Abstract:
Disclosed is a photo diode. The photo diode includes: at least two branched waveguides configured to receive beating signals; absorbing layers disposed in vertical directions to the waveguides, and disposed while being spaced apart from distal ends of the waveguides by a predetermined interval; and one or more intermediate layers formed based on the distal ends of the waveguides and disposed with the absorbing layers at upper end of the one or more intermediate layers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0061839, filed on Apr. 30, 2015, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a photo diode, which enables a broadband terahertz continuous wave to have a high output. 
     2. Description of the Related Art 
     In a generation of a terahertz (THz) continuous wave by using photo-mixing, a photo mixer serves to convert an incident beating signal into a terahertz continuous wave. Significant variables determining a characteristic of the photo mixer include a fast response rate, large dark-resistance, high carrier mobility, and the like. 
     Efficiency, a radiation pattern, and the like of the generated terahertz wave have a characteristic of being dependent on an integrated antenna, but in a case where the terahertz wave is applied to a broadband, an absorbing layer of the photo mixer needs to basically have the aforementioned characteristic. 
     The currently known photo mixer may be generally classified into two elements. One is an element using a low-temperature grown III-V semiconductor having a short carrier lifetime, and includes, for example, low-temperature grown indium-gallium-arsenic (InGaAs), gallium-arsenic (GaAS), indium-gallium-arsenic/indium-aluminum-arsenic (InGaAs/InAlAs) multi-layer, and erbium-arsenic/(indium)gallium-arsenic (ErAs/(In)GaAs). The other is an element using a photo diode structure having a short transit-time, and includes, for example, a unit-travelling carrier photo diode and a pin photo diode. When IR power of several tens of milliwatt (mW) is used in a level of about 10 −3  of Terahertz/infrared (THz/IR) power conversion efficiency of the photo mixer using the photo diode structure, the photo mixer has a terahertz output of several tens of microwatt (μW). Compared to the photo mixer using the low-temperature grown III-V semiconductor, the photo mixer using the photo diode structure has a disadvantage in a band width aspect, but an output of the photo mixer using the photo diode structure in a low frequency band is considerably excellent, so that the photo mixer using the photo diode structure is advantageous to be applied to a system having up to about 1.5 THz. 
     A surface incident pin photo diode among the photo diodes has a characteristic in that responsivity is decreased when a rate of the photo diode is increased by using a thin absorbing layer. In contrast to this, a waveguide photo diode may improve photoelectric conversion efficiency while using a thin absorbing layer, but when light of large intensity is input, a response rate of the waveguide photo diode is decreased by a saturation effect. Due to the aforementioned problems, there is a problem in that it is difficult to satisfy both a broadband and a high output which are the conditions of the photo diode for the application of THz. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in an effort to solve the above-described problems associated with the prior art, and provides a photo diode, which is capable of generating a broadband high output THz continuous wave in consideration of a limit of a single absorbing layer determining an operation characteristic. 
     An exemplary embodiment of the present disclosure provides a photo diode, including: at least two branched waveguides configured to receive beating signals; absorbing layers disposed in vertical directions to the waveguides, and disposed while being spaced apart from distal ends of the waveguides by a predetermined interval; and one or more intermediate layers formed based on the distal ends of the waveguides and disposed with the absorbing layers at upper end of the one or more intermediate layers. 
     The waveguides, the absorbing layers, and the intermediate layer may be disposed so that beating sources input through the waveguides physically have the same length until the beating sources reach the absorbing layers. 
     The photo diode may further include antennas disposed while being spaced apart from the absorbing layers at a predetermined interval. 
     The photo diode may further include a substrate, on which the waveguides, the absorbing layers, the antennas, and the one or more intermediates are integrated and disposed. 
     The one or more intermediate layers may have relatively larger refractivity than that of the waveguides. 
     According to the photo diode of the present invention, in order to overcome a low output due to a light absorbing saturation phenomenon by a single absorbing layer, a plurality of photo diodes having the same absorbing layer is arranged at the same phase, so that it is possible to generate broadband high output terahertz continuous waves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the example embodiments to those skilled in the art. 
