David Jackrel - PhD Orals Announcement

David Bryan Jackrel djackrel at stanford.edu
Thu Jul 21 19:35:57 PDT 2005



"InGaAs and GaInNAs(Sb) 1064 nm Photodetectors and Solar Cells"

Speaker:          David Jackrel
Research Advisor: Professor James S. Harris, Jr.

Date / Time:      Wednesday, July 27th 2005, 2:00 PM
Location:         CIS-X Auditorium


The dilute-nitrides GaInNAs and GaInNAsSb show great promise in becoming
the next choice for 1064 nm photodetectors and multijunction solar cells
because these materials can be grown lattice-matched to GaAs and Ge
substrates. One application where 1064 nm photodetectors with superior
properties are required is LIGO, The Laser Interferometer Gravitational
Wave Observatory. LIGO is designed to be one of the first instruments
capable of detecting gravitational waves from astrophysical sources, such
as black holes and neutron stars. The ability to tune the bandgap of the
dilute nitrides between 1.0 eV and 1.4 eV while maintaining the lattice
constant of GaAs (or Ge) also makes them ideal candidates for the second
smallest junction in multijunction solar cells grown on Ge substrates. The
current solution for growing 1 eV materials on GaAs or Ge substrates is to
grow lattice-mismatched (metamorphic) InGaAs using a graded buffer layer;
however, these structures are plagued by higher threading dislocation
densities and rough surfaces.

In this thesis, metamorphic InGaAs, lattice-matched GaInNAs and GaInNAsSb,
p-i-n double heterostructures grown on GaAs substrates by molecular beam
epitaxy with a RF-plasma nitrogen cell are presented. GaInNAs structures
grown with and without deflection plates on the nitrogen source, which
serve to protect the growing surface from ion damage caused by the high
energy plasma, are also compared. The lattice constants and film
relaxation are investigated by high-resolution x-ray diffraction, the
dislocation density is determined through spectral cathodoluminescence
imaging, and the bandgap of the materials is established by
photoluminescence. The structures were processed into back-illuminated
photodetectors, which have their absorbing layer less than 1 micron from
the heat sink to improve damage threshold, and solar cells. As a result of
this work, the first dilute nitride rear-illuminated 1 eV photodiodes and
the first GaInNAsSb solar cells have been produced.

GaIn(N)As(Sb) photodiode and solar cell properties can be summarized as
follows. The internal quantum efficiency (IQE) of back-illuminated GaInNAs
and GaInNAs(Sb) photodiodes is somewhat lower than comparable metamorphic
InGaAs devices due only to a thinner absorbing region (IQE is 60% and 75%,
respectively, and absorbing layers are 1 micron and 2 microns,
respectively). Recombination loss between the dilute nitride and
metamorphic devices is similar. Likewise, the efficiency of the dilute
nitride photodiodes with and without Sb is also similar. The device
efficiencies are limited by free-carrier absorption in the substrate,
which was thinned to 100 microns but not completely removed. If this loss
were eliminated, device efficiency would increase to 90% and 75% for the
InGaAs and GaInNAs(Sb) devices respectively, which indicates that both
materials systems could yield photodiodes with properties comparable with
commercially available InP-based detectors. The solar cell study is
focused on the dilute nitride materials. All aspects of the GaInNAs solar
cell performance are improved drastically when the deflection plates are
employed during growth. Additionally, the GaInNAsSb solar cells show
substantially higher internal quantum efficiency (79%) than the GaInNAs
devices grown using the deflection plates (67%), but reduced power
conversion efficiency due to depressed fill-factor and open-circuit
voltage. These results are encouraging however, and suggest that with
further optimization it may be possible to improve dilute nitride solar
cell properties through the incorporation of antimony.

David Jackrel                                           djackrel at stanford.edu
Dept. of Materials Science and Engineering
Stanford University

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