BALKAN PHYSICS LETTERS Bogazici University Press 22 February 2011 BPL, 19, , pp , (2011)

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1 BALKAN PHYSICS LETTERS Bogazici University Press 22 February 2011 BPL, 19, , pp , (2011) TRANSMISSION ELECTRON MICROSCOPY, RAMAN AND ELECTRON SPIN RESONANCE CHARACTERIZATION OF Ge AND SiGe NANOCRYSTALS IN THIN FILMS SYNTHESIZED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION B. SAHIN Physics Department, Kirikkale University, Graduate School of Natural and Applied Sciences Kirikkale, TURKEY, S. AGAN Physics Department, Kırıkkale University, Kirikkale, TURKEY, and B.ASIK Physics Department, Karatekin University, Cankiri, TURKEY, Abstract: In this study, we have proposed to produce nano size metarials which are promised for technological aplications by many sciencities. In this aim, thin films have been grown using by Plasma Enchanced Chemical Vapor Deposition (PECVD). After that, grown these films have been annealed to form nanocrystals inside the matrix for different annealing time. Due to annealing processes size and size distiributions of Ge and SiGe nanocrystals in SiO 2 martix have been investigated using by Transmission Electron Microscopy (TEM) and Raman Spectroscopy. The results show that nanocrystals sizes depends on annealing time, temperature and flow rates clearly. At the same time, defects have been obtained from Electron Spin Resonance (ESR) spectroscopy using - source and broken bands defects with g-parameters measuruments inside the nanocrystals defect centers intensity were determined. 1. Introduction It's known that nano technology is used in many areas. One of these areas is semiconductor quantum dot structures. And there are many related research areas. Findings derived from the studies on morphological character and structure of these quantum structures have a great importance on design of electronic and optical

2 B. SAHIN et al.: TRANSMISSION ELECTRON MICROSCOPY 209 materials[1]. Because of their optical and electronic properties, Ge nanocrystals have a wide range of application area such as production of transistors [2], light emmiters [3] and photo detectors [4]. On the other hand, in case the quantum dot structures are used as memory elements, it was observed that ritention time has a direct corelation with the shape and size of quantum spot structures [5]. Ritention time was also related with the order of Si energy levels [6]. This study aims to analyse the change of size and distribution of Ge quantum dot structures which are reated in SiO 2 Matrix with PECVD technique related to annealing time and temperature. Formation of Ge nanocrystals in the obtained multilayer films is observed by the help of Raman Spektroscopy. Raman Spectoscopy is going to help us to determinate the size of nanocrystals and to gather information about the chemical structure. [7]. Moreover, situation of nano structured materials, which is in the study area of many scientists because of it's wide application area in optics and electronics, is also researched in our study by the help of ESR technique. 2. Experimental Procedures The germanosilicate film was grown in a PECVD reactor (model PlasmaLab 8510C) on Si substrates using 250 sccm SiH 4, 15 sccm NO 2, 60 sccm rate GeH 4 as precursor gases, at a sample temperature of C, a process pressure of 1000 mtorr under and an applied rf power of 10 W. Germanium doped SiO 2 :Ge layer is where nanocrystals form upon annealing. The samples were then annealed in N 2 atmosphere in a quartz oven at temperature 900 C for 15 and 60 minutes varying time. Figure 1: Plasma Enchanced Chemical Vapor Deposition (PECVD) systems.

3 210 BALKAN PHYSICS LETTERS Raman scattering experiments were carried out using a 1-m double monochromator with GaAs photomultiplier and photon counting electronics. Various lines of an Ar + laser, in particular 488 nm, were used to excite the spectra. Care was taken to minimize the heating of the sample during the experiment by using a cylindrical lens to focus the light onto the sample. The dangling bond defect density of the films was determined by electron spin resonance (ESR) measurement ESR measurements were performed at room temperature using an X-band spectrometer. 3. Results and Discussions TEM studies of several samples were caried out at (JEOL) 300kv. High- resoluton micrographs and selected area diffraction confirm that Ge nanocrystals are formed in our samples. Figure 2: (a) shows High Resolution Transmission Electron Microscopy (HRTEM) image of Ge nanocrystals with a size of 10 nm formed in the germanosilicate thin films. Sample is annealed at for 15 minutes. Figure 2: (a) HRTEM image of Ge and SiGe nanocrystals with a size of 5-10 nm formed in the SiO 2 matrix. (Annealed time 15 min.) (b) HRTEM image of Ge and SiGe nanocrystals with a size of nm formed in the SiO 2 matrix. (Annealed time 60 min.) It can clearly be seen that the most of the nanocrystals spherical shapes are formed. But some of thr them look like elipsoidal. Nanocriystals sizes are estimated to vary in the range 5-10 nm. Similar results are otained Figure 2: (b). Figure 2: (b) shows HRTEM image of Ge and SiGe nanocrystals with a size of nm formed 210

