Room-temperature optical absorption data in the 1.5–2.5-eV range are reported for indium nitride thin films prepared by reactive radio-frequency sputtering. The fundamental absorption edge in high-purity material is located at 1.89±0.01 eV and corresponds to a direct transition at k=0, in agreement with band-structure calculations. A significant Moss-Burstein shift is noted for carrier concentrations in excess of 1019 cm−3 and obeys the empirical relationship EG =1.89+2.1×10−8 n1/3 eV.

Electron mobilities in GaN and InN are calculated, by variational principle, as a function of temperature for carrier concentrations of 1016, 1017, and 1018 cm−3 with compensation ratio as a parameter. Both GaN and InN have maximum mobilities between 100 and 200 K, depending on the electron density and compensation ratio, with lower electron density peaking at lower temperature. This is due to the interplay of piezoelectric acoustic phonon scattering at low carrier concentrations and ionized impurity scattering at higher carrier concentrations. Above 200 K, polar mode optical phonon scattering is the mobility limiting process. The 300 and 77 K electron and Hall mobilities as functions of carrier concentration in the range of 1016–1020 cm−3 and compensation ratio are also calculated. The theoretical maximum mobilities in GaN and InN at 300 K are about 1000 and 4400 cm2 V−1 s−1, respectively, while at 77 K the limits are beyond 6000 and 30 000 cm2 V−1 s−1, respectively. We compare the results with experimental data and find reasonable correlation, but with evidence that structural imperfection and heavy compensation play important roles in the material presently available. Only phonon limited scattering processes are considered in the calculation of the mobility in AlN since it is an insulator of extremely low carrier concentration. We find a phonon limited electron mobility of about 300 cm2 V−1 s−1 at 300 K.

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The electronic transport mechanism in polypyrrole is discussed in terms of Mott variable range hopping (VRH) in samples with a wide range of conductivities and which have been formed using different doping techniques. Samples were synthesized in both aqueous and organic media and samples were either doped during polymerization or dedoped afterwards giving a three order of magnitude range of conductivities at 300 K and a range of sample morphologies. No difference in behavior is observed for materials with different morphologies, suggesting that transport predominantly involves monomer units and occurs independent of structure. The various transport parameters obtained appear reasonable with the exception of the apparent ‘‘hopping attempt frequency,’’ related to the phonon frequency in VRH, some values of which are anomalously high. The density of states at the Fermi level was found to be between 5×1018 and 1×1022 eV−1 cm−3 for a range of samples and the mean hopping distances ranged between 2 and 34 monomer units. The minimum hopping distance of 2 monomer units is consistent with electron delocalization on individual monomer units. An upper limit of conductivity in polypyrrole of no more than 400 S cm−1 is suggested in the limit of the VRH regime in which hopping occurs between adjacent monomer units.