Ion Irradiation Enhanced Formation and Luminescence of Silicon Nanoclusters from a-sio x

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1 Journal of the Korean Physical Society, Vol. 39, December 2001, pp. S83 S87 Ion Irradiation Enhanced Formation and Luminescence of Silicon Nanoclusters from a-sio x Yohan Sun, Se-Young Seo and Jung H. Shin Department of Physics, Korea Advanced Institute of Science and Technology, Taejon T. G. Kim and C. N. Whang Department of Physics, Yonsei University, Seoul J. H. Song Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul (Received 1 November 2000, in final form 22 May 2001) Effect of irradiation on the formation of Si nanoclusters from a-sio x films and their luminescence properties are investigated. 1 µm thick SiO x films with Si content ranging from 33 to 50 at.% and were deposited by electron cyclotron resonance enhanced chemical vapor deposition of SiH 4 and O 2. Prior to anneal, some samples were implanted with 380 kev Si to a dose ranging from cm 2 to cm 2. All films were rapid thermal annealed under flowing Ar environment, and hydrogenated after anneals to passivate defects. For films with Si content less than 40 at.%, ion irradiation slightly reduces the PL intensity and induces a slight blueshift of the luminescence. For films with Si cotent greater than 40 at.%, ion irradiation greatly increases the PL intensity. Based on the effect of the irradiation dose and the ion specie, we rule out chemical effects due to the implanted ions, and propose that irradiation damage greatly accelerates nucleation of small Si clusters from the a-sio x matrix. I. INTRODUCTION Si nanocrystals, due to quantum confinement effects, possess optical and electrical characteristics which allow realization of Si-based devices with novel functionalities [1 7]. A widely used method to prepare a dense and robust array of well-passivated Si nanocrystals is precipitating them out of SiO x (x < 2) [8 10]. While this method has the advantage of being compatible with the standard Si processing technology, it often requires long anneals at temperatures in the excess of 1100 C [8], which is undesirable from the process point of view. The thermal budget can be reduced by increasing the excess Si content of the a-sio x films, but this leads to larger Si cluster sizes and lower luminescence efficiencies. Thus, there is very little possibility for control over the formation of Si nanoclusters and their luminescence properties once the Si content of the a-sio x is fixed. In this paper, we investigate the effect of pre-anneal ion irradiation upon the formation of Si nanocluster from a-sio x films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition, and their lumines- jsun@mail.kaist.ac.kr cence properties. We find that the effect of ion irradiation is dependent upon the Si content. For Si-poor a-sio x films, ion irradiation slightly reduces the PL intensities, but also blueshifts the PL spectra. For Si-rich a-sio x films, ion irradiation greatly increases the PL intensities. We have also investigated the effect of varying the ion dose, annealing temperature, and annealing time. Based upon the results, we identify irradiation induced damage as a possible candidate responsible for the observed effects. II. EXPERIMENT Films were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR- PECVD) of SiH 4 and O 2. The base pressure and the deposition pressures were torr and torr, respectively. The microwave power was 400 W, and the deposition temperature was 450 C. The O 2 flow rate was fixed at 2 sccm, and the SiH 4 flow rate was varied from 1.7 to 2 sccm. The Si content of the deposited films were verified using Rutherford backscattering spectroscopy to vary from 33 to 50 at.%. Some -S83-

