CA1180092A - Photodiode with separate absorption and avalanche zones - Google Patents
Photodiode with separate absorption and avalanche zonesInfo
- Publication number
- CA1180092A CA1180092A CA000418122A CA418122A CA1180092A CA 1180092 A CA1180092 A CA 1180092A CA 000418122 A CA000418122 A CA 000418122A CA 418122 A CA418122 A CA 418122A CA 1180092 A CA1180092 A CA 1180092A
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- Canada
- Prior art keywords
- layer
- avalanche
- forbidden band
- substrate
- type
- Prior art date
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- 238000010521 absorption reaction Methods 0.000 title claims abstract description 21
- 239000000758 substrate Substances 0.000 claims abstract description 21
- 239000000463 material Substances 0.000 claims abstract description 18
- 238000000576 coating method Methods 0.000 claims description 6
- 239000002184 metal Substances 0.000 claims description 6
- 239000011248 coating agent Substances 0.000 claims description 4
- 229910005542 GaSb Inorganic materials 0.000 claims description 3
- 239000013078 crystal Substances 0.000 claims description 3
- 230000009102 absorption Effects 0.000 claims 3
- 230000005684 electric field Effects 0.000 abstract description 16
- 230000003321 amplification Effects 0.000 abstract description 7
- 238000003199 nucleic acid amplification method Methods 0.000 abstract description 7
- 230000000694 effects Effects 0.000 abstract description 5
- 238000001514 detection method Methods 0.000 abstract description 4
- 230000007423 decrease Effects 0.000 abstract description 2
- 230000003287 optical effect Effects 0.000 abstract description 2
- 239000010410 layer Substances 0.000 description 64
- 238000010586 diagram Methods 0.000 description 5
- 239000004065 semiconductor Substances 0.000 description 5
- 238000001465 metallisation Methods 0.000 description 4
- 238000009792 diffusion process Methods 0.000 description 3
- 239000013307 optical fiber Substances 0.000 description 3
- 238000013459 approach Methods 0.000 description 2
- 150000001875 compounds Chemical class 0.000 description 2
- 238000002513 implantation Methods 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 230000003595 spectral effect Effects 0.000 description 2
- 229910001218 Gallium arsenide Inorganic materials 0.000 description 1
- 229910000530 Gallium indium arsenide Inorganic materials 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 239000002800 charge carrier Substances 0.000 description 1
- 238000000151 deposition Methods 0.000 description 1
- 230000008021 deposition Effects 0.000 description 1
- 239000002019 doping agent Substances 0.000 description 1
- 238000009499 grossing Methods 0.000 description 1
- 230000031700 light absorption Effects 0.000 description 1
- 230000004807 localization Effects 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- 230000010287 polarization Effects 0.000 description 1
- 238000001179 sorption measurement Methods 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
- H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
- H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
- H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier
- H01L31/107—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier the potential barrier working in avalanche mode, e.g. avalanche photodiode
- H01L31/1075—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier the potential barrier working in avalanche mode, e.g. avalanche photodiode in which the active layers, e.g. absorption or multiplication layers, form an heterostructure, e.g. SAM structure
Abstract
ABSTRACT OF THE DISCLOSURE
The invention relates to a photodiode made from materials from group III-V, whose detection effect by photon absorption is amplified by an avalanche, which occurs in a layer separate from the absorption layer.
In order to obtain a low amplification noise, it is necessary that the ionization coefficients differ significantly for the electrons and the holes, i.e. the electric field is weak. This field must be exercised over a considerable length to obtain a high gain. The diode according to the invention comprises a substrate, an absorption layer, a field decrease layer, an avalanche layer, all having the same conductivity type, as well as a contact layer with the opposite conductivity type. The field reducing layer and the avalanche layer both satisfy the condition "concentration x length = constant".
Application to photodetecting diodes in telecommunications by optical fibres.
(Fig 2).
The invention relates to a photodiode made from materials from group III-V, whose detection effect by photon absorption is amplified by an avalanche, which occurs in a layer separate from the absorption layer.
In order to obtain a low amplification noise, it is necessary that the ionization coefficients differ significantly for the electrons and the holes, i.e. the electric field is weak. This field must be exercised over a considerable length to obtain a high gain. The diode according to the invention comprises a substrate, an absorption layer, a field decrease layer, an avalanche layer, all having the same conductivity type, as well as a contact layer with the opposite conductivity type. The field reducing layer and the avalanche layer both satisfy the condition "concentration x length = constant".
Application to photodetecting diodes in telecommunications by optical fibres.
(Fig 2).
Description
PHOTODIODE ~ITH SE,PARATE ABSORPTION AND AVALANCHE ZONES
BACKGROUND OF ~H;E INVENTION
~ he present invention relates to a photodiode with separate absorption and avalanche zones, operating in the range 1.2 - 1.6 microns. It is made from materi-als in groups III-V and has a high avalanche gain, although the noise factor is low.
The invention more specifically relates to the structure of semiconductor layers forming an avalanche photodiode. The optimization of this structure, by geometrical modifications of the layers or contacts for example forms part of the knowledge of the art and certain improvements to avalanche photodiodes are already known, e.g. from Canadian Patents 1 106 483, 15 issued August ~, 1981 and 1 125 423, issued June 8, 1982 to the present Applicant.
The invention will be described with reference to the example of a photodiode having two InP/GaAs hetero-junctions. However, other materials such as GaAsSb, GaAlAsSb and GaSb fall within the scope of the in-vention, provided that there is reciprocal matching of the crystal meshes.
An avalanche photodiode is a semiconductor device, adding to the photodetector effect an amplifier effect due to the avalanche. This type of device is widely used in the field of telecommunications by opti-cal fibres.
~ -1- ~
~8~
A photodiode with separate absorption and avalanche zones comprises, inter alia, a photon ab-sorption layer in which the incident photons create -la-. :
excitons. Under the effect of a reverse bias in a layer having a doping concentration and a thickness such that it is at least partly in space charge or the diffusion length of the carriers is high, a current is produced by exchange between the electrons and the holes. The polariza~ion potential applied is assumed to be adequate, so that the electrons travel towards the first region forming a first heterojunction with the absorption layer, whilst the holes travel in the opposite direction towards a second region forming a second heterojunction with the absorption layer. In this second region, the avalanche phenomenon is initiated as a result of a powerful electric field.
The particular interest of initia~ing the avalanche by the holes in the case o~ group III-V
materials and especially InP is that the noise resulting from the amplification is lower than in the case when it is initiated by electrons.
It is now well known that for producing avalanche photodiodes based on III-V compounds~ in the spectral range 1.2 - 1.6 micrometers, it is necessary to use heterostructures such that the powerful electric field zone ~f approximately 10 V/cm in which the avalanche is produced, occurs in a material with a large forbidden band, the absorption of light taking place in an adjacent area with a small forbidden band.
~ Moreover, the measurement of the ionization ; 30 coefficients of the charge carriers in the se~iconductor
BACKGROUND OF ~H;E INVENTION
~ he present invention relates to a photodiode with separate absorption and avalanche zones, operating in the range 1.2 - 1.6 microns. It is made from materi-als in groups III-V and has a high avalanche gain, although the noise factor is low.
The invention more specifically relates to the structure of semiconductor layers forming an avalanche photodiode. The optimization of this structure, by geometrical modifications of the layers or contacts for example forms part of the knowledge of the art and certain improvements to avalanche photodiodes are already known, e.g. from Canadian Patents 1 106 483, 15 issued August ~, 1981 and 1 125 423, issued June 8, 1982 to the present Applicant.
The invention will be described with reference to the example of a photodiode having two InP/GaAs hetero-junctions. However, other materials such as GaAsSb, GaAlAsSb and GaSb fall within the scope of the in-vention, provided that there is reciprocal matching of the crystal meshes.
An avalanche photodiode is a semiconductor device, adding to the photodetector effect an amplifier effect due to the avalanche. This type of device is widely used in the field of telecommunications by opti-cal fibres.
~ -1- ~
~8~
A photodiode with separate absorption and avalanche zones comprises, inter alia, a photon ab-sorption layer in which the incident photons create -la-. :
excitons. Under the effect of a reverse bias in a layer having a doping concentration and a thickness such that it is at least partly in space charge or the diffusion length of the carriers is high, a current is produced by exchange between the electrons and the holes. The polariza~ion potential applied is assumed to be adequate, so that the electrons travel towards the first region forming a first heterojunction with the absorption layer, whilst the holes travel in the opposite direction towards a second region forming a second heterojunction with the absorption layer. In this second region, the avalanche phenomenon is initiated as a result of a powerful electric field.
The particular interest of initia~ing the avalanche by the holes in the case o~ group III-V
materials and especially InP is that the noise resulting from the amplification is lower than in the case when it is initiated by electrons.
It is now well known that for producing avalanche photodiodes based on III-V compounds~ in the spectral range 1.2 - 1.6 micrometers, it is necessary to use heterostructures such that the powerful electric field zone ~f approximately 10 V/cm in which the avalanche is produced, occurs in a material with a large forbidden band, the absorption of light taking place in an adjacent area with a small forbidden band.
~ Moreover, the measurement of the ionization ; 30 coefficients of the charge carriers in the se~iconductor
-2-.
material show that they become increasingly asymmetrical for the electrons and the holes as the electric field decreases. However, as the noise due to amplification by avalanche increases as the ionization ccefficient ratio approaches unity, the useful gain of the known structures is limited to low values of approximately 10.
Thus, in order to be able to increase the gain of an avalanche photodiode~ it is necessary to operate with a lower electric field, so that the ionization coefficients are more asymmetrical, whilst maintaining relatively limited noise due to amplification.
BRIEF SUMMARY OF THE INVENTION.
The object of the present inventi~n is to provide a photodiode with separate absorption and avalanche areas having a high avalanche gain and simultaneously a low noise factor.
According to the invention, an appreciable gain is obtained with a weak field by increasing the length on which said field is exerted, i.e. by replacing the avalanche region of the known structures and which is not very thick, but is under a powerful electric fieldS by a longer avalanche region under a weak electric field. Thus, statistically the gain obtained by the avalanche is the same. This is obtained by 2S inserting a layer which is weakly doped and has a large forbidden band between the electric field reducing layer and the window layer or contacting layer ~y metalliæation on the body of the diode, in order that the avalanche zone extends over an adequate distance to obtain a high gain under a weak electric
material show that they become increasingly asymmetrical for the electrons and the holes as the electric field decreases. However, as the noise due to amplification by avalanche increases as the ionization ccefficient ratio approaches unity, the useful gain of the known structures is limited to low values of approximately 10.
Thus, in order to be able to increase the gain of an avalanche photodiode~ it is necessary to operate with a lower electric field, so that the ionization coefficients are more asymmetrical, whilst maintaining relatively limited noise due to amplification.
BRIEF SUMMARY OF THE INVENTION.
The object of the present inventi~n is to provide a photodiode with separate absorption and avalanche areas having a high avalanche gain and simultaneously a low noise factor.
According to the invention, an appreciable gain is obtained with a weak field by increasing the length on which said field is exerted, i.e. by replacing the avalanche region of the known structures and which is not very thick, but is under a powerful electric fieldS by a longer avalanche region under a weak electric field. Thus, statistically the gain obtained by the avalanche is the same. This is obtained by 2S inserting a layer which is weakly doped and has a large forbidden band between the electric field reducing layer and the window layer or contacting layer ~y metalliæation on the body of the diode, in order that the avalanche zone extends over an adequate distance to obtain a high gain under a weak electric
-3-.
field.
l~ore specifically, the present invention relates to a photodiode with separate absorption and avalanche zones comprising on a substrate carrying a first electric contact metal coating, a hetero-structure formed by a first light absorbing layer made from a material with a small forbidden band~
and by a second layer made from a material with a large forbidden band, the assembly of the substrate and the two heterostructure layers being of a first conductivity type, said diode also comprising a layer having a second type of conductivity and carrying a second electric contact metal coating~ wherein the avalanche phenomenon is confined in an avalanche layer with a small forbidden band and of the first type of conductivity forming a junction with the layer o~ the second type of conductivity, said avalanche layer being separated from the first light absorbing layer having a small forbidden band by the second layer of the heterostructure and having a large forbidden band and whose thickness is such that the product "doping concentration x thickness"
is below 1.3 . 102 a-toms/cm 2, whilst the layer with the large forbidden band has a thickness such that the product "doping concentration .c thickness" is sub~tantially equal to 2.5 . 1012 atoms/cm 2.
BRIEF DESCRIPTION OF THE DRAWINGS.
. . . ~
The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein show:
Fig 1 the structure and profile of the electric field for a prior art avalanche photodiode.
