Overview of the Fabrication Processes - Nano-Fabrication Process of PhC slab

Chapter 1 Introduction 1

2.3 Nano-Fabrication Process of PhC slab

2.3.1 Overview of the Fabrication Processes

At first, we show the nano-fabrication flow chart of PhC slab, as shown in Fig 2-3. The details of each step are illustrated in the following sections.

Fig. 2-3: Nano-fabrication flow chart of PhC slab structure.

2.3.2

SiNx

At first, we deposit a 200 nm SiN

Hard Mask Deposition

x layer on the epitaxial wafer plasma-enhanced chemical vapor deposition (PECVD) by Oxford Instruments Plasma Technology Plasmalab 80 Plus, as shown in Fig 2.3. The SiNx layer is served as the hard mask for the following selective dry-etching process. The SiH₄ / NH₃ / N₂ mixed gas with flow rate of SiH₄ / NH₃ / N2 is 8 sccm / 8 sccm / 250 sccm is used at chamber temperature of 200 ℃, pressure of 1000 mTorr, and 20 W RF power. The deposition rate of SiN_x under this condition is about 38 nm/min. In addition, the quality of the SiNx film controlled by the ratio of SiH4 / NH3 / N2 is optimized, which guarantee that we can remove the SiN_x residue by buffer-oxide-etchant (BOE) wet-rtching after the dry-etching processes.

Fig. 2-4: Picture of PECVD system (Oxford Instruments Plasma Technology Plasmalab 80 Plus), facilities of Center for Nano-Science and Technology (CNST), National Chiao Tung University (NCTU).

2.3.3 Electron-Beam Lithography for Defining Patterns

Then we use the ELS - 7500EX electron beam lithography (EBL) system shown in Fig 2-4 for defining the PhC pattern. In this process, there are three steps. First, a 240 nm positive resist, poly-merthyl methacrylate (PMMA), is spin-coated on the MQWs with SiNx mask by a spin-coater. Second, the PhC pattern is defined on the PMMA-coated sample according to our computer-aided design (CAD) via the EBL system. Several dosages of electron beam are applied to include suitable dosage for the desired parameters of the PhCs. The beam position resolution of ELS-7500EX is 0.625 nm under the 150 × 150 μm writing field. Third, written PhC patterns on PMMA are developed and fixed by immersing in the methyl-isobutyl ketone (MIBK) and iso-propyl alcohol (IPA) solution at 24~25 ℃ in sequence. Then, the pattern on PMMA can be served as the first mask for the following dry-etching processes.

Fig. 2-5: EBL system (ELS - 7500EX), facilities of CNST, NCTU.

2.3.4 Dry Etching Processes for Transferring PhC Patterns

The following dry etching procedures for transferring pattern into the SiNx hard mask and MQWs are done by the inductively-coupled plasma reactive-ion etching (ICP-RIE), Oxford Instruments Plasma Technology Plasmalab 100 shown in Fig 2-5(a). In this process, there are two dry-etching steps. The first dry etching process transfers the patterns from the PMMA to SiNx hard mask. The CHF3 / O2 mixed gas with the flow rate of CHF3 / O2 is 50 sccm / 5 sccm is used at chamber temperature of 20 ℃, pressure of 55 mTorr, and 150 W RF power. The etching rate of SiNx under this condition is about 80 nm/min. The second dry etching process then transfers the patterns from the SiN_x hard mask to InGaAsP / InP. The Cl₂ / H2 / CH4 mixed gas with flow rate of 6.5 sccm / 13.5 sccm / 11.5 sccm is used at chamber temperature of 150 ℃, pressure of 4 mTorr, and 85 W / 1000 W RF power / ICP power. The etching rate of InP / InGaAsP under this condition is about 440 nm/min and the etching selectivity ratio to SiN_x is 4. Scanning electron microscope (SEM) picture of the cross-section of the PhCs on InGaAsP MQWs is shown in Fig. 2-5(b), where the sidewall angle is about 86°

- 87°.

Fig. 2-6: ICP-RIE system (Oxford Instruments Plasma Technology Plasmalab 100), facilities of CNST, NCTU. (b) SEM picture of cross-section of the PhCs after a series of dry etching processes.

2.3.5 Wet Etching for Suspended Slab Structure Formation

Then the dry-etching processes are followed by the BOE and HCl selective wet-etching.

The former one is used to remove the SiN_x residue and the latter one is used to form the suspended slab structure via removing the InP below the InGaAsP MQWs. According to the selective wet-etching chemistry [37], the solution of HCl : H2O = 3 : 1 at 2 ℃ is applied to form the suspended InGaAsP MQWs slab and smooth the InGaAsP slab surface.

2.4 Measurement Setup

To characterize the PhC lasers and emitters, a near-infrared (NIR) confocal mico-photoluminescence (micro-PL) is setup, as shown in Figs. 2-7 and 2-8.

In this system, a 845 nm transistor-transistor logic (TTL) diode laser (Power Technology Inc., APMT-60) is served as the pump source. To operate in the pulsed mode, a function generator (Stanford Inc., DG-535) is connected to a DC power source for modulating the output driving current. From this function generator, the pulse width and duty cycle of output signal are set to 15 ns and 100 kHz. The pump laser beam is split into two beams via a 50 / 50 beam splitter (BS), it. One beam will be reflected into a 100X long working distance NIR objective lens with numerical aperture of 0.42 and then focused on the sample. Another beam will transmit the BS to the power meter (Newport Inc., 1815-C). By reading the power value shown in the power meter, the incident power can be estimated.

To precisely pump the cavity region of our devices, a microscope system with a white light source and a charge-couple device (CCD) camera is setup in the micro-PL system. In addition, the sample is mounted on an 3-axes piezo-stage with minimum moving step of 30 nm and the efficient pumping on the cavity can be achieved via the microscope system.

Once the cavity is pumped, the emission will be collected via the 100X objective lens. A both-side polished Si wafer is placed in front of the receiving end to block the reflected pump source. Then the emission is feed into a multimode fiber (MMF) via a 10X objective lens. The MMF is connected into optical spectrum analyzer (OSA) (Ando Inc., AQ-6315A) and InGaAs power meter (Advantest Inc., Q8221) to analyze the emission properties.

Fig. 2-7: Configuration of the NIR confocal micro-PL system.

Fig. 2-8: Picture of the NIR confocal micro-PL system.

2.5 Summary

In this chapter, we have described the theories of numerical methods used for analyzing the PhC structures, including PWE, FDTD, and FEM. Subsequently, the nano-fabrication process for PhC slab structure based on InGaAsP MQWs are introduced, including the techniques of PECVD deposition, EBL system, ICP-RIE dry etching, and selective wet etching. Finally, we setup a NIR con-focal micro-PL system to characterize the emission properties of the PhC devices.

Chapter 3 1D PhC Nanocavities on Nanbeam

3.1 Introduction

In this chapter, at first, we will introduce two different one-dimensional (1D) photonic crystal (PhC) nanobeam (NB) nanocavity designs. Via numerical simulations, we will investigate the characteristics of defect modes in 1D PhC NB nanocavities and the structure will be optimized for large quality (Q) factor and small mode volume (V), that is high Q/V.

For serving as an active laser, these nanocavity designs with all-gradual PhC mirror are fabricated on InGaAsP multi-quantum-wells (MQWs) suspended slab structure. The measurement results will be addressed by FDTD and FEM simulation. Finally, we will summarize our work on 1D PhC NB nanocavities.

3.2 Design of 1D PhC Nanocavity on Nanobeam

We de sign two different 1D PhC NB nanocavities on suspended nanobeam (NB), the air-centered and the dielectric-centered nanocavities, whose center region is air-hole and vein (dielectric region between air holes) respectively, as shown in Fig. 3-1.

The width (w), thickness (t), and refractive index (n_NB) of the NB are 600 nm, 220 nm, and 3.4 respectively. The air-hole radius of 1D PhC is fixed at 126nm. To form the 1D PhC mirror with mode-gap effect, the lattice constant (a) increases digitally from 360 m (a_c) with 5 nm increment (Δa) away from the cavity, as shown in Fig. 3-2(a). The mode-gap effect via tuning lattice constant is illustrated by the band diagrams of 1D PhC on NBs with a₁ and a₂ (a2 > a1) in Fig. 3-2(b). For 1D PhCs with a1 and a2 on NB, the propagating mode frequency in a₁ is higher than a₂, which means the mode propagates in a₁ will be forbidden in a₂ and the 1D PhC with a2 acts like a mirror. To further reduce the optical scattering losses, multi-hetero-interface with mode-gap effect is usually applied, for example, digitally tuning the lattice constant shown in Fig. 3-2(a).

Fig. 3-1: Dielectric- and air-centered 1D PhC NB nanocavities on a suspended InGaAsP MQWs suspended slab structure.

Fig. 3-2: (a) Digital tuning of lattice constant in our nanocavity design. (b) Illustration of mode-gap effect formation via 1D PhCs with different lattice constants on NB.

In our nanocavity design, we apply all-gradual-mirror (all-GM), which is different from the generally-used hybrid GM / periodic mirror (PM). To show their difference, we simulate the 1D PhC NB nanocavities (dielectric-centered) with all-GM and GM / PM under fixed 1D PhC period numbers of eight for comparison, as shown in Figs. 3-3(a) and (b). The r/ac, w, ac,

△a, and t are 0.35, 600 nm, 360 nm, 5 nm, and 220 nm. The simulated mode profiles in electric-fields of these two nanocavities are simulated in Figs. 3-3(a) and (b). From the simulated results shown in Table 3-1, nanocavity with all-GMs shows better modal performance than that with GM / PM, including larger Q and smaller V.

Fig. 3-3: 1D PhC NB nanocavities with (a) all-GM and (b) GM / PM under fixed PhC period numbers of eight.

Table 3-1: Simulation results of the two nanocavities shown in Fig. 3-3.

Types of mirror design Modal Properties of the 0^th–order mode

All-GM

Q : 5.41×10³

V : 3.64×10^-1(λ/nNB)³ λ : 1563.5 nm

GM / PM

Q : 1.68×10³

V : 4.06×10^-1(λ/nNB)³ λ : 1560.9 nm

To further confirm the PBG effect formed by the 1D PhC all-GM on NB with mode-gap effect, we calculate the transmission spectra of all GM with different period numbers, as shown in Fig. 3-4. The input signal is an impulse with Gaussian mode profile, whose central wavelength is 1.5 μm. When the GM period increases, a loss suppression of more than two orders of magnitude between 4 and 10 periods can be achieved. Under the reflection provided by the 1D PhC GM on NB, we then discuss several parameters, including w, GM period number, and r/ac, for optimizing the 1D PhC NB nanocavities with large Q/V value in the following section.

Fig. 3-4: Transmission spectra of 1D PhC GMs with different periods from 4 to 10. The lattice constant is varied from 360 nm with 5 nm increment under fixed r, w, and t of 126, 600, and 220 nm.

3.3 1D PhC NB nanocavities

3.3.1 Simulated Modal Properties

Via 3D FDTD simulation, the modal properties of the air- and dielectric-centered 1D PhC NB nanocavities are investigated. Different structural parameters, including w, GM period number, and r/a_c, are tuned for optimizing the properties of 0^th–order mode in nanocavities. At first, Q and V of the 0^th–order mode in these two nanocavities when the beam width w is varied from 500 nm to 700 nm are simulated. The parameters a_c, △a, t, GM period number, and r/a_c, are set as 360 nm, 5 nm, 220 nm, 10, and 0.35. The simulated results are shown in Fig. 3-5. In Fig. 3-5(a), Q of the two nanocavities both decrease when w increases. This is because that the increased w increases the effective index and leads to enlarged scattering losses in GMs. In addition, the difference in Q between these two nanocavities can be explained via their time-averaged power flow in z component (S_z) shown in Fig. 3-6. Because the mode distribution Sz in air-centered nanocavity shows nodal lines in symmetric plane according to the far field cancellation [38], the vertical radiation loss can be effectively suppressed. Thus, the air-centered nanocavity shows higher Q than that of dielectric-centered nanocavity. And the simulated V f the dielectric- and air- centered nanocavities reach their smallest value when w = 550 nm (V ~ 0.36 (λ/nNB)³) and 600 nm (V ~ 0.39 (λ/nNB)³) respectively. Considering for the smallest V, we choose w = 600 nm in the following optimization work. Within the tuning range of w, the highest Q/V value for each air- and dielectric-centered NB nanocavities are 4.5 × 10⁵ (λ/nNB)^-3 and 2.4 × 10⁵ (λ/nNB)^-3.

Fig. 3-5: The simulated (a) Q and (b) V of 0^th–order mode as the function of w in dielectric- and air-centered 1D PhC NB nanocavities.

Fig. 3-6: The simulated mode profiles in Sz

Then we investigate the modal properties of these nanocavities with different GM periods, as shown in Fig. 3-7. The parameters a

of air- and dielectric-centered 1D PhC NB nanocavities in x-z plane.

c, △a, t, w, and r/ac, are set as 360 nm, 5 nm, 220 nm, 600 nm, and 0.35. In Fig. 3-7, Q increases with the GM periods and saturates when GM periods > 13. Moreover, λ and V also increase with the GM periods increases and saturates when GM periods > 7. The saturations in λ and V also imply that the 0^th–order mode in 1D PhC NB nanocavities is a localized mode, as shown in Fig. 3-7(d). Thus the further addition of GM periods larger than seven cannot be experienced by the 0^th–order mode. During the optimization of the GPs, the highest Q/V value for air- and dielectric-centered 1D PhC NB nanocavities are 1 × 10⁶ (λ/nNB)^-3 and 4 × 10⁶ (λ/nNB)^-3 respectively.

Fig. 3-7: The simulated (a) Q, (b) V, and (c) λ of 0^th–order mode as the function of GM periods in air- and dielectric-centered 1D PhC NB nanocavities. (d) The simulated |𝐸𝐸�⃗|² of 0^th–order mode in electric field in air-centered 1D PhC NB nanocavity with 14 GM periods.

Furthermore, we also investigate the influence of r/a_c on the modal properties of air- and dielectric- centered 1D PhC NB nanocavities, as shown in Figs. 3-8(a) and (b). The parameters ac, △a, t, w, and GM period number are set as 360 nm, 5 nm, 220 nm, 600 nm, and 14. In Figs. 3-8(a) and (b), the Qs of these two nanocavity designs reach their optimized values of 4.98 × 10⁶ and 1.38 × 10⁶ both when r/a_c = 0.29. Two mechanisms are responsible for the Q variation. One is that the modulation strength of effective index becomes smaller as r/a_c increase and Q would increase due to the reduced mode mismatch. Another one is that the total effective index of NB structure would decrease as r/ac increase and Q would decrease due to the reduction of total-internal-reflection confinement. The simulated V of these two nanocavity designs both decrease when r/ac increases, which is owing to the volume

shrinkage of the dielectric material in cavity region. During the optimization of r/ac, the highest Q/V value for air- and dielectric-centered 1D PhC NB nanocavities are 2.9 × 10⁶ (λ/nNB)^-3 and 9.9 × 10⁶ (λ/nNB)^-3.

Fig. 3-8: The Q and V of 0^th–order mode as the function of r/ac in (a) air- and (b) dielectric-centered 1D PhC NB nanocavities.

3.3.2 Measurement Results and Discussions for Laser Application

Referring to the simulation results, the air- and dielectric-centered 1D PhC NB nanocavities are realized on a InGaAsP MQWs suspended NB. To characterize the devices, a near-infrared micro-photoluminescence system is utilized. Experimental results will be confirmed via FDTD and FEM simulations.

To keep the device size compact, we choose 1D PhC NB nanocavities with 8 GM periods in our experiment. The length of the NB is only 6 μm, which is very beneficial for serving as a nanolaser in condensed photonic integrated circuits (PICs). The scanning electron microscope (SEM) pictures of air- and dielectric-centered 1D PhC NB nanocavities are shown in Figs.

3-9(a) and (b). Their r/ac and w are 0.37 and 690 nm.

In measurements, the lasing spectrum of air-centered 1D PhC NB nanocavity in Fig.

3-10(a) shows single mode lasing with side mode suppression ratio (SMSR) of 14 dB at 1555.5 nm. The light-in and light-out (L-L) curve in Fig. 3-10(b) shows a low threshold of 350 μW. Moreover, the lasing spectra under different pump power from 1.67 mW to 3.8 mW are shown in Fig. 3-11. Only one lasing peak is observed, which confirms the single mode lasing.

Fig. 3-9: SEM pictures of (a) air- and (b) dielectric-centered 1D PhC NB nanocavities with 8 GM periods.

Fig. 3-10: Lasing characterization of air-centered 1D PhC NB nanocavity with r/ac = 0.37 and w = 690nm. (a) Under the pump power of 1.5 mW, the lasing spectrum is shown and its inset shows the SMSR. (b) The L-L curve shows the lasing threshold of 350 μW.

Fig. 3-11: Lasing spectra under different pump powers, which confirms the single mode lasing.

In addition, we also show the lasing properties of dielectric-centered nanocavity design for comparison. In Fig. 3-12(a), single mode lasing with higher SMSR of 24 dB at 1546.5 nm is observed. In Fig. 3-12(b), the L-L curve shows a lasing threshold of 461 μW. Comparing these values with those of air-centered nanocavity in Fig. 3-10, we can see that the air-centered nanocavity has smaller SMSR but lower threshold than that of dielectric-centered nanocavity. This is mainly attributed to the higher Q of air-centered nanocavity than that of dielectric-centered nanocavity. To confirm this, we compare the thresholds between air- and

dielectric-centered nanocavities under the same parameters, as shown in Fig. 3-13. In each comparison, the lasing threshold of air-centered nanocavity is always smaller than that of dielectric-centered nanocavity, which confirms this phenomenon. Among these laser devices, the lowest threshold of 292 μW is observed from an air-centered nanocavity.

