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Silicon (Si)-based light emitting thin film has been a key ingredient for all-Si-based optoelectronics. Besides material engineering, adopting a novel 3D photonic architecture represents an effective strategy to boost light excitation and extraction from Si-based thin film material. We here explore the use of a nanowires (NW) framework, grown via vapor-liquid-solid mode, to achieve strongly enhanced yellow-green luminescence from SiNxOy/NW core-shell structure, with an order of magnitude enhancement compared to co-deposited planar references. We found that choosing geometrically-identical but different NW cores (Si or SiO2) can lead to profound influence on the overall light emission performance. Combining parametric investigation and theoretical modeling, we have been able to evaluate the key contributions arising from different mechanisms that include near-field enhancement, 3D light trapping and enhanced light extraction. These new findings indicate a new and effective strategy for strong Si-based thin film light emitting source, while being generic enough to be applicable in a wide variety of other thin film materials.
© 2015 Optical Society of America
1. Introduction
Efficient light-emitting thin film has been a critical ingredient to achieving all Si-based optoelectronics [1–4]. While a great deal of efforts have been devoted to engineering the material properties of nanostructured thin films, for instance by tailoring the grain size and the band-gap in Si nano crystallites embedded in various dielectrics [2, 5–7], Si nanowires (SiNW) of high-refractive-index, in singly and array form, bring in also unique opportunities to enhance the light emission performance from Si-based thin film materials. These benefits could arise from a strong near field in the close proximity of NW cores [8–10], a strong light trapping/photonic effect that will enhance light in-coupling or excitation [11–14] and an effective light extraction from the light emitting medium [15, 16]. Very recently, H. Kallel et al. have demonstrated that a strong near-field around a single Si NW, lying on top of SiO2/Si QDs/SiO2/Si multilayer stack, can help to excite beneath Si QDs leading to a stronger PL emission [8], indicating an antenna-like behavior of the Si NW in analogy to that achieved with metal plasmonics. Analysis based on the concept of leaky-mode-resonance (LMR) effect has provided a theoretical basis to address the photonic behaviors in single Si or Ge NW cavities [8, 17]. However, few research has ever explored these benefits among a dense matrix of SiNWs. Though NW arrays have provided an ideal 3D framework for building high-efficient light emitting or photovoltaics applications [18–23], applying this strategy to boost light emission of Si-based thin film materials has yet to be experimentally testified. A more comprehensive understanding upon the key controlling parameters/mechanism also needs to be established.
In this work, we choose a matrix of self-assembled SiNWs fabricated via a Sn nanoparticle catalyzed vapor-liquid-solid (VLS) method [24], which provides an ideal testing bed to assess the light emission enhancement of amorphous silicon oxynitride (SiNxOy) thin films coated over a 3D SiNW architecture. Schematic illustration of the whole structure has been depicted in Fig. 1(f). SiNxOy thin film is a promising candidate to serve as a Si compatible light source [7, 25] that gives a strong broad-band yellow-green luminescence under photo- (PL) or electro- (EL) excitations. A series of researches have been carried out in our previous studies to optimize the light emission from SiNxOy thin film materials [7, 25]. In this work, the 3D architecture is provided by a matrix of the VLS-grown SiNWs, which are catalyzed by a low-melting-point metal droplet of Sn and fabricated in a low-temperature plasma enhanced chemical vapor deposition (PECVD) [26, 27]. Combining these advanced techniques and concepts in this work, we here focus on developing a new strategy to boost the luminescence performance of SiNxOy thin film materials upon a properly designed nanowire core matrix. We will also assess the impact of various key parametric variations and distinguish the contributions from different mechanisms with the aid of a systematic simulation investigation.
