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Investigation on the surface treatments of CdMnTe single crystals
Materials Science in Semiconductor Processing 31 (2015) 536–542
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Investigation on the surface treatments of CdMnTe single crystals Min Shen, Jijun Zhang n, Linjun Wang, Jiahua Min, Lin Wang, Xiaoyan Liang, Jian Huang, Ke Tang, Wei Liang, Hua Meng School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China
a r t i c l e in f o
Keywords: CdMnTe Surface treatments Roughness Specific contact resistivity
abstract The surface quality of CdMnTe crystal plays an important role in the performance of CdMnTe nuclear radiation detectors. In this paper, the surfaces of CdMnTe samples treated by Mechanical Polishing (MP), Chemical Polishing (CP), Chemical–Mechanical Polishing (CMP), and CMP followed by CP treatment (CMPþ CP) were investigated. The structural properties of CdMnTe surfaces were characterized by a High Resolution Scanning Electron Microscope (HRSEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). The Circular Transmission Line Model (CTLM) was adopted to reveal the specific contact resistivity (ρc ) of Au/CdMnTe contacts after various surface treatments. The AFM measurements shown an ultra-smooth surface was achieved after the CMPþCP treatment with the roughness value of 0.84 nm, and the surfaces by MP, CP and CMP were with the roughness of 8.2 nm, 5.1 nm and 1.4 nm, respectively. The XPS analysis indicated that the (TeþTe4 þ )/(Cdþ Mn) ratios of the CdMnTe samples after the CMPþ CP and MP treatment were 1.1 and 1.07 which are close to stoichiometric composition, while after the CP and CMP treatment were 2.00 and 1.40 which are enriched with Te. The CMPþ CP treatment decreased the Te4 þ /(Teþ Te4 þ ) ratio from 7% in CMP to 1%, which reveals the surface with few Te oxides. The specific contact resistivity (ρc ) was used to determine the ohmic characteristic of Au/CdMnTe contacts by the CTLM. The calculated ρc values of Au/CdMnTe contacts on the CP, CMP and CMPþ CP treated surfaces are 383, 112 and 15 Ω cm2, respectively. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, Cd1 xMnxTe (CdMnTe) is demonstrated to be a good candidate to compete with Cd1 xZnxTe (CdZnTe) in room-temperature X-ray and γ-ray nuclear detector applications. The potential use of CdMnTe for room-temperature radiation detector was first reported by Burger et al. in 1999 [1]. Then large improvement has been achieved in the crystal growth of CdMnTe single crystal for detector applications [2–4]. The performance of CdMnTe detector depends not only on the quality of CdMnTe wafer material, but also on the
n
Corresponding author. Tel.: þ 86 21 66136126; fax: þ 86 21 66138023. E-mail address: [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.mssp.2014.12.051 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
surface state of CdMnTe and metal/CdMnTe contact properties [5]. The surface states of CdMnTe wafers can significantly contribute to leakage currents, which in turn affect a detector's signal-to-noise ratio and energy resolution [6–8]. To achieve high-performance CdMnTe-based detectors, the defect-free CdMnTe surfaces with stoichiometric ratio should be realized. Usually, the CdMnTe wafers were mechanically polished (MP) to achieve smooth surfaces, and then the wafers were chemically polished (CP) in bromine methanol (BM) solution of varying concentrations, which further decrease the roughness and microstructural damage of the wafer surfaces [9– 11]. However, the CP treatment leaves a Te-rich surface and oxide of Te, which influence the contact properties to CdMnTe wafers. With regard to the preparation of high-performance
M. Shen et al. / Materials Science in Semiconductor Processing 31 (2015) 536–542
nuclear detectors, the chemical–mechanical polishing (CMP) was used, especially in CdZnTe wafer treatment. Zhang et al. [12] had studied CMP technology on CdZnTe wafers using a CMP method, and the polished surface of CdZnTe wafers was very smooth with the roughness of 1.68 nm, free of scratches and imbedding. For the progress on the fabrication of CdMnTe radiation detector, a few works have been performed on the surface treatment of CdMnTe wafers and the influence of surface state on the electrical contact to CdMnTe. In this work, the surface conditions of CdMnTe wafers were investigated after MP, CP, CMP, and CMP followed by CP treatments, and the contact resistivity of Au contact on CdMnTe wafers after various surface treatments was calculated to reveal the ohmic characteristic of Au/CdMnTe contact. The surface morphologies were characterized by atomic force microscopy (AFM) and the High Resolution Scanning Electron Microscope (HRSEM). The X-ray photoelectron spectroscopy (XPS) was used to study the surface stoichiometry and contamination. The circular transmission line model (CTLM) was used to determine the ohmic contact and get the specific contact resistivity (ρc ) of Au/ CdMnTe contact [13,14]. 2. Experimental procedure The Cd1 xMnxTe (x ¼0.1) crystal used in this study was grown by the vertical Bridgman method. Raw materials of Cd (7N), Mn (5N) and Te (7N) were stoichiometrically weighed for the growth of CdMnTe ingot. The CdMnTe samples with dimension of 10 10 2 mm3 were sliced along the (111) plane from the ingot. The CdMnTe samples were prepared by four kinds of surface treatment methods, as the MP, CP, CMP and CMP followed by the CP method (CMP þCP). The MP was done on a mechanical polisher (UNIPOL-802, Shenyang Kejing Auto-instrument Co., Ltd.) in two steps with 1-mm and 0.05-mm particle size alumina suspensions which were formed by mixing the ionized water and alumina powders with a weight ratio of 10:1. The polishing pressure and polishing velocity were 120 g/cm2 and 0.3 μm/min, respectively. The polishing plate is a non-woven fabric pad, and the polishing duration was four hours. The CP was finished by immerging the CdMnTe samples in 2% bromine methanol (BM) solution for 2 min. The CMP was carried out using the MP process except the polishing solution was made of 8% BM solution and alumina powder (0.05 mm size) suspension with a volume ratio of 20:1. The CMP followed by CP surface treatment included the CMP step as described above and then the CP etching treatment with the etching solution of 2% BM for 20 s and 5% hydrochloric acid (HCl) for 15 s. The CdMnTe wafers after each surface treatment were rinsed with methanol before the measurements. The surface roughness and morphology were characterized by an atomic force microscope (AFM, SHIMAZU SPM-9600). The morphology of surface and defect was obtained by the High Resolution Scanning Electron Microscope (HRSEM, JA JSM-6700F). The chemical composition of polished surfaces was investigated by XPS measurements (Thermo Scientific ESCALAB 250Xi). The XPS experiment was performed in ultrahigh-vacuum conditions at a pressure better than 5 10 10 mbar. Along with ports for
