Time-resolved high energy ionoluminescence of Al₂O₃

Abstract

We have compared the luminescence decay from intact and pre-damaged Al₂O₃ crystals registered during single swift heavy ion and photo- picosecond laser pulse excitation (λ_ex. = 440 nm). The decay curves measured during 1.2 ÷ 1.6 MeV/amu C, Ar, V, Kr, Xe ion irradiation at room temperature are composed of three components – fast (τ₁ < 1 ns), τ₂ = 1.8 ns (F⁺-centers) and τ₃ = 54–80 ns (E-luminescence, the nature of emission is under discussion). The measurements performed on pre-irradiated samples have demonstrated that accumulated radiation damage suppresses the E-luminescence and only τ₁ and τ₂ components are observed.

It was found that photoexcitation at 440 nm induces strong emission of F₂²⁺- centers with a lifetime of about 8 ns which is not detected during high energy ion irradiation. This is ascribed to the quenching effect of high density excitations in a particle track.

Introduction

The spectral content and intensity of optical emission generated by energetic ions (ionoluminescence, IL) are strongly dependent on structural changes and this may be used for real-time characterization of irradiated materials. All works related the IL measurements can be divided into two groups. The first and largest one concerns time-integrated measurements when the evolution of the IL spectra affected by accumulated radiation damage and associated mechanical stresses, is studied. A large number of such experiments have been performed over a wide ion energy range - from single keV to hundreds of MeV (see, for example, [1], [2], [3], [4], [5]). Much less works are devoted to time-resolved IL measurements, especially to those in which the luminescence decay is registered after single ion impact. The temporal resolution of single ion techniques, not limited by beam pulse duration, may reach picoseconds. This facilitates the study of new interesting features in the dynamics of dense electronic excitations in the vicinity of a swift ion trajectory, as demonstrated by K. Kimura et al., (for example, [6], [7], [8], [9], [10], [11]) and M. Koshimizu et al. [12]. In particular, novel ultra-short-lived luminescence (several tens of ps) generated by 2 MeV/amu N, Ar, Kr and Xe ions, has been registered over UV–VIS wavelength range in some alkali halides, alkaline-earth halides, oxide and semiconductor crystals [10]. The luminescence was not detected during He-ion irradiation and exhibited a super-linear increase with excitation density. This and other features like non-tailing decay, sample dependence and non-temperature sensitivity, led to the conclusion that emission may be associated with electron–hole plasma as a transient stage of relaxation of extremely high-density-excitation. It is important to note that the fastest ionoluminescence component (t < 1 ns) could also be related to the so-called intraband luminescence (spread in a wide spectral region) connected with radiative relaxation of hot valence holes and detected in a number of metal oxides, including Al₂O₃ [13], [14].

A strong increase in the luminescence efficiency of α-Al₂O₃ in the vacuum ultraviolet wavelength region for 2 MeV/amu Xe ions in comparison with the same energy N and Ar ions has also been observed in [12]. It was found that the decay rate of luminescence in the band peaked at ~170 nm and ascribed to self-shrunk excitons [15], increased with linear energy transfer. This effect was suggested to be due to the enhanced radiative rate caused by close interactions among excited states.

The above mentioned experiments [6], [7], [8], [9], [10], [11], [12] have been performed using intact crystals, since the single ion excitation assumes a minimal damage production rate thus excluding possible effects of newly formed radiation defects on properties of time-resolved luminescence. The aim of this work is to compare the luminescence decay registered in intact and pre-damaged Al₂O₃ crystals under single swift heavy ion and photo- picosecond laser pulse excitation. In conclusion of the Introduction section, we give some spectral parameters of single and dimer F-type centers in aluminum oxide to be used in discussion of the experimental results (Table 1).

It should be mentioned that one of two components of the recombination luminescence of self-trapped excitons (STE), so called E-emission, also contribute in the band centered around 330 nm [15].