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Crystallization from amorphous structure to hexagonal quantum dots induced by an electron beam on CdTe thin films
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1245–1249
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystallization from amorphous structure to hexagonal quantum dots induced by an electron beam on CdTe thin films M. Becerril a,, O. Zelaya-Angel a, A.C. Medina-Torres b, J.R. Aguilar-Herna´ndez b, R. Ramı´rez-Bon c, F.J. Espinoza-Beltran c a
´n y de Estudios, Avanzados del IPN, Apartado Postal 14-740, Me´xico D.F. 07360, Mexico Departamento de Fı´sica, Centro de Investigacio ´ticas, Instituto Polite´cnico Nacional, Me ´xico D.F. 07738, Mexico Escuela Superior de Fı´sica y Matema c ´n y de Estudios, Avanzados del IPN, Unidad Quere ´taro, Apartado Postal 1-798, Quere ´taro, Qro. 76001, Mexico Centro de Investigacio b
a r t i c l e in fo
abstract
Article history: Received 28 November 2007 Received in revised form 5 December 2008 Accepted 20 December 2008 Communicated by K.H. Ploog Available online 13 January 2009
Amorphous cadmium–telluride films were prepared by rf sputtering on Corning 7059 glass substrates at room temperature. The deposition time was 10 and 12 h with a thickness of 400 and 480 (740 nm), respectively. As-prepared films were amorphous according to X-ray diffraction (XRD) patterns, but a win-fit-software analysis of the main XRD broad band suggests a wurtzite structure at short range. Transmission electron microscopy (TEM) at 200 keV produces crystallization of the amorphous CdTe. The TEM-electron beam induces the formation of CdTe quantum dots with the wurtzite hexagonal structure (the metastable structure of CdTe) and with 6 nm of average grain size. As effect of a probable distortion of the CdTe crystalline lattice, the unit cell volume (UCV) shrinks to about 30% with respect to the bulk-UCV of CdTe. Besides, the energy band gap increases as expected, according to literature data on quantum confinement. & 2009 Elsevier B.V. All rights reserved.
PACS: 63.37.Lp 68.65.Hb 71.70.Ej 78.30.Fs Keywords: A1. Crystal morphology A1. Nanostructures A1. X-ray diffractions B2. Semiconducting II–VI materials
1. Introduction Nowadays, semiconductor nanostructures are a research area of increasing interest because of the novel phenomena, which have been observed on these physical systems. Typically, the exciton Bohr radius in many semiconductor materials, such as II–VI compounds, is about a few nanometers, thus, the reduction in the size of semiconductor particles to nanometric dimensions yields to interesting quantum effects due to the confinement regime of excitons. The properties of nanocrystalline semiconductors in the quantum confinement regime undergo strong modifications, for example, it has been observed a strong blue-shift in the energy band gap as the particle size decreases in many types of semiconductors. Nanocrystalline CdTe has been prepared by employing different techniques such as sputtering [1–3], ball-milling [4], molecular beam epitaxy (MBE) [5] and dielectric-CdTe-comelting [6–8], among others. The technique
Corresponding author. Tel. +52 55 5061 3800; fax: +52 55 5747 7097.
E-mail address: becerril@fis.cinvestav.mx (M. Becerril). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.12.056
of rf sputtering has been successfully employed to grow size-controlled CdTe quantum dots [9]. It has also been fruitful to deposit amorphous CdTe films [10]. Transmission electron microscopy (TEM) is one of the most widely employed experimental techniques for the characterization of nanostructured systems, because it can provide directly the size of the nanoparticles [11]. Some other techniques, such as X-ray diffraction (XRD) and Raman spectroscopy, can also provide indirectly the size of nanoparticles by using the full-width at half-maximum (FWHM) of the peaks of XRD patterns and/or the FWHM of the bands of Raman spectra, while in the last case, after suitable corrections [12]. However, the exposure to the energetic TEM electron beam can modify amorphous or nanocrystalline structures of solid materials. Metallic [13] and semiconducting [14] thin films increase their average nanoparticle size as a consequence of the incident electron beam during TEM measurements. Crystallization of amorphous materials because of TEM-electron beam has been observed by other authors [15–18]. The aim of this work is to analyze the modification that amorphous CdTe thin films experiences under a TEM electronbeam exposure.
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M. Becerril et al. / Journal of Crystal Growth 311 (2009) 1245–1249
2. Experimental procedure By utilizing a magnetron sputtering system and argon as working gas, amorphous CdTe films were grown on 7059 Corning glass substrates at room temperature (RT) during 10 and 12 h. The rf power used in the grown process was 40 W and the distance between the target and substrate was 4 cm. High-purity argon gas was used to obtain the sputtering plasma. The initial pressure in the chamber was 1 105 Torr, and the final pressure of the argon plasma was about 103 Torr. The CdTe (99.999% pure) target had an area of 4.92 cm2. The thickness, as measured by means of a Dektak II profilometer, was 400 and 480 nm (740 nm), which were denoted by A1 and A2, respectively. The slow rate of the growing process (4074 nm/h) could influence the amorphization of CdTe films. When we have used a deposition rate in the range 50–65 nm/h the CdTe films grow into polycrystalline structure with the grain size in the interval 40–65 nm [19,20]. Auger mass spectroscopy measurements, achieved employing an ESCA-SAM PHI-560 system, indicated (49.770.5)% Te and (50.370.5)% Cd as atomic concentrations. No impurities were detected with the resolution limit level of the Auger spectroscopy. X-ray diffraction patterns were obtained through Siemens D5000 diffractometer, with the CuKa line. TEM apparatus employed was a Jeol 2010 working at 200 keV. CdTe layers were detached from the substrate and attached onto the TEM-grills. For the XRD broad bands deconvolution and the grain size calculation of the Win-Fit software was used. For Raman spectra registration a Labram-Dilor system was used and for the Raman band line shape fitting the MathCad program was run. UV–vis absorption spectra measurements were performed in a Unicam 8700 spectrometer system over the 190–900 nm wavelength range. Energy band gap (Eg) measurements were carried out by means of optical absorption spectra by using the Tauc’s formula for amorphous semiconductors: (ahn)p(hnEg)m [10–21], where a is the absorption coefficient, h is the Planck’s constant, n is the frequency of the light, and the exponent m takes the values 12 and 2 for allowed direct and indirect transitions [10–21]. Actually, as the absorption coefficient is proportional to the optical density (O.D.), one can directly plot the (O.D. hn)m versus the photon energy hn to get Eg.
