Abstract:
Disclosed herein is a photodetector suitable for use in an optical pickup reproducing apparatus, which is capable of detecting short-wavelength light (e.g., light of about 405 nm) from storage media having large capacity, such as BD, with a high efficiency at a high speed, and a method of manufacturing the same.

Description:
INCORPORATION BY REFERENCE  
       [0001]     The present application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 2004-103122 filed on Dec. 8, 2004. The content of the application is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a photodetector and a method of manufacturing the same. More specifically, the present invention relates to a photodetector suitable for use in an optical pickup reproducing apparatus, which is capable of detecting short-wavelength light (e.g., light of about 405 nm) from storage media having large capacity, such as BD (Blue-ray Disc), with a high efficiency at a high speed, and a method of manufacturing the same.  
         [0004]     2. Description of the Related Art  
         [0005]     In recent years, optical storage techniques have advanced toward high density, high speed and miniaturization while technically competing with memory devices, hard discs and magnetic discs. Further, the above techniques are becoming increasingly important owing to characteristics that distinguish them from other storage media.  
         [0006]     The optical storage technique uses optical storage media (e.g., optical disc) which are removable from a disc drive and have advantages, such as lower prices and permanent data storage, compared to other storage media. In particular, the optical storage media are known to have much higher resistance to temperature and impact than other storage media.  
         [0007]     Although the optical storage technique is disadvantageous because of low transmission rate and small storage capacity, it has recently been developed to realize high capacity and high speed comparable to magnetic discs in accordance with rapid technical progress. Nowadays, thorough research into photodetector integrated circuits to transform the received light into electric signals in the optical storage media is being conducted.  
         [0008]      FIG. 1  is a view schematically showing a general photodetector integrated circuit.  
         [0009]     In the photodetector integrated circuit shown in  FIG. 1 , a photodetector  1  absorbs light  3  to generate current I P . The current I P  is transformed into the voltage through an amplifier  2 , such as TIA (Trans-Impedance Amplifier), and then amplified. For example, when the current I P  is applied to the TIA, the voltage V OUT  discharged from the TIA is calculated as represented by Equation 1, below:  
               V   OUT     =       (     1   +       R   2       R   1         )     ⁢     (       V   C     +       I   P     ⁢     R   V         )               Equation   ⁢           ⁢   1             
 
         [0010]     Wherein R V  is a variable resistance of an I-V amplifier (I-V AMP), R 1  and R 2  are resistances of a driving device (DRV), and V C  is a driving voltage.  
         [0011]     Of the optical storage techniques manifesting high capacity and high speed, extensive and intensive research into photodetectors of photodetector integrated circuits to absorb light of about 405 nm to be transformed into electric signals is being conducted.  
         [0012]      FIG. 2  is a sectional view of a conventional photodetector which is disclosed in Japanese Patent Laid-open Publication No. 2001-320075.  FIG. 3  is a graph showing optical efficiency and frequency characteristics varying with finger spaces in the conventional photodetector, in which the frequency characteristics are obtained by measuring the frequency of 3 dB at which a gain varying with the frequency is halved.  
         [0013]     As shown in  FIG. 2 , the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075 comprises an N − -type semiconductor layer  10  containing an N-type impurity at low concentration, a P + -type semiconductor layer  11  completely embedded in the N − -type semiconductor layer  10  and containing a P-type impurity at high concentration, and a protective film formed on the whole upper surface of the N − -type semiconductor layer  10  and the P + -type semiconductor layer  11 . The P + -type semiconductor layer  11  has a width La, and the P + -type semiconductor layers  11  have spaces Lb therebetween. The photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075 is advantageous because it effectively detects light of 780 nm or 650 nm.  
         [0014]     As shown in  FIG. 3 , in the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075, the region able to absorb light is enlarged in proportion to increasing the finger spaces (that is, spaces Lb between the P + -type semiconductor layers  11 ). Thus, the above photodetector can exhibit high optical efficiency  31  for light of about 405 nm. However, the wider finger spaces result in increasing the moving distance of electron-hole pairs created by light absorption, and inducing a low electric field between the fingers (P + -type semiconductor layers  11 ). Hence, since the moving time of electrons or holes lengthens, the above photodetector cannot be used for a high frequency. Consequently, the frequency characteristics  32  become decreased due to the wider finger spaces.  
