The superior carrier mobility of SiGe alloys make them a highly desirable channel material in complementary metal-oxide-semiconductor (CMOS) transistors. Passivation of the SiGe surface and the associated minimization of interface defects between SiGe channels and high- k dielectrics continues to be a challenge for fabrication of high-performance SiGe CMOS. A primary source of interface defects is interfacial GeO . This interfacial oxide can be decomposed using an oxygen-scavenging reactive gate metal, which nearly eliminates the interfacial oxides, thereby decreasing the amount of GeO at the interface; the remaining ultrathin interlayer is consistent with a SiO -rich interface. Density functional theory simulations demonstrate that a sub-0.5 nm thick SiO-rich surface layer can produce an electrically passivated HfO/SiGe interface. To form this SiO -rich interlayer, metal gate stack designs including Al/HfO/SiGe and Pd/Ti/TiN/nanolaminate (NL)/SiGe (NL: HfO-AlO) were investigated. As compared to the control Ni-gated devices, those with Al/HfO/SiGe gate stacks demonstrated more than an order of magnitude reduction in interface defect density with a sub-0.5 nm SiO -rich interfacial layer. To further increase the oxide capacitance, the devices were fabricated with a Ti oxygen scavenging layer separated from the HfO by a conductive TiN diffusion barrier (remote scavenging). The Pd/Ti/TiN/NL/SiGe structures exhibited significant capacitance enhancement along with a reduction in interface defect density.

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Combinations of sub 1 μm absorber films with high-work-function back surface contact layers are expected to induce large enough internal fields to overcome adverse effects of bulk defects on thin-film photovoltaic performance, particularly in earth-abundant kesterites. However, there are numerous experimental challenges involving back surface engineering, which includes exfoliation, thinning, and contact layer optimization. In the present study, a unique combination of nanocharacterization tools, including nano-Auger, Kelvin probe force microscopy (KPFM), and cryogenic focused ion beam measurements, are employed to gauge the possibility of surface potential modification in the absorber back surface via direct deposition of high-work-function metal oxides on exfoliated surfaces. Nano-Auger measurements showed large compositional nonuniformities on the exfoliated surfaces, which can be minimized by a brief bromine-methanol etching step. Cross-sectional nano-Auger and KPFM measurements on Au/MoO/CuZnSn(S,Se) (CZTSSe) showed an upward band bending as large as 400 meV within the CZTSSe layer, consistent with the high work function of MoO, despite Au incorporation into the oxide layer. Density functional theory simulations of the atomic structure for bulk amorphous MoO demonstrated the presence of large voids within MoO enabling Au in-diffusion. With a less diffusive metal electrode such as Pt or Pd, upward band bending beyond this level is expected to be achieved.

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