Abstract:
An optoelectronic device includes an input waveguide structure that receives an input optical signal. A GeSi/Si waveguide structure receives from the input waveguide the input optical signal and performs selective optoelectronic operations on the input optical signal. The GeSi/Si waveguide structure outputs an optical or electrical output signal associated with the selective optoelectronic operations performed on the input optical signal. An output waveguide structure receives the output optical signal from the GeSi/Si waveguide structure and provides the optical output signal for further processing.

Description:
PRIORITY INFORMATION  
       [0001]     This application claims priority from provisional application Ser. No. 60/738,845 filed Nov. 22, 2005, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to the field of optical modulators, and in particular to a high speed and low loss GeSi/Si electro-absorption light modulator.  
         [0003]     Si-based modulators are highly required for photonic integrated circuits. However, the indirect bandgap of Si prevent any electric-field induced light modulation. Ge-rich GeSi (Ge composition&gt;50%) devices grown on Si are becoming more utilized because of the relatively small difference between the indirect and direct bandgaps of Ge, which can address some of the difficulties Si devices have faced. However, adequate device design and fabrication are very important in forming EA modulators which work efficiently.  
         [0004]     There are several challenges in making workable GeSi/Si modulators. The key goal of GeSi based electro-absorption (EA) modulators is to achieve lowest possible loss at voltage off-state with highest possible extinction ratio at voltage on-state. Because GeSi EA modulators inevitably have material loss due to the absorption of indirect band gap, the length of GeSi active region has to be very short, typically less than the order of ˜150 μm, to achieve low insertion loss at off-sate. Therefore, it has to be coupled to a low loss waveguide, such as Si or SiN x  wavguides, for on-chip applications. Since GeSi/SiO 2  material system has a high index contrast and its single mode dimensional is very small (the single mode cut-off dimension is less than 1 μm), it is a big challenge to achieve an efficient waveguide-modulator coupling. Another issue is that standard reactive ion etching (RIE) of GeSi material usually results in rough sidewalls that increases the scattering loss in the GeSi EA modulator. It would be desirable to circumvent this issue.  
       SUMMARY OF THE INVENTION  
       [0005]     According to one aspect of the invention, there is provided an optoelectronic device on Si or SOI wafer. The optoelectronic device includes an input waveguide structure that receives an input optical signal. A GeSi/Si waveguide structure receives from the input waveguide the input optical signal and performs selective optoelectronic operations on the input optical signal. The GeSi/Si waveguide structure outputs an optical or electrical output signal associated with the selective optoelectronic operations performed on the input optical signal. An output waveguide structure receives the output optical signal from the GeSi/Si waveguide structure and provides the optical output signal for further processing.  
         [0006]     According to another aspect of the invention, there is provided a method of forming an optoelectronic device. The method includes providing a Si or SOI substrate and forming on the substrate an input waveguide structure that receives an input optical signal. A GeSi/Si waveguide structure is formed that receives from the input waveguide the input optical signal and performs selective optoelectronic operations on the input optical signal. The GeSi/Si waveguide structure outputs an optical or electrical output signal associated with the selective optoelectronic operations performed on the input optical signal. Also, the method includes forming an output waveguide structure that receives the output optical signal from the GeSi/Si waveguide structure and provides the output optical signal for further processing.  
