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Surface states and passivation of p-Cd0.9Zn0.1Te crystal
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471 www.elsevier.com/locate/nima
Surface states and passivation of p-Cd0.9Zn0.1Te crystal Li Qiang, Jie Wanqi College of Materials Science and Engineering, Northwestern Polytechnical University, Xian 710072, China Received 16 December 2005; received in revised form 26 December 2005; accepted 1 February 2006 Available online 2 March 2006
Abstract Angle-resolved photoemission spectroscopy is used to characterize the surface states of the clean p-CdZnTe surface. A 0.8 eV surface band with the peak at 0.9 eV below the Fermi level is identified. The surface electron density is about 6.9 1014 electrons/cm2. By comparing the X-ray photoelectron spectroscopy spectrum of etched and passivated surfaces of p-CdZnTe, it is found that passivation with NH4F/H2O2 introduced TeO2 oxide film with the thickness about 3.1 nm on p-CdZnTe surface. Meanwhile, Photoluminescence spectra confirmed that passivation treatment minimized the surface trap states density and decreased the deep level impurity defects related to Cd vacancies. r 2006 Elsevier B.V. All rights reserved. PACS: 71.55.Gs; 73.20.r; 78.55.m Keywords: CdZnTe crystal; Passivation; ARPS; XPS; PL
1. Introduction The CdZnTe has shown great promise as a material for large-volume room-temperature X-ray and g-ray spectrometers because of its wider band gap and high absorption coefficients [1–3]. However, the performance of CdZnTe Xray and g-ray spectrometers is often limited by its surface leakage currents. Thus, reducing the surface leakage currents and the noise effects is very important for the fabrication of CdZnTe detectors. Meanwhile, defects at the metal–semiconductor interfaces increase the recombination rates, which reduce the charge carrier collection of the detectors [4]. They also cause the bending of the applied electric field, and thus, seriously affect the charge-collection efficiency, which can be improved through surface passivation. Surface passivation technologies include deposition of dielectric materials (ZnS, SiO2), preparation of the native films (oxides, sulphides, fluorides), and in situ growth of heterostuctures of wide band gap II–VI compound [5]. Wright et al. [6] evaluated the effectiveness of NH4F/H2O2 as a surface passivation agent for CdZnTe crystals, and found that NH4F/H2O2 surface passivation significantly Corresponding author. Tel.: +86 29 88486065; fax: +86 29 88495414.
E-mail address: [email protected] (L. Qiang). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.006
decreased the leakage current and improved the sensitivity and energy resolution of CdZnTe detectors. In our recent researches, the surface states of the clean CdZnTe surface were studied using angle-resolved photoemission spectroscopy (ARPS). ARPS, X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) analyses were used to study the effects of the surface passivation of p-CdZnTe with NH4F/H2O2. 2. Experimental methods High-purity raw materials Cd, Zn, Te (all 7 N) were used. A boule of Cd1xZnxTe (x ¼ 0:1) grown by Bridgman method and the growth parameters were as below: the temperature gradient was 15 k/cm, the lower rate of crucible was 1 mm/h. The grown crystals were cut into wafers of 10 10 2.5 mm3 with the surface orientation of (1 1 1). These wafers were mechanically polished using diamond paste and MgO powders of 0.5 mm in diameter. The samples were placed in a vacuum chamber, where Ar+ was used to clean CdZnTe surface. The vacuum of the chamber was 1.7 105 Torr. The ionization voltages for etching were 1.5, 1.2, 1.0, 0.8 and 0.6 kV respectively and the etching time was 20 min to remove absorbed O and
ARTICLE IN PRESS L. Qiang, J. Wanqi / Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471
3. Results and discussions
according to the energy of the exciting photons, is from bulk-state direct (k-conserving) transitions. Peak B at 0.9 eV below the Fermi energy is corresponding to the initial-state energy and is not change with the energy of exciting photons. The 0.8 eV surface band with the peak 0.6 to 1.4 eV below the Fermi level was found. Similar experiments validated that this surface band structure was caused by effects of the surface states [7]. Conducting two-dimensional surface electron according to first-order approximation, we calculated that the surface electron density is about 6.9 1014 electrons/cm2 [8] in Eq. (1). The results agree well with that of Wanger’s observation on single crystal silicon [9]. Ns ¼ 4 ð2:94 1022 Þ ð5:5=10:8Þ 0:096 ð1:2 109 Þ 6:9 1014 electrons=cm2 .
