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Semimetal Electronics
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
Alfonso Sanchez-Soares, Christian König, Conor O’Donnell, Jean-Pierre Colinge, James C. Greer
In two-dimensional nanostructures, confinement is introduced in one spatial dimension, allowing electrons to move along the remaining two unconstrained ones. While the impact of confinement in these thin films or quantum wells is generally lower than in other types of nanostructures discussed previously, exploitation of quantum-size effects for optoelectronic devices was first proposed in this type of nanostructures: in the 1970s, while the field of integrated optics was gaining momentum, research in using thin films as optical waveguides led to elucidation of an application that later became known as quantum well lasers [7,8]. Figure 10.2 shows the band profile associated with thin-film heterostructures typically employed in semiconductor lasers, illustrating this type of device name’s etymology: arranging alternating layers of two materials with different bandgaps EG,1 and EG,2 effectively creates a band profile in which electrons (holes) located in thin regions composed of the material with the lower bandgap are confined to wells at the conduction (valence) band edge. The distribution of discretized energy levels in these wells can then be tuned by varying chemical compositions and layer thickness, allowing great control over the optical properties of these heterostructures. Quantum well lasers were extensively investigated in the coming years, eventually becoming one of the most popular and versatile laser designs with applications in several mass-produced devices such as fiber optic communication links, optical disc players, and laser printers.
Quantum Well
Published in Kenichi Iga, Yasuo Kokubun, Encyclopedic Handbook of Integrated Optics, 2018
An ultra-thin semiconductor layer with the thickness upto a maximum of about 10 nm is sandwiched between layers with higher band gap energies. The lineup of the conduction and valence band edges forms well potentials, and electrons and holes are confined in the wells. Since the well thickness is as thin as the de Broglie wavelength of electrons in a semiconductor, the energy of electrons in the wells is discrete according to the quantum mechanics. Semiconductor lasers utilizing quantum wells as the active layers are called quantum well lasers. These lasers have superior characteristics, such as ultra-low threshold and less sensitivity to temperature. Today, quantum well lasers are used so widely that semiconductor lasers has become synonymous with quantum well lasers.
Low threshold 2.37μm InGaAsSb/GaSb QW lasers: Towards the ideal Quantum Well laser?
Published in J Kono, J Léotin, Narrow Gap Semiconductors, 2006
K. O’Brien, S.J. Sweeney, A.R. Adams, S.R. Jin, B.N. Murdin, C.N. Ahmad, A. Salhi, Y. Rouillard, A. Joullié
A threshold current density of 126 Acm−2 (42 Acm−2 per QW) has been measured for these devices, which is extremely low for a quantum well laser. This threshold is significantly lower than that of a typical near infrared 1.5 μm InP-based laser as shown in figure 1(a). It has previously been shown [8] that the temperature dependence of the 1.5 μm devices is dominated by Auger recombination, accounting for 80% of the threshold at room-temperature. The wavelength dependence of the threshold current of NIR devices, as shown in figure 1(b) is attributable to the band gap dependent non-radiative Auger recombination process. As the band gap decreases in the MIR wavelength range, we would expect the threshold current density to increase sharply, following the Auger dominated trend of figure 1(b). The low threshold measured for the 2.37 μm device can be explained, by considering how the different recombination processes depend on the band gap, Eg [11]. The radiative current at threshold, Jrad, is proportional to Eg2, hence Jrad for the 2.37 μm device should be 40% of Jrad for a 1.5 μm device. Jrad is known to account for 20% of Jth (250 Acm−2) in the 1.5 μm device, hence we would expect (through the Jrad ∝ Eg2 relationship) that Jrad of the 2.37 μm device would be ~20 Acm−2. Since Jth is > 20 Acnf2 for the 2.37 μm device, we must consider the non-radiative processes which influence the threshold of the device through an analysis of its spontaneous emission characteristics, as follows.
Bandgap tailoring and optical response of InAlAs/InGaAs/GaAsSb double quantum well heterostructures: the impact of uniaxial strain and well width variations
Published in Journal of Modern Optics, 2022
Md. Riyaj, Amit Rathi, A. K. Singh, P. A. Alvi
Bandgap tailoring is a powerful technique for the design and optimization of semiconductor heterostructures to obtain high-performance laser structures with characteristics such as low threshold current, longer wavelength and higher material gain. In a quantum well laser diode, when the carriers are in the optical region (i.e. active region), they can recombine radiatively for the process of photon amplification to occur [1–3]. As the optical gain is a function of the probability of occupation factor and carrier density, confinement of charge carriers is realized by a single or multiple quantum well active region materials separated by barrier layers. Since the energy bandgap of the barrier region material is larger, the light produced in the active region will not have an adequate amount of photon energy to be absorbed in them. In consequence, the non-radiative recombination rate shifts towards lower values, the radiative recombination rate shifts towards higher values and concurrently a fall in the recombination lifetimes is noticed [4–7]. When an external strain is applied, the mixing of wavefunctions between continuous sub-bands is reduced. It also affects the transition matrix elements [8–11]. To understand the nature of semiconductor optoelectronic devices, it is necessary to know their optical characteristics under different situations. If we apply an external field to the semiconductor heterostructure, the potential profiles are slanted and the positions of the energy states are changed. Therefore, the material gain spectra can be altered through an external field [12,13]. The field changes the band offsets and wavefunction forms, disfigures the electronic positions in the atoms or alters the crystal structure, resulting in the modification of the optical characteristics [14–18]. In semiconductor heterostructures, bandgap tailoring is an important design aid for optoelectronic devices [19–22]. In the present work, our motive behind this study of such nano-scale heterostructures is to attract attention to their potential applications in a variety of upcoming applications including optical tweezers and optical interconnects.