Hybrid light-emitting polymer/SiNx platform for photonic integration

Organic semiconductors have potentials for a broad range of applications; however, it is difficult to be integrated with traditional inorganic material to meet the need of further application. Based on low-temperature silicon nitride (SiNx) deposition technique, here we demonstrate a hybrid structure fabricated by directly depositing high-quality SiNx on organic polymer film Poly[2-(2',5'-bis(2"-ethylhexyloxy)- phenyl) -1,4-phenylene vinylene] (BEHP-PPV). Stacked BEHP-PPV/SiNx hybrid structures with different periods are obtained and their optical properties are systematically characterized. Moreover, a group of BEHP/PPV embedded SiNx micro-disk is fabricated and amplification of spontaneous emission (ASE) is observed under optical pumping, further confirming that the gain properties of BEHP/PPV are well preserved. Our technique offers a platform to fabricate organic/inorganic hybrid optical devices compatible with integrated components.


Introduction
Recent research and progress on organic semiconductors has sprouted interest in the fields of organic LED and LET [1-4], organic lasers [5-9], organic sensors [10], and organic telecommunication devices [11,12]. Organic semiconductors have many novel optoelectronic properties, such as ultra-high photoluminescence efficiency [4], high gain [5], and tenability [6], among their broad fluorescence spectrum, making them suitable as laser materials [13,14]. Especially, conjugated polymers, one kind of organic semiconductors, exhibit desired features of simple fabrication and lost-cost, while it can be adapted to fabricate devices easily using solution process techniques such as spin-coating and ink-jet printing [13]. The advantageous optical gain properties and fabrication flexibility of conjugated polymer make it appealing to be combined with passive materials to fabricate active components compatible with photonic integrated circuits (PICs) [15], which is an essential issue for building fully on-chip integrated systems. Over the past decades, on-chip organic/inorganic hybrid devices have been demonstrated for different applications, for instance, hybrid organic/inorganic lasers and silicon-polymer modulators [16][17][18].
Considering material properties and fabrication techniques, silicon nitride (SiN x ) is a promising material as inorganic component for hybrid devices. SiN x has been widely used in the complementary metal-oxide-semiconductor (CMOS) industry as electrical and chemical isolation [19]. Furthermore, SiN x is transparent from the visible to near-infrared spectral region, making it a high performance solution for PICs [20][21][22]. Commonly, SiN x can be deposited with conventional techniques including low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) [23], but high deposition temperature may conflict with many applications related to materials with poor heat stability [19,24]. Recently, inductively coupled plasma chemical vapor deposition (ICP-CVD) technique has been developed to deposit SiN x film under ultra-low temperature [25][26][27]. In our previous work, we have reported that a high quality SiN x film with thickness up to 2 μm can be achieved in a single growth process under ultra-low temperature circumstance [27], which shows great potential for active passive hybrid photonic integration [28,29].
In this work, we fabricate a hybrid structure consisting of stacked SiN x layer and BEHP-PPV spin-coating film using ultra-low temperature SiN x deposition technique in ICP-CVD. Here, we have succeeded to deposit high quality SiN x with thickness of up to 380 nm directly on spin-coated BEHP-PPV film while optical properties of the hybrid structure have been measured. The results confirm that the activity of BEHP-PPV remains stable after the whole fabrication process. Moreover, we fabricate multi-layer micro-disks with different radii containing a 367 nm-thick SiN x layer on BEHP-PPV film on the basis of the hybrid structure, in which amplified spontaneous emission (ASE) is observed during the measurement of the multi-layer micro-disk. Our work provides a new strategy for constructing hybrid lightemitting polymer/SiN x platform which could be applied in integrated photonics.

Fabrication of multi-layer polymer/SiN x structure
Conjugated polymer powder BEHP-PPV is purchased from Sigma-Aldrich and dissolved in toluene in 10 mg/mL proportion. The solution is stirred using a vertex mixer until the BEHP-PPV powder is totally dissolved in toluene. Then it is spin-coated to obtain a film with thickness of about 50 nm on a cleaned and pretreated quartz substrate. After that, the spincoated film is dried under nitrogen atmosphere at 90 °C for 180 min. A thin SiN x layer with thickness of around 55 nm is deposited on the spin-coated polymer film by means of inductively coupled plasma chemical vapor deposition (ICP-CVD, Oxford Instrument Plasmalab System 100 ICP180) under 75 °C ambience and a single period of PPV/SiN x structure (SPS) is obtained. Repeating the spin-coating and SiN x deposition process, we also fabricate double period PPV/SiN x structure (DPS) and triple period PPV/SiN x structure (TPS). The image of the cross section of TPS under scanning electron microscope (SEM) [ Fig. 1(b)] reveals a smooth interface between BEHP-PPV spin-coating film (green layer) and SiN x layers (blue layer). The photograph of the SPS, DPS and TPS without any crack exposed in UV light around 380 nm is shown in Fig. 1(c). The absorption and fluorescence spectra of SPS, DPS, TPS and spin-coated BEHP-PPV film have been measured using an EMCCD (Andor Technology Newton DU907P) attached to a Shamrock SR500i spectrometer. Two obvious peaks around 489 nm and 521 nm can be identified in the fluorescence spectrum of the neat spin-coated BEHP-PPV film [ Fig. 2(a)]. As Fig. 2(b) shows, the fluorescence spectrum of DPS keeps consistent with the neat spincoated BEHP-PPV film. For further characterization, samples are excited by a tunable laser source (Vibrant 355 II, OPOTEK Inc.) which operates at a wavelength of 420 nm with a repetition rate of 10 Hz and a pulse duration of 5 ns. The emission is collected by a fiber coupled multichannel spectrometer platform (Avantes model AVS-DESKTOP-USB2) with 0.34 nm resolution. An obvious vibrational peak around 516 nm can be observed in emission spectrum of DPS [ Fig. 2(b)], which indicates a threshold behavior because peak intensity and full-width at half-maximum (FWHM) of the emission peak evolve as a function of energy density of the pump pulse laser [ Fig. 2(c)]. The emission peak rises and narrows rapidly when the pump energy density exceeds a threshold around 0.083 mJ/cm 2 [ Fig. 2(d)]. Moreover, DPS has a larger saturation gain coefficient [ Fig. 2(e)] and lower threshold [vertical dashed line in Fig. 2(e)] than both SPS and TPS. Gain coefficient of BEHP-PPV film is measured with variable stripe length (VSL) technique [30]. As shown in Fig. 2(e), SPS and DPS have gain saturation coefficient of up to 22 and 32 cm −1 , respectively, which is close to film of other PPV derivatives [5], implying that gain characteristics of BEHP-PPV is preserved well during the fabrication process. Compared with SPS, the total thickness of the active polymer film in DPS is larger, leading to stronger absorption and emission. However, surface of a spin-coating PPV film could not be completely smooth, especially for the film prepared with toluene [31]. Because the center of our spin-coated film is thicker than the edge, as structure period increases, the quality of spin-coated BEHP-PPV film and SiN x layer decreases, leading to the drop of gain coefficient of TPS compared to that of DPS [ Fig. 2(e)].
Lifetimes of samples are measured with an imaging spectrograph (C5094, HAMAMATSU) and a streak camera system (High speed streak unit M1952, HAMAMATSU), pumped by a Nd:YAG laser (EKSPLAPL2143 B/SS) and an OPG system (EKSPLA PG401SH/DFG2-10) operating at the wavelength of 420 nm with a repetition rate of 10 Hz and a pulse duration of 21 ps. As Fig. 2(f) shows, the lifetime of all samples stabilizes to around 23 ps as energy per pulse of the pump laser increases. In addition, photoluminescence (PL) quantum efficiency of BEHP-PPV before and after SiNx deposition is measured with a transient fluorescence spectrometer (FLS 980, EDINBURGH Instrument). Although the PL quantum efficiency of BEHP-PPV film slightly decreases from 7.07% to 6.16% as a result of SiN x deposition, the gain property and uniformity of the lifetime among SPS, DPS and TPS indicate that BEHP-PPV in the hybrid structure remains stable after our process flow.

