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Microstructural, electrical, and optical properties of undoped and Nd3+-doped SiOx/SiNy multilayers fabricated by reactive radio frequency magnetron co-sputtering have been investigated with regard to thermal treatment. This letter demonstrates the advantages of using SiNy as the alternating sublayer instead of SiO2. A high density of silicon nanoclusters of the order 1019 nc/cm3 is achieved in the SiOx sublayers. Enhanced conductivity, emission, and absorption are attained at low thermal budget, which are promising for photovoltaic applications. Furthermore, the enhancement of Nd3+ emission in these multilayers in comparison with the SiOx/SiO2 counterparts offers promising future photonic applications.
PACS: 88.40.fh (Advanced materials development), 81.15.cd (Deposition by sputtering), 78.67.bf (Nanocrystals, nanoparticles, and nanoclusters).
Silicon nanoclusters [Si-ncs] with engineered band gap  have attracted the photonic and the photovoltaic industries as potential light sources, optical interconnectors, and efficient light absorbers [2-5]. Multilayers [MLs] of silicon-rich silicon oxide [SiOx] alternated with SiO2 became increasingly popular due to the precise control on the density and size distribution of Si-ncs [6,7]. Moreover, the efficiency of light emission from SiOx-based MLs exceeds that of the single SiOx layers with equivalent thickness due to the narrower Si-nc size distribution. The ML approach is also a powerful tool to investigate and control the emission of rare-earth [RE] dopants, for example, Er-doped SiOx/SiO2 MLs . It also allows us to control the excitation mechanism of the RE ions by adjusting the optimal interaction distance between the Si-ncs and the RE ions. However, achieving electroluminescence and hence extending its usage for photovoltaic applications are problematic due to the high resistivity caused by SiO2 barrier layers . Hence, replacement of the SiO2 sublayer by alternative dielectrics becomes interesting. Due to the lower potential barrier and better electrical transport properties of silicon nitride [Si3N4] in comparison to SiO2, multilayers like SiOx/Si3N4 , Si-rich Si3N4 (SiNy)/Si3N4 , and Si-rich Si3N4/SiO2  were proposed and investigated  for their optical and electrical properties.
In this letter, we investigate SiOx/SiNy MLs and compare them with the SiOx/SiO2 counterparts reported earlier [9,14]. We demonstrate that an enhancement in the conductive and light-emitting properties of SiOx/SiNy MLs can be achieved with a reduced thermal budget. We also report a pioneering study on Nd-doped SiOx/SiNy MLs. A comparison between the properties of Nd3+-doped SiOx/SiO2 and SiOx/SiNy MLs are presented, and we show the benefits of using SiNy sublayers to achieve enhanced emission from Nd3+ ions.
Undoped and Nd-doped 3.5-nm SiOx/5-nm SiNy (50 periods) MLs were deposited at 500°C on a 2-inch p-Si substrate by radio frequency [RF] magnetron co-sputtering of Si and SiO2 targets in hydrogen-rich plasma for the SiOx sublayers and a pure Si target in nitrogen-rich plasma for the SiNy sublayers. An additional Nd2O3 target was used to dope the SiOx and SiNy sublayers by Nd3+ ions. More details on the growth process can be found elsewhere . The excess Si content in the corresponding SiOx and SiNy single layers obtained from RBS studies are calculated to be 25 and 11 at.%, respectively (i.e., SiOx = 1 and SiNy = 1.03). Conventional furnace annealing under nitrogen atmosphere at different temperatures, TA = 400 to 1,100°C, and times, tA = 1 to 60 min, was performed on the MLs. X-ray diffraction analysis was performed using a Phillips XPERT HPD Pro device (PANalytical, Almelo, The Netherlands) with CuKα radiation (λ = 0.1514 nm) at a fixed grazing angle incidence of 0.5°. Asymmetric grazing geometry was chosen to increase the volume of material interacting with the X-ray beam and to eliminate the contribution of the Si substrate. Photoluminescence [PL] spectra were recorded in the 550- to 1,150-nm spectral range using the Triax 180 Jobin Yvon monochromator (HORIBA Jobin Yvon SAS, Longjumeau, Paris, France) with an R5108 Hamamatsu PM tube (Hamamatsu, Shizuoka, Japan). The 488-nm Ar+ laser line served as the excitation source. All the PL spectra were corrected by the spectral response of the experimental setup. Top and rear-side gold contacts were deposited on the MLs by sputtering for electrical characterization. Current-voltage measurements were carried out using a SUSS Microtec EP4 two-probe apparatus (SUSS Microtec, Germany) equipped with Keithley devices (Keithly, Cleveland, OH, USA). Energy-filtered transmission electron microscopy [EFTEM] was carried out on a cross-sectional specimen using a TEM-FEG microscope Tecnai F20ST (FEI, Eindhoven, The Netherlands) equipped with an energy filter TRIDIEM from Gatan (Gatan, München, Germany). The EFTEM images were obtained by inserting an energy-selecting slit in the energy-dispersive plane of the filter at the Si (17 eV) and at the SiO2 (23 eV) plasmon energy, with a width of ± 2 eV.
