Fluence–dependent lifetime variations in neutron irradiated MCZ Si measured by microwave probed photoconductivity and dynamic grating techniques E.Gaubas, A.Kadys, A.Uleckas, and J.Vaitkus Vilnius university, Institute of Materials Science and Applied Research, Sauletekio av. 10, LT-10223, Vilnius, Lithuania Outline  Objectives of investigations  Setup of MWR and DG experiments  Fluence and heat-treatment dependent lifetime variations  Characteristics of lifetime cross-sectional profiles  Summary

Objectives of investigation - Direct measurements of recombination lifetime:  combined investigations of of MWR and DG applied in the range of the highest fluence; - Control of possible anneal of defects:  heat treatments 80C 5 min, 30 min and 24 h,- WODEAN standard; - Cross-sectional scans within wafer depth to control of oxygen out-diffusion

Measurement techniques and instruments Microwave probed photoconductivity (MW-PCD) in MW reflection mode (MWR) Dynamic gratings (DG) Diffraction efficiency (  =I -1 /I 0 ) on light induced dynamic grating is a measure   (  N) 2 of excess carrier density, while its variations in time  (t)  exp(-2t/  G ) by changing a grating spacing (  ) enable one to evaluate directly the parameters of grating erase 1/  G = 1/  R + 1/  D through carrier recombination (  R ) and diffusion  D =  2 /(4  2 D) with D as a carrier diffusion coefficient. K.Jarasiunas, J.Vaitkus, E.Gaubas, et al. IEEE Journ. QE, QE-22, (1986)  (t) MWR >100  m   0 =(4  /c  )  dc, transient:  (t)    (t)   FC n exFC (t) E.Gaubas. Lith. J. Phys., 43 (2003) 145. The microwave probed photoconductivity (MW- PCD) technique is based on the direct measurements of the carrier decay transients by employing MW absorption by excess free carriers. Carriers are photoexcited by 1062 nm light generated by pulsed (700 ps) laser and probed by 22 GHz cw microwave probe.

Direct measurements of recombination lifetime by MWR   R   t| U ~ exp(-1)   R  g exc  Rs /g exc  RL (U MWRs 5 ns )

Direct measurements of recombination lifetime by DG D (cm 2 /s)  R (ps) 15.4   3 D (cm 2 /s)  R (ps) 15.4   10 1  n/cm 2 3  n/cm 2

Recombination lifetime in wafer and diode samples measured by MWR

Lifetime under heat treatments at 80C for 5 min, 30 min and 24 h Lifetime variation with neutron irradiation fluence in MCZ Si 61,62,63,64, 54,56,58, and 60 wafers for the as-received and heat treated material

Cross-sectional scans within wafer depth fiber MW coaxial needle-tip antenna sample

SUMMARY  Lifetime decreases from few  s to about of 200 ps with enhancement of neutron irradiation fluence ranging from to 3  n/cm 2, as measured directly by exploiting microwave probed photoconductivity transients and verified by dynamic grating technique.  Lifetime values are nearly the same for wafer and diode samples.  Small increase of lifetime values under annealing can be implied.  Lifetime values are nearly invariable within wafer thickness, as determined from the lifetime cross-sectional scans within wafer depth.  Investigation of the lifetime – temperature variations in the range of more than RT is anticipated, to determine the activation factors for the dominant recombination centers. However, annealing and instability of defects would be a problem for precise extraction of these parameters.

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