Structure and electrical transport in films of Ge nanoparticles ...

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The films containing Ge nanoparticles embedded in SiO2 matrix were prepared by RF ... The electrical transport was shown to take place through the network of ...
J Nanopart Res (2012) 14:930 DOI 10.1007/s11051-012-0930-5

RESEARCH PAPER

Structure and electrical transport in films of Ge nanoparticles embedded in SiO2 matrix Ionel Stavarache • Ana-Maria Lepadatu • Adrian V. Maraloiu • Valentin S. Teodorescu Magdalena Lidia Ciurea



Received: 27 September 2011 / Accepted: 16 May 2012 / Published online: 17 June 2012 Ó Springer Science+Business Media B.V. 2012

Abstract The films containing Ge nanoparticles embedded in SiO2 matrix were prepared by RF magnetron sputtering and subsequently by thermal annealing. Their structure was investigated by conventional transmission electron microscopy and high resolution transmission electron microscopy together with energy-dispersive X-ray spectroscopy. The electrical behavior of films was studied by measuring current–temperature and current–voltage characteristics. The structure investigation reveals two kinds of features: a low density of big Ge nanoparticles with sizes from 20 to 50 nm and a network of small amorphous Ge nanoregions/nanoparticles (5 nm size or less) with high density, both being embedded in amorphous SiO2 matrix. The electrical transport was shown to take place through the network of amorphous Ge nanoregions. At low temperature, the T-1/4 dependence of the current was evidenced, while at high temperature, the T-1 Arrhenius dependence was found. At both low and high temperatures, the conductivity is nearly constant. The behavior at low

All the authors contributed equally to this article. I. Stavarache  A.-M. Lepadatu  A. V. Maraloiu  V. S. Teodorescu  M. L. Ciurea (&) National Institute of Materials Physics, 105 bis Atomistilor Street, 077125 Magurele, Romania e-mail: [email protected]

temperature was explained by the hopping mechanism on localized states located in a band near the Fermi energy, while at high temperature by the charge excitation to the extended states. Keywords Nanoparticles  Magnetron sputtering  TEM  Electron irradiation  Conduction mechanisms  Nanostructures

Introduction Nanostructures formed by Ge and Si nanoparticles embedded in dielectric matrix, particularly amorphous SiO2, have been intensively studied in the last decades for their potential applications in optoelectronics (Tzeng and Li 2008), photovoltaics (Zhang et al. 2011), and non-volatile memory devices (Beyer et al. 2007; Chen et al. 2007; Peibst et al. 2010). In addition, amorphous GexSi1-xOy thin films are investigated for micro-machined uncooled bolometer applications (Ahmed and Tait 2003), and amorphous Ge/SiO2 multilayered structures are attractive for their property of ferromagnetism at room temperature (RT) (Zhen et al. 2010). Also, metal–insulator–semiconductor photodetector with high efficiency, based on amorphous Ge quantum dots embedded in a SiO2 matrix, were reported (Cosentino et al. 2011a). The deposition method commonly used for the preparation of structures formed by Si or Ge nanoparticles embedded in amorphous SiO2 matrix is the

