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Growth of oriented polycrystalline α-HgI2 films by ultrasonic-wave-assisted physical vapor deposition
Journal of Crystal Growth 324 (2011) 149–153
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of oriented polycrystalline a-HgI2 films by ultrasonic-wave-assisted physical vapor deposition Weiguang Yang a, Lei Nie a, Dongmei Li a, Yali Wang b, Jie Zhou a, Lei Ma a, Zhenhua Wang a, Weimin Shi a,n a b
School of Materials Science and Engineer, Shanghai University, Shanghai 200072, China Nano-Science and Nano-Technology Research Center, School of Materials Science and Engineer, Shanghai University, Shanghai 200444, China
a r t i c l e i n f o
abstract
Article history: Received 27 September 2010 Received in revised form 27 March 2011 Accepted 7 April 2011 Communicated by R.S. Feigelson Available online 21 April 2011
Polycrystalline a-HgI2 thick films have been grown on ITO-coated glass substrates using ultrasonicwave-assisted vapor phase deposition (UWAVPD) with the different source temperatures and ultrasonic frequencies. The influence of the assisted ultrasonic wave and source temperature on the structural and electrical properties of the polycrystalline a-HgI2 films is investigated. It is found that the assisted ultrasonic wave plays an important role in the improvement of the structural and electrical properties. An uniformly oriented polycrystalline a-HgI2 film with clear facets and narrow size distribution can be obtained at the source temperature of 80 1C under the assistance of 59 KHz ultrasonic frequency with the ultrasonic power of 200 W, which has the lowest value of r ¼2.2 1012 O cm for E-field parallel to c-axis, approaching to that of high quality a-HgI2 single crystals (4.0 1012 O cm). & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Morphology A2. Growth from vapor A3. Physical vapor phase deposition processes B1. Inorganic compounds B2. Semiconducting mercury compounds
1. Introduction Mercuric iodide is one of the most suitable semiconductor materials for gamma ray and X-ray detectors operating at room temperature because of its favorable characteristics such as high atomic number of its constituent elements and large band gap (2.13 eV), resulting in a high photopeak efficiency [1–5]. Though detector properties of polycrystalline HgI2 are not yet comparable to that of single crystal HgI2, polycrystalline films of HgI2 are of renewed interest to be used as one of the promising direct converters in X-ray digital radiography because of the lower fabrication cost for large area detectors and practical experience gained with HgI2 material in space and commercial applications [6–8]. Therefore, in recent years, the research emphasis of HgI2 has been transferred from single crystal films to low cost and large area polycrystalline ones. The polycrystalline HgI2 films can be prepared on Si or ITO-coated glass substrates by different fabrication methods, such as vapor phase deposition (PVD), screen printing (SP), laser ablation (AL), and hot pressing (HP). Among them, the most widely used is PVD method [9]. The detector applications demand a lot of characteristics of polycrystalline films, such as film thickness and film growth
n
Corresponding author. Tel.: þ86 21 56334007; fax: þ86 21 56332694. E-mail address: [email protected] (W. Shi).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.04.005
orientation. Among them, the film growth orientation plays an important role in sensitivity and transport properties of the final devices fabricated with it. The better the orientation, the better the charge transport along c-axis of microcrystals when they are irradiated. Oriented polycrystalline a-HgI2 films have been grown through varying the growing conditions of PVD [5,10,11]. In this present work, the highly oriented polycrystalline a-HgI2 films with clearer facets and more narrow size distribution have been grown onto ITO-coated glass substrates by ultrasonic-waveassisted PVD method at relatively lower deposition temperature ( o90 1C) compared with other research [12]. The influence of assisted ultrasonic wave on the morphology, grain size, degree of crystallinity, growth rate, texture, and resistivity of the polycrystalline a-HgI2 films is also discussed.
