- Email: [email protected]
Iodine detection in iodine-doped CdS single crystals
Journal of Crystal Growth 37 (1977)9 0 North-Holland Publishing Company
12
IODINE DETECTION IN IODINE-DOPED CdS SINGLE CRYSTALS C. PAORICI, C. PELOSI and G. ATTOLINI Laboratorio MASPEC NCR, 43100 Par,na, Italy Received 14 Juli 1976
The detection of iodine traces in the 80 1600 ppm range in small size 15 115 mg CdS Crystals has been proved feasible by a DFP version of argentometric titration. The silver ions, used as a titrant, were directly introduced in the solution to be titrated by coulo-generation. The analitical method described, accurate within + 10%, is particularly suitable for low-cost routine analyses of small samples such as crystals grown by vapour-phase chemical transport. As an application of the method, the analytical results were used to evaluate the iodine distribution constant between solid and vapour phase. A value of 1.3 X 10 10 3lmol CdS] 3latm] 1 at a growth temperature of 1143 K was found, in reasonable agreement with previously [g—atof iodine) reported values.
1. Introduction
sic drawback is the difficulty to separate the halogen not only from the matrix, but also from interfering ions often present at very low concentrations. The subsequent concentration of the separated halogen is still a difficult operation in the analytical procedure. This being the case, our attention was turned to finding an analytical method of sufficient simplicity, accuracy and sensitivity for detecting halogen impurities in low-weighing samples of cadmium calcogenides. Analyses of halogen traces in II VI semiconductor compounds have already been described, where use was made of tracers (1131 in CdS [5,6]), neutron activation (I in CdS [7], I, Br, Cl in CdS and ZnSe [81), optic spectrophotometry (Cl in ZnS [9]). In this article we refer to a simple analytical method suitable to reveal halogen traces in the range of 80 1600 ppm in CdS samples of 15 115 mg. The calibration of the method for Cl and Br analysis in CdS, CdSe and CdTe was previously described [10,11] by the authors. Here we present an extension of the method for iodine detection in CdS single crystals grown by iodine chemical transport from the vapour phase.
Halogen detection is of particular interest when vapour-phase chemical-transport reactions which make use of gaseous halogenides for metal purification (Van Arkel and De Boer’s method [1] or for single-crystal growth [21) are employed. In both cases a small portion of halogen remains incorporated in the solid matrix, in a concentration range generally between fractions of ppm up to about 2000 ppm. The presence of halogen impurities can strongly affect many properties of the crystallized solid phase, such as, e.g., the optic and electric properties, or the growth mechanisms of a growing interface. This notwithstanding analyses of halogen impurities in crystals grown by chemical transport (generally of small size and often weighing no more than a few milligrams) are rarely reported. The reason can be seen from the fact that, although many highly sensitive detection methods have been developed, based on mass spectrometry, neutron activation and radioactive tracers, such methods are not easily available to the grower, especially when a low-cost routine analysis is required. If we consider easily available analytical techniques, such as polarography, atomic absorption spectrometry and optic ~spectrophotometry, only this last technique could in principle be suitable for halogen detections [2—41,but an intrin-
2. Outline of the method The method is based on differential electrolytical potentiometry (DEP) or bipotentiometry [12,13]) 9