         [0015]     On the other hand, in the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075, while the finger spaces are reduced, the mobile distance of electrons or holes formed between the fingers  104  and  105  is decreased and a high electric field is induced therebetween, therefore increasing the frequency characteristics  32 . However, since the region able to absorb light diminishes in proportion to reducing the finger spaces, the optical efficiency  31  for light of about 405 nm is remarkably lowered.  
         [0016]     Therefore, the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075, which has optical efficiency  31  and the frequency characteristics  32  that vary with the finger spaces as mentioned above, is applicable to low speed (e.g., 1× speed) BD optical reproducing apparatuses, however it cannot be used in high speed (e.g., 2× speed or more) BD optical reproducing apparatuses requiring high optical efficiency and high frequency characteristics.  
       SUMMARY OF THE INVENTION  
       [0017]     Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a photodetector which can manifest high optical efficiency and high frequency characteristics for the short-wavelength light of about 405 nm.  
         [0018]     Another object of the present invention is to provide a method of manufacturing such a photodetector.  
         [0019]     In order to accomplish the above objects, the present invention provides a photodetector, comprising a substrate to support upper layers; an epitaxial layer formed on the substrate; at least one heavily doped first type finger partially embedded in the epitaxial layer to a small depth; at least one heavily doped second type finger partially embedded in the epitaxial layer to a small depth; a first type well formed in the epitaxial layer which is disposed outside the heavily doped first type fingers and the heavily doped second type fingers; a heavily doped first type electrode unit partially embedded in the first type well to a small depth; and a circuit unit formed on the heavily doped first type electrode unit, wherein the first type and the second type are doped with opposite type elements.  
         [0020]     Preferably, the photodetector according to the present invention further comprises a regrown epitaxial layer formed on the epitaxial layer, the heavily doped first type fingers and the heavily doped second type fingers.  
         [0021]     More preferably, in the photodetector according to the present invention, the at least one heavily doped first type finger and the at least one heavily doped second type finger are alternately partially embedded in the epitaxial layer to a small depth.  
         [0022]     More preferably, the photodetector according to the present invention comprises a substrate to support upper layers; an epitaxial layer formed on the substrate; N heavily doped first type fingers partially embedded in the epitaxial layer to a small depth; N+1 heavily doped second type fingers partially embedded in the epitaxial layer to a small depth to alternate with the N heavily doped first type fingers; and a regrown epitaxial layer formed on the epitaxial layer, the N heavily doped first type fingers and the N+1 heavily doped second type fingers, wherein N is a natural number, and the first type and the second type are doped with opposite type elements.  
         [0023]     Further, the present invention provides a method of manufacturing a photodetector, comprising (A) forming an epitaxial layer on a substrate; and (B) forming at least one heavily doped first type finger and at least one heavily doped second type finger partially embedded in the epitaxial layer to a small depth, wherein the first type and the second type are in opposite states of being doped.  
         [0024]     Preferably, the above method further comprises (C) forming a regrown epitaxial layer on the epitaxial layer, the heavily doped first type fingers and the heavily doped second type fingers.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0026]      FIG. 1  is a view schematically showing a general photodetector integrated circuit;  
         [0027]      FIG. 2  is a sectional view showing a conventional photodetector;  
         [0028]      FIG. 3  is a graph showing optical efficiency and frequency characteristics varying with finger spaces in the conventional photodetector;  
         [0029]      FIG. 4   a  is a top plan view showing a photodetector according to the present invention;  
         [0030]      FIG. 4   b  is a sectional view taken along the line A-A′ of  FIG. 4   a;    
         [0031]      FIG. 5  is a graph showing frequency characteristics varying with finger spaces in the conventional photodetector and the photodetector according to the present invention;  
         [0032]      FIG. 6  is a graph showing optical efficiency varying with finger spaces in the conventional photodetector and the photodetector according to the present invention;  
         [0033]      FIG. 7  is an energy diagram showing an energy level varying with the depth from the surface of the photodetector according to the present invention; and  
         [0034]      FIGS. 8   a  to  8   i  are sectional views showing a process of manufacturing the photodetector according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Hereinafter, a detailed description will be given of a photodetector and a method of manufacturing the photodetector, according to the present invention, with reference to the appended drawings.  
