Abstract:
An avalanche photodiode according to this invention include a light receiving region  101  surrounded by a ring-shaped trench  13 , a first electrode  11  formed on the light receiving region  101 , a second electrode  12  formed on the periphery of the ring-shaped trench  13  surrounding the light receiving region, a first semiconductor layer lying just under the first electrode  11 , and a second semiconductor layer lying just under the second electrode  12 . Conductivity types of the first semiconductor and the second semiconductor are identical.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to avalanche photodiodes that are excellent in high-frequency characteristics and can be manufactured in a high yield.  
         [0003]     2. Description of the Related Art  
         [0004]     A conventional avalanche photodiode, for example, as represented in U.S. Patent Publication 5,552,629, includes crystalline layers grown on a p +  type InP substrate  11 , which are each a p +  type InP buffer layer  12 , a p −  type InGaAs light absorption layer  13 , a p +  type InP field-buffer layer  14 , an n −  type InAlGaAs/InAlAs superlatice multiplication layer  15 , an n +  type InAlAs cap layer  16 , and an n +  type InGaAs contact layer  17 . A ring-shaped trench is formed around the n +  type InAlAs cap layer  16  and the n +  type InGas contact layer  17 . Ap +  type conductivity region that reaches the p +  type InP field-buffer layer  14  is formed outside of the ring-shaped trench by Zn selective thermal diffusion. A circular n-type electrode  18  is provided on the top of a light receiving region composed of the superlatice multiplication layer  15 , cap layer  16 , and contact layer  17 .  
         [0005]     The above described avalanche photodiode, which has the ring-shaped p +  type conductivity region provided around the n-type light receiving region, prevents a depletion layer that extends from their pn junction from reaching the side face portion of the light receiving region, when a reverse bias voltage is applied, thereby realizing low-dark-current and high-reliability. The ring-shaped p +  type conductivity region is formed by the Zn selective thermal diffusion process, which requires high cost and decrease yield of the device fabrication and. It is therefore, a primary object of the invention to provide an avalanche photodiode that is excellent in high-frequency characteristics and can be easily produced in a high-yield.  
       SUMMARY OF THE INVENTION  
       [0006]     An avalanche photodiode according to the present invention includes: a light receiving region surrounded by a ring-shaped trench; a first electrode formed on the light receiving region; a second electrode formed on the periphery of the ring-shaped trench surrounding the light receiving region; a first semiconductor layer lying just under the first electrode; and a second semiconductor layer lying just under the second electrode, the conductivity type of the first semiconductor and the conductivity type of the second semiconductor being identical. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIGS. 1 and 8  are diagrams illustrating various parts of an avalanche photodiode according to Embodiment 1 of the present invention;  
         [0008]      FIGS. 2-4  are diagrams illustrating a method of producing the avalanche photodiode according to Embodiment 1 of the present invention;  
         [0009]      FIGS. 5-7  are diagrams illustrating device characteristics of the avalanche photodiode according to Embodiment 1 of the present invention;  
         [0010]      FIGS. 9-12  are diagrams illustrating avalanche photodiodes according to Embodiment 2 of the present invention 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [0011]     FIGS.  1 ( a ) and ( b ) are diagrams illustrating a cross-sectional view and a top view of an avalanche photodiode according to Embodiment 1 of the present invention. The avalanche photodiode illustrated in  FIG. 1  is includes a semi-insulating semiconductor substrate  1 , an n +  type buffer layer  2 , a p +  type buffer layer  3 , a light absorption layer  4 , a first-conductivity-type field-buffer layer  5 ; an avalanche multiplication layer  6 , a stop-etch layer  7 , a second-conductivity-type cap layer  8 , a second-conductivity-type contact layer  9 , a passivation film  10 , a first electrode  11 , and a second electrode  12 . A ring-shaped trench  13  is formed in the second-conductivity-type cap  8 .  
