Manufacturing method of photodiode structure and photodiode structure

Abstract

To fabricate a photodiode structure, a substrate (1) is provided having an upper layer (2) formed from a semiconductor material doped with cadmium. A first HgCdTe-based layer (3) is formed by liquid phase epitaxy from the upper layer (2) using a bath containing an n-type electroactive dopant to electrically dope the first layer (3). Cadmium diffuses from the upper layer (2) into the first layer (3), thereby forming a decreasing cadmium concentration gradient starting at the interface to the upper layer (2) and moving away from the interface. The cadmium concentration gradient induces a decreasing band gap width gradient in the first layer (3) starting at the interface, and an n-type dopant concentration gradient in the first layer (2) starting at the interface.

Description

[Technical field]

[0001] The present invention relates to a method for fabricating a photodiode structure, and more generally to optoelectronic devices. [Background technology]

[0002] In many technical fields, photodetectors are known that provide electrical signals representative of the observed scene. In particular, there exist photodetectors that are specifically configured to capture infrared signals, and such photodetectors can be used in night vision devices and in many other fields of activity where the majority of signals of interest are in the infrared region.

[0003] For example, it is known to produce infrared detectors from semiconductor materials that are alloys of HgCdTe or of MCT, which stands for mercury cadmium telluride. Such materials are particularly attractive because they have a direct gap associated with a large band gap energy value that is tunable depending on the cadmium composition. Such materials are produced by epitaxy from a substrate that acts as a support and is totally transparent in the wavelength range under investigation.

[0004] It is particularly advantageous to use a photodetector in the form of a photodiode, more particularly in the form of a pn or pin junction. Electromagnetic radiation passing through the photodiode with an energy greater than the band gap value is absorbed and converted into electron-hole pairs. The charges are collected and processed as a signal representative of the observed scene.

[0005] Different photodiode architectures are used to capture infrared radiation, and several differences exist between these architectures aimed at improving operation, for example, signal-to-noise ratio.

[0006] The construction of a photodiode is presented in the document US 2014 / 0217540, which describes a layer stack comprising, in sequence, starting from a substrate, a passivation buffer layer of a first conductivity type, an active layer of the first conductivity type or undoped, a covering layer, a second passivation layer and a junction layer of the second conductivity type for forming a pn junction together with the active layer and together with the second passivation layer.

[0007] The document US Patent No. 4,376,663 describes the formation of a HgCdTe layer by liquid phase epitaxy on a CdTe substrate. The substrate can be undoped to form a p-type doped HgCdTe layer, or the substrate can be doped with indium to form an n-type doped HgCdTe layer by diffusing indium atoms during epitaxial growth. The n-type doped HgCdTe layer can also be formed by diffusing a gas atmosphere. The HgCdTe layer comprises a thickness of 20 micrometers to 30 micrometers, even 40 micrometers. The CdTe layer is epitaxied on the HgCdTe layer and comprises a thickness of 5 micrometers to 10 micrometers.

[0008] The publication by Martyniuk et al. ("Utmost response time of long-wave HgCdTe photodetectors operating under zero voltage condition") discloses an infrared detector formed by layers of HgCdTe to define an npn-type structure, in which a centrally located absorption layer is 10 17 cm -3 The structure is doped with p+ type with an electrical acceptor concentration equal to the layer that receives the incident radiation (X Cd =0.25) and the absorbing layer (X Cd=0.19) and a decreasing cadmium composition gradient from the other end of the absorbing layer toward the contact layer. The contact layer has the opposite conductivity to the absorbing layer and is formed with an increasing concentration gradient back to the cadmium concentration of the absorbing layer. The photodetector has a highly doped absorbing layer for high frequency operation. Summary of the Invention

[0009] One object of the present invention is to provide a method for fabricating a photodiode structure that is easy to implement while at the same time ensuring better channeling of photogenerated carriers.

[0010] The object is to provide a method for manufacturing a photodiode structure, comprising the steps of: - providing a substrate having at least an upper layer formed from CdZnTe or CdTe, the upper layer having a first concentration of a first electrical dopant of a first conductivity type and a first cadmium concentration, the upper layer being of the first conductivity type; - growing a first layer of HgCdTe or of at least one of its quaternary derivatives from the top layer by liquid phase epitaxy using a single bath containing precursors of the first layer including cadmium and possibly at least one electrical dopant, the bath having a second cadmium concentration smaller than the first cadmium concentration and possibly a second electrical dopant concentration smaller than the first concentration of the first electrical dopant, the at least one electrical dopant being selected from the first electrical dopant and / or the second electrical dopant, and forming a first cadmium concentration gradient that decreases continuously away from the interface between the top layer and the first layer, the first concentration gradient having a minimum atomic cadmium concentration value between 10 atomic % and 25 atomic %, thereby forming a second concentration gradient of at least one electrical dopant in the first layer, the second concentration gradient decreasing, the first layer being of a first conductivity type, and a thickness of the first layer being less than 6 microns. - forming at least one junction layer formed from a second semiconductor material doped with a second conductivity type to form a pn junction or a pin junction with the first layer, the first cadmium concentration gradient and the second concentration gradient of the first electrical dopant being preserved in the first layer after formation of the junction layer; This is easily achieved by a method which sequentially includes:

[0011] In one development, the thickness of the first layer, after formation of the second layer, is greater than or equal to 3 microns, preferably less than or equal to 5 microns.

[0012] In certain embodiments, the first concentration gradient extends within the first layer until the cadmium concentration is constant over a distance of between 500 nm and 1.5 microns from the interface.

[0013] Advantageously, the cadmium concentration difference within the first concentration gradient is at least equal to 10 atomic %, preferably at least equal to 25 atomic %.

