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Cathodoluminescence study of plastically deformed bulk ZnO single crystal
ARTICLE IN PRESS
Physica B 366 (2005) 185–191 www.elsevier.com/locate/physb
Cathodoluminescence study of plastically deformed bulk ZnO single crystal Z. Takkouka,, N. Brihia, K. Guergouria, Y. Marfaingb a
Laboratoire de physique-chimie des semiconducteurs, De´partement de physique, Faculte´ des Sciences, Universite´ Mentouri, Constantine 25000, Algeria b Laboratoire de Physique des solides et de Crystalloge´ne`se, CNRS Meudon 1 place A. Briand, 92195 Meudon Cedex, France Received 1 October 2004; received in revised form 17 May 2005; accepted 22 May 2005
Abstract ¯ ZnO single crystal has been studied by means of a scanning microscope Vickers microindentation of bulk ð0 0 0 1Þ using cathodoluminescence (CL) monochromatic imaging and spectral modes. A main feature of the sample’s deformation is the propagation of the latter away from the indenter contact diameter, in well-defined directions, characterizing the hexagonal crystal systems. An attempt is made to understand plastic deformation mechanism, which is responsible for the observed deformation behavior. The deformation-produced dislocations act as nonradiative recombination centers, which is confirmed by the absence of any additional emission band and the diminution of the band edge luminescence intensity in the Cl spectra after micro-indentation. r 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Lk; 78.60.Hr; 62.20.Fe; 71.55.Gs Keywords: n-ZnO; Dislocations; Cathodoluminescence; Plastic deformation; II–VI semiconductors
1. Introduction Wide band gap semiconductors are of great interest for several applications in for light emitting, electron emitting and high temperature, high power devices. After the successful use of zinc oxide (ZnO) for the manufacture of surface and Corresponding author. Tel./fax: ++213344 74897.
E-mail addresses: [email protected] (Z. Takkouk), [email protected] (N. Brihi).
bulk acoustic wave (SAW, BAW) devices, piezoelectric devices, varistors, and solar cell windows, this material is also promising for devices that emit light in the ultra violet, visible, and blue, and for high-power and high-temperature devices, in particular after recent advances in the growth of high-quality single crystals [1,2]. The above-mentioned applications are related to numerous ZnO properties, especially a direct wide band gap (3.37 eV at room temperature) and a high melting point (20001C). In addition we can mention
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.05.027
ARTICLE IN PRESS 186
Z. Takkouk et al. / Physica B 366 (2005) 185–191
attractive properties such as a large free exciton binding energy (60 meV for ZnO versus 28 meV only for GaN at room temperature), low threshold voltage, high doping concentrations (1021 cm3), and more resistance to radiation damage greater than GaN. The growth of bulk ZnO can be obtained using several methods, such as vaporphase transport [1,3], hydrothermal growth [4,5], and flux method [6,7]. Another important factor that in concentrates much effort on ZnO is the lattice mismatch between ZnO and GaN which is only 2.2% (compared with 14–16% between GaN and sapphire substrate), which allows ZnO to be a preferable material as substrate for GaN epitaxial growth. For these reasons and others (related recently with magnetic properties) ZnO has long been subject to many investigations. The practical use of materials requires a knowledge of their mechanical behavior and the effect of mechanical damage on their desired properties, because through the fabrication or handling processes the material may be submitted to mechanical loads which cause damage, or the mechanical loads arise from shock loading in mobile systems. Vickers microindentation, which produces highly localized deformation, is an excellent method for studying mechanical properties and understanding the mechanism of plastic deformation. In spite of the plastic deformation in some materials enhancing the emission in some spectral emission regions as the 550 nm band emission in plastically deformed GaN, the indentation-induced defects (e.g. dislocations) acting as efficient nonradiative recombination centers result in a drastic diminution of the intensity of another band emission such as 360 nm band emission in GaN [8]. This is a non-desirable effect for devices that work in that spectral region. Czernuszka [9] have not obtained a well-defined rosette structure after deformation of the (0 0 0 1) Zn face by Vickers microhardness, which can be probably due to the misorientation of (0 0 0 1) plane and/or the non-uniform distribution of the stress under their Vickers indenter. Previous work [10,11] on deformed bulk single crystals of ZnO using a spherical indenter showed that the slip along the basal and pyramidal planes is the major mode of plastic deformation.
In this work we have investigated the defect structure of plastically deformed ZnO bulk single crystal, using monochromatic and spectrally resolved cathodoluminescence (CL) microscopy in a scanning electron microscope (SEM/CL). This technique is a powerful tool for studying the microscopic plastic deformation of materials caused by microindentation, and related introduced dislocations.
2. Experimental procedure The used material was provided by Cermet Inc; ¯ ZnO it is a bulk n-type hexagonal wurtzite ð0 0 0 1Þ single crystal (the lattice parameters are a ¼ 3.2499 A˚, c ¼ 5.20568 A˚ at room temperature) with a carrier concentration and a Hall resistivity nearly equal to 1.5 1017 cm3 and 0.02 O cm, respectively. The samples were provided with a polished O face containing an etch pits density of about 4 104 cm2. Micro-hardness indentations were performed at room temperature and atmospheric pressure for 30 s using a Vickers diamond indenter with applying loads of 5, 15, 40, and 90 g respectively. In order to avoid the interference of plastic deformations (the arms of the indentation rosette) which extend away from the center of the imprint, the distance between two neighboring indentations has been chosen equal to 0.5 mm. The hardness, H, is given by the following formula: HðPa sÞ ¼ 1:8544P=d 2 ðN=m2 Þ; where d is the diagonal length of the imprint and P the applied load. CL measurements were performed in a JEOL 840 scanning electron microscope equipped with an Oxford elliptic mirror for light collection.
3. Results and discussion Mechanically Table 1 presents some characteristics of indentations which appear near to values reported in Refs. [10,11]. However, we note a small difference probably due to the shape of the used indenter, because material hardness H depends on
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Table 1 Characteristics of the indentation rosettes Load (103 kg) Central zone diagonal length (106 m) Arms length (106 m) Penetration (109 m) Hardness (H) (GP)
5 8 50 1142 1.421
15 14 68 1714 1.895
40 18 70 2258 2.842
90 20 90 2857 4.093
Fig. 1. Monochromatic CL image of indentation at loads of (a) 5 g and (b) 15 g, l ¼ 387 nm, beam voltage 20 kV. Inset: corresponding SE image.
several experimental conditions, such as indenter matter and its shape. After indentation the residual imprints were directly imaged using secondary electron (SE) and CL. CL images were taken using a near-gap emission peak as shown in Fig. 1. We clearly observe an indentation figure called rosette, with a structure symmetry reflecting the hexagonal symmetry of the (0 0 0 1) ZnO planes. It appears as two well separated regions (a) a strongly perturbed central zone, appearing as a very dark region with non-uniform distribution damage, has an average size of about 8 mm; (b) six double arms, nearly parallel to axes separated by an angle of 601, confirming the hexagonal symmetry. Each double arm, clearly separated, is composed of two branches with comparable extensions. The extension of the arms is about 50 mm measured from the center of the imprint. Dark spots are visible in each arm; they correspond to the emergences of dislocations on the
surface which act as nonradiative recombination centers and therefore quench locally the luminescence. Nothing is observed between and within the arms. The length of each arm increases with increasing load as shown in Table 1. These two fundamental features have been observed previously for ZnO using a spherical indenter [11]. This is contrary to GaN, a structurally similar material, where the quenching of luminescence is not extended beyond the indenter contact diameter [8, 12–14]. The intensity of the near gap CL emission is drastically suppressed after indentation because of induced defects (dislocations). The central zone of the imprint clearly appeared as a star rosette, which has previously been reported for other material with similar structure [8,12]. Fig. 2 is a schematic diagram of the ZnO structure, viewed along the ½1 1 2¯ 0 direction. It is shown that f0 0 0 1g basal and f1¯ 1 0 1g pyramidal planes are terminated by either zinc atoms or
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
188
[0001] [1102] [1101] Pyramidal 1
[1120]
[1100]
Pyramidal 2
α(g) Basal
β(g) β
(g
)
α
(g)
Prismatic
[0001] 0
Zn
Fig. 2. Projection of the ZnO wurtzite structure along ½1 1 2¯ 0 direction.
Contrary, there is no detailed study on the slip geometry of dislocations around microindentation in ZnO. XTEM studies of ZnO revealed that deformation-introduced defects extend deep into the bulk of the material as well as lateral extension [11]. We expect that aðgÞ and bðgÞ dislocations extend into the crystal along the edges of Thompson’s tetrahedron (Fig. 3(a)). The rosette glide is characterized by the arms in ½1 1 2¯ 0, ½2 1¯ 1¯ 0, ½1¯ 2 1¯ 0 directions. The possible tetrahedral glide with inclined Burgers vectors, which is the principal factor in the determination of the hardness, does occur in directions possessing certain symmetry as in the rosette glide as shown in Fig. 3(b). [2203] [0223]
oxygen atoms. Thus dislocations lying in these planes possess a and b character, analogous to (1 1 1) planes in sphalerite structure. We suppose that dislocations move in the glide set planes. Sphalerite(1 1 1) and wurtzite(0 0 0 1) planes have hexagonal symmetry. This clearly appeared in the rosette structure obtained after Vickers microindentation, as has been reported previously for CdTe [15–17] and our results. ZnO can be considered as a hexagonal-closed-packed (HCP) of Zn atoms with O atoms in tetrahedral interstitial position of Zn. The atoms of ZnO structure are tetrahedrally bonded with a nearestneighbor environment similar to that of CdTe structure (sphalerite), but the geometry of the second neighbor is different. We expect that two kinds of defects (dislocations) have been created by indentation (at the impact point) and then propagated in well-defined directions. The first related to Zn atoms (aðbÞg ), while the second related to O atoms (bðaÞg ). These two kinds of dislocations act as non-radiative recombination centers which appeared in the CL image as dark lines. As in previous studies on ZnO the slip process is the major mode of plastic deformation. The slip process is composed of a rosette glide with surface parallel Burgers vectors and a tetrahedral glide with inclined Burgers vectors. The glide prism model has been successfully applied to interpret the results obtained for CdTe [15–17].
s
[1210] c a
[1120] b [2110]
(a)
T
[2110]
[1210]
[1120]
[2023]
(b)
[2203]
[0223]
β(α)(g) α(β)(g)
Fig. 3. (a) Thompson’s tetrahedron in the HCP structure and (b) expected slip directions in 3D of indented ZnO samples.
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
With increased loading, the central zone becomes larger and more perturbed, accompanied by shorter and very dark arms (denoted by arrows) appearing between double arms, which becomes less clear at the low loads as shown in Figs. 4(a) and (b). In addition, two ‘‘slip’’ triangles, opposite
189
in their positions and surrounding the central zone, appeared. Each triangle was made up of many triangles lying inside each other (cross-slip). The edges of these triangles are parallel to the rosette arms. The appearance of these opposite triangles has been reported in other material with
Fig. 4. Monochromatic CL image of indentation at loads of (a) 40 g and (b) 90 g, l ¼ 387 nm, beam voltage 20 kV. Inset: corresponding SE image. (c) Schema of an indentation figure as imaged by SEM/CL at relatively high load (90 g).
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
190
similar structure [8,12–14] and for CdTe [15]. We believe that other slip systems (slip bands) have been activated. The pop-in events observed elsewhere are related to the nucleation of other slip bands [10]. We note that dark lines appearing near the rosettes are not associated with the deformation itself, but with the polishing or/and handling processes. Fig. 4(c) is a schematic drawn of the CL image at relatively high load (as example 90 g). Each triangle is formed by the intersection of three rosette arms. The existence of these triangles as in CdTe [15] reveals that the tetrahedral glide is more probable. Fig. 5 shows typical room temperature CL spectra. It is clearly seen that the near band edge luminescence intensity decreased after indentation with the absence of any additional emission band. This means that deformation-produced
dislocations act as nonradiative recombination centers. The quenching of luminescence in the deformed areas (central zone and arms of the rosettes) is clearly visible in all CL images taken at the near gap emission peak (387 nm). The damaged area increased with increasing load; this implies that the band edge emission intensity in the CL spectra decreases with increasing load. However, additional XTEM and XCL studies seem to be needed to better understand the physical mechanism of plastic deformation in ZnO single crystal and to set a detailed model explaining the observed behavior. Further investigations on the effect of hydrogenation on the deformationproduced dislocations are under way and will be published elsewhere.
