Semiconductor photo-detector having a two-stepped impurity profile

Abstract: A semiconductor photo-detector having a two-stepped impurity profile comprises a semiconductor substrate (10), a light absorption layer (14) of a first conductivity type formed on a semiconductor substrate, a multiplication layer (18) of a first conductivity type formed on the light absorption layer to multiply a photocurrent, a semicortductor region (22) of a second conductivity type formed on the mtiitiplication layer and constituting an abrupt pn lunction with the multiplication layer, and a guard ring area (38) of a second conductivity type formed around a periphery of the semiconductor region, whereby the carrier concentration profile of the guard ring region is sharp at its surface and flat below that surface.

Description: This invention relates to a semiconductor photo-detector for use, for example, in an optical communication system. A p⁺in⁺ type photodiode and a so-called avalanche photodiode (APD) for allowing a light-induced carrier to be multiplied through the application of a reverse bias to a pn junction are known as a semiconductor photo-device. In order to properly employ these photo-diodes as a photo-detector it is required that the concentration gradient and concentration profile in a semiconductor layer be controlled in a proper fashion. In a planar type APD, for example, a photo-detecting section is so formed as to provide an abrupt junction and guard ring section for preventing edge breakdown is so formed as to provide a graded junction. In this connection it is known that the lower the background carrier concentration level, the greater the curvature radius of a diffusion layer in a planar device and the lower the carrier concentration gradient, then the greater the breakdown voltage of the graded junction. This is disclosed in IEEE Transactions on Electronic Devices, vol. ED-16, No.11, P924. The carrier concentration of a multiplication layer is normally of the order of 1 to 3 x 10¹⁶/cm³. When the concentration gradient of a diffusion layer is reduced below 2 x 10²⁰/cm⁴, then a breakdown voltage becomes greater abruptly, and does not depend on the curvature radius of the diffusion layer and a fluctuation in the background concentration. In the conventional InP type APD, however, in order to form an abrupt pn junction at the photo-detecting section, cadmium, for example, was diffused at high temperature for a brief period. In order to form a graded pn junction at the guard ring section, zinc or cadmium, for example, was diffused as a P type impurity into an n type InP multiplication layer at a lower temperature for a long time period or beryllium was ion implanted followed by a heat treatment. In these methods it has been difficult to adequately attain a greater withstand voltage difference between the guard ring section and the photo-detecting section. In Fig. 2, the broken line shows a carrier concentration profile of a device obtained when beryllium was ion implanted into an InP substrate at an acceleration voltage of 150 KeV and dosage of 5 x 10¹³/cm² and the resultant structure was annealed under a partial pressure of phosphorus within an ampoule. This carrier concentration profile is disclosed in "Extended Abstructs of the 45th Autumn Meeting (1984) of the Japan Society of Applied Physics," 13a-K-6 (P. 568). In the semiconductor photo-detector fabricated by this method, the carrier concentration gradient is reduced to as small as 0.6dec./µm in semilogarithmic scale with the concentration as a common logarithm. In the neighborhood of 1 x 10¹⁶/cm³ in background carrier concentration the concentration gradient becomes around 1 x 10²⁰/cm⁴, but with gradual approximation to a surface the concentration was increased exponentially with the result that the withstand voltage of the guard ring layer in the device was not adequate. In a P⁺in⁺type photodiode, in order to obtain a high-speed response characteristic with a depletion layer adequately extended, the carrier concentration of the i region (intrinsic region) is selected to be below 1 x 10¹⁶ cm⁻³ normally at a temperature of 20°C (this temperature is referred as such unless otherwise indicated to the contrary). In order to form an epilayer of high purity whose carrier concentration level is below 1 x 10¹⁶ cm⁻³, a baking step of a growth reactor is necessary over a longer time period for both the vapor phase growth and the liquid phase growth. Furthermore, the selection of high purity material or the purification of the material is necessary for the intended purpose. This is one cause for a rise in production costs. Within the growth reactor used in the epitaxial growth of P⁺ and n⁺ high concentration layers there exists a residual impurity. It is therefore difficult to properly control the growth of an i layer and thus to perform a continuous growth of a high purity layer and high concentration layer within the same reactor. This constitutes an obstacle to a complex photosemiconductor device, such as an opto-electronic integrated circuit (OEIC). Thus, the epitaxial growth of a high-purity layer of below 1 x 10¹⁶ cm in carrier concentration in a P⁺in⁺ type photodiode poses problems in relation to the productivity, cost, and manufacturing flexibility. A planar APD with an InGaAs(P)/InP heterostructure, and having the features defined in the pre-characterising portion of Claim 1 below, is known from Fujitsu Scientific and Technical Journal, vol.21, no.1, March 