10
C. Paoru , ci at.
/ Iodine detection
where use is made of argentontetric titrations. The titrant (silver ions) is supplied coulonietrically [13, 14] by means of a silver working electrode. The equivalence point is given by the time necessary fom the obtaining of a iiiaxiniunt (titration peak) in the ctirve “DEP voltage-titration time”. The equipment employed for iodine analysis consisted of a titration cell, a coulometric circuit and DEP electrodes, hasically the same as previously described foi bromine titration [111. The analytical procedure is fairly simple. A calibration of the method if first carried out as follows, in order to define with what accumacy iodine can he recovered. Iodine, as iodide ion, is added to previously weighed powdered CdS samples, by using 1 10 X 10 ~ M standard solutions prepared from previously dried NH4I. The iodine-added CdS sample is then dissolved, at room temperature, within a closed glass tube, by employing the minimum amount of concentrated (657) HNO3. The tube is then cooled, opened, and the solution thus obtained is neutralized with concentrated (25~o)NH4OH, some drops of 25~N2H5OH are added and the solution is vigorously stirred withofa magnetic stirrer up to cornplete decomposition N 2H5OH. The addition of N2H5OH is needed to reduce iodine completely to iodide ions. This solution is now poured into the titration cell together with a base electrolyte solution, consisting of 0.01 M HNO3 in 80 : 20 (volume ratio) methanol water, to give 10 ml volume. After addition of 3 M HNO3 up to a pH of about 3 4, the DFP titration is started. Since the base electrolyte solution often contains impurities which could be titrated by silver ions, a DEP pretitration of this solution is always required, and the time which elapsod between the obtaining of the pretitration peak down to the pretitration end is finally added to the equivalence time of the base-electrolyte-plus iodine solution. Care should be taken since a sulphur peak can sometimes precede the equivalence peak [101 because of an incomplete oxidation of the sulphur ions during closed-tube dissolution. Some comments will now be made about these DEP argentometric titrations for the iodine detection. Previous attempts were reported [13] to have failed because of a probable formation of high resistance AgI layers at the electrode interfaces, which prevented the electrolytic processes from occurring. In those cases, standard solutions prepared with KI
in zodme doped CdS single crystals
weic used. In our case, the presence of NH
4 ions due to the hydrolysis of NH4NO3 in the final solution seems to favour the dissolution of the Ael barriens by the probable formation of Ag(Nll3)~complexes. By using NH4I standard solutions, the iodine detection up to 1 .0 X 10 6 M concentration was found to he feasible within +51/ recovery, with a DEP current of ~0 nA and a Agtgenerating current of 50 A.
3. Results The calibration of the method with iodine-added CdS powders is reported in table I . The maximum deviation between added and found iodide is within +l0~ recovery. Such recovery is assumed as the experimental error of the method. Once the procedure was calibrated,CdS single crystals were analyzed. The crystals were grown by iodine chemical transport in quartz ampoules 18 cm long and 2 cm in inner diameter. The source and deposit temperatures were fixed at 900°C and 870°C for each run, while 3the of iodine amount was varied from 1 to 5 mg per cm ampoule. The deposition time was fixed at 120 h for
fable I Deternimnation of iodide in CdS samples. Calibration (‘dS (mg)
15.8 22.4 22.5 27.0 27.6 29.0 21.5 23.1 41.2 37.5 63.4 42.6 64.0 85.7 93.2 99.1
Iodide Added (ppm) 1658.0 1170.0 1165.0 970.0 949.6 903.0 812.5 756.0 636.2 465.0 413.4 410.0 267.8 203.0 187.0 99.18
I ound (ppm) 1580.6 1042.5 1154.5 889.0 1024.5 941.0 718.8 825.5 686.3 455.5 414.9 370.5 246.0 184.1 197.0 89.5 88.5
~ Recovery 4.6 10.8 0.9 8.3 +7.8 +4.2 11.5 +9.2 +7.8 2.0 +0.3 9.6 8.1 9 3 +5.3 9.7 ±5.3
Gcner ,iting current (pA) lOt) 100 100 100 100 100 100 100 100 101) 100 100 100 100 100 50 100
DOP surrcnt (nA) S 5 5 5 S ~ 5 5 5 5 5
5 5 S 5 5 5
C. Paorici et a!.
/ Iodine detection
in iodine doped CdS single crystals
11
Table 2 Iodine in the ampoule (mg/cm3)
CdS saniple (mg)
Iodine found in the sample (ppm)
~cdI, (atm)
P (atm)
K x lOb 3lmol CdSl ([g/at II
1
40.4 30.6 35.0
269 215 234
0.31
0.54
0.46 0.24 0.30
2
30.6 37.0 59.4
338
0.57
1.00
0.50 0.09 0.38
43.2 26.6 53.9
426 347 525
0.90
1.60
0.63 0.34
4
46.7 49.0 23.0 50.7
556 456 457 551
1.25
2.14
1.01 0.56 0.56 0.99
5
31.0 31.1 48.6
636 486 677
1.45
2.61
1.31 0.58 1.57
3
191
310
i)
1.18
each run. After opening the ampoules, the crystals were washed with an aqueous Na 2S2O3 solution, to
by the relationship
remove any trace of Cd12 and free iodine, and finally
K = [I
rinsed with bidistilled water. The amount of iodine found in the various samples is given in table 2.
