Using planar conductance measurements, we have recently studied (p)a-Si:H/(i) a-Si:H/(n) c-Si heterostructures where the thickness of the (i)a-Si:H buffer layer (t_(i)aSi:H) varies between 2 and 50 nm, well beyond the values used in heterojunction solar cells [1]. By comparing experimental results obtained at room temperature with 1D analytical calculations and 2D numerical modelling, we were able to demonstrate that: 1) the planar conductance of the heterostructure is linked to the conductance of the hole inversion channel present at the heterointerface and 2) the density of deep defects linked to dangling bonds in the (i)a-Si:H layer increases strongly from 1×10¹⁷ to 4×10¹⁸ cm^-3 when t_(i)aSi:H is reduced from 50 to 2 nm [1]. This result was interpreted in terms of defect formation and the dependence of defect density on the position of the Fermi level relative to the valence band edge [1]. 

We are extending the measurements and analyses to the temperature dependence of the planar conductance over a wide range [80K-450K]. These confirm the trend observed for measurements carried out at room temperature. The limitation of the 1D calculation is discussed with the help of 2D modelling. We then highlight non-negligible contributions from additional conductances linked to (i) thermionic transport at the heterointerface, (ii) the p-type a-Si:H layer and (iii) electrons from the c-Si bulk. These make a significant contribution to the total conductance of the structure when the inversion layer becomes weaker. Furthermore, the experimental evolution of the planar conductance as a function of temperature for the different heterostructures is fairly well reproduced in 2D simulation over a more restricted temperature range [250K-400K] for structures where t_(i)aSi:H is greater than 5 nm. Outside this temperature range and for heterostructures with lower t_(i)aSi:H values, several hypotheses will be modelled and tested in order to better interpret the experimental results.

[1] A. Levtchenko, S. Le Gall, R. Bruggemann, J.-P. Kleider, Phys. Status Solidi RRL 2019, 1900411