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Effect of gamma irradiation on the optical properties of nano-crystalline InP thin films
Applied Surface Science 255 (2009) 9439–9443
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of gamma irradiation on the optical properties of nano-crystalline InP thin films M.M. El-Nahass, A.A.M. Farag *, F. Abd-El-Salam Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Cairo, 11757, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 November 2008 Received in revised form 19 July 2009 Accepted 20 July 2009 Available online 25 July 2009
Thin films of InP were prepared onto glass and quartz substrates using laser ablation technique. Some of the prepared films were irradiated using a 60Co g -ray source irradiation with a total dose of 100 kGy at room temperature. The as deposited and irradiated films were identified by scanning electron microscopy, SEM and X-ray diffraction, XRD. The SEM images have shown a nano-flower like structure for the as deposited films and influenced by the irradiation dose. The Optical characterizations of the as deposited and irradiated InP films were studied using spectrophotometric measurements of transmittance T(l) and reflectance, R(l) at normal incidence of light in the spectral range from 200 nm to 2500 nm. The refractive index, n, and the absorption index, k values were calculated using a modified computer program based on minimizing (DT)2 and (DR)2 simultaneously, within the desired accuracy. Analysis of the dispersion of the refractive index in the range 900 l 2500 was discussed in terms of the single oscillator model. The optical parameters, such as the dispersion energy, Ed, the oscillator energy, Eo, the high frequency dielectric constant, e1 and the lattice dielectric constant, eL were evaluated for the as deposited and irradiated films. The allowed optical transitions were found to be direct for the as deposited and irradiated films with energy gaps of 1.35 eV and 1.54 eV, respectively. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Irradiation InP Nanocrystalline Optical properties
1. Introduction InP is a material of considerable importance for fabrication of high electron mobility transistors [1], optical gratings [2], and semiconductor lasers [3]. The synthesis of InP nano-particles has been reported by various chemical methods [4–6] and their synthesis by in situ generation of phosphine gas has also been highlighted [7]. The use of phosphine gas and/or organo-metallic phosphine compounds as binary and single molecular sources for indium and phosphorus are reported to be highly effective for the synthesis but these reagents have limitations due to the high degree of toxicity. Organometallic indium precursors need special skills in handling through preparation of precursors and is often very difficult due to their sensitivity to moisture and air. Pulsed laser deposition (PLD) is a thin film deposition technique, which has been applied to a wide range of materials [8]. Growth of compound semiconductor films by PLD has been of interest due to its ability to preserve stoichiometry and the possibility of reactive deposition in a low-pressure background gas [9]. The main characteristic of interest in some solid state materials, such as InP is their sensitivity to ionizing radiation (i.e., g and X-rays,
b-particles, etc.), which constitute the largest source of background for a wide applications in various domains of science and technology, such as geological dating, environmental sciences (radon), life sciences (radiobiology, dosimetry, etc.), radiation hardness of the npn transistors as well as nuclear and astro-physics [10]. In particular, InP nanowires have been extensively investigated in recent years for their potential applications including photonics, nanoelectronics, and thermoelectrics [11]. Moreover flower-like structures were observed on surfaces irradiated with more than 103 pulses for InP [12]. Besides these experimental observations, optical characterizations of InP flower-like structure still remains a significant issue. To the best of our knowledge, the detailed studies on the effects of irradiation on the optical characterizations of InP prepared by laser ablation technique have not been extensively reported. The objective of the present work is to produce nano crystalline InP thin films by laser ablation technique and to investigate the influence of g-irradiation on their structural and optical properties. 2. Experimental procedures 2.1. Film preparation
* Corresponding author. Tel.: +20 0233 518705. E-mail address: [email protected] (A.A.M. Farag). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.051
InP films were deposited on clean glass and quartz substrates by laser ablation deposition technique. A high purity (99.999%) powder
M.M. El-Nahass et al. / Applied Surface Science 255 (2009) 9439–9443
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of InP was compressed and used as a target. A pulsed Nd:YAG laser(Model PL-7010, CA 95051), with a fundamental wavelength 1064 nm was used. The experimental set-up of the deposition system was described in a previous work [13]. Briefly, the substrate was placed in an evacuated chamber (residual pressure of 104 Pa) and the laser beam was focused onto a rotating target at an oblique angle of 458 to allow the plume expansion in the direction normal to the target surface. All the preparation processes have been carried out in National Institute of Laser Enhanced Science, Laser System Department, Cairo University, Egypt. 2.2. Irradiation process InP films were exposed to the gamma radiation from a 60Co source at a dose rate of 6.316 kGy/h using a gamma cell available in National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority (AEA), Cairo, Egypt. The irradiation process was performed in air, at room temperature, where a cooling system was used in the irradiation chamber to avoid heating of the samples during irradiation. Samples of InP films were irradiated at a total absorbed dose of 100 kGy because lower doses than which have no remarkable effect on the properties of InP and the higher doses can affect the stoichiometry of InP. 2.3. Structural measurements Scanning electron microscopy, SEM (Model JXA 8400, JEOL, Japan) was used to record the scanning electron micrograph images of the InP thin films. The structure of the as deposited and irradiated InP films was analyzed by using Philips X-ray diffraction (XRD) system (model X, Pert Pro) equipped with Cu target and a filter giving CuKa radiation (l = 1.5408 A˚). The X-ray tube voltage and current were 40 kV and 30 mA, respectively. The speed of the detector was 18 per min.
