Photo-sensing device

Abstract: A bottom-incidence type photo-sensing device has a pn junction, as a photo-sensing region (10), formed by selectively providing a first region (3) of a second conductivity tpye in a portion of a semiconductive layer (2) of a first conductivity type. The first region (3) is surrounded by a second region (4) of the second conductivity type formed in the semiconductive layer (2), and the second region (4) is of the same or larger depth as or than that of the first region (3). Even when a light is directed to outside of the photo-sensing region (10), extra charges generated therein are absorbed by the second region (4) and the flow of the extra charges into the photo-sensing region (10) is prevented.

Description: Background of the Invention (Field of the Invention) The present invention relates to a photo-sensing device used as a photo-sensor of a light communication system. (Related Background Art) Figs. 1A and 1B respectively show a structure of a prior art photo-sensing device. Fig. 1A shows a top view and Fig. 1B shows an X-X sectional view. As shown, in the prior art photo-sensing device, an electrode 8 having a center opening through which light is directed is formed on an underside of a semiconductor substrate 1 of a first conductivity type which is transparent to the light to be detected, and an anti-reflection film 9 is formed in the opening. A semiconductive crystal layer 2 of the first conductivity type for absorbing the incident light is formed on the surface of the semiconductor substrate 1. The semiconductive crystal layer 2 is lamination of a buffer layer 2a, a photo-sensing layer 2b and a cap layer 2c in sequence. Impurities are selectively diffused into the semiconductive crystal layer 2 to form a first region 3 of a second conductivity type. This is a pin photo-diode structure where the semiconductor substrate 1 is an n layer (or a p layer), the semiconductive crystal layer 2 is an i layer and the first region 3 is a p layer (or an n layer), and a photo-sensing region 10 is formed in the i layer. An electrode 5 for taking out a photo-current is formed on the first region 3, the surface of the semiconductive crystal layer 2 around the electrode 5 is covered with a protection film (i.e. passivation film) 7. When a reverse bias is applied to the semiconductor device thus constructed, a depletion layer is created in a pn junction area in the semiconductive crystal layer 2. Thus, an electric field is developed in the depletion layer and electrons and holes generated by a light applied to the photo-sensing region 10 are directed to the semiconductor substrate 1 and the first region 3 of different conductivity types, respectively, and accelerated thereby. In this manner, a photo-current is take out and a light signal is detected. A similar structure of a photo-sensing device to that described above is disclosed in U.S.P. 4,093,174 (issued on '84,05.11). In the structure shown in Figs. 1A and 1B, when the light is applied to the photo-sensing region 10, light generating carriers are captured by the depletion layer and a good response characteristic is offered. However, when the light is directed to the outside of the region 10, the generated carriers reach the pn junction while they are diffused by a density gradient and are taken out as a photo-current. As a result, the response characteristic is adversely affected. Fig. 2A shows a response characteristic of the photo-sensing device. Since the movement of the carriers by the diffusion is slow, a response waveform to a light pulse includes a tail at the end as shown in Fig. 2A. When such a photo-sensing device is used for the light communication, a light emitted from an optical fiber is focused so that it is directed to the photo-sensing region 10. However, when a portion of light leaks out of the photo-sensing region 10, it leads to the reduction of the response speed of the photo-sensing device by the reason described above. In a high speed photo-sensing device, the area of the photo-sensing region 10 is reduced for reduction of a junction capacitance. As a result, a ratio of light directed to the outside of the photo-sensing region 10 increases and a diffused component which has a low response speed increases. This leads to the degradation of the response speed. A rear-entry type structure as shown is suitable for use as a high response speed device because of a small junction area. However, when it is coupled to a single mode fiber (core diameter 10µm), a portion of an incident light leaks to the outside of the photo-sensing region 10 due to a deviation of an optical axis or an aberration of a lens. The length of diffusion of carriers is in the order of several tens µm (approximately 40 µm in the n-type indium-gallium-arsenide layer with the indium-phosphide cap layer), and the carriers generated at a distant point from the pn junction also contribute to the photo-current by the diffusion. This also leads to the reduction of the response speed. Besides, Japanese Journal of Applied Physics, Vol. 28, No. 10, pp. 1843 - 1846 discloses a photodiode which is constituted by a plurality of compound semiconductor layers. Furthermore, IEE PROCEEDINGS, Vol.137. Pt.J., No. 3, June 1990, pages 171-173 discloses a high quantum efficiency photodiode which comprises a first p + region in a semiconductor substrate and a second p + region in the semiconductor substrate. The first p + region includes a "π type layer" (a shallow p + region) with an area of 5.3 mm 2 and a depth of 0.1 µm. This π type layer is also called the "active region". The π type layer is surrounded by a "guard ring" with a junction depth about 1,5 µm. Fig.1 shows, that they are directly connected. Outside the guard ring, there is a second p + region surrounding the first p + region, spaced from the first p + region and called the "separate annular diode", this second p + region is formed "to make a separate annular diode for dispersing the surface leakage current. The problem to be solved with the above-mentioned photodiode is to provide a high efficiency photodiode for short wavelengths, particularly for UV light. Summary of the Invention It is an object of the present invention to provide a photo-sensing device which has an improved response speed. This object is solved by the features described in claim 1. Particular embodiments of the invention are set out in the dependent claims 2 and 3. Accordingly, even if the light is directed to the outside of the photo-sensing region, the charges are absorbed by the second region and the flow of the charges into the photo-sensing region is prevented. Thus, only the required photo-current is taken out to an external circuit. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Brief Description of the Drawings Figs. 1A and 1B respectively show a structure of a prior art photo-sensing device; Figs. 2A and 2B respectively show a light pulse response characteristic measured for a prior art structure and the structure of the present invention; Figs. 3A and 3B respectively show a basic structure of a photo-sensing device of the present invention, Figs. 4A and 4B show a structure of a photo-sensing device in accordance with a further embodiment of the present invention, and Fig. 5 shows a band gap energy chart of the pin structure. Description of the Preferred Embodiment The embodiments of the present invention are now explained with reference to the drawings. Figs. 3A and 3B respectively show a basic structure of a photo-sensing device of the present invention. Fig. 3A shows a top view and Fig. 3B shows an X-X sectional view. As shown, an n electrode 8 having a center opening through which a light is directed is formed on an underside of substrate 1 of a heavily doped first conductivity type, and the opening is covered by an anti-reflection film 9 to eliminate a reflection loss of the light. A lightly doped first conductivity type photo-sensing layer 2 is formed on the surface of the semiconductor substrate 1. A first region 3 of a second conductivity type is formed on the surface of the photo-sensing layer 2 at a position facing the opening, by the selective diffusion using a sealed ampoule method (impurity doping method using a sealed silica tube including a semiconductor wafer and an impurity material), and a second region 4 of a second conductivity type is formed in a similar manner with a spacing of 5µm from the first region 3 so as to surround the first region 3. The deeper the second region 4 becomes than the first region 3, the higher a capture effect for extra charges is, because the extra charges are generated in the photo-sensing layer outside the photo-sensing region and diffuse into the photo-sensing region 10 by the density gradient. Accordingly, the second region 4 may be deep enough to reach the buffer layer 2a. However, it is not necessary to be so deep. For example, when the first region 3 and the second region 4 are simultaneously formed, they are of the same depth. Even in this case, the same effect as that of the present embodiment is attained. However, when the second region 4 is shallower than the first region 3, the above effect becomes lower. A p-electrode 5 is formed on the first region 3 on the surface of the photo-sensing layer 2, and an auxiliary electrode 6 to which a reverse voltage is applied is formed on a portion of the second region 4 to take out the charges collected to the second region 4. The surface of the photo-sensing layer 2, excluding the electrode 5 and 6, is covered with a device protection film 7. In the structure such as the above embodiment in which the semiconductive crystal layer 2 is used as an i layer of a pin photo-diode, generally, the addition of the impurities is not performed in the crystal growth. But the semiconductive crystal layer may become the first conductive type semiconductor layer by locally stoichiometric shift in some kind of the material or by mixture of the impurities from a crystal growing apparatus etc. Further, in order to improve the electrical characteristics of an device, the impurities maybe added in formation of the semiconductor crystal layer 2. Therefore, in the present application, the meaning of "lightly doped" also includes a case that "the intentional addition of the impurities is not performed". A second embodiment of the present invention is now explained with reference to Figs. 4A and 4B. Fig. 4A shows a top view and Fig. 4B shows an X-X sectional view. Like in the first embodiment, an n-electrode 8 having a light incidence opening is formed on an underside of an n + -type InP (Indium-phosphide) substrate 1 (n = 2 x 10 18 cm -3 ), and the opening is covered with an anti-reflection film 9. A non-doped InP buffer layer 2a (n = 2 x 10 15 cm -3 , thickness 2 µm), a non-doped InGaAs (Indium-gallium-arsenide) photo-sensing layer 2b (n = 2 x 10 15 cm -3 , thickness 3.5 µm) and a lightly doped InP cap layer 2c (n = 2 x 10 16 cm -3 , thickness 1 µm) are sequentially laminated, as a semiconductor crystal layer 2, on the n + -type InP substrate 1. A first region 3 and a second region 4 of p-type are formed by the selective diffusion of Zn using the sealed ampoule method. The