Photodiode structure and method for manufacturing the same

Abstract

A substrate (1) having an upper layer (2) made of a cadmium-doped semiconductor material is provided to form a photodiode structure. A first layer (3) made of HgCdTe is formed by liquid-phase epitaxy starting 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) to the first layer (3), forming a cadmium concentration gradient that decreases from the interface with the upper layer (2) to the interface. The cadmium concentration gradient causes a bandgap gradient that decreases from the interface to the first layer (3) and causes an n-type dopant concentration gradient in the first layer (2) starting from the interface.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a photodiode structure, and more specifically, to an optoelectronic device. Background Technology

[0002] In many technical fields, photodetectors that provide electrical signals representing observed scenes are known. In particular, there exist photodetectors configured to specifically capture infrared signals that can be used in night vision devices or in numerous other fields of activity where most of the signals to be studied are located in the infrared range.

[0003] For example, it is known that infrared detectors are fabricated from semiconductor materials such as alloys of HgCdTe or MCT representing mercury cadmium telluride. These materials are particularly attractive because they have a direct gap associated with high bandgap energy values ​​that are tunable depending on the cadmium composition. These materials are produced by epitaxy from a substrate that acts as a support and is generally transparent in the studied wavelength range.

[0004] It is particularly advantageous to use photodetectors in the form of photodiodes, more specifically in the form of pn or pin junctions. Electromagnetic radiation passing through the photodiode at energies higher than the bandgap value is captured and converted into electron-hole pairs. The charges are collected and processed into signals representing the observed scene.

[0005] Different photodiode architectures are used to capture infrared radiation, and numerous differences exist between these architectures to improve their operation, for example, the signal-to-noise ratio.

[0006] A photodiode configuration is disclosed in the literature US 2014 / 0217540, which describes a stack comprising, starting from a substrate, a passivating buffer layer of a first conductive type, an active layer of a first conductive type or undoped active layer, a covering layer, a second passivating layer and an active layer, and a junction layer of a second conductive type for forming a pn junction with the second passivating layer.

[0007] Reference US 4,376,663 describes the formation of an HgCdTe layer by liquid-phase epitaxy on a CdTe substrate. The substrate may not be doped to form a p-doped HgCdTe layer, or the substrate may be doped with indium to form an n-doped HgCdTe layer by the diffusion of indium atoms when epitaxial growth occurs. It is also possible to form an n-doped HgCdTe layer by diffusion in a gaseous atmosphere. The HgCdTe layer has a thickness between 20 and 30 micrometers, or even equal to 40 micrometers. The CdTe layer is deposited by epitaxy on the HgCdTe layer and has a thickness between 5 and 10 micrometers.

[0008] The literature by Martyniuk et al. ("Utmost response time of long-wave HgCdTe photodetectors operating under zero voltage condition") states that the central absorption layer is 10 17 An infrared detector formed by multiple layers of HgCdTe defining a p+-doped npn type structure with an electroreceptor concentration equal to *cm-3 is disclosed. The structure comprises a layer (X) that receives incident radiation. Cd =0.25) and absorption layer (X CdIt has a decreasing cadmium concentration between =0.19) and a decreasing cadmium composition gradient from the other end of the absorption layer toward the contact layer. The contact layer has conductivity opposite to that of the absorption layer and is formed by an increasing concentration gradient that returns to the cadmium concentration of the absorption layer. The photodetector has a highly-doped absorption layer to enable operation at high frequencies. The problem to be solved

[0009] One objective of the present invention is to provide a method for manufacturing a photodiode structure that is easy to implement while ensuring better channeling of photogenerated carriers. means of solving the problem

[0010] This objective is intended to be achieved by a method for manufacturing a photodiode structure comprising the following in succession:

[0011] A step of providing a substrate having at least an upper layer made from CdZnTe or CdTe and having a first concentration of a first electrical dopant of a first conductive type and a first cadmium concentration, wherein the upper layer is of a first conductive type, the step of providing said substrate.

[0012] A step of growing a first layer of HgCdTe or at least one quaternary derivative thereof from an upper layer by liquid-phase epitaxy using a single bath comprising precursors of a first layer comprising cadmium and possibly at least one electric dopant, wherein the bath has a second cadmium concentration lower than a first cadmium concentration and possibly a second electric dopant concentration lower than a first concentration of the first electric dopant, and at least one electric dopant is selected from the first electric dopant and / or the second electric dopant, and the liquid-phase epitaxy is performed at a temperature that achieves diffusion of some of the first electric dopant and cadmium atoms from the upper layer to the first layer, thereby forming a first cadmium concentration gradient that decreases continuously from the interface in a direction away from the interface between the upper layer and the first layer—the minimum cadmium atomic concentration value in the first concentration gradient is contained between 10 atomic% and 25 atomic%—and a second concentration gradient of at least one electric dopant in the first layer— A step of growing the first layer, wherein the second concentration gradient decreases to form, the first layer is of the first conductive type, and the thickness of the first layer is less than 6 microns,

[0013] A step of forming at least one junction layer made from a second semiconductor material doped with a second conductive type to form a first layer and a pn or pin junction, wherein a first cadmium concentration gradient and a second concentration gradient of a first electric dopant are preserved in the first layer after the formation of the junction layer.

[0014] In one embodiment, after the formation of the second layer, the thickness of the first layer is 3 microns or more and preferably 5 microns or less.

[0015] In a specific embodiment, the first concentration gradient extends in the first layer until the cadmium concentration is constant over a distance between 500 nm and 1.5 microns from the interface.

[0016] In an advantageous manner, the difference in cadmium concentration in the first concentration gradient is equal to at least 10 atomic percent, and preferably equal to at least 25 atomic percent.

[0017] Over a distance included between 500 nm and 1.5 microns from the interface, and the first electro-dopant concentration is 2 × 10 15 at / cm 3 It is advantageous to provide a second concentration gradient extending from the first layer until it is less than

[0018] 5*10 before the liquid phase epitaxy deposition step 15 and 1*10 19 at / cm 3 It is additionally advantageous to provide the first dopant concentration of the top layer included in between.

[0019] In a preferred embodiment, the first electric dopant concentration gradient in the first layer is 5*10 18 at / cm 3 2*10 from excess concentration 15 at / cm 3 It decreases to a concentration below that level.

[0020] Advantageously, the total thickness of the first layer and the bonding layer is less than 6 microns, preferably less than 5 microns.

[0021] In a specific configuration, the substrate is removed at least partially or wholly after the formation of electrically conductive contacts on the second layer.

[0022] Preferably, the first electric dopant is iodine, and the second concentration gradient of iodine decreases continuously from the interface in a direction away from the interface between the upper layer and the first layer.

[0023] Another objective of the present invention is to provide a photodiode structure that offers better performance than conventional configurations while achieving better control of the channeling of photogenerated charge carriers.

[0024] The photodiode structure comprises the following in succession:

[0025] A first layer of HgCdTe or at least one quaternary derivative thereof, wherein the first layer comprises, in a direction away from the interface between the upper layer and the first layer, a first cadmium concentration gradient that decreases continuously from the interface—the minimum cadmium atomic concentration value in the first concentration gradient is contained between 10 atomic% and 25 atomic%—and a second concentration gradient of at least one electrical dopant in the first layer—the second concentration gradient decreases—the first layer is of a first conductive type and the thickness of the first layer is less than 6 microns, said first layer

[0026] A second layer of HgCdTe or at least one quaternary derivative thereof, wherein the second layer is a layer of a second conductive type opposite to the first conductive type, the first and second layers form a junction that performs conversion of electromagnetic signals into electron-hole pairs, and the second layer has at least the same Hg, Cd, and Te composition as the first layer at the interface between the first layer and the second layer. Brief explanation of the drawing

[0027] Other advantages and features will become more apparent from the following description of specific embodiments and modes of implementation of the invention shown in the accompanying drawings, which are given solely for non-limiting illustrative purposes: - FIG. 1 schematically illustrates, in cross-section, the first step of a method for manufacturing a photodiode structure according to the present invention; - FIG. 2 schematically illustrates, in cross-section, the second step of a method for manufacturing a photodiode structure according to the present invention; - FIG. 3 schematically illustrates the third step of a method for manufacturing a photodiode structure according to the present invention in cross-section. Specific details for implementing the invention

[0028] To perform the detection of electromagnetic radiation in the infrared range, it is preferable to use a photodiode, which is a pn diode or pin diode, that converts the collected electromagnetic signal into an electrical signal. The photodiode has a junction formed by an n-type conductive layer and a p-type conductive layer.

[0029] 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 are in direct contact and define an interface.

[0030] A PIN diode is formed by a first layer of an n-type doped semiconductor material, a second layer of a p-type doped semiconductor material, and a third layer of a semiconductor material having an extrinsic doping that is unintentionally doped or close to the doping value of the unintentionally doped semiconductor material layer. The third layer separates the first and second layers of the n-type or p-type doped semiconductor material, respectively, and has an interface between them.

[0031] Another more complex structure having one or more doped or undoped passivation layers is used, and these two layers can be inserted between a first layer of an n-type doped semiconductor material and a second layer of a p-type doped semiconductor material, as long as they form a junction. The formation of the junction ensures the creation of a depletion region within the photodiode. The depletion region extends partially from the p-type doping layer and the n-type doping layer. Unlike the structure disclosed by Martyniuk, the photodetector has only a single pn or pin junction. That is, the photodetector is not an npn or pnp structure. The photodetector is structurally different and has a different charge carrier collection mode, which means that the operating mode of the photodetector is different.

[0032] To capture infrared radiation, photodiodes fabricated from semiconductor materials with low bandgap values ​​within the infrared range, for example, materials whose general composition is represented by the chemical formula HgCdTe, can be used. The bandgap value varies depending on the composition of the alloy, which allows the wavelength range collected by the diode to be adjusted. In the case of HgCdTe-based alloys, the bandgap value varies depending on the cadmium and mercury concentrations. A layer fabricated from HgCdTe material is a layer in which the major components are Hg, Cd, and Te. The precise composition of each component is not defined unless otherwise specified.

[0033] In order to manufacture a photodiode exhibiting good performance and preferably a plurality of photodiodes exhibiting good performance, it is advantageous to form a photodiode on a substrate, more specifically, to form at least one active layer of the photodiode by a liquid phase epitaxy step.

[0034] In a specific configuration, a first layer of n-doped semiconductor material is formed on a substrate before a second layer of p-doped material is formed. In another configuration, a first layer of p-doped material is formed on a substrate before a second layer of n-doped material is formed.

[0035] To obtain good channeling of photogenerated charge carriers, it is advantageous to form a photodiode in which an electric doping gradient exists within a first layer of an n-doped semiconductor material and / or a first layer of a p-doped semiconductor material. It is equally advantageous for a second layer of semiconductor material. However, since the electric doping level affects optical detection and electro-optical performance (particularly dark current), it is advantageous to provide the electric doping gradient associated with the bandwidth gradient so that regions electro-doped at the highest concentration are less electro-optically active.

