Ohmic contact formation mechanism of Ta∕Al∕Mo∕Au and Ti∕Al∕Mo∕Au metallizations on AlGaN∕GaN HEMTs

Abstract

high electron mobility transistors is presented. By the optimization of surface treatment schemes and annealing temperature, contact resistances of 0.172 and 0.228 ⍀ mm, and specific contact resistivities of 2.96ϫ 10 −7 and 1.09ϫ 10 −6 ⍀ cm 2 were obtained for Ti/ Al/ Mo/ Au and Ta/ Al/ Mo/ Au, respectively. Auger electron spectroscopy ͑AES͒, x-ray diffraction ͑XRD͒, and transmission electron microscopy ͑TEM͒ were utilized to study microstructural changes occurring in the metallization layers as a result of heat treatment. Results indicate dynamic systems of severe intermixing between atoms from the metallization layers and epilayers resulting in changes in surface morphology, as determined by atomic force microscopy ͑AFM͒. The formation of intermetallics and interfacial compounds was observed. The combined effect of surface treatment and annealing-induced reaction is proposed as the mechanisms for low-resistance Ohmic contact formation.

Formation mechanism of Ohmic contacts on AlGaN∕GaN heterostructure: Electrical and microstructural characterizations

“…Ti/ Al-based multilayer structures are regarded as the standard Ohmic contacts for n-type GaN. In sharp contrast to the copious and systematic efforts to optimize Ohmic contact performance on AlGaN / GaN heterostructures for different metallizations, 2,7,8,10,11,[17][18][19] studies on the interfacial structural evolutions associated with underannealing and overannealing are paucid. 2-9͒ at elevated temperatures due to an exchange reaction mechanism.…”

First-layer Si metallizations for thermally stable and smooth Ohmic contacts for AlGaN∕GaN high electron mobility transistors

“…Although Ti/ Al/ metal/ Au have been shown to yield low resistance contact formation, reliability concerns regarding high temperature thermal stability still exist. Recent Auger electron spectroscopy ͑AES͒ and transmission electron microscopy ͑TEM͒ investigations have indicated that the highmelting-point metal layer hardly acts as a diffusion barrier 14 and plays an active role in dictating Ohmic performance of Ti/ Al/ metal/ Au schemes. 15 Also contacts annealed at optimal conditions have been demonstrated to contain complex a͒ Author to whom correspondence should be addressed; electronic mail: iadesida@uiuc.edu microstructures in which high degree of intermixing between the epilayers and metallization layers was observed along with varying degrees of interfacial reactions and grain formation in the metallization matrix.…”

Ohmic contacts to Al‐rich AlGaN heterostructures

“…The low Schottky barrier contacts can be formed during high temperature anneals of the ohmic metallization, in which interfacial reaction between the metals and the underlying semiconductor results in reaction islands, or spikes (Fig. a), driving down to the heterostructure interface resulting in direct two‐dimensional electron gas (2DEG) 2DEG contact .…”

“…The fabrication scheme that requires the simplest structure is that of planar metallization. Several different metallization stacks, anneal conditions, and surface treatments have been investigated and successfully utilized over the years for GaN‐based devices , with reports of low contact resistance of <0.2 Ω‐mm. Contact resistance in this regime enables a very low on‐resistance, crucial for superior device performance.…”

“…Contact resistance in this regime enables a very low on‐resistance, crucial for superior device performance. Anneal temperatures for Ti/Al/Ni/Au of ∼850 °C have been reported to show rough surface morphology and significant inter‐metallic mixing, ultimately causing metal spike formation down into the heterostructure . In this work, the current–voltage characteristics of several different metal stacks and anneal temperatures were examined for possible implementation as an ohmic contact (Table ) to Al 0.85 Ga 0.15 N/Al 0.66 Ga 0.34 N heterostructure.…”