ð1Þ
0
Te 3d5/2
0
Te 3d3/2
Intensity / arb.uni.
enriched Te on the surface. Then, the ideally clean CdZnTe surface was obtained. The samples were chemically etched with 5%Br–MeOH, rinsed with methanol and dried with N2. For the passivation, the prepared samples were etched in 10 wt% NH4F–10 wt% H2O2–H2O solution for 15 min and rinsed with de-ionized water. The surface states of the ideally clean CdZnTe surface was analyzed with ARPS VGX900 in an ultra highvacuum chamber (1 1010 Torr). The effect of passivation with NH4F/H2O2 agent on p-CdZnTe surface was studied by XPS. The experiments were carried out at the National Synchrotron Radiation Laboratory in Hefei, China. XPS surface analysis used radiations generated by a magnesium anode with MgKa of 1253.6 eV. This was confirmed by the variation of XPS spectra Cd, Te, O and Zn contents after the passivation. In the PL measurements, the samples were attached on a cold copper finger in a closed-cycle cryostat with grease to keep the temperature at 10 K. An argon ion laser with the wavelength of 488 nm was used to excite the PL spectrum. A Triax 550 tri-grating monochromator with a photomultiplier tube (PMT) possessing the spectral resolution better than 0.3 nm was employed to collect and to analyze the signals emitted from the samples. The wavelength of argon laser was 488 nm and intensity was 20 mW.
469
3.1. Surface states measurements In ARPS analysis, the photon energy was changed from 19 to 33 eV, and kept the incidence angle at 151. The energy distribution curves from the clean CdZnTe surface are shown in Fig. 1. Peak A, with the variable state energy
595
590
(a)
585 580 Binding Energy /eV
40 A
35
570
4+
surfaces states
Te 3d5/2
B
*
4+
30 33eV
25 0.8 eV
20
*
15 10
*
*
Te 3d3/2
Intensity / arb.uni.
Intensity /arb.uni.
575
25eV
0
0
Te 3d3/2
Te 3d5/2
23eV
5
21eV
0 -5 -8
19eV
-7
-6
-5 -4 -3 -2 Binding Energy /eV
-1
0
595
1
Fig. 1. Electron energy distribution curves recorded at normal emission for various photonenergies from CdZnTe surface. The angle of incidence is a ¼ 151.
(b)
590
585 580 Binding Energy /eV
575
570
Fig. 2. (a) XPS spectra of CdZnTe wafer surfaces before NH4F/H2O2 passivation and (b) XPS spectra of CdZnTe wafer surfaces after NH4F/ H2O2 passivation.
ARTICLE IN PRESS 470
L. Qiang, J. Wanqi / Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471
Table 1 The surface composition of CdZnTe wafers before and after passivation Surface treatment states
Before passivation After passivation
Concentration (at%)
[Cd]/[Te]
Cd3d5
Te3d5
O1s
Zn2p3
41.60 24.99
50.35 26.97
4.86 45.26
3.19 2.78
3.2. XPS spectra
0.826 0.927
d ¼ l ln½ðTe4þ 3d 5=2 =Te0 3d 5=2 Þ þ 1,
(2) 4þ
0
where l is the non-elastic free radius, Te 3d 5=2 =Te 3d 5=2 is the ratio of Te atom in the form of TeO2 to Te atom in Te substance. For the MgKa radiation source of 1253.6 eV, l ¼ 200 nm, the thickness of oxide after passivation was determined to be 3.1 nm. Meanwhile, the I–V curves of before and after passivation were measured by Agilent 4155c and the total resistivity (surface +bulk) of crystals calculated. The results were about 3.3 108 O cm for non-passivated crystal and about 1 109 O cm for passivated crystal in Fig. 3. Passivation increased the total resistivity of crystals and decreased the leakage current of crystals. 3.3. Photoluminescence (PL) spectrum PL spectroscopy is a sensitive technique for characterizing the type and distribution of defects and impurities in crystals. The PL spectra at a low temperature of 10 K for the etched and passivated surface of CdZnTe are shown in Fig. 4. The near band edge emission includes donor-bound exciton (D0, X) and acceptor-bound exciton (A0, X). The dominant peak is the neutral donor-bound exciton (D0, X), in contrast to CdTe and Cd0.96Zn0.4Te, where (A0, X) is
Before passivaton
1.0 × 10-7
After passivaton
8.0 × 10-8 6.0 × 10-8 4.0 × 10-8 2.0 × 10-8 0.0 0
2
4 6 Bias Voltage / V
8
10
Fig. 3. The I–V curves: before and after passivation of CdZnTe crystals.