4. Discussion and conclusion Analyses of iodine traces in chemical transportgrown CdS crystals,have been previously reported by
Schafer Odenbach [6], who used the analytical data forand evaluating an iodine distribution constant between solid and vapour phase. In order to compare our results with theirs, the same distribution constant was calculated by assuming the same incorporation mechanism, given by Cd1
3latml
2 = 2 CdS (s) + 2 1 + D (Cd2~). 2 (g) + S Here, S2 and I are sulphur and iodine ions in lattice sulphur sites, and D(Cd2~)is a substitutional Cd vacancy. Then the distribution constant will be given
]
3/2PcdI 2
where [I ] is the iodine concentration in the solid phase and ~cdI2 is the partial pressure of CdI2 at the deposition temperature. ~cdI2 values, calculated as described in ref. [15] are given in table 2 together with the overall pressure P (calculated) and the distribution constant. Since the pressure dependence of K is small, an CdS] average 3value 5.9 be X 10 ~ at[g-at 3 [mol [atm]K = can given T iodine] 1143 K. Schafer and Odenbach’s analytical results lead to an iodine distribution constantK4.5 X 10 12 at 1073 K. The agreement between Schafer and Odenbach’s results and ours can be considered reasonably good, especially if we take into account that different iodine solubilities can be observed in the presence of a second impurity in the solid matrix. The amount of stoichiometric defects in CdS crystals can also affect the iodine solubility. Since Schafer and Odenbach’s standardization of the starting CdS powders is not described, a more precise comparison cannot be
12 made. Furthermore, the
C. Paoricm ci al. Iodine detection to iodine doped C’dS single crystals
P( d12 values are slightly d’if
ferent in the two cases because of a different calcula lion approach.
[4] W. Kemula, A. Hulanicki and A. Janoss ski. Talanta 7 (1960) 6S. [SI J. ‘.. Iteun, It. NitsL lie and IlL. Boesterli, P1i~sica 28 (1962) 184.
16! Fl. Schafer and H. Odenbach, Z. Anorg. Ailgem. Chem.
Acknowledgements This research was supported h~a financial con tribution from the G.N.S.M. of ihe C.N.R. The authors are indebted to Mr. G. Zuccalli for his assiSlance in the preparation of C dS ci ystals.
References [I] R.l . Rolsten, Iodide Metals and Metal lodides (Wiley, Ness York, 1961). 121 H. Schdfer, Chemical Transport Reactions (Academic Press, New “~ork, 1964). 131 Spectrophotometric Data for Colotimetric Analysis (Butter~orths,London, 1963).
346 (1966) 127.
[71 C. Paorici, J. Co stal Gross tls S (1969) 315. [81 C.K. Kini and S.S. Voris, J. Phvs. (hem. Solids 3S (1 97~ 1381. ‘i . Kotera, J. Crystal (,ro\s th 11) (1971) 320 110! C. Pelosi, C. l’aorici. 0. Attolini and 0. /uccalli. 1. flee tronanal. (‘hem. 57 (1974) 259. [11! ( . Paorici, ( . Pclosi and 0, \ttolini. J. I lectroarmal. Chem.61 (l97S)213. [1211. Bishop. Analyst 83(1958)212. 11311 . Bishop and R.(,. Danesliwar, Anal. ( Item. 36 (1964) 727. [14] D.G. Davis, Anal. (‘hem. 44 (1972) 79R; 46 (1974) 21R. [151 C. Paorici and C. Pehosi, 1. Crystal Grossth 3S (1976) 65. [161 I’ A. Kroger, The C hemistry of Imperfect Cr) stals (North Holland, Amsterdam, 1964) PP. 622. 716.
