Optimized ohmic contacts for InAlGaN/GaN HEMTs

International audienceDuring the last years, the most significant improvement of the contact resistance forAlInN/GaN high electron mobility has been the use of a highly doped n+ GaN layer grown bymolecular beam epitaxy for the source and drain terminals. In a first report [1], the ohmiccontact processing was carried out as follows, first the device surface was passivated using aSiN layer and then protected by SiO2. Then the contact windows were opened through theprotection and the InAlN(5.6 nm) /GaN heterostructures was then etched out to a depth of 12nm followed by a regrowth of a 40 nm thick n+ doped GaN layer by MOCVD. After removalof the residual polycrystalline GaN from the SiO2 surface, a Ti/Al/Ni/Au metal stack wasdeposited by electron-beam evaporation and annealed at 850°C for 30s in N2 which resultedin an improved contact resistance of 0.40 Ohm.mm. In a more recent work using an identicalprocedure [2], the same authors deposited, a different metallic stack based on Mo/Au andmeasured a lower contact resistance of 0.16 Ohm.mm. In this work, the Ohmic contact isfabricated in a more conventional way by first removing the passivation layers through anoptimized surface chemical cleaning procedure in ohmic region, next a Ti/Al/Ni/Au metallicstack is deposited by electron-beam evaporation and the sample is submitted to rapid thermalannealing at 875°C for 30 seconds. In our report, we shall discuss the results measured on twosamples; S1 was processed using standard surface cleaning procedure which leads routinely toa reproducible contact resistance of about 0.5-0.6 Ohm.mm. For S2, we carefully optimizedthe AlInGaN barrier surface preparation prior to the metal stack deposition resulting incontact resistances as low as 0.15-0.16 Ohm.mm. Through a detailed analysis by transmissionelectron microscopy, it is shown that, for both samples the metallic stack is transformed into amultiphase alloy, although in a different way. In S1, there is an extensive phase separationwith two dominant alloys, one AuAl rich, and the second NiAl rich. In S2, the metallicreaction leads to a homogeneous alloy of the four starting metals. Moreover, at the interfacewith the barrier extended epitaxial relationships are present with the metallic particles.Although a diffusion of Ti, Ni, and Au can be seen along screw or mixed type dislocations,the surface layer of the barrier presents extended reaction with the metals. All thiscombination is probably at the origin of the measured low contact resistance, which becomecomparable to the case where the localized MBE growth of n+ GaN has been used