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Phase transformation and disorder effect on optical and electrical properties of Zn3P2 thin films
Spectrochimica Acta Part A 94 (2012) 378–383
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Phase transformation and disorder effect on optical and electrical properties of Zn3 P2 thin films I.K. El Zawawi a,∗ , A. Abdel Moez a , T.R. Hammad b , R.S. Ibrahim a a b
Solid State Physics Department, National Research Center, 12622 Dokki, Cairo, Egypt Physics Department, Faculty of Science, Helwan University, Helwan, Egypt
a r t i c l e
i n f o
Article history: Received 29 January 2012 Received in revised form 16 March 2012 Accepted 22 March 2012 Keywords: Zn3 P2 thin films X-ray diffraction Optical properties Electrical properties
a b s t r a c t The phase transformation of zinc phosphide (Zn3 P2 ) thin films was detected through isochronal annealing process. The effects on isochronal annealing on the internal structural, optical and electrical properties of deposited Zn3 P2 thin films have been discussed. The films were prepared by thermal evaporation under constant preparation conditions of vacuum 1.3 × 10−5 Torr, substrate temperature (300 K), rate of deposition (∼1 nm/s) and film thickness (480 nm). The annealing process was carried out under vacuum for 2 h at different temperatures ranging from 373 to 623 K. X-ray diffraction patterns showed that the asdeposited films and those annealed at temperatures less than 623 K exhibit amorphous structure, while the films annealed at 623 K showed tetragonal polycrystalline structure. The optical transmission and reflection spectra were measured at the wavelength range of 190–2500 nm. The absorption coefficient spectra and the degree of disorder as measured from the absorption edge were determined. The indirect and direct optical energy band gaps were evaluated for indirect allowed and direct allowed transitions for amorphous and polycrystalline films, respectively. The refractive index no increases with raising the annealing temperature which refers to more condensation in the material. The electrical resistivity for Zn3 P2 films decreases exponentially with raising the annealing temperature up to 623 K as influenced by structure transformation and decreasing the degree of disorder in the films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zn3 P2 thin film is one of the important components in many industrial electronic devices. Zn3 P2 is a member of II–V compounds with a direct band gap of about 1.5 eV [1,2], which matches with the solar spectrum in the visible region [3]. This allows Zn3 P2 to be one of the most promising materials for the production of solar cells [4,5]. One of the major advantages of this material is its availability in sufficient abundance that fulfills the requirements of large-scale power generation [6]. In addition Zn3 P2 films have important application in infrared (IR) and ultraviolet (UV) sensors [7]. The most important electronic application of Zn3 P2 glass is that the fabrication of this material as a wave guide [8]. Recently some physical studies were done for special uses of Zn3 P2 [9–11]. The preparation of high-quality thin films of Zn3 P2 is of particular importance to provide superior optoelectronic devices [12]. In addition Zn3 P2 thin films have a long minority carrier diffusion length (13 m) and large optical absorption coefficient [7,13,14].
Zn3 P2 thin films of both polycrystalline and amorphous structures forms have been prepared by different techniques [15,16] such as CVD [17,18], vacuum evaporation [19], hot wall epitaxy [20] and RF sputtering [21]. So far the problem faced with Zn3 P2 films is the occurrence of micro-cracks and production of high quality deposited material [22]. No studies have been reported on electrical conduction of thermally evaporated Zn3 P2 thin films. However, some optical properties studies and optical band gap determination due to direct allowed transitions were reported [23]. The optical properties were studied and it was found that the refractive index of glass Zn3 P2 could be changed using femtosecond laser [24]. The performance of the Zn3 P2 thin films in any electronic device is fundamentally dependent on their physical properties. Thus, controlling the optical and electrical properties of deposited films that are mainly influenced by its internal structure is a basic goal for better performance device. This work aims to study phase transformation through the effects of isochronal post-deposition annealing on the structure, optical and electrical properties of Zn3 P2 thin films. 2. Experimental details
∗ Corresponding author. Tel.: +20 2 25164693/33322412; fax: +20 2 33370931. E-mail address: [email protected] (I.K. El Zawawi). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.03.072
Zn3 P2 thin films were prepared by vacuum thermal evaporation technique using coating unit type Edwards E 306. The thin
I.K. El Zawawi et al. / Spectrochimica Acta Part A 94 (2012) 378–383
379
70 60
(4 0 0)
40
0 10
30
50
(6 2 4)
(6 0 6)
(4 2 7)
(417 )
10
(4 2 3) (5 2 1 ) (4 4 0) (5 2 2) (5 2 3) (5 0 5)
(102) (2 0 1)
20
(3 2 3)
30 (2 1 1) (2 0 2) (2 1 2) (2 0 3) (2 2 0) (301 ) (3 0 2) (303 )
I (A.U.)
50
70
90
2 (degree) Fig. 2. X-ray diffraction patterns for Zn3 P2 powder.
(6 2 4)
(4 0 7)
(5 2 1) (4 4 0) (5 2 3)
(3 2 3)
(40 4)
(4 0 0)
(3 0 1) (3 0 2)
(2 2
films were deposited under a vacuum of 1.3 × 10−5 Torr onto freshly well-cleaned glass substrates at constant substrate temperature of 300 K from Zn3 P2 ingot powder (99.999%). The thickness of the films and rate of evaporation were controlled by a quartz crystal thickness monitor (Edwards FTM4) attached to the evaporation system. The film thickness and deposition rate were kept constant at 480 nm and ∼1 nm/s, respectively with an error of 2%. Postdeposition isochronal annealing for as-deposited Zn3 P2 films was done under vacuum of 10−2 Torr in furnace for 2 h at temperatures up to 623 K. The structure of the ingot powder and thin films was examined by X-ray diffraction analysis (XRD) using powder X-ray diffractometer (Philips X’pert). In order to measure electrical properties, four silver electrodes forming ohmic contacts were deposited at the top of the films [25–27]. The electrical resistivity measurements were done using four-point probe technique employing an electrometer (Keithely 617). The optical reflection and transmission spectra were measured at ambient atmosphere using spectrophotometer (Jasco V-570) at wavelength ranging 190–2500 nm. The error in optical measurements is 1%.
I (A.U.)
Fig. 1. The SEM images of amorphous (a) and polycrystalline (b) Zn3 P2 thin films of thickness 480 nm.
as-deposited and annealed at 623 K, are shown in Figs. 2 and 3a and b, respectively. It could be observed that the ingot Zn3 P2 powder has polycrystalline structure with tetragonal unit cell as matched with JCPDS cards (No. 74-1156) as shown in Fig. 2. The as-deposited Zn3 P2 film at room temperature exhibits amorphous structures as seen in Fig. 3a. These results are correlated with those reported by Peiteado et al. [12]. With ascending annealing temperature, the film structure transformed to polycrystalline film and the crystallinity increased [12]. All as-deposited films annealed at temperatures lower than 623 K have been examined by XRD and showed featureless XRD patterns, as it still acquire amorphous structure. The films annealed at 623 K for 2 h at vacuum showed polycrystalline structure of tetragonal unit cell (Fig. 3b). The optical transmission and reflection spectra of as-deposited and annealed thin films with temperature up to 623 K were measured. The transmission spectra showed maxima T+ and minima T− of interference fringes. The absorption coefficient as function of wavelength was calculated [28]. The absorption coefficient spectra ˛(h) of as-deposited and annealed films at 623 K are represented in Fig. 4. Different absorption behaviors were observed depending on the internal structure of the films. It could be observed that the amorphous film has lower absorption values than polycrystalline one at fundamental absorption region which could be due to some little change in the reddish brown color of the films. The absorption edge for polycrystalline film was sharper than amorphous one due to the change in the structure and the decrease the degree of disorder in the Zn3 P2 film material. Defect sites are shown in
3. Results and discussion High quality Zn3 P2 films were deposited without any microcracks as seen in the scanning electron microscope (SEM) images of amorphous and polycrystalline films as shown in Fig. 1. XRD patterns of Zn3 P2 powder and thin films of thickness (480 nm),
(B)
(A)
10
20
30
40
50
60
70
80
90
2 (degree) Fig. 3. X-ray diffraction patterns for: (a) as-deposited film of thickness 480 nm and (b) film annealed at 623 K of thickness 480 nm.
380
I.K. El Zawawi et al. / Spectrochimica Acta Part A 94 (2012) 378–383
Fig. 4. The absorption coefficient ln(˛) as a function of photon energy (h) for asdeposited and annealed Zn3 P2 thin films at 623 K of thickness ∼480 nm.
absorption tail at lower energy values for high annealing temperature films. The optical energy gap (Eg ) is determined from the absorption spectra curves at fundamental absorption region near the absorption edge using the empirical equation [29]: ˛ = A(h − Eg )
p
(1)
where A is a constant, Eg is the energy band gap, is the frequency of the incident radiation and h is Plank’s constant. The constant p takes the value 0.5 for direct energy gap due to direct allowed transition and the best linearity is obtained for annealed film at 623 K. The constant p has the value 2 for indirect energy gap due to indirect allowed transition and the best linearity is obtained for asdeposited Zn3 P2 films and those annealed at temperatures lower than 623 K. Figs. 5 and 6 show the relation between (˛h)1/2 , (˛h)2 and h for indirect transition of as-deposited and direct transition of annealed films at 623 K. The optical indirect energy gap Egindirec and optical direct energy gap Egdirec are estimated from the extrapolation of the linear part of the curves in Figs. 5 and 6, respectively.
Fig. 6. Relation between (˛h)2 and photon energy h (eV) for Zn3 P2 annealed at 623 K of thickness ∼480 nm.
For as-deposited amorphous Zn3 P2 film the energy gap was indirect gap of 1.0 eV, while for the film annealed at 623 K of polycrystalline structure has the direct energy gap of 1.56 eV in agreement with other reported data for Zn3 P2 thin films [30]. The indirect energy gap was estimated as before for all amorphous films which exhibit annealing temperatures up to 623 K and indicated in Fig. 7. It could be noted that the indirect energy gap Egindirec increases with the annealing temperature Tann as influenced by the change in the slope of the absorption edge. The value of the Urbach energy Eo was calculated according to Urbach equation [31] ˛ = ˛o eh/Eo
(2)
The Eo value is calculated from the slope of the linear part of the logarithmic dependence of absorption coefficient on photon energy
2.5
Egindirect (eV)
2
1.5
1
0.5
0 200
300
400
500
600
Tann (K) Fig. 5. Relation between (˛h)1/2 and photon energy h (eV) for Zn3 P2 as-deposited thin film of thickness ∼480 nm.
