We report on the photoluminescence enhancement of 1.3 μm InAs quantum dots (QDs) epitaxially grown on an ultrathin 250 nm GaAs buffer on a Si substrate. Decreasing the GaAs buffer thickness from 1000 to 250 nm was found to not only increase the coalesced QD density from 6.5 × 10 8 to 1.9 × 10 9 cm −2 but also decrease the QD photoluminescence emission intensity dramatically. Inserting an Al 0.4 Ga 0.6 As potential barrier layer maintained strong photoluminescence from the QDs by effectively suppressing carrier leakage to the GaAs/Si interfacial region even when the GaAs buffer was thinned to 250 nm. We then fabricated a light-emitting diode using the ultrathin 250 nm GaAs buffer on Si and confirmed strong electroluminescence peaking at 1.28 μm without interfacial defect emission at room temperature. We believe that this work is promising for monolithically integrated evanescent Si lasers using InAs/GaAs QDs.
Tunnel junctions (TJs) are essential for high-performance multijunction solar cells to act as transparent low resistance paths for carriers to travel between adjacent cells. However, TJs typically exhibit highly degraded tunneling performance due to unwanted dopant out-diffusion during top cell growth. In this study, GaAs TJs with Si and Te delta-doping (δ-doping) were grown via solid source molecular beam epitaxy to investigate the tunneling performance and thermal stability. While Si δ-doped TJs exhibited typical tunneling characteristics with an Esaki peak current density of 173 A/cm2, Te δ-doped TJs revealed 1.5 A/cm2 at Vbias = 100 mV without negative resistance. It was found that the performance degradation after annealing at 600 °C for 90 min was significantly higher for TJs with Si δ-doping than for Te. Secondary ion mass spectroscopy measurements reveal that Te shows no clear signs of dopant diffusion while Si exhibited significant out-diffusion in the active TJ layer after thermal annealing. The superior thermal stability of Te compared to Si proves to be advantageous as an alternative n-type dopant for high temperature and long duration grown multi-junction solar cells.
Fabrication of high quantum efficiency nanoscale device is challenging due to increased carrier loss at surface. Low dimensional materials such 0D quantum dots and 2D materials have been widely studied to mitigate the loss. Here, we demonstrate a strong photoluminescence enhancement from graphene/III−V quantum dot mixed-dimensional heterostructures. The distance between graphene and quantum dots in the 2D/0D hybrid structure determines the degree of radiative carrier recombination enhancement from 80% to 800% compared to the quantum dot only structure. Time-resolved photoluminescence decay also shows increased carrier lifetimes when the distance decreases from 50 to 10 nm. We propose that the optical enhancement is due to energy band bending and hole carrier transfer, which repair the imbalance of electron and hole carrier densities in quantum dots. This 2D graphene/0D quantum dot heterostructure shows promise for high performance nanoscale optoelectronic devices.
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