[0014] A second concentration gradient is formed within the first layer, the first electrical dopant concentration being 2×10 15 at / cm 3 Advantageously, the distance between the first and second electrodes is extended until the distance is less than 1 mm.

[0015] The first dopant concentration in the upper layer is set to 5×10 15 at / cm 3 ~1×10 19 at / cm 3 It is further advantageous to

[0016] In a preferred embodiment, the first electrical dopant concentration gradient in the first layer is greater than 5×10 18 at / cm 3 From a concentration of over 2 x 10 15 at / cm 3 The concentration of 1000 to 1500 mg / kg is reduced to less than 100 mg / kg.

[0017] Advantageously, the combined thickness of the first layer and the bonding layer is less than 6 microns, preferably less than 5 microns.

[0018] In certain configurations, the substrate is at least partially or entirely removed after forming the conductive contacts on the second layer.

[0019] Preferably, the first electrical dopant is iodine and the second concentration gradient of iodine originates at the interface between the top layer and the first layer and decreases continuously away from the interface.

[0020] It is a further object of the present invention to provide a photodiode structure that exhibits superior performance over prior art configurations while achieving better control over the channeling of photogenerated charge carriers.

[0021] The photodiode structure is - a first layer consisting of HgCdTe or at least one of its quaternary derivatives, the first layer including a first cadmium concentration gradient that starts at an interface between an upper layer and the first layer and decreases continuously away from the interface, the minimum atomic cadmium concentration within the first cadmium concentration gradient being between 10 atomic % and 25 atomic %; and a second concentration gradient of at least one electrical dopant within the first layer, the second concentration gradient decreasing, the first layer being of a first conductivity type, and a thickness of the first layer being less than 6 microns; a second layer of HgCdTe or at least one of its quaternary derivatives, of a second conductivity type opposite to the first conductivity type, the first layer and the second layer forming a junction for converting electromagnetic signals into electron-hole pairs, the second layer having at least the same composition of Hg, Cd and Te as the first layer at the interface between the first layer and the second layer; sequentially includes

[0022] Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention and modes of implementation thereof, given for non-restrictive illustrative purposes only and illustrated in the accompanying drawings, in which: [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 illustrates diagrammatically in cross section a first step in a method for manufacturing a photodiode structure according to the invention. [Diagram 2] FIG. 2 illustrates diagrammatically in a cross-sectional view a second step in the method for manufacturing a photodiode structure according to the invention. [Diagram 3] FIG. 3 illustrates diagrammatically in a cross-sectional view a third step in the method for manufacturing a photodiode structure according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] To detect electromagnetic radiation in the infrared range, it is preferable to use a photodiode, be it a pn or pin diode, which converts the collected electromagnetic signal into an electrical signal. A photodiode has a junction formed by a layer of n-type conductivity and a layer of p-type conductivity.

[0025] A pn diode is formed by a first layer of n-type doped semiconductor material and a second layer of p-type doped semiconductor material, the two layers of semiconductor material being in direct contact and defining an interface.

[0026] A pin diode is formed from a first layer of n-type doped semiconductor material, a second layer of p-type doped semiconductor material, and a third layer of unintentionally doped semiconductor material or semiconductor material having an external doping close to the doping value of the unintentionally doped semiconductor material layer, the third layer separating the first layer of n-type doped semiconductor material from the second layer of p-type doped semiconductor material and having an interface to the first and second layers.

[0027] Other more complex structures can be used, such as having one or more doped or undoped passivation layers, interposed between the first layer of n-type doped semiconductor material and the second layer of p-type doped semiconductor material, insofar as these two layers form a junction. The formation of the junction ensures that a depletion region is formed in the photodiode. The depletion region extends partially into the p-type doped layer and partially into the n-type doped layer. Unlike the structure disclosed by Martyniuk, the photodetector only has a single pn junction or a single pin junction. In other words, the photodetector is neither an npn nor a pnp structure. The photodetectors are structurally different and have different modes of charge carrier collection, which means that the operation modes of the photodetectors are different.

[0028] To capture infrared radiation, photodiodes made of semiconductor materials with small band gap values ​​in the infrared range can be used, for example made of materials with a typical composition represented by the chemical formula HgCdTe. The band gap value varies depending on the composition of the alloy, making it possible to adjust the wavelength range collected by the diode. In the case of HgCdTe-based alloys, the band gap value varies depending on the cadmium concentration and on the mercury concentration. Layers made of HgCdTe material are layers with Hg, Cd and Te as the main components. The exact composition of each component is not specified unless otherwise specified.

[0029] In order to manufacture photodiodes exhibiting good performance, and preferably a large number of photodiodes exhibiting good performance, it is advantageous to form the photodiodes on a substrate, and more particularly to form at least one active layer of the photodiode by a liquid phase epitaxy process.

[0030] In a particular configuration, a first layer of n-type doped semiconductor material is formed on a substrate, followed by a second layer of p-type doping, while in another configuration, a first layer of p-type doped material is formed on a substrate, followed by a second layer of n-type doping.

[0031] To obtain good channeling of the photogenerated charge carriers, it is advantageous to form the photodiode in which there is an electrical doping gradient within the first layer of n-type doped semiconductor material and / or within the first layer of p-type doped semiconductor material. The same is advantageous in the second layer of semiconductor material. However, since the electrical doping level has an influence on the photodetection and on the electro-optical performance (especially the dark current), it is advantageous to relate the electrical doping gradient to a bandwidth gradient, so that the most electrically doped regions are the less electro-optically active.