Fig. 1. Raman spectrum of A1 as-grown CdTe film. The inset exhibits the normalized band at 166 cm1 (points) and the fitting (solid line) for grain size determination.
3. Results Raman spectrum of the as-grown CdTe layer is shown in Fig. 1, where the longitudinal optical (LO) mode at the frequency of 166 cm1, with two phonon replicas, characteristic of the cubic CdTe lattice [22], is observed. The broadening of the Raman bands reveals the grade of disorder present in the structure of the material. The diffractogram of the as-grown CdTe film is displayed in Fig. 2, which shows the two typical broad bands of the XRD pattern of an amorphous semiconductor material (at 2y~251 and 421, respectively). The recorded XRD signal also evidences an unusual small shoulder at 2yffi15.51. This third shoulder precisely ensures that the diffractogram is not an XRD pattern of a complete amorphous material, because of XRD spectra of amorphous materials only exhibit two broad bands. Deconvolution of the XRD pattern in Gaussian curves makes clear the presence of nine broad bands. As established, the envelope curve reproduces the experimental pattern. Besides, the broad band centered at 2y=15.52, which could not be associated to any crystalline plane of the different structural phases of CdTe, neither with planes of elemental Cd nor Te, the remaining eight Gaussian curves can be identified with the XRD peaks of the hexagonal wurtzite (W) crystalline structure of CdTe [23]. Evidently, there are some differences in positions because of, in practice, the amorphous
Fig. 2. XRD pattern of A1 as-grown CdTe amorphous film. Noisy line represents the experimental data. Solid continuous smooth lines represent the envelop of the deconvolution process applied to the pattern. Straight line is the zero reference line.
structural character. However, peaks belonging to cubic zincblende (ZB) CdTe do not reproduce the whole pattern. After all, the presence of a component of the ZB-CdTe phase cannot be discarded. This result is not in opposition to the Raman measurements of Fig. 1 whose phonon mode has been associated with the LO mode of CdTe in cubic phase, because the LO modes of ZB and W phases of CdTe can coincide at the same frequency, in a similar way as reported for CdS [24]. Until now the Raman spectra of hexagonal CdTe has not been reported.
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Since the interplanar distance (dl) corresponding to planes located at 2y=15.521 is 5.70 A˚ and the interplanar distance (dc) to the position of the central band is 2.80 A˚, i.e., dlffi2dc, one can associate this particular feature with some particular crystalline structure of CdTe existing at short-range distance into the bulk of the film. The most common crystalline structures of CdTe are the stable ZB and the metastable W phases. Actually, the interplanar distances along the (0 0 0 1) and (111) directions, the staking directions in both phases, are 3.750 and 3.742 A˚ [23] for W and ZB crystalline lattices of CdTe, respectively. Both are larger than 3.72 A˚ found for planes associated with the corresponding Gaussian band at 2y=23.86 of Fig. 2. As it will be pointed out below, the crystalline lattice of the sputtered CdTe films as a whole undergoes a shrinkage owing, with good probability, to a distortion of the crystalline entities (‘‘grains’’ in the amorphous layer). The unit cell of the ZB-CdTe involves three (111)interplanar distances, and the unit cell of W-CdTe involves only two (0 0 0 1) interplanar distances. In some way, the XRD band at 2y=15.521 represents the diffraction of planes of a special polytype of CdTe. Applying the Debye–Scherrer formula on the FWHM of the Gaussians bands of the deconvoluted XRD pattern of Fig. 2, an average diameter of 2.670.7 nm for the grains was calculated. From the dependence of the FWHM of the Raman bands as a function of the inverse grain size [25], the particle size of asgrown CdTe samples can be calculated from the fitting of the normalized band of Fig. 1. The inset shows the fit used here with a prediction of a particle size of 2.070.2 nm, which, within the error scattering data, is in agreement with the results obtained from XRD measurements. A picture of the TEM-magnification can be observed in Fig. 3. The existence of crystalline structures of nanometric dimensions is evident. The statistical analysis on the size of the crystalline nanostructures measured directly on the picture yields an average value of 6.071.5 nm. The experimental result evidences that the effect of the electron-beam exposure on the film during the TEM measurement process were: firstly, the crystallization of the amorphous material; and, secondly, the increase of the grain size. Fig. 4 displays the TEM diffraction of a CdTe layer. Obviously, the film has lost the amorphous character. The well-defined rings of the pattern undoubtedly demonstrate the polycrystalline nature of the film, with nanometric dots in W structure. Table 1 lists the data, calculated from the TEM diffractogram, of the
Fig. 3. TEM picture taken at 200 keV of the A1 sample detached from the glass substrate.
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Fig. 4. TEM diffraction pattern obtained at 200 keV of the A1 sample.
Table 1 Accepted values of 2y-positions and dhkl interplanar distances for bulk-CdTe and those obtained here for CdTe films. hkl
2y (XRD cards19) (A˚)
100 002
22.319 23.707
101 102 110 103 200 201 202
25.281 32.716 39.222 42.717 45.425 47.331 52.068
2y (films)
dhkl (XRD cards19) (A˚)
dhkl (films)
23:720
3.980 3.750
3:700
32.77 41.93 45.17 50.26 54.87 58.21
3.520 2.735 2.295 2.115 1.995 1.919 1.755
2.730 2.153 2.01 1.81 1.672 1.584
2y-positions and the interplanar distances (dhkl) of the nanocrystallized lattice. The first ring in Fig. 4 includes the initial three rings of the wurtzite structure, which could not be resolved with our TEM equipment. Experimental data of the diameter of these three rings are merged into an only average value, indicated in Table 1 with superlined figures. The 2y values calculated from the TEM diffractograms are larger than accepted data. Consequently, from the Bragg condition, experimental dhkl values result smaller. In Table 1 it can be checked out how dhkl decreases continuously as Miller indexes go to higher values. This phenomenon can be probably ascribed to some distortion of the CdTe lattice. The unit cell volume (UCV) of the lattice, considered as a hexagonal structure, after an average of the unit cell parameters a and c obtained from the dhkl values, 11578 A˚3. The UCV of bulk-CdTe, calculated from the accepted a=4.58 A˚ and c=7.50 A˚ lattice parameters values [23], is 157.33 A˚3. The UCV in the CdTe film has been reduced approximately 30% with respect to UCV of bulk-CdTe. With high probability, the compressive stress exerted by the amorphous substrate on the as-grown film induced the shrinkage of the lattice, which could not be completely relaxed by the incident energetic electrons, i.e., the
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Fig. 6. Optical transmission (T) and reflection (R) versus l of the as-grown CdTe sample. The inset displays the first derivative of T with respect to l, as a function of l.
Fig. 5. Optical absorption of as-grown CdTe film. The inset exhibits the determination of (a) the direct band gap energy and (b) the indirect one.
‘‘TEM-annealing’’, and the lattice stayed with reduced dimensions and probably distorted. From the optical absorption spectra the Eg values of as-grown films were determined. Fig. 5 shows the optical absorption as a function of the photon wavelength (l) measurements in the CdTe film. The insets exhibit the (O.D. hn)m versus hn plots when: (a) m=2 for direct transitions and (b) m ¼ 12 for indirect transitions. The extrapolation of the linear part of the curves to intercept the hn axis gives the Eg values, which equal 1.9570.04 eV for the direct Eg and 1.6770.03 eV for the indirect one. The direct Eg value coincides with experimental results reported by Potter and Simmons [1] for sputtered ZB-CdTe nanocrystals of 2.5 nm diameter, where the increase of Eg was assigned to quantum confinement effects. We suppose both ZB and W phases in CdTe have approximately the same values of Eg, in a similar way to CdS [26]. Indirect band gaps in CdTe polycrystalline films have been observed in amorphous and polycrystalline CdTe thin films by Marafi et al. [10]. In Fig. 6 optical transmission (T) and reflection (R) versus l the as-grown CdTe films are plotted. The T spectrum evidences how the layer starts showing significant transmission, since hnffi3.1 eV (l=400 nm), and R indicates that there is very low reflection in the entire visible spectrum. The inset displays the first derivative of T with respect to l as a function of l. Two features arisen in this spectrum at 2.0070.05 and 2.8870.05 eV, which can be associated with Eg and the first DEg i.e. here separated by 0.88710 eV from Eg. DEg due to the spin–orbit (Dso) unfolding (the G7vG6c transition) has the value of 0.8770.08 eV in bulk-ZB-CdTe [22,27]. In non-cubic lattices this unfolding should include, besides Dso, the crystal-field interaction (Dcf). In case of DEg value found for W-CdTe, its value is practically the
Fig. 7. XRD pattern of A2 as-grown CdTe amorphous film.
same value of Dso for ZB-CdTe. This result means that Dcf in W-CdTe has a small value (lies within the error bar, i.e., 0.10 eV). Dcf equals 0.028 and 0.040 eV in CdS and CdSe, respectively [22]. One can realize that the quantum confinement effect, as expected, shifts Eg and DEg around the same value (0.9 eV) than in bulk CdTe. The narrow feature at l=660 in Fig. 6 nm has been attributed here to an artifact, since it appears in the T and R spectra. To demonstrate the reproducibility of the crystallization from amorphous structure to hexagonal quantum dots induced by an electron beam on CdTe thin film, second sample was studied. Fig. 7 shows the XRD pattern of the A2 as-grown sample. The diffractogram is the typical amorphous material. A picture of the TEM magnification of this sample can be observed in Fig. 8. A statistical analysis of the particle size measured directly on the picture yield an average value of 10.072.5 nm. The inset displays
ARTICLE IN PRESS M. Becerril et al. / Journal of Crystal Growth 311 (2009) 1245–1249
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because of the relative high energy of the electron beam. Cautions must be taken in order to determine the actual size of nanocrystals by means of TEM studies.
Acknowledgements The authors thank to Ings. N. Castillo, M. Guerrero, Z. Rivera and A. B. Soto for their technical assistance. This work was partially supported by the CONACyT—Me´xico.
References
Fig. 8. TEM picture taken at 200 keV of the A2 sample detached from the glass substrate. The inset displays TEM diffraction pattern obtained at 200 keV of the same sample.
TEM diffraction pattern obtained at 200 keV of the same sample. The well-defined rings in the pattern undoubtedly demonstrate the polycrystalline nature of the films. The experimental results evidences that the effects of the electron-beam exposure on the films during the TEM measurement process were: first, the nanocrystallization of the amorphous material; and second, the increase of the grain size from amorphous to nanocrystalline films. The Raman and optical absorption data of the A2 sample is similar to that of the A1 sample. Hexagonal CdTe nanocrystals have been produced by Lefevre et al. [4] by utilizing the mechanical milling on CdTe powder. Strains generated inside the nanoparticles were the main logical cause claimed for the origin of the metastable W structure. Even though in case nanocrystals are not free, the influence of the amorphous substrate can be the responsible factor for strains arising in CdTe quantum dots, and hence the hexagonal structure origin. Grain growth of nanocrystals after TEM-annealing from 50 keV onwards has been observed in GaAs by Jencˇicˇ et al. [14]. They assumed that the electron beam provokes a fast thermal annealing on the thin samples, which modified the original size of nanocrystals. It is important to mention that ‘‘burning’’ of films of several types of materials produced by TEM exposure, has been observed by us.
4. Conclusions We have demonstrated how TEM alters the CdTe either amorphous or nanocrystalline structure of the samples studied
[1] B.G. Potter, J.H. Simmons, J. Appl. Phys. 68 (1990) 1218. [2] K. Tsunemoto, H. Nasu, H. Kitayama, A. Kawabuchi, Y. Osaka, K. Takiyama, Jpn. Appl. Phys. 28 (1990) 1928. [3] H. Arizpe-Cha´vez, R. Ramı´rez-Bon, F.J. Espinoza-Beltra´n, O. Zelaya-Angel, J.L. Marı´n, R. Riera, J. Phys. Chem. Solids 61 (2000) 511. [4] P. Lefevre, T. Richard, J. Alle`gre, H. Mathieu, A. Combette-Roos, W. Granier, Phys. Rev. B 53 (1996) 15440. [5] Y. Terai, S. Kuroda, K. Takita, T. Okuno, Y. Masumoto, Appl. Phys. Lett. 73 (1998) 3757. [6] V. Esch, B. Fluegel, G. Khitrova, H.M. Gibbs, X. Jiajin, K. Kang, S.W. Koch, L.C. Liu, S.H. Risbud, N. Peyghambarian, Phys. Rev. B 42 (1990) 7450. [7] R. Ramı´rez-Bon, F.J. Espinoza-Beltra´n, H. Arizpe-Cha´vez, O. Zelaya-Angel, F. Sa´nchez-Sinencio, J. Appl. Phys. 77 (1995) 5461. [8] I.V. Bodnar, V.S. Gurin, A.P. Molochko, N.P. Solovei, P.V. Prokoshin, K.V. Yumashev, Semiconductors 36 (2002) 298. [9] S.K. Mandal, S. Chauduri, A.K. Pal, Thin Solid Films 357 (1999) 102. [10] M. Marafi, F. El Akkad, B. Pradeep, J. Mater. Sci. Mater. Electron. X 21 (2003). [11] A. Ekimov, J. Lumines, Phys. Solid State 70 (1996) 1479. [12] W.H. Li, S.W. Wu, C.C. Yang, S.K. Lai, K.C. Lee, H.L. Huang, H.D. Yang, Phys. Rev. Lett. 89 (2002) 135504. [13] R. Dannenberg, E. Stach, J.R. Groza, B.J. Dresser, Thin Solid Films 379 (2000) 133. [14] I. Jencˇicˇ, M.W. Bench, I.M. Robertson, M.A. Kirk, J. Appl. Phys. 78 (1995) 974. [15] A.G. Fitzgerald, C.P. McHardy, J. Mater. Sci. Lett. 3 (1984) 91. [16] M. Kaiser, L. van Pieterson, M.A. Verheijen, J. Appl. Phys. 96 (2004) 3193. [17] J.S. Romero, A.G. Fitzgerald, M.J. Rose, Appl. Surf. Sci. 234 (2004) 369. [18] Y. Zhang, C.M. Wang, M.H. Engelhard, W.J. Weber, J. Appl. Phys. 100 (2006) 113533. [19] O. Alvarez-Fregoso, O. Zelaya-Angel, J.G. Mendoza-Alvarez, F. SanchezSinencio, M.H. Farı´as, L. Cota-Araiza, J. Phys. Chem. Solids 56 (1995) 117. [20] H. Arizpe-Chavez, F.J. Espinoza-Beltran, R. Ramirez-Bon, O. Zelaya-Angel, J.G. Gonza´lez-Herna´ndez, Solid State Commun. 101 (1997) 39. [21] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903. [22] O. Madelung, M. Schulz, H. Weiss (Eds.), Landolt–Bo¨rnstein New Series. Numerical Data and Functional Relationships in Science and Technology, vol. III/17b, Springer, Berlin, 1982. [23] JCPDS-ICDD Cards Numbers 19-193 for Wurtzite and 15-770 for Zincblende. ˜ o-Lo´pez, A. Escamilla[24] O. Zelaya-Angel, F. de L. Castillo-Alvarado, J. Avendan Esquivel, G. Contreras-Puente, R. Lozada-Morales, G. Torres-Delgado, Solid State Commun. 104 (1997) 161. [25] I. Kosacki, T. Suzuki, H.U. Anderson, P. Colomban, Solid State Ionics 149 (2002) 99. [26] O. Zelaya-Angel, R. Lozada-Morales, Phys. Rev. B 62 (2000) 13064. [27] O. Madelung, M. Schulz, H. Weiss (Eds.), Landolt–Bo¨rnstein New Series. Numerical Data and Functional Relationships in Science and Technology, vol. III/22a, Springer, Berlin, 1982.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystallization from amorphous structure to hexagonal quantum dots induced by an electron beam on CdTe thin films M. Becerril a,, O. Zelaya-Angel a, A.C. Medina-Torres b, J.R. Aguilar-Herna´ndez b, R. Ramı´rez-Bon c, F.J. Espinoza-Beltran c a
´n y de Estudios, Avanzados del IPN, Apartado Postal 14-740, Me´xico D.F. 07360, Mexico Departamento de Fı´sica, Centro de Investigacio ´ticas, Instituto Polite´cnico Nacional, Me ´xico D.F. 07738, Mexico Escuela Superior de Fı´sica y Matema c ´n y de Estudios, Avanzados del IPN, Unidad Quere ´taro, Apartado Postal 1-798, Quere ´taro, Qro. 76001, Mexico Centro de Investigacio b
a r t i c l e in fo
abstract
Article history: Received 28 November 2007 Received in revised form 5 December 2008 Accepted 20 December 2008 Communicated by K.H. Ploog Available online 13 January 2009
Amorphous cadmium–telluride films were prepared by rf sputtering on Corning 7059 glass substrates at room temperature. The deposition time was 10 and 12 h with a thickness of 400 and 480 (740 nm), respectively. As-prepared films were amorphous according to X-ray diffraction (XRD) patterns, but a win-fit-software analysis of the main XRD broad band suggests a wurtzite structure at short range. Transmission electron microscopy (TEM) at 200 keV produces crystallization of the amorphous CdTe. The TEM-electron beam induces the formation of CdTe quantum dots with the wurtzite hexagonal structure (the metastable structure of CdTe) and with 6 nm of average grain size. As effect of a probable distortion of the CdTe crystalline lattice, the unit cell volume (UCV) shrinks to about 30% with respect to the bulk-UCV of CdTe. Besides, the energy band gap increases as expected, according to literature data on quantum confinement. & 2009 Elsevier B.V. All rights reserved.
PACS: 63.37.Lp 68.65.Hb 71.70.Ej 78.30.Fs Keywords: A1. Crystal morphology A1. Nanostructures A1. X-ray diffractions B2. Semiconducting II–VI materials
1. Introduction Nowadays, semiconductor nanostructures are a research area of increasing interest because of the novel phenomena, which have been observed on these physical systems. Typically, the exciton Bohr radius in many semiconductor materials, such as II–VI compounds, is about a few nanometers, thus, the reduction in the size of semiconductor particles to nanometric dimensions yields to interesting quantum effects due to the confinement regime of excitons. The properties of nanocrystalline semiconductors in the quantum confinement regime undergo strong modifications, for example, it has been observed a strong blue-shift in the energy band gap as the particle size decreases in many types of semiconductors. Nanocrystalline CdTe has been prepared by employing different techniques such as sputtering [1–3], ball-milling [4], molecular beam epitaxy (MBE) [5] and dielectric-CdTe-comelting [6–8], among others. The technique
Corresponding author. Tel. +52 55 5061 3800; fax: +52 55 5747 7097.
E-mail address: becerril@fis.cinvestav.mx (M. Becerril). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.12.056
of rf sputtering has been successfully employed to grow size-controlled CdTe quantum dots [9]. It has also been fruitful to deposit amorphous CdTe films [10]. Transmission electron microscopy (TEM) is one of the most widely employed experimental techniques for the characterization of nanostructured systems, because it can provide directly the size of the nanoparticles [11]. Some other techniques, such as X-ray diffraction (XRD) and Raman spectroscopy, can also provide indirectly the size of nanoparticles by using the full-width at half-maximum (FWHM) of the peaks of XRD patterns and/or the FWHM of the bands of Raman spectra, while in the last case, after suitable corrections [12]. However, the exposure to the energetic TEM electron beam can modify amorphous or nanocrystalline structures of solid materials. Metallic [13] and semiconducting [14] thin films increase their average nanoparticle size as a consequence of the incident electron beam during TEM measurements. Crystallization of amorphous materials because of TEM-electron beam has been observed by other authors [15–18]. The aim of this work is to analyze the modification that amorphous CdTe thin films experiences under a TEM electronbeam exposure.
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2. Experimental procedure By utilizing a magnetron sputtering system and argon as working gas, amorphous CdTe films were grown on 7059 Corning glass substrates at room temperature (RT) during 10 and 12 h. The rf power used in the grown process was 40 W and the distance between the target and substrate was 4 cm. High-purity argon gas was used to obtain the sputtering plasma. The initial pressure in the chamber was 1 105 Torr, and the final pressure of the argon plasma was about 103 Torr. The CdTe (99.999% pure) target had an area of 4.92 cm2. The thickness, as measured by means of a Dektak II profilometer, was 400 and 480 nm (740 nm), which were denoted by A1 and A2, respectively. The slow rate of the growing process (4074 nm/h) could influence the amorphization of CdTe films. When we have used a deposition rate in the range 50–65 nm/h the CdTe films grow into polycrystalline structure with the grain size in the interval 40–65 nm [19,20]. Auger mass spectroscopy measurements, achieved employing an ESCA-SAM PHI-560 system, indicated (49.770.5)% Te and (50.370.5)% Cd as atomic concentrations. No impurities were detected with the resolution limit level of the Auger spectroscopy. X-ray diffraction patterns were obtained through Siemens D5000 diffractometer, with the CuKa line. TEM apparatus employed was a Jeol 2010 working at 200 keV. CdTe layers were detached from the substrate and attached onto the TEM-grills. For the XRD broad bands deconvolution and the grain size calculation of the Win-Fit software was used. For Raman spectra registration a Labram-Dilor system was used and for the Raman band line shape fitting the MathCad program was run. UV–vis absorption spectra measurements were performed in a Unicam 8700 spectrometer system over the 190–900 nm wavelength range. Energy band gap (Eg) measurements were carried out by means of optical absorption spectra by using the Tauc’s formula for amorphous semiconductors: (ahn)p(hnEg)m [10–21], where a is the absorption coefficient, h is the Planck’s constant, n is the frequency of the light, and the exponent m takes the values 12 and 2 for allowed direct and indirect transitions [10–21]. Actually, as the absorption coefficient is proportional to the optical density (O.D.), one can directly plot the (O.D. hn)m versus the photon energy hn to get Eg.