0
etched CdZnTe
(D , X)
passivated CdZnTe
PL Intensity /arb. uni.
Fig. 2 shows the XPS spectra on the surface of CdZnTe wafer before and after passivation with NH4F/H2O2 agent. After the passivation, the peaks Te4þ 3d 3=2 and Te4þ 3d 5=2 are higher, while Te0 3d 3=2 and Te0 3d 5=2 are lower. The binding energies of two higher peaks are 586.2 and 576.1 eV, respectively. The lower peaks with binding energies of 582.8 and 572.7 eV in Fig. 2(b) have the same position as that of main peaks in Fig. 2(a), which are supposed to be from Te element or CdZnTe. Table 1 shows the results of surface composition analysis of CdZnTe wafers before and after passivation. After passivation, the majority of Te was oxided by NH4F+H2O2 and the [Cd]/ [Te] was 0.927 by XPS measurement. ([Cd]+[Zn])/[Te] ratio approximates to 1, which means the stoichiometry is restored and the crystallinity is improved near the surface region. Fig. 2(b) indicates that the only oxide on p-CdZnTe surface after passivation is TeO2. Chen et al. [10] calculated the oxide layer thickness produced through heat oxidation on the CdTe surface under overpressure O2. The results fit the following relation:
Leakage Current /A
1.2 × 10-7
1.44
(shallow-level DAP)
(deep-level DAP)
0
(A , X)
(1 LO) (2 LO)
1.47
1.50
1.53 1.56 1.59 Energy /eV
1.62
1.65
1.68
Fig. 4. The typical PL spectra of etched and passivated CdZnTe at 10 K.
typically dominant [11]. The acceptor bound exciton (A0, X) located at the shoulder of the lower energy side has the peak at 1.643 eV, which is coincident to the one reported by Rzepka et al. [12] and Oettinger et al. [13]. The emission at 1.610 eV is the zero-phonon line (ZPL) of the shallow donor–acceptor pair (DAPs) with the longitudinal optical phonon replica. The energy difference between ZPL and 1 LO, 1 LO and 2 LO has the interval of about 22.0 mV. This agrees with the longitudinal optical (LO) phonon energy of 21.3 mV in CdTe [14,15].
ARTICLE IN PRESS L. Qiang, J. Wanqi / Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471
471
Table 2 PL results for CdZnTe surface treated by etching and passivation Surface treatment states
(D0, X)
(A0, X)
DAP
DAP
IðDAPÞ IðD0 ;XÞ
IðDAPÞ IðD0 ;XÞ
FWHMðD0 ;XÞ
Etched Passivated
(eV) 1.656 1.657
(eV) 1.643 1.640
(eV) 1.610 1.610
(eV) 1.510 1.510
(arb.) 0.545 0.632
(arb.) 0.595 0.348
(meV) 9.2 7.5
The deep-level donor–acceptor pair (DAPd) possesses the peak energy of 1.510 eV. The intensity ratio of the DAPd emission to the donor-bound exciton (D0, X), IðDAPd Þ=IðD0 ; XÞ can be related to the concentration of acceptor Cd vacancies [15,16]. The intensity ratio is smaller for the passivated CdZnTe surface than that of etched surface. The full-width at half-maximum (FWHM) of the (D0, X) peak for passivated p-CdZnTe is narrower than that of etched surface. The DAPd emission peak minimized after passivation. It is concluded that passivation minimized the surface trap state density and decreased the deeplevel impurities related to the recombination of Cd vacancies and the vacancy complexes known as A-centers (VCdDCd, D ¼ impurities element). The major peak locations and intensity ratios of PL spectra at 10 K are shown in Table 2.
