Study of contact formation in AlGaN/GaN heterostructures

Abstract

The contact formation of Ti/Al and Ti metallization on AlGaN/GaN heterojunction field effect transistors (HFET) was investigated. It was found that ohmic contact formation is related to the low work function of the Ti contacting layer and the formation of a TiN phase at the Ti/nitride interface. Contact resistance as low as 1 Ω mm or less can be obtained on HFET samples with a nsμ product of ∼0.8×1016/V s and on n-GaN with a carrier concentration of 1.5×1018/cm3. Ti/Al bilayer contact scheme is superior to Ti-only contact due to a surface Al3Ti layer in the bilayer contact, which may reduce the oxidation problem when annealed in N2 at high temperatures. Preannealing the HFET samples at 850 °C for 1 h in N2 appears to improve the ohmic contact in general, but not always observed. Our results indicate that Ti/Al contact scheme yields sufficiently low contact resistance on HFET structures for microwave applications.

Investigation of Ta∕Ti∕Al∕Ni∕Au ohmic contact to AlGaN∕GaN heterostructure field-effect transistor

“…[8][9][10] Liu et al first reported that the Ti/ Al metal scheme on AlGaN / GaN showed contact resistivity of 3.2ϫ 10 −6 ⍀ cm 2 with 950°C heat treatment. 11 Qiao et al recently reported that the addition of a Ta layer below the Ti/ Al metal layer leads to contact resistivity as low 5 ϫ 10 −7 ⍀ cm 2 at the 950°C annealing temperature. 12,13 Because high-temperature annealing has some drawbacks, such as surface roughness, it is necessary to develop new methods which reduce contact resistivity at moderate annealing temperatures.…”

Effect of Various Pre-Treatments on Ti/Al/Ti/Au Ohmic Contacts for AlGaN/GaN HFET Devices

“…The improvement in r c in the case of GaN layers [3] was postulated to be due to the loss of N 2 from the annealed surface, generating N vacancies and leaving the surface heavily n-type doped. In the previous work on AlGaN/GaN HFET structures [4], pre-annealing also resulted in an increase in R s , but the decrease in r c was more significant, leading to an overall decrease in R c . That anneal was for one hour at 850 C. The present results suggest that while preannealing may sometimes be used to improve the contact resistance for AlGaN/GaN HFET structures, this improvement may come with the cost of a deterioration in the electrical properties of the 2D electron gas, and may not be beneficial overall.…”

“…Inductively coupled plasma N 2 discharges [1], or O 2 reactive ion etching [2] have been demonstrated to reduce the contact resistance in some work. Pre-annealing, prior to metal deposition, has been also shown to reduce the contact resistance for GaN [3], and also in some cases, for AlGaN/GaN HFET structures [4], although very long pre-anneals were used in the latter case.…”

Dependence of the electrical and morphological properties on the Ti and Al content in Mo-based ohmic contacts for III–V nitrides

“…Among the variety of ohmic contacts proposed for III-V nitrides, the low resistivity metallizations are predominantly based on the Ti/Al film combination [5]- [15]. Usually Ti is deposited as a first layer to the heterostructure since it reacts with N to form TiN at the interface, which contributes to the barrier lowering [16], [17].…”