- Email: [email protected]
A new ZnSe1-xTex scintillator: luminescence mechanism
Nucl. Tracks Radiat. Meas., Vol. 21, No. 1, pp. 53-54, 1993 Printed in Great Britain
0735-245X/93 $6.00 + .00 Pergamon Press Ltd
A NEW ZnSe~_xTe~ SCINTILLATOR: LUMINESCENCE MECHANISM V. D. RYZHIKOV,V. I. SILINand N. G. STARZHINSKY Institute for Single Crystals, Lenin av. 60, 310141 Kharkov, Ukraine Abstract--The spectral and kinetic luminescence characteristics of the new scintillator ZnSet _~Te~ are investigated. The conversion efficiency of this material is 1.5 times larger in comparison with that of CsI:T1. It is shown that the luminescencemechanism of ZnSet xTe~crystals is defined by the ensemble of intrinsic point defects. In these frames the amphoteric complexes [Vz.Tes~Znd are the radiative recombination centres responsible for the "working" luminescenceband with a 640 nm maximum (L-640). The high radiation stability (~ 107 rad) of these crystals and their optoelectronic characteristics have allowed the development of a new generation of ,scintillation detectors unsurpassed in the field of introscopy and dosimetry both of weak and superstrong radiation flows.
1. INTRODUCTION
2. OPTOELECTRONIC CHARACTERISTICS OF ZnSe~_xTex: HEAT TREATMENT EFFECT
RYZIKOV AND STARZrtINSK+(1988) have shown that zinc excited with high-energy radiation or doped with various metals/halogens has luminescence properties similar to those of ZnSe~ _ xTex crystals. This suggests that the intrinsic point defects of the ZnSe~_xTe, lattice participate in the formation of the L-640 band. An essential difference in ionic radius between Te and Se leads to considerable local stresses and lattice deformation in the vicinity of Te, thus initiating vacancy generation in the cationic sublattice (Gal'chinetskii et al., 1989). The formation of Vzn in the neighbourhood of Te has been observed in experiments using EPR and ODMR methods, the activation energy Ea of the Vzn level being 0.7 and 1.1 eV (Watkins, 1970). The interstitial Zn~ is pushed in the crystallographic direction [001] or [111]. It may remain in one of the nearest configuration spheres bonded with the formed vacancy by means of the Coulomb forces and be part of the complex [VznTes~Znl] playing the role of the radiative recombination centre (Dmitriyev and Ryzhikov, 1991). A high conversion efficiency of ZnSe I xTe~ exceeding that of Csl :TI crystal (Gal'chinetskii et al., 1988), as well as the practical realization of "scintillatorphotodiode" type detectors on the base of ZnSe~_xTex crystals for computer tomography and radiation monitoring (Ryzhikov, 1989; Ryzhikov and Yakovlev, 1990; Danshin et al., 1991) have stimulated further investigations of their properties, To extend the application sphere of the crystals under consideration as scintillators both for detectors with a high time resolution and devices which require slow decay kinetics, the effect of non-stoichiometric components on the formation of the luminescence parameters of the materials mentioned is studied.
The crystals grown from the extra pure raw material ZnSe-ZnTe with the addition of ~ 2 % of the elementary components Zn, Se or Te and the crystals obtained from the stoichiometric raw material, were investigated. At the final preparation stage the samples were annealed in zinc vapour or in liquid zinc. While studying the spectral and kinetic characteristics, the crystals were excited with 40-120keV X-radiation. The main characteristics of the heat-treated crystals are presented in Table 1. Here 2max is the position of the maximum in the emission spectrum; z is the decay time ( l ( z ) = Imam~e); tl is the afterglow level in a 10ms interval ( r / = l ( t = 10ms)/Imax × 100%); IXL is the integrated intensity of X-ray luminescence (XL). It should be noted that the crystals Z1-Z4 were annealed in Zn vapour, the annealing medium for Z5 crystals being liquid zinc. The Z2 crystals are characterized by a low intensity of the edge emission (which is an order of magnitude smaller than that of Z1 crystals). For Z5 samples such an emission is absent.
