EP4272264A1

EP4272264A1 - Photodiode structure and method for producing same

Info

Publication number: EP4272264A1
Application number: EP21844784.5A
Authority: EP
Inventors: Nicolas Pere-Laperne; Alexandre Kerlain; Vincent Destefanis; Paul FOUGERES
Original assignee: Lynred SAS
Current assignee: Lynred SAS
Priority date: 2020-12-31
Filing date: 2021-12-30
Publication date: 2023-11-08
Also published as: WO2022144419A1; FR3118532B1; IL304045A; FR3118532A1; JP2024501694A; KR20230122673A; US20240072183A1

Abstract

A substrate (1) having an upper layer (2) made of a cadmium-doped semiconductor material is provided in order to produce a photodiode structure. A first layer (3) made of HgCdTe is formed by liquid phase epitaxy starting from the upper layer (2) with a bath containing an n-type electrically active dopant for electrically doping the first layer (3). The cadmium diffuses from the upper layer (2) to the first layer (3) to form a cadmium concentration gradient that decreases from the interface with the upper layer (2) moving away from the interface. The cadmium concentration gradient causes a band gap gradient which decreases in the first layer (3) from the interface and causes an n-type dopant concentration gradient in the first layer (2) starting from the interface.

Description

METHOD FOR MANUFACTURING A PHOTODIODE STRUCTURE AND PHOTODIODE STRUCTURE

Technical area

The invention relates to a process for manufacturing a photodiode structure and more generally to an optoelectronic device.

Prior technique

It is known to produce photodetectors in multiple technical fields which provide an electrical signal representative of an observed scene.

In particular, there are photodetectors that are configured to specifically pick up an infrared signal whose use can be found in night vision devices or in many other fields of activity where a significant part of the signal to be studied is in the infrared spectrum.

It is, for example, known to produce infrared detectors in semiconductor materials which are alloys of HgCdTe or MCT for Mercury Cadmium Telluride. These materials are particularly interesting because they have a direct gap associated with a significant and adjustable band gap energy value depending on the Cadmium composition. These materials are produced by epitaxy from a substrate which serves as a support and which is generally transparent in the wavelength range studied.

It is particularly advantageous to use photodetectors in the form of photodiodes and more particularly in the form of a pn or pin junction. The electromagnetic radiation which passes through the photodiode with an energy greater than the bandgap value is captured and transformed into an electron-hole pair. The electrical charges are captured to be processed as a signal representative of the observed scene. Different architectures of photodiodes are used to capture infrared radiation and the many differences that exist between these architectures seek to improve their operation, for example the signal-to-noise ratio.

A photodiode configuration is presented in document US 2014/0217540 which describes a stack comprising successively, from the substrate, a passivation buffer layer of a first type of conductivity, an active layer of the first type of conductivity or undoped, a cover layer, a second passivation layer and a junction layer of the second conductivity type to form a pn junction with the active layer and the second passivation layer.

Document US 4,376,663 describes the formation of an HgCdTe layer by liquid phase epitaxy on a CdTe substrate. The substrate may be undoped to form a p-type doped HgCdTe layer or the substrate may be indium doped to form an n-type doped HgCdTe layer by diffusion of the indium atoms during epitaxy growth. It is also possible to form the n-type doped HgCdTe layer by diffusion of a gaseous atmosphere. The HgCdTe layer has a thickness of between 20 and 30 micrometers or even equal to 40 micrometers. A CdTe layer is deposited by epitaxy on the HgCdTe layer and has a thickness of between 5 and 10 micrometers.

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 several HgCdTe layers to define an npn type structure whose central absorbing layer is doped with type pi- with a concentration of electrical acceptors equal to 1 O ¹⁷ .cm-3. The structure has a decreasing cadmium concentration gradient between the layer receiving the incident radiation (Xcd=0.25) and the absorbing layer (Xcd=0.19) as well as a decreasing cadmium concentration gradient from the other end of the absorbent layer towards the contact layer. The contact layer has a conductivity opposite to that of the absorbent layer and is formed by an increasing concentration gradient to return to the cadmium concentration of the absorbent layer. The photodetector has a heavily doped absorbing layer in order to be able to operate at high frequency.

Object of the invention

The subject of the invention is a method for manufacturing a photodiode structure which is easy to produce while ensuring better channeling of the photogenerated carriers.

We tend towards this object by means of a method of manufacturing a photodiode structure comprising successively

- providing a substrate having at least one upper layer made of CdZnTe or CdTe and having a first concentration of a first electrical dopant of a first type of conductivity and a first concentration of cadmium, the upper layer being of the first type of conductivity,

- growing a first layer of HgCdTe or one of its at least quaternary derivatives from the upper layer, by liquid phase epitaxy using a single bath containing the precursors of the first layer including cadmium and optionally at least one electrical dopant, the bath having a second cadmium concentration lower than the first cadmium concentration and optionally a second electric dopant concentration lower than the first electric dopant concentration, the at least one electric dopant being chosen from the first electric dopant and/or a second electric dopant, the liquid-phase epitaxy being carried out at a temperature causing the diffusion of a part of the cadmium atoms and of the first electric dopant from the upper layer towards the first layer to form a first cadmium concentration gradient continuously decreasing from the interface between the upper layer and the first layer e away from the interface, the minimum atomic cadmium concentration value in the first concentration gradient being between 10% and 25 atomic%, and to form a second concentration gradient of at least one electric dopant in the first layer, the second concentration gradient being decreasing, the first layer being of the first type of conductivity, the thickness of the first layer being less than 6 microns, - forming at least a junction layer of second semiconductor material doped with a second type of conductivity to produce a pn or pin junction with the first layer, the first cadmium concentration gradient and the second concentration gradient of first electrical dopant being retained in the first layer after formation of the junction layer.

In a development, the thickness of the first layer is greater than or equal to 3 microns and preferably less than or equal to 5 microns, after formation of the second layer.

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

Advantageously, the difference in cadmium concentration in the first concentration gradient is at least equal to 10 atomic % and preferably at least equal to 25 atomic %.

It is advantageous to provide that the second concentration gradient extends, in the first layer, over a distance of between 500 nm and 1.5 microns from the interface and until the concentration of first electric dopant is less than 2.10 ¹⁵ at/cm ³ .

It is also beneficial to provide for the concentration of first dopant in the upper layer to be between 5.10 ¹⁵ and 1.10 ¹⁹ at/cm ³ before the liquid phase epitaxy deposition step. In a preferred embodiment, the first electrical dopant concentration gradient in the first layer decreases from a concentration greater than 5.10 ¹⁸ at/cm ³ to a concentration less than 2.10 ¹⁵ at/cm ³ ..

Advantageously, the total thickness of the first layer and of the joining layer is less than 6 microns, preferably less than 5 microns.

In a particular configuration, the substrate is at least partially or completely removed after the formation of an electrically conductive contact on the second layer.

Preferably, the first electrical dopant is iodine and the second iodine concentration gradient is continuously decreasing from the interface between the upper layer and the first layer as it moves away from the interface.

The invention also relates to a photodiode structure which is more efficient than the configurations of the prior art by ensuring better control of the channeling of the photogenerated charge carriers.

The photodiode structure successively comprises:

- a first layer of HgCdTe or one of its at least quaternary derivatives, the first layer comprising a first cadmium concentration gradient continuously decreasing from the interface between the upper layer and the first layer as it moves away from the interface, the minimum value of atomic concentration of cadmium in the first concentration gradient being between 10% and 25 atomic%, a second concentration gradient of at least one electric dopant in the first layer, the second concentration gradient being decreasing, the first layer being of the first conductivity type, the thickness of the first layer being less than 6 microns,

- a second layer of HgCdTe or one of its at least quaternary derivatives, the second layer being a layer of a second type of conductivity opposite to the first type of conductivity, the first and second layers forming a junction ensuring the transformation of an electromagnetic signal into electron-hole pairs, the second layer having at least the same composition in Hg, Cd and Te as the first layer at the interface between the first layer and the second layer.

Brief description of the drawings

Other advantages and characteristics will emerge more clearly from the following description of particular embodiments and implementations of the invention given by way of non-limiting examples and represented in the appended drawings, in which:

- Figure 1 shows, schematically, in section, a first step of a method of manufacturing a photodiode structure according to the invention;

- Figure 2 shows, schematically, in section, a second step of a method of manufacturing a photodiode structure according to the invention;

- Figure 3 shows, schematically, in section, a third step of a method of manufacturing a photodiode structure according to the invention.

Description of embodiments

For the detection of electromagnetic radiation in the infrared range, it is preferable to use a photodiode which is a PN diode or a PIN diode to transform the electromagnetic signal picked up into an electrical signal. The photodiode has a junction formed by a layer whose conductivity is n-type and a layer whose conductivity is p-type.

The 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. The PIN diode is formed by a first layer of n-type doped semiconductor material, a second layer of p-type doped semiconductor material, a third layer of semiconductor material which is not intentionally doped or which has an extrinsic doping close to the value of the doping of the unintentionally doped semiconductor material layer. The third layer separates and has an interface with respectively the first layer and the second layer in semiconductor materials and n-type or p-type doped.

Other more complex structures with one or more doped or undoped passivation layers can be used and introduced between the first n-type doped semiconductor material layer and the second p-type doped semiconductor material layer insofar as these two layers form a junction. The formation of a junction ensures the creation of a depletion zone in the photodiode. The depletion zone extends partly in the p-type doped layer and in the n-type doped layer. Unlike the structure disclosed by Martyniuk, the photodetector has only one PN or PIN junction. In other words, the photodetector is not an npn or pnp structure. The photodetector is structurally different and its charge carrier collection mode is different, which means that the mode of operation of the photodetector is different.

To capture infrared radiation, it is possible to use a photodiode in a semiconductor material with a low bandgap value in the infrared range, for example a material whose general composition is represented by the formula HgCdTe. The value of the forbidden band evolves according to the composition of the alloy which makes it possible to modulate the range of wavelength picked up by the diode. For an alloy based on HgCdTe, the band gap value changes with the cadmium concentration and with the mercury concentration. A layer of material of the HgCdTe type is a layer whose main constituents are Hg, Cd and Te. The precise composition of each of the constituents is not defined unless otherwise indicated. In order to produce a high-performance photodiode and, preferably, a multitude of high-performance photodiodes, it is advantageous to form the photodiode on a substrate and more particularly to form at least one active layer of the photodiode by a liquid phase epitaxy step.

In a particular configuration, a first layer of n-type doped semiconductor material is formed on a substrate before forming a second p-type doped layer. In another configuration, a first p-type doped layer is formed on the substrate before forming a second n-type doped layer.