4. Conclusion Non-indented area
Intensity (a.u)
Local plastic deformation of bulk ZnO single crystal has been investigated. Cathodoluminescence scanning electron microscopy has been used to observe the defect configuration after microdeformation at room temperature. The following conclusions arise from the work described here:
3500 (a)
4000
4500
5000 5500 Wavelength (A˚)
6000
6500
5000 5500 Wavelength (A˚)
6000
6500
Intensity (a.u)
indented area
3500 (b)
4000
4500
Fig. 5. Cathodoluminescence spectra recorded from (a) undeformed and (b) deformed (i.e. dark) areas of indented sample at a load of 40 g.
(1) ZnO is a relatively soft material compared with GaN(HGaN15.5 GPa [12]). (2) CL images of indentations showed a very welldefined rosette structure with hexagonal symmetry, which extends to long distance (50 mm) as double arms in directions at 601 intervals ( ½1 1 2¯ 0, ½2 1¯ 1¯ 0, ½1¯ 2 1¯ 0). Each double arm is composed probably from aðgÞ and bðgÞ dislocations. (3) The slip systems occurring at microindentation of ZnO is composed from those with Burgers vectors parallel to the indented surface and those with inclined Burgers vectors. (4) At relatively high load, cross-slip is clearly observed, and more slip systems are activated which lead to more complicated deformation configuration. (5) CL spectra recorded at room temperature showed a decrease of the near band edge luminescence intensity and the absence of any additional emission band after indentation.
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
Acknowledgments The authors would like to express their sincere thanks to J.P. Rivie´re for his help in the deformation of the samples and F. Jomard for cathodoluminescence experiments. References [1] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun 105 (1998) 399. [2] D.C. Look, Materials Science and Engineering B 80 (2001) 383. [3] S. Hassani, A. Tromson-Carli, A. Lusson, G. Didier, R. Triboulet, Phys. Stat. Sol. (b) 229 (2) (2002) 835. [4] R.A. Laudice, E.D. Colbardn, A.J. Caporaso, J. Am. Soc. 47 (1964) 9. [5] T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, N. Sakagami, J. Cryst. Growth 214/215 (2000) 72. [6] T. Sekiguchi, N. Ohashi, Y. Terada, Jpn. J. Appl. Phys. 36 (1997) L289.
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[7] N. Ohashi, T. Ohgaki, T. Nakata, T. Tsumuri, T. Sekiguchi, H. Haneda, J. Tanaka, J. Kore, Phys. Soc. 35 (1999) S287. [8] M.H. Zaldivar, P. Fernandez, J. Piqueras, Semicond. Sci. Technol. 13 (1998) 900. [9] J.T. Czernuszka, N. Pratt, Phil. Mag. Lett. 61 (3) (1990) 83. [10] S.O. Kucheyev, J.E. Bradby, J.S. Williams, C. Jagadish, Appl. Phys. Lett. 80 (6) (2002) 956. [11] J.E. Bradby, S.O. Kucheyev, J.S. Williams, C. Jagadish, Appl. Phys. Lett. 80 (24) (2002) 4537. [12] S.O. Kucheyev, J.E. Bradby, J.S. Williams, C. Jagadish, Appl. Phys. Lett. 77 (21) (2000) 3373. [13] H. Lei, H.S. Leipner, J. Schreiber, T. Wosinski, I. Grezegory, J. Appl. Phys. 92 (11) (2002) 6666. [14] J.E. Bradby, S.O. Kucheyev, J.S. Williams, Wong-Leung, Appl. Phys. Lett. 80 (3) (2002) 383. [15] A. Rivie`re, R. Sieber, J.P. Rivie`re, Microsc. Microanal. Microstruct. 2 (1991) 257. [16] H.S. Leipner, J. Shreiber, H. Uniewski, S. Hildbrandt, Scanning Microscopy 12 (1) (1998) 149. [17] J. Schreiber, L. Horing, H. Uniewski, S. Hildbrandt, Phys. Stat. Sol. (a) 171 (1999) 89.
