1. Introduction

The investigation of Si nanocrystals (Si-ncs)-based light source was believed to show excited impact in the future Si photonic circuits [1,2]. Unfortunately, the light emission from Si-nc-incorporated devices usually has a weak emission intensity with broadened spectral linewidth up to 150–200 nm. The wide luminescent response is mainly attributed to the nonuniform size distribution of Si-ncs [3]. Many research groups emphasize on engineering the Si material to enhance the quantum efficiency of Si-nc-based metal-oxide-semiconductor light emitting diode [4,5]. Later on, the spectral linewidth narrowing of the Si-based light source also becomes an intriguing topic by employing distributed Bragg reflector (DBR) or one-dimensional photonic crystal structure, which is often used to form the cavity with selective resonant wavelength. The high-reflective DBR precisely controls the constructively reflected interference at a desired wavelength by detuning the thickness and the refractive index of the cascaded SiO₂/Si₃N₄ quarter-wave plates, which has been employed to integrate with the broadband Si-nc emitter for narrowing down the spectral linewidth to 20 nm or less [3,6–10].

Iacona et al. obtained an extremely narrow linewidth with 1.5 nm by embedding the Si-nc layer in the a-Si/SiO₂ DBR cavity; the Q-factor and the PL peak enhancement are 500 and 40, respectively [6]. With similar materials, the thermally tunable filter was demonstrated to show a tuning range of 26 nm by varying temperature (ΔT) up to 339 K [7]. Another SiO₂/TiO₂-based DBR cavity system was also employed to reduce the linewidth of Si-nc-based light emitter to 13 nm, and the conical angle of emission was determined as small as 15° to show non-Lambertian behavior [8]. The first electrically pumped (a-Si/SiO₂) DBR resonant cavity Si-nc LED was proposed by Muscara et al. They enhanced the device Q-factor from 17 to 150 (with Δλ decreasing from 50 to 5 nm) by increasing DBR pairs to achieve 20-time PL/EL enhancement at resonant wavelength [9]. In addition, an Si-nc-embedded Si-rich SiN_x light emitter integrated with SiN_x/SiO₂ DBR cavity was demonstrated to obtain narrow linewidth and enhanced PL [10]. However, a possibility of Si-nc-based in situ DBR emitter is considered by synthesizing the SiO₂/Si₃N₄ multilayer DBR with Si-rich composition ratio. If each SiN_x/SiO_x pair serves as both the light emitter and the DBR layer, the broadband PL spectrum from Si-nc-embedded SiN_x/SiO_x layer can concurrently be self-filtered.

In this work, we demonstrate for the first time the in situ SiN_x/SiO_x multilayer DBR-based Si-nc self-luminescent light emitter. The DBR structure is designed and simulated by commercial software; the high-reflection filter response is measured by the reflectance spectrometer. The SiN_x/SiO_x self-luminescent DBR filter shows narrow linewidth by extending its pair number to 24.

2. Experiment Setup

In experiment, the Si-rich SiN_x and SiO_x layers were grown by PECVD with the reactant gas of NH₃/SiH₄ and N₂O/SiH₄ at inductively coupled plasma power of 70 and 40 W, respectively. The refractive index of SiN_x and SiO_x are 1.96 and 1.62, as determined from the reflectance spectrum. Afterwards, the Si-rich SiN_x/SiO_x multilayer DBR is consecutively deposited on a quartz substrate by PECVD at a chamber pressure of 120 mtorr and substrate temperature of 350°C; each SiN_x/SiO_x pair induces broadened PL in visible wavelength from buried Si-ncs. Up to 24 SiN_x/SiO_x pairs are designed as the quarter-wave stack to form the in situ high-reflection filter.

