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Surface microstructure of pure and doped ZnS cubic crystals
Journal ofCrystal Growth 55(1981) 526 —530 North-Holland Publishing Company
SURFACE MICROSTRUCTURE OF PURE AND DOPED ZnS CUBIC CRYSTALS M.I. TOACSAN, I. ZBEREA and S.V. NISTOR Central Institute of Physics, PO.B. MG- 7, Magurele-Bucuresti, Romania R- 76900 Received 26 March 1981
The structure of the basal growth surfaces of cubic ZnS crystals giown from PbC1
2 flux was studied by both optical microscopy and multiple-beam interferometry. The surface microstructure was analysed in connection with the growth rate and nature of added impurities.
I. Introduction The wide-bandgap semiconductors based on Il—VI compounds, well know as luminescent materials, are also promising materials for light emitters and integrated optics. ZnS single crystals, which are usually grown [1,2] from the melt or by vapour transport, contain besides cubic and hexagonal phases a high density of dislocations and stacking faults. According to some authors [3] the presence of such defects affects the growth and physical properties such as luminescence, photovoltaic effect, etc. It has been shown [4] that crystal growth of ZnS by the. gradient method from molten PbC12 leads to relatively large crystals free of non-cubic phases, offering the possibility to verify these assumptions. Along this line of research a study of segregation in the crystal growth process of the most common impurities was recently performed. It has been found [5] that no direct relationship exist between the segregation and ionic radii, In this paper we report further results concerning the surface microstructure of ZnS single crystals, undoped or doped with either copper of maganese. The experimental results are correlated with the rate of crystal growth and the previous results on segregation effects during the growth process. 2. Experimental ZnS crystals exhibiting only sphalerite structure were grown from molten PbC12 in sealed silica am0022-0248/8 1/0000—0000/$02.50 © 1981 North-Holland
poules by the gradient technique [4]. The ampoules of identical size containing as starting materials reagent grade ZnS (Görlitz) and PbC12 (Reactivul) were placed in a vertical tube furnace at such position to assume identical growing conditions, i.e., a temperature of 575 ±1°Cat the bottom of the ampoules in the nucleation zone and a temperature gradient between the nucleation zone and the nutrient source of 7°C cm’. The doping was performed by adding reagent grade CuC12 or MnC12 to the flux. The initial concentration of copper and manganese were 0.1—0.5 at% and 0.01—0.25 at% respectively. The growth rate was determined as the ratio of the weight of all ZnS crystals produced after a growth run to its duration in days. Each growth rate reported here represents the average value over several growth runs performed in similar conditions. The surface microstructure of ZnS single crystals was analysed by conventional optical microscopy and by multiple-beam interference technique using the monochromatic radiation of thallium (X = 5350 A). The measurements of step heights by multiple-beam interference could be done under relatively good conditions due to the presence of a thin homogeneous layer with metallic luster covering the individual ciystals [4]. The measurements were performed on plate-like specimens of ZnS, ZnS: Cu and ZnS : Mn single crystals. The examination was made on the basal growth faces. As revealed by X-ray diffraction [4], the {1 1 1} planes lay in, or nearly in, these basal faces.
M.I. Toacsan eta!. / Surface microstructure of pure and doped ZnS
527
3. Results As expected, the growth rate of ZnS crystals depends on the type of impurity initially added to the flux. Indeed, the growth rate for pure ZnS under the above mentioned conditions had a value of 25 mg day’ compared with 34 mg day’ for ZnS : Cu and 19.3 mg day’ for ZnS : Mn. Moreover, ZnS and ZnS: Cu crystals occur as both plate-like and polyhedral forms. The ZnS : Mn crystals occur only as plate-like forms, some of them of larger basal faces but thinner than the pure or Cu doped crystals. The impurity concentration of the starting materials and resulting crystals from a typical growth run, determined by spectrographic analysis, was previously reported [5]. Further analysis by mass spectroscopy has been performed on the crystals studied in the present work. Values of concentration (table 1) were sought only for those elements which exhibited significant amounts in the early determinations as well as for manganese and copper. The sensitivity of the mass spectroscopy analysis was limited by the small amount available of the crystals which were studied. The detection limits for Pb and Mn were 10 ppm. However, in the case of manganese a determination of relative concentrations has been performed by monitoring the Electron Spin Resonance absorptions of Mn2~ions. The concentration of Mn2~in the manganese doped crystals was found to be 2000 times larger than in the undoped or copper doped crystals. An optical microscopy examination of the platelike crystals indicates the presence of step-like structures on large surfaces. These structures develop from closed-ring forms (fig. 1) and sometimes from the
Fig. 1. Closed-ring growth
pattern on
a
ZnS crystal, X
140.
