Crystallization effects in annealed thin Ge Se films photodiffused with Ag

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1 Journal of Non-Crystalline Solids 32 (200) Crystallization effects in annealed thin Ge films photodiffused with Ag M. Mitkova, M.N. Kozicki, H.C. Kim, T.L. Alford Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 2-20, United States Abstract Ge glasses with composition ranging from Ge 20 0 to Ge 40 0 photodoped with Ag and annealed at moderate temperatures are investigated. Raman spectroscopy suggests that Ag photodiffusion brings about the formation of a Ge backbone that is deficient in and that the structure is depolymerized due to the extraction of during the formation of crystalline products. X-ray diffraction shows that after Ag diffusion into the glass, nanocrystals of Ag 2 and Ag Ge form, depending on the composition of the host. The crystal size is affected by the molecular clustering of the host. Ó 200 Elsevier B.V. All rights reserved. PACS:.30. j; 1.10.Nz; 1.43.Fs Keywords: Amorphous semiconductors; Raman scattering; X-ray diffraction; Raman spectroscopy 1. Introduction Photodiffusion of silver is one of the most interesting effects that occurs in chalcogenide glass films as it dramatically changes the properties of the starting material and forms a ternary that has a multitude of potential applications [1]. One system that has attracted significant community attention in the research is Ag-diffused Ge. The kinetics of Ag photodiffusion in Ge has been shown to be dependent on the hosting backbone [2]. The form of the resulting structure has been proposed [3]. X-ray diffraction and scattering studies have been carried out on Ge and the stoichiometric composition Ge 2 showing the formation of Ag 2 and in some cases Ag 2 O 4 [4] or Ag 2 and Ag Ge on a very fine scale of phase separation []. Our recent studies show that the introduction of Ag in chalcogenide glass films by photodiffusion results in significant changes of the hosting backbone s structure []. This Corresponding author. Tel.: ; fax: address: mmitkova@asu.edu (M. Mitkova). is caused not only by the interaction of the material with light but also by the reaction between the Ag and the hosting material []. The result of this is a higher amount of Ag being able to be incorporated into the film compared with the case of bulk material []. Furthermore, we have found a correlation between the composition of the hosting material and the morphology of Ag electrodeposits grown on these photodiffused films [9]. In our opinion, one of the most important unresolved issues regarding the photodiffusion process is the relationship between the hosting backbone composition and the composition and morphology of the diffusion product. Closely related to this problem is crystal growth during isothermal annealing of the diffusion product since, in real world of applications of these materials, thermal processing is often applied or they are operated at elevated temperatures. In this work, we investigate these topics. 2. Experimental Samples were prepared by sequential thermal evaporation of Ge x 1 x thin films (x being 0.2; 0.3; 0.33 and /$ - see front matter Ó 200 Elsevier B.V. All rights reserved. doi:10.101/j.jnoncrysol

