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Ratcliff Dissertation Final 11-3.pdf (10.5 MB)

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Growth and Characterization of III-Phosphide Materials and Solar Cells for III-V/Si Photovoltaic Applications

Ratcliff, Christopher
http://rave.ohiolink.edu/etdc/view?acc_num=osu1414760746

Abstract Details

2014, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
Utilizing major advances in III-V/Si epitaxy is a promising approach to realize high-efficiency, cost efficient terrestrial solar power. The successful demonstration of planar GaP epitaxial layers free of nucleation-related defects grown on Si by both MBE and MOCVD establishes extremely fertile ground for the development of high quality III V devices on Si substrates. Solar cell devices in particular stand to benefit, and not only by the merging of the best performing photovoltaic materials with the cost, availability, and scalability advantages of Si substrates, but also because of the advanced state of Si PV itself. With this motivation in mind, several aspects of a III-V/Si multijunction solar cell are investigated in this dissertation. Such a device requires many advanced and novel components. The use of GaP as the bridge from Si to the wide range of III-V materials renews interest in a material whose activity in research efforts had faded significantly since the advent of modern growth techniques and GaAs-based heterostructures in the 1980s. GaP-based research is also motivated by the fact that it represents the far lesser studied binary component of both GaxIn1-xP and GaAsyP1-y, two alloys essential to the wider scope of high efficiency PV research. Efforts to advance the ability to grow GaP by MBE and MOCVD were undertaken. Post-growth surface morphologies of homoepitaxial GaP films grown by MBE and MOCVD have been studied. Smooth, stepped surface morphologies of MBE-grown layers, measured by atomic force microscopy, were found for a wide range of substrate temperatures and P2:Ga flux ratios. A MOCVD-based growth study performed under similar conditions to MBE-grown samples show a nearly identical smooth, step-flow surface morphology, presenting a convergence of growth conditions for the two different methods. Electrical characterization of these films was then carried out by Hall effect measurements and DLTS to determine transport properties and to test for the presence of defect levels. Si-doped GaP showed mobilities that compare well to previous studies concerning n-type GaP grown by vapor- and liquid-phase growth methods. An activation energy greater than 80 meV, confirming the relatively deep position of the Si donor atom in these GaP films, was extracted from fits of temperature dependent Hall effect measurements, agreeing with similar measurements carried out several decades ago on vapor-phase grown samples. DLTS measurements revealed seven distinct majority-carrier deep levels for the films grown by MBE and current experiments to compare MBE and MOCVD films are discussed. The other major material of interest in this work is GaxIn1-xP. A significant effort is currently being made throughout the PV area to develop high bandgap top junction subcells which are needed for a variety of technologies to continue to push the boundaries of high-efficiency solar cells. Ga-rich compositions of GaxIn1-xP are prime candidates that can achieve direct bandgaps of up to about 2.2 eV. Prototype Ga0.57In0.43P solar cells have been grown by MBE and MOCVD on GaAsyP1-y compositionally graded buffers in order to optimize the cell structures, compare performance between the two growth methods, and to investigate the effects of threading dislocation density on solar cell performance metrics and material properties. Major improvements were made in the subcells by both growth techniques. The MOCVD-grown subcells, at this juncture, have shown superior performance likely due to the greater degree of freedom in growth parameters, most notably the ability to grow at higher temperature to increase crystalline quality; a conclusion supported by the fact that post-growth annealing of the MBE-grown subcells resulted in better material properties as measured by photoluminescence as well as lower levels of depletion recombination current in the solar cell junction itself and better photovoltaic performance: larger VOC and FF. In addition to standard photovoltaic characterization methods, DLTS and DLOS were used to determine the presence of bandgap states in the MBE-grown solar cell devices and to observe their sensitivity to threading dislocation density. Three total majority carrier levels were found within the Ga0.57In0.43P bandgap, one by DLTS (EV + 0.70 eV) and two by DLOS (EV + 0.90 eV, EV + 1.82 eV). The results obtained during the course of this research present significant strides in the understanding of several challenging components essential to the development of a III-V/Si multijunction device. Experience gained for the growth and material properties of GaP epitaxial layers and for the development of high bandgap GaxIn1-xP solar cells are of interest to a wide range of semiconductor technologies. The comparison of the materials and devices between two of the most important and common modern semiconductor growth methods in both the research and commercial worlds expands the impact of the work.
Steven Ringel, Dr. (Advisor)
218 p.
Electrical Engineering
Semiconductor, III-V, Si, InGaP, GaP, Gallium Phosphide, MBE, MOCVD, Epitaxy, Metamorphic, Solar, Photovoltaics

Recommended Citations

Citations

  • Ratcliff, C. (2014). Growth and Characterization of III-Phosphide Materials and Solar Cells for III-V/Si Photovoltaic Applications [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1414760746

    APA Style (7th edition)

  • Ratcliff, Christopher. Growth and Characterization of III-Phosphide Materials and Solar Cells for III-V/Si Photovoltaic Applications. 2014. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1414760746.

    MLA Style (8th edition)

  • Ratcliff, Christopher. "Growth and Characterization of III-Phosphide Materials and Solar Cells for III-V/Si Photovoltaic Applications." Doctoral dissertation, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1414760746

    Chicago Manual of Style (17th edition)

osu1414760746
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© 2014, some rights reserved.Creative Commons License Growth and Characterization of III-Phosphide Materials and Solar Cells for III-V/Si Photovoltaic Applications by Christopher Ratcliff is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.
Release 3.2.12