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Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Journal of Luminescence 80 (1999) 213—216 Dose dependence of room temperature photoluminescence from Si implanted SiO S. Cheylan*, N.B. Manson, R.G. Elliman Research School of Physical Science and Engineering, Institute of Advanced Studies, Australian National University, Canberra, ACT 0200, Australia Abstract Photoluminescence from Si implanted silica is studied as a function of Si fluence and Si concentration profile in order to assess the effect of particle size and size distribution on emission spectra. Peaked (skewed Gaussian) concentration profiles were produced by implanting with 400 keV Si ions and uniform Si profiles were produced by a multi-energy implant sequences. Both as-implanted and annealed samples are shown to exhibit a distinct maximum in the emission intensity as a function of ion fluence, with the intensity increasing with fluence up to the maximum and then decreasing at higher fluences. Samples with a uniform Si profile are also shown to produce emission which is significantly red-shifted relative to that of samples with a peaked Si profile. This is consistent with the fact that such samples are expected to have a narrower particle size distribution (i.e. a greater fraction of larger particles). 1999 Elsevier Science B.V. All rights reserved. PACS: 73.61.Tm; 78.55.!m; 81.20.!n; 68.55.Ln Keywords: Nanocrystal; Silica; Photoluminescence; Ion-implantation; Light emission; Photonics 1. Introduction The optical properties of Si nanocrystals in SiO have been studied extensively since visible room temperature photoluminescence (PL) was first ob- served in such systems [1]. However, despite in- tense study, the physical origins of the PL remain unclear. For example, it is still not known whether the luminescence results from electronic transitions within the nanocrystal itself or whether it results from defect centres outside the nanocrystal [2—4]. * Corresponding author. Further work is clearly required in order to fully understand these processes. Si nanocrystals are readily produced in SiO by ion-implantation and annealing. The excess Si, in- troduced by ion implantation, forms small precipi- tates during annealing. This is a relatively simple process which is compatible with standard elec- tronic and optoelectronic processing technologies. It also has the advantage that it allows independent control over the concentration and depth of the implanted impurity, and the size of the nanocrys- tals can be controlled by adjusting the implantation (dose, temperature) and annealing (temperature, time) parameters. 0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 0 0 - 8 Samples annealed at high temperature typically exhibit a broad PL peak at around 750 nm, with an increasing red shift with increasing implant dose [4—6]. The interpretation of the emission spectra is somewhat complicated by the fact that ion-im- plantation also produces radiation damage in the SiO and by the fact that the resulting nanocrystals have a broad size distribution. The latter is ex- pected to result from the Gaussian concentration profile produced by ion-implantation. It is there- fore of interest to compare the PL emission from samples implanted in a conventional manner with that from samples containing nanocrystals with a more uniform size distribution. In this study, the PL emission is studied as a function of implant dose and Si concentration profile in order to assess the significance of nanoc- rystal size and size distribution on the emission spectrum. The effect of annealing ambient is also briefly discussed. 2. Experimental Fused silica plates, 1 mm thick, were implanted with Si ions at room temperature. One set of sam- ples was implanted with 400 keV Si ions to doses of 0.6, 1.0, 2.0, 3.0, 4.0 and 6.0;10 Si cm \ , whilst a second set was implanted with five successive implants at energies of: 400, 300, 200, 150 and 100 keV to produce a uniform Si distribution over the range 100—500 nm. In the latter case, implant doses were chosen to give a uniform implant profile with a concentration equal to the peak concentra- tion of the four lowest dose single-energy implants. According to calculations using the PROFILE code, the single-energy implants give rise to a peaked profile, as shown in Fig. 1a, and the multiple energy implants give rise to a uniform profile, as shown in Fig. 1b. Implanted samples were subsequently annealed to 1000°C for 1 h using a quartz tube furnace with a flowing Ar ambient. Photoluminescence measure- ments were performed at room temperature using PROFILE is a commercially available code for calculating high-dose implant profiles. Fig. 1. Implant profiles calculated using the PROFILE code for a single 400 keV implant to a dose of 3;10 Si cm\ (dotted) and a multi-energy sequence designed to give approximately the same peak concentration as the single-implant profile (solid). the 488 nm line of an Ar ion laser as the excitation source. The PL emission was collected by a quartz light pipe and analysed using a grating spectrom- eter (Digikrom model DK480) and a GaAs photo- multiplier tube (Hamamatsu R943-02). Standard lock-in detection techniques were used to improve the signal-to-noise ratio and all spectra were cor- rected for the system response. 