2. Experimental details The polycrystalline red mercuric iodide (a-HgI2) films were deposited on ITO-coated glass substrates via ultrasonic-waveassisted physical vapor deposition method (UWAPVD) in the whole process. Fig. 1 schematically depicts the UWAPVD system. The system is made up of (i) a growth tube with an inner diameter of 20 mm, an outer diameter of 25 mm and a length of 120 mm, (ii) a sealing lid and an air-pumping branch, (iii) an ultrasonic wave generator, (iv) a substrate temperature controller, and (v) a heating
150
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
distribution for each polycrystalline a-HgI2 film. All polycrystalline a-HgI2 films have microcrystalline structures. With the increase of
Fig. 1. Schematic diagram of the UWAPVD system in a water-bath oven used for growing polycrystalline a-HgI2: (1) substrate temperature controller; (2) cooling water; (3) growth tube; (4) air pumping branch; (5) ultrasonic wave generator; (6) heating wall and temperature controller; and (7) substrate holder.
wall and temperature controller. In preparation for polycrystalline a-HgI2 film growth, the growth tube was put inside water bath oven with the sealing lid and an air pumping branch while the substrate was cooled at the same separate controlled temperature of 25 1C. Prior to loading a-HgI2 source powder, the growth tube was cleaned with aqua regia, rinsed with deionized water for several times, and dried for 12 h at 300 1C. After the typical cleaning processes, the a-HgI2 powder was placed inside the bottom of the dust-proofing quartz glass growth tube and the ITO-coated glass substrate was put on the substrate holder, then the lid on the top of the apparatus was sealed. And then the air-pumping branch was connected with the vacuum pumping system. The heating preparation process started when the level of vacuum reached 3–6 10 3 Pa, and this pressure was maintained during the whole preparation process. Type a-d polycrystalline a-HgI2 films were obtained at the different waterbath temperatures (source temperatures) arranging from 50 to 80 1C without ultrasonic wave. Type e–h polycrystalline a-HgI2 films were obtained at the different source temperatures arranging from 50 to 80 1C with the assistance of ultrasonic wave of 40 KHz with the ultrasonic power of 100% (200 W). Type i–l polycrystalline a-HgI2 films were obtained at the different source temperatures arranging from 50 to 80 1C with ultrasonic wave of 59 KHz with a maximum power output of 200 W. The deposition time for all the a-HgI2 polycrystalline films is 240 min. The morphology, thickness, grain size, texture determination, and degree of crystallinity of the as-deposited polycrystalline a-HgI2 films were characterized by scanning electron microscopy (SEM: FEI Quanta 400) and X-ray diffraction (XRD: Rigaku D/max 2550 diffractometer, using Cu Ka radiation). HgI2 tends to chemically react with various metals (Cu, Zn, Mg, Al, Au, and Ag) [2]. In order to avoid such chemical reaction, an 1-mm-thick non-reactive parylene layer as a blocking layer was evaporated on polycrystalline a-HgI2 films, and then Au as contact material was deposited by thermal evaporation method. For electrical measurements with a sandwiched (ITO–HgI2–Au) configuration, Keithley 4200-SCS was used to measure the dark current– voltage (I–V) characteristics. The current vs. voltage (7V) was recorded immediately after applying the bias.
3. Results and discussion Fig. 2 shows typical SEM images of type a–l polycrystalline a-HgI2 films obtained with different source temperatures (50–80 1C) and ultrasonic frequencies (0–59 KHz). The inset shows grain size
source temperatures from 50 to 80 1C, their average grain sizes increase from 30 mm for type a obtained under the source temperature of 50 1C without ultrasonic wave to 158 mm for type l obtained with the source temperature of 80 1C and ultrasonic frequency of 59 KHz. Compared to type c and d films obtained at the synthesis temperature of 70 and 80 1C without ultrasonic wave, type g, h, k, and l films obtained at the same source temperature with the ultrasonic frequency of 40 and 59 KHz are compactly formed by separated rectangle monocrystalline a-HgI2 with clearer facets, more narrow size distribution, and more uniform orientation along c-axis direction also confirmed by XRD. The density of the polycrystalline a-HgI2 films plays an important role in their X-ray absorption properties. For polycrystalline a-HgI2 