CA1298640C - Avalanche photodiodes and methods for their manufacture - Google Patents
Avalanche photodiodes and methods for their manufactureInfo
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- CA1298640C CA1298640C CA000598773A CA598773A CA1298640C CA 1298640 C CA1298640 C CA 1298640C CA 000598773 A CA000598773 A CA 000598773A CA 598773 A CA598773 A CA 598773A CA 1298640 C CA1298640 C CA 1298640C
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- 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 potential barriers, 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
- H01L31/107—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier the potential barrier working in avalanche mode, e.g. avalanche photodiodes
- H01L31/1075—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier the potential barrier working in avalanche mode, e.g. avalanche photodiodes in which the active layers, e.g. absorption or multiplication layers, form an heterostructure, e.g. SAM structure
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Abstract
Abstract of the Disclosure AVALANCHE PHOTODIODES AND METHODS FOR THEIR MANUFACTURE
An avalanche photodiode has separate absorption and multiplication regions. The photodiode has a first charge sheet located between the absorption region and a central portion of a pn junction of the multiplication region, and a second charge sheet located between the absorption region and edges of the pn junction. The second charge sheet has a lower doping concentration per unit area than the first charge sheet. The first and second charge sheets may be formed by forming a heavily doped semiconductor sublayer, and preferentially removing a portion of the sublayer to leave thicker portion of the sublayer which defines the first charge sheet and a thinner portion of the sublayer which defines the second charge sheet. Another sublayer may then be formed over the charge sheets, and the pn junction may be formed in that sublayer. The photodiode is useful for optical signal detection in optical fiber telecommunications systems.
An avalanche photodiode has separate absorption and multiplication regions. The photodiode has a first charge sheet located between the absorption region and a central portion of a pn junction of the multiplication region, and a second charge sheet located between the absorption region and edges of the pn junction. The second charge sheet has a lower doping concentration per unit area than the first charge sheet. The first and second charge sheets may be formed by forming a heavily doped semiconductor sublayer, and preferentially removing a portion of the sublayer to leave thicker portion of the sublayer which defines the first charge sheet and a thinner portion of the sublayer which defines the second charge sheet. Another sublayer may then be formed over the charge sheets, and the pn junction may be formed in that sublayer. The photodiode is useful for optical signal detection in optical fiber telecommunications systems.
Description
~sa~o AVALANCHE PHOTODIODES AND METHODS FOR THEIR MANUFACTURE
Field o~ the Invention This invention relates to avalanche photodiodes and methods for their manufacture.
Backqround of the Invention Avalanche photodiodes (APDs) are commonly used in optical fiber telecommuncations systems to convert optical signals to electrical signals. APDs used for this purpose should be capable of very high speed operation since optical fiber telecommunications systems may be operated at very high data rates. Such APDs should also have a high internal gain for conversion of weak optical signals to electrical signals of usable amplitude. Moreover, such APDs should conduct very little current when no optical signal is present since such "dark current" is a noise component which limits the usable sensitivity of the APD.
The most basic APD structure comprises a pn junction formed in a semicondu~tor material having a band gap which is less than the energy of the photons to be detected.
The pn junction is reverse biased to set up a large electric field at the junction. Photons which are absorbed in the semiconductor material generate carriers which are swept `~ through the large electric field at the junction and detected ; as a photocurrent. If the reverse bias and the resulting electric field at the junction are sufficiently large, the photogenerated carriers acquire enough energy while drifting through the pn junction region to generate further carriers.
The generation of further carriers provides a photocurrent gain mechanism which is called "avalanche multiplication".
The avalanche multiplication gain mechanism also `` operates on leakage currents which result directly from the reverse bias applied to the pn junction. If the reverse bias is sufficiently large, the pn junction will "break down" and conduct very large avalanche multiplied leakage currents. As ~ 35 these leakage currents do not depend on photogeneration of ; carriers, multiplied leakage currents are "dark currents"
which flow even in the absence of an optical signal and which ~k ~98640 therefore degrade the usablé sensitivity of the ~PD. As the leakage currents increase with increasing reverse bias of the pn junction, the reverse bias must be made large enough to support avalanche multiplication of the photocurrent, but small enough to ensure that the avalanche multiplied leakage currents do not swamp the avalanche multiplied photocurrent.
In particular, the reverse bias must be less than the reverse bias required to support avalanche breakdown of the pn junction.
Unfortunately, the large electric fields which are required to support avalanche multiplication in the semiconductor materials which have band gaps appropriate for absorption at wavelengths commonly used for optical fiber transmission also tend to cause "tunneling" of carriers at the reverse biased pn junction. The current due to this tunneling is a leakage current which is not dependent on photogeneration of carriers. Thus the tunneling current is a dark current which degrades the usable sensitivity of the APD.
Improved APD structures employ separate absorption and multiplication regions to reduce the dark current due to tunneling. The absorption region comprises a layer of semiconductor material which has a band gap appropriate for absorption at wavelengths commonly used for optical fiber transmission. The multiplication region comprises a pn junction formed in a semiconductor material having a band gap which is wider than the band gap of the semiconductor material forming the absorption region. The pn junction is reverse biased to set up a large electric field at the pn junction. Carriers which are photogenerated in the absorption region drift into the multiplication region where they are swept through the reverse biased pn junction by the large electric field and detected as a photocurrent. If the reverse bias and the resulting electric field are sufficiently large, avalanche multiplication of the photocurrent occurs near the pn junction. The wider band gap semiconductor which forms the multiplication region is less prone to tunneling than the narrower band gap semiconductor ~L29~
of the absorption region, even at the relatively larger electric fields required to support avalanche multiplication in the wider band gap semiconductor.
In devices having separate absorption and multiplication regions, the multiplication region can be made by forming a doped well of one conductivity type in a semiconductor layer of an opposite conductivity type. The doped well has finite lateral dimensions and therefore defines a pn junction having a central portion which extends generally parallel to the major dimensions of the semiconductor layer in which the doped well is formed, and peripheral portions which extend generally perpendicular to the major dimensions of the semiconductor layer in which the doped well is formed. The peripheral portions of the pn junction meet the central portion at "corners" or "edges".
When this pn junction is reverse biased, the electric field is greater at the edges than at the central portion of the pn junction. When the electric field is large enough to support a desired level of avalanche multiplication at the edges of the pn junction, it may not be large enough to support avalanche mul~iplication at the central portion of the pn junction, and the overall photocurrent gain will be limited.
Moreover, when the electric field is large enough to support avalanche multiplication of the photocurrent at the central portion of the pn junction for large overall gain, the electric field at the edges of the pn junction may be large enough to cause avalanche breakdown at the edges of the pn junction. Thus, while separation of the absorption and multiplication regions reduces dark current due to tunneling at the pn junction, it does not eliminate dark current due to premature breakdown at the edges of the pn junction.
Moreover, the electric field in the absorption region beneath the edges of the pn junction may be large enough to support tunneling in this part of the absorption region, and this localized tunneling is a further source of dark current.
The concentration of electric field at the edges of the pn junction has been reduced by etching the multiplication region around the central region of the pn ~29~;4~
junction to remove the peripheral portions and edges of the pn junction, leaving a mesa structure with a plane pn junction extending across the mesa. Unfortunately, the etched surfaces are difficult to passivate reliably, and the required etching steps complicate integration of mesa type APDs with other electronic and optoelectronic devices.
Moreover, the etched mesas generally have sloped sidewalls, and charge balancing requirements at the pn junction near the sloped sidewalls cause some electric field concentration near the sloped sidewalls even though the edges of the pn junction have been removed. Such electric field concentration may cause premature avalanche breakdown near the sloped sidewalls of the mesa when the APD is reverse biased for avalanche multiplication at a central portion of the pn junction.
Alternatively, the concentration of electric field at the edges of the pn junction has been reduced by forming one or more a~nular doped wells at the periphery of the doped well which defines the pn junction. These annular doped wells are called "guard rings" and effectively round the edges of the pn junction by providing a less abrupt doping junction. Unfortunately, the guard rings must be very accurately placed relative to the doped well which defines the central portion of the pn junction, and this is very difficult to achieve in practice.
P.P. Webb et al have reported another APD structure employing separate absorption and multiplication regions (SPIE, Vol. 839, Components for Fiber Optic Applications II
(1987)). In this structure, the multiplication region is very lightly doped except for a heavily doped well which defines a pn junction as described above, and a highly doped charge sheet which is located between the central portion of the pn junction and the absorption region. The charge sheet does not extend between the edges of the pn junction and the absorption region, so that the multiplication reglon has a relatively high concentration of carriers per unit area between the central portion of the pn junction and the absorption region due to the charge sheet, and a relatively low concentration of carriers per unit area between the edges ~9~o of the pn junction and the absorption region. As will be explained in greater detail below, this distribution of carriers in the multiplication region between the pn junction and the absorption region can be arranged so as to ensure that there is a larger electric field at the central portion of the pn junction than at the edges of the pn junction when the pn junction is reverse biased. Consequently, the pn junction can ~e reverse biased for good avalanche gain near the central portion of the pn junction without causing avalanche breakdown at the edges of the pn junction.
Webb et al note that the charge sheet in combination with the light background doping of the multiplication region is e~fective in eliminating dark current due to avalanche breakdown at the edges of the pn junction only if the curvature of those edges is fairly gradual. Webb et al used junction depths exceeding 3 micrometers ts achieve the required gradual curvature at the edges of the pn junctions.
While the distribution of carriers in the multiplication region of the Webb et al APD ensllres that there is a larger electric field at the central por~ion of the pn junction than at the edges of the pn junction, this carrier distribution also ensures that there is a larger electric field in peripheral portions of the absorption region which are aligned with the edges of the pn junction than in a central portion of the absorption region which is aligned with the central portion of the pn junction and the charge sheet. In particular, the electric field in the peripheral portions of the absorption region may be large enough to cause tunneling in these portions when the reverse bias on the pn junction is larye enough to support good avalanche gain near the central portion of the pn junction.
Such tunneling is a source of dark current which degrades the usable sensitivity of the APD.
Webb et al also disclose a method for making the APD structure which is described above. According to the disclosed method, an n~ InGaAs absorption layer is grown on - an n-type InP substrate, and a thin n~ InP layer is grown on ~98~
the InGaAs layer. Si is implanted into a central portion of the thin InP layer to define an n+ charge sheet in the thin InP layer. Further n~ InP is then grown to thicken the InP
layer, and Cd is diffused into the InP layer to define a p~
doped well in the thickened InP layer over the Si implant.
The doped well defines a pn junction which, when reverse biased, provides the large electric fields necessary to make the InP layer a multiplication layer. The pn junction has a central portion which extends generally parallel to the major dimensions of the InP layer and-peripheral portions which extend generally perpendicular to the major dimensions of the InP layer and which meet the central portion at "corners" or "edges". The doped well is sized and placed so that the central portion of the pn junction is over the implanted charge sheet, but the edges of the pn junction are not over the implanted charge sheet.
Summar~v of the Invention This invention provides an APD design which permits further reduction of dark currents due to tunneling~ This invention also provides novel methods for making APDsO
One aspect of the invention provides an avalanche photodiode having separate absorption and multiplication ; regions, said photodiode having a first charge sheet located between the absorption region and a central portion of a pn junction of the multiplication region, and a second charge sheet located between the absorption region and edges of the pn junction, said second charge sheet having a lower doping concentration per unit area than said first charge sheet.
The use of two charge sheets facilitates independent control of the electric fields at central and peripheral portions of the multiplication region and absorption region for optimized design. In particular, the use of two charge sheets facilitates the design and manufacture of an APD in which good avalanche gain can be achieved in the central portion of the multiplication region without avalanche breakdown in the peripheral portions of the multiplication region or tunneling in the peripheral portions of the absorption region.
Stated in more detailed structural terms, one aspect of the invention pxovides an avalanche photodiode comprising an absorption layer of a semiconductor having a first band gap and a first conductivity type and a multiplication layer of a semiconductor having a second band gap exceeding the first band gap. The multiplication layer contains a first region of the first conductivity type which has a first doping concentration per unit volume. The multiplication region also contains a second region of a second conductivity type opposite to the first conductivity type which is surrounded by the first region. The first and second regions together define a pn junction having a central portion which extends generally parallel to major dimensions of the multiplication layer and peripheral portions which extend generally perpendicular to the major dimensions of the multiplication layer. The peripheral portions meet the central portion at edges of the pn junction. The multiplication layer further contains a third region of the first conductivity type having a third doping concentration per unit volume which exceeds the first doping concentration per unit volume and a first doping concentration per unit area. The third region is located between the edges of the pn junction and the absorption layer. The multiplication layer also contains a fourth region of the first conductivity type which has a fourth doping concentration per unit volume which exceeds the first doping concentration per unit volume and a second doping concentration per unit area which exceeds the first doping concentration per unit area. The fourth region is located between the central portion of the pn junction and the absorption layer. The avalanche photodiode further comprises an electrical contact to the second region of the multiplication layer and an electrical contact to the absorption layer.
Another aspect of the invention provides a method for making an avalanche photodiode according to the invention. The method comprises the steps of forming an 9L;29~6~
absorption layer of a semiconductor having a first band gap and a first conductivity type, forming a multiplication layer of a semiconductor having a second band gap exceeding the first band gap, with the multiplication layer containing first, second, third and fourth regions as described above, and forming electrical contacts to the second reyion of the multiplication layer and to the absorption layer.
The third doping concentration per unit volume may equal the fourth doping concentration per unit volume, and the fourth region may be thicker-than the third region, so that the second doping concentration per unit area exceeds the first doping concentration per unit area.
Another aspect of the invention provides a method for ma~ing an avalanche photodiode which comprises the ~teps of forming an absorption layer of a semiconductor having a first band gap and a first conductivity type and forming a multiplication layer of a semiconductor having a second band gap exceeding the first band gap. The multiplication layer is formed by forming a first sublayer of the semiconductor having the second band gap and a first doping concentration per unit volu~e of the first conductivity type and preferentially removing a peripheral portion of the first sublayer. A second sublayer of semiconductor having the second band gap and a second doping concentration per unit volume of the first conductivity type is formed over a remaining portion of the first sublayer. The second doping concentration per unit volume is less than the first doping concentration per unit volume. A doped well of a second conductivity type which is opposite to the first conductivity type is formed in the second sublayer. The doped well has lateral boundaries which are disposed over a region where the first sublayer was preferentially removed, and a central portion which is disposed over a region where the first sublayer was not preferentially removed. The method further comprises the step of forming electrical contacts to the absorption region and to the doped well.
In one embodiment of this method, a peripheral portion of the first sublayer is preferentially removed to ~L~9~
leave a central portion of the first sublayer surrounded by a thinner peripheral portion of the first sublayer. The doped well is formed in the second sublayer with lateral boundaries disposed over the thinner peripheral portion of the first sublayer and a central portion disposed over the central portion of the first sublayer.
The semiconductor sublayers comprising the multiplication layer may be formed directly on an absorption layer of semiconductor having the first band gap and the first conductivity type. Preferably however, a gra~ing layer is formed on the absorption layer, and the multiplication layer is formed on the grading layer. The grading layer, which may comprise one or more carefully selected semiconductor layers facilitates movement of carriers between the absorption layer and the multiplication layer.
Brief Description of the Drawinas Embodiments of the invention are described below by way of example only. Reference is made to the accompanying drawings, in which:
Figure 1 is a cross-sectional view of an APD as disclosed by Webb et al;
Figure 2 is a series of plots showing the doping profile, fixed charge profile and electric field profiles for the APD of Figure 1 along section line II-II of Figure 1 when the APD is reverse biased;
Figure 3 is a series of plots showing the doping profile, fixed charge profile and electric field profiles for the APD of Figure 1 along section line III-III of Figure 1 when the APD is reverse biased;
Figure 4 is a cross-sectional view of an APD
according to a first embodiment;
Figure 5 is a series of plots showing the doping profile, fixed charge profile and electric field profiles for the APD of Figure 3 along section line V-V of Figure 4 when the APD is reverse biased; and 1298~4~
Figure 6 is a series of cross-sectional views of the APD of Figure 4 at successive stages of its manufacture.
Detailed Description of Embodiments Figure 1 is a cross-sectional view of an APD 100 as disclosed by Webb et al. The APD 100 comprises an n+ InP
substrate 110 including an n+ InP buffer layer 112, an absorption region in the fcrm of an n~ InGaAs layer 120 on the buffer layer 112, and a multiplication region in the form of an InP layer 130 on the absorption layer 120. The absorption layer 120 and the multiplication layer together define a heterojunction 125 at their interface.
The multiplication layer 130 contains a p-type region in the form of a doped well 132 which is surrounded by an n-type region 133 of the multiplication layer 130. The p-15 type doped well 132 and the surrounding n-type region 133 together define a pn junction 135 which has a central portion 136 extending generally parallel to major dimensions of the multiplication layer 130, and peripheral portions 137 extending generally perpendicular to the major dimensions of 20 the multiplication layer 130. The peripheral portions 137 meet the central portion 136 at edges 138 of the pn junction 135. An n+ region or "charge sheet" 139 is located in the multiplication layer 130 between the central portion 136 of the pn junction 135 and the absorption layer 120.
The APD further comprises an annular electrical contact 140 to the doped well 132 and an electrical contact 150 to the absorption layer 120 via the substrate 110 and buffer layer 112.
In operation of the APD 100, a reverse bias is applied to the pn junction 135 v~a the electrical contacts 140, 150, and an optical signal is introduced into the absorption layer 120 through the opening defined by the annular contact 140. Photons which are absorbed in the absorption layer 120 generate electrons and holes. Under the influence of electric fields set up by the reverse bias, the photogenerated electrons drift to the lower electrical contact 150 via the buffer layer 112 and substrate 110, while ~ ;~9~40 the photogenerated holes drift into the multiplication layer 130 and across the pn junction 135 to the upper electrical contact 140. Thus, the photogenerated carriers are detected at the contacts 140, 150 as a photocurrent.
If the reverse bias and the resulting ~lectric field at the pn junction 135 are large enough, the photogenerated holes acquire enough energy while drifting through the junction region to generate further holes and electrons, which also drift to the upper and lower contacts 140, 150 respectively. This generation of further carriers provides a photocurrent gain mechanism which is called "avalanche multiplication".
Fiyure 2(a) illustrates the doping profile of the APD 100 along line II-II in Figure 1 which passes through the central portion 136 of the pn junction 135. The pn junction 135 and the heterojunction 125 are modeled as abrupt junctions for the purposes of this discussion. In practice they will be less abrupt than this illustration suggests.