[9l S. Ujnc and
12
IODINE DETECTION IN IODINE-DOPED CdS SINGLE CRYSTALS C. PAORICI, C. PELOSI and G. ATTOLINI Laboratorio MASPEC NCR, 43100 Par,na, Italy Received 14 Juli 1976
The detection of iodine traces in the 80 1600 ppm range in small size 15 115 mg CdS Crystals has been proved feasible by a DFP version of argentometric titration. The silver ions, used as a titrant, were directly introduced in the solution to be titrated by coulo-generation. The analitical method described, accurate within + 10%, is particularly suitable for low-cost routine analyses of small samples such as crystals grown by vapour-phase chemical transport. As an application of the method, the analytical results were used to evaluate the iodine distribution constant between solid and vapour phase. A value of 1.3 X 10 10 3lmol CdS] 3latm] 1 at a growth temperature of 1143 K was found, in reasonable agreement with previously [g—atof iodine) reported values.
1. Introduction
sic drawback is the difficulty to separate the halogen not only from the matrix, but also from interfering ions often present at very low concentrations. The subsequent concentration of the separated halogen is still a difficult operation in the analytical procedure. This being the case, our attention was turned to finding an analytical method of sufficient simplicity, accuracy and sensitivity for detecting halogen impurities in low-weighing samples of cadmium calcogenides. Analyses of halogen traces in II VI semiconductor compounds have already been described, where use was made of tracers (1131 in CdS [5,6]), neutron activation (I in CdS [7], I, Br, Cl in CdS and ZnSe [81), optic spectrophotometry (Cl in ZnS [9]). In this article we refer to a simple analytical method suitable to reveal halogen traces in the range of 80 1600 ppm in CdS samples of 15 115 mg. The calibration of the method for Cl and Br analysis in CdS, CdSe and CdTe was previously described [10,11] by the authors. Here we present an extension of the method for iodine detection in CdS single crystals grown by iodine chemical transport from the vapour phase.
Halogen detection is of particular interest when vapour-phase chemical-transport reactions which make use of gaseous halogenides for metal purification (Van Arkel and De Boer’s method [1] or for single-crystal growth [21) are employed. In both cases a small portion of halogen remains incorporated in the solid matrix, in a concentration range generally between fractions of ppm up to about 2000 ppm. The presence of halogen impurities can strongly affect many properties of the crystallized solid phase, such as, e.g., the optic and electric properties, or the growth mechanisms of a growing interface. This notwithstanding analyses of halogen impurities in crystals grown by chemical transport (generally of small size and often weighing no more than a few milligrams) are rarely reported. The reason can be seen from the fact that, although many highly sensitive detection methods have been developed, based on mass spectrometry, neutron activation and radioactive tracers, such methods are not easily available to the grower, especially when a low-cost routine analysis is required. If we consider easily available analytical techniques, such as polarography, atomic absorption spectrometry and optic ~spectrophotometry, only this last technique could in principle be suitable for halogen detections [2—41,but an intrin-
2. Outline of the method The method is based on differential electrolytical potentiometry (DEP) or bipotentiometry [12,13]) 9