[0032] If a layer of n-type doped semiconductor material is formed on a substrate followed by a layer of p-type doped semiconductor material, it is advantageous for the concentration of electrically active n-type dopants to decrease from the substrate towards the layer of p-type doped semiconductor material, and the band gap width in the layer of n-type doped semiconductor material also decreases from the interface to the substrate towards the layer of p-type doped semiconductor material.

[0033] If a layer of p-type doped semiconductor material is formed on a substrate followed by a layer of n-type doped semiconductor material, it is advantageous for the concentration of electrically active p-type dopants to decrease from the substrate towards the layer of n-type doped semiconductor material, and the band gap width in the layer of p-type doped semiconductor material also decreases from the interface to the substrate.

[0034] It is particularly advantageous for the concentration of the n-type or p-type dopant to decrease continuously or, possibly, for there to be one or more plateaus in the concentration of the n-type or p-type dopant. In an advantageous embodiment, the concentration of the n-type or p-type electrical dopant strictly decreases from the interface to the substrate before becoming constant or substantially constant in the p-type doped layer or adjacent to the n-type doped layer to form a pn junction or pin junction.

[0035] Achieving such a concentration gradient by liquid phase epitaxy procedures is particularly complicated due to the need to provide multiple growth baths with different dopant concentrations and different main component concentrations. The multiple baths are sequential to each other to form successive layers with different compositions to form the gradient. Furthermore, it is common practice to melt a part of the upper layer of the substrate to facilitate the formation of an interface with good crystallographic quality. If the successive layers are formed at high temperatures, the previously formed layers will be totally or partially dissolved, and the final concentration profile will deviate significantly from the initially desired profile. This drawback limits the advantages of sequential liquid phase epitaxy using successive baths. This problem can be more severe due to the fact that the gradient is formed over a small thickness.

[0036] It is known to grow a HgCdTe layer on a CdTe substrate by liquid phase epitaxy. Citation may be made to the teachings of the above-mentioned document US Pat. No. 4,376,663, which proposes liquid phase epitaxial growth of a HgCdTe layer on a CdTe substrate and then growing a CdTe layer on the HgCdTe layer.

[0037] It has been proposed to use the diffusion phenomenon of substrate atoms in the liquid phase epitaxy process to easily achieve n-type or p-type electrical doping gradients. The atoms undergo the same thermal history during the liquid phase epitaxy process, allowing a better correlation of the doping profile for the components in the HgCdTe alloy with the n-type or p-type electrical dopant profile from the interface to the substrate. In a preferred embodiment, the liquid phase epitaxy is carried out with a tellurium supersaturation in order to form a tellurium-rich layer.

[0038] As shown in FIG. 1, a substrate 1 is provided with at least a top layer 2 made of a first semiconductor material. The top layer 2 of the first semiconductor material is chosen from CdTe and CdZnTe. The thickness of the top layer 2, when different from the substrate, is advantageously greater than 500 nm, more preferably less than 2 microns. The first semiconductor material has a first cadmium concentration. In an advantageous configuration, cadmium represents at least 30 atomic %, more advantageously at least 40 atomic %, and even more advantageously at least 45 atomic % of the top layer 2 in a CdZnTe type material. Preferably, the CdZnTe type material has a tellurium content equal to 50 atomic %, a cadmium content equal to or greater than 45% and a zinc content equal to or less than 5%.

[0039] The top layer 2 has a first electrical dopant. The first electrical dopant can be an n-type dopant, such as chlorine, iodine, and indium, or a p-type dopant, such as lithium, sodium, potassium, copper, silver, and gold. The top layer 2 can have a single n-type or p-type electrical dopant, or can have multiple different electrical dopants to take advantage of different diffusion rates. When the top layer 2 contributes to current flow in the completed diode, the first electrical dopant is the electrically active dopant of the top layer 2, and at least a portion of the first electrical dopant is located in a substitutional position.

[0040] Before the liquid phase epitaxy step, the introduction of the first electrical dopant into the upper layer 2 can be carried out by an implantation step advantageously followed by a recrystallization anneal on the surface of the substrate 1 in order to promote the formation of a good quality crystallographic seed for the subsequent growth of a good quality monocrystalline first layer 3. The recrystallization anneal is advantageously carried out at a temperature of 400° C. or higher. The doping of the first layer 2 can also be carried out by annealing the substrate 1 in an atmosphere containing a precursor of the first electrical dopant. Furthermore, the upper layer 2 can also be doped when it is formed, for example by crystal growth, more particularly when pulling an ingot which is subsequently sliced ​​to form the substrate 1.

[0041] During the process of forming the first layer 3 by liquid phase epitaxy, a part of the cadmium content and a part of the first electrical dopant content will migrate from the upper layer 2 into the first layer 3. The first layer 3 is formed from a semiconductor material that is a ternary or at least ternary, e.g. quaternary, alloy in which cadmium is one of the main components.

[0042] In one embodiment, the top layer 2 corresponds to the top region of the substrate 1, i.e., the top layer 2 is made of the same semiconductor material and has the same cadmium concentration as the substrate 1. In an alternative embodiment, the top layer 2 has a cadmium concentration that is greater than, or possibly less than, the cadmium concentration of the substrate 1 to better control the cadmium profile in the first layer 3 during epitaxy. Also, the concentration of the first electrical dopant can be the same between the substrate and the top layer 2.

[0043] As shown in Figure 2, a first layer 3 of a second semiconductor material is grown by liquid phase epitaxy from an upper layer 2 of the first semiconductor material. The first layer 3 has an interface to the upper layer 2. The thickness of the first layer 3 is preferably greater than 500 nm and advantageously less than 6 microns. More preferably, the thickness is greater than 1 micron and advantageously less than or equal to 6 microns or even less than 5 microns. The thickness of the first layer 3 is preferably greater than 3 microns, so that the final structure has an absorbing region extending over at least 2 microns.

[0044] The second semiconductor material is an alloy containing at least Hg, Cd, and Te. The semiconductor material formed is single crystalline and has the chemical formula Hg 1-x CD x The second semiconductor material may be represented by the formula HgCdTe, or more generally by the formula HgCdTe. The second semiconductor material may be at least a quaternary derivative of HgCdTe, such as Hg 1-x-y CD x Zinc y Te, Hg 1-x-y CD x Mn y Te, or Hg 1-x CD x Te 1-z Se z In a typical embodiment, the second semiconductor material is a material having a band gap energy value that varies with at least the cadmium concentration. The first semiconductor material, although different from the second semiconductor material, allows for tuning of the lattice constant to effect single crystal growth of the first layer 3 from the surface of the top layer 2.

[0045] The liquid phase epitaxy process uses a single bath containing all the elements involved in the formation of the second semiconductor material, including Hg, Cd, Te, and possibly Zn, Mn, Se, or other necessary materials. The bath may also contain at least one electrical dopant to electrically dope the first layer 3 of the second semiconductor material. The at least one electrical dopant may be a first electrical dopant and / or a second electrical dopant of the same conductivity type. The bath is not modified during growth, e.g. no components are added to the bath.

[0046] The material forming the first layer 3 is selected such that a monocrystalline first layer 3 of a second semiconductor material can be formed by liquid phase epitaxy. The bath has a cadmium concentration that is smaller than the cadmium concentration of the top layer 2. If the bath contains an electrical dopant, its concentration is smaller than the concentration of the first electrical dopant in the top layer 2.

[0047] When liquid phase epitaxial growth takes place, the at least one electrical dopant and the cadmium diffuse from the upper layer 2 into the first layer 3 of the second semiconductor material.

[0048] The growth from the top layer 2 creates a cadmium concentration gradient from the interface to the top layer 2, which creates a bandgap width gradient in the first layer 3 starting at the interface to the top layer 2 and decreasing with distance from the interface. The minimum cadmium concentration in the first layer 3, i.e. at the bottom and / or constant part of the gradient, is between 10 atomic % and 25 atomic % to obtain good absorption of the targeted infrared radiation. It is also preferred that the cadmium gradient represents a concentration difference at least equal to 10 atomic % or at least equal to 25 atomic % to provide a good separation of the electro-optical behavior.

[0049] As the growth proceeds, the cadmium present in the top layer 2 becomes more and more difficult to move, since it has to diffuse over larger distances and is itself consumed. An equilibrium is created between the cadmium present in the bath and the cadmium coming from the top layer 2. The cadmium concentration on the growth front of the first layer 3 will decrease until it finally reaches a value defined by the bath, where the cadmium concentration is constant or nearly constant, for example with a gradient of less than 1%. A constant cadmium concentration means that the concentration has a variation of less than 1 atomic % over 50 nm.

[0050] The growth conditions for the first layer 3 are preferably selected to form a first layer 3 having a constant cadmium concentration over at least 100 nm, advantageously over at least 500 nm or 1 micron or 2 microns, and advantageously the constant cadmium concentration is located at a distance of 500 nm to 1.5 microns from the interface.

[0051] At the same time, an electrical dopant concentration gradient is formed in the first layer 3. The electrical dopants are mainly or almost exclusively located in substitutional positions. This electrical dopant concentration gradient is not related to a change in composition in the growth bath, but to the progress of incorporation / diffusion of a first electrical dopant from the upper layer 2, possibly from the bath, into the crystal lattice, and also to the possible incorporation of a second electrical dopant. Preferably, the electrical dopant concentration gradient extends over at least 500 nm, more preferably between 500 nm and 1.5 microns. Also, the first electrical dopant concentration is preferably greater than 2×10 15 at / cm 3 Preferably, the gradient extends until it is less than .

[0052] In a particular case, the electrical dopant is indium and the concentration of the electrical dopant decreases starting at the interface between the top layer 2 and the first layer 3. In another particular case, the electrical dopant is iodine and there is a peak of the first electrical dopant at or very close to the interface, such that the concentration can increase slightly and then decrease over the remainder of the first layer 3.

[0053] In a preferred embodiment, the concentration of the electrical dopant in the bath is zero. Advantageously, the growth conditions are such that the concentration of the first electrical dopant in the bath is, for example, 2×10 15 cm -3 A concentration such as 0.1% by weight is selected to define the required minimum concentration in the first layer 3. As the growth of the first layer 3 takes place, the electrical dopant concentration decreases and can then remain constant at a bath defined value. A constant electrical dopant concentration means that the concentration has a relative variation of less than 10% over 50 nm.

[0054] Advantageously, the growth of the first layer 3 is carried out until it exhibits a constant cadmium concentration over at least 500 nm and an electrical dopant concentration that varies by less than 10% over at least 50 nm. If the bath does not contain any first electrical dopant, the growth is preferably carried out until the first layer 3 is unintentionally doped and until the cadmium concentration is constant. Preferably, the first layer 3 is formed until it exhibits an at least constant cadmium concentration over at least 2 microns.

[0055] The temperature of the liquid phase epitaxy step is greater than the threshold temperature to ensure diffusion of the cadmium and the first electrical dopant. The thermal budget (integrated time and temperature) of the liquid phase epitaxy step is less than the threshold thermal budget to achieve homogenization of the cadmium and / or the first electrical dopant over the thickness of the first layer 3. If the temperature is too low, the cadmium and / or the first dopant cannot diffuse and the required reduction profile cannot be formed. Conversely, if the thermal budget is too high, the cadmium and / or the first electrical dopant will diffuse too much and the concentration will be homogenous over the thickness of the deposited first layer 3. The liquid phase epitaxy temperature can be between 400°C and 500°C.

[0056] In another technical field, it is known from the document US 5861321 to grow a HgCdTe layer on a CdTe substrate or on a CdZnTe substrate by liquid phase epitaxy. The thickness of the HgCdTe layer is at least 30 microns. The substrate is doped n-type or p-type and the HgCdTe layer is formed undoped. An anneal is then performed to form a uniform doping in the HgCdTe layer. In another embodiment, the substrate is doped with dopants present in the substrate. A uniformization anneal is performed during the growth.

[0057] The growth step of the first layer 3 by liquid phase epitaxy is arranged so that the thickness of the first layer 3 is advantageously less than 5 microns, preferably less than 4 microns. The growth step of the first layer 3 by liquid phase epitaxy is arranged so that the thickness of the first layer 3 is advantageously greater than 1 micron, preferably greater than 2 microns. By forming a first layer 3 with reduced thickness, the thermal budget during growth is reduced, which limits diffusion phenomena that tend towards concentration homogenization.

[0058] It is particularly advantageous to form the first layer 3 with a cadmium gradient of less than 2 microns, and even less than 1.5 microns, the gradient within the first layer 3 being measured from the interface with the upper layer to the portion where the concentration of the first element is constant.

[0059] In an advantageous embodiment, the extent of the cadmium gradient is less than 1.5 microns, more preferably less than 1 micron. It is particularly advantageous to form the first layer 3 such that the extent of the cadmium gradient is greater than 100 nm, preferably greater than 500 nm.

[0060] In an advantageous embodiment, the first layer 3 has a minimum cadmium concentration, i.e. a non-cadmium-rich portion in the first gradient, at least equal to 10 atomic %, preferably less than or equal to 30 atomic % or less than or equal to 20 atomic %.

[0061] The first electrical dopant and the cadmium penetrate into the first layer 3 by diffusion, so that the cadmium and the first electrical dopant have similar and well-controlled doping profiles, meaning that the shapes of the profiles are similar to each other, while at the same time corresponding to very different concentration levels when the concentration profiles are observed on a semi-logarithmic scale. The ranges of the first and second concentration gradients are preferably identical or exhibit a difference of less than 1 micron, or even less than 500 nm, over the thickness.

[0062] Advantageously, prior to the liquid phase epitaxy step, the upper layer 2 is 5×10 16 at / cm 3 Advantageously, 5×10 17 at / cm 3 More like 5×10 18 at / cm 3 or 1×10 19 at / cm 3The concentration of the first electrical dopant is preferably 5×10 15 at / cm 3 ~1×10 19 at / cm 3 It is.

[0063] The concentration of the first electrical dopant in the upper layer 2, for example made of indium or iodine, is adjusted so that the active electrical dopant is introduced into the first layer 3 at a concentration of at least 5×10 18 at / cm 3 and preferably at least 1×10 19 at / cm 3 It is particularly advantageous to select the ion exchange material so as to ensure that it is captured by the ion exchange material.

[0064] In preferred embodiments, the second electrical dopant concentration gradient exhibits a concentration ratio between its highest and lowest concentrations that is at least equal to 10, or at least equal to 50, or at least equal to 100, or even equal to 1000. Such a profile is obtained in a single liquid phase epitaxy operation using a single bath.

[0065] The concentration of the first electrical dopant in the upper layer 2 , the concentration of the dopant in the liquid bath and the operating conditions in the liquid phase epitaxy process define the doping profile in the first layer 3 .

[0066] It is advantageous to select the growth operating conditions such that a constant cadmium concentration is reached before the first layer 3 reaches a constant electrical dopant concentration. A minimum concentration of the first electrical dopant is present only at the constant or nearly constant cadmium content of the first layer 3 and is preferably less than 5×10 13 at / cm 3 ~2×10 15 at / cm 3 It is said.

[0067] In the growth bath, a concentration that ensures the incorporation of n-type or p-type electrically active dopants is set to 5×1015 at / cm 3 Less than 2 x 10 15 at / cm 3 less than 5×10 14 at / cm 3 It is particularly advantageous to select less than

[0068] In a preferred embodiment, the first layer 3 is at least 1×10 18 at / cm 3 or 5×10 18 at / cm 3 from the first dopant concentration equal to 2×10 15 at / cm 3 to less than 4 x 10, preferably at least 18 at / cm 3 from a value equal to 8 x 10 14 at / cm 3 The concentration of electrically active dopants is strictly graded to a value less than 1000 nm. By using a doping gradient that achieves weak doping in the absorbing region, or a doping gradient that tends to a level corresponding to depletion in the layer, the charge carriers can be guided more efficiently. Such a configuration reduces crosstalk and improves the modulation transfer function (MTF).

[0069] During the liquid phase epitaxy process, the cadmium concentration is modified, which changes the band gap width of the deposited material. At the same time, the concentration of the first electrical dopant changes and tends to decrease. The characteristics of the two concentration gradients are not defined by the growth baths, but by the cadmium concentration and the first electrical dopant concentration in the upper layer and in the bath in relation to the thermal history of the liquid phase epitaxy process. The two concentration gradients can be defined by performing one or more preliminary simulations, which take into account the cadmium diffusion coefficient, the first electrical dopant diffusion coefficient, the respective concentrations in the bath and in the upper layer 2, and the depletion of the two components as the growth progresses.

[0070] The modulation of the cadmium concentration, which represents a modulation of the band gap width and a modulation of the electrical dopant, makes it possible to functionalize several individual portions of the first layer 3 in a single step to convert an electromagnetic signal into an electrical signal and further to facilitate the conduction of this electrical signal.

