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Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures. M.Merano et al. Nature, vol.438, 479-482 (2005) M.Merano et al. Appl. Phys. B, 84, 343-350 (2006). Itoh Lab. Takahiro DOKI. Outline. ・ Introduction Quantum Structures Complex System

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Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

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  1. Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures M.Merano et al. Nature, vol.438, 479-482 (2005) M.Merano et al. Appl. Phys. B, 84, 343-350 (2006) Itoh Lab. Takahiro DOKI

  2. Outline • ・Introduction • Quantum Structures Complex System • Cathodoluminescence • ・Motivation • ・Experimental Setup • ・Sample • ・Results • ・Summary

  3. Complex Quantum Structures T-shaped quantum wire Pyramidal quantum structure ex.) Quantum dot (QD) Quantum wire (QWR) Quantum well (QW) Vertical QWR (VQWR) Vertical QW (VQW) Quantum well (QW) Quantum wire (QWR) Interest in carrier dynamics in the complex quantum structures Necessity of high spatial resolution

  4. Electron beam Auger electron Secondary electron Cathodoluminescence （CL） X-ray Sample High spatial resolution Cathodoluminescence ・Cathodoluminescence (CL) ：Luminescence under electron beam irradiation Wavelength of electron beam is very short. Small excitation spot (to ~10nm) ・Secondary electron 　→Observation of image with scanning electron microscope (SEM)

  5. Motivation ●Cathodoluminescence is useful for observing electronic and optical properties of each nanostructure. ●For time-resolved measurement, picosecond or femtosecond laser pulses are used. However the spatial resolution of photo-excitation measurement is not enough to investigate each nanostructure. Time resolution High spatially resolved CL ・Original time resolved cathodoluminescence (TRCL) set-up ・Investigation of the time resolved luminescence of quantum structures located in InGaAs/AlGaAs tetrahedral pyramids.

  6. Experimental Setup Ti:Sa mode-locked laser Gold particles deposited on an amorphous carbon film GaN Spatial resolution: ＜50 nm Temporal resolution: 10 ps Temperature: 90 K Accelerating voltage: 10 kV Beam probe current: 10 pA

  7. Sample; InGaAs/AlGaAs tetrahedral pyramid ・GaAs substrate is patterned of tetrahedral recesses using photolithography and wet etching. ・InGaAs/AlGaAs heterostructures are grown by MOCVD (Metal-Organic Chemical Vapor Deposition). 45 nm Al0.75Ga0.25As 130 nm Al0.55Ga0.45As 140 nm Al0.30Ga0.70As & 0.5 nm In0.10Ga0.90As 130 nm Al0.55Ga0.45As (111)A facet GaAs substrate

  8. 2μm Results The secondary electron image CL spectrum from one pyramid Spectrally resolved CL images VQW QWR QW VQWR QD

  9. 6 4 5 1 2 3 Results TRCL of QW emission Excitation volume (radius of 400 nm) 1μm ・The rise time represents the carrier capture from the AlGaAs barrier into the InGaAs QWs. ・For dark regions in CL image, carriers from the QWs are trapped in theQWRs or QD. Since the QWs are all grown on the (111)A facet Expected equal rise time Shorter decay time

  10. :Carrier Results Carriers in QW Rise time Decay time QWR Barrier Diffusion Capture QW

  11. 6 4 5 1 2 3 Results TRCL of VQW emission Excitation volume (radius of 400 nm) 1μm VQWs exist above and below QWRs Decay time increases with the distance of the exciting point from the edge Rise time The time the carriers take to leave the VQW Decay time The time the carriers arrive in the VQW Time for carriers to leave the quantum structure ＜ Time to access it

  12. 6 4 5 1 2 3 Results TRCL of QWR emission Excitation volume (radius of 400 nm) 1μm Rise time: three points along the pyramid edge the carrier capture from the excitation volume three points on the pyramid facet the path that carriers travel to get trapped into QWR Decay time: Considering the carrier lifetime in the QWR the carrier diffusion towards the QD

  13. 6 4 5 1 2 3 Results TRCL of VQWR emission Excitation volume (radius of 400 nm) 1μm ・A clear variation of the rise times luminescence correlated with the excitation point distancefrom the pyramid centre. QD emission TRCL of QD emission is constant in time. The carrier lifetime in the QD is long. Carriers transfer to the QD from VQWR.

  14. :Carrier Results Carriers in the QWR, VQW, VQWR & QD QWR VQW QD VQWR QWR 1 electron ↓ a few thousand of electron-hole pairs Carriers in the QD are saturated. High density excitation

  15. :Carrier Results Carriers in the QWR, VQWR & QD QD VQWR QWR QD: The long carrier lifetime & High carrier density The VQWR constantly fills carriers in the QD . VQWR: Saturation in the QD ↓ QWR: Few carriers can transfer to the QD Saturation Decay time ↓ Mainly dependent on carrier lifetime

  16. Summary • ■TRCL has been used to investigate the optical properties of quantum structures in InGaAs/AlGaAs tetrahedral pyramids. • ■From spectrally resolved CL images, several CL bands from the pyramid were related with the each quantum structure. • ■By TRCL measurements, rise and decay times for the different emission energies are strongly dependent on the excitation point location and can be correlated to the carrier capture and relaxation processes in the different nanostructures and to the carrier diffusion mechanism in the pyramid.

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