The optical fiber communication, which was mainly used for business, has also been widely used for home use. As a result, an optical communication device of high performance has been required. As an optical communication device for use in various optical communication systems, such as an optical fiber communication system for home use and a local area network (LAN) system, there is a silicon-based optical communication device which functions at optical fiber communication wavelengths of 1330 nm and 1500 nm. The silicon-based optical communication device is a very promising device in which an optical functional element and an electronic circuit can be integrated on a silicon platform by using a CMOS (Complementary Metal Oxide Semiconductor) technique.
As silicon-based optical communication devices, passive devices, such as a waveguide, an optical coupler, and a wavelength filter, have been widely investigated. Further, active devices, such as a silicon-based optical modulator and a silicon-based optical switch, are listed as important elements of means for operating an optical signal in the above-described optical communication system, and have been attracting much attention. The optical modulator and the optical switch, in which the refractive index is changed by using the thermo-optic effect of silicon, have a low optical modulation speed, and hence can be used only for a device having an optical modulation frequency of 1 Mb/s or lower. An optical modulator using the electro-optic effect is required for a device having an optical modulation frequency higher than 1 Mb/s.
Pure silicon does not exhibit any change in its refractive index due to the Pockels effect and exhibits a very small change in its refractive index due to the Franz-Keldysh effect and the Kerr effect. For this reason, many of the optical modulators that is proposed at present and that use the electro-optic effect utilize the carrier plasma effect. That is, the phase and intensity of light are changed by changing the real part and imaginary part of the refractive index by changing the density of free carriers in a silicon layer.
The density of free carriers in the optical modulator can be changed by the injection, accumulation, depletion or inversion of the free carriers. Many of such optical modulators, which have been investigated to date, have a low optical modulation efficiency, and require, for optical phase modulation, a length of 1 mm or more and an injection current density of 1 kA/cm3 or more. In order to realize miniaturization, high integration, and low power consumption in the optical modulator, a device structure having high optical modulation efficiency is required. When a device structure having high optical modulation efficiency is realized, the length required for the optical phase modulation can be reduced. Further, in the case where an optical communication device has a large size, it is also conceivable that the optical communication device is liable to be affected by the temperature on the silicon substrate, and thereby the electro-optic effect to be originally obtained is cancelled by a change in the refractive index of the silicon layer resulting from the thermo-optic effect.
FIG. 1 shows an example of a related art of a silicon-based electro-optical optical modulator using a rib waveguide formed on an SOI (Silicon on Insulator) substrate. Embedded oxide layer 32 and intrinsic semiconductor 31 including a rib-shaped portion are laminated in order on substrate 33. P+-doped semiconductor 34 and n+-doped semiconductor 35 are respectively formed on both sides of the rib-shaped portion of the intrinsic semiconductor 31 so as to be separated from each other by a distance. P+-doped semiconductor 34 and n+-doped semiconductor 35 are formed by performing a high concentration doping process to respective parts of intrinsic semiconductor 31. The optical modulator shown in FIG. 1 is configured as a PIN (P-intrinsic-N) diode. When a forward bias voltage or a reverse bias voltage is applied to the PIN diode, the density of free carriers in intrinsic semiconductor 31 is changed, so that the refractive index is changed by the carrier plasma effect. In this example, electrode contact layer 36 is arranged on one side of the rib-shaped portion of intrinsic semiconductor 31, and p+-doped semiconductor 34 described above is formed at a position facing electrode contact layer 36. Similarly, electrode contact layer 36 is arranged also on the other side of the rib-shaped portion of intrinsic semiconductor 31, and n+-doped semiconductor 35 is formed at a position facing electrode contact layer 36. Further, the waveguide including the rib-shaped portion is covered by oxide clad 37. In the above-described PIN diode structure, the doping process can be performed at a high concentration so that the density of carriers in semiconductors 34 and 35 becomes about 1020/cm3.
When an optical modulation operation is performed, forward bias voltage is applied to the PIN diode from a power source connected to electrode contact layers 36, so that free carriers are injected into the waveguide. At this time, free carriers are increased, and thereby the refractive index of intrinsic semiconductor 31 is changed, so that the light propagated through the waveguide is phase-modulated. However, the speed of the optical modulation operation is restricted by the lifetime of free carriers in the rib-shaped portion of intrinsic semiconductor 31, and by the carrier diffusion at the time when the application of the forward bias voltage is stopped. The electro-optical optical modulator having the related art PIN diode structure as described above usually has an operating speed in the range from 10 to 50 Mb/s at the time when the forward bias voltage is applied. On the other hand, by introducing impurities into intrinsic semiconductor 31 in order to reduce the lifetime of carriers, the switching speed of the electro-optical optical modulator can be increased. However, there is a problem that the introduced impurities lower the optical modulation efficiency. Further, the major factor that influences the operation speed is the RC time constant. The electrostatic capacitance at the time when the forward bias voltage is applied is significantly increased by the reduction in the carrier depletion layer of the PN (Positive-Negative) junction portion. Theoretically, the high-speed operation of the PN junction portion can be realized by applying a reverse bias voltage. However, a comparatively high drive voltage or a large element size is required in this case.
As another example of the related art, Patent literature 1 discloses a silicon-based electro-optical optical modulator having a capacitor structure in which embedded oxide layer 32 and a first conductivity type main body region are laminated in order on substrate 33, in which a second conductivity type gate region is further laminated so as to partially overlap the main body region, and in which thin dielectric layer 41 is then formed at the lamination interface between the main body region and the gate region. Note that hereinafter, “thin” is intended to mean a thickness of submicron order (less than 1 μm).
FIG. 2 shows a silicon-based electro-optical optical modulator having an SIS (silicon-insulator-silicon) structure according to the related art. The electro-optical optical modulator is formed on an SOI substrate configured by substrate 33, embedded oxide layer 32, and a main body region. The main body region is configured by p-doped semiconductor 38 formed by performing a doping process on the silicon layer of the SOI substrate, p+-doped semiconductor 34 formed by performing a high concentration doping process on the silicon layer of the SOI substrate, and electrode contact layer 36. The gate region is configured by n-doped semiconductor 39 formed by performing a doping process on a thin silicon layer laminated on the SOI substrate, n+-doped semiconductor 35 formed by performing a high concentration doping process on the thin silicon layer, and electrode contact layer 36. Oxide clad 37 is provided in the gap formed by embedded oxide layer 32, the main body region and the gate region, and is also provided on the main body region and the gate region.
The region subjected to the doping process is configured such that the change in carrier density is controlled by an external signal voltage. Further, when a voltage is applied between electrode contact layers 36, free carriers are accumulated, depleted or inverted on both sides of dielectric layer 41. Thereby, the optical phase modulation is performed. For this reason, it is preferred that the region of optical signal electric field be made to coincide with the region in which the carrier density is dynamically controlled from the outside.