Fig. 1. Raman spectrum of A1 as-grown CdTe film. The inset exhibits the normalized band at 166 cm1 (points) and the fitting (solid line) for grain size determination.
3. Results Raman spectrum of the as-grown CdTe layer is shown in Fig. 1, where the longitudinal optical (LO) mode at the frequency of 166 cm1, with two phonon replicas, characteristic of the cubic CdTe lattice [22], is observed. The broadening of the Raman bands reveals the grade of disorder present in the structure of the material. The diffractogram of the as-grown CdTe film is displayed in Fig. 2, which shows the two typical broad bands of the XRD pattern of an amorphous semiconductor material (at 2y~251 and 421, respectively). The recorded XRD signal also evidences an unusual small shoulder at 2yffi15.51. This third shoulder precisely ensures that the diffractogram is not an XRD pattern of a complete amorphous material, because of XRD spectra of amorphous materials only exhibit two broad bands. Deconvolution of the XRD pattern in Gaussian curves makes clear the presence of nine broad bands. As established, the envelope curve reproduces the experimental pattern. Besides, the broad band centered at 2y=15.52, which could not be associated to any crystalline plane of the different structural phases of CdTe, neither with planes of elemental Cd nor Te, the remaining eight Gaussian curves can be identified with the XRD peaks of the hexagonal wurtzite (W) crystalline structure of CdTe [23]. Evidently, there are some differences in positions because of, in practice, the amorphous
Fig. 2. XRD pattern of A1 as-grown CdTe amorphous film. Noisy line represents the experimental data. Solid continuous smooth lines represent the envelop of the deconvolution process applied to the pattern. Straight line is the zero reference line.
structural character. However, peaks belonging to cubic zincblende (ZB) CdTe do not reproduce the whole pattern. After all, the presence of a component of the ZB-CdTe phase cannot be discarded. This result is not in opposition to the Raman measurements of Fig. 1 whose phonon mode has been associated with the LO mode of CdTe in cubic phase, because the LO modes of ZB and W phases of CdTe can coincide at the same frequency, in a similar way as reported for CdS [24]. Until now the Raman spectra of hexagonal CdTe has not been reported.
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Since the interplanar distance (dl) corresponding to planes located at 2y=15.521 is 5.70 A˚ and the interplanar distance (dc) to the position of the central band is 2.80 A˚, i.e., dlffi2dc, one can associate this particular feature with some particular crystalline structure of CdTe existing at short-range distance into the bulk of the film. The most common crystalline structures of CdTe are the stable ZB and the metastable W phases. Actually, the interplanar distances along the (0 0 0 1) and (111) directions, the staking directions in both phases, are 3.750 and 3.742 A˚ [23] for W and ZB crystalline lattices of CdTe, respectively. Both are larger than 3.72 A˚ found for planes associated with the corresponding Gaussian band at 2y=23.86 of Fig. 2. As it will be pointed out below, the crystalline lattice of the sputtered CdTe films as a whole undergoes a shrinkage owing, with good probability, to a distortion of the crystalline entities (‘‘grains’’ in the amorphous layer). The unit cell of the ZB-CdTe involves three (111)interplanar distances, and the unit cell of W-CdTe involves only two (0 0 0 1) interplanar distances. In some way, the XRD band at 2y=15.521 represents the diffraction of planes of a special polytype of CdTe. Applying the Debye–Scherrer formula on the FWHM of the Gaussians bands of the deconvoluted XRD pattern of Fig. 2, an average diameter of 2.670.7 nm for the grains was calculated. From the dependence of the FWHM of the Raman bands as a function of the inverse grain size [25], the particle size of asgrown CdTe samples can be calculated from the fitting of the normalized band of Fig. 1. The inset shows the fit used here with a prediction of a particle size of 2.070.2 nm, which, within the error scattering data, is in agreement with the results obtained from XRD measurements. A picture of the TEM-magnification can be observed in Fig. 3. The existence of crystalline structures of nanometric dimensions is evident. The statistical analysis on the size of the crystalline nanostructures measured directly on the picture yields an average value of 6.071.5 nm. The experimental result evidences that the effect of the electron-beam exposure on the film during the TEM measurement process were: firstly, the crystallization of the amorphous material; and, secondly, the increase of the grain size. Fig. 4 displays the TEM diffraction of a CdTe layer. Obviously, the film has lost the amorphous character. The well-defined rings of the pattern undoubtedly demonstrate the polycrystalline nature of the film, with nanometric dots in W structure. Table 1 lists the data, calculated from the TEM diffractogram, of the
Fig. 3. TEM picture taken at 200 keV of the A1 sample detached from the glass substrate.
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Fig. 4. TEM diffraction pattern obtained at 200 keV of the A1 sample.
Table 1 Accepted values of 2y-positions and dhkl interplanar distances for bulk-CdTe and those obtained here for CdTe films. hkl
2y (XRD cards19) (A˚)
100 002
22.319 23.707
101 102 110 103 200 201 202
25.281 32.716 39.222 42.717 45.425 47.331 52.068
2y (films)
dhkl (XRD cards19) (A˚)
dhkl (films)
23:720
3.980 3.750
3:700
32.77 41.93 45.17 50.26 54.87 58.21
3.520 2.735 2.295 2.115 1.995 1.919 1.755
2.730 2.153 2.01 1.81 1.672 1.584
2y-positions and the interplanar distances (dhkl) of the nanocrystallized lattice. The first ring in Fig. 4 includes the initial three rings of the wurtzite structure, which could not be resolved with our TEM equipment. Experimental data of the diameter of these three rings are merged into an only average value, indicated in Table 1 with superlined figures. The 2y values calculated from the TEM diffractograms are larger than accepted data. Consequently, from the Bragg condition, experimental dhkl values result smaller. In Table 1 it can be checked out how dhkl decreases continuously as Miller indexes go to higher values. This phenomenon can be probably ascribed to some distortion of the CdTe lattice. The unit cell volume (UCV) of the lattice, considered as a hexagonal structure, after an average of the unit cell parameters a and c obtained from the dhkl values, 11578 A˚3. The UCV of bulk-CdTe, calculated from the accepted a=4.58 A˚ and c=7.50 A˚ lattice parameters values [23], is 157.33 A˚3. The UCV in the CdTe film has been reduced approximately 30% with respect to UCV of bulk-CdTe. With high probability, the compressive stress exerted by the amorphous substrate on the as-grown film induced the shrinkage of the lattice, which could not be completely relaxed by the incident energetic electrons, i.e., the
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Fig. 6. Optical transmission (T) and reflection (R) versus l of the as-grown CdTe sample. The inset displays the first derivative of T with respect to l, as a function of l.
Fig. 5. Optical absorption of as-grown CdTe film. The inset exhibits the determination of (a) the direct band gap energy and (b) the indirect one.
‘‘TEM-annealing’’, and the lattice stayed with reduced dimensions and probably distorted. From the optical absorption spectra the Eg values of as-grown films were determined. Fig. 5 shows the optical absorption as a function of the photon wavelength (l) measurements in the CdTe film. The insets exhibit the (O.D. hn)m versus hn plots when: (a) m=2 for direct transitions and (b) m ¼ 12 for indirect transitions. The extrapolation of the linear part of the curves to intercept the hn axis gives the Eg values, which equal 1.9570.04 eV for the direct Eg and 1.6770.03 eV for the indirect one. The direct Eg value coincides with experimental results reported by Potter and Simmons [1] for sputtered ZB-CdTe nanocrystals of 2.5 nm diameter, where the increase of Eg was assigned to quantum confinement effects. We suppose both ZB and W phases in CdTe have approximately the same values of Eg, in a similar way to CdS [26]. Indirect band gaps in CdTe polycrystalline films have been observed in amorphous and polycrystalline CdTe thin films by Marafi et al. [10]. In Fig. 6 optical transmission (T) and reflection (R) versus l the as-grown CdTe films are plotted. The T spectrum evidences how the layer starts showing significant transmission, since hnffi3.1 eV (l=400 nm), and R indicates that there is very low reflection in the entire visible spectrum. The inset displays the first derivative of T with respect to l as a function of l. Two features arisen in this spectrum at 2.0070.05 and 2.8870.05 eV, which can be associated with Eg and the first DEg i.e. here separated by 0.88710 eV from Eg. DEg due to the spin–orbit (Dso) unfolding (the G7vG6c transition) has the value of 0.8770.08 eV in bulk-ZB-CdTe [22,27]. In non-cubic lattices this unfolding should include, besides Dso, the crystal-field interaction (Dcf). In case of DEg value found for W-CdTe, its value is practically the
Fig. 7. XRD pattern of A2 as-grown CdTe amorphous film.
same value of Dso for ZB-CdTe. This result means that Dcf in W-CdTe has a small value (lies within the error bar, i.e., 0.10 eV). Dcf equals 0.028 and 0.040 eV in CdS and CdSe, respectively [22]. One can realize that the quantum confinement effect, as expected, shifts Eg and DEg around the same value (0.9 eV) than in bulk CdTe. The narrow feature at l=660 in Fig. 6 nm has been attributed here to an artifact, since it appears in the T and R spectra. To demonstrate the reproducibility of the crystallization from amorphous structure to hexagonal quantum dots induced by an electron beam on CdTe thin film, second sample was studied. Fig. 7 shows the XRD pattern of the A2 as-grown sample. The diffractogram is the typical amorphous material. A picture of the TEM magnification of this sample can be observed in Fig. 8. A statistical analysis of the particle size measured directly on the picture yield an average value of 10.072.5 nm. The inset displays
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because of the relative high energy of the electron beam. Cautions must be taken in order to determine the actual size of nanocrystals by means of TEM studies.
Acknowledgements The authors thank to Ings. N. Castillo, M. Guerrero, Z. Rivera and A. B. Soto for their technical assistance. This work was partially supported by the CONACyT—Me´xico.
References
Fig. 8. TEM picture taken at 200 keV of the A2 sample detached from the glass substrate. The inset displays TEM diffraction pattern obtained at 200 keV of the same sample.
