Ian R. Sellers

Robust magnetic polarons in type-II (Zn,Mn)Te/ZnSe magnetic quantum dots

We present a magneto-optical study of magnetic polarons in type-II Zn,Mn Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to 150 K, with a formation energy of 40 meV. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field 2 meV at 8 T and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperaturedependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.

Wide depletion width of 1 eV GaInNAs solar cells by thermal annealing

We present high quality GaInNAs p-i-n solar cells with depletion widths in excess of 1 μm for material absorbing in the practically important 1 eV band gap regime. This is achieved through optimization of post-growth rapid thermal annealing at a temperature of ∼ 910 °C. Despite the improvements in material quality evidenced by a low background impurity concentration and improved minority carrier collection, the external quantum efficiency remains limited to ∼50%. This is attributed to losses due to efficient radiative recombination in the bulk GaInNAs intrinsic region enhanced via localization of carriers in alloy fluctuations.

Effects of localization on hot carriers in InAs/AlAsxSb1–x quantum wells

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.

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

Probing the nature of carrier localization in GaInNAs epilayers by optical methods

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Measurement of the valence band-offset in a PbSe/ZnO heterojunction by x-ray photoelectron spectroscopy

Sb

Improved performance in GaInNAs solar cells by hydrogen passivation

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Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH3NH3)2CdX4 (X = Cl, Br, I)

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.

High-density InAs/GaAs1−xSbx quantum-dot structures grown by molecular beam epitaxy for use in intermediate band solar cells

Photoluminescence (PL), optical pumping, and reflectance studies of nominally undoped and p-type GaInNAs epilayers are presented. The PL peak energy of the nominally undoped sample exhibits an S-shaped dependence on temperature for T < 50 K. This is attributed to recombination of bound excitons localized on traps. The energy of the PL circular-polarization maximum coincides with the energy of the free-exciton related reflectance feature at all temperatures. In heavily p-type samples the S-shaped temperature-dependence of the PL energy disappears, and the PL peak and circular polarization maximum coincide with the reflectance feature at all temperatures, indicating that the PL is free-exciton-like.

Photoluminescence study of Be-acceptors in GaInNAs epilayers

A heterojunction of PbSe/ZnO has been grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was used to directly measure the valence-band offset (VBO) of the heterojunction. The VBO, ΔEV, was determined as 2.51 ± 0.05 eV using the Pb 4p3/2 and Zn 2p3/2 core levels as a reference. The conduction-band offset, ΔEC, was, therefore, determined to be 0.59 ± 0.05 eV based on the above ΔEV value. This analysis indicates that the PbSe/ZnO heterojunction forms a type I (Straddling Gap) heterostructure.