         [0036]      FIG. 4   a  is a top plan view of the photodetector according to the present invention, and  FIG. 4   b  is a sectional view of the photodetector taken along the line A-A′ of  FIG. 4   a.    
         [0037]     As shown in  FIGS. 4   a  and  4   b,  the photodetector  100  of the present invention includes a substrate  101 , a heavily doped first type buried layer  102  disposed on the substrate  101 , an epitaxial layer  103  disposed on the heavily doped first type buried layer  102 , at least one heavily doped first type finger  104  and at least one heavily doped second type finger  105  partially embedded in the epitaxial layer  103  to a small depth, and a regrown epitaxial layer  106  disposed on the epitaxial layer  103 , the heavily doped first type fingers  104  and the heavily doped second type fingers  105 . Further, the photodetector  100  has a first type well  107  formed in the epitaxial layer  103  and the regrown epitaxial layer  106  which are disposed outside the heavily doped first type fingers  104  and the heavily doped second type fingers  105  to be connected to the heavily doped first type buried layer  102 . In addition, a heavily doped first type electrode unit  108  partially embedded in the first type well  107  to a small depth, and a circuit unit  109  connected to the heavily doped first type electrode unit  108  to externally transmit electric signals, are provided. As such, the first type and the second type are in opposite states of being doped (e.g., if the first type is a P-type, the second type is an N-type). Also, the photodetector  100  of the present invention further comprises an anti-reflection coating layer  110  disposed on the regrown epitaxial layer  106  so that the light is not reflected from the surface thereof.  
         [0038]     In the photodetector  100  of the present invention, the substrate  101  functions to support the upper layers. Preferably, the substrate  101  includes a silicon-based substrate, and more preferably, a substrate doped in the same type as the heavily doped first type buried layer  102  formed thereon.  
         [0039]     The heavily doped first type buried layer  102  is formed by ion-implanting a Group III or V element on the substrate  101 .  
         [0040]     The heavily doped first type buried layer  102  includes an impurity at a concentration of about 10 15 -10 21  cm −3 , and preferably, about 10 16 -10 17  cm −3 . If the impurity in the heavily doped first type buried layer  102  has a concentration less than 10 15  cm −3 , resistance of the heavily doped first type buried layer  102  increases, and thus, the frequency characteristics of the photodetector  100  are decreased. On the other hand, if the impurity in the heavily doped first type buried layer  102  has a concentration exceeding 10 21  cm −3 , an energy level may be deformed into an impurity band structure, and thus a structure thereof becomes undesirable.  
         [0041]     Alternatively, in the cases where the substrate  101  is doped in the same type as the heavily doped first type buried layer  102  and includes an impurity having a sufficiently high concentration (about 10 15 -10 21  cm −3 ), the substrate  101  may act as the heavily doped first type buried layer  102 , and therefore, the heavily doped first type buried layer  102  need not be formed.  
         [0042]     The epitaxial layer  103  results from epitaxial growth on the heavily doped first type buried layer  102  using a CVD (Chemical Vapor Deposition) process.  
         [0043]     In this case, to achieve a lattice match between the heavily doped first type buried layer  102  and the epitaxial layer  103 , the epitaxial layer  103  is formed of silicon, silicon carbide (SiC) or diamond, having a lattice constant similar to silicon crystals.  