Fig. 1. (a) SEM image of the as deposited InP film, (b) SEM image of the irradiated InP film.
2.4. Optical measurements The measurements of transmittance, T(l), and reflectance, R(l), of the films deposited on fused quartz substrates were carried out using a double beam spectrophotometer (JASCO, V-570 UV-VISNIR), at normal incidence of light and in the wavelength range between 200 and 2500 nm. All the measurements were carried out at room temperature. If Ift and Iq are the intensities of light passing through the film-quartz system and through the reference quartz, respectively, then [14,15] I T exp ¼ ft ð1 Rq Þ; (1) Iq
in this method is carried out in two operations, the bi-variant search operation and the step-length optimization operation. The experimental errors were taken into account as follows: 2.2% for film thickness measurements, 1% for T and R calculations, 3% for refractive index and 2.5% for absorption index measurements.
where Rq is the reflectance of quartz. In addition, if the intensity of light reflected from the sample reaching the detector is Ifr and that reflected from the reference aluminum mirror is IAl, then I (2) Rexp ¼ fr RAl ½1 þ ð1 Rq Þ2 T 2 Rq IAl
The surface morphology of the as deposited and irradiated InP films was investigated by the scanning electron microscopy (SEM). SEM image of the as deposited InP thin films, Fig. 1a, shows clearly a flower-like structure morphology (which are so named due to their similarity in appearance to flower). The flowers are fundamentally composed of a number of nano round flakes in clusters. For the present research, the type of laser used and the pulse duration have an influence on the nucleation and growth of the crystals [12]. The phase with the lowest Gibbs free energy is thermodynamically stable, and has more chance to exist in the process and then reflect the product morphology [16,17]. The SEM image of the irradiated InP films, Fig. 1b, suggests that there are some morphological modifications/changes in the like flower structure after irradiation. Interesting features of surface morphology observed in SEM, actually exhibits a fine nanostructure with the same stoichiometry which could be attributed to the general roughness/grain boundary caused by swift heavy
From the experimentally determined values of Texp, Rexp and the film thickness d the values of the refractive index n and the extinction coefficient k were computed by a special computer program [14,15] based on minimizing (DT)2 and (DR)2 simultaneously, where: 2
ðDTÞ ¼ jT ðn;kÞ T exp j2 2
ðDRÞ ¼ jRðn;kÞ Rexp j2 ;
(3) (4)
where T(n,k) and R(n,k) are the calculated values of T and R, obtained using the well known Murmann’s equations. The search
3. Results and discussion 3.1. Surface morphological and nano-structural properties
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Fig. 2. X-ray diffraction pattern of the as deposited and irradiated InP film.
irradiation. The change in the grain structure can be attributed to the interfacial free energy and temperature induced by irradiation, which collapses the grain boundary [18] Fig. 2 shows the X-ray diffraction patterns, XRD for the as deposited and irradiated InP thin films on glass substrates. It is evident from the XRD pattern that the nature of InP is less crystalline, as contrast to the highly crystalline patterns obtained by Won Jun et al. [13]. The diffraction pattern is in good agreement with that of crystalline zinc-blend indium phosphide, exhibiting the major broad peak corresponding to the (1 1 1) plane. The peak is very broad for the irradiated films as compared to the as deposited one. This confirms the presence of fine or nano crystallites of InP in their matrix which may induce significant disorder in the InP film. The average particle sizes, using the Scherrer’s equation, are estimated to be a few nanometers for the as deposited and irradiated films. Though this does not seem to be so precise due to the error in measuring the half-widths of XRD peak but the average crystallite size supports the above results of the SEM. 3.2. Optical properties
Fig. 4. Dispersion curves of the refractive index n (l) for the as-deposited and irradiated films.