To obtain good channeling of the photogenerated charge carriers, it is advantageous to form a photodiode in which there is an electrical doping gradient inside the first layer of n-type semiconductor material and/or inside the first layer of p-type semiconductor material. It is advantageously the same in the second layer of semiconductor material. However, as the level of the electric doping has an influence on the optical detection and on the electro-optical performances (in particular the dark current), it is advantageous to provide that the gradient of electric doping is associated with a gradient of width of forbidden band so that the most electrically doped zones are less electro-optically active.

When the n-type doped semiconductor material layer is formed on a substrate before the p-type doped semiconductor material layer, it is advantageous for the concentration of n-type electrically active dopants to decrease from the substrate to the layer in p-type semiconductor material. The width of the band gap in the n-type doped semiconductor material layer also decreases from the interface with the substrate to the p-type semiconductor material layer. When the layer of p-type doped semiconductor material is formed on a substrate before the layer of n-type doped semiconductor material, it is advantageous for the concentration of electrically active p-type dopants to decrease from the substrate to the layer in n-type semiconductor material. The width of the forbidden band in the p-type doped semiconductor material layer also decreases from the interface with the substrate.

It is particularly advantageous for the concentration of n- or p-type dopant to be continuously decreasing or possibly to have one or more levels in the concentration of n- or p-type dopants. In an advantageous embodiment, the concentration of n- or p-type electrical dopants strictly decreases from the interface with the substrate before becoming constant or substantially constant in the adjacent portion of the p- or n-type doped layer to form the junction. pn or pine.

The realization of such a concentration gradient by means of a liquid phase epitaxy operation is particularly complex because it is necessary to provide several growth baths which have different dopant concentrations as well as different concentrations of the main constituents. The baths follow one another to form successive layers having different compositions forming the gradient. Furthermore, to facilitate the formation of an interface with good crystallographic quality, it is common to melt part of the upper layer of the substrate. The realization, at high temperature, of successive layers results in a total or partial dissolution of the layers carried out previously so that the final concentration profile deviates notably from the profile initially sought. This drawback limits the interest of a succession of epitaxies carried out in the liquid phase and using several successive baths. The problem can be all the more critical that the gradient is carried out on a weak thickness.

The growth of an HgCdTe layer on a CdTe substrate by liquid phase epitaxy is known. It is possible to cite the teaching of the US document 4376663 which shows the growth by liquid phase epitaxy of an HgCdTe layer on a CdTe substrate then the growth of a CdTe layer on the HgCdTe layer.

In order to facilitate the realization of the n- or p-type electric doping gradient, it is proposed to use phenomena of diffusion of the atoms of the substrate during a step of epitaxy in the liquid phase. The atoms are subjected to the same thermal budget during the liquid phase epitaxy step, which makes it possible to better correlate the doping profiles of the constituents in the alloys based on HgCdTe and the profile of an n-type electrical dopant or of p-type from the interface with the substrate. Preferably, the liquid phase epitaxy is carried out with a supersaturation of tellurium in order to form a layer rich in tellurium.

As illustrated in Figure 1, a substrate 1 is provided which has at least one top layer 2 formed from a first semiconductor material. The upper layer 2 of first semiconductor material is chosen 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 it is different from the substrate. The first semiconductor material has a first cadmium concentration. In an advantageous configuration, the cadmium represents at least 30 atomic % of the upper layer 2, more advantageously at least 40 atomic % and even more advantageously at least 45 atomic % in a material of the CdZnTe type. Preferably, the CdZnTe type material has a tellurium content equal to 50 atomic%, a cadmium content greater than or equal to 45% and a zinc content less than or equal to 5%.

Upper layer 2 has a first electrical dopant. The first electrical dopant can be an n-type dopant for example chlorine, iodine and indium or a p-type dopant for example lithium, sodium, potassium, copper, silver and gold. The upper layer 2 can have a single n-type or p-type electrical dopant or several different electrical dopants in order to take advantage of speeds of different distribution. When the upper layer 2 participates in the circulation of an electric current in the finalized diode, the first electric dopant is an electrically active dopant of the upper layer 2 and at least part of the first electric dopant is in a substitutional position.

Before the liquid phase epitaxy step, the introduction of the first electrical dopant into the upper layer 2 can be carried out by an implantation step advantageously followed by 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 first monocrystalline layer 3 of good quality. The recrystallization annealing is advantageously carried out at a temperature greater than or equal to 400°C. It is also possible to do the doping of the first layer 2 by means of an annealing of the substrate 1 with an atmosphere which contains a precursor of the first electrical dopant. It is still possible to dope the upper layer 2 during the formation, for example by crystal growth and more particularly during the drawing of an ingot subsequently cut out to form the substrate 1 .

During the step of forming the first layer 3 by liquid phase epitaxy, part of the cadmium and first electrical dopant content will migrate from the upper layer 2 to a first layer 3. The first layer 3 is produced in a semiconductor material which is a ternary or at least ternary alloy, for example quaternary, and of which cadmium is one of the main constituents.

In one embodiment, upper layer 2 corresponds to an upper zone of substrate 1, ie upper layer 2 is formed in the same semiconductor material as substrate 1 and with the same cadmium concentration. In an alternative embodiment, upper layer 2 has a cadmium concentration which is higher than that of substrate 1 or possibly lower than that of substrate 1 to better control the cadmium profile in first layer 3 during epitaxy. It is also possible to provide for the concentration of first electrical dopant to be identical between the substrate and the upper layer 2.