films, their densities are determined by the polycrystalline a-HgI2 column densities and the thickness of the films. Based on the SEM observation, the polycrystalline a-HgI2 films deposited with ultrasonic assistance have higher densities compared to the polycrystalline a-HgI2 films obtained without ultrasonic assistance. X-ray diffraction patterns for type a–l polycrystalline a-HgI2 films grew at different source temperatures with different ultrasonic frequencies are shown in Fig. 3, which reflects consistent results with SEM images shown in Fig. 2. All the XRD patterns show obvious peaks of (0 0 2), (0 0 4), (0 0 6), and (0 0 8), indicating a very strong (0 0 1) growth-preference. From Fig. 3, the polycrystalline a-HgI2 films texture can be estimated according P P to [ (0 0 l)/ (h k l)] [13,14]. In Fig. 4, this relation is plotted against source temperature and a correlation between the preferred orientation of the a-HgI2 crystal with c-axis perpendicular to the substrate and source temperature can be deduced. Fig. 4 indicates an increase in preferred orientation with c-axis perpendicular to the substrate with increasing source temperature. At the same source temperature for growing a-HgI2 films, an increase in preferred orientation with c-axis perpendicular to the substrate is also observed with increasing ultrasonic power. Compared with type d film obtained at 80 1C without ultrasonic wave, for type h and l films, fully oriented (0 0 1) peaks are observed, indicating uniformly oriented polycrystalline a-HgI2 film can be obtained with the assistance of ultrasonic wave. Due to the inherent layer structure of the tetragonal red phase of a-HgI2 crystal, the aggregated HgI2 molecules tend to align themselves with c-axis parallels to the growth direction if the surface diffusion energy is high enough [5]. As a result, the crystallographic orientation of the film and texture can be improved by using ultrasonic wave. Fig. 5 shows the dependence of growth rate on the source temperature. It is found that the growth rate is slightly increased below 70 1C with or without the ultrasonic wave. But it is sharply enhanced at the temperature of 80 1C under the assistance of ultrasonic wave. This change is due to the decrease in the enthalpy of sublimation of more than 40% at 340 K [15]. However, the growth rate increases with the increase of the assisted ultrasonic frequency. In the growth process of polycrystalline a-HgI2 films, ultrasonic wave plays an important role: (i) promotion of HgI2 molecules escape from the HgI2 source powder and (ii) increase of HgI2 molecules diffusion energy. Using the ultrasonic wave during deposition process, the HgI2 molecules with high energy can diffuse on the substrate surface, thus resulting in (0 0 l) growth preference corresponding to the lowest energy configuration. The HgI2 molecules may orderly diffuse in the region between the source and the substrate with the assistance of directional ultrasonic wave. With the increase of the ultrasonic wave frequency from 40 to 59 KHz, the HgI2 molecules can have enough surface diffusion energy to complete the Ostwald ripening
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
151
Fig. 2. SEM images: (a)–(i) corresponding to type a–l polycrystalline a-HgI2 films, respectively (inset: grain size distribution of each polycrystalline a-HgI2 film and side SEM images).
and Sintering step, and the type l film with good quality and uniform orientation along c-axis direction was achieved. Different with conventional PVD process, the assisted ultrasonic wave vibrates the HgI2 molecules on the surface of the powder, making them easier to escape from the surface of the powder at a relatively lower deposition temperature, and compensates the energy loss of the escaped molecules during their vapor transport.
Therefore, the growth rate increases obviously with the increase of ultrasonic frequency with all other parameters unchanged. The highly oriented polycrystalline a-HgI2 thick films were grown under the assistance of 59 KHz ultrasonic wave and relatively lower deposition temperature ( o90 1C) compared with other research [12]. At the relatively low deposition temperature, it may do less harm to the amorphous-Si thin film transistor (TFT)
152
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
Fig. 3. X-ray diffraction diagrams for type a–l polycrystalline a-HgI2 films grew at different synthesis temperatures with different ultrasonic frequencies.