irradiation which results as an increase in the amorphous nature of the InP structure. The reflection spectrum of the as deposited films was found to be decrease as the wavelength increase up to 280 nm after which it increases with increasing wavelength till 500 nm. The spectral dependence of the reflection for the irradiated films shows two peaks lies in the wavelengths at 430 nm and 770 nm, respectively. These peaks may be attributed to the presence of two modes of oscillations as a result of the irradiation. The calculations of the optical constants n and k were performed for the as deposited and irradiated InP films. The dispersion curves shown in Fig. 4 represent normal dispersion at l 900 nm, in which a single oscillator model can be applied. At l < 900 nm, the anomalous dispersion and appearance of many peaks in the refractive index spectrum, especially for the irradiated films which can be interpreted by using the multi-oscillator model [19]. The dispersion of refractive index n(l) was analyzed using the concept of the single oscillator model and can be expressed by the Wemple-Didomenico relationship [20,21] as E0 Ed
The spectral behavior of the transmittance, T(l), and reflectance, R(l) for the as deposited and irradiated InP films of thickness 112 nm, as a representative example, in the spectral wavelength range of 200–2500 nm, are shown in Fig. 3. As observed, T(l) for the as deposited and irradiated InP films, in the absorbing region, increases with increasing wavelength up to 850 nm. The transmission for the irradiated films shows a step in the wavelength range 400–570 nm. This step may be attributed to the induced effect of
where E is the photon energy hy, Ed is the dispersion energy, a direct measure of the oscillator strength, which in turn is a measure of the probability of a given transition; and the effective oscillation energy E0 which is typically near the main peak of the imaginary part of dielectric function e2 and reflects the overall
Fig. 3. Spectral behavior of transmittance, T, and reflectance, R, for as-deposited and irradiated InP films.
Fig. 5. Dpendence of (n2 1) irradiated films.
n2 1 ¼
E20 E2
(5)
;
1
on photon energy (hn)2 for the as-deposited and
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Table 1 The calculated values of the dispersion parameters and energy gaps of the as deposited and irradiated InP films.
e1 As deposited InP Irradiated InP Ref. [23] Ref. [24] Ref. [25]
5.8 9.5 12.35 12.5 9.52
Ed (eV) 8.65 19.95 – – –
eL
E0 (eV)
E0/Eg
1.798
1.33
8.05
2.34 – – –
1.52 – – –
12.25 – – –
N/m* (cm3 g1)
Eg (eV)
9.85 1047
1.35
1.5 1048 – – –
1.54 1.35 1.34 1.344
band structure information. The parameter Ed is found dependent on the coordination number and atomic valency uniquely [22]. It is considered that the dispersion energy Ed that measures the average strength of inter-band optical transitions is associated with the changes in the structural order of the materials [22]. Fig. 5 shows the relation of (n2 1)1 against (hy)2 for the as deposited and irradiated InP films. At long wavelengths, a positive curvature deviation from linearity is observed due to the negative contribution of lattice vibrations to the refractive index. At short wavelengths, a negative curvature deviation is observed due to proximity of the band edge or excitonic absorption The values of E0 and Ed are directly determined from the slope, (E0Ed)1 and the intercept, (E0/Ed) of the linear fit portion. The calculated values of the dispersion parameters E0 and Ed, as well as the corresponding high frequency dielectric constant (e1 = n2) for the as deposited and irradiated films are listed in Table 1. It is noted that the high frequency dielectric constant of the as deposited and irradiated InP films are much lower than that of published in the literature [23–25]. This behavior may be attributed to the strong effect of the nano-like flower structure of InP on the dielectric property of InP [12,26]. The oscillator energy, E0 is, to a fair approximation, related empirically to the direct band gap, Eg by E0 1.5 Eg as deduced by Wemple and DiDominico [20] and was found to be applicable in our results as listed in Table 1 for the as deposited and irradiated InP films. Another parameters can be deduced from the relation as in [27] to obtain the lattice dielectric constant via a procedure that describes the contribution of the free carriers and the lattice vibration modes of the dispersion using the determined n2 = e values as
e ¼ eL
e2 N l2 4p2 eo c2 m
(6)
where eL is the lattice dielectric constant, c is the light speed, and N/ m* is the ratio of the carrier concentration to the electron effective
Fig. 7. The photon energy dependence of (aE) 2 for the as-deposited and irradiated films.
mass. The dependence of n2 on l2 is linear at longer wavelength for the as deposited and irradiated InP films as shown in Fig. 6. The lattice dielectric constant, eL and the ratio of carrier concentration to the effective mass for the as deposited and irradiated InP films are obtained from Fig. 6 and listed in Table 1. The absorption coefficient, a, was calculated from the average absorption index as a = 4pk/l. The films have a highest absorption coefficient >105 cm1 in the wavelength up to 800 nm. At a given wavelength in the absorption range, the absorption coefficient of the as deposited films is lower than that of the irradiated films. To determine the energy gap, Eg, and the type of optical transition, the absorption due to band-to-band transition can be described using the following relation [28]
ahy ¼ Aðhy Eg Þx
(7)
where x = 1/2 and 3/2 for the direct allowed and forbidden transitions, respectively, x = 2 and 3 for indirect allowed and forbidden transitions, respectively. The types of transition and the value of the optical energy gap are illustrated in Fig. 7. This figure shows a direct allowed transition with energy gaps for the as deposited and irradiated InP films listed in Table 1. As Observed the calculated energy gap for the as deposited InP films is in good agreement with the values deduced by other researchers [23–25]. Moreover, the increase in the value of the energy gap for the irradiated InP films than that of the as deposited films may be attributed to the effect of g-dose irradiation on the degree of crystallinity of InP [29]. Thus, the large influence of g-irradiation on the optical properties is related to the higher degree of disorder caused in the amorphous material [30,31]. The increase of structural disorder will consequently increase the number of localized states into the gap and hence the depth of localized states. This will, in turn, increase the transition probabilities through the localized states to the conduction band. According to the above discussion, the absorption coefficient increases, the absorption edge shifts to the lower energies and the width of tail increases. 4. Conclusion From the present work we concluded that:
Fig. 6. variation of n2 versus l2 for the as-deposited and irradiated films
InP thin films prepared by laser ablation technique have a flowerlike structure and exhibited a change in their surface morphology with influencing of g-irradiation of 100 kGy. The dispersion parameters of the films are affected by girradiation of 100 kGy. The g-irradiation leads to long-wavelength shift of the fundamental absorption edge which connected with the bond rearrangements of the atomic disorder in the investigated films.
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The g-irradiation stimulated the increase of absorption coefficient which can be used for industrial purposes. References [1] T. Sonoda, Y. Yamamoto, N. Hayafuji, H. Yoshida, H. Sasaki, T. Kitano, S. Takamiya, M. Ostubo, Solid-State Electron 41 (1997) 1621. [2] M. Ezaki, H. Kumagai, K. Toyoda, M. Obara, IEEE J. Sel. Top. Quant. Electron 1 (1995) 841. [3] Y. Kashima, T. Nozawa, T. Munakata, J. Cryst. Growth 204 (1999) 429. [4] M. Green, P.O. Brien, J. Mater. Chem. 14 (2004) 629. [5] J. Lee, V.C. Sunder, J.R. Heine, M.G. Bawendi, K.F. Jensen, Adv. Mater. 12 (2000) 102. [6] S. Gao, J. Lu, N. Chen, Y. Zhao, Y. Xie, Chem. Commun. 250 (2002) 3064. [7] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin films, Wiley, New York, 1994. [8] A. Borowiec, H.F. Tiedje, H.K. Haugen, Appl. Surf. Sci. 243 (2005) 129. [9] J.C. Miller, R.F. Haglund, Laser Ablation: and Deposition, Academic Press, London, 1998. [10] M. Fromm, F. Vaginay, D. Pusset, G. Meesen, A. Chambaudet, A. Poffijn, 3D Confocal microscopy of etched nuclear tracks CR-39, in: First International Workshop on Space Microscopy Radiation Research and 11th Annual NASA Space Radiation Health Investigators, Workshop Arona, Italy, 2000.
9443
[11] Kosuke Sano, Toru Akiyama, Kohji Nakamura, Tomonori Ito, J. Cryst. Growth 301– 302 (2007) 862. [12] H.X. Qian, W. Zhou, H.Y. Zheng, G.C. Lim, Surf. Sci. 595 (2005) 49. [13] A.M.A. El-Barry, M.M. El-Nahass, Optoelectron Adv. Mater. 1 (2007) 322. [14] M.M. El-Nahass, J. Mater. Sci. 27 (1992) 6592. [15] A.M. Bakry, A.H. El-Naggar, Thin Solid Films 360 (2000) 293. [16] K. Sopunna, T. Thongtem, M. McNallan, S. Thongtem, J. Mater. Sci. 41 (2006) 4654. [17] S. Biswas, S. Kar, S. Chaudhuri, J. Cryst. Growth 299 (2007) 94. [18] J. Molla, A. Ibaraa, Nucl. Instr. Meth. B 218 (2004) 189. [19] A. Stendal, U. Beckars, S. Wilbrand, O. Stenzel, C. Von Borczskowski, J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 2589. [20] S.H. Wemple, M. Didomenico, J. Phys. Rev. B 3 (1971) 1338. [21] S.H. Wemple, M. Didomenico, J. Phys. Rev. B 7 (1973) 3767. [22] M. Modreanu, P.K. Hurley et al., Proceedings of the SPIE, 4876 (2003) 1236. [23] Z. Maojun, Z. Lide, L.I. Guanghai, J. Zhi, Chin. Sci. Bull. 46 (2001) 1. [24] C. Zhang, B. Jin, J. Chen, P. Wu, J. Optical Soc. Am. B 26 (2009) A1. [25] R. Brazis, R. Raguotis, Ph. Moreau, M.R. Siegrist, Int J. Infrared Millimetre Waves 658 (2000) 1. [26] D. Yu-Wen, Z.Ru.L. Pei-Lin, Optoelectron. Lett. 5 (2009) 37. [27] M.M. El-Nahass, Z. El-Gohary, H.S. Soliman, Opt. Laser Technol. 35 (2003) 52. [28] M.P. Singh, C.S. Thakur, K. Shalini, S. Banerjee, N. Bhat, S.A. Shivashankar, J. Appl. Phys. 96 (2004) 5631. [29] S. Satyam, S. Parashari, S. Kumar, Auluck, Solid State Electron 52 (2008) 749. [30] R.A. Street, Solid State Commun. 24 (1977) 363. [31] M.I. Klinger, Solid State Commun. 45 (1983) 949.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of gamma irradiation on the optical properties of nano-crystalline InP thin films M.M. El-Nahass, A.A.M. Farag *, F. Abd-El-Salam Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Cairo, 11757, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 November 2008 Received in revised form 19 July 2009 Accepted 20 July 2009 Available online 25 July 2009
Thin films of InP were prepared onto glass and quartz substrates using laser ablation technique. Some of the prepared films were irradiated using a 60Co g -ray source irradiation with a total dose of 100 kGy at room temperature. The as deposited and irradiated films were identified by scanning electron microscopy, SEM and X-ray diffraction, XRD. The SEM images have shown a nano-flower like structure for the as deposited films and influenced by the irradiation dose. The Optical characterizations of the as deposited and irradiated InP films were studied using spectrophotometric measurements of transmittance T(l) and reflectance, R(l) at normal incidence of light in the spectral range from 200 nm to 2500 nm. The refractive index, n, and the absorption index, k values were calculated using a modified computer program based on minimizing (DT)2 and (DR)2 simultaneously, within the desired accuracy. Analysis of the dispersion of the refractive index in the range 900 l 2500 was discussed in terms of the single oscillator model. The optical parameters, such as the dispersion energy, Ed, the oscillator energy, Eo, the high frequency dielectric constant, e1 and the lattice dielectric constant, eL were evaluated for the as deposited and irradiated films. The allowed optical transitions were found to be direct for the as deposited and irradiated films with energy gaps of 1.35 eV and 1.54 eV, respectively. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Irradiation InP Nanocrystalline Optical properties