With the schematic diagram shown in the inset of Fig. 1(a) , the PL analysis is performed by off-axis (tilted by 40° from surface normal) pumping of the SiN_x/SiO_x single-pair and DBR samples with GaN laser diode at 405 nm and by collimating the luminescence into a spectrometer (Ocean Optics USB4000). The PL of the single-pair SiN_x/SiO_x is centered at 495 nm with 3 dB linewidth of 150 nm, as shown in Fig. 1(a), which is strongly correlated with the Gaussian size distribution of Si-ncs synthesized within the sample [11,12], as confirmed by the high-resolution transmission electron microscopy (HRTEM) shown in Fig. 1(b). Similar PL response contributed by Si-ncs precipitated in as-grown SiN_x without high-temperature annealing process was also reported in previous work [13]. To narrow down the emission linewidth by eliminating the UV-band PL, the DBR architecture is designed to block the luminescence ranging between 400 and 500 nm. The thickness of SiN_x and SiO_x layers is determined as 45 nm and 86 nm, respectively, through the simulation by Rosft. The central forbidden wavelength of the 24-pair SiN_x/SiO_x DBR is located at 459 nm with a corresponding reflectance of 0.99. Figure 1(c) illustrates the schematic diagram of the multilayer in situ SiN_x/SiO_x DBR emitter with its HRTEM structural image shown in Fig. 1(d). The actual thickness of the deposited SiN_x and SiO_x layers are strictly controlled at 45 nm and 86 nm to coincide with the simulation.

 

Fig. 1 (a) The PL of single SiN_x/SiO_x layer; (b) TEM image of Si-ncs; (c) The DBR architecture; (d) TEM image of SiN_x/SiO_x pair.

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3. Results and Discussion

With the xenon-lamp illumination at a tilted angle of 10° and the monochromator-PMT (CVI DK240) detection, the reflectance spectrum of the in situ self-luminescent SiN_x/SiO_x DBR filter is shown in Fig. 2(a) . Not only the maximum reflectance and peak wavelength window but also the high-order ripples at longer wavelengths are in good agreement with the simulation; however, the slightly mismatched multilayer interference, which degrades the slope on trailing edge of the DBR filter function, is also observed. The fabricated DBR filter exhibits a reflection bandwidth of 95 nm, which is much broader than that of 53 nm simulated by using Δλ = 4(λ₀/π)sin⁻¹[(n_SiOx-n_SiNx)/(n_SiOx + n_SiNx)] [14], where λ₀, n_{SiNx ,} and n_SiOx are the central wavelength, and the refractive indices of SiN_x and SiO_x, respectively. The measured DBR reflectance and simulated DBR reflectance obtained by pumping at incident angles of 10°, 20°, 30° and 40° are demonstrated in Fig. 2(b). By increasing the incident angle from 10° to 40°, the reflectance peak shifts from 459 to 420 nm due to the variation of layer thickness. Nonetheless, such a filter bandwidth broadening could positively contribute to a narrower emission spectrum as described below.

 

Fig. 2 (a). Experimental (black) and simulation (red) results on the reflectance spectra of the in situ self-luminescent DBR filter and the schematic diagram of reflectance spectroscopy. (b) The measured and the simulated DBR reflectance obtained by pumping at incident angles of 10°, 20°, 30° and 40°

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Figure 3(a) demonstrates the PL of single-pair SiN_x/SiO_x (black solid line) and the measured transmittance (blue dashed line) of the 24-pair DBR filter, which clearly shows a possibility on narrowing the emission linewidth by broadening the filter linewidth in this case. When pumping the single-pair SiN_x/SiO_x structure and probe the PL at same side, the PL spectra obtained from top SiN_x and bottom SiO_x surfaces reveal almost identical shape and intensity with that of the single SiN_x layer shown in Fig. 1(a). Since there is not a significant difference on the reflection of single-pair SiN_xSiO_x layers, it is not expected that any distinguished PL response can be observed between pump-and-probe analyses at top and bottom surfaces. Figure 3(b) shows the measured PL spectrum centered at 533 nm for the in situ self-luminescent DBR filter (black line) with its green pattern given as the inset photograph. By pumping the DBR filter to measure the transmitted PL response, the expected sharpened PL spectrum is demonstrated with its linewidth even narrower than that obtained from single-pair response, as shown in Fig. 3(b).

 

Fig. 3 (a) The PL of single-pair SiN_x/SiO_x layer (black line) and the transmittance of the 24-pair DBR filter (blue dash line). (b) The simulated (red dash line) and measured (black line) PL of 24-pair in situ self-luminescent DBR filter with the inset photograph showing its luminescence pattern.

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Figure 3(b) demonstrates that the measured peak PL wavelength is almost the same as that of the simulated PL (at 532 nm). In addition, the dip around 459 nm makes good agreement with the experimental result. With the simulated PL response given by simply subtracting the PL and transmittance spectra in Fig. 3(a), we further observe that such a cascaded emission response is mainly attributed to the PL emitted and filtered by different SiN_x/SiO_x pairs within the DBR filter. Because of the filtered transmission, the transmitted PL response of the in situ self-luminescent DBR filter at 400-510 nm degrades dramatically when compared with that of the single-pair SiN_x/SiO_x sample. The peak PL wavelength of such a self-luminescent DBR filter located at 533 nm is exactly the intersection point of the PL of the single-layer SiN_x/SiO_x and the transmittance edge of the 24-pair DBR filter function. With such an integrated emitter/filter design, the PL linewidth of SiN_x/SiO_x layer is greatly reduced from 150 nm to 10 nm. The combination of gain medium and DBR filter simplifies the design for Si-nc-based LED with purified emission spectrum, in which the emission wavelength can be selected by fitting the PL and filter response at the very beginning.

The PL spectra of 24-pair SiN_x/SiO_x DBR sample with different pump power as well as the relationship between the output PL intensity and the pump power are demonstrated in Fig. 4(a) and Fig. 4(b). The PL intensity linearly increases from 400 to 3157 counts with enlarging pumping power from 10 to 70 mW, the linear enhancement indicates that the emitting mechanism is still dominated by the spontaneous emission of the buried Si-ncs within the SiN_x/SiO_x materials. The high-order peaks are attributed to the transmitted ripples of the DBR filter. During the variation of the pumping power, the 3-db linewidth and central wavelength of the principle PL peak for the 24-pair DBR sample remain at 10 nm and 533 nm, respectively. From our experimental results, the dominated emission behavior of Si-ncs in our device is still the spontaneous emission as same as that in the bulk Si-nc film. There is only a difference on the luminescent wavelength caused by the quantum confinement effect.

 

Fig. 4 (a) The PL of DBR sample obtained with different pump power. (b) The PL intensity versus pumping power.

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As a matter of fact, the mechanism responsible for the PL response from the 24-pair self-luminescent DBR filter is more complicated owing to the cascaded luminescence contributed by each SiN_x/SiO_x pair. The PL contributed by the upper SiN_x/SiO_x pairs experience less quarter-wave plates, which suffers from a degraded filtering function to provide a broader transmitted luminescence. To realize the changed filtering effect caused by varying transmittance of the alternating DBR pairs, the Fig. 5 demonstrates the simulation results of the filtering spectra for the DBR filter with alternating SiN_x/SiO_x pair number. With decreasing SiN_x/SiO_x pair number from 24 to 1, the filter bandwidth becomes gradually broadened but smoothened, providing a rejection-band linewidth enlarged from 53 nm to 117 nm and the degradation on its pass-band reflectance from 1% to 53%. The emitting photons propagating through less DBR pairs may produce residual luminescent peaks within the rejection band at 400-500 nm to result in a less-purified PL response. In contrast, the cascaded PL passing through plenty of DBR pairs would emit greatly narrowing PL, which causes a deviation from the simulation result, including the different FWHM of PL in the region of 500-600 nm, and the presence of second-order luminescent peaks around 408 nm.