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the energy analyzer and X-ray sources, the chamber is equipped with ports for Auger electron spectroscopy, UV lamp, ion guns, etc. This allows the XPS system to be used as a true multi-technique facility. The circular transmission line model (CTLM) was adopted to reveal the contact resistivity of Au/CdMnTe after various surface treatments. The contact graphics on each CdMnTe wafer were created through the use of photomask and photolithography. The Au contacts on CdMnTe wafers were deposited with the electroless AuCl3 technique [15]. Finally, the I/V test was applied on the special electrode with Keithley 4200-SCS/F. 3. Results and discussions 3.1. HRSEM measurement of CdMnTe wafers The HRSEM revealed the surface morphology of the CdMnTe samples treated by MP, CP, CMP and CMPþCP. Fig. 1(a) shows that there was a high density of abrasive scratches and defects on the surface treated by the MP. After the MP, the CdMnTe samples were treated by CP and CMP. Fig. 1(b) shows the CdMnTe surface after CP, and it is found that the 2% Br–MeOH solution etched away the scratches left by the MP. However, the CP treatment left a high density of etching pits and bumps randomly on the surface. It is considered that the etching pits are caused by the removal of Te inclusion and the bumps are the oxide of Te. In Fig. 1(c), the CMP treatment removed the etching pits and bumps on the surface, which accords with material removal mechanism of CMP. During the CMP treatment, the abrasive particles in the CMP solution help to maintain a flat surface and the chemical component of the CMP solution reduces the surface roughness. However, a few scratches and defects remained on the surface. After the CMP, another CP process including dilute BM and HCl solutions etching was adopted, which further improved the surface morphology. As seen in Fig. 1(d), after the CMPþCP treatment, the CdMnTe surfaces did not show any scratches and defects even at 22,000 magnification times under the HRSEM microscope. 3.2. AFM measurement of CdMnTe wafers AFM images of the CdMnTe samples after the four surface treatment methods are shown in Fig. 2. The average value of the surface roughness was given by Ra, the average deviation of surface height within a given area is as follows: P ðZ i Z ave Þ ð1Þ Ra ¼ N where Zi is the topographic height value at point i, Zave is the average value of height within the area, and N is the number of points measured within the area. Roughness values Ra have been determined from an imaged area of 10 10 μm2 which was representative of the topography of the samples. Table 1 summarizes the roughness Ra of the CdMnTe wafer surfaces after different surface treatments. For the MP treatment, as shown in Fig. 2(a), scratches resulting from the alumina abrasive were clearly seen, and the Ra value was found to be 8.2 nm. The CP etching process in 2% BM solution etched away most scratches, but the Ra value
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M. Shen et al. / Materials Science in Semiconductor Processing 31 (2015) 536–542
Fig. 1. HRSEM images of the treated CdMnTe surfaces: (a) MP, (b) CP, (c) CMP, (d) CMP þCP.
of the CP treated sample was still about 5.1 nm, which attributed to the formation of a large density of small particulates in surface, as shown in Fig. 2(b). In Fig. 2(c), the CMP treated away the residual particles with a few light scratches and the Ra value was 1.4 nm. Fig. 2(d) shows the CMPþ CP treated surface free of scratches and particles, and the Ra value minimized to 0.84 nm. From the AFM view, the CMPþCP surface treatment resulted the lowest roughness among the four surface treatment methods, and an ultra-smooth CdMnTe surface was obtained.
3.3. XPS measurement of CdMnTe wafers XPS is a nondestructive technique to study the chemical composition and electronic properties of the surface of CdMnTe samples. In the XPS spectra, chemical state information in the surface region was assessed by determining the exact positions of the constituent peaks. Fig. 3 shows the XPS spectra of Te3d peaks of the CdMnTe samples after different surface treatments. The positions of the Te3d5/2 and Te3d3/2 peaks corresponding to the Te elemental state were found to be 583.4 eV and 573.0 eV, while the positions of the TeO3d3/2 and TeO3d5/2 corresponding to the Te oxide
state were found to be 586.9 eV and 576.5 eV, respectively. Some results were calculated from the XPS images. Table 2 summarizes the results of XPS evaluation of the CdMnTe samples by different surface treatments. The atomic percentage of (TeþTe4 þ )/(Cdþ Mn) from the MP, CP, CMP and CMPþCP surface was 1.07, 2.00, 1.40 and 1.10, respectively. The XPS analysis indicated that the CdMnTe samples after the CMPþCP and MP treatment were close to stoichiometric composition, while the surfaces after the CP and CMP treatment were enriched with Te. Compared to the CP treated surface, the CMP treatment decreased the (TeþTe4 þ )/(Cdþ Zn) ratio from 2.0 to 1.40. In the CMP process, the removal rates of chemical reaction and mechanical polishing get to the equilibrant when the physical polishing process remove the enriched Te produced during chemical etching, thus the surface left less Te as compared to the CP treatment. The CMPþCP process further reduced the (TeþTe4 þ )/(CdþMn) ratio to 1.10, which was due to the reaction between etching solution and enriched Te on the surface. The Te4 þ /(TeþTe4 þ ) ratios of samples treated with MP, CP, CMP and CMPþCP were 12%, 25%, 7% and 1%, respectively. It is found that, during the CMP þCP process, the final BM and HCl solution etching was effective at removing the thin Te oxide layers
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Fig. 2. AFM images of the CdMnTe samples by the four surface treatments: (a) MP, (b) CP, (c) CMP, (d) CMPþ CP.
Table 1 The roughness of CdMnTe samples after different treatments. Treatments
MP
CP
CMP
CMP þ CP
Ra
8.2 nm
5.1 nm
1.4 nm
0.84 nm
and the Te4 þ /(TeþTe4 þ ) ratio decreased to only 1%. As shown in Fig. 3(d), the satellite peaks of Te3d5/2 and Te3d3/2 almost disappeared. The XPS spectra also showed the Cd3d peaks of the CdMnTe samples after various surface treatments, as shown in Fig. 4. There were no peaks corresponding to the Cd oxide state, which indicated that the Cd oxides were not formed. Therefore, a stoichiometric CdMnTe surface was achieved after the CMPþCP treatment, which is suitable for metal electrode deposition. 3.4. The specific contact resistivity of Au/CdMnTe Ohmic contacts between metal and CdMnTe crystal are extremely important for both the electrical properties and performances of CdMnTe detector. Low contact resistance shares less voltage and is advantageous in collecting carriers with low barrier height, which are in favor of improving the energy resolution of CdMnTe detector [16,17]. The circular transmission line model (CTLM) was used to get the specific contact resistivity (ρc ), which can quantify a metal– semiconductor ohmic contact [18–20]. The CTLM structure on the wafers after various surface treatments was prepared by the photolithography with the photomask shown in Fig. 5. The CTLM method was described by the following
two equations [18,19]: RT ¼ Rin þ Rp þRout ¼ ρc ¼ LT 2 Rsh
Rsh ðd þ 2LT Þ 2πr 0
ð2Þ ð3Þ
where RT is the total resistance consisting the central dot contact resistance, Rin is the resistance of the semiconductor layer surrounding the central dot electrode (Rp ) and outer electrode contact resistance (Rout ), Rsh is the sheet resistance of CdMnTe and LT is the transfer length in linear transmission line model. Fig. 6 shows the current–voltage (I–V) profile of CdMnTe samples with various surface treatments. The linearity of the I–V profile from different surfaces indicated that they were typical ohmic contact, which means that the CTLM method is applicable. Fig. 7 reveals the variation of RT with increasing spacing gap d for the CdMnTe samples. The resistances RT extracted from the I–V curves were used to draw the resistance–spacing gap curves. According to Eq. (2), with the linear-fitting of the RT –d profile, the Rsh and LT were obtained from the slope and intercept, respectively. Then, the specific contact resistivity ρc was calculated by Eq. (3). Table 3 summarizes the specific contact resistivity ρc of the Au/CdMnTe contact with the surfaces treated by different methods. For the MP treatment, it is hard to get a linear RT –d profile curve so the Rsh and LT could not be extracted. Thus, the ρc of the Au/CdMnTe with MP treatment could not be calculated. This is due to the nonuniform surface and the high leakage current caused by
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Fig. 3. XPS images of Te3d5/2 from the surfaces treated by: (a) MP, (b) CP, (c) CMP, (d) CMP þ CP. Table 2 The ratios of (Teþ Te4 þ )/(Cdþ Mn) and Te4 þ /(Te þTe4 þ ) are listed, and the values are concluded from the XPS data. Surface treatment
(Teþ Te4 þ )/(Cdþ Mn) atomic ratio
Te4 þ /(Teþ Te4 þ ) (%)
MP CP CMP CMP þCP
1.07 2.0 1.4 1.1
12 25 7 1
the defects as shown in Fig. 1. The ρc value of the Au/ CdMnTe contact with the CP treated surface was 383 Ω cm2, since there was a lot of heterogeneous Te oxide on the CP surfaces by the XPS which contributed to the increase of the contact resistivity. The CMP treatment reduced the Te enrichment and the roughness value of the CdMnTe sample, and the ρc value of the Au/CdMnTe contact decreased to 112 Ω cm2. After the CMPþCP treatment, a near stoichiometric surface with the (TeþTe4 þ )/(CdþMn) ratio of 1.1 and an ultra-smooth surface with the roughness value of 0.84 nm was achieved. Therefore, the ρc value of the Au/ CdMnTe contact with the CMPþ CP surface was 15 Ω cm2.
Fig. 4. The typical XPS image of Cd3d of CdMnTe wafers by various surface treatment methods.
Obviously, the CMPþCP treatment is beneficial for the low specific contact resistivity of Au/CdMnTe contact, and an ideal ohmic Au/CdMnTe contact was achieved.
M. Shen et al. / Materials Science in Semiconductor Processing 31 (2015) 536–542
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4. Conclusions
Fig. 5. The photomask of CTLM structure on the CdMnTe wafers: the inner radius of the ring is r 0 (r 0 ¼150 μm) and the outer radius of the ring is r n (r 1 r 9 ¼155 μm, 160 μm, 165 μm, 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm). The varying spacing gap between inner and outer electrode is d ¼ r n r0 . The metal electrodes are assumed to be equipotentials in the model.