[0036] When an n-doped semiconductor material layer is formed on a substrate prior to a p-doped semiconductor material layer, it is advantageous for the concentration of electrically active n-type dopants to decrease from the substrate to the p-doped semiconductor material layer. The bandgap width in the n-doped semiconductor material layer also decreases from the interface with the substrate to the p-doped semiconductor material layer.

[0037] When a p-doped semiconductor material layer is formed on a substrate prior to an n-doped semiconductor material layer, it is advantageous for the concentration of electrically active p-type dopants to decrease from the substrate to the n-doped semiconductor material layer. The bandgap width in the p-doped semiconductor material layer also decreases from the interface with the substrate.

[0038] It is particularly advantageous to have the concentrations of n-type or p-type dopants decrease continuously, or possibly to present one or more plateaus in the concentrations of n-type or p-type dopants. In an advantageous embodiment, the concentrations of n-type or p-type electric dopants decrease strictly from the interface with the substrate before becoming constant or substantially constant in an adjacent portion of the p-type or n-type doped layer to form a pn or pin junction.

[0039] Achieving such a concentration gradient by liquid-phase epitaxy is particularly complex because it requires providing multiple growth baths with different dopant concentrations and different concentrations of major components. The baths follow one another to form successive layers with different compositions that form the gradient. Additionally, it is common practice to melt a portion of the top layer of the substrate to facilitate the formation of an interface of good crystallographic quality. The formation of successive layers at high temperatures results in the complete or partial melting of previously formed layers, causing the final concentration profile to deviate significantly from the initially required profile. This disadvantage limits the advantages of a series of epitaxies performed in the liquid phase using several successive baths. This issue can be even more critical because the gradient occurs over a small thickness.

[0040] The growth of an HgCdTe layer on a CdTe substrate by liquid phase epitaxy is known. The teaching of US 4376663 may be cited as presenting the growth of a CdTe layer on an HgCdTe layer following the liquid phase epitaxial growth of an HgCdTe layer on a CdTe substrate.

[0041] To facilitate the achievement of an n-type or p-type electric doping gradient, it is proposed to utilize the diffusion phenomenon of substrate atoms during the liquid-phase epitaxy stage. The atoms receive the same thermal budget during the liquid-phase epitaxy stage, thereby enabling better correlation between the profile of the n-type or p-type electric dopant from the interface with the substrate and the doping profiles of the components within the HgCdTe alloys. In a preferred manner, liquid-phase epitaxy is performed with supersaturation of tellurium to form a tellurium-rich layer.

[0042] As illustrated in FIG. 1, a substrate (1) having at least an upper layer (2) formed from a first semiconductor material is provided. The upper layer (2) of the first semiconductor material is selected from CdTe and CdZnTe. The thickness of the upper layer (2) is advantageously greater than 500 nm and more preferably less than 2 microns when different from the substrate. 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 upper layer (2) in a CdZnTe type material. Preferably, the CdZnTe type material has a tellurium content equal to 50 atomic%, a cadmium content of 45 atomic% or more, and a zinc content of 5 atomic% or less.

[0043] The upper layer (2) has a first electric dopant. The first electric dopant may be an n-type dopant, e.g., chlorine, iodine, and indium, or a p-type dopant, e.g., lithium, sodium, potassium, copper, silver, and gold. The upper layer (2) may have a single n-type or p-type electric dopant or several different electric dopants to utilize different diffusion rates. When the upper layer (2) participates in the flow of current in the final diode, the first electric dopant is the dopant of the electrical activity of the upper layer (2), and at least a portion of the first electric dopant is located at a substitution position.

[0044] Prior to the liquid phase epitaxy step, the introduction of the first electric dopant in the upper layer (2) can be advantageously performed by an injection step, followed by recrystallization annealing of the surface of the substrate (1) to promote the formation of a seed of good crystallographic quality for the subsequent growth of a good quality single crystal first layer (3). Recrystallization annealing is advantageously performed at a temperature of 400°C or higher. It is also possible to perform doping of the first layer (2) by annealing the substrate (1) in an atmosphere containing a precursor of the first electric dopant. It is further possible to dope the upper layer (2) when formation occurs, for example by crystal growth, more specifically when drawing an ingot that is subsequently sliced ​​to form the substrate (1).

[0045] During the formation step of the first layer (3) by liquid epitaxy, a portion of the cadmium content and the first electric dopant content will be transferred from the upper layer (2) to the first layer (3). The first layer (3) is made from a ternary or at least ternary, for example quaternary semiconductor material, and such a cadmium alloy is one of the main components.

[0046] In one embodiment, the upper layer (2) corresponds to the upper region of the substrate (1), that is, the upper layer (2) is formed from the same semiconductor material as the substrate (1) and has the same cadmium concentration. In an alternative embodiment, the upper layer (2) has a cadmium concentration higher than or possibly lower than the cadmium concentration of the substrate (1) to better control the cadmium profile in the first layer (3) during epitaxy. It is also possible to provide that the concentration of the first electrodopant is the same between the substrate and the upper layer (2).

[0047] As illustrated in FIG. 2, a first layer (3) of the second semiconductor material is grown from an upper layer (2) of the first semiconductor material by liquid-phase epitaxy. The first layer (3) has an interface with 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 6 microns or even 5 microns or less. The thickness of the first layer (3) is preferably greater than 3 microns so that the final structure has an absorption region extending over at least 2 microns.

