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Polarization of an exciton in a ZnO layer using a split gate potential

2003; American Physical Society; Volume: 68; Issue: 15 Linguagem: Inglês

10.1103/physrevb.68.155334

1095-3795

P. A. Sundqvist, Q. X. Zhao, M. Willander,

Spectroscopy and Quantum Chemical Studies

In this paper we focus on structural and optical transitions of an exciton in a Zinc Oxide (ZnO) layer, which could be widely controlled by a split gate potential. We have solved the exciton problem by a self-consistent Schr\"odinger-Poisson technique, where the Hamiltonian includes the boundary conditions for the split gate structure. The gate voltage creates a paraboliclike potential, which at a typical threshold voltage separates or polarizes the exciton strongly. This sharp structural transition brings the exciton from being strongly correlated with a large overlap to a regime where the correlation is very small (with small overlap). The resulting structure for the exciton at negative gate voltages is a structure where the hole is located like a ring around a dotlike electron. For positive values of the gate voltage the situation is opposite. We have especially studied the ground-state binding energy and the optical transitions of the exciton. We found that the ground-state energy for ZnO could be tuned and the decrease of the ground-state energy can be as large as the double of the bulk exciton energy (60 meV for ZnO) with a gate voltage of $\ensuremath{-}5\mathrm{V}.$ The ground-state energy is almost constant for small values of the gate voltage but at a typical threshold voltage (approximately $\ensuremath{-}2\mathrm{V})$ the energy suddenly changes and becomes linear with the gate voltage. We also analyze the lifetime for the exciton, which is shown to increase from nanoseconds to beyond milliseconds. This was shown to be an effect of the small overlap between the hole and the electron when the gate voltage increased above the threshold voltage. Stimulated by the long lifetime of the ground state of the exciton we also calculated the optical transition frequency and the corresponding oscillator strength for the transition between the ground state and the dominating excited (self-consistent) exciton states. The transition frequency was found to occur in the THz region and the oscillator strength in the range of 0.3--0.4 for gate voltages between $\ensuremath{-}2\mathrm{V}$ and $\ensuremath{-}5\mathrm{V}.$ In addition, we have also analytically described polarization and especially total charge densities for excitons in small linear electric fields.