Fig. 7. The indirect energy gap Egindirec in as a function of annealing temperature Tann (K) for amorphous Zn3 P2 films.
I.K. El Zawawi et al. / Spectrochimica Acta Part A 94 (2012) 378–383
2500
0.5
2000
cm)
0.6
(109) (
Eo (eV)
0.4
0.3
381
1500
1000
0.2
500 0.1
0 0
300 20 0
300
400
500
600
Fig. 8. Relation between the degree of disorder (Eo ) and annealing temperature for Zn3 P2 thin film of thickness 480 nm.
that is drawn as representative for the as-deposited and annealed films. The Eo value dependence on annealing temperature Tann is depicted in Fig. 8, which shows a parabolic relation. It is clear from Fig. 8 that value of Eo decreases with raising the annealing temperature. The decrease in Eo is referring to the decrease in the degree of disorder in the film material as the annealing temperature increases and consequently the structure transformed gradually from amorphous to polycrystalline tetragonal structure. The refractive index (n) of these films was calculated using the following equation [32] 0.5 0.5
]
500
600
700
Tann (K)
Tann (K)
n = [N + (N 2 − n2s )
400
700
(3)
where 1 + n2s T+ − T− + 2ns · N= 2 T+ · T− and ns is the refractive index of the used substrate, T+ is the value of the transmission of the upper limit of the envelope, T− is the value of the transmission of the lower envelope. Fig. 9 shows the refractive index at wavelength = 2000 nm (no ) as function of annealing temperature Tann for all examined films. It is clear that the refractive index for film annealed at 623 K is higher than that for the as-deposited one. It could be observed that the refractive index
Fig. 10. Relation between the resistivity () and annealed temperature (Tann ) for Zn3 P2 thin film of thickness 480 nm.
increases generally with increasing annealing temperature. This is attributed to the gradual phase transformation from amorphous to polycrystalline structure. This could reflect more condensation in thin film material as the annealing temperature increases. Fig. 10 shows the electrical resistivity () dependence of Zn3 P2 thin films on its annealing temperature for 2 h in vacuum. It could be observed that the electrical resistivity for as-deposited thin film has high value of 2 × 109 cm due to its amorphous structure and high degree of disorder. The resistivity sharply decreases with increasing the annealing temperature to 448 K then slower decrease is observed from 448 to 623 K following exponential behavior tending to stability at high temperature range. The decrease of film resistivity could be attributed to the gradual change of film structure from amorphous to polycrystalline structure which confirmed by X-ray diffraction investigation (Table 1). The gradual decrease in disorder degree due to the increase in the annealing temperature showed a decrease in resistivity about 2 orders of magnitude to reach 1 × 108 cm for polycrystalline films annealed at 623 K. The annealing temperature as a function of resistivity shows nearly exponential behavior and could be expressed by the relation between ln and 1/T seen in Fig. 11. Thus, for evaluating the
22 y = 1724.x + 1.4E-7
21.5
ln (( ) (Ohm Cm))
4 3.5 3
no
2.5 2 1.5 1
20.5 20 19.5 19 18.5
0.5 0 200
21
18 300
400
500
600
700
Tann (K) Fig. 9. Relation between refractive index at ( = 2000 nm) and annealed temperature for Zn3 P2 thin film of thickness ∼480 nm.
0
0.001
0.002
0.003
0.004
-1
1/Tann (K ) Fig. 11. Relation between ln() and the inverse of annealing temperature.
382
I.K. El Zawawi et al. / Spectrochimica Acta Part A 94 (2012) 378–383
Table 1 X-ray diffraction data of Zn3 P2 powder and thin film annealed at 623 K. Powder
Thin film annealed at 623 K
d (Å)
I/Io (%)
4.675 3.812 3.445 3.301 3.056 2.857 2.771 2.622 2.438 2.199 2.021 1.931 1.743 1.723 1.650 1.634 1.600 1.556 1.512 1.488 1.453 1.427 1.398 1.320 1.308 1.277 1.255 1.211 1.167 1.100
4 6 14 27 14 35 32 16 11 6 100 1 2 6 6 4 2 3 4 2 6 8 2 2 1 1 1 3 10 3
d (Å)
I/Io (%)
2.840
78
2.625 2.485
49 63
2.025 1.921
100 26
1.628
13
1.473
10
1.422 1.378
8 11
1.273
8
1.160
6
(h k l) (1 0 2) (2 0 1) (2 1 1) (2 0 2) (2 1 2) (2 2 0) (2 0 3) (3 0 1) (3 0 2) (3 0 3) (4 0 0) (3 2 3) (4 1 3) (4 2 2) (4 0 4) (4 2 3) (4 3 1) (4 3 2) (4 0 5) (5 2 1) (5 2 2) (4 4 0) (5 2 3) (5 0 5) (5 3 3) (4 0 7) (4 1 7) (4 2 7) (6 2 4) (6 0 6)
electrical resistivity for Zn3 P2 thin films at any annealing temperature, a general equation is considered as: =
1 a
e
b/T
(4)
where a and b are constants, from curve fitting (Fig. 10), their values were estimated as a = 1.4 × 10−7 and b = 1724.7. The decrease in the degree of disorder is correlated to the decrease in the resistivity of the annealed films. Fig. 12 shows the relation between the degree of disorder referred by Eo and the
2500
( Ohm cm)
2000
1500
1000
500
0 0
0 .2
0.4
0.6
Eo (eV) Fig. 12. The relation between E0 and the resistivity of Zn3 P2 annealing thin film.
resistivity of the annealed films. A linear relationship observed in Fig. 10 indicates that the degree of disorder is the main factor influencing the resistivity of the films (in the phase transformation region).
