Nuclear Instruments and Methods in Physics Research A283 (1989) 167-171 North-Holland, Amsterdam
POSITRON ANNIHILATION IN (x-H91 2 Y.F . NICOLAU D. LETZ/D. OPT, CEN-G, 85 X, F-38041 Grenoble Cédex, France
P. MOSER DRF, SPh, CEN-G, 85 X, F-38041 Grenoble Cédex, France
INSTN, CEN-Saclay, F-91191 Gif-sur- Yvette Cédex, France
Received 26 May 1989 The lifetime of the delocalized positrons in a-HgIZ crystals is found to be 320±4 ps. Some crystals grown from the vapor phase in an excess of iodine have native mercury vacancies. The lifetime of positrons localized in mercury vacancies is 360±5 ps . Some crystals grown in unstabilized solutions with respect to added iodine show shorter lifetime components of 180-200 ps attributed to positron annihilation inside mercury-rich and/or impurity-rich clusters . Other crystals show both shorter and longer lifetime components than the lifetime in the bulk, indicating the coexistence of mercury vacancies with clusters nch in mercury interstitials and/or impurities . The energy resolution of the y-ray detectors is correlated to the positron-lifetime spectra of the crystals . 1. Introduction As is well known, the performance of (Y-1-1 91 2 -y-ray spectrometers is diminished by trapping of the charge carriers produced by absorption of -y-photons in different point defects of the crystal lattice . Many trapping levels have been determined in undoped crystals of different origin by thermally stimulated current, photoconductivity, photovoltaic effect and by mobility-trapping time product measurements [2-7]. Some levels could be identified as hole traps, others as electron traps [3,5,6] . Whited and Van den Berg  related some different traps to possible point defects by investigating both undoped, Hg-doped and I-doped crystals grown from the vapor phase. Nicolau and Rolland  studied the deviation from stoichiometry in a-HgI Z crystals by means of lattice constants, density and transition temperature measurements . Among the techniques that can be used to directly determine the interstitial or vacancy nature of a defect, positron annihilation provides a powerful method to investigate vacancies on an atomic scale. The positrons are trapped by vacancies and are subsequently annihilated with characteristic lifetimes. The generation of
* Now with DRF, SPh, DSPE, CEN-G, 85X, F-38041 Grenoble Cédex, France. 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
vacancies in metals has been widely studied by this technique [9-11] . The generation of vacancies in alkali halides by irradiation [12-15] or by plastic deformation  has also been investigated by positron annihilation . Only a few studies have been devoted to semiconductors. As grown, silicon is free of native vacancies but vacancies can be produced by irradiation [16-18] or by plastic deformation  . In contrast to silicon, compound semiconductors like GaAs [20-26] or Cd x Hg,- xTe  contain native vacancies. The concentration of the native vacancies is estimated to be rather high, in the order of 10 17 -1018 cm -3 . In this study an attempt to detect, characterize and identify such defects by measurements of positron annihilation lifetime spectra in different a-HgI Z crystals, grown in solution or from the vapor phase, was carried out. 2. Experimental A 10 ltCi positron source prepared by evaporating a ZZ NaCl solution on a Ni foil (1 Wm thickness) was sandwiched between two identical HgI Z samples of 5 X 5 X 0.5 mm3. Positron-lifetime spectra with 10 6 counts have been recorded at 77 K using a fast-fast coincidence system having a time resolution of 300 ps at the FWHM of the peak .
Y. E. Nicolau
The lifetime spectra have been fitted to the sum of exponential decay components : n(t)=F_I, exp(-t/T,),
convoluted with the Gaussian resolution function of the spectrometer . Here I, and T, are the relative intensities and decay constants, respectively. Only one or two exponential components have been resolved in our experimental spectra after background and source corrections . The average lifetime T is defined as : T
= Il T1
The fitting has been made so that the variance is minimum and so that the average lifetime, T, coincides with the center of mass of the spectrum within f 1 ps. The samples have been sawn with a wet (1 :1 vol. solution of dimethylsulfoxide-ethanol) thread saw from large single crystal slides, rinsed with methanol and dried . The slides were sawn in the same way from single crystals, usually parallel to the basal plane (001), chemo-mechanically polished with a 1 :1 vol . solution of dimethylsulfoxide-methanol on a silk cloth, rinsed with methanol and dried . In order to prevent damage of the soft sample surfaces great care was taken in handling the slides . Since the crystals could have inhomogeneously distributed point defects two positron-lifetime spectra have been recorded with the same samples, the second spectrum has been recorded after turning the samples up against the source .