Fig. 1 (a) SEM image of VLS-grown silicon nanowires (SiNWs) and (b) the SiNxOy/SiNWs core-shell structure after PECVD deposition for 20 min, while (c) shows the SEM image of SiO2 nanowires (SoNWs) by oxidizing SiNWs, and (d) the SiNxOy/SoNW structure; (e) presents a SEM cross section view of the SiNxOy/SiNWs forest with a dashed-white line contouring such a single unit, which has an inner core-shell structure as schematically illustrated in (f).
2. Experiments
The SiNWs matrix was grown upon Si wafers, where a thin layer of Sn (~2 nm thick) was evaporated prior to loading into PECVD chamber. A H2 plasma treatment for 2 min was conducted at 250 °C that transforms the Sn layer into separate Sn droplets. Typical RF power density, chamber pressure and H2 flow are 14 mW/cm2, 600 mTorr and 100 standard cubic centimeters per minute (SCCM), respectively. After then, 10 SCCM SiH4 flow was introduced to initiate the growth of SiNWs at a higher substrate temperature of 400 °C. After 8 min growth, the SiNWs are usually 500nm long on average with a tapering tip of around 20 nm and a base diameter of around 40 nm, as seen in Fig. 1(a), and a typical density of ~5 x 108/cm2. More details in the growth control of Sn-catalyzed SiNWs are available in our previous papers [26–29]. In the next step, a SiNxOy thin film was deposited with a mixture of 5 SCCM SiH4 and 40 SCCM NH3 at 250 °C, at a chamber pressure of 600 mTorr and RF power density of 260mW/cm2. The elemental composition of the SiNxOy thin film has been determined by X-ray Photoelectron Spectroscopy (XPS) to be Si0.52N0.43O0.05. After 10 min, 20 min and 30 min deposition, the thickness of the SiNxOy coating layer over the top segment of SiNWs were determined by scanning electron microscopy (SEM) observations to be around 45 nm, 79 nm and 125 nm, respectively. For example, a SEM image of such core-shell SiNW/SiNxOy forest after 20 min coating is shown in Fig. 1(b). Meanwhile, it is also important to note that, as the plasma deposition over a dense matrix of SiNW is not always conformal, we thus check this coating thickness variation in a typical side-view SEM image as presented in Fig. 1(e). It shows that the coating SiNxOy layer is indeed much thinner at the root of NWs, roughly one third of that measured at the top ends. This feature, as depicted in Fig. 1(f), is thus taken into account in our following modeling and simulation. PL luminescence spectra were characterized by using Jobin-Yvon Fluorolo-3 system with a filtered 450W Xe lamp excitation at 325 nm, and a photomultiplier tube (Hamamatsu 928 PMT) was used as a detector. Optical reflection spectra were recorded over a spectral range of 300 to 1,000 nm using Shimadzu UV-3600 spectrophotometer (Shimadzu Corp., Kyoto, Japan).
3. Results and discussions
Figures 2(a) - 2(c) show the PL spectra of the SiNxOy thin films deposited over SiNWs matrix, as well as those on planar substrate (without SiNWs), with deposition times of 10 min, 20 min and 30 min on Si wafer substrates, respectively. It is noteworthy that neither the VLS-grown SiNWs nor the oxidized SiO2 NW (SoNW) matrices, by themselves, give any detectable PL luminescence before coating them with the SiNxOy thin film. As we can see, the PL emissions from SiNW/SiNxOy structures can be greatly enhanced by a factor of 3 to 4, compared to their planar references, while preserving basically similar line shape. We also notice that the PL spectra of SiNxOy thin film over NW forest demonstrate a blue shifting of 40~50 nm compared to planar one. This could be attributed to the somewhat different plasma deposition condition on the vertical sidewalls of NWs, which could modify the precise SiNxOy composition and peak position. In this work, we focus on the changes of the relative peak emission intensities. The PL emission intensity enhancement factors (EF), defined as , are also extracted at corresponding maximum PL emission wavelengths and plotted in Fig. 2(d) as a function of the SiNxOy coating thicknesses on NW sidewall.
