SiOx/SiNy multilayers for photovoltaic and photonic applications.

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).

Introduction

Silicon nanoclusters [Si-ncs] with engineered band gap [1] 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 [SiO] 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 SiO-based MLs exceeds that of the single SiOlayers 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 SiO/SiO2 MLs [8]. 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 [9]. 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 SiO/Si3N4 [10], Si-rich Si3N4 (SiN)/Si3N4 [11], and Si-rich Si3N4/SiO2 [12] were proposed and investigated [13] for their optical and electrical properties.

In this letter, we investigate SiO/SiNMLs and compare them with the SiO/SiO2 counterparts reported earlier [9,14]. We demonstrate that an enhancement in the conductive and light-emitting properties of SiO/SiNMLs can be achieved with a reduced thermal budget. We also report a pioneering study on Nd-doped SiO/SiNMLs. A comparison between the properties of Nd3+-doped SiO/SiO2 and SiO/SiNMLs are presented, and we show the benefits of using SiNsublayers to achieve enhanced emission from Nd3+ ions.

Experimental details

Undoped and Nd-doped 3.5-nm SiO/5-nm SiN(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 SiOsublayers and a pure Si target in nitrogen-rich plasma for the SiNsublayers. An additional Nd2O3 target was used to dope the SiOand SiNsublayers by Nd3+ ions. More details on the growth process can be found elsewhere [15]. The excess Si content in the corresponding SiOand SiNsingle layers obtained from RBS studies are calculated to be 25 and 11 at.%, respectively (i.e., SiOand SiN). 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.

Results and discussions

Effect of annealing on the PL

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 SiOsingle layers [7] or in SiO/SiO2 MLs [16], such treatment was first employed on SiO/SiNMLs. The X-ray diffraction [XRD] broad peak centered around 2θ = 28° is the signature of the Si nanoclusters' formation in the SiO/SiO2 (Figure 1, curve 1) and SiO/SiNMLs (Figure 1, curve 2) as already observed by means of atomic scale studies on similar multilayers [17]. However, contrary to the PL emission obtained from the SiO/SiO2 MLs, no PL emission was observed in the SiO/SiNMLs after such annealing (Figure 2a). This stimulated a deeper investigation of the post-fabrication processing to achieve efficient light emission from the SiO/SiNMLs.

Figure 1

XRD spectra of annealed Si-based MLs. (curve 1) SiO/SiO2 1 h, 1,100°C; (curve 2) SiO/SiN1 h, 1,100°C; (curve 3) SiO/SiN1 min, 1,000°C; and (curve 4) SiO/SiO2 1 min, 1,000°C.

Figure 2

Photoluminescence. (a) Maximum PL intensity [IPL] of SiO/SiNMLs vs TA and tA, and SiO/SiO2 at 1,100°C; (b) PL spectra of STA SiO/SiNMLs; (c) IPL vs TA for tA = 1 min. The asterisk represents the peak from second order emission of laser.

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 2a).

The highest PL intensity in SiO/SiNMLs was obtained with TA = 1,000°C and tA = 1 min (Figure 2b,c), whereas the SiO/SiO2 MLs showed no emission after such short-time annealing treatment (Figure 2a). Corresponding XRD pattern of this short-time annealed [STA] (STA = 1 min, 1,000°C) SiO/SiNy showed a broad peak in the range 2θ = 20° to 30° which is absent in STA SiO/SiO2 MLs (Figure 1, curves 3 and 4). This suggests the presence of small Si clusters in the SiO/SiNMLs, with lower sizes (broader peak) by comparison with higher annealing temperature (1,100°C; Figure 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 SiOsublayers and the localized bandtail defect states in the SiNsublayers.

Absorption and electrical studies

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 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 SiOsublayers 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 SiO/SiNMLs is quenched after an increase in the time and temperatures of annealing (Figure 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 1, curve 2), which is not seen in the short-time annealed sample (Figure 1, curve 3).

Figure 3

Absorption coefficient and current-voltage behavior. (a) Evolution of absorption coefficient with annealing; (b) Comparison of current-voltage behavior of SiO/SiNand SiO/SiO2 MLs.