1. Introduction InP is a material of considerable importance for fabrication of high electron mobility transistors [1], optical gratings [2], and semiconductor lasers [3]. The synthesis of InP nano-particles has been reported by various chemical methods [4–6] and their synthesis by in situ generation of phosphine gas has also been highlighted [7]. The use of phosphine gas and/or organo-metallic phosphine compounds as binary and single molecular sources for indium and phosphorus are reported to be highly effective for the synthesis but these reagents have limitations due to the high degree of toxicity. Organometallic indium precursors need special skills in handling through preparation of precursors and is often very difficult due to their sensitivity to moisture and air. Pulsed laser deposition (PLD) is a thin film deposition technique, which has been applied to a wide range of materials [8]. Growth of compound semiconductor films by PLD has been of interest due to its ability to preserve stoichiometry and the possibility of reactive deposition in a low-pressure background gas [9]. The main characteristic of interest in some solid state materials, such as InP is their sensitivity to ionizing radiation (i.e., g and X-rays,
b-particles, etc.), which constitute the largest source of background for a wide applications in various domains of science and technology, such as geological dating, environmental sciences (radon), life sciences (radiobiology, dosimetry, etc.), radiation hardness of the npn transistors as well as nuclear and astro-physics [10]. In particular, InP nanowires have been extensively investigated in recent years for their potential applications including photonics, nanoelectronics, and thermoelectrics [11]. Moreover flower-like structures were observed on surfaces irradiated with more than 103 pulses for InP [12]. Besides these experimental observations, optical characterizations of InP flower-like structure still remains a significant issue. To the best of our knowledge, the detailed studies on the effects of irradiation on the optical characterizations of InP prepared by laser ablation technique have not been extensively reported. The objective of the present work is to produce nano crystalline InP thin films by laser ablation technique and to investigate the influence of g-irradiation on their structural and optical properties. 2. Experimental procedures 2.1. Film preparation
* Corresponding author. Tel.: +20 0233 518705. E-mail address: [email protected] (A.A.M. Farag). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.051
InP films were deposited on clean glass and quartz substrates by laser ablation deposition technique. A high purity (99.999%) powder
M.M. El-Nahass et al. / Applied Surface Science 255 (2009) 9439–9443
9440
of InP was compressed and used as a target. A pulsed Nd:YAG laser(Model PL-7010, CA 95051), with a fundamental wavelength 1064 nm was used. The experimental set-up of the deposition system was described in a previous work [13]. Briefly, the substrate was placed in an evacuated chamber (residual pressure of 104 Pa) and the laser beam was focused onto a rotating target at an oblique angle of 458 to allow the plume expansion in the direction normal to the target surface. All the preparation processes have been carried out in National Institute of Laser Enhanced Science, Laser System Department, Cairo University, Egypt. 2.2. Irradiation process InP films were exposed to the gamma radiation from a 60Co source at a dose rate of 6.316 kGy/h using a gamma cell available in National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority (AEA), Cairo, Egypt. The irradiation process was performed in air, at room temperature, where a cooling system was used in the irradiation chamber to avoid heating of the samples during irradiation. Samples of InP films were irradiated at a total absorbed dose of 100 kGy because lower doses than which have no remarkable effect on the properties of InP and the higher doses can affect the stoichiometry of InP. 2.3. Structural measurements Scanning electron microscopy, SEM (Model JXA 8400, JEOL, Japan) was used to record the scanning electron micrograph images of the InP thin films. The structure of the as deposited and irradiated InP films was analyzed by using Philips X-ray diffraction (XRD) system (model X, Pert Pro) equipped with Cu target and a filter giving CuKa radiation (l = 1.5408 A˚). The X-ray tube voltage and current were 40 kV and 30 mA, respectively. The speed of the detector was 18 per min.