Fabrication of polymer/SiN x micro-disks
On basis of the multi-layer polymer/SiN x structure, micro-disks are fabricated by optolithography and the fabrication process is described as follows. Firstly, a layer of 120 nmthick BEHP-PPV film is spin-coated on a clean quartz substrate, followed by a deposition of a SiN x layer with thickness of around 370 nm. Then, negative photo-resist AZnLoF2035 (Clariant Corporation) is spin-coated onto the SiN x layer and exposed with a direct-writing lithography system (Heidelberg InstrumentsuPG501). At last, we fabricate the multi-layer micro-disk with reactive ion etch system (PlamsaPro System 100RIE). The SEM image of the multi-layer micro-disk with radius of 76.6 μm [ Fig. 3(b)] shows that the as-fabracated micro-disk has obvious sandwich-like structure with smooth edge and sidewall. The polymer-embedded structure not only provides solid protection for the polymer but also effectively enhances the spatial overlap between gain medium and the resonating modes.
Characterization setup of the multi-layer micro-disk is schematically illustrated in Fig.  3(a). Before the laser pulse is focused onto the backside of the micro-disk, it propagates through a cylindrical lens, a Glan-Taylor prism, a polarization beam splitter, a 450-nmcentered band-pass filter and a 50:50 beam splitter. A lens collects the signals emitted from the micro-disk into the fiber-coupled spectrometer while images of the micro-disk are captured by a CMOS camera (Thorlabs, Inc). Here, we fabricate six groups of micro-disks with different radii, 52.3, 76.6, 104.6,156.9, 209.2, and 261.5 μm. Photograph of the whole sample under UV light illumination [ Fig. 4(a)] shows that BEHP-PPV in the sample can still emit strong fluorescence. Fluorescence spectrum of the micro-disk with radius of 261.5 nm under 405 nm light illumination [ Fig. 4(b)] keeps consistence with the neat spin-coated BEHP-PPV film [ Fig. 2(a)]. Moreover, when pumped by the 420-nm and 10-Hz laser pulses, ASE can be observed in every group of micro-disks. As for the micro-disk with radius of 261.5 μm, there is an emission peak around 516 nm [ Fig. 4(c)]. On the one hand, the emission peak rises rapidly when pump energy density exceeds 6.61 mJ/cm 2 , indicating a threshold behavior [ Fig. 4(d)]. On the other hand, this emission peak blue shifts slightly as the pump density increases, which is related with the self-absorption of BEHP-PPV film. Compared with micro-disk with radius of 52.3 μm, micro-disk with radius 261.5 μm shows more obvious threshold behavior and thus has higher gain. Fluorescence spectrum of the micro-disk with radius of 261.5 μm. (c) Emission spectrum of micro-disk with radius of 261.5 μm pumped by a 420-nm and 10-Hz pulse laser with different energy density. The inset picture shows the micro-disk under white light illumination (top) and the pulse laser pump (bottom). (d) Peak intensity of the emission peaks of micro-disks with radius of 261.5 and 52.3 μm, respectively, indicating a threshold around 6.61 mJ/cm 2 for the micro-disk with its radius of 261.5 μm.

Conclusion
We have fabricated an organic/inorganic hybrid structure consisting of spin-coated BEHP-PPV film and SiN x layer. During the whole fabrication processes, activity of BEHP-PPV in the hybrid structure shows no obvious degradation. Typically, saturation gain coefficient of DPS sample can be up to 32 cm −1 . Then we fabricate six groups of multi-layer hybrid microdisks with different radius on the basis of such hybrid structure and ASE is observed when the micro-disks are pumped by nanosecond laser pulses. Thus, optical properties of BEHP-PPV remain stable in the hybrid structure and the micro-disk, which indicates that the organic/inorganic hybrid structure could provide a new platform for constructing novel hybrid integrated components in many photonic applications.