[0048] The second semiconductor material is an alloy comprising at least Hg, Cd, and Te. The formed semiconductor material is a single crystal, and has the chemical formula Hg 1-x CD x It can be represented as Te or, more generally, HgCdTe. The second semiconductor material is 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 Generally, the second semiconductor material is a material having a bandgap energy value that varies with at least the cadmium concentration. The first semiconductor material is different from the second semiconductor material but allows for the adjustment of lattice parameters to perform single-crystal growth of the first layer (3) from the surface of the upper layer (2).

[0049] The liquid phase epitaxy step uses a single bath containing all elements involved in forming a second semiconductor material having 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 a 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, for example, no components are added thereto.

[0050] The material forming the first layer (3) is selected to allow the formation of a single crystal first layer (3) of the second semiconductor material by liquid phase epitaxy. The bath has a cadmium concentration lower than the cadmium concentration of the upper layer (2). If the bath contains an electric dopant, its concentration is lower than the concentration of the first electric dopant in the upper layer (2).

[0051] When liquid epitaxy growth occurs, at least one electric dopant and cadmium diffuse from the upper layer (2) into the first layer (3) of the second semiconductor material.

[0052] Growth from the upper layer (2) creates a cadmium concentration gradient from the interface with the upper layer (2), which creates a bandgap width gradient in the first layer (3) that decreases as it moves away from the interface with the upper layer (2). The minimum cadmium concentration in the first layer (3), i.e., the bottom portion and / or a certain portion of the gradient, is contained between 10 atomic% and 25 atomic% to obtain good absorption of the desired infrared radiation. Additionally, a cadmium gradient exhibiting a concentration difference equal to at least 10% atomic or at least 25 atomic% is preferred to appropriately dissociate the electro-optical behaviors.

[0053] As growth progresses, the cadmium present in the upper layer (2) must diffuse over a greater distance and becomes increasingly difficult to move as it is consumed. A balance is created between the cadmium present in the bath and the cadmium originating from the upper layer (2). The cadmium concentration at the growth front of the first layer (3) decreases until the cadmium concentration finally reaches a value defined by the bath, and the cadmium concentration becomes constant or nearly constant, having, for example, a gradient of less than 1%. A constant cadmium concentration means a concentration having a deviation of less than 1 atomic% over 50 nm.

[0054] The growth conditions of 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. It is also advantageous for the constant cadmium concentration to be located at a distance between 500 nm and 1.5 microns from the interface.

[0055] Simultaneously, an electric dopant concentration gradient is formed in the first layer (3). The electric dopant is located primarily or almost exclusively at the substitution site. The electric dopant concentration gradient does not lead to modification of the components within the growth bath, but leads to the progression of incorporation / diffusion of the first electric dopant into the crystal lattice from the upper layer (2) and possibly the bath, and possibly to the possible incorporation of the second electric dopant. Preferably, the electric dopant concentration gradient extends over at least 500 nm, more preferably between 500 nm and 1.5 microns. The first electric dopant concentration is 2*10 15 at / cm 3 It is also desirable for the gradient to be extended until it is less than

[0056] In a specific case, the electric dopant is indium, and the concentration of the electric dopant decreases from the interface between the upper layer (2) and the first layer (3). In another specific case, the electric dopant is iodine, and the first electric dopant peak is at or very close to the interface, so there is a slight increase in concentration followed by a decrease over the rest of the first layer (3).

[0057] In a preferred embodiment, the concentration of the electric dopant in the bath is 0. In an advantageous manner, the concentration of the first electric dopant in the bath is the minimum required concentration in the first layer (3), e.g., 2*10 15 cm -3 Growth conditions are selected to define the following concentrations. When the growth of the first layer (3) occurs, the electrical dopant concentration decreases and then may remain constant at a value defined by the bath. A constant electrical dopant concentration means a concentration having a relative deviation of less than 10% over 50 nm.

[0058] Advantageously, the growth of the first layer (3) is carried out until the first layer (3) exhibits at least 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 the cadmium concentration is constant. Preferably, the first layer (3) is manufactured until the first layer (3) exhibits at least a constant cadmium concentration over at least 2 microns.

[0059] The temperature of the liquid phase epitaxy step is higher than the critical temperature that ensures the diffusion of cadmium and the first electro-dopant. The thermal budget (time-temperature pair) of the liquid phase epitaxy step is lower than the critical thermal budget that performs homogenization of cadmium and / or the first electro-dopant across the thickness of the first layer (3). If the temperature is too low, cadmium and the first dopant cannot diffuse to form the desired reduction profile. Conversely, if the thermal budget is too high, cadmium and / or the first electro-dopant diffuse too much, and the concentration becomes homogenized across the thickness deposited on the first layer (3). The liquid phase epitaxy temperature may be between 400°C and 500°C.

[0060] In different technical fields, growing an HgCdTe layer on a CdTe or CdZnTe substrate by liquid-phase epitaxy is known from the document US 5861321. The thickness of the HgCdTe layer is at least 30 microns. The substrate is doped with n-type or p-type, and the HgCdTe layer is formed in an undoped state. Subsequently, annealing is performed to form homogeneous doping in the HgCdTe layer. In another embodiment, the substrate is doped with dopants present within the substrate. Homogenous annealing is performed during growth.

