Temperature dependence of the specific resistance in Ti∕Al∕Ni∕Au contacts on n-type GaN

Abstract

The temperature dependence of the specific resistance ρc in annealed Ti∕Al∕Ni∕Au contacts on n-type GaN was monitored, obtaining information on the current transport mechanisms. After annealing at 600°C, the contacts exhibited a rectifying behavior and became Ohmic only after high temperature processes (>700°C), with ρc in the low 10−5Ωcm2 range. The results demonstrated that the current transport is ruled by two different mechanisms: thermoionic field emission occurs in the contacts annealed at 600°C, whereas field emission dominates after higher temperature annealing. The significant physical parameters related to the current transport, i.e., the Schottky barrier height and the carrier concentration under the contact, could be determined. In particular, a reduction of the Schottky barrier from 1.21eV after annealing at 600°Cto0.81eV at 800°C was determined, accompanied by a strong increase of the carrier concentration, i.e., from 2×1018cm−3 in the as-prepared sample to 4.6×1019cm−3 in the annealed contacts. The electrical properties were correlated to the microstructure of the interfacial region, providing a scenario to explain the transition from Schottky to Ohmic behavior in annealed Ti∕Al∕Ni∕Au contacts.

Electrical behavior of AlGaN/GaN heterostuctures upon high-temperature selective oxidation

“…Thereafter, Ohmic contacts were formed using a Ti/Al/Ni/Au multilayer annealed above 750°C. 15,16 The test patterns fabricated to evaluate the electrical insulation efficiency of the oxidized layer consisted of two Ohmic contacts ͑60ϫ 120 m 2 ͒, at a distance of 60 m, isolated by a rectangular frame ͑20 m wide͒, defined by selective oxidation ͑"isolated" structure͒. For a quantitative comparison, a second test pattern was fabricated, without the selectively oxidized region between the Ohmic contacts ͑"reference"͒.…”

Effects of cap layer on ohmic Ti/Al contacts to Si+ implanted GaN

“…The formation of TiN at the metal/ GaN interface results in the generation of N vacancies, and, is often used to improve the ohmic contact resistivity by the creation of a heavily doped GaN surface. Moreover Ti/Al ratio of 1/3 is commonly reported, to promote the formation of Al 3 Ti alloy during annealing, also known to improve the thermal stability due to its higher resistance to oxidation than either Al or Ti [21].…”

“…The formation of TiN at the interface is reported to reduce effectively the barrier height [21]. The mechanism is that the formation of TiN creates N vacancies in the GaN surface during the contact anneal that act as donors, enhance the doping level at the interface, then reducing the contact resistance.…”

“…To verify this, a reported successfully Ti/Al metal scheme was chosen due to its low work function [18][19][20][21]. Ti helps to improve the adhesion and reacts easily with N forming a TiN interface often even only after deposition [15].…”

“…Since we have not detected TiN, we believe that our measurement may be not sensitive enough to detect the formation of a thin TiN layer at the interface. First, according to the literature the interface TiN layer should be 4-6 nm thick [21], as revealed by TEM analysis and likely of not very good crystalline quality. In addition, the u-2u reported TiN (1 1 1) reflection [ICDD-PDF #65-0565] is expected at 36.78.…”