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] Photodetectors are known in many technical fields that provide electrical signals representative of the observed scene. In particular, there are photodetectors that are specifically designed 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 fabricate infrared detectors from semiconductor materials that are alloys of HgCdTe or MCT (mercury cadmium telluride). Such materials are particularly attractive due to their direct gap associated with large bandgap energy values ​​that can be adjusted depending on the cadmium content. Such materials are produced by epitaxy on a substrate that serves as a support and is totally transparent in the wavelength range of interest.

[0004] It is particularly advantageous to use a photodetector in the form of a photodiode, more particularly in the form of a pn junction or a pn junction. Electromagnetic radiation passing through the photodiode with an energy greater than the bandgap 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 to improve performance, 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 including, 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 with the second passivation layer.

[0007] U.S. Patent No. 4,376,663 describes the formation of an HgCdTe layer on a CdTe substrate by liquid phase epitaxy. A p-type doped HgCdTe layer can be formed by using an undoped substrate, or an n-type doped HgCdTe layer can be formed by doping the substrate with indium and diffusing indium atoms during epitaxial growth. An n-type doped HgCdTe layer can also be formed by diffusing a gas atmosphere. The HgCdTe layer can have a thickness of 20 micrometers to 30 micrometers, or even 40 micrometers. A CdTe layer can be grown on the HgCdTe layer by epitaxy and have 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 multiple 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 p+ 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 the cadmium composition gradient decreases from the other end of the absorber layer toward the contact layer. The contact layer has the opposite conductivity to the absorber layer and is formed with an increasing concentration gradient back to the cadmium concentration of the absorber layer. The photodetector has a heavily doped absorber 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: - 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 from the top layer by liquid phase epitaxy a first layer consisting of HgCdTe or at least one of its quaternary derivatives 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 less than the first cadmium concentration and possibly a second electrical dopant concentration less 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 with distance from the interface between the upper layer and the first layer, the first concentration gradient being between 10 atomic % and 25 atomic %. The first layer is of a first conductivity type, and the thickness of the first layer is 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, wherein the first cadmium concentration gradient and the second concentration gradient of the first electrical dopant are preserved in the first layer after the formation of the junction layer; This is likely to be achieved by a method which sequentially includes:

[0011] In one development, the thickness of the first layer, after the 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 500 nm to 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 from a first electrical dopant concentration of 2×10 to 1.5 microns from the interface. 15 at / cm 3 Advantageously, the distance is extended until it is less than 1 / 2.

[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 even more 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 HCl is reduced to less than 1000 ppm.

[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 starts 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 made of HgCdTe or at least one of its quaternary derivatives, the first layer including a first cadmium concentration gradient that begins at an interface between the top 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 made of HgCdTe or at least one of its quaternary derivatives, of a second conductivity type opposite to the first conductivity type, the first and second layers 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 and second layers; 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 purposes only and illustrated in the accompanying drawings, in which: [Brief explanation of the drawings]

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

[0024] To detect electromagnetic radiation in the infrared region, it is preferable to use a photodiode, such as a pn or pn diode, which converts the collected electromagnetic signal into an electrical signal and 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-doped semiconductor material and a second layer of p-doped semiconductor material in direct contact, 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 providing an interface to the first and second layers.

[0027] Other, more complex structures can be used, such as those with one or more doped or undoped passivation layers, interposed between a first layer of n-type doped semiconductor material and a second layer of p-type doped semiconductor material, as long as the two layers form a junction. The formation of the junction ensures the formation of a depletion region within 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 photodetector's operating modes are different.

[0028] To capture infrared radiation, photodiodes made of semiconductor materials with small band gaps in the infrared range, such as those typically represented by the chemical formula HgCdTe, can be used. Because the band gap varies with the alloy composition, it is possible to adjust the wavelength range collected by the diode. In the case of HgCdTe-based alloys, the band gap varies with the cadmium concentration and with the mercury concentration. Layers made of HgCdTe material are primarily composed of Hg, Cd, and Te. 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 certain configurations, a first layer of n-type doped semiconductor material is formed on a substrate, followed by a second layer of p-type doping. In other configurations, a first layer of p-type doping is formed on a substrate, followed by a second layer of n-type doping.

[0031] To obtain good channeling of photogenerated charge carriers, it is advantageous to form the photodiode in which an electrical doping gradient exists 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 for the second layer of semiconductor material. However, since the electrical doping level has an impact on photodetection and on electro-optical performance (especially dark current), it is advantageous to relate the electrical doping gradient to a bandwidth gradient, so that the most electrically doped regions are 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 toward the layer of p-type doped semiconductor material, and the bandgap width within the layer of n-type doped semiconductor material also decreases from the interface to the substrate toward the layer of p-type doped semiconductor material.

[0033] When 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 toward the layer of n-type doped semiconductor material, and the bandgap width within 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 decreases strictly from the interface to the substrate before becoming constant or substantially constant in the adjacent portion to the p-type or n-type doped layer to form a pn-junction or pin-junction.

[0035] Achieving such a concentration gradient through liquid phase epitaxy is particularly complicated due to the need to provide multiple growth baths with different dopant concentrations and different major component concentrations. The multiple baths are sequentially layered to form successive layers with different compositions to form the gradient. Furthermore, it is common to melt a portion of the upper layer of the substrate to facilitate the formation of an interface with good crystallographic quality. When successive layers are formed at high temperatures, the previously formed layers are completely or partially dissolved, causing the final concentration profile to deviate significantly from the initially desired profile. This drawback limits the benefits of sequential liquid phase epitaxy using successive baths. This problem can be exacerbated when the gradient is formed over a thin 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, U.S. Pat. No. 4,376,663, which proposes liquid phase epitaxial growth of a HgCdTe layer on a CdTe substrate, followed by growth of 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 for a better correlation between the doping profile of the components within the HgCdTe alloy and 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 tellurium supersaturation to form a tellurium-rich layer.

[0038] As shown in FIG. 1, a substrate 1 is provided having at least an upper layer 2 made of a first semiconductor material. The upper layer 2 of the first semiconductor material is selected from CdTe and CdZnTe. The thickness of the upper 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 accounts for at least 30 atomic %, more advantageously at least 40 atomic %, and even more advantageously at least 45 atomic % of the upper 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] Top layer 2 includes 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. Top layer 2 can include a single n-type or p-type electrical dopant, or multiple different electrical dopants to take advantage of different diffusion rates. When top layer 2 contributes to current flow in the completed diode, the first electrical dopant is the electrically active dopant of top layer 2, and at least a portion of the first electrical dopant is located in a substitutional position.

[0040] Prior to the liquid phase epitaxy step, the introduction of a 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 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. 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 that is subsequently sliced ​​to form the substrate 1.

[0041] During the process of forming the first layer 3 by liquid phase epitaxy, part of the cadmium content and 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 upper layer 2 corresponds to the top region of the substrate 1, i.e., the upper layer 2 is formed from the same semiconductor material and has the same cadmium concentration as the substrate 1. In an alternative embodiment, the upper 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. The concentration of the first electrical dopant can also be the same between the substrate and the upper 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 not more than 6 microns, or even not more 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 formed semiconductor material is single crystalline and has the chemical formula Hg 1-x CD x The second semiconductor material can be represented by HgCdTe or more generally by the chemical formula HgCdTe. The second semiconductor material can be at least a quaternary derivative of HgCdTe, for example, Hg 1-x-y CD x Zn 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 has a bandgap 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 for single crystal growth of the first layer 3 from the surface of the upper layer 2.

[0045] The liquid phase epitaxy process uses a single bath containing all elements involved in the formation of the second semiconductor material, including Hg, Cd, and Te, and possibly Zn, Mn, Se, or other necessary materials. The bath can 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 can 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 so that a monocrystalline first layer 3 of the second semiconductor material can be formed by liquid phase epitaxy. The bath has a cadmium concentration that is less than the cadmium concentration of the top layer 2. If the bath contains an electrical dopant, its concentration is less than the concentration of the first electrical dopant in the top layer 2.

[0047] When liquid phase epitaxial growth is carried out, 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] Growth from the upper layer 2 creates a cadmium concentration gradient from the interface to the upper layer 2, which in turn creates a bandgap width gradient within the first layer 3 that starts at the interface to the upper layer 2 and decreases with increasing distance from the interface. The minimum cadmium concentration within the first layer 3, i.e., at the base and / or constant portion of the gradient, is between 10 atomic % and 25 atomic % to achieve good absorption of the targeted infrared radiation. Preferably, the cadmium gradient exhibits a concentration difference at least equal to 10 atomic % or at least equal to 25 atomic % to achieve adequate isolation of the electro-optical behavior.

[0049] As growth progresses, the cadmium present in the top layer 2 becomes increasingly difficult to move, as it must diffuse over larger distances and is 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 eventually reaches a value defined by the bath, at which point the cadmium concentration becomes constant or nearly constant, e.g., with a gradient of less than 1%. A constant cadmium concentration means that the concentration varies by 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 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 located primarily or almost exclusively in substitutional positions. This electrical dopant concentration gradient is not related to a change in composition in the growth bath, but rather to the progressive incorporation / diffusion of a first electrical dopant from the upper layer 2, possibly from the bath, into the crystal lattice, and possibly also to the 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 1 / 2.