Fig. 4. Source temperature dependence of the texture of polycrystalline a-HgI2 films obtained with or without ultrasonic wave.
arrays and lower the stress between them when the polycrystalline a-HgI2 film is directly deposited on the TFT for preparation of the direct digital X-ray detectors. To investigate electric transport properties of the prepared polycrystalline a-HgI2 films, the I–V characteristics were measured, which are expressed in terms of J–E curves (J ¼I/S and E¼ V/L, S (0.071 cm2) is the effective area of Au contact and L is the film thickness). Fig. 6 demonstrates the J–E curves are rather linear, suggesting fine ohmic contact. Resistivity (r ¼E/J) is the most important parameter defining the quality of the polycrystalline a-HgI2 films, whose values are estimated from the J–E characteristics. Fig. 6(a) shows that the values of r for type i–l polycrystalline a-HgI2 films grown under 59 KHz ultrasonic wave decrease with the increase of the source temperature. As is shown in Fig. 6(b), the values of r for type d, h and l polycrystalline a-HgI2 films prepared at the source temperature of 80 1C decrease with the increase of the assisted ultrasonic frequency. The highly
Fig. 5. Source temperature dependence of the growth velocity of polycrystalline a-HgI2 films obtained with or without ultrasonic wave.
oriented type l polycrystalline a-HgI2 thick film grown under the assistance of 59 KHz ultrasonic wave with the ultrasonic power of 200 W exhibits the lowest value of r ¼2.2 1012 O cm for E-field parallel to c-axis, which is very close to that of high quality a-HgI2 single crystals (4.0 1012 O cm) with Ref. [16], indicating a good quality of the (0 0 1)-oriented polycrystalline a-HgI2 film.
4. Conclusion Polycrystalline a-HgI2 films have been grown on ITO-coated glass substrates using UWAVPD system with the different source temperatures and ultrasonic frequencies. The morphology, grain size, degree of crystallinity, growth rate, texture, and resistivity of the polycrystalline a-HgI2 films can be adjusted by controlling the source temperature and assisted ultrasonic frequency. It is found
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
153
Fig. 6. J–E curves for type i (131 mm), j (177 mm), k (251 mm), l (380 mm) polycrystalline a-HgI2 films prepared under 59 KHz ultrasonic wave with different source temperatures (a) and type d (278 mm), h (332 mm), and l polycrystalline a-HgI2 films prepared at the same source temperature of 80 1C with different ultrasonic frequencies (0, 40, and 50 KHz) (b).
that the assisted ultrasonic wave plays an important role in the improvement of the morphology, crystallographic orientation, and electrical properties of polycrystalline a-HgI2 films. The structural and electrical properties can be improved through increasing the assisted ultrasonic frequency at the same source temperature. An uniformly oriented polycrystalline a-HgI2 film with clear facets and narrow size distribution can be obtained at the source temperature of 80 1C under the assistance of 59 KHz ultrasonic frequency with the ultrasonic power of 200 W, which has the lowest value of r ¼2.2 1012 O cm for E-field parallel to c-axis, approaching to that of high quality a-HgI2 single crystals (4.0 1012 O cm).
References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10]
Acknowledgments The authors gratefully acknowledge the financial support of the project from the National Science Foundation of China (Grant No. 10775096), China Postdoctoral Science Foundation (No. 20100480579), Nature Science Foundation of Shanghai (No. 06ZR14035) and Shanghai Leading Academic Disciplines (T0101).