As illustrated in FIG. 2, a first layer 3 of second semiconductor material is grown by liquid phase epitaxy from the upper layer 2 of first semiconductor material. 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. Even more preferentially, the thickness is greater than 1 micron and advantageously less than or equal to 6 microns or even 5 microns. The thickness of the first layer 3 is preferably greater than 3 microns so that the final structure has an absorbent zone extending over at least 2 microns

The second semiconductor material is an alloy which comprises at least Hg, Cd and Te. The semiconductor material formed is monocrystalline and can be represented by the formula Hg-i-xCdxTe or more generally HgCdTe. The second semiconductor material can be an at least quaternary derivative of HgCdTe, for example Hg- _xy CdxZn _y Te, Hg-i- _xy CdxMn _y Te or Hgi- _x CdxTei-zSe _z . In general, the second semiconductor material is a material whose bandgap energy value varies at least according to the cadmium concentration. The first semiconductor material is different from the second semiconductor material while allowing an adaptation of the lattice parameter to achieve single-crystal growth of the first layer 3 from the surface of the upper layer 2.

The liquid phase epitaxy step uses a single bath containing all the elements which are used to form the second semiconductor material with Hg, Cd, Te and optionally Zn, Mn and Se or other necessary materials. The bath may also contain at least one electrical dopant so as to electrically dope the first layer 3 of second semiconductor material. The at least one electrical dopant may be the first electrical dopant and/or a second electrical dopant of the same conductivity type. The bath is not modified during growth, for example no constituent is added.

The material forming the first layer 3 is chosen so as to allow the formation of a first layer 3 in first monocrystalline semiconductor material by liquid phase epitaxy. The bath has a cadmium concentration which is lower than the cadmium concentration of the upper layer 2. If the bath has an electric dopant, its concentration is lower than the concentration of the first electric dopant in the upper layer 2.

During growth by liquid phase epitaxy, the at least one electrical dopant and the cadmium diffuse from the upper layer 2 to the first layer 3 of second semiconductor material.

The growth from the upper layer 2 generates a cadmium concentration gradient from the interface with the upper layer 2 which generates a gradient of forbidden band width which is decreasing in the first layer 3 from the interface with the layer upper 2, moving away from the interface. The minimum cadmium concentration in the first layer 3, that is to say the lower part of the gradient and/or the constant part, is between 10% and 25 atomic% in order to have good absorption of the desired infrared radiation. It is also preferable for the cadmium gradient to represent a difference in concentration at least equal to 10 atomic % or even at least equal to 25 atomic % in order to properly dissociate the electro-optical behaviors.

As the growth progresses, the cadmium present in the upper layer 2 has more and more difficulty in moving because it must diffuse over a greater distance and it is consumed. An equilibrium is created between the cadmium present in the bath and the cadmium from the upper layer 2. The cadmium concentration on the growth front of the first layer 3 decreases until the cadmium concentration eventually reaches the value defined by the bath and the cadmium concentration becomes constant or quasi-constant, for example with a gradient of less than 1%. By constant cadmium concentration is meant a concentration whose variation is less than 1 atomic % over 50 nm.

The growth conditions of the first layer 3 are preferably chosen to form a first layer 3 which has a constant cadmium concentration over at least 100 nm, advantageously over at least 500 nm or even 1 micron or 2 microns. It is also advantageous for the constant cadmium concentration to be located at a distance of between 500 nm and 1.5 microns from the interface.

Simultaneously, an electric dopant concentration gradient forms in the first layer 3. The electric dopant is mainly or almost exclusively in the substitutional position. The electric dopant concentration gradient is not linked to a modification of the constituents in the growth bath but to the evolution of the incorporation/diffusion of the first electric dopant into the crystal lattice from the upper layer 2, and possibly from the bath as well as the incorporation of a possibly second electric dopant. Preferably, the electric dopant concentration gradient extends over at least 500 nm, even more preferably between 500 nm and 1.5 microns. It is also preferable for the gradient to extend until the concentration of first electrical dopant is less than 2.10 ¹⁵ at/cm ³ .

In one case, the electric dopant is indium, the concentration of electric dopant is decreasing from the interface between the upper layer 2 and the first layer 3. In another case, the electric dopant is l iodine, a first electric dopant peak is present at the interface or very close to the interface so that there may be a slight increase in concentration then a decrease on the rest of the first layer 3. In a preferred embodiment, the concentration of electric dopant in the bath is zero. Advantageously, the growth conditions are chosen so that the concentration of first electrical dopant in the bath defines the desired minimum concentration in the first layer 3, for example a concentration less than or equal to 2×10 ¹⁵ cirr ³ . During the growth of the first layer 3, the concentration of electrical dopant decreases and then can be constant at the value defined by the bath. By constant concentration of electric dopant is meant a concentration whose relative variation is less than 10% over 50 nm.

Advantageously, the growth of the first layer 3 is carried out until the first layer 3 has at least a constant cadmium concentration over at least 500 nm and an electric dopant concentration which varies by less than 10% over at least 50 nm. If the bath is devoid of 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 made until the first layer 3 has at least a constant cadmium concentration over at least 2 microns.

The temperature of the liquid phase epitaxy step is above a threshold temperature which ensures the diffusion of the cadmium and of the first electrical dopant. The thermal budget (time pair, temperature) of the liquid phase epitaxy step is less than a threshold thermal budget which achieves the homogenization of the cadmium and/or of the first electric dopant over the thickness of the first layer 3. If the temperature is too low, the cadmium and the first dopant cannot diffuse and form the desired decreasing profile. On the contrary, if the thermal budget is too high, the cadmium and/or the first electric dopant diffuse too much and the concentration becomes homogeneous over the thickness of the first deposited layer 3. The temperature of the liquid phase epitaxy which can be comprised between 400°C and 500°C. In a different technical field, it is known from document US 5861321 to grow by liquid phase epitaxy an HgCdTe layer on a CdTe or CdZnTe substrate. The thickness of the HgCdTe layer is at least 30 microns. The substrate is n- or p-type doped and the HgCdTe layer is formed undoped. Annealing is then carried out so as to form a homogeneous doping in the HgCdTe layer. In another embodiment, the substrate is doped from the dopants present in the substrate. The homogenization annealing is carried out during the growth.

The growth step of the first layer 3 by liquid phase epitaxy is configured 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 configured 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 of reduced thickness, the thermal budget during growth is reduced, which limits the diffusion phenomena which tend towards the homogenization of the concentrations.

It is particularly advantageous to form a first layer 3 in which the extent of the cadmium gradient is less than 2 microns or even 1.5 microns, the extent of the gradient in the first layer 3 is measured from the interface with the upper layer up to the portion where the concentration of the first element becomes constant.

Advantageously, the extent of the cadmium gradient is less than 1.5 microns and even more preferably less than 1 micron. It is particularly advantageous to form a first layer 3 in which the extent of the cadmium gradient is greater than 100 nm and preferably greater than 500 nm. In an advantageous embodiment, the first layer 3 has a minimum cadmium concentration, that is to say the less rich part of the first gradient, which is at least equal to 10 atomic %, and preferably less than or equal to 30% atomic or even 20% atomic.

As the first electric dopant and the cadmium are introduced into the first layer 3 by diffusion, it is possible to have similar and well-controlled doping profiles for the cadmium and the first electric dopant. By similar, we mean that the shapes of the profiles are close while corresponding to very different concentration levels when we observe their concentration profiles in a semi-logarithmic scale. The extents of the first concentration gradient and of the second concentration gradient are preferably identical or have a difference of less than 1 micron or even less than 500 nm over the thickness.

Advantageously, before the liquid phase epitaxy step, the upper layer 2 has a concentration of first electrical dopant greater than or equal to 5.10 ¹⁶ at/cm ³ , advantageously greater than or equal to 5.10 ¹⁷ at/cm ³ , or even greater than 5.10 ¹⁸ at/cm ³ or 1.10 ¹⁹ at/cm ³ . The concentration is preferably between 5.10 ¹⁵ and 1.10 ¹⁹ at/cm ³ .

It is particularly advantageous to choose a concentration of first electrical dopant in the upper layer 2, for example indium or iodine, which ensures an incorporation of electrically active dopant at least equal to 5×10 ¹⁸ at/cm ³ in the first layer 3 and preferably at least equal to 1.10 ¹⁹ at/cm ³ .

Preferably, the second electric dopant concentration gradient has a concentration ratio at least equal to 10, or at least equal to 50, or at least equal to 100 or even 1000 between its highest concentration and its lowest concentration. . Such a profile is obtained in a single liquid phase epitaxy operation with a single bath. The concentration of first electrical dopant in upper layer 2, of dopants in the liquid bath and the operating conditions of the liquid phase epitaxy step define the doping profile in first layer 3.

It is advantageous to choose operating growth conditions for which the first layer 3 reaches a constant concentration of cadmium before reaching a constant concentration of electric dopant. The minimum concentration of first electrical dopant is present only in the portion of the first layer 3 with a constant or quasi-constant cadmium content and is preferably between 5.10 ¹³ at/cm ³ and 2.10 ¹⁵ at/cm ³ .

In the growth bath, it is particularly advantageous to choose a concentration which ensures an incorporation of electrically active dopants, of n or p type, of less than 5.10 ¹⁵ at/cm ³ in the constant or quasi-constant portion of the first layer 3, preferably less than 2.10 ¹⁵ at/cm ³ and even more preferably less than 5.10 ¹⁴ at/cm ³ .

Preferably, the first layer 3 comprises a concentration of electrically active dopant with a concentration gradient which decreases strictly from a concentration of first dopant at least equal to 1.10 ¹⁸ or 5.10 ¹⁸ at/cm ³ down to a concentration of less than 2.10 ¹⁵ at/cm ³ , preferably, with a value at least equal to 4.10 ¹⁸ at/cm ³ down to a value of less than 8.10 ¹⁴ at/cm ³ . The use of a doping gradient which reaches a low doping in the absorbent zone or even which tends towards a level corresponding to the depletion in the layer makes it possible to guide the electric charge carriers more effectively. Such a configuration makes it possible to reduce crosstalk as well as to improve the Modulation Transfer Function (MTF).

