Bismuth tri-iodide polycrystalline films for digital X-ray ... - IEEE Xplore

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 1, FEBRUARY 2004

Bismuth Tri-Iodide Polycrystalline Films for Digital X-Ray Radiography Applications L. Fornaro, E. Saucedo, L. Mussio, A. Gancharov, and A. Cuña

Abstract—Bismuth tri-iodide polycrystalline films were grown 1 in size by the physical vapor deposition method. Glass 1 was used as the substrate. Palladium was deposited previously onto the substrates as the rear contact. For growth, bismuth tri-iodide 99.999% was heated at 130–170 C under high vacuum atmosphere 5 mmHg) or under Ar pressure for 20 hours. Film thick( ness was measured by the transmission of 59.5 keV 241 Am emission, giving values ranging from 90 to 130 m (5%). Film grain size was measured by scanning electron microscopy, and it gave an average of m. Detectors were made with the films by depositing palladium as the front contact (contact area 4 mm2 ) and then performing acrylic encapsulation. Resistivities of 12 .cm and current densities of 240 pA/cm2 at 20 V were obtained for these detectors. The electron mobility and lifetime and the electron mobility-lifetime product were measured by the transient charge technique, which gave values of 4.4 cm2 /V.s, 7 s and 6 cm2 /V respectively. X-ray film response was checked by irradiating the films with a 241 Am source and with an X-ray beam, for different beam energies and intensities and for several bias voltages applied to the detector. A linear response with exposure rate was obtained. Finally, the results were compared with previous ones for monocrystals of bismuth tri-iodide and polycrystalline films of alternative materials like lead iodide and mercuric iodide.

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TABLE I COMPARISON OF THE PROPERTIES OF BiI WITH THE ONES OF THE SEMICONDUCTORS USED FOR GROWING FILMS INTENDED FOR X-RAY IMAGING. Z: ATOMIC NUMBER; d: DENSITY; : PHOTON ATOMIC ABSORPTION : ENERGY BAND GAP AT ROOM COEFFICIENT AT 20 KeV [33]; TEMPERATURE; : RESISTIVITY AT ROOM TEMPERATURE

1E

(50 20)

6

10

10

1 4 10

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TABLE II RELEVANT REPORTED RESULTS FOR FILMS OF THE HEAVY METAL IODIDES GROWN BY THE PVD METHOD

Index Terms—Bismuth tri-iodide, compound semiconductor films, X-ray imaging.

I. INTRODUCTION OWADAYS, X-ray imaging is applied in many fields, such as medicine, industry, science, cargo inspection, and astronomical observation. For medical diagnosis, radiography has been performed for more than 100 years using photographic films. Afterwards, phosphor screens with Gd O S:Tb or CsI(Tl) have also been used as detectors, coupled to photomultipliers, and, recently, to amorphous silicon arrays [1]. Both photographic films and phosphor screens are examples of indirect approaches to performing X-ray imaging, the first as a nondigital and the second as a digital method. Although both processes perform acceptably, several drawbacks remain unsolved, especially poor spatial resolution because of a significant signal spread. In recent years, a direct approach to X-ray imaging, which may completely eliminate the indirect deficiencies, has been under development. In this approach, crystalline films of high atomic number and high radiation absorption coefficient semiconductors absorb the X-rays and

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Manuscript received March 24, 2003; revised May 5, 2003. This work was supported by Projects CHLCC 132 and CONICYT CE 6054, Uruguay. The authors are with the Compound Semiconductors Group, Radiochemistry Department, Faculty of Chemistry, University of Uruguay, Montevideo 11800, Uruguay (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Digital Object Identifier 10.1109/TNS.2004.824821

convert them directly into electrical signals, which can be read, for example, by a thin film transistor (TFT) [2]–[23]. This direct digital approach has several advantages over the indirect methods, such as convenient image acquisition and retrieval, digital image processing, computer-assisted diagnosis, easy image storage and transmission, real-time images, a better spatial resolution and higher X-ray detection efficiency, which may reduce the radiation dose for equivalent images [24]. TFT readout arrays are leading flat panel imaging technology, and they are at a well developed stage. Therefore, the deposition of appropriate semiconductor films onto them has become the main challenge for the development of a direct digital approach for X-ray imaging. Several semiconductor materials are being studied for X-ray digital imaging; Table I compares their main properties, together with the ones of bismuth tri-iodide, which is the subject of this study. Table II summarizes some relevant reported results for films of the heavy metal iodides, grown by the physical vapor deposition (PVD) method. Evidently, films of mercuric iodide and lead iodide are under development, and as yet it is too early to be sure which of them is more suitable for X-ray digital imaging. Nevertheless, the fact that they are among the best

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FORNARO et al.: BISMUTH TRI-IODIDE POLYCRYSTALLINE FILMS FOR X-RAY RADIOGRAPHY APPLICATIONS

candidates for the described application is undeniable. The current success with these two materials leads us to consider the whole series of heavy metal iodides as possible semiconductors for X-ray imaging, and bismuth tri-iodide, for example, might be studied as well. Bismuth tri-iodide, like mercuric and lead iodide, is a layered compound whose crystal lattices are built from three layer packages (I-Hg-I; I-Pb-I; I-Bi-I) with weak van der Waals bonding between adjacent planes of iodine atoms and perpendicular to the c axis [25]–[27]. This weak bonding determines that the three materials are soft, and have to be handled carefully. Other analogies between bismuth tri-iodide and lead iodide are the metal ion outer electron configuration (5d 6s ), and the melting point close to 408 C, depending on material purity. We should now consider the material stability. The phase diagram of the Bi-I system [28], [29] shows that hexagonal polymorph BiI is stable within the range from the melting point to room temperature. The vapor pressure of BiI equals 70 Torr at the melting point [30], [31], and, although BiI vapor does not dissociate significantly below about 600 C, the material tends toward dissociation, which is a few percent at 250 C–300 C [30]. That tendency will lead to a lack of stoichiometry—even for high purity material—which will cause charge transport deterioration. These considerations show another similarity between bismuth tri-iodide and lead iodide, since they both present similar decomposition and stoichiometry problems [19], [32]. On the other hand, BiI presents good values of the properties important for X-ray detection, as can be seen in Table I. Attempts have been made at growing bismuth tri-iodide single crystal platelets by open flow sublimation and recrystallization [35], and bulk monocrystals by PVD [26] and also by the vertical Bridgman method [25], [27], [36]. Only one reference was found to the growth of bismuth tri-iodide films, but those were grown onto a monocrystalline substrate (lead iodide) and had thicknesses between 0.01 and 0.1 m [37]. This background is not useful for films intended for X-ray imaging, which must be grown onto a TFT and with thicknesses in the order of 100 m for adequate radiation absorption. Although counting results obtained for bismuth tri-iodide monocrystals up to now are not encouraging [26], [27], [36], if we consider the material similarities with mercuric iodide and lead iodide, as well as the results obtained for films of these two materials, promising results for bismuth tri-iodide films may be expected. Therefore, the purpose of the present work is to study the capability of bismuth tri-iodide to give polycrystalline films suitable for X-ray digital imaging, and to compare this capability with those previously obtained for films of the other heavy metal iodides. II. EXPERIMENTAL Bismuth tri-iodide Alfa Aesar 99.999% was used as the starting material. Experiments were performed in quartz ampoules 5 cm in diameter and 20 cm in length (quartz from Dow Corning Inter-America, New York, NY), cleaned with aqua regia for 12 h, rinsed with distilled water and outgassed for mmHg. Glass substrates 12 h at 800 C and