3 . Results and discussion 3.1 . Crystals showing one positron
Most of the crystals exhibit positron-lifetime spectra showing only one lifetime. This lifetime varies from 316 to 324 ps . We consider that the same single annihilation state yields these lifetimes and we attribute a lifetime of 320 ± 4 ps to this state . This lifetime, corresponding to annihilation of positrons in a delocalized state is referred to as the positron lifetime in the bulk . Many crystals grown from the vapour phase, such as EG&G crystals V5-2, S2-26, N18-2, 78-1-ED (grown at the Louis Pasteur University in Strasbourg), V-II-9 (grown at the Hebrew University of Jerusalem), and in solution, such as LETI crystals 32, 33 and 39, show only this positron lifetime in the bulk . These are crystals representative of the state of the art reached in different laboratories in growing detector crystals . The methods used for the purification and growth of these different crystals and their analyses have been published elsewhere [28-33] . (With regard to purity, LETI crystal 32 is similar to 29 ; crystals 33 and 39 are similar to 30 ; EG&G crystal V5-2 is similar to S2-26 and N18-2 to
annihilation in a-HgI2
N8-7.) Compared with other crystals they have a rather low impurity content and a composition close to the stoichiometric composition. They generally yielded -y-ray detectors having a good energy resolution . 3.2. Crystals showing two or more positron ponents
Two crystals give positron-lifetime spectra showing a long lifetime of 360 f 5 ps. The intensity of this long lifetime is rather high (see table 1), with the OT-22 crystal giving a higher 12 intensity than the OT-19 crystal. The average lifetime is higher than 320 ps for both crystals . This long T2 lifetime is about 11% longer than the lifetime in the bulk. We assign this long lifetime to annihilation of positrons localized in a vacancy-type defect. It is well established that positrons can be trapped in vacancies [9-111 and the decomposition of the positron-lifetime spectra of crystals with vacancies can be explained in the framework of the positron trapping model [341 . By applying this model to analyse the positron-lifetime spectra of crystals OT-19 and OT22, we calculate the positron lifetime in the bulk assuming that the positrons find only one kind of trap in the crystals . We obtain 324 ps for the OT-19 crystal and 322 ps for the OT-22 crystal, which are quite close to the lifetime of 320 ± 4 ps that we have assigned to the bulk. This agreement ascertains that the positrons are annihilated in these crystals either delocalized or localized in vacancies. The increase of the positron lifetime from 320 to 360 ps in these crystals is comparable to the increase in the positron lifetime found in other semiconductors for annihilation of positrons trapped in monovacancies produced by irradiating the crystals with electrons. For example, the increase is about 22% in Si [16-18], 26% in Ge , 10% and 26% in GaAs [20-26]. Based on this comparison we assume that the defect is a monovacancy . Positively charged defects in «-HgI2, as in other crystals, are most likely unable to trap positrons owing to electrostatic repulsion. It has been shown, for inTable 1 Positron lifetime components 'rt [PSI
CRNS OT-19 CRNS OT-22 CRNS-S-06 a) a) EG&G N8-2 a)
Imposed lifetime component (see text) .
1 70 1
Y F. Nicolau et al. / Positron annihilation in a-HgI2
stance, that arsenic vacancies do not trap any more positrons when they become positively charged . Thus these vacancies in a-Hg12 should be either neutral or negatively charged. The OT-22 and OT-19 crystals have been grown from the vapor phase at the CRN in Strasbourg. Both crystals have been grown from a Baker commercial material melted with I (iodine) addition and subsequently five times sublimated . 0.5 g I and 1 g I have been added to 100 g HgI 2 before melting the starting material used to grow the OT-19 and OT-22 crystals respectively. The I added to the melt changes the stoichiometry deviation of the Hg-rich commercial material into I-rich . The starting material remained I-rich although most of the excess of I was pumped away during sublimations . Thus both crystals have been grown from an I-rich vapor phase. Moreover, the OT-22 crystal has been grown in a large excess of I by directly introducing 0.1 g iodine in the (0.5 1) growth ampoule, corresponding to a mole ratio of I 2/HgI 2 in the gas phase of about 900 . Under these conditions OT-19 and OT-22 crystals have been grown that are rich in I. a to ß phase transition temperature (Tr) measurements on powders determined by differential scanning calorimetry under a heating rate of 10 K/min gave a T of 105.0 K for the OT-19 crystal and a T, r of 402.0 for the OT-22 crystal . At comparable purity, these T,r 's, which are lower than 407 .4 K, indicate a departure from stoichiometry larger for the OT-22 crystal than for the OT-19 crystal . We arrive at the same conclusion comparing the intensities 12 of the 360 ps lifetime (see table 1) considering that the departure from the stoichiometry involves Hg vacancies. Hg vacancies (VHg, VHg and most probably VHg ) are predicted in I-rich crystals [36,4] . The presence of Hg vacancies in these crystals does not exclude the possible presence of incorporated I interstitials  but the positron annihilation is known to be insensitive to the presence of non-agglomerated interstitials. Applying the trapping model we calculate the total positron trapping rate k in the vacancy. This rate is given by the relationship : I2 1 1 _ k= 1-I2I Tb - T2 for 12 and T2 given in table 1, Tb being the lifetime in the bulk (320 ps). Assuming a specific trapping rate at the vacancy of 2 x 10-9 s -1 cm3 we estimate the total concentration of vacancies to be equal to 1017-10 18 cm -3, as in other compound semiconductors . Other crystals (see table 2) give rise to positron-lifetime spectra which exhibit the 320 ps lifetime in addition to a shorter lifetime of about 190 ps, which varies from one sample to another . The average lifetime in these spectra is always longer than 290 ps. The presence