Fig 2 the structure and profile of the electric field for an avalanche diode according to the invention.
5 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.
Fig 1 shows the structure of a photodiode having separate absorption and avalanche zones according to the prior art. As the geome-try of such a photodiode lends itself to this, the representation of the diagram or profile of the electric field has been superimposed on the representation of the structure by semiconductor layers. With a view to simplifying the explanations, this prior art avalanche diode is made froin InP/ÇaInAs, whose different layers are of the n and p types.
Such an avalanche photodlode comprises a n~
substrate made from InP and designated 1. In reality, layer 1 is formed from the actual substrate and a buffer layer of the same material and conductivity type, said buffer layer serving to smooth the relatively rough surface of the substrate. Onto substrate 1 is deposited a ~aInAs layer 2 of conductivity n, which will be called ~, because it is a slightly doped n zone. A type n InP layer 3 forms a hetero-junction with a large forbidden band with the photon-absorbing zone, the absorption layer having a small forbidden band. Finally, a InP contact layer 4 of conductivity p~ ensures the contacting with the electrode metallization. Such a diode is completed by two electrode metallizations 5 on the side of the ~3L80~
substrate and 6 on the side of the window by which enters the light detected by the avalanche diode.
The contact metal coatings and the various layers forr.ling the structure of such a diode are shown in a highly diagrammati~ manner.
The surface region on the side o~ contact 6 constitutes a window, which can serve as a filter limiting towards short wavelengths the spectrum of`
the light received by the device used as a photo-detector Due to its high doping, it at the same timeforms a contacting region a~ its localization makes it possible to limit the absorption region of the device to the area which is strictly necessary, ~.g the very small area corresponding to the cross-section of an optical fibre or an optical fibre beam. When a photQnv penetrates the photodiode by the window formed in metal coatings 6, it traverses the surface layers 3 and 4 and is detected and absorbed in the absorption layer 2 in which it creates an exciton~
Under the action of an electric ~ield, exerted by the reverse bias applied to the diode terminals, the electrons and holes separate, the electrons being directed towards substrate 1, whilst the holes are directed towards layer 3 and the sur~ace region.
The horizontal axis of the design representing the distances or thicknesses of the layers, so that the electric ~ield E in absolute value, developed by the polarization is shown on the vertical axis.
This field is of little importance in the substrate, 0 but in absorption layer 2 it reaches a value of max.
.
'' , -' 10 V/cm, beyond which there would be a risk of thediode breaking down by the tunnel effect. This is followed in layer 3 having a large forbidden band by a rise in the field up to a maximum in which the avalanche occurs, whereas in the contacting layer ~
there is a sudden drop in the field beyond the junction between layers 3 and 4 to a very low value. The photon detection and absorption zone is represented by zone 7, whilst the avalanche zone is represented by zone 8 in the field diagram superimposed on the structural diagram of Fig 1. The thickness of the avalanche zone is approximately 100 ~.
The sudden variation of the field in layer 3 can be considered~ as a function of the particular case, lS as a rise in the field in the case of holes passing - from zone 2 to zone 3 or a drop in the field on i considering the path of the photons. However, in all cases this field varies stron~ly as a function of the distance x and consequently, as has been stated hereinbefore7 statistically there is a high noise due to the sudden amplification by avalanche. This noise increases in proportion to the approach of the ionization coefficient ratio to unity, i.e. the electric field is high.
According to the invention, the noise is reduced, whilst retaining a high gain due to the avalanche, the avalanche zone extending over an adequate distance to obtain a gain under a weak electric field. This is shown in Fig 2 representing the structural diagram of an avalanche diode according , ", ~ ' . ' .
to the invention.
Fig 2 can be compared with Fig 1 in the sense that it gives along the x axis the different semiconductor layers forming an avalanche photodiode and, superimposed along the y axis, the curve of the electric field E in absolute values in the same layers.
So as to be comparable to the aforementioned example according to the prior art, the present embodiment of the invention uses the III-V compound Ga In As, which makes it possible to cover the x l-x spectral range 1 to 1.7 micrometers, when it is adapted in crystal mesh to a InP subskrate. However, as has been stated hereinbefore, other materials are usable within the scope of the present in-~ention.
