Younghyun Kim

Strain-induced enhancement of plasma dispersion effect and free-carrier absorption in SiGe optical modulators

The plasma dispersion effect and free-carrier absorption are widely used to change refractive index and absorption coefficient in Si-based optical modulators. However, the weak free-carrier effects in Si cause low modulation efficiency, resulting in large device footprint and power consumption. Here, we theoretically and experimentally investigate the enhancement of the free-carrier effects by strain-induced mass modulation in silicon-germanium (SiGe). The application of compressive strain to SiGe reduces the conductivity effective mass of holes, resulting in the enhanced free-carrier effects. Thus, the strained SiGe-based optical modulator exhibits more than twice modulation efficiency as large as that of the Si modulator. To the best of our knowledge, this is the first demonstration of the enhanced free-carrier effects in strained SiGe at the near-infrared telecommunication wavelength. The strain-induced enhancement technology for the free-carrier effects is expected to boost modulation efficiency of the most Si-based optical modulators thanks to high complementary metal-oxide-semiconductor (CMOS) compatibility.

Numerical Analysis of Carrier-Depletion Strained SiGe Optical Modulators With Vertical p-n Junction

The modulation characteristics of carrier-depletion strained SiGe optical modulators with a vertical p-n junction are numerically analyzed by technology computer-aided design simulation and finite-difference optical mode analysis. In addition to the strong optical confinement in the vertical direction for the fundamental transverse electric field mode, the vertical p-n junction effectively depletes the strained SiGe layer in which the plasma dispersion effect is enhanced owing to the mass modulation of holes by strain. We predict that a Si0.7Ge0.3 optical modulator exhibits VπL at 1.55 μm of as small as 0.31 V-cm at a bias voltage of -2 V, which is ~1.8 times smaller than that of a Si optical modulator. The product of VπL and the phase-shifter loss (αVπL) is also expected to be as low as 18.3 V-dB at -2 V, enabling optical modulation with a 5-dB extinction ratio in a symmetric Mach-Zehnder modulator.

Ge-rich SiGe-on-insulator for waveguide optical modulator application fabricated by Ge condensation and SiGe regrowth

We have numerically analyzed plasma dispersion effect in a Ge-rich SiGe layer for optical modulator applications. Since strain induces reduction in effective masses of electron and hole, we expect enhanced plasma dispersion effect in a strained Ge-rich SiGe layer. The plasma dispersion effects of Si(0.15)Ge(0.85) on Si(0.2)Ge(0.8) for hole and electron are expected to be approximately 3.0 and 1.5 times larger than those of Si. To realize Ge-rich SiGe-based waveguide optical modulators, we have also investigated the fabrication procedure of SiGe-on-insulator (SGOI) wafers. We have successfully fabricated Ge-rich SGOI wafers without any thick SiGe buffer layers by using Ge condensation in conjunction with the SiGe regrowth technique. We have evaluated the SGOI by Raman spectroscopy, atomic force microscopy (AFM), reflected high energy electron diffraction (RHEED) and transmission electron microscopy (TEM). Ge-rich SiGe waveguides have been fabricated on the SGOI wafer. The propagation loss was found to be approximately 13 dB/mm, which can be reduced to be below 2 dB/mm by optimizing the Ge condensation process. We expect that strained SiGe grown on the fabricated SGOI exhibits more than 2.3 times higher plasma dispersion than Si in case of a carrier injection type, suitable for high-performance waveguide optical modulators.

First demonstration of SiGe-based carrier-injection Mach-Zehnder modulator with enhanced plasma dispersion effect.

We demonstrate a strained Si0.91Ge0.09-based carrier-injection Mach-Zehnder (MZ) optical modulator using the enhanced plasma dispersion effect in strained SiGe through mass modulation for the first time. The SiGe modulator has an injection current of 1.47 mA for a phase shift of π which is lower than that for a Si modulator. Also, it is expected that the injection current can be further reduced by increasing the strain and Ge fraction, enabling operation at an injection current of less than 1 mA. As an example of the dynamic characteristics, 10 Gbps modulation with clear eye opening was obtained by the pre-emphasis method.

