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5CONDUCTIBILITY AND IRRADIATION STABILITY OF MONOCRYSTALS ZnlnSA
2 A
E. ARAMA1, V. PiNTEA2
Republic of Moldova, Chisinau city 1Nicolae Testemitanu State University of Medicine and Pharmacy 2Technical University of Moldova
Abstract: This study presents the results of investigations on the conductivity and irradiation stability of single crystals ZnIn2S4 in a wide range of incident electron energies (30 ^ 75keV) and
the respective doses (1014 ^1020 cm"2). It considers the possibilities to manufacture accelerated electron detctors and assesses their parameters. Considering that the energy values of the order 102 keV are near the threshold of structural defects of intensive formation, th einfluence of this phenomenon on the detector parameters is subject to the analysis.
Keywords: ternary semiconductors, irradiational recombination, conductivity, accelerated electron, photo-conductivity, optical transitions.
1. INTODUCTION
The materials having semiconductor properties are quite sensitive and change their essential physical properties under the action of external radiation and elementary particles bombarding them with high energy. Usually, under the action of external radiation or different high energy particles in crystals, following the ionization, additional scattering centers of charge carriers appear. Experimental and theoretical study of these processes is up-to-date in terms of characteristics stability of microelectronic devices and prevention of degradation processes on different devices made of different materials with semiconductor properties. Therefore, it is quite important to perform researches related to the influence of electron beams with energies up to 100 keV on physical properties of semiconductors. To highlight the changes that occur in optical, photoelectric and irradiation properties, investigations were carried out on optical absorption spectra, photo-conductibility for non-irradiated samples and for those irradiated with different doses of electrons at certain energies. Depending on the doping element, the impurity concentration is in the range of (1,2-1019 ^2-1020) cm-3. Growth technology of ZnIn2SA is described in detail in [l -2].
2. EXPERIMENTAL RESULTS ON IRRADIATION CONDUCTIVITY
This paper describes the experimental rasults regarding the change of photoelectric properties and irradiation under the influence of electron beam accelerated in ternary combinations of the type A11 B^Cl1, based on the example of typical ternary compound ZnIn2S4. For research we chose perfect slabs in terms of defect density, with an outer surface that had quality optical thickness from 0,20 to 1,0 mm, obtained from gas phase using iodine as a carrier agent.
Irradiation conductivity was measured in vacuum at the temperature of 296K, both in the stationary as well as modulated conditions according to the method described in [3]. Current density of the
electron beam was provided by the flow of 1016 cm-2s-1 particles and the bombardment energy was 30; 40; 75 keV.
R /
Based on the dependence of the relative resistance change Rn and the current density of the
/ R0
electron beam, the three samples of ZnIn S at 40 keV (Figure 1), one may observe that this dependence for all samples is basically a linear function throughout the study period. The dependence
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R /
between relаtive resistance () and irrаdiation dоse at diffеrent levеls оf excitatiоn of elесtron
bеаm energy wаs a^ investigаted. Aссоrding tо this dependence, it is observed that at 75keV (curve 1, Figure 2), the resistance decreases slightly, in а narrow range оf vаriatiоn оf thе irrаdiаtiоn dоse. Аt thе elесtron bеаm enеrgy 40keV (curve 2) this pаssage is оbservеd tо bе vеry stow, which is аpprоximаtely linеаr, аnd at thе enеrgy оf 30keV thе depеndence оn thе dоse is а linеаr functiоn in ^ studiеd rangе.
Fоr аll mеаsuremеnts dеscribеd abоve, а cоnstаnt cоnductivity оf thе sаmples in ^ dаrk wаs obsеrved, ^t of (107 ^ 109)Q. Аt high dоses of radiatton (1018 ^1020)cm"2, irrеversiblе chаnges оf thе rеsistancе in ^ dаrk werе оbservеd, which indites а sudden increase in the concentration of balanced charge carriers, in the volume of monocrystal (at ^st to the dеpth оf penetratiоn of thе etoctrons ~ 5 ^m).
Fig.1. Dеpendenсе оf relаtive rеsistancе R / R upоn thе sizе оf the elеctrоn beam current for the samples of ZnIn2S4, sаmple thick^ss: 1 - 0,20mm, 2 - 0,5 mm, 3 -1,0 mm.
Fig.2. Dependence of relative resistance R / R0 and radiation dose at different values of the electron beam energy: 1-75, 2-40, 3-30 keV.
Fig.3. The dependence AR / R0 of single crystals ZnIn2 according to the dose of irradiation, T = 296K, 1 - 50keV, 2 -100keV.
AR.
In figure 3, it is shown the dependence according to the dose of irradiation, wherein
AR = R0 - R, R is dark resistance of the sample after irradiation dose of (1018 ^ 1020) cm-2. According
ТЕХНИЧЕСКИЕ НАУКИ
to this dependency one may conclude that, at a radiation dose of 1020 cm 2, the resistance in the dark becomes very small, it is approximately 102 Q.
Figure 4 represents the temperature dependence of the parameter 9 for accelerated electrons with energy: 1 - 50keV, and 2 - 100keV at the radiation dose 5-1018 cm-2, wherein= = cm-2
. Following the above it can be mentioned that the activation energy of the conductivity decreases along with the decrease of sample resistance.
