Influence of the annealing temperature and silicon concentration on ...

JOURNAL OF APPLIED PHYSICS 102, 043104 共2007兲

Influence of the annealing temperature and silicon concentration on the absorption and emission properties of Si nanocrystals A. Podhorodecki,a兲 G. Zatryb, and J. Misiewicz Institute of Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

J. Wojcik and P. Mascher Centre for Emerging Device Technologies, Department of Engineering Physics, McMaster University, Hamilton, Ontario L8S 4L7, Canada

共Received 23 February 2007; accepted 6 July 2007; published online 23 August 2007兲 Silicon nanocrystals embedded in a silicon-rich silicon-oxide matrix have been fabricated at different silicon contents 共38%, 40%, and 49%兲 using plasma-enhanced chemical vapor deposition and annealing at different temperatures in the range from 900 ° C to 1100 ° C. Their optical properties have been investigated by photoluminescence and transmittance measurements. Strong, room-temperature emission bands at ⬃1.6 eV have been observed for all samples, with intensities dependent on the annealing temperature and Si content of the samples. From transmittance measurements, a redshift of the absorption edge has been detected when increasing the annealing temperature or Si content. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2772501兴 centration, on optical 共emission as well absorption兲 properties of silicon nanocrystals embedded in the SiO2 matrix.

I. INTRODUCTION

In the last few years, nanocrystalline materials, i.e., nanocrystalline powders,1 nanocrystalline films,2 or nanocrystals embedded inside different insulating matrices,3 have been intensively studied due to their broad application perspectives. The dominant interest in these materials comes from quantum confinement effects and nanocrystalline surface chemistry, factors that create many possible applications, not only in optoelectronics but also in biology and medicine. In the case of Si nanocrystals 共Si-nc兲, this interest originates mostly from the observation of room-temperature, nanocrystalline-size-dependent photoluminescence in the visible spectral range. This gives possibilities for lightemitting devices and lasers based on silicon. Although this interest is impressive, a debate as to whether the observed visible emission band is caused by quantum confinement,4 defects at the nanocluster surface,5 surface states, or by oxide-related defect states6 is still ongoing. This debate is mostly due to the fact that the influence of the wide variety of synthesis processes, such as cosputtering,7,8 sol-gel chemistry, or plasma-enhanced chemical vapor deposition 共PECVD兲,9–12 might be one reason for the different explanations of the origin of the emission from Si-nc based systems. Thus, there still exists a strong demand for optimizing the technological processes to obtain both high quality and good reproducibility of siliconrich silicon-based 共SRSO兲 films containing Si-nc. In addition, most of the recent results focus only on investigation of the emission properties of Si-nc, and, as a consequence, their absorption properties are even less well understood. Thus, in this work, we report the influence of technological parameters, i.e., annealing temperature and silicon cona兲

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II. EXPERIMENTAL DETAILS

SRSO films, 2 ␮m thick, with different Si contents 共y = 0.38, 0.40, and 0.49 in SiyO1−y兲 were deposited on quartz wafers by inductively coupled plasma chemical vapor deposition 共ICP-CVD兲 reactor manufactured by Johnsen Ultravac Inc. of Burlington, Ontario using Ar+ 30%SiH4 and Ar + 90%O2 plasma gases. The substrate temperature during deposition was fixed at 120 ° C. The y values were determined by Rutherford backscattering 共RBS兲 measurements. After growth, wafers were diced using a Loadpoint dicing saw and then annealed in the Lindberg quartz tube furnace under flowing Ar for 2 h and temperatures 共Ta兲 of 900, 1000, and 1100 ° C. Each sample was annealed only once at a specific temperature; no cumulative annealing experiments were performed. Details about the deposition process can be found elsewhere.13 Inside the ICP plasma source, plasma gases such as oxygen and argon are introduced and excited to form plasma. At the tip of the plasma source, an interchangeable ceramic cap with different aperture patterns can be chosen to guide the plasma into the main chamber, where a sample stage with a heater underneath could travel 40 mm and rotate at a speed of 21 rpm during deposition. Between the ICP source and the sample stage, silane 共SiH4兲 is introduced through a circular dispersion ring. The sufficient distance between the plasma source, dispersion ring, and sample surface ensures that the reactions of the chemical vapor deposition happen far away from the plasma region so as to achieve a “clean” deposition with minimized plasma damage on the sample surface. The whole deposition process is monitored using a J.A. Woollam M-44 spectroscopic ellipsometer, providing realtime measurement of thickness and refractive index of the

