Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method

G Model APSUSC-30010; No. of Pages 5

ARTICLE IN PRESS Applied Surface Science xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method Tamara Potlog a,∗ , Dumitru Duca a , Marius Dobromir b a b

Department of Physics and Engineering, Moldova State University, MD 2009 Chisinau, Republic of Moldova Faculty of Physics, Alexandru Ioan Cuza University, 11 Carol I Blvd., Iasi 700506, Romania

a r t i c l e

i n f o

Article history: Received 12 November 2014 Received in revised form 17 March 2015 Accepted 21 March 2015 Available online xxx Keywords: Close space sublimation method ZnTe Ag-doping XRD XPS

a b s t r a c t Zinc telluride (ZnTe) thin films were sublimated on a glass substrate using closed space sublimation (CSS) technique. The influence of the substrate temperature on the physical properties is studied. The deposited films were immersed in AgNO3 solution with different concentrations, and then annealed in air. The structure and composition are studied using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction patterns of as-deposited ZnTe thin films exhibited polycrystalline behavior. The preferred orientation of (1 1 1) having cubic phase irrespective of the substrate temperature was observed. The XPS analysis confirmed the presence of Ag in the ZnTe thin films after doping by immersion in the AgNO3 solution of different concentrations. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

1. Introduction Zinc telluride can absorb photons in the visible region without any phonon assisted mechanism that makes it useful in several electro-optic and optoelectronic applications. ZnTe is a direct band gap semiconductor having band gap 2.26 eV at 300 K, high absorption coefficient of 105 cm−1 and usually is a p-type semiconductor [1]. Recently, interest has increased considerably, because of their potential application for CdS/CdTe and multi-junction tandem solar cells [2–5]. Due to the high work function (∼5.9 eV) of CdTe and the difficulty in p-type CdTe doping to a high level, it is difficult to form low resistance ohmic contacts with metals [6,7]. Therefore in the case of the n-/CdS/p-CdTe superstrate cells a potential solution is the insertion of ZnTe film with work function (∼5.4–5.75 eV in function of the thickness) as an interlayer between CdTe and the back metal electrode. This low work function of ZnTe thin film helps to form ohmic contact in CdTe solar cells [4]. Also, ZnTe is known to be an excellent window heterojunction partner of n/CdTe [8] and a good absorber of n-/ZnSe [9]. The commonly used methods for depositing ZnTe films are molecular beam epitaxy (MBE), metalorganic vapor phase epitaxy (MOVPE), electron beam gun evaporation, rf sputtering, thermal evaporation, hot wall epitaxy, pulsed-laser deposition and electrodeposition [14–20]. Out of

∗ Corresponding author. Tel.: +373 22577827; fax: +373 2244248. E-mail address: [email protected] (T. Potlog).

these techniques, close space sublimation (CSS) is economic and so most promising for large scale use [10]. CSS is an attractive process since it offers high depositions rates, it is able to produce films with larger grains than those deposited by other techniques and can be easily scaled up for manufacturing purposes. In this paper, ZnTe thin films prepared by CSS at different substrates temperatures were investigated. Since the surface is primarily involved in the initial interaction, in this paper we also report on X-ray photoelectron spectroscopy investigation of the surface ZnTe thin films doped with Ag. 2. Material and method Close space sublimated thin film properties are sensitive to various deposition conditions such as substrate–source temperatures, the source–substrate separation and composition of gases in the deposition chamber [9]. The CSS growth system designed and developed by our group used in the present study consists of two heated graphite blocks. The 20-mm substrate–source distance of our evaporation system was employed to reduce the substrate temperature. The temperature and growth rate are maintained using BPT-3 temperature controllers that allow keeping a constant temperature with a precision of ±0.5 ◦ C. The pressure during deposition was below 5 × 10−6 Torr. The substrate temperature was varied in the interval (300–420) ◦ C and a source temperature was maintained at 570 ◦ C. The polycrystalline films were obtained in a short time (about 40 min) without an additional transport agent gas. For the

http://dx.doi.org/10.1016/j.apsusc.2015.03.133 0169-4332/Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: T. Potlog, et al., Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.133

G Model APSUSC-30010; No. of Pages 5

ARTICLE IN PRESS T. Potlog et al. / Applied Surface Science xxx (2015) xxx–xxx

2

Fig. 1. The SEM images of ZnTe thin films deposited at different substrate temperatures.

increase of the substrate temperatures the grain sizes of crystallites increase. The cross-sectional images indicate that the ZnTe layer grows in a columnar morphology. The grain size of the crystallites increases from about 772 nm (Ts = 320 ◦ C) to 1889 nm (Ts = 420 ◦ C). The X-ray diffraction (XRD) analysis was performed using Rigaku software PDXL. The X-ray diffraction studies indicated that the films are polycrystalline having the fcc zinc blende structure irrespective of the substrate temperature. Fig. 2 shows the XRD pattern of the films deposited at different substrate temperatures. The more intensive ZnTe phase is found at 2 = 25.24◦ and corresponds to the 216: F-43m space group. It is observed that the films exhibit highly preferred orientation along the (1 1 1) direction. The same diffraction maxima with lower intensity, corresponding to

Ts=300 C o Ts=340 C o Ts=380 C o Ts=420 C

(422)

(422) (422)

(331) (331) (331)

(400) (400)

(222)

(400)

(222)

(311)

(200)

(220)

(222)

4.0x10

(222)

3.1. Effect of substrate temperature on the morphology and structure of ZnTe thin films

(311)

8.0x10

(311)

6

1.2x10

(311)

(111)

6

1.6x10

(220)

6

2.0x10

(220)

(111)

6

2.4x10

(220)

6

2.8x10

(200)

6

3.2x10

(200)

6

3.6x10

3. Results and discussion

The as-deposited ZnTe thin films show uniform surface without cracks or pinholes and with good adhesion to the glass substrate. The SEM images illustrated in Fig. 1 show that the films are densely packed with no visible voids or pinholes. It can be seen that with the

(111)

o

Intensity, arb.un.

10

20

30

40

50

60

(331)

(422)

0.0

(400)

5

(200)

5

(111)

optimization of the technological regime a set of ZnTe thin films at different substrate temperatures was grown from the same source of evaporation. Another 2 sets of ZnTe layers prepared at 380 ◦ C were investigated: first – a thick one (10 ␮m) for the application as absorber in ZnTe/CdTe heterojunction and the second – much thinner (1 ␮m) for the application as window in ZnSe/ZnTe heterojunction. As a dopant we used a solution of AgNO3 of different mass concentration, while the time of doping remained constant. After the chemical doping the ZnTe films were annealed at 400 ◦ C in the air atmosphere for 1 h. For the first set the structure and surface morphology were recorded using a Rigaku X-ray diffractometer with CuK␣ radiation ˚ Ni filter, and a scanning electron microscope (SEM) ( = 1.54056 A), with energy dispersive X-ray (EDX), respectively. For the second set the elemental composition in the surface region was investigated by X-ray photoelectron spectroscopy (XPS). The spectra were measured using a Physical Electronics PHI 5000 Versa Probe instrument, equipped with a monochromated AlK␣ X-ray source (1486.6 eV). The photoelectrons were collected at a take-off angle of 45◦ . The surface quantification was done following the standard procedure [11] using the Zn, Te, O and Ag XPS spectra. Peak deconvolution has been done using the PHI-MULTIPAK software, and the elemental atomic concentration was calculated from the peak surface areas, taking into account the sensitivity factors of the analyzed elements [12].

70

80

2θ, degrees Fig. 2. The X-ray diffraction patterns of as-deposited ZnTe thin films at different substrates temperatures.

Please cite this article in press as: T. Potlog, et al., Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.133

G Model

ARTICLE IN PRESS

APSUSC-30010; No. of Pages 5

T. Potlog et al. / Applied Surface Science xxx (2015) xxx–xxx

a, A˚

0.07 0.04 – –

6.1052 6.1053 6.1042 6.1046

the following crystallographic planes (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0) and (4 2 2) confirms the formation of single phase ZnTe. The associated micro structural parameters for the ZnTe films deposited at different substrate temperatures are presented in Table 1. The grain size determined from XRD patterns is observed to increase from about 56 nm to 68 nm with substrate temperature. The reduction in the strain with increase of the substrate temperature may be due to the movement of interstitial Zn atoms from the bulk of the grain to its boundary region which dissipate to larger area leading to reduction in concentration of lattice imperfections and decreases in lattice parameter. We suppose the somewhat larger lattice parameter of the as-deposited ZnTe films in comparison with bulk value 6.1034 A˚ [13] might be attributed to grain coarsening. Similar trend has been observed earlier in ZnSe and CdTe thin films obtained by thermal evaporation [14]. 3.2. Effect of doping on the composition of the ZnTe thin films The X-ray photoelectron spectroscopy was used to estimate the composition of two sets of as-deposited and Ag-doped ZnTe thin films. The first set was for ZnTe films with thickness 10 ␮m. A second set was for the ZnTe films with thickness 1 ␮m. Both sets were prepared at 380 ◦ C substrate temperature and were doped with the same different concentration of the Ag. The survey XPS spectra for both sets in the whole binding energy region contain the following elements: Zn2p, Te3d, Ag3d and O1s. The C1s peak of the as-deposited and 20% Ag-doped ZnTe films for the first set is located at a binding energy value of 284.6 eV, while for the sample doped with 1% Ag shifts to 288.9 eV. For the second set the position of the C1s peak is located at 284.2 eV and 284.8 eV, respectively. The detected carbon is related to the carbon adsorbed on the surface during the exposure of the samples to the ambient atmosphere. All binding energies were corrected for the charge shift using the C1s peaks. Figs. 3 and 4 show the high-resolution Zn2p doublets for both sets of ZnTe films. The Zn spectra for both sets of as-deposited

1% AgNO3

2.08x10

4

1.82x10

5% AgNO3

4

1.04x10

4

Zn2p1/2 4

2.0x10

4

1.6x10

4

1050

1040 1030 1020 Binding energy,eV

1010

Fig. 4. XPS spectra of Zn2p region of as-deposited ZnTe (1 ␮m) film and Ag-doped with different concentrations air-annealed at 400 ◦ C.

ZnTe thin films consist of a characteristic Zn2p3/2 ground state situated at 1021.5 eV which correspond to Zn metal [15] and Zn2p1/2 excited state at 1045.3 eV [16]. The shift of the peak position is generally associated with the change in chemical environment. The position of the Zn doublets in Ag environment for both sets shifted toward higher binding energies. For example, the position of the Zn doublet in the alloy environment with 5% concentration of Ag for the first set shifted toward higher binding energies with 0.3 eV, while for the second set with 0.6 eV. In the as-deposited thicker layer Zn is bonded mostly to Te and O forming ZnTe [17] and ZnO [18]. The binding energy of Zn2p3/2 and Zn2p5/2 at 1022.5 eV and 1045.4 eV peaks in doped films confirmed the association of Zn with oxygen in the completely oxidized state. The surface compositions of all elements for the both sets are presented in Table 2. As one can see in thinner as-deposited ZnTe film the surface atomic concentration of Zn is greater comparing to the thicker ones. For the layers of greater thickness there is the same effect, the only difference is that the concentration of Zn–O is smaller than for the thinner layers. The O1s XPS peak consisted of two to three components (Fig. 7). The peak located at 530.2 eV corresponds to O–Zn binding originating from the ZnO lattice. Figs. 5 and 6 display the XPS spectra of Te3d region of as-deposited ZnTe films and Ag-doped air-annealed at 400 ◦ C ZnTe films with different concentrations for both sets. The four Gaussian peaks in the Te3d spectra of the binary system are due to different bonding states of Te atoms [19,20]. The peaks at binding energy of 572.6 eV and 582.9 eV correspond to Zn Te bond. Another two peaks at 576.9 eV and 585.7 correspond to oxide tellurium, composed of TeO2 and 4

4.32x10

as-deposited Zn2p1/2

1.30x10

as-deposited

Zn2p3/2

4

4

2.4x10

1.2x10 1060

20% AgNo3

1.56x10

20% AgNo3

4

XPS signals, counts

XPS signals, counts

2.34x10

4

5% AgNo3

Te3d3/2

563 591 643 676

Strain, %

Zn2p3/2

1% AgNo3

1% AgNO3 5% AgNO3 20% AgNO3

4

3.60x10

as-deposited 4

2.88x10

4

Te3d3/2

1–300 ◦ C 2–340 ◦ C 3–380 ◦ C 4–420 ◦ C

DXRD , A˚

2.8x10

Te3d5/2

Thickness of ZnTe layers, ␮m

4



Te3d5/2



XPS signals, counts

Table 1 Structural parameters of ZnTe thin films.