Fig. 1. (a) SEM image of the as deposited InP film, (b) SEM image of the irradiated InP film.
2.4. Optical measurements The measurements of transmittance, T(l), and reflectance, R(l), of the films deposited on fused quartz substrates were carried out using a double beam spectrophotometer (JASCO, V-570 UV-VISNIR), at normal incidence of light and in the wavelength range between 200 and 2500 nm. All the measurements were carried out at room temperature. If Ift and Iq are the intensities of light passing through the film-quartz system and through the reference quartz, respectively, then [14,15] I T exp ¼ ft ð1 Rq Þ; (1) Iq
in this method is carried out in two operations, the bi-variant search operation and the step-length optimization operation. The experimental errors were taken into account as follows: 2.2% for film thickness measurements, 1% for T and R calculations, 3% for refractive index and 2.5% for absorption index measurements.
where Rq is the reflectance of quartz. In addition, if the intensity of light reflected from the sample reaching the detector is Ifr and that reflected from the reference aluminum mirror is IAl, then I (2) Rexp ¼ fr RAl ½1 þ ð1 Rq Þ2 T 2 Rq IAl
The surface morphology of the as deposited and irradiated InP films was investigated by the scanning electron microscopy (SEM). SEM image of the as deposited InP thin films, Fig. 1a, shows clearly a flower-like structure morphology (which are so named due to their similarity in appearance to flower). The flowers are fundamentally composed of a number of nano round flakes in clusters. For the present research, the type of laser used and the pulse duration have an influence on the nucleation and growth of the crystals [12]. The phase with the lowest Gibbs free energy is thermodynamically stable, and has more chance to exist in the process and then reflect the product morphology [16,17]. The SEM image of the irradiated InP films, Fig. 1b, suggests that there are some morphological modifications/changes in the like flower structure after irradiation. Interesting features of surface morphology observed in SEM, actually exhibits a fine nanostructure with the same stoichiometry which could be attributed to the general roughness/grain boundary caused by swift heavy
From the experimentally determined values of Texp, Rexp and the film thickness d the values of the refractive index n and the extinction coefficient k were computed by a special computer program [14,15] based on minimizing (DT)2 and (DR)2 simultaneously, where: 2
ðDTÞ ¼ jT ðn;kÞ T exp j2 2
ðDRÞ ¼ jRðn;kÞ Rexp j2 ;
(3) (4)
where T(n,k) and R(n,k) are the calculated values of T and R, obtained using the well known Murmann’s equations. The search
3. Results and discussion 3.1. Surface morphological and nano-structural properties
M.M. El-Nahass et al. / Applied Surface Science 255 (2009) 9439–9443
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Fig. 2. X-ray diffraction pattern of the as deposited and irradiated InP film.
irradiation. The change in the grain structure can be attributed to the interfacial free energy and temperature induced by irradiation, which collapses the grain boundary [18] Fig. 2 shows the X-ray diffraction patterns, XRD for the as deposited and irradiated InP thin films on glass substrates. It is evident from the XRD pattern that the nature of InP is less crystalline, as contrast to the highly crystalline patterns obtained by Won Jun et al. [13]. The diffraction pattern is in good agreement with that of crystalline zinc-blend indium phosphide, exhibiting the major broad peak corresponding to the (1 1 1) plane. The peak is very broad for the irradiated films as compared to the as deposited one. This confirms the presence of fine or nano crystallites of InP in their matrix which may induce significant disorder in the InP film. The average particle sizes, using the Scherrer’s equation, are estimated to be a few nanometers for the as deposited and irradiated films. Though this does not seem to be so precise due to the error in measuring the half-widths of XRD peak but the average crystallite size supports the above results of the SEM. 3.2. Optical properties
Fig. 4. Dispersion curves of the refractive index n (l) for the as-deposited and irradiated films.
irradiation which results as an increase in the amorphous nature of the InP structure. The reflection spectrum of the as deposited films was found to be decrease as the wavelength increase up to 280 nm after which it increases with increasing wavelength till 500 nm. The spectral dependence of the reflection for the irradiated films shows two peaks lies in the wavelengths at 430 nm and 770 nm, respectively. These peaks may be attributed to the presence of two modes of oscillations as a result of the irradiation. The calculations of the optical constants n and k were performed for the as deposited and irradiated InP films. The dispersion curves shown in Fig. 4 represent normal dispersion at l 900 nm, in which a single oscillator model can be applied. At l < 900 nm, the anomalous dispersion and appearance of many peaks in the refractive index spectrum, especially for the irradiated films which can be interpreted by using the multi-oscillator model [19]. The dispersion of refractive index n(l) was analyzed using the concept of the single oscillator model and can be expressed by the Wemple-Didomenico relationship [20,21] as E0 Ed
The spectral behavior of the transmittance, T(l), and reflectance, R(l) for the as deposited and irradiated InP films of thickness 112 nm, as a representative example, in the spectral wavelength range of 200–2500 nm, are shown in Fig. 3. As observed, T(l) for the as deposited and irradiated InP films, in the absorbing region, increases with increasing wavelength up to 850 nm. The transmission for the irradiated films shows a step in the wavelength range 400–570 nm. This step may be attributed to the induced effect of
where E is the photon energy hy, Ed is the dispersion energy, a direct measure of the oscillator strength, which in turn is a measure of the probability of a given transition; and the effective oscillation energy E0 which is typically near the main peak of the imaginary part of dielectric function e2 and reflects the overall
Fig. 3. Spectral behavior of transmittance, T, and reflectance, R, for as-deposited and irradiated InP films.
