Abstract:
The present invention provides the photodetector comprising a lower cladding layer including a n-type doped region, an absorbing layer, an upper cladding layer including a p-type doped region, and ohmic electrodes connected to said lower cladding layer and said upper cladding layer, wherein said p-type doped region extends to be formed into said absorbing layer by a predetermined length. 
     In accordance with present invention, by reducing effect of the hetero junction barrier where holes move in the intrinsic region, the operating voltage can be decreased and the bandwidth can be improved.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a photodetector, in particular, to the photodetector with high power operation and large bandwidth in optical communication system. 
   2. Description of the Prior Art 
   As an information technology has been developed, an amount of information, which transmits through optical fibers, has been significantly increased. According to this trend, the photodetector requires a large bandwidth to receive a large amount of information and a high power operation. 
   When quantum wells are utilized in the photodetector, cladding layers having relatively small refractive index are stacked at both sides thereto for optical confinement. Thus, materials of large band gap are used for the cladding layer. InGaAsP material is generally used on a InP substrate in a device operable in wavelength band of 1.55 μm,. 
   However, energy band structure has a tendency of a discontinuity due to a difficulty of crystalline growth, and excited charges are subject to passing some barriers, so that it results in an increment of operating voltage and decrement of bandwidth. 
   SUMMARY OF THE INVENTION 
   Therefore, to solve the above problem, it is an object of the present invention is to provide a photodetector capable of removing an influence on the bandwidth and operating voltage caused by heterojunction. 
   To achieve the above object, one aspect of the present invention provides a photodetector having a heterojunction structure in an intrinsic region, comprising a lower cladding layer including an n-type doped region, an absorbing layer, an upper cladding layer including a p-type doped region and ohmic electrodes connected to said lower cladding layer and said upper cladding layer, respectively, wherein said p-type doped region extends into said absorbing layer by a predetermined length. 
   The photodetector to which the present invention is applied is one that has a heterojunction structure in an intrinsic region, and is not limited to specific kind, and for example, the photodetector is preferably one of a waveguide type PIN structure photodetector, a travelling wave photodetector, or an avalanche photodetector. 
   Preferably, a distance between said n-type doped region and said p-type doped region can be 0.6 to 1.2 μm, and when absorbing layer has a structure that a quantum well barrier and a quantum well layer are alternatively formed, the p-type doped region can extend to be formed to some portion of the quantum well barrier. 
   Hereinafter, embodiments of the present invention will be explained with reference to the accompanying drawings to provide those who are skilled in the art with detailed explanations of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic configuration of the optical waveguide type photodetector having a PIN structure in accordance with one embodiment of the present invention. 
       FIG. 2  shows a band diagram for the PIN photodetector having a quantum well absorbing layer without performing any doping process. 
       FIG. 3  shows a band diagram for the PIN photodetector having a quantum well absorbing layer when performing doping process of the prior art. 
       FIG. 4  shows a band diagram for the PIN photodetector having a quantum well absorbing layer when performing doping process in accordance with one embodiment of the present invention. 
       FIGS. 5A and 5B  show graphs for measuring driving voltages of the photodetector of the present invention and of the prior art, respectively. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic configuration of the optical waveguide type photodetector having a PIN structure in accordance with one embodiment of the present invention. 
   A non-conductive InP substrate  11  having a low loss of microwave can be used for high-speed operation, and n+ InP layers  12 ,  18  are grown on the substrate  11 . And then a cladding layer  17 , an absorbing layer  16  and a cladding layer  15  are consequently formed thereon. They are etched in ridge shape for the lateral optical confinement, n+ ohmic electrodes  13  are formed on both sides of the surface of the exposed n+ InP layer  12  and used as a ground line, and a p+ ohmic electrode  14  is formed on the p+ cladding layer  15  and used as a signal wiring. N+ ohmic electrode  13  and p+ ohmic electrode  14  constitute a coplanar waveguide and functions as propagating microwave safely. 
   Those n+ InP layers  12 ,  18  can be grown with doping concentration of more than about 10 18 /cm 3 , and the cladding layer  17  can be grown with the InP material and has a highly doped n+ region. The cladding layer  15  may be grown with doping concentration of about 10 18 /cm 3 . Meanwhile, the absorbing layer  16  may have a structure such as quantum well, multi quantum well or bulk type. For example, in quantum well structure, the absorbing layer  16  may have a lower quantum well barrier of InGaAsP with a thickness of about 2000 Å, a quantum well of InGaAs with a thickness of about 2000 Å, and an upper quantum well barrier of InGaAsP with a thickness of about 2000 Å. 
   In this case, a starting position of p-type doping region is within the upper quantum well barrier of InGaAsP, in accordance with the preferred embodiment of the present invention. In other words, the doping region for the cladding layer  15  extends into the quantum well barrier of InGaAsP. For example, it may extend into about 1000 Å. On the contrary, n-type doped region does not extend into the cladding layer  17 . This will be explained later in detail. 
   The most important factor in high-speed operation of the PIN photodetector is electrostatic capacitance of the photodetector. Especially, the amount of the electrostatic capacitance is determined depending on the area of the photodetector and thickness of intrinsic region thereof in the optical waveguide type PIN photodetector. 
   The thickness of the intrinsic region is changed in accordance with the position of the doped region. The thicker the intrinsic region is, the smaller the electrostatic capacitance is, and it causes a limitation in bandwidth. Thus, in case the intrinsic region is significantly thick, the limitation in bandwidth is decreased, so that it is preferable that the intrinsic region has a thickness of not more than about 6000 Å. 
