Thermal Strain Effects in Germanium Thin Films on Silicon Travis Willett-Gies Nalin Fernando Stefan Zollner

Outline Introduction Strain Calculation Technique: Spectroscopic Ellipsometry Results Future Studies

Germanium Thoroughly studied group-IV semiconductor Many applications in space-based electronics, e.g.: High frequency transistors, solar cells, optical detectors Knowledge of the T dependence of the optical constants is vital for reliability and design Using spectroscopic ellipsometry, we can determine the optical properties of Germanium as a function of temperature and strain

E1E1 E1+Δ1E1+Δ1 E0’E0’ E2E2 Ge Band Structure Bauer et al., SSC 127 (2003) Chelikowsky et al., PRB 14 (1976) Optical constants directly related to the T-dependence of electronic states. Ge on Si(100) 4

Strained Films

At 300 (K) Thermal expansivity α L (K -1 ) Lattice parameter (Å) Ge5.80 x Si2.56 x Roucka et al., PRB 81, (2010) Ge Films on Si(100) Substrate at growth temperature T g = 700 K Ge ε = 0 Dislocations T = 100 K Ge; ~2α Si; α Si Commensurate growth (Pseudomorphic) Incommensurate growth (relaxed at growth temperature) Ge on Si (100) substrate 6

Thermal expansion mismatch of Ge and Si 7 Ge Si Reeber and Wang, Material Chemistry and Physics 46, 259 (1996)

Strain on Ge films Grown on Si (100) Cannon et al., Appl. Phys. Lett. 84, 906 (2004) C 11, C 12 – elastic constants of Ge In-plane strain Out-of-plane strain Theoretical strain calculations Hydrostatic strain 8

Strain on Ge films Grown on Si (100) Comparison with X-ray Diffraction 9

Critical Point Shift due to Strain in Ge Layer “Silicon-Germanium Carbon Alloys, Growth, Properties and Applications”, S. Pantelides and S. Zollner, Chapter 12 E 1 CP energy shifts due to strain: (E 1 +∆ 1 ) CP energy shifts due to strain: ∆ 1 - spin orbit splitting (=198 meV) assumed to be constant 10

Technique

Instrument

Fundamentals of Ellipsometry Measure the change in polarization state of light Transform to obtain the dielectric function or index of refraction 13

In this work we are particularly concerned with ε 1 and ε 2 from which we determine critical point parameters. Using a cryostat attachment, we measure these parameters as a function of temperature. Temperature-dependent Spectroscopic Ellipsometry E1E1 E1+Δ1E1+Δ1 E0’E0’ E2E2 Ge on 300 K 14

Instrumentation Issues Light Source Temperature Measurement 15

Light Sources 16

Light Sources 17

Temperature Measurements Turbo Pump torr (UHV) LN2 Arm 77K Heater Control 800K Heater and built-in TC up here Second Thermocouple behind here 18

Thermocouple Comparison Built-in thermocouple reading (K) New thermocouple reading (K) 19

Results 20

77 K Result from transitions with momentum along. E 1, E 1 +∆ 1 : 2D critical points CP parameters; E- energy, A- amplitude Γ- broadening, φ- phase E1E1 E1+Δ1E1+Δ1 E0’E0’ E2E2 2 nd Derivative Analysis of ε 1 and ε 2 Near E 1 and E 1 +∆ 1 21 Vina et al., PRB 30, 1979 (1984) 21

Temperature Dependence of the CP Critical points shift to lower energies as T increases. Strain generated by thermal expansion causes a difference between Ge layer on Si and bulk Ge. E 1 +∆ 1 E1E1 E1E1 22

Energy Difference due to Strain 23 Very small temperature dependence of spin-orbit splitting ∆ 1. Experimentally observed energy difference 6-8 meV larger than the theory. May be caused by impurities and threading dislocations.

Summary 24 We used Spectroscopic Ellipsometry to determine the strain dependence of E 1 and E 1 +∆ 1 CP of Ge on Si. We modified the apparatus to provide a smoother, more accurate spectrum and more accurate temperature readings. Strain generated due to the thermal expansivity mismatch moves the E 1 and E 1 +∆ 1 CP to lower energies. A small difference (not due to strain) may be due to threading dislocations.

Future Work 25 Determine the cause of the difference between bulk and film Analyze GeSn alloys to observe the effect of tin in controlling the band-gap

Acknowledgements The New Mexico Space Grant consortium Air Force Office of Scientific Research: Grant # FA The ellipsometry group under Dr. Stefan Zollner

Thank You!