Demonstration of record-low injection-current variable optical attenuator based on strained SiGe with optimized lateral pin junction

We demonstrate a strained SiGe variable optical attenuator (VOA) with a lateral pin junction which exhibits record-low injection-current for 20-dB attenuation. We optimize the distance between the highly doped p + and n + regions in the lateral pin junction to effectively inject electrons and holes, taking into account the propagation loss. In conjunction with the enhanced free-carrier absorption in strained SiGe, the SiGe VOA with the optimized lateral pin junction exhibits 20-dB attenuation by 20-mA/mm injection current, which is 1.5 times lower current density than that of the Si VOA. The SiGe VOA also shows better RF response than the Si VOA due to the short carrier lifetime in SiGe, allowing us to achieve efficient and fast attenuation modulation simultaneously. Furthermore, 2-GHz switching and error-free transmission of 4 × 12.5 Gbps WDM signal have been also achieved.

Low temperature Al 2 O 3 surface passivation for carrier-injection SiGe optical modulator

Surface passivation by Al(2)O(3) 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 SiO(2)/Si and Al(2)O(3)/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 Al(2)O(3) passivated surface is more than one order of magnitude less than that of SiO(2) passivated one. As a result, the modulation efficiency is improved by 1.3 by inserting Al(2)O(3) layer prior to SiO(2) deposition by PECVD owing to superior interface between Al(2)O(3) and Si. The Al(2)O(3) passivated device exhibits comparable modulation efficiency to a thermally-grown SiO(2) passivated one formed by dry oxidation. Hence, the ALD Al(2)O(3) passivation is effective to passivate SiGe optical modulators for which low temperature processes are required.

Record-low Injection-current Strained SiGe variable optical attenuator with optimized lateral PIN junction

We demonstrate record-low injection-current VOAs using strain-enhanced free-carrier absorption in SiGe. The strained SiGe VOA with optimized lateral PIN junction exhibits 20-dB attenuation by 20-mA/mm injection current. 2-GHz switching and error-free transmission of 50-Gbps WDM signal are also successfully demonstrated.

High-speed and highly efficient Si optical modulator with strained SiGe layer

We developed a high speed and high efficiency of depletion type Si optical modulator (Si-MOD) with a pn junction by applying a p-type-doped strained SiGe layer which was slacked on the lateral pn junction type Si-MOD. We designed the optimum Si-MOD structure and demonstrated a very high modulation efficiency of 0.81 Vcm. which is one of die mosl efficient in Si-MODs with a pn junction. We also demonstrated u high speed operation of 25 Gbps for the Si-MOD al around 1.3 μm wavelength.

Heterogeneous CMOS Photonics Based on SiGe/Ge and III–V Semiconductors Integrated on Si Platform

The heterogeneous integration of SiGe, Ge, and III–V semiconductors on Si provides many opportunities to develop high-performance photonic integrated circuits through complementary metal oxide semiconductor (CMOS) processes. We found that strained SiGe possesses greater free-carrier effects than Si, contributing to the improved modulation efficiency of Si-based optical modulators. In addition to low-dark-current Ge photodetectors (PDs) with GeO 2 passivation, we investigated Ge CMOS photonics platform for midinfrared wavelengths. We demonstrated Ge passive waveguides and carrier-injection variable optical attenuators (VOAs) on a Ge-on-insulator wafer. We also investigated III–V CMOS photonics platform on a III–V-on-insulator (III–V-OI) wafer. The strong optical confinement in the III–V-OI structure enabled the realization of ultrasmall III–V passive waveguides similarly to those in Si photonics. Carrier-injection InGaAsP optical switches and VOAs as well as InGaAs waveguide PDs were also demonstrated on III–V-OI wafers. We discuss the opportunities and challenges of heterogeneous CMOS photonics technologies to develop high-performance electronic–photonic integrated circuits for near-infrared and midinfrared applications.

CMOS photonics technologies based on heterogeneous integration of SiGe/Ge and III-V on Si

In this paper, we present heterogeneous integration of SiGe/Ge and III-V semiconductors on Si for electronic-photonic integrated circuits through CMOS photonics technologies. The introduction of high-mobility channel materials, which is promising for achieving high-performance MOSFETs, are also beneficial to photonics for off-chip/on-chip optical interconnection and bio/medical sensors. As for SiGe CMOS photonics, strained SiGe is shown to enhance modulation efficiency for optical modulators. We demonstrated that the plasma dispersion effect is enhanced by strain application to SiGe owing to a decrease in the effective hole mass. As for Ge CMOS photonics, Ge-based photonic-wire waveguides are demonstrated for Mid-IR applications by using photonic Ge-on-Insulator wafers for the first time. As for III-V CMOS photonics, we developed high-quality photonic III-V on Insulator wafers by using direct wafer bonding. InGaAsP photonic-wire devices including optical switches and InGaAs photodetectors are demonstrated. We have also successfully demonstrated wafer-size-scalable III-V-OI wafers by using a III-V epi on Si wafer.