Yushuai Dai

Abstract

Self-assembled quantum dots (QDs) and nanowires (NWs) are currently the subjects of extensive study due to their promising applications in optoelectronic devices. In order to enhance understanding of the short circuit current improvement in InAs/GaAs quantum dots solar cell (QDSC), the mechanisms of carrier escape by thermal activation and tunneling from InAs quantum dots (QDs) confinements in InAs/GaAs QDSCs are investigated. The fitted activation energy of electrons from temperature dependent photoluminescence (TDPL) is 114 meV. Using this fitted activation energy, calculated thermal escape time and tunneling time of electrons from the ground state of the QDs are 10-12 seconds and 10-6 seconds at 300K, respectively. These results indicate that at room temperature thermal escape is dominant for electrons escape from ground state. At low temperature (8K), tunneling mainly affects the electrons escape from ground state, since thermal energy cannot support electrons to overcome the fitted activation energy (barrier, 114 meV).

In addition, in order to describe the new physics and achieve the final success in nanowire device for photovoltaic applications, the first step is to develop high-quality semiconductor nanowires on the selected substrate. Morphological and crystal structure characterizations were performed via SEM and TEM for InAs nanowire samples grown with and without Au seed on GaAs substrate using metal organic vapor phase expitaxy (MOVPE). Several major factors affect the NW growth in terms of shape, density, etc. For nanowire growth with Au seed, its growth direction mainly depends on the substrate, while its uniformity is initially related to the Au seed coverage. III/V ratio affects the NW aspect ratio (length/bottom width), ranged from 12.00 to 38.93. Increasing temperature accelerates the growth rate in both axial and radial directions. NWs grown without Au seed using a pattern mask show no tapering along the growth direction with an average diameter of 26 nm. All defects stop in the buffer layer when InAs nanowires grown with an Au seed, but a mix of ZB and WZ crystal phases were observed along the growth direction of nanowire. InAs NWs grown without Au seeds also show a mixture of different crystal phases along the growth direction. The diameter of InAs nanowire should be further reduced to 3-6 nm as to achieve PL response between 1000~1300 nm.

Publication Date

8-2013

Document Type

Thesis

Student Type

Graduate

Degree Name

Materials Science and Engineering (MS)

Department, Program, or Center

Center for Materials Science and Engineering

Advisor

Seth M. Hubbard

Comments

Physical copy available through RIT's The Wallace Library at: QC611.6.Q35 D34 2013

Campus

RIT – Main Campus

Plan Codes

MSENG-MS

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