Low thermal budget Hf/Al/Ta ohmic contacts for InAlN/GaN-on-Si HEMTs with enhanced breakdown voltage

Abstract

Articles you may be interested inTi/Al/Ti/Ni/Au ohmic contacts on AlGaN/GaN high electron mobility transistors with improved surface morphology and low contact resistanceThe authors have studied the electrical characteristics of Hf/Al/Ta ohmic contacts on In 0.18 Al 0.82 N/GaN heterostructure grown on Si (111) substrate. With annealing at 600 C in vacuum (which is $200 C lower than that for traditional Ti/Al/Ni/Au contacts), a minimum ohmic contact resistance of $0.58 XÁmm and specific contact resistivity of $6.75 Â 10 À6 XÁcm 2 are obtained. The minimum contact resistance of Hf/Al/Ta contacts is comparable to that of Ti/Al/Ni/Au contacts. Owing to the lower annealing temperature, Hf/Al/Ta contacts exhibit better surface morphology and edge acuity. More importantly, Hf/Al/Ta contacts show a smooth interface with In 0.18 Al 0.82 N/GaN, whereas spike structures that penetrate the In 0.18 Al 0.82 N layer are observed for Ti/Al/Ni/Au contacts. As a result, the source-carrier-injection induced breakdown mechanism is reduced in the In 0.18 Al 0.82 N/GaN-on-Si high electron mobility transistors (HEMTs) with Hf/Al/Ta ohmic contacts, thereby leading to an improved three-terminal off-state breakdown voltage by about 100 V ($53.5% improvement), in comparison to Ti/Al/Ni/Au based HEMTs.

Mechanisms of Ohmic Contact Formation and Carrier Transport of Low Temperature Annealed Hf/Al/Ta on In0.18Al0.82N/GaN-on-Si

“…With increasing annealing temperature, Hf/Al/Ta contacts became ohmic 16 The Hf-Al alloy formation has also been reported to form at a low temperature of 350 • C. 20 Furthermore, similar to the formation of Ti-Al alloy in the Ti/Al based ohmic contacts on GaN based heterostructures, the Hf-Al alloy formation could play a critical role (i.e., to limit HfN formation) to yield good ohmic contact properties, as reported by us earlier. 16 The formation of HfN has been reported by different methods in the temperature range of 450-1200 • C. [21][22][23] In our case, the possible formation of HfN can be linked with the observed out-diffusion of N, which can be understood as follows. The out diffusion of N at 600 • C as seen in Figure 2(b) is possibly due to the breaking of In-N bonds rather than Al-N bonds, since the bond strength of In-N bond (1.93 eV) is lower than that of Al-N bond (2.88 eV).…”

“…The Hf/Al/Ta metal layer thickness ratio is optimal, based on our earlier work reported elsewhere. 16 The contacts were given post deposition thermal annealing at different temperatures (from 500 to 700 • C) for 60 s in vacuum. The current-voltage (I-V) characterizations of Hf/Al/Ta ohmic contacts on In 0.18 Al 0.82 N/GaN heterostructure were carried out at room-temperature using a semiconductor parameter analyzer (Agilent B1500A) by the four-point method.…”

“…On the other hand, these contact inclusions can also lead to lower breakdown voltage for GaN based HEMTs. 14,15 In our recent study, 16 we have proposed a low thermal budget (≤600 • C) Hf/Al/Ta ohmic contact scheme on unintentionally doped In 0.18 Al 0.82 N/GaN grown on Si substrate, which has demonstrated low contact resistivity, smooth surface morphology and metalsemiconductor interface. In addition, the In 0.18 Al 0.82 N/GaN HEMTs fabricated with Hf/Al/Ta ohmic contacts have shown significantly improved breakdown voltage, compared to devices with traditional Ti/Al/Ni/Au ohmic contacts, owing to the source-carrier-injection mechanism suppressed by the smooth metal-semiconductor interface in the former.…”

Investigation of InAlN Layers Surface Reactivity after Thermal Annealings: A Complete XPS Study for HEMT

“…J. Palisaitis et al have done deep investigations by STEM (Scanning Transmission Electron Microscopy) characterization on InAlN thin layers during in situ thermal annealing. 41,42 Their work points out the fact that In-rich layers decompose at 750 • C, starting from the formation of metallic In clusters at grain boundaries, whereas Al-rich layers show few signs of decomposition and remain chemically stable even at 950 • C. Consequently, different approaches are considered by the community, such as operating at slower growth rate in MOVPE to increase thermal stability, 43 or processing ohmic contact at a lower annealing temperature around 600 • C. [44][45][46] Actually, the growth of a thermal oxide layer on III-V nitrides like InAlN is still hard to control and not fully understood.…”

Overcoming the poor crystal quality and DC characteristics of AlGaN/GaN/AlGaN double‐heterostructure high electron mobility transistors

“…As V DS keeps on increasing, the electric field by V DS becomes strong enough to attract electrons directly from the source electrode to the drain electrode through the buffer layer under the depletion region, then the I S becomes negative, and the inflection point appears, and I D = | I S | + | I G |. Finally, breakdown happens due to the source carrier injection . There are two factors resulting in the promoted BV of the DH.…”