4 B. SAHIN et al.: TRANSMISSION ELECTRON MICROSCOPY 211 in the germanosilicate thin films. Sample is annealed at for 60 minutes. Figure 2: (b) show that nanocrystals sizes depends on annealing time, temperature clearly. The images show that not all nanocrystals are of the same size, there is size distiribution due to annealing temperature and time. The size depend spatial seperation of Ge nanocrystals has been observed. This may be crucial for memory device aplications. PECVD allows fort he control of the thiekness of the SiO 2 tunnel oxide underneath the nanocrystal layer as well as the density and size of the Ge nanocrystals [8]. Several smaller gray regions are seen in the Figure 2, We could explain them as a combination af crystallized and amorphous germanium clusters. Similar TEM data have been obtained for 15 and 60 min. annealed samples. Samples are show that anneal temperature time increases, size and density of the nanocrystals increases [9,10]. Typical Raman spectra of the SiO 2 :Ge samples grown with 60 sccm rates GeH 4 flow and annealed in nitrogen environment is shown in Figure :3 (a) and (b). Figure : 3 (a) Raman spectra from SiO 2 : Ge films with 250 sccm SiH 4 60 sccm GeH 4 gas flow rate at C annealing temperature for 15 min and (b) for 60 min Figure 3 shows the Raman measuruments from the samples upon the annealing time in the 15 and 60 min. We observe broad asymmetric peak centered around 299 cm -1 indicative of the quasi amorphus nature of the Ge- Ge and 435 cm -1 around Ge-Si scatter for samples annealed at C for 15 min. İf the annealing temperature time raised to 60 min. a sharp peak was appeared at 299 cm -1 and 435 cm -1. This peak stronger, narrower and becomes weaker as the annealing temperature time is increased up to 60 min. This is due to phonon confinement in small cyrstals [11]. We report experimental investigation by ESR measurements of room temperature -ray irradiation effects in PECVD Ge doped amorphous SiO 2. Figure 4 shows the relation between SiO 2 :Ge samples

5 212 BALKAN PHYSICS LETTERS grown varying temperature time with difference of ESR signals annealing at C temperature. Figure 4: The ESR spectrum as recorded in the sample at C temperature annealed for 15 and 30 min. The characteristic splittings are observed, which can be attributed to a typical a-center, identical to Si dangling bonds (DB) in the amorphous silicon/oxide matrix surrounding the crystalline core. Ge-related paramagnetic point defects (GECs) previously reported in the literature [12,13]. Paramagnetic defect called the Pb centers the Si/SiO 2 interface. Pb centers are quite important Si/SiO 2 interface state centers in as processed Si/SiO 2 systems [14,15]. Its intensity reflects the total number of these centers and linearly decreases with increasing annealing time. 4. Conclusions We have embedded Ge nanocrystals in silicon oxide thin films have been fabricated using by Plasma Enchanced Chemical Vapor Deposition (PECVD).Their HRTEM, Raman scattering and Electron Spin Resonance (ESR) properties have been studied. We have shown the formation Ge, SiGe nanocrystals in SiO 2 matrices by using this technicue. With increasing annealing time, nanocrystals sizes increasing and dangling bond defect densities decrease. 212

6 B. SAHIN et al.: TRANSMISSION ELECTRON MICROSCOPY 213 Acknowledgements: This work is a support ed by TUBİTAK (106T043) and Kırıkkale University (BAB- 2008/31). We thank to Prof. A. Aydınlı of Bilkent University to grow the samples. REFERENCES [1] S.V. Gaponenko, Optical properties of semiconductor nanocrystal, Cambridge University pres, [2] I. Shlimak, V. Vagner and V. Safarov, Proc. 25th Int. Conf. On the Phiysics of Semiconductors (Osake:Springer), [3] Y.Q. Wang, G.L. Kong, W.D. Chen, H.W. Diao, C.Y. Chen, S.B. Zhang and X.B. Liao, Appl. Phys. Lett. 81, [4] K.L. Wang, J.L. Liu, and Jin, G. J., Cryst. Growth, , [5] J. Nishii, K. Kintaka, H., Hosono, H. Kawazone, M. Kato and K. Muta, Phys. Rev. B 60, [6] R. Oha, N. Sugiyama, and K. Uchida, IEDM Tech. Dig. 557, [7] X.L. Wu, T. Gao, X.M. Bao, F. Yan, S.S. Jiang and D.J. Feng, Appl. Phys. 82, [8] A. Dana, S. Ağan, S. Tokay, A. Aydınlı and T.G. Finstad, Phys. Stat.Sol.(c) 4, p: , [9] Y.X. Jie, A.T.S. Wee, C.H.A. Huan, W.X. Sun, Z.X. Shen and S.J. Chua, Metarial science and Engineering, B 107, 8-13, [10] S. Ağan, A. Dana and A. Aydınlı, J. Phys. Condens. Matter 18, , [11] Y. Maeda, Phys. Rev. B51, 1658, [12] C.W. Chang, T. Matsui and M. Konto, Journal of Non-Crystalline Solids 354, [13] A. Alessi, S. Agnello, F.M. Gelardi, S. Grandi, A. Magistris, and R. Boscino, (Optical Society of America) Optics Express, Vol. 16, p 4895, [14] Y. Nishi, Jpn. J. Appl. Phys. 10, 52, [15] Y. Nishi, T. Tanaka and A. Ohwada, Jpn. J. Appl. Phys. 11, 85, 1972.