4. Conclusions
(1) The clean p-CdZnTe surface possesses a 0.8 eV surface band with the peak at 0.9 eV below the Fermi level, where the surface electron density is about 6.9 1014 electrons/cm2. (2) By using surface passivation agent, NH4F/H2O2, the native oxide TeO2 film with the thickness about 3.1 nm on p-CdZnTe surface was obtained. Meanwhile, [Cd]/ [Te] ratio approximates to 1 after passivation. (3) Donor-bound exciton (D0, X), DAPs and DAPd are major peaks in typical PL spectra of CdZnTe at 10 K. FWHM of the (D0, X) peak for passivated CdZnTe is narrower than that of etched CdZnTe. This indicates that passivation minimizes the surface trap state density and decreases the deep-level impurities recombination with Cd vacancies.
Acknowledgements The financial support of the Key Project of National Natural Science Foundation of China under Grant no. 50336040 is acknowledged. We are also grateful to Zhang Wenhua and Xu Faqiang for their kindly help in experimental researches in National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei. References [1] Y. Eisen, A. Shor, J. Crystal Growth 184 (1998) 1302. [2] A. Burger, H. Chen, K. Chattopadhyay, J.O. Ndap, S.U. Egatievwe, R.B. James, SPIE 3446 (1998) 154. [3] Y. Nemirovsky, A. Ruzin, G. Asa, Y. Gorelik, L. Li, J. Electron. Mater. 26 (6) (1997) 756. [4] A. Burger, H. Chen, J. Tong, 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. NS-44 (3) (1997) 934. [5] Y.-H. Kim, S.-H. Kim, I.-J. Kim, S.Y. An, K.H. Kim, Curr. Appl. Phy. 5 (2005) 392. [6] G.W. Wright, R.B. James, D. Chinn, B.A. Brunett, R.W. Olsen, J. Van Scyoc, M. Clift, A. Burger, K. Chattopadhyay, D. Shi, R. Wingfield, SPIE 4141 (2000) 324. [7] H. Qu, P.O. Nilsson, J. Kanski, L. Ilver, Phys. Rev. B 39 (1989) 5276. [8] L.F. Eastman, W.D. Grobman, Phys. Rev. Lett. 28 (1972) 1378. [9] L.F. Wagner, W.E. Spicer, Phys. Rev. Lett. 28 (1972) 1381. [10] K.T. Chen, D.T. Shi, H. Chen, B. Granderson, M.A. George, W.E. Collins, R.B. James, J. Vac. Sci. Technol. A 15 (3) (1997) 850. [11] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, R.B. James, Mater. Sci. Eng. Rep. 32 (2001) 103. [12] E. Rzepka, A. Lusson, A. Riviere, A. Aoudia, Y. Marfaing, R. Triboulet, J. Cryst. Growth 161 (1996) 286. [13] K. Oettinger, D.M. Hofmann, Al.L. Efros, B.K. Meyer, M. Salk, K.W. Benz, J. Appl. Phys. 71 (1992) 4523. [14] J. Lee, N.C. Giles, D. Rajavel, C.J. Summers, J. Appl. Phys. 78 (1995) 5669. [15] Y.-H. Kim, S.-Y. An, J.-Y. Lee, I.J. Kim, K.-N. Oh, S.-U. Kim, M.-J. Park, T.-S. Lee, J. Appl. Phys. 85 (1999) 7370. [16] B. Hu, A. Yin, G. Karczewski, H. Luo, S.W. Short, N. Samarth, M. Dobrowolska, J.K. Furdyna, J. Appl. Phys. 74 (1993) 4153.
Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471 www.elsevier.com/locate/nima
Surface states and passivation of p-Cd0.9Zn0.1Te crystal Li Qiang, Jie Wanqi College of Materials Science and Engineering, Northwestern Polytechnical University, Xian 710072, China Received 16 December 2005; received in revised form 26 December 2005; accepted 1 February 2006 Available online 2 March 2006
Abstract Angle-resolved photoemission spectroscopy is used to characterize the surface states of the clean p-CdZnTe surface. A 0.8 eV surface band with the peak at 0.9 eV below the Fermi level is identified. The surface electron density is about 6.9 1014 electrons/cm2. By comparing the X-ray photoelectron spectroscopy spectrum of etched and passivated surfaces of p-CdZnTe, it is found that passivation with NH4F/H2O2 introduced TeO2 oxide film with the thickness about 3.1 nm on p-CdZnTe surface. Meanwhile, Photoluminescence spectra confirmed that passivation treatment minimized the surface trap states density and decreased the deep level impurity defects related to Cd vacancies. r 2006 Elsevier B.V. All rights reserved. PACS: 71.55.Gs; 73.20.r; 78.55.m Keywords: CdZnTe crystal; Passivation; ARPS; XPS; PL
1. Introduction The CdZnTe has shown great promise as a material for large-volume room-temperature X-ray and g-ray spectrometers because of its wider band gap and high absorption coefficients [1–3]. However, the performance of CdZnTe Xray and g-ray spectrometers is often limited by its surface leakage currents. Thus, reducing the surface leakage currents and the noise effects is very important for the fabrication of CdZnTe detectors. Meanwhile, defects at the metal–semiconductor interfaces increase the recombination rates, which reduce the charge carrier collection of the detectors [4]. They also cause the bending of the applied electric field, and thus, seriously affect the charge-collection efficiency, which can be improved through surface passivation. Surface passivation technologies include deposition of dielectric materials (ZnS, SiO2), preparation of the native films (oxides, sulphides, fluorides), and in situ growth of heterostuctures of wide band gap II–VI compound [5]. Wright et al. [6] evaluated the effectiveness of NH4F/H2O2 as a surface passivation agent for CdZnTe crystals, and found that NH4F/H2O2 surface passivation significantly Corresponding author. Tel.: +86 29 88486065; fax: +86 29 88495414.
E-mail address: [email protected] (L. Qiang). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.006
decreased the leakage current and improved the sensitivity and energy resolution of CdZnTe detectors. In our recent researches, the surface states of the clean CdZnTe surface were studied using angle-resolved photoemission spectroscopy (ARPS). ARPS, X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) analyses were used to study the effects of the surface passivation of p-CdZnTe with NH4F/H2O2. 2. Experimental methods High-purity raw materials Cd, Zn, Te (all 7 N) were used. A boule of Cd1xZnxTe (x ¼ 0:1) grown by Bridgman method and the growth parameters were as below: the temperature gradient was 15 k/cm, the lower rate of crucible was 1 mm/h. The grown crystals were cut into wafers of 10 10 2.5 mm3 with the surface orientation of (1 1 1). These wafers were mechanically polished using diamond paste and MgO powders of 0.5 mm in diameter. The samples were placed in a vacuum chamber, where Ar+ was used to clean CdZnTe surface. The vacuum of the chamber was 1.7 105 Torr. The ionization voltages for etching were 1.5, 1.2, 1.0, 0.8 and 0.6 kV respectively and the etching time was 20 min to remove absorbed O and
ARTICLE IN PRESS L. Qiang, J. Wanqi / Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471
3. Results and discussions
according to the energy of the exciting photons, is from bulk-state direct (k-conserving) transitions. Peak B at 0.9 eV below the Fermi energy is corresponding to the initial-state energy and is not change with the energy of exciting photons. The 0.8 eV surface band with the peak 0.6 to 1.4 eV below the Fermi level was found. Similar experiments validated that this surface band structure was caused by effects of the surface states [7]. Conducting two-dimensional surface electron according to first-order approximation, we calculated that the surface electron density is about 6.9 1014 electrons/cm2 [8] in Eq. (1). The results agree well with that of Wanger’s observation on single crystal silicon [9]. Ns ¼ 4 ð2:94 1022 Þ ð5:5=10:8Þ 0:096 ð1:2 109 Þ 6:9 1014 electrons=cm2 .