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FIG. 1. Room-temperature photoluminescence spectra obtained for SRSO films with 38% silicon at different annealing temperatures under 488 nm excitation.

deposited thin films in the wavelength range from 600 to 1100 nm. The optical properties of the generated Si-nc were analyzed by room-temperature photoluminescence 共PL兲 and transmittance using an HR4000 Ocean Optics spectrometer as a detection system. The 488 and 300 nm lines of an Ar+ cw laser were used as the excitation sources, with output pump powers of about 6 and 10 mW, respectively, and with a laser spot size with an area of about 1 mm2. For the transmission measurements, a deuterium-tungsten lamp was used, which was coupled to an HR4000 spectrometer by an optical fiber. III. RESULTS AND DISCUSSION

Room-temperature photoluminescence spectra for an unannealed sample and for samples after annealing at 900 ° C, 1000 ° C, and 1100 ° C obtained at 488 nm excitation wavelength are shown in Fig. 1. All presented results have been obtained for samples with 38% Si since the interpretation of results obtained for 40% and 49% is very similar to those obtained for 38% Si. Compared with the unannealed sample 共emission band centered at 2.1 eV兲, the sample annealed at 900 ° C has an emission band centered at ⬃1.6 eV whose intensity increases by a factor of 6 for 1000 ° C and decreases by a factor of 1.3 for 1100 ° C. These changes could be due to the fact that a different annealing temperature yields a different 共a兲 nanocrystal size; 共b兲 nanocrystal density; and 共c兲 structural phase 共crystalline or amorphous兲, which will be reflected in the optical properties of the samples. This has been confirmed in our previous work,14 where similar structures have been investigated. It has been demonstrated by x-ray diffraction 共XRD兲 measurements that crystalline Si precipitates are formed within an amorphous SiO2 matrix during the annealing at 900 ° C. For T = 1000 ° C and 1100 ° C, the Si XRD peaks become narrower, which indicates that the average size of Si-nc increases with increasing T. Similar results have been obtained by increasing the amount of Si excess. As can be observed in Fig. 2, the broad emission band centered at ⬃1.6 eV has a complex structure with at least

J. Appl. Phys. 102, 043104 共2007兲

FIG. 2. 共Color online兲 Room-temperature photoluminescence spectrum obtained for SRSO film with 38% silicon at 1100 ° C annealing temperature under 488 nm excitation, together with fitting curves. Right axis: transmission spectrum obtained for the same sample.

five bands centered at 2.10, 1.71, 1.55, 1.38, and 1.31 eV. The band centered at 2.10 eV could be related to the lowenergy part of the emission band from the matrix, since this emission band is also present in the unannealed sample, where the high-energy part of this band has been cut off by the edge filter. Moreover, the intensity of this emission band is strongly reduced after annealing. In addition, Fig. 2 shows the transmittance spectrum for the sample annealed at 1100 ° C with a clear interference pattern, which means that interference conditions in our samples are fulfilled for the investigated spectral range. This indicates that interference effect could be also reflected in the emission spectrum and modify their shape. Figure 3 shows the results obtained with a 300 nm excitation wavelength for the samples discussed above. A similar behavior is observed for the emission band centered at ⬃1.6 eV. Additionally, an emission band centered between 2 and 3 eV has been observed for all annealed samples. This band could be related to radiative recombination from optically active centers in the matrix, since it covers the spectral range of the emission band for the unannealed sample. Figures 4 and 5 show PL results obtained for samples with different Si concentrations and at different excitation

FIG. 3. Room-temperature photoluminescence spectra obtained for SRSO films with 38% silicon at different annealing temperatures under 300 nm excitation.