1985, pages 19-30; T. Sakurai: "Optical semiconductor devices operating in the lmm wavelength region". An object of this invention is to provide a semi-conductor photo-detector having a proper impurity concentration profile and concentration gradient. Another object of this invention is to provide a semiconductor photo-detector having a guard ring layer which can provide a greater withstand voltage difference in relation to a photo-detecting section and can be formed by a simpler manufacturing step with a better reproduction. Another object of this invention is to provide a method for manufacturing a semiconductor photo-detector having a proper impurity concentration profile and concentration gradient. In order to attain the aforementioned objects, the invention provides a semiconductor photo-detector as defined in claim 1, and a method of manufacturing a photo-detector as defined in claim 3. With the benefit of this invention, an APD can be implemented which has a heretofore-unattainable high withstand voltage difference between the photo-detecting section and the guard ring section. In an APD embodying the invention and having an n type InP layer as the multiplication layer a pn junction of the photo-detecting section is formed as an abrupt junction through the thermal diffusion of cadmium. The guard ring layer is formed through the implantation of a magnesium ion so that its carrier concentration gradient is relatively flat, below 0.2 dec./µm in semi-logarithmic representation, and the pn junction of the guard ring layer is located at that flat area. It is thus possible to obtain an APD having a withstand voltage difference of over 100V between the photo-detecting section and the guard ring section. Also in a preferred embodiment of this invention, a carrier concentration profile is obtained by implanting a magnesium ion under some condition into an n type InP layer of Group III-V compound semiconductor and performing a heat treatment. According to the method of this invention a guard ring layer of an adequately high withstand voltage can be obtained through a single ion implantation step and subsequent heat treatment step. The withstand voltage of the guard ring layer depends on neither its curvature radius nor the background concentration level owing to a lower concentration gradient. Thus this invention assures a high yield during the manufacture of photo-devices. Other objects and advantages of this invention will be apparent from the following description taken in conjunction with the following drawings in which: Fig. 1 is a cross-sectional view showing a semiconductor photo-detector according to a first embodiment of this invention; Fig. 2 is a graph showing the carrier profile of a guard ring portion of a semiconductor photo-detector of this invention and that of the prior art; Figs. 3A to 3E are views showing the steps of manufacturing a semiconductor photo-detector, according to a second embodiment of this invention; and Fig. 4 is a view showing a carrier profile of a guard ring portion of the second embodiment of this invention. Fig. 1 shows an InGaAs/InP-structured avalanche photodiode (APD) according to a first embodiment of this invention. The steps of manufacturing such an avalanche photo-diode will be explained below. n Type InP buffer layer 12, n⁻type InGaAs or InGaAsP optical absorption layer 14, n type InGaAsP buffer layer 16 and n type InP multiplication layer 18 are sequentially epitaxially grown on n⁺type InP substrate 10 in that order. InP multiplication layer 18 is 5 µ in thickness and 1 x 10¹⁶/cm³ in carrier concentration. With an insulation film such as SiO₂, metal film such as Au or the like used as masks, magnesium is ion implanted into such a wafer at an acceleration voltage of 200 KeV and dosage of 1 x 10¹⁴/cm². Then the mask is removed and over 300nm-thick (3000A-thick) PSG film is deposited by a CVD (Chemical Vapor Deposition) on the resultant structure. Then that structure is heat treated for 20 minutes at 750°C to form P type guard ring layer 20. pn-Junction 21 of guard ring layer 20 is about 4 µm in thickness deep from the surface of the structure. Then Cd is diffused by an ordinary method into the resultant structure to form P⁺ type InP layer 22 about 3 µm with p⁺n junction 24 defined relative to multiplication layer 18. Thereafter, passivation film 26 is formed which serves also as an antireflection film. The passivation film is pierced to provide an opening over which P type electrode 28 is formed. n Type electrode 30 is formed on the reverse surface of the wafer to complete an InGaAs/InP-Structured APD. Fig. 2 shows a carrier concentration profile (see solid line) of the guard ring layer in the APD of this embodiment in which case the concentration profile is very sharp at its surface and substantially flat below that surface. The carrier concentration profile is a two-stepped curve, noting that in Fig. 2 the concentration gradient at the sharp portion is over 3 decade/µm and the flat portion is below 0.2 decade/ µm in the same scale, the unit being the exponent to the base 10 of the carrier concentration in atom/cm³ per µm of depth in the guard ring. At this time a graded pn junction is formed at the flat portion of the carrier concentration profile. Let it be assumed that this pn junction is of a linear graded type. Since that background carrier concentration is 1 x 10¹⁶/cm³, the concentration gradient is below 1.5 x 10²⁰/cm⁴. In this