SAM PHOTODIODE WITH MULTIPLICATION OF A SINGLE TYPE OF CARRIERS IN A PERIODIC MULTILAYER REGION

Abstract

SAM PHOTODIODE WITH MULTIPLICATION OF A SINGLE TYPE OF CARRIERS IN A PERIODIC MULTILAYER REGION The present description relates to an avalanche photodiode (5) comprising, in order, an absorption region (10) intended to receive radiation (6), a first multiplication region (20a), a second multiplication region (20b), and a collection region (30). The first and second multiplication regions have a lower dopant concentration than those of the absorption region and the collection region. The first multiplication region comprises from two to five multilayer structures (22) where each multilayer structure comprises a first layer having a first bandgap width and then a second layer having a second bandgap width strictly smaller than the first bandgap width.The second multiplication region has a band gap that is constant or varies in one direction from the first multiplication region to the collection region. Figure for abstract: Fig. 1.

Description

Title of the invention: SAM PHOTODIODE WITH MULTIPLICATION OF A SINGLE TYPE OF CARRIERS IN A PERIODIC MULTILAYER REGION Technical field

[0001] The present description relates generally to avalanche photodiodes, and more particularly to separate absorption and multiplication region photodiodes (SAM-APD, acronym for "Separate Absorption and Multiplication region - Avalanche PhotoDiode") which carry out a multiplication of carriers by ionization by impact of a single type of carrier. Prior art

[0002] Certain applications of light detection using an avalanche photodiode, such as LIDAR ("Light Detection And Ranging") laser remote sensing, quantum optics or telecommunications in free space, require the detection of a small number of photons per observation time. However, the conservation of the information contained by a photon flux is fundamentally limited by the quantum efficiency of the photodiode which describes its capacity to transform each incident photon into an electrical signal, and by the excess noise factor of the photodiode which reflects the noise added by the multiplication of the electrical charges carried out by the photodiode.

[0003] Document US11322637B2 describes a photodiode with separate absorption and multiplication zones based on mercury-cadmium tellurides CdHgTe having a reduced excess noise factor comprising an amplification region comprising the repetition of several multilayer structures. Such a structure makes it possible to localize the multiplication events and to reduce the excess noise factor. A disadvantage is that the production of a stack comprising a large number of multilayer structures can be complex. Another disadvantage is that high and / or fluctuating doping can be observed in the multilayer structures.This results in a spatial variation of the operating electric field in the amplification region which can induce a strong variation in the probability of multiplication of electric charges in the amplification region and can, in an extreme case, block the transport of carriers in layers with a small band gap. Summary of the invention.

[0004] One embodiment overcomes all or part of the drawbacks of known photodiodes with separate absorption and multiplication zones.

[0005] An object of an embodiment is to provide a photodiode with absorption zones and separate multiplication which has a reduced excess noise factor.

[0006] One embodiment provides an avalanche photodiode comprising, in order, an absorption region for receiving radiation and doped with a first conductivity type, a first multiplication region, a second multiplication region, and a collection region doped with a second conductivity type different from the first conductivity type, the first and second multiplication regions performing carrier multiplication by impact ionization of a single carrier type and having a dopant concentration lower than that of the absorption region and the collection region, the first multiplication region comprising from two to five multilayer structures where each multilayer structure comprises, from the absorption region to the collection region, a first layer having a first bandgap width and then a second layer having a second bandgap width,the first forbidden band width being strictly greater than the second forbidden band width, the second multiplication region having a constant forbidden band width or which varies in one direction from the first multiplication region to the collection region and having a thickness greater than each second layer.

[0007] According to one embodiment, the first multiplication region comprises from two to four multilayer structures, preferably three multilayer structures.

[0008] According to one embodiment, the second multiplication region has a forbidden band width different from the second forbidden band width.

[0009] According to one embodiment, the second multiplication region has a forbidden band width strictly less than the second forbidden band width.

[0010] According to one embodiment, the second multiplication region has a forbidden band width strictly greater than the second forbidden band width.

[0011] According to one embodiment, the concentration of dopants in the absorption region and the collection region is strictly greater than 1016 atoms / cm3 and the concentration of dopants in the first and second multiplication regions is strictly less than 1016 atoms / cm3.

[0012] According to one embodiment, the first layer of the multilayer structure in contact with the absorption region is thicker than the first layer of the other multilayer structure or structures.

[0013] According to one embodiment, each multilayer structure comprises, from the absorption region to the collection region, a layer with a negative bandgap gradient between the first layer and the second layer and a layer with a positive bandgap gradient after the second layer, the negative bandgap gradient passing from the first bandgap to the second bandgap.

[0014] According to one embodiment, the positive bandgap gradient changes from the second bandgap to the first bandgap for at least the multilayer structure closest to the absorption region.

[0015] According to one embodiment, the positive gradient bandgap layer is thicker than the negative gradient bandgap layer.

[0016] According to one embodiment, the absorption region has a forbidden band width which decreases towards the first multiplication region. Brief description of the drawings

[0017] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0018] [Fig.l] is a partial and schematic sectional view of an embodiment of an avalanche photodiode with separate absorption and multiplication zones;

[0019] [Fig.2] represents a variation curve of the proportion of cadmium and a variation curve of the cut-off wavelength in the layers of the photodiode of [Fig.l];

[0020] [Fig.3] illustrates the variation of the forbidden band width within a multilayer structure of the multiplication region of the photodiode [Fig.l];

[0021] [Fig.4] represents curves of evolution of the total excess noise factor of an amplification region of the photodiode of [Fig.l] for different values ​​of the gain of an amplification region; and

[0022] [Fig.5], [Fig.6], [Fig.7], and [Fig.8] are partial and schematic sectional views of other embodiments of an avalanche photodiode with separate absorption and multiplication zones. Description of the embodiments

[0023] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0024] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the electronic circuits implemented to polarize an avalanche photodiode with separate absorption and multiplication zones and to process the signals provided by the photodiode are known and are not described subsequently.

[0025] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") between them, this means that these two elements can be connected or be linked through one or more other elements.

[0026] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0027] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0028] According to one embodiment, a SAM-APD photodiode with separate absorption and multiplication zones is considered which performs a multiplication of carriers by impact ionization of a single type of carrier (multiplication called SCM for "Single Carrier Multiplication"). By "multiplication of carriers by impact ionization of a single type of carrier", it is meant that the multiplication by impact of one of the types of carriers (holes for example) is negligible compared to the multiplication by impact of the other type of carrier (electrons in this example), that is to say that the ratio between the two multiplication rates is greater than 50, preferably 100, or even 1000.

[0029] [Fig.l] is a partial and schematic sectional view of an embodiment of an avalanche photodiode 5 intended for the detection and measurement of electromagnetic radiation 6. The wavelength of the radiation 6 is included in a given wavelength range, for example a wavelength included in the near infrared wavelength range, for example equal to approximately 1.55 pm.

[0030] The photodiode 5 is for example produced by means of molecular beam epitaxy growth of the CdHgTe material whose cadmium composition can be modulated in order to vary the forbidden bandgap width of the material.

[0031] The photodiode 5 comprises an absorption region 10, a collection region 30 and a multiplication region 20 between the absorption region 10 and the collection region 30. The multiplication region 20 carries out a multiplication of carriers by impact ionization which is predominant for the electrons (i.e. the multiplication of the holes is negligible compared to the multiplication of the electrons). The absorption region 10 comprises a face 11 receiving the radiation 6.

[0032] The photodiode 5 is made from a material which makes it possible to obtain a predominant multiplication for one type of carrier. According to one embodiment, the photodiode 5 is made from mercury-cadmium telluride, that is to say from a material comprising tellurium and at least one element chosen from cadmium and mercury and respecting the following formulation CdxHgi ​​xTe with the value x, also called xCd hereinafter, corresponding to the proportion of cadmium relative to mercury which is between 0 and 1, 0 and 1 inclusive. According to another embodiment, the photodiode 5 is made from indium arsenide-antimonide of the InAs, v Sby type, with the value y corresponding to the proportion of antimony relative to arsenic which is between 0 and 1, 0 and 1 inclusive.

[0033] In operation, the photodiode 5 is subjected to a reverse bias, such as a bias comprised between 10 V and 15 V, for example equal to 13 V. The electrons generated in the absorption region 10 are transported by drift or diffusion towards the multiplication region 20. The application of a high potential difference between the absorption region 10 and the collection region 30 will cause a multiplication of the carriers in the multiplication region 20. This multiplication amplifies the initial photocurrent and corresponds to the establishment of a gain, also called multiplication gain or avalanche gain.

[0034] [Fig.2] represents a curve of variation of the proportion of cadmium xCd relative to mercury and a curve of variation of the cut-off wavelength Xc in the layers of the photodiode 5 of [Fig.l] as a function of the distance D measured from the face 11 in a direction orthogonal to the face 11 when the photodiode 5 is made from mercury-cadmium telluride.

[0035] The absorption region 10 has a first conductivity type, for example the P type, and thickness and band gap characteristics adapted to absorb the majority of the light radiation 6 received by the photodiode 5. According to one embodiment, the absorption region 10 has a doping level typically greater than 1016 atoms / cm3, for example between 1016 atoms / cm3 and 1018 atoms / cm3. Preferably, the doping is obtained by adding arsenic (As), gold (Au) or antimony (Sb). The thickness and the proportion of cadmium in the absorption region 10 are adapted to the wavelength targeted for each application. To detect photons with a wavelength equal to 1.55 pm, the thickness is typically between 1 pm and 3 pm, preferably between 1 pm and 2 pm. The proportion of cadmium xCd is typically greater than 0.3, for example greater than 0.45.According to one embodiment, the proportion of cadmium xCd in the absorption region 10 decreases from the face 11 to the multiplication region 20. According to one embodiment, the proportion of cadmium xCd is between 0.6 and 0.42.

[0036] The multiplication region 20 is characterized by a low doping level relative to the absorption region 10, typically less than 1016 atoms / cm3 and preferably less than 1015 atoms / cm3, in order to establish a uniform electric field across the multiplication region 20.

[0037] According to one embodiment, the multiplication region 20 comprises the stack of a first multiplication region 20a and a second multiplication region 20b. The second multiplication region 20b is in direct physical contact with the first multiplication region 20a. The first multiplication region 20a is in direct physical contact with the absorption region 10 and the second multiplication region 20b is in direct physical contact with the collection region 30. According to one embodiment, the first multiplication region 20a is not in direct physical contact with the collection region 30 and the second multiplication region 20b is not in direct physical contact with the absorption region 10.

[0038] According to one embodiment, the first multiplication region 20a comprises a plurality of multilayer structures 22. The characteristics of the materials of the multilayer structures 22 promote localization of the impact ionization events in a fraction of the thickness of each multilayer structure 22 and the multiplication of a single type of carrier corresponding to the minority carriers in the absorption region 10, for example electrons. The multilayer structure 22 of the first multiplication region 20a closest to the absorption region 10 is called the first multilayer structure 22 and the multilayer structure 22 of the first multiplication region 20a closest to the second multiplication region 20b is called the last multilayer structure 22.

[0039] [Fig. 3] illustrates the variation of the forbidden band width which separates the valence Ev and conduction Ec bands of an embodiment of a multilayer structure 22 of the first unpolarized multiplication region 20a as a function of the distance D measured in a direction orthogonal to the face 11 from the edge of the multilayer structure 22 located on the side of the absorption region 10.

[0040] Each multilayer structure 22 successively comprises, in a direction going from the absorption region 10 towards the collection region 30, a first constant bandgap layer 22a having a substantially constant bandgap equal to a first value AEb, a first transition layer 22b, a second constant bandgap layer 22c having a second substantially constant bandgap equal to a second value AE2, and a second transition layer 22d. In the first transition layer 22b, the bandgap varies from the first bandgap AEi to the second bandgap AE2.In the second transition layer 22d, the band gap varies from the second band gap AE2 to the first band gap AEi when the multilayer structure 22 is followed by another multilayer structure 22 and varies from the second band gap AE2 to the band gap of the second multiplication region 20b for the last multilayer structure 22.

[0041] According to one embodiment, the first forbidden band width AEi is su greater than the second bandgap width AE2. In such a way, the second constant bandgap layer 22c has an average carrier multiplication rate per micrometer greater than the average carrier multiplication rate per micrometer of the first constant bandgap layer 22a. To do this, according to one embodiment, the first and second constant bandgap layers 22a, 22c are made of mercury-cadmium tellurides of the CdxHgi ​​xTe type with different cadmium proportions x from each other, and more particularly a maximum proportion of cadmium xCdi of the first constant bandgap layer 22a greater than the minimum proportion of cadmium xCd2 of the second constant bandgap layer 22c.According to one embodiment, the maximum proportion of cadmium xCdi of the first constant bandgap layer 22a is between 0.4 and 0.5, for example equal to 0.42. According to one embodiment, the minimum proportion of cadmium xCd2 of the second constant bandgap layer 22c is between 0.33 and 0.4, for example equal to 0.34.

[0042] According to a possible embodiment, the first transition layer 22b has a negative bandgap gradient and the second transition layer 22d has a positive bandgap gradient. The negative bandgap gradient changes from the first bandgap AEi to the second bandgap AE2 and the positive bandgap gradient changes from the second bandgap AE2 to the first bandgap AEi when the multilayer structure 22 is followed by another multilayer structure 22 and changes from the second bandgap AE2 to the bandgap of the second multiplication region 20b for the last multilayer structure 22.

[0043] According to one embodiment, the second transition layer 22d with a positive gradient of forbidden band width is thicker than the first transition layer 22b with a negative gradient of forbidden band width. The negative gradient is thus steeper than the positive gradient. According to one embodiment, between the first forbidden band width AEi and the second forbidden band width AE2, the relative variation, in absolute value, of the conduction band Ec is greater than the relative variation, in absolute value, of the valence band Ev. This makes it possible to guarantee fast drift transport and a fast response time of the photodiode. According to one embodiment, the variation of the forbidden band width of the second transition layer 22d is substantially linear.

[0044] According to one embodiment, the thickness of each multilayer structure 22 is between 400 nm and 800 nm. According to one embodiment, for each multilayer structure 22, the ratio between the thickness of the first constant bandgap layer 22a and the thickness of the multilayer structure 22 is between 45% and 80%, for example equal to approximately 60%. According to one embodiment, for each multilayer structure 22, the ratio between the thickness of the second constant bandgap layer 22c and the thickness of the multilayer structure 22 is between 20% and 50%, for example equal to approximately 25%. According to one embodiment, for each multilayer structure 22, the ratio between the thickness of the second transition region 22d and the thickness of the multilayer structure 22 is between 5% and 20%, for example equal to approximately 10%. According to one embodiment, for each multilayer structure 22, the ratio between the thickness of the first transition region 22b and the thickness of the multilayer structure 22 is between 1% and 10%, for example less than 5%.

[0045] According to one embodiment, the number Nb of multilayer structures 22 of the first absorption region 20a is greater than or equal to 2 and less than or equal to 5, preferably equal to 3. The reduced number Nb of multilayer structures 22 makes it possible to obtain a more homogeneous electric field in this part of the structure. This makes it possible to obtain a similar multiplication probability in the different multilayer structures 22, in favor of a reduction in the excess noise factor which can approach unity. According to one embodiment, the thickness of the first multiplication region 20a is between 0.8 μm and 4 μm. The first multiplication region 20a makes it possible to obtain a high gain and a low excess noise factor with reduced sensitivity to a variation in the residual doping in the first multiplication region 20a.

[0046] According to a possible embodiment, the positive bandgap gradient changes from the second bandgap AE2 to the first bandgap AEi, such that the multilayer structures 22 have the same bandgap widths AEi and AE2 within the first and second constant bandgap layers 22a and 22c.

[0047] According to one embodiment, the last multilayer structure 22 of the first multiplication region 20a differs from the other multilayer structures 22 by the characteristic of the second transition region 22d. In this region, the thickness and the cadmium proportion gradient must be adapted to allow the carriers to transit between this region and the second multiplication region 20b.

[0048] The second multiplication region 20b has a substantially constant proportion of cadmium xCd or a proportion of cadmium xCd which has a gradient in one direction only from the first multiplication region 20a to the collection region 30. According to one embodiment, the thickness of the second multiplication region 20b is between 1 pm and 5 pm. The proportion of cadmium xCd and the thickness of the second multiplication region 20b are adapted to obtain a level of gain and dark noise adapted for each application. According to one embodiment, in the case of an application that requires a high operating temperature, the forbidden band width of the second multiplication region 20b may be greater than the second forbidden band width AE2. According to another embodiment, in the case where one seeks to maximize the gain, for example in the case of applications that require the detection of single photons, the forbidden band width of the second multiplication region 20b may be less than the second forbidden band width AE2.

[0049] The concentration of dopants in the first multiplication region 20a and in the second multiplication region 20b is lower than the concentration of dopants in the absorption region 10 and in the collection region 30, preferably lower than 101 5 atoms / cm3. The first multiplication region 20a can be doped with the same conductivity type as the second multiplication region 20b or be doped with the opposite conductivity type to that of the second multiplication region 20b. According to one embodiment, the level of doping in the first multiplication region 20a and in the second multiplication region 20b corresponds to the level of residual doping imposed by the material composing these regions and the growth method implemented for the formation of the multiplication region 20. In the case where the photodiode 5 is based on CdHgTe, the residual doping is of type N at a level lower than 1015 atoms / cm3.The level of residual doping will influence the profile of the field through the multiplication regions 20a and 20b. Advantageously, the impact of this modulation of the electric field through the photodiode 5 is reduced compared to a photodiode whose multiplication region 20 comprises a number Nb of multilayer structures greater than 4. The presence of the second multiplication region 20b makes it possible to reduce the sensitivity of the photodiode 5 to a variation of the residual doping in the first multiplication region 20a.

[0050] The collection region 30, with which the multiplication region 20 is in direct physical contact in the embodiment of [Fig.l], is characterized by a second type of conductivity, for example the N type. According to one embodiment, the collection region 30 has a doping level typically greater than 1016 atoms / cm3, preferably greater than 1017 atoms / cm3. The proportion of cadmium xCd of the collection region 30 is for example between 0.35 and 0.5. According to one embodiment, the thickness of the collection region 30 is between 0.5 pm and 2 pm, and is for example equal to 0.8 pm. According to one embodiment, the band gap in the collection region 30 is preferably greater than the band gap of the second multiplication region 20b in order to reduce the generation of a dark current on defects or by tunneling in this region.In this case, a band gap gradient is also achieved between the second multiplication region 20b and the collection region 30 to facilitate charge transport. between the two regions.

[0051] In operation, the photodiode 5 is subjected to a reverse bias, such as a bias comprised between 10 V and 15 V, for example equal to 13 V. The electrons generated in the absorption region 10 are transported by drift or diffusion towards the multiplication region 20. The application of a high potential difference between the absorption region 10 and the collection region 30 will cause a multiplication of the carriers in the multiplication region 20. This multiplication amplifies the initial photocurrent and corresponds to the establishment of a so-called avalanche gain.

[0052] In operation, the first constant bandgap layer 22a forms a carrier acceleration layer that increases the electron energy without initiating electron multiplication events while the second constant bandgap layer 22c forms a multiplication layer within which the electron multiplication events are localized. The first multiplication region 20a with a plurality of multi-layer structures 22 thus allows significant localization of the multiplication events within the second constant bandgap layers 22c. In such a way, the multiplication randomness is reduced and consequently the excess noise factor is decreased.

[0053] The reduced number Nb of multilayer structures 22 in the first multiplication region 20a reduces the achievable gain M2a in this part of the structure. For example, the gain M2a will approach 8 for Nb equal to 3 in the optimal case with a multiplication probability equal to 1 per multilayer structure 22.

[0054] The second multiplication region 20b of the multiplication region 20 is designed to introduce a high gain M2b, which makes it possible to increase the total gain Mtot in the photodiode 5 which is equal to the product of the gain M2a of the first multiplication region 20a and the gain M2b of the second multiplication region 20b.

[0055] Due to the order of implementation of the first multiplication region 20a and the second multiplication region 20b and due to the fact that only one type of carriers is multiplied in the first multiplication region 20a, the first multiplication region 20a acts as a preamplifier whose gain and excess noise factor fluctuation characteristics are dominant for the total gain fluctuations across both multiplication regions 20a and 20b of the photodiode 5.

[0056] Calling F2a the excess noise factor of the first multiplication region 20a and F2b the excess noise factor of the second multiplication region 20b, the total excess noise factor Ftot of the amplification region 20 considering independent multiplication processes in the two multiplication regions 20a and 20b is given by the following formula: [Math 1] <h2 style=";text-align:left;direction:ltr">T? _1 ,%«_1 , , ( tj l\ , (P2b'D<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr"> ^tot“ M?o, + MiM, -1+kr2a-JJ+ M2a<h2 style=";text-align:left;direction:ltr">

[0057] [Fig.4] represents curves C1, C2, C3, C4 and C5 of the evolution of an estimate of the total excess noise factor Ftot of the amplification region 20 of the photodiode 5 for different values ​​of the gain M2a of the first amplification region 20a and for different values ​​of the excess noise factor F2b of the second multiplication region 20b. The curves C1, C2, C3, C4 and C5 are obtained with the gain M2b of the second multiplication region 20b equal to 30 and the excess noise factor F2a of the first multiplication region 20a equal to 1.01. The curves C1, C2, C3, C4, and C5 are obtained with the excess noise factor F2b equal to 10, 3, 2, 1.5, and 1.24 respectively. The values ​​obtained on line A correspond to a number Nb equal to 1, which corresponds to a gain M2a equal to 2. The values ​​obtained on line B correspond to a number Nb equal to 2, which corresponds to a gain M2a equal to 4.The values ​​obtained on line C correspond to a number Nb equal to 3, which corresponds to a gain M2a equal to 8. The values ​​obtained on line D correspond to a number Nb equal to 4, which corresponds to a gain M2a equal to 16.

[0058] The estimates of [Fig.4] highlight that a low excess noise factor Ftot can be obtained even with a second multiplication region 20b with a very random multiplication characterized by a multiplication of the two types of carriers and, consequently, an excess noise factor F2b greater than 5. In this typical case, for a multiplication layer made with a III-V semiconductor, the use of at least 3 multilayer structures 22 allows the total excess noise factor at Ftot to be less than 1.2. In the case of implementing a second multiplication region 20b with less random multiplication with an excess noise factor F2b less than 2, as is the case when the photodiode 5 is made with the semiconductor HgCdTe, the total excess noise factor at Ftot is less than 1.1 already when Nb is equal to 2 and tends towards Fia, i.e. close to unity, when Nb is greater than or equal to 3.

[0059] The estimates of [Fig.4] show that the multiplication region 20 comprising the first multiplication region 20a and the second multiplication region 20b allows to have a randomness dominated by the gain fluctuations in the first multiplication region 20a. The second multiplication region 20b can thus be adjusted to obtain the total gain Mtot required depending on the application and without paying attention to the potential excess noise values ​​generated in this region. For example, for single photon detection applications, it is important to obtain a very high total gain Mtot to generate signals above the noise of the electronic circuits connected to the photodiode and to obtain, consequently, a good detection efficiency.

