Photoluminescence studies on MBE grown Co-doped ZnO thin films fabricated through ion implantation and swift heavy ion irradiation Basavaraj Angadi a , Ravi Kumar b , Dong-Hee Park c , Ji-Won Choi c , Won-Kook Choi c,⇑ a Department of Physics, Bangalore University, Bangalore-560 056, India b Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New-Delhi-110 067, India c Optoelectronic Materials Center, Korea Institute of Science and Technology, Cheongryang, P.O. Box 131, Seoul 130-650, Republic of Korea article info Article history: Available online 2 February 2011 Keywords: DMS ZnO Photoluminescence Swift heavy ion abstract The temperature dependant photoluminescence of the Co-doped ZnO thin films, prepared by ion implantation on the MBE grown ZnO thin films followed by swift heavy ion irradiation, were investigated. The phenomenon of negative thermal quenching (NTQ), where the photoluminescence (PL) intensity increases with temperature, in contrast to the usual behavior of decrease in intensity with temperature, has been observed. The I 3 peak and the peaks (a, b, c, d, and e), corresponding to t 2g and e g levels of the crystal field split Co d orbitals exhibit the NTQ behavior. The NTQ temperature range 35–45 K observed in un-doped ZnO shifts towards lower temperature with the Co doping. The increased number of dopant related and/or the vibrational/rotational resonance states with lower activation energies, from which the thermal excitation of the electrons takes place to the initial state of the PL transition, are responsible for the NTQ behavior. Ó 2011 Published by Elsevier B.V. 1. Introduction Recently ZnO, a II–VI semiconductor, has been emerged as a good candidate for high efficient UV/Blue emitting LED owing to its large exciton binding energy of 60 meV [1,2]. It has also been considered as a potential candidate for making oxide based dilute magnetic semiconductor (DMS) through doping with a small con- centration of transition metal ions [3]. Among these, the Co-doped ZnO (Co:ZnO) is a well known DMS material with a ferromagnetic ordering at room temperature [3,4]. However, there are reports of difficulties in achieving the single phase Co:ZnO, due to the forma- tion of Co clusters. A novel method of achieving a single phase Co:ZnO, through ion implantation followed by swift heavy ion (SHI) irradiation, was introduced and their magnetic and optical properties were studied in detail for their applications in spintron- ics devices [5,6]. The SHI irradiation, known for depositing large amount of energy to the lattice in a short span of time through electron–phonon interactions [7,8], help dissolving the Co clusters in Co:ZnO. The thermal quenching phenomenon is commonly observed in the temperature dependant photoluminescence (PL) studies of many solids such as semiconductors and the ionic crystals [9,10]. This is mainly due to the temperature induced increase in the non-radiative recombination probability of electrons and holes. As a result, the intensity of the PL peaks decrease with increase in the measuring temperature. However, in contrast to this usual behavior some semiconducting materials such as GaAs [11,12] and ZnS [12,13] exhibit an increase in the PL intensity with an increment of temperature in some temperature range. This phe- nomenon is called ‘Negative thermal quenching (NTQ)’. There have been various explanations for this unusual behavior [11,12,14]. Ini- tially, Williams and Eyring [14] proposed analytical formulae for the observed phenomenon, which were not in agreement with some of the experimental results. Later, Bebb and co-workers [11,12] proposed a different mechanism for the NTQ observed in the (e-A o ) emission of GaAs, which was ascribed to the dissociation of the (D o , X) system, which resulted in the ejection of a free elec- tron into the conduction band. Recently, Shibata [15] suggested the principal mechanism of NTQ as a thermal excitation of electrons into the initial state of the PL transition from the eigen-states with smaller energy eigen-values. The NTQ was also observed in single crystals, nanorods and bulk samples of un-doped ZnO and in Ga-doped ZnO thin films [16–18]. Meyer et al. [19] explained the NTQ behavior of I 6B /I 8B donor bound exciton PL peaks at 15–20 K in ZnO crystal, through fitting an ana- lytical equation, in terms of the emergence of excited states involv- ing B-valence band. In a similar way, Jung et al. [20] explained the enhancement of I 10 peak intensity at 40–50 K in un-doped ZnO thin film grown by plasma-assisted MBE, in terms of both the thermal- 0168-583X/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.nimb.2011.01.088 ⇑ Corresponding author. Tel.: +82 2 9585562. E-mail addresses: brangadi@gmail.com (B. Angadi), ranade65@gmail.com (R. Kumar), pdmtime@kist.re.kr (D.-H. Park), jwchoi@kist.re.kr (J.-W. Choi), wkchoi@kist.re.kr (W.-K. Choi). Nuclear Instruments and Methods in Physics Research B 272 (2012) 305–308 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb