Reconfigurable Field-Effect Transistor Technology via Heterogeneous Integration of SiGe with Crystalline Al Contacts

Abstract

Si1-xGex is a key material in modern complementary metal-oxide-semiconductor and bipolar devices. Importantly SiGe and Ge are promising materials to enable higher drive currents, reduced power consumption and enhanced switching speeds. However, despite considerable efforts in metal-silicide and-germanide compound material systems, reliability concerns have so far hindered the implementation of metal-Si1-xGex junctions that are vital for diverse emerging "More than Moore" and quantum computing paradigms. In this respect, we report on the systematic structural and electronic properties of Al-Si1-xGex heterostructures, obtained from a thermally induced exchange between ultra-thin Si1-xGex nanosheets and Al layers. Remarkably, no intermetallic phases were found after the exchange process, alleviating process variability compared to Ni-silicide/Ni-germanide contacts. Instead, abrupt, flat and void-free junctions of high structural quality could be obtained. Interestingly, ultra-thin interfacial Si layers formed between the metal and Si1-xGex segments, explaining the morphologic stability. Integrated into omega-gated Schottky barrier transistors with the channel length being defined by the selective transformation of Si1-xGex into single-elementary Al leads, a detailed analysis of the transport was conducted.

Thereby, the vertical Si-Si0.67Ge0.33 heterostructure showed high potential for reconfigurable field-effect transistors (RFETs). This emerging device concept is capable of dynamically switching between p-and n-type operation during run-time, overcoming the static nature of conventional CMOS and reducing the transistor count and the circuit path delay. Further, RFETs enable an efficient implementation of dynamically reconfigurable logic gates, allowing e.g. switching between NAND to NOR functions, or intrinsic XOR functionality. The here proposed top-down fabricated SiGe-based reconfigurable transistor technology comprising a vertical Si-Si0.67Ge0.33 heterostructure enables high and symmetric on-currents of both n-and p-type operation, which has so far fallen short in Ge based RFETs due to interface instability to their contacts and gate oxides. The implementation of a three top-gate transistor in combination with a hysteresis-free SiO2/HfO2 gate insulator stack, enhances polarity control and leakage current suppression to limit static power dissipation. Importantly, the obtained Al-Si-SiGe multi-heterojunction and advanced reconfigurable transistor design is the first Ge based technology showing the envisioned stability and performance enhancements.