High-performance optical modulators on silicon platform - Integration of compound semiconductors and silicon, providing compactness, low-power consumption, and low propagation loss -

1.Background

Data traffic is expected to continue increasing over a wide range of transmission distances from inter-datacenters to core network systems. One of the driving forces contributing to the increase in network traffic will be the introduction of "5G"--the fifth-generation mobile network system. Mobile network traffic in 2020s is expected to be 1000 times larger than it was in 2010 ^[1]. It is essential to build large-capacity optical backbone network systems to support this increasing traffic, and, for this purpose compact, high-speed, and low-power-consumption optical devices are indispensable. In addition, low-cost mass-production technologies are crucial because a large number of devices are required. Silicon-based large-scale integration is the foundation of electric circuits like microprocessors^*4, while InP-based compound semiconductors are now commonly used in photonic active devices for optical communications. However, InP-based devices are not suitable for mass production because of the small wafer size compared to silicon. Silicon-based optical-device manufacturing is therefore promising for sustainable development of optical communications technologies.
Mach-Zehnder optical modulators^*5 (MZMs) are key devices for transmitters. They modulate continuous laser light to convert electrical signals to optical ones. The applied voltage in phase-shifter regions controls the refractive index in those regions to generate optical signals. The modulation efficiency, defined as the product of the half-wavelength voltage^*6 and the phase shifter length, is the fundamental characteristic for evaluating the performance of an optical modulator. In addition, insertion loss is critical with regards to power consumption because large insertion loss requires high laser power. LiNbO₃ (lithium niobate), compound semiconductors, and silicon have been applied to MZMs. Silicon has a bigger trade-off between modulation efficiency and absorption loss than the other materials do. The trade-off limits the application range of silicon photonics as the future photonic integration platform; therefore, breaking through the tradeoff is strongly required (Fig. 1).

2.Results

To break through the limitation of silicon photonics, NTT developed carrier-accumulation^*7 MZMs by heterogeneously integrating InP-based compound semiconductors with silicon (Fig. 2). Carrier-accumulation modulators^*8 provide higher modulation efficiency than the more common silicon-based carrier-depletion ones. Previously reported carrier-accumulation MZMs employed silicon and polysilicon. However, the optical loss caused by the increasing carrier densities and optical absorption in the polysilicon layer makes it difficult to break through the limitation of silicon-based modulators. To achieve high efficiency and low optical loss while maintaining high compatibility with silicon photonics technologies, NTT used an n-type InGaAsP membrane layer, which provides both higher modulation efficiency and lower optical loss than a polysilicon layer. The modulation efficiency of the fabricated modulators is 0.09 Vcm, which is ten times larger than that of carrier-depletion MZMs. The insertion loss of a device with a modulation length of 0.25 millimeters is 1 dB (transmittance of about 80%). These results exceed the previous limitations of silicon-based MZMs. The modulator also operates at the modulation speed of 32 Gbit/s, indicating the feasibility of using it for practical applications like 100-gigabit Ethernet^*9 (Fig. 3).

3.Future developments

Large-scale integration of optical modulators and filters, making use of the features of silicon photonics technology, will provide optical transmitters with transmission capacity of over one terabit^*10 per port. In addition, low-power-consumption transmitters will reduce the power consumed by intra-datacenter networks in the future.

4.Technical features

(1)Materials for phase-shifter regions: n-type InGaAsP

Electrons and holes (carriers) are used to control the refractive index of semiconductors. N-type InGaAsP provides a high-efficiency carrier-induced refractive index change because of its small effective mass compared to n-type silicon. In addition, the bandgaps^*11 of InP-based compound semiconductors can be flexibly controlled by changing the composition of In, Ga, As, and P. Proper bandgap design provides both a large refractive index change and small optical loss.

(2)Device design: reducing optical loss by optimizing InGaAsP-silicon membrane layer thickness and introducing tapered structures (Fig. 4)

To improve modulation efficiency, it is essential to design the cross section so that the overlap between the area where the refractive index changes and the optical field is maximized. The refractive index change is largest at the MOS interface due to carrier accumulation; therefore, minimizing the layer thickness while maintaining strong optical confinement is essential. However, a thin layer has the drawback of large resistance, which limits modulation speed. The layer thicknesses of Si and InGaAsP were optimized to 110 and 100 nm, respectively. Taking advantage of the silicon platform, NTT introduced silicon-waveguides and tapered structures with small optical losses and reflections for input and output ports to the phase shifters. The propagation modes can be converted smoothly between the silicon wire waveguides and MOS phase shifters via 80 micrometer-long two-stage tapers.

(3)Fabrication: InGaAsP-MOS interface and heterogeneous integration (Fig. 5)

Fabricating MOS structures with both InP-based semiconductors and silicon has been a challenge. Good InGaAsP-silicon MOS interfaces were obtained by using electron cyclotron resonance (ECR) sputtering to form flat insulating (SiO₂) layers, followed by O₂-plasma-assisted direct wafer bonding process.

1. [1]https://www.nttdocomo.co.jp/english/corporate/technology/whitepaper_5g/

Fig. 1: Modulation efficiency and insertion loss of Mach-Zehnder modulators (MZMs)