Physica B 366 (2005) 185–191 www.elsevier.com/locate/physb
Cathodoluminescence study of plastically deformed bulk ZnO single crystal Z. Takkouka,, N. Brihia, K. Guergouria, Y. Marfaingb a
Laboratoire de physique-chimie des semiconducteurs, De´partement de physique, Faculte´ des Sciences, Universite´ Mentouri, Constantine 25000, Algeria b Laboratoire de Physique des solides et de Crystalloge´ne`se, CNRS Meudon 1 place A. Briand, 92195 Meudon Cedex, France Received 1 October 2004; received in revised form 17 May 2005; accepted 22 May 2005
Abstract ¯ ZnO single crystal has been studied by means of a scanning microscope Vickers microindentation of bulk ð0 0 0 1Þ using cathodoluminescence (CL) monochromatic imaging and spectral modes. A main feature of the sample’s deformation is the propagation of the latter away from the indenter contact diameter, in well-defined directions, characterizing the hexagonal crystal systems. An attempt is made to understand plastic deformation mechanism, which is responsible for the observed deformation behavior. The deformation-produced dislocations act as nonradiative recombination centers, which is confirmed by the absence of any additional emission band and the diminution of the band edge luminescence intensity in the Cl spectra after micro-indentation. r 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Lk; 78.60.Hr; 62.20.Fe; 71.55.Gs Keywords: n-ZnO; Dislocations; Cathodoluminescence; Plastic deformation; II–VI semiconductors
1. Introduction Wide band gap semiconductors are of great interest for several applications in for light emitting, electron emitting and high temperature, high power devices. After the successful use of zinc oxide (ZnO) for the manufacture of surface and Corresponding author. Tel./fax: ++213344 74897.
E-mail addresses: [email protected] (Z. Takkouk), [email protected] (N. Brihi).
bulk acoustic wave (SAW, BAW) devices, piezoelectric devices, varistors, and solar cell windows, this material is also promising for devices that emit light in the ultra violet, visible, and blue, and for high-power and high-temperature devices, in particular after recent advances in the growth of high-quality single crystals [1,2]. The above-mentioned applications are related to numerous ZnO properties, especially a direct wide band gap (3.37 eV at room temperature) and a high melting point (20001C). In addition we can mention
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.05.027
ARTICLE IN PRESS 186
Z. Takkouk et al. / Physica B 366 (2005) 185–191
attractive properties such as a large free exciton binding energy (60 meV for ZnO versus 28 meV only for GaN at room temperature), low threshold voltage, high doping concentrations (1021 cm3), and more resistance to radiation damage greater than GaN. The growth of bulk ZnO can be obtained using several methods, such as vaporphase transport [1,3], hydrothermal growth [4,5], and flux method [6,7]. Another important factor that in concentrates much effort on ZnO is the lattice mismatch between ZnO and GaN which is only 2.2% (compared with 14–16% between GaN and sapphire substrate), which allows ZnO to be a preferable material as substrate for GaN epitaxial growth. For these reasons and others (related recently with magnetic properties) ZnO has long been subject to many investigations. The practical use of materials requires a knowledge of their mechanical behavior and the effect of mechanical damage on their desired properties, because through the fabrication or handling processes the material may be submitted to mechanical loads which cause damage, or the mechanical loads arise from shock loading in mobile systems. Vickers microindentation, which produces highly localized deformation, is an excellent method for studying mechanical properties and understanding the mechanism of plastic deformation. In spite of the plastic deformation in some materials enhancing the emission in some spectral emission regions as the 550 nm band emission in plastically deformed GaN, the indentation-induced defects (e.g. dislocations) acting as efficient nonradiative recombination centers result in a drastic diminution of the intensity of another band emission such as 360 nm band emission in GaN [8]. This is a non-desirable effect for devices that work in that spectral region. Czernuszka [9] have not obtained a well-defined rosette structure after deformation of the (0 0 0 1) Zn face by Vickers microhardness, which can be probably due to the misorientation of (0 0 0 1) plane and/or the non-uniform distribution of the stress under their Vickers indenter. Previous work [10,11] on deformed bulk single crystals of ZnO using a spherical indenter showed that the slip along the basal and pyramidal planes is the major mode of plastic deformation.