4. Conclusions The surface topography of the Zn3 P2 thin films was studied using SEM, and structure of Zn3 P2 films was found to be amorphous for as-deposited films and those annealed at temperatures less than 623 K for 2 h, while changed to tetragonal polycrystalline structure for films annealed at temperature of 623 K for 2 h as examined by XRD technique. The optical absorption spectra was measured for amorphous and polycrystalline films. A change from an indirect transition for the as-deposited film to direct transition was reported as the annealing temperature increases to 623 K due to the phase transformation. The degree of disorder was estimated from the absorption edge and shows a gradual decrease with raising the annealing temperature. The refractive index shows higher value for polycrystalline Zn3 P2 films due to more condensation in the material affected by the progress of annealing temperature of the deposited Zn3 P2 films. The electrical resistivity for Zn3 P2 films decreases with ascending the annealing temperature due to change in structure from amorphous to polycrystalline which is associated by decrease in the degree of disorder. This was confirmed by decreasing in degree of disorder referred by Eo linearly with the resistivity of the annealing films. References [1] M. Pawlikowski Janusz, J. Misiewicz, N. Mirowska, Journal of Physics and Chemistry of Solids 40 (1979) 1027–1033. [2] E.A. Fagen, Journal of Applied Physics 50 (1979) 6505–6520. [3] J. Loferski Joseph, Journal of Applied Physics 27 (1956) 777–785. [4] A.M. Barnett, Proc. European Communities Photovoltaic Solar Energy Conference, Reidel, Dordrecht, 1979, pp. 440–445. [5] G. Sberveglieri, N. Romeo, Thin Solid Films 83 (1981) L133–L136. [6] A. Catalano, V. Dalal, W.E. Deavancy, Zn3 P2 : a promising photovoltaic material, in: Proceedings of the 13th IEEE (PVSC), 1978, 288 pp. [7] Y. Kato, S. Kurita, T. Suda, Journal of Applied Physics 62 (1987) 3733–3740. [8] L.B. Fletcher, J.J. Witcher, N. Troy, S.T. Reis, R.K. Brow, D.M. Krol, Optical Express 19 (9) (2011) 7929–7936. [9] T. Suda, T. Miyakawa, S. Kurita, Journal of Crystal Growth 86 (1988) 423–429. [10] S. Sudhakar, K. Baskar, Journal of Crystal Growth 310 (2008) 2707–2711. [11] A. Mokhtari, Journal of Physics: Condensed Matter 21 (27) (2009) 2802. [12] M. Peiteado, T. Jardiel, F. Rubio, A.C. Caballero, Materials Research Bulletin 45 (2010) 1586–1592. [13] K. Kakishita, S. Ikeda, T. Suda, Journal of Crystal Growth 115 (1991) 793–797. [14] N. Convers Wyeth, A. Catalano, Journal of Applied Physics 50 (1979) 1403–1408. [15] T. Suda, K. Kakishta, Journal of Applied Physics 71 (1992) 3039–3042. [16] L. Bryja, K. Jezierski, J. Misiewicz, Thin Solid Films 229 (1993) 11–13. [17] T.L. Chu, S. Chu Shirley, K. Murthy, E.D. Stokes, P.E. Russell, Journal of Applied Physics 54 (1983) 2063–2069. [18] E. Papazoglou, T.W.F. Russell, Journal of Vacuum Science and Technology A 5 (1987) 3378–3383. [19] L. Bryja, M. Ciorga, K. Jezierski, A. Bohdziewicz, J. Misiewicz, Vacuum 50 (1998) 97–98. [20] S. Fuke, S. Kawarabayashi, K. Kuwahara, T. Imai, Journal of Applied Physics 60 (1986) 2368–2372. [21] A. Weber, P. Sutter, H. Von Kanel, Thin Solid Films 239 (1994) 205–210. [22] L. Bryja, K. Jezierski, M. Ciorga, A. Bohdziewicz, J. Misiewicz, Vacuum 50 (1–2) (1998) 5–7. [23] R. Sathyamoorthy, C. Sharmila, K. Natarajan, S. Velumani, Materials Characterization 58 (2007) 745–749. [24] D.J. Little, M. Ams, P. Dekker, G.D. Marshall, M.J. Withford, Journal of Applied Physics 108 (3) (2010) 033110. [25] J. Misiewicz, N. Mirowska, Z. Gumienny, Physica Status Solidi A 83 (1984) K51. [26] N. Mirowska, J. Misiewicz, Semiconductor Science and Technology 7 (11) (1992) 1332–1337.