[0060] According to one embodiment, in the case where the second multiplication region 20b is made of HgCdTe, the gain in the second multiplication region 20b can be increased by decreasing the forbidden bandwidth in this region compared to the minimum forbidden bandwidth AE2 in the first multiplication region 20a. In this case, a very high total gain Mtot is obtained associated with a total excess noise factor Ftot close to unity. The use of a lower forbidden bandwidth in the second multiplication region 20b will however lead to an increase in the generation of thermal generation events which will induce an increase in the noise of the detector in the dark.The impact of this noise will be limited by the architecture of the photodiode 5 which provides the advantage of less amplification of the generation events, thermal or photonic, in the second multiplication region 20b compared to the signals which are amplified by the total gain. This makes it possible to obtain a better signal-to-noise ratio compared to a homogeneous multiplication layer with a low forbidden bandwidth.

[0061] [Fig.5], [Fig.6], [Fig.7], and [Fig.8] are partial and schematic sectional views of other embodiments of a photodiode.

[0062] [Fig. 5] is a partial and schematic sectional view of another embodiment of a photodiode 60. The photodiode 60 comprises all of the elements of the photodiode 5 shown in [Fig. 1] and further comprises a passivation layer 40 covering the collection region 30 on the side opposite the second multiplication region 20b and making it possible to protect the photodiode 60 from electrical degradation induced by a mechanical or chemical mechanism. The photodiode 70 further comprises a metal pad 50 located in and around a hole 42 in the passivation layer 40, in order to have an electrical contact, ideally ohmic, with the collection region 30. An electrical contact (not shown) is also made with the absorption region 10 in order to apply a bias between the absorption region 10 and the collection region 30 through the multiplication region 20.

[0063] [Fig. 6] is a partial and schematic sectional view of another embodiment of a photodiode 70. The photodiode 70 comprises all of the elements of the photodiode 60 shown in [Fig. 5] with the difference that the multiplication region 20 and the collection region 30 have a mesa-type structure on the absorption region 10, i.e. the stack 62 of the multiplication region 20 and the collection region 30 forms a raised element with inclined sides 64 resting on the absorption region 10 with a lateral dimension, measured parallel to the face 6, smaller than the lateral dimension of the absorption region 10. The passivation layer 40 further extends over the inclined sides 64 of the collection region 30 and the multiplication region 20, and extends over the absorption region 10 in direct physical contact with the absorption region 10 around the stack 62. The stack 62 may be formed by an etching defining a mesa-shaped pillar.

[0064] [Fig. 7] is a partial and schematic sectional view of another embodiment of a photodiode 80. The photodiode 80 comprises all of the elements of the photodiode 70 shown in [Fig. 6] with the difference that the collection region 30 comprises a central region 31 and a peripheral region 32 surrounding the central region 31 and interposed between the flanks 64 and the central region 31. The doping level of the peripheral region 32 is lower than the doping level of the central region 31. According to one embodiment, the doping level of the peripheral region 32 is lower than 1015 atoms / cm3, and is for example substantially equal to the level of the residual doping imposed by the material making up the collection region 30 and the growth method used for the formation of the collection region 30.In the case where the photodiode 80 is based on CdHgTe, the residual doping in the peripheral region 32 is of type N at a level lower than 1015 atoms / cm3. This advantageously makes it possible to limit the concentration of the electric field on the sides 64 of the stack. According to one embodiment, the doping level of the central region 31 is greater than 1017 atoms / cm3. The central region 31 can be formed by ion implantation.

[0065] [Fig.8] is a partial and schematic sectional view of another embodiment of a photodiode 90. The photodiode 90 comprises all of the elements of the photodiode 60 shown in [Fig.5], the collection region 30 further comprising the central region 31 surrounded by the peripheral region 32 as for the photodiode 80 shown in [Fig.7].

[0066] According to one embodiment, a P-type doping by mercury vacancy in the structure is first induced in the entire photodiode following the growth by molecular beam epitaxy of the layers composing the absorption region 10, the multiplication region 20, and the collection region 30. When the photodiode 90 is based on CdHgTe, this doping can be obtained by vacuum annealing at a temperature close to 300°C in order to obtain a doping level close to 1016 atoms / cm 3. The formation of the central N-type region 31 at a level typically greater than 1017 atoms / cm3 and located in the collection region 30 will induce a suppression of the mercury vacancies and, consequently, the formation of a zone 91 in the multiplication region 20 and the collection region 30 characterized by an N-type doping at a level lower than 1015 atoms / cm3, corresponding to the residual doping of the material making up the photodiode 90.the remaining area 92 of the absorption region 10, the multiplication region 20 and the collection region is still characterized by P-type doping. The boundary between areas 91 and 92 is schematically represented by a dashed line 93 in [Fig.8]. In this case, the multiplication function with very low excess noise takes place in the part of the multiplication region 20 contained . in area 91, that is to say in the center of the photodiode 90. This embodiment has the advantage of eliminating the need for a mesa-type structure to delimit the active area of ​​the photodiode.

[0067] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art.

[0068] Finally, the practical implementation of the embodiments and variants described is within the reach of the person skilled in the art from the functional indications given above.

Claims

Claims

1. An avalanche photodiode (5; 60; 70; 80; 90) comprising, in order, an absorption region (10) for receiving radiation (6) and doped with a first conductivity type, a first multiplication region (20a), a second multiplication region (20b), and a collection region (30) doped with a second conductivity type different from the first conductivity type, the first and second multiplication regions (20a, 20b) performing carrier multiplication by impact ionization of a single type of carrier and having a lower dopant concentration than the absorption region (10) and the collection region (30), the first multiplication region (20a) comprising from two to five multilayer structures (22) where each multilayer structure comprises, from the absorption region (10) to the collection region (30),a first layer (22a) having a first forbidden band width (AEJ then a second layer (22c) having a second forbidden band width (AE2), the first forbidden band width being strictly greater than the second forbidden band width, the second multiplication region (20b) having a constant forbidden band width or which varies in a single direction from the first multiplication region (20a) to the collection region (30) and having a thickness greater than each second layer (22c).,

2. An avalanche photodiode according to claim 1, wherein the first multiplication region (20a) comprises two to four multilayer structures (22), preferably three multilayer structures (22).

3. An avalanche photodiode according to claim 1 or 2, wherein the second multiplication region (20b) has a band gap different from the second band gap (AE2).

4. An avalanche photodiode according to claim 3, wherein the second multiplication region (20b) has a band gap strictly less than the second band gap (AE2).

5. An avalanche photodiode according to claim 3, wherein the second multiplication region (20b) has a band gap strictly greater than the second band gap (AE2).

6. An avalanche photodiode according to any one of claims 1 to 5, wherein the dopant concentration of the absorption region (10) and the collection region (30) is strictly greater than 1016 atoms / cm3 and in which the dopant concentration of the first and second multiplication regions (20a, 20b) is strictly less than 1016 atoms / cm3.

7. An avalanche photodiode according to any one of claims 1 to 6, wherein the first layer (22a) of the multilayer structure (22) in contact with the absorption region (10) is thicker than the first layer of the other multilayer structure or structures.

8. An avalanche photodiode according to any one of claims 1 to 7, wherein each multilayer structure (22) comprises, from the absorption region (10) to the collection region (30), a layer (22b) with a negative bandgap gradient between the first layer (22a) and the second layer (22c) and a layer (22d) with a positive bandgap gradient after the second layer (22c), the negative bandgap gradient changing from the first bandgap (AEi) to the second bandgap (AE2).

9. An avalanche photodiode according to claim 8, wherein the positive bandgap gradient changes from the second bandgap (AE2) to the first bandgap (AEi) for at least the multilayer structure (22) closest to the absorption region (10).

10. An avalanche photodiode according to claim 8 or 9, wherein the positive bandgap gradient layer (22d) is thicker than the negative bandgap gradient layer (22b).

11. An avalanche photodiode according to one of claims 1 to 10, wherein the absorption region (10) has a band gap which decreases towards the first multiplication region (20a).

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