Figure 2(b) illustrates the charge density profile of the APD 100 along line II-II in Figure 1 when a reverse bias is applied to the APD 100. This charge density profile, which is determined by the doping profile and the applied reverse bias, sets up an electric field profile as illustrated in Figure 2(c). The maximum electric field occurs at the pn junc~ion 135. The total area under the electric field profile corresponds to the applied reverse bias.
The reverse bias is selected so as to set up an electric field at the pn junction 135 which is at least EM, the minimum electric field required to support avalanche multiplication, but less than EB, the minimum electric field at which the pn junction 135 breaks down.
Figures 3(a), (b) and (c) illustrate the doping profile, charge density profile and electric field profile 35 for the APD 100 taken along a line III-III in Figure 1 which passes through an edge 138 of the pn junction. As illustrated in Figure 2(f), the curvature of the pn junction 135 at the edges 138, and the absence of the charge sheet 139 ~z9~
in the region of the multiplication layer 130 which is between the edges 138 of the pn junction 135 and the absorption layer 120 affect the electric field profile. In particular, the electric field at the edges 138 of the pn junction 135 ls smaller than the electric field at the central portion 136 of the pn junction 135, so that the electric field at the edges 138 of the pn junction 135 is less than both EB, the minimum electric field required to support avalanche breakdown at the pn junction 135, and EM, the minimum electric field requi-red to support avalanche multiplication when the electric field at the central portion 136 of the pn junction 135 is EM. However, the electric field in a peripheral portion of the absorption layer 120 beneath the edges 138 of the pn junction 135 is larger than the electric field in a central portion of the absorption layer 120 beneath the central portion 136 of the pn junction 135. Thus, the electric field in the peripheral portion of the absorption layer 120 beneath the edges 138 of the pn junction 135 may be larger than ET, the minimum electric field required to support tunneling in the absorption layer, even though the electric field in the central portion of the absorption layer 120 beneath the central portion 136 of the pn junction 135 is less than ET. The resulting dark current degrades the usable sensitivity of the APD 100.
: 25 Figure 4 is a cross-sectional view of an APD 200 according to a first embodiment of the invention. The APD
200 is similar to the Webb et al APD 100, except that an n+
; region or "charge sheet" 210 is located in the multiplication layer 130 between the edges 138 of the pn junction 135 and the absorption layer 120. The n+ region 210 has a lower doping concentration per unit area than the n+ region 139 which is located in the multiplication layer between the central portion 136 of the pn junction 135 and the absorption layer 120.
The APD 200 according to the first embodiment also includes a grading layer 220 between the absorption layer 120 and the multiplication layer 130. The structure of the grading layer 220, which facilitates movement of holes ~9~69~
between the absorption layer 120 and the multiplication layer 130, is discussed in greater detail below.
As the doping profile for the APD 200 along line II-II in Figure 4 is identical to the doping profile along the line II-II for the Webb et al APD 100, the corresponding charge density profile and electric field profile for the modified APD 200 should also be identical to the corresponding charge density profile and electric field profile for the Webb et al APD 100 under identical reverse - 10 bias conditions. In particular, the doping profile, charge density profile ancl electric field profile for the modified APD 2û0 along the line II-II of Figure 4 are as shown in Figures 2(a), (b) and tc~ respectively if the same reverse bias is applied to the APD 200 as is applied to the APD 100.
Consequently, the same reverse bias should provide the same avalanche gain at the central portion 136 of the pn junction 135 for both the Webb et al APD 100 and the APD 200 according to the first embodiment.
Figure 6(a) illustrates the doping profile for the APD 200 taken along a line V-V in Figure 4 which passes ~: through an edge 138 of the pn junction 135. As this doping profile differs from the doping profile shown in Figure 3 (a) for the Webb et al APD 100, the corresponding charge density - profile and electric field profile for the APD 200 willdiffer from those shown in Figures 3 (b) and (c) respectively if the same reverse bias is applied to the APD 200 as is applied to the Webb et al APD 100. In particular, the APD
200 will have a somewhat higher electric field at edges 138 of the pn junction 135 than the Webb et al APD 100 when they are reverse biased so as to cause the same amount of avalanche multiplication across the central portiDn 136 of the pn junction 135. This higher electric field will cause breakdown at the edges 13~ of the pn junction 135 only if it exceeds EB, and will cause significant avalanche gain at the edges 138 of the pn junction 135 only if it approaches EM.
Because of the n+ region 210, the APD 200 will also have a lower electric field in the absorption layer 120 beneath the edges 138 of the pn junction 135 than the Webb et al APD 100 ~;~98~4~9 when they are reverse biased so as to cause the same amount of avalanche multiplication across the central portion 136 of the pn junction 135. With appropriate selection of the doping concentration per unit area for the region 210, the electric field in the absorption region 120 beneath the edges 138 of the pn junction 135 can be brought below ET, the minimum electric field for significant tunneling in the absorption layer 120 without raising the electric field at the edges 139 of the pn junction 136 above EB, the minimum electric field for avalanche breakdown at the pn junction, or EM, the mlnimum electric field required to support avalanche multiplication at the pn junction 135. Thus, inclusion of the n~ region 210 may permit reduction in dark currents due to tunneling in the absorption layer 120 without causing dark currents due to avalanche breakdown, and, in any case, provides a further degree of design freedom for minimization of dark currents.
Figures 6(a) to 6(e) illustrate the APD 200 according to the first embodiment at successive stages in its manufacture.
The n InP buffer layer 112, n InGaAs absorption layer 120, grading layer 220, and an n+ InP sublayer 510 are successively grown on the n+ InP substrate 110 by MetalOrganic Chemical Vapour Deposition (MOCVD) to provide the structure shown in Figure 6(a).
The n+ substrate 110 has a Sn doping concentration of approximately 1019 cm~3. The buffer layer 112 is approximately 1.3 micrometers thick, and is not intentionally doped, but has a background doping concentration of approximately 1015 cm~3. The absorption layer 120 is approximately 2.5 micrometers thick, has an In to Ga ratio of 0.53 to 0.47, and is not intentionally doped, but has a background doping concentration of approximately 1015 cm~3.
The grading layer 220 comprises a series of thin alternating sublayers of InP and InGaAs. The InP layers are grown progressively thicker from a thickness of 5 Angstrom units to a thickness of 45 Angstrom units over 18 sublayers, and the InGaAs sublayers are grown progressively thinner from ~186~L~
a thlckness of 45 Angstrom units to a thickness of 5.5 Angstrom units over 18 sublayers to provide a graded interface between the InGaAs absorption layer 120 and the InP
multiplication layer. The graded interface smooths band edge discontinuities which can impede movement of photogenerated ; holes across the interface.
The n+ InP sublayer 510 has a Si doping concentration of approximately 2.6xl017 cm~3 and is approximately 0.1 micrometers thick.
A central portion 139 of the n+ InP sublayer 510 is masked with photoresist and an unmasked peripheral portion of the layer 510 is preferentially etched by wet etching or reactive ion etching to remove approximately 0.07 micrometers of the thickness of the peripheral portion, leaving the central portion 139 surrounded by a thinner peripheral portion 210. The structure which remains after the etching step is shown in Figure 6(b). The central portion 139 of the remaining n+ InP sublayer 510 defines the n+ region or "charge sheet" 139 which is between the central portion 136 of the pn junction 135 and the absorption layer 120 in the finished APD 200. The thinner peripheral portion 210 of the remaining n+ InP sublayer 510 defines the n+ region or "charge sheet" 210 which is between the edges 138 of the pn junction 135 and the absorption layer 120 in the finished APD
200. Because the central and peripheral portions 210, 139 have the same doping concentration per unit volume, but the peripheral portion 210 is thinner than the central portion 139, the peripheral portion 210 has a lower doping concentration per unit area than the central portion 139.
The central portion 139 is approximately 0.1 micrometers thick and approximately 26 micrometers wide, and has a doping concentration per unit volume of approximately 2.6x1017 cm~3 for a doping concentration per unit area of approximately 2.6x1012 cm~2. The peripheral portion 210 is approximately 0.03 micrometers thick for a doping concentration per unit area of approximately 7.8xlOll cm~2.
An n~ InP sublayer 520 approximately 2.5 micrometers thick is then grown on the remaining central and ~98~
peripheral portions 139, 210 of the n+ InP sublayer 510 by MOCVD to form the structure shown in Figure 6(c). No n-type dopant is intentionally added to the InP during growth of the n~ sublayer 520, but unavoidable impurities provide an n~
doping of approximately 1015 cm~3 in the grown InP. The n~
InP sublayer 520 has a lower doping concentration per unit area than the central and peripheral portions 139, 210 of the n+ InP sublayer 510 which are intentionally doped during growth.
The upper surface of the structure shown in Figure 6(c) is masked with Si3N4 530 deposited by Electron Cyclotron Resonance (ECR). The Si3N4 530 is patterned using standard photolithographic techniques, and Zn is diffused through an opening in the Si3N4 mask 530 to form a p+ doped well 132 approximately 2 micrometers deep and approximately 44 micrometers wide in the n~ InP sublayer 520, as shown in Figure 6(d). The opening in the Si3N4 mask 530 is aligned with the central portion 139 of the n+ InP sublayer 510 so that the doped well 132 has lateral boundaries disposed over the peripheral portions 210 of the n+ InP sublayer and a central portion disposed over the central portion 139 of the n+ InP sublayer.
The upper surface of the structure shown in Figure 6(d) is then passivated with an antireflective coating of Si3N4 540, an annular contact hole is opened in the Si3N4 passivation 540 over the doped well 132, and Cr/Au is vacuum evaporated onto the InP surface which is exposed by the contact hole to form a contact 140 to the doped well, as shown in Figure 6(e). Cr/Au is also vacuum evaporated onto the back surface of the n+ InP substrate 110 to form a contact 150 to the absorption layer 120 via the substrate 100 ; and buffer layer 112.
Numerous variations of the APD structure and fabrication method described above will be apparent to those familiar with the design and manufacture of APDs. For example, other crystal growth techniques such as Molecular Beam Epitaxy (MBE) and Liquid Phase Epitaxy (LPE) could be used instead of MOCVD to form the semiconductor layers 112, ' ~g~6~0 120, 220, 510, 520. Other semiconductor materials systems such as the GaAs/AlGaAs system could be used instead of the InP/InGaAs system. Other p and n type dopants could be used, the doping polarities could be reversed, or the doping concentrations and device dimensions could be altered. For example, the central charge sheet 139 could be disposed between 0.2 micrometers and 0.5 micrometers below the pn junction 135. Other dielectrics could be used for passivation and diffusion masks 530, 540, and other metal systems could be used for the electrical contacts 140, 150.
The lower contact 150 could be made annular instead of the upper contact 140 to permit illumination of the absorption layer through the lower contact 150 instead of the upper contact 140.
The "charge sheets" 139, 210 could be formed by implantation into undoped or lightly doped semiconductor, although this approach would require additional masking, implantation and annealing steps.
In the growth and etch back method described above, the peripheral portion 210 of the sublayer 510 could be entirely removed to leave a single charge sheet 139 under the central portion 136 of the pn junction 135. This variation provides a novel method for making the Webb et al APD 100.
In one advantageous variation of the growth and etch back method described above, the sublayer 510 is formed as three successively grown strata. The first of these strata is 0.04 micrometers thick and has a doping concentration per unit volume of 1017 cm~3. The second stratum is 0.08 micrometers thick and has a doping concentration per unit volume of 5X1016 cm~3. The third stratum is 0.18 micrometers thick and has a doping concentration per unit volume of 1017 cm~3. A peripheral portion of the sublayer is etched back 0.2 micrometers to define a central unetched portion having a doping concentration per unit area of 2.6x1012 cm~2 and peripheral etched portion having a doping concentration per unit area of 8Xloll cm~3 Because the etching is terminated in the relatively lightly doped second stratum, any errors in the ~986~
8X1o11 cm~3 Because the etching is terminated in the relatively lightly doped second stratum, any errors in the etching depth will have a relatively small effect on the doping density per unit area of the peripheral portion.
Moreover, because the doping concentrations per unit volume of all three strata are smaller than in the fabrication method described above, the etching depth required to define the desired doping concentrations per unit area is larger.
The larger etching depth provides a taller and more visible mesa, which is more readily used for alignment of masks in subsequent processing steps.
Selective epitaxy could also be used instead of growth and etch back techniques to form the charge sheets 139, 210. For example, a first stratum of the sublayer 510 could be grown across the entire device to form the entire peripheral charge sheet 210 and part of the thickness of the central charge sheet 139. A second stratum of the sublayer 510 could then be selectively grown only where the central charge sheet 139 is desired to complete the central charge sheet 139. This variation avoids the need for etching back the sublayer 510.
Other known forms of grading layer could be used in place of the grading layer 220 described above, or the grading layer 220 could be omitted, though omission of the grading layer would likely degrade the high speed performance of the resulting APD.
These and other variations are within the scope of the invention as claimed below.
Field o~ the Invention This invention relates to avalanche photodiodes and methods for their manufacture.
Backqround of the Invention Avalanche photodiodes (APDs) are commonly used in optical fiber telecommuncations systems to convert optical signals to electrical signals. APDs used for this purpose should be capable of very high speed operation since optical fiber telecommunications systems may be operated at very high data rates. Such APDs should also have a high internal gain for conversion of weak optical signals to electrical signals of usable amplitude. Moreover, such APDs should conduct very little current when no optical signal is present since such "dark current" is a noise component which limits the usable sensitivity of the APD.
The most basic APD structure comprises a pn junction formed in a semicondu~tor material having a band gap which is less than the energy of the photons to be detected.
The pn junction is reverse biased to set up a large electric field at the junction. Photons which are absorbed in the semiconductor material generate carriers which are swept `~ through the large electric field at the junction and detected ; as a photocurrent. If the reverse bias and the resulting electric field at the junction are sufficiently large, the photogenerated carriers acquire enough energy while drifting through the pn junction region to generate further carriers.
The generation of further carriers provides a photocurrent gain mechanism which is called "avalanche multiplication".
The avalanche multiplication gain mechanism also `` operates on leakage currents which result directly from the reverse bias applied to the pn junction. If the reverse bias is sufficiently large, the pn junction will "break down" and conduct very large avalanche multiplied leakage currents. As ~ 35 these leakage currents do not depend on photogeneration of ; carriers, multiplied leakage currents are "dark currents"
which flow even in the absence of an optical signal and which ~k ~98640 therefore degrade the usablé sensitivity of the ~PD. As the leakage currents increase with increasing reverse bias of the pn junction, the reverse bias must be made large enough to support avalanche multiplication of the photocurrent, but small enough to ensure that the avalanche multiplied leakage currents do not swamp the avalanche multiplied photocurrent.
In particular, the reverse bias must be less than the reverse bias required to support avalanche breakdown of the pn junction.
Unfortunately, the large electric fields which are required to support avalanche multiplication in the semiconductor materials which have band gaps appropriate for absorption at wavelengths commonly used for optical fiber transmission also tend to cause "tunneling" of carriers at the reverse biased pn junction. The current due to this tunneling is a leakage current which is not dependent on photogeneration of carriers. Thus the tunneling current is a dark current which degrades the usable sensitivity of the APD.
Improved APD structures employ separate absorption and multiplication regions to reduce the dark current due to tunneling. The absorption region comprises a layer of semiconductor material which has a band gap appropriate for absorption at wavelengths commonly used for optical fiber transmission. The multiplication region comprises a pn junction formed in a semiconductor material having a band gap which is wider than the band gap of the semiconductor material forming the absorption region. The pn junction is reverse biased to set up a large electric field at the pn junction. Carriers which are photogenerated in the absorption region drift into the multiplication region where they are swept through the reverse biased pn junction by the large electric field and detected as a photocurrent. If the reverse bias and the resulting electric field are sufficiently large, avalanche multiplication of the photocurrent occurs near the pn junction. The wider band gap semiconductor which forms the multiplication region is less prone to tunneling than the narrower band gap semiconductor ~L29~
of the absorption region, even at the relatively larger electric fields required to support avalanche multiplication in the wider band gap semiconductor.
In devices having separate absorption and multiplication regions, the multiplication region can be made by forming a doped well of one conductivity type in a semiconductor layer of an opposite conductivity type. The doped well has finite lateral dimensions and therefore defines a pn junction having a central portion which extends generally parallel to the major dimensions of the semiconductor layer in which the doped well is formed, and peripheral portions which extend generally perpendicular to the major dimensions of the semiconductor layer in which the doped well is formed. The peripheral portions of the pn junction meet the central portion at "corners" or "edges".
When this pn junction is reverse biased, the electric field is greater at the edges than at the central portion of the pn junction. When the electric field is large enough to support a desired level of avalanche multiplication at the edges of the pn junction, it may not be large enough to support avalanche mul~iplication at the central portion of the pn junction, and the overall photocurrent gain will be limited.
Moreover, when the electric field is large enough to support avalanche multiplication of the photocurrent at the central portion of the pn junction for large overall gain, the electric field at the edges of the pn junction may be large enough to cause avalanche breakdown at the edges of the pn junction. Thus, while separation of the absorption and multiplication regions reduces dark current due to tunneling at the pn junction, it does not eliminate dark current due to premature breakdown at the edges of the pn junction.
Moreover, the electric field in the absorption region beneath the edges of the pn junction may be large enough to support tunneling in this part of the absorption region, and this localized tunneling is a further source of dark current.
The concentration of electric field at the edges of the pn junction has been reduced by etching the multiplication region around the central region of the pn ~29~;4~
junction to remove the peripheral portions and edges of the pn junction, leaving a mesa structure with a plane pn junction extending across the mesa. Unfortunately, the etched surfaces are difficult to passivate reliably, and the required etching steps complicate integration of mesa type APDs with other electronic and optoelectronic devices.
Moreover, the etched mesas generally have sloped sidewalls, and charge balancing requirements at the pn junction near the sloped sidewalls cause some electric field concentration near the sloped sidewalls even though the edges of the pn junction have been removed. Such electric field concentration may cause premature avalanche breakdown near the sloped sidewalls of the mesa when the APD is reverse biased for avalanche multiplication at a central portion of the pn junction.
Alternatively, the concentration of electric field at the edges of the pn junction has been reduced by forming one or more a~nular doped wells at the periphery of the doped well which defines the pn junction. These annular doped wells are called "guard rings" and effectively round the edges of the pn junction by providing a less abrupt doping junction. Unfortunately, the guard rings must be very accurately placed relative to the doped well which defines the central portion of the pn junction, and this is very difficult to achieve in practice.
P.P. Webb et al have reported another APD structure employing separate absorption and multiplication regions (SPIE, Vol. 839, Components for Fiber Optic Applications II
(1987)). In this structure, the multiplication region is very lightly doped except for a heavily doped well which defines a pn junction as described above, and a highly doped charge sheet which is located between the central portion of the pn junction and the absorption region. The charge sheet does not extend between the edges of the pn junction and the absorption region, so that the multiplication reglon has a relatively high concentration of carriers per unit area between the central portion of the pn junction and the absorption region due to the charge sheet, and a relatively low concentration of carriers per unit area between the edges ~9~o of the pn junction and the absorption region. As will be explained in greater detail below, this distribution of carriers in the multiplication region between the pn junction and the absorption region can be arranged so as to ensure that there is a larger electric field at the central portion of the pn junction than at the edges of the pn junction when the pn junction is reverse biased. Consequently, the pn junction can ~e reverse biased for good avalanche gain near the central portion of the pn junction without causing avalanche breakdown at the edges of the pn junction.
Webb et al note that the charge sheet in combination with the light background doping of the multiplication region is e~fective in eliminating dark current due to avalanche breakdown at the edges of the pn junction only if the curvature of those edges is fairly gradual. Webb et al used junction depths exceeding 3 micrometers ts achieve the required gradual curvature at the edges of the pn junctions.
While the distribution of carriers in the multiplication region of the Webb et al APD ensllres that there is a larger electric field at the central por~ion of the pn junction than at the edges of the pn junction, this carrier distribution also ensures that there is a larger electric field in peripheral portions of the absorption region which are aligned with the edges of the pn junction than in a central portion of the absorption region which is aligned with the central portion of the pn junction and the charge sheet. In particular, the electric field in the peripheral portions of the absorption region may be large enough to cause tunneling in these portions when the reverse bias on the pn junction is larye enough to support good avalanche gain near the central portion of the pn junction.
Such tunneling is a source of dark current which degrades the usable sensitivity of the APD.
Webb et al also disclose a method for making the APD structure which is described above. According to the disclosed method, an n~ InGaAs absorption layer is grown on - an n-type InP substrate, and a thin n~ InP layer is grown on ~98~
the InGaAs layer. Si is implanted into a central portion of the thin InP layer to define an n+ charge sheet in the thin InP layer. Further n~ InP is then grown to thicken the InP
layer, and Cd is diffused into the InP layer to define a p~
doped well in the thickened InP layer over the Si implant.
The doped well defines a pn junction which, when reverse biased, provides the large electric fields necessary to make the InP layer a multiplication layer. The pn junction has a central portion which extends generally parallel to the major dimensions of the InP layer and-peripheral portions which extend generally perpendicular to the major dimensions of the InP layer and which meet the central portion at "corners" or "edges". The doped well is sized and placed so that the central portion of the pn junction is over the implanted charge sheet, but the edges of the pn junction are not over the implanted charge sheet.
Summar~v of the Invention This invention provides an APD design which permits further reduction of dark currents due to tunneling~ This invention also provides novel methods for making APDsO
One aspect of the invention provides an avalanche photodiode having separate absorption and multiplication ; regions, said photodiode having a first charge sheet located between the absorption region and a central portion of a pn junction of the multiplication region, and a second charge sheet located between the absorption region and edges of the pn junction, said second charge sheet having a lower doping concentration per unit area than said first charge sheet.
The use of two charge sheets facilitates independent control of the electric fields at central and peripheral portions of the multiplication region and absorption region for optimized design. In particular, the use of two charge sheets facilitates the design and manufacture of an APD in which good avalanche gain can be achieved in the central portion of the multiplication region without avalanche breakdown in the peripheral portions of the multiplication region or tunneling in the peripheral portions of the absorption region.
Stated in more detailed structural terms, one aspect of the invention pxovides an avalanche photodiode comprising an absorption layer of a semiconductor having a first band gap and a first conductivity type and a multiplication layer of a semiconductor having a second band gap exceeding the first band gap. The multiplication layer contains a first region of the first conductivity type which has a first doping concentration per unit volume. The multiplication region also contains a second region of a second conductivity type opposite to the first conductivity type which is surrounded by the first region. The first and second regions together define a pn junction having a central portion which extends generally parallel to major dimensions of the multiplication layer and peripheral portions which extend generally perpendicular to the major dimensions of the multiplication layer. The peripheral portions meet the central portion at edges of the pn junction. The multiplication layer further contains a third region of the first conductivity type having a third doping concentration per unit volume which exceeds the first doping concentration per unit volume and a first doping concentration per unit area. The third region is located between the edges of the pn junction and the absorption layer. The multiplication layer also contains a fourth region of the first conductivity type which has a fourth doping concentration per unit volume which exceeds the first doping concentration per unit volume and a second doping concentration per unit area which exceeds the first doping concentration per unit area. The fourth region is located between the central portion of the pn junction and the absorption layer. The avalanche photodiode further comprises an electrical contact to the second region of the multiplication layer and an electrical contact to the absorption layer.
Another aspect of the invention provides a method for making an avalanche photodiode according to the invention. The method comprises the steps of forming an 9L;29~6~
absorption layer of a semiconductor having a first band gap and a first conductivity type, forming a multiplication layer of a semiconductor having a second band gap exceeding the first band gap, with the multiplication layer containing first, second, third and fourth regions as described above, and forming electrical contacts to the second reyion of the multiplication layer and to the absorption layer.
The third doping concentration per unit volume may equal the fourth doping concentration per unit volume, and the fourth region may be thicker-than the third region, so that the second doping concentration per unit area exceeds the first doping concentration per unit area.
Another aspect of the invention provides a method for ma~ing an avalanche photodiode which comprises the ~teps of forming an absorption layer of a semiconductor having a first band gap and a first conductivity type and forming a multiplication layer of a semiconductor having a second band gap exceeding the first band gap. The multiplication layer is formed by forming a first sublayer of the semiconductor having the second band gap and a first doping concentration per unit volu~e of the first conductivity type and preferentially removing a peripheral portion of the first sublayer. A second sublayer of semiconductor having the second band gap and a second doping concentration per unit volume of the first conductivity type is formed over a remaining portion of the first sublayer. The second doping concentration per unit volume is less than the first doping concentration per unit volume. A doped well of a second conductivity type which is opposite to the first conductivity type is formed in the second sublayer. The doped well has lateral boundaries which are disposed over a region where the first sublayer was preferentially removed, and a central portion which is disposed over a region where the first sublayer was not preferentially removed. The method further comprises the step of forming electrical contacts to the absorption region and to the doped well.
In one embodiment of this method, a peripheral portion of the first sublayer is preferentially removed to ~L~9~
leave a central portion of the first sublayer surrounded by a thinner peripheral portion of the first sublayer. The doped well is formed in the second sublayer with lateral boundaries disposed over the thinner peripheral portion of the first sublayer and a central portion disposed over the central portion of the first sublayer.
The semiconductor sublayers comprising the multiplication layer may be formed directly on an absorption layer of semiconductor having the first band gap and the first conductivity type. Preferably however, a gra~ing layer is formed on the absorption layer, and the multiplication layer is formed on the grading layer. The grading layer, which may comprise one or more carefully selected semiconductor layers facilitates movement of carriers between the absorption layer and the multiplication layer.
Brief Description of the Drawinas Embodiments of the invention are described below by way of example only. Reference is made to the accompanying drawings, in which:
Figure 1 is a cross-sectional view of an APD as disclosed by Webb et al;
Figure 2 is a series of plots showing the doping profile, fixed charge profile and electric field profiles for the APD of Figure 1 along section line II-II of Figure 1 when the APD is reverse biased;
Figure 3 is a series of plots showing the doping profile, fixed charge profile and electric field profiles for the APD of Figure 1 along section line III-III of Figure 1 when the APD is reverse biased;
Figure 4 is a cross-sectional view of an APD
according to a first embodiment;
Figure 5 is a series of plots showing the doping profile, fixed charge profile and electric field profiles for the APD of Figure 3 along section line V-V of Figure 4 when the APD is reverse biased; and 1298~4~
Figure 6 is a series of cross-sectional views of the APD of Figure 4 at successive stages of its manufacture.
Detailed Description of Embodiments Figure 1 is a cross-sectional view of an APD 100 as disclosed by Webb et al. The APD 100 comprises an n+ InP
substrate 110 including an n+ InP buffer layer 112, an absorption region in the fcrm of an n~ InGaAs layer 120 on the buffer layer 112, and a multiplication region in the form of an InP layer 130 on the absorption layer 120. The absorption layer 120 and the multiplication layer together define a heterojunction 125 at their interface.
The multiplication layer 130 contains a p-type region in the form of a doped well 132 which is surrounded by an n-type region 133 of the multiplication layer 130. The p-15 type doped well 132 and the surrounding n-type region 133 together define a pn junction 135 which has a central portion 136 extending generally parallel to major dimensions of the multiplication layer 130, and peripheral portions 137 extending generally perpendicular to the major dimensions of 20 the multiplication layer 130. The peripheral portions 137 meet the central portion 136 at edges 138 of the pn junction 135. An n+ region or "charge sheet" 139 is located in the multiplication layer 130 between the central portion 136 of the pn junction 135 and the absorption layer 120.
The APD further comprises an annular electrical contact 140 to the doped well 132 and an electrical contact 150 to the absorption layer 120 via the substrate 110 and buffer layer 112.