3

2.16x10

4

1.44x10

3

7.20x10

1060

1050

1040 1030 1020 Binding energy, eV

1010

Fig. 3. XPS spectra of Zn2p region of as-deposited ZnTe (10 ␮m) film and Ag-doped with different concentrations air-annealed at 400 ◦ C.

590

585

580 575 Binding energy,eV

570

565

Fig. 5. XPS spectra of Te3d region of as-deposited ZnTe (10 ␮m) film and Ag-doped with different concentrations air-annealed at 400 ◦ C.

Please cite this article in press as: T. Potlog, et al., Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.133

G Model

ARTICLE IN PRESS

APSUSC-30010; No. of Pages 5

T. Potlog et al. / Applied Surface Science xxx (2015) xxx–xxx

4

Table 2 The atomic concentration of all elements for the both sets of ZnTe thin films.

Atomic concentration (%), d = 1 ␮m 26.4 As-deposited 1% AgNO3 12.1 5% AgNO3 10.7 17 20% AgNO3 Atomic concentration (%), d = 10 ␮m 21.9 As-deposited 3.8 1% AgNO3 5% AgNO3 0.2 20% AgNO3 13.1

Te

O

Ag

Total

40.3 33.3 37.1 40.8

33.3 52.3 49.5 38.9

0 2.3 2.7 3.3

100 100 100 100

30.6 62.2 37.6 48.6

47.5 27.8 55 29.7

0 6.2 7.2 8.6

100 100 100 100

1% AgNO 4

1x10 XPS signals, counts

Zn

O1s

4

1x10

5% AgNO

3

9x10

3

8x10

20% AgNO as-deposited

3

7x10

3

6x10

3

5x10

3

4x10

3

XPS signals, counts

Te3d3/2

4

2.87x10

Te3d3/2

4

3.28x10

Te3d5/2

Te3d5/2

3x10 1% AgNO

Binding energy, eV

5% AgNO 20% AgNO

4

Fig. 8. XPS spectra of O1s region of as-deposited ZnTe (1 ␮m) film and Ag-doped with different concentrations air-annealed at 400 ◦ C.

as-deposited

2.46x10

4

2.05x10

4

1.64x10

4

1.23x10

3

8.20x10

3

4.10x10 595

590

585 580 575 Binding energy, eV

570

565

Fig. 6. XPS spectra of Te3d region of as-deposited ZnTe (1 ␮m) film and Ag-doped with different concentrations air-annealed at 400 ◦ C.

TeO3 [21]. The differences in binding energies suggest that the composition depend on the structural species responsible for various Te3d peaks [22,23]. A shift in the binding energy value of oxide related peaks of Ag-doped ZnTe thin films suggests a fact that Te has donated electrons to oxygen and therefore the binding energy corresponding to 3d5/2 and 3d3/2 transitions has been increased. With the increase of Ag concentration the amount of Te O bonds increases. For the ZnTe thicker layer annealed in the 20% AgNO3 solution it is evident that oxide tellurium signal is stronger than that in as-deposited ZnTe, but for the 5% concentration of Ag in the thinner the concentration of Te O bonds are minimal. The O1s region (Fig. 7) for the thicker film shows three peaks for as-deposited and two peaks with stronger signal and a small shifting toward higher binding energy for Ag-doped films.

O1s

The first component with the lower binding energy in asdeposited films is attributed to O ions in the ZnO lattice and the second and third ones with the higher binding energy are ascribed to TeO2 (575.6 ± 0.3 eV) eV and TeO3 (576.7 ± 0.3 eV). The concentration of the oxygen in the thicker film is smaller than in the thinner films, while for the Ag-doped films the situation is contrary. The peak which corresponds to the lowest binding energy in doped film with 20% Ag concentration is attributed to Te O Ag bond forming (Ag2 O)0.5(TeO2 )0.5 (572.9 eV) oxide [19]. All the studied thinner samples (Fig. 8) also, had variable concentrations of O therefore the possibility of the formation of auxiliary oxides exists. The peaks at 532.4–532.6 eV correspond to hydroxyl species, indicating the presence of hydrated oxides [19]. In the O1s XPS spectrum of 1% Ag concentration of ZnTe thin film appear two peaks with binding energies situated at 535.2 eV and 536.9 eV which we suppose that can be attributed to sodium Auger peak (Na KLL) originated from the glass substrates during annealing [24]. Figs. 9 and 10 show the high-resolution Ag3d doublets for both sets of ZnTe films with binding energies situated at 368.2 ± 0.2 eV and 374.4 ± 0.2 eV which were correspond to Ag3d3/2 and Ag3d5/2 transitions. According to Zhang et al. [25] these peaks can be attributed to metallic silver (Ag0 ). In the 5% Ag doped ZnTe thinner film appears the binding energy of oxidized Ag+ which is consistent with the results in the literature [25] and forming disilver(I) oxide tellurium(IV) [21]. It is observed that the line width of metallic silver is smaller than that of oxidized silver. Ag0 is expected to have several roles in these types of films. It can substitute for Zn forming

