Chapter 7 Conclusion and Prospective
7.2 Prospective
In principle, MgxZn1-xO (MZO) could be engineered to achieve any bandgap in the range of 3.37 to 7.7 eV. However, the solubility of MgO in ZnO is limited in 0 < x
< 0.33 that restricts the band gap to the range of 3.37 eV < Eg < 3.99 eV [1]. The
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nonpolar ZnO QW based on ZnO/MZO structures have been reported without QCSE in literatures by MBE [2, 3] and laser-MBE [4, 5]. Photonic devices with nonpolar ZnO, such as a-plane (1120) and m-plane (1100), have been proposed to improve the quantum efficiency suffered by the quantum-confinement Stark effect (QCSE). The nonpolar QWs structures not only reduce the QCSE effect but also increase the polarized emission rate. We will grow the more nonpolar a-plane (1120) and m-plane (1100) ZnO QWs on r- and m-plane sapphires with various well widths and
Mg contents. In addition, the surface roughness between the wells and barriers is also important to the QWs emission. The sharper the interface has the more efficient emission that also benefits for combining with the optical cavities. Many interesting physical properties in nonpolar QWs still need to investigate that are about excitons binding energy, exciton and phonon coupling interaction, phonon coupling with barriers, localized state, lifetime study, carriers dynamic and design barrier band engineered, etc.
If high density of excitons is excited in semiconductor materials such that average separation of excitons becomes shorter than the de Broglie wavelength of the exciton, then the excitons as quasi-Bosons would all condense to the lowest energy state. It is called the exciton Bose-Einstein condensates (BECs) [6]. The excitons BEC could generate the coherent emission as a laser does with high internal and extraction efficiencies having extreme low energy consumption. It is named the exciton polariton laser that has been recently realized in CdTe single quantum well within a microcavity at 19K under optical excitation 50 times below the lasing threshold [6].
A lot of polariton BEC effects have been observed in the semiconductor microcavity (MC) systems which often combine QWs and MCs to enhance electron and photon coupling generating polariton, including a bimodal momentum-space distribution with a narrow peak at zero momentum, long-range off-diagonal order [6,7], spatial
condensation in a macroscopic trap [7,8], spontaneous symmetry breaking [9], flow without dispersion [10], and a dramatic increase of coherence as measured in first-order and second-order correlation measurements [11-13]. Recently, L.
Sapienza, et al. [14] have realized an electroluminescent device operating in the light-matter strong-coupling regime based on a GaAs/AlGaAs quantum cascade structure embedded in a planar MC. They experimentally demonstrated that the electrons can be selectively injected into polariton states up to RT. Intrinsic decoherence mechanisms in the MC polariton condensate have been studied limited by the combination of number fluctuations and interactions. [15,16] RT low-threshold transition to a coherent polariton state has been observed as polariton lasing in bulk wide bandgap GaN MC in the strong-coupling regime. [17] The observation of polariton lasing over such a broad range of temperatures reveals a clear transition from a kinetic to a thermodynamic regime with increasing temperature.
Similar to GaN, ZnO is an environmental-friendly wide direct bandgap semiconductor.
Therefore, dominant exciton emission can be constantly observed in ZnO even at RT.
Due to its fast carrier cooling rate (< 200 fs) [18] and high Mott density (3.7 x 1019 cm-3, exciton Bohr radius ~2.34 nm) and large Rabi splitting of 120 meV [19] that have been theoretically show that as a consequence of the broadening of the upper branch by the continuum, Rabi oscillations should not be observed in ZnO MCs which nevertheless remain good candidates for polariton-based effects (polariton BEC) involving the lower polariton branch. [20]
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