 

Fig. 5 The simulated transmittance of different DBR pairs

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To precisely simulate the filtering mechanism for transmitted PL from such an in situ emissive DBR with integrated emitter/filter function, each SiN_x/SiO_x pair is considered to serve as the individual emitter with the PL spectrum of each single SiN_x/SiO_x pair denoted as PL(λ). The DBR-dependent luminescence of the nth SiN_x/SiO_x pair with its PL intensity denoting as P_n, which is proportional to the pump intensity (power/area) and the transmittance of the top DBR filter with (n-1) layer number. Therefore, the total output power of the emissive SiN_x/SiO_x DBR (P_total) equals to the summation of P_n(λ) from each single luminescent pair through the filtering by DBR with changing pair number. The simulation is illustrated in Fig. 6(a) with the following equation given by the newly developed model,

 

Fig. 6 (a) The schematic diagram of PL simulation. (b) The experimental and simulated PL spectra.

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where P_pump and w²(z) are the pump power and the Gaussian beam radius of the excitation source at depth z, α_SiOx and α_SiNx are the absorption coefficients of SiO_x and SiN_x layer determined as 3 × 10³ cm⁻¹ and 1 × 10⁴ cm⁻¹ [15,16], respectively, d_SiOx and d_SiNx are the thickness of SiO_x and SiN_x layer, T_pump is the transmittance of the excitation source calculated by 1-[(n_SiOx-n_SiNx)/(n_SiOx + n_SiNx)]², and T_DBR(n-1) is the transmittance of the DBR filter with (n-1) pair number.

As a result, the experimental and simulated PL spectra are compared in Fig. 6(b). Our simulation for the PL response of the Si-nc embedded DBR device confirms that the emission is the combination of the PL from each single-pair SiN_x/SiO_x suffering from the different DBR responses in different layers. The simulation correlates well with the experimental result with the PL spectrum of each single-pair SiN_x/SiO_x layer set to be coincident with that of the Si-nc, which validates our consideration that both the emission behaviors of the Si-nc embedded SiN_x/SiO_x DBR and the bulk Si belong to the spontaneous emissions at different wavelengths. The simulated linewidth of 15 nm is correlated with the experimental result, indicating the accuracy of our hypothesis that the transmitted PL is a summation from each emissive SiN_x/SiO_x pair passing through the cascaded DBR filter. The simulation also interprets that the main reason makes the experimental PL wavelength red-shifted from the simulation is attributed to the much broader reflectance spectrum of the fabricated DBR than its original design, as the slightly mismatched multilayer interference inevitably results in a degraded slope to smoothen the trailing edge of the transmitted PL.

When pumping at the bottom side with SiO_x surface layer, the PL peak is located at 533 nm with a 3-dB linewidth of 10 nm and a higher extinction ratio. In contrast, the PL peak is located slightly blue-shifts to 531 nm with a 3-dB linewidth of 68 nm and a lower extinction ratio is observed by the arising fringe PL pattern, the results are shown in Fig. 7(a) . This phenomenon is induced by the slightly different reflection characteristics from both sides of the DBR structure as shown in Fig. 7(b). It is seen that the reflectance spectrum probed at the bottom side of the DBR structure with SiO_x top layer shows a similar shape with enlarged fringes around the passband. Such fringes with a larger extinction ratio inevitably cause a transmitted PL with a degraded extinction ratio.

 

Fig. 7 (a) The PL spectra obtained from the bottom SiO_x (upper plot) and the top SiN_x (lower plot) sides of the 24-pair DBR structure. (b) The experimental and simulated reflection spectra obtained from the bottom SiO_x (upper pot) and the top SiN_x (lower plot) sides of the 24-pair DBR structure.

