Defects in Irradiated ZnO Thin Films Studied by Photoluminescence and Photoconductivity
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1268-EE01-05
Defects in Irradiated ZnO Thin Films Studied by Photoluminescence and Photoconductivity Reinhard Schwarz1, Rachid Ayouchi1, Marta Brandão1, Carlos Marques2, Eduardo Alves2, Melanie Pinnisch3, Detlef Hofmann3, and Bruno Meyer3 1 Physics Department and ICEMS, Instituto Superior Técnico, Lisbon, Portugal 2 Instituto Tecnológico e Nuclear, ITN, Lisbon, Portugal 3 I. Physics Department, University of Giessen, Giessen, Germany ABSTRACT Pulsed-laser-deposited ZnO thin films were exposed to a 1.5 MeV helium ion beam to study the changes in radiative and non-radiative recombination. We first measured photoluminescence (PL) spectra at 4.2 K excited with the 325 nm line of a HeCd laser. The asdeposited films showed a donor-bound exciton peak at 3.3567 eV attributed to Zn interstitials. After irradiation the donor-bound-exciton dominated PL spectra shifted to acceptor-bound behaviour with a signal at 3.3519 eV, tentatively attributed to Li or Na acceptors. In contrast to the approximately 30 % decrease of the PL signal near the band edge, we observed a strong concomitant enhancement of the green/orange PL band, located between 2.1 eV and 2.8 eV, by a factor of over 4. Candidates for those transitions are Li impurities and/or O vacancies. For comparison, the steady-state photocurrent decreased strongly in the irradiated region, which can also be attributed to increased non-radiative recombination through oxygen-related defects.
INTRODUCTION Zinc oxide (ZnO) has been investigated in recent years since it could become an alternative for other wide-band-gap materials like GaN, with the advantage of high abundance of the required elements in nature. However, applications in optoelectronics device structures has been hampered by several problems, like difficulty in p-type doping [1], the presence of structural defects, mostly related to oxygen [2], and the observation of persistent photoconductivity (PPC) in virtually all non-intentionally doped films prepared by different methods [3,4]. We have previously looked at optoelectonic properties of irradiated GaN films prepared by pulsed laser deposition (PLD) and found that GaN shows somewhat higher radiation resistance [5] when compared to particle detectors based on amorphous silicon films [6,7] or crystalline silicon (c-Si) wafers [8]. Apart from the classical c-Si based detectors several “new” semiconductor materials have been tested, like stoichiometric SiC, CVD diamond, CdTe, or GaN [9]. What is new in most cases, compared to the state of knowledge a few decades ago, is the enormous progress in the development of preparation techniques for high purity crystals or high quality epitaxial films. In the case of GaN the development of new film growth processes has led to a breakthrough in UV light-emitting diodes or blue lasers [10]. In general, semiconductor materials with a relatively high atomic number show increased the electron-hole pair creation rate at given thickness. In addition GaN has a very large band gap of 3.4 eV and should therefore allow building detectors
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