In operation of the APD 100, a reverse bias is applied to the pn junction 135 v~a the electrical contacts 140, 150, and an optical signal is introduced into the absorption layer 120 through the opening defined by the annular contact 140. Photons which are absorbed in the absorption layer 120 generate electrons and holes. Under the influence of electric fields set up by the reverse bias, the photogenerated electrons drift to the lower electrical contact 150 via the buffer layer 112 and substrate 110, while ~ ;~9~40 the photogenerated holes drift into the multiplication layer 130 and across the pn junction 135 to the upper electrical contact 140. Thus, the photogenerated carriers are detected at the contacts 140, 150 as a photocurrent.
If the reverse bias and the resulting ~lectric field at the pn junction 135 are large enough, the photogenerated holes acquire enough energy while drifting through the junction region to generate further holes and electrons, which also drift to the upper and lower contacts 140, 150 respectively. This generation of further carriers provides a photocurrent gain mechanism which is called "avalanche multiplication".
Fiyure 2(a) illustrates the doping profile of the APD 100 along line II-II in Figure 1 which passes through the central portion 136 of the pn junction 135. The pn junction 135 and the heterojunction 125 are modeled as abrupt junctions for the purposes of this discussion. In practice they will be less abrupt than this illustration suggests.
Figure 2(b) illustrates the charge density profile of the APD 100 along line II-II in Figure 1 when a reverse bias is applied to the APD 100. This charge density profile, which is determined by the doping profile and the applied reverse bias, sets up an electric field profile as illustrated in Figure 2(c). The maximum electric field occurs at the pn junc~ion 135. The total area under the electric field profile corresponds to the applied reverse bias.
The reverse bias is selected so as to set up an electric field at the pn junction 135 which is at least EM, the minimum electric field required to support avalanche multiplication, but less than EB, the minimum electric field at which the pn junction 135 breaks down.
Figures 3(a), (b) and (c) illustrate the doping profile, charge density profile and electric field profile 35 for the APD 100 taken along a line III-III in Figure 1 which passes through an edge 138 of the pn junction. As illustrated in Figure 2(f), the curvature of the pn junction 135 at the edges 138, and the absence of the charge sheet 139 ~z9~
in the region of the multiplication layer 130 which is between the edges 138 of the pn junction 135 and the absorption layer 120 affect the electric field profile. In particular, the electric field at the edges 138 of the pn junction 135 ls smaller than the electric field at the central portion 136 of the pn junction 135, so that the electric field at the edges 138 of the pn junction 135 is less than both EB, the minimum electric field required to support avalanche breakdown at the pn junction 135, and EM, the minimum electric field requi-red to support avalanche multiplication when the electric field at the central portion 136 of the pn junction 135 is EM. However, the electric field in a peripheral portion of the absorption layer 120 beneath the edges 138 of the pn junction 135 is larger than the electric field in a central portion of the absorption layer 120 beneath the central portion 136 of the pn junction 135. Thus, the electric field in the peripheral portion of the absorption layer 120 beneath the edges 138 of the pn junction 135 may be larger than ET, the minimum electric field required to support tunneling in the absorption layer, even though the electric field in the central portion of the absorption layer 120 beneath the central portion 136 of the pn junction 135 is less than ET. The resulting dark current degrades the usable sensitivity of the APD 100.
: 25 Figure 4 is a cross-sectional view of an APD 200 according to a first embodiment of the invention. The APD
200 is similar to the Webb et al APD 100, except that an n+
; region or "charge sheet" 210 is located in the multiplication layer 130 between the edges 138 of the pn junction 135 and the absorption layer 120. The n+ region 210 has a lower doping concentration per unit area than the n+ region 139 which is located in the multiplication layer between the central portion 136 of the pn junction 135 and the absorption layer 120.
The APD 200 according to the first embodiment also includes a grading layer 220 between the absorption layer 120 and the multiplication layer 130. The structure of the grading layer 220, which facilitates movement of holes ~9~69~
between the absorption layer 120 and the multiplication layer 130, is discussed in greater detail below.
As the doping profile for the APD 200 along line II-II in Figure 4 is identical to the doping profile along the line II-II for the Webb et al APD 100, the corresponding charge density profile and electric field profile for the modified APD 200 should also be identical to the corresponding charge density profile and electric field profile for the Webb et al APD 100 under identical reverse - 10 bias conditions. In particular, the doping profile, charge density profile ancl electric field profile for the modified APD 2û0 along the line II-II of Figure 4 are as shown in Figures 2(a), (b) and tc~ respectively if the same reverse bias is applied to the APD 200 as is applied to the APD 100.
Consequently, the same reverse bias should provide the same avalanche gain at the central portion 136 of the pn junction 135 for both the Webb et al APD 100 and the APD 200 according to the first embodiment.
Figure 6(a) illustrates the doping profile for the APD 200 taken along a line V-V in Figure 4 which passes ~: through an edge 138 of the pn junction 135. As this doping profile differs from the doping profile shown in Figure 3 (a) for the Webb et al APD 100, the corresponding charge density - profile and electric field profile for the APD 200 willdiffer from those shown in Figures 3 (b) and (c) respectively if the same reverse bias is applied to the APD 200 as is applied to the Webb et al APD 100. In particular, the APD
200 will have a somewhat higher electric field at edges 138 of the pn junction 135 than the Webb et al APD 100 when they are reverse biased so as to cause the same amount of avalanche multiplication across the central portiDn 136 of the pn junction 135. This higher electric field will cause breakdown at the edges 13~ of the pn junction 135 only if it exceeds EB, and will cause significant avalanche gain at the edges 138 of the pn junction 135 only if it approaches EM.
Because of the n+ region 210, the APD 200 will also have a lower electric field in the absorption layer 120 beneath the edges 138 of the pn junction 135 than the Webb et al APD 100 ~;~98~4~9 when they are reverse biased so as to cause the same amount of avalanche multiplication across the central portion 136 of the pn junction 135. With appropriate selection of the doping concentration per unit area for the region 210, the electric field in the absorption region 120 beneath the edges 138 of the pn junction 135 can be brought below ET, the minimum electric field for significant tunneling in the absorption layer 120 without raising the electric field at the edges 139 of the pn junction 136 above EB, the minimum electric field for avalanche breakdown at the pn junction, or EM, the mlnimum electric field required to support avalanche multiplication at the pn junction 135. Thus, inclusion of the n~ region 210 may permit reduction in dark currents due to tunneling in the absorption layer 120 without causing dark currents due to avalanche breakdown, and, in any case, provides a further degree of design freedom for minimization of dark currents.
Figures 6(a) to 6(e) illustrate the APD 200 according to the first embodiment at successive stages in its manufacture.
The n InP buffer layer 112, n InGaAs absorption layer 120, grading layer 220, and an n+ InP sublayer 510 are successively grown on the n+ InP substrate 110 by MetalOrganic Chemical Vapour Deposition (MOCVD) to provide the structure shown in Figure 6(a).
The n+ substrate 110 has a Sn doping concentration of approximately 1019 cm~3. The buffer layer 112 is approximately 1.3 micrometers thick, and is not intentionally doped, but has a background doping concentration of approximately 1015 cm~3. The absorption layer 120 is approximately 2.5 micrometers thick, has an In to Ga ratio of 0.53 to 0.47, and is not intentionally doped, but has a background doping concentration of approximately 1015 cm~3.
The grading layer 220 comprises a series of thin alternating sublayers of InP and InGaAs. The InP layers are grown progressively thicker from a thickness of 5 Angstrom units to a thickness of 45 Angstrom units over 18 sublayers, and the InGaAs sublayers are grown progressively thinner from ~186~L~
a thlckness of 45 Angstrom units to a thickness of 5.5 Angstrom units over 18 sublayers to provide a graded interface between the InGaAs absorption layer 120 and the InP
multiplication layer. The graded interface smooths band edge discontinuities which can impede movement of photogenerated ; holes across the interface.
The n+ InP sublayer 510 has a Si doping concentration of approximately 2.6xl017 cm~3 and is approximately 0.1 micrometers thick.
A central portion 139 of the n+ InP sublayer 510 is masked with photoresist and an unmasked peripheral portion of the layer 510 is preferentially etched by wet etching or reactive ion etching to remove approximately 0.07 micrometers of the thickness of the peripheral portion, leaving the central portion 139 surrounded by a thinner peripheral portion 210. The structure which remains after the etching step is shown in Figure 6(b). The central portion 139 of the remaining n+ InP sublayer 510 defines the n+ region or "charge sheet" 139 which is between the central portion 136 of the pn junction 135 and the absorption layer 120 in the finished APD 200. The thinner peripheral portion 210 of the remaining n+ InP sublayer 510 defines the n+ region or "charge sheet" 210 which is between the edges 138 of the pn junction 135 and the absorption layer 120 in the finished APD
200. Because the central and peripheral portions 210, 139 have the same doping concentration per unit volume, but the peripheral portion 210 is thinner than the central portion 139, the peripheral portion 210 has a lower doping concentration per unit area than the central portion 139.
The central portion 139 is approximately 0.1 micrometers thick and approximately 26 micrometers wide, and has a doping concentration per unit volume of approximately 2.6x1017 cm~3 for a doping concentration per unit area of approximately 2.6x1012 cm~2. The peripheral portion 210 is approximately 0.03 micrometers thick for a doping concentration per unit area of approximately 7.8xlOll cm~2.