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RF magnetron sputtering (Choi et al. 2001; Ray and Das 2005; Chen et al. 2007; Teodorescu et al. 2008; Gao et al. 2008; Aktag˘ et al. 2010; Jie et al. 2011; Cosentino et al. 2011b). Two approaches are commonly used: one consists in the conventional Ge nanocrystal fabrication method using a thick SiO2 matrix (Kolobov et al. 2003; Choi et al. 2006a; Jensen et al. 2006) and the other one is the fabrication of super lattices (Li et al. 2004; Gao et al. 2008; Zhang et al. 2010a; Pinto et al. 2011). A SiO2 diffusion barrier is deposited on the Si substrate in order to prevent Ge diffusion into it (Li et al. 2004; Chew et al. 2006; Choi et al. 2006a; Basa et al. 2008; Aktag˘ et al. 2010), and a top SiO2 layer is deposited in order to impede the diffusion of Ge atoms through the film surface (Li et al. 2004; Basa et al. 2008; Aktag˘ et al. 2010). The film deposition is made either on non-heated substrates (Inoue et al. 1998; Shen et al. 2002; Li et al. 2004) or on heated ones, the structure of the films being dependent on the substrate temperature (Cosentino et al. 2011a; Zhang et al. 2010a, 2011; Pinto et al. 2011). In order to form Ge nanoparticles, subsequently to the deposition, an annealing process is performed in a furnace (Chew et al. 2006, 2007; Jensen et al. 2006; Aktag˘ et al. 2010), by a rapid thermal annealing (Choi et al. 2001, 2006a; Zhang et al. 2010b) or in microwave field (Srinivasa Rao et al. 2011). Ge nanocrystals with diamond structure are the most frequently obtained, as it results from literature (Fujii et al. 1998; Inoue et al. 1998; Shen et al. 2002; Gao et al. 2008; Cosentino et al. 2011b). Several key factors strongly influence the nanocrystal formation. They are the annealing conditions (the ambient, temperature, and duration) and the Ge concentration related to Si (or SiO2) (Choi et al. 2006b; Chew et al. 2007). Thus, different shapes of nanocrystals (faceted, spherical, and even with multiple twinning defects) can be obtained (Choi et al. 2001; Kolobov et al. 2003). The nanocrystals usually experience compressive stress in the oxide matrix, which is related to the density, size, and distribution of Ge nanocrystals (Chew et al. 2007; Ciurea et al. 2011). This compressive stress can lead to the distortion of Ge lattice from the cubic structure to the orthorhombic one (Li et al. 2004). Our previous work also evidenced the (high pressure) tetragonal phase of Ge, appearing in rather thick (2.5 lm) RF GeSiO films (Stavarache et al. 2011).

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There is extensive literature about the structural and optical properties of films containing Ge nanoparticles embedded in amorphous SiO2 (Maeda 1995; Takeoka et al. 1998; Choi et al. 1999, 2001; Shen et al. 2002; Zheng et al. 2008; Zhang et al. 2010a; Srinivasa Rao et al. 2011; Pinto et al. 2011; Jie et al. 2011). However, few papers were dedicated to the electrical behavior of these films (Fujii et al. 1996, 1998; Inoue et al. 1998; Zhao et al. 2002; Zhang et al. 2010b, 2011). Current–voltage (I–V) characteristics measured on Ge nanocrystals embedded in SiO2 matrix, at temperatures lower than 77 K, present aperiodic steplike features dependent on nanocrystal size (Inoue et al. 1998). These features are explained by resonant tunneling through the discrete energy levels of Ge nanocrystals in interplay with Coulomb blockade effect. The authors discuss the influence of the asymmetry between source and drain junctions upon the aperiodic steplike features in the I–V curves. The variable range hopping (VRH) mechanism on localized states associated with Ge clusters is found to be the dominant mechanism in as-prepared co-sputtered films of SiO2 containing Ge clusters (Fujii et al. 1996). The films exhibit the conductivity–temperature (r–T) dependence according to the Mott law (ln r / T 1=4 ) up to about RT. On contrary, in the annealed films (Fujii et al. 1998), the r–T characteristics have a ln r / T 1=2 dependence up to RT too, and the I–V curves are nearly Ohmic (r nearly constant). This electrical behavior is explained by a thermally activated tunneling of carriers between adjacent nanocrystals. An Arrhenius temperature dependence of the conductivity is also reported (Zhang et al. 2010b, 2011), explained by a thermally activated charge hopping between the nearest neighbor nanocrystals. In this article, we present the structure and electrical investigations of films containing Ge nanoparticles embedded in SiO2 matrix, prepared by magnetron sputtering method. For this, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were taken, and I–T and I–V characteristics were measured. The films contain big Ge nanocrystals with low density embedded in amorphous oxide matrix. The amorphous oxide matrix also contains a network of small amorphous Ge nanoregions. The dominant charge transport takes place through the network of small amorphous Ge nanoregions.