Fig. 5. Dpendence of (n2 1) irradiated films.
n2 1 ¼
E20 E2
(5)
;
1
on photon energy (hn)2 for the as-deposited and
M.M. El-Nahass et al. / Applied Surface Science 255 (2009) 9439–9443
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Table 1 The calculated values of the dispersion parameters and energy gaps of the as deposited and irradiated InP films.
e1 As deposited InP Irradiated InP Ref. [23] Ref. [24] Ref. [25]
5.8 9.5 12.35 12.5 9.52
Ed (eV) 8.65 19.95 – – –
eL
E0 (eV)
E0/Eg
1.798
1.33
8.05
2.34 – – –
1.52 – – –
12.25 – – –
N/m* (cm3 g1)
Eg (eV)
9.85 1047
1.35
1.5 1048 – – –
1.54 1.35 1.34 1.344
band structure information. The parameter Ed is found dependent on the coordination number and atomic valency uniquely [22]. It is considered that the dispersion energy Ed that measures the average strength of inter-band optical transitions is associated with the changes in the structural order of the materials [22]. Fig. 5 shows the relation of (n2 1)1 against (hy)2 for the as deposited and irradiated InP films. At long wavelengths, a positive curvature deviation from linearity is observed due to the negative contribution of lattice vibrations to the refractive index. At short wavelengths, a negative curvature deviation is observed due to proximity of the band edge or excitonic absorption The values of E0 and Ed are directly determined from the slope, (E0Ed)1 and the intercept, (E0/Ed) of the linear fit portion. The calculated values of the dispersion parameters E0 and Ed, as well as the corresponding high frequency dielectric constant (e1 = n2) for the as deposited and irradiated films are listed in Table 1. It is noted that the high frequency dielectric constant of the as deposited and irradiated InP films are much lower than that of published in the literature [23–25]. This behavior may be attributed to the strong effect of the nano-like flower structure of InP on the dielectric property of InP [12,26]. The oscillator energy, E0 is, to a fair approximation, related empirically to the direct band gap, Eg by E0 1.5 Eg as deduced by Wemple and DiDominico [20] and was found to be applicable in our results as listed in Table 1 for the as deposited and irradiated InP films. Another parameters can be deduced from the relation as in [27] to obtain the lattice dielectric constant via a procedure that describes the contribution of the free carriers and the lattice vibration modes of the dispersion using the determined n2 = e values as
e ¼ eL
e2 N l2 4p2 eo c2 m
(6)
where eL is the lattice dielectric constant, c is the light speed, and N/ m* is the ratio of the carrier concentration to the electron effective
Fig. 7. The photon energy dependence of (aE) 2 for the as-deposited and irradiated films.
mass. The dependence of n2 on l2 is linear at longer wavelength for the as deposited and irradiated InP films as shown in Fig. 6. The lattice dielectric constant, eL and the ratio of carrier concentration to the effective mass for the as deposited and irradiated InP films are obtained from Fig. 6 and listed in Table 1. The absorption coefficient, a, was calculated from the average absorption index as a = 4pk/l. The films have a highest absorption coefficient >105 cm1 in the wavelength up to 800 nm. At a given wavelength in the absorption range, the absorption coefficient of the as deposited films is lower than that of the irradiated films. To determine the energy gap, Eg, and the type of optical transition, the absorption due to band-to-band transition can be described using the following relation [28]
ahy ¼ Aðhy Eg Þx
(7)
where x = 1/2 and 3/2 for the direct allowed and forbidden transitions, respectively, x = 2 and 3 for indirect allowed and forbidden transitions, respectively. The types of transition and the value of the optical energy gap are illustrated in Fig. 7. This figure shows a direct allowed transition with energy gaps for the as deposited and irradiated InP films listed in Table 1. As Observed the calculated energy gap for the as deposited InP films is in good agreement with the values deduced by other researchers [23–25]. Moreover, the increase in the value of the energy gap for the irradiated InP films than that of the as deposited films may be attributed to the effect of g-dose irradiation on the degree of crystallinity of InP [29]. Thus, the large influence of g-irradiation on the optical properties is related to the higher degree of disorder caused in the amorphous material [30,31]. The increase of structural disorder will consequently increase the number of localized states into the gap and hence the depth of localized states. This will, in turn, increase the transition probabilities through the localized states to the conduction band. According to the above discussion, the absorption coefficient increases, the absorption edge shifts to the lower energies and the width of tail increases. 4. Conclusion From the present work we concluded that:
Fig. 6. variation of n2 versus l2 for the as-deposited and irradiated films
InP thin films prepared by laser ablation technique have a flowerlike structure and exhibited a change in their surface morphology with influencing of g-irradiation of 100 kGy. The dispersion parameters of the films are affected by girradiation of 100 kGy. The g-irradiation leads to long-wavelength shift of the fundamental absorption edge which connected with the bond rearrangements of the atomic disorder in the investigated films.
