1. Introduction

Si-based optical modulators have been attracted much attention as an essential component for on-chip optical interconnect in conjunction with electronics [1]. The Si optical modulators based on the plasma dispersion effect have been demonstrated [2] by means of carrier injection [3–6], depletion [7–14], and accumulation [15] using p-i-n, p-n, junctions, and MOS capacitor. However, the weak free-carrier effects in Si cause the low modulation efficiency, resulting in the longer device length [9]. The plasma dispersion effect and free-carrier absorption are expressed by the Drude model. According to the Drude model, the changes in refractive index and absorption coefficient are described by a change in the plasma frequency of free carriers, and the plasma frequency is dependent on not only the number of free carriers but also their conductivity effective masses. The change in the refractive index induced by the plasma dispersion effect is inversely proportional to the conductivity effective masses of electrons and holes [16–18]. Therefore, the lighter the conductivity masses become, the greater the plasma dispersion is. In terms of that, we have proposed carrier injection type SiGe modulator in which the plasma dispersion effect is enhanced by introducing a compressively strained SiGe layer as shown in Fig. 1 [19,20].

 

Fig. 1 Schematic device structure of carrier injection type strained SiGe optical modulator.

Download Full Size | PDF

On the other hand, it is known that the modulation efficiency of the carrier injection type modulators is deteriorated by surface recombination due to interface traps between Si and a passivation layer. When carriers are injected to a waveguide core from p + and n + regions by forward bias, the injected carriers are trapped by interface traps, and recombined at dry-etched Si surfaces, decreasing carrier concentrations in the waveguide core where the light interacts with carriers [21,22]. A thermally grown SiO₂ layer is one of the best solutions for passivating etched Si surfaces with low interface trap densities; however it requires the high temperature oxidation process above 900 °C, which causes undesirable strain relaxation of strained SiGe [23]. Thus, a low temperature surface passivation process is indispensable for the strained SiGe optical modulators.

In this paper, we investigate a low temperature passivation process by using Al₂O₃ deposited by atomic layer deposition (ALD) to suppress surface recombination by using Si optical modulators. If SiGe modulators were used, we had to take into account the Ge diffusion impact on the modulation efficiency. Thus, it might be difficult to evaluate the surface passivation effect independently. Since interface traps on the etched Si slab region mainly degrade the efficiency of carrier injection in both cases of Si and SiGe modulators, Si modulators are more preferable to investigate the surface passivation effect between the thermally grown SiO₂ and Al₂O₃. Evaluation of interface traps by Capacitance-Voltage (C-V) measurements of metal-oxide-semiconductor (MOS) capacitors reveals that Al₂O₃ passivation is effective to reduce the interface traps even with 200 °C process temperature. By introducing Al₂O₃ passivation prior to SiO₂ passivation, we demonstrate the improvement in the modulation efficiency of a carrier-injection Si optical modulator.

2. Evaluation of interface traps

To evaluate Al₂O₃ passivation effect, we have fabricated Al/Al₂O₃/Si MOS capacitors by ALD. A B-doped p-Si (N_A ~1 × 10¹⁵ cm⁻³) substrate was used to fabricate the MOS capacitors. We have also fabricated Al/Al₂O₃/etched Si MOS capacitors to investigate interface traps generated by reactive ion etching (RIE) of Si surfaces. For comparison, Al/SiO₂/Si and etched Si MOS capacitors have also been fabricated by SiO₂ deposition by plasma enhanced chemical vapor deposition (PECVD). The interface trap density (D_it) distributions of the MOS capacitors are evaluated by the C-V measurements. Figure 2 shows the process flow of Al/Al₂O₃/etched Si MOS capacitor. First, the Si substrate was etched by RIE with CF₄ gas after organic cleaning. After cleaning Si surfaces by buffered HF, the 5-nm-thick Al₂O₃ was deposited at 200 °C. Then, Al electrode was formed by thermal evaporation. Finally, the forming gas annealing (FGA) was carried out for 30 min at 400 °C. Although a 1-nm thick Al₂O₃ layer is enough for passivating the Si surface, it is difficult to evaluate C-V curves when the Al₂O₃ thickness is less than 3 nm because of gate leakage current through quantum tunneling. Thus, we chose 5-nm thickness in Al₂O₃ to evaluate the interfacial properties by C-V measurements. The Al/SiO₂/etched Si MOS capacitor was fabricated by 45-nm-thick SiO₂ deposition at 350 °C instead of Al₂O₃. Since it is difficult to deposit 5-nm SiO₂ with keeping good uniformity, we chose 45-nm thickness in PECVD SiO₂. The difference in thickness between Al₂O₃ and SiO₂ does not affect the evaluation of interface trap density.

