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Growth of ZnS : Tm thin films by MOCVD
Journal of Crystal Growth 208 (2000) 259}263
Growth of ZnS : Tm thin "lms by MOCVD Chin-Tsar Hsu Department of Electrical Engineering, National Huwei Institute of Technology, 64 Wun-Hua Rd., Huwei, Yun Lin 632, Taiwan, ROC Received 3 May 1999; accepted 13 July 1999 Communicated by R. James
Abstract Thin "lms of thulium-active ZnS have been deposited on ITO coated glass substrates by a low-pressure vertical metalorganic chemical vapor deposition (MOCVD) system. Dimethylzinc (DMZn) and hydrogen sul"de (H S) are used 2 as reactants, and Tris(hexa#uoroacetonato-thulium) (Tm(HFAA) , molecular formula Tm(C HF O ) ) is used as 3 5 6 23 dopant source. Energy dispersive spectrometer (EDS), X-ray di!raction (XRD), scanning electron microscopy (SEM), Auger electron spectra (AES), and secondary ion mass spectra (SIMS) were used to determine the elemental composition, surface morphologies and depth pro"les. Optical properties of ZnS : Tm thin "lms were characterized by photoluminesence (PL). The best growth conditions are substrate temperature at 2253C, DMZn bubbler temperature at !83C, Tm(HFAA) bubbler temperature at 703C, growth pressure at 30 Torr, #ow rate of DMZn 2.5]10~5 mol/min, 3 #ow rate of H S 5]10~4 mol/min, #ow rate of Tm(HFAA) 0.1 SLM, and VI/II molar ratio of 20. ( 2000 Elsevier 2 3 Science B.V. All rights reserved. Keywords: ZnS : Tm; EDS; SIMS; DLTS
1. Introduction ZnS : Tm is a very e$cient photoluminescence (PL) and cathodoluminescence (CL) blue phosphor. It has been considered as an important candidate for blue emitting thin-"lm electroluminescent (TFEL) devices. Ma et al. [1] investigated the excitation mechanism of ZnS : Tm TFEL by measuring the time-resolved spectra and showed that the excitation in ZnS : Tm is due to the energy transfer from the excited host to the luminescence center. Thulium is of particular interest among all rareearth ions, because it shows the most e$cient cathodoluminescence (0.216 W/W) in the blue emission region around 478 nm, and the line centered around 800 nm has a power e$ciency of 0.59 W/W as reported by Shrader et al. [2]. This means that if the electrons can be accelerated to ballistic energies,
more e$cient devices can be fabricated which use direct impact excitation of the rare earth ions [3]. Tm3` emission was "rst observed by Rothschild [4], who found emission lines centered at about 478 nm. Ibuki and Langer [5}7] reported four groups of emission lines around 365, 480, 650 and 800 nm. The e$ciency of thin-"lm phosphors depends strongly on the crystallinity of the "lms [5,8}10]. In this paper, the properties of ZnS : Tm thin "lms deposited on ITO coated glass substrates by MOCVD were investigated.