         [0044]     The epitaxial layer  103  functions to form a fingered photodiode, along with the heavily doped first type buried layer  102  and the heavily doped second type finger  105 , or the heavily doped first type buried layer  102  and the heavily doped first type finger  104 , so as to absorb light of about 405 nm to be transformed into electric signals. Commonly, light of about 405 nm is mostly absorbed in the range of a depth of about 0.1 μm or less from the surface of a silicon layer. Accordingly, to sufficiently absorb light of about 405 nm, the epitaxial layer  103  has a thickness of 0.2-5 μm, and preferably, about 1-3 μm. If the thickness of the epitaxial layer  103  exceeds 5 μm, it is difficult to manufacture a BJT (Bipolar Junction Transistor) to externally transmit the electric signals. Meanwhile, if the thickness of the epitaxial layer  103  is less than 0.2 μm, the light absorption region diminishes, thus lowering the optical efficiency.  
         [0045]     The epitaxial layer  103  may grow by adding a small amount of impurity thereto during the epitaxial growth, so long as it has sufficient resistance. At this time, the impurity in the epitaxial layer  103  has a concentration of about 5×10 15  cm −3  or less, and preferably, about 10 12 -10 15  cm −3 . If the impurity in the epitaxial layer  103  has a concentration exceeding 5×10 15  cm −3 , the optical efficiency of the photodetector  100  is decreased.  
         [0046]     The heavily doped first type finger  104  is formed by ion-implantation of a Group III or V element in the epitaxial layer  103  to be partially embedded therein to a small depth.  
         [0047]     Also, the heavily doped first type finger  104  has a width W 1  in the range of about 0.09-5 μm, and preferably, about 0.09-0.6 μm. Even if the heavily doped first type finger  104  is manufactured to have a width W 1  less than 0.09 μm, it does not negatively affect the characteristics of the photodetector  100 . However, since such a finger is smaller than a minimal size required in the semiconductor manufacturing process, it is difficult to actually manufacture. Meanwhile, if the width W 1  of the heavily doped first type finger  104  exceeds 5 μm, the size of the finger is much larger than that of the photodetector  100 , and the light absorption region diminishes, therefore resulting in lost characteristics of the fingered photodiode.  
         [0048]     Moreover, the impurity in the heavily doped first type finger  104  has a concentration of about 10 18 -10 21  cm −3 , and preferably, about 10 20 -10 21  cm −3 . When the impurity in the heavily doped first type finger  104  has a concentration less than 10 18  cm −3 , the resistance of the heavily doped first type finger  104  increases, thus deteriorating the performance of the photodetector  100 . Conversely, if the impurity in the heavily doped first type finger  104  has a concentration exceeding 10 21  cm −3 , an energy level may be deformed into an impurity band structure, and thus a structure thereof becomes undesirable.  
         [0049]     The heavily doped second type finger  105  is obtained by ion-implanting the element of opposite type in the heavily doped first type finger  104  in the epitaxial layer  103  to be partially embedded therein to a small depth.  
         [0050]     Additionally, the heavily doped second type finger  105  has a width W 2  in the range of about 0.09-5 μm, and preferably, about 0.09-0.6 μm, like the heavily doped first type finger  104 . Even if the heavily doped second type finger  105  is manufactured to have a width W 2  less than 0.09 μm, it does not negatively affect the characteristics of the photodetector  100 . However, since such a finger is smaller than a minimal size required in the semiconductor manufacturing process, it is difficult to actually manufacture. Meanwhile, if the width W 2  of the heavily doped second type finger  105  is larger than 5 μm, the finger has a much larger size than the photodetector  100 , and thus, the light absorption region diminishes, and the characteristics of the fingered photodiode become lost.  
         [0051]     An impurity concentration in the heavily doped second type finger  105  is in the range of about 10 18 -10 21  cm −3 , and preferably, about 10 20 -10 21  cm −3 . When the heavily doped second type finger  105  has an impurity concentration less than 10 18  cm −3 , resistance of the heavily doped second type finger  105  increases, thus deteriorating the performance of the photodetector  100 . However, if the heavily doped second type finger  105  has an impurity concentration higher than 10 21  cm −3 , an energy level may be deformed into an impurity band structure, and thus a structure thereof becomes undesirable.  
