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

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)

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

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:

•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

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 >c

•Number of passes through Sn layer:

cott

lN

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