Fig. 2 (a)-(c) the photoluminescence (PL) spectra measured on SiNxOy thin films coated upon SiNWs (black), SoNWs (red) and flat (green) structures for various deposition times of 10 min, 20 min and 30 min, respectively; (d) PL enhancement factors as a function of SiNxOy coating layer thickness for the two NW core cases.
Then, we come to examine the major mechanisms that could contribute to an enhanced PL emission from the SiNxOy thin film over a 3D architecture: 1) First, the SiNW forest is known to be beneficial for achieving a strong light trapping and anti-reflection effects that will enhance the excitation of incident light field of the SiNxOy thin film medium [11, 30–32]; 2) Second, the high-refractive-index SiNW cores could establish strong near field around its vicinity that will help to excite the sidewall coating SiNxOy layer effectively for stronger PL emission [8–10]; 3) Third, the 3D structured light emitting medium introduces a gradient index variation interface, from the high-index NW/SiNxOy matrix to the air, which is advantageous for minimizing internal reflection and thus boosting the PL extraction efficiency [15, 16]. The first and the third mechanisms are closely related to the geometry and distribution of the nanowire array, while the second one relies more on the internal layer structure.
Considering that the crystalline SiNW cores can also absorb both the incident photons (@325 nm) and the PL signals (around 480 nm), we designed a new experiment where the VLS-grown SiNWs on top of Si wafer were oxidized completely into SiO2 NWs (SoNW) before loading again into the PECVD chamber for SiNxOy thin film coating. The oxidation was carried out in a tube furnace with dry O2 ambient at 1000 °C for 1 hour, which transformed as well a superficial c-Si substrate layer of ~50 nm into SiO2. (In order to confirm a complete oxidation of the SiNWs into SoNWs, a reference sample with SiNWs grown upon quartz substrate was also prepared and oxidized with the same conditions mentioned above. Then, we characterized the oxidized sample with Raman spectroscopy and found that all the Raman peaks corresponding to c-Si or a-Si have disappeared. This indicates that the SiNWs have been completely converted into SiO2 NWs.) It is important to note that, during this process, the geometry, density and shape of the nanowire were not changed much as witnessed in the SEM image of SoNWs in Fig. 1(c) compared to that of SiNWs in Fig. 1(a), except that a roughly 40% lateral expansion in diameter of the nanowire was observed. After coating with SiNxOy for 20 min, the overall core-shell structure of SoNW/SiNxOy [in Fig. 1(d)] is found to be basically identical to that over SiNW structure (in Fig. 1b).
In Figs. 2(a)-2(c), we show that by using SoNWs as a 3D framework, the PL emission from the co-deposited SiNxOy thin film can be further augmented by a factor of 1.8, 2.8, 2.9 for the deposition time of 10 min, 20 min and 30 min, respectively. In the meantime, the reflection spectra measured upon the SiNW/SiNxOy or SoNW/SiNxOy core-shell structures, as shown in Fig. 3(a), are almost the same over a wide range from 200 nm to 750 nm. These observations indicate that i) the different PL emission performances for the SiNW-core or SoNW-core samples could not be explained based on the light trapping or anti-reflection effects among the 3D NW structures; and ii) the change of NWcore material, from a high-refractive-index SiNW-cores to a relatively low-refractive-index SoNW-cores has a quite significant impact on the final PL emission performance.
Fig. 3 (a) Reflectance characterizations of SiNxOy on flat Si wafer (black), SiNxOy/SiNWs (red) and SiNxOy/SoNWs (blue) structures; (b) shows the simulated absorption power realized within the SiNxOy active medium when deposited upon flat Si wafer (black), SiNWs (red) and SoNWs (green).