[11] [12] [13] [14] [15] [16]
L. van den Berg, S.W. Pauly, Proc. SPIE 3769 (1999) 206. M. Perez, I. Noguerol, L. Fornaro, Nucl. Instr. Meth. A 610 (2009) 328. L. Fornaro, L. Mussio, M. Koncke, L. Luchini, E. Saucedo, Mater. Res. 2 (1999) 81. M. Schieber, H. Hermon, A. Zuck, A. Vilensky, L. Meiekhov, R. Shatunovsky, E. Meerson, Y. Saado, M. Lukach, E. Pinkhasy, S.E. Ready, R.A. Street, J. Cryst. Growth 225 (2001) 118. C.T. Shih, T.J. Huang, Y.Z. Luo, S.M. Lan, K.C. Chiu, J. Cryst. Growth 280 (2005) 442. M. Schieber, H. Hermon, A. Zuck, A. Vilensky, L. Melekhov, R. Shatunovsky, E. Meerson, H. Saado, Nucl. Instr. Meth. A 458 (2001) 41. J.S. Iwanczyk, B.E. Patt, C.R. Tull, L.R. MacDonald, N. Skinner, E.J. Hoffman, L. Fornaro, L. Mussio, E. Saucedo, A. Gancharov, Proc. SPEI 4508 (2001) 28. M. Schieber, A. Zuck, J. Optoelectron. Adv. Mater. 5 (2003) 1299. A.L. Noguera, M.E. Perez, L. Fornaro, Sci. Plena 3 (2007) 30. Y.T. Shiu, T.J. Huang, C.T. Shih, C.F. Su, S.M. Lan, K.C. Chiu, J. Phys. D: Appl. Phys. 40 (2007) 2170. P.J. Sellin, Nucl. Instr. Meth. A 563 (2006) 1. G. Zentai, M. Schieber, L. Partain, R. Pavlyuchkova, C. Proano, J. Cryst. Growth 275 (2005) e1327. L. Fornaro, E. Saucedo, L. Mussio, A. Gancharov, F. Guimaraes, A.C. Hernandes, Proc. SPEI 4507 (2001) 99. D. Chateigner, B. Erler, Mater. Sci. Eng. B 45 (1997) 152. M. Piechotka, E. Kaldis, J. Less-Comm. Metals 115 (1986) 315. H.T. Zhou, C.H. Lee, J.M. Chung, C.T. Shin, K.C. Chiu, S.M. Lan, Mater. Res. Bull. 38 (2003) 1987.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of oriented polycrystalline a-HgI2 films by ultrasonic-wave-assisted physical vapor deposition Weiguang Yang a, Lei Nie a, Dongmei Li a, Yali Wang b, Jie Zhou a, Lei Ma a, Zhenhua Wang a, Weimin Shi a,n a b
School of Materials Science and Engineer, Shanghai University, Shanghai 200072, China Nano-Science and Nano-Technology Research Center, School of Materials Science and Engineer, Shanghai University, Shanghai 200444, China
a r t i c l e i n f o
abstract
Article history: Received 27 September 2010 Received in revised form 27 March 2011 Accepted 7 April 2011 Communicated by R.S. Feigelson Available online 21 April 2011
Polycrystalline a-HgI2 thick films have been grown on ITO-coated glass substrates using ultrasonicwave-assisted vapor phase deposition (UWAVPD) with the different source temperatures and ultrasonic frequencies. The influence of the assisted ultrasonic wave and source temperature on the structural and electrical properties of the polycrystalline a-HgI2 films is investigated. It is found that the assisted ultrasonic wave plays an important role in the improvement of the structural and electrical properties. An uniformly oriented polycrystalline a-HgI2 film with clear facets and narrow size distribution can be obtained at the source temperature of 80 1C under the assistance of 59 KHz ultrasonic frequency with the ultrasonic power of 200 W, which has the lowest value of r ¼2.2 1012 O cm for E-field parallel to c-axis, approaching to that of high quality a-HgI2 single crystals (4.0 1012 O cm). & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Morphology A2. Growth from vapor A3. Physical vapor phase deposition processes B1. Inorganic compounds B2. Semiconducting mercury compounds
1. Introduction Mercuric iodide is one of the most suitable semiconductor materials for gamma ray and X-ray detectors operating at room temperature because of its favorable characteristics such as high atomic number of its constituent elements and large band gap (2.13 eV), resulting in a high photopeak efficiency [1–5]. Though detector properties of polycrystalline HgI2 are not yet comparable to that of single crystal HgI2, polycrystalline films of HgI2 are of renewed interest to be used as one of the promising direct converters in X-ray digital radiography because of the lower fabrication cost for large area detectors and practical experience gained with HgI2 material in space and commercial applications [6–8]. Therefore, in recent years, the research emphasis of HgI2 has been transferred from single crystal films to low cost and large area polycrystalline ones. The polycrystalline HgI2 films can be prepared on Si or ITO-coated glass substrates by different fabrication methods, such as vapor phase deposition (PVD), screen printing (SP), laser ablation (AL), and hot pressing (HP). Among them, the most widely used is PVD method [9]. The detector applications demand a lot of characteristics of polycrystalline films, such as film thickness and film growth
n
Corresponding author. Tel.: þ86 21 56334007; fax: þ86 21 56332694. E-mail address: [email protected] (W. Shi).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.04.005
orientation. Among them, the film growth orientation plays an important role in sensitivity and transport properties of the final devices fabricated with it. The better the orientation, the better the charge transport along c-axis of microcrystals when they are irradiated. Oriented polycrystalline a-HgI2 films have been grown through varying the growing conditions of PVD [5,10,11]. In this present work, the highly oriented polycrystalline a-HgI2 films with clearer facets and more narrow size distribution have been grown onto ITO-coated glass substrates by ultrasonic-waveassisted PVD method at relatively lower deposition temperature ( o90 1C) compared with other research [12]. The influence of assisted ultrasonic wave on the morphology, grain size, degree of crystallinity, growth rate, texture, and resistivity of the polycrystalline a-HgI2 films is also discussed.