[0061] The growth stage of the first layer (3) by liquid epitaxy is configured such that the thickness of the first layer (3) is advantageously less than 5 microns, preferably less than 4 microns. The growth stage of the first layer (3) by liquid epitaxy is configured such that the thickness of the first layer (3) is advantageously greater than 1 micron, preferably greater than 2 microns. By forming the first layer (3) of reduced thickness, the thermal budget when growth occurs is reduced, thereby limiting diffusion phenomena that tend toward homogenization of concentrations.

[0062] It is particularly advantageous to form a first layer (3) in which the range of the cadmium gradient is less than 2 microns or even 1.5 microns, and the range of the gradient in the first layer (3) is measured from the interface with the upper layer to the part where the concentration of the first element becomes constant.

[0063] In an advantageous manner, the range of the cadmium gradient is less than 1.5 microns, more preferably less than 1 micron. It is particularly advantageous to form a first layer (3) in which the range of the cadmium gradient is greater than 100 nm, preferably greater than 500 nm.

[0064] In a favorable embodiment, the first layer (3) has a minimum cadmium concentration, i.e., a less abundant portion of the first gradient, which is equal to at least 10 atomic percent, preferably 30 atomic percent or less or 20 atomic percent.

[0065] As the first electro-dopant and cadmium penetrate into the first layer (3) by diffusion, it is possible to have similar and well-controlled doping profiles for cadmium and the first electro-dopant. Similarity means that when their concentration profiles are observed on a semi-logarithmic scale, the shapes of the profiles are similar to each other and at the same time correspond to very different concentration levels. The ranges of the first concentration gradient and the second concentration gradient preferably show the same or a difference of less than 1 micron or even less than 500 nm over the thickness.

[0066] In an advantageous way, prior to the liquid epitaxy step, the upper layer (2) is 5*10 16 at / cm 3 Ideally, 5*10 17 at / cm 3 More than, or even 5*10 18 at / cm 3 Excess or 1*10 19 at / cm 3 It has a concentration of the first electrical dopant. The concentration is preferably 5*10 15and 1*10 19 at / cm 3 It is included between.

[0067] At least 5*10 in the first layer (3) 18 at / cm 3 , preferably at least 1*10 19 at / cm 3 It is particularly advantageous to select a concentration of the first electric dopant in the upper layer (2), which is made from, for example, indium or iodine, to ensure the incorporation of the same active electric dopant.

[0068] In a preferred manner, the second electro-dopant concentration gradient is shown to have a concentration ratio between its highest concentration and its lowest concentration equal to at least 10, at least 50, or at least 100 or even 1,000. This profile is obtained in a single liquid-phase epitaxy operation using a single bath.

[0069] The concentration of the first electric dopant in the upper layer (2) among the dopants in the liquid bath and the operating conditions of the liquid phase epitaxy step limit the doping profile in the first layer (3).

[0070] It is advantageous to select growth operating conditions in which the first layer (3) reaches a constant cadmium concentration before reaching a constant electric dopant concentration. The minimum concentration of the first electric dopant exists only in the portion of the first layer (3) with a constant or nearly constant cadmium content, preferably 5*10 13 at / cm 3 and 2*10 15 at / cm 3 It is included between.

[0071] In the growth bath, 5*10 in a constant or nearly constant portion of the first layer (3) 15 at / cm 3 Less than, preferably 2*10 15 at / cm 3 Less than, even more preferably 5*10 14 at / cm3 It is particularly advantageous to select a concentration that ensures the incorporation of electrically active dopants of less than n-type or p-type.

[0072] In a preferred manner, the first layer (3) is at least 1*10 18 or 5*10 18 at / cm 3 2*10 from the same first dopant concentration 15 at / cm 3 At a concentration of less than 4*10 18 at / cm 3 8*10 from the same value 14 at / cm 3 It includes the concentration of an electrically active dopant having a concentration gradient that strictly decreases to a value below a certain threshold. The use of a doping gradient that achieves weak doping in the absorption region or even tends toward a level corresponding to depletion in the layer allows charge carriers to be guided more efficiently. This configuration reduces crosstalk and improves the modulation transfer function (MTF).

[0073] During the liquid phase epitaxy stage, the cadmium concentration is modified, which causes a change in the bandgap width of the deposited material. At the same time, the concentration of the first electro-dopant changes and tends to decrease. The characteristics of the two concentration gradients are defined not by multiple growth baths, but by the concentrations of the first electro-dopants and cadmium in the baths and in the upper layer in relation to the thermal budget of the liquid phase epitaxy stage. The two concentration gradients can be defined by running one or more prior simulations. The simulations consider the cadmium diffusion coefficient, the first electro-dopant diffusion coefficient, the respective concentrations in the baths and in the upper layer (2), and the depletion of these two components as growth progresses.

[0074] The control of the cadmium concentration, which indicates the control of the bandgap width and the control of the electric dopant, allows several distinct parts of the first layer (3) to be functionalized in a single step to convert the electromagnetic signal into an electric signal and promote the conduction of this electric signal.

[0075] The first layer (3) has a first part (3a) designed to facilitate the movement of electrical signals and a second part (3b) designed to capture only or mainly electromagnetic signals over at least part of its thickness.