2 M. Mitkova et al. / Journal of Non-Crystalline Solids 32 (200) ) and Ag, using the technique described in [], at a pressure of 2 10 Torr on silicon substrates. The thickness of the Ge films was 20 nm while the thickness of the Ag was 0 nm. To promote silver diffusion, the samples were illuminated with 43 nm light at room temperature for 10 min with an optical power density of. mw/cm 2. These experimental conditions were established as being sufficient to saturate the films with Ag while leaving little residual Ag on the surface []. Any remaining Ag was dissolved in 1 mol solution of Fe(NO 3 ) 3. The compositions of the films before (i.e., only the Ge glass film) and after the diffusion process were studied using Rutherford backscattering spectrometry (RBS) analysis, performed with a 2 MeV 4 He with the beam at normal incidence to the sample and a backscattering angle of at a reduced charge of around 0.2 lc/mm 2. Raman spectra were obtained to provide information on the short range order occurring in the hosting films following the diffusion processes since they retain an amorphous component following all treatments. Because of the light sensitivity of the materials investigated, the need to excite Raman scattering at low energy is of paramount importance. For this reason, resonant enhancement of the scattering by tuning the laser energy close to the optical band gap of the glass is particularly desirable. Raman studies were therefore performed in the micro Raman mode with the following conditions: illumination with the 4.1 nm wavelength of a Kr ion laser, 1 accumulations of 1 s with 1. mw of light power at the sample. The phases formed in the films were characterized using X-ray diffraction (XRD) performed with Cu Ka radiation with k = nm in the 2h range from 23 to 100 at a step size of 0.0. The fabricated Ag-diffused films were annealed at C, 110 C, 12 C and 10 C in an inert atmosphere to avoid oxidation for either 1 or 120 min. After annealing, the XRD spectra were measured at room temperature and the crystal size of the diffused products was evaluated using the Scherrer equation; t ¼ ð0:9 kþ=b cos h; where t is crystallite size, k is X-ray wavelength, B is the full width at half maximum of the peak, h is the angle at full width at half maximum. This approach is suitable to estimate the particle size of very small crystals. We used X-Pert Peak Find software to identify the peaks since the spectra were noisy due to the very small amount of crystalline inclusions and small crystallite size. In this particular case, the diffraction peaks were rather broad. The calculated data give average sizes in the case where several peaks are available for a particular crystalline product. 3. Results The RBS data confirmed that the initial composition of the hosting backbone was within ±2 at.% of the composition of the source material, which corresponds well with ð1þ the results of our earlier studies []. As far as the structure of the hosting glass backbone is concerned, the spectra of non-diffused glasses show the specific Raman features closely matching those of bulk materials with the same composition, as illustrated in Fig. 1(a) (d). However, after diffusion, the spectra of all samples show a vibrational band at 10 cm 1 and a higher frequency band at 200 cm 1 independent of the composition (Fig. 1(e)), suggesting the formation of a structure containing ethane-like units with Ge Ge bond as well as the Ge tetrahedra. These spectra remained unchanged following the moderate annealing applied in this study. Fig. 2 gives representative curves of the XRD spectra of the photodiffused glasses for an initial composition of Ge 33. In all cases, the hosting Ge glass remained amorphous during the annealing while the silver containing species formed nanocrystals. We found orthorhombic Ag 2 with peaks at 2h = 33.10, 34.2, JCPDS card ; 39.03, 41.3 and 42.3 JCPDS card 2-0; as well as cubic Ag 2 with 2h = 3.91 JCPDS Intensity, Arb. Units a b c d e 100 Ge 20 0 Ge 30 0 Ge 33 Ge 40 0 Film resulting after photodiffusion Raman shift, cm -1 Fig. 1. Raman spectra of the undoped Ge glasses and spectrum of the photodiffused material. Compositions are noted in the figure. Intensity, Arb. units ^ ^ ^ ^ Theta, Deg. Fig. 2. Representative XRD plots of Ge 33 glass photodiffused with Ag annealed (a) at C for 1 min, (b) at C for 120 min, (c) at 10 C for 1 min and (d) at 10 C for 120 min; peaks characteristic for Ag Ge ; ^ peaks characteristic for aag 2 ; peaks characteristic for bag 2. Some peaks were reduced to fit on a single graph. d c b a

3 19 M. Mitkova et al. / Journal of Non-Crystalline Solids 32 (200) card and Ag Ge with 2h 2.139, 2.00, 4.9 and 49.3 JCPDS card Crystal growth during annealing is presented in Figs. 3 for different compositions of the hosting glass. One can see not only fluctuations in the nanocluster size but also in the composition of the crystal fractions depending on the chalcogenide host. β Ag 2 α Ag 2 Fig. 3. Cluster sizes for annealed Ge Ag 4.2 for the conditions shown on the figure. 9 βag 2 Fig. 4. Cluster sizes for annealed Ge 1 42 Ag 40 for the conditions shown on the figure β Ag Ge Fig.. Cluster sizes for annealed Ge Ag 33 for the conditions shown on the figure. When Ag is diffused in Ge 20 0 only Ag 2 forms while for diffusion in more Ge-rich compositions (over 33 at.% Ge) the formation of Ag Ge occurs as well. A progressive increase in nanocrystal size with the Ge content of the hosting backbone occurs until the stoichiometric composition Ge 2 is reached, beyond which the crystal size decreases to that seen in the more -rich glasses. Of course, the annealing temperature and time also have an influence over the crystal size and these results are discussed next. 4. Discussion 9 Ag Ge Fig.. Cluster sizes for annealed Ge 2 42 Ag 30 for the conditions shown on the figure. First, we would like to stress the fact that the photodiffused films contain more Ag than can be introduced in bulk glasses while retaining a glassy character of the films. During our studies of the Ge Ag system in bulk or thin film form, we have consistently found Ag to be chemically bonded. Furthermore, illumination with light causes formation of a number of charged defects that react with Ag. This fact has important consequence since some is extracted from the initial Ge backbone to react with the diffused Ag. So the remaining chalcogenide glass backbone becomes deficient, as demonstrated by the appearance of a Raman signature that is characteristic of a Ge-rich glass, independent of the initial composition of the host. In this composition the underlying molecular phase consists of face-sharing quasi one-dimensional ethane-like Ge 2 ( 1/2 ) chain fragments whose presence is manifested on the Raman spectra by the appearance of the mode at 10 cm 1 [10] depicted in Fig. 1(e). The Raman spectrum of the resulting material shows a lower intensity ratio between the modes at 10 cm 1 and the mode of the Ge-tetrahedral units at 200 cm 1 when compared to the intensity ratio of these modes for a Ge 40 0 initial glass film indicating that the number of ethane like units is lower than in Ge 40 0 glass. However this structure still contains Ge Ge bonds. They are the result of the spontaneous reaction of Ag with charged metastable states on the chalcogen [11] initiated by light illumination and with charged defects

