Investigation of SiC by Positrons

Transcription

1 nd/march/000/erlangen Investigation of SiC by Positrons Atsuo KAWASUSO Martin-Luther-Universität Halle-Wittenberg (Humboldt Research Fellow) Japan Atomic Energy Research Institute Takasaki Establishment contents: - Positron Study of Solid Materials - Defects in SiC induced by e - -irradiation

2 Positron (e+) Study of Solid Materials e+ source commercial RI Ion beam RI e+e- pair creation e+ Lifetime Meas. γ-ray Energy Meas. Vacancy-type defects Band structure e+ energy Depth ~MeV ~30keV ~10eV e+ Beam technique Spatial resolution ~µm ~mm Bulk Defect Surface Defect Positron Auger Electron Spect. Positron Diffraction e+ e+ e- e+ e- e+ e+ spectroscopy Other methods combination Probable characterization of defects in materials

3 1. Defects in SiC Fundamental defects Extended defects (dislocation, micro- and nano-pipes ) Surface and interface (oxide, metallic overlayers.) Impurities Point defects (vacancies, interstitials, anti-site, complexes) Functional Carrier lifetime control Impurity diffusion Harmful Scattering and recombination centers for carriers Establishment of Selective Doping by Ion Implantation Control of Minority Carrier Lifetime

4 e-irradiation induced defects in SiC DLTS studies (6H) Zhang et al. (1989) Ballandovich et al. (1986) Conduction band PL studies Patrick&Choyke (1970~) E1/E Ec-0.35eV E3/E4 Ec-0.57eV Z1/Z Ec-0.6/0.64eV? Ec-1.1eV.86eV Valence band E1 E4.disappear at 1450 o C Z1/Z remain even at 1700 o C D1 lines remain even at 1700 o C

5 Recent progress 6H SiC D1 peaks E1/E levels (negative-u) 4349A peak Z1/Z levels (positive-u) 4H SiC D1 peaks Z1/Z levels (negative-u) Hemmingsson et al. APL 74(1999)839, PRB 58(1998)R Frank et al. Proc. of ICSCRM 99. Origin of Deep Levels?

6 Positron Annihilation Spec. (PAS) Isotope, Na Positron Beam e+ γ 511keV e+ Lifetime -1 3 l = t = prc d ry y e Vacancy Presence! Vacancy Size ò + - e+eγ 511keV m Trapping Rate k = mc V p å PM i if d Ei E f h if = ( - ) Doppler broadening measurement State of impurities bound at vacancies

7 Analysis of Lifetime Spectrum COUNT (arb. units) n I i Lt ( ) = exp( -t / t ) å i = 1 t n i å I i = 100% i = 1 i CHANNEL NUMBER (5.6 ps/ch) Trapping Model e+ Trapping κ τ B Bulk annihilation τ V Defect annihilation t t 1 t = 1/ = t V 1 t B + k I = ( 1/ -1/ ) k t B t V I1

8 e-irradiated n-type 3C SiC Kawasuso et al. AP A67(1998)09. INTENSITY (arb. units) 1MeV(1.1x10 18 e - /cm n =8.981 GHz H // <100> RT C Si vacancy MAGNETIC FIELD (mt) =.003, isotropic d symmetry à V Si - Itoh et al. IEEE Trans.Nucl.Sci.37(1990)173.

9 Balona and Loubser, J. Phys. C3(1970)344.

10 Positron Lifetime Spectrum COUNTING RATE (arb. units) unirradiated Two-comp. analysis τ d =189psI=7 irradiated TIME (ns) Theoretical e+ lifetime (Brauer et al) Bulk V C V Si V Si V C 138ps 153ps 191ps 1ps

11 Annealing of e+ lifetime and trapping rate 6 1. INTENSITY (%) τ d =189 ±4ps e+ trapping rate(ns -1 ) V-I recombination V Si àsinks T1-intensity (arb.units) LIFETIME (ps) τ 1 =1/(τ B -1 +κ) ANNEALING TEMPERATURE ( o C) e+ trapping rate: κ=µc V µ~6x10 16 s ANNEALING TEMPERATURE ( o C) Determination of defect concentration

12 1E17 e+ trapping coefficient v.s. defect charge state SiC:V Si Si:V Kawasuso e + TRAPPING COEFFICIENT (s -1 ) 1E16 1E15 1E14 1E13 + Si:VP Kawasuso Si:V GaP: V Krause P Si: V Kawasuso Si: VP Makinen Si:V Mascher Si:V Mascher Si:VP Makinen GaP:V Krause P GaP: V Krause P Si: V Kawasuso,Mascher Si: VP Kawasuso DEFECT CHARGE STATE V - V 0 k B T~Ze /4πε r V + Dielectric constant ε 6.7 (SiC) < 11.9 (Si)

13 e-irradiated n-type 6H-SiC (Cree Res.) 10 0 Defects exist in as-grown state COUNT (arb. units) unirradiated <τ>=14ps 3MeV, 1x10 18 e-irradiated <τ>= 178ps COUNTING RATE (arb. units) 10 0 unirradiated n-type 6H SiC TIME (ns) CHANNEL NUMBER (1.3 ps/ch)

14 Annealing of e+ lifetime 100 INTENSITY (%) C-SiC I LIFETIME (ps) t t 1 V Si V C V Si V C Bulk Theoretical e+ lifetime 50 t 1 TM ANNEALING TEMPERATURE ( o C)

15 ESR spectra NA: g=.003, isotropic T1 signal in 3C-SiC NB: g c-axis symmetry r spin =4.04A 3MeV, 1x10 18 e-irradiated NC: Principal axes of D deviates ~45 o from c-axis r spin =3.6A

16 Proposed atomic models for ESR centers NA signal NC signal C Si vacancy 1.89A 3.15A NB signal

17 e+ detected vacancies and ESR signals POSITRON TRAPPING RATE (ns -1 ) e+ vacancies ESR NA NB NC ESR SIGNAL INTENSITY (arb. units) NA NC NB ESR signals are related to vacancy type defects ANNEALING TEMPERATURE ( o C) Related to V Si e.g., V si + impurities

18 3MeV, 1x10 18 e-irradiated As for D1 luminescence D1 lines remains after 1500 o C annealing e+ detected vacancies and ESR centers vanish

19 4H SiC (Cree Research) H SiC as-grown 100 I COUNTING RATE (arb. units) p-type τ d =16p I=6 n-type INTENSITY (%) LIFETIME(ps) t t TIME (ns) ANNEALING TEMPERATUER ( o C) Grown-in vacancies, No change up to 1500 o C

20 Summary of Part I Electron-irradiated 3C SiC Isolated V Si is major e+ annihilation center. Agreement with ESR T1 signal. V Si is annealed at 00 o C and 800 o C. Electron-irradiated 6H SiC V Si and V Si V C are e+ annihilation centers. ESR NA,NB&NC centers: vacancy type defects. No correlation between D1 peaks and e+ detected vacancies 4H SiC Grown-in vacancies Origin of Optical and Electrical Centers PL e+ annihilation DLTS Detailed annealing experiment High quality epilayer

21 Conclusions Positron annihilation is a superior tool to study vacancytype defects in SiC. Complementary study of positron and the other methods is necessary to elucidate origin of optical and electrical centers. Application of positron beam gives us more sophisticated knowledge concerning with defects in epilayers. Talk is published as pdf-file at: