Journal of the Korean Physical Society, Vol. 58, No. 6, June 2011, pp. 1577∼1580

Development of InSb Semiconductor Detector for High Resolution Radiation Measurement

Se-Hwan Park,∗ Han Soo Kim, Hee-Sung Shin and Ho-Dong Kim Korea Atomic Energy Research Institute, Daejeon 305-353, Korea

Yun-Ho Cho and Yong Kyun Kim Department of Nuclear Engineering, Hanyang University, Seoul 133-791, Korea

(Received 27 December 2010, in final form 29 March 2011)

Indium Antimonide (InSb) has possibility to be developed as the next generation radiation de- tector due to its small energy bandgap and high electron mobility. In general, the InSb crystal has been used for infrared applications, and a studies to grow the InSb crystal for the radiation detector application are rare. The dependency of the crystal growth speed on the crystal quality was studied in the present work. The InSb crystal was grown using the Bridgman method at various crystal growth speeds. The grown crystal was cut into 2-mm-thick wafers, and the defects in the lattice structure of the crystal were analyzed with X-Ray diffraction (XRD) and Fourier transform infraRed spectroscopy (FT-IR). The wafer was made into a Schottky-type diode, and the I-V curves were measured at various temperatures. An InSb detector was also made with a commercial InSb wafer, and the crystal characteristics were measured and compared with the grown one. Our work could be helpful in developing an InSb radiation detector.

PACS numbers: 29.40.Wk, 85.30.De, 81.05.Ea Keywords: InSb, Bridgman, I-V curve, Schottky diode DOI: 10.3938/jkps.58.1577

I. INTRODUCTION are rare [6]. Since the impurity in the crystal can be easily controlled with a Bridgman method, the Bridg- man method was selected for crystal growth to obtain Conventional semiconductors such as silicon or germa- high-quality crystals. The crystals were grown at var- nium have been developed as radiation detector and used ious crystal growth speeds, and the dependency of the in many research and application areas. However, new crystal quality on the growth speed was studied. An semiconductor is still necessary especially in high reso- InSb detector was also fabricated with a wafer supplied lution X-ray and γ-ray spectroscopy [1,2]. It is known from a commercial vendor, and the I-V curve and the that the InSb may be developed as radiation detector energy spectrum were measured with the detector. The due to its small band gap and large electron mobility. result was compared with that of the detector made with The energy band gap of InSb is 0.165, which is 1/6 that the grown crystal. of Si and 1/4 that of Ge. The electron mobility of InSb is 78000 cm2V−1s−1, which is 40 and 30 times those of Si and Ge. McHarris pointed out the possibility of using InSb as an ultra-high-resolution radiation detector ma- II. EXPERIMENTS terial [3]. Kanno et al. developed the InSb detector and measured α and γ-rays [4]. However, the detector per- 1. Crystal Growth formance should be still improved further to reach the required resolution. The main limitation in the detector performance is its low resistivity. The InSb crystals were grown with the Bridgman In the present work, an InSb crystal was grown for ra- method. The growth procedure is as follows: High-purity diation detector applications. Previously, InSb crystals materials (6N) were inserted into a quartz ampule. The were grown with the Czochralski method [5]. Studies ampule was semiconductor- grade quartz. The outer di- to grow InSb crystals for radiation detector applications ameter of the ampule was 2 cm, and the length of the ampule was ∼20 cm. One end of the ampule was pointed, ∗E-mail: [email protected]; Fax: +82-42-862-7313 and the crystal was started to grow from the pointed end. -1577- -1578- Journal of the Korean Physical Society, Vol. 58, No. 6, June 2011

Fig. 1. InSb crystal grown with the Bridgman method.

Before the crystal growth, the ampule was cleaned. The ampule was cleaned with distilled water several times, and the ampule was dipped into aqua regia overnight to remove the contaminants on the ampule Fig. 2. XRD spectrum of the grown InSb crystal. surface. After that, the ampule was cleaned with run- ning distilled water several times to remove the remnants on the ampule. The ampule was installed in a furnace, which was designed to coat a carbon film on the inner surface of the ampule. Argon gas was flowed inside the ampule at a flowing rate of 30 cc/min. The tempera- ture of the ampule was increased to 1100 ◦C. It took 8 hours. The ampule temperature was kept at 1100 ◦C for 5 hours. After that, the temperature was lowered to room temperature, which took for 8 hours. The ampule was detached from the furnace, and the high purity (6N grade) In and Sb were inserted into the ampule, considering the stoichiometry. The raw materi- als were kept in a glove box and inserted into the ampule in the same glove box to minimize the contamination. The ampule was moved to a vacuum sealing system. The Fig. 3. (Color online) FT-IR spectrum of the grown InSb vacuum inside the ampule was kept at 10−6 Torr when crystal. the ampule was sealed. The sealed ampule was installed in a Bridgman fur- nace to grow the crystal. The Bridgman furnace had 3 was continuously cleaned with running water. After the heating zone: Zone 1 was the high-temperature region, grinding process, the crystal surface was polished with and zones 2 and 3 were the low-temperature regions. At alumina powder. Alumina of 1-µm was used at the be- the beginning of the crystal growth, the temperatures ginning of the polishing process, and 0.05-µm alumina of all the zones were raised to the same temperature, was used at the end of the polishing process. A pol- 640 ◦C and kept at the same temperature for 24 hours. ishing with 0.3-µm alumina was done as an intermediate In this procedure, the materials in the ampule could be process. At the end of each polishing process, the crystal melted and become a compound material. The temper- was cleaned with the isopropyl alcohol and methanol. ature of each heater was changed to grow the crystal. X-Ray diffraction (XRD) and Fourier transform in- The temperature of zone 1 was lowered to 545 ◦C and fraRed (FT-IR) analyses were done to analyze the crys- the temperature of zones 2 and 3 was lowered to 475 ◦C. tallography and the defect density of the crystal. The The ampule was moved from zone1 to zone 2, and a crys- XRD analysis result can be seen in Fig. 2, and the FT-IR tal was grown inside the ampule. We grew the crystal result can be seen in Fig. 3. From the Fig. 2, the crystal with three different growth rate, 6 mm/h, 2 mm/h, and was not singly oriented. The transmittance of the crys- 0.8 mm/h, to study the effect of crystal growth speed tal increased with decreasing growth speed, which can on the crystal quality. A grown crystal can be seen in be seen in Fig. 3. Fig. 1. Metal electrodes were deposited on both sides of the The ingot was cut with a diamond-wire saw to a thick- crystal to measure the resistivity of the crystal. The ness of 2 mm. Each side of the crystal surface was me- crystal was chemically etched with an acid solution (10% chanically grinded with SiC paper (4000 grit). The final nitric acid and 90% lactic acid). The crystal was dipped thickness of the InSb crystal was ∼1.5 mm. The dam- into the acid solution for 10 min. A gold electrode was aged layers on both sides of the crystal could be removed deposited on one side of the crystal, and an indium elec- in the procedure. In the grinding process, the crystal trode was deposited on the opposite side of the crystal. Development of InSb Semiconductor Detector for High Resolution Radiation Measurement – Se-Hwan Park et al. -1579-

The deposition was done with a thermal evaporator. The vacuum in the evaporation chamber was kept at 10−6 Torr, and the deposition rate was 10 nm/s. The final thickness of the metal electrode was ∼500 nm. Signal wires were connected to both sides of the electrode with silver paste. The I-V curve of the crystal was measured at various crystal temperatures. The crystal was placed in a cryostat system (Janis Research Co. Model No.: STVP-100). Helium gas flowed in the cryostat system, and the temperature of the crystal could be controlled in the cryostat system by using a heating wire.

2. Diode Fabrication

We fabricated InSb detectors with n-type and undoped wafers (from Wafer Technology Ltd.). The wafer thick- Fig. 4. (Color online) I-V data taken with the InSb crystals ness was 450 µm. In n-type wafer, Te was included as with a Schottky structure. a doping element. Both sides of the wafer were already polished when the wafer was obtained from the vendor. The preliminary study to study the optimization of the fabrication procedure can be found in Ref. 7. Schottky diodes were made with the wafers. The wafers were cut into a square shape, the size of which was 10 × 10 mm2. Since the wafers were very brittle, the cutting speed was kept below 1.0 mm/sec. The fab- rication process to make the InSb detector was almost similar to previous ones. The fabrication was done in a class-1000 clean room. The wafers were dipped in isopropyl alcohol and methanol for 5 min, respectively, to remove the remnants on the surface. The wafer surface was etched with a chemical acid solution (nitric acid and lactic acid). The etching time was 10 min, and the etching process could Fig. 5. I-V curve of the Schottky detector with an undoped remove the damaged layer on the wafer surface. Metal InSb crystal. electrodes were evaporated on both sides of the wafer. Gold was evaporated on one side of the wafer, and in- dium was evaporated on the opposite side of the wafer. The electrode diameter was 3 mm, and the electrode thickness was 500 nm. A signal wire was connected to both sides with silver paste.

3. Measurement of Diode Characteristics

Figure 4 shows the I-V curves measured with the grown crystals. One can see that the leakage current became smaller as the crystal temperature was lowered. Fig. 6. I-V curve of the Schottky detector with an n-type Also, the crystal grown with a lower speed had lower InSb crystal. leakage current. This result is consistent with the FT- IT result. The crystal grown with lower speed has lower defect density. undoped-crystal detector, and Fig. 6 was obtained with The leakage currents of the detectors, which were made the n-type crystal detector. The I-V curve shows the with the commercial wafers, were measured in the tem- diode characteristics; the leakage current was lower at perature range from 10 to 150 K. I-V curves can be seen one bias polarity region, and higher at the other bias po- in Fig. 5 and Fig. 6. Figure 5 was obtained with the larity region. Also, the leakage current decreased with -1580- Journal of the Korean Physical Society, Vol. 58, No. 6, June 2011

Because high-purity crystals can be grown with the Bridgman method, the Bridgman method was selected as a growth method. An InSb detector was also made with an InSb wafer obtained from a vendor. We found that the growth speed was an important parameter to obtain a crystal for high resolution detector. We also found that the wafer with n-type doping showed a lower leakage current. Therefore, a doping process is necessary to obtain a detector with a low leakage current. The energy spectrum of α particles could be measured with the InSb detector.

Fig. 7. α energy spectra measured with an InSb detector at 25 and 60 K. ACKNOWLEDGMENTS decreasing detector temperature. From a comparison be- This work has been carried out under the nuclear R&D tween Fig. 5 and Fig. 6, the Schottky detector made with program of the Ministry of Education, Science and Tech- the n-type wafer showed a lower leakage current. nology (MEST) of Korea. This work was supported by The energy spectrum was obtained with the InSb de- a Korea Research Foundation Grant funded by the Ko- tector (n-type wafer). The α particles (energy: 5.4 MeV) rean Government (MOEHRD, Basic Research Promo- from 241Am were measured. The signal from the de- tion Fund) (KRF-2008-313-D01255). tector was processed through a preamplifier (Canberra Model 2003BT), and the signal from the preamplifier was shaped and amplified with a shaping amplifier (ORTEC REFERENCES 575A). The resistor (100 MΩ) in the preamplifier was replaced with a 2-MΩ resistor. The spectrum is shown in Fig. 7. One can see the bump in the spectrum, and [1] I. Kanno, S. Hishiki, O. Sugiura, R. Xiang, T. Nakamura this low resolution caused by the low resistivity of the and M. Katagiri, Nucl. Instrum. Methods Phys. Res., crystal. Sect. A 568, 416 (2006). [2] E. Silver, M. LeGros, N. Madden, J. Beeman and E. Haller, X-ray Spectrom. 25, 115 (1996). [3] W. C. McHarris, Nucl. Instrum. Methods Phys. Res., III. CONCLUSION Sect. A 242, 373 (1986). [4] I. Kanno, F. Yoshihara, R. Nouchi, O. Sugiura, T. Naka- mura and M. Katagira, Rev. Sci. Instrum. 73, 2533 InSb may be a possible candidate semiconductor for an (2002). next generation radiation detector. Most previous works [5] J. A. Godines, R. Castillo, J. Mart´ınez, M. E. Navarro, concerning the InSb crystal were concerned with infrared F. De Anda, A. Canales, J. G´uzmanand D. Rios-Jara, J. applications, and a studies to grow InSb crystals for ra- Cryst. Growth 178, 422 (1997). diation detector applications are rare. In the present [6] P. Mohan, N. Senguttuvan, S. M. Babu, P. Santhanaragh- work, an InSb crystal was grown to obtain a method to van and P. Ramasamy, J. Cryst. Growth 200, 96 (1999). grow high-resistivity crystals for radiation detector ap- [7] S. H. Park, T. Y. Song, H. S. Kim, J. H. Ha and Y. K. plications. Kim, J. Korean Phys. Soc. 53, 1854 (2008).