2. Experiments The samples in this study were grown on glass substrates (HOYA NA-40) coated with an
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 4 1 0 - 8
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C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
Table 1 The growth conditions of ZnS : Tm Substrates temp. DMZn bubbler temp. Tm(HFAA) bubbler temp. 3 Growth pressure Flow rate of DMZn Flow rate of H S 2 Flow rate of Tm(HFAA) 3 VI/II molar ratio
2253C !83C 703C 30 Torr 2.5]10~5 mol/min 5]10~4 mol/min 0.1 SLM 20
indium}tin oxide (ITO}In O : SnO ) transparent 2 3 2 electrode by MOCVD. Dimethylzinc (DMZn) was chosen as the zinc source. DMZn was introduced into a rector by passing a carrier gas through a stainless steel bubbler that was maintained at !83C in order to obtain an appropriate vapor concentration. The #ow rate of DMZn was calculated by assuming that the carrier gas was saturated with DMZn during bubbling. Hydrogen sul"de (H S) diluted with 10% hydrogen was used 2 as the sulfur (S) source, and hydrogen was the carrier source. The dopant source was Tm(HFAA) 3 (Tris(hexa#uoroacetonato-thulium) (Tm(HFAA) , 3 molecular formula Tm(C HF O ) )). The growth 5 6 23 conditions are summarized in Table 1. The crystallinity of the ZnS : Tm thin "lms was measured by XRD. The surface morphologies of the ZnS : Tm thin "lms were observed by scanning electron microscopy (SEM). The atomic ratios of Zn, S and Tm in the thin "lms are evaluated by energy dispersive spectrometer (EDS) based on the SEM system. The depth pro"le of the ZnS : Tm thin "lm was analyzed by secondary ion mass spectra (SIMS). The character of epitaxial layer was evaluated by photoluminescence (PL) using a He}Cd laser (3250 As ) with an intensity of 50 mW as an excitation source. The bulk defects were examined by deep level transient spectroscopy (DLTS).
3. Results and discussion The samples with di!erent Tm concentrations were studied. The Tm concentrations were 0.30, 0.36, 0.51 and 0.56 at% for Samples 1}4, respective-
Fig. 1. X-ray di!raction patterns of samples.
ly. The X-ray di!raction patterns of ZnS : Tm thin "lms and ITO coating glass substrate were shown in Fig. 1. In ZnS : Tm thin "lm, XRD patterns have a major peak at 2h"28.53C which corresponds to ZnS(1 1 1). The full-width at half-maximum (FWHM) is about 0.173. The composition of ZnS : Tm thin "lms was determined by energy dispersive spectrometer (EDS). The EDS analysis spectrum is shown in Fig. 2. From the EDS analysis spectrum, the atomic ratio of S/Zn for the samples always approaches unity. These results indicated that the thin-"lms have good stoichiometry. The depth pro"le of the ZnS : Tm thin "lms was analyzed by secondary ion mass spectra (SIMS). The detectable limit of this SIMS is about 10~4 at%. The SIMS spectra of ZnS : Tm are shown in Fig. 3. From the depth pro"le analysis,
C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
Fig. 2. EDS spectrum of composition.
Fig. 3. SIMS depth pro"le of ZnS : Tm thin "lm.
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C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
the distribution of Zn and S in the "lms is approximately uniform. The transition between the "lm and ITO is abrupt. These results indicated little or no interaction or interdi!usion occurred. The results of photoluminescence measurement at 300 K are shown in Fig. 4. The PL spectra have
three peaks at 470, 700 and 800 nm and a broad peak at about 900 nm. These three peaks correspond to 1G }3H , 1G }3F and 3H }3H , 4 6 4 4 4 6 respectively. The broad peak is possibly due to
Fig. 4. PL spectra of samples at 300 K.
Fig. 5. PL spectra of ZnS : Tm thin "lms at varied temperatures.
Fig. 6. DLTS spectra of samples.
C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
incorporation of impurities. The PL spectra at varied temperatures are shown in Fig. 5. The positions of the peaks are the same and the blue-emitting peak becomes sharper when the measurement temperature decreases. The deep level transient spectroscopy (DLTS) measurement examined the existence of bulk defects in the samples. The results are shown in Fig. 6. A deep hole trap (designed as E ) was consistently 51 observed in ZnS : Tm thin "lms regardless of the Tm doping concentration. The DLTS signal intensity of the E trap increases with Tm doping con51 centration.
