Vacancy-like defects in SI GaAs: post-growth treatment

Transcription

1 Vacancy-like defects in SI : post-growth treatment V. Bondarenko, R. Krause-Rehberg Martin-Luther-University Halle-Wittenberg, Halle, Germany B. Gruendig-Wendrock, J.R. Niklas TU Bergakademie Freiberg, Institut for Experimental Physics, Freiberg, Germany Motivation Results of previous investigations on n-type Undoped - positron annihilation results - defects identification Summary

2 Motivation Idea: investigation of the native point defects configuration in different equilibrium states Material: semi-insulating Continuation of the work done on n-type : :Si, :Te H. Wenzl et al., J. Cryst. Growth 109, 191 (1991).

3 Native point defects in Vapor Solid system has F = C P + 2 = 2 degrees of freedom Six native point defects demand six reactions: 1 vap, vap I I + + I I I I 1 ( T ) I K ( T ) = 2 P + V K ( T ) = [ I 3 + V K ( T ) = [ I K + + I I [ I = P K K 5 6 ] 1/ ( T ) ( T ) ][ V ][ V ] ] [ ][ I = [ I ] [ = [ I ][ I ] ] ] [ I [ I [ V [ V [ [ For given T ] ] ] ] P P P P ] ] 1/ 1/ 1/ 1/ P P 1/ 2 1/ 2

4 Scheme of the experiment Use of two-zone oven to control the samples temperature and pressure control two necessary degrees of freedom to fix the equilibrium state (T and P ) Sample T: 1100 C Ampoule: Cu-free quartz ([Cu] < 0.02 ppm) cleaned with 3HCl:1HNO 3 T Defines Pressure Annealing during 2 hours; Quenching into the water; Etching in 2% Bromine Methanol

5 Previous investigations :Si well-known Si V defect complex 260 :Si Average positron lifetime (ps) [Si] = cm -3 temperature: 700 C 660 C 600 C 566 C 500 C 66 C 00 C Substrate Measurement temperature (K) F.Redmann degree work (1999)

6 Previous investigations Si -V Te -V :Si :Te Vacancy concentration (cm -3 ) Linear fit 0,01 0, Arsenic pressure (bar) Thermodynamic reaction: 1/ gas + V Vacancy concentration (cm -3 ) [Te] in cm -3 6x x10 18 x x , Arsenic pressure (bar) τ av at 550 K (ps) J. Gebauer et al., Physica B , 705 (1999) Mass action law: [V ] = K VG p 1/ Fit: [V -Dopant] ~ p n n = 1/

7 Undoped Average positron lifetime (ps) Measurement temperature (K) pressure (bar): reference Defects: - Vacancy complex - Shallow traps Reciprocal dependence [Vacancies] pressure 2-component decomposition: τ 2 = 293±10 ps at 300 K I 2 = 0 70 % V -? complex

8 Defect identification: vacancy complex Concentration (cm -3 ) SI- :Si µ=10 15 s -1 Copper 0,01 0, Arsenic pressure (bar) Thermodynamic reaction: V + 1/ gas Mass action law: [V ] = K V p -1/ Fit: [V-complex] ~ p n n = -1/ vacancy Cu is the first candidate for the complex, due to unavoidable contamination - confirmed by titration and photoluminescence measurements

9 Hall measurements Carrier concentration (cm -3 ) 8x x x x10 11 x x x x10 11 hole concentration All the samples became p-type: p ~ cm -3 independent on the -pressure Mobility cm 2 /V s Arsenic pressure (bar) Observed vacancy complex is electrically inactive

10 Defect identification According to all theoretical calculations V are always positive in SI an p-type not visible for positrons

11 Doppler Coincidence measurements 1,2 1,1 :Si :Cu SI annealed at 0.2 bar Most popular candidates: V Cu V Cu V Cu V Cu Ratio to the bulk 1,0 0,9 0,8 0,7 0,6 but Cu cannot be the nearest neighbor in our case 0, p L (10-3 m 0 c)

12 Defect identification: shallow traps Temperature-dependent Hall-effect measurements Acceptor level: E A = E V ev usually attributed to copper defect complex Cu acts as an acceptor and as a shallow trap for positrons

13 Annealing and optical sensitivity of the defect Average positron lifetime (ps) Before illumination During illumination After illumination T measurement=266 K Annealing stage at about K During illumination with white light a certain fraction of defects is recharged Annealing temperature (K)

14 Average positron lifetime (ps) Average positron lifetime (ps) reference -Pressure (bar): 0,2 bar 2.62 bar 9.68 bar 1 Measurement temperature (K) 2 reference pressures (bar): Reproducibility Average positron lifetime (ps) pressure (bar): Re-Annealing Measurement temperature (K) In spite of bad reproducibility the reciprocal dependence on -pressure is clearly seen Possible reasons for the results deviations: uncontrolled copper contamination not the same cooling rate at each quenching Measurement temperature (K)

15 Re-annealing effect 2 pressure (bar): 0.2 Average positron lifetime (ps) Re-Annealing Experiment: 1. Quenching from 1100 C at 0.2 bar; 2. Annealing of defects at 600 С; 3. Second quenching from 1100 C at 0.2 bar Result: Positron signal disappears completely The samples became more p-type [p]~10 16 cm Measurement temperature (K) The defect complex is not seen due to the lowering of the Fermi level

16 Summary Defect concentration are defined by the equilibrium state of the system by means of mass action laws A reciprocal dependence of the vacancy-complex concentration on the pressure in SI was observed Such a dependence points to the V defect complex In spite of copper contamination observed the vacancy-like defect is not connected to the copper atom The exact nature of the observed complex can t be established from the positron annihilation data alone and is the matter of further investigations