         [0052]     In a preferable embodiment, spaces S between the heavily doped first type fingers  104  and the heavily doped second type fingers  105  range from about 1 to 20 μm, and preferably, from about 1.4 to 9.4 μm. Even if the fingers  104  and  105  are manufactured to have the spaces S less than 1 μm therebetween, they do not negatively affect the characteristics of the photodetector  100  of the present invention, however, they are difficult to actually manufacture. On the other hand, if the spaces S between the fingers  104  and  105  exceed 20 μm, a low electric field is induced between the heavily doped first type finger  104  and the heavily doped second type finger  105 , and hence, the frequency characteristics of the photodetector  100  are decreased.  
         [0053]     In a more preferable embodiment, the heavily doped first type fingers  104  and the heavily doped second type fingers  105  are alternately partially embedded in the epitaxial layer  103  to a small depth. This is because the frequency characteristics of the photodetector  100  are related to the spaces S between the fingers  104  and  105  and the electric field induced therebetween, as represented by Equation 2, below:  
             Frequency   ⁢           ⁢     Characteristics   ⁡     (     mobility   ⁢           ⁢   of   ⁢           ⁢   electrons   ⁢           ⁢   or   ⁢           ⁢   holes     )       =       (     Electric   ⁢           ⁢   Field   ⁢           ⁢   between   ⁢           ⁢   the   ⁢           ⁢   Fingers     )       (     Space   ⁢           ⁢   between   ⁢           ⁢   the   ⁢           ⁢   Fingers     )               Equation   ⁢           ⁢   2             
 
         [0054]     In the cases where the heavily doped first type fingers  104  and the heavily doped second type fingers  105  are alternately formed, the high electric field is induced in the epitaxial layer  103  and the regrown epitaxial layer  106  which are disposed between the heavily doped first type fingers  104  and the heavily doped second type fingers  105 , thus improving the frequency characteristics of the photodetector  100 .  
         [0055]     In a still more preferable embodiment, in the cases where the number of heavily doped first fingers  104  is N (wherein, N is a natural number), N+1 heavily doped second type fingers  105  are partially embedded in the epitaxial layer  103  to a small depth to alternate with the N heavily doped first type fingers  104 . Thereby, the high electric field is induced in the epitaxial layer  103  and the regrown epitaxial layer  106  which are disposed between the outermost second type finger  105  and the first type well  107 , and thus, the frequency characteristics of the photodetector  100  can be further increased.  
         [0056]     The regrown epitaxial layer  106  results from epitaxial growth on the epitaxial layer  103 , the heavily doped first type fingers  104  and the heavily doped second type fingers  105  using CVD. In this case, to achieve the lattice match of the epitaxial layer  103 , the heavily doped first type finger  104  and the heavily doped second type finger  105  with the regrown epitaxial layer  106 , the epitaxial layer  103  is formed of silicon, silicon carbide (SiC) or diamond having a lattice constant similar to the silicon crystals.  
         [0057]     In addition, the regrown epitaxial layer  106  acts to form a fingered photodiode, together with the heavily doped first type finger  104  and the heavily doped second type finger  105 , so as to absorb light of about 405 nm to be transformed into electric signals. Commonly, light of about 405 nm is mostly absorbed in the range of a depth of about 0.1 μm or less from the surface of a silicon layer. Accordingly, the regrown epitaxial layer  106  has a thickness of about 0.01-0.5 μm, and preferably, about 0.05-0.2 μm. Even if the regrown epitaxial layer  106  is manufactured to be thinner than 0.01 μm, it does not negatively affect the characteristics of the photodetector  100  of the present invention, however it is difficult to actually manufacture. Meanwhile, if the regrown epitaxial layer  106  has a thickness exceeding 0.5 μm, the regrown epitaxial layer  106  is outside the range of depletion region formed in the regrown epitaxial layer  106  by the heavily doped first type fingers  104  and the heavily doped second type fingers  105 . Thus, the electron-hole pair created in the regrown epitaxial layer  106  may be eliminated by surface recombination (e.g., combination of a carrier by a dangling bond).  
