Band bending and determination of band offsets in amorphous/crystalline silicon heterostructures from planar conductance measurements

Abstract

An analytical model for the calculation of the band bending in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions is presented and validated by comparison with full numerical simulations. The influence of the various structure properties and parameters, such as the density of states in bulk a-Si:H or at interface defects, the position of the Fermi level in a-Si:H, the temperature dependence of band gaps, is investigated. Significant band offsets imply the presence of a strong inverted layer at the c-Si surface of both (p)a-Si:H/(n)c-Si and (n)a-Si:H/(p)c-Si structures, forming two-dimensional hole and electron gases, respectively. This leads to high sheet carrier densities that have been evidenced from planar conductance measurements. Experimental data obtained on samples coming from various research institutes are analyzed with our model in order to extract the band offsets. We find that the valence band offset ranges between 0.32 and 0.42 eV with an average value at 0.36 eV; the conduction band offset is found between 0.08 and 0.26 eV with a mean value at 0.15 eV. These values are discussed in the frame of the branch point theory for band line-up; they imply that the branch point energy in a-Si:H is almost independent of doping and lies close to mid-gap.

Revisiting the theory and usage of junction capacitance: Application to high efficiency amorphous/crystalline silicon heterojunction solar cells

“…However in the low temperature range one has to take care of transport in a-Si:H and to the TCO/aSi:H interface that can also have an impact on the transition temperature above which the regime fully determined by strong inversion prevails. Note that the obtained range for the conduction band offset agrees with the values determined in our group from planar conductance measurements [22] and in other groups using other techniques [23]; to pay again tribute to Cohen, we can notice that he also published early results on the a-Si:H/c-Si band offsets from capacitance measurements, however not directly on solar cells and using another experimental method and procedure (namely from a voltage filling pulse method) and he also concluded that most of the band offset between a-Si:H and c-Si occurs at the valence band [27]. The depletion layer approximation had been first criticized by Gummel and Scharfetter from calculations on p þ n step homojunctions where these authors showed that the intercept voltage in highly dissymmetric junctions is not as expected from the simple depletion approximation [28].…”

“…(12), one then obtains that the valence band offset is almost zero. This is much less than values reported recently in the literature: about 0.4 eV obtained from planar conductance measurements performed on (p) a-Si:H/ (n) c-Si interfaces coming from various institutes [22] and similar values (or even larger for higher hydrogen content and thus larger band gap of a-Si:H) obtained from photoelectron spectroscopy [23]. In addition, we can note that the value of V int ¼ 0.56 V is much lower than the open circuit voltage measured under normalized AM1.5G conditions, V oc ¼ 725 mV.…”

Understanding inversion layers and band discontinuities in hydrogenated amorphous silicon/crystalline silicon heterojunctions from the temperature dependence of the capacitance

“…11 We then find a simple expression for the potential drop in c-Si as a function of the total potential drop V d…”

Analysis of lateral transport through the inversion layer in amorphous silicon/crystalline silicon heterojunction solar cells

“…14,15 Until now, the inversion layer has been considered only as a tool to characterize the band offsets at the a-Si:H/c-Si interface 16,17 and its influence on lateral transport in operating devices has not yet been analyzed. Lateral transport through an inversion layer is successfully leveraged in modulationdoped field-effect transistors 18 and in metal-insulatorconductor inversion layer solar cells.…”