         [0007]     According to another aspect of the invention, there is provided a monolithically integrated optoelectronic circuit on Si or SOI wafer. The optoelectronic circuit includes an input waveguide structure that transmits an input optical signal and a GeSi/Si waveguide structure that receives from the input waveguide the input optical signal and modulates the input optical signal. An output waveguide structure receives the modulated optical signal from the GeSi/Si waveguide structure and transfers the modulated optical signals for further processing. An optoelectronic function module receives the modulated optical signals from the output waveguide structure and performs optoelectronic functions such as demultiplexing or filtering. A second GeSi/Si waveguide structure receives the optical signal from the optoelectronic function module and converts it to electrical signals to be processed by an electronic integrated circuit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic diagram demonstrating the basic concept of the invention;  
         [0009]      FIG. 2  is a schematic diagram demonstrating an example of an electro-absorption (EA) based optical modulator that includes a short Si/GeSi/Si p-i-n diode waveguide structure;  
         [0010]      FIG. 3  is a schematic diagram demonstrating another embodiment of the inventive modulator structure;  
         [0011]      FIG. 4  is a schematic diagram illustrating an embodiment of the invention where the inventive GeSi EA modulator, waveguide and GeSi photodetector are integrated;  
         [0012]      FIG. 5  is a flowchart demonstrating an example of the steps in fabricating and integrating the inventive modulator and photodetector devices on a Si substrate; and  
         [0013]      FIG. 6  is a flowchart demonstrating an example of the steps in fabricating and integrating the inventive modulator and photodetector devices on a Si on insulator (SOI) substrate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  illustrates the basic concept of the invention. The inventive optical modulator  2  generally comprises an input waveguide  4 , an output waveguide  6 , and a modulator waveguide structure  8 . Sturcture  8  is the active light modulation region of the device. The input waveguide  4  and the output waveguide  6  can comprise Si, SiN x  or SiO x N y  waveguides. The modulator waveguide structure  8  can comprise Ge or GeSi with adequate composition. The composition of the GeSi material is chosen such that the relative change in the absorption coefficient at the on-state compared to the off-state (i.e., |α on −α off |/α off , where α on  and α off  are the absorption coefficients of GeSi at on and off states, respectively) is maximized around the operating wavelength. One of the advantages of this invention is the ability to integrate the components  4 ,  6  and  8  together with high efficiency butt-coupling. The invention relies on the fact that the off-state of the modulator is operated at relatively low absorption regime of the GeSi material and the length of modulator waveguide  6  is less than the order of ˜150 μm so as to keep a low absorption loss at off-state. The on-state is operated with a high electric field applied on the GeSi material to change its absorption coefficient and create enough extinct ratio for light modulation at on-state.  
         [0015]      FIG. 2  illustrates an example of an electro-absorption (EA) based optical modulator  10  that includes a very short GeSi-based waveguide  12 . The optical modulator includes the GeSi modulator waveguide  12 , an input waveguide  14 , an output waveguide  16 , a p(or n)-type Si substrate  18 , an n(or p)-type poly-Si or epitaxial Si layer  20 , and various SiO 2  layers  22 ,  24 ,  26 ,  28 . Basically, the p(n)-type Si mesa  30 , the GeSi modulator waveguide  12  and the n(p)-type poly-Si or epitaxial Si layer  20  form a Si/GeSi/Si p-i-n diode waveguide structure, which allows a high electric field to be applied on the GeSi active layer  12  at reverse bias. The applied electric field significantly changes the absorption coefficient of the GeSi material at the operating wavelength so that the intensity of the input light can be modulated. In this embodiment, the cross-section perpendicular to the light propagation direction of the input and output waveguides  14  and  16  is 0.5×0.2 μm, but it can be varied as long as they keep single mode. The cross-section perpendicular to the light propagation direction of the GeSi modulator waveguide  12  is 0.6 μm×0.4 μm, but the dimension can vary to achieve optimal coupling efficiency with the input and output waveguides,  14  and  16 . In this embodiment, the input waveguide  14  and output waveguide  16  comprise Si. However, they can also comprise SiO x N y  or SiN x  waveguides. The input/output waveguides  14  and  16  are butt-coupled to the GeSi modulator waveguide  12 . One great advantage of butt coupling is that the coupling efficiency is high, greater than 90% when input and output waveguides  14  and  16  comprise Si. The thickness of the bottom SiO 2  layers  26  and  28  is 2 μm, however, in other embodiments the size can also vary as long as the light in waveguides  12 ,  14  and  16  do not leak to the substrate. The blocks of SiO 2  layers  22 ,  24 ,  26 , and  28  can also be substituted with other dielectric materials like SiO x N y  or SiN x , as long as the refractive indexes of the materials in blocks  22 ,  24 ,  26  and  28  are smaller than the core materials of input and output waveguides  14  and  16 . The poly Si or epi-Si layer  20  and the Si mesa  30  have opposite types of doping, i.e., if the former is n-type, the latter is p-type, and visa verse. The Si substrate  18  can also be substituted with SOI substrate, as will be described later.  