M.M. El-Nahass et al. / Applied Surface Science 255 (2009) 9439–9443
The g-irradiation stimulated the increase of absorption coefficient which can be used for industrial purposes. References [1] T. Sonoda, Y. Yamamoto, N. Hayafuji, H. Yoshida, H. Sasaki, T. Kitano, S. Takamiya, M. Ostubo, Solid-State Electron 41 (1997) 1621. [2] M. Ezaki, H. Kumagai, K. Toyoda, M. Obara, IEEE J. Sel. Top. Quant. Electron 1 (1995) 841. [3] Y. Kashima, T. Nozawa, T. Munakata, J. Cryst. Growth 204 (1999) 429. [4] M. Green, P.O. Brien, J. Mater. Chem. 14 (2004) 629. [5] J. Lee, V.C. Sunder, J.R. Heine, M.G. Bawendi, K.F. Jensen, Adv. Mater. 12 (2000) 102. [6] S. Gao, J. Lu, N. Chen, Y. Zhao, Y. Xie, Chem. Commun. 250 (2002) 3064. [7] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin films, Wiley, New York, 1994. [8] A. Borowiec, H.F. Tiedje, H.K. Haugen, Appl. Surf. Sci. 243 (2005) 129. [9] J.C. Miller, R.F. Haglund, Laser Ablation: and Deposition, Academic Press, London, 1998. [10] M. Fromm, F. Vaginay, D. Pusset, G. Meesen, A. Chambaudet, A. Poffijn, 3D Confocal microscopy of etched nuclear tracks CR-39, in: First International Workshop on Space Microscopy Radiation Research and 11th Annual NASA Space Radiation Health Investigators, Workshop Arona, Italy, 2000.
9443
[11] Kosuke Sano, Toru Akiyama, Kohji Nakamura, Tomonori Ito, J. Cryst. Growth 301– 302 (2007) 862. [12] H.X. Qian, W. Zhou, H.Y. Zheng, G.C. Lim, Surf. Sci. 595 (2005) 49. [13] A.M.A. El-Barry, M.M. El-Nahass, Optoelectron Adv. Mater. 1 (2007) 322. [14] M.M. El-Nahass, J. Mater. Sci. 27 (1992) 6592. [15] A.M. Bakry, A.H. El-Naggar, Thin Solid Films 360 (2000) 293. [16] K. Sopunna, T. Thongtem, M. McNallan, S. Thongtem, J. Mater. Sci. 41 (2006) 4654. [17] S. Biswas, S. Kar, S. Chaudhuri, J. Cryst. Growth 299 (2007) 94. [18] J. Molla, A. Ibaraa, Nucl. Instr. Meth. B 218 (2004) 189. [19] A. Stendal, U. Beckars, S. Wilbrand, O. Stenzel, C. Von Borczskowski, J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 2589. [20] S.H. Wemple, M. Didomenico, J. Phys. Rev. B 3 (1971) 1338. [21] S.H. Wemple, M. Didomenico, J. Phys. Rev. B 7 (1973) 3767. [22] M. Modreanu, P.K. Hurley et al., Proceedings of the SPIE, 4876 (2003) 1236. [23] Z. Maojun, Z. Lide, L.I. Guanghai, J. Zhi, Chin. Sci. Bull. 46 (2001) 1. [24] C. Zhang, B. Jin, J. Chen, P. Wu, J. Optical Soc. Am. B 26 (2009) A1. [25] R. Brazis, R. Raguotis, Ph. Moreau, M.R. Siegrist, Int J. Infrared Millimetre Waves 658 (2000) 1. [26] D. Yu-Wen, Z.Ru.L. Pei-Lin, Optoelectron. Lett. 5 (2009) 37. [27] M.M. El-Nahass, Z. El-Gohary, H.S. Soliman, Opt. Laser Technol. 35 (2003) 52. [28] M.P. Singh, C.S. Thakur, K. Shalini, S. Banerjee, N. Bhat, S.A. Shivashankar, J. Appl. Phys. 96 (2004) 5631. [29] S. Satyam, S. Parashari, S. Kumar, Auluck, Solid State Electron 52 (2008) 749. [30] R.A. Street, Solid State Commun. 24 (1977) 363. [31] M.I. Klinger, Solid State Commun. 45 (1983) 949.