Paper EM-ThP7
SiGe_x (100) (x=0.25, 0.5, 0.75) and Ge (100) MOSCaps with Aqueous Ammonium Sulfide Passivation

Thursday, November 10, 2016, 6:00 pm, Room Hall D

SiGe_x is a potential semiconductor for next generation transistors because it could be incorporated into current, silicon-based semiconductor manufacturing processes and it would improve transistor performance due to the high carrier mobility of Ge. Despite these advantages, one major challenge is to reduce the number of Ge defects at the SiGe/dielectric interface because they degrade electrical performance of the transistor. While a relatively stable SiO₂ layer can be grown on Si with relatively few defects, the same is not true for Ge. One approach to forming a high quality interface is to remove Ge oxides by passivating the Ge atoms on the surface. SiGe and Ge metal oxide semiconductor capacitors (MOSCaps) were fabricated (10 nm Al₂O₃) and tested to evaluate the effect of sulfur-based chemical passivation on electrical performance. The (100) faces of three SiGe_x substrates – x = 0.25, 0.5, 0.75 – and one Ge substrate were cleaned and treated with one of two ammonium sulfide, (NH₄)₂S, wet chemistries: (NH₄)₂S/H₂O (1:100 v/v) or (NH₄)₂S/HCl/HF/H₂O (1:0.15:0.15:100 v/v). The surfaces of control MOSCaps were cleaned only. The contact area on each device was 12.6 µm².

Average capacitance in accumulation for SiGe_x MOSCaps (x = 0.25, 0.5) was 199 ± 5.4 and 205 ± 22 pF, with a DC bias sweep from -2 to +2 V at 1 MHz. The same capacitance was 42 ± 1.3 pF for SiGe_x (x = 0.75) and 457 ± 12 pF for the Ge MOSCaps. Both (NH₄)₂S treatments increased the accumulation capacitance by 6% and 34%, on average, for SiGe_x (x = 0.25, 0.5), 8% for SiGe_x (x = 0.75), and 8% for Ge MOSCaps. Similarly, V_FB shifted -2 V (SiGe_x (x = 0.25, 0.5)) and -.14 V (SiGe_x (x = 0.75) and Ge) with respect to the controls. While flatband shifting is at least due to reduction of oxide defects in the Al₂O₃ layer, the low magnitude of accumulation capacitance (according to oxide thickness calculations) suggests that there are other oxide layers present.

Half of the SiGe_x MOSCaps (x = 0.25, 0.75) were annealed in forming gas. V_FB shifted +3 V for SiGe_x MOSCaps (x = 0.25) with respect to non-annealed results, which is indicative of a reduction in negatively charged bulk oxide defects . SiGe_x (x = 0.75), and Ge MOSCaps possibly had less bulk oxide defects because their V_FB were within ± 0.5 V before and after annealing. Less bulk oxide defects in these MOSCaps suggest that nucleation and growth of the Al₂O₃ layer on these surfaces may differ from that of the SiGe_x (x = 0.25) surface. Among all three SiGe_x (x = 0.25) MOSCaps, the one treated with (NH₄)₂S and acid and annealed resulted in the flatband voltage closest to 0 V, as well as the lowest capacitance.