[0076] By forming a first portion (3a) of a second semiconductor material that is richer in the first electric dopant and overall richer in cadmium, a weakly resistive layer with a wide bandgap can be formed, thereby ensuring good transmission of electrical signals. The first portion (3a) is less electro-optically active, thereby reducing the influence of crystal defects on the quality of the signal provided by the photodiode. The second portion (3b) forms the absorption portion of the photodiode structure, that is, the portion designed to capture electromagnetic radiation. The second portion (3b) exhibits a bandgap width lower than the threshold value for collecting the desired electromagnetic radiation. The second portion (3b) exhibits a smaller bandgap width and is either weakly doped or undoped, providing a photodetector with good optoelectronic performance. The use of weak doping makes it possible to reduce the value of the dark current, which is a parameter that impedes the performance of the photodetector, thereby facilitating the use of a thicker absorption layer.

[0077] The second portion (3b) preferably exhibits a cadmium concentration lower than a threshold value equal to 25 atomic%, and more preferably a constant cadmium concentration. The second portion (3b) preferably exhibits a thickness between 1 and 4.5 microns, more preferably between 1.5 and 4 microns, or even between 2 and 4 microns. The thickness of the second portion can be selected according to the formation mode of the second layer (4) designed to complete the junction. The use of an absorption layer greater than 1 micron, preferably greater than 1.5 microns, facilitates its use in the imaging field by enabling better quantum efficiency to be achieved. Advantageously, the second portion (3b) is 2*10 15 at / cm 3 It has a concentration of less than the first electric dopant. For example, the second portion (3b) of the first layer (3) made from the second semiconductor material includes a portion having a constant or nearly constant cadmium concentration, or is formed exclusively by it, and forms the most optically active n-type or p-type doped layer.

[0078] The first part (3a) preferably represents a minimum cadmium concentration corresponding to the maximum value of the second part (3b). The first part (3a) preferably represents a cadmium concentration equal to the maximum value of at least 30 atomic%, advantageously at least 40 atomic% or 50 atomic%. Advantageously, the second part (3b) is 2*10 15 at / cm 3 It has an excess concentration of the first electric dopant. The first portion (3a) may have an area with a constant cadmium concentration. The first portion (3a) preferably has a thickness between 1 and 4 microns, and more preferably has a thickness between 2 and 4 microns.

[0079] The presence of a first electric dopant concentration gradient ensures the formation of an electric field or pseudo-field in the first layer (3), thereby allowing photogenerated charge carriers to be channeled from the first part (3a) to a second part (3b) which is weakly resistive and capable of performing efficient signal transmission. Good signal transmission is ensured by the absence of a growth interface between the second part (3b) and the first part (3a).

[0080] In an advantageous manner, the upper layer (2) and possibly the bath contains only the first electric dopant, or the first electric dopant represents the majority thereof. If the conditions of the liquid phase epitaxy phase and the concentration of the first electric dopant do not allow the first electric dopant concentration profile to be matched with the cadmium concentration profile, it is advantageous to dope the upper layer (2) with a second electric dopant of the same type of conductivity as the first electric dopant. The second electric dopant is selected to exhibit a different diffusion rate from the first electric dopant during the formation of the first layer (3). For example, in the case of n-type doping, it is possible to use iodine in relation to indium to match the electric doping profile with the bandgap width profile defined by cadmium.

[0081] Variation of the first dopant content is performed continuously over the thickness of the first layer (3), which makes it easier to achieve continuous variation of the concentration of n or p-type electroactive dopants and thus enables the appearance of an electric field that allows photogenerated charge carriers to be efficiently channeled.

[0082] As illustrated in FIG. 3, once the first layer (3) of the first semiconductor material is formed, the remainder of the photodiode can be formed, and in particular, a second layer (4) of opposite conductivity can be formed to define a pn or pin junction. If the first layer (3) is n-doped, the second layer (4) is p-doped. Conversely, if the first layer (3) is p-doped, the second layer (4) is n-doped. As previously mentioned, one or more other layers may be deposited before the second layer (4) to form a pin junction. It is also possible to form an unintentionally doped third layer (5) that separates the p-doped and n-doped layers defining the junction. The layer (5) may be formed by one or more different materials.

[0083] The second layer (4) is manufactured from a third semiconductor material having a bandgap width smaller than a threshold value. The active or most active optical portion of the photodiode is formed by a material having a bandgap width smaller than a threshold value so that the photodiode is sensitive to a specific range of electromagnetic radiation different from that of the first portion (3a). This electromagnetic radiation is not collected by the first portion (3a) of the first layer (2) having a larger bandgap width. The third semiconductor material is advantageously, preferably a ternary or at least a ternary alloy based on HgCdTe or HgCdTe. In an advantageous manner, the total thickness of the first layer (3) and the second layer (4) is 6 microns or less, or even 5 microns or less.

[0084] For example, the second layer (4) made from the third semiconductor material is deposited by epitaxy, advantageously by liquid phase epitaxy, by molecular beam epitaxy (MBE), or by organometallic chemical vapor deposition (MOCVD).

[0085] In an alternative embodiment, a second layer (4) designed to form a pn junction is obtained by the injection of a dopant of opposite electrical conductivity or by diffusion using an atmosphere containing a precursor of the dopant. A third semiconductor material may be the same as the second semiconductor material. The second layer (4) is formed by extrinsic doping of the first layer (3). The thickness of the junction formed by the layers (3 and 4) corresponds to the initial thickness of the first layer (3) formed by liquid phase epitaxy.

[0086] The total 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 controlled depending on whether the second layer is deposited on the first layer (3) or formed within the first layer (3). In a specific embodiment, after the formation of the second layer (4), the thickness of the first layer (3) is 3 microns or more and preferably 5 microns or less.

[0087] The first cadmium concentration gradient and the second gradient of the first electric dopant are preserved in the first layer (3) when the formation of the second layer (4) occurs.

[0088] In a preferred manner, absorption of electromagnetic radiation within the photodiode occurs mainly in the second portion (3b) of the first layer (3). In order to preserve the region having good electro-optical properties, when the second layer is formed on the first layer, 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.

[0089] It is particularly advantageous to form an electrically conductive contact (6) on the second layer (4) or at least in electrical contact with the second layer (4). The electrically conductive contact (6) may be formed by a contact layer preferably made of a pure metal material or a metal alloy. The contact layer may be etched after deposition to define the contact (6). When multiple photodiodes are formed on the same substrate (1), a specific contact is formed on each photodiode. A first cadmium concentration gradient and a second gradient of the first electric dopant are preserved in the first layer (3) when the formation of the contact or contacts (6) occurs.

[0090] The photodiode is advantageously partially covered by a covering layer (7) made of, for example, silicon nitride (Si3N4) or silicon oxide (SiOx) or a layer of ZnS to protect the photodiode from an external environment, for example, moisture. The first cadmium concentration gradient and the second gradient of the first electric dopant are preserved in the first layer (3) when the covering layer (7) is formed.

[0091] If present, the growth of the first layer (3), the second layer (4), and the third layer (5) is advantageously performed in a manner that forms more than one photodiode. It is advantageous to form multiple photodiodes in the form of a photodiode array.

[0092] The electrically conductive contact (6) is designed to be connected to a read circuit that will apply bias to the photodiode and receive an electrical signal representing the observed scene. It is advantageous to associate each photodiode with a read circuit. Multiple read circuits are also connected as a read circuit array such that multiple read circuits are hybridized to multiple photodiodes to form a focal plane array (FPA).

[0093] It is particularly advantageous to operate the photodiode at a low temperature, preferably at a temperature below 0°C, and more preferably in the range of 130K to 250K.

[0094] After the photodiode is formed, for example, after the electrically conductive contact (6) is formed, or after the deposition of the covering layer (7), or after hybridization with the read circuit, the substrate (1) may be considered to be removed. Alternatively, the substrate (1) may be maintained at its initial thickness or thinned. Electromagnetic radiation enters the photodiode structure through the substrate (1).

[0095] A photodiode structure is proposed comprising 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 manufactured from a first semiconductor material and defines a first portion (3a) that is highly doped and a second portion (3b) that is weakly doped.

[0096] The photodiode structure comprises a first portion of a first layer manufactured from a first semiconductor material, a second portion of a first layer manufactured from a first semiconductor material, and a second layer manufactured from a second semiconductor material in succession. The first and second layers (3 and 4) form a junction that performs the conversion of an electromagnetic signal into electron-hole pairs.

[0097] When the second layer (4) is formed by injecting a second conductive type dopant into the first layer (3) to form a junction, the same semiconductor material exists on each side of the junction, except for the type of doping.

[0098] A photodiode having a first layer (3) made of HgCdTe or at least one of its quaternary derivatives is obtained. The first layer (3) includes a first cadmium concentration gradient and a second concentration gradient of a first electric dopant. Both the first concentration gradient and the second concentration gradient decrease from one end of the first layer (3). The end may be coated with a material having predefined optical properties that is not covered when the substrate (1) and the upper layer (2) are removed. Depending on the configuration, the substrate (1) is at least partially removed after the formation of an electrically conductive contact (6) on the second layer (4), or the substrate (1) is entirely removed after the formation of an electrically conductive contact (6) on the second layer (4).

[0099] The first layer (3) is of the first conductive type and has a thickness of less than 6 micrometers. The second layer (4) is also made from HgCdTe or at least one of its quaternary derivatives. The second layer (4) is a layer of the second conductive type opposite to the first conductive type, and the first and second layers form a junction that performs the conversion of electromagnetic signals into electron-hole pairs. The second layer (4) has at least the same Hg, Cd, and Te composition as the first layer (3) at the interface between the first layer (3) and the second layer (4).

[0100] The first layer (3) provides a decreasing cadmium concentration and a decreasing electrically active dopant concentration between the first part (3a) and the second part (3b), thereby enabling a signal of better quality to be provided. The decreasing concentrations are not discontinuous.

[0101] The precautionary measures taken during the liquid phase epitaxy step to form a concentration gradient are maintained until the end of the method for forming the diode. The method for manufacturing the diode does not include annealing to homogenize the cadmium concentration and / or the concentration of the first electrodopant over the total thickness of the first layer.

[0102] In conventional methods, the electroactive dopant concentration profile is obtained by a number of crenels corresponding to many successive baths. The profile formed by the multiple crenels subsequently undergoes annealing designed to cause the crenels to disappear. Since the diffusion rate of cadmium differs from that of the n-type electroactive dopant, it is particularly difficult to obtain the same profile as that obtained by the aforementioned method. Since doping is obtained when growth occurs via liquid-phase epitaxy, it generates significantly fewer defects than the injection step, particularly in the weakly doped region to be electro-optically active.

[0103] The photodiode structure can allow for better control of photocharge carriers, which, for example, can reduce blur in an acquired image when multiple photodiodes are connected to form an array.

