Results

Study of Carrier Dynamics in ZnSe Based Scintillators by Frequency Domain Lifetime Measurements

J.Mickevičius, P.Vitta, G.Tamulaitis, A. ŽukauskasInstitute of Materials Science and Applied Research, Vilnius University, Saulėtekio 9-III, LT-10222 Vilnius, Lithuania

N.Starzhinskiy, K.Katrunov, V.RyzhikovSTC for Radiation Instruments, ST Complex ‘‘Institute for Single Crystals’’ of the National Academy of Sciences of Ukraine, 60 Lenin Ave., 61001 Kharkov, Ukraine

Introduction Experimental technique

Temperature evolution of decay components

Conclusions

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.

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%.

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):

A 375-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.

2

1221)(),()(

marctg

.)(),/()( 22 DNmDNarctg

00

00

)(cos)(

,)(sin)(

dttItdttID

dttItdttIN

0

2)(

0

2)( ,)(14exp)( dRReRWdRReNtI tRWtRW

,2

exp)( max

Da

RWRW

101

102

103

104

105

106

DAP exp1 exp2 exp3

Life

time

(ns)

0 20 40 60 80 100 120 1400.01

0.1

1

Wei

ght

coef

ficie

nt (

arb.

u.)

Temperature (K)

~300 ns(f)

~400 ns(e)

~200 s(d)

~80 ns(a) (c)

conduction band

valence band

~4 ns(b)

Object

Modeling of decay

i o

ii

o

ii

i

dttIA

dttIA

C

)(

)(

i

ii tIAtIAtIAtIAtI )(...)()()()( 332211

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:

The PL of ZnSe(Te) consisted of two distinct spectral bands: near-band-edge (NBE) emission and deep-level related PL.

These two bands were separated by using optical filters and analyzed separately by using FDLM technique.

Each decay component has been assigned to a possible recombination channel.

The DAP component (cyan diamonds) dominant in the deep level PL is attributed to the self-activated PL. The exponential component (magenta triangles) can be assigned to carrier transfer from/to the donor-acceptor associate responsible for the so-called M-band.

The NBE part of PL is determined by recombination of shallow DAP (black squares) and bound excitons (red dots) at low temperatures. With increasing temperature, recombination of free carriers (green triangles) starts dominating.

Carrier trapping/detrapping processes (blue and yellow triangles) play an important role in both deep-level related and NBE emission.

Conclusions Application of frequency domain lifetime measurements technique enabled us to investigate luminescence decay in ZnSe(Te) scintillator

crystal at extremely low excitation and in a wide time window ranging from milliseconds to nanoseconds.

The two components responsible for deep-level-related luminescence have been shown to have different decay dynamics: the decay rate of one component is governed by DAP recombination rate, while the second component decays exponentially, which indicates strong influence of carrier transfer from/to corresponding recombination centers.

Deep level emission Near-band-edge emission