intersection point of growth defects with the crystal surface, as the emergence point of a group of dislocations (fig. 2). From interferometric measurements performed on such structures, values of step height up to 1400 A were inferred for ZnS and ZnS : Cu crystals (figs. 3 and 4). In the case of ZnS : Mn crystals (figs. 5a and
Table 1 The content of several impurities in the as-grown ZnS crystals; the analysis has been performed with a JEOL, JMS-01 BM mass spectrometer; an estimated error of ±10% was given for the detected elements Material
Co
Fe
Cu
Pb
Mn
ZnS 105 50 <10 <10 <10 ZnS : Cu 113 50 2.000 <10 <10 ZnS : Mn 108 50 10
______
~f_ ~
Fig. 2. Growth patterns formed at the emergence point of dislocations with a (111) surface of a ZnS Cu crystal, x 140.
528
M.I. Toacsan eta!.
/ Surface
microstntcture ofpure and doped ZnS
U.
Fig. 3. Step-like form ofgrowth on a ZnS crystal: (a) picture taken by conventional optical microscopy, X 140; (b) multiple-beam interferogram ofthe same area, X 260.
Sb), the steps were higher (1600 A). Sometimes, the step height exceeded 2000 A giving rise to indistinct interference fringes, 4. Discussions
A tentative explanation of the above-mentioned results is suggested by taking into consideration some
earlier results concerning the growth of crystals from melt. It was shown [6] that by firing zinc sulphide in the presence of copper, a separate phase of copper sulphide precipitates and in doing so, supplies the energy necessary for nucleation of cubic zinc sulphide. Moreover studies of ZnS: Cu crystals grown from the melt in high pressure inert atmosphere have shown [7] that during the crystal growth, cubic Cu2S
Fig. 4. Step-like form of growth on a ZnS: Cy crystal: (a) picture taken by conventional optical microscopy. X 140; (b) multiplebeam interferogram of the same area, X 140.
M.L Toacsan et aL / Surface microstructure ofpure and doped ZnS
Fig. 5. Step-like form of growth on a ZnS: Mn crystal: (a) picture taken by conventional optical microscopy, beam interferogram on the same crystal, x 260.
is precipitated and forms the seed centres for sphalerite crystallization of ZnS. The precipitation of Cu2S would supply, in part, the activation energy necessary for the ZnS nucleation. As the cubic a-Cu2S is stable above 450°C [8,9], a similar mechanism would explain theimpurities. higher growth of ZnSofinCu the presence of copper The rate presence 2S precipitates would determine the production of supplementary dislocations in the ZnS lattice,in agreement with the experimentally observed larger density of such defects as growth defects and emergent dislocations in the ZnS: Cu specimens. It2~can is known that sulphides ofZnS divalent elements like be incorporated into without coactivaMn tion [6]; consequently, it would be expected that the entry of impurities into the adsorption layer during crystal growth would cause a decrease in the growth rate [10]. A high concentration of such impurity would make possible the incorporation at the solid— liquid interface of a manganese-rich layer. Such layers of impurities would decrease the absorption of subsequent ZnS. Consequently, the growth would be mainly directed into the (111) face direction and hindered in the perpendicular direction, until the incorporation of such layers into the lattice would allow the growth rate to attain again its maximum value along this direction. The final result of such a
529
x
140; (b) multiple-
growing process will be plate-like crystals with a higher area-to-thickness ratio and higher step heights on the larger {l 1 l} faces. Moreover this non-steady state growth would reduce the overall growth rate compared with pure ZnS crystals. factsituated that ESR 2~ionsThe being at spectra indicate [5] the Mn isolated sites of the ZnS crystal lattice suggests the impurities in the adsorbed layer to be subjected, as expected, to a diffusion process after being incorporated in the lattice. The similar values of step heights in ZnS and ZnS : Cu are easily explained by accepting the idea of Cu2S precipitating at lattice defects. would The absence adsorbed layers of copper impurities renderof a non-steady state growth impossible. Consequently from the point of view of the aspect of crystal faces, the crystal growth would proceed like in pure crystals. It should be mentioned that to the best of our knowledge, it is the first time that such high steps due to the successive deposition of ZnS layers have been measured. As revealed by impurity level analysis, a relatively high concentration of Co and Fe is present in our crystals. However, as their amount is the same in all types of crystal, their influence on the growth process would be an added effect, which can be neglected in
530
M.I. Toacsanet al.