Since an annealing at TA = 1,100°C and tA = 60 min is the most suitable to achieve an efficient PL from Si-ncs either in sputtered SiOx single layers  or in SiOx /SiO2 MLs , such treatment was first employed on SiOx/SiNy MLs. The X-ray diffraction [XRD] broad peak centered around 2θ = 28° is the signature of the Si nanoclusters' formation in the SiOx /SiO2 (Figure (Figure1,1, curve 1) and SiOx/SiNy MLs (Figure (Figure1,1, curve 2) as already observed by means of atomic scale studies on similar multilayers . However, contrary to the PL emission obtained from the SiOx/SiO2 MLs, no PL emission was observed in the SiOx/SiNy MLs after such annealing (Figure (Figure2a).2a). This stimulated a deeper investigation of the post-fabrication processing to achieve efficient light emission from the SiOx/SiNy MLs.
It was observed that the PL signals from the MLs annealed during tA = 60 min are significant only at lower temperatures (TA = 400°C to 700°C), and high intensities are obtained when the samples are annealed at high temperatures for a short time (TA = 900°C to 1,000°C, tA = 1 min). It is interesting to note that an interplay between TA and tA can yield similar PL efficiencies, as can be seen for TA = 900°C and tA = 1 min, and TA = 700°C and tA = 15 min (Figure (Figure2a2a).
The highest PL intensity in SiOx/SiNy MLs was obtained with TA = 1,000°C and tA = 1 min (Figure 2b,c), whereas the SiOx/SiO2 MLs showed no emission after such short-time annealing treatment (Figure (Figure2a).2a). Corresponding XRD pattern of this short-time annealed [STA] (STA = 1 min, 1,000°C) SiOx/SiNy showed a broad peak in the range 2θ = 20° to 30° which is absent in STA SiOx/SiO2 MLs (Figure (Figure1,1, curves 3 and 4). This suggests the presence of small Si clusters in the SiOx/SiNy MLs, with lower sizes (broader peak) by comparison with higher annealing temperature (1,100°C; Figure Figure1,1, curves 1 and 2). However, we cannot distinguish which of the sublayer is at the origin of the PL emission. Consequently, the recorded PL may be a combined contribution of the Si-ncs in the SiOx sublayers and the localized bandtail defect states in the SiNy sublayers.
The absorption studies show similar absorption coefficients for as-grown and STA MLs, whereas annealing at TA = 1,100°C and tA = 60 min results in an absorption enhancement (Figure (Figure3a).3a). One can say that, at such temperature, an increase in density and size of the Si-ncs occurs due to phase separation of the SiOx sublayers into Si and SiO2 phases. The formation of Si nanocrystals is complete at TA = 1,100°C and tA = 60 min and leads to this enhancement. This reasoning is supported by the results obtained from the PL and the XRD analysis of the samples annealed at such temperature. The PL in the SiOx/SiNy MLs is quenched after an increase in the time and temperatures of annealing (Figure (Figure2a),2a), and this can be attributed to the increase in the size leading to the loss of quantum confinement effect. The formation of Si nanoclusters can be witnessed from the appearance of the XRD peak at 2θ = 28° (Figure (Figure1,1, curve 2), which is not seen in the short-time annealed sample (Figure (Figure1,1, curve 3).
Considering a balance between light emission and absorption for photovoltaic applications, we chose to study STA SiOx/SiNy MLs with a total thickness of 850 nm for electrical measurements. Figure Figure3b3b compares the dark current curves of 3.5-nm SiOx/5-nm SiNy with our earlier reported 3.5-nm SiOx/3.5-nm SiO2 (140 nm) MLs . The resistivity was calculated at 7.5 V to be 2.15 and 214 MΩ·cm in the SiOx/SiNy and SiOx/SiO2 MLs, respectively. Since the thickness of the SiOx sublayer is the same in both cases (3.5 nm), this decrease in the resistivity of the SiOx/SiNy MLs can be ascribed to the substitution of 3.5-nm SiO2 by 5-nm SiNy sublayers. This hundred-times enhanced conductivity at low voltage paves way for further improvement of the SiOx/SiNy MLs' conductivity, for example, by decreasing the thickness of this SiNy sublayer.