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in size were cleaned with high purity acetone, and palladium was deposited onto them as the rear contact by thermal evaporation using a Denton Vacuum DV-502 evaporator. Typical mm Hg and a deposition time deposition conditions were of 10 s, with the distance Pd source—film at about 10 cm. Substrates were mounted in a holder especially designed for this experiment. For film growth, 3 g of powdered BiI were charged at the bottom of the ampoule and this was evacuated at mmHg and sealed, or cleaned with high purity Ar and finally sealed under an initial pressure of 100–600 mmHg of Ar. Films were grown by the physical vapor deposition method (PVD). The charged extreme of the ampoule was put inside a vertical resistance furnace and kept at 280 C ( 5 C). Substrate temperature was 130 C–170 C and the growth time was 20 h. Am Film thickness was measured by absorption of the 59.5 keV radiation, selecting the photopeak with an EG&G Ortec Solid Scintillation NaI(Tl) spectrometry system. Film surface and grain size were studied by scanning electron microscopy (SEM) using a JEOL 5900 system with an EDS microprobe Vantage NORAN. For electrical properties and X-ray response measurements, detectors were assembled by deposition of palladium onto selected films as the front contact, with areas in the order of 4 mm , under the conditions mentioned above. Palladium leads 0.001” in diameter were attached using “aquadag” (from Acheson Inc., Amsterdam, The Netherlands), and finally, the detector was capsulated with a protective coating (“Humiseal” from Chase Corp., Woodside, NY). Room temperature measurements of current density through the films as a function of the detector bias were carried out using a Keithley electrometer (Model 614) and an EG&G Ortec (Model 556) dc high voltage power supply. Detector response to X-rays was studied by Am source (35 mR/hr) and also with an irradiation with a X-ray tube, using the same arrangement. The charge transport properties of the films were measured using the transient charge technique by irradiation with the Am source. Pulses from interactions were processed by a fast preamplifier and then displayed and recorded on a digital oscilloscope Tektronix TDS 380 (400 MHz). III. RESULTS AND DISCUSSION Taking into account the high vapor pressure and the tendency of the material toward dissociation under temperatures close to the melting point, especial care was taken to prevent or at least diminish decomposition, which would strongly damage stoichiometry. PVD by sublimation of the starting material below the melting point, at temperatures in the range 200 C–300 C, seemed to be an appropriate method to perform film growth. These temperatures also appear to be just high enough to ensure that sublimation occurs at a practical rate. Evaporation conditions, with temperatures above the melting point, were not used, because they would need excessive inert gas pressure to prevent decomposition. When film growth was performed at mmHg, vapor iodine was seen in the ampoule. This fact has also been verified by other researchers when growing bismuth tri-iodide monocrystals [26]. When film growth was performed under

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Fig. 3. SEM image of a BiI film grown under an initial argon pressure of 100 mmHg. Fig. 1. Picture of a polycrystalline bismuth tri-iodide film grown on a glass 1 in size. substrate 1

2

Fig. 2.

SEM image of a BiI polycrystalline film grown at 5

2 10

Fig. 4. Dark current density as a function of detector bias for a representative detector made with a bismuth tri-iodide film 130 m in thickness. mmHg.

an argon initial pressure of 100–600 mmHg, decomposition was not observed. This recourse has been employed successfully for purifying lead iodide and growing films from it [19], [21]–[23], [32], precisely because of lead iodide decomposition under vacuum conditions. As material decomposition can damage stoichiometry, measurements are being carried out to elucidate the possible influence of film composition on its properties. The best film growth conditions were a source temperature of mmHg and a growth 280 C, an initial Ar pressure of time of 20 h, with a substrate temperature of 150 C. Under these conditions, homogeneous films were obtained, as can be seen in Figs. 1 and 2. Conversely, the growth under argon atmosphere gave nonhomogeneous films, which are shown in Fig. 3. As the growth time was 20 hours for all the films, a possible explanation of that nonhomogeneity might be that argon decreases the growth rate, and films grown under such condition need more time to become homogeneous. Films grown under vacuum have thicknesses from 90 to 130 m, suitable for the stopping of X-rays used for medical radiography (165 m of bismuth tri-iodide absorb 99% of 20 keV radiation). Thicknesses are in the range of previous results reported for lead iodide [17]–[19], [21], [22] and for mercuric iodide [11], [12] films. The thickness nonuniformity

in the best layers was less than 5% } over the whole area of the film. SEM images show that, whatever the growth conditions, the film microcrystals tend to grow along the plane of the substrate surface, which will give a large area exposed to radiation. Considering the layered nature of the material, a growth preferred orientation with the c axis perpendicular to the substrate should be inferred. X-ray diffraction studies are being performed for further verification of such an orientation. For films grown under vacuum, average grain size was m. The grain sizes achieved are in the order of or smaller than the previous ones for PbI films (20–80 m [23]), (1–5 m [18]) and for mercuric iodide films (5–10 m [11], 50–100 m [9], up to 50 m [12] and 11–160 m [15]). This grain size is well suited to TFT pixellation and should provide a spatial resolution commensurate with the needs of digital radiography. Fig. 4 shows dark current density as a function of the detector bias for a representative film grown under vacuum. Dark current density values of 240 pA/cm at 20 V and resistivity .cm were measured. As these are values up to the first bismuth tri-iodide films grown up to now, it is not possible to compare the obtained results with previous ones. Nevertheless, comparison can be made with results obtained for bismuth tri-iodide monocrystals. Higher dark current densities, of about 200 nA/cm , have been measured for crystals grown