Table 2 Positron lifetime components Crystal
EG&G N8-7 LETI-27 LETI-30
296+1 297±1 296±1
192±18 186±15 177±15
322+6 315±5 317±6
20±7 14±5 15±5
80±7 86±6 85±5
of these short lifetimes is inconsistent with the idea that the lifetime in the bulk is 320 ps. However, the LETI crystals 27 and 30 which give rise to these short lifetime components are shown, by SEM and by optical microscopy examination, to contain lens-shaped clusters , rich in Hg and/or in other impurities, having diameters ranging from 3 to 500 Wm and thicknesses ranging from 0.3 to 50 pm . The positrons once directly implanted in such clusters are annihilated inside the clusters with lifetimes reflecting the electronic density of the clusters. So, we assign a lifetime of 180-200 ps to positron annihilation inside these clusters. The intensity I, is about 15%. These crystals have been grown from newly prepared solutions . In order to correct the departure from the stoichiometry of the dissolved starting material that regularly turns Hg-rich during successive distillations [29,38] an excess of I was added. But the excess of I reacts slowly with the Hg2I2 . Thus these crystals are expected to be lightly Hg-rich as they were grown from an unstable (1-consuming) solution . Hg-rich crystals can contain Hg interstitials and/or I vacancies. Neutral I vacancies, if detected by positrons, would introduce a long lifetime component and positively charged I vacancies V, [4,36,39] are not detected by positrons. Hg interstitials have been indirectly determined in Hg-rich crystals . Consequently we assume an increased concentration of Hg interstitials inside the clusters. Finally, in two crystals, CRNS-S-06 and EG&G N8-2, the lifetime spectra contain more than one component, but the resolution of components is difficult . Long lifetimes of about 355 f 10 ps appear. We have fixed the lifetime at about 360 ps in order to obtain a two-lifetime decomposition with precise intensities . The average lifetime in these crystals is lower or equal to 320 ps (see table 1). In these crystals, clusters rich in Hg interstitials and native Hg vacancies coexist, resulting lifetime spectra which are difficult to analyse with two free components. A better analysis would be made with three lifetime components but the analysis in three components is difficult because the lifetimes in the clusters (180-200 ps) and in the bulk in the presence of vacancies (about 230 ps) are too close. LETI crystals 27 and 30 and CRNS crystals S-06 and OT-22 gave detectors that do not satisfactorily resolve the 241Am peak at 59.6 keV, but the OT-19 crystal gave detectors showing an energy resolution of 4
YF Nicolau et al / Positron annihilation in a-HgIZ
keV FWHM for this peak. The EG&G N8-2 crystal gave detectors having a rather poor energy resolution, whereas the EG&G N8-7 crystal gave detectors having a rather good energy resolution . The samples used for positron annihilation measurements from this last crystal have been sawn from a slide kept in a nontightly closed plastic box for many years and were not etched . The positron annihilation spectrum of this crystal, credited as quasistoichiometric according to the quality of detectors that it gave and showing 20% of a short 192 ps lifetime, indicates Hg enrichment at its surfaces by ageing in air. The positron annihilation spectrum obtained using samples of a LETI 34 crystal, sawn parallel to its natural (001) faces, aged in air for many years and unetched, also shows a short lifetime component . This short lifetime vanishes after etching the samples . To all appearances ageing of crystals in air forms a Hg-rich surface layer either by noncongruent evaporation  or by print-out mode of photolysis , or by reduction with organics . Both nonstoichiometric types of Hg-rich and I-rich crystals, containing Hg interstitials (giving rise to hole trapping) and Hg vacancies (giving rise to electron trapping) respectively, give detectors having poor energy resolution . 3.3. Effect of annealing, of quenching and of plastic deformation Samples of the quasistoichiometric LETI-39 crystal, giving a positron annihilation spectrum showing only the lifetime in the bulk have been : (1) bent arround the a-axis : (2) quenched at 20 ° C from 125 ° C; (3) annealed for one week in I vapors at 125 ° C under atmospheric pressure : (4) annealed for two days in Hg vapors at 125 ° C. The samples annealed in Hg vapors should be etched before positron annihilation measurements in order to eliminate a polycrystalline H9 2 1 2 surface layer formed during annealing. The positron annihilation spectra of all treated samples remained unchanged . From these last experiments we draw the following conclusions : (1) The most common edge dislocations introduced by bending, running parallel to the  direction and having the Burgers vector b , are not negatively charged. (2) The native Hg interstitials and Hg vacancies that we identified are more probably introduced during the growth from a nonstoichiometric mother phase than resulting from a thermal equilibrium of a Frenkel type : Hg ;=:t VHg + Hg r . The self-diffusion coefficients of Hg and of I at 125 ° C appear to be very low, so that redressing the departure of some nonstoichiometric crystals from
stoichiometry by annealing in I or in Hg vapors at 125 ° C is inefficient, in agreement with other reports [39,42] .
Acknowledgements The authors are indebted to P . Remy for his technical assistance in carrying out the positron annihilation experiments and to M. Schieber, C. Ortale and P . Siffert for sending us samples of different crystals .
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