The surfaces of CdMnTe wafers grown by the vertical Bridgman method were treated by MP, CP, CMP and CMPþCP methods. The surface roughness Ra of the CdMnTe sample treated by the CMPþCP method was 0.84 nm. Based on the CMP treatment which attains the roughness Ra of 1.4 nm, the final CP treatment consisting of 2% BM and HCl solution etching effectively eliminated the scratches after CMP and created an ultra-smooth surface. The XPS measurements showed that the (TeþTe4 þ )/(CdþMn) ratios of the CdMnTe samples after the MP, CP, CMP and CMPþCP treatment are 1.07, 2.0, 1.4 and 1.1, respectively. It is shown that the surfaces of CdMnTe after the CMPþCP and MP are close to stoichiometric composition, while after CP and CMP treatment are enriched with Te and Te oxides. The circular transmission line model (CTLM) was used to get the specific contact resistivity (ρc ) of Au/CdMnTe contacts. The calculated ρc values of Au/CdMnTe contacts on the CP, CMP and CMPþCP treated surfaces are 383, 112 and 15 Ω cm2, respectively. The CMPþ CP treatment of CdMnTe samples results the lowest ρc of Au/CdMnTe contacts among the four surface treatments, which has advantage to improve the performance of CdMnTe detector.
Acknowledgments This work was supported by the National Natural Science Foundations of China (Nos. 51472155, 11375112 and 11275122), and Innovative Foundation of Shanghai University. Fig. 6. The I–V curves of the CdMnTe samples with various surface treatments at the same distance (30 μm).
Fig. 7. The total resistance RT between the adjacent electrodes varied with the length of the gap for CdMnTe samples.
Table 3 The specific contact resistivity (ρc ) on the four kinds of surfaces. Treatment method 2
ρc (Ω cm )
MP
CP
CMP
CMP þ CP
–
383
112
15
References [1] A. Burger, K. Chattopadhyay, H. Chen, D. Shi, S.H. Morgan, W.E. Collins, J. Cryst. Growth 872 (1) (1999) 198–199. [2] J.J. Zhang, W.Q. Jie, T. Wang, D.M. Zeng, S.Y. Ma, H. Hua, Mater. Res. Bull. 43 (5) (2008) 1239–1245. [3] K. Kim, G. Jeng, P. Kim, J. Choi, A.E. Bolotnikov, G.S. Camarda, R.B. James, J. Appl. Phys. 114 (2013) 063706. [4] R. Rafiei, M.I. Reinhard, K. Kim, D.A. Prokopovich, D. Boardman, A. Sarbutt, G.C. Watt, A.E. Bolotnikov, L.J. Bignell, R.B. James, IEEE Trans. Nucl. Sci. 60 (2) (2013) 1450–1456. [5] A. Burger, H. Chen, J. Tong, D. Shi, M.A. George, K.T. Chen, W.E. Collins, R.B. James, C.M. Stahle, L.M. Bartlett, IEEE Trans. Nucl. Sci. 44 (6) (1997) 934–938. [6] Q. Li, W.Q. Jie, L. Fu, X.Q. Wang, X.G. Zhang, Appl. Surf. Sci. 253 (3) (2006) 1190–1193. [7] E.J. Morton, M.A. Hosain, P. De Antonis, Nucl. Instrum. Method A 458 (5) (2001) 558–562. [8] X.Q. Wang, W.Q. Jie, Q. Li, Z. Gu, Mat. Sci. Semicond. Process. 8 (6) (2005) 615–621. [9] S. Tari, F. Aqariden, Y. Chang, C. Grein, J. Li, N. Kioussis, J. Electron. Mater. 42 (11) (2013) 3252–3258. [10] S. Babar, P.J. Sellin, J.F. Watts, M.A. Baker, Appl. Surf. Sci. 264 (2013) 681–686. [11] H. Bensalaha, J.L. Plaza, J. Crocco, Appl. Surf. Sci. 257 (10) (2011) 4633–4636. [12] Z.Y. Zhang, Y.W. Meng, D.M. Guo, R.K. Kang, H. Gao, Int. J. Adv. Manuf. Technol. 47 (9–12) (2010) 1105–1112. [13] L.P. Galchinetskyi, V.M. Koshkin, V.M. Kumakov, Sov. Solid State Phys. 14 (1972) 646–648. [14] G.K. Reeves, H.B. Harrison, IEEE Electron Device Lett. 3 (5) (1982) 111–113. [15] Anand Singha, A.K. Shuklab, Sumit Jaina, Brijesh S. Yadava, R. Pala, Mat. Sci. Semicond. Process. 26 (2014) 294–300.
542
M. Shen et al. / Materials Science in Semiconductor Processing 31 (2015) 536–542
[16] M. Witkowska-Baran, A. Mycielski, D. Kochanowska, A.J. Szadkowski, R. Jakieła, B. Witkowska, W. Kaliszek, J. Domagała, E. Lusakowska, V. Domukhovski, K. Dybko, Y. Cui, R.B. James, IEEE Trans. Nucl. Sci. 58 (1) (2011) 347–353. [17] A. Bobby, N. Shiwakoti, S. Verma, P.S. Gupta, B.K. Antony, Mat. Sci. Semicond. Process. 21 (2014) 116–121.
