Metal/Semiconductor Barrier Properties of Non-Recessed Ti/Al/Ti and Ta/Al/Ta Ohmic Contacts on AlGaN/GaN Heterostructures

Tungsten carbide (WC) contacts have been investigated as a novel gold-free Schottky metallization for AlGaN/GaN heterostructures. The evolution of the electrical and structural/compositional properties of the WC/AlGaN contact has been monitored as a function of the annealing temperature in the range from 400 to 800°C. The Schottky barrier height (ΦB) at WC/AlGaN interface, extracted from the forward current-voltage characteristics of the diode, decreased from ∼0.8 eV in the asdeposited and 400°C annealed sample, to 0.56 eV after annealing at 800 °C. This large reduction of ΦB was accompanied by a corresponding increase of the reverse bias leakage current. Transmission electron microscopy coupled to electron energy loss spectroscopy analyses revealed the presence of oxygen (O) uniformly distributed in the WC layer, both in the as-deposited and 400°C annealed sample. Conversely, oxygen accumulation in a 2-3 nm thin W-O-C layer at the interface with AlGaN was observed after the annealing at 800 °C, as well as the formation of W2C grains within the film (confirmed by X-ray diffraction analyses). The formation of this interfacial W-O-C layer is plausibly the main origin of the decreased ΦB and the increased leakage current in the 800°C annealed Schottky diode, whereas the decreased O content inside the WC film can explain the reduced resistivity of the metal layer. The results provide an assessment of the processing conditions for the application of WC as Schottky contact for AlGaN/GaN heterostructures.

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Thermal annealing effect on electrical and structural properties of Tungsten Carbide Schottky contacts on AlGaN/GaN heterostructures

Greco

Franco

Bongiorno

et al. 2020

“…Valuable information concerning the annealed ohmic contact (a new scheme at 790 • C and conventional scheme at 820 • C) is extracted using x-ray diffraction (XRD) analysis. Extra conducting metal phases between inter-metals have been observed with the Si/Au-based metal scheme including TiN, TiAl, AlAu 2 and TiAl in comparison to the conventional metal Ti/Al-based ohmic contact metal scheme [1,6,15]. The conventional ohmic contact in GaN or a GaNbased system is based on the formation of titanium nitride.…”

Section: Ohmic Contact Surface and Xrd Characterizationsmentioning

confidence: 99%

Ohmic contact morphology improvement with reduced resistance using Si/Au/Ti/Al/Ni/Au (AlGaN) and Si/Au/Ti/Al/Ni/Au (InAlN) stack layers in III-Nitride HEMTs

Visvkarma

Laishram

Kapoor

et al. 2022

This article reports Ti/Al-based ohmic contact utilizing a thin interfacial Au layer for improved morphology, edge acuity, and low contact resistance for applications to III-Nitride HEMTs. Conventional Ti/Al contacts are based on a metal stack of the form Ti/Al/X/Au, where X is any suitable barrier layer such as Ni. The formation of ohmic contact to GaN and its alloy systems is governed by the formation of TiN during annealing; Au inter-diffusion during annealing also assists the formation of TiN islands and aids in reducing the contact resistance. Further, inter-metallic phase formation between Ti and Al lowers the contact resistance. It is observed that the contact surface morphology strongly depends on the top Au layer thickness. Insertion of a very thin Au layer (2-3 nm) has drastically improved the surface quality with the achievement of contact resistance as low as 0.36Ω-mm and specific contact resistance 2.4 × 10-6 Ω-cm2 on AlGaN/GaN HEMT. The reduction in contact resistance is due to the formation of more conducting inter-metallic phases during annealing which is assisted due to the thin Au layer insertion. Further reduction in contact resistance to 0.13Ω-mm was also achieved with the introduction of Si as a dopant layer in the reported metal scheme. The measured rms surface roughness was reduced to ~8 nm from 0.12 µm in comparison to the conventional ohmic contact.

“…For these reasons, Au-free and low temperature ohmic contacts have been investigated by several groups [12][13][14][15][16][17][18]. Few reports on Au-free Ta/Al-based metal stacks typically fabricated by evaporation describe low-resistive ohmic contacts even for annealing temperatures lower than 650 • C [16,17]. Recently, the latter approach was successfully combined with a recess technique of the AlGaN barrier to obtain low-resistive ohmic contacts [19].…”

Section: Introductionmentioning

confidence: 99%

Material investigations for improving stability of Au free Ta/Al-based ohmic contacts annealed at low temperature for AlGaN/GaN heterostructures

Calzolaro

Hentschel

Edokam

et al. 2020

Gold-free Ta/Al-based ohmic contacts fabricated by sputtering on AlGaN/GaN heterostructures and annealed at low temperature were investigated. The presence of a thin AlN spacer layer at the AlGaN/GaN heterojunction is demonstrated to prevent the ohmic contact formation as shown by rectifying behavior after annealing. Ta as an additional capping layer on Al leads to a severe morphology degradation and subsequent deterioration of the metal stack after annealing at 600 • C due to strong Ta-Al alloying verified by transmission electron microscopy. Using the compound metal TiN as capping layer circumvents the alloy formation, thereby making the contacts much more stable under annealing. A low contact resistance of 0.8 Ω mm was obtained using Ta/Al/TiN metal layers annealed at only 500 • C. The nature of the current transport was investigated by analyzing the temperature dependence of the specific contact resistance, which points towards a current path through the AlGaN barrier by thermionic field emission.