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FIG. 6. Transmission spectra obtained for 共a兲 different silicon content at 1100 ° C annealing temperature, and 共b兲 at different annealing temperatures with 38% Si. FIG. 4. Room-temperature photoluminescence spectra obtained for SRSO films at 1100 ° C annealing temperature with different Si content under 488 nm excitation.

wavelengths i.e., 488 and 300 nm. As can be seen, the strongest emission was observed for the sample with the lowest Si concentration. Moreover, while the intensity of the emission band centered at 1.6 eV increases, the PL intensity of the band centered at ⬃2.5 eV decreases. This may be due to the fact that, in the case of the smallest Si concentration 共i.e., 38%兲, the smallest nanocrystals appear in the sample. It is well known that the absorption cross-section coefficient depends on the nanocrystal size, and reaches the biggest values for the smallest nanocrystals. This absorption efficiency should also be reflected in the emission properties, which is observed in our case where the strongest emission has been seen for the smallest nanocrystals. Moreover, in Fig. 5 a distinct structure in the emission bands can be observed when the silicon concentration increases. The complex nature of all these emission peaks 共also for the emission bands for samples annealed at different temperatures兲 could be caused by the formation of Si-nc with different sizes or by the existence of some additional, opti-

FIG. 5. Room-temperature photoluminescence spectra obtained for SRSO films at 1100 ° C annealing temperature with different Si contents under 300 nm excitation.

cally active recombination centers, activated by annealing. However, as was shown elsewhere,15 this complex emission band may also be influenced by interference effects as was already suggested. To further investigate this, the transmission spectra were obtained for all samples and are presented in Fig. 6. The annealing of SRSO films at different temperatures does not change the transmission spectra significantly 共the shift of the absorption edge will be discussed in subsequent paragraphs兲. On the other hand, the interference pattern of the transmission spectra depends strongly on the Si concentration in the film. Since the film thickness is approximately the same for all samples, this strong dependence could be due to the changes of the refractive index, which will change the conditions for interference effects. For this reason, the shape of the emission band of samples with different Si content changes significantly when the Si concentration increases. The obtained results could be interpreted as follows. Before the annealing, the SiyO1−y film has an amorphous structure, with a high amount of defect states 共also optically active兲. After the annealing at 900 ° C, the Si atoms start to form small clusters. At this low temperature, most of the clusters are in the amorphous phase, which is characterized by long tails of densities of states and low emission/ absorption efficiency. When the annealing temperature increases, the dominant Si phase becomes a crystalline phase, and many small nanocrystals 共with broad size distribution and high absorption/emission coefficient兲 with good crystallinity are formed in the matrix. When the temperature increases further, the nanocrystals begin to agglomerate and form bigger nanocrystals with a narrower size distribution and lower emission probability, similar to bulk silicon in nature. The growth of Si-nc is believed to be a diffusionlimited type process,16 and thus, this behavior can be interpreted in the framework of the diffusivity of Si in SiO2, which is very low at 900 ° C, whereas at 1000 ° C − 1100 ° C it is of the order of 10−16 cm2 / s.17 Similar results and discussions have been presented elsewhere.18,19 To confirm this scenario, the absorption spectra were obtained for all samples and presented in Fig. 7. The figure shows results only for samples with 38% Si because results obtained for 40% and 49% Si have very similar behavior.


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FIG. 8. 共Color online兲 Values of the absorption edges for all samples obtained for fitting parameters m = 2 and m = 1 / 2 in 共␣ប␻兲 = A共ប␻ − Eo兲m.

FIG. 7. Absorption spectra obtained from the transmission spectra for samples with 38% Si at different annealing temperatures, together with PL spectrum obtained at 1100 ° C.

By treating the SRSO films as an effective medium composed of Si-nc and the silica matrix, the optical absorption edge was analyzed by the following relation, known as the Tauc plot: 共␣ប␻兲 = A共ប␻ − E0兲m ,