[18] G.S. Marlow, D.B. Mukunda, Solid-State Electron. 25 (2) (1982) 91–94. [19] Y. Pan, G.K. Reeves, P.W. Leech, A.S. Holland, IEEE Trans. Electron Devices 60 (3) (2013) 1202–1207. [20] H. Krishna Deepak, J. Electron. Mater. 36 (5) (2007) 598–605.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Investigation on the surface treatments of CdMnTe single crystals Min Shen, Jijun Zhang n, Linjun Wang, Jiahua Min, Lin Wang, Xiaoyan Liang, Jian Huang, Ke Tang, Wei Liang, Hua Meng School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China
a r t i c l e in f o
Keywords: CdMnTe Surface treatments Roughness Specific contact resistivity
abstract The surface quality of CdMnTe crystal plays an important role in the performance of CdMnTe nuclear radiation detectors. In this paper, the surfaces of CdMnTe samples treated by Mechanical Polishing (MP), Chemical Polishing (CP), Chemical–Mechanical Polishing (CMP), and CMP followed by CP treatment (CMPþ CP) were investigated. The structural properties of CdMnTe surfaces were characterized by a High Resolution Scanning Electron Microscope (HRSEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). The Circular Transmission Line Model (CTLM) was adopted to reveal the specific contact resistivity (ρc ) of Au/CdMnTe contacts after various surface treatments. The AFM measurements shown an ultra-smooth surface was achieved after the CMPþCP treatment with the roughness value of 0.84 nm, and the surfaces by MP, CP and CMP were with the roughness of 8.2 nm, 5.1 nm and 1.4 nm, respectively. The XPS analysis indicated that the (TeþTe4 þ )/(Cdþ Mn) ratios of the CdMnTe samples after the CMPþ CP and MP treatment were 1.1 and 1.07 which are close to stoichiometric composition, while after the CP and CMP treatment were 2.00 and 1.40 which are enriched with Te. The CMPþ CP treatment decreased the Te4 þ /(Teþ Te4 þ ) ratio from 7% in CMP to 1%, which reveals the surface with few Te oxides. The specific contact resistivity (ρc ) was used to determine the ohmic characteristic of Au/CdMnTe contacts by the CTLM. The calculated ρc values of Au/CdMnTe contacts on the CP, CMP and CMPþ CP treated surfaces are 383, 112 and 15 Ω cm2, respectively. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, Cd1 xMnxTe (CdMnTe) is demonstrated to be a good candidate to compete with Cd1 xZnxTe (CdZnTe) in room-temperature X-ray and γ-ray nuclear detector applications. The potential use of CdMnTe for room-temperature radiation detector was first reported by Burger et al. in 1999 [1]. Then large improvement has been achieved in the crystal growth of CdMnTe single crystal for detector applications [2–4]. The performance of CdMnTe detector depends not only on the quality of CdMnTe wafer material, but also on the
n
Corresponding author. Tel.: þ 86 21 66136126; fax: þ 86 21 66138023. E-mail address: [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.mssp.2014.12.051 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
surface state of CdMnTe and metal/CdMnTe contact properties [5]. The surface states of CdMnTe wafers can significantly contribute to leakage currents, which in turn affect a detector's signal-to-noise ratio and energy resolution [6–8]. To achieve high-performance CdMnTe-based detectors, the defect-free CdMnTe surfaces with stoichiometric ratio should be realized. Usually, the CdMnTe wafers were mechanically polished (MP) to achieve smooth surfaces, and then the wafers were chemically polished (CP) in bromine methanol (BM) solution of varying concentrations, which further decrease the roughness and microstructural damage of the wafer surfaces [9– 11]. However, the CP treatment leaves a Te-rich surface and oxide of Te, which influence the contact properties to CdMnTe wafers. With regard to the preparation of high-performance
M. Shen et al. / Materials Science in Semiconductor Processing 31 (2015) 536–542
nuclear detectors, the chemical–mechanical polishing (CMP) was used, especially in CdZnTe wafer treatment. Zhang et al. [12] had studied CMP technology on CdZnTe wafers using a CMP method, and the polished surface of CdZnTe wafers was very smooth with the roughness of 1.68 nm, free of scratches and imbedding. For the progress on the fabrication of CdMnTe radiation detector, a few works have been performed on the surface treatment of CdMnTe wafers and the influence of surface state on the electrical contact to CdMnTe. In this work, the surface conditions of CdMnTe wafers were investigated after MP, CP, CMP, and CMP followed by CP treatments, and the contact resistivity of Au contact on CdMnTe wafers after various surface treatments was calculated to reveal the ohmic characteristic of Au/CdMnTe contact. The surface morphologies were characterized by atomic force microscopy (AFM) and the High Resolution Scanning Electron Microscope (HRSEM). The X-ray photoelectron spectroscopy (XPS) was used to study the surface stoichiometry and contamination. The circular transmission line model (CTLM) was used to determine the ohmic contact and get the specific contact resistivity (ρc ) of Au/ CdMnTe contact [13,14]. 2. Experimental procedure The Cd1 xMnxTe (x ¼0.1) crystal used in this study was grown by the vertical Bridgman method. Raw materials of Cd (7N), Mn (5N) and Te (7N) were stoichiometrically weighed for the growth of CdMnTe ingot. The CdMnTe samples with dimension of 10 10 2 mm3 were sliced along the (111) plane from the ingot. The CdMnTe samples were prepared by four kinds of surface treatment methods, as the MP, CP, CMP and CMP followed by the CP method (CMP þCP). The MP was done on a mechanical polisher (UNIPOL-802, Shenyang Kejing Auto-instrument Co., Ltd.) in two steps with 1-mm and 0.05-mm particle size alumina suspensions which were formed by mixing the ionized water and alumina powders with a weight ratio of 10:1. The polishing pressure and polishing velocity were 120 g/cm2 and 0.3 μm/min, respectively. The polishing plate is a non-woven fabric pad, and the polishing duration was four hours. The CP was finished by immerging the CdMnTe samples in 2% bromine methanol (BM) solution for 2 min. The CMP was carried out using the MP process except the polishing solution was made of 8% BM solution and alumina powder (0.05 mm size) suspension with a volume ratio of 20:1. The CMP followed by CP surface treatment included the CMP step as described above and then the CP etching treatment with the etching solution of 2% BM for 20 s and 5% hydrochloric acid (HCl) for 15 s. The CdMnTe wafers after each surface treatment were rinsed with methanol before the measurements. The surface roughness and morphology were characterized by an atomic force microscope (AFM, SHIMAZU SPM-9600). The morphology of surface and defect was obtained by the High Resolution Scanning Electron Microscope (HRSEM, JA JSM-6700F). The chemical composition of polished surfaces was investigated by XPS measurements (Thermo Scientific ESCALAB 250Xi). The XPS experiment was performed in ultrahigh-vacuum conditions at a pressure better than 5 10 10 mbar. Along with ports for
