Departmental Seminar

The Nanoscopic Electronic Structure Shift Induced by Anions at Surfaces (NESSIAS) effect: Theory, Experimental Evidence, Materials, Applications

Dr Dirk König
Integrated Material Design Centre (IMDC), UNSW, Sydney
Integrated Materials Design Lab (IMDL), ANU, Canberra

The NESSIAS effect induces a considerable energy offset for valence band (VB) and conduction band (CB) edges in low-nanoscale (lns-) Si as a function of SiO2- vs. Si3N4-embedding, resulting in an electronic band structure in analogy to a p/n junction. As a true alternative to impurity doping for Si-based ultra-large scale integration (ULSI) [3-5], NESSIAS provides an excellent platform for cryo-electronics, ultra-low power ULSI, high gain opto-electronic nanodevices, and Si-fin/-nanowire (NWire) field effect transistors (FETs) with physical gate lengths down to ca. 3 nm.

I will elucidate the nature of the NESSIAS by delimiting it against other nanoscopic interface effects, followed by a close look on the localization of the electronic charge Dq transferred from lns-Si into SiO2 vs. Si3N4 as a function of the respective anion (O, N). The quantum chemical parameters of O and N relative to lns-Si as the charge source (extrinsic cations) are crucial in establishing a strong energy offset between VBs and CBs of lns-Si embedded in SiO2 vs. Si3N4 [1-5]. I introduce the analytical parameter L which is proportional to the energy of the highest occupied molecular orbital (HOMO); L ~ EHOMO. This parameter enables us not only to quantitatively predict the electronic structure of lns-Si as a function of anions of the 1st and 2nd row of the periodic table which compose embedding dielectrics, but also predicts the NESSIAS to work at lns-C (diamond) and -Ge. Evidence is provided by density functional theory (DFT) calculations, using several DFs and MO basis sets as test cases, whereby Dq(lns-Si) is the only DFT-derived parameter applied to L as per combination of lns-Si and terminating anion group. Further DFT calculations on Si-NWire FETs embedded in OH/NH2/OH show that excellent NWire-FETs can be accomplished when exploiting the NESSIAS effect with CMOS-compatible materials. NWires embedded in F/NH2/F show the clear application of band-to-band tunnelling for full ballistic transport in BTBT-NWire-FETs. Introducing a positive/negative charge density of 1.3 ´ 1020 cm-3 into the NWire approximants shows that their electronic structure as set by NESSIAS is very robust and allows for full-scale device operation.

Experimental data were obtained from Si-nanowells (NWells) embedded in SiO2 vs. Si3N4 using synchrotron UV photoelectron spectroscopy (UPS) and synchrotron X-ray absorption spectroscopy in total-fluorescence yield modus (XAS-TFY). These data confirm the considerable energy offset of VB and CB edges of NWells when embedded in SiO2 vs. Si3N4. The optimum NWell thickness for a maximum NESSIAS can be readily transferred to NWires with technologically relevant thickness by a new analytic crystallographic description.

Dr Dirk König is Deputy Director (research), Integrated Material Design Centre (IMDC) at the University of New South Wales, Sydney.  He is the Principal Investigator / Partner Investigator / Key Staff Member in successful project applications, total acquired funding of AU-$ 13.2 Million and the author of Two book chapters, 67 journal papers (HSI = 18, total citations  » 2.63 k), five national patents (DE, AU), five worldwide patents (WO), 89 conference contributions, 19 international research seminars.


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