In this work we have investigated the defect structure of plastically deformed ZnO bulk single crystal, using monochromatic and spectrally resolved cathodoluminescence (CL) microscopy in a scanning electron microscope (SEM/CL). This technique is a powerful tool for studying the microscopic plastic deformation of materials caused by microindentation, and related introduced dislocations.
2. Experimental procedure The used material was provided by Cermet Inc; ¯ ZnO it is a bulk n-type hexagonal wurtzite ð0 0 0 1Þ single crystal (the lattice parameters are a ¼ 3.2499 A˚, c ¼ 5.20568 A˚ at room temperature) with a carrier concentration and a Hall resistivity nearly equal to 1.5 1017 cm3 and 0.02 O cm, respectively. The samples were provided with a polished O face containing an etch pits density of about 4 104 cm2. Micro-hardness indentations were performed at room temperature and atmospheric pressure for 30 s using a Vickers diamond indenter with applying loads of 5, 15, 40, and 90 g respectively. In order to avoid the interference of plastic deformations (the arms of the indentation rosette) which extend away from the center of the imprint, the distance between two neighboring indentations has been chosen equal to 0.5 mm. The hardness, H, is given by the following formula: HðPa sÞ ¼ 1:8544P=d 2 ðN=m2 Þ; where d is the diagonal length of the imprint and P the applied load. CL measurements were performed in a JEOL 840 scanning electron microscope equipped with an Oxford elliptic mirror for light collection.
3. Results and discussion Mechanically Table 1 presents some characteristics of indentations which appear near to values reported in Refs. [10,11]. However, we note a small difference probably due to the shape of the used indenter, because material hardness H depends on
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
187
Table 1 Characteristics of the indentation rosettes Load (103 kg) Central zone diagonal length (106 m) Arms length (106 m) Penetration (109 m) Hardness (H) (GP)
5 8 50 1142 1.421
15 14 68 1714 1.895
40 18 70 2258 2.842
90 20 90 2857 4.093
Fig. 1. Monochromatic CL image of indentation at loads of (a) 5 g and (b) 15 g, l ¼ 387 nm, beam voltage 20 kV. Inset: corresponding SE image.
several experimental conditions, such as indenter matter and its shape. After indentation the residual imprints were directly imaged using secondary electron (SE) and CL. CL images were taken using a near-gap emission peak as shown in Fig. 1. We clearly observe an indentation figure called rosette, with a structure symmetry reflecting the hexagonal symmetry of the (0 0 0 1) ZnO planes. It appears as two well separated regions (a) a strongly perturbed central zone, appearing as a very dark region with non-uniform distribution damage, has an average size of about 8 mm; (b) six double arms, nearly parallel to axes separated by an angle of 601, confirming the hexagonal symmetry. Each double arm, clearly separated, is composed of two branches with comparable extensions. The extension of the arms is about 50 mm measured from the center of the imprint. Dark spots are visible in each arm; they correspond to the emergences of dislocations on the
surface which act as nonradiative recombination centers and therefore quench locally the luminescence. Nothing is observed between and within the arms. The length of each arm increases with increasing load as shown in Table 1. These two fundamental features have been observed previously for ZnO using a spherical indenter [11]. This is contrary to GaN, a structurally similar material, where the quenching of luminescence is not extended beyond the indenter contact diameter [8, 12–14]. The intensity of the near gap CL emission is drastically suppressed after indentation because of induced defects (dislocations). The central zone of the imprint clearly appeared as a star rosette, which has previously been reported for other material with similar structure [8,12]. Fig. 2 is a schematic diagram of the ZnO structure, viewed along the ½1 1 2¯ 0 direction. It is shown that f0 0 0 1g basal and f1¯ 1 0 1g pyramidal planes are terminated by either zinc atoms or
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
188
[0001] [1102] [1101] Pyramidal 1
[1120]
[1100]
Pyramidal 2
α(g) Basal
β(g) β
(g
)
α
(g)
Prismatic
[0001] 0
Zn
Fig. 2. Projection of the ZnO wurtzite structure along ½1 1 2¯ 0 direction.