       In the drawing figures, dimensions may be exaggerated for clarity of illustration. It will be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout. 
         FIG. 1  is a diagram illustrating a photo diode according to an exemplary embodiment of the present disclosure. 
         FIG. 2  is an enlarged diagram illustrating a part, in which an absorbing layer of the photo diode illustrated in  FIG. 1  is positioned. 
         FIG. 3  is a diagram illustrating a cross-section of a region around the absorbing layer of  FIG. 1 . 
         FIG. 4  is a diagram illustrating a structure of a photo diode including one absorbing layer according to an exemplary embodiment of the present disclosure. 
         FIG. 5  is a diagram illustrating a structure of a photo diode including two absorbing layers according to an exemplary embodiment of the present disclosure. 
         FIG. 6  is a diagram illustrating a structure of a photo diode including four absorbing layers according to an exemplary embodiment of the present disclosure. 
         FIGS. 7A and 7B  are diagrams illustrating photo diodes implemented by using a single waveguide and dual waveguides according to an exemplary embodiment of the present disclosure. 
         FIG. 8  is a graph illustrating a comparison of output power of a terahertz continuous wave according to  FIGS. 7A and 7B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments according to the present disclosure will be described in detail with reference to the accompanying drawings. In the description below, it should be noted that only parts necessary for understanding operations according to various exemplary embodiments of the present disclosure will be described, and descriptions of other parts may be omitted so as to avoid unnecessarily obscuring the subject matter of the present disclosure. 
     The present disclosure provides a photo diode for generating a broadband high output terahertz continuous wave. 
       FIG. 1  is a diagram illustrating a photo diode according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 1 , a photo diode  100  includes a substrate  110 , waveguides  120 , waveguide forming recesses  130 , antennas  140 , and an intermediate layer (or an N-layer)  150 . Further, the photo diode  100  includes an absorbing layer positioned on the intermediate layer, and a structure of the photo diode including the absorbing layer  101  will be described in detail with reference to  FIG. 2  below. 
     The waveguides  120 , the antennas  130 , and the intermediate layer  150  are integrated on the substrate  110 . 
     The waveguides  120  include, for example, a Y-distributor waveguide as a waveguide in a form of “Y”. Here, two waveguides  120  are formed based on both side surfaces of the substrate  110 , but for convenience of the description, the present disclosure will be described based on one waveguide  120 . 
     The waveguide forming recesses  130  may be formed in both side surfaces of the waveguide  120  and may be recesses, in which the waveguides  120  are formed on the substrate  110 . 
     The antenna  140  radiates electromagnetic waves. The antenna  140  may be implemented by one module or one or more modules. Here, two antenna modules formed in both side surfaces of the two waveguides  120  are illustrated. 
     The intermediate layer  150  is formed based on a branched point of a distal end of the waveguide  120 , and has relatively higher refractivity than that of the waveguide  120 . The intermediate layer  150  has a structure, which is gradually widened based on the branched point of the waveguide  120 . 
       FIG. 2  is an enlarged diagram illustrating a part, in which an absorbing layer of the photo diode illustrated in  FIG. 1  is positioned. 
     Referring to  FIG. 2 , a part  101 , in which an absorbing layer  160  of the photo diode  100  is positioned, is enlarged. The waveguide  120  is formed on the substrate  110 , and the intermediate layer  150  is formed at an upper end of a part, at which the waveguide  120  ends. In this case, the absorbing layer  160  is formed at an upper end of the intermediate layer, and the absorbing layer  160  is formed based on the branched point of the distal end of the waveguide  120 . 
     The absorbing layer  160  is formed, and a P-layer  170  is formed at an upper end of the absorbing layer  160 . One antenna  140  is connected to the upper end of the P-layer  170  to radiate electromagnetic waves. The other antenna  140  is disposed at the upper end of the intermediate layer  150 , which is spaced apart from the absorbing layer  160  by a predetermined interval, to radiate electromagnetic waves. 
     Accordingly, the photo diode has a structure, in which the waveguides  120 , the antennas  140 , the intermediate layer  150 , the absorbing layer  160 , and the P-layer  170  are integrated on the substrate  110 . 