/ Surface microstructure ofpure and
doped ZnS
our discussion as a constant parameter. Finally, we should like to express our belief that the above discussion is neither detailed nor exhaustive, being limited by the amount of data available.
[2] A. Pajaczkowska, Progr. Crystal Growth Characteriza-
Acknowledgements
[61M.
The authors would like to thank Professor I. Ursu for continuous encouragement and support, Dr. L.C. Nistor for helpful discussion and Dr. P. Dumitrescu for performing the mass spectroscopy analysis.
References [11 M.
Aven and J.S. Prener, Eds., Physcis and Chemistry of Il—VI Compounds (North-Holland, Amsterdam, 196 7).
tion 1 (1978) 289. [3] E. Lendvay, J. Crystal Growth 10 (1971) 77. [4] L.C. Nistor, S.V. Nistor and M.L. Toacsan, J. Crystal Growth 50 (1980) 557. [5] S.V. Nistor and M.I. Toacs~n,Rev. Roum. Phys. 25 6 (1980) 707. Aven and J.A. Parodi, J. Phys. Chem. Solids 13
(1960) 56. [7] L.G. Suslina 21(1974) 389.and D.L. Fedorov, Phys. Status Solidi (a) [8] R. Ueda, J. Phys. Soc. Japan 4 (1949) 287. [9] G.B. Abdullaev, Z.A. Aliyaxova, E.H. Zamanova and G.A. Asadov, Phys. Status Soildi 26 (1968) 65. [10] R.J. Favey and J.W. Muffin, J. Crystal Growth 23 (1974) 89.
SURFACE MICROSTRUCTURE OF PURE AND DOPED ZnS CUBIC CRYSTALS M.I. TOACSAN, I. ZBEREA and S.V. NISTOR Central Institute of Physics, PO.B. MG- 7, Magurele-Bucuresti, Romania R- 76900 Received 26 March 1981
The structure of the basal growth surfaces of cubic ZnS crystals giown from PbC1
2 flux was studied by both optical microscopy and multiple-beam interferometry. The surface microstructure was analysed in connection with the growth rate and nature of added impurities.
I. Introduction The wide-bandgap semiconductors based on Il—VI compounds, well know as luminescent materials, are also promising materials for light emitters and integrated optics. ZnS single crystals, which are usually grown [1,2] from the melt or by vapour transport, contain besides cubic and hexagonal phases a high density of dislocations and stacking faults. According to some authors [3] the presence of such defects affects the growth and physical properties such as luminescence, photovoltaic effect, etc. It has been shown [4] that crystal growth of ZnS by the. gradient method from molten PbC12 leads to relatively large crystals free of non-cubic phases, offering the possibility to verify these assumptions. Along this line of research a study of segregation in the crystal growth process of the most common impurities was recently performed. It has been found [5] that no direct relationship exist between the segregation and ionic radii, In this paper we report further results concerning the surface microstructure of ZnS single crystals, undoped or doped with either copper of maganese. The experimental results are correlated with the rate of crystal growth and the previous results on segregation effects during the growth process. 2. Experimental ZnS crystals exhibiting only sphalerite structure were grown from molten PbC12 in sealed silica am0022-0248/8 1/0000—0000/$02.50 © 1981 North-Holland
poules by the gradient technique [4]. The ampoules of identical size containing as starting materials reagent grade ZnS (Görlitz) and PbC12 (Reactivul) were placed in a vertical tube furnace at such position to assume identical growing conditions, i.e., a temperature of 575 ±1°Cat the bottom of the ampoules in the nucleation zone and a temperature gradient between the nucleation zone and the nutrient source of 7°C cm’. The doping was performed by adding reagent grade CuC12 or MnC12 to the flux. The initial concentration of copper and manganese were 0.1—0.5 at% and 0.01—0.25 at% respectively. The growth rate was determined as the ratio of the weight of all ZnS crystals produced after a growth run to its duration in days. Each growth rate reported here represents the average value over several growth runs performed in similar conditions. The surface microstructure of ZnS single crystals was analysed by conventional optical microscopy and by multiple-beam interference technique using the monochromatic radiation of thallium (X = 5350 A). The measurements of step heights by multiple-beam interference could be done under relatively good conditions due to the presence of a thin homogeneous layer with metallic luster covering the individual ciystals [4]. The measurements were performed on plate-like specimens of ZnS, ZnS: Cu and ZnS : Mn single crystals. The examination was made on the basal growth faces. As revealed by X-ray diffraction [4], the {1 1 1} planes lay in, or nearly in, these basal faces.
