Phase analysis and thermal stability of thin films synthesized via solid state reaction of Ni with Si1 − xGex substrate

• The reaction of ultrathin Ni film with Si1 − xGex layer (x: 25 and 55 at.%) is studied.
• Depending of Ge%, NiSi or NiGe is obtained but with different formation temperatures.
• The morphology, roughness and resistivity were found to be the same for NiSi and NiGe.
• NiGe on Si45Ge55 has limited thermal stability compared to NiSi on Si75Ge25.

The products of the solid state reaction involving ultra-thin Ni film (6 nm) and Si1 − xGex layers (Ge 25 and 55 at.%), were analysed using sheet resistance (Rs), glancing angle X-ray diffraction (GIXRD), scanning electron and atomic force microscopy (SEM, AFM) techniques. The reaction was carried out via rapid thermal process (RTP) annealing using two different steps (RTP1 and RTP2) while applying a selective etch (SE) in between them. The intermediate and the end reaction products resulting after RTP1 and RTP2 were found to be dependent on the Ge content, forming Ni-rich silicide (Ni2Si) and NiSi on Si75Ge25, while Ni-rich germanide (Ni5Ge3) and NiGe were obtained by using Si45Ge55. Though the onset of intermediate Ni-rich silicide or germanide phase formation occurs at similar RTP1 temperature (275 °C), the reaction completion to yield low resistive phase NiSi or NiGe phase results at different RTP2 temperatures (400 °C vs 350 °C). Based on the volume expansion, a resistivity value of 25 μΩ cm was obtained for the synthesized NiGe (12 nm) and NiSi (14 nm) layers. Independent of the phases obtained, the films were found to be closed and homogeneous and exhibit rms roughness of 0.5–0.8 nm as evidenced by SEM and AFM analysis. Thermal stability studies carried out on NiSi and NiGe thin films, post RTP1/RTP2, show the latter phase to have limited stability and result in Rs degradation starting already at 475 °C due to phase decomposition.

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