Contrary, there is no detailed study on the slip geometry of dislocations around microindentation in ZnO. XTEM studies of ZnO revealed that deformation-introduced defects extend deep into the bulk of the material as well as lateral extension [11]. We expect that aðgÞ and bðgÞ dislocations extend into the crystal along the edges of Thompson’s tetrahedron (Fig. 3(a)). The rosette glide is characterized by the arms in ½1 1 2¯ 0, ½2 1¯ 1¯ 0, ½1¯ 2 1¯ 0 directions. The possible tetrahedral glide with inclined Burgers vectors, which is the principal factor in the determination of the hardness, does occur in directions possessing certain symmetry as in the rosette glide as shown in Fig. 3(b). [2203] [0223]
oxygen atoms. Thus dislocations lying in these planes possess a and b character, analogous to (1 1 1) planes in sphalerite structure. We suppose that dislocations move in the glide set planes. Sphalerite(1 1 1) and wurtzite(0 0 0 1) planes have hexagonal symmetry. This clearly appeared in the rosette structure obtained after Vickers microindentation, as has been reported previously for CdTe [15–17] and our results. ZnO can be considered as a hexagonal-closed-packed (HCP) of Zn atoms with O atoms in tetrahedral interstitial position of Zn. The atoms of ZnO structure are tetrahedrally bonded with a nearestneighbor environment similar to that of CdTe structure (sphalerite), but the geometry of the second neighbor is different. We expect that two kinds of defects (dislocations) have been created by indentation (at the impact point) and then propagated in well-defined directions. The first related to Zn atoms (aðbÞg ), while the second related to O atoms (bðaÞg ). These two kinds of dislocations act as non-radiative recombination centers which appeared in the CL image as dark lines. As in previous studies on ZnO the slip process is the major mode of plastic deformation. The slip process is composed of a rosette glide with surface parallel Burgers vectors and a tetrahedral glide with inclined Burgers vectors. The glide prism model has been successfully applied to interpret the results obtained for CdTe [15–17].
s
[1210] c a
[1120] b [2110]
(a)
T
[2110]
[1210]
[1120]
[2023]
(b)
[2203]
[0223]
β(α)(g) α(β)(g)
Fig. 3. (a) Thompson’s tetrahedron in the HCP structure and (b) expected slip directions in 3D of indented ZnO samples.
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
With increased loading, the central zone becomes larger and more perturbed, accompanied by shorter and very dark arms (denoted by arrows) appearing between double arms, which becomes less clear at the low loads as shown in Figs. 4(a) and (b). In addition, two ‘‘slip’’ triangles, opposite
189
in their positions and surrounding the central zone, appeared. Each triangle was made up of many triangles lying inside each other (cross-slip). The edges of these triangles are parallel to the rosette arms. The appearance of these opposite triangles has been reported in other material with
Fig. 4. Monochromatic CL image of indentation at loads of (a) 40 g and (b) 90 g, l ¼ 387 nm, beam voltage 20 kV. Inset: corresponding SE image. (c) Schema of an indentation figure as imaged by SEM/CL at relatively high load (90 g).
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
190
similar structure [8,12–14] and for CdTe [15]. We believe that other slip systems (slip bands) have been activated. The pop-in events observed elsewhere are related to the nucleation of other slip bands [10]. We note that dark lines appearing near the rosettes are not associated with the deformation itself, but with the polishing or/and handling processes. Fig. 4(c) is a schematic drawn of the CL image at relatively high load (as example 90 g). Each triangle is formed by the intersection of three rosette arms. The existence of these triangles as in CdTe [15] reveals that the tetrahedral glide is more probable. Fig. 5 shows typical room temperature CL spectra. It is clearly seen that the near band edge luminescence intensity decreased after indentation with the absence of any additional emission band. This means that deformation-produced
dislocations act as nonradiative recombination centers. The quenching of luminescence in the deformed areas (central zone and arms of the rosettes) is clearly visible in all CL images taken at the near gap emission peak (387 nm). The damaged area increased with increasing load; this implies that the band edge emission intensity in the CL spectra decreases with increasing load. However, additional XTEM and XCL studies seem to be needed to better understand the physical mechanism of plastic deformation in ZnO single crystal and to set a detailed model explaining the observed behavior. Further investigations on the effect of hydrogenation on the deformationproduced dislocations are under way and will be published elsewhere.