[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, whereby 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% or less is selected to define the minimum required concentration within the first layer 3. As the growth of the first layer 3 occurs, 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, 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, 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 at least a constant cadmium concentration over at least 2 microns.

[0055] The temperature of the liquid phase epitaxy process is greater than the threshold temperature required to ensure diffusion of the cadmium and the first electrical dopant. The thermal budget (time-temperature integration) of the liquid phase epitaxy process is less than the threshold thermal budget required to achieve uniformity of the cadmium and / or the first electrical dopant throughout the thickness of the first layer 3. If the temperature is too low, the cadmium and / or the first dopant will not be able to diffuse and the required reduction profile will not be achieved. Conversely, if the thermal budget is too high, the cadmium and / or the first electrical dopant will diffuse too much, resulting in a uniform concentration throughout 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, U.S. Pat. No. 5,861,321 discloses growing 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 achieve uniform doping in the HgCdTe layer. In another embodiment, the substrate is doped with dopants present in the substrate. The uniformization anneal is performed during 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 so that the degree of cadmium gradient is less than 2 microns, or even less than 1.5 microns, where the degree of gradient within the first layer 3 is measured from the interface with the upper layer to the point where the concentration of the first element is constant.

[0059] Advantageously, 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 of 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 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. By similar, we mean that the profiles are similar in shape 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 differ by less than 1 micron, or even less than 500 nm, across the thickness.

[0062] Advantageously, prior to the liquid phase epitaxy step, the upper layer 2 is 16 at / cm 3 Advantageously, 5×10 17 at / cm 3 More like, even 5 x 10 18 at / cm 3 or 1×10 19 at / cm 3The concentration of the first electrical dopant is preferably 5×10 or more. 15 at / cm 3 ~1×10 19 at / cm 3 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 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 in the portion of the first layer 3 where the cadmium content is constant or nearly constant, and is preferably less than 5×10 13 at / cm 3 ~2×10 15 at / cm 3 It is said that.

[0067] In the growth bath, a concentration ensuring the incorporation of n-type or p-type electrically active dopants is set to 5×10 in a constant or approximately constant portion of the first layer 3.15 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 has a density of at least 1×10 18 at / cm 3 or 5×10 18 at / cm 3 from the first dopant concentration equal to 2 x 10 15 at / cm 3 to less than, and preferably at least 4 x 10 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 . By using a doping gradient that achieves weak doping in the absorption region, or a doping gradient that tends to a level corresponding to depletion within the layer, 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 altered, thereby changing the bandgap width of the deposited material. At the same time, the concentration of the first electrical dopant changes and tends to decrease. These two concentration gradients are not determined by multiple growth baths, but rather by the cadmium and first electrical dopant concentrations 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 determined by performing one or more preliminary simulations, which take into account the cadmium diffusion coefficient, the first electrical dopant diffusion coefficient, their respective concentrations in the bath and in the upper layer 2, and the depletion of these 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, in a single step, several individual portions of the first layer 3 to convert the 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 part 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 cadmium-rich, a low-resistivity layer with a wide bandgap can be formed, ensuring good electrical signal transmission. The first portion 3a is less electro-optically active, thereby reducing the impact 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 electromagnetic radiation. The second portion 3b exhibits a bandgap width smaller than the threshold to collect the required electromagnetic radiation. The second portion 3b exhibits a smaller bandgap width and is weakly doped or undoped to provide good optoelectronic performance for the photodetector. The use of weak doping can reduce the value of dark current, a parameter that impairs photodetector performance, thereby facilitating 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 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, and even more preferably between 2 microns and 4 microns. The thickness of the second portion can be selected depending on the formation mode 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, thereby facilitating use in the imaging field. Advantageously, the second portion 3b has a thickness of 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 has 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 . 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 the first electrical dopant concentration gradient ensures the formation of an electric or pseudo-electric field within the first layer 3, which allows the transfer of photo-generated charge carriers from the first portion 3a to the second portion 3b, which has low resistance and can perform efficient signal transfer. There is no growth interface between the second portion 3b and the first portion 3a, ensuring good signal transfer.