4. Conclusions The best growth conditions are substrate temperature at 2253C, DMZn bubbler temperature at !83C, Tm(HFAA) bubbler temperature at 703C, 3 growth pressure at 30 Torr, #ow rate of DMZn 2.5]10~5 mol/min, #ow rate of H S 5]10~4 mol/ 2 min, #ow rate of Tm(HFAA) 0.1 SLM, and VI/II 3 molar ratio at 20. Based on X-ray di!raction tests, cubic (1 1 1) ZnS : Tm thin "lms are grown having good crystallinity. The lowest FWHM was 0.173. The PL spectra at room temperature showed that ZnS : Tm thin "lms have a strong blue emission peak and narrow shape.
263
Acknowledgements The II}VI LP}MOCVD activities are supported by Department of Electrical Engineering, National Cheng Kung University, and the experiments are conducted by Jiann-Haur Lin. Furthermore, the author would like to thank Professor Meiso Yokoyama and M.S. Jiann-Haur Lin. This project was supported by the National Science Council, Republic of China, under contract NSC 87-2215E-150-005. References [1] G. Ma, L. Zhong, S.H. Xu, Lumin. Display 6 (1985) 192. [2] R.E. Shrader, S. Larach, P.N. Yocom, J. Appl. Phys. 42 (1971) 4529. [3] H.J. Lozykowski, Spring Proceeding in Physics, Vol. 38, 1989, p. 60. [4] S. Rothschild, Proceedings of the International Conference on Solid-State Physics in Electronics and Telecommunication, Brussels, 1958, Vol. 4, Academic Press, New York, 1960, p. 705. [5] S. Ibuki, D.W. Langer, Appl. Phys. Lett. 2 (1963) 95. [6] S. Ibuki, D.W. Lander, J. Chem. Phys. 40 (1969) 796. [7] Y. Charreire, P. Porcher, J. Chem. Phys. 130 (1983) 175. [8] S. Ibuki, D.W. Langer, J. Chem. Phys. 40 (1964) 796. [9] Y. Charreier, H. Dexpert, J. Loviers, Mater. Res. Bull. 15 (1980) 657. [10] L. Jastrabik, J. Mares, S. Pacesova, M.V. Fock, N.N. Grigoriev, J. Lumin. 24/25 (1981) 293.
Growth of ZnS : Tm thin "lms by MOCVD Chin-Tsar Hsu Department of Electrical Engineering, National Huwei Institute of Technology, 64 Wun-Hua Rd., Huwei, Yun Lin 632, Taiwan, ROC Received 3 May 1999; accepted 13 July 1999 Communicated by R. James
Abstract Thin "lms of thulium-active ZnS have been deposited on ITO coated glass substrates by a low-pressure vertical metalorganic chemical vapor deposition (MOCVD) system. Dimethylzinc (DMZn) and hydrogen sul"de (H S) are used 2 as reactants, and Tris(hexa#uoroacetonato-thulium) (Tm(HFAA) , molecular formula Tm(C HF O ) ) is used as 3 5 6 23 dopant source. Energy dispersive spectrometer (EDS), X-ray di!raction (XRD), scanning electron microscopy (SEM), Auger electron spectra (AES), and secondary ion mass spectra (SIMS) were used to determine the elemental composition, surface morphologies and depth pro"les. Optical properties of ZnS : Tm thin "lms were characterized by photoluminesence (PL). The best growth conditions are substrate temperature at 2253C, DMZn bubbler temperature at !83C, Tm(HFAA) bubbler temperature at 703C, growth pressure at 30 Torr, #ow rate of DMZn 2.5]10~5 mol/min, 3 #ow rate of H S 5]10~4 mol/min, #ow rate of Tm(HFAA) 0.1 SLM, and VI/II molar ratio of 20. ( 2000 Elsevier 2 3 Science B.V. All rights reserved. Keywords: ZnS : Tm; EDS; SIMS; DLTS
1. Introduction ZnS : Tm is a very e$cient photoluminescence (PL) and cathodoluminescence (CL) blue phosphor. It has been considered as an important candidate for blue emitting thin-"lm electroluminescent (TFEL) devices. Ma et al. [1] investigated the excitation mechanism of ZnS : Tm TFEL by measuring the time-resolved spectra and showed that the excitation in ZnS : Tm is due to the energy transfer from the excited host to the luminescence center. Thulium is of particular interest among all rareearth ions, because it shows the most e$cient cathodoluminescence (0.216 W/W) in the blue emission region around 478 nm, and the line centered around 800 nm has a power e$ciency of 0.59 W/W as reported by Shrader et al. [2]. This means that if the electrons can be accelerated to ballistic energies,
more e$cient devices can be fabricated which use direct impact excitation of the rare earth ions [3]. Tm3` emission was "rst observed by Rothschild [4], who found emission lines centered at about 478 nm. Ibuki and Langer [5}7] reported four groups of emission lines around 365, 480, 650 and 800 nm. The e$ciency of thin-"lm phosphors depends strongly on the crystallinity of the "lms [5,8}10]. In this paper, the properties of ZnS : Tm thin "lms deposited on ITO coated glass substrates by MOCVD were investigated.