   However, by adjusting the regions of cladding layers  15 ,  17  and the position in which the p-type doping is started, barrier effect may be reduced. Therefore, holes can pass through the barrier under a low voltage of reverse bias. It is possible to implement the intrinsic region having more than 6000 Å. In other words, the intrinsic region may be formed with 0.6˜1.2 μm in length. Hereinafter, the operation principle will be described in detail with reference to  FIGS. 2 ,  3 , and  4 . 
   In the PIN photodetector having the quantum well absorbing layer,  FIG. 2  shows a band diagram for the PIN photodetector having the quantum well absorbing layer without doping process,  FIG. 3  shows a band diagram for the PIN photodetector having the quantum well absorbing layer through doping process of the prior art, and  FIG. 4  shows a band diagram for the PIN photodetector having the quantum well absorbing layer through doping process in accordance with one embodiment of the present invention. 
   Referring to  FIG. 2 , A represents a conduction band, B represents a balance band, and numerical references  101  and  105  show n+ and p+ cladding layers, respectively. When the quantum well is used as the absorbing layer, cladding layers  101 , 105  are used for optical confinement in the waveguide type photodetector. In this case, a material having small index of refraction is used for the cladding layer and has a resultant large band gap. Numerical references  102  and  104  show upper and lower barriers of the quantum well, respectively. 
   A band diagram when the doping process is performed in accordance with the prior art will be described with reference to  FIG. 3 . In the prior art, some portions ( 201   b ,  205   b ) of two cladding layers ( 201   a ,  201   b ,  205   a ,  205   b ) are included in the intrinsic region. 
   Therefore, electrons and holes move toward the doped region due to reverse voltage applied to the photodetector and reach junction region between cladding layers  201   b ,  205   b  and barriers  202 ,  204  of the quantum well  203 . Thus, for those electrons and holes to move to the doped region, those electrons and holes should jump the potential barriers by thermal energy. It increases the operating voltage and reduces the bandwidth. 
   On the contrary,  FIG. 4  shows a band diagram for the PIN photodetector having a quantum well absorbing layer through doping process in accordance with one embodiment of the present invention, wherein the reverse voltage is applied to the photodetector. A numerical reference  303  represents the quantum well layer,  301   a  and  305  show the doped lower cladding layer and the doped upper cladding layer, respectively, and  302  and  304  show lower and upper quantum well barriers, respectively. The difference between  FIG. 4  and  FIG. 3  is that p+ doped region extends into the barrier of the upper quantum well, which is divided into the doped region ( 304   a ) and the undoped region ( 304   b ) in  FIG. 4 . 
   In this case, holes and electrons are generated from the quantum well layer  303  and move toward p-layer and n-layer, respectively. However, electrons are not easily induced to the doped region due to the energy barrier, which is generated at the interface between the lower quantum well barrier  302  and undoped cladding layer  301   b . The holes reach the doped layer  304   a  before the interface between the cladding layer  305  and upper quantum well barrier  304 . For these reasons, holes can pass through barrier through tunneling or jumping a thin energy barrier so that moving speed of holes is significantly not reduced by barrier. 
   Both electrons and holes reach the doped region, before reaching the cladding layer, due to a limitation of electrostatic capacitance. It can be applied for holes having a large effective mass and low moving speed. In other words, because electrons have small effective mass, the barrier effect is not dominant. 
   Since the electrostatic capacitance is inverse proportional to the thickness of the intrinsic region, the electrostatic capacitance of not more than a predetermined value can be maintained. 
   (Comparative Example) 
   Hereinafter, a comparative result of the photodetector of the prior art and that of the present embodiment will be described. 
   First, the PIN photodetector consists of the InP cladding layer, and the lower quantum well barrier InGaAsP with about 2000 Å, the quantum well InGaAs layer with about 2000 Å, and the upper quantum well barrier InGaAsP layer with about 2000 Å as the absorbing layer on the non-conductive InP substrate, and p+ doped cladding layer with about 10 18 /cm 3  is formed thereon. 
   The n+ region having doping concentration of more than 10 18 /cm 3  and the p+ region having doping concentration of more than 10 18 /cm 3  are positioned within the n cladding layer and p cladding layer, respectively, in accordance with the photodetector of the prior art, however, the p+ region extends into the absorbing layer by a predetermined length (e.g. 1000 Å) in accordance with the present embodiment. This difference can be readily acknowledged by comparing  FIG. 3  and  FIG. 4 . 
     FIGS. 5A and 5B  show graphs for measuring applied voltages of the photodetector of the present invention and of the prior art, respectively. The present embodiment shows the characteristic that O/E reaction (optical/electric reaction) is saturated at the reverse voltage of −2V, however, the O/E reaction is saturated at more than the reverse voltage of −4V in the prior art. In other words, these results represent that the applied voltage to the photodetector is decreased. 
   Although the present invention has been described in conjunction with the preferred embodiment, the present invention is not limited to the embodiments, and it will be apparent to those skilled in the art that the present invention can be modified in variation within the scope of the invention. 
   As described above, the photodetector of the present invention uses quantum well as absorbing layer to achieve a high output current. Small junction capacitance is provided by thick intrinsic region, and the reduced effect of energy barrier along the path of holes suppresses the reduction of bandwidth and increase in operating voltage. Therefore, this invention can be applied to high output power-wide bandwidth photodetector.