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Such an in situ emitter/filter design is preliminarily demonstrated by directly using the Si-nc-embedded SiN_x/SiO_x pair instead of standard oxide and nitrite film stacks. In earlier works, using the Si-based DBR structure as a purely resonant cavity, the resonant luminescence with 3-dB linewidth narrowing to 20 nm (by Creazzo el al.) or even 1.5 nm (by Iacona et al.) was approached. In comparison with previous investigations [4,12–15], the contribution of this work is its unique design with all Si-nc-embedded dielectric layers and a comparable line-shape sharpening performance. Such an emitter/filter integrated architecture excludes the need for additional cavity design and fabrication for resonant wavelength selection. The self-luminescent DBR combines the emitter and the filter to block the broadband spontaneous emission and leaves sharpened PL contributed by the transmitted fringe aside the stop-band. With increasing pair number, both the stop-band and the transmitted fringe of the DBR reflectance spectrum becomes sharpened to narrow the spectral linewidth and to enlarge high extinction ratio. Furthermore, the PL intensity is linearly proportional to the pair number. The combination of gain medium and DBR filter simplifies the design, only three parameters (PL of the single pair, layer thickness and the refractive index of two materials) are needed to determine the structure of the self-luminescent DBR. With our developed model, The transmitted PL response of the device including the peak wavelength and the narrowed linewidth can be accurately simulated. In particular, the Si-nc is synthesized in the as-grown SiN_x layer without employing high-temperature annealing.

Previously, there are many distinguished works describing the electroluminescence of the similar DBR device [3,9], Our proposed device is not suitable for electrically pumping due to its large thickness (> 3 μm) design at current stage, since the metal-oxide-semiconductor light emitting diode (MOSLED) configuration meets a serious problem on the efficient thermal dissipation. Reducing its pair number by increasing the difference of refractive index of two materials can shorten the thickness of the DBR, such that the associated effects including the decreasing reflectance and broadening bandwidth must take into consideration. In the case of the separated Si-nc gain medium between two DBR structure (DBR/gain /DBR), the radiative decay rate is influenced by the resonant cavity [17]. The luminescence from the Si-ncs with DBR cavity can be enlarged by designing the certain cavity parameters (for example: the reflectance of two DBRs). In our designed self-luminescence DBR, the peak intensity is proportional to the total volume of the active materials. Since the self-luminescent DBR only exists one DBR architecture without separated gain medium, and each single-pair SiN_x/SiO_x layer experiences different filtering effect during propagation through the whole DBR structure, the summation of the reshaped PL could inevitably attenuate its intensity as compared to the typical DBR cavity with separated gain medium. Nevertheless, this is a preliminary design of such a unique device and further improvement can be made after a thorough consideration and optimization on the device parameters.

4. Conclusion

In conclusion, the 24-pair multilayer self-photoluminescent SiN_x/SiO_x DBR emitter with green PL pattern is demonstrated by filtering the UV and blue luminescence at 400–510 nm to narrow down the transmitted emission linewidth. With the SiN_x and SiO_x layer thickness of 45 and 86 nm and the refractive index of 1.96 and 1.62, respectively, the measured reflectance, the peak wavelength, and the 3-dB linewidth of the DBR filter spectrum are 0.99, 459 nm, and 95 nm, respectively. Such an integrated emitter/filter design facilitates a transmitted PL linewidth to greatly reduce from 150 nm to 10 nm. The transmitted PL exhibits a central peak at 533 nm as prediction. The PL intensity linearly increases with enlarging pumping power, which indicates that the emitting mechanism is dominated by the spontaneous emission of the buried Si-ncs within the SiN_x/SiO_x materials. The complicated PL response of the 24-pair self-luminescent DBR filter mainly results from the cascaded luminescence contributed by each SiN_x/SiO_x pair. When decreasing SiN_x/SiO_x pair number from 24 to 1 to cause a broadening but smoothened filter bandwidth, the rejection-band linewidth enlarges from 53 to 117 nm with its pass-band reflectance degradation from 1% to 53%. This inevitably leads to the degraded spectral lineshape at 500-600 nm and the presence of second-order luminescent peaks around 408 nm owing to the incomplete filtering of the transmitted PL when passing through the DBR of insufficient pair number. By considering each SiN_x/SiO_x pair as independent luminescence center except a sole DBR filter element, a new emissive DBR model is established to simulate the transmitted PL mechanism with coincident luminescent shape and linewidth.