2. Experimental details The polycrystalline red mercuric iodide (a-HgI2) films were deposited on ITO-coated glass substrates via ultrasonic-waveassisted physical vapor deposition method (UWAPVD) in the whole process. Fig. 1 schematically depicts the UWAPVD system. The system is made up of (i) a growth tube with an inner diameter of 20 mm, an outer diameter of 25 mm and a length of 120 mm, (ii) a sealing lid and an air-pumping branch, (iii) an ultrasonic wave generator, (iv) a substrate temperature controller, and (v) a heating
150
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
distribution for each polycrystalline a-HgI2 film. All polycrystalline a-HgI2 films have microcrystalline structures. With the increase of
Fig. 1. Schematic diagram of the UWAPVD system in a water-bath oven used for growing polycrystalline a-HgI2: (1) substrate temperature controller; (2) cooling water; (3) growth tube; (4) air pumping branch; (5) ultrasonic wave generator; (6) heating wall and temperature controller; and (7) substrate holder.
wall and temperature controller. In preparation for polycrystalline a-HgI2 film growth, the growth tube was put inside water bath oven with the sealing lid and an air pumping branch while the substrate was cooled at the same separate controlled temperature of 25 1C. Prior to loading a-HgI2 source powder, the growth tube was cleaned with aqua regia, rinsed with deionized water for several times, and dried for 12 h at 300 1C. After the typical cleaning processes, the a-HgI2 powder was placed inside the bottom of the dust-proofing quartz glass growth tube and the ITO-coated glass substrate was put on the substrate holder, then the lid on the top of the apparatus was sealed. And then the air-pumping branch was connected with the vacuum pumping system. The heating preparation process started when the level of vacuum reached 3–6 10 3 Pa, and this pressure was maintained during the whole preparation process. Type a-d polycrystalline a-HgI2 films were obtained at the different waterbath temperatures (source temperatures) arranging from 50 to 80 1C without ultrasonic wave. Type e–h polycrystalline a-HgI2 films were obtained at the different source temperatures arranging from 50 to 80 1C with the assistance of ultrasonic wave of 40 KHz with the ultrasonic power of 100% (200 W). Type i–l polycrystalline a-HgI2 films were obtained at the different source temperatures arranging from 50 to 80 1C with ultrasonic wave of 59 KHz with a maximum power output of 200 W. The deposition time for all the a-HgI2 polycrystalline films is 240 min. The morphology, thickness, grain size, texture determination, and degree of crystallinity of the as-deposited polycrystalline a-HgI2 films were characterized by scanning electron microscopy (SEM: FEI Quanta 400) and X-ray diffraction (XRD: Rigaku D/max 2550 diffractometer, using Cu Ka radiation). HgI2 tends to chemically react with various metals (Cu, Zn, Mg, Al, Au, and Ag) [2]. In order to avoid such chemical reaction, an 1-mm-thick non-reactive parylene layer as a blocking layer was evaporated on polycrystalline a-HgI2 films, and then Au as contact material was deposited by thermal evaporation method. For electrical measurements with a sandwiched (ITO–HgI2–Au) configuration, Keithley 4200-SCS was used to measure the dark current– voltage (I–V) characteristics. The current vs. voltage (7V) was recorded immediately after applying the bias.