     In this case, an area of the absorbing layer  160  is a significant factor for generating a broadband and high frequency terahertz continuous wave. The photo diode  100  has a value of a capacitor C, which is in proportional to the area of the absorbing layer  160 , and a cutoff frequency (f 3dB =1/(2πRC), which is in inverse-proportional to the area of the absorbing layer  160 , is determined, so that the absorbing layer is designed so as not to have a large area. 
       FIG. 3  is a diagram illustrating a cross-section of a region around the absorbing layer of  FIG. 1 . 
     Referring to  FIG. 3 , the waveguide  120 , the intermediate layer  150 , the absorbing layer  160 , and the P-layer  170  are sequentially disposed on a cross-section of the photo diode  100 . 
     A beating source (or a beating signal (excitation light))  10  may have a wavelength of about 1.3 λm. A transmission direction by the beating source is illustrated by an arrow. The beating source is coupled or inductively transmitted up to a region around the absorbing layer  160  through the waveguide  120  in a form of a shallow ridge, and has a structure, which is evanescently coupled in an up direction through the intermediate layer  140 , which is positioned around an end region of the waveguide  120  and has relatively larger refractivity than that of the waveguide  120 , and is considerably absorbable in the thin absorbing layer  160 . The absorbed beating source has a structure in which the absorbed beating source is converted into a current, and then is radiated in a form of electromagnetic waves through the integrated antennas  140 . A start point of the beating source is another end part of the waveguide  120 , in which the intermediate layer  150  is not positioned. 
     A BPM simulation result is illustrated at a lower end of the cross-section of the photo diode  100 , and coupling efficiency of 0.3 ampere/watt (A/W) can be seen. Accordingly, the waveguide  120 , the intermediate layer  150 , and the absorbing layer  160  consider an influence according to a structure variable therebetween. 
     As described above, in order to generate a broadband terahertz continuous wave, it is necessary to select the small area absorbing layer  160 , and thus, a lower absorption saturation phenomenon is generated in the absorbing layer  160 , compared to the beating source of a high input (30 mW or more (&gt;30 mW)). For the generation of a high output, it is necessary to overcome a low absorption saturation phenomenon, and to this end, a plurality of absorbing layers having small areas may be included. A structure including one absorbing layer will be described with reference to  FIG. 4  below, and the structures including the plurality of absorbing layers based on  FIG. 4  are illustrated in  FIGS. 5 and 6 . 
     Hereinafter, for convenience of the description, a waveguide, an intermediate layer, an absorbing layer, and a distal end of an antenna in a photo diode will be described. 
       FIG. 4  is a diagram illustrating a structure of a photo diode including one absorbing layer according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 4 , one waveguide  121  is illustrated, and an absorbing layer  161  having a small area is positioned at a distal end of the waveguide. 
     A part  201 , in which the absorbing layer  161  is disposed, is enlarged and illustrated at a right side. An intermediate layer  151  is formed based on the distal end of the waveguide  121 , and the absorbing layer  161  is positioned at an upper end of the intermediate layer  151 . An antenna  141  is connected to one side surface of the absorbing layer  161 . 
       FIG. 5  is a diagram illustrating a structure of a photo diode including two absorbing layers according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 5 , a waveguide  122  branched into two waveguides is illustrated, and absorbing layers  162  and  163  having small areas are positioned at distal parts of Y-shaped branched waveguides  1221  and  1222 . 
     A part  202 , in which the absorbing layers  162  and  163  are disposed, is enlarged and illustrated at a right side. Intermediate layers  152  and  153  are formed based on end parts of the waveguides  1221  and  1222 . In this case, the absorbing layer  162  is positioned at an upper end of the intermediate layer  152 , and an antenna  142  is connected to one side surface of the absorbing layer  162 . The absorbing layer  163  is positioned at an upper end of the intermediate layer  153 , and an antenna  413  is connected to one side surface of the absorbing layer  162 . In this case, the absorbing layers  162  and  163  are adjacently disposed to each other, and the antennas  142  and  143  are disposed at a relatively longer distance compared to a distance between the absorbing layers  162  and  163 . 