An n~ InP sublayer 520 approximately 2.5 micrometers thick is then grown on the remaining central and ~98~
peripheral portions 139, 210 of the n+ InP sublayer 510 by MOCVD to form the structure shown in Figure 6(c). No n-type dopant is intentionally added to the InP during growth of the n~ sublayer 520, but unavoidable impurities provide an n~
doping of approximately 1015 cm~3 in the grown InP. The n~
InP sublayer 520 has a lower doping concentration per unit area than the central and peripheral portions 139, 210 of the n+ InP sublayer 510 which are intentionally doped during growth.
The upper surface of the structure shown in Figure 6(c) is masked with Si3N4 530 deposited by Electron Cyclotron Resonance (ECR). The Si3N4 530 is patterned using standard photolithographic techniques, and Zn is diffused through an opening in the Si3N4 mask 530 to form a p+ doped well 132 approximately 2 micrometers deep and approximately 44 micrometers wide in the n~ InP sublayer 520, as shown in Figure 6(d). The opening in the Si3N4 mask 530 is aligned with the central portion 139 of the n+ InP sublayer 510 so that the doped well 132 has lateral boundaries disposed over the peripheral portions 210 of the n+ InP sublayer and a central portion disposed over the central portion 139 of the n+ InP sublayer.
The upper surface of the structure shown in Figure 6(d) is then passivated with an antireflective coating of Si3N4 540, an annular contact hole is opened in the Si3N4 passivation 540 over the doped well 132, and Cr/Au is vacuum evaporated onto the InP surface which is exposed by the contact hole to form a contact 140 to the doped well, as shown in Figure 6(e). Cr/Au is also vacuum evaporated onto the back surface of the n+ InP substrate 110 to form a contact 150 to the absorption layer 120 via the substrate 100 ; and buffer layer 112.
Numerous variations of the APD structure and fabrication method described above will be apparent to those familiar with the design and manufacture of APDs. For example, other crystal growth techniques such as Molecular Beam Epitaxy (MBE) and Liquid Phase Epitaxy (LPE) could be used instead of MOCVD to form the semiconductor layers 112, ' ~g~6~0 120, 220, 510, 520. Other semiconductor materials systems such as the GaAs/AlGaAs system could be used instead of the InP/InGaAs system. Other p and n type dopants could be used, the doping polarities could be reversed, or the doping concentrations and device dimensions could be altered. For example, the central charge sheet 139 could be disposed between 0.2 micrometers and 0.5 micrometers below the pn junction 135. Other dielectrics could be used for passivation and diffusion masks 530, 540, and other metal systems could be used for the electrical contacts 140, 150.
The lower contact 150 could be made annular instead of the upper contact 140 to permit illumination of the absorption layer through the lower contact 150 instead of the upper contact 140.
The "charge sheets" 139, 210 could be formed by implantation into undoped or lightly doped semiconductor, although this approach would require additional masking, implantation and annealing steps.
In the growth and etch back method described above, the peripheral portion 210 of the sublayer 510 could be entirely removed to leave a single charge sheet 139 under the central portion 136 of the pn junction 135. This variation provides a novel method for making the Webb et al APD 100.
In one advantageous variation of the growth and etch back method described above, the sublayer 510 is formed as three successively grown strata. The first of these strata is 0.04 micrometers thick and has a doping concentration per unit volume of 1017 cm~3. The second stratum is 0.08 micrometers thick and has a doping concentration per unit volume of 5X1016 cm~3. The third stratum is 0.18 micrometers thick and has a doping concentration per unit volume of 1017 cm~3. A peripheral portion of the sublayer is etched back 0.2 micrometers to define a central unetched portion having a doping concentration per unit area of 2.6x1012 cm~2 and peripheral etched portion having a doping concentration per unit area of 8Xloll cm~3 Because the etching is terminated in the relatively lightly doped second stratum, any errors in the ~986~
8X1o11 cm~3 Because the etching is terminated in the relatively lightly doped second stratum, any errors in the etching depth will have a relatively small effect on the doping density per unit area of the peripheral portion.
Moreover, because the doping concentrations per unit volume of all three strata are smaller than in the fabrication method described above, the etching depth required to define the desired doping concentrations per unit area is larger.
The larger etching depth provides a taller and more visible mesa, which is more readily used for alignment of masks in subsequent processing steps.
Selective epitaxy could also be used instead of growth and etch back techniques to form the charge sheets 139, 210. For example, a first stratum of the sublayer 510 could be grown across the entire device to form the entire peripheral charge sheet 210 and part of the thickness of the central charge sheet 139. A second stratum of the sublayer 510 could then be selectively grown only where the central charge sheet 139 is desired to complete the central charge sheet 139. This variation avoids the need for etching back the sublayer 510.
Other known forms of grading layer could be used in place of the grading layer 220 described above, or the grading layer 220 could be omitted, though omission of the grading layer would likely degrade the high speed performance of the resulting APD.
These and other variations are within the scope of the invention as claimed below.
Claims (19)
1. An avalanche photodiode having separate absorption and multiplication regions, said photodiode having a first charge sheet located between the absorption region and a central portion of a pn junction of the multiplication region, and a second charge sheet located between the absorption region and edges of the pn junction, said second charge sheet having a lower doping concentration per unit area than said first charge sheet.
2. An avalanche photodiode comprising:
an absorption layer of a semiconductor having a first band gap and a first conductivity type;
a multiplication layer of a semiconductor having a second band gap exceeding the first band gap, said multiplication region containing:
a first region of the first conductivity type having a first doping concentration per unit volume;
a second region of a second conductivity type opposite to the first conductivity type, said second region being surrounded by the first region and together with the first region defining a pn junction having a central portion which extends generally parallel to major dimensions of the multiplication layer and peripheral portions which extend generally perpendicular to the major dimensions of the multiplication layer, said peripheral portions meeting said central portion at edges of the pn junction;
a third region of the first conductivity type having a third doping concentration per unit volume exceeding the first doping concentration per unit volume, said third region having a first doping concentration per unit area and being located between the edges of the pn junction and the absorption layer; and a fourth region of the first conductivity type having a fourth doping concentration per unit volume exceeding the first doping concentration per unit volume, said fourth region having a second doping concentration per unit area exceeding the first doping concentration per unit area and being located between the central portion of the pn junction and the absorption layer;
an electrical contact to the second region of the multiplication layer; and an electrical contact to the absorption layer.
an absorption layer of a semiconductor having a first band gap and a first conductivity type;
a multiplication layer of a semiconductor having a second band gap exceeding the first band gap, said multiplication region containing:
a first region of the first conductivity type having a first doping concentration per unit volume;
a second region of a second conductivity type opposite to the first conductivity type, said second region being surrounded by the first region and together with the first region defining a pn junction having a central portion which extends generally parallel to major dimensions of the multiplication layer and peripheral portions which extend generally perpendicular to the major dimensions of the multiplication layer, said peripheral portions meeting said central portion at edges of the pn junction;
a third region of the first conductivity type having a third doping concentration per unit volume exceeding the first doping concentration per unit volume, said third region having a first doping concentration per unit area and being located between the edges of the pn junction and the absorption layer; and a fourth region of the first conductivity type having a fourth doping concentration per unit volume exceeding the first doping concentration per unit volume, said fourth region having a second doping concentration per unit area exceeding the first doping concentration per unit area and being located between the central portion of the pn junction and the absorption layer;
an electrical contact to the second region of the multiplication layer; and an electrical contact to the absorption layer.
3. An avalanche photodiode as defined in claim 2, wherein the first conductivity type is n-type and the second conductivity type is p-type.
4. An avalanche photodiode as defined in claim 3, wherein the semiconductor of the absorption layer is InGaAs, and the semiconductor of the multiplication layer is InP.
5. An avalanche photodiode as defined in claim 4, wherein the first doping concentration per unit volume is approximately 1015 cm-3, the first doping concentration per unit area is approximately 2.6x1012 cm-2, and the second doping concentration per unit area is approximately 8x1011 cm-2.
6. An avalanche photodiode as defined in claim 5, wherein the third and fourth regions have a doping concentration per unit volume of approximately 2.6x1017 cm-3, the third region has a thickness of approximately 0.1 micrometers, and the fourth region has a thickness of approximately 0.03 micrometers.
7. An avalanche photodiode as defined in claim 6, wherein the second region is at least 18 micrometers wider than the fourth region.
8. An avalanche photodiode as defined in claim 7, wherein the second region is approximately 2 micrometers deep, and the fourth region is disposed between 0.2 micrometers and 0.5 micrometers below the second region.
9. An avalanche photodiode as defined in claim 2, comprising a grading layer between the absorption layer and the multiplication layer.
10. An avalanche photodiode as defined in claim 9, wherein the grading layer comprises a plurality of semiconductor sublayers.
11. An avalanche photodiode as defined in claim 10, wherein:
the semiconductor of the absorption layer is InGaAs and the semiconductor of the multiplication layer is InP;
the grading layer comprises a series of alternating sublayers of InP and InGaAs;
the InP sublayers are progressively thicker from the absorption layer to the multiplication layer; and the InGaAs sublayers are progressively thinner from the absorption layer to the multiplication layer.
the semiconductor of the absorption layer is InGaAs and the semiconductor of the multiplication layer is InP;
the grading layer comprises a series of alternating sublayers of InP and InGaAs;
the InP sublayers are progressively thicker from the absorption layer to the multiplication layer; and the InGaAs sublayers are progressively thinner from the absorption layer to the multiplication layer.