共1兲

where E0 is the band gap corresponding to a particular transition occurring in the film, A is a constant, and the exponent m characterizes the nature of band transition. m = 1 / 2 and 3/2 correspond to direct allowed and direct forbidden transitions, and m = 2 and 3 correspond to indirect allowed and indirect forbidden transitions, respectively.20,21 In Fig. 7, it can be seen that when the structures are annealed, the absorption edge is redshifted. This redshift increases when the annealing temperature increases. This, however, is not a linear process. While a clear redshift of the absorption edge can be observed after annealing of the asgrown sample at 900 ° C, there is no evident change in the absorption edge when the annealing temperature increases to 1000 ° C. When the structure is annealed at 1100 ° C, again a redshift of the absorption edge is observed. Assuming that the absorption edge is dominated by the optical properties of the silicon nanoclusters, these results are in good agreement with previous interpretations, since at the beginning of annealing, the redshift is related to the Si-nc formation and then to the agglomeration process, and as a consequence, changes with the Si-nc band-gap 共size兲 due to quantum confinement effects. Moreover, analyzing results presented in Fig. 7, it can be concluded that the absorption signal rapidly increases by means of the direct transition for m = 2, with increasing photon energy starting at around 3.60, 3.24, 3.22, and 2.93 eV for the unannealed sample and samples annealed at 900 ° C, 1000 ° C, and 1100 ° C, respectively. These values are quite different from the optical direct transitions in bulk silicon, where one finds values of 3.4 and 4.2 eV for the ⌫25⬘ − ⌫15 and ⌫25⬘ − ⌫2⬘ transitions, respectively. In fact, in the case of Si-nc, these values should even be blueshifted compared to bulk Si, as was presented in previous theoretical and experimental studies.22–24

However, assuming m = 1 / 2, which is a typically used value in the case of an indirect energy gap or amorphous material, the absorption edges become 2.86, 2.32, 2.25, and 2.0 eV for the unannealed sample and samples annealed at 900 ° C, 1000 ° C, and 1100 ° C, respectively. These values correspond to the first dipole-allowed transition, which coincides with the HOMO-LUMO gap, defined as the difference between the lowest unoccupied state 共LUMO兲 and the highest occupied state 共HOMO兲 energy. The horizontal axis in Fig. 7 represents the size dependence of the energy of the absorption HOMO-LUMO edge.25 The correlation between the energy and the nanocrystalline size has been obtained based on the recent results presented by Niquetet al.,25 and was estimated for annealed samples as ⬃2.1, 2.2, and 2.6 nm for absorption edges at 2.32, 2.25, and 2.0 eV, respectively. Figure 8 summarizes the absorption results obtained for all samples. The results indicate that the annealing of SRSO films, as well as increasing silicon content in the SRSO films, produces a redshift in the absorption edge, which is probably due to the formation of silicon clusters with subsequent agglomeration to bigger grains. Additionally, Fig. 9 shows values of the Urbach energy 共EU兲 estimated from tails of the absorption spectra. The value of EU is related to the amount of structural disorder, electron-phonon interactions 共indirect transitions兲, amount of

FIG. 9. Values of the Urbach energy 共EU兲 estimated from the tails of the absorption spectra as ␣ = A exp共ប␻ / EU兲.


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dangling bonds, and/or size/phase distribution in the SRSO film. A decrease of this value with increased annealing temperature or silicon content could be due to the fact that annealing improves the structural order in the sample as an effective medium and/or reduces the size/phase distribution in cases where the edge is dominated by optical properties of the Si-nc. IV. CONCLUSIONS

In conclusion, it has been demonstrated how the annealing temperature and Si concentration influence the emission as well as the absorption properties of SRSO films. It has been shown that annealing redshifts the absorption edge and increases the emission intensity when the sample is annealed up to 1000 ° C. It has also been shown that annealing does not change the emission shape significantly. Moreover, it has been shown that an increase in Si content in the SRSO films also redshifts the absorption edge. At the same time, it reduces the emission intensity and changes dramatically its shape due to interference effects caused by changes in the refractive index. The interpretation of the obtained results could be given in the framework of Si nanocrystal formation, where an increase in annealing temperature or Si content increases the nanocrystal size and reduces their energy band gap. Also, the reduction of nanocrystal size will increase the oscillator strength and influence the absorption/emission intensity. This has been observed in all emission spectra. However, due to the complex nature of the emission band, which is additionally influenced by the interference effect, it is not possible to clearly indicate that the observed emission band at ⬃1.6 eV is directly related to the recombination via the Si nanocrystals core. ACKNOWLEDGMENTS

The authors wish to thank D. Comedi, E.A. Irving, and T.R. Roschuk for useful discussions. In Canada, this work has been supported by the Ontario Research and Development Challenge Fund 共ORDCF兲 under the auspices of the Ontario Photonics Consortium 共OPC兲 and by Ontario Centres of Excellence 共OCE兲 Inc.

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