embodiment a greater withstand voltage difference exists between the guard ring section and the photo-detecting section and thus the withstand voltage does not depend upon the curvature radius of the guard ring pn junction and fluctuation in the background carrier concentration. In actual practice, a diode is manufactured by implanting a magnesium ion, under the aforementioned condition, into the n type InP substrate of 1 x 10¹⁶/cm³ in carrier concentration in which case it has a withstand voltage of over 250 V. On the other hand, the pn junction formed through the thermal diffusion of Cd has a withstand voltage of about 130 V. Thus, the guard ring of the APD of this invention has a withstand voltage of more than 100 V as great as that obtained in the light receiving section. First, the appearance of that high withstand voltage at the guard ring layer is due to the fact that the guard ring layer has a low concentration gradient region at the depth of over 3 µm. In the second place, the high concentration region is formed at the surface of the guard ring layer and thus serves to permit a depletion layer which extends due to the application of a reverse bias to be absorbed at the surface portion, with the result that the sweep-out effect is suppressed at the surface portion. By the way, the main purpose is to form a pn junction at the flat portion of the carrier concentration profile and this structure may be made free from the high concentration region. The curvature radius of the guard ring pn junction and background carrier concentration can be determined by properly selecting the acceleration voltage and dosage of the ion implantation as well as the temperature and time of a heat treatment. In this embodiment the guard ring layer is formed through the implantation of a magnesium ion into the n type InP layer. However this invention is not restricted to this embodiment. For example, the guard ring layer may be formed with the use of Zn or Cd. As a semiconductor material use may also be made of a compound semiconductor other than InGaAs/InP. The carrier concentration profile of said guard ring area (20) has a large gradient portion at a location adjacent to the device surface and a small gradient portion below the large gradient portion, the carrier concentration gradient of said large gradient portion being above 3 decade/µm, and the carrier concentration gradient of said small gradient portion being below 0.2 decade/µm, the unit decade/µm being defined as the exponent to the base 10 of the carrier concentration in atom/cm³ per µm of depth in said guard ring area; said small gradient portion at least reaching to the level of a pn junction (21) between said guard ring area and the semiconductor layer (18) adjacent to said guard ring area. A semiconductor photo-detector according to a second embodiment of this invention will be explained below with reference to Figs. 3A to 3E and 4. Figs. 3A to 3E are cross-sections showing the steps of manufacturing a photo-detector and Fig. 4 is a graph showing a hole concentration profile when a magnesium ion is implanted into an InP crystal. As shown in Fig. 3A, n type InP layer 12 of, for example, 2 x 10¹⁶ cm⁻³ in electron density is formed by a liquid phase growth method on a major surface of n type InP substrate 10. Then with a photoresist as a mask, InP layer 12 and portion of InP substrate 10 are selectively etched as shown in Fig. 3B to provide a circular area 32 as a photo-detecting section. Then, n type InP layer 34 is formed by a liquid growth method to cover the resultant structure. The electron density of n type InP layer 34 so controlled as to obtain, for example, 6 x 10¹⁵cm⁻³. As another means for forming InP layer 34, an InP layer may be selectively grown on the etched away area and InP layer may be grown again to planarize the main surface. Then an SiO₂ and photoresist are selectively formed on the main surface of the InP layer and with these as masks magnesium ions are implanted into the resultant structure. This ion implantation was performed at, for example, an acceleration voltage of 150 KeV and dosage of 5 x 10¹³cm⁻². After the removal of the SiO₂ and photoresist, a PSG layer is formed on the major surface of the resultant structure, followed by a heat treatment for 20 minutes at 750°C for instance. As a result, P type InP layer 36 and low concentration InP layer 38 are formed simultaneously. Then as shown in Fig. 3E, Si₃N₄ film 40 is formed on the major surface at an area where the PSG layer was formed. Electrodes 42 and 44 are formed by a vacuum deposition method to complete a semiconductor photodetector. The pn junction of the so constructed photo-detector has been found to be of two types in spite of a single magnesium ion implantation step. Furthermore the photodetector so manufactured has been found to have an excellent characteristic, the reason for which will be explained below with reference to Fig. 4. The hole concentration produced in the InP crystal is found to have two types: a sharp variation portion 50 and gradual variation portion 52. Where pn junction 46 of the photo-detecting section is formed through the utilization of the portion 50, an abrupt type pn junction is provided. Where, on the other hand, pn junction 48 of the guard ring section is formed through the utilization of area 52, a graded type pn junction varying in a very gradual carrier concentration profile is formed or a pn junction is formed with a low concentration layer formed therebetween. The abrupt type pn junction 46 of the photo-detecting section is formed by P type InP layer 36 and n type InP layer 32 at a carrier concentration level of over 1 x 10¹⁶cm⁻³ but less than 1 x 10¹⁷ cm⁻³ where no adverse effect is exerted over the noise characteristic. The guard ring area is formed in InP layer 34 of 1 x 10¹⁵ to 1 x 10¹⁶cm⁻³ in carrier concentration. Not shown in the above Figure, the pn junction of the photo-detecting section and guard ring area has been confirmed, through the evaluation of an EBIC (Electron Beem Induced Current) characteristic, voltage/capacity variation characteristic and breakdown voltage characteristic, to be of an abrupt type or a type with a low concentration layer formed therebetween. The breakdown voltage difference between two such type of pn junctions was about 40V in which case the uniform breakdown of the photo-detecting section first occurred. At the pn junction of the guard ring area no local breakdown due to the concentration of an electric field applied occurred, in spite of the curvature radius of the guard ring area, thus obtaining a stable device characteristic. Fig. 4 is one form of a carrier concentration profile when a magnesium ion is implanted into the InP crystal. Then concentration profile takes a variety of forms, depending upon the conditions of the magnesium ion implantation and subsequent heat treatment. The abrupt junction and graded type pn junctions can be obtained through the utilization of sharp and flat portions of the concentration profile, respectively. According to the method and apparatus of this invention, since the carrier concentration level of the epitaxial growth layer is an ordinary value of the order of 2.5 x 10¹⁶ cm⁻³ it can satisfactorily be attained under an ordinary service condition of the growth reactor. Thus no particular consideration needs be given to the baking step at a high temperature for a longer period of time as well as to the purification step for material. It is therefore possible to obtain a semiconductor photo-detector in high quantity and at a lower cost.

Claims: A semiconductor photo-detector comprising: a semiconductor substrate (10); a light absorption layer (14) of a first conductivity type formed on the semiconductor substrate; a multiplication layer (18) of said first conductivity type, formed on the light absorption layer, for multiplying a photocurrent; a semiconductor region (22) of a second conductivity type formed on the multiplication layer and constituting an abrupt pn junction (24) with said multiplication layer; and a guard ring area (20) of a second conductivity type formed around a periphery of the semiconductor region characterised in that the carrier concentration profile of said guard ring area (20) has a large gradient portion at a location adjacent to the device surface (26) and a small gradient portion below the large gradient portion, the carrier concentration gradient of said large gradient portion being above 3 decade/µm, and the carrier concentration gradient of said small gradient portion being below 0.2 decade/µm, the unit decade/µm being defined as the exponent to the base 10 of the carrier concentration in atom/cm³ per µm of depth in said guard ring area; said small gradient portion at least reaching to the level of a pn junction (21) between said guard ring area and the semiconductor layer (18) adjacent to said guard ring area. A semiconductor photo-detector according to claim 1, characterized in that said light absorption layer (14) is InGaAs or InGaAsP, said multiplication layer (18) is InP, and said guard ring area (20) includes magnesium as an impurity. A method for manufacturing a semiconductor photo-detector, comprising the steps of: (1) sequentially epitaxially growing a light absorption layer (14) of a first conductivity type and a multiplication layer (18) of said first conductivity type on a semiconductor substrate (10); (2) selectively forming a semiconductor region (22) of a second conductivity type on the surface of the multiplication layer (18) to form an abrupt pn junction (24) therebetween; (3) implanting Mg ions as an impurity of the second conductivity type into a selected region, in which a guard ring (21) is formed, of said semiconductor region; and characterised by: (4) heat-treating the resultant structure at a temperature of 650° to 750°C to form a guard ring area (20) such that the carrier concentration profile of said guard ring area has a large gradient portion at a location adjacent to the device surface and a small gradient portion below the large gradient portion, the carrier concentration gradient of said large gradient portion being above 3 decade/µm, and the carrier concentration gradient of said small gradient portion being below 0.2 decade/µm, the unit decade/µm being defined as the exponent to the base 10 of the carrier concentration in atom/cm³ per µm of depth in said guard ring area; said small gradient portion at least reaching to the level of a pn junction (21) between said guard ring area and the semiconductor layer adjacent to said guard ring area.

Inventors: KURODA FUMIHIKO C O PATENT DIV, NAKAMURA MASARU C O PATENT DIV, SADAMASA TETSUO C O PATENT DIV, SUZUKI NOBUO C O PATENT DIVISI, KURODA, FUMIHIKO C/O PATENT DIVISION, NAKAMURA, MASARU C/O PATENT DIVISION, SADAMASA, TETSUO C/O PATENT DIVISION, SUZUKI, NOBUO C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01L31
IPC codes: H01L31, H01L21