Study of Carrier Dynamics in ZnSe Based Scintillators by Frequency Domain Lifetime Measurements. J.Mickevi čius, P.Vitta , G.Tamulaitis, A. Žukauskas Institute of Materials Science and Applied Research, Vilnius University, Saul ė tekio 9-III, LT-10222 Vilnius, Lithuania.
Study of Carrier Dynamics in ZnSe Based Scintillators by Frequency Domain Lifetime Measurements
J.Mickevičius, P.Vitta, G.Tamulaitis, A. Žukauskas
Institute of Materials Science and Applied Research, Vilnius University, Saulėtekio 9-III, LT-10222 Vilnius, Lithuania
N.Starzhinskiy, K.Katrunov, V.Ryzhikov
STC for Radiation Instruments, ST Complex ‘‘Institute for Single Crystals’’ of the National Academy of Sciences of Ukraine, 60 Lenin Ave., 61001 Kharkov, Ukraine
Semiconductor scintillators based on ZnSe crystals activated with Te, Cd, Al, or O emit in a convenient for detection spectral region (600-640 nm) and have conversion efficiency of up to 22% and radiation stability of more than 500 Mrad. These materials might be the best choice for radiation detectors in X-ray medical and industrial tomography and other applications. However, the further development of these radiation detectors requires more detailed study of the origin and properties of radiative and nonradiative recombination centers in this material. Emission decay kinetics is a very informative parameter for characterization of scintillators and, usually, time-resolved photoluminescence is employed for such characterization.
We report on a study of PL decay kinetics using the frequency domain fluorescence lifetime measurement technique. This technique enabled us to perform the characterization under extremely low excitation power densities, which are typical for scintillator operation in sensitive radiation detectors.
In response to amplitude modulation of the excitation source, the sample luminescence is also modulated with the same frequency ω. However, due to the finite luminescence decay time τ, the detected signal has a phase shift φ and its modulation depth is lower by a factor of 1/m. For a single-exponent decay, the phase shift and modulation depth can be expressed as:
For nonexponential decay, the phase shift and modulation depth can be expressed as:
where Nω and Dω are sine- and cosine-transforms of the luminescence intensity decay function I(t):
A375-nm UV LED from NICHIA was used as an excitation source in our experiments carried out in the frequency range from 5 Hz to 200 MHz. The measurements were performed in the temperature range from 8 K to 300 K.
The ZnSe(Te) sample under study was fabricated at the STC Institute for Single Crystals, Kharkov, Ukraine. Concentration of tellurium in the sample was 0.2 wt%.
Deep level emission
Modeling of decay
The luminescence decay was described by a function consisting of a set of terms with characteristic decay times and weight coefficients:
The decay could not be described by using only exponential terms, therefore two types of terms were used: exponential and terms corresponding to donor-acceptor pair (DAP) recombination.
The decay of donor-acceptor pairs luminescence is described by the function:
where N is the concentration of the majority constituent of DAPs. The DAP recombination rate depends on distance R between the donor and acceptor involved as:
The weight coefficients were further normalized to the entire number of carriers to reflect the influence of the recombination channel by integrating the corresponding component in time:
Temperature evolution of decay components