4 M. Mitkova et al. / Journal of Non-Crystalline Solids 32 (200) occurring at bond conversion [12]. This reaction will be preferred since the energy that it requires is less than the energy for the Ge bonding (4.4 kcal/mol versus 113 kcal/mol). We suggest that this is the reason for the extraction of some from the Ge backbone for the formation of Ag 2 in addition to the reaction of Ag with the initially available free chains. Formation of Ag Ge also brings about this effect since its six atoms are related to the extraction of one Ge atom from the glassy material while in the case of initial glass there are on average two atoms for each one Ge atom in the Ge tetrahedral structure excluding face sharing units. The structure of Ge backbone formed after photodoping is depolymerized to some extent due to the extraction of and formation of crystalline products. It is for this reason that the organization of the photodiffused hosting glass does not change with the moderate annealing applied, as happens with pure Ge films [13] where the local stressed configurations with a high free energy relax through breaking of the Ge Ge bonds and formation of Ge cornersharing units due to reaction with wrong bonds. Considering the results from the XRD analysis, the crystals forming after diffusion are relatively small because they can only form in the free interspaces available in the matrix of the hosting glass. Although in the case of Ge 20 0 glass the initial structure is floppy, following the initial silver inclusion and formation of Ag 2, the glass structure becomes depleted in and stiffer. The internal space limitation produces the same effects as elevated pressure, stabilizing some clusters in the high temperature form which has the closest packing. With Ge-enrichment of the hosting backbone, the intensity of the peaks of aag 2 becomes higher, suggesting reflectance from a larger number of planes. At the same time, Ag Ge clusters are formed and we assume that these occur at terminal defects on the Ge tetrahedra in the case of the Ge 33 host or develop within the volume of the films when Ag is diffused in a Ge 40 0 host. Indeed, Mössbauer spectroscopy definitely shows that replacement of Ge by Ag occurs in Gerich glasses [] so a combined effect could be the reason for the development of the three-component composition. During annealing at 10 C, additional cubic Ag 2 likely forms but transforms back to the orthorhombic structure after cooling to room temperature. The other specific feature that we would like to discuss is the average size of the crystal nanoclusters and how this is related to the host composition. Within the limits of accuracy of the method, there is a noticeable fluctuation in the crystal size as shown in the results section (see Figs. 3 ). We suggest that this effect is related to the molecular clustering of the hosting glass. As pointed out by Feltz et al. [14] the evolution of molar volume with composition has a maximum at x = 0.33, i.e., for Ge 2, where the structure is less densified. Although the lattice parameters of Ag 2 are smaller, in the cases when Ag Ge crystallizes in a more dense and rigid backbone, its initial clusters are smaller than the Ag 2 clusters developed in a structure with larger free volume and higher flexibility of the hosting backbone. One last point for discussion is the development of the crystal growth at elevated temperatures. The conditions discussed in this work are relatively mild since our final goal is to deduce information useful for the processing of these materials for applications in semiconductor technologies and particularly in the formation of programmable metallization cell (PMC) memory devices []. The annealing results showed that there are no changes in the composition of the crystalline products with time at the temperatures chosen. In the case of Ag 2 clusters, the crystalline phases are different than the surrounding material and we assume that their growth is limited by atomic diffusion, which is somewhat restricted considering the heterogeneous character of the medium, and allows only at about % growth after annealing. In the case of the formation of Ag Ge clusters, their growth is much more affected by annealing and their size changes by about 10%. This situation corresponds closely to homogeneous growth. We suggest that the adjacent clusters fuse with Ge atoms cross-linking the cluster edges and leading to the formation of a common structure combining the three elements. The consequence of the crystallization nature in photodiffused films is that the structure that forms is very stable and there are no compositional changes caused by the temperatures used.. Conclusions We have investigated the products of Ag diffusion in Ge films with varying initial composition and also collected information about their development during isothermal annealing. The main results of our work can be summarized as follows. 1. Regardless of the initial composition of the hosting glass, the photodiffused material shows Raman features characteristic of Ge-rich material. The glassy component becomes -deficient due to consumption of in the formation of the diffusion products. 2. The diffusion products are nanocrystalline regions dispersed into the glassy matrix and their composition is dependent upon the hosting glass composition and develops from Ag 2 to a combination of Ag 2 and Ag Ge with enrichment of the host in Ge. 3. The cluster size of the crystalline products depends on the molar volume of the host in close relation to its rigidity. 4. Isothermal annealing at moderate temperatures results in diffusion limited slow growth of the Ag 2 clusters and homogeneous growth of the Ag Ge clusters. Acknowledgement This work was sponsored by Axon Technologies Corp.

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