3. LUMINESCENCE CENTRES OF ZnSej_,Te,~ CRYSTALS
Our investigations have confirmed the complex structure of the "working" band L-640 for ZnSe I _ xTex crystals and have shown the possibilities of cardinal changes in the kinetic characteristics with the simultaneous shift of 2m~x. The observed peculiarities of the appearance of the L-640 band may be explained by the amphoteric nature of the emission centre which is the complex 53
54
' , I). RYZHIKOV e't a l i~bte
Spectral and kinetic characteristics or" ZnSe:Te crystals wi)i~ isovalent impurities
i.
Sample Raw material Zl Z2 Z3 Z,4 Z~
Stoichiometric Excessof zinc Excessof selenium Excessof tellurium Stoichiometric
!.~,
r
,*l
/Xl
!nmi
tmcs)
(%)
(rel. units)
640 630 640 640 595
100 > 5000 180 140 ~3
[VznTeseZn~] with the following recharge kinetics (Vakulenko et al., 1988): d N - /dt = C,,, N°n - C+,~N p d N + / d t = ('~0N°p -- (',. N ~n
N = N ~j+N
<~0.05 31 0.05 ~0.05 <-0.05
100 62 83 58
centres, their high stability providing good thermal and radiation stabilities of the scintillator. Different types of heat treatment allow variation of the spectral and kinetic characteristics of ZnSe~ _xTex crystals over a wide range.
4-N+.
where N O, N and N ' are the concentrations of neutral, electron and hole centres, respectively; N is the total concentration of amphoteric centres; C,0, (Cp0~) are the coefficients of electron (hole) capture by neutral and positively (negatively) charged centres. In this case the quantum luminescence yield K 4 in L-640 is defined as KA=(I 4-('.). :V ~r:(',.,,N~)), and this explains the observed complex spectral and kinetic dependences for Z1 -Z4 samples, as well as the linear and non-linear parts of the luminescence intensity dependence on the excitation density characterizing ZnSe~ ~Te~ crystals (Vakulenko et al., 1990). The spectral and kinetic XL characteristics of the Z5 samples are conditioned by the high electron density (n > 10~gcm 3) in the conductivity band. This defines the short recharge times of the radiative recombination centres and the inefficiency of the exciton mechanism of the edge emission caused by free charge carrier screening (Seeger, 1973). 4. CONCLUSION In ZnSe~ ~Texcrystals, the complexes [VznTes~Znl] play the role of amphoteric radiative recombination
REFERENCES Danshin E., Piven' L. and Ryzhikov V. (1991) "Scintillatorphotodiode" detector as a radiactive radiation counter. Prib. Techn. Experim. 4, 65-69. Dmitriyev Yu. and Ryzhikov V. (1991) On radiation stability of ZnSe(Te) crystals. Atomnaya Energia 70, 119-121. Gal'chinetskii L., Dmitriyev Yu., Rzyhikov V. and Starzhinsky N. (1988) Evaporation peculiarities of zinc selenide crystals doped with isovalent impurities, lzv. An SSSR. Neorg. Mater. 25, 1632-1636. Ryzhikov V. (1989) Scintillation Crystals Based on A ItBvl Compounds. Growth, Properties, Application, pp. 1-127. NIITEKhlM, Moscow. Ryzhikov V. and Starzhinsky N. (1988) On red luminescence mechanism for doped zinc selenide crystals. Ukr. Fiz. Zh. 33, 818-824. Ryzhikov V. and Yakovlev Yu. (1990) Application of "scintillator-photodiode" detectors for dosimetry control. Atomnaya Energia 69, 392-394. Seeger K. (1973) Semiconductor Physics. Springer, Vienna. Vakulenko O., Ryzhikov V. and Starzhinsky N. (1990) Nonlinear photoluminescence of ZnSe(Te) crystals. Zh. Prikl. Spektrosk. 52, 231-234. Vakulenko O., Veretennikov A. and Ryzhikov V. (1988) X-ray luminescencekinetics ofZnSe(Te). Zh. Tekhn. Fiz. 68, 632~35. Watkins J. (1970) Lattice Dejects in AUB ~ Compounds, pp. 221-242. Mir, Moscow.
