ρs=1.2×10−3 Ω cm2. Treating the RIE etched sample in boiling aqua regia for 5 min yielded a contact resistivity on the order of 3.6×10−4 Ω cm2. Dramatic improvement in current–voltage characteristics was observed after boiling RIE etched samples in KOH for 5 min. It yielded contact resistivity on the order of ρs∼7×10−5 Ω cm2, which is comparable to the contact resistivity of alloyed contacts. It was demonstrated, for the first time, that chemical treatment of postetched GaN surfaces employing KOH is very promising for new generation of nonalloyed ohmic contacts. This study suggested that the reduction of contact resistivity results from the effective removal of surface oxides and hydroxides, which act as an impeding barrier for electron transport.