The InP substrate 1 of conductivity type n+ is formed from the actual substrate layer and a buffer layer for smoothing the substrate surface. Onto substrate 1 is epitaxially deposited in the vapour phase a layer 2, which is the Ga~Inl xAs absorption layer of composition x = 0.47 and of doping n C2.10 ionized impurity atoms/cm3, the thickness being approximately 2 microns.
This is followed by the deposition of a nl doped InP layer of thickness xl such that nlxl~2l51012 ionized impurity atoms/cm . For example, nl= 5.10 atoms/cm3 and xl= 0.5 microns. This field reducing layer is designated 9 in Fig 2 to differentiate it rom layer 3 in Fig 1. Layer 9 must satisfy condition nlx~2r51012 atoms/cm2. Onfield reducing layer 9 is deposited a n2 doped InP layer 10 and, prior to the operations to be described`hereinafter, this thick layer extends up to the contacting metallization 6. Like layer 2~ layer 10 is designated by v, which 5 indicates that it is only slightly doped.
The pn electrical junction necessary for fo~ming a diode of this type is formed by implantation or diffusion into the InP layer lO of a type p dopant through an oxide mask and up to a depth such that the - 10 following condltion is fulfilled n2.x~ 1.3 1012 atoms/
cm , x2 being the distance separating the junction `
from the interface between the two InP layers 9 and 10. This implantation or diffusion operation thus forms the type p+ layer ll of Fig 2 in the type n2 layer 10.
The ohmic contacts are then formed by ~own metallization means with a geometry permitting the transmission of light on at least one of the faces.
The ohmic contact 5 on the n+ layer, which is the substrate, is not transparen~ to light and it is the ohmic contact 6 on the p~ layer 11 which has a window for the passage of e.g. on optical fibre.
On now considering the electric field diagram related to and superimposed on this diode structure, the detection zone is, as in the case of the prior art diode, zone 7 in absor~ion layer 2. This detection zone corresponds to a maximum electric field of 105 V/cm. The avalanche zone is designated 12 and corresponds to a minimum of 4.5 x 10 V/cm.
3Q This avalanche zone extends through the InP avalanche _g_ .
~.8~
layer 10 ;n which there is little variation in the field. However, it varies over an adequate distance for the product of the amplification per distance unit multiplied by the thickness of the layer to give a large gain.
The structure described is of type nv n~ p.
However, if the materials used in forming an avalanche diode are such that the ionization coefficient of the electrons is greater than that of the holes, it is then sufficient to have a type p~p~n structure in order to obtain an avalanche diode having the sarn~
high gain and low noise characteristics in excess of the amplification. However9 as a limited noise in excess is obtained for very different ion,zation coeff;cients, it is preferable to inject in~o the avalanche zone the carrier having the highest ionization coefficient and for this reason the most general case is a n ~n ~p.
Finally, the invention has been described, for simpli~ication reasons only, with reference to the materials InP and GaInAs, but it is also possible to use other materials such as GaAISb, GaAlAsSb, GaSb.
Moreover, the structure of the diode has been described in the most general case corresponding to layers of semiconductor m~erials. However, there is nothing to prevent the diode being improved by giving the different layers fo.-ms making it possible to optimi~e the diode according to the invention and in accordance with the folL~wing claims.
- ' .
field.
l~ore specifically, the present invention relates to a photodiode with separate absorption and avalanche zones comprising on a substrate carrying a first electric contact metal coating, a hetero-structure formed by a first light absorbing layer made from a material with a small forbidden band~
and by a second layer made from a material with a large forbidden band, the assembly of the substrate and the two heterostructure layers being of a first conductivity type, said diode also comprising a layer having a second type of conductivity and carrying a second electric contact metal coating~ wherein the avalanche phenomenon is confined in an avalanche layer with a small forbidden band and of the first type of conductivity forming a junction with the layer o~ the second type of conductivity, said avalanche layer being separated from the first light absorbing layer having a small forbidden band by the second layer of the heterostructure and having a large forbidden band and whose thickness is such that the product "doping concentration x thickness"
is below 1.3 . 102 a-toms/cm 2, whilst the layer with the large forbidden band has a thickness such that the product "doping concentration .c thickness" is sub~tantially equal to 2.5 . 1012 atoms/cm 2.
BRIEF DESCRIPTION OF THE DRAWINGS.
. . . ~
The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein show:
Fig 1 the structure and profile of the electric field for a prior art avalanche photodiode.
Fig 2 the structure and profile of the electric field for an avalanche diode according to the invention.
5 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.
Fig 1 shows the structure of a photodiode having separate absorption and avalanche zones according to the prior art. As the geome-try of such a photodiode lends itself to this, the representation of the diagram or profile of the electric field has been superimposed on the representation of the structure by semiconductor layers. With a view to simplifying the explanations, this prior art avalanche diode is made froin InP/ÇaInAs, whose different layers are of the n and p types.
Such an avalanche photodlode comprises a n~
substrate made from InP and designated 1. In reality, layer 1 is formed from the actual substrate and a buffer layer of the same material and conductivity type, said buffer layer serving to smooth the relatively rough surface of the substrate. Onto substrate 1 is deposited a ~aInAs layer 2 of conductivity n, which will be called ~, because it is a slightly doped n zone. A type n InP layer 3 forms a hetero-junction with a large forbidden band with the photon-absorbing zone, the absorption layer having a small forbidden band. Finally, a InP contact layer 4 of conductivity p~ ensures the contacting with the electrode metallization. Such a diode is completed by two electrode metallizations 5 on the side of the ~3L80~
substrate and 6 on the side of the window by which enters the light detected by the avalanche diode.
The contact metal coatings and the various layers forr.ling the structure of such a diode are shown in a highly diagrammati~ manner.
The surface region on the side o~ contact 6 constitutes a window, which can serve as a filter limiting towards short wavelengths the spectrum of`
the light received by the device used as a photo-detector Due to its high doping, it at the same timeforms a contacting region a~ its localization makes it possible to limit the absorption region of the device to the area which is strictly necessary, ~.g the very small area corresponding to the cross-section of an optical fibre or an optical fibre beam. When a photQnv penetrates the photodiode by the window formed in metal coatings 6, it traverses the surface layers 3 and 4 and is detected and absorbed in the absorption layer 2 in which it creates an exciton~
Under the action of an electric ~ield, exerted by the reverse bias applied to the diode terminals, the electrons and holes separate, the electrons being directed towards substrate 1, whilst the holes are directed towards layer 3 and the sur~ace region.
The horizontal axis of the design representing the distances or thicknesses of the layers, so that the electric ~ield E in absolute value, developed by the polarization is shown on the vertical axis.
This field is of little importance in the substrate, 0 but in absorption layer 2 it reaches a value of max.
.
'' , -' 10 V/cm, beyond which there would be a risk of thediode breaking down by the tunnel effect. This is followed in layer 3 having a large forbidden band by a rise in the field up to a maximum in which the avalanche occurs, whereas in the contacting layer ~
there is a sudden drop in the field beyond the junction between layers 3 and 4 to a very low value. The photon detection and absorption zone is represented by zone 7, whilst the avalanche zone is represented by zone 8 in the field diagram superimposed on the structural diagram of Fig 1. The thickness of the avalanche zone is approximately 100 ~.
The sudden variation of the field in layer 3 can be considered~ as a function of the particular case, lS as a rise in the field in the case of holes passing - from zone 2 to zone 3 or a drop in the field on i considering the path of the photons. However, in all cases this field varies stron~ly as a function of the distance x and consequently, as has been stated hereinbefore7 statistically there is a high noise due to the sudden amplification by avalanche. This noise increases in proportion to the approach of the ionization coefficient ratio to unity, i.e. the electric field is high.
According to the invention, the noise is reduced, whilst retaining a high gain due to the avalanche, the avalanche zone extending over an adequate distance to obtain a gain under a weak electric field. This is shown in Fig 2 representing the structural diagram of an avalanche diode according , ", ~ ' . ' .
to the invention.
Fig 2 can be compared with Fig 1 in the sense that it gives along the x axis the different semiconductor layers forming an avalanche photodiode and, superimposed along the y axis, the curve of the electric field E in absolute values in the same layers.
So as to be comparable to the aforementioned example according to the prior art, the present embodiment of the invention uses the III-V compound Ga In As, which makes it possible to cover the x l-x spectral range 1 to 1.7 micrometers, when it is adapted in crystal mesh to a InP subskrate. However, as has been stated hereinbefore, other materials are usable within the scope of the present in-~ention.