         [0012]      FIGS. 2-4  are diagrams illustrating a method for producing the avalanche photodiode represented in  FIG. 1 . As illustrated in  FIG. 2 , each of the layers from the n +  type buffer layer  2  made of n +  type InP to the contact layer  9  made of n +  type GaInAs is sequentially formed on the semi-insulating substrate  1  made of InP by an epitaxial method. The n +  type InP buffer layer  2  having thickness of 0.2-1.0 μm is at first formed on the semi-insulating InP substrate  1 , then the buffer layer  3 , having its thickness of 0.5-1.0 μm and being made of p +  type InP, is formed on the n +  type InP buffer layer  2 . The light absorption layer  4 , having thickness of 1.0-1.5 μm and being made of non-doped i-type or p −  type GaInAs with carrier density of approximately 1×10 16  cm −3 , is formed on the p +  type InP buffer layer  3 . The first-conductivity-type field-buffer layer  5 , having thickness of 0.01-0.05 μm and being made of p +  type InP with carrier density of  0.5- 1×10 18  cm −3 , is formed on the GaInAs light absorption layer  4 . The avalanche multiplication layer  6 , having thickness of 0.1-0.3 μm and being non-doped i-type or n type AlInAs with carrier density of approximately 5×10 15  cm −3 , is formed on the p + -type-InP first-conductivity-type field-buffer layer  5 . The stop-etch layer  7 , having thickness of 0.005-0.05 μm and being made of non-doped i-type InP, is formed on the n − -type-AlInAs avalanche multiplication layer  6 . The second-conductivity-type cap layer  8 , having thickness of 0.5-1.0 μm and being made of n +  type AlInAs with carrier density of 1-10×10 19  cm −3 , is formed on the i-type InP stop-etch layer  7 . The second-conductivity-type contact layer  9 , having its layer thickness of 0.1-0.5 m and being made of n +  type GaInAs with carrier density of 1-10×10 19  cm −3 , is formed on the n + -type-AlInAs second-conductivity-type cap layer  8 . Each layer can be formed with a molecular beam epitaxy (MBE) method or metal-organic vapor-phase epitaxy (MO-VPE) method, etc. using a solid source or gas source.  
         [0013]     Next, as illustrated in  FIG. 3 , the n + -type-GaInAs second-conductivity-type contact layer  9  and the n + -type-AlInAs second-conductivity-type cap layer  8  are selectively removed by etching using mixed solution of an organic acid such as a citric acid and a hydrogen peroxide solution, so as to form the ring-shaped trench  13 . A resist pattern formed by a well known lithography technique, or an SiN x  or SiO 2  pattern formed by the resist can be used for etching mask.  
         [0014]     Next, as illustrated in  FIG. 4 , titanium (Ti) ions for removing the p-type characteristics is implanted in the ring-shaped trench  13  by using heat treatment at 600° C., to form an ion implanted region  102  reaching the first-conductivity-type field-buffer layer  5 . Due to the ion implanted region  102 , the carrier density of the first-conductivity-type field-buffer layer  5  in the perimeter portion of a light receiving region  101  decreases. Such ion implanted region  102  functions as a guard ring. The ion implanted region  102  can be formed using H, He, N, C, O, Ar, B, or Fe.  
         [0015]     Next, the second-conductivity-type contact layer  9  is selectively removed by an etching procedure, and the passivation film  10  such as a SiN x  film used for preventing reflection is formed on the wafer surface. The passivation film  10  formed on the second-conductivity-type contact layer  9  is selectively removed. Then, the first electrode  11  is formed on the surface of the light receiving region  101  of the second-conductivity-type contact layer  9 , and the second electrode  12  is formed on the surface of the second-conductivity-type contact layer  9  outside of the light receiving region  101 . The first electrode  11  and the second electrode  12  are formed using an alloy such as AuZn, AuTi, AuTiPt, and AuGeNi.  
         [0016]     As illustrated in  FIG. 1 ( b ), on the avalanche photodiode device, a first-electrode bonding pad  11   a  and a second-electrode bonding pad  12   a  are provided for connecting to exterior circuits the first electrode  11  and the second electrode  12 , respectively. The first electrode  11  and the first-electrode bonding pad  11   a  are connected with a first-electrode lead  11   b , and the second electrode  12  and the second-electrode bonding pad  12   a  are connected with a second-electrode lead  12   b . The main portion of the avalanche photodiode illustrated in  FIG. 1  is completed with above described procedures.  
         [0017]     The avalanche photodiode according to the present invention illustrated in  FIG. 1  has n-p-n structure in between the first electrode  11  and the second electrode  12 , where reverse bias voltage is applied. The first “n” is formed by the second-conductivity-type contact layer  9  in the light receiving region  101 , on which the first electrode  11  is provided, and the second-conductivity-type cap layer  8  in the light receiving region  101 . The last “n” is formed by the second-conductivity-type contact layer  9  outside of the light receiving region  101 , on which the second electrode  12  is provided, and the second-conductivity-type cap layer  8  outside of the light receiving region  101 . The “p” is formed by the first-conductivity-type field-buffer layer  5 . With the n-p-n structure created between the first electrode  11  and the second electrode  12 , the selective thermal diffusion process of p-type impurities into an n-type region, an essential process in conventional general p-n structured avalanche photodiodes, can be omitted, thereby simplifying the process significantly. The accurate control of the selective thermal diffusion is difficult, and the impurities once having been diffused may diffuse again in a later heat process. So the conventional avalanche photodiodes with p-n structure is extremely difficult to manufacture in a high yield.  