2 -S84- Journal of the Korean Physical Society, Vol. 39, December 2001 Fig. 1. The PL spectra of the film with Si content of (a) 33, (b) 36, (c) 38, (d) 40, (e) 42 and (f) 50 at.% that have been implanted with 380 kev Si ions to a dose of cm 2, annealed for 1 min anneal at 1000 C, and hydrogenated. films were irradiated with 380 kev Si to a dose ranging from cm 2 to cm 2. All films were rapid thermal annealed under flowing Ar environment for either 1 or 30 min at temperatures ranging from 800 to 1000 C. Annealed samples were hydrogenated by annealing in ultra-high purity forming gas (10 % H 2 balanced with 90 % N 2 ) to passivate defects. Photoluminescence (PL) spectra were measured at room temperature using the 488 nm line of an Ar laser, a grating monochromator, and employing the standard lock-in technique. Si and InGaAs photodioces were used as detectors in the visible and infrared range, respectively. All spectra were corrected for the system response. III. RESULTS Figures 1 (a)-(f) show the PL spectra of the films with Si content of 33, 36, 38, 40, 42, and 50 at.% that have been implanted with 380 kev Si ions to a dose of cm 2, annealed for 1 min anneal at 1000 C, and hydrogenated. Also shown are the spectra of the same films that did not undergo ion irradiation but otherwise underwent the same treatments. In all other cases, we observe luminescence in the nm range and another luminescence peak centered near 1100 nm. We note that without hydrogenation, very little luminescence could be observed [9,12 14]. Since Si nanoclusters are known to luminesce in the nm range, the luminescence due to the nanoclusters were deconvoluted by fitting three gaussian peaks centered near 700, 900, and 1100 nm, and subtracting the luminescence peak centered near the 1100 nm. From the our published result, after the 900 C anneal, silicon crystallites from in Si-rich a-sio x films, as evidenced by Si [111] reflection peaks (X-ray diffraction measurment). From the width of the diffraction peak, we estimate the crystal size to be in the nm range [15]. We also note that many others have also reported that CVD deposit a-sio x films often contain Si nanocrystals which have PL spectra in nm range [10,16]. The result of Fig. 2. (a) The effective peak position defined to be the weighted average of the two PL peak, and (b) the effect of ion irradiation on the PL intensities at the effective peak position. this deconvolution is shown in Fig. 2(a) which shows the effective peak position, defined to be the weighted average of the two PL peak positions. We find that that PL peak position increases with increasing Si content. Furthermore, for films with Si content less than 40 at.%, ion irradiation induces a slight blueshift. However, the PL intensity of these ion irradiated films are slightly reduced compared to that of non-irradiated films. The effect of ion irradiation on the PL intensities are shown in Fig. 2(b). We find that for film with Si content of 36 at.%, ion irradiation reduces the PL intensity slightly. For films with higher concentration of Si, however, ion irradiation increases the PL intensity. The greatest degree of enhancement is observed from the film with 42 at.%. The effect of ion dose is shown in Fig. 3, which shows the PL spectra of the film with Si content of 42 at. % that has been implanted with 380 kev Si ions to doses ranging from cm 2 to cm 2 and annealed at 1000 C for 1 min. We find that a dose of at least cm 2 is needed to observe enhancement of PL. If the dose exceeds cm 2, however, ion irradiation no longer greatly enhances the PL intensity, even though it does modify the PL spectrum. The effect of annealing temperature is shown in Fig.

3 Ion Irradiation Enhanced Formation and Luminescence of Yohan Sun et al. -S85- Fig. 3. The PL spectra of the film with Si content of 42 at.% that has been implanted with 380 kev Si ions to doses ranging from cm 2 to cm 2 and annealed at 1000 C for 1 min. 4, which shows the PL spectra of the film with 42 at.% Si that has been implanted with 380 kev Si ions to a dose of cm 2 and annealed for 1 min at temperatures ranging from 800 to 1000 C. We find that once the annealing temperature exceeds 850 C, further increase in the annealing temperature has little effect. We also observe a rather strong luminescence from the film that was not annealed. However, we note that because of the hydrogenation step, this film can be though of as having undergone a 700 C anneal. IV. DISCUSSION First of all, the fact that the peak position of the lu- Fig. 4. The PL spectra of the film with 42 at.% Si that has been implanted with 380 kev Si ions to a dose of cm 2 and annealed for 1 min. at temperatures ranging from 800 to 1000 C. Fig. 5. PL spectra of the films with 42 at.% Si that has been implanted with (a) 380 kev Si ions to a dose of cm 2 or (b) 1050 kev Ge ions to a dose of cm 2. For comparison, the PL spectra of the films with same Si contents that has been (c) not implanted. All samples are annealed at 1000 C for 1min. minescence spectra increases with increasing Si content as shown in Fig. 2(a), and that the luminescence intensity increases with hydrogenation is consistent with, and indicates that the luminescence we observe is indeed due to the nanoclusters and not due to defects in the oxide. This conclusion is further supported by the fact that no luminescence is observed from the film with Si content of 33 at.% even after irradiation. Therefore, the figures shown above indicate that irradiating the deposited a-sio 2 film prior to anneal greatly enhances the formation and luminescence of Si nanoclusters, at least for those films with Si content greater than 40 at.%. Given the very small dose necessary for this effect to occur (< 0.1 at.% peak concentration), however, this is not likely to be due to the excess Si introduced during ion implantation. This is further confirmed by Fig. 5, which shows that enhancement of PL is observed even when Ge was implanted instead of Si. Such enhancement, however, can be at least partially explained by the damage induced by ion irradiation. Ion irradiation at room temperature can displace atoms and thus create a very large concentration of point defects and network defects [17,18]. Presence of such defects are known to enhance the diffusion of atoms at elevated temperatures [11], and such enhanced initial diffusion would be consistent with the observed fast, initial formation of Si nanoclusters. But creation of defects does not explain all of the effects of ion irradiation. This is demonstrated in Fig. 6, which shows the PL spectra of the films after a 30 min anneal at 1000 C. We observe that implantation-induced enhancement of luminescence from the film with Si content of 42 at. % persists even though most of irradiationinduced defects are expected to annihilate quickly at this elevated temperatures. In fact, the most intense lumines-

