No abstract

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.

The temperature dependence of a InAs/AlAs0.84Sb0.16 multi-quantum-well sample is studied using continuous wave photoluminescence. An “s-shape” shift in peak energy is observed and attributed to low energy localization states. High incident power density photoluminescence measurements were performed to probe the nature of such localization. The results opposed the possibility of a type-II band structure and supported the idea of low energy localization states. The effect of such localization on hot carriers in our system was studied and an improvement in their stability due to hole mobility at elevated temperature is presented.

InAs/AlAs x Sb 1−x 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.

An InGaAsP quantum well with a type‐II band alignment is studied using continuous wave power and temperature dependent photoluminescence (PL) spectroscopy. The small energy separation between the ground state and first excited state results in significant thermal carrier redistribution and excited state occupation, particularly, with increasing excitation power and temperature. This state filling is evident as a high‐energy shoulder in the PL spectra, the same energy region where in the simplest Planck‐description the gradient is considered inversely proportional to carrier temperature. The outcome of an excited state occupation in broadening the high‐energy PL tail is to perturb the temperature extracted using this analysis; therefore, the true temperature of carriers is not properly evaluated when significant state filling occurs. In addition, broadening of the PL due to phonons at higher temperatures also distorts (or falsely increases) the non‐equilibrium “hot” carrier temperature and as such should be considered when using Planck's relation. The role of these two effects is considered and their mutual effect on the analysis of the extracted hot carrier temperature discussed. Copyright © 2017 John Wiley & Sons, Ltd.