1. Tin Based Absorbers for Infrared Detection, Part 2 Presented By: Justin Markunas 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 Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, MS 128-95, Pasadena, CA 91125, USA

2. 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)

3. Si 1-x Sn x Alloys Motivations: Many advantages of growing on a silicon substrate Cost considerations Thermally compatible to read-out circuitry Si 1-x Sn x predicted to become direct bandgap for x >.9 HgCdTe Detector Array CdZnTe Substrate Si Read-Out Circuitry In Bump Bond Contact Metallization

4. Si 1-x Sn x Alloys Drawbacks: Mismatch between Si and Sn is large (a Si = 5.43 Å a Sn = 6.48 Å ) 19.5% mismatch Makes pseudomorphic growth nearly impossible Solubility of Sn in Si is low (~5x10 19 cm -3 ) Results in an x-value ~.01 This changes Si electronic band structure very little Surface segregation occurs under normal MBE growth conditions

5. Si 1-x Sn x Quantum Dots Solution: Grow thin Si 1-x Sn x 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 Si 1-x Sn x 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 Si 1-x Sn x : 1-4nm Anneal Si Buffer Layer Si Substrate Si Cap Layer: 14nm Sn quantum dots

6. TEM Analysis Cross-sectional bright field TEM images shown 2nm thick Si.95 Sn.05 layer Annealed at 800°C for 30 minutes

7. TEM Analysis Plan-view bright field TEM images shown 2nm thick Si.9 Sn.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

8. 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:

9. IR Absorption Results from a 2nm Si.9 Sn.1 sample : E g ~.27eV Absorption doubles after annealing the sample at 800 ° C Absorption is consistent with direct interband transitions

10. Dot Growth Measurement: Anneal a Si 1-x Sn x 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 Si 1-x Sn x and nucleation of Sn nanocrystals After annealing: coarsening occurs, where larger dots grown at the expense of smaller ones

11. 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