M.I. Toacsan eta!. / Surface microstructure of pure and doped ZnS
527
3. Results As expected, the growth rate of ZnS crystals depends on the type of impurity initially added to the flux. Indeed, the growth rate for pure ZnS under the above mentioned conditions had a value of 25 mg day’ compared with 34 mg day’ for ZnS : Cu and 19.3 mg day’ for ZnS : Mn. Moreover, ZnS and ZnS: Cu crystals occur as both plate-like and polyhedral forms. The ZnS : Mn crystals occur only as plate-like forms, some of them of larger basal faces but thinner than the pure or Cu doped crystals. The impurity concentration of the starting materials and resulting crystals from a typical growth run, determined by spectrographic analysis, was previously reported [5]. Further analysis by mass spectroscopy has been performed on the crystals studied in the present work. Values of concentration (table 1) were sought only for those elements which exhibited significant amounts in the early determinations as well as for manganese and copper. The sensitivity of the mass spectroscopy analysis was limited by the small amount available of the crystals which were studied. The detection limits for Pb and Mn were 10 ppm. However, in the case of manganese a determination of relative concentrations has been performed by monitoring the Electron Spin Resonance absorptions of Mn2~ions. The concentration of Mn2~in the manganese doped crystals was found to be 2000 times larger than in the undoped or copper doped crystals. An optical microscopy examination of the platelike crystals indicates the presence of step-like structures on large surfaces. These structures develop from closed-ring forms (fig. 1) and sometimes from the
Fig. 1. Closed-ring growth
pattern on
a
ZnS crystal, X
140.
intersection point of growth defects with the crystal surface, as the emergence point of a group of dislocations (fig. 2). From interferometric measurements performed on such structures, values of step height up to 1400 A were inferred for ZnS and ZnS : Cu crystals (figs. 3 and 4). In the case of ZnS : Mn crystals (figs. 5a and
Table 1 The content of several impurities in the as-grown ZnS crystals; the analysis has been performed with a JEOL, JMS-01 BM mass spectrometer; an estimated error of ±10% was given for the detected elements Material
Co
Fe
Cu
Pb
Mn
ZnS 105 50 <10 <10 <10 ZnS : Cu 113 50 2.000 <10 <10 ZnS : Mn 108 50 10
______
~f_ ~
Fig. 2. Growth patterns formed at the emergence point of dislocations with a (111) surface of a ZnS Cu crystal, x 140.
528
M.I. Toacsan eta!.
/ Surface
microstntcture ofpure and doped ZnS
U.
Fig. 3. Step-like form ofgrowth on a ZnS crystal: (a) picture taken by conventional optical microscopy, X 140; (b) multiple-beam interferogram ofthe same area, X 260.
Sb), the steps were higher (1600 A). Sometimes, the step height exceeded 2000 A giving rise to indistinct interference fringes, 4. Discussions
A tentative explanation of the above-mentioned results is suggested by taking into consideration some
earlier results concerning the growth of crystals from melt. It was shown [6] that by firing zinc sulphide in the presence of copper, a separate phase of copper sulphide precipitates and in doing so, supplies the energy necessary for nucleation of cubic zinc sulphide. Moreover studies of ZnS: Cu crystals grown from the melt in high pressure inert atmosphere have shown [7] that during the crystal growth, cubic Cu2S
Fig. 4. Step-like form of growth on a ZnS: Cy crystal: (a) picture taken by conventional optical microscopy. X 140; (b) multiplebeam interferogram of the same area, X 140.
