The steady and transient photoconductivity, and related phenomena in the neutron irradiated Si.

1. The steady and transient photoconductivity, and related phenomena in the neutron irradiated Si. J.Vaitkus, E.Gaubas, A.Kadys, V.Kalendra, V.Kazukauskas, A.Mekys, J.Storasta, E.Zasinas Vilnius University, Institute of Materials Science and Applied Research J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

2. Outline: • A few general words about the photoconductivity & related transport phenomena • The results: • general data • the peculiarities • Attempts to conclude J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

3. Definition: If the sample is uniform, then the chosen effects:photoconductivity, light induced transient gratings, Hall and photo-Hall effects, magnetoresistance effects allow to characterize the behavior of free carriers and the local levels in the sample and their parameters . The inhomogegenities differently influence these effects, therefore the complex application gives a possibility to recognize what happens in the sample after a certain treatment. J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

4. EC EM  ET  EOpt EV  EOpt  ET Photoconductivity Photoconductivity spectra allows to identify the deep level andcharacterize a role of electron-phonon interaction in its environment. The ionizing energy of deep centre in thermal and optical experiments are different J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

5. Spectra in irradiated Si J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

6. EC ET ER EV Transitions J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

7. Intrinsic photoconductivity According to the classical theory of photoconductivity spectral dependence (T.Smith. Semiconductors. 1959) Where jpis the photocurrent, jpis the photocurrent independent on the light aborption coefficient, s – surface recombination velocity, pfree carrier lifetime. This expression that can be transformed into the relation A=b+ck. s Recombination rate J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

8. Si 3 1016 cm-2 and summary • Recombination at the surface mush faster; • 2. Induced PC from deep levels ~0.5 and 0.62-4 eV J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

9. Steady state lifetime dependence on fluence The lifetime extracted from the peak photocurrent dependence on the irradiation by neutrons depends as a square root from the fluence. It can be proposed that it means that the lifetime depends on the distance between the defects in the plane. (50 V bias). J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

10. Extrinsic photoconductivity photoresponse @1200 meV @ 18 K inverse peak value of photo-conductivity - effect of saturation of the level 0.82 eV inverse photoresponse value (steady state lifetime) (~to RC1 concentration) inverse lifetime by MW (~to RC2 concentration) At high centre concentration the two step PC can appear, then signal becomes dependent on the lifetime J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

11. Transient photoconductivity (TP) Depends on free carrier concentration, mobility, internal electric field distribution • Light pulse excitation • TP can be measured by: • DC circuit(contact problems) • Microwave technique • Free carrier absorption(~ non-sensitive to mobility) • Transient gratings: measure free carrier concentration profile amplitude, i.e. allows to measure concentration decay law and diffusitivity, separately. • TP gives information about recombination cannels and traps J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

12. Il sample I-1 I I0 L I+1 Transient gratings • Two features: • The decrease of generated by the light pulse carrier concentration with the fluence. • The decay of the signal may consist two versions: • A simple decay with one time constant; • The decay can be separated into the two components (fast a few ns and a “slow” – tens of ns or more) J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

13. Transient gratings The recombination channel which causes the capture time less than 20 ps. We plan direct measurement this trapping with fs pulses A peculiarity of this capture: These levels (we propose – the disordered region of cluster) it becomes filled and the remaining e-h pairs participate in the recombination and ambipolar diffusion. J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

14. Transport phenomena: Hall, photo-Hall, magnetoresistance effects • Comparison Hall and magnetoresistance effects – a role of inhomogeneity • Temperature dependence of mobility – depends on scattering mechanism J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

15. Hall mobility and magnetoresistance vs temperature J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

16. Hall mobility, annealing, excitation effects J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

17. Conductivity vs T J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

18. Photoconductivity decay J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

19. Compare the dependences on fluence the data measured by photoconductivity and by microwaves technique The lifetime measured by microwave technique linearly depends on the fluence while from the photoconductivity – as a square root of fluence!!! It shows that the recombination process is more complicated but the MW measurements were performed at room temperature, and PC – at 18 K (no retraping). J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

20. Conclusions: 1. Spectral dependences, different transient phenomena reveals the details of the roles of radiation induced changes in semiconductor. 2. It seems that clusters can be seen by a specific trapping and by the appearance of percolation. 3. A lot of work ahead … J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

21. Thanks for Your attention !!! J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

22. The peculiarities of dark conductivity, photoconductivity and other transport phenomena were investigated in the neutron irradiated Si. The photoconductivity mechanism, observed deep levels and the effects related to the nano- and micro- defects are discussed. • At low temperature (18K) • The measurement pf photoconductivity spectral dependence in the high absorption region shows the decrease of photoconductivity related with an increase of recombination at the surface, but the comparison with a standard surface velocity model show that the main recombination is going not at the surface, but in the layer near to surface. Probably there is different doping of the bulk and the layer near to surface. • The photoconductivity at the absorption edge linearly depends on the intensity of light that shows that in the investigated range of intensities on type of recombination canters plays the main role. The steady state lifetime depends as a square root on the fluence. The microwave date showed its linear dependence, therefore it is necessary to predict the more complicated process of recombination. It could be proposed that this peculiarity is related with the very fast recombination process observed in the measurements with the picosecond resolution. • The extrinsic photoconductivity shows two different regions: on – clearly expressed impurity photoconductivity band from the level at 0,8-0,9 eV that at low fluence linearly depends on the fluence and saturate at high fluence. The second band (level ~0,5-0,6 eV) is caused by the induced photoconductivity by capture of carriers generated from the deeper level or the intrinsic photogeneration. These levels can be related with clusters because they demonstrate the accumulation effect that cause the partial compensation of the deeper level (the impurity photoconductivity becomes linear to the excitation). • Near to the room temperature and down to the liquid nitrogen temperature. • The measurement of Hall effect mobility by Hall effect and by magnetoresistance shows more or less the same data in the lowest fluence sample. For higher fluences the difference becomes high that can be explained only by model of inhomogeneous sample and percolation type of conductivity. The data of photo-Hall effect support this model. The heating cycle up to 80 C shows the significant change of the percolation and the important role of the overlap of the space charge regions in the bulk of sample. The clusters with the effective charge of a few electron charge was evaluated. J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

23. Si bandgap f(T)_ J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

24. J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

25. Dark current vs T J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06