Patent Publication Number: US-2021193726-A1

Title: Optoelectronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of Application Ser. No. 16/540,267, filed Aug. 14, 2019, which is a continuation of Application Ser. No. 15/552,945, filed Aug. 23, 2017, which is a national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/EP2016/053993 which has an International filing date of Feb. 25, 2016, which claims priority to European Application No. 15305288.1, filed Feb 25, 2015, the entire contents of each of which are hereby incorporated by reference. 
    
    
     FIELD 
     This invention concerns the field of implantable optoelectronic devices that can be used in particular in retinal prostheses designed to offset the deterioration of the photoreceptor cells of the human eye. 
     BACKGROUND 
     The human eye, or eyeball, is a hollow structure with a globally spherical form. The innermost layer of the back part of the eye is the retina. The retina is a nervous structure, comprising many photoreceptors and neurones that process and channel visual information to the brain via the optic nerve. At the point where the optic nerve comes out, the retina is interrupted: this is the blind spot, close to which is the yellow spot, or macula, containing a central pit, known as the fovea. 
     Specialised photoreceptor nerve cells line the inner wall of the back of the eye; cones and rods, thus named due to their shape, which contain photo-sensitive substances. The rods, sensitive to light intensity, are photoreceptors that are designed specifically for twilight vision and the cones, responsible for colour vision, are designed more specifically for daylight vision. Cones are divided into three families of cells, each with its sensitivity peak in a determined zone of the spectrum (blue-purple, green and yellow-green). 
     The deterioration of the photoreceptor cells of the human eye may be due for example to age-related macular degeneration (AMD) or to genetically inherited retinitis pigmentosa. The photoreceptors (cones and rods) are the cells of the retina that are sensitive to light, whereas the other neurones that process signals captured by photoreceptors send information to the brain via the optic nerve. When photoreceptor cells deteriorate, the retina can no longer respond to light. However, a sufficient number of other neurones remain so that their electrical stimulation produces the perception of light by the brain. 
     In order to treat the deficiencies of the photoreceptor cells, two methods have been explored: implanting retinal prostheses and the optogenetic approach. 
     The optogenetic technique involves changing the neurones to make them sensitive to light, by incorporating a light-sensitive protein into the cellular membrane. By making each cell sensitive to light, vision can potentially be restored to near-normal acuity. However, artificial vision based on the optogenetic approach presents a major drawback. The modified cells require blue (460 nm) and very bright light to be activated, and the light intensity required is around seven times greater than the light sensitivity threshold normally observed in healthy individuals. 
     Retinal prostheses have an optoelectronic device that includes a matrix of optoelectronic components that are activated, either by light entering the eye in the case of a “subretinal” prosthesis, or by an electric signal from a micro-camera fitted outside the eye, in the case of an “epiretinal” prosthesis. The different types of implants used require a silicon-based technology which is easy to use and allows the development of nanometric devices. However silicon is a material that is opaque in the visible optic field. 
     The epiretinal solution involves placing an electronic implant in the front of the retina to stimulate the neurones. The epiretinal implant itself is not sensitive to light and must be connected to a micro-camera fitted outside the eye. The epiretinal implant requires a “coder” whose function is to fulfil the role of the neurones in the inner layer of the retina, which perform the preliminary processing of visual information. 
     In the United States, in February 2013 the Food and Drug Administration (FDA) authorised the use of the first epiretinal prosthesis designed to treat patients with advanced retinitis pigmentosa. This epiretinal prosthesis known as the “Argus II Retinal Prosthesis System” is manufactured by the company Second Sight Medical Products Inc. This epiretinal prosthesis is a gold standard with a matrix of some sixty electrodes, and has already been tested on patients throughout the world. 
     The sub-retinal prosthesis is placed beneath the retina to replace the destroyed photoreceptor nerve cells, which is surgically more difficult to perform but allows the neurones to be stimulated in a more natural position. The subretinal prosthesis converts incidental light to an electric signal which is transmitted to the neurones (bipolar cells). The subretinal prosthesis is itself sensitive to light and does not need an external device. The reference subretinal prosthesis “Retina” is manufactured by the company Retina Implant AG. 
     It is considered that to read a text, to move about independently and to recognise a face, minimum resolution must be more than 1,000 pixels. The subretinal prosthesis is sensitive to light with a matrix of 1,500 pixels. With an epiretinal prosthesis, resolution is only 60 pixels. However, clinical tests performed with the two types of prosthesis give equivalent results, whereas there are a significantly larger number of electrodes in a subretinal prosthesis. When the number of optoelectronic components increases, the matrices become denser with smaller optoelectronic components. More precise positioning of the individual optoelectronic components becomes essