Ta-based interface ohmic contacts to AlGaN/GaN heterostructures

Abstract

Cross-plane electronic and thermal transport properties of p-type La0.67Sr0.33MnO3/LaMnO3 perovskite oxide metal/semiconductor superlattices J. Appl. Phys. 112, 063714 (2012) Polarization Coulomb field scattering in In0.18Al0.82N/AlN/GaN heterostructure field-effect transistors J. Appl. Phys. 112, 054513 (2012) Modulation doping to control the high-density electron gas at a polar/non-polar oxide interface Appl. Phys. Lett. 101, 111604 (2012) Ultra low-resistance palladium silicide Ohmic contacts to lightly doped n-InGaAs Al/Ti based metallization is commonly used for ohmic contacts to n-GaN and related compounds. We have previously reported an ohmic contact scheme specially designed for AlGaN/GaN heterostructure field-effect transistors ͑HFETs͒ ͓D. Qiao et al., Appl. Phys. Lett. 74, 2652 ͑1999͔͒. This scheme, referred to as the ''advancing interface'' contact, takes advantage of the interfacial reactions between the metal layers and the AlGaN barrier layer in the HFET structure. These reactions consume a portion of the barrier, thus facilitating carrier tunneling from the source/drain regions to the channel region. The advancing interface approach has led to consistently low contact resistance on Al 0.25 Ga 0.75 N/GaN HFETs. There are two drawbacks of the Al/Ti based advancing interface scheme, ͑i͒ it requires a capping layer for the ohmic formation annealing since Ti is too reactive and is easily oxidized when annealing is performed in pure N 2 or even in forming gas, and ͑ii͒ the atomic number of Al and that of Ti are too low to yield efficient backscattered electron emission for e-beam lithographic alignment purposes. In this work, we investigated a Ta based advancing interface contact scheme for the HFET structures. We found that the presence of Ta in this ohmic scheme leads to ͑1͒ a specific contact resistivity as low as 5ϫ10 Ϫ7 ⍀ cm 2 , ͑2͒ efficient electron emission for e-beam lithographic alignment, and ͑3͒ elimination of the capping layer for the ohmic annealing.

V/Al/Pt/Au Ohmic contact to n-AlGaN/GaN heterostructures

“…The best Ti/Al/Pt/Au contact, yielding C р2.2 ϫ10 Ϫ6 ⍀ cm 2 and R C р0.51 ⍀ mm, is observed with x Ti ϭ0.31 after annealing at 850°C. The formation of Ohmic contacts with C р3ϫ10 Ϫ4 ⍀ cm 2 using Ti/Al/Pt/Au is thus shown to require an annealing temperature of at least 800°C, which is in excellent agreement with published studies, [1][2][3][4][5][6][7][8][9][11][12][13][14][15][16][17][18]20,21 whereas a C of (1.4Ϯ0.3) ϫ10 Ϫ5 ⍀ cm 2 and R C ϭ0.82Ϯ0.09 ⍀ mm are readily obtained after annealing V͑15 nm͒/Al͑85 nm͒/Pt͑50 nm͒/Au͑50 nm͒ at 650°C. These values of C and R C are, respectively, almost two and one orders of magnitude lower than the best Ti/Al/Pt/Au contact annealed up to 650°C.…”

“…However, annealing at high temperatures has been suspected of causing degradation in device performance, e.g., deteriorating edge acuity of electrical contacts 10,16 and relaxation of the AlGaN/GaN heterostructure. 13 Studies using Pd, 7 Si, 17 and Ta 20,21 in the contacts have also demonstrated good Ohmic contacts but did not alleviate the need for high annealing temperatures. Improvements of Ohmic contacts have been demonstrated by regrowth, 1,3,8,10 implantation, 12 plasma pretreatment, 4,18 and annealing prior to metalization, 6 but these improvements increase the number of processing steps.…”

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

“…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. Many attempts have been made to find a way of lowering the contact resistivity and improving the surface characteristics.…”

Effect of Carbon Doping and Crystalline Quality on the Vertical Breakdown Characteristics of GaN Layers Grown on 200-mm Silicon Substrates

“…Schematic cross-sections of the growth structures are shown in Fig. 1, and Table I presents the detailed growth pressure and temperature conditions for layer II. To investigate the influence of the carbon doping level on the buffer leakage properties, 100 lm 9 100 lm surface electrode contacts were made using a non-Au-based metal stack Ta/Al/Ta 18,19 annealed at 575°C for 30 s in N 2 atmosphere. The vertical leakage current behavior of all samples was measured using the setup illustrated in the insets of Fig.…”