0735-245X/93 $6.00 + .00 Pergamon Press Ltd
A NEW ZnSe~_xTe~ SCINTILLATOR: LUMINESCENCE MECHANISM V. D. RYZHIKOV,V. I. SILINand N. G. STARZHINSKY Institute for Single Crystals, Lenin av. 60, 310141 Kharkov, Ukraine Abstract--The spectral and kinetic luminescence characteristics of the new scintillator ZnSet _~Te~ are investigated. The conversion efficiency of this material is 1.5 times larger in comparison with that of CsI:T1. It is shown that the luminescencemechanism of ZnSet xTe~crystals is defined by the ensemble of intrinsic point defects. In these frames the amphoteric complexes [Vz.Tes~Znd are the radiative recombination centres responsible for the "working" luminescenceband with a 640 nm maximum (L-640). The high radiation stability (~ 107 rad) of these crystals and their optoelectronic characteristics have allowed the development of a new generation of ,scintillation detectors unsurpassed in the field of introscopy and dosimetry both of weak and superstrong radiation flows.
1. INTRODUCTION
2. OPTOELECTRONIC CHARACTERISTICS OF ZnSe~_xTex: HEAT TREATMENT EFFECT
RYZIKOV AND STARZrtINSK+(1988) have shown that zinc excited with high-energy radiation or doped with various metals/halogens has luminescence properties similar to those of ZnSe~ _ xTex crystals. This suggests that the intrinsic point defects of the ZnSe~_xTe, lattice participate in the formation of the L-640 band. An essential difference in ionic radius between Te and Se leads to considerable local stresses and lattice deformation in the vicinity of Te, thus initiating vacancy generation in the cationic sublattice (Gal'chinetskii et al., 1989). The formation of Vzn in the neighbourhood of Te has been observed in experiments using EPR and ODMR methods, the activation energy Ea of the Vzn level being 0.7 and 1.1 eV (Watkins, 1970). The interstitial Zn~ is pushed in the crystallographic direction [001] or [111]. It may remain in one of the nearest configuration spheres bonded with the formed vacancy by means of the Coulomb forces and be part of the complex [VznTes~Znl] playing the role of the radiative recombination centre (Dmitriyev and Ryzhikov, 1991). A high conversion efficiency of ZnSe I xTe~ exceeding that of Csl :TI crystal (Gal'chinetskii et al., 1988), as well as the practical realization of "scintillatorphotodiode" type detectors on the base of ZnSe~_xTex crystals for computer tomography and radiation monitoring (Ryzhikov, 1989; Ryzhikov and Yakovlev, 1990; Danshin et al., 1991) have stimulated further investigations of their properties, To extend the application sphere of the crystals under consideration as scintillators both for detectors with a high time resolution and devices which require slow decay kinetics, the effect of non-stoichiometric components on the formation of the luminescence parameters of the materials mentioned is studied.
The crystals grown from the extra pure raw material ZnSe-ZnTe with the addition of ~ 2 % of the elementary components Zn, Se or Te and the crystals obtained from the stoichiometric raw material, were investigated. At the final preparation stage the samples were annealed in zinc vapour or in liquid zinc. While studying the spectral and kinetic characteristics, the crystals were excited with 40-120keV X-radiation. The main characteristics of the heat-treated crystals are presented in Table 1. Here 2max is the position of the maximum in the emission spectrum; z is the decay time ( l ( z ) = Imam~e); tl is the afterglow level in a 10ms interval ( r / = l ( t = 10ms)/Imax × 100%); IXL is the integrated intensity of X-ray luminescence (XL). It should be noted that the crystals Z1-Z4 were annealed in Zn vapour, the annealing medium for Z5 crystals being liquid zinc. The Z2 crystals are characterized by a low intensity of the edge emission (which is an order of magnitude smaller than that of Z1 crystals). For Z5 samples such an emission is absent.