The high-resolution transmission electron microscope [HRTEM] and EFTEM observations on STA SiOx/SiNy show Si-ncs in the SiOx sublayers with an average diameter of 3.4 nm. Only a couple of Si nanocrystals were observed in the HRTEM (Figure (Figure4a),4a), whereas a high density of Si-nanoclusters of about 1019 nc/cm3 can be witnessed from the EFTEM images taken at the Si plasmon energy (Figure (Figure4c)4c) implying that they are predominantly amorphous. Interestingly, this density of the Si-ncs in the SiOx/SiNy MLs is an order of magnitude higher than the Si-ncs formed in the SiOx/SiO2 MLs fabricated under similar conditions. The brighter SiOx sublayers are distinguished from the darker SiNy sublayers by filtering the SiO2 plasmon energy (Figure (Figure4b).4b). No evidence of Si-ncs within the SiNx sublayers was obtained. The STA could favor the formation of Si-ncs only in SiOx and not in SiNy sublayers. This could be attributed to the different mechanism of Si-ncs formation in SiOx and SiNy in MLs as opposed to that in single layers  and/or the low Si-excess content in SiNy.
Understanding the microstructure of MLs and considering the enhancement of absorption and emission properties in SiOx/SiNy MLs compared to the SiOx/SiO2 MLs, we investigate the effect of using SiNy sublayer on the PL emission from Nd3+ ions. For this purpose, the SiOx-Nd/SiNy-Nd and SiOx-Nd/SiO2-Nd MLs were fabricated, and their PL properties were compared. No PL emission was detected from the Nd3+-doped SiNy single layers at the different annealing treatments investigated here. Figure Figure55 shows the PL spectra of the Nd3+-doped as-grown MLs under non-resonant excitation with peaks corresponding to the 4F3/2→4I9/2 and 4F3/2→4I11/2 transitions at 1.37 and 1.17 eV, respectively. The comparison between the PL properties of undoped (Figure (Figure2c)2c) and Nd3+-doped MLs (Figure (Figure5,5, inset) clearly shows the quenching of visible PL emission and the appearance of two Nd3+-related PL peaks in the Nd-doped MLs. Moreover, the intensity of Nd3+ PL from the doped SiOx/SiNy MLs exceeds that of the SiOx/SiO2 MLs (Figure (Figure5,5, inset). Thus, we deal with the efficient energy transfer towards Nd3+ ions not only in SiOx but also in SiNy sublayers. Since this emission is observed for as-grown MLs, when no Si-ncs were formed in these MLS, it is obvious that the emission from the Nd3+ ions in the SiNx-Nd sublayers is due to an efficient energy transfer from SiNy-localized defect states towards the Nd3+ ions [19,20]. PL observed from the doped MLs after STA was not intense, and it was quenched with increasing annealing time. The same behavior was observed for the 900°C annealing. This could be due to the decrease in the number of defect-related sensitizers in SiNy and the formation of Nd2O3 clusters in the SiOx sublayers . On the other hand, annealing at TA = 400°C to 700°C, discussed above for the undoped SiOx/SiNy MLs, enhance Nd3+ PL emission when applied to the doped counterparts (Figure (Figure3).3). Thus, we attain intense PL at a low thermal budget with TA (400°C to 700°C) and tA (1 min). To optimize Nd3+ emission, the effect of the thickness of each sublayer in SiOx/SiNy MLs is under consideration now.
In conclusion, we show that SiOx/SiNy MLs fabricated by RF magnetron sputtering can be engineered as structures for photovoltaic and photonic applications. The as-grown and STA SiOx/SiNy MLs show enhanced optical and electrical properties than the SiOx/SiO2 counterparts. Besides achieving a high density of Si-ncs at a reduced thermal budget, we show that high emission and absorption efficiencies can be achieved even from amorphous Si-ncs. The Nd-doped MLs, as-grown and those annealed at lower thermal budgets, demonstrate efficient emission from rare-earth ions. We also show that our STA SiOx/SiNy MLs have about a hundred times higher conductivity compared to the SiOx/SiO2 MLs. These results show the advantages of SiOx/SiNy MLs as materials for photovoltaic and photonic applications and open up perspectives for a detailed study.
MLs: multilayers; PL: photoluminescence; Si-nc: silicon nanoclusters; SiNy: silicon-rich silicon nitride; SiOx: silicon-rich silicon oxide; STA: short time annealing at 1,000°C for 1 min.
The authors declare that they have no competing interests.
RPN fabricated the undoped multilayers under investigation and carried out the characterization studies. LK and OD fabricated the Nd-doped layers and studied the effect of Nd doping on the MLs. JC and CD made contributions to the optical studies. MC performed the EFTEM measurements. FG conceived of the study and participated in the coordination of the manuscript. All authors read and approved the final manuscript.
This study is supported by the DGA (Defense Procurement Agency) through the research program no. 2008.34.0031. The authors acknowledge J. Pierriére for the RBS measurements done with the SAFIR accelerator (INSP, UPMC) and X. Portier (CIMAP) for the TEM image.