Free carrier effects in gallium nitride epilayers: the valence band dispersion

The dispersion of the A-valence-band in GaN has been deduced from the observation of high-index magneto-excitonic states in polarised interband magneto-reflectivity and is found to be strongly non-parabolic with a mass in the range 1.2-1.8 m_{e}. It matches the theory of Kim et al. [Phys. Rev. B 56, 7363 (1997)] extremely well, which also gives a strong k-dependent A-valence-band mass. A strong phonon coupling leads to quenching of the observed transitions at an LO-phonon energy above the band gap and a strong non-parabolicity. The valence band was deduced from subtracting from the reduced dispersion the electron contribution with a model that includes a full treatment of the electron-phonon interaction.Comment: Revtex, 4 pages, 5 figure

Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown remarkable progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance due to spontaneous free carrier generation following light absorption. Additionally, we determine the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties

Magnetophotoluminescence of GaN/Al_xGa_1-xN quantum wells: valence band reordering and excitonic binding energies

A reordered valence band in GaN/AlxGa1-xN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g factor (g*similar to3) in magnetoluminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlxGa1-xN barriers thus giving different barker heights for the different quantum well hole states. From k-p calculations in the quasicubic approximation, a change in the valence band ordering will account for the observed values for the g factors. We have also observed the well-width dependence of the in-plane extent of the excitonic wave function from which we infer an increase in the exciton binding energy with the reduction of the well width in general agreement with theoretical calculations of Bigenwald et al. [Phys. Status Sokidi B 216, 371 (1999)] that use a variational approach in the envelope function formalism that includes the effect of the electric field in the wells