 

Fig. 2 Process flow of Al/Al₂O₃/etched Si MOS capacitor. (a) Dry etching, (b) Deposition of 5-nm-thick Al₂O₃ (c) Gate formation by Al electrode.

Download Full Size | PDF

Since there are many interface traps at the PECVD SiO₂/Si interface, an additional annealing for 30 min at 1000 °C in N₂ was carried out after SiO₂ deposition.

To investigate electrical properties of the MOS interfaces, we measured the C-V curves of the MOS capacitors at 1 kHz to 1 MHz. Figures 3 show the C-V characteristics of (a) the Al/Al₂O₃/Si, (b) the Al/Al₂O₃/etched Si, (c) Al/SiO₂/Si, and (d) Al/SiO₂/etched Si MOS capacitors. The solid line and dotted line show the voltage sweep of accumulation (V_acc) to inversion (V_inv) and inversion to accumulation, respectively. The C-V curves of the Al/Al₂O₃/Si MOS capacitor shows a well-behaved characteristic with no frequency dispersion, meaning that ALD Al₂O₃ can passivate Si surfaces well. It is found in Fig. 3(b) that the etched Si surface is also well passivated by Al₂O₃. On the other hand, Figs. 3(c) and (d) reveal that PECVD SiO₂ cannot passivate the etched Si surface because the Al/SiO₂/etched Si MOS capacitor exhibits stretch-out C-V curves with large frequency dispersion and hysteresis due to interface traps. Hence, it is shown that Al₂O₃ passivation is more effective to minimize etching damage of the Si surface than PECVD SiO₂ passivation. For the quantitative estimation, the energy distributions of D_it evaluated by the conductance method [24] are shown in Fig. 4. Although the etched Si samples exhibits higher D_it than the bulk Si sample, the ALD Al₂O₃ passivated surface shows D_it of less than 2 × 10¹¹ cm⁻² eV⁻¹. On the other hand, the PECVD SiO₂ passivated surface has D_it of larger than 10¹² cm⁻²eV⁻¹. Hence, the Al₂O₃ passivation is more effective to suppress surface recombination due to low interface traps.

 

Fig. 3 C-V characteristics of (a) Al₂O₃/Si, (b) Al₂O₃/etched Si, (c) SiO₂/Si, and (d) SiO₂/etched Si MOS capacitors.

Download Full Size | PDF

 

Fig. 4 Interface trap density distributions of the SiO₂/etched Si, the Al₂O₃/etched Si, and the Al₂O₃/Si MOS interfaces.

Download Full Size | PDF

3. Device fabrication and characterization

To evaluate the effect of surface passivation on the modulation characteristics, the pin-junction-based in-line optical modulators with the Al₂O₃ or SiO₂ passivation were fabricated by using a 220-nm-thick silicon-on-insulator (SOI) with N_A of ~1 × 10¹⁵ cm⁻³ wafer as shown in Fig. 5. To estimate carrier injection efficiency precisely, it is necessary to evaluate the modulator characteristic in the wide range of injection current. However, it is not easy to precisely estimate the carrier injection efficiency from refractive index change in a Mach-Zehnder interferometer (MZI) due to the unavoidable free-carrier absorption in an MZI’s arm which degrades interference properties. In the case of an in-line modulator, there is not such a problem to evaluate attenuation properties in the wide range of injection current. Therefore, an in-line modulator based on straight waveguide was used for the attenuation characteristics. First, the straight rib waveguide was formed by deep ultraviolet lithography and dry etching by reactive ion etching with CF₄ gas, followed by 20-nm-thick SiO₂ deposition by PECVD as a hard mask in Fig. 5(a). The width and mesa of waveguide is 2 μm and 120 nm, respectively. We chose the width of waveguides for obtaining enough optical output power in the experiments. Since we fabricated the waveguide by using contact photolithography, the propagation loss is large when the waveguide width is less than 1 μm due to the large sidewall roughness. In addition, the coupling loss is increased by narrowing waveguide width due to end coupling. The 2-μm-wide waveguide has theoretically high-order optical modes. However, we have tried to couple the light to the fundamental mode of the 2-μm-wide waveguide as much as possible by monitoring the output image through InGaAs camera. Thus, the 2-μm-wide waveguide does not have significant impact on evaluating the attenuation properties. Although the waveguide width should be scaled down to 500 nm for high speed operation, we have focused on carrier injection efficiency which is affected by interface traps on the Si surfaces. Therefore, it is not a significant matter to use the 2-μm-wide waveguide for evaluating the static characteristics of output power by carrier injection in this paper. Then, ion implantations of boron and phosphorus (dose: 1 × 10¹⁵ cm⁻², energy: 10 keV and 30 keV, respectively) were carried out to make the p + and n + regions to form a lateral pin-junction, followed by activation annealing at 1000 °C for 30 min in N₂ atmosphere in Fig. 5(b). Here, the distance between implanted regions and waveguide edge is approximately 2 μm. Then, the contact pads for the p + and n + regions were formed by thermal evaporation of aluminum. The final structure of a SiO₂-passivated device is shown in Fig. 4(c). In the case of the Al₂O₃-passivated device, the SiO₂ passivation layer was removed by wet etching using BHF after activating the implanted dopants. Then, a 1-nm-thick Al₂O₃ layer was deposited on Si as a passivation layer by ALD at 200 °C and a 280-nm-thick SiO₂ layer was deposited on Al₂O₃ layer by PECVD as shown in Fig. 5(e). Finally, the contact pads regions were formed. The final structure of a Al₂O₃ passivated device is shown in Fig. 5(f). Forming gas annealing was implemented for all devices to decrease the contact resistance, which is for 30 min at 400 °C. Figure 6 shows a top-view of the fabricated device passivated by Al₂O₃. The structure of in-line modulator has been used to estimate the optical attenuation occurred by free-carrier absorption.

