Nominally undoped ZnTe and CdTe crystals were implanted with radioactive $^{111}\!$Ag, which decays to $^{111}\!$Cd, and investigated by photoluminescence spectroscopy (PL). In ZnTe, the PL lines caused by an acceptor level at 121 meV are observed: the principal bound exciton (PBE) line, the donor-acceptor pair (DAP) band, and the two-hole transition lines. In CdTe, the PBE line and the DAP band that correspond to an acceptor level at 108 meV appear. Since the intensities of all these PL lines decrease in good agreement with the half-life of $^{111}\!$Ag of 178.8 h, both acceptor levels are concluded to be associated with defects containing a single Ag atom. Therefore, the earlier assignments to substitutional Ag on Zn- and Cd-lattice sites in the respective II-VI semiconductors are confirmed. The assignments in the literature of the S$_1$, S$_2$, and S$_3$ lines in ZnTe and the X$\scriptstyle^\textrm{Ag}_{1}\,\,,$ X$\scriptstyle^\textrm{Ag}_{2}$/ C$\scriptstyle^\textrm{Ag}_{1}\,$  and C$\scriptstyle^\textrm{Ag}_{2}\,$ lines in CdTe to Ag-related defect complexes are not confirmed

Burchard, A

Deicher, M

Filz, T

Hamann, J

Lany, S

Ostheimer, V

Strasser, F

Wichert, T

Wolf, H

Nominally undoped ZnTe and CdTe crystals were implanted with radioactive /sup 111/Ag, which decays to /sup 111/Cd, and investigated by photoluminescence spectroscopy (PL). In ZnTe, the PL lines caused by an acceptor level at 121 meV are observed: the principal bound exciton (PBE) line, the donor-acceptor pair (DAP) band, and the two-hole transition lines. In CdTe, the PBE line and the DAP band that correspond to an acceptor level at 108 meV appear. Since the intensities of all these PL lines decrease in good agreement with the half-life of /sup 111/Ag of 178.8 h, both acceptor levels are concluded to be associated with defects containing a single Ag atom. Therefore, the earlier assignments to substitutional Ag on Zn- and Cd-lattice sites in the respective II-VI semiconductors are confirmed. The assignments in the literature of the S/sub 1/, S /sub 2/, and S/sub 3/ lines in ZnTe and the X/sub 1//sup Ag/, X/sub 2 //sup Ag//C/sub 1//sup Ag/, and C/sub 2//sup Ag/ lines in CdTe to Ag- related defect complexes are not confirmed. (16 refs)

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Identification of Ag-acceptors in $^{111}Ag^{111}Cd$ doped ZnTe and CdTe

1EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN-EP-2000-02511 February 2000Identification of Ag-acceptors in 111Ag/111Cd dopedZnTe and CdTeJ. Hamanna, A. Burchardb, M. Deicherb, T. Filza, S. Lanya, V. Ostheimera, F. Strassera, H.Wolfa, ISOLDE Collaborationc, and Th. WichertaAbstractNominally undoped ZnTe and CdTe crystals were implanted with radioactive 111Ag, whichdecays to 111Cd, and investigated by photoluminescence spectroscopy (PL). In ZnTe, the PLlines caused by an acceptor level at 121 meV are observed: the principal bound exciton (PBE)line, the donor-acceptor pair (DAP) band, and the two-hole transition lines. In CdTe, the PBEline and the DAP band that correspond to an acceptor level at 108 meV appear. Since theintensities of all these PL lines decrease in good agreement with the half-life of 111Ag of178.8 h, both acceptor levels are concluded to be associated with defects containing a singleAg atom. Therefore, the earlier assignments to substitutional Ag on Zn- and Cd-lattice sites inthe respective II-VI semiconductors are confirmed. The assignments in the literature of the S1,S2, and S3 lines in ZnTe and the XAg1 , XAg2 / CAg1 , and CAg2  lines in CdTe to Ag-relateddefect complexes are not confirmed.(IS369)Presented at: 9th International Conference on II-VI Compounds, Kyoto, November 1-5, 1999Accepted for publication in: J. Cryst. Growth                                                aTechnische Physik, Universität des Saarlandes, D-66041 Saarbrücken, GermanybFakultät für Physik, Universität Konstanz, D-78134 Konstanz, GermanycPPE ISOLDE, CERN, CH-1211 Genève, Switzerland2PACS numbers: 61.72.Vv, 71.55.Gs, 78.55.Et.Key words: Ag, ZnTe, CdTe, photoluminescence, radioactive isotope 111Ag, implantation.1. IntroductionPhotoluminescence (PL) spectra reveal a lot of defect levels in semiconductors, but it is oftendifficult to identify the chemical nature of the impurity causing a particular defect level. Anunambiguous chemical identification can be achieved if radioactive isotopes are used asdopants. Depending on whether the parent or the daughter isotope is the dopant element, thedopant concentration decreases or increases with the characteristic half-life of the radioactivedecay, respectively. If the intensity of a PL line is correlated with the half-life of theradioactive decay, the parent or daughter isotope can be identified as a constituent of thedefect causing the observed level [1].The element Ag is an unintentional impurity in both ZnTe and CdTe, which is important forunderstanding the electrical and optical properties of these materials [2]. In the PL spectra ofundoped ZnTe, lines corresponding to an acceptor level at 121 meV are observed: theprincipal bound exciton line (PBE) at 2.3740 eV ( AAg1 ), which is caused by therecombination of excitons bound to this acceptor, and the corresponding two hole transitions(THT) [3, 4, 5]. The THT, excitonic recombinations that leave the remaining hole in anexcited state, are observed at 2.2737 eV ( AAg2 ), 2.2630 eV ( AAg3 ), and in the energy rangebetween 2.2587 eV and 2.2530 eV ( AnAg‡ 4 ). Magnea et al. show the intensities of these PLtransitions to be enhanced by diffusion of Ag into ZnTe crystals and assign the observedacceptor level to substitutional Ag on a Zn-lattice site (AgZn) [4]. The same series of PL lines,however, appear after annealing the crystal at 750 °C for 7 days in a Zn or Te atmosphere [3].Additionally, the bound exciton lines S1 at 2.3149 eV, S2 at 2.3486 eV, and S3 at 2.365 eVare created by diffusion of a high concentration of Ag into ZnTe. S1 is assigned to pairs ofsubstitutional and interstitial Ag. S2 and S3 are tentatively identified with defects which arecomposed of three atoms involving Ag [6, 7].In CdTe, the situation is very similar: Molva et al. report on PL investigations regarding anacceptor level at 108 meV in undoped CdTe. The AAg1  line at 1.5885 eV, the AAg2  line at1.5010 eV, and the donor-acceptor pair (DAPAg) band at 1.491 eV are observed [8]. Again,the intensities of these PL lines are shown to be enhanced by diffusion of Ag into CdTe, and3the observed acceptor level is assigned to AgCd defects [9]. Using radioactive 111Ag, thisassignment is confirmed for the DAPAg band, but not for the excitonic lines AAg1  and AAg2[10]. Storing Ag-doped CdTe crystals at room temperature for several weeks creates the linesXAg1  at 1.5880 eV [11], XAg2  at 1.5815 eV [11], also called CAg1  [12], and CAg2  at1.5583 eV [12]. The CAg1  line is tentatively attributed to pairs of substitutional and interstitialAg or to complexes of Cd vacancies and interstitial Ag [11].All these assignments in both ZnTe and CdTe are not conclusive, though, because thediffusion process used for doping also produces intrinsic defects and possibly defectcomplexes containing impurities already present in the crystals. In order to verify the aboveassignments, ZnTe and CdTe were doped