1% AgNO3

4

1.04x10

Ag3d5/2

5% AgNO3

4

1.44x10

as-deposited

3

9.10x10

3

7.80x10

3

6.50x10

3

5.20x10

XPS signals,counts

20% AgNO3

XPS signals, counts

540 538 536 534 532 530 528 526

1% AgNO

3

5% AgNO 4

1.20x10

3 20% AgNO Ag3d3/2 3

3

9.60x10

3

7.20x10

3

4.80x10 3

3.90x10

3

540

538

536

534

532

530

528

526

Binding energy, eV Fig. 7. XPS spectra of O1s region of as-deposited ZnTe (1 ␮m) film and Ag-doped with different concentrations air-annealed at 400 ◦ C.

2.40x10

382 380 378 376 374 372 370 368 366 364

Binding energy, eV Fig. 9. XPS spectra of Ag3d region of ZnTe (10 ␮m) films.

Please cite this article in press as: T. Potlog, et al., Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.133

G Model

ARTICLE IN PRESS

APSUSC-30010; No. of Pages 5

T. Potlog et al. / Applied Surface Science xxx (2015) xxx–xxx

References

Ag3d5/2 3

9x10

1% AgNO 3

3

XPS signals,counts

8x10

Ag3d3/2

5% AgNO 3 20% AgNO 3

3

7x10

5

3

6x10

3

5x10

3

4x10

3

3x10

3

2x10

382 380 378 376 374 372 370 368 366 364 362

Binding energy, eV Fig. 10. XPS spectra of Ag3d region of ZnTe (1 ␮m) films.

the AgZn acceptor. For example, by substitution of Zn2+ by Ag1+ the following interaction is possible [26]. Zn + 2(Ag+ + e) → Zn2+ + 2e + 2Ag Alternatively, Ag might be incorporated interstitially in ZnTe. 4. Conclusions Closed space sublimation method was selected as a deposition technique for depositing ZnTe thin films because is economical and a simple technique as compared to other techniques. The XRD analysis of as-deposited ZnTe thin films showing the cubic phase with (1 1 1) preferred orientation. It was observed that crystallite size increased with the increasing of the substrate temperature. Scanning electron microscopy (SEM) was used to observe the change of as-deposited grains sizes. The XPS results suggest that due to the immersion of ZnTe films into AgNO3 solution and annealing at 400 ◦ C for 1 h silver was incorporated in the ZnTe in the different concentrations depending on the thickness of ZnTe films. The incorporation of the silver causes appreciable shift in the binding energies of the XPS peaks. The decreasing of Zn concentration in the films, as Ag increased, leads to verify that Ag replaced Zn. The doublet splitting of Te3d region arises through complex ionization process in which ejection of an inner shell electron is apparently coupled simultaneously with excitation of one or more of the other electrons. These results demonstrate that annealing in AgNO3 solution of different concentration causes the presence of unwanted oxides and modify composition of the original films. Acknowledgment