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Experimental details The films were deposited by RF magnetron sputtering, in high-purity Ar atmosphere, on quartz substrates, using a UVN-75R1 equipment (Stavarache et al. 2011). A SiO2 target with Ge pieces attached on it was used. The Ge/Si ratio is about 0.7. The asdeposited films were then annealed in H2 at 500 °C. In order to perform electrical measurements, 7 9 2 9 130 nm (length 9 width 9 height) rectangular aluminum electrodes with a planar geometry, separated with 1.5 mm from each other, were thermally evaporated on the annealed films. The TEM specimens were prepared by conventional cross-section (XTEM) method, by cutting the sample in pieces of 0.5 9 2 mm with a diamond wire saw, gluing each two pieces face to face with M-bond, followed then by mechanical polishing and finally by ion milling in a Gatan PIPS 691 instrument. The conventional TEM measurements were performed on a Jeol 200CX electron microscope, while the energydispersive X-ray spectroscopy (EDX) measurements were done using a JEM ARM200F instrument. For electrical investigations, I–T and I–V measurements were performed using a Janis CCS-450 cryostat, a Keithley 6517A electrometer, and a Lakeshore 331 temperature controller.

Results and discussion Structure investigations The investigated films formed by Ge nanoparticles embedded in amorphous SiO2 matrix have about 2.5 lm thickness. The low-magnification XTEM image illustrated in Fig. 1 shows the film structure

Fig. 1 Low-magnification XTEM image of the film with SAED pattern inserted

Fig. 2 TEM image showing the characteristic features of the film structure: a low density of big nanoparticles and a high density of small nanoparticles with dark contrast

over the whole thickness. The film structure is amorphous as the selected area electron diffraction (SAED) pattern taken on a large area (1,000 nm diameter) reveals. Some quite big (from 20 to 50 nm) nanoparticles are visible with higher density in the top part of the film, but they are also present in all film volume as is revealed by Fig. 2. These big nanoparticles are non-uniformly distributed in the film thickness. They are more visible at the top part of the film, due to the TEM contrast conditions and the local thickness of the XTEM specimen. The big nanoparticles TEM contrast suggests the presence of crystallinity. However, the SAED patterns obtained from small areas (300 nm diameter) containing big nanoparticles reveal only traces of crystallinity. Similar nanoparticles were identified by SAED on TEM specimens prepared by extraction of small fragments from the film, in our previous paper (Stavarache et al. 2011). These nanoparticles consist of Ge with tetragonal structure (high pressure phase). In the present study, the TEM specimens were prepared by conventional ion milling. This process relaxes the stress field in the very thin region of the XTEM specimen, where the HRTEM imaging is possible. Also, it can induce the amorphization of the tetragonal high-pressure phase, through the stress relaxation of the surrounding oxide matrix, as it was evidenced in the high pressure studies of the bulk Ge (Coppari et al. 2009). However, some crystallinity remains. This subject will be analyzed in a separate paper. We must point out that the big nanoparticles are distributed in the film volume at distances between 20 and 50 nm.

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Figure 2 also shows the presence of amorphous dark contrast nanoregions with high density, which are uniformly distributed in the film volume. A closer look at the nanometric structure, using high-resolution multibeam mode, shows the presence of a network of globe-shaped nanoregions with dark contrast (HRTEM image in Fig. 3). By exploring the very thin regions of the TEM specimen (less than 10 nm), we observe that the most of the dark contrast nanoregions (nanoparticles) are separated. The average size of the dark contrast nanoregions can be estimated at 5 nm. Similar distances (or less) separate these nanoregions from each other, but some of them are really connected. This is the main structural feature of the film nanostructure which can be related with the electrical transport properties as one will see afterward. The HRTEM observations for several minutes induce the crystallization of the amorphous nanoregions in cubic Ge nanocrystallites as shown in Figs. 4 and 5. It is difficult to estimate the local temperature of the irradiated area during the HRTEM observations, but it is probably of the same order of magnitude as the temperature during the film annealing. In our opinion, the crystallization process is mainly controlled by the electron irradiation enhanced diffusion effect. It is also interesting to point out that the crystallization effect indirectly shows that some of the Ge amorphous nanoregions are really connected. In Fig. 4, the crystallites A and B show the same crystalline orientations, which can be an indication that the initial

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Fig. 4 Crystallization of the dark contrast nanoregions under the electron irradiation during the HRTEM observations, forming Ge nanocrystallites with cubic structure. The majority of the lattice fringes visible in the image corresponds to the (111) lattice planes of cubic Ge (0.33 nm). The particles A and B show the same crystalline orientation, suggesting that the initial amorphous Ge nanoregions were connected