FORNARO et al.: BISMUTH TRI-IODIDE POLYCRYSTALLINE FILMS FOR X-RAY RADIOGRAPHY APPLICATIONS

Fig. 5. Voltage pulse obtained by irradiation with emissions of 5.5 MeV from a Am source, from a fast rise-time preamplifier and collected by a digital oscilloscope (electron collection). Film thickness: 60 m; detector bias: 50 V.

by the Bridgman method [36]. Bismuth tri-iodide crystals have .cm given resistivity values along the c axis of (2–5) 10 .cm [27] and .cm [36]. [26], (0.5–2.0) 10 Therefore, the resistivity measured for our films is the highest obtained up to now for this material. Also, film electrical properties can be compared to the results obtained for films of other heavy metal iodides. Resistivity values are in the order of some .cm [19], of the reported results for lead iodide films ( .cm [17]), although lower than others ( .cm [21]–[23]). A similar conclusion can be deduced from the comparison with resistivity values obtained for mercuric iodide .cm [9], (1–6) 10 .cm [15]). Dark films ( current density values are lower than some reported results (10000 pA/cm , [18]) although higher than others (2 pA/cm , [21]–[23]). At a detector bias of 30 V, dark current densities of about 353 pA/cm were measured, and can be compared with results for HgI films (5000 pA/cm [11], 600 pA/cm [12], 20 pA/cm [15], all values at 30 V of detector bias). Resistivity and dark current density results for BiI layers, although not the best, are already suitable for the X-ray digital imaging application. Taking into account that this is the first attempt at growing films of this material, it will surely be possible to improve electrical properties in the future. The electrical properties of films grown under Ar pressure could not be checked because of their nonhomogeneity. One captured pulse from the generation and transport of electrons in the BiI layers grown under vacuum is shown in Fig. 5. From this figure, an electron mobility of 4.4 cm /V.s and s, and therefore an electron an electron lifetime of cm /V, were obtained. mobility-lifetime product of Although there are no data for these parameters for bismuth tri-iodide films, they can be compared with results obtained for BiI monocrystals, for which an electron mobility-lifetime cm /V was obtained [27]. Comparison product of indicates that the transport properties of these BiI layers are similar to those for monocrystals of the same material. Also, mobility-lifetime product is in the order of the values obtained cm /V, [18] and mercuric iodide for lead iodide films ( cm /V, [15]) films. (

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=I ) as a function of detector bias for a representative Fig. 6. (I : current density through the detector irradiated with Am source; film. I I : dark current density. Film thickness: 60 m.

Fig. 7. X-ray response of the films. I : current density through the film : dark current density. Radiation dose of irradiated by the X-ray beam; I the Am source: 3.5 mRem/h (measured with an ionization chamber). Film thickness: 60 m; detector bias: 50 V.

When detectors made with the films are irradiated with a Am source, current density significantly increases. Fig. 6 shows the relation between current densities for the irradiated as a function of the deand nonirradiated films tector bias, which gives signals up to eight times greater than the dark response. This is a very good result, taking into account the low count previously reported for detectors made with bismuth tri-iodide monocrystals, which have not given any response to X, or radiations [26], have given a poor response to radiation [36] or to X-rays from a pulsed X-ray machine [27], always with irradiation through the negative electrode, that is, with electron collection. An acceptable linearity of response to an X-ray beam for several kilovoltages of the X-beam, and therefore sensitivity almost constant with the exposure rate, can be observed in Fig. 7. These results are worse than the ones obtained for HgI and PbI films [10], [14]. [23].

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IV. CONCLUSION Polycrystalline bismuth tri-iodide films have been grown for the first time on amorphous substrates. Films grow at lower temperatures than lead iodide films, which better agrees with TFT specifications. Films have thickness and grain size suitable for the absorption and the spatial resolution required for X-ray digital imaging applications. Dark current density and resistivity values were found to be better than values reported for BiI monocrystals, and comparable with data obtained for lead and mercuric iodide layers. Transport properties, signal to dark current relation and linearity of response to X-rays are also similar to the parameters obtained for the other heavy metal iodide layers. Taking into account that this has been the first attempt at growing bismuth tri-iodide films, the results are very encouraging. Future efforts will be focused on optimizing the film growth conditions, and seeking better electrical and transport properties, as well as higher linearity and sensitivity to X-rays.

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