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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 264 (2007) 249–253 www.elsevier.com/locate/nimb
Structural studies of Ge nanocrystals embedded in SiO2 matrix N. Srinivasa Rao a, S. Dhamodaran a, A.P. Pathak a,*, P.K. Kulriya b, Y.K. Mishra b, F. Singh b, D. Kabiraj b, J.C. Pivin c, D.K. Avasthi b a
School of Physics, University of Hyderabad, Central University (P.O.), Hyderabad 500 046, India b Inter University Accelerator Centre, P.O. Box 10502, New Delhi 110 067, India c CSNSM, IN2P3-CNRS, Batiment 108, F-91405 Orsay Campus, France Received 12 April 2007; received in revised form 17 August 2007 Available online 19 September 2007
Abstract Germanium nanoparticles embedded in SiO2 matrix were prepared by atom beam sputtering on a p-type Si substrate. The as-deposited films were annealed at temperatures of 973 and 1073 K under Ar + H2 atmosphere. The as-deposited and annealed films were characterized by Raman, X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). Rutherford backscattering spectrometry was used to quantify the concentration of Ge in the SiO2 matrix of the composite thin films. The formation of Ge nanoparticles were observed from the enhanced intensity of the Ge mode in the Raman spectra as a function of annealing, the appearance of Ge(3 1 1) peaks in the X-ray diffraction data and the Ge vibrational mode in the FTIR spectra. We have irradiated the films using 100 MeV Au8+ ions with a fluence of 1 · 1013 ions/cm2 and subsequently studied them by Raman and FTIR. The results are compared with the ones obtained by annealing. 2007 Elsevier B.V. All rights reserved. PACS: 61.46.Df; 79.20.Rf; 78.30. j Keywords: Ge nanoparticles; Atom beam sputtering; Raman
1. Introduction Si and Ge nanocrystals (nc-Ge, nc-Si) embedded in SiO2 have recently attracted much attention due to their possible applications in integrated optoelectronic devices [1–3]. In particular it has been suggested that direct optical transitions are possible in small size group-IV nanocrystals [4]. Although porous Si is expected to be the most promising Si-based light emitting material, Ge nano crystalline (ncGe) embedded in Silica glasses have their own advantages. These Ge dots are useful in infrared detectors [5]. Ge has smaller electron and hole effective masses and larger dielectric constant than Si. The effective Bohr radius of the exci-
*
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[email protected] (A.P. Pathak). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.094
ton in Ge is larger than that in Si. Hence Ge is much easier to change the electronic structure around the band gap than Si due to its larger exciton Bohr radius [6,7]. To date, a series of techniques have been used to prepare nc-Ge, including sol–gel [8], inorganic solution-phase synthetic routes [9], ion-implantation [10,11], UV-assisted oxidation [12] and sputtering [13,14]. The present work investigates the structure of the Ge nanoparticles grown by atom beam sputtering (ABS). This technique has been used to prepare metal nanoparticles embedded in a silica matrix and found to be suitable for synthesis of nanoparticles embedded in silica matrix [15,16]. ABS has been already used to prepare Ge nanoparticles in SiO2 matrix and to compare the luminescence properties with samples prepared by ion-implantation [17]. The present study concentrates on the structural characterization of the films as a function of annealing temperature and ion irradiation induced crystallization using Raman, XRD and FTIR.
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content in the films and was found to be close to the nominal values.
2. Experiments The samples were prepared by the fast ABS method where small pieces of high purity Ge were placed on a SiO2 target and co-sputtered by an Argon atom source. Thin films of the mixture of Ge and SiO2 were deposited on Si substrates. Thin films of Ge and SiO2 nanocomposites with various Ge concentrations of 6%, 15% and 30% were prepared by co-sputtering of Ge and silica using 1.5 keV Ar atoms. The Ar atom source was mounted at an angle of 45 facing the sputtering target inside a vacuum chamber, as shown in Fig. 1. The uniformity of the films was achieved by rotating the substrate with a DC motor. The beam delivered by the atom source was around 5 cm in diameter when placed 10 cm away from the sputtering target. The source delivers a maximum current density of 30 lA/cm2 on the target with a variable energy ranging from 0.8 to 2 keV. The variation of the semiconductor fraction in the films is determined by the relative area and sputtering rates of Ge and SiO2. Annealing of the samples was performed at 973 and 1073 K in a quartz tube furnace in Ar + H2 (5%) reducing atmosphere. The duration of annealing was kept constant for 30 min in all cases. These annealing temperatures were chosen because in the literature it was reported that in a few cases at very high temperature films exhibit an amorphous nature [18]. We also irradiated the as-deposited films with 100 MeV Au8+ ions with a fluence of 1 · 1013 ions/cm2 and compared with the results of annealing. The Raman measurements were carried out at room temperature using the 514.5 nm line of an Ar ion laser as an excitation source. X-ray diffraction measurements were carried out with the Cu Ka line, k = 0.154 nm in a glancing angle incidence geometry. FTIR spectra were recorded using a Nexus 670 FTIR spectrometer in the wave number region 400–1500 cm 1 with a resolution of 4 cm 1. RBS was used for quantifying the Ge
3. Results and discussion Raman spectra of a Si wafer have line shapes similar to Ge nanoparticles. In particular, the mode observed around 300 cm 1 from the Si wafer is identified as two-phonon TA modes of Si which overlaps with the single-phonon LO mode of Ge [19–21]. The mode around 300 cm 1 from the Si wafer is identified as two-phonon transverse acoustic (TA) modes of Si which overlaps with the single-phonon longitudinal optical (LO) mode of Ge. To remove the contribution of the Si peak, first a Raman spectrum was recorded from a Si wafer which showed a weak signal around 300 cm 1. Then the Raman spectrum was recorded from the Ge + SiO2 composite film on a Si wafer. After subtracting the Si peak contribution, we observed a high intensity peak attributed to Ge in the films. The full spectrum is shown in Fig 2. Figs. 3 and 4 show the Raman spectra of the as-deposited and of the samples annealed at 973 and 1073 K of 6%, 15% and 30%. For the as-deposited samples, we observed a very low intensity Ge peak around 300 cm 1, indicating early stage of Ge formation. For higher concentrations it is replaced by a broad hump around 270 cm 1, indicating the presence of amorphous Ge. The integrated intensity of the mode at 300 cm 1 increases as a function of annealing, supporting our argument that is mode is from Ge and not the 2-TA phonon mode of the Si substrate. The Raman modes around 410 and 480 cm 1 correspond to the Ge–Si and Si–Si local optical phonon modes respectively. The sharp peak position at 521 cm 1 is from the TO/LO phonon mode of the c-Si substrate. The peak around 300 cm 1 from the annealed films appears asymmetric in the low-frequency region, which is due to the smaller size
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Fig. 1. Schematic set-up of ABS.
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Fig. 2. Raman spectra of Si wafer, Ge + SiO2 composite annealed at 800 C and Ge + SiO2 composite after removing the Si contribution.
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Ge (6%)
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Fig. 3. Raman spectra of (a) Ge 6% as-deposited and annealed samples and (b) Ge 15% as-deposited and annealed samples.
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Fig. 4. Raman spectra of (a) Ge 30% as-deposited and annealed samples and (b) Ge 30% as-deposited and irradiated.
of the Ge particles. The position of the Ge mode red shifts (3 cm 1) to lower frequency upon annealing for the 6% and 15% samples. The position of the Ge mode for the as-deposited 30% sample is observed around 270 cm 1, which is blue shifted to higher frequency up to 300 cm 1 (Fig. 3(a)) upon annealing. Compared with the available literature, the width of the Ge mode indicates a particle size of 10–15 nm [21]. In fact a weak signature of Ge is present in all spectra for the as-deposited samples. Fig. 4(b) shows the Raman spectra of Ge 30% as-deposited and irradiated samples. The irradiated sample exhibits a sharp peak around 300 cm 1 together with a shoulder at low-frequency (270 cm 1), corresponding to crystalline and amorphous Ge, respectively. This is in contrast to the annealed results where only sharp crystalline Ge mode
was seen and no signal of amorphous Ge was observed. The Raman spectra of annealed and irradiated samples were comparable with those of the samples prepared by implantation and r.f. puttering and subsequent high temperature annealing [13,22,23]. The energy loss of high energy ions is mainly through electronic stopping mechanism (in the present case 17 keV/nm) and hence the irradiation results in local heating of the samples within a nanometric track. The material within the ion tracks is molten for a short duration of time 10 12 s if the input energy is large enough. The result obtained indicates that the heating is sufficient for promoting short range rearrangements leading to Ge crystallization. Fig. 5 shows the XRD patterns of the as-deposited and annealed films. No Ge peak is observed from the
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Ge (15%)
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2θ (degrees) Fig. 5. GIXRD of (a) Ge 6% as-deposited and annealed (b) Ge 15% as-deposited, annealed and irradiated (c) Ge 15% annealed.