Fig. 5 TEM image showing the crystallization effect in the irradiated area: recorded after several minutes of HRTEM observation. The SAED patterns from irradiated and nonirradiated areas are inserted

Fig. 3 High-magnification HRTEM image of the dark contrast nanoregion network in the film. The majority of the nanoregions (nanoparticles) are amorphous, but some of them show the lattice fringe contrast corresponding to the (111) reflections of the Ge cubic structure

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amorphous nanoregions were connected and the crystallization started from a single nucleation center. The EDX spectrum (Fig. 6) obtained from a large area (300 nm) of the film shows a Ge/Si atomic ratio of about 0.73, similar with the value from our previous paper (Stavarache et al. 2011). The EDX measurements performed in nanoprobe mode with an electron probe of 3 nm diameter, centered on a dark contrast nanoregion, show a ratio of Ge/Si between 3.5 and 4, depending on the place, indicating the massive presence of Ge atoms in the dark nanoregions (see Fig. 7). The EDX measurements also

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Fig. 6 EDX spectrum obtained from a large area (300 nm diameter), showing the average atomic concentrations of elements in the film

Fig. 7 EDX spectrum obtained with a probe diameter of 3 nm centered in the middle of a dark contrast nanoregion

using a 3 nm probe located between the dark nanoregions (i.e., in amorphous SiO2 matrix) show a much less Ge/Si ratio of 0.4 or 0.5. We can conclude that the dark contrast nanoregions mainly consist of amorphous clusters of Ge atoms. It is also difficult to estimate if the traces of crystallinity observed in Fig. 3 can be considered to be present in the film or appear in several seconds after the beginning of the high resolution observations. In our

opinion, the Ge clusters forming the network of dark contrast nanoregions are initially amorphous. Electrical investigations Current–temperature characteristics were measured at different biases between 6 and 50 V (6, 15, 30, and 50 V). These characteristics have two parts with different temperature dependences, meaning that

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different mechanisms take place at low temperature (approximately up to the RT) compared with high temperature. The analysis of the curves shows that the best fit for them in the low temperature interval is  pffiffiffiffiffiffiffiffiffiffiffi obtained for a I / exp  4 T0 =T law, where the characteristic temperature T0 is a constant (Pollak et al. 1973; Elliott et al. 1974; Fujii et al. 1996; Lepadatu et al. 2011). At the same time, for the high temperature interval, the best fit is obtained for an Arrhenius dependence, I / expðEa =kB T Þ. The experimental curves and corresponding fit ones are presented in Figs. 8 and 9. As one can see from Fig. 8, the curves lg I = f(T-1/4) are parallel, and the characteristic temperature we determined is T0 = 2.2 9 109 K. From Fig. 9, the curves lg I = f(1000/T) are also parallel, with an activation energy Ea = 0.36 eV. The correlation coefficients obtained for all curves presented in Figs. 8 and 9, for both dependences on each curve are greater than 0.999. In the literature, the T-1/4 dependence of the conductivity (and current, respectively) in amorphous materials was explained by both Mott and Pollak et al. models. The model of VRH in the absence of Coulomb interaction was proposed by Mott (Mott 1969) and considers that the hopping takes place between localized states with a very high concentration located

Fig. 8 Experimental I–T curves and corresponding fit ones in the representation lg I = f(T-1/4)

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Fig. 9 Experimental I–T curves and corresponding fit ones in the representation lg I = f(1000/T)

at the Fermi energy. This model is valid at low temperature, but cannot explain the experimental behavior of T-1/4 dependence at high temperatures (up to RT). In the model proposed by Pollak et al., the hopping takes place between localized states that form a band (with 2E0 width) near the Fermi energy, thus explaining the extension of I–T curves with T-1/4 dependence to high temperatures in amorphous Ge (Pollak et al. 1973). I–T characteristics with a T-1/4 dependence up to 320 K (in amorphous carbon nitride) can also be explained by a VRH-related model (Godet 2001), where the hopping takes place between localized states in the band tails. Taking into account the microstructure of the films, it is logical to assume that the charge transport takes place mainly through the network of small amorphous Ge nanoregions with size of 5 nm or less and separated by a similar distance or less. We consider this because the transport between the crystalline big nanoparticles (with diameters from 20 to 50 nm, see Figs. 1, 2) with tetragonal symmetry has to be excluded, because the average distance between them is too large (20–50 nm or more) to be tunneled or ‘‘hopped’’ by the electrons (Iancu et al. 2004). In other words, the presence of the big nanoparticles which are distributed in the film volume at large distances does not really influence the electrical properties of the films.