         [0058]     Also, so long as having sufficient resistance, the regrown epitaxial layer  106  may grow by adding a small amount of impurity thereto during the epitaxial growth. As such, the impurity in the regrown epitaxial layer  106  has a concentration of about 5×10 15  cm −3  or less, and preferably, about 10 12 -10 15  cm −3 . If the regrown epitaxial layer  106  has an impurity concentration higher than 10 15  cm −3 , the optical efficiency of the photodetector  100  is reduced.  
         [0059]     Alternatively, in the cases where the spaces S between the fingers  104  and  105  are sufficiently large, the depletion region able to absorb light between the heavily doped first type fingers  104  and the heavily doped second type fingers  105  is formed to have a relatively large area, thereby exhibiting high optical efficiency for light of about 405 nm. Hence, the regrown epitaxial layer  106  need not be formed in the photodetector  100 .  
         [0060]     The first type well  107  is formed by ion-implantation of a Group III or V element in the epitaxial layer  103  and the regrown epitaxial layer  106  (or the epitaxial layer  103  in the absence of the regrown epitaxial layer  106 ) disposed outside the heavily doped first type fingers  104  and the heavily doped second type fingers  105 . Preferably, the first type well  107  is connected to the heavily doped first type buried layer  102  (or the substrate  101  doped in the first type when the first type impurity doped in the substrate  101  has a sufficiently high concentration).  
         [0061]     The heavily doped first type electrode unit  108  is obtained by ion-implantation of a Group III or V element in the first type well  107  to be partially embedded therein to a small depth.  
         [0062]     The circuit unit  109  is formed on the heavily doped first type electrode unit  108 , and acts to externally transmit the electron-hole pair (that is, electric signal) created by light-absorption of the epitaxial layer  103  or the regrown epitaxial layer  106 , along with the first type well  107  and the heavily doped first type electrode unit  108 .  
         [0063]     The anti-reflection coating layer  110  is formed in an appropriate thickness using silicon nitride on the regrown epitaxial layer  106  (or the epitaxial layer  103 , the heavily doped first type fingers  104  and the heavily doped second type fingers  105  in the absence of the regrown epitaxial layer  106 ), so that light of about 405 nm is not reflected from the surface of the photodetector  100 .  
         [0064]     Preferably, the first type of the photodetector  100  is a P-type, and the second type thereof is an N-type. The reason is that the electrons functioning as a majority carrier when the first type is a P-type and the second type is an N-type have higher carrier mobility than the holes functioning as a majority carrier when the first type is an N-type and the second type is a P-type. Thereby, the frequency characteristics become superior.  
         [0065]      FIG. 5  is a graph showing the frequency characteristics varying with the finger spaces in the inventive photodetector and the conventional photodetector, in which a photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075 shown in  FIG. 2  is used as the conventional photodetector, and the frequency characteristics are determined by measuring the frequency of 3 dB at which a gain varying with the frequency is halved.  
         [0066]     As shown in  FIG. 5 , the inventive photodetector  100  exhibits frequency characteristics  200  for light of about 405 nm at all the finger spaces S, superior to frequency characteristics  32  of the conventional photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075.  
         [0067]     In particular, at the wide finger spaces S causing poor frequency characteristics due to the larger mobile distance of electrons or holes, the frequency characteristics  200  of the inventive photodetector  100  are better than those 32 of the conventional photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075.  
         [0068]     As seen in Equation 2, since the heavily doped first type finger  104  and the heavily doped second type finger  105  are doped with opposite type elements, the electric field is induced in the epitaxial layer  103  and the regrown epitaxial layer  106  which are disposed between the heavily doped first type finger  104  (or the first type well  107 ) and the heavily doped second type finger  105 .  
         [0069]      FIG. 6  is a graph showing the optical efficiency varying with the finger spaces in the inventive photodetecor and the conventional photodetector.  FIG. 7  is an energy diagram showing the energy level varying with the depth from the surface of the photodetector of the present invention. As such, a photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075 shown in  FIG. 2  is used as the conventional photodetector.  