TEM diffraction pattern obtained at 200 keV of the same sample. The well-defined rings in the pattern undoubtedly demonstrate the polycrystalline nature of the films. The experimental results evidences that the effects of the electron-beam exposure on the films during the TEM measurement process were: first, the nanocrystallization of the amorphous material; and second, the increase of the grain size from amorphous to nanocrystalline films. The Raman and optical absorption data of the A2 sample is similar to that of the A1 sample. Hexagonal CdTe nanocrystals have been produced by Lefevre et al. [4] by utilizing the mechanical milling on CdTe powder. Strains generated inside the nanoparticles were the main logical cause claimed for the origin of the metastable W structure. Even though in case nanocrystals are not free, the influence of the amorphous substrate can be the responsible factor for strains arising in CdTe quantum dots, and hence the hexagonal structure origin. Grain growth of nanocrystals after TEM-annealing from 50 keV onwards has been observed in GaAs by Jencˇicˇ et al. [14]. They assumed that the electron beam provokes a fast thermal annealing on the thin samples, which modified the original size of nanocrystals. It is important to mention that ‘‘burning’’ of films of several types of materials produced by TEM exposure, has been observed by us.
4. Conclusions We have demonstrated how TEM alters the CdTe either amorphous or nanocrystalline structure of the samples studied
[1] B.G. Potter, J.H. Simmons, J. Appl. Phys. 68 (1990) 1218. [2] K. Tsunemoto, H. Nasu, H. Kitayama, A. Kawabuchi, Y. Osaka, K. Takiyama, Jpn. Appl. Phys. 28 (1990) 1928. [3] H. Arizpe-Cha´vez, R. Ramı´rez-Bon, F.J. Espinoza-Beltra´n, O. Zelaya-Angel, J.L. Marı´n, R. Riera, J. Phys. Chem. Solids 61 (2000) 511. [4] P. Lefevre, T. Richard, J. Alle`gre, H. Mathieu, A. Combette-Roos, W. Granier, Phys. Rev. B 53 (1996) 15440. [5] Y. Terai, S. Kuroda, K. Takita, T. Okuno, Y. Masumoto, Appl. Phys. Lett. 73 (1998) 3757. [6] V. Esch, B. Fluegel, G. Khitrova, H.M. Gibbs, X. Jiajin, K. Kang, S.W. Koch, L.C. Liu, S.H. Risbud, N. Peyghambarian, Phys. Rev. B 42 (1990) 7450. [7] R. Ramı´rez-Bon, F.J. Espinoza-Beltra´n, H. Arizpe-Cha´vez, O. Zelaya-Angel, F. Sa´nchez-Sinencio, J. Appl. Phys. 77 (1995) 5461. [8] I.V. Bodnar, V.S. Gurin, A.P. Molochko, N.P. Solovei, P.V. Prokoshin, K.V. Yumashev, Semiconductors 36 (2002) 298. [9] S.K. Mandal, S. Chauduri, A.K. Pal, Thin Solid Films 357 (1999) 102. [10] M. Marafi, F. El Akkad, B. Pradeep, J. Mater. Sci. Mater. Electron. X 21 (2003). [11] A. Ekimov, J. Lumines, Phys. Solid State 70 (1996) 1479. [12] W.H. Li, S.W. Wu, C.C. Yang, S.K. Lai, K.C. Lee, H.L. Huang, H.D. Yang, Phys. Rev. Lett. 89 (2002) 135504. [13] R. Dannenberg, E. Stach, J.R. Groza, B.J. Dresser, Thin Solid Films 379 (2000) 133. [14] I. Jencˇicˇ, M.W. Bench, I.M. Robertson, M.A. Kirk, J. Appl. Phys. 78 (1995) 974. [15] A.G. Fitzgerald, C.P. McHardy, J. Mater. Sci. Lett. 3 (1984) 91. [16] M. Kaiser, L. van Pieterson, M.A. Verheijen, J. Appl. Phys. 96 (2004) 3193. [17] J.S. Romero, A.G. Fitzgerald, M.J. Rose, Appl. Surf. Sci. 234 (2004) 369. [18] Y. Zhang, C.M. Wang, M.H. Engelhard, W.J. Weber, J. Appl. Phys. 100 (2006) 113533. [19] O. Alvarez-Fregoso, O. Zelaya-Angel, J.G. Mendoza-Alvarez, F. SanchezSinencio, M.H. Farı´as, L. Cota-Araiza, J. Phys. Chem. Solids 56 (1995) 117. [20] H. Arizpe-Chavez, F.J. Espinoza-Beltran, R. Ramirez-Bon, O. Zelaya-Angel, J.G. Gonza´lez-Herna´ndez, Solid State Commun. 101 (1997) 39. [21] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903. [22] O. Madelung, M. Schulz, H. Weiss (Eds.), Landolt–Bo¨rnstein New Series. Numerical Data and Functional Relationships in Science and Technology, vol. III/17b, Springer, Berlin, 1982. [23] JCPDS-ICDD Cards Numbers 19-193 for Wurtzite and 15-770 for Zincblende. ˜ o-Lo´pez, A. Escamilla[24] O. Zelaya-Angel, F. de L. Castillo-Alvarado, J. Avendan Esquivel, G. Contreras-Puente, R. Lozada-Morales, G. Torres-Delgado, Solid State Commun. 104 (1997) 161. [25] I. Kosacki, T. Suzuki, H.U. Anderson, P. Colomban, Solid State Ionics 149 (2002) 99. [26] O. Zelaya-Angel, R. Lozada-Morales, Phys. Rev. B 62 (2000) 13064. [27] O. Madelung, M. Schulz, H. Weiss (Eds.), Landolt–Bo¨rnstein New Series. Numerical Data and Functional Relationships in Science and Technology, vol. III/22a, Springer, Berlin, 1982.