4 -S86- Journal of the Korean Physical Society, Vol. 39, December 2001 mation of smaller clusters, thus explaining the observed blueshift. For the silicon-poor films, however, the final luminescence intensities are expected to be lower than that observed from pre-existing clusters, since the anneal temperature is too low. Only for the silicon-rich films for which the annealing temperature of 1000 C is sufficient for formation of Si nanoclusters will this amorphization result in enhanced luminescence intensity, as is observed. V. CONCLUSION In conclusion, we have investigated the effect of ion irradiation on the formation and luminescence properties of Si nanoclusters from a-sio x films deposited by ECR- PECVD. We find that for films with Si content greater than 40 at.%, ion irradiation greatly enhances the luminescence intensity. Based on the effects of Si concentration, ion specie, and ion dose, we identify irradiationinduced defect creation and amorphization as likely factors contributing to the observed effects. REFERENCES Fig. 6. The PL spectra of the film with (a) Si content of 42 at.% that has been implanted with 380 kev Si ions to a dose of cm 2 or cm 2, and annealed for 1 or 30 min. at 1000 C, and (b) Si contents ranging from 36 to 42 at.% that have been not implanted, annealed for 1 or 30 min at 1000 C. cence among all films considered in this paper is observed from the film with Si content of 42 at.% that has been ion implanted and annealed at 1000 C for 30 min. We note, however, that the dose required to observe implantation-enhanced luminescence coincides with the known threshold for amorphization of Si [11]. And such amorphization, we propose, is an important factor for the observed effects of ion irradiation. First, 1000 C is much lower than what has been observed to be necessary to induce nucleation of Si nanoclusters from a-sio x if the Si content is less than 40 at.% [8]. The fact that we observe luminescence from the film with Si content of 36 at.% that did not undergo ion irradaition after an anneal of only 1 min at 1000 C indicates that there already exist some clusters in our as-deposited films. Implanting such as-deposited film with ions will eliminate much of the pre-exisiting clusters, and thus raise the degree of supersaturation of Si in the surrounding oxide matrix. The nucleation rate is a very sensitive function of the degree of supersaturation. Thus amorphization is expected to result in increased nucleation rate and thus faster for- [1] See, for example, Microcrystalline and Nanocrystalline Semiconductors , Mat. Res. Soc. Symp. Proc. 536, Materials Research Society, Warrendale, Pennsylvania (1999). [2] L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990). [3] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe and K. Chan, Appl. Phys. Lett. 68, 1377 (1996). [4] SuK-Ho Choi and Jin Jang, J. Korean Phys. Soc. 32, 718 (1998). [5] H. J. Lee, Y. H. Seo, D. H. Oh, K. S. Nahm, Y. H. Lee, E.-K. Suh, H. J. Lee, Y. G. Hwang, K. H. Park, S. H. Chang and E. H. Lee, J. Korean Phys. Soc. 26, S103 (1993). [6] Y. H. Seo, K. S. Nahm, H. I. Jeon, E.-K. Suh, Y. H. Lee, H. J. Lee and Y. G. Hwang, J. Korean Phys. Soc. 28, S75 (1995). [7] K. Kim, M. S. Suh, D. H. Oh, Y. H. Lee, C. J. Youn, K. B. Lee and H. J. Lee, J. Korean Phys. Soc. 30, 580 (1997). [8] F. Iacona, G. Franozò and C. Spinella, J. Appl. Phys. 87, 1295 (2000). [9] K. S. Min, K. V. Shcheglov, C. M. Yang, Harry A. Atwater, M. L. Brongersma and A. Polman, App. Phys. Lett. 69, 2033 (1996). [10] Keunjoo Kim, M. S. Suh, T S Kim, C. J. Youn, E. K. Suh, Y. J. Shin, K. B. Lee, H. J. Lee, M. H. An, H. J. Lee and H. Ryu, Appl. Phys. Lett. 69, 3908 (1996). [11] P. A. Stolk, F. W. Saris, A. J. M. Berntsen, W. F. van der Weg, L. T. Sealy, R. C. Baklie, G. Krötz and G. Müller, J. Appl. Phys. Lett. 75, 7266 (1994). [12] S. Cheylan and R. G. Elliman, Nucl. Instrum. Methods Phys. Res. B 148, 986 (1999). [13] S. Cheylan and R. G. Elliman, Appl. Phys. Lett. 78, 1225 (2001).

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