537
the energy analyzer and X-ray sources, the chamber is equipped with ports for Auger electron spectroscopy, UV lamp, ion guns, etc. This allows the XPS system to be used as a true multi-technique facility. The circular transmission line model (CTLM) was adopted to reveal the contact resistivity of Au/CdMnTe after various surface treatments. The contact graphics on each CdMnTe wafer were created through the use of photomask and photolithography. The Au contacts on CdMnTe wafers were deposited with the electroless AuCl3 technique [15]. Finally, the I/V test was applied on the special electrode with Keithley 4200-SCS/F. 3. Results and discussions 3.1. HRSEM measurement of CdMnTe wafers The HRSEM revealed the surface morphology of the CdMnTe samples treated by MP, CP, CMP and CMPþCP. Fig. 1(a) shows that there was a high density of abrasive scratches and defects on the surface treated by the MP. After the MP, the CdMnTe samples were treated by CP and CMP. Fig. 1(b) shows the CdMnTe surface after CP, and it is found that the 2% Br–MeOH solution etched away the scratches left by the MP. However, the CP treatment left a high density of etching pits and bumps randomly on the surface. It is considered that the etching pits are caused by the removal of Te inclusion and the bumps are the oxide of Te. In Fig. 1(c), the CMP treatment removed the etching pits and bumps on the surface, which accords with material removal mechanism of CMP. During the CMP treatment, the abrasive particles in the CMP solution help to maintain a flat surface and the chemical component of the CMP solution reduces the surface roughness. However, a few scratches and defects remained on the surface. After the CMP, another CP process including dilute BM and HCl solutions etching was adopted, which further improved the surface morphology. As seen in Fig. 1(d), after the CMPþCP treatment, the CdMnTe surfaces did not show any scratches and defects even at 22,000 magnification times under the HRSEM microscope. 3.2. AFM measurement of CdMnTe wafers AFM images of the CdMnTe samples after the four surface treatment methods are shown in Fig. 2. The average value of the surface roughness was given by Ra, the average deviation of surface height within a given area is as follows: P ðZ i Z ave Þ ð1Þ Ra ¼ N where Zi is the topographic height value at point i, Zave is the average value of height within the area, and N is the number of points measured within the area. Roughness values Ra have been determined from an imaged area of 10 10 μm2 which was representative of the topography of the samples. Table 1 summarizes the roughness Ra of the CdMnTe wafer surfaces after different surface treatments. For the MP treatment, as shown in Fig. 2(a), scratches resulting from the alumina abrasive were clearly seen, and the Ra value was found to be 8.2 nm. The CP etching process in 2% BM solution etched away most scratches, but the Ra value
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Fig. 1. HRSEM images of the treated CdMnTe surfaces: (a) MP, (b) CP, (c) CMP, (d) CMP þCP.
of the CP treated sample was still about 5.1 nm, which attributed to the formation of a large density of small particulates in surface, as shown in Fig. 2(b). In Fig. 2(c), the CMP treated away the residual particles with a few light scratches and the Ra value was 1.4 nm. Fig. 2(d) shows the CMPþ CP treated surface free of scratches and particles, and the Ra value minimized to 0.84 nm. From the AFM view, the CMPþCP surface treatment resulted the lowest roughness among the four surface treatment methods, and an ultra-smooth CdMnTe surface was obtained.
3.3. XPS measurement of CdMnTe wafers XPS is a nondestructive technique to study the chemical composition and electronic properties of the surface of CdMnTe samples. In the XPS spectra, chemical state information in the surface region was assessed by determining the exact positions of the constituent peaks. Fig. 3 shows the XPS spectra of Te3d peaks of the CdMnTe samples after different surface treatments. The positions of the Te3d5/2 and Te3d3/2 peaks corresponding to the Te elemental state were found to be 583.4 eV and 573.0 eV, while the positions of the TeO3d3/2 and TeO3d5/2 corresponding to the Te oxide
state were found to be 586.9 eV and 576.5 eV, respectively. Some results were calculated from the XPS images. Table 2 summarizes the results of XPS evaluation of the CdMnTe samples by different surface treatments. The atomic percentage of (TeþTe4 þ )/(Cdþ Mn) from the MP, CP, CMP and CMPþCP surface was 1.07, 2.00, 1.40 and 1.10, respectively. The XPS analysis indicated that the CdMnTe samples after the CMPþCP and MP treatment were close to stoichiometric composition, while the surfaces after the CP and CMP treatment were enriched with Te. Compared to the CP treated surface, the CMP treatment decreased the (TeþTe4 þ )/(Cdþ Zn) ratio from 2.0 to 1.40. In the CMP process, the removal rates of chemical reaction and mechanical polishing get to the equilibrant when the physical polishing process remove the enriched Te produced during chemical etching, thus the surface left less Te as compared to the CP treatment. The CMPþCP process further reduced the (TeþTe4 þ )/(CdþMn) ratio to 1.10, which was due to the reaction between etching solution and enriched Te on the surface. The Te4 þ /(TeþTe4 þ ) ratios of samples treated with MP, CP, CMP and CMPþCP were 12%, 25%, 7% and 1%, respectively. It is found that, during the CMP þCP process, the final BM and HCl solution etching was effective at removing the thin Te oxide layers
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Fig. 2. AFM images of the CdMnTe samples by the four surface treatments: (a) MP, (b) CP, (c) CMP, (d) CMPþ CP.
