The rapid development of the modern optoelectronic information industry has increased the requirements for semiconductors in optoelectronic devices. Low-dimensional semiconductor materials with smaller sizes, adjustable performance, and greater integration flexibility are the basis for developing a new generation of nano-optoelectronic devices and systems. Quantum dots occupy an important position in low-dimensional quantum structure materials owing to their unique structural and physical properties, which make them a promising, low-dimensional structural material for the next generation of optoelectronic devices. However, the performance of quantum dot-based devices has not matched the theoretical expectations of the material because quantum dots are difficult to fabricate at a high standard. As quantum dots are very small, the number of hot carriers is increased, which significantly affects the modulation speed and results in a decrease of the carrier collection efficiency of the device.
In a quantum well, another well-known low-dimensional structure, the carriers are constrained only in the direction of the well width, and the density of the state presents a ladder-like distribution, which is significantly larger than that of quantum dots. Thus, a quantum well is more suitable for the collection and storage of carriers than quantum dots. This makes it possible to study and develop a composite quantum structure that combines quantum dots and wells. As a new type of semiconductor low-dimensional quantum structure, the indium-based well-dot composite quantum structure inherits the advantages of both traditional quantum dots and wells while overcoming their inherent constraints due to its composite nature as well as the available control over the structure. Furthermore, well-dot composite quantum structures can be used to implement energy-band engineering more effectively, improve the physical and optical properties of traditional low-dimensional semiconductor materials, and expand optoelectronic device applications.
This paper describes in detail the structural properties, optical characteristics, and application prospects of low-dimensional indium-based well-dot composite quantum structures. The content covers three types of structures: an indium-based well-dot tunnel-coupled quantum structure, a dots-in-a-well quantum structure, and a self-assembled indium-rich cluster composite quantum structure. For an indium-based well-dot tunnel-coupled composite quantum structure, the well and dots are generally separated by a spacer. The coupling degree between the well and dots is dependent on the material, component, and thickness of the spacer. The flexibility of this approach makes it possible to achieve better device performance.
For a dots-in-a-well composite quantum structure, the larger absorption cross section of the quantum well increases the well’s ability to capture carriers, which limits their presence around the quantum dots. Thus, this type of composite quantum structure exhibits the significant fluorescence enhancement phenomenon. Because a quantum well can form a potential barrier, the probability of carriers escaping from the dots is reduced in this composite quantum structure.
A self-assembled indium-rich cluster composite quantum structure is constructed based on the unique indium-rich cluster effect which occurs during the epitaxial growth of the InGaAs/GaAs materials. The material has an irregularly strained quantum structure with different bandgaps and researches on this structure reveal that it possesses outstanding optical characteristics that have not been achieved by other quantum structures. The ultra-broad and uniform gain spectra in bipolarization, from this structure, indicate an opportunity for the development of high-performance semiconductor lasers, such as polarized dual-wavelength laser diodes, polarization-independent semiconductor optical amplifier devices, and ultra-broadband tunable semiconductor lasers with uniform spectral output power for all tunable wavelengths.
In conclusion, owing to the introduction of tunnel injection technology, well-dot tunnel-coupled composite quantum structures significantly improve the performance of optical devices, resulting in higher gains, lower currents, better temperature stability, higher speed modulation performance, and a lower linewidth enhancement factor. The structures show good application prospects in the development of high-performance semiconductor lasers and semiconductor optical amplifiers. In addition, the dots-in-a-well composite quantum structure effectively overcomes the inherent limitations of quantum dots, improves the density of quantum dots, enhances the carrier capture ability of the structure, and significantly improves optical performance. This composite structure shows great application value for optimizing the performances of quantum dot lasers, infrared detectors, solar cells, and other devices. Furthermore, the self-assembled indium-rich cluster composite quantum structure exhibits excellent optical properties, such as ultra-wide and uniform gain, as well as bimodal spontaneous emission and absorption. The structure has shown broad development prospects for the realization of a new generation of ultra-wide tunable semiconductor lasers with uniform spectral power, polarized dual-wavelength lasers, and polarized independent semiconductor optical amplifiers. Simultaneously, as the lasing wavelength tuning range of the composite structure is in the terahertz band, it can be developed into a new tunable terahertz source in the future. In addition, we believe that in the future, by using an InGaAs indium-rich cluster composite quantum material as a new activation medium, combined with second-harmonic technology, we can achieve continuous high-power and wide-tuning short-wave laser output that is difficult for wide band gap materials. Because each well-dot composite structure has its own unique advantages and disadvantages, the structure can be selected according to the desired device’s performance requirements, to maximize the advantages of the individual structure.
