Tin Based Absorbers for Infrared
Detection, Part 2
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
Presented By: Justin Markunas
Recap
•Attempting to use a-phase tin for IR
detection
•Bandgap separation achieved by growing
a thin film layer
• a-phase/b-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 a-phase tin (lattice matched
materials)
Si1-xSnx Alloys
Motivations:
Si Read-Out Circuitry
•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
Contact
Metallization
In Bump Bond
HgCdTe Detector Array
CdZnTe Substrate
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
Si1-xSnx Quantum Dots
Solution:
Si Cap Layer: 14nm
Si1-xSnx: 1-4nm
Si Buffer Layer
•Grow thin Si1-xSnx layers on Si by MBE
(1-4 nm thick)
Si Substrate
•Attempted x-values: .05 - .2
•Growth performed at 170°C
Anneal
•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 Cap Layer: 14nm
Sn quantum dots
Si Buffer Layer
Si Substrate
TEM Analysis
Cross-sectional bright field TEM images
shown
•2nm thick Si.95Sn.05 layer
•Annealed at 800°C for 30 minutes
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
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 q>qc
•Number of passes through Sn layer:
N
l
t cot q
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
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
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