         [0018]     In the avalanche photodiode according to the present invention, a portion of the second-conductivity-type cap layer  8  having the first electrode  11 , which forms the first “n”, and a portion of the second-conductivity-type cap layer  8  having the second electrode  12 , which forms the second “n”, are separated by the ring-shaped trench  13 . The p-type layer of the first-conductivity-type field-buffer layer  5 , which forms “p”, is provided underneath these n-type layers. This n-p-n configuration does not require selective thermal diffusion process of the p-type impurities. Therefore, high yield manufacturing can be realized.  
         [0019]      FIGS. 5-7  are diagrams illustrating device characteristics of the avalanche photodiode according to the embodiment.  FIG. 5  represents characteristics of photo-current I photo , dark current I dark , and their magnification M in response to reverse bias voltage −V b . As represented in  FIG. 5 , in the avalanche photodiode according to the embodiment, neither the edge break-down nor the tunnel break-down occurs until the reverse bias voltage −V b  reaches in the proximity of 27 V. In addition, the avalanche magnification M of approximately 50 times is realized. That is, the device characteristics represented in  FIG. 5  indicates that the avalanche photodiode according to the present invention employing the n-p-n structure has an excellent performance. This result also shows that the guard ring of the ion implanted region  102  functions enough.  
         [0020]      FIG. 6  is a diagram showing S11 parameters of the avalanche photodiode with conventional p-n structure and the avalanche photodiode with the n-p-n structure according to this invention. In  FIG. 6 , the vertical axis represents the S11 parameter, and the horizontal axis represents frequency. The electrode capacity of the equivalent circuit constructed based on the characteristics illustrated in  FIG. 6  proved that the electrode pad capacity in the avalanche photodiode according to this embodiment can be reduced by approximately 20%, compared with conventional structure one. This means that the avalanche photodiode according to this embodiment has excellent high-frequency characteristics.  
         [0021]      FIG. 7  is a diagram representing reliability test results of the avalanche photodiode according to this embodiment, which is operated with a 100 μA constant-current at 175° C. In  FIG. 7 , the horizontal axis represents operation time, and the vertical axis represents dark current values when the magnification M becomes  10 . As illustrated in  FIG. 7 , after the dark current values of the device drastically decrease at the initial aging step and stabilize, the device operates normally. The activation energy of the avalanche photodiode mainly composed of InP is estimated to be equal to or higher than 1.0 eV. This value is equivalent to an operation time of one million hours at operational temperature of 50° C. This result represents that the n-p-n structured avalanche photodiode according to this embodiment has adequate reliability in practical use.  
         [0022]     In the avalanche photodiode illustrated in  FIG. 1 , a light is incident from the upper side of the semiconductor substrate  1 , however, the avalanche photodiode maybe structured so that the light is incident from back face side of the semiconductor substrate  1 . In this case, the first electrode  11  does not need to be connected from the light receiving region  101  to the outside of the ring-shaped trench  13 , so the flip-chip bonding method becomes applicable.  
       Embodiment 2  
       [0023]      FIGS. 9-12  are diagrams illustrating other configurations of the avalanche photodiode illustrated in  FIG. 1 .  
         [0024]     In a device structure illustrated in  FIG. 9 , an exterior ring-shaped trench  15  is provided around the second electrode  12 . In a device structure illustrated in  FIG. 10 , a trench  16  is provided around the second-electrode bonding pad  12   a . In a device structure illustrated in  FIG. 11 , a ring-shaped trench  17  is provided around the first-electrode bonding pad  11   a . In a device structure illustrated in  FIG. 12 , a wiring trench  18  is provided along both the sides of the first-electrode lead  11   b.    
         [0025]     Each of the trenches illustrated in  FIGS. 9-12  is formed by removing the second-conductivity-type contact layer  9  and the second-conductivity-type cap layer  8  with a selective etching process. By forming each of the trenches and the ring-shaped trench  13  at the same time, the manufacturing process becomes simpler. The effective electrode capacity can be further reduced by forming the above described trenches, so a higher-speed operation can be realized.  
         [0026]     Either A p-type semiconductor substrate or an n-type semiconductor substrate can be used for the semiconductor substrate  1 . When a p-type semiconductor substrate is used, the second electrode  12  outside of the light receiving region  101  may be provided on the back face side of the semiconductor substrate  1  without providing the n +  type InP buffer layer  2 . When an n-type semiconductor substrate is used, the second electrode  12  outside of the light receiving region may be provided on the back face side of the semiconductor substrate  1  without providing the p +  type InP buffer layer  3 . In the avalanche photodiode according to the present invention, the ionizing rate of electrons in the avalanche multiplication layer  6  should be higher than that of holes. The avalanche multiplication layer  6  can be formed with a superlattice structure composed of Al x Ga y In 1-x-y , Ga x In 1-x As y F 1-y , a compound semiconductor including antimony (Sb), or their mixture.  
         [0027]     Furthermore, by reversing the n-type layer and p-type of the avalanche multiplication layer  6 , InP which has higher ionizing rates of the holes, can be used for a p-type avalanche multiplication layer  6 .