with 111Ag, which decays to 111Cd with a half-lifeof t1/2 = 178.8 h [13], and investigated by PL.2. Experimental procedureBridgman-grown, nominally undoped ZnTe and CdTe crystals of typically 3· 3· 0.5 mm3 wereimplanted with 111Ag ions at the ISOLDE mass-separator facility (CERN, Geneva). Theimplantation was performed with a dose of 5 × 1013 cm-2 and an energy of 60 keV, resulting inan average implantation depth of 30 nm. In order to remove implantation-induced damage, theZnTe crystals were annealed at 700 K for 30 min in a Te atmosphere and the CdTe crystalswere annealed at 620 K in vacuum. Since Ag is a fast diffuser in both ZnTe and CdTe [3, 14],the diffusion occurring during this annealing process resulted in a broad depth distributionand, as a consequence, in a low concentration of the 111Ag atoms.The PL measurements were carried out at 1.8 K. The 351.1 nm line of an Ar laser, which wasattenuated to 10 mW and focussed to a diameter of about 200 µm, was used for excitation.The luminescence was analysed by a 0.5 m grating monochromator and detected by a CCDcamera.3. Experimental results and discussionIn order to look for the changes of the PL spectra due to the element transmutation, PLmeasurements were performed at various times after annealing. Between these measurementsthe temperature never exceeded 150 K and, therefore, modifications of the crystals, whichwere not induced by the radioactive decay, probably did not occur.4523 522      43 h  261 h  623 h1488 h ZnTe:111Ag fi  111Cd2.370 2.375AAg1 555 550 545AAg2ACu1 ,ALi1DAPLi-2LOAAgn‡ 4DAPAgAAg2 -LOACu2DAPCuAAg3Wavelength (nm)PL-Intensity (arb. units)Energy (eV)2.24 2.26 2.28·  500 Fig. 1. PL spectra of a ZnTe crystal implanted with 111Ag, which were recorded at differenttimes after annealing (700 K, Te atmosphere, 30 min).a) ZnTeA subset of the PL spectra obtained from ZnTe is shown in Fig. 1. It consists of two parts, thePBE region near the band edge and the THT region at lower energies. In each part, theintensities of the spectra are normalised to the intensity of the dominating PL line, i.e. theintensities in the PBE region are normalised to the intensity of the unresolvable ACu1  andALi1  lines at 2.3746 eV [4, 5] and the intensities in the THT region are normalised to theDAPCu band at 2.234 eV [4]. Cu and Li are usually present as unintentional impurities in bothZnTe and CdTe. The intensities of all lines that correspond to the acceptor level at 121 meVmonotonously decrease with time, whereas the intensities of the Cu- and Li-related linesremain constant. The seeming decrease of the intensities of the DAPLi-2LO band and theACu2  line is due to the decreasing intensities of the neighbouring Ag-related lines. Ifcompared with Ref. [4], the PL spectra show that the lines AnAg‡ 4  are not resolved into singlelines and the DAPAg band at 2.260 eV is clearly visible. The observation of the DAPAg bandin this experiment is probably due to a higher donor concentration in our crystals. The S1, S2,and S3 lines are not observed (see below).Figure 2 shows the intensities of a) the AAg1  line, b) the AAg2  line, and c) the sum of theAnAg‡ 3  lines and the DAPAg band, logarithmically plotted as a function of time. The intensitiesare determined by integrating the intensities in the energy ranges between 2.3735 eV and52.3740 eV, between 2.2718 eV and 2.2739 eV, and between 2.2516 eV and 2.2643 eV,respectively. The exponential fits yield half-lives t1/2 = 170– 10 h, t1/2 = 188– 30 h, andt1/2 = 213– 30 h, which are in good agreement with the half-life t1/2 = 178.8 h of 111Ag.  (b)  AAg2t1/2 = 188 ± 30 hPL-Intensity (arb. units)ZnTe:111Ag fi  111Cd(a)  AAg1t1/2 = 170 ± 10 h 0 200 400 600 800 1000 1200 1400t1/2 = 213 ± 30 h   Data   Exponential fit(c)  AAgn ‡  3 and DAPAg  Time (h)Fig. 2. Intensity of (a) the AAg1  line, (b) the AAg2  line, and (c) the sum of the AnAg‡ 3  lines andthe DAPAg band in ZnTe, logarithmically plotted as a function of time after annealing. Theconstant background, determined from the exponential fit, is already subtracted from theexperimental data.Normalising the data to other lines in the PL spectrum yields similar results, but with a largerscatter of the extracted intensities. In accordance with the literature [3, 14], the diffusionoccurring during the annealing process resulted in a low concentration of 111Ag, which isconcluded from the low intensities of the Ag-related lines. They are not stronger than the linesrelated to the Li and Cu impurities, which are usually present in the order of 1015 cm-3.6Therefore, the indirect effects discussed in Ref. [15] and in Ref. [1], which are connected withthe change of the doping concentration through the radioactive decay and affect the observedintensities as well, are not relevant, because these effects are only important at high dopinglevels of radioactive isotopes. Hence, the observed insensitivity to the normalisation is due tothe