Sangeetha Vijeyaragunathan

Suppression of phonon‐mediated hot carrier relaxation in type‐II InAs/AlAsxSb1 − x quantum wells: a practical route to hot carrier solar cells

InAs/AlAs

Determination of time-reversal symmetry breaking lengths in an InGaAs interferometer array.

_{x}

Enhanced hot electron lifetimes in quantum wells with inhibited phonon coupling

Sb

Evidence of suppressed hot carrier relaxation in type-II InAs/AlAs1-xSbx quantum wells

_{1-x}

Evidence of hot carriers at elevated temperatures in InAs/AlAs0.84Sb0.16 quantum wells

quantum wells are investigated for their potential as hot carrier solar cells. Continuous wave power and temperature dependent photoluminescence indicate a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation below 90 K to a regime where inhibited radiative recombination dominates the hot carrier relaxation at elevated temperatures. At temperatures below 90 K photoluminescence measurements are consistent with type-I quantum wells that exhibit hole localization associated with alloy/interface fluctuations. At elevated temperatures hole delocalization reveals the true type-II band alignment; where it is observed that inhibited radiative recombination due to the spatial separation of the charge carriers dominates hot carrier relaxation. This decoupling of phonon-mediated relaxation results in robust hot carriers at higher temperatures even at lower excitation powers. These results indicate type-II quantum wells offer potential as practical hot carrier systems.

Valence band states in an InAs/AlAsSb multi-quantum well hot carrier absorber

Quantum interference oscillations due to the Aharonov-Bohm phase were measured in a ring interferometer array fabricated on a two-dimensional electron system in an InGaAs/InAlAs heterostructure. Coexisting oscillations with magnetic flux periodicity h/e and h/2e were observed and their amplitudes compared as function of applied magnetic field. The h/2e oscillations originate in time-reversed trajectories with the ring interferometers operating in Sagnac-type mode, while the h/e oscillations result from Mach-Zehnder operation. The h/2e oscillations require time-reversal symmetry and hence can be used to quantify time-reversal symmetry breaking, more particularly the fundamental mesoscopic dephasing length associated with time-reversal symmetry breaking under applied magnetic field, an effective magnetic length. The oscillation amplitudes were investigated over magnetic fields spanning 2.2 T, using Fourier transforms over short segments of 40 mT. As the magnetic field increased, the h/2e oscillation amplitude decreased due to time-reversal symmetry breaking by the local magnetic flux in the interferometer arms. A dephasing model for quantum-coherent arrays was used to experimentally quantify effective magnetic lengths. The data was then compared with analytical expressions for diffusive, ballistic and confined systems.

Accurate determination of the temperature dependent thermalization coefficient (

Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

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Hot carrier solar cells (HCSCs) have been proposed as devices, which can increase the conversion efficiency of a single junction solar cell above the Shockley-Queisser limit. For practical implementation of such systems, solar cells operating with efficient hot carrier extraction must circumvent two fundamental challenges: 1. Find an absorber material in which hot carriers are sustained either via inhibiting or circumventing phonon relaxation pathways; 2. Implement energy selective contacts in which only a narrow range of energy within the hot carrier distribution is extracted; thereby, reducing cooling losses in the contacts. Here, type-II InAs/AlAs0.16Sb0.84 quantum-wells are investigated as a candidate system for hot carrier absorbers. Continuous wave power and temperature dependent photoluminescence measurements are presented that indicate: a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation − below 90 K − to a regime where inhibited radiative recombination dominates the hot carrier relaxation − at higher temperatures1. The reduction in the PL efficiency is strongly coupled to an increase in the hot carrier temperature extracted from the measurements. This behavior is attributed to a build-up of electrons in the QWs, which appears to inhibit electron-phonon relaxation2.

) in InAs/AlAsSb quantum wells

InAs/AlAs0.84Sb0.14 quantum wells (QWs) are investigated as a potential system for applications in hot carrier solar cells. Temperature and power dependent photoluminescence (PL) measurements show evidence of carrier localization. Evidence of for the presence of hot carriers is provided through the broadening of the high-energy tail in PL with increasing excitation power. Moreover, with increasing temperature, the stability of the hot carriers appears to improve despite the increased contribution of phonons at elevated temperatures. This is attributed to the reduced radiative recombination rate driven by the type-II band offset inherent in this system; which is suggested to result in inhibited hot carrier relaxation through electron pile-up in the conduction band