The InP substrate 1 of conductivity type n+ is formed from the actual substrate layer and a buffer layer for smoothing the substrate surface. Onto substrate 1 is epitaxially deposited in the vapour phase a layer 2, which is the Ga~Inl xAs absorption layer of composition x = 0.47 and of doping n C2.10 ionized impurity atoms/cm3, the thickness being approximately 2 microns.
This is followed by the deposition of a nl doped InP layer of thickness xl such that nlxl~2l51012 ionized impurity atoms/cm . For example, nl= 5.10 atoms/cm3 and xl= 0.5 microns. This field reducing layer is designated 9 in Fig 2 to differentiate it rom layer 3 in Fig 1. Layer 9 must satisfy condition nlx~2r51012 atoms/cm2. Onfield reducing layer 9 is deposited a n2 doped InP layer 10 and, prior to the operations to be described`hereinafter, this thick layer extends up to the contacting metallization 6. Like layer 2~ layer 10 is designated by v, which 5 indicates that it is only slightly doped.
The pn electrical junction necessary for fo~ming a diode of this type is formed by implantation or diffusion into the InP layer lO of a type p dopant through an oxide mask and up to a depth such that the - 10 following condltion is fulfilled n2.x~ 1.3 1012 atoms/
cm , x2 being the distance separating the junction `
from the interface between the two InP layers 9 and 10. This implantation or diffusion operation thus forms the type p+ layer ll of Fig 2 in the type n2 layer 10.
The ohmic contacts are then formed by ~own metallization means with a geometry permitting the transmission of light on at least one of the faces.
The ohmic contact 5 on the n+ layer, which is the substrate, is not transparen~ to light and it is the ohmic contact 6 on the p~ layer 11 which has a window for the passage of e.g. on optical fibre.
On now considering the electric field diagram related to and superimposed on this diode structure, the detection zone is, as in the case of the prior art diode, zone 7 in absor~ion layer 2. This detection zone corresponds to a maximum electric field of 105 V/cm. The avalanche zone is designated 12 and corresponds to a minimum of 4.5 x 10 V/cm.
3Q This avalanche zone extends through the InP avalanche _g_ .
~.8~
layer 10 ;n which there is little variation in the field. However, it varies over an adequate distance for the product of the amplification per distance unit multiplied by the thickness of the layer to give a large gain.
The structure described is of type nv n~ p.
However, if the materials used in forming an avalanche diode are such that the ionization coefficient of the electrons is greater than that of the holes, it is then sufficient to have a type p~p~n structure in order to obtain an avalanche diode having the sarn~
high gain and low noise characteristics in excess of the amplification. However9 as a limited noise in excess is obtained for very different ion,zation coeff;cients, it is preferable to inject in~o the avalanche zone the carrier having the highest ionization coefficient and for this reason the most general case is a n ~n ~p.
Finally, the invention has been described, for simpli~ication reasons only, with reference to the materials InP and GaInAs, but it is also possible to use other materials such as GaAISb, GaAlAsSb, GaSb.
Moreover, the structure of the diode has been described in the most general case corresponding to layers of semiconductor m~erials. However, there is nothing to prevent the diode being improved by giving the different layers fo.-ms making it possible to optimi~e the diode according to the invention and in accordance with the folL~wing claims.
- ' .
Claims (5)
1. A photodiode with separate absorption and avalanche zones comprising on a substrate carrying a first electric contact metal coating? a hetero-structure formed by a first light absorbing layer made from a material with a small forbidden band, and also by a second layer made from a material with a large forbidden band, the assembly of the substrate and the two heterostructure layers being of a first conductivity type, said diode also comprising a layer having a second type of conductivity and carrying a second electric contact metal coating, wherein the avalanche phenomenon is confined in an avalanche layer with a small forbidden band and of the first type of conductivity forming a junction with the layer of the second type of conductivity, said avalanche layer being separated from the first light absorbing layer having a small forbidden band by the second layer of the heterostructure and having a large forbidden band and whose thickness is such that the product "doping concentration x thickness"
is below 1.3 . 102 atoms/cm-2, whilst the layer with the large forbidden band has a thickness such that the product "doping concentration x thickness" is substantially equal to 2.5 . 1012 atoms/cm
is below 1.3 . 102 atoms/cm-2, whilst the layer with the large forbidden band has a thickness such that the product "doping concentration x thickness" is substantially equal to 2.5 . 1012 atoms/cm
2. A photodiode according to claim 1, wherein its structure is of type n+vnvp+: n+ substrate, v absorp-tion layer, n layer with a large forbidden band, v avalanche layer, p+ junction layer, whilst the avalanche is initiated by the holes.
