skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: EXAMINATION OF DISLOCATIONS IN LATTICE-MISMATCHED GaInAs/BUFFER LAYER/GaAs FOR III-V PHOTOVOLTAICS

Abstract

Dislocations act as sites for nonradiative electron/hole pair recombination, which reduces the effi ciency of photovoltaics. Lattice-matched materials can be grown on top of one another without forming a high density of dislocations. However, when the growth of lattice-mismatched (LMM) materials is attempted, many dislocations result from the relaxation of strain in the crystal structure. In an attempt to reduce the number of dislocations that propagate into a solar device when using LMM materials, a compositionally step-graded buffer is placed between the two LMM materials. In order to confi ne the dislocations to the buffer layer and therefore increase material quality and device effi ciency, the growth temperature and thickness of the buffer layer were varied. A GaInP compositionally graded buffer and GaInAs p-n junction were grown on a GaAs substrate in a metal-organic chemical vapor deposition (MOCVD) system. A multibeam optical stress sensor (MOSS) and X-ray diffraction (XRD) were used to characterize the strain in the epilayers. Electrical and optoelectronic properties were measured using a probe station and multimeter setup, solar simulator, and a quantum effi ciency instrument. It was determined that device functionality was highly dependent on the growth temperature of the graded buffer. As growth temperature increased,more » so did the dislocation density in the device despite an increase in the dislocation velocity, which should have increased the dislocation annihilation rate and the diffusion of dislocations to the edge of the crystal. The thickness of the graded buffer also affected device effi ciency with thinner samples performing poorly. The thinner graded buffer layers had high internal resistances from reduced carrier concentrations. In terms of effi ciency, the empirically derived recipe developed by the scientists at the National Renewable Energy Laboratory (NREL) produced the highest quality cells.« less

Authors:
;
Publication Date:
Research Org.:
DOESC (USDOE Office of Science (SC) (United States))
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1051828
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Undergraduate Research; Journal Volume: 7
Country of Publication:
United States
Language:
English