[0076] Advantageously, the bath for the upper layer 2 preferably contains only or predominantly the first electrical dopant. If the liquid phase epitaxy process conditions 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 to exhibit 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 to match the electrical doping profile to the bandgap 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 makes it easy to achieve a continuous variation of the concentration of n-type or p-type electrically active dopants, thus allowing efficient channeling of the photo-generated 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, particularly the second layer 4 of opposite conductivity to define the p-n or pin junction. When the first layer 3 is n-doped, the second layer 4 is p-doped. Conversely, when the first layer 3 is p-doped, the second layer 4 is n-doped. As mentioned above, one or more other layers can be deposited before the second layer 4 to form the pin junction. Alternatively, an unintentionally doped third layer 5 can be formed to separate the p-doped and n-doped layers that define the junction. 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 bandgap width smaller than the threshold. The active or most active optical part of the photodiode is made of a material with a bandgap width smaller than the threshold, so that the photodiode is sensitive to a specific range of electromagnetic radiation different from that of the first part 3a. This electromagnetic radiation is not collected by the first part 3a of the first layer 2, which has a larger bandgap width. The third semiconductor material is advantageously a ternary alloy or at least a ternary alloy, preferably of or based on HgCdTe. Advantageously, the total thickness of the first layer 3 and the second layer 4 is 6 microns or less, or even 5 microns or less.

[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 vapor 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 dopant precursor. 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 greater than or equal to 3 microns, and advantageously less than or equal to 5 microns.

[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 a conductive contact 6 on the second layer 4, or at least in electrical contact with the second layer 4. The conductive contact 6 can be formed by a contact layer, preferably made of a pure metallic material or of a metal alloy. The contact layer can be etched after deposition to form the contacts 6. When multiple 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, for example, from silicon nitride Si3N4 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, for example 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 covering layer 7 is formed.

[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 are formed. Advantageously, more than one photodiode is formed in the form of a photodiode array.

[0088] The conductive contacts 6 are designed to be connected to a readout circuit that applies a bias to the photodiode and receives an electrical signal representative of the observed scene. Advantageously, each photodiode is associated with a readout circuit. Multiple readout circuits may 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 kept at its initial thickness. Electromagnetic radiation enters the photodiode structure via the substrate 1.

[0091] A photodiode structure is proposed that 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 heavily 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, with the first layer 3 and the second layer 4 forming a junction that converts electromagnetic signals 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 the junction, the same semiconductor material is present on both sides of the junction 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 at one end of the first layer 3. This end can be uncovered or uncoated with a material having predefined optical properties when the substrate 1 and top layer 2 are removed. In some configurations, the substrate 1 is at least partially removed after forming a conductive contact 6 on the second layer 4, or the substrate 1 is completely removed after forming a 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 at least one of its quaternary derivatives. The second layer 4 is of a second conductivity type opposite the first conductivity type, and the first and second layers form 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 and second layers 3 and 4.

[0096] The first layer 3 exhibits a decreasing cadmium concentration and a decreasing electroactive dopant concentration between the first portion 3a and the second portion 3b, which allows for providing 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 throughout 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 prior art methods, the electrically active dopant concentration profile is achieved by multiple crenels, corresponding to the number of successive baths. The profile formed by the multiple crenels is then subjected to an anneal designed to eliminate the crenels. Because the diffusion rate of cadmium differs from that of n-type electrically active dopants, it is particularly difficult to obtain a profile identical to that achieved by the above-described method. Because doping is achieved during growth by liquid phase epitaxy, this doping method produces significantly fewer defects than 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, for example, 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), wherein a minimum atomic cadmium concentration within the first cadmium concentration gradient is 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) presents a thickness greater 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 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 thickness of the first layer (3) is greater than or equal to 3 microns.

7. A photodiode structure according to any one of claims 1 to 6, wherein the thickness of the first layer (3) is 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 total thickness of the first layer (3) and the bonding layer (4) is less than 5 microns.

10. 10. 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 fabricating a photodiode structure according to claim 1, comprising: 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, by liquid phase epitaxy, from said upper layer (2) a first layer (3) consisting of HgCdTe or at least one of its quaternary derivatives, 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 lower than said first cadmium concentration and, possibly, a second concentration with respect to the electrical dopant lower 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 said first electrical dopant; and a temperature at which diffusion of the at least one electrical dopant from the upper layer (2) into the first layer (3) is achieved, thereby forming a first cadmium concentration gradient that continuously decreases from the interface between the upper 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 that of the first portion (3a), thereby forming a second concentration gradient of the at least one electrical dopant 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) from the interface to the upper layer (2) over a distance of 500 nm to 1.5 microns, 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), wherein the first cadmium concentration gradient and the second concentration gradient of the first electrical dopant are 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 upper layer (2) and the first layer (3) towards the bonding layer (4), the first layer (3) exhibiting a thickness greater than the bonding layer (4), and the total thickness of the first layer (3) and the bonding layer (4) being less than 6 microns; The method of claim 1, wherein

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