Surface passivation by Al2O3 deposited by atomic layer deposition (ALD) at 200 °C is examined to suppress surface recombination for carrier-injection SiGe optical modulators. We have investigated the interface trap densities at SiO2/Si and Al2O3/Si interfaces formed by plasma enhanced chemical vapor deposition (PECVD) and ALD, respectively. By evaluating metal-oxide-semiconductor (MOS) capacitors formed on Si surfaces after dry etching, we found that the interface trap density of Al2O3 passivated surface is more than one order of magnitude less than that of SiO2 passivated one. As a result, the modulation efficiency is improved by 1.3 by inserting Al2O3 layer prior to SiO2 deposition by PECVD owing to superior interface between Al2O3 and Si. The Al2O3 passivated device exhibits comparable modulation efficiency to a thermally-grown SiO2 passivated one formed by dry oxidation. Hence, the ALD Al2O3 passivation is effective to passivate SiGe optical modulators for which low temperature processes are required.
©2014 Optical Society of America
Si-based optical modulators have been attracted much attention as an essential component for on-chip optical interconnect in conjunction with electronics . The Si optical modulators based on the plasma dispersion effect have been demonstrated  by means of carrier injection [3–6], depletion [7–14], and accumulation  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 . 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].
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 . 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 (NA ~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 (Dit) 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.
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 (Vacc) to inversion (Vinv) 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 Dit evaluated by the conductance method  are shown in Fig. 4. Although the etched Si samples exhibits higher Dit than the bulk Si sample, the ALD Al2O3 passivated surface shows Dit of less than 2 × 1011 cm−2 eV−1. On the other hand, the PECVD SiO2 passivated surface has Dit of larger than 1012 cm−2eV−1. Hence, the Al2O3 passivation is more effective to suppress surface recombination due to low interface traps.
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 NA 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.
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 VπL 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, VπL 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.
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.
This work was partly supported by the Strategic Information and Communications R&D Promotion Programme of the Ministry of Internal Affairs and Communications.
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