[0071] The first layer 3 has a first portion 3a designed to facilitate the passage of electrical signals and a second portion 3b, over at least a portion of its thickness, designed to capture only or mainly electromagnetic signals.

[0072] By forming the first portion 3a of the second semiconductor material to be rich in the first electrical dopant and further entirely rich in cadmium, a low resistance layer with a wide bandgap can be formed, which ensures good transmission of the electrical signal. The first portion 3a is less electro-optically active, which reduces the effect of crystal defects on the signal quality provided by the photodiode. The second portion 3b forms the absorbing portion of the photodiode structure, i.e. the portion designed to capture the electromagnetic radiation. The second portion 3b presents a bandgap width smaller than the threshold in order to collect the required electromagnetic radiation. The second portion 3b presents a smaller bandgap width and is weakly doped or undoped in order to provide good optoelectronic performance for the photodetector. The use of weak doping allows the value of the dark current, a parameter that impairs the performance of the photodetector, to be reduced, which facilitates the use of a thicker absorbing layer.

[0073] The second portion 3b preferably exhibits a cadmium concentration less than a threshold value equal to 25 atomic %, and even more preferably a constant cadmium concentration. The second portion 3b preferably comprises a thickness between 1 micron and 4.5 microns, more preferably between 1.5 microns and 4 microns, or even between 2 microns and 4 microns. The thickness of the second portion can be chosen depending on the mode of formation of the second layer 4 designed to complete the bond. The use of an absorber layer of more than 1 micron, preferably more than 1.5 microns, makes it possible to achieve a better quantum efficiency, thus facilitating the use in the imaging field. Advantageously, the second portion 3b has a thickness of less than 2×10 15 at / cm 3 For example, the second portion 3b of the first layer 3 made of the second semiconductor material contains or is formed exclusively by portions with a constant or nearly constant cadmium concentration, forming the most optically active n-type or p-type doped layer.

[0074] The first portion 3a preferably exhibits a minimum value for cadmium concentration corresponding to the maximum value of the second portion 3b. The first portion 3a preferably exhibits a cadmium concentration whose maximum value is at least 30 atomic %, advantageously at least 40 atomic % or 50 atomic %. Advantageously, the second portion 3b exhibits a cadmium concentration of at least 2×10 15 at / cm 3 The first portion 3a has a first electrical dopant concentration greater than 1000 nm. The first portion 3a may have a region with a constant cadmium concentration. The first portion 3a preferably has a thickness of 1 micron to 4 microns, and more preferably has a thickness of 2 microns to 4 microns.

[0075] The presence of a first electrical dopant concentration gradient ensures the formation of an electric or pseudo-electric field in the first layer 3, which allows the transfer of photo-generated charge carriers from the first portion 3a to the second portion 3b, which can perform an efficient transfer of signals with low resistance. There is no growth interface between the second portion 3b and the first portion 3a, which ensures good transfer of signals.

[0076] In an advantageous embodiment, the upper layer 2 is doped with a bath, possibly containing only the first electrical dopant, or the first electrical dopant being predominant. If the conditions of the liquid phase epitaxy process and the concentration of the first electrical dopant do not allow the first electrical dopant concentration profile to match the cadmium concentration profile, it is advantageous to dope the upper layer 2 with a second electrical dopant of the same conductivity type as the first electrical dopant. The second electrical dopant is selected so that it exhibits a different diffusion rate than the first electrical dopant during the formation of the first layer 3. For example, in the case of n-type doping, iodine can be used in conjunction with indium in order to match the electrical doping profile to the band gap width profile defined by cadmium.

[0077] The variation of the first dopant content is carried out continuously across the thickness of the first layer 3, which allows for a continuous variation of the concentration of n-type or p-type electrically active dopants to be easily achieved, thus allowing efficient channelling of the photogenerated charge carriers upon the appearance of an electric field.

[0078] As shown in FIG. 3, after the first layer 3 of the first semiconductor material is formed, the remainder of the photodiode can be formed, in particular a second layer 4 of opposite conductivity to define a pn or pin junction. When the first layer 3 is doped n-type, the second layer 4 is doped p-type. Conversely, when the first layer 3 is doped p-type, the second layer 4 is doped n-type. As mentioned above, one or more other layers can be deposited before the second layer 4 to form the pin junction. Also, a third layer 5 can be formed that is unintentionally doped, separating the p-type and n-type doped layers that define the junction. The layer 5 can be formed of one or more different materials.

[0079] The second layer 4 is made of a third semiconductor material with a band gap width smaller than the threshold. The active or most active optical part of the photodiode is made of a material with a band gap width smaller than the threshold, so that the photodiode is sensitive to a specific range of electromagnetic radiation different from the first part 3a. This electromagnetic radiation is not collected by the first part 3a of the first layer 2 with a larger band gap width. The third semiconductor material is advantageously a ternary alloy or at least a ternary alloy, preferably of HgCdTe or based on HgCdTe. Advantageously, the total thickness of the first layer 3 and the second layer 4 is less than or equal to 6 microns, or even less than 5 microns.

[0080] For example, the second layer 4 made of a third semiconductor material is deposited by epitaxy, advantageously by liquid phase epitaxy, by molecular beam epitaxy (MBE) or by metalorganic chemical vapour deposition (MOCVD).

[0081] In an alternative embodiment, the second layer 4 designed to form a pn junction is obtained by implantation of a dopant with opposite electrical conductivity or by diffusion using an atmosphere containing a precursor of the dopant. The third semiconductor material can be identical to the second semiconductor material. The second layer 4 is formed in the first layer 3 by external doping. The thickness of the junction formed by layers 3 and 4 corresponds to the initial thickness of the first layer 3 formed by liquid phase epitaxy.