ð1Þ
0
Te 3d5/2
0
Te 3d3/2
Intensity / arb.uni.
enriched Te on the surface. Then, the ideally clean CdZnTe surface was obtained. The samples were chemically etched with 5%Br–MeOH, rinsed with methanol and dried with N2. For the passivation, the prepared samples were etched in 10 wt% NH4F–10 wt% H2O2–H2O solution for 15 min and rinsed with de-ionized water. The surface states of the ideally clean CdZnTe surface was analyzed with ARPS VGX900 in an ultra highvacuum chamber (1 1010 Torr). The effect of passivation with NH4F/H2O2 agent on p-CdZnTe surface was studied by XPS. The experiments were carried out at the National Synchrotron Radiation Laboratory in Hefei, China. XPS surface analysis used radiations generated by a magnesium anode with MgKa of 1253.6 eV. This was confirmed by the variation of XPS spectra Cd, Te, O and Zn contents after the passivation. In the PL measurements, the samples were attached on a cold copper finger in a closed-cycle cryostat with grease to keep the temperature at 10 K. An argon ion laser with the wavelength of 488 nm was used to excite the PL spectrum. A Triax 550 tri-grating monochromator with a photomultiplier tube (PMT) possessing the spectral resolution better than 0.3 nm was employed to collect and to analyze the signals emitted from the samples. The wavelength of argon laser was 488 nm and intensity was 20 mW.
469
3.1. Surface states measurements In ARPS analysis, the photon energy was changed from 19 to 33 eV, and kept the incidence angle at 151. The energy distribution curves from the clean CdZnTe surface are shown in Fig. 1. Peak A, with the variable state energy
595
590
(a)
585 580 Binding Energy /eV
40 A
35
570
4+
surfaces states
Te 3d5/2
B
*
4+
30 33eV
25 0.8 eV
20
*
15 10
*
*
Te 3d3/2
Intensity / arb.uni.
Intensity /arb.uni.
575
25eV
0
0
Te 3d3/2
Te 3d5/2
23eV
5
21eV
0 -5 -8
19eV
-7
-6
-5 -4 -3 -2 Binding Energy /eV
-1
0
595
1
Fig. 1. Electron energy distribution curves recorded at normal emission for various photonenergies from CdZnTe surface. The angle of incidence is a ¼ 151.
(b)
590
585 580 Binding Energy /eV
575
570
Fig. 2. (a) XPS spectra of CdZnTe wafer surfaces before NH4F/H2O2 passivation and (b) XPS spectra of CdZnTe wafer surfaces after NH4F/ H2O2 passivation.
ARTICLE IN PRESS 470
L. Qiang, J. Wanqi / Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471
Table 1 The surface composition of CdZnTe wafers before and after passivation Surface treatment states
Before passivation After passivation
Concentration (at%)
[Cd]/[Te]
Cd3d5
Te3d5
O1s
Zn2p3
41.60 24.99
50.35 26.97
4.86 45.26
3.19 2.78
3.2. XPS spectra
0.826 0.927
d ¼ l ln½ðTe4þ 3d 5=2 =Te0 3d 5=2 Þ þ 1,
(2) 4þ
0
where l is the non-elastic free radius, Te 3d 5=2 =Te 3d 5=2 is the ratio of Te atom in the form of TeO2 to Te atom in Te substance. For the MgKa radiation source of 1253.6 eV, l ¼ 200 nm, the thickness of oxide after passivation was determined to be 3.1 nm. Meanwhile, the I–V curves of before and after passivation were measured by Agilent 4155c and the total resistivity (surface +bulk) of crystals calculated. The results were about 3.3 108 O cm for non-passivated crystal and about 1 109 O cm for passivated crystal in Fig. 3. Passivation increased the total resistivity of crystals and decreased the leakage current of crystals. 3.3. Photoluminescence (PL) spectrum PL spectroscopy is a sensitive technique for characterizing the type and distribution of defects and impurities in crystals. The PL spectra at a low temperature of 10 K for the etched and passivated surface of CdZnTe are shown in Fig. 4. The near band edge emission includes donor-bound exciton (D0, X) and acceptor-bound exciton (A0, X). The dominant peak is the neutral donor-bound exciton (D0, X), in contrast to CdTe and Cd0.96Zn0.4Te, where (A0, X) is
Before passivaton
1.0 × 10-7
After passivaton
8.0 × 10-8 6.0 × 10-8 4.0 × 10-8 2.0 × 10-8 0.0 0
2
4 6 Bias Voltage / V
8
10
Fig. 3. The I–V curves: before and after passivation of CdZnTe crystals.