3. Results and discussion Fig. 2 shows typical SEM images of type a–l polycrystalline a-HgI2 films obtained with different source temperatures (50–80 1C) and ultrasonic frequencies (0–59 KHz). The inset shows grain size
source temperatures from 50 to 80 1C, their average grain sizes increase from 30 mm for type a obtained under the source temperature of 50 1C without ultrasonic wave to 158 mm for type l obtained with the source temperature of 80 1C and ultrasonic frequency of 59 KHz. Compared to type c and d films obtained at the synthesis temperature of 70 and 80 1C without ultrasonic wave, type g, h, k, and l films obtained at the same source temperature with the ultrasonic frequency of 40 and 59 KHz are compactly formed by separated rectangle monocrystalline a-HgI2 with clearer facets, more narrow size distribution, and more uniform orientation along c-axis direction also confirmed by XRD. The density of the polycrystalline a-HgI2 films plays an important role in their X-ray absorption properties. For polycrystalline a-HgI2 films, their densities are determined by the polycrystalline a-HgI2 column densities and the thickness of the films. Based on the SEM observation, the polycrystalline a-HgI2 films deposited with ultrasonic assistance have higher densities compared to the polycrystalline a-HgI2 films obtained without ultrasonic assistance. X-ray diffraction patterns for type a–l polycrystalline a-HgI2 films grew at different source temperatures with different ultrasonic frequencies are shown in Fig. 3, which reflects consistent results with SEM images shown in Fig. 2. All the XRD patterns show obvious peaks of (0 0 2), (0 0 4), (0 0 6), and (0 0 8), indicating a very strong (0 0 1) growth-preference. From Fig. 3, the polycrystalline a-HgI2 films texture can be estimated according P P to [ (0 0 l)/ (h k l)] [13,14]. In Fig. 4, this relation is plotted against source temperature and a correlation between the preferred orientation of the a-HgI2 crystal with c-axis perpendicular to the substrate and source temperature can be deduced. Fig. 4 indicates an increase in preferred orientation with c-axis perpendicular to the substrate with increasing source temperature. At the same source temperature for growing a-HgI2 films, an increase in preferred orientation with c-axis perpendicular to the substrate is also observed with increasing ultrasonic power. Compared with type d film obtained at 80 1C without ultrasonic wave, for type h and l films, fully oriented (0 0 1) peaks are observed, indicating uniformly oriented polycrystalline a-HgI2 film can be obtained with the assistance of ultrasonic wave. Due to the inherent layer structure of the tetragonal red phase of a-HgI2 crystal, the aggregated HgI2 molecules tend to align themselves with c-axis parallels to the growth direction if the surface diffusion energy is high enough [5]. As a result, the crystallographic orientation of the film and texture can be improved by using ultrasonic wave. Fig. 5 shows the dependence of growth rate on the source temperature. It is found that the growth rate is slightly increased below 70 1C with or without the ultrasonic wave. But it is sharply enhanced at the temperature of 80 1C under the assistance of ultrasonic wave. This change is due to the decrease in the enthalpy of sublimation of more than 40% at 340 K [15]. However, the growth rate increases with the increase of the assisted ultrasonic frequency. In the growth process of polycrystalline a-HgI2 films, ultrasonic wave plays an important role: (i) promotion of HgI2 molecules escape from the HgI2 source powder and (ii) increase of HgI2 molecules diffusion energy. Using the ultrasonic wave during deposition process, the HgI2 molecules with high energy can diffuse on the substrate surface, thus resulting in (0 0 l) growth preference corresponding to the lowest energy configuration. The HgI2 molecules may orderly diffuse in the region between the source and the substrate with the assistance of directional ultrasonic wave. With the increase of the ultrasonic wave frequency from 40 to 59 KHz, the HgI2 molecules can have enough surface diffusion energy to complete the Ostwald ripening
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
151
Fig. 2. SEM images: (a)–(i) corresponding to type a–l polycrystalline a-HgI2 films, respectively (inset: grain size distribution of each polycrystalline a-HgI2 film and side SEM images).
and Sintering step, and the type l film with good quality and uniform orientation along c-axis direction was achieved. Different with conventional PVD process, the assisted ultrasonic wave vibrates the HgI2 molecules on the surface of the powder, making them easier to escape from the surface of the powder at a relatively lower deposition temperature, and compensates the energy loss of the escaped molecules during their vapor transport.