         [0016]     The Si/GeSi/Si p-i-n diode waveguide modulator off-state is operated at 0 bias or a small forward bias with low electric field in the GeSi active material. At the off-state, the GeSi modulator  12  has an absorption coefficient of approximately 100/cm, but the length of the GeSi modulator waveguide  12  is kept short (&lt;150 μm) such that a low insertion loss of &lt;4 dB can be achieved. The on-state is operated at a reverse bias with a high electric field in the GeSi active layer to significantly change its absorption coefficient at the operating wavelength for a high extinct ratio (˜10 dB) of light modulation. The composition of the GeSi material is chosen such that the relative absorption coefficient change at the on-state compared to the off-state (i.e., |α on −α off |/α off ) is maximized around the operating wavelength.  
         [0017]     The GeSi modulator waveguide structure  12  can be multimode, where the dimensions of Ge modulator waveguide structure  12  is larger than its single mode cut-off dimension. Larger feature sizes make fabrication easier. Though the GeSi modulator waveguide structure  12  is multimode, the operation of the modulator can keep single mode. In order to achieve this, the input waveguide  14  and output waveguide  16  have to be center aligned and butt-coupled to the GeSi modulator waveguide structure  12  to only excite the fundamental mode in the GeSi modulator waveguide  12 .  
         [0018]      FIG. 3  illustrates another embodiment of the inventive modulator structure  31  on an SOI substrate. The modulator  31  includes an SOI substrate comprising of a bottom Si substrate  32 , a SiO 2  insulator layer  33  and a p(n)-type silicon layer on insulator  34 . The modulator  31  further includes a Ge or GeSi modulator waveguide structure  35 , an n(p)-type Si layer  36  on top of the GeSi modulator waveguide, an input waveguide  37  and an output waveguide  38  that are butt coupled to the GeSi modulator waveguide  35 , and SiO 2  blocks  39 ,  40 ,  41 , and  42 . The thickness of the SiO 2  layer  33  in the SOI wafer is 2 μm in this embodiment, but it can be adjusted to other thicknesses as long as the light in waveguides  35 ,  37  and  38  does not leak to the substrate  32 . The dimensions of GeSi modulator waveguide  35 , the input waveguide  37  and the output waveguide  38  are the same as the corresponding waveguides  12 ,  14  and  16  in  FIG. 2 , respectively. The n-type Si layer  36  on top of the GeSi modulator waveguide can be changed to p-type, and in that case the Si layer on insulator  34  at the bottom of the GeSi modulator waveguide should be n-type. The input and output waveguides  37  and  38  comprise Si waveguides in this embodiment. However, they can also comprise SiO x N y  or SiN x  waveguides. The SiO 2  blocks  39 ,  40 ,  41  and  42  can also be can also be substituted with other dielectric materials like SiO x N y  or SiN x , as long as the refractive indexes of the materials in blocks  39 ,  40 ,  41  and  42  are smaller than the core materials of input and output waveguides  37  and  38 .  
         [0019]     In addition, the same inventive optoelectronic device can also be used as a butt-coupled GeSi detector structure. The only difference is that the Si/GeSi/Si p-i-n diode waveguide structure in the photodetector device is usually longer than that in the modulator device in order to increase light absorption. At reverse bias, the electrons and holes excited by the absorbed photons are accelerated by the electric field applied on the GeSi layer through the Si/GeSi/Si p-i-n diode waveguide structure, and are collected by the electrodes. This way, optical signals are transformed into electrical ones for further processing in an electronic integrated circuit.  
         [0020]      FIG. 4  illustrates an embodiment of the invention where a monolithically integrated optoelectronic circuit is formed, including a GeSi EA modulator, a GeSi photodetector, waveguides and other optoelectronic function module. In this embodiment, the modulator structure  43  comprises an input waveguide structure  44 , an output waveguide structure  45 , and a GeSi modulator waveguide structure  46 . The output waveguide  45  is coupled to an optoelectronic function module  48 . The detector  50  comprises an input waveguide  52 , an output waveguide  54 , and a GeSi photodetector structure  56 . In some cases the output waveguide of the photodetector  54  is not necessary and can be omitted. Note the GeSi detector waveguide structure  56  is almost the same as the GeSi modulator waveguide structure  46  except that the length of the GeSi detector waveguide structure  56  can be longer. The GeSi EA modulator and photodetector are of exactly the same GeSi material, so they can be formed together in a single selective growth of GeSi in pre-defined regions exposed on Si or SOI substrate. This design greatly simplifies the fabrication process of modulator/waveguide/photodetector integration, which is another big advantage of this invention.  