During the liquid phase epitaxy step, the cadmium concentration evolves which causes a variation in the width of the forbidden band of the deposited material. Simultaneously, the concentration of the first electrical dopant evolves and tends to lessen. The characteristics of the two concentration gradients are not defined by multiple growth baths, but by the cadmium and first electrical dopant concentrations in the upper layer and in the bath in relation to the thermal budget of the epitaxy step. in the liquid phase. It is possible to define the two concentration gradients by carrying out one or more preliminary simulations. The simulations take into account the diffusion coefficients of cadmium, of the first electrical dopant, the respective concentrations in the bath and in the upper layer 2 as well as the depletion of these two constituents as the growth progresses.

The modulation of the cadmium concentration representing the modulation of the forbidden band width and the modulation of the electrical dopant make it possible, in a single technological step, to functionalize several distinct portions of the first layer 3 to transform an electromagnetic signal into an electrical signal and promote the conduction of this electrical signal.

The first layer 3 has a first portion 3a intended to facilitate the transit of an electrical signal and a second portion 3b intended to capture exclusively or mainly the electromagnetic signal at least over part of its thickness.

By forming a first portion 3a of the second semiconductor material richer in first electrical dopant and generally richer in cadmium, it is possible to form a low resistive layer with a wide band gap which makes it possible to ensure good transfer of the electrical signal. The first portion 3a is less electro-optically active, which reduces the impact of crystalline defects on the quality of the signal supplied by the photodiode. The second portion 3b forms the absorbing part of the photodiode structure, that is to say the portion intended to capture the electromagnetic radiation. The second portion 3b has a forbidden band width less than the threshold value for capturing the electromagnetic radiation sought. The second portion 3b has a smaller forbidden band width and it is lightly doped or undoped to ensure good optoelectronic performance at the photodetector. The use of low doping makes it possible to reduce the value of the dark current, which is a parameter which degrades the performance of the photodetector, which facilitates the use of a thicker absorbent layer.

The second portion 3b preferably has a cadmium concentration below the threshold value equal to 25 atomic % and even more preferably a constant cadmium concentration. The second portion 3b preferably has a thickness between 1 and 4.5 microns and even more preferably between 1.5 and 4 microns or even between 2 and 4 microns. The thickness of the second portion can be chosen according to the mode of formation of the second layer 4 intended to finalize the junction. The use of an absorbent layer greater than 1 micron, preferably greater than 1.5 microns, makes it possible to achieve better quantum efficiency, which facilitates its use in the field of imaging. Advantageously, the second portion 3b has a concentration of first electrical dopant of less than 2.10 ¹⁵ at/cm ³ . For example, the second portion 3b of the first layer 3 of second semiconductor material contains or is exclusively formed by the portion having a constant or quasi-constant concentration of cadmium and forms the most optically active n- or p-type doped layer.

The first portion 3a preferably has a cadmium concentration whose minimum value corresponds to the maximum value of the second portion 3b. The first portion 3a preferably has a cadmium concentration whose maximum value is at least equal to 30 atomic %, advantageously at least 40 atomic % or 50 atomic %. Advantageously, the second portion 3b has a concentration of first electrical dopant greater than 2.10 ¹⁵ at/cm ³ . The first portion 3a may have an area where the cadmium concentration is constant. The first portion 3a preferentially has a thickness comprised between 1 and 4 microns and even more preferentially comprised between 2 and 4 microns. The existence of the first electric dopant concentration gradient ensures the production of a field or a pseudo electric field in the first layer 3 which makes it possible to channel the photogenerated charge carriers from the first portion 3a into the second portion 3b which is weakly resistive and which is able to ensure effective signal transfer. There is no growth interface between the second portion 3b and the first portion 3a, which ensures good transit of the signal.

Advantageously, the upper layer 2 and possibly the bath comprises only the first electric dopant or the first electric dopant is predominant. If the conditions of the liquid phase epitaxy step and the concentration of first electrical dopant do not make it possible to adapt the concentration profile of the first electrical dopant to 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 chosen so as to present a different diffusion rate from the first electrical dopant during the formation of the first layer 3. For example, for n-type doping, it is possible to use iodine in combination with indium in order to adapt the profile of the electrical doping to the profile of the forbidden band width defined by the cadmium.

The evolution of the content of first dopant is carried out continuously over the thickness of the first layer 3 which facilitates the realization of a continuous evolution of the concentration of electrically active dopant of n or p type and therefore the appearance of an electric field making it possible to efficiently channel the photogenerated charge carriers.

As illustrated in FIG. 3, once the first layer 3 of first semiconductor material has been formed, the rest of the photodiode can be formed and in particular the second layer 4 of opposite conductivity to define the pn or pin junction. When the first layer 3 is n-type doped, the second layer 4 is p-type doped. Conversely, when the first layer 3 is p-type doped, the second layer 4 is n-type doped. As indicated above, one or more other layers can be deposited before the second layer 4 to form a pin junction. It is also possible to form a third layer 5 which is not intentionally doped and which separates the n-type and p-type doped layers defining the junction. Layer 5 can be formed by one or more different materials.

The second layer 4 is made of a third semiconductor material which has a forbidden band width less than the threshold value. The active or most active optical part of the photodiode is formed by materials whose forbidden band width is less than the threshold value so that the photodiode is sensitive to a particular range of electromagnetic radiation distinct from that of the first portion 3a . This electromagnetic radiation is not picked up by the first portion 3a of the first layer 2 which has a larger forbidden band width. The third semiconductor material is advantageously a ternary or at least ternary alloy which is preferably HgCdTe or based on HgCdTe. Advantageously, the total thickness of the first layer 3 and of the second layer 4 is less than or equal to 6 microns or even less than or equal to 5 microns.

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

In an alternative embodiment, the second layer 4 intended to form the pn junction is obtained by implantation of a dopant of opposite electrical conductivity or by diffusion using an atmosphere containing a precursor of the dopant. The third semiconductor material may be identical to the second semiconductor material. The second layer 4 is formed in the first layer 3 by extrinsic doping. 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.

The total thickness of the first layer 3 and of the second layer 4 is advantageously less than 5 microns. The thickness of the first layer 3 is adapted depending on whether the second layer is deposited on the first layer 3 or formed in the first layer 3. In a particular embodiment, the thickness of the first layer 3 is greater than or equal to 3 microns and preferably less than or equal to 5 microns, after formation of the second layer 4.

The first cadmium concentration gradient and the second gradient in first electrical dopant are retained in the first layer 3 during the formation of the second layer 4.

Preferably, the absorption of electromagnetic radiation inside 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 third semiconductor material when the second layer is formed in the first layer in order to maintain a zone having good electro-optical properties.

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 can be formed by a contact layer advantageously made of a pure metallic material or an alloy of metallic materials. The contact layer can be deposited then etched to define the contact 6. When several photodiodes are formed on the same substrate 1, a specific contact is formed on each photodiode. The first cadmium concentration gradient and the second gradient in the first dopant electricity are kept in the first layer 3 during the formation of the contact(s) 6.

The photodiode is advantageously partially covered by a covering layer 7, for example of silicon nitride Si3N4 or a silicon oxide SiOx or else a layer of ZnS in order to protect the photodiode from the external environment, for example from humidity. The first cadmium concentration gradient and the second gradient in first electrical dopant are retained in the first layer 3 during the formation of the covering layer 7.

The growth of the first layer 3, of the second layer 4 and if necessary of the third layer 5 is advantageously carried out so as to form more than one photodiode. It is advantageous to form a plurality of photodiodes as a photodiode array.

The electrically conductive contact 6 is intended to be connected to a read circuit which will apply the bias to the photodiode and will receive the electrical signal representative of the observed scene. It is advantageous to associate each photodiode with a read circuit. The plurality of readout circuits are also connected in an array of readout circuits such that the plurality of readout circuits are hybridized to the plurality of photodiodes to form a Focal Plane Array (FPA).

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

It is possible to remove the substrate 1 after the formation of the photodiode, for example after the formation of the electrically conductive contact 6 or after the deposition of the covering layer 7 or after the hybridization to the read circuit. Alternatively, the substrate 1 can be thinned or kept with its thickness initial. The electromagnetic radiation enters the photodiode structure through the substrate 1.

A photodiode structure is proposed which comprises a junction formed from a first doped layer 3 whose conductivity is n-type and a second doped layer 4 whose conductivity is p-type. The first layer is in a first semiconductor material and defines a heavily doped first portion 3a and a lightly doped second portion 3b.

The photodiode structure successively comprises a first portion of first layer in a first semiconductor material, a second portion of first layer in the first semiconductor material and a second layer in second semiconductor material. The first and second layers 3 and 4 form the junction which ensures the transformation of the electromagnetic signal into electron-hole pairs.

When the second layer 4 is formed by implanting a dopant of the second conductivity type in the first layer 3 to form the junction, the same semiconductor material is present on each side of the junction, except for the type of doping.

A photodiode is obtained which has a first layer 3 of HgCdTe or one of its at least quaternary derivatives. The first layer 3 comprising a first cadmium concentration gradient and a second concentration gradient of first electrical dopant. The first concentration gradient and the second concentration gradient are each decreasing from an end of the first layer 3. The end can be uncovered or covered by a material having predefined optical properties when the substrate 1 and the upper layer 2 have been removed. According to the configurations, 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 totally removed after the formation of an electrically conductive contact 6 on the second layer 4.

The first layer 3 is of a first type of conductivity and it has a thickness of less than 6 microns. The second layer 4 is also made of HgCdTe or one of its at least quaternary derivatives. The second layer 4 is a layer of the second type of conductivity opposed to the first type of conductivity, the first and second layers forming a junction ensuring the transformation of an electromagnetic signal into electron-hole pairs. The second layer 4 has at least the same composition in Hg, Cd and Te as the first layer 3 at the interface between the first layer 3 and the second layer 4.

The first layer 3 has a continuously decreasing concentration of cadmium and of electrically active dopant between the first portion 3a and the second portion 3b, which allows the supply of a signal of better quality.

The precautions taken during the liquid phase epitaxy step to form the concentration gradients are maintained until the end of the diode formation process. The diode manufacturing process does not require annealing which homogenizes the concentration of cadmium and/or of the first electrical dopant over the entire thickness of the first layer.

In the methods of the prior art, the electrically active dopant concentration profile is obtained by a multitude of slots which correspond to as many successive baths. Subsequently, the profile formed by the plurality of slots is subjected to annealing which is intended to make the slots disappear. Since the diffusion rate of cadmium is different from the diffusion rate of the n-type electrically active dopant, it is particularly difficult to obtain a profile identical to that obtained with the method described above. Since the doping is obtained during growth by liquid phase epitaxy, the doping process generates significantly fewer defects than an implantation step and especially in the lightly doped zone which will be electro-optically active. The photodiode structure makes it possible to better direct the photogenerated charge carriers, which makes it possible, for example, to reduce the blurring of the image obtained when several photodiodes are connected in a network.