3. LUMINESCENCE CENTRES OF ZnSej_,Te,~ CRYSTALS
Our investigations have confirmed the complex structure of the "working" band L-640 for ZnSe I _ xTex crystals and have shown the possibilities of cardinal changes in the kinetic characteristics with the simultaneous shift of 2m~x. The observed peculiarities of the appearance of the L-640 band may be explained by the amphoteric nature of the emission centre which is the complex 53
54
' , I). RYZHIKOV e't a l i~bte
Spectral and kinetic characteristics or" ZnSe:Te crystals wi)i~ isovalent impurities
i.
Sample Raw material Zl Z2 Z3 Z,4 Z~
Stoichiometric Excessof zinc Excessof selenium Excessof tellurium Stoichiometric
!.~,
r
,*l
/Xl
!nmi
tmcs)
(%)
(rel. units)
640 630 640 640 595
100 > 5000 180 140 ~3
[VznTeseZn~] with the following recharge kinetics (Vakulenko et al., 1988): d N - /dt = C,,, N°n - C+,~N p d N + / d t = ('~0N°p -- (',. N ~n
N = N ~j+N
<~0.05 31 0.05 ~0.05 <-0.05
100 62 83 58
centres, their high stability providing good thermal and radiation stabilities of the scintillator. Different types of heat treatment allow variation of the spectral and kinetic characteristics of ZnSe~ _xTex crystals over a wide range.
4-N+.
where N O, N and N ' are the concentrations of neutral, electron and hole centres, respectively; N is the total concentration of amphoteric centres; C,0, (Cp0~) are the coefficients of electron (hole) capture by neutral and positively (negatively) charged centres. In this case the quantum luminescence yield K 4 in L-640 is defined as KA=(I 4-('.). :V ~r:(',.,,N~)), and this explains the observed complex spectral and kinetic dependences for Z1 -Z4 samples, as well as the linear and non-linear parts of the luminescence intensity dependence on the excitation density characterizing ZnSe~ ~Te~ crystals (Vakulenko et al., 1990). The spectral and kinetic XL characteristics of the Z5 samples are conditioned by the high electron density (n > 10~gcm 3) in the conductivity band. This defines the short recharge times of the radiative recombination centres and the inefficiency of the exciton mechanism of the edge emission caused by free charge carrier screening (Seeger, 1973). 4. CONCLUSION In ZnSe~ ~Texcrystals, the complexes [VznTes~Znl] play the role of amphoteric radiative recombination
REFERENCES Danshin E., Piven' L. and Ryzhikov V. (1991) "Scintillatorphotodiode" detector as a radiactive radiation counter. Prib. Techn. Experim. 4, 65-69. Dmitriyev Yu. and Ryzhikov V. (1991) On radiation stability of ZnSe(Te) crystals. Atomnaya Energia 70, 119-121. Gal'chinetskii L., Dmitriyev Yu., Rzyhikov V. and Starzhinsky N. (1988) Evaporation peculiarities of zinc selenide crystals doped with isovalent impurities, lzv. An SSSR. Neorg. Mater. 25, 1632-1636. Ryzhikov V. (1989) Scintillation Crystals Based on A ItBvl Compounds. Growth, Properties, Application, pp. 1-127. NIITEKhlM, Moscow. Ryzhikov V. and Starzhinsky N. (1988) On red luminescence mechanism for doped zinc selenide crystals. Ukr. Fiz. Zh. 33, 818-824. Ryzhikov V. and Yakovlev Yu. (1990) Application of "scintillator-photodiode" detectors for dosimetry control. Atomnaya Energia 69, 392-394. Seeger K. (1973) Semiconductor Physics. Springer, Vienna. Vakulenko O., Ryzhikov V. and Starzhinsky N. (1990) Nonlinear photoluminescence of ZnSe(Te) crystals. Zh. Prikl. Spektrosk. 52, 231-234. Vakulenko O., Veretennikov A. and Ryzhikov V. (1988) X-ray luminescencekinetics ofZnSe(Te). Zh. Tekhn. Fiz. 68, 632~35. Watkins J. (1970) Lattice Dejects in AUB ~ Compounds, pp. 221-242. Mir, Moscow.