 

Fig. 5 Process flow of carrier-injection type Si in-line optical modulator. (a) Waveguide formation (b) p + and n + region formation by ion implantation and activation. (c) Final structure of SiO₂-passivated device after Al electrode formation, (d) Removal of SiO₂ passivation layer by wet etching (e) Al₂O₃ and SiO₂ passivation, and (f) Final structure of Al₂O₃-passivated device after Al electrode formation

Download Full Size | PDF

 

Fig. 6 Top-view of Al₂O₃-passivated device observed by optical microscopy.

Download Full Size | PDF

Figure 7 shows the measured optical attenuation properties of the Si in-line intensity modulators with and without the Al₂O₃ layer as a function of injected current. The optical attenuation was measured by injecting current to evaluate the effect of Al₂O₃ passivation. Since we have not focused on high-speed operation, the static characteristics of attenuation are measured to evaluate the carrier injection efficiency. Continuous-wave (CW) TE-polarized light with a wavelength of 1550 nm was coupled to the waveguide through a lensed fiber. Then, the output power was monitored using an InGaAs photodetector while changing the injection current. The length of the phase-shifters is 1 mm. It can be seen that the attenuation of the Al₂O₃-passivated modulator is larger than that of the SiO₂-passivated modulator. As a result, the injection current required for the 20-dB attenuation can be reduced from 64 mA/mm to 47 mA/mm owing to the reduction in the interface trap density using the Al₂O₃ passivation layer. Also, we have fabricated the Si in-line intensity modulator passivated by the thermally oxidized SiO₂ at 1000 °C. It is worth noting that the performance of the Al₂O₃-passivated device is comparable to the SiO₂ passivated device formed by dry oxidation, indicating that Al₂O₃ passivation is as good as SiO₂ passivation by dry oxidation in terms of surface recombination. We can estimate the modulation efficiency in V_πL from the attenuation results. Since the voltage necessary for 20-dB attenuation is reduced from 2.2 V to 1.8 V by using Al₂O₃ passivation as compared with PECVD SiO₂ passivation, V_πL of the Al₂O₂-passivated device is expected to be improved by approximately 20%, which is comparable to the thermally grown SiO₂ passivation. We have also found no significant impact of the passivation layers on the propagation loss. Thus, the total insertion loss is not changed even in the Al₂O₃-passivated device.

 

Fig. 7 Attenuation characteristics of Si modulators passivated by PECVD SiO₂ (black), dry-oxidized SiO₂ (blue), and Al₂O₃ (red).

Download Full Size | PDF

6. Conclusion

We have investigated the low-temperature surface passivation, which is especially necessary for carrier-injection SiGe optical modulators due to the restriction of thermal budge, for suppressing surface recombination. We find that Al₂O₃ passivation formed by ALD at 200 °C is effective to decrease interface traps on the etched Si surfaces at low temperature as compared with PECVD SiO₂ passivation. The interface trap density of the Al₂O₃-passivated etched Si surface is less than 2 × 10¹¹ cm⁻²eV⁻², which is one order magnitude lower than that of the PECVD SiO₂-passivated surface. The modulation efficiency is also improved by introducing the Al₂O₃ passivation layer prior to SiO₂ deposition. The injection current density for 20-dB attenuation is improved by approximately 40%, which is comparable to the thermally grown SiO₂-passivated device. Thus, ALD Al₂O₃ is promising for passivating the strained SiGe optical modulator at low temperature.