3. A photodiode according to claim 19 wherein its structure is of the p+.pi.p.pi. n+ type: p+
substrate, .pi. absorption layer, p layer with a large forbidden band, p avalanche layer, n+ junction layer, the avalanche being initiated by electrons.
substrate, .pi. absorption layer, p layer with a large forbidden band, p avalanche layer, n+ junction layer, the avalanche being initiated by electrons.
4. A photodiode according to claim l, wherein the material of the substrate and the avalanche and jtmction layers with a large forbidden band are formed from InP, so that the heterostructure is formed by a layer with a small forbidden band of GaxIn1-xAs, with x = 0.47.
5. A photodiode according to claim 13 wherein the material of the substrate and the avalanche and junction layers with a large forbidden band are of GaSb, so that the heterostructure is formed by a layer with a small forbidden band of a material adapted with regards to the crystal mesh parameter from among GaAlSb or GaAlAsSb.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR8124164A FR2518817A1 (en) | 1981-12-23 | 1981-12-23 | PHOTODIODE WITH SEPARATE ABSORPTION AND AVALANCHE ZONES |
| FR8124164 | 1981-12-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1180092A true CA1180092A (en) | 1984-12-27 |
Family
ID=9265362
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000418122A Expired CA1180092A (en) | 1981-12-23 | 1982-12-20 | Photodiode with separate absorption and avalanche zones |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0082787B1 (en) |
| JP (1) | JPS58114472A (en) |
| CA (1) | CA1180092A (en) |
| DE (1) | DE3276560D1 (en) |
| FR (1) | FR2518817A1 (en) |
Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0150564A3 (en) * | 1983-10-26 | 1986-05-14 | AT&T Corp. | Electronic device comprising a heterojunction |
| JP3141080B2 (en) * | 1994-06-22 | 2001-03-05 | ケイディディ株式会社 | Semiconductor functional element |
| US6720588B2 (en) | 2001-11-28 | 2004-04-13 | Optonics, Inc. | Avalanche photodiode for photon counting applications and method thereof |
| US8269222B2 (en) | 2010-05-25 | 2012-09-18 | The United States Of America As Represented By The Secretary Of The Army | Semiconductor photodetector with transparent interface charge control layer and method thereof |
| US8269223B2 (en) | 2010-05-27 | 2012-09-18 | The United States Of America As Represented By The Secretary Of The Army | Polarization enhanced avalanche photodetector and method thereof |
| US9893227B2 (en) | 2013-05-24 | 2018-02-13 | The United States Of America As Represented By The Secretary Of The Army | Enhanced deep ultraviolet photodetector and method thereof |
| US9379271B2 (en) | 2013-05-24 | 2016-06-28 | The United States Of America As Represented By The Secretary Of The Army | Variable range photodetector and method thereof |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS52101990A (en) * | 1976-02-21 | 1977-08-26 | Hitachi Ltd | Semiconductor device for photoelectric transducer and its manufacture |
| JPS5513907A (en) * | 1978-07-17 | 1980-01-31 | Kokusai Denshin Denwa Co Ltd <Kdd> | Avalnche photo diode with semiconductor hetero construction |
| JPS5793585A (en) * | 1980-12-02 | 1982-06-10 | Fujitsu Ltd | Semiconductor photoreceiving element |
-
1981
- 1981-12-23 FR FR8124164A patent/FR2518817A1/en active Granted
-
1982
- 1982-12-20 CA CA000418122A patent/CA1180092A/en not_active Expired
- 1982-12-21 JP JP57224814A patent/JPS58114472A/en active Pending
- 1982-12-21 DE DE8282402340T patent/DE3276560D1/en not_active Expired
- 1982-12-21 EP EP82402340A patent/EP0082787B1/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE3276560D1 (en) | 1987-07-16 |
| EP0082787A2 (en) | 1983-06-29 |
| JPS58114472A (en) | 1983-07-07 |
| EP0082787B1 (en) | 1987-06-10 |
| EP0082787A3 (en) | 1984-09-05 |
| FR2518817A1 (en) | 1983-06-24 |
| FR2518817B1 (en) | 1985-05-17 |
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