         [0070]     As is apparent from  FIG. 6 , the inventive photodetector  100  has higher optical efficiency  300  for light of about 405 nm at all the finger spaces S, compared to the optical efficiency  31  of the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075.  
         [0071]     Particularly, it can be shown that the optical efficiency  300  of the inventive photodetector  100  is better than that 31 of the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075, at the narrow finger spaces S causing poor optical efficiency due to the small light absorption region.  
         [0072]     This is because the regrown epitaxial layer  106  is formed on the epitaxial layer  103 , the heavily doped first type fingers  104  and the heavily doped second type fingers  105 , whereby the region able to absorb light of about 405 nm can be enlarged.  
         [0073]     As shown in  FIG. 7 , since the photodetector  100  of the present invention uses the heavily doped first type fingers  104  and the heavily doped second type fingers  105 , the energy level of a conduction band  410  and a valence band  420  near the surface of the photodetector  100  of the present invention is higher than that of a conduction band  41  and a valence band  42  of the photodetector disclosed in Japanese Patent Laid-open Publication No. 2001-320075. Thus, a high electric field is induced in the epitaxial layer  103  or the regrown epitaxial layer  106 . Thereby, the depletion region in the epitaxial layer  103  or the regrown epitaxial layer  106  is enlarged, and hence, the light absorption region becomes larger, resulting in increased optical efficiency for light of about 405 nm.  
         [0074]     Turning now to  FIGS. 8   a  to  8   i,  there is illustrated a process of manufacturing the photodetector of the present invention.  
         [0075]     In  FIG. 8   a,  a silicon-based substrate  101  is prepared.  
         [0076]     In  FIG. 8   b,  a Group III or V element is ion-implanted on the substrate  101  to form a heavily doped first type buried layer  102 .  
         [0077]     As such, it is preferable that a Group III or V element be implanted so that the heavily doped first type buried layer  102  has an impurity concentration of about 10 15 -10 21  cm −3 .  
         [0078]     Alternatively, in the cases where the substrate  101  is doped in the same type as the heavily doped first type buried layer  102  and includes an impurity in a sufficiently high concentration (e.g., 10 15 -10 21  cm −3 ), the substrate  101  can act as the heavily doped first type buried layer  102 , and thus, the heavily doped first type buried layer  102  need not be formed.  
         [0079]     In  FIG. 8   c,  the upper surface of the heavily doped first type buried layer  102  (or the substrate  101  doped in a first type having a high impurity concentration) is subjected to epitaxial growth using CVD, to form an epitaxial layer  103 .  
         [0080]     In this case, it is preferable that the epitaxial layer  103  be formed to include an impurity of about 5×10 15  cm −3  or less so as to exhibit sufficient resistance. Further, the epitaxial layer  103  is about 0.2-5 μm thick.  
         [0081]     In  FIG. 8   d,  a Group III or V element is ion-implanted in the epitaxial layer  103  to be partially embedded therein to a small depth, thereby forming at least one heavily doped first type finger  104 .  
         [0082]     The heavily doped first type finger  104  is preferably formed by implanting a Group III or V element at a concentration of about 10 18 -10 21  cm −3 . In addition, the first type finger  104  has a width W 1  of about 0.09-5 μm.  
         [0083]     In  FIG. 8   e,  the element of opposite type to the element in the heavily doped first type finger  104  is ion-implanted in the epitaxial layer  103  to be partially embedded therein to a small depth, to obtain at least one heavily doped second type finger  105 .  
         [0084]     As in the heavily doped first type finger  104 , the heavily doped second type finger  105  is preferably formed by implanting a Group III or V element at a concentration of about 10 18 -10 21  cm −3 . In addition, the second type finger  105  has a width W 2  of about 0.09-5 μm.  
         [0085]     In a preferable embodiment, the heavily doped first type fingers  104  and the heavily doped second type fingers  105  are formed to have spaces S of about 1-20 μm therebetween.  
         [0086]     In a more preferable embodiment, the heavily doped first type fingers  104  and the heavily doped second type fingers  105  are alternately partially embedded in the epitaxial layer  103  to a small depth.  