       FIG. 6  is a diagram illustrating a structure of the photo diode including four absorbing layers according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 6 , a waveguide  123  branched into four waveguides is illustrated, and absorbing layers  164 ,  165 ,  166 , and  167  having small areas are positioned at distal parts of Y-shaped branched waveguides  1231 ,  1232 ,  1233 , and  1234 . 
     A part  203 , in which the absorbing layers  164 ,  165 ,  166 , and  167  are disposed, is enlarged and illustrated at a right side. Intermediate layers  154  and  155  are formed based on end parts of the waveguides  1231 ,  1232 ,  1233 , and  1234 . 
     In this case, the intermediate layer  154  is formed to be connected to the waveguides  1231  and  1232 , and the intermediate layer  155  is formed to be connected to the waveguides  1233  and  1234 . Accordingly, one intermediate layer may be formed in a form sharing two waveguides. 
     The two absorbing layers  164  and  165  are positioned so as to correspond to the waveguides  1231  and  1232 , respectively, at an upper end of the intermediate layer  154 , and the two absorbing layers  166  and  167  are positioned so as to correspond to the waveguides  1233  and  1234 , respectively, at an upper end of the intermediate layer  155 . 
     An antenna  144  is connected between the two absorbing layers  164  and  165 , and the other antenna  154  is connected between the other two absorbing layers  166  and  167 . 
     In  FIGS. 5 and 6 , the waveguides are branched in the Y-form, and the waveguide is branched one time in  FIG. 5 , and the waveguide is branched two times in  FIG. 6 . 
     In the meantime, the absorbing layers  161  to  167  in  FIGS. 4 to 6  may be disposed in a vertical direction to the waveguides  122 ,  1221 ,  1222 ,  1231 ,  1232 ,  1233 , and  1234  in parallel, respectively. 
     Here, the plurality of waveguides may be arranged in a vertical direction based on one waveguide, and each of the waveguides may be disposed in parallel to the waveguide, which serves as a reference. Accordingly, the waveguide enables the first incident beating source to have the same physical length until the beating source reaches each absorbing layer. This is for the purpose that currents generated in other absorbing layers have the same phase. 
       FIGS. 4 and 5  are illustrated for convenience of the description, and the diode may be implemented in various forms, in addition to the aforementioned structures. 
     The present disclosure makes a current generated by the beating signal incident through a Y distributor have the same phase by disposing the single absorbing layer having a small area for a broadband operation side by side in a vertical direction to the waveguide of the beating signal. Accordingly, it is possible to overcome an output limit of the existing single photo diode. 
       FIGS. 7A and 7B  are diagrams illustrating photo diodes implemented by using a single waveguide and dual waveguides according to an exemplary embodiment of the present disclosure. 
     Referring to  FIGS. 7A and 7B ,  FIG. 7A  is an implementation example of a photo diode having a single waveguide and a single absorbing layer, and  FIG. 7B  is an implementation example of a photo diode having duel waveguides and two absorbing layers. 
       FIG. 8  is a graph illustrating a comparison of output power of a terahertz continuous wave according to  FIGS. 7A and 7B . 
     Referring to  FIG. 8 , a horizontal axis of the graph represents a frequency (THz), and a vertical axis of the graph represents a ratio of output power having dual waveguides and a single waveguide. 
     The graph illustrates a value obtained by dividing output power of the photo diode having the dual waveguides by output power of the photo diode having the single waveguide. Referring to a measurement result  300 , it can be seen that performance is improved by about 1.7 times in a frequency band of about 220 THz. 
     The existing terahertz application systems have very low photoelectric conversion efficiency and low output, so that it is difficult to apply the terahertz application system to an existing industry or a new industry field. The present disclosure provides a high output and broadband terahertz continuous wave generator through the arrangement of the photo diode at the same phase, thereby being substantially applicable to the development of a terahertz application system. Further, the photo diode of the present disclosure may be expanded and applied to photo diodes having the similar structure. 
     In the detailed description of the present disclosure, the particular exemplary embodiment has been described, but various modifications are available without departing from the scope of the present disclosure. Therefore, the scope of the present disclosure is not limited to the exemplary embodiments described, but shall be defined by the claims to be described below and the equivalents to the claims.