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Schematic (a) and band energy diagram (b) of the InGaAs MQW structure. The red dots in panel (b) show the positions of discrete energy levels within the QWs.
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Context 1
... we investigate hot carrier properties of an InGaAs MQW structure with a type-I band alignment under 980 nm laser excitation at room temperature. The schematic and band energy diagram of the QW structure are shown in Figure 2. The active region of the sample is made by five identical In0.53Ga0.47As/In0.8Ga0.2As0.44P0.56 ...
Context 2
... isolated by InP cladding layers. Figure 2 (b) indicates the existence of multiple discrete energy levels within the QW, which are shown by red dotted lines. Figure 3 shows the PL spectra of the InGaAs MQW structure at various excitation powers collected by an objective lens with a numerical aperture (NA) of 0.95 and registered by a high sensitivity InGaAs CCD camera. ...
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Citations
... In one widely used technique, the carrier temperature is determined in the range of energies where the absorptivity is constant, known as the linear fitting method. 2 However, if the absorptivity changes above the bandgap energy, which is the case for nanostructured materials, the temperature extracted by the linear fitting may be inaccurate. 27 There are important questions about hot carrier effects in quantum-well (QW) structures, such as how they depend on lattice temperature and excitation power. In this work, we report experimental investigations of these questions based on hyperspectral luminescence imaging, which allows for an effective power-dependent measurement in a single shot. ...
... The determination of the carrier temperature in nanostructured materials by this common technique can be challenging and may lead to erroneous results. 27,30,31 In addition, depending upon the range of energies to which the linear fitting is applied, extracted carrier temperatures will be different. This effect is associated with their absorptivity, which has an energy-dependent behavior above the bandgap. ...
... This comprehensive fitting method provides a more accurate estimate of the temperature and the quasi-Fermi level splitting of carriers in the system. 8,27 In addition, in the full fitting of the PL spectrum, it is possible to include the band filling effect, which occurs under high illumination, and to determine separate temperatures for electrons and holes. 8,26 At intense excitation powers, when the quasi-Fermi level splitting is equal to or greater than the bandgap, (Δμ ! ...
Observation of robust hot carrier effects in quantum-well structures has prompted hopes to increase the efficiency of solar cells beyond the Shockley–Queisser limit (33% for single junction solar cells at AM1.5G). One of the main studies in hot carrier effects is the determination of carrier temperature, which provides information on the thermalization mechanisms of hot carriers in semiconductor materials. Here, we investigate the spatial distribution of photo-generated hot carriers in a InGaAs multi-quantum-well structure via hyperspectral luminescence imaging. We discuss proper methods of extracting the temperature of carriers from a photoluminescence spectrum. Robust hot carrier effects are observed at the center of the laser spot at various lattice temperatures. In addition, it is seen that the local carrier temperature scales linearly with the local laser intensity as long as the illumination exceeds a threshold power; the carrier temperature at regions with local intensities below the threshold drops to the lattice temperature, i.e., experiences no hot carrier effects. Moreover, at large distances from the concentrated light, where the level of illumination is negligible, evidence of carrier radiative recombination is observed, which is attributed to carrier diffusion in the planar structure. The results of this study can be applied to investigate the influence of carrier diffusion and thermoelectric effects on the thermalization of hot carriers.
Semiconductor quantum tunneling heterostructures offer a wide range of applications as photosensors, lasers, and circuit elements. However, fundamental questions related to energy transfer mechanisms and the correlation between some electrical and optical responses remain challenging. In order to provide insights into these problems, the nature of the carrier dynamics in these structures has been investigated by exploring the role of majority and minority carriers in the optoelectronic responses and the limiting factors responsible for their modulation. Thus, the transport and optical properties of different n-type samples built on Sb- and As-based double-barrier quantum well architectures have been analyzed using transport measurements and luminescence spectroscopies in continuous- and pulsed-wave mode. The characterization has been carried out by tuning various external parameters such as temperature, illumination, and magnetic fields. Our observations revealed resonant tunneling of carriers from cryogenics up to room temperature. The inclusion of III-V quaternary alloys enhances the photodetection of infrared wavelengths. Quaternary layers serve as absorbers that allow a thorough tuning of the photosensor capabilities. The optical response of the devices allowed unveiling complex dynamics of non-equilibrium carriers, pointing out the formation of independent electron and hole populations that do not thermalize. This characteristic enables the mapping of thermalization mechanisms of hot carriers along with the structures. In all cases, it was found that the optoelectronic characteristics of the devices are strongly intertwined. In this regard, bistable optical and transport characteristics associated with an intrinsic magnetoresistance of some samples can be modulated simultaneously with temperature and magnetic fields. Models were proposed to simulate the charge dynamics, discerning the main ingredients that control the electrical response and, in some cases, the photosensor abilities. These models were complemented considering coherent and incoherent transport channels, which demonstrates how the transport and optical attributes correlate to produce the peculiar quantum response. This approach allows discussing the role of minority carriers in the overall dynamics of the systems, the segmentation of the relaxation mechanisms, and the optimization of the optoelectronic properties. The results are intended to offer a comprehensive theoretical and experimental analysis of the complex quantum phenomena behind these quantum tunneling systems.
Current greenhouse gas emissions predict catastrophic climate change, calling for a rapid switch towards sustainable energy production. Harvesting the energy from the Sun is among the main ways to enable this new low-carbon economy. Improving the efficiency of solar cells is crucial to accelerate their deployment, as it increases the power output of a system without changing the cost of its infrastructure. Thanks to recent progress, the record efficiency of conventional solar cells is now beyond 26%, getting close to their 33% efficiency limit. The hot-carrier solar cell is a promising candidate to push this limit further, with an efficiency limit of 85% under concentrated sunlight.
In a hot-carrier solar cell, the carriers (electrons and holes) are at a higher temperature than the lattice. This higher temperature can be converted into extra voltage through a thermoelectric conversion, leading to higher cell efficiency. However, hot-carrier solar cells face challenges, the main one being the generation of hot carriers in the light absorber region. Despite significant progress, hot-carrier solar cells are still a long way from operating efficiently under sunlight illumination.
Hot carriers appear under continuous illumination when the absorbed power density is higher than what carriers can dissipate through so-called thermalization. Therefore, there are two ways to generate hotter carriers. The first way is to reduce the thermalization rate, for example by using thin quantum wells rather than bulk materials. The second way is to increase the power density generation. To do so, we can concentrate the incident sunlight onto a smaller surface, but only up to a limit. To go beyond, we also need to reduce the thickness of the light-absorbing region.
The issue with ultrathin quantum-well absorbers is that they do not absorb light as well. Under laser illumination, increasing the incident power is enough to observe hot carriers, which is why few works involving hot carriers have focused on absorption. However, this is ultimately not acceptable for solar cells: broadband light trapping is necessary to achieve high absorption in thin absorbers. Multi-resonant light trapping, which involves coupling light with resonant modes in the absorber, is particularly well suited for this application. This thesis aims to understand better the mechanisms of thermalization and light trapping and their interaction. The objective is to combine both to generate hot carriers under concentrated sunlight illumination.
