Formation of non-alloyed Ti/Al/Ni/Au low-resistance ohmic contacts on reactively ion-etched n-type GaN by surface treatment for GaN light-emitting diodes applications

Abstract

Ti/Al/Ni/Au (30/120/20/100 nm) non-alloyed ohmic contacts on reactively ion-etched n-GaN have been obtained by thermal annealing in N₂ + O₂ ambient prior to metal contacts deposition. After surface treatment, contacts show the linear current–voltage (I–V) characteristics with specific contact resistance (SCR) of 2.91 × 10⁻⁵ Ω-cm² without any post-deposition annealing, whereas, SCR of untreated sample has been found 1.02 × 10⁻⁴ Ω-cm². The improvement in SCR of Ti/Al/Ni/Au contacts with this process may be attributed to formation of n⁺-highly doped layer due to presence of oxygen during thermal annealing. A further increase in SCR of non-treated sample from 1.02 × 10⁻⁴ Ω-cm² to 3.54 × 10⁻⁴ Ω-cm², has been observed on further annealing of contacts at 550 °C in N₂ + O₂ ambient for 5 min, which is generally an essential condition for formation of low-resistance highly transparent p-contact on p-GaN during fabrication of conventional GaN light-emitting diodes (LEDs). Based on X-ray photoelectron spectroscopy results, we found that there is increase in Ga 3d photoelectron binding energy with this surface treatment, which indicates a shift of Fermi level closure to conduction band, resulting in decrease of Schottky barrier height (SBH). The reduction in SBH is responsible for formation of low-resistance Ohmic contacts on n-GaN. Such non-alloyed n-contacts formed on thermally annealed reactively etched n-GaN surface may open for fabrication of more efficient conventional GaN LEDs. In addition, the forward voltage of GaN LED fabricated by present suggested process lowered by an amount of 0.37 V at 100 mA compared to standard process.