M.L Toacsan et aL / Surface microstructure ofpure and doped ZnS
Fig. 5. Step-like form of growth on a ZnS: Mn crystal: (a) picture taken by conventional optical microscopy, beam interferogram on the same crystal, x 260.
is precipitated and forms the seed centres for sphalerite crystallization of ZnS. The precipitation of Cu2S would supply, in part, the activation energy necessary for the ZnS nucleation. As the cubic a-Cu2S is stable above 450°C [8,9], a similar mechanism would explain theimpurities. higher growth of ZnSofinCu the presence of copper The rate presence 2S precipitates would determine the production of supplementary dislocations in the ZnS lattice,in agreement with the experimentally observed larger density of such defects as growth defects and emergent dislocations in the ZnS: Cu specimens. It2~can is known that sulphides ofZnS divalent elements like be incorporated into without coactivaMn tion [6]; consequently, it would be expected that the entry of impurities into the adsorption layer during crystal growth would cause a decrease in the growth rate [10]. A high concentration of such impurity would make possible the incorporation at the solid— liquid interface of a manganese-rich layer. Such layers of impurities would decrease the absorption of subsequent ZnS. Consequently, the growth would be mainly directed into the (111) face direction and hindered in the perpendicular direction, until the incorporation of such layers into the lattice would allow the growth rate to attain again its maximum value along this direction. The final result of such a
529
x
140; (b) multiple-
growing process will be plate-like crystals with a higher area-to-thickness ratio and higher step heights on the larger {l 1 l} faces. Moreover this non-steady state growth would reduce the overall growth rate compared with pure ZnS crystals. factsituated that ESR 2~ionsThe being at spectra indicate [5] the Mn isolated sites of the ZnS crystal lattice suggests the impurities in the adsorbed layer to be subjected, as expected, to a diffusion process after being incorporated in the lattice. The similar values of step heights in ZnS and ZnS : Cu are easily explained by accepting the idea of Cu2S precipitating at lattice defects. would The absence adsorbed layers of copper impurities renderof a non-steady state growth impossible. Consequently from the point of view of the aspect of crystal faces, the crystal growth would proceed like in pure crystals. It should be mentioned that to the best of our knowledge, it is the first time that such high steps due to the successive deposition of ZnS layers have been measured. As revealed by impurity level analysis, a relatively high concentration of Co and Fe is present in our crystals. However, as their amount is the same in all types of crystal, their influence on the growth process would be an added effect, which can be neglected in
530
M.I. Toacsanet al.
/ Surface microstructure ofpure and
doped ZnS
our discussion as a constant parameter. Finally, we should like to express our belief that the above discussion is neither detailed nor exhaustive, being limited by the amount of data available.
[2] A. Pajaczkowska, Progr. Crystal Growth Characteriza-
Acknowledgements
[61M.
The authors would like to thank Professor I. Ursu for continuous encouragement and support, Dr. L.C. Nistor for helpful discussion and Dr. P. Dumitrescu for performing the mass spectroscopy analysis.
References [11 M.
Aven and J.S. Prener, Eds., Physcis and Chemistry of Il—VI Compounds (North-Holland, Amsterdam, 196 7).
tion 1 (1978) 289. [3] E. Lendvay, J. Crystal Growth 10 (1971) 77. [4] L.C. Nistor, S.V. Nistor and M.L. Toacsan, J. Crystal Growth 50 (1980) 557. [5] S.V. Nistor and M.I. Toacs~n,Rev. Roum. Phys. 25 6 (1980) 707. Aven and J.A. Parodi, J. Phys. Chem. Solids 13
(1960) 56. [7] L.G. Suslina 21(1974) 389.and D.L. Fedorov, Phys. Status Solidi (a) [8] R. Ueda, J. Phys. Soc. Japan 4 (1949) 287. [9] G.B. Abdullaev, Z.A. Aliyaxova, E.H. Zamanova and G.A. Asadov, Phys. Status Soildi 26 (1968) 65. [10] R.J. Favey and J.W. Muffin, J. Crystal Growth 23 (1974) 89.