ta‐C/Si heterojunction diodes with apparently giant ideality factors

Abstract

A common feature of many wide band gap heterojunction diodes is an unexplained large ideality factor n > 2. In this context we investigate the diode characteristics of heterojunction diodes consisting of a crystalline semiconductor material such as Si covered with a thin semiconducting film of amorphous or disordered material. As thin amorphous film we use tetrahedral amorphous carbon (ta‐C). These heterojunctions exhibit a pronounced rectifying behavior, low saturation current and low parasitic currents. Moreover, we observe an apparently giant ideality factor reaching values of n > 75. As a consequence, the turn on voltage is around 3 – 10 V and the I‐V curves can be measured for bias up to 40 V without reaching saturation or electrical breakdown. We present a quantitative model for the unusual diode characteristics of these Metal – Amorphous Semiconductor – Semiconductor diodes (MASS‐diodes). We demonstrate that the I‐V characteristics of the heterojunctions are well described by a serial arrangement of an ideal Schottky‐diode, a Frenkel‐Poole type resistance and an Ohmic contact resistance, emulating a p‐n‐ or Schottky diode characteristic with giant ideality factor and referred to as the FPID‐model. We propose that heterojunctions exhibiting apparently large ideality factors n ≫ 2 may possess an interfacial disordered or amorphous layer with Frenkel‐Poole conduction properties. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An Explicit Multiexponential Model as an Alternative to Traditional Solar Cell Models With Series and Shunt Resistances

“…It is worth pointing out that the very large values of n 1 reported for this particular solar cell [34] and indicated in Table II, which apparently are incompatible with classical diode theory [38], might be a nonphysical artifact. On the other hand, a high illumination-enhanced interface state density, due to buffer-absorber heterojunction lattice mismatch, has been alleged as a possible justification for such large ideality factor values [39], [40].…”

A novel approach of immittance-spectra analysis and how it resolves a decade-old deviation of the Frenkel-Poole model

“…The resistive part is described by the Frenkel-Poole model. This current-voltage (and -temperature) behaviour of ta-C has been identi ed in numerous publications (e. g. [38], [168,91] 8 , [90], [53], [132]), in general, and for samples synthesised in the same set-up by Ronning et al [156], Hofsäss [68] and Brötzmann et al [15], in particular. Especially for the dynamic permittivity, in the barrier-lowering coe cient, the systematic deviation from literature values, obtained through optical measurements, is a well known problem for the Frenkel-Poole model in general.…”

“…While for the depletion layer in silicon both, capacitive and resistive, properties are dependent on voltage, the bulk-capacitance of the amorphous thin lm material, with solely one atom species consequently connected by non-polar bonds, is not expected to be dependent on it. The resistance, on the other hand, is known to exhibit Frenkel-Poole conduction (e. g. confer to the voltage and often also temperature-dependent currentvoltage analysis of [38], [168,91] 6 , [90], [53], [132] and in particular Ronning et al [156], Hofsäss [68] and Brötzmann et al [15] who used the identical set-up for the synthesis of the ta-C lm) which is again a process dependent on voltage. For this speci c model a quantitative discrepancy between the experimentally obtained and theoretically predicted barrier-lowering coe cient exists.…”

“…To sum it up, deviations between the Schottky-barrier heights extracted either from capacitance-voltage analysis and current-voltage measurements are not unusual. In fact, the frequency-dependence of shift and stretch-out of the at-band voltage (corresponding to the built-in potential derived also from the Schottky-barrier height) was thoroughly investigated for decades to understand the consequences of interface traps or charge defects (mobile or immobile) in the insulator and to extract the corresponding concentrations (for this quite extensive eld it is conferred to its 'standard reference' by Nicollian and Brews [138] 15 in which many examples of how this apparent shift in the Schottky-barrier height is converted into defect densities are given and newer works of Schroder [162,161] which use a pedagogically more pleasant description and add some more recent insights). The di ferent barrier heights may today only be neglected because the knowledge on metallisation of silicon (which includes 'soft' processes that do not create disorder at the interface, annealing and passivation) has advanced to such a point that above described interface defects may be ignored [162, p. 90].…”

“…The analytical models that were applied in this work and describe the depletion layer, are still regularly applied to analyse experimental data as well as extract certain key parameters and, further, include more e fects as some models most often applied to analyse the characteristics of metal/semiconductor interfaces. For example, many publications do not include the e fect of Schottky-barrier lowering, though, precisely accounting for that phenomenon solved the discrepancy between the capacitive and resistive part and, furthermore, lead to a reasonably good description of the measured reverse bias current-voltage behaviour (other authors compensated for this discrepancy by including parallel 'leakage' resistors [15] which can due to its missing voltage dependence, only be a crude approximation). On the other hand, it is still worth mentioning that to arrive at those analytical descriptions of the diode certain approximations and restrictions are 58 2.4 The benefit of combining resistive and capacitive models unavoidable.…”