To develop further insights upon these findings, we carried out a set of finite element simulations by using RF module in COMSOL MULTIPHYSICS package to evaluate the incident light field intensity distribution, absorption profile and extraction within the radial core–shell structure. In this modeling, the nanowire and core-shell structure was represented by a cone standing upon a silicon substrate for SiNWs (SiO2 substrate for SoNWs), with a radius of 5 nm (15 nm) at the tip (base) and a height of 480 nm. Incident light field of 325 nm in wavelength, polarized along y-axis is introduced normally from the top plane with an initial electric field strength of . Taking advantage of the geometric symmetry in a periodic radial core-shell array, the simulation task over the whole periodic structure (with a nanowire spacing or periodicity of 2*Wsub = 600 nm, according to the estimated average SiNW spacing observed in experiment) can be reduced into a quarter region bounded by a set of proper boundary settings. The simulation is based on the n-k data/curves for SiNxOy, c-Si and glass materials that were determined by spectroscopic ellipsometry characterizations on corresponding thin film materials deposited on flat surface. For the sake of straightforward comparison, the volume expansion of SiNW in oxidation was not taken into account in the simulation.
In Fig. 4(a) [Fig. 4(b)], we show the incident light field intensity mappings, from the top-left to bottom-right, of a bare SiNW (SoNW) core, those of 10 min and 20 min SiNxOy depositions over SiNWs (SoNWs). As we can see in Fig. 4(a), a strong near-field can be clearly seen around the bare SiNW core, with at least times local enhancement, in strong contrast to that around a bare SoNW in Fig. 4(b), where the maximum electric field enhancement factor falls below 1.3 due to a much lower index of SiO2 compared to c-Si. However, this near field enhancement has been limited to a very close proximity around the SiNWs, with a spatial extension less than 30 nm from the SiNW sidewall. When coated with SiNxOy thin film after 10 min deposition (corresponding to a layer of ~40 nm), as seen in Fig. 4(a) and 4(b), the near field distribution patterns around both the SiNWs and SoNWs are largely distorted, as the whole SiNW or SoNW/SiNxOy radial structure behave like a new wider cavity with somewhat similar cavity field distribution and less impact from the different core materials. This is especially the case when increasing SiNxOy deposition time to 20 min for a coating layer of ~80 nm on the nanowire, where our simulations indicate that the contribution from the enhanced near field around the SiNW core, if still any, becomes far less important for the overall light absorption/excitation, while a thicker SiNxOy coating layer does lead to a stronger confinement of the incident light field into the active SiNxOy medium. Assuming that the local excitation of the SiNxOy medium is proportional to the effective local light absorption (power dissipation) in the matrix, as
where is the speed of light in vacuum, the local incident light intensity and the wavelength-dependent extinction coefficient of SiNxOy medium. The effective incident light excitation of the light emitting SiNxOy layer can be estimated by evaluating the integral of the overall absorption power realized within the SiNxOy volume byThe absorption power realized within the SiNxOy layers deposited over planar substrates, SiNWs and SoNWs have been calculated and shown in Fig. 3(b), with three marked shadow zones corresponding to roughly the sidewall SiNxOy layer thicknesses after 10 min, 20 min and 30 min depositions. For comparison, we show also the absorption power, marked by the blue inverse triangles in Fig. 3(b), realized in the SiNW cores as a function of different SiNxOy coating layer thickness. Obviously, both the SiNW and SoNW structures lead to a much strong light absorption/excitation enhancement compared to the flat thin films (with roughly the same SiNxOy volume), which explains well the observed PL emission gain of the 3D coated thin film over the planar reference. It is also noticeable that, to benefit from the near field enhancement around the high-index SiNW cores, the sidewall coating thin film has to be thin enough to fit in the near field around SiNWs (see for example in Fig. 4a). For example, in our case, a thin sidewall-coating layer below 40 nm is predicted to have a stronger light absorption and excitation. However, as the near field enhancement usually exists around the surface of a high-refractive-index medium (for instance, Si), when the SiNW core is replaced by a SoNW that features a much lower refractive index, the basis for benefiting from near field enhancement is lost. Nevertheless, the experimental observations in Figs. 2(a) to 2(c) show that SoNW cores lead to even stronger PL emissions than that upon SiNW cores. According to our simulations, with the increase of the coating layer thickness, the choice of inner nanowire core materials should have little impact upon the effective absorption/excitation in the radially coated outer and much thicker SiNxOy layers, as seen in Fig. 3(b).Fig. 4 (a) or (b) shows the profiles of the norm of |Ey| component (on y-z plane cutting through the center origin) around a bare SiNW or SoNW core, and those after coating with 10 min and 20 min-deposited SiNxOy thin films upon the NW cores.
