Picosecond time-resolved photoluminescence of zinc oxide single crystals, films and nanoparticles /

by 1976- Wilkinson, John Henry

Zinc oxide (ZnO) is a refractory semiconductor material whose band gap is both wide (3.39 eV) and direct. It is under consideration as a promising material for blue/uv semiconducting light emitting diodes (LEDs) and lasers. High purity macroscopic single crystals are available, along with epitaxial films grown by chemical vapor deposition, molecular beam epitaxy or pulsed-laser deposition. The exciton has a very fast spontaneous radiative lifetime, which makes it an interesting case for understanding what limits exciton oscillator strength. The 60 meV exciton binding energy is large relative to most other semiconductors, so exciton effects remain important at room temperature. In this thesis, results from time-dependent photoluminescence spectra of ZnO single crystals, films and powders from 16 to 296 K are reported. The main lifetime of the thermally shifted free exciton emission at room temperature is 440 ps. Temperature dependence shows this to be the radiative lifetime, and it decreases to 290 ps at 85 K. I measure the lifetime of the neutral donor bound exciton (D0X) in ZnO to be 50 ps. These are unusually fast spontaneous radiative lifetimes, and I have applied theories of exciton oscillator strength to understand the role of exciton coherence volume and look for possible ways to engineer very short lifetimes. I observed amplified spontaneous emission and so-called random media lasing in thin films and powder samples. The very short radiative lifetime for excitonic luminescence in this and other wide-gap semiconductors, and especially the “reverse quenching” temperature dependence of lifetime, can be explained in terms of the oscillator strength of the exciton, which scales with wavefunction size (coherence volume). The lifetime is then inversely ix proportional to the coherence volume of the exciton. This so-called giant oscillator strength has been described for CuCl and ZnO nanocrystals. Polariton transport was also studied in this dissertation. Energy transport at the D0X wavelength was found to occur at a velocity of 4.2 x 108 cm/s, and this was interpreted as polariton transport. 1