2. Experiments The samples in this study were grown on glass substrates (HOYA NA-40) coated with an
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 4 1 0 - 8
260
C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
Table 1 The growth conditions of ZnS : Tm Substrates temp. DMZn bubbler temp. Tm(HFAA) bubbler temp. 3 Growth pressure Flow rate of DMZn Flow rate of H S 2 Flow rate of Tm(HFAA) 3 VI/II molar ratio
2253C !83C 703C 30 Torr 2.5]10~5 mol/min 5]10~4 mol/min 0.1 SLM 20
indium}tin oxide (ITO}In O : SnO ) transparent 2 3 2 electrode by MOCVD. Dimethylzinc (DMZn) was chosen as the zinc source. DMZn was introduced into a rector by passing a carrier gas through a stainless steel bubbler that was maintained at !83C in order to obtain an appropriate vapor concentration. The #ow rate of DMZn was calculated by assuming that the carrier gas was saturated with DMZn during bubbling. Hydrogen sul"de (H S) diluted with 10% hydrogen was used 2 as the sulfur (S) source, and hydrogen was the carrier source. The dopant source was Tm(HFAA) 3 (Tris(hexa#uoroacetonato-thulium) (Tm(HFAA) , 3 molecular formula Tm(C HF O ) )). The growth 5 6 23 conditions are summarized in Table 1. The crystallinity of the ZnS : Tm thin "lms was measured by XRD. The surface morphologies of the ZnS : Tm thin "lms were observed by scanning electron microscopy (SEM). The atomic ratios of Zn, S and Tm in the thin "lms are evaluated by energy dispersive spectrometer (EDS) based on the SEM system. The depth pro"le of the ZnS : Tm thin "lm was analyzed by secondary ion mass spectra (SIMS). The character of epitaxial layer was evaluated by photoluminescence (PL) using a He}Cd laser (3250 As ) with an intensity of 50 mW as an excitation source. The bulk defects were examined by deep level transient spectroscopy (DLTS).
3. Results and discussion The samples with di!erent Tm concentrations were studied. The Tm concentrations were 0.30, 0.36, 0.51 and 0.56 at% for Samples 1}4, respective-
Fig. 1. X-ray di!raction patterns of samples.
ly. The X-ray di!raction patterns of ZnS : Tm thin "lms and ITO coating glass substrate were shown in Fig. 1. In ZnS : Tm thin "lm, XRD patterns have a major peak at 2h"28.53C which corresponds to ZnS(1 1 1). The full-width at half-maximum (FWHM) is about 0.173. The composition of ZnS : Tm thin "lms was determined by energy dispersive spectrometer (EDS). The EDS analysis spectrum is shown in Fig. 2. From the EDS analysis spectrum, the atomic ratio of S/Zn for the samples always approaches unity. These results indicated that the thin-"lms have good stoichiometry. The depth pro"le of the ZnS : Tm thin "lms was analyzed by secondary ion mass spectra (SIMS). The detectable limit of this SIMS is about 10~4 at%. The SIMS spectra of ZnS : Tm are shown in Fig. 3. From the depth pro"le analysis,
C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
Fig. 2. EDS spectrum of composition.
