Thermal Effect on the Luminescence Properties of Quantum Dots
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Semiconductor quantum dots (QDs) are similar to atoms in having a very sharp density of states distribution. However, what makes QDs such unusual objects are that we can control their shape, size, and hence the detailed characteristics of their energy levels and the number of confined states. The controllable shape, size, spatial distribution, and density of states endow the QDs with many novel properties, which have promising applications in optoelectronic devices. For example, QD-based injection lasers are predicted to have improved performance characteristics, such as a lower threshold current density, higher characteristic temperature, and higher deferential gain in comparison with the conventional quantum well lasers. The intersubband absorption of QDs can be used to design QD-based photodetectors operated in the mid-infrared spectral region. As compared to quantum wells, the reduced electron–phonon scattering in QDs provides the photoexcited carriers to be swept away as photocurrent before relaxing to the ground state and therefore makes the QDs to be more efficient in detection. Moreover, unlike the quantum-well photodetectors, QD photodetectors are intrinsically sensitive to normal photon incidence due to the breaking of the polarization selection rule.
Because of these promising applications, the growth of semiconductor QDs has been extensively investigated in recent years. A major breakthrough in the fabrication of high-quality quantum dots is the growth of self-assembled quantum dots (SAQDs), typically formed by strained-layer epitaxy in systems having a large lattice-mismatch. The growth in such a system is usually through the Stranski–Krastanow (S–K) growth mode or by the Volmer–Weber (V–W) growth mode. For S–K growth, the growth is initially through a layer-by-layer mode, which results in a thin wetting layer; after the wetting layer exceeds a critical thickness, defect-free three-dimensional (3-D) islands nucleate coherently on top of the strained wetting layer. For V–W growth, 3-D islands nucleate directly on the substrate without the formation of wetting layer. In the past years, laser diodes and photodetectors based on SAQDs with improved operation have been demonstrated. However, many factors, which fundamentally limit the performance of these devices, are still not well understood. Thermal effects have a great influence on the population of carriers in the energy states of QDs, and thus, such effects strongly influence the device performance. In order to realize room-temperature operation of the QD-based devices, the influence of temperature on their emission and absorption properties must be well understood. In this article, we will review the impact of thermal effects on the optical properties of SAQDs. We first show how thermal excitation affects the optical properties of bulk semiconductors. We believe this knowledge for bulk semiconductors will be helpful for understanding the optical properties of QDs. After that, recent temperature-dependent emission studies of SAQDs are reviewed. In high-quality SAQDs, evidence of thermally excited carrier transfer between QDs is shown. An analytical model based on rate equations is used to interpret the emission properties. Finally, a conclusion and a brief future vision are given.