Low-contact-resistance (Rc) non-gold Ta/Si/Ti/Al/Ni/Ta ohmic contacts were realized on an undoped AlGaN/GaN high-electron-mobility transistor (HEMT) grown on a silicon substrate. Optimization of the rapid thermal process reveals that Rc decreases drastically from the annealing temperature of 700 to 850 °C and slightly increases from 875 to 900 °C. The sample annealed at 850 °C exhibited the lowest Rc of 0.22±0.03 Ω·mm [specific contact resistivity, ρc=(0.78±0.22)×10-6 Ω·cm2] with a smooth surface morphology (RMS roughness ∼5.5 nm). The low Rc is due to the formation of TixSiy and the intermixing of TixSiy with the bottom Ta layer at the metal/semiconductor interface.
We have demonstrated 0.17-µm gate-length In 0.17 Al 0.83 N/GaN high-electron-mobility transistors (HEMTs) on Si (111) substrates using a non-gold metal stack (Ta/Si/Ti/Al/Ni/Ta) with a record-low ohmic contact resistance (R c ) of 0.36 Ω-mm. This contact resistance is comparable to the conventional gold-based (Ti/Al/Ni/Au) ohmic contact resistance (R c = 0.33 Ω-mm). A non-gold ohmic contact exhibited a smooth surface morphology with a root mean square surface roughness of ~2.1 nm (scan area of 5×5 µm 2 ). The HEMTs exhibited a maximum drain current density of 1110 mA/mm, a maximum extrinsic transconductance of 353 mS/mm, a unity current gain cutoff frequency of 48 GHz, and a maximum oscillation frequency of 66 GHz. These devices exhibited a very small (<8%) drain current collapse for the quiescent biases (V gs0 = -5 V, V ds0 = 10 V) with a pulse width/period of 200 ns/1 ms. These results demonstrate the feasibility of using a non-gold metal stack as a low R c ohmic contact for the realization of high-frequency operating InAlN/AlN/GaN HEMTs on Si substrates without using recess etching and regrowth processes.
This work investigates the conduction mechanism of non-gold Ta/Si/Ti/Al/Ni/Ta ohmic contact in un-doped AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on Si. The temperature dependent I-V measurement reveals that the conduction mechanism is primarily via Thermionic Emission(TE). The extracted mean barrier height(Φ B ) values are 0.113 and 0.121 eV and the mean contact resistance(R c ) values are 0.24 and 0.28 Ω•mm respectively for annealing temperature at 850 ºC and 900 ºC. The low R c is attributed to the formation of low work function Ti x Si y at the metal-semiconductor interface. The HR-TEM and EDX analysis also provide structural evidence to support the TE mechanism.
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