10
C. Paoru , ci at.
/ Iodine detection
where use is made of argentontetric titrations. The titrant (silver ions) is supplied coulonietrically [13, 14] by means of a silver working electrode. The equivalence point is given by the time necessary fom the obtaining of a iiiaxiniunt (titration peak) in the ctirve “DEP voltage-titration time”. The equipment employed for iodine analysis consisted of a titration cell, a coulometric circuit and DEP electrodes, hasically the same as previously described foi bromine titration [111. The analytical procedure is fairly simple. A calibration of the method if first carried out as follows, in order to define with what accumacy iodine can he recovered. Iodine, as iodide ion, is added to previously weighed powdered CdS samples, by using 1 10 X 10 ~ M standard solutions prepared from previously dried NH4I. The iodine-added CdS sample is then dissolved, at room temperature, within a closed glass tube, by employing the minimum amount of concentrated (657) HNO3. The tube is then cooled, opened, and the solution thus obtained is neutralized with concentrated (25~o)NH4OH, some drops of 25~N2H5OH are added and the solution is vigorously stirred withofa magnetic stirrer up to cornplete decomposition N 2H5OH. The addition of N2H5OH is needed to reduce iodine completely to iodide ions. This solution is now poured into the titration cell together with a base electrolyte solution, consisting of 0.01 M HNO3 in 80 : 20 (volume ratio) methanol water, to give 10 ml volume. After addition of 3 M HNO3 up to a pH of about 3 4, the DFP titration is started. Since the base electrolyte solution often contains impurities which could be titrated by silver ions, a DEP pretitration of this solution is always required, and the time which elapsod between the obtaining of the pretitration peak down to the pretitration end is finally added to the equivalence time of the base-electrolyte-plus iodine solution. Care should be taken since a sulphur peak can sometimes precede the equivalence peak [101 because of an incomplete oxidation of the sulphur ions during closed-tube dissolution. Some comments will now be made about these DEP argentometric titrations for the iodine detection. Previous attempts were reported [13] to have failed because of a probable formation of high resistance AgI layers at the electrode interfaces, which prevented the electrolytic processes from occurring. In those cases, standard solutions prepared with KI
in zodme doped CdS single crystals
weic used. In our case, the presence of NH
4 ions due to the hydrolysis of NH4NO3 in the final solution seems to favour the dissolution of the Ael barriens by the probable formation of Ag(Nll3)~complexes. By using NH4I standard solutions, the iodine detection up to 1 .0 X 10 6 M concentration was found to he feasible within +51/ recovery, with a DEP current of ~0 nA and a Agtgenerating current of 50 A.
3. Results The calibration of the method with iodine-added CdS powders is reported in table I . The maximum deviation between added and found iodide is within +l0~ recovery. Such recovery is assumed as the experimental error of the method. Once the procedure was calibrated,CdS single crystals were analyzed. The crystals were grown by iodine chemical transport in quartz ampoules 18 cm long and 2 cm in inner diameter. The source and deposit temperatures were fixed at 900°C and 870°C for each run, while 3the of iodine amount was varied from 1 to 5 mg per cm ampoule. The deposition time was fixed at 120 h for
fable I Deternimnation of iodide in CdS samples. Calibration (‘dS (mg)
15.8 22.4 22.5 27.0 27.6 29.0 21.5 23.1 41.2 37.5 63.4 42.6 64.0 85.7 93.2 99.1
Iodide Added (ppm) 1658.0 1170.0 1165.0 970.0 949.6 903.0 812.5 756.0 636.2 465.0 413.4 410.0 267.8 203.0 187.0 99.18
I ound (ppm) 1580.6 1042.5 1154.5 889.0 1024.5 941.0 718.8 825.5 686.3 455.5 414.9 370.5 246.0 184.1 197.0 89.5 88.5
~ Recovery 4.6 10.8 0.9 8.3 +7.8 +4.2 11.5 +9.2 +7.8 2.0 +0.3 9.6 8.1 9 3 +5.3 9.7 ±5.3
Gcner ,iting current (pA) lOt) 100 100 100 100 100 100 100 100 101) 100 100 100 100 100 50 100
DOP surrcnt (nA) S 5 5 5 S ~ 5 5 5 5 5
5 5 S 5 5 5
C. Paorici et a!.
/ Iodine detection
in iodine doped CdS single crystals
11
Table 2 Iodine in the ampoule (mg/cm3)
CdS saniple (mg)
Iodine found in the sample (ppm)
~cdI, (atm)
P (atm)
K x lOb 3lmol CdSl ([g/at II
1
40.4 30.6 35.0
269 215 234
0.31
0.54
0.46 0.24 0.30
2
30.6 37.0 59.4
338
0.57
1.00
0.50 0.09 0.38
43.2 26.6 53.9
426 347 525
0.90
1.60
0.63 0.34
4
46.7 49.0 23.0 50.7
556 456 457 551
1.25
2.14
1.01 0.56 0.56 0.99
5
31.0 31.1 48.6
636 486 677
1.45
2.61
1.31 0.58 1.57
3
191
310
i)
1.18
each run. After opening the ampoules, the crystals were washed with an aqueous Na 2S2O3 solution, to
by the relationship
remove any trace of Cd12 and free iodine, and finally
K = [I
rinsed with bidistilled water. The amount of iodine found in the various samples is given in table 2.
