Time-resolved photoluminescence spectroscopy at 2 K was used to measure the radiative recombination lifetime of the allowed (Γ5) and forbidden (Γ6) free excitons in ZnO. The measurements were made on a sample containing internal strain, which altered the sample symmetry, and resulted in relaxed selection rules, allowing the Γ6 exciton to be observed. A radiative recombination lifetime of 259 ps was measured for the Γ5 exciton and 245 ps for the Γ6 exciton. The decay of the free excitons was of single-exponential form, and the decay times were obtained using a least-squares fit of the data.
The crystal structure of ZnO is wurtzite and the stacking sequence of atomic layers along the ''c'' axis is not symmetric. As a result, a ZnO crystal surface that is normal to the c axis exposes one of two distinct polar faces, with ͑0001͒ being considered the O face and ͑0001͒ the Zn face. Photoluminescence ͑PL͒ measurements on the two faces reveal a striking difference. Two transitions are observed in PL that are dominant from the O face and barely observed in PL from the Zn face. These lines are identified as phonon replicas of a particular D 0 ,X transition using energy separations, excitation dependence, and time-resolved PL measurements. In addition, PL emission from free excitons is found to be more intense from the O face than from the Zn face.
High quality GaN crystals can be grown on sapphire by hydride vapor phase epitaxy. The thermal expansion mismatch between sapphire and GaN produces strain in the GaN crystal as it is cooled from the growth temperature to room temperature. The strain is evidenced by shifts in the photoluminescence and reflectance line positions. By analyzing the surface strain as the crystal thickness is increased, the thickness required to obtain zero surface strain can be estimated. This structure might provide a lattice matched and thermally matched substrate for further epitaxial growth of GaN.
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