[1] A.K.S. Aqili, Z. Ali, A. Mazsood, Optical and structural properties of two-sourced evaporated ZnTe thin films, Appl. Surf. Sci. 167 (2000) 1–11. [2] T.A. Gessert, P. Sheldon, X. Li, D. Dunlavy, D. Niles, R. Sasala, S. Albright, B. Zadler, Studies of ZnTe back contacts to CdS/CdTe solar cells, in: Proceedings of the 26th Photovoltaic Specialists Conference, 1997, http://dx.doi.org/ 10.1109/PVSC. 1997.654117. [3] L.H. Feng, W. Cai, J.G. Zheng, et al., Effects of ZnTe complex back contacts on CdTe solar cells, Acta Energy Solar Sin. 22 (4) (2001) 403–408. [4] First Solar Inc., http://investor.firstsolar.com/releasedetail.cfm?ReleaseID= 828273, 2014. [5] D. Xu, T. Biegala, M. Carmody, J.W. Garland, C. Grein, S. Sivananthan, Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrates, Appl. Phys. Lett. 96 (2010), 073508-1-3. [6] S.H. Demtsu, J.R. Sites, Effect of back-contact barrier on thin-film CdTe solar cells, Thin Solid Films 510 (2006) 320–324. [7] A.L. Fahrenbruch, Exploring back contact technology to increase CdS/CdTe solar cell efficiency, Mater. Res. Soc. Symp. Proc. 1012 (2007), abstr. 1012Y07-05. [8] B. Späth, J. Fritsche, A. Klein, W. Jaegermann, p-ZnTe for back contacts to CdTe thin film solar cells, MRS Proc. 865 (2005) F8.3, http://dx.doi.org/ 10.1557/PROC-865-F8.3. [9] T. Potlog, N. Spalatu, V. Fedorov, N. Maticiuc, C. Antoniuc, V. Botnariuc, J. Hiie, T. Raadik, V. Valdna, The performance of thin film solar cells employing photovoltaic ZnSe/CdTe, CdS/CdTe and ZnTe/CdTe heterojunctions, in: 2011 37th IEEE Photovoltaic Specialists Conference (PVSC), 19–24 June 2011, 2011, http://dx.doi.org/10.1109/PVSC. 2011.6186211. [10] S.N. Alamri, The growth of CdTe thin film by close space sublimation system, Phys. Stat. Sol. (a) 200 (2003) 352–360. [11] J.F. Watts, J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES, John Wiley & Sons, Chichester, 2003. [12] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, ULVAC-PHI Japan, Physical Electronics, USA, 1995. [13] W.M. Haynes (Ed.), CRC Handbook of Chemistry and Physics, 92nd ed., 2011. [14] S. Venkatachalam, D. Mangalaraj, Sa. K. Narayandass, K. Kim, J. Yi, Structure, optical and electrical properties of ZnSe thin films, Physica B 358 (2005) 27–35. [15] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of XPS, Perkin Elmer Corporation, Eden Prairie, MN, USA, 1979. [16] D.W. Langer, C.J. Vesely, Electronic core levels of zinc chalcogenides, Phys. Rev. B 2 (1970) 4885–4892. [17] W. Wang, G. Xia, J. Zheng, L. Feng, R. Hao, Study of polycrystalline ZnTe (ZnTe:Cu) thin films for photovoltaic cells, Mater. Sci.: Mater. Electron. 18 (2007) 427–431. [18] B. Bozzini, C. Lenardi, N. Lovergine, Electrodeposition of stoichiometric polycrystalline ZnTe on n+ GaAs and Ni–P, Mater. Chem. Phys. 66 (2000) 219–228. [19] B.V.R. Chowdari, P.P. Kumari, Thermal, electrical and XPS studies of Ag2 O TeO glasses, J. Non-Cryst. Solids 197 (1996) 31–40. [20] E. De La Rosa, S. Sepulveda-Guzman, B. Reeja-Jayan, A. Torres, P. Salas, N. Elizondo, M.J. Yacaman, J. Phys. Chem. C 111 (2007) 8489–8495. [21] S. Bhumia, P. Bhattcharya, D.N. Bose, Pulsed laser deposition of ZnTe thin films, Mater. Lett. 27 (1996) 307–311. [22] M. Nveumann-Spallart, et al., Thin Solid Films (1995) 233–265. [23] http://xpssimplified.com/elements/oxygen.php [24] H. Zhang, G. Wang, D. Chen, X.J. Lv, J.H. Li, Chem. Mater. 20 (2008) 6543–6549. [25] J.S. Hammond, S.W. Gaarenstroom, N. Winograd, X-ray photoelectron spectroscopic studies of cadmium- and silver-oxygen surfaces, Anal. Chem. 47 (13) (1975) 2193–2199. [26] A.K.S. Aqiliet, et al., Ag doped ZnTe films prepared by closed space sublimation and an ion exchange process, J. Alloys Compd. 520 (2012) 83–88.

This research was supported by the Bilateral Moldova-Romania grant no. 13.820.15.19/RoA.

Please cite this article in press as: T. Potlog, et al., Temperature-dependent growth and XPS of Ag-doped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.03.133