Charge transport at the poly(pyrrole)|indium phosphide interface: Influence of barrier heterogeneities

The doped conjugated polymer hybrid poly(pyrrole) phosphomolybdate (PMH) contacted to the inorganic semiconductor indium phosphide (InP) serves as a useful example to contrast charge transport in organic electronic devices with more conventional inorganic devices. This interface shares similarities to metal-semiconductor Schottky contacts, with non-trivial differences due to the unique physical and electronic properties of conjugated polymers. We establish, under careful consideration of existing transport theory, quantitative parameters and methods of rigorous extraction of these parameters that can be used to characterize charge transport across the interface; the two most important parameters being the barrier height and the transmission coefficient. Upon analysis of devices made with low-doped (∼1015--10 16cm-3) n- and p-type InP, it is shown that the transport follows closely with classic thermionic emission near room temperature, with barrier heights of 0.78 eV and 0.62 eV on n-type and p-type InP respectively, and transmission coefficients of 0.004 and 0.2 respectively. In contrast, common metal contacts to n-type InP have typically been 0.6 eV or lower, and the transmission coefficient is taken to be 1.; As the temperature of the device is lowered, the current transport deviates away from thermionic emission. The best fit model for this behavior is a laterally inhomogeneous barrier, in which small voltage-dependent low-barrier regions dominate the current at low temperature. Theory predicts that the bias dependence of the low barrier is a result of interaction with higher barrier background.; The relatively high barrier of the PMH | n-InP contact and our ability to model and extract the appropriate charge-transport parameters makes this system ideal to test the theory behind inhomogeneous barriers and to examine transport as the contacts are scaled down to the nanoscale regime. To accommodate this study, we use e-beam lithography to engineer low-barrier gold or chromium contacts in parallel with the PMH contact to InP. Results show that there are inconsistencies in modeling the current based on known parameters, and that there are anomalous barrier height measurements that are well below what is predicted by a small fractional coverage of the engineered metal contacts.