Defect chemistry in GaAs studied by two-zone annealings under defined As vapor pressure. Outlook:

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1 Defect chemistry in studied by two-zone annealings under defined vapor pressure V. Bondarenko 1, R. Krause-Rehberg 1, J. Gebauer 2, F. Redmann 1 1 Martin-Luther-University Halle-Wittenberg, Halle, Germany 2 Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 9720, USA Introduction Thermodynamics of Role of positron annihilation Experimental Results and discussion Outlook: Vacancy concentration as function of stoichiometry Vacancy origin and charge state

2 Introduction Market continues growing remains an important material for production of semiconductor devices Decrease in ICs production is compensated by the increase in A (ower Amplifiers) modules for wireless local-area Networks MMIC for automotive radars Detailed knowledge on native point defects formation is crucial in engeneering of electrical properties

3 Defects concentrations Thermodynamics of Defect chemistry evaluate the equilibrium defect concentrations as function of doping, temperature and chemical potential (stoichiometry) Major achievments of thermodynamic analysis of Demonstration of the key role of V + for dopant solubility in and of V m- for annealing and diffusion of n-doped prediction of negative T-dependence for V (Fermi-level effect) Role of ositron annihilation Experimental proove of Fermi-level effect D.T.J. Hurle, Journ. of Appl. hys. 85 (1999) T.Y.Tan et al, Appl. hys. A 56 (1993) Determination of formation enthalpy and entropy of the uncharged V J. Gebauer et al., hysical Review B 67 (2003)

4 Thermodynamic model of undoped Native point-defects 1 six native defects formation is described by six thermodynamic massaction law reactions: vap, vap I I + + I I I I 1 ( T ) I K ( T ) = 2 + V K ( T ) = [ I 3 + V K ( T ) = [ I K + + I I [ I = K K 5 6 ] 1/ ][ V ][ V ] ] [ ][ I ( T ) = [ I ] [ ][ I ( T ) = [ I ] ] ] [ I [ I [ V [ V [ [ ] ] ] ] ] ] 1/ 1/ 1/ 1/ 1/ 2 1/ 2

5 Experimental Annealing in a two-zone oven at 1100 C High-purity quartz ampoule ([Cu] < 0.02 ppm) 2 hours heating Sample T: 1100 C T Defines ressure Quenching into the water at room temperature ALS measurements at K

6 Results: ositron lifetime spectroscopy Si-doped Vacancy + shallow traps τ 2 = 260±5 ps Si V defect complex Si as shallow trap [Si V ] increases with Average positron lifetime (ps) :Si [Si] = cm -3 druck (bar): Substrate increasing p Semi-insulating Vacancy + shallow traps Origin unknown τ 2 = 293±10 ps; I 2 = 0-70% Reciprocal dependence [Vacancy] - p Average positron lifetime (ps) Measurement temperature (K) pressure (bar): reference Measurement temperature (K)

7 Results: Hall-effect measurements (SI ) Hall-measurements at room temperature All SI samples became p-type No correlation between and [p], bar [p], cm Neutral vacancy defect Temperature-dependent measurement Acceptor level: E A = E V ev Usually attributed to Cu acceptor µ, cm 2 /Vs ρ, Ωcm Act as positron shallow trap defects

8 Discussion: evidence of V in annealed SI p concentration dependence Concentrations were determined at RT with µ = s -1 Different slopes for :Si and SI Different vacancy sublattices Thermodynamic reactions V : 1/ gas + V [V ] = K V p 1/ V : V + 1/ gas [V ] = K V p -1/ Concentration (cm -3 ) µ=10 15 s SI- :Si Copper 0,01 0, Arsenic pressure (bar) Fit: ln[v] = n ln(p ) :Si: n = 0.25±0.03 SI : n = - 0.2±0.02

9 Discussion: charge state of V Theoretical calculations for an arsenic vacancy ionisation energies are lying in the upper part of the band gap V are positive in SI or p-type

10 Discussion: charge state of V -defect Hall measurements Showed no correlations with V concentrations and hence imply they are in the neutral charge state ositron lifetime spectroscopy Strong supression of uncontrolled contamination by advanced annealing procedure yielded The plateau at K is 250 a clear sign of neutral vacancy The plateau is not seen in case 25 of higher concentration of shallow traps 20 [p]~10 9 cm -3 Average positron lifetime (ps) 235 SI T Ann = 1100 C p = 0.2 bar Measurement temperature (K)

11 Discussion: formation energy of V Concentration of neutral V [ V 0 ] = K V / p 1/ B gas pressure constant G f = H f TS f 1/ = ( B / p ) exp[ ( H TS ) / kt] f f Gibbs formation energy 1 B 2 3 / 2 5 / 2 = ( 2πm / h ) ( kt) 131.5T = 5 / 2 Fitting to the concentrations G f as only fitting parameter G f = 3.9 ± ev V concentration /10 16 (cm -3 ) G f =3.9 ev Arsenic pressure (bar) 8 10

12 Summary ositron annihilation + annealing experiments Formation of monovacancy-like defects at 1100 C in SI τ 2 = 293 ± 10 ps Shallow traps Hall-measurements Vacancy-like defect is neutral Cu acceptor-like impurity, acting as the positron shallow traps E A = E V ev ressure dependence Arsenic vacancy V is observed Gibbs free energy of formation of the V was obtained: G f = 3.9 ± ev