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The model proposed by Pollak et al. is suitable to explain the T-1/4 dependence of the current obtained in our films (Fig. 8). The theoretical band width 2E0 evaluated by Pollak et al. in amorphous Ge is between 0.14 and 0.20 eV being strongly dependent on its shape, while the experimental value also obtained by them on amorphous Ge is 2E0 = 0.36 eV. The density   of states at the Fermi energy is g0 ¼ C 4 kB T0 n3 , where C is a numerical constant, kB is the Boltzmann constant, T0 is the characteristic temperature, and n is the localization length. We calculated g0 in our films and we found it to be 6.04 9 1016 cm-3 eV-1. For this, we used our experimental value for the characteristic temperature T0 = 2.2 9 109 K, we considered n = 1 nm and took for the C constant the value of 1.84 (Pollak et al. 1973). Consequently, in the amorphous Ge nanoregions, the total concentration of localized states, which we presume to be located in a band of h i RE 0.36 eV width, is N ¼ E0 0 g0 1  ðE=E0 Þ2 dE ¼ 1:43  1016 cm3 . This value obtained for the network of small amorphous Ge nanoparticles is less than the value of 2.2 9 1017 cm-3 obtained by Pollak in amorphous Ge, because our films were annealed. The annealing diminishes the density of states g0 (Elliott et al. 1974) and, consequently, the total concentration of localized states. As it is shown in Fig. 9, the current presents an Arrhenius dependence at high temperatures. This can be produced by the excitation of carriers from localized states to the extended ones (at absolute energy higher than the mobility edge). We cannot precise if the transport is of p-type, as usually happens in amorphous Ge (Lomas et al. 1973; Zhang et al. 2010b, 2011), because Hall measurements could not be performed, due to the very high resistivity of the films. The conductivity–voltage (r–V) characteristics (Fig. 10) were measured at 370 and 193 K in the -50 to ?50 V voltage interval (low field regime, E \ 3.3 9 102 V/cm). These curves show that the conductivity is nearly constant at both temperatures. These results are in good agreement with both proposed transport mechanisms, namely conduction on extended states at the mobility edge at high temperatures and hopping between localized states that form a band near the Fermi energy at low temperatures (Elliott et al. 1974).

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Fig. 10 Experimental r–V characteristics measured at 370 and 193 K

Conclusions In this article, we prepared and characterized films of Ge nanoparticles embedded in amorphous SiO2 matrix. The film structure presents two kinds of features. On the one hand, a low density of big Ge nanoparticles was observed in the TEM image contrast on specimens prepared by ion milling. Most of them are amorphous as the SAED pattern reveals, the areas containing such particles revealing only traces of crystallinity. In our opinion, the ion milling relaxes the film matrix, leading to the amorphisation of the tetragonal high pressure phase, as it was observed in our previous paper. On the other hand, a high density of small amorphous Ge globe-shaped nanoregions/ nanoparticles embedded in the amorphous SiO2 matrix was evidenced in HRTEM images. The size of small amorphous Ge nanoparticles is about 5 nm or less, and the distance between them is also 5 nm or less, they being sometimes connected. The network of small amorphous Ge nanoregions is the main responsible for the charge transport in the investigated films. The I–T characteristics have different temperature dependences at low (up to RT) and high temperatures. The T-1/4 dependence of the current, evidenced in the low-temperature interval, is explained by a hopping mechanism on localized states located in a band near the Fermi energy with a band width of 0.36 eV, while the T-1 Arrhenius dependence, found at high temperature, is explained by charge excitation above the mobility edge (extended states). The conductivity is

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nearly constant over both temperature intervals, supporting the proposed mechanisms. Acknowledgments This work was supported from Project No. 471/2009 (ID 918/2008), Ideas Program, National Research, Development and Innovation Plan 2007–2013.

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