0
etched CdZnTe
(D , X)
passivated CdZnTe
PL Intensity /arb. uni.
Fig. 2 shows the XPS spectra on the surface of CdZnTe wafer before and after passivation with NH4F/H2O2 agent. After the passivation, the peaks Te4þ 3d 3=2 and Te4þ 3d 5=2 are higher, while Te0 3d 3=2 and Te0 3d 5=2 are lower. The binding energies of two higher peaks are 586.2 and 576.1 eV, respectively. The lower peaks with binding energies of 582.8 and 572.7 eV in Fig. 2(b) have the same position as that of main peaks in Fig. 2(a), which are supposed to be from Te element or CdZnTe. Table 1 shows the results of surface composition analysis of CdZnTe wafers before and after passivation. After passivation, the majority of Te was oxided by NH4F+H2O2 and the [Cd]/ [Te] was 0.927 by XPS measurement. ([Cd]+[Zn])/[Te] ratio approximates to 1, which means the stoichiometry is restored and the crystallinity is improved near the surface region. Fig. 2(b) indicates that the only oxide on p-CdZnTe surface after passivation is TeO2. Chen et al. [10] calculated the oxide layer thickness produced through heat oxidation on the CdTe surface under overpressure O2. The results fit the following relation:
Leakage Current /A
1.2 × 10-7
1.44
(shallow-level DAP)
(deep-level DAP)
0
(A , X)
(1 LO) (2 LO)
1.47
1.50
1.53 1.56 1.59 Energy /eV
1.62
1.65
1.68
Fig. 4. The typical PL spectra of etched and passivated CdZnTe at 10 K.
typically dominant [11]. The acceptor bound exciton (A0, X) located at the shoulder of the lower energy side has the peak at 1.643 eV, which is coincident to the one reported by Rzepka et al. [12] and Oettinger et al. [13]. The emission at 1.610 eV is the zero-phonon line (ZPL) of the shallow donor–acceptor pair (DAPs) with the longitudinal optical phonon replica. The energy difference between ZPL and 1 LO, 1 LO and 2 LO has the interval of about 22.0 mV. This agrees with the longitudinal optical (LO) phonon energy of 21.3 mV in CdTe [14,15].
ARTICLE IN PRESS L. Qiang, J. Wanqi / Nuclear Instruments and Methods in Physics Research A 562 (2006) 468–471
471
Table 2 PL results for CdZnTe surface treated by etching and passivation Surface treatment states
(D0, X)
(A0, X)
DAP
DAP
IðDAPÞ IðD0 ;XÞ
IðDAPÞ IðD0 ;XÞ
FWHMðD0 ;XÞ
Etched Passivated
(eV) 1.656 1.657
(eV) 1.643 1.640
(eV) 1.610 1.610
(eV) 1.510 1.510
(arb.) 0.545 0.632
(arb.) 0.595 0.348
(meV) 9.2 7.5
The deep-level donor–acceptor pair (DAPd) possesses the peak energy of 1.510 eV. The intensity ratio of the DAPd emission to the donor-bound exciton (D0, X), IðDAPd Þ=IðD0 ; XÞ can be related to the concentration of acceptor Cd vacancies [15,16]. The intensity ratio is smaller for the passivated CdZnTe surface than that of etched surface. The full-width at half-maximum (FWHM) of the (D0, X) peak for passivated p-CdZnTe is narrower than that of etched surface. The DAPd emission peak minimized after passivation. It is concluded that passivation minimized the surface trap state density and decreased the deeplevel impurities related to the recombination of Cd vacancies and the vacancy complexes known as A-centers (VCdDCd, D ¼ impurities element). The major peak locations and intensity ratios of PL spectra at 10 K are shown in Table 2.
