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.

A relationship between conformation and photophysics of poly[2-methoxy-5-(2‘-ethylhexoxy)-p-phenylenevinylene] (MEH−PPV) in dilute solution was investigated by utilizing UV/vis absorption, excitation, and emission spectroscopy. By tuning polymer−solvent interactions, a control of conjugation length which relates to state of chain collapse is achieved. Position of absorption and emission spectra can be systematically moved within 60 nm by using a series of alcohols and aromatic solvents as well as mixed solvents. In addition to a decrease of conjugation length, the collapsed chain exhibit optical characteristics different from that of the extended counterpart. While a single type of emitter is present in the extended chain, separate emission from multiple emitters with various conjugation lengths is detected from the collapsed coils. Fluorescence decay measurements support the presence of multiple emitters. Studies of UV/vis absorption and photoluminescence upon increasing temperature detect a blue shift of transition energies and a decrease of absorption and emission efficiency. In addition, rate of the shift is found to decrease with increasing magnitude of chain collapse.

No abstract

No abstract