4. Discussion and conclusion Analyses of iodine traces in chemical transportgrown CdS crystals,have been previously reported by
Schafer Odenbach [6], who used the analytical data forand evaluating an iodine distribution constant between solid and vapour phase. In order to compare our results with theirs, the same distribution constant was calculated by assuming the same incorporation mechanism, given by Cd1
3latml
2 = 2 CdS (s) + 2 1 + D (Cd2~). 2 (g) + S Here, S2 and I are sulphur and iodine ions in lattice sulphur sites, and D(Cd2~)is a substitutional Cd vacancy. Then the distribution constant will be given
]
3/2PcdI 2
where [I ] is the iodine concentration in the solid phase and ~cdI2 is the partial pressure of CdI2 at the deposition temperature. ~cdI2 values, calculated as described in ref. [15] are given in table 2 together with the overall pressure P (calculated) and the distribution constant. Since the pressure dependence of K is small, an CdS] average 3value 5.9 be X 10 ~ at[g-at 3 [mol [atm]K = can given T iodine] 1143 K. Schafer and Odenbach’s analytical results lead to an iodine distribution constantK4.5 X 10 12 at 1073 K. The agreement between Schafer and Odenbach’s results and ours can be considered reasonably good, especially if we take into account that different iodine solubilities can be observed in the presence of a second impurity in the solid matrix. The amount of stoichiometric defects in CdS crystals can also affect the iodine solubility. Since Schafer and Odenbach’s standardization of the starting CdS powders is not described, a more precise comparison cannot be
12 made. Furthermore, the
C. Paoricm ci al. Iodine detection to iodine doped C’dS single crystals
P( d12 values are slightly d’if
ferent in the two cases because of a different calcula lion approach.
[4] W. Kemula, A. Hulanicki and A. Janoss ski. Talanta 7 (1960) 6S. [SI J. ‘.. Iteun, It. NitsL lie and IlL. Boesterli, P1i~sica 28 (1962) 184.
16! Fl. Schafer and H. Odenbach, Z. Anorg. Ailgem. Chem.
Acknowledgements This research was supported h~a financial con tribution from the G.N.S.M. of ihe C.N.R. The authors are indebted to Mr. G. Zuccalli for his assiSlance in the preparation of C dS ci ystals.
References [I] R.l . Rolsten, Iodide Metals and Metal lodides (Wiley, Ness York, 1961). 121 H. Schdfer, Chemical Transport Reactions (Academic Press, New “~ork, 1964). 131 Spectrophotometric Data for Colotimetric Analysis (Butter~orths,London, 1963).
346 (1966) 127.
[71 C. Paorici, J. Co stal Gross tls S (1969) 315. [81 C.K. Kini and S.S. Voris, J. Phvs. (hem. Solids 3S (1 97~ 1381. ‘i . Kotera, J. Crystal (,ro\s th 11) (1971) 320 110! C. Pelosi, C. l’aorici. 0. Attolini and 0. /uccalli. 1. flee tronanal. (‘hem. 57 (1974) 259. [11! ( . Paorici, ( . Pclosi and 0, \ttolini. J. I lectroarmal. Chem.61 (l97S)213. [1211. Bishop. Analyst 83(1958)212. 11311 . Bishop and R.(,. Danesliwar, Anal. ( Item. 36 (1964) 727. [14] D.G. Davis, Anal. (‘hem. 44 (1972) 79R; 46 (1974) 21R. [151 C. Paorici and C. Pehosi, 1. Crystal Grossth 3S (1976) 65. [161 I’ A. Kroger, The C hemistry of Imperfect Cr) stals (North Holland, Amsterdam, 1964) PP. 622. 716.
[9l S. Ujnc and