Dissertations, Theses, and Capstone Projects

Date of Degree

2-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Maria C. Tamargo

Committee Members

Carlos Meriles

Aidong Shen

Christophe Testelin

Igor L. Kuskovsky

Subject Categories

Condensed Matter Physics | Semiconductor and Optical Materials

Keywords

II-VI, Quantum Dots, Semiconductors

Abstract

In this thesis, we discuss the growth procedure and the characterization results obtained for epitaxially grown submonolayer type-II quantum dot superlattices made of II-VI semiconductors. We have investigated the spin dynamics of ZnSe layers with embedded type-II ZnTe quantum dots and the use of (Zn)CdTe/ZnCdSe QDs for intermediate band solar cell (IBSC). Samples with a higher quantum dot density exhibit longer electron spin lifetimes, up to ~1 ns at low temperatures. Tellurium isoelectronic centers, which form in the ZnSe spacer regions as a result of the growth conditions, were also probed. A new growth sequence for type-II (Zn)CdTe/ZnCdSe (QDs) was developed in order to avoid the formation of a parasitic strain-inducing ZnSe interfacial layer. Elimination of the ZnSe interfacial layer allows for simplifications in the fabrication of the IBSC cell device structure based on these QDs, since different Cd compositions in the buffer layer and barriers is no longer required to grow stress free structures. Careful consideration was taken during the initiation of the growth process of QDs by migration enhanced epitaxy, in order to avoid the formation of the undesirable interfacial layer. The use of CdTe fractional monolayer QDs in a ZnCdSe host matrix was also explored for potential application in the IBSC device. This completely eliminated the formation of any interfacial layer, and also allowed for strain engineering of the QD superlattices. Simple arguments are used following continuum elastic theory to deduce the size of the dots and the strain within the superlattice from XRD data. This is further verified using PL and used in the energy calculations that yield the values of the intermediate band energy. The results suggest that the optimized materials are highly suitable for these high efficiency solar cells.

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