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Fig. 6. FTIR spectra of (a) Ge 30% as-deposited and annealed and (b) Ge 30% as-deposited and irradiated.
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as-deposited films while high temperature annealing results in a broad Ge(3 1 1) peak at 54.5 [24,25]. In few cases Ge(1 1 1) around 27.3 and GeO2 peak around 26.5 (Fig. 5(c)) are observed. For irradiated samples the Ge(3 1 1) peak was sharp along with Si(3 1 1) peak and no other Ge peaks were observed. FTIR spectra of samples as a function of annealing and irradiation are shown in Fig. 6. The structure of amorphous GeO2 can be described as Ge–GeyO4 y tetrahedral connected by bridging oxygen atoms. The asymmetric stretching vibration mode of Ge– O–Ge in stoichiometric GeO2 is at 885 cm 1. The vibration frequency of oxygen deficient structures, for y = 0–4 changes linearly from 885 to 740 cm 1 according to the empirical equation [26], m(x) = 72.4x + 743 cm 1, where x is the oxygen content which ranges from two to zero. In the case of as-deposited films the Ge–Ge mode was also observed (740 cm 1) along with Si–O–Ge and GeO2 vibrational modes around 1020 and 840 cm 1, respectively [27,28]. Upon annealing in a reducing atmosphere the Si– O–Ge mode merges with the Si–O–Si asymmetric stretching mode (1108 cm 1). Moreover the Si–O–Si mode sharpens which indicates the densification of the SiO2 matrix. The stretching mode of Si–Si observed around 614 cm 1 becomes sharper upon annealing. Apart from Ge–Ge, the Si–Si peak also became sharp. At the higher temperature it is possible to diffuse Si atoms from the substrate to the matrix. In the case of irradiation the Si–O–Ge mode has totally vanished and the Ge–Ge mode becomes intense and sharper. This may be due to either diffusion of Si or reduction of oxygen content in the film or both. These observations are consistent with the Raman studies. 4. Conclusions Ge nanoparticles embedded in SiO2 matrix were prepared by Atom beam Co-sputtering from a combined target of Ge and SiO2. The Ge modes in the Raman spectra of annealed samples are sharp, indicating the existence of crystalline Ge. In the spectra of irradiated samples the sharp mode was observed with a broad shoulder peak indicating the existence of both amorphous and crystalline phases of Ge. XRD shows a broad Ge(3 1 1) peak in all the cases and a GeO2 and Ge(1 1 1) peak in some cases. FTIR complements the Raman results. As a function of annealing the Si–O–Ge mode merges with the Si–O–Si stretching mode. The Si–O–Si mode becomes sharper upon annealing indicating densification of the matrix. The Si–O– Ge mode vanishes upon irradiation and a more intense and sharp Ge–Ge mode has been observed. The possible mechanism for irradiation induced crystallization is discussed. The results have been compared with the other growth methods like sputtering and implantation.
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Acknowledgements N.S.R. and S.D. thanks CSIR, New Delhi for award of JRF and SRF respectively. A.P.P. thanks IUAC for UFUP project. We would like to acknowledge the help of S.R. Abhilash, Dr. S. Mohapatra, S.A. Khan and D.C. Agarwal for help during the experiments. D.K.A. is thankful to DST for providing XRD under IRPHA project. References [1] [2] [3] [4] [5]
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