4. Conclusion Non-indented area
Intensity (a.u)
Local plastic deformation of bulk ZnO single crystal has been investigated. Cathodoluminescence scanning electron microscopy has been used to observe the defect configuration after microdeformation at room temperature. The following conclusions arise from the work described here:
3500 (a)
4000
4500
5000 5500 Wavelength (A˚)
6000
6500
5000 5500 Wavelength (A˚)
6000
6500
Intensity (a.u)
indented area
3500 (b)
4000
4500
Fig. 5. Cathodoluminescence spectra recorded from (a) undeformed and (b) deformed (i.e. dark) areas of indented sample at a load of 40 g.
(1) ZnO is a relatively soft material compared with GaN(HGaN15.5 GPa [12]). (2) CL images of indentations showed a very welldefined rosette structure with hexagonal symmetry, which extends to long distance (50 mm) as double arms in directions at 601 intervals ( ½1 1 2¯ 0, ½2 1¯ 1¯ 0, ½1¯ 2 1¯ 0). Each double arm is composed probably from aðgÞ and bðgÞ dislocations. (3) The slip systems occurring at microindentation of ZnO is composed from those with Burgers vectors parallel to the indented surface and those with inclined Burgers vectors. (4) At relatively high load, cross-slip is clearly observed, and more slip systems are activated which lead to more complicated deformation configuration. (5) CL spectra recorded at room temperature showed a decrease of the near band edge luminescence intensity and the absence of any additional emission band after indentation.
ARTICLE IN PRESS Z. Takkouk et al. / Physica B 366 (2005) 185–191
Acknowledgments The authors would like to express their sincere thanks to J.P. Rivie´re for his help in the deformation of the samples and F. Jomard for cathodoluminescence experiments. References [1] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun 105 (1998) 399. [2] D.C. Look, Materials Science and Engineering B 80 (2001) 383. [3] S. Hassani, A. Tromson-Carli, A. Lusson, G. Didier, R. Triboulet, Phys. Stat. Sol. (b) 229 (2) (2002) 835. [4] R.A. Laudice, E.D. Colbardn, A.J. Caporaso, J. Am. Soc. 47 (1964) 9. [5] T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, N. Sakagami, J. Cryst. Growth 214/215 (2000) 72. [6] T. Sekiguchi, N. Ohashi, Y. Terada, Jpn. J. Appl. Phys. 36 (1997) L289.
191
[7] N. Ohashi, T. Ohgaki, T. Nakata, T. Tsumuri, T. Sekiguchi, H. Haneda, J. Tanaka, J. Kore, Phys. Soc. 35 (1999) S287. [8] M.H. Zaldivar, P. Fernandez, J. Piqueras, Semicond. Sci. Technol. 13 (1998) 900. [9] J.T. Czernuszka, N. Pratt, Phil. Mag. Lett. 61 (3) (1990) 83. [10] S.O. Kucheyev, J.E. Bradby, J.S. Williams, C. Jagadish, Appl. Phys. Lett. 80 (6) (2002) 956. [11] J.E. Bradby, S.O. Kucheyev, J.S. Williams, C. Jagadish, Appl. Phys. Lett. 80 (24) (2002) 4537. [12] S.O. Kucheyev, J.E. Bradby, J.S. Williams, C. Jagadish, Appl. Phys. Lett. 77 (21) (2000) 3373. [13] H. Lei, H.S. Leipner, J. Schreiber, T. Wosinski, I. Grezegory, J. Appl. Phys. 92 (11) (2002) 6666. [14] J.E. Bradby, S.O. Kucheyev, J.S. Williams, Wong-Leung, Appl. Phys. Lett. 80 (3) (2002) 383. [15] A. Rivie`re, R. Sieber, J.P. Rivie`re, Microsc. Microanal. Microstruct. 2 (1991) 257. [16] H.S. Leipner, J. Shreiber, H. Uniewski, S. Hildbrandt, Scanning Microscopy 12 (1) (1998) 149. [17] J. Schreiber, L. Horing, H. Uniewski, S. Hildbrandt, Phys. Stat. Sol. (a) 171 (1999) 89.