3. Results and discussion Fig. 2 shows typical PL emission spectra for samples as-implanted with single-energy ions and after annealing at 1000°C in Ar. As previously re- ported [7—9], the spectra from as-implanted sam- ples exhibit a single broad peak spread over the range from 550 to 900 nm, centred at around 660 nm. This emission is believed to result prim- arily from defect centres created in the SiO by ion implantation [10,11]. Interestingly, however, the emission intensity is observed to increase with in- creasing ion dose for doses up to 6;10 Si cm \ and to decrease with increasing dose thereafter. The initial increase is consistent with an increase in the concentration of optically active defects, however, the decrease at higher doses suggests that either the nature of these defects is altered by subsequent irradiation or that competing relaxation paths 214 S. Cheylan et al. / Journal of Luminescence 80 (1999) 213— 216 Fig. 2. PL emission spectra for samples: (a) as-implanted with 400 keV Si ions to various doses, and (b) after annealing to 1000°C in Ar for 1 h. Doses are indicated in the figure. are created. (i.e. either the creation of non-radiative defects or defects with emission wavelength outside the region of observation.) This complex depend- ence on ion dose is even more evident after annealing, as shown by the spectra in Fig. 2b and summarised in Fig. 3a. In this case, the emission intensity exhibits a distinct maximum at a dose of 4;10 Si cm \ compared to 6;10 Si cm \ for as-implanted samples. The strongest emission is observed for a relatively narrow range of Si doses, corresponding to peak Si concentrations around 2;10 Si cm \ . The stronger emission after an- nealing is consistent with previous reports and is generally attributed to the annealing of defects in SiO [4—6]. However, preliminary results have shown that the emission intensity can be increased significantly by annealing at low temperatures (500°C) in forming gas (H /N ). This implies that many alternative relaxation channels exist for excit- ed carriers, even after high temperature annealing. The reduced emission intensity at higher doses Fig. 3. Emission intensity as a function of peak concentration for samples with: (a) single-energy implants (peaked profiles) and (b) multi-energy implants (uniform profiles). Circles indicate as-implanted samples and diamonds represent samples annealed at 1000°C in Ar. could therefore result from a complex interplay between different defects as a function of dose. However, other possibilities also exist. For example, interaction between Si nanocrystals is ex- pected to increase with increasing Si concentration (i.e. quantum tunnelling between crystallites, large scale physical clustering). More work is clearly re- quired in order to assess these possibilities. PL emission spectra for samples as-implanted with multi-energy Si ions are similar to those depic- ted in Fig. 2a. However, after annealing, spectra from these samples exhibit distinct differences from those implanted with a peaked Si profile, as shown in Fig. 3b and Fig. 4. First, the emission intensity is much lower (approx. 30%) than that from samples irradiated with a single-energy implant to the same peak concentration, and second, the spectra exhibit a significant red-shift relative to the single-implant samples. Since the size distribution of precipitated S. Cheylan et al. / Journal of Luminescence 80 (1999) 213— 216 215 Fig. 4. Peak emission wavelength as a function of total implant dose for samples with: (a) single-energy implants (peaked pro- files) and (b) multi-energy implants (uniform profile). Circles indicate as-implanted samples and diamonds represent samples annealed at 1000°C in Ar. nanocrystallites is expected to be narrower (i.e. greater fraction of larger particles) in the multi- energy case it might be concluded that the domi- nant emission comes from smaller particles and that the emission is lower for samples with a uni- form Si profile because of the reduced fraction of smaller particles. However, if this were the case, the emission intensity would be expected to increase for lower concentration samples. As shown in Fig. 3b, this is not the case. The lower emission intensity must therefore relate to some other effect as well. Interestingly, the emission intensity for samples im- planted with multi-energy ions is enhanced 20 fold by annealing in forming gas (H /N ) at 500°C, whereas only a factor of 7 is observed for single- energy implants. After such an anneal the emission intensity is therefore similar in both cases. This again suggests that many defects survive high-tem- perature annealing. Fig. 4 compares the peak of the emission spec- trum for single and multi-energy implants as a func- tion of total ion dose. For as-implanted samples a slight redshift is observed with increasing dose for sampleswithbothpeakedanduniformSiprofiles. This effect is more pronounced for annealed sam- ples, where the nanocrystals are more fully de- veloped. Significantly, the red-shift is greater for samples implanted with a uniform Si profile. This is consistent with the fact that such a profile should result in a greater fraction of larger particles. 4. Conclusions It has been shown that the PL emission intensity of as-implanted samples increases with increasing dose to a maximum value before decreasing at high- er doses, and that such samples exhibit a monotonic redshift with increasing dose. In addition, it has been shown that Si nanocrystals precipitated within a uniform Si distribution exhibit PL emission which is red-shifted relative to nanocrystallites precipitated within a peaked profile. This is consistent with the expected narrowing of the nanocrystal size distribu- tion in the uniform case. Measurements are in pro- gress to correlate the measured nanocrystal size dis- tribution and PL emission spectrum. References [1] L.T. Cahnam, Appl. Phys. Lett. 57 (1990) 1046. [2] G. Qin, G.G. Qin, J. Appl. Phys. 82 (1997) 2572. [3] S K. 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