Ultrafast carrier dynamics studies have been carried out on samples with single layers of self-assembled In0.4Ga0.6As/GaAs quantum dots (QDs). Measurements were made using femtosecond degenerate pump-probe differential reflectivity with an 800-nm, 28-fs Ti-sapphire oscillator as the source. The QDs were grown via modified Stranski-Krastanov growth. This modified growth process consists of two steps: low-temperature growth and high-temperature annealing. Specifically, the InGaAs QD structures are fabricated on n-type GaAs(001) using molecular beam epitaxy. The InGaAs layer is deposited at 350-3700C followed by QD self assembly at 420-4900C. Finally, these QDs are capped with 10 nm or 100 nm of GaAs. The measured width and height of these QDs are typically 25 nm and 8 nm, respectively. Dots annealed at higher temperature have larger base area (width and length) and reduced height, as compared to those annealed at lower temperature. We have used a model consisting of a linear combination of an exponential decaying function to describe the carrier dynamics and fit the reflectivity data, revealing trends in the carrier capture and relaxation times associated with the InGaAs layer versus laser excitation level and QD morphology. Capture times are ~ 1 ps for the 100-nm capped samples, but slightly shorter for the 10-nm capped thin samples, indicating carrier transport plays a role in dynamics. The carrier dynamics in 10-nm capped samples are correlated with sample annealing temperature, indicating QD morphology affects carrier capture. Versus laser intensity, and thus carrier excitation level, the dynamics generally become slower, suggesting state filling is important in both the capture and relaxation of excited carriers in these samples.
| Identifer | oai:union.ndltd.org:UTAHS/oai:digitalcommons.usu.edu:etd-2471 |
| Date | 01 May 2013 |
| Creators | Chauhan, Kripa Nidhan |
| Publisher | DigitalCommons@USU |
| Source Sets | Utah State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | All Graduate Theses and Dissertations |
| Rights | Copyright for this work is held by the author. Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user. For more information contact Andrew Wesolek (andrew.wesolek@usu.edu). |
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