Enhancing The Hole Conductivity of an Undoped Silicon Channel by Defect Engineering of Surrounding SiO₂ Dielectric

The road to miniaturization in semiconductor microelectronic technology has led to an ongoing demand for identification of suitable approaches for device performance enhancements and raised requirements of new materials and device structures [1].<br/><br/>Here, we report the electronic device performance improvement opportunity of silicon nanostructures without direct channel doping by adding Al acceptor states into a thin SiO₂ layer surrounding the silicon [2]. This novel concept is analogous to modulation doping in III-V semiconductors. The manifestation of the dielectric by incorporation of trivalent Al impurities gives the possibility for electrons from silicon to tunnel to the acceptor states in SiO₂ yielding silicon with holes as majority carriers. Our method removes the disadvantages of impurity doping, namely a required thermal ionization, inelastic impurity scattering, problematic dopant activation vs. out-diffusion and statistical fluctuations of dopant density [3-4]. The SiO₂doping also facilitates interface passivation by discharging the dangling bond defects at Si/SiO₂ interface [5]. To this end, the doping approach is demonstrated on silicon channels fabricated on a silicon-on-insulator (SOI) substrate with nickel silicide contacts. The same layers sequence is extended to a transfer length measurement (TLM) structure.<br/><br/>It was experimentally observed that the energetic state of Al-acceptors in SiO₂ formed by atomic layer deposition of Al₂O₃ on thin thermally grown SiO₂ induces a corresponding shift in the Fermi level of silicon towards the valence band. A variation in the density of hole carriers in the silicon channel is regulated by the variation of number of ALD Al₂O₃ cycles [6] which governs both the silicon resistivity and the specific contact resistivity. The effect of addition of a monolayer of ALD Al₂O₃ is reflected in the reduction of silicon resistivity to a value as low as 0.04-0.06 Ω●cm and a contact resistivity as low as 3.5E-7 Ω●cm² [7].<br/><br/>A single segment of the fabricated TLM structure can also perform equivalently as a transistor with the body contact operated as a back gate. The performance of the transistor is improved by an enhanced hole carrier transport with an I_ON/I_OFFratio of up to 10⁶at a drain voltage of -1V. The NiSi source/drain contacts placed directly within the SOI region also undergoes a transition from Schottky to low-resistance contact as an effect of the barrier height reduction [8]. The drawbacks of conventional impurity doped devices such as dopant scattering, fluctuating device performance due to dopant diffusion, and even requirements of lightly or heavily doped source/drain regions can be circumvented by the new method. The aforesaid impurity free doping of the channel would be ideal for a high performance, high mobility device and enables new advancements encompassing areas of low-temperature physics and quantum technology.<br/><br/>[1] Wong. H, and Iwai. H, <i>Physics World</i> 18.9 (2005): 40.<br/>[2] König. D, et al., Scientific Reports 7.1 (2017): 46703.<br/>[3] Norris. D. J., et al., Science 319.5871, 1776 (2008).<br/>[4] Nagarajan. S, et al., <i>Solid-State Electronics</i> 208 (2023): 108739.<br/>[5] Hiller. D, et al., J. App. Phys. 125.1 (2019): 015301.<br/>[6] Ratschinski. I, et al., <i>physica status solidi (a)</i> (2023): 2300068.<br/>[7] Nagarajan. S, et al., Advanced Materials Interfaces, Oct 2023 (Accepted).<br/>[8] Nagarajan. S, et al., Device Research Conference. IEEE, 2023.