Fig. 3-12: Lasing characterization of dielectric-centered 1D PhC NB nanocavity with r/a_c = 0.37 and w = 690nm. (a) Under the pump power of 1.5 mW, the lasing spectrum is shown and its inset shows the SMSR. (b) The L-L curve shows the lasing threshold and its inset shows polarization of laser emission.

Fig. 3-13: Lasing threshold comparison between the air-center and dielectric-center nanocavity designs.

Fig. 3-14: The measured polarizations of (a) air- and (b) dielectric-centered 1D PhC NB nanocavities. The simulated mode profiles in Ex and Ey

Moreover, the measured polarization of air- and dielectric-centered 1D PhC NB nanocavity are both y-polarized with polarization degree of 1 : 11 and 1 : 36, as shown in Figs.

3-14(a) and (b). The y-polarization is the nature of waveguide-based nanocavity, which is owing to the dominant E

are shown as the insets of (a) and (b)

y than E_x of the mode, as shown in the insets of Figs. 3-14(a) and (b).

In addition, the measured lasing wavelength of these two nanocavities under different r/a_c and w also show good matching with the simulation results, as shown in Figs. 3-15 (a)-(d). The increasing w and decreasing r/a will increase the effective refractive index of the nanocavity. Therefore, we observe that lasing wavelength increases when w increases and r/a_c decreases. The difference between the simulated and measured results may result from the fabrication imperfections, including surface roughness, slight hole-size variation, and so on.

Fig. 3-15: The lasing wavelength of 0^th

We also investigate on the effect of adding the outer PM with two and six periods in dielectric-centered 1D PhC NB nanocavities with all-GMs. The parameters a

–order mode as the function of (a), (b) r/a and (c), (d) w in air- and dielectric-centered 1D PhC NB nanocavities.

c, △a, t, w, GM period number and r/ ac

, are 360 nm, 5 nm, 225 nm, 670 nm, 8, and 0.39. Some interesting observations are illustrated in the following. First, extra PM increases the total NB length,

Fig. 3-16: The SEM pictures of dielectric-centered 1D PhC NB nanocavities with different extra PM periods of zero, two, and six.

Fig. 3-17: The lasing spectra of dielectric-centered 1D PhC NB nanocavities with different PM periods under the pump power of 1.5 mW. The 1^st–and 2^nd

which leads to a fragile structure, as shown in Fig. 3-16. This also lets this structure not suitable for the optofluidic application due to the external force from the environment. Second, high order modes will appear and lead to multimode lasing. As shown in Fig. 3-17, for the nanocavity with outer PMs (two and six periods), the high-order modes, including 1

–order mode lasing can be observed in sequence when the PM period increase from zero to six.

st–and 2^nd–order modes are observed in the lasing spectra, results from the Q enhancements for the high-order modes due to extra PMs.

The simulated Q of different order modes in dielectric-centered 1D PhC NB nanocavities with zero, two, and six PM periods are listed in table 3-2. The Qs of different order modes become larger than 10³ when the PM period increases from zero to six. Thus, the 0^th–, 1^st–and 2^nd– order modes in lasing spectra are observed in sequence when the PM period increases. Third, moving the pump spot along the NB (x-direction) will lead to 0^th

Table 3-2: The simulated Q of different order modes in dielectric-centered 1D PhC NB nanocavities with different PM periods.

–order

mode suppression. The size of pump spot is approximately 4.2 μm and the dielectric-centered 1D PhC NB nanocavity with ten PM periods is used, as shown in Fig. 3-18(a). The parameters of nanocavity including a_c, △a, t, w, GM period number, and r/ a_c, are 360 nm, 5 nm, 225 nm, 660 nm, 8, and 0.36.When the pump spot is moved from 0 nm (center of the cavity) to 1350 nm, the 0^th–order mode is suppressed while the 1^st–order mode is still observed. The varied spectra are shown in Figs. 3-18(b)-(g). The mode suppression is caused by the different mode distribution of the 0^th– and 1^st–order modes shown in Figs. 3-19(a) and (b). When the pump

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