Table 1 The roughness of CdMnTe samples after different treatments. Treatments
MP
CP
CMP
CMP þ CP
Ra
8.2 nm
5.1 nm
1.4 nm
0.84 nm
and the Te4 þ /(TeþTe4 þ ) ratio decreased to only 1%. As shown in Fig. 3(d), the satellite peaks of Te3d5/2 and Te3d3/2 almost disappeared. The XPS spectra also showed the Cd3d peaks of the CdMnTe samples after various surface treatments, as shown in Fig. 4. There were no peaks corresponding to the Cd oxide state, which indicated that the Cd oxides were not formed. Therefore, a stoichiometric CdMnTe surface was achieved after the CMPþCP treatment, which is suitable for metal electrode deposition. 3.4. The specific contact resistivity of Au/CdMnTe Ohmic contacts between metal and CdMnTe crystal are extremely important for both the electrical properties and performances of CdMnTe detector. Low contact resistance shares less voltage and is advantageous in collecting carriers with low barrier height, which are in favor of improving the energy resolution of CdMnTe detector [16,17]. The circular transmission line model (CTLM) was used to get the specific contact resistivity (ρc ), which can quantify a metal– semiconductor ohmic contact [18–20]. The CTLM structure on the wafers after various surface treatments was prepared by the photolithography with the photomask shown in Fig. 5. The CTLM method was described by the following
two equations [18,19]: RT ¼ Rin þ Rp þRout ¼ ρc ¼ LT 2 Rsh
Rsh ðd þ 2LT Þ 2πr 0
ð2Þ ð3Þ
where RT is the total resistance consisting the central dot contact resistance, Rin is the resistance of the semiconductor layer surrounding the central dot electrode (Rp ) and outer electrode contact resistance (Rout ), Rsh is the sheet resistance of CdMnTe and LT is the transfer length in linear transmission line model. Fig. 6 shows the current–voltage (I–V) profile of CdMnTe samples with various surface treatments. The linearity of the I–V profile from different surfaces indicated that they were typical ohmic contact, which means that the CTLM method is applicable. Fig. 7 reveals the variation of RT with increasing spacing gap d for the CdMnTe samples. The resistances RT extracted from the I–V curves were used to draw the resistance–spacing gap curves. According to Eq. (2), with the linear-fitting of the RT –d profile, the Rsh and LT were obtained from the slope and intercept, respectively. Then, the specific contact resistivity ρc was calculated by Eq. (3). Table 3 summarizes the specific contact resistivity ρc of the Au/CdMnTe contact with the surfaces treated by different methods. For the MP treatment, it is hard to get a linear RT –d profile curve so the Rsh and LT could not be extracted. Thus, the ρc of the Au/CdMnTe with MP treatment could not be calculated. This is due to the nonuniform surface and the high leakage current caused by
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Fig. 3. XPS images of Te3d5/2 from the surfaces treated by: (a) MP, (b) CP, (c) CMP, (d) CMP þ CP. Table 2 The ratios of (Teþ Te4 þ )/(Cdþ Mn) and Te4 þ /(Te þTe4 þ ) are listed, and the values are concluded from the XPS data. Surface treatment
(Teþ Te4 þ )/(Cdþ Mn) atomic ratio
Te4 þ /(Teþ Te4 þ ) (%)
MP CP CMP CMP þCP
1.07 2.0 1.4 1.1
12 25 7 1
the defects as shown in Fig. 1. The ρc value of the Au/ CdMnTe contact with the CP treated surface was 383 Ω cm2, since there was a lot of heterogeneous Te oxide on the CP surfaces by the XPS which contributed to the increase of the contact resistivity. The CMP treatment reduced the Te enrichment and the roughness value of the CdMnTe sample, and the ρc value of the Au/CdMnTe contact decreased to 112 Ω cm2. After the CMPþCP treatment, a near stoichiometric surface with the (TeþTe4 þ )/(CdþMn) ratio of 1.1 and an ultra-smooth surface with the roughness value of 0.84 nm was achieved. Therefore, the ρc value of the Au/ CdMnTe contact with the CMPþ CP surface was 15 Ω cm2.
Fig. 4. The typical XPS image of Cd3d of CdMnTe wafers by various surface treatment methods.
Obviously, the CMPþCP treatment is beneficial for the low specific contact resistivity of Au/CdMnTe contact, and an ideal ohmic Au/CdMnTe contact was achieved.
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4. Conclusions
Fig. 5. The photomask of CTLM structure on the CdMnTe wafers: the inner radius of the ring is r 0 (r 0 ¼150 μm) and the outer radius of the ring is r n (r 1 r 9 ¼155 μm, 160 μm, 165 μm, 170 μm, 175 μm, 180 μm, 185 μm, 190 μm, 195 μm). The varying spacing gap between inner and outer electrode is d ¼ r n r0 . The metal electrodes are assumed to be equipotentials in the model.
