Abstract

In this work, InAs quantum dots grown by organometallic vapor-phase epitaxy (OMVPE) are investigated for application in III − V material solar cells. The first focus is on the opti- mization of growth parameters to produce high densities of uniform defect-free quantum dots via growth on 2" vicinal GaAs substrates. Parameters studied are InAs coverage, V/III ratio and growth rate. QDs are grown by the Stranski-Krastanov (SK) growth mode on (100) GaAs substrates misoriented toward (110) or (111) planes with various degrees of misorientation from 0◦ to 6◦. Atomic force microscopy results indicated that as misorientation angle increased toward(110),criticalthicknessforquantumdotformationincreasedwithθc =1.8ML,1.9ML and 2.0ML corresponding to 0◦, 2◦ and 6◦, respectively. Results for quantum dots grown on (111) misoriented substrates indicated, on average, that higher densities of quantum dots were achieved, compared with similar growths on substrates misoriented toward (110). Most no- tably, a stable average number density of 8 × 1010cm−2 was observed over a range of growth rates of 0.1ML/s − 0.4ML/s on (111) misoriented substrates compared with a decreasing number density as low as 2.85 × 1010cm−2 corresponding to a growth rate of 0.4ML/s grown on (110) misoriented substrates. p-i-n solar cell devices with a 10-layer quantum dot super- lattice imbedded in the i-region were also grown on (100) GaAs substrates misoriented 0◦, 2◦ and 6◦ toward (110) as well as a set of devices grown on substrates misoriented toward (111). Device results showed a 1.0mA/cm2 enhancement to the short-circuit current for a v 2◦ misoriented device with 2.2ML InAs coverage per quantum dot layer. Spectral response measurements were performed and integrated spectral response showed sub-GaAs bandgap short-circuit contribution which increased with increasing InAs coverage in the quantum dot layers from 0.04mA/cm2/ML, 0.28mA/cm2/ML and 0.19mA/cm2/ML corresponding to 0◦, 2◦ and 6◦ misorientation, respectively.

The second focus of this study was on the OMVPE growth of InAs quantum dots in a large-area commercial reactor. Quantum dot growth parameters require careful balancing in the large-scale reactor due to different thermodynamic and flow profiles compared with smaller- area reactors. The goal of the work was to control the growth process in order to produce high densities of uniform quantum dots for inclusion in double and triple junction III − V material solar cells. Initial growth proved unsuccessful due to lack of familiarity with the process but through balancing of injector flows of alkyl gasses, coherent and optically active quantum dots were able to first be formed at low densities (0.5 − 0.7 × 1010cm−2). Further optimization included increased quantum dot growth times leading to number densities in the (2.1−2.7×1010cm−2) with improved optical performance as measured by photoluminescence (PL) spectroscopy. Finally, an investigation of GaAs spacer layer thickness for improved opti- cal coupling was performed, indicating that a combined low temperature and high temperature GaAs thickness of 9.3nm led to strong PL intensity indicating good optical coupling of QD layers. Ge/(In)GaAs double junction solar cells were grown and fabricated with and without quantum dots in the (In)GaAs cell to investigate the effect of quantum dot inclusion on device performance. AM 0 measurements showed an average increase of 1.0mA/cm2 in short-circuit current for these devices. Integrated spectral response measurements revealed a contribution to vi short-circuit current of 0.02mA/cm2/QDlayer which is consistent with reports seen in literature. The current improvement for the double junction solar cells motivated the investigation of quantum dot inclusion in the (In)GaAs junction of a Ge/(In)GaAs/InGaP triple junction solar cell. AM0 measurements on these cells did not reveal any increase in current for quantum dot enhanced devices over a baseline device. Integrated spectral response for each junction revealed an increase of 0.3mA/cm2 in current for the middle junction and the top junction, respectively, compared with baseline results for these junctions, but also that the InGaP top junction was current limiting. This potentially is due to poor material quality in the InGaP junction as a result of quantum dot inclusion in the junction beneath it or to strain effects re- sulting from quantum dot inclusion. This current limiting nature of the top junction may have led to a reduced efficiency for quantum dot devices compared with a baseline and further opti- mization is required in order increase the efficiency of the quantum dot device compared with a baseline device.

Library of Congress Subject Headings

Solar cells--Materials; Quantum dots; Indium arsenide

Publication Date

8-2013

Document Type

Thesis

Student Type

Graduate

Degree Name

Materials Science and Engineering (MS)

Advisor

Seth M. Hubbard

Advisor/Committee Member

David V. Forbes

Advisor/Committee Member

Michael S. Pierce

Comments

Physical copy available from RIT's Wallace Library at TK2960 .P64 2014

Campus

RIT – Main Campus

Plan Codes

MSENG-MS

Download

Share

COinS