To understand this discrepancy, we consider the light extraction from the 3D NW matrix which could also have a large impact on the final PL emission: when a photon is emitted within SiNxOy medium, it has to escape the nanowire forest to be extracted and collected by the detector positioned right above. To address this situation qualitatively, we place a dipole point light source (emitting at ) within a SiNxOy layer of ~80 nm thick in the middle segment, with different distance from the NW core ranging from 5 nm to 45 nm. Then, we integrate the out-going power flux of Poynting vector normal to the top plane, and plot the percentage of the extracted power (with respect to the dipole emission power) against the different distance of the point light source to NW core in Fig. 5(a). Corresponding light field strength distributions in a log scale, for both SiNW cores (left-half) and SoNW cores (right-half), are shown in Fig. 5(b). As we can see, the PL emission that could be eventually extracted or detected increases when the light source point moves from the inner NW/SiNxOy interface to the outer SiNxOy/air interface, and adopting SoNW cores always leads to a stronger PL emission compared to that with SiNW cores. This behavior can be understood more clearly in Fig. 5(b), where we find that the high-index SiNWs cores by themselves are absorptive and have directed more PL emission towards the bottom Si substrate. In contrast, the PL emission around the SoNW cores can escape more easily towards the top plane. This mechanism could provide a reasonable but still qualitative explanation to the observed stronger PL emission from the SiNxOy thin film coated over SoNWs [as seen in Fig. (2)], compared to that over SiNW cores. On the other hand, there has been recent report that such a conical and trumpet-like cavity is beneficial for achieving a broadband coupling between the core-shell cavity modes and the free-space propagating (emission) modes [33]. This mechanism could also have a positive contribution to the enhanced light extraction from the taper SiNxOy/NW core-shell structure, which will continue to be investigated in our future work.
Fig. 5 (a) the calculated light-extraction power percentage (integrals of the detected out-flow power on the top plane), when placing a dipole point light source [emitting at λ = 480 nm and marked as green stars in (b)] within the SiNxOy matrix with various distance away from a SiNW or a SoNW core, ranging from 5 nm to 45 nm, while (b) shows off the corresponding light field strength distributions for the cases of SiNW or SoNW cores in a log-scale plot.
4. Conclusion
In summary, we have explored a new strategy to boost the light emission from SiNxOy thin film, more than an order of magnitude, by adopting a properly designed 3D nanowire architecture. In a series of experiments and theoretical modeling, we found that the choice of the nanowire core material has a large impact over the light emission and extraction from the nanowire/thin film core-shell structure. Combining parametric investigations and simulation, we have been able to evaluate the key contributions arising from different mechanisms that include near-field enhancement, 3D light trapping and enhanced light extraction. Our results suggest that the VLS-grown nanowire matrix could serve as an effective and generic architecture/approach for boosting the light emission form various thin film materials.
Acknowledgements
The authors Z. YU, L. YU, J. Wang, S. Qian, J. Xu and Y. Shi thank the financial supports from the followings: Jiangsu Province Natural Science Foundation (Young Talent Program No. BK20130573) and National Basic Research 973 Program under Grant Nos 2013CB632101, 2014CB921101, 2013CB932900, and NSFC Nos 61036001 and 61204050.
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