A photoluminescence study of Cd, In and Sn in ZnO using radioisotopes Joseph Cullen, Martin Henry, Enda McGlynn Dublin City University Karl Johnston Universitat des Saarlandes and CERN/ISOLDE

The nature of ZnO ZnO - semiconductor at room temperature, energy gap ~ 3.4 eV - universally n-type as-grown - p-type conduction can be obtained, but not readily Difficult to purify for growth of large single crystal boules Questions: Origin(s) of dominant n-type conduction Understanding impurities / defects

Group III impurity on Zn site should: provide one excess electron act as donor: n-type conductivity provide binding centre for e-h pairs under optical excitation Group V impurity on O site should: create a free hole act as acceptor: p-type conductivity BUT these prefer to occupy Zn sites and/or form complex defects The neighbourhood of ZnO

Proof of common donor impurity identifications Wider study of Zn-site and O-site impurities Principal experimental technique : Photoluminescence at low temperatures - in conjunction with other techniques/partners in ISOLDE collaboration Our research programme:

PL intensity Photon energy (eV) Multiplicity of lines - Various impurities - Various transition types I-lines D 0 X – neutral donor-bound excitons D + X – ionised donor bound excitons Also - DAP, eA, A 0 X

Proof of identity of common donor impurities

72 Zn 72 Ga 72 Ge 73 Ga -- -- - hr 14.1 hr 4.86 hr 73 Ge ZnO:Ga I-line identification

ZnO: 73 As → 73 Ge decay Half-life: 80.3 days

Decay of Ga-related I 8 (and I 1 )Growth of Ge-related DD 2

New results from Ga → Ge decay Ge-related luminescence observed for the first time Large spectral binding energy compared to III impurities Low thermal binding energy Similar to I-lines under stress

Nature of Ge-related luminescence? Ge on Zn site: two extra electrons per Ge atom several electron-hole recombination paths are possible we are pursuing this using Zeeman/stress Theory: Ge and Si should act as shallow double-donors Lyons et al (2009)

ZnO:In I-line identification Muller at al – APL (2007) I-9 identified with In D 0 X No evidence for D + X line Possible Cd-related weak band Is I-9 the D o X for In ? Any evidence for D + X line ? Also: does Sn behave like Ge? 111 In → 111 Cd

We examined the reverse decay path 117 Ag 72 s 117 Cd 3 h 117 In 43 m

I-9 line I-2 line Confirm Muller et al result for I-9 New results: I-2 is D + X for In No Cd or Sn signals observed ZnO: 117 Cd/In/Sn

IsotopeFitted τ ½ Tabulated τ ½ 72 Ga12.7 ± 1.5 h14.1 h 73 Ga4.9 ± 0.2 h4.76 h 73 As78 ± 3 d80.3 d 117 In43 ± 2 m43.2 m Fits to experimental data

Summary Lines I-1 and I-8 due to Ga Lines I-2 and I-9 due to In New PL line due to Ge For Zn-site impurities in ZnO Not observed:Sn counterpart of Ge line Cd counterpart of Hg line (Agne et al 2003)

Thank you! ENSAR

Zn-site impurities

R. J. Mendelsberg et al. J. Vac. Sci. Technol. B 27(3) (2009) Pb in ZnO….