Fig. 3. SIMS depth pro"le of ZnS : Tm thin "lm.
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the distribution of Zn and S in the "lms is approximately uniform. The transition between the "lm and ITO is abrupt. These results indicated little or no interaction or interdi!usion occurred. The results of photoluminescence measurement at 300 K are shown in Fig. 4. The PL spectra have
three peaks at 470, 700 and 800 nm and a broad peak at about 900 nm. These three peaks correspond to 1G }3H , 1G }3F and 3H }3H , 4 6 4 4 4 6 respectively. The broad peak is possibly due to
Fig. 4. PL spectra of samples at 300 K.
Fig. 5. PL spectra of ZnS : Tm thin "lms at varied temperatures.
Fig. 6. DLTS spectra of samples.
C.-T. Hsu / Journal of Crystal Growth 208 (2000) 259}263
incorporation of impurities. The PL spectra at varied temperatures are shown in Fig. 5. The positions of the peaks are the same and the blue-emitting peak becomes sharper when the measurement temperature decreases. The deep level transient spectroscopy (DLTS) measurement examined the existence of bulk defects in the samples. The results are shown in Fig. 6. A deep hole trap (designed as E ) was consistently 51 observed in ZnS : Tm thin "lms regardless of the Tm doping concentration. The DLTS signal intensity of the E trap increases with Tm doping con51 centration.
4. Conclusions The best growth conditions are substrate temperature at 2253C, DMZn bubbler temperature at !83C, Tm(HFAA) bubbler temperature at 703C, 3 growth pressure at 30 Torr, #ow rate of DMZn 2.5]10~5 mol/min, #ow rate of H S 5]10~4 mol/ 2 min, #ow rate of Tm(HFAA) 0.1 SLM, and VI/II 3 molar ratio at 20. Based on X-ray di!raction tests, cubic (1 1 1) ZnS : Tm thin "lms are grown having good crystallinity. The lowest FWHM was 0.173. The PL spectra at room temperature showed that ZnS : Tm thin "lms have a strong blue emission peak and narrow shape.
263
Acknowledgements The II}VI LP}MOCVD activities are supported by Department of Electrical Engineering, National Cheng Kung University, and the experiments are conducted by Jiann-Haur Lin. Furthermore, the author would like to thank Professor Meiso Yokoyama and M.S. Jiann-Haur Lin. This project was supported by the National Science Council, Republic of China, under contract NSC 87-2215E-150-005. References [1] G. Ma, L. Zhong, S.H. Xu, Lumin. Display 6 (1985) 192. [2] R.E. Shrader, S. Larach, P.N. Yocom, J. Appl. Phys. 42 (1971) 4529. [3] H.J. Lozykowski, Spring Proceeding in Physics, Vol. 38, 1989, p. 60. [4] S. Rothschild, Proceedings of the International Conference on Solid-State Physics in Electronics and Telecommunication, Brussels, 1958, Vol. 4, Academic Press, New York, 1960, p. 705. [5] S. Ibuki, D.W. Langer, Appl. Phys. Lett. 2 (1963) 95. [6] S. Ibuki, D.W. Lander, J. Chem. Phys. 40 (1969) 796. [7] Y. Charreire, P. Porcher, J. Chem. Phys. 130 (1983) 175. [8] S. Ibuki, D.W. Langer, J. Chem. Phys. 40 (1964) 796. [9] Y. Charreier, H. Dexpert, J. Loviers, Mater. Res. Bull. 15 (1980) 657. [10] L. Jastrabik, J. Mares, S. Pacesova, M.V. Fock, N.N. Grigoriev, J. Lumin. 24/25 (1981) 293.