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[30] N. Mirowska, J. Misiewicz, Materials Science and Engineering B 130 (2006) 49–56. [31] F. Urbach, Physical Review 92 (1953), 1324-1324. [32] A. Nayak, D.R. Rao, H.D. Banerjee, Materials Science and Engineering 10 (1991) 403.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Phase transformation and disorder effect on optical and electrical properties of Zn3 P2 thin films I.K. El Zawawi a,∗ , A. Abdel Moez a , T.R. Hammad b , R.S. Ibrahim a a b
Solid State Physics Department, National Research Center, 12622 Dokki, Cairo, Egypt Physics Department, Faculty of Science, Helwan University, Helwan, Egypt
a r t i c l e
i n f o
Article history: Received 29 January 2012 Received in revised form 16 March 2012 Accepted 22 March 2012 Keywords: Zn3 P2 thin films X-ray diffraction Optical properties Electrical properties
a b s t r a c t The phase transformation of zinc phosphide (Zn3 P2 ) thin films was detected through isochronal annealing process. The effects on isochronal annealing on the internal structural, optical and electrical properties of deposited Zn3 P2 thin films have been discussed. The films were prepared by thermal evaporation under constant preparation conditions of vacuum 1.3 × 10−5 Torr, substrate temperature (300 K), rate of deposition (∼1 nm/s) and film thickness (480 nm). The annealing process was carried out under vacuum for 2 h at different temperatures ranging from 373 to 623 K. X-ray diffraction patterns showed that the asdeposited films and those annealed at temperatures less than 623 K exhibit amorphous structure, while the films annealed at 623 K showed tetragonal polycrystalline structure. The optical transmission and reflection spectra were measured at the wavelength range of 190–2500 nm. The absorption coefficient spectra and the degree of disorder as measured from the absorption edge were determined. The indirect and direct optical energy band gaps were evaluated for indirect allowed and direct allowed transitions for amorphous and polycrystalline films, respectively. The refractive index no increases with raising the annealing temperature which refers to more condensation in the material. The electrical resistivity for Zn3 P2 films decreases exponentially with raising the annealing temperature up to 623 K as influenced by structure transformation and decreasing the degree of disorder in the films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zn3 P2 thin film is one of the important components in many industrial electronic devices. Zn3 P2 is a member of II–V compounds with a direct band gap of about 1.5 eV [1,2], which matches with the solar spectrum in the visible region [3]. This allows Zn3 P2 to be one of the most promising materials for the production of solar cells [4,5]. One of the major advantages of this material is its availability in sufficient abundance that fulfills the requirements of large-scale power generation [6]. In addition Zn3 P2 films have important application in infrared (IR) and ultraviolet (UV) sensors [7]. The most important electronic application of Zn3 P2 glass is that the fabrication of this material as a wave guide [8]. Recently some physical studies were done for special uses of Zn3 P2 [9–11]. The preparation of high-quality thin films of Zn3 P2 is of particular importance to provide superior optoelectronic devices [12]. In addition Zn3 P2 thin films have a long minority carrier diffusion length (13 m) and large optical absorption coefficient [7,13,14].
Zn3 P2 thin films of both polycrystalline and amorphous structures forms have been prepared by different techniques [15,16] such as CVD [17,18], vacuum evaporation [19], hot wall epitaxy [20] and RF sputtering [21]. So far the problem faced with Zn3 P2 films is the occurrence of micro-cracks and production of high quality deposited material [22]. No studies have been reported on electrical conduction of thermally evaporated Zn3 P2 thin films. However, some optical properties studies and optical band gap determination due to direct allowed transitions were reported [23]. The optical properties were studied and it was found that the refractive index of glass Zn3 P2 could be changed using femtosecond laser [24]. The performance of the Zn3 P2 thin films in any electronic device is fundamentally dependent on their physical properties. Thus, controlling the optical and electrical properties of deposited films that are mainly influenced by its internal structure is a basic goal for better performance device. This work aims to study phase transformation through the effects of isochronal post-deposition annealing on the structure, optical and electrical properties of Zn3 P2 thin films. 2. Experimental details
∗ Corresponding author. Tel.: +20 2 25164693/33322412; fax: +20 2 33370931. E-mail address: [email protected] (I.K. El Zawawi). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.03.072
Zn3 P2 thin films were prepared by vacuum thermal evaporation technique using coating unit type Edwards E 306. The thin
I.K. El Zawawi et al. / Spectrochimica Acta Part A 94 (2012) 378–383
379
70 60
(4 0 0)
40
0 10
30
50
(6 2 4)
(6 0 6)
(4 2 7)
(417 )
10
(4 2 3) (5 2 1 ) (4 4 0) (5 2 2) (5 2 3) (5 0 5)
(102) (2 0 1)
20
(3 2 3)
30 (2 1 1) (2 0 2) (2 1 2) (2 0 3) (2 2 0) (301 ) (3 0 2) (303 )
I (A.U.)
50
70
90
2 (degree) Fig. 2. X-ray diffraction patterns for Zn3 P2 powder.