The surfaces of CdMnTe wafers grown by the vertical Bridgman method were treated by MP, CP, CMP and CMPþCP methods. The surface roughness Ra of the CdMnTe sample treated by the CMPþCP method was 0.84 nm. Based on the CMP treatment which attains the roughness Ra of 1.4 nm, the final CP treatment consisting of 2% BM and HCl solution etching effectively eliminated the scratches after CMP and created an ultra-smooth surface. The XPS measurements showed that the (TeþTe4 þ )/(CdþMn) ratios of the CdMnTe samples after the MP, CP, CMP and CMPþCP treatment are 1.07, 2.0, 1.4 and 1.1, respectively. It is shown that the surfaces of CdMnTe after the CMPþCP and MP are close to stoichiometric composition, while after CP and CMP treatment are enriched with Te and Te oxides. The circular transmission line model (CTLM) was used to get the specific contact resistivity (ρc ) of Au/CdMnTe contacts. The calculated ρc values of Au/CdMnTe contacts on the CP, CMP and CMPþCP treated surfaces are 383, 112 and 15 Ω cm2, respectively. The CMPþ CP treatment of CdMnTe samples results the lowest ρc of Au/CdMnTe contacts among the four surface treatments, which has advantage to improve the performance of CdMnTe detector.
Acknowledgments This work was supported by the National Natural Science Foundations of China (Nos. 51472155, 11375112 and 11275122), and Innovative Foundation of Shanghai University. Fig. 6. The I–V curves of the CdMnTe samples with various surface treatments at the same distance (30 μm).
Fig. 7. The total resistance RT between the adjacent electrodes varied with the length of the gap for CdMnTe samples.
Table 3 The specific contact resistivity (ρc ) on the four kinds of surfaces. Treatment method 2
ρc (Ω cm )
MP
CP
CMP
CMP þ CP
–
383
112
15
References [1] A. Burger, K. Chattopadhyay, H. Chen, D. Shi, S.H. Morgan, W.E. Collins, J. Cryst. Growth 872 (1) (1999) 198–199. [2] J.J. Zhang, W.Q. Jie, T. Wang, D.M. Zeng, S.Y. Ma, H. Hua, Mater. Res. Bull. 43 (5) (2008) 1239–1245. [3] K. Kim, G. Jeng, P. Kim, J. Choi, A.E. Bolotnikov, G.S. Camarda, R.B. James, J. Appl. Phys. 114 (2013) 063706. [4] R. Rafiei, M.I. Reinhard, K. Kim, D.A. Prokopovich, D. Boardman, A. Sarbutt, G.C. Watt, A.E. Bolotnikov, L.J. Bignell, R.B. James, IEEE Trans. Nucl. Sci. 60 (2) (2013) 1450–1456. [5] A. Burger, H. Chen, J. Tong, D. Shi, M.A. George, K.T. Chen, W.E. Collins, R.B. James, C.M. Stahle, L.M. Bartlett, IEEE Trans. Nucl. Sci. 44 (6) (1997) 934–938. [6] Q. Li, W.Q. Jie, L. Fu, X.Q. Wang, X.G. Zhang, Appl. Surf. Sci. 253 (3) (2006) 1190–1193. [7] E.J. Morton, M.A. Hosain, P. De Antonis, Nucl. Instrum. Method A 458 (5) (2001) 558–562. [8] X.Q. Wang, W.Q. Jie, Q. Li, Z. Gu, Mat. Sci. Semicond. Process. 8 (6) (2005) 615–621. [9] S. Tari, F. Aqariden, Y. Chang, C. Grein, J. Li, N. Kioussis, J. Electron. Mater. 42 (11) (2013) 3252–3258. [10] S. Babar, P.J. Sellin, J.F. Watts, M.A. Baker, Appl. Surf. Sci. 264 (2013) 681–686. [11] H. Bensalaha, J.L. Plaza, J. Crocco, Appl. Surf. Sci. 257 (10) (2011) 4633–4636. [12] Z.Y. Zhang, Y.W. Meng, D.M. Guo, R.K. Kang, H. Gao, Int. J. Adv. Manuf. Technol. 47 (9–12) (2010) 1105–1112. [13] L.P. Galchinetskyi, V.M. Koshkin, V.M. Kumakov, Sov. Solid State Phys. 14 (1972) 646–648. [14] G.K. Reeves, H.B. Harrison, IEEE Electron Device Lett. 3 (5) (1982) 111–113. [15] Anand Singha, A.K. Shuklab, Sumit Jaina, Brijesh S. Yadava, R. Pala, Mat. Sci. Semicond. Process. 26 (2014) 294–300.
542
M. Shen et al. / Materials Science in Semiconductor Processing 31 (2015) 536–542
[16] M. Witkowska-Baran, A. Mycielski, D. Kochanowska, A.J. Szadkowski, R. Jakieła, B. Witkowska, W. Kaliszek, J. Domagała, E. Lusakowska, V. Domukhovski, K. Dybko, Y. Cui, R.B. James, IEEE Trans. Nucl. Sci. 58 (1) (2011) 347–353. [17] A. Bobby, N. Shiwakoti, S. Verma, P.S. Gupta, B.K. Antony, Mat. Sci. Semicond. Process. 21 (2014) 116–121.
[18] G.S. Marlow, D.B. Mukunda, Solid-State Electron. 25 (2) (1982) 91–94. [19] Y. Pan, G.K. Reeves, P.W. Leech, A.S. Holland, IEEE Trans. Electron Devices 60 (3) (2013) 1202–1207. [20] H. Krishna Deepak, J. Electron. Mater. 36 (5) (2007) 598–605.