Claims

Claim 1 A photodiode structure comprising a junction designed to convert infrared electromagnetic radiation into electron-hole pairs, wherein the junction comprises a first layer (3) made from HgCdTe or one of at least quaternary derivatives of a first conductive type thereof, and a second layer (4) made from HgCdTe or one of at least quaternary derivatives of a second conductive type opposite to the first conductive type thereof, thereby forming a pn or pin junction; wherein the first layer (3) comprises: - a first end and a second end located opposite to the first end, wherein the second end is located in the direction of the second layer (4) from the first layer (3); - a first cadmium concentration gradient decreasing in the direction of the second layer (4) from the first end of the first layer (3), wherein the minimum value of the cadmium atomic concentration in the first cadmium concentration gradient is A first cadmium concentration gradient included between 10 atomic% and 25 atomic%, wherein the first cadmium concentration gradient defines a first portion (3a) and a second portion (3b), wherein the second portion (3b) has a smaller bandgap width than the first portion (3a); a second concentration gradient of at least one electrodopant, wherein the second concentration gradient decreases in the direction of the second layer (4) from the first end, and the total thickness of the first layer (3) and the second layer (4) is less than 6 microns; wherein the first cadmium concentration gradient decreases continuously in the form of a diffusion profile, wherein the second concentration gradient of at least one electrodopant decreases continuously in the form of a diffusion profile, wherein the second concentration gradient extends over a distance included between 500 nm and 1.5 microns from the first end in the first layer (3), and wherein the second The concentration gradient is 5*10 18 at / cm 3 2*10 from excess concentration 15 at / cm 3 A photodiode structure characterized by a concentration that decreases to less than the first layer (3) and the first layer (3) having a greater thickness than the second layer (4). Claim 2 In claim 1, the electrically conductive contact (6) is electrically connected to the second layer (4), and the electrically conductive contact (6) is manufactured from a pure metal material or from an alloy of a metal material, a photodiode structure. Claim 3 In claim 1, the first layer (3) is a photodiode structure having a constant cadmium concentration over at least 500 nm. Claim 4 In claim 3, the first layer (3) is a photodiode structure having a constant cadmium concentration over at least 1 micrometer. Claim 5 In claim 1, the first layer (3) is a photodiode structure having a constant cadmium concentration and representing a portion that is not intentionally doped. Claim 6 A photodiode structure according to claim 1, wherein the thickness of the first layer (3) is 3 microns or more. Claim 7 A photodiode structure according to any one of claims 1 to 6, wherein the thickness of the first layer (3) is 5 microns or less. Claim 8 A photodiode structure according to claim 1, wherein the difference in cadmium concentration in the first cadmium concentration gradient is equal to at least 25 atomic percent. Claim 9 A photodiode structure according to claim 1, wherein the total thickness of the first layer (3) and the second layer (4) is less than 5 microns. Claim 10 A photodiode structure according to claim 1, wherein the second concentration gradient of at least one electric dopant comprises an iodine concentration gradient and a chlorine concentration gradient. Claim 11 A method for manufacturing a photodiode structure according to claim 1, comprising: a step of providing a substrate (1) having at least an upper layer (2) made from CdZnTe or CdTe and having a first concentration of a first electrical dopant of a first conductive type and a first cadmium concentration, wherein the upper layer (2) is of the first conductive type; a step of providing the substrate (1), wherein the upper layer (2) is of the first conductive type; and a step of growing the first layer (3) of HgCdTe or at least quaternary derivatives thereof from the upper layer (2) by liquid phase epitaxy using a single bath comprising precursors of the first layer (3) comprising cadmium and possibly at least one electrical dopant, wherein the bath has a second cadmium concentration lower than the first cadmium concentration and possibly a second concentration of an electrical dopant lower than the first concentration of the first electrical dopant, wherein the at least one electrical dopant is selected from the first electrical dopant and / or the second electrical dopant, and the liquid phase epitaxy is the Performed at a temperature that achieves the diffusion of cadmium atoms and a portion of the first electric dopant from the upper layer (2) to the first layer (3), thereby forming a first cadmium concentration gradient that decreases continuously from the interface as it moves away from the interface between the upper layer (2) and the first layer (3)—wherein the minimum cadmium atom concentration value in the first concentration gradient is between 10 atomic% and 25 atomic%, and the first concentration gradient defines a first portion (3a) and a second portion (3b), wherein the second portion (3b) has a smaller bandgap width than the first portion (3a)—and a second concentration gradient of the at least one electric dopant in the first layer (3)—wherein the second concentration gradient of the at least one electric dopant is 5*10 18 at / cm 3 2*10 from excess concentration 15 at / cm 3 A step of growing the first layer (3), wherein the first layer (3) is of the first conductive type, wherein the second concentration gradient is continuously reduced to a concentration less than that, and the second concentration gradient extends over a distance between 500 nm and 1.5 microns from the interface with the upper layer (2), and the first layer (3) is of the first conductive type, wherein the first layer (3) is of the first conductive type, and the step of forming at least a second layer (4) made from HgCdTe or one of at least quaternary derivatives thereof of the second conductive type opposite to the first conductive type to form a pn or pin junction with the first layer (3), wherein the first cadmium concentration gradient and the second concentration gradient of the first electric dopant are preserved in the first layer (3) after the formation of the second layer (4), and the second concentration gradient decreases in the direction of the second layer (4) from the interface between the upper layer (2) and the first layer (3), and the first layer (3) is the second A method for manufacturing a photodiode structure, comprising the step of forming the second layer (4) having a thickness greater than that of the first layer (3) and the second layer (4), wherein the total thickness of the first layer (3) and the second layer (4) is less than 6 microns.

Images