(6 2 4)
(4 0 7)
(5 2 1) (4 4 0) (5 2 3)
(3 2 3)
(40 4)
(4 0 0)
(3 0 1) (3 0 2)
(2 2
films were deposited under a vacuum of 1.3 × 10−5 Torr onto freshly well-cleaned glass substrates at constant substrate temperature of 300 K from Zn3 P2 ingot powder (99.999%). The thickness of the films and rate of evaporation were controlled by a quartz crystal thickness monitor (Edwards FTM4) attached to the evaporation system. The film thickness and deposition rate were kept constant at 480 nm and ∼1 nm/s, respectively with an error of 2%. Postdeposition isochronal annealing for as-deposited Zn3 P2 films was done under vacuum of 10−2 Torr in furnace for 2 h at temperatures up to 623 K. The structure of the ingot powder and thin films was examined by X-ray diffraction analysis (XRD) using powder X-ray diffractometer (Philips X’pert). In order to measure electrical properties, four silver electrodes forming ohmic contacts were deposited at the top of the films [25–27]. The electrical resistivity measurements were done using four-point probe technique employing an electrometer (Keithely 617). The optical reflection and transmission spectra were measured at ambient atmosphere using spectrophotometer (Jasco V-570) at wavelength ranging 190–2500 nm. The error in optical measurements is 1%.
I (A.U.)
Fig. 1. The SEM images of amorphous (a) and polycrystalline (b) Zn3 P2 thin films of thickness 480 nm.
as-deposited and annealed at 623 K, are shown in Figs. 2 and 3a and b, respectively. It could be observed that the ingot Zn3 P2 powder has polycrystalline structure with tetragonal unit cell as matched with JCPDS cards (No. 74-1156) as shown in Fig. 2. The as-deposited Zn3 P2 film at room temperature exhibits amorphous structures as seen in Fig. 3a. These results are correlated with those reported by Peiteado et al. [12]. With ascending annealing temperature, the film structure transformed to polycrystalline film and the crystallinity increased [12]. All as-deposited films annealed at temperatures lower than 623 K have been examined by XRD and showed featureless XRD patterns, as it still acquire amorphous structure. The films annealed at 623 K for 2 h at vacuum showed polycrystalline structure of tetragonal unit cell (Fig. 3b). The optical transmission and reflection spectra of as-deposited and annealed thin films with temperature up to 623 K were measured. The transmission spectra showed maxima T+ and minima T− of interference fringes. The absorption coefficient as function of wavelength was calculated [28]. The absorption coefficient spectra ˛(h) of as-deposited and annealed films at 623 K are represented in Fig. 4. Different absorption behaviors were observed depending on the internal structure of the films. It could be observed that the amorphous film has lower absorption values than polycrystalline one at fundamental absorption region which could be due to some little change in the reddish brown color of the films. The absorption edge for polycrystalline film was sharper than amorphous one due to the change in the structure and the decrease the degree of disorder in the Zn3 P2 film material. Defect sites are shown in
3. Results and discussion High quality Zn3 P2 films were deposited without any microcracks as seen in the scanning electron microscope (SEM) images of amorphous and polycrystalline films as shown in Fig. 1. XRD patterns of Zn3 P2 powder and thin films of thickness (480 nm),
(B)
(A)
10
20
30
40
50
60
70
80
90
2 (degree) Fig. 3. X-ray diffraction patterns for: (a) as-deposited film of thickness 480 nm and (b) film annealed at 623 K of thickness 480 nm.
380
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Fig. 4. The absorption coefficient ln(˛) as a function of photon energy (h) for asdeposited and annealed Zn3 P2 thin films at 623 K of thickness ∼480 nm.
absorption tail at lower energy values for high annealing temperature films. The optical energy gap (Eg ) is determined from the absorption spectra curves at fundamental absorption region near the absorption edge using the empirical equation [29]: ˛ = A(h − Eg )
p
(1)
where A is a constant, Eg is the energy band gap, is the frequency of the incident radiation and h is Plank’s constant. The constant p takes the value 0.5 for direct energy gap due to direct allowed transition and the best linearity is obtained for annealed film at 623 K. The constant p has the value 2 for indirect energy gap due to indirect allowed transition and the best linearity is obtained for asdeposited Zn3 P2 films and those annealed at temperatures lower than 623 K. Figs. 5 and 6 show the relation between (˛h)1/2 , (˛h)2 and h for indirect transition of as-deposited and direct transition of annealed films at 623 K. The optical indirect energy gap Egindirec and optical direct energy gap Egdirec are estimated from the extrapolation of the linear part of the curves in Figs. 5 and 6, respectively.
Fig. 6. Relation between (˛h)2 and photon energy h (eV) for Zn3 P2 annealed at 623 K of thickness ∼480 nm.
For as-deposited amorphous Zn3 P2 film the energy gap was indirect gap of 1.0 eV, while for the film annealed at 623 K of polycrystalline structure has the direct energy gap of 1.56 eV in agreement with other reported data for Zn3 P2 thin films [30]. The indirect energy gap was estimated as before for all amorphous films which exhibit annealing temperatures up to 623 K and indicated in Fig. 7. It could be noted that the indirect energy gap Egindirec increases with the annealing temperature Tann as influenced by the change in the slope of the absorption edge. The value of the Urbach energy Eo was calculated according to Urbach equation [31] ˛ = ˛o eh/Eo
(2)
The Eo value is calculated from the slope of the linear part of the logarithmic dependence of absorption coefficient on photon energy
2.5
Egindirect (eV)
2
1.5
1
0.5
0 200
300
400
500
600
Tann (K) Fig. 5. Relation between (˛h)1/2 and photon energy h (eV) for Zn3 P2 as-deposited thin film of thickness ∼480 nm.