Therefore, the growth rate increases obviously with the increase of ultrasonic frequency with all other parameters unchanged. The highly oriented polycrystalline a-HgI2 thick films were grown under the assistance of 59 KHz ultrasonic wave and relatively lower deposition temperature ( o90 1C) compared with other research [12]. At the relatively low deposition temperature, it may do less harm to the amorphous-Si thin film transistor (TFT)
152
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
Fig. 3. X-ray diffraction diagrams for type a–l polycrystalline a-HgI2 films grew at different synthesis temperatures with different ultrasonic frequencies.
Fig. 4. Source temperature dependence of the texture of polycrystalline a-HgI2 films obtained with or without ultrasonic wave.
arrays and lower the stress between them when the polycrystalline a-HgI2 film is directly deposited on the TFT for preparation of the direct digital X-ray detectors. To investigate electric transport properties of the prepared polycrystalline a-HgI2 films, the I–V characteristics were measured, which are expressed in terms of J–E curves (J ¼I/S and E¼ V/L, S (0.071 cm2) is the effective area of Au contact and L is the film thickness). Fig. 6 demonstrates the J–E curves are rather linear, suggesting fine ohmic contact. Resistivity (r ¼E/J) is the most important parameter defining the quality of the polycrystalline a-HgI2 films, whose values are estimated from the J–E characteristics. Fig. 6(a) shows that the values of r for type i–l polycrystalline a-HgI2 films grown under 59 KHz ultrasonic wave decrease with the increase of the source temperature. As is shown in Fig. 6(b), the values of r for type d, h and l polycrystalline a-HgI2 films prepared at the source temperature of 80 1C decrease with the increase of the assisted ultrasonic frequency. The highly
Fig. 5. Source temperature dependence of the growth velocity of polycrystalline a-HgI2 films obtained with or without ultrasonic wave.
oriented type l polycrystalline a-HgI2 thick film grown under the assistance of 59 KHz ultrasonic wave with the ultrasonic power of 200 W exhibits the lowest value of r ¼2.2 1012 O cm for E-field parallel to c-axis, which is very close to that of high quality a-HgI2 single crystals (4.0 1012 O cm) with Ref. [16], indicating a good quality of the (0 0 1)-oriented polycrystalline a-HgI2 film.
4. Conclusion Polycrystalline a-HgI2 films have been grown on ITO-coated glass substrates using UWAVPD system with the different source temperatures and ultrasonic frequencies. The morphology, grain size, degree of crystallinity, growth rate, texture, and resistivity of the polycrystalline a-HgI2 films can be adjusted by controlling the source temperature and assisted ultrasonic frequency. It is found
W. Yang et al. / Journal of Crystal Growth 324 (2011) 149–153
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Fig. 6. J–E curves for type i (131 mm), j (177 mm), k (251 mm), l (380 mm) polycrystalline a-HgI2 films prepared under 59 KHz ultrasonic wave with different source temperatures (a) and type d (278 mm), h (332 mm), and l polycrystalline a-HgI2 films prepared at the same source temperature of 80 1C with different ultrasonic frequencies (0, 40, and 50 KHz) (b).
that the assisted ultrasonic wave plays an important role in the improvement of the morphology, crystallographic orientation, and electrical properties of polycrystalline a-HgI2 films. The structural and electrical properties can be improved through increasing the assisted ultrasonic frequency at the same source temperature. An uniformly oriented polycrystalline a-HgI2 film with clear facets and narrow size distribution can be obtained at the source temperature of 80 1C under the assistance of 59 KHz ultrasonic frequency with the ultrasonic power of 200 W, which has the lowest value of r ¼2.2 1012 O cm for E-field parallel to c-axis, approaching to that of high quality a-HgI2 single crystals (4.0 1012 O cm).
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Acknowledgments The authors gratefully acknowledge the financial support of the project from the National Science Foundation of China (Grant No. 10775096), China Postdoctoral Science Foundation (No. 20100480579), Nature Science Foundation of Shanghai (No. 06ZR14035) and Shanghai Leading Academic Disciplines (T0101).
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