[0082] The combined thickness of the first layer 3 and the second layer 4 is advantageously less than 5 microns. The thickness of the first layer 3 is adjusted depending on whether the second layer is deposited on or within the first layer 3. In certain embodiments, the thickness of the first layer 3, after the formation of the second layer 4, is 3 microns or more, and advantageously 5 microns or less.

[0083] The first cadmium concentration gradient and the second gradient of the first electrical dopant are preserved in the first layer 3 when the formation of the second layer 4 takes place.

[0084] In a preferred embodiment, the absorption of electromagnetic radiation within the photodiode takes place mainly in the second portion 3b of the first layer 3. The thickness of the second portion 3b of the first layer 3 is greater than the thickness of the second layer 4 of the third semiconductor material when the second layer is formed within the first layer, so that an area with good electro-optical properties is preserved.

[0085] It is particularly advantageous to form an electrically conductive contact 6 on or at least in electrical contact with the second layer 4. The electrically conductive contact 6 can be formed by a contact layer, which can advantageously consist of a pure metallic material or of a metal alloy. The contact layer can be etched after deposition in order to form the contacts 6. When several photodiodes are formed on the same substrate 1, a specific contact is formed on each photodiode. The first cadmium concentration gradient and the second gradient of the first electrical dopant are preserved in the first layer 3 when the formation of the contact(s) 6 takes place.

[0086] The photodiode is advantageously made of silicon nitride, for example Si 3 N 4 From or silicon oxide SiO x The first layer 3 is partially covered by a covering layer 7 formed from a ZnS layer or from a ZnS layer, which protects the photodiode from the external environment, e.g. from moisture. The first cadmium concentration gradient and the second gradient of the first electrical dopant are preserved in the first layer 3 when the formation of the covering layer 7 takes place.

[0087] The growth of the first layer 3, the second layer 4 and, if present, the third layer 5 is advantageously carried out in such a way that a plurality of photodiodes is formed. It is advantageous to form more than one photodiode in the form of a photodiode array.

[0088] The conductive contacts 6 are designed to be connected to a readout circuit which will bias the photodiode and receive an electrical signal representative of the observed scene. Advantageously, each photodiode is associated with a readout circuit. Multiple readout circuits can also be connected as a readout circuit array, hybridizing multiple readout circuits to multiple photodiodes to form a focal plane array (FPA).

[0089] It is particularly advantageous to operate the photodiode at low temperatures, preferably below 0°C, more preferably in the range of 130K to 250K.

[0090] It is conceivable to remove the substrate 1 after the photodiode has been formed, for example after the conductive contacts 6 have been formed or after the covering layer 7 has been deposited or after hybridization with the readout circuitry has been carried out. Alternatively, the substrate 1 can be thinned or it can be kept at its initial thickness. Electromagnetic radiation is incident through the substrate 1 into the photodiode structure.

[0091] A photodiode structure is proposed which includes a junction formed from a first layer 3 doped with n-type conductivity and a second layer 4 doped with p-type conductivity. The first layer is formed from a first semiconductor material and defines a first highly doped portion 3a and a second weakly doped portion 3b.

[0092] The photodiode structure sequentially comprises a first portion of a first layer formed from a first semiconductor material, a second portion of the first layer formed from the first semiconductor material, and a second layer formed from a second semiconductor material, the first layer 3 and the second layer 4 forming a junction that converts an electromagnetic signal into electron-hole pairs.

[0093] When the second layer 4 is formed by implanting dopants of the second conductivity type into the first layer 3 to form a junction, both sides of the junction have the same semiconductor material, except for the doping type.

[0094] A photodiode is obtained having a first layer 3 formed from HgCdTe or at least one of its quaternary derivatives. The first layer 3 includes a first cadmium concentration gradient and a second concentration gradient of a first electrical dopant. Both the first and second concentration gradients decrease starting from one end of the first layer 3. This end can be uncovered when the substrate 1 and the top layer 2 are removed, or can be uncoated with a material having predefined optical properties. In some configurations, the substrate 1 is at least partially removed after forming the conductive contact 6 on the second layer 4, or the substrate 1 is completely removed after forming the conductive contact 6 on the second layer 4.

[0095] The first layer 3 is of a first conductivity type and has a thickness of less than 6 microns. The second layer 4 is also formed from HgCdTe or from at least one of its quaternary derivatives. The second layer 4 is of a second conductivity type opposite to the first conductivity type, the first and second layers forming a junction that converts electromagnetic signals into electron-hole pairs. The second layer 4 has at least the same composition of Hg, Cd and Te as the first layer 3 at the interface between the first layer 3 and the second layer 4.

[0096] The first layer 3 exhibits a decreasing cadmium concentration and a decreasing electrically active dopant concentration between the first portion 3a and the second portion 3b, making it possible to provide a better quality signal. The decrease in concentration does not have discontinuities.

[0097] The precautions taken in the liquid phase epitaxy process to form the concentration gradient are maintained until the end of the method for forming the diode. The method for fabricating the diode does not include an anneal to homogenize the cadmium concentration and / or the first electrical dopant concentration throughout the thickness of the first layer.

[0098] In the prior art method, the electrically active dopant concentration profile is obtained by a number of crenels corresponding to the number of successive baths. The profile formed by the crenels is then subjected to an anneal designed to eliminate the crenels. Since the diffusion rate of cadmium is different from that of the n-type electrically active dopant, it is particularly difficult to obtain a profile identical to that obtained by the method described above. Since the doping is obtained when growth is carried out by liquid phase epitaxy, this doping method produces significantly fewer defects compared to implantation processes, especially in the weakly doped regions that are electro-optically active.