Citation Formats

Levander, A., and Geisz, J. EXAMINATION OF DISLOCATIONS IN LATTICE-MISMATCHED GaInAs/BUFFER LAYER/GaAs FOR III-V PHOTOVOLTAICS. United States: N. p., 2007. Web.
Levander, A., & Geisz, J. EXAMINATION OF DISLOCATIONS IN LATTICE-MISMATCHED GaInAs/BUFFER LAYER/GaAs FOR III-V PHOTOVOLTAICS. United States.
Levander, A., and Geisz, J. Mon . "EXAMINATION OF DISLOCATIONS IN LATTICE-MISMATCHED GaInAs/BUFFER LAYER/GaAs FOR III-V PHOTOVOLTAICS". United States. doi:. https://www.osti.gov/servlets/purl/1051828.
@article{osti_1051828,
title = {EXAMINATION OF DISLOCATIONS IN LATTICE-MISMATCHED GaInAs/BUFFER LAYER/GaAs FOR III-V PHOTOVOLTAICS},
author = {Levander, A. and Geisz, J.},
abstractNote = {Dislocations act as sites for nonradiative electron/hole pair recombination, which reduces the effi ciency of photovoltaics. Lattice-matched materials can be grown on top of one another without forming a high density of dislocations. However, when the growth of lattice-mismatched (LMM) materials is attempted, many dislocations result from the relaxation of strain in the crystal structure. In an attempt to reduce the number of dislocations that propagate into a solar device when using LMM materials, a compositionally step-graded buffer is placed between the two LMM materials. In order to confi ne the dislocations to the buffer layer and therefore increase material quality and device effi ciency, the growth temperature and thickness of the buffer layer were varied. A GaInP compositionally graded buffer and GaInAs p-n junction were grown on a GaAs substrate in a metal-organic chemical vapor deposition (MOCVD) system. A multibeam optical stress sensor (MOSS) and X-ray diffraction (XRD) were used to characterize the strain in the epilayers. Electrical and optoelectronic properties were measured using a probe station and multimeter setup, solar simulator, and a quantum effi ciency instrument. It was determined that device functionality was highly dependent on the growth temperature of the graded buffer. As growth temperature increased, so did the dislocation density in the device despite an increase in the dislocation velocity, which should have increased the dislocation annihilation rate and the diffusion of dislocations to the edge of the crystal. The thickness of the graded buffer also affected device effi ciency with thinner samples performing poorly. The thinner graded buffer layers had high internal resistances from reduced carrier concentrations. In terms of effi ciency, the empirically derived recipe developed by the scientists at the National Renewable Energy Laboratory (NREL) produced the highest quality cells.},
doi = {},
journal = {Journal of Undergraduate Research},
number = ,
volume = 7,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • High-quantum-efficiency, lattice-mismatched, 1.0-eV GaInAs solar cells grown by organometallic vapor phase epitaxy have been developed for ultimate integration into AlGaAs/GaAs/GaInAs 3-junction, 2-terminal monolithic devices. The more standard n/p junction was replaced with an n-i-p structure in the GaInAs cell in order to increase the short-circuit current by overcoming the material deficiencies which arise as a result of accommodating the lattice mismatch. This led to single junction 1.0-eV GaInAs cells with internal quantum efficiencies >90% and short-circuit-current densities that match or closely approach those needed to current match the upper AlGaAs and GaAs cells. A 4.1% (1-sun, air mass 0, 25[degrees]C)more » power conversion efficiency was achieved with a developmental structure, indicating the potential of the lattice-mismatched n-i-p 1.0-eV GaInAs cell. An analogous device designed to allow direct monolithic integration with the upper AlGaAs and GaAs cells, with a modified grading layer of AlGaInAs in place of the usual GaInAs, achieved an efficiency of 2.2%, primarily due to a lower open-circuit voltage. The open-circuit voltage is perhaps limited by structural defects revealed in transmission electron micrographs. 26 refs., 7 figs., 2 tabs.« less
  • The project focused on development of new approaches and materials combinations to expand and improve the quality and versatility of lattice-mismatched (LMM) III-V semiconductor epilayers for use in high-efficiency multijunction photovoltaic (PV) devices. To address these goals, new capabilities for materials synthesis and characterization were established at SDSM&T that have applications in modern opto- and nano-electronics, including epitaxial crystal growth and transmission electron microscopy. Advances were made in analyzing and controlling the strain profiles and quality of compositional grades used for these technologies. In particular, quaternary compositional grades were demonstrated, and a quantitative method for characteristic X-ray analysis was developed.more » The project allowed enhanced collaboration between scientists at NREL and SDSM&T to address closely related research goals, including materials exchange and characterization.« less
  • The results of the analysis of the infrared lattice reflectance spectra of multiperiod ZnTe/CdTe superlattices with CdTe quantum dots are reported. The samples are grown by molecular beam epitaxy on the GaAs substrate with the ZnTe buffer layer. Due to the large number of periods of the superlattices, it is possible to observe CdTe-like vibration modes in the quantum dots, i.e., the dislocation-free stressed islands formed during the growth due to relaxation of elastic stresses between the ZnTe and CdTe layers are markedly different in their lattice parameters. From the frequency shifts of the CdTe- and ZnTe-like vibration modes withmore » respect to the corresponding modes in the unstressed materials, it is possible to estimate the level of elastic stresses.« less
  • The high balance-of-system costs of photovoltaic (PV) installations indicate that reductions in cell $/W costs alone are likely insufficient for PV electricity to reach grid parity unless energy conversion efficiency is also increased. Technologies which yield both high-efficiency cells (>25%) and maintain low costs are needed. GaAs and related III-V semiconductors are used in the highest-efficiency single- and multi-junction photovoltaics, but the technology is too expensive for non-concentrated terrestrial applications. This is due in part to the difficulty of scaling the metal-organic chemical vapor deposition (MOCVD) process, which relies on expensive reactors and employs toxic and pyrophoric gas-phase precursors suchmore » as arsine and trimethyl gallium, respectively. In this study, we describe GaAs films made by an alternative close-spaced vapor transport (CSVT) technique which is carried out at atmospheric pressure and requires only bulk GaAs, water vapor, and a temperature gradient in order to deposit crystalline films with similar electronic properties to that of GaAs deposited by MOCVD. CSVT is similar to the vapor transport process used to deposit CdTe thin films and is thus a potentially scalable low-cost route to GaAs thin films.« less