4. Conclusions
(1) The clean p-CdZnTe surface possesses a 0.8 eV surface band with the peak at 0.9 eV below the Fermi level, where the surface electron density is about 6.9 1014 electrons/cm2. (2) By using surface passivation agent, NH4F/H2O2, the native oxide TeO2 film with the thickness about 3.1 nm on p-CdZnTe surface was obtained. Meanwhile, [Cd]/ [Te] ratio approximates to 1 after passivation. (3) Donor-bound exciton (D0, X), DAPs and DAPd are major peaks in typical PL spectra of CdZnTe at 10 K. FWHM of the (D0, X) peak for passivated CdZnTe is narrower than that of etched CdZnTe. This indicates that passivation minimizes the surface trap state density and decreases the deep-level impurities recombination with Cd vacancies.
Acknowledgements The financial support of the Key Project of National Natural Science Foundation of China under Grant no. 50336040 is acknowledged. We are also grateful to Zhang Wenhua and Xu Faqiang for their kindly help in experimental researches in National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei. References [1] Y. Eisen, A. Shor, J. Crystal Growth 184 (1998) 1302. [2] A. Burger, H. Chen, K. Chattopadhyay, J.O. Ndap, S.U. Egatievwe, R.B. James, SPIE 3446 (1998) 154. [3] Y. Nemirovsky, A. Ruzin, G. Asa, Y. Gorelik, L. Li, J. Electron. Mater. 26 (6) (1997) 756. [4] A. Burger, H. Chen, J. Tong, 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. NS-44 (3) (1997) 934. [5] Y.-H. Kim, S.-H. Kim, I.-J. Kim, S.Y. An, K.H. Kim, Curr. Appl. Phy. 5 (2005) 392. [6] G.W. Wright, R.B. James, D. Chinn, B.A. Brunett, R.W. Olsen, J. Van Scyoc, M. Clift, A. Burger, K. Chattopadhyay, D. Shi, R. Wingfield, SPIE 4141 (2000) 324. [7] H. Qu, P.O. Nilsson, J. Kanski, L. Ilver, Phys. Rev. B 39 (1989) 5276. [8] L.F. Eastman, W.D. Grobman, Phys. Rev. Lett. 28 (1972) 1378. [9] L.F. Wagner, W.E. Spicer, Phys. Rev. Lett. 28 (1972) 1381. [10] K.T. Chen, D.T. Shi, H. Chen, B. Granderson, M.A. George, W.E. Collins, R.B. James, J. Vac. Sci. Technol. A 15 (3) (1997) 850. [11] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, R.B. James, Mater. Sci. Eng. Rep. 32 (2001) 103. [12] E. Rzepka, A. Lusson, A. Riviere, A. Aoudia, Y. Marfaing, R. Triboulet, J. Cryst. Growth 161 (1996) 286. [13] K. Oettinger, D.M. Hofmann, Al.L. Efros, B.K. Meyer, M. Salk, K.W. Benz, J. Appl. Phys. 71 (1992) 4523. [14] J. Lee, N.C. Giles, D. Rajavel, C.J. Summers, J. Appl. Phys. 78 (1995) 5669. [15] Y.-H. Kim, S.-Y. An, J.-Y. Lee, I.J. Kim, K.-N. Oh, S.-U. Kim, M.-J. Park, T.-S. Lee, J. Appl. Phys. 85 (1999) 7370. [16] B. Hu, A. Yin, G. Karczewski, H. Luo, S.W. Short, N. Samarth, M. Dobrowolska, J.K. Furdyna, J. Appl. Phys. 74 (1993) 4153.