tin based absorbers for infrared detection, part 2
DESCRIPTION
Tin Based Absorbers for Infrared Detection, Part 2. Direct energy gap group IV semiconductor alloys and quantum dot arrays in Sn x Ge 1-x /Ge and Sn x Si 1-x /Si alloy systems Regina Ragan, Kyu S. Min, Harry A. Atwater - PowerPoint PPT PresentationTRANSCRIPT
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Tin Based Absorbers for Infrared Detection, Part 2
Presented By: Justin Markunas
Direct energy gap group IV semiconductor alloys and quantum dot arrays in SnxGe1-x/Ge and SnxSi1-x/Si alloy systems
Regina Ragan, Kyu S. Min, Harry A. Atwater
Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, MS 128-95, Pasadena, CA 91125, USA
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Recap
•Attempting to use -phase tin for IR detection
•Bandgap separation achieved by growing a thin film layer
• -phase/-phase transition temperature raised by pseudomorphic epitaxial growth
•For necessary absorption and correct bandgap, superlattices required
•Both CdTe and InSb failed as superlattice materials with -phase tin (lattice matched materials)
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Si1-xSnx Alloys
Motivations:
•Many advantages of growing on a silicon substrate
•Cost considerations•Thermally compatible to read-out circuitry
•Si1-xSnx predicted to become direct bandgap for x > .9
HgCdTe Detector Array
CdZnTe Substrate
Si Read-Out Circuitry
In Bump BondContactMetallization
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Si1-xSnx Alloys
Drawbacks:
•Mismatch between Si and Sn is large (aSi= 5.43 Å aSn= 6.48 Å)•19.5% mismatch •Makes pseudomorphic growth nearly impossible
•Solubility of Sn in Si is low (~5x1019 cm-3)•Results in an x-value ~.01•This changes Si electronic band structure very little
•Surface segregation occurs under normal MBE growth conditions
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Si1-xSnx Quantum Dots
Solution:
•Grow thin Si1-xSnx layers on Si by MBE (1-4 nm thick)
•Attempted x-values: .05 - .2•Growth performed at 170°C
•Anneal sample at 500 – 800°C •Si1-xSnx layer segregates and forms Sn quantum dots•Quantum confinement effects of dots create a nonzero Sn bandgap
Si Buffer Layer
Si Substrate
Si Cap Layer: 14nm
Si1-xSnx: 1-4nm
Anneal
Si Buffer Layer
Si Substrate
Si Cap Layer: 14nm
Sn quantum dots
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TEM Analysis
Cross-sectional bright field TEM images shown
•2nm thick Si.95Sn.05 layer•Annealed at 800°C for 30 minutes
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TEM Analysis
Plan-view bright field TEM images shown•2nm thick Si.9Sn.1 layer•One sample annealed at 500°C for 3 hours•Another at 800°C for 30 minutes
Results:•Phase separation evident in as-grown film•Sample annealed at 500°C shows formation of quantum dots with gradually varying background contrast•Sample annealed at 800°C results in larger dots with little variation in background contrast
RBS Result:•Dot composition was estimated to be pure Sn
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IR Absorption
Key unknown: •Which allotrope of Sn the dots are composed of•Can determine by taking IR absorption spectrum
Measurement Setup:•Shape sample into a trapezoid•Measurement taken by a FTIR spectrometer•Incident radiation at angle >c
•Number of passes through Sn layer:
cott
lN
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IR Absorption
Results from a 2nm Si.9Sn.1 sample :•Eg ~ .27eV•Absorption doubles after annealing the sample at 800°C •Absorption is consistent with direct interband transitions
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Dot Growth
Measurement:•Anneal a Si1-xSnx sample at 650°C and plot dot size as time elapses
Results:•Dots trend to larger sizes and lower density as time progresses
Growth Mechanisms:•Before annealing: decomposition of Si1-xSnx and nucleation of Sn nanocrystals•After annealing: coarsening occurs, where larger dots grown at the expense of smaller ones
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Conclusions
•Sn quantum dots in Si have been fabricated and shown to absorb IR radiation
•Bandgap adjusted by controlling dot size
•Still many issues to resolve before making a detector•Dot size controllability•Doping•Absorber thickness