Temperature Dependent Characterization of Imbedded InAs Quantum Dots in GaAs Superlattice Solar Cell Structures by High
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Temperature Dependent Characterization of Imbedded InAs Quantum Dots in GaAs Superlattice Solar Cell Structures by High Resolution X-ray Diffraction Josephine J. Shenga, David. C. Chapmanb, David M. Wiltb, Stephen J. Pollyc, Christopher G. Baileyc, Christopher Kerestesc, Seth M. Hubbardc, Sang M. Hana,d. a
Department of Nanoscience and Microsystems, University of New Mexico, Albuquerque, New Mexico. b Air Force Research Laboratory, Albuquerque, New Mexico. c NanoPower Research Laboratory, Rochester Institute of Technology, Rochester, New York. d Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico. ABSTRACT The insertion of nanostructured materials (such as quantum wells, wires, and dots) into the intrinsic region of p-i-n solar cells introduces an intermediate band within the bandgap of the host material. It has been shown that the sub-bandgap conversion provided by the nanostructured materials, enhances the short circuit current as well as the overall efficiency of InAs quantum dots (QD) imbedded in GaAs superlattice (SL) solar cells [1]. As a contender for space applications, it is necessary to subject these solar cell structures to temperatures encountered in the Low Earth Orbit (LEO), probing for any material degradation. Herein, we focus on temperature dependent characterization using high resolution X-ray diffraction (HRXRD) of InAs QD enhanced GaAs solar cell structures with varying growth parameters. The structures characterized can be classified into three groups: (1) GaP strain compensation coverage, (2) GaAs barrier coverage, and (3) InAs coverage for QD formation. HRXRD rocking curves of each structure focusing around the GaAs peak are analyzed at a range of temperatures up to 200˚C. Although no noticeable shifts in the SL peaks are detected, interfacial diffusion decreased the resolution of fringes produced by reflections at the SL interfaces in test structures with varying InAs QD coverage. Unbalanced strain in the same structures shows a distortion in the GaAs peaks. INTRODUCTION For space applications, solar cells with a high power-to-weight ratio is desired, hence the need to increase power conversion efficiencies. One way to do so is through bandgap engineering with the insertion of multiple layers of quantum dots (QD) in the intrinsic region of p-i-n solar cells [1]. The QD layers form an isolated intermediate band within the bandgap of the host. Fig. 1 shows a cross-sectional band diagram of an InAs/GaAs QD solar cell structure with the QD structure repeated five times, with the QDs confined within the high electric field of the i-region. Photon absorption is believed to occur in two steps where incident photons with energies below the GaAs host bandgap result in absorption in the QD states. Secondary excitation causes carriers in the QD states to be excited into the conduction band where it is subsequently collected. As a contender for space applications, it is necessary to subject these solar cell structures to temperatures encountered in the Low E
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