         [0087]     In a still more preferable embodiment, in the cases where the number of heavily doped first type fingers  104  is N (wherein N is a natural number), N+1 heavily doped second type fingers  105  are partially embedded in the epitaxial layer  103  to a small depth to alternate with the N heavily doped first type fingers  104 .  
         [0088]     In  FIG. 8   f,  the upper surfaces of the epitaxial layer  103 , the heavily doped first type fingers  104  and the heavily doped second type fingers  105  are subjected to epitaxial growth using the CVD process, to obtain a regrown epitaxial layer  106 .  
         [0089]     It is preferable that the regrown epitaxial layer  106  be formed to have an impurity of about 5×10 15  cm −3  or less so as to exhibit sufficient resistance. Further, the regrown epitaxial layer  106  has a thickness of about 0.01-0.5 μm.  
         [0090]     Alternatively, in the cases where the spaces S between the fingers  104  and  105  are sufficiently large, the depletion region able to absorb light between the heavily doped first type fingers  104  and the heavily doped second type fingers  105  is formed to have a relatively large area, and thus, the regrown epitaxial layer  106  need not be formed.  
         [0091]     In  FIG. 8   g,  a Group III or V element is ion-implanted in the epitaxial layer  103  and the regrown epitaxial layer  106  (or the epitaxial layer  103  in the absence of the regrown epitaxial layer  106 ) disposed outside the heavily doped first type fingers  104  and the heavily doped second type fingers  105 , thereby forming a first type well  107 .  
         [0092]     The first type well  107  is preferably connected to the heavily doped first type buried layer  102  (or the substrate  101  doped in a first type having a high impurity concentration).  
         [0093]     In  FIG. 8   h,  a Group III or V element is ion-implanted in the first type well  107  to be partially embedded therein to a small depth, to form a heavily doped first type electrode unit  108 .  
         [0094]     In  FIG. 8   i,  a circuit unit  109  is formed on the heavily doped first type electrode unit  108  to externally transmit the electric signals, and also, an anti-reflection coating layer  110  is formed using silicon nitride on the regrown epitaxial layer  106  (or the epitaxial layer  103 , the heavily doped first type fingers  104  and the heavily doped second type fingers  105  in the absence of the regrown epitaxial layer  106 ) so that light of about 405 nm is not reflected from the surface of the photodetector  100 .  
         [0095]     Alternatively, the first type well  107 , the heavily doped first type electrode unit  108  and the circuit unit  109  may not be formed. For example, a circuit may be formed to transmit electric signals through a side surface or a lower surface of the heavily doped first type buried layer  102  (or the substrate  101  doped in a first type when the first type impurity in the substrate  101  has a sufficiently high concentration) of the photodetector  100 . At this time, light of about 405 nm is absorbed to the epitaxial layer  103  or the regrown epitaxial layer  106  to create the electric signals, which are then externally transmitted through the heavily doped first type buried layer  102  or the substrate  101 .  
         [0096]     As described above, the present invention provides a photodetector and a method of manufacturing the photodetector, in which a high electric field is induced in the epitaxial layer or regrown epitaxial layer by the two types of fingers, and thus, the frequency characteristics can be further improved even at the wide finger spaces as well as the narrow finger spaces.  
         [0097]     According to the photodetector and the manufacturing method thereof of the present invention, since the regrown epitaxial layer for absorption of the short wavelength light of about 405 nm is formed on the two-type fingers, the optical efficiency can be further increased even at the narrow finger spaces as well as the wide finger spaces.  
         [0098]     Additionally, according to the photodetector and the manufacturing method thereof of the present invention, the high electric field is induced by the two-type fingers, whereby the depletion region in the epitaxial layer or regrown epitaxial layer is enlarged, thus increasing the optical efficiency regardless of the finger spaces.  
         [0099]     Moreover, according to the photodetector and the manufacturing method thereof of the present invention, the optical efficiency and the frequency characteristics are suitable for light of about 405 nm and all the finger spaces, which satisfy the requirements for use in high speed BD optical reproducing apparatuses.  
         [0100]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.