diameter of the first region 3 is 100 µm, and the width of the n-type region between the region 3 and the surrounding region 4 is 10 µm. A p electrode 5 is formed on the first region 3 on the semiconductive crystal layer 2, and an auxiliary electrode 6 for taking out the charges collected to the second region 4 is formed on a portion of the second region 4. The periphery thereof is covered with a device protection film 7. Since the cap layer 2c is formed by the material having a wider band gap than that of the photo-sensing layer 2b, a surface leakage current is minimized. Further, since undesired charges are absorbed by the region 4, only the current required for the detection of the light signal is taken out. A band gap energy chart of the pin structure is shown in Fig. 5. In the present embodiment, the composition of InGaAs of the photo-sensing layer 2 is In 0.53 Ga 0.47 As. This composition provides the smallest band gap energy, that is, can sense the longest wavelength light among the InGaAsP semiconductors which can be lattice-matched with the InP layer. It is preferable that the thickness of the photo-sensing layer 2 is between 1µm and 7 µm to attain efficient absorption of the incident light, although it is not limited thereto. It is further preferable that the spacing between the first region 3 and the second region 4 is between 5µm and 30µm in order to attain good response characteristic and electrical characteristic, although it is not limited thereto. In the photo-sensing device of the above structure, charges generated by the light directed to the outside of the photo-sensing area 10 are collected by the depletion layer created by the second region 4. Accordingly, only the photo-current enquired for the detection of the light signal is taken out. The response speed of the photo-sensing device was measured. Figs. 2B shows a light pulse response characteristic measured in the pin photo diode of the present invention as shown in Figs. 4A and 4B. As shown in Fig. 2B, the end of the waveform includes no tail and no degradation of the response speed by the stray light directed to the outside of the photo-sensing region 10 was confirmed. In the present embodiment, the incident light has a wavelength of 1.3µm. Since the photo-sensing layer 2 is the In 0.53 Ga 0.47 As layer, the same effect is attained for an incident light of a long wavelength such as 1.55µm. A structure for absorbing the undesired charges by an impurity layer is disclosed in Japanese Laid-Open Patent Application No. 53-96719/1978. It teaches the provision of the impurity layer in order to prevent interference between a photo-sensing device and a scan circuit in an image sensor. However, no undesired charge absorbing region is formed around the photo-sensing device and the improvement of the response speed is not attained. The semiconductor materials and the dimension thereof are examples and they are changed depending on applications and wavelengths. For example, the semiconductor materials may be compound semiconductors such as GaAs (gallium-arsenide), AlGaAs (aluminum-gallium-arsenide), CdTe (cadmium-telluride), HgCdTe (mercury-cadmium-telluride), InSb (indium-antimonide), or Si (silicon) or Ge (germanium). The impurities to be selectively diffused may be Be (beryllium) or Cd (cadmium). The impurity diffusion to form the first and second regions 3 and 4 may be done by an ion implantation method. In accordance with the present invention, the charges generated by the light directed to the outside of the photo-sensing region are collected by the simple structure of forming the region of the second conductivity type in the rear-entry type photo-sensing device to surround the photo-sensing region, and the degradation of the response speed is prevented. It should be noted that a backward voltage is applied between said first region 3 and said semiconductive layer 2.

Claims: A rear-entry type photo-sensing device for receiving light on a bottom surface thereof, said device comprising: a semiconductor substrate (1); a semiconductor layer (2) of a first conductivity type provided on a first main surface of said semiconductor substrate (1); a first region (3) of a second conductivity type provided in said semiconductor layer (2), said semiconductor layer (2) and said first region (3) being in contact to define a pn junction which provides a photo-sensing region (10); and a second region (4) of said second conductivity type surrounding said first region (3), being spaced apart from said first region (3), and being deeper than said first region (3). A rear-entry type photo-sensing device according to claim 1, further comprising: a first electrode (8) located on a second main surface of said semiconductor substrate (1) and having a window (9) for passing light incident on the second main surface side of said semiconductor substrate (1) to said pn junction; and a second electrode (5) in electrical communication with said first region (3), wherein light passing through said window (9) from said second main surface side of said semiconductor substrate (1) is incident on said photo-sensing region (10) and is converted into an electric signal. A rear-entry type photo-sensing device according to claim 1 or 2, wherein the depth of said first region (3) is essentially constant in a direction from said first main surface of said semiconductor substrate (1) to said second main surface thereof.

Inventors: TONAI ICHIRO, TONAI, ICHIRO, Tonai, Ichiro, c/o Osaka Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1991
Main classification: H01L31
IPC codes: H01L31