low concentration of 111Ag.781.0 780.5 780.0 779.5ACu1ALi1 , ANa1AAg1   48 h146 h214 h312 hCdTe:111Ag fi  111CdWavelength (nm)PL-Intensity (arb. units)Energy (eV)1.588 1.589 1.590 Fig. 3. PL spectra of a CdTe crystal implanted with 111Ag, which were recorded at differenttimes after annealing (620 K, vacuum, 30 min).b) CdTeIn Fig. 3 a subset of the PL spectra obtained from CdTe is shown. The intensities arenormalised to the intensity of the ACu1  line at 1.5896 eV [9]. Obviously, the intensity of theAAg1  line at 1.5885 eV decreases with time. Its intensity is determined by integration of theintensities between 1.5883 eV and 1.5887 eV and logarithmically plotted in Fig. 4. The bestfit is obtained with t1/2 = 150– 40 h, which agrees well with the half-life of 111Ag. For theDAPAg band at 1.491 eV, the agreement with the half-life of 111Ag has been shownpreviously [10]. The AAg2  line is not observable because it is covered by the DAPLi, Natransitions (not shown). The XAg1 , XAg2 / CAg1 , and CAg2  lines are not observed, either (seebelow).For both ZnTe and CdTe, the presented results unambiguously confirm that the acceptorlevels at 121 meV and 108 meV are associated with defects containing a single Ag atom.750 100 150 200 250 300   Data   Exponential fitCdTe:111Ag fi  111Cd            AAg1  lineT1/2 = 150 ± 40 h  PL-Intensity (arb. units)Time (h)Fig. 4. Intensity of the AAg1  line in CdTe, logarithmically plotted as a function of time afterannealing. The background, determined for each spectrum individually, is already subtractedfrom the experimental data.Taking into account the Td symmetry of these acceptors, which was previously shown [5, 16],the present data verify the assignments to the AgZn and AgCd defects in ZnTe and CdTe,respectively. In addition to the lines caused by these acceptor levels, no lines are observed thatchange their intensities during the decay of 111Ag. In particular, the S1, S2, and S3 lines inZnTe and the XAg1 , XAg2 / CAg1 , and CAg2  lines in CdTe are not observed. Since the absenceof these lines might be due to the low Ag concentrations in the crystals, both Bridgman-grownbulk crystals and MOCVD-grown films of the two materials ZnTe and CdTe were implantedwith a higher dose of 5 × 1014 cm-2 stable Ag ions and annealed in various atmospheres atvarious temperatures. Although the Ag concentration in these crystals is higher, especially inthe MOCVD-grown films being only 1 µm thick, the lines looked for are not observed, either,not even after storing the crystals and films at room temperature for several weeks. Therefore,the assignments of the S1, S2, and S3 lines in ZnTe and the XAg1 , XAg2 / CAg1 , and CAg2  linesin CdTe to Ag-related defects are not confirmed.The financial support by the Bundesministerium für Bildung und Forschung under grant No.WI04SAA is gratefully acknowledged. It is a pleasure to thank R. Grötzschel and U. Hornauer(FZ Rossendorf) for the implantation of stable Ag.8References[1] R. Magerle, Mat. Res. Soc. Symp. Proc. 442 (1997) 3.[2] E. Molva, J.L. Pautrat, K. Saminadayar, G. Milchberg, and N. Magnea, Phys. Rev. B30 (1984) 3344.[3] D. Bensahel, N. Magnea, J.L. Pautrat, J.C. Pfister, and L. Revoil, Inst. Phys. Conf. Ser.46 (1979) 421.[4] N. Magnea, D. Bensahel, J.L. Pautrat, and J.C. Pfister, Phys. Stat. Sol. (b) 94 (1979)627.[5] H. Venghaus, P.J. Dean, Phys. Rev. B 21 (1980) 1596.[6] B. Monemar, W.M. Chen, P.O. Holtz, and H.P. Gislason, Phys. Rev. B 36, (1987)4831.[7] B. Monemar, P.O. Holtz, H.P. Gislason, N. Magnea, Ch. Uihlein, and P.L. Liu, Phys.Rev. B 32 (1985) 3844.[8] E. Molva, J.P. Chamonal, and J. L. Pautrat, Phys. Status Solidi B 109 (1982) 635.[9] J.P. Chamonal, E. Molva, and J.L. Pautrat, Solid State Commun. 43 (1982) 801.[10] J. Hamann, A. Burchard, M. Deicher, T. Filz, V. Ostheimer, C. Schmitz, H. Wolf,Th. Wichert, and the ISOLDE Collaboration, Appl. Phys. Lett. 72 (1998) 3029.[11] B. Monemar, E. Molva, and Le Si Dang, Phys. Rev. B 33 (1986) 1134.[12] J.P. Chamonal, E. Molva, J.L. Pautrat, and L. Revoil, J. Cryst. Growth 59 (1982) 297.[13] see e.g. E. Browne, J.M. Dairiki, R.E. Doebler, in: Table of Isotopes, Ed.C.M. Lederer, V.S. Shirley (John Wiley & Sons, New York, 1978).[14] I. Lyubornirsky, M.K. Rabinal, D. Cahen, J. Appl. Phys. 81 (1997) 6684.[15] J. Hamann, A. Burchard, M. Deicher, T. Filz, V. Ostheimer, F. Strasser, H. Wolf,ISOLDE Collaboration, and Th. Wichert, submitted to Physica B.[16] E. Molva and Le Si Dang, Phys. Rev. B 27, 6222 (1983).

Identification of Ag-acceptors in $^{111}\!$Ag $^{111}\!$Cd doped ZnTe and CdTe

Identification of Ag-acceptors in $^{111}\!$ Ag $^{111}\!$ Cd doped ZnTe and CdTe

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Identification of Ag-acceptors in 111 ⁣^{111}\!111Ag 111 ⁣^{111}\!111Cd doped ZnTe and CdTe

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Identification of Ag-acceptors in $^{111}\!$ Ag $^{111}\!$ Cd doped ZnTe and CdTe