         [0021]     In other embodiments, the optical function module  48  is not needed and there is a direct connection from the modulator  43  to the detector  50 . In that case, the output waveguide  45  of the optical modulator  43  becomes the input waveguide  52  of the detector  50 . Also, the input waveguides  44 ,  52  and output waveguides  45 ,  54  can comprise Si, SiO x N y , or SiN x  waveguides.  
         [0022]      FIG. 5  demonstrates an example of the steps in fabricating the inventive optical modulator and photodetector devices on a Si substrate. There is provided a p or n type Si substrate, as shown in step  58 . Si mesas can be formed on the Si substrate by etching part of the wafer via lithography, as shown in step  68 . Then an oxide layer is deposited on the etched Si wafer and the surface can be planarized by chemical mechanical polishing (CMP), shown in step  70 . In other embodiments, these dielectric layers can also comprise other materials like SiO x N y , or SiN x . Alternatively, the structure in step  70  can be formed by selective growth of Si. In that case, an oxide layer is grown on the Si wafer and patterned by lithography, as shown in step  62 . Then, epitaxial Si with the same type of doping as the Si substrate can be grown in the area where the Si substrate is exposed, as shown in step  66 . The top of the Si mesa can be planarized by CMP if necessary. The doping of Si epitaxial layer can be achieved either by in situ doping during the growth or ex situ by ion implantation. After step  66 , an oxide layer can be deposited on the top to form the structure in step  70 . In other embodiments, these dielectric layers can also comprise other materials like SiO x N y , or SiN x . From step  70 , we deposit a layer of Si and pattern it into the cores of Si waveguides. In other embodiments, these waveguide cores can also comprise SiO x N y  or SiN x  as long as its refractive index is larger than the cladding material. Then an oxide layer is deposited on top to form the upper cladding of Si waveguide, followed by CMP planarization (see step  72 ). In other embodiments, this upper cladding layer can also comprise other materials like SiO x N y , or SiN x . Trenches are subsequently etched into this structure to expose the tops of said Si mesas, shown in step  74 . Then GeSi material is selectively grown into these trenches and the top is planarized by CMP. A Si layer with opposite type of doping to the Si substrate is further deposited and patterned on top of the structure, as shown in step  76 . Metal electrodes can be used to contact the doped Si regions on tops and bottoms of the GeSi waveguides. The shorter GeSi waveguides are used as modulators, while the longer ones following them are used as photodetectors. A great advantage is that the modulators and photodetectors are of the same GeSi material and can be grown at the same time, which greatly simplifies the fabrication process of modulator/waveguide/photodetector monolithic integration. Furthermore, the fabrication method presented here provides integrated modulators, waveguides and photodetectors on a Si wafer, an important step to the integration of optical and electrical components on Si platform.  
         [0023]      FIG. 6  demonstrates an example of the steps in fabricating the inventive optical modulator and photodetector devices on a SOI substrate. There is provided a SOI substrate, with the top Si layer doped p or n type, as shown in step  78 . Si mesas are then formed by patterning the top Si layer on the SiO 2  insulator layer, shown in step  80 . An oxide layer is deposited on top of the structure and planarized by CMP, as shown in step  82 . In other embodiments, this dielectric layer can also comprise other materials like SiO x N y , or SiN x . We deposit a layer of Si and pattern it into the cores of Si waveguides. In other embodiments, these waveguide cores can also comprise SiO x N y  or SiN x  as long as its refractive index is larger than the cladding materials. Then an oxide layer is deposited on top to form the upper cladding of the Si waveguides, followed by CMP planarization, as shown in step  84 . In other embodiments, this upper cladding layer can also comprise other materials like SiO x N y , or SiN x . Trenches are subsequently etched into this structure to expose the tops of the Si mesas, shown in step  86 . Then GeSi material is selectively grown into these trenches and the top is planarized by CMP. A Si layer with opposite type of doping to the Si mesas underneath GeSi waveguide structures is further deposited and patterned on top of the structure, as shown in step  88 . Metal electrodes can be used to contact the doped Si regions on tops and bottoms of the GeSi waveguide structures. The shorter GeSi waveguides are used as modulators, while the longer ones following them are used as photodetectors. In this way, monolithic integration of waveguides, GeSi modulators and GeSi photodetectors can be achieved.  
         [0024]     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.