Based on the experimental results concerning the cathode conductivity of single crystals ZnIn2S4, it can be concluded that the conductivity type of the single crystals in this case does not change. We assume that the concentration excess of balanced carriers occurs due to the activation of small donors, which are probably formed at an intensive irradiation. It was experimentally established that the increase in dose leads to reduction of activation energy. For example, for the dose 5 • 1018 cm-2 , the activation energy was 0,006 eV, that is comparable to the ionization energy of Zn atoms between nodes Zn for the network ZnO according to the data [4]. So, we could say that following
the interaction of medium-energy electrons in ZnIn S , the zinc occurs between the nodes of crystalline network.
Based on previous results, we will examine some exploitation parameters of electron detectors with energies up to 50keV made on the basis of ZnIn2S4 . For electrons with 50keVenergies having the number of excited electrons per second «1015 s-1, the lifetime of free carriers is approximately ( r« 103s) and the interior propagation coefficient of carriers at calculated contact voltage of (5 ^20) V, reaches the value of «103 [5,6], which is obviously lower in comparison to 108 for the binary compounds. Being experimentally determined, the detector's power reached the value of 4 Wxcm-2 , as for the detectors based on CdS and CdSe, it is 103 W• cm-2 [7].
To develop detectors of high-energy particle and X-ray irradiation, it is necessary to take into account other advantages: simple manufacturing of detectors and their operation in steady-state and
12-
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modulated conditions; high stability to the action of high energy electron beams and X- rays. Investigations on the development of new-type detectors or of their new modifications allow obtaining some performances of exploitation parameters compared to the existing ones. Having high stability at irradiation, the investigated compounds have many practical applications including for the construction of Roentgen radiation detector. In this way, the optimum parameters make it possible to use X-ray detector in medicine, ecology as well as in agriculture.
3. EXPERIMENTAL RESULTS OF IRADIATION STABILITY
As a result of experimental investigations, there have been determined photo-conductibility spectra (FC) of layered compounds ZnIn2S4 for the initial sample (Figure 5 the curve 1) and irradiated with different doses of accelerated electrons 1018 cm-2, 1020 cm-2 with the energy 60keV (curves 2 and 3, Figure 5). From the comparison of these curves, it is observed that after irradiation with the dose 1018 cm-2, highest spectrum is moving towards the area of low energies located at 2,68 eV (curve 2, Figure 5). On the high-energy wing of the spectrum, it is clearly perceived a platform feature at 2,85 eV. It should be noticed that at the radiation dose 1018 cm-2, signal intensity FC in the maximum located at 2,85 eV increases, which corresponds to the fundamental absorption.
In the research it was found that the spectrum FC of mono-crystals ZnIn2S4 undergoes radical changes by increasing the dose of irradiation (curve 3, Figure 5). At the irradiation of ZnIn2S4 with accelerated electron of 1020 cm-2 dose, maximum value of sensitivity is shifted even further to the area of low energies and is set at 2,34 eV. On the high-energy wing of the spectrum, an additional maximum to the energy 2,63 eK is highlighted, giving way, in terms of intensity, to initial spectrum (curve 1). At high doses of radiation 1020 cm 2, the samples' resistance to the dark decreases irreversibly up to 102 Qcm and photosensitivity decreases accordingly. Similar studies have been carried out on the crystals CdGa2S4 that have demonstrated that at their irradiation with D « 1020cm-2 dose, significant changes occur in optical and radiation properties.
Fig.5. Photo-conductivity spectra of ternary compounds ZnIn2S4 :
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original sample (curve 1) and irradiated with different doses of accelerated electrons D «1018 cm-2,
D ^102° cm-2 (curves 2 and 3), with energy 60 keV.
4. ANALYSIS OF RESULTS
To summarize, based on these results, we could notice about high stability of optical and radiation properties of semiconductor compounds CdGa2S4 and ZnIn2S4, which is also demonstrated by the results of investigations of the optical absorption spectrum of ZnIn S samples, irradiated with accelerated electrons with energy 50, and respectively 100 keV, at a density of electron beam 1,5 40"2 A • cm-2 and at the temperature of 296K (Figure 6). Exponential sector of the absorption spectrum can be explained by the presence of „tails" of state densities of the conduction band conditioned by the disorder of cationic subnet. This fact is described in detail in the case of ( ZnIn2 S4) [2].
Based on the study of the bibliographic data, it can be noticed that significant changes of the properties of elementary or binary semiconductors [8] at their irradiation with the electrons of
energies up to 100 keV occur starting by the doses (1014 ^ 1016 j cm-2 and in the case of compounds
ZnIn S and CdGa S essential changes in optical properties occur starting by doses higher than
1019 cm-2 . In the case of ternary semiconductor compounds, which have a forbidden energy band, higher than 3eV, while radiating them with dose « 1014cm-2, it shows a reduction of optical absorption in the ultraviolet diapason of spectrum, which is called - the effect of low doses. These results correlate with the data presented in [9] .
The absorption increase is recorded at doses higher than 1015cm-2 . The results we obtained and presented in this work concerning the nature of clear structure of the spectrum FC of single crystals
ZnIn2S4 irradiated at dose 1018cm-2 are consequences of state improvement of crystals' surface at the initial stage of irradiation, which we suppose, influences the recombination of free charge carriers at the sample's surface.
At the same time, we find that the formation mechanism of the threshold defects in layer compounds ZnIn2S4 is initiated at doses higher than 1018cm-2.
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1A 2,6 2.8
lit», eV
Fig.6. Optical absorption for single crystals ZnIn2S4 irradiated with accelerated electrons: 1-50keV ; 2-100keV ; j = 1,5 ■1Qr2A • cm-2 ; T=296K.
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