Claims

28 Claims

1 . Photodiode structure comprising a junction intended to convert infrared electromagnetic radiation into electron-hole pairs, the junction comprising a first layer (3) made of HgCdTe or one of its at least quaternary derivatives of a first type of conductivity and a second layer (4) made of HgCdTe or one of its at least quaternary derivatives of a second type of conductivity opposite to the first type of conductivity to form a p-n or p-i-n junction; wherein the first layer (3) comprises:

- a first and a second end, the first end being separated from the second layer (4) by the second end;

- a first cadmium concentration gradient decreasing from the first end of the first layer (3) towards the second layer (4), the minimum value of atomic cadmium concentration in the first concentration gradient being between 10% 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 smaller than the first portion (3a);

- a second concentration gradient of at least one electric dopant, the second concentration gradient decreasing from the first end in the direction of the second layer (4); wherein the total thickness of the first layer (3) and the junction layer (4) is less than 6 microns; characterized that:

- the first cadmium concentration gradient is continuously decreasing in the form of a diffusion profile,

- the second concentration gradient of at least one electrical dopant is continuously decreasing in the form of a diffusion profile,

- the second concentration gradient extends, in the first layer (3), over a distance of between 500 nm and 1.5 microns from the first end,

- the second concentration gradient decreases from a concentration greater than 5.10 ¹⁸ at/cm ³ to a concentration less than 2.10 ¹⁵ at/cm ³ ;

- the first concentration gradient extends until the concentration of first electrical dopant is less than 2.10 ¹⁵ at/cm ³ and in that the first layer (3) has a greater thickness than the second layer (4) .

2. Photodiode structure according to claim 1, in which an electrically conductive contact (6) is electrically connected to the second layer (4), the contact (6) being made of pure metallic material or of an alloy of metallic materials.

3. Photodiode structure according to either of claims 1 and 2, in which the first layer has a constant cadmium concentration over at least 500 nm.

4. Photodiode structure according to claim 3 wherein the first layer (3) has a constant cadmium concentration over at least one micrometer.

5. Photodiode structure according to any one of claims 1 to 3 wherein the first layer (3) has a portion with a constant cadmium concentration and which is unintentionally doped.

6. Photodiode structure according to any one of the preceding claims, in which the thickness of the first layer (3) is greater than or equal to 3 microns and preferably less than or equal to 5 microns.

7. Photodiode structure according to any one of the preceding claims, in which the difference in cadmium concentration in the first concentration gradient is at least equal to 25 atomic %.

8. Photodiode structure according to any one of the preceding claims, in which the total thickness of the first layer (3) and of the junction layer (4) is less than 5 microns.

9. Photodiode structure according to any one of the preceding claims, in which the second concentration gradient of at least one electrical dopant comprises a gradient of an iodine concentration and a gradient of a chlorine concentration.

10. A method of manufacturing a photodiode structure according to any one of claims 1 to 9 successively comprising:

- providing a substrate (1) having at least one upper layer (2) made of CdZnTe or CdTe and having a first concentration of a first electrical dopant of a first type of conductivity and a first concentration of cadmium, the upper layer ( 2) being of the first conductivity type,

- growing a first layer (3) in HgCdTe or one of its at least quaternary derivatives from the upper layer (2), by liquid phase epitaxy using a single bath containing the precursors of the first layer (3) including cadmium and optionally at least one electric dopant, the bath having a second cadmium concentration lower than the first cadmium concentration and optionally a second electric dopant concentration lower than the first electric dopant concentration, the at least one electric dopant being chosen from the first electric dopant and/or a second electric dopant, the liquid phase epitaxy being carried out at a temperature which achieves the diffusion of a part of the cadmium atoms and of the first electric dopant from the upper layer (2) towards the first layer (3) to form a first continuously decreasing cadmium concentration gradient from the interface between the upper layer (2) and the first layer (3) moving away from the interface, the minimum atomic cadmium concentration value in the first concentration gradient being between 10% and 25 atomic%, the first concentration gradient defining a first portion (3a) and a second portion (3b), the second portion (3b) having a smaller band gap than the first portion (3a) and to form a second concentration gradient of the at least one electrical dopant in the first layer (3), the second concentration gradient of at least one electrical dopant being continuously decreasing from a concentration higher than 5.10 ¹⁸ at/cm ³ to a concentration lower than 2.10 ¹⁵ at/cm ³ , the second gradient of concentration extending, in the first layer (3), over a distance between 500 nm and 1.5 microns from the interface with the top layer (2), the first layer (3) being of the first conductivity type, the first concentration gradient extending until the concentration of first electrical dopant is less than 2.10 ¹⁵ at/cm ³ , - forming at least one junction layer (4) of HgCdTe or one of its at least quaternary derivatives of a second type of conductivity opposite to the first type of conductivity to form a pn or pin junction with the first layer (3), the first cadmium concentration gradient and the second concentration gradient of first electrical dopant being retained in the first layer (3) after formation of the junction layer (4), the second concentration gradient decreasing from the interface between the upper layer (2) and the first layer (3) in the direction of the second layer (4), the first layer (3) having a greater thickness than the second layer (4), the total thickness of the first re layer (3) and the junction layer (4) being less than 6 microns.