[0099] The photodiode structure allows for better control of photo-generated charge carriers, e.g., when multiple photodiodes are connected to form an array, allowing for reduced blurring of the resulting image.

Claims

1. 1. A photodiode structure comprising a junction designed to convert infrared electromagnetic radiation into electron-hole pairs, said junction comprising a first layer (3) of a first conductivity type formed from HgCdTe or at least one of its quaternary derivatives, and a junction layer (4) of a second conductivity type opposite to said first conductivity type formed from HgCdTe or at least one of its quaternary derivatives, thereby forming a p-n junction or a pin junction, The first layer (3) is a first end and a second end, said first end being separated from said bonding layer (4) by said second end, a first cadmium concentration gradient decreasing in a direction from the first end of the first layer (3) towards the bonding layer (4), the first cadmium concentration gradient having a minimum atomic cadmium concentration of between 10 atomic % and 25 atomic %, the first cadmium concentration gradient defining a first portion (3a) and a second portion (3b), the second portion (3b) having a smaller band gap width than the first portion (3a); a second concentration gradient of at least one electrical dopant, said second concentration gradient decreasing in the direction from said first end towards said bonding layer (4); Including, the total thickness of the first layer (3) and the bonding layer (4) is less than 6 microns; said first cadmium concentration gradient is continuously decreasing in the form of a diffusion profile; said second concentration gradient of at least one electrical dopant decreases continuously in the form of a diffusion profile; - said second concentration gradient extends within said first layer (3) over a distance of between 500 nm and 1.5 microns from said first end; said second concentration gradient is 5×10 １８ at / cm ３ From a concentration of over 2 x 10 １５ at / cm ３ to concentrations below A photodiode structure, characterized in that said first layer (3) exhibits a greater thickness than said bonding layer (4).

2. 2. The photodiode structure of claim 1, wherein a conductive contact (6) is electrically connected to the bonding layer (4), the conductive contact (6) being formed from a pure metallic material or from a metallic material alloy.

3. 2. The photodiode structure of claim 1, wherein the first layer (3) has a constant cadmium concentration over at least 500 nm.

4. 4. The photodiode structure of claim 3, wherein the first layer (3) has a constant cadmium concentration over at least 1 micrometer.

5. 2. The photodiode structure of claim 1, wherein the first layer (3) has a constant cadmium concentration and exhibits unintentionally undoped portions.

6. 2. The photodiode structure of claim 1, wherein the first layer (3) has a thickness of 3 microns or more.

7. A photodiode structure according to any one of the preceding claims, wherein the first layer (3) has a thickness less than or equal to 5 microns.

8. 2. The photodiode structure of claim 1, wherein a difference in cadmium concentration within said first cadmium concentration gradient is at least equal to 25 atomic percent.

9. 2. The photodiode structure of claim 1, wherein the combined thickness of the first layer (3) and the bonding layer (4) is less than 5 microns.

10. 2. The photodiode structure of claim 1, wherein the second concentration gradient of at least one electrical dopant comprises an iodine concentration gradient and a chlorine concentration gradient.

11. 10. A method for manufacturing a photodiode structure according to claim 1, comprising the steps of: providing a substrate (1) having at least an upper layer (2) made of CdZnTe or CdTe, said upper layer (2) having a first concentration of a first electrical dopant of a first conductivity type and a first cadmium concentration, said upper layer (2) being of said first conductivity type; growing said first layer (3) of HgCdTe or of at least one of its quaternary derivatives from said upper layer (2) by liquid phase epitaxy using a single bath containing precursors of said first layer (3) including cadmium and possibly at least one electrical dopant, said bath having a second cadmium concentration smaller than said first cadmium concentration and possibly a second concentration with respect to the electrical dopant smaller than said first concentration of said first electrical dopant, said at least one electrical dopant being selected from said first electrical dopant and / or a second electrical dopant, said liquid phase epitaxy being carried out with respect to a portion of the cadmium atoms and with respect to the first electrical dopant, and a temperature at which diffusion of the at least one electrical dopant from the top layer (2) into the first layer (3) is obtained, whereby a first cadmium concentration gradient is formed that decreases continuously from the interface between the top layer (2) and the first layer (3) with increasing distance from the interface, the minimum atomic cadmium concentration value within the first concentration gradient being between 10 atomic % and 25 atomic %, the first concentration gradient defining a first portion (3a) and a second portion (3b), the second portion (3b) having a band gap width smaller than the first portion (3a), whereby a second concentration gradient of the at least one electrical dopant is formed within the first layer (3), the second concentration gradient of the at least one electrical dopant being between 5×10 １８ at / cm ３ From a concentration of over 2 x 10 １５ at / cm ３ the second concentration gradient continuously decreases to a concentration less than 500 nm, the second concentration gradient extending within the first layer (3) over a distance of 500 nm to 1.5 microns from the interface to the upper layer (2), the first layer (3) being of the first conductivity type; forming at least a bonding layer (4) of a second conductivity type opposite to the first conductivity type, made of HgCdTe or at least one of its quaternary derivatives, to form a p-n junction or a pin junction with the first layer (3), the first cadmium concentration gradient and the second concentration gradient of the first electrical dopant being preserved in the first layer (3) after the formation of the bonding layer (4), the second concentration gradient decreasing in the direction from the interface between the top layer (2) and the first layer (3) towards the bonding layer (4), the first layer (3) exhibiting a greater thickness than the bonding layer (4), the total thickness of the first layer (3) and the bonding layer (4) being less than 6 microns; The method of claim 1,

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