12. An avalanche photodiode as defined in claim 11, wherein:
the InP sublayers are progressively thicker from a thickness of 5 Angstrom units to a thickness of 45 Angstrom units over 18 layers from the absorption layer to the multiplication layer; and the InGaAs sublayers are progressively thinner from a thickness of 45 Angstrom units to a thickness of 5.5 Angstrom units over 18 layers from the absorption layer to the multiplication layer.
the InP sublayers are progressively thicker from a thickness of 5 Angstrom units to a thickness of 45 Angstrom units over 18 layers from the absorption layer to the multiplication layer; and the InGaAs sublayers are progressively thinner from a thickness of 45 Angstrom units to a thickness of 5.5 Angstrom units over 18 layers from the absorption layer to the multiplication layer.
13. A method for making an avalanche photodiode, the method comprising:
forming an absorption layer of a semiconductor having a first band gap and a first conductivity type;
forming a multiplication layer of a semiconductor having a second band gap exceeding the first band gap, said multiplication layer containing:
a first region of the first conductivity type having a first doping concentration per unit area;
a second region of a second conductivity type opposite to the first conductivity type, said second region being surrounded by the first region and together with the first region defining a pn junction having a central portion which extends generally parallel to major dimensions of the multiplication layer and peripheral portions which extend generally perpendicular to the major dimensions of the multiplication layer, said peripheral portions meeting said central portion at edges of the pn junction;
a third region of the first conductivity type having a third doping concentration per unit area exceeding the first doping concentration per unit area, said third region being located between the edges of the pn junction and the absorption layer;
and a fourth region of the first conductivity type having a fourth doping concentration per unit area exceeding the first and second doping concentrations per unit area, said fourth region being located between the central portion of the pn junction and the absorption layer; and forming an electrical contact to the second region of the multiplication layer and an electrical contact to the absorption layer.
forming an absorption layer of a semiconductor having a first band gap and a first conductivity type;
forming a multiplication layer of a semiconductor having a second band gap exceeding the first band gap, said multiplication layer containing:
a first region of the first conductivity type having a first doping concentration per unit area;
a second region of a second conductivity type opposite to the first conductivity type, said second region being surrounded by the first region and together with the first region defining a pn junction having a central portion which extends generally parallel to major dimensions of the multiplication layer and peripheral portions which extend generally perpendicular to the major dimensions of the multiplication layer, said peripheral portions meeting said central portion at edges of the pn junction;
a third region of the first conductivity type having a third doping concentration per unit area exceeding the first doping concentration per unit area, said third region being located between the edges of the pn junction and the absorption layer;
and a fourth region of the first conductivity type having a fourth doping concentration per unit area exceeding the first and second doping concentrations per unit area, said fourth region being located between the central portion of the pn junction and the absorption layer; and forming an electrical contact to the second region of the multiplication layer and an electrical contact to the absorption layer.
14. A method for making an avalanche photodiode, the method comprising:
forming an absorption layer of a semiconductor having a first band gap and a first conductivity type:
forming a first multiplication sublayer of a semiconductor having a second band gap exceeding the first band gap and a first doping concentration per unit volume of the first conductivity type;
preferentially removing a peripheral portion of the first multiplication sublayer:
forming over a remaining portion of the first multiplication sublayer a second sublayer of the semiconductor having the second band gap and a second doping concentration per unit volume which is less than the first doping concentration per unit volume;
forming in the second multiplication sublayer a doped well of a second conductivity type which is opposite to the first conductivity type, said doped well having lateral boundaries which are disposed over a region where the first sublayer was preferentially removed, and a central portion which is disposed over a region where the first sublayer was not preferentially removed; and forming electrical contacts to the absorption region and to the doped well.
forming an absorption layer of a semiconductor having a first band gap and a first conductivity type:
forming a first multiplication sublayer of a semiconductor having a second band gap exceeding the first band gap and a first doping concentration per unit volume of the first conductivity type;
preferentially removing a peripheral portion of the first multiplication sublayer:
forming over a remaining portion of the first multiplication sublayer a second sublayer of the semiconductor having the second band gap and a second doping concentration per unit volume which is less than the first doping concentration per unit volume;
forming in the second multiplication sublayer a doped well of a second conductivity type which is opposite to the first conductivity type, said doped well having lateral boundaries which are disposed over a region where the first sublayer was preferentially removed, and a central portion which is disposed over a region where the first sublayer was not preferentially removed; and forming electrical contacts to the absorption region and to the doped well.
15. A method as defined in claim 14, wherein:
the step of preferentially removing a peripheral portion of the first multiplication layer comprises preferentially removing a peripheral portion of the first multiplication sublayer to leave a central portion of the first multiplication sublayer surrounded by a thinner peripheral portion of the first multiplication sublayer; and the step of forming a doped well comprises forming a doped well of the second conductivity type in the second multiplication sublayer, said doped well having lateral boundaries disposed over the thinner peripheral portion of the first multiplication sublayer, and having a central portion disposed over the central portion of the first multiplication sublayer.
the step of preferentially removing a peripheral portion of the first multiplication layer comprises preferentially removing a peripheral portion of the first multiplication sublayer to leave a central portion of the first multiplication sublayer surrounded by a thinner peripheral portion of the first multiplication sublayer; and the step of forming a doped well comprises forming a doped well of the second conductivity type in the second multiplication sublayer, said doped well having lateral boundaries disposed over the thinner peripheral portion of the first multiplication sublayer, and having a central portion disposed over the central portion of the first multiplication sublayer.
16. A method as defined in claim 15, wherein:
the step of forming a first multiplication sublayer comprises forming at least two strata which together comprise the first multiplication sublayer, one of the strata having a smaller doping concentration per unit volume than the other strata; and the step of preferentially removing a peripheral portion of the first multiplication layer comprises preferentially etching a peripheral portion of the first multiplication sublayer to leave a central portion of the first multiplication sublayer surrounded by a thinner peripheral portion of the first multiplication sublayer, the etching being terminated within the stratum having the smaller doping concentration per unit volume.
the step of forming a first multiplication sublayer comprises forming at least two strata which together comprise the first multiplication sublayer, one of the strata having a smaller doping concentration per unit volume than the other strata; and the step of preferentially removing a peripheral portion of the first multiplication layer comprises preferentially etching a peripheral portion of the first multiplication sublayer to leave a central portion of the first multiplication sublayer surrounded by a thinner peripheral portion of the first multiplication sublayer, the etching being terminated within the stratum having the smaller doping concentration per unit volume.
17. A method as defined in claim 15, comprising:
forming an absorption layer of a semiconductor having the first band gap and the first conductivity type on a semiconductor substrate of the first conductivity type;
forming a grading layer on the absorption layer;
and forming the semiconductor sublayers defining the multiplication layer on the grading layer.
forming an absorption layer of a semiconductor having the first band gap and the first conductivity type on a semiconductor substrate of the first conductivity type;
forming a grading layer on the absorption layer;
and forming the semiconductor sublayers defining the multiplication layer on the grading layer.
18. A method as defined in claim 17, wherein the step of forming the grading layer comprises forming a plurality of semiconductor sublayers on the semiconductor layer defining the absorption layer.
19. A method as defined in claim 18, wherein the step of forming the grading layer comprises forming a series of semiconductor sublayers alternating between sublayers of a semiconductor having the second band gap and sublayers of a semiconductor having the first band gap, with successive sublayers of the semiconductor of the second band gap being progressively thicker and successive sublayers of the semi-conductor of the first band gap being progressively thinner.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000598773A CA1298640C (en) | 1989-05-04 | 1989-05-04 | Avalanche photodiodes and methods for their manufacture |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000598773A CA1298640C (en) | 1989-05-04 | 1989-05-04 | Avalanche photodiodes and methods for their manufacture |
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| CA1298640C true CA1298640C (en) | 1992-04-07 |
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| CA000598773A Expired - Fee Related CA1298640C (en) | 1989-05-04 | 1989-05-04 | Avalanche photodiodes and methods for their manufacture |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2014102353A2 (en) * | 2012-12-31 | 2014-07-03 | Commissariat à l'énergie atomique et aux énergies alternatives | Avalanche photodiode semiconductor structure having a high signal-to-noise ratio and method for manufacturing such a photodiode |
| CN112018142A (en) * | 2016-06-21 | 2020-12-01 | 深圳帧观德芯科技有限公司 | Avalanche photodiode based image sensor |
-
1989
- 1989-05-04 CA CA000598773A patent/CA1298640C/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2014102353A2 (en) * | 2012-12-31 | 2014-07-03 | Commissariat à l'énergie atomique et aux énergies alternatives | Avalanche photodiode semiconductor structure having a high signal-to-noise ratio and method for manufacturing such a photodiode |
| FR3000609A1 (en) * | 2012-12-31 | 2014-07-04 | Commissariat Energie Atomique | SEMICONDUCTOR STRUCTURE OF THE AVALANCHE PHOTODIODE TYPE AT HIGH SIGNAL TO NOISE RATIO AND METHOD OF MANUFACTURING SUCH PHOTODIODE |
| WO2014102353A3 (en) * | 2012-12-31 | 2014-10-09 | Commissariat à l'énergie atomique et aux énergies alternatives | Avalanche photodiode semiconductor structure having a high signal-to-noise ratio and method for manufacturing such a photodiode |
| CN112018142A (en) * | 2016-06-21 | 2020-12-01 | 深圳帧观德芯科技有限公司 | Avalanche photodiode based image sensor |
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