Considering a balance between light emission and absorption for photovoltaic applications, we chose to study STA SiO/SiNMLs with a total thickness of 850 nm for electrical measurements. Figure 3b compares the dark current curves of 3.5-nm SiO/5-nm SiNwith our earlier reported 3.5-nm SiO/3.5-nm SiO2 (140 nm) MLs [14]. The resistivity was calculated at 7.5 V to be 2.15 and 214 MΩ·cm in the SiO/SiNy and SiO/SiO2 MLs, respectively. Since the thickness of the SiOsublayer is the same in both cases (3.5 nm), this decrease in the resistivity of the SiO/SiNy MLs can be ascribed to the substitution of 3.5-nm SiO2 by 5-nm SiNsublayers. This hundred-times enhanced conductivity at low voltage paves way for further improvement of the SiO/SiNMLs' conductivity, for example, by decreasing the thickness of this SiNsublayer.

Microstructural studies

The high-resolution transmission electron microscope [HRTEM] and EFTEM observations on STA SiO/SiNshow Si-ncs in the SiOsublayers with an average diameter of 3.4 nm. Only a couple of Si nanocrystals were observed in the HRTEM (Figure 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 4c) implying that they are predominantly amorphous. Interestingly, this density of the Si-ncs in the SiO/SiNMLs is an order of magnitude higher than the Si-ncs formed in the SiO/SiO2 MLs fabricated under similar conditions. The brighter SiOsublayers are distinguished from the darker SiNsublayers by filtering the SiO2 plasmon energy (Figure 4b). No evidence of Si-ncs within the SiNsublayers was obtained. The STA could favor the formation of Si-ncs only in SiOand not in SiNsublayers. This could be attributed to the different mechanism of Si-ncs formation in SiOand SiNin MLs as opposed to that in single layers [18] and/or the low Si-excess content in SiN.

Figure 4

HRTEM (a) and EFTEM (b, c) images. SiO/SiNML annealed at TA = 1,000°C, tA = 1 min by filtering the energy at SiO2 plasmon (b) and Si plasmon (c) energies, respectively.

Effect of Nd3+-doping

Understanding the microstructure of MLs and considering the enhancement of absorption and emission properties in SiO/SiNMLs compared to the SiO/SiO2 MLs, we investigate the effect of using SiNsublayer on the PL emission from Nd3+ ions. For this purpose, the SiO-Nd/SiN-Nd and SiO-Nd/SiO2-Nd MLs were fabricated, and their PL properties were compared. No PL emission was detected from the Nd3+-doped SiNsingle layers at the different annealing treatments investigated here. Figure 5 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 2c) and Nd3+-doped MLs (Figure 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 SiO/SiNMLs exceeds that of the SiO/SiO2 MLs (Figure 5, inset). Thus, we deal with the efficient energy transfer towards Nd3+ ions not only in SiObut also in SiNsublayers. 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 SiN-Nd sublayers is due to an efficient energy transfer from SiN-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 SiNand the formation of Nd2O3 clusters in the SiOsublayers [21]. On the other hand, annealing at TA = 400°C to 700°C, discussed above for the undoped SiO/SiNy MLs, enhance Nd3+ PL emission when applied to the doped counterparts (Figure 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 SiO/SiNMLs is under consideration now.

Figure 5

PL intensity with annealing time and temperature. Evolution of the Nd3+ PL intensity at 1.37 eV for doped SiO/SiNMLs with annealing temperature and time. (Inset) PL spectra of as-grown Nd3+-doped SiO/SiNand SiO/SiO2 MLs with equal number of periods. The thicknesses of the SiO, SiO2, and SiNsublayers are 3.5, 5.0, and 5.0 nm, respectively.

Conclusion

In conclusion, we show that SiO/SiNMLs fabricated by RF magnetron sputtering can be engineered as structures for photovoltaic and photonic applications. The as-grown and STA SiO/SiNMLs show enhanced optical and electrical properties than the SiO/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 SiO/SiNMLs have about a hundred times higher conductivity compared to the SiO/SiO2 MLs. These results show the advantages of SiO/SiNMLs as materials for photovoltaic and photonic applications and open up perspectives for a detailed study.

Abbreviations

MLs: multilayers; PL: photoluminescence; Si-nc: silicon nanoclusters; SiN: silicon-rich silicon nitride; SiO: silicon-rich silicon oxide; STA: short time annealing at 1,000°C for 1 min.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

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.