Fig. 7. The indirect energy gap Egindirec in as a function of annealing temperature Tann (K) for amorphous Zn3 P2 films.
I.K. El Zawawi et al. / Spectrochimica Acta Part A 94 (2012) 378–383
2500
0.5
2000
cm)
0.6
(109) (
Eo (eV)
0.4
0.3
381
1500
1000
0.2
500 0.1
0 0
300 20 0
300
400
500
600
Fig. 8. Relation between the degree of disorder (Eo ) and annealing temperature for Zn3 P2 thin film of thickness 480 nm.
that is drawn as representative for the as-deposited and annealed films. The Eo value dependence on annealing temperature Tann is depicted in Fig. 8, which shows a parabolic relation. It is clear from Fig. 8 that value of Eo decreases with raising the annealing temperature. The decrease in Eo is referring to the decrease in the degree of disorder in the film material as the annealing temperature increases and consequently the structure transformed gradually from amorphous to polycrystalline tetragonal structure. The refractive index (n) of these films was calculated using the following equation [32] 0.5 0.5
]
500
600
700
Tann (K)
Tann (K)
n = [N + (N 2 − n2s )
400
700
(3)
where 1 + n2s T+ − T− + 2ns · N= 2 T+ · T− and ns is the refractive index of the used substrate, T+ is the value of the transmission of the upper limit of the envelope, T− is the value of the transmission of the lower envelope. Fig. 9 shows the refractive index at wavelength = 2000 nm (no ) as function of annealing temperature Tann for all examined films. It is clear that the refractive index for film annealed at 623 K is higher than that for the as-deposited one. It could be observed that the refractive index
Fig. 10. Relation between the resistivity () and annealed temperature (Tann ) for Zn3 P2 thin film of thickness 480 nm.
increases generally with increasing annealing temperature. This is attributed to the gradual phase transformation from amorphous to polycrystalline structure. This could reflect more condensation in thin film material as the annealing temperature increases. Fig. 10 shows the electrical resistivity () dependence of Zn3 P2 thin films on its annealing temperature for 2 h in vacuum. It could be observed that the electrical resistivity for as-deposited thin film has high value of 2 × 109 cm due to its amorphous structure and high degree of disorder. The resistivity sharply decreases with increasing the annealing temperature to 448 K then slower decrease is observed from 448 to 623 K following exponential behavior tending to stability at high temperature range. The decrease of film resistivity could be attributed to the gradual change of film structure from amorphous to polycrystalline structure which confirmed by X-ray diffraction investigation (Table 1). The gradual decrease in disorder degree due to the increase in the annealing temperature showed a decrease in resistivity about 2 orders of magnitude to reach 1 × 108 cm for polycrystalline films annealed at 623 K. The annealing temperature as a function of resistivity shows nearly exponential behavior and could be expressed by the relation between ln and 1/T seen in Fig. 11. Thus, for evaluating the
22 y = 1724.x + 1.4E-7
21.5
ln (( ) (Ohm Cm))
4 3.5 3
no
2.5 2 1.5 1
20.5 20 19.5 19 18.5
0.5 0 200
21
18 300
400
500
600
700
Tann (K) Fig. 9. Relation between refractive index at ( = 2000 nm) and annealed temperature for Zn3 P2 thin film of thickness ∼480 nm.
0
0.001
0.002
0.003
0.004
-1
1/Tann (K ) Fig. 11. Relation between ln() and the inverse of annealing temperature.
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Table 1 X-ray diffraction data of Zn3 P2 powder and thin film annealed at 623 K. Powder
Thin film annealed at 623 K
d (Å)
I/Io (%)
4.675 3.812 3.445 3.301 3.056 2.857 2.771 2.622 2.438 2.199 2.021 1.931 1.743 1.723 1.650 1.634 1.600 1.556 1.512 1.488 1.453 1.427 1.398 1.320 1.308 1.277 1.255 1.211 1.167 1.100
4 6 14 27 14 35 32 16 11 6 100 1 2 6 6 4 2 3 4 2 6 8 2 2 1 1 1 3 10 3
d (Å)
I/Io (%)
2.840
78
2.625 2.485
49 63
2.025 1.921
100 26
1.628
13
1.473
10
1.422 1.378
8 11
1.273
8
1.160
6
(h k l) (1 0 2) (2 0 1) (2 1 1) (2 0 2) (2 1 2) (2 2 0) (2 0 3) (3 0 1) (3 0 2) (3 0 3) (4 0 0) (3 2 3) (4 1 3) (4 2 2) (4 0 4) (4 2 3) (4 3 1) (4 3 2) (4 0 5) (5 2 1) (5 2 2) (4 4 0) (5 2 3) (5 0 5) (5 3 3) (4 0 7) (4 1 7) (4 2 7) (6 2 4) (6 0 6)
electrical resistivity for Zn3 P2 thin films at any annealing temperature, a general equation is considered as: =
1 a
e
b/T
(4)
where a and b are constants, from curve fitting (Fig. 10), their values were estimated as a = 1.4 × 10−7 and b = 1724.7. The decrease in the degree of disorder is correlated to the decrease in the resistivity of the annealed films. Fig. 12 shows the relation between the degree of disorder referred by Eo and the
2500
( Ohm cm)
2000
1500
1000
500
0 0
0 .2
0.4
0.6
Eo (eV) Fig. 12. The relation between E0 and the resistivity of Zn3 P2 annealing thin film.
resistivity of the annealed films. A linear relationship observed in Fig. 10 indicates that the degree of disorder is the main factor influencing the resistivity of the films (in the phase transformation region).
4. Conclusions The surface topography of the Zn3 P2 thin films was studied using SEM, and structure of Zn3 P2 films was found to be amorphous for as-deposited films and those annealed at temperatures less than 623 K for 2 h, while changed to tetragonal polycrystalline structure for films annealed at temperature of 623 K for 2 h as examined by XRD technique. The optical absorption spectra was measured for amorphous and polycrystalline films. A change from an indirect transition for the as-deposited film to direct transition was reported as the annealing temperature increases to 623 K due to the phase transformation. The degree of disorder was estimated from the absorption edge and shows a gradual decrease with raising the annealing temperature. The refractive index shows higher value for polycrystalline Zn3 P2 films due to more condensation in the material affected by the progress of annealing temperature of the deposited Zn3 P2 films. The electrical resistivity for Zn3 P2 films decreases with ascending the annealing temperature due to change in structure from amorphous to polycrystalline which is associated by decrease in the degree of disorder. This was confirmed by decreasing in degree of disorder referred by Eo linearly with the resistivity of the annealing films. References [1] M. Pawlikowski Janusz, J. Misiewicz, N. Mirowska, Journal of Physics and Chemistry of Solids 40 (1979) 1027–1033. [2] E.A. Fagen, Journal of Applied Physics 50 (1979) 6505–6520. [3] J. Loferski Joseph, Journal of Applied Physics 27 (1956) 777–785. [4] A.M. Barnett, Proc. European Communities Photovoltaic Solar Energy Conference, Reidel, Dordrecht, 1979, pp. 440–445. [5] G. Sberveglieri, N. Romeo, Thin Solid Films 83 (1981) L133–L136. [6] A. Catalano, V. Dalal, W.E. Deavancy, Zn3 P2 : a promising photovoltaic material, in: Proceedings of the 13th IEEE (PVSC), 1978, 288 pp. [7] Y. Kato, S. Kurita, T. Suda, Journal of Applied Physics 62 (1987) 3733–3740. [8] L.B. Fletcher, J.J. Witcher, N. Troy, S.T. Reis, R.K. Brow, D.M. Krol, Optical Express 19 (9) (2011) 7929–7936. [9] T. Suda, T. Miyakawa, S. Kurita, Journal of Crystal Growth 86 (1988) 423–429. [10] S. Sudhakar, K. Baskar, Journal of Crystal Growth 310 (2008) 2707–2711. [11] A. Mokhtari, Journal of Physics: Condensed Matter 21 (27) (2009) 2802. [12] M. Peiteado, T. Jardiel, F. Rubio, A.C. Caballero, Materials Research Bulletin 45 (2010) 1586–1592. [13] K. Kakishita, S. Ikeda, T. Suda, Journal of Crystal Growth 115 (1991) 793–797. [14] N. Convers Wyeth, A. Catalano, Journal of Applied Physics 50 (1979) 1403–1408. [15] T. Suda, K. Kakishta, Journal of Applied Physics 71 (1992) 3039–3042. [16] L. Bryja, K. Jezierski, J. Misiewicz, Thin Solid Films 229 (1993) 11–13. [17] T.L. Chu, S. Chu Shirley, K. Murthy, E.D. Stokes, P.E. Russell, Journal of Applied Physics 54 (1983) 2063–2069. [18] E. Papazoglou, T.W.F. Russell, Journal of Vacuum Science and Technology A 5 (1987) 3378–3383. [19] L. Bryja, M. Ciorga, K. Jezierski, A. Bohdziewicz, J. Misiewicz, Vacuum 50 (1998) 97–98. [20] S. Fuke, S. Kawarabayashi, K. Kuwahara, T. Imai, Journal of Applied Physics 60 (1986) 2368–2372. [21] A. Weber, P. Sutter, H. Von Kanel, Thin Solid Films 239 (1994) 205–210. [22] L. Bryja, K. Jezierski, M. Ciorga, A. Bohdziewicz, J. Misiewicz, Vacuum 50 (1–2) (1998) 5–7. [23] R. Sathyamoorthy, C. Sharmila, K. Natarajan, S. Velumani, Materials Characterization 58 (2007) 745–749. [24] D.J. Little, M. Ams, P. Dekker, G.D. Marshall, M.J. Withford, Journal of Applied Physics 108 (3) (2010) 033110. [25] J. Misiewicz, N. Mirowska, Z. Gumienny, Physica Status Solidi A 83 (1984) K51. [26] N. Mirowska, J. Misiewicz, Semiconductor Science and Technology 7 (11) (1992) 1332–1337.
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