The optical properties of a high quality bulk ZnO, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at 3.3605 eV having a linewidth of 0.7 meV and dominating the PL spectrum at 10 K. This bound exciton transition was visible only below 150 K, whereas the A-free exciton transition at 3.3771 eV persisted up to room temperature. A-free exciton binding energy of 60 meV is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of ϳ170 and ϳ864 ps for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

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Abstract

Single crystal zinc oxide is a wide band gap semiconductor with great potential for a variety of commercial applications including substrates, UV photodetectors, acoustic wave devices, light emitting diodes, laser diodes, and high frequency electronic devices. ZnO is unique in that it has a very high exciton binding energy (60 meV) enabling stability at higher device operating temperatures, and it is highly resistant to radiation damage compared even to GaN. Bulk growth of ZnO single crystals is being conducted using the following primary methods: hydrothermal solution growth, seeded sublimation growth, and pressurized melt growth. Cermet, Inc. has employed the pressurized melt growth approach with much success. ZnO dissociates upon heating into a defective ZnO 1−x structure, which is addressed by providing an overpressure of oxygen in the growth environment. Single crystals nucleate and grow from the stoichiometric ZnO melt, which is contained in a thin layer of cooled, polycrystalline ZnO, eliminating crucible-introduced impurities. From these large ingots, high quality (∼10 4 defects cm −2 , linewidths as low as 49 arcsec), high purity ZnO crystals have been crystallized, oriented, and shaped into round or square boules and eventually processed into epitaxial-ready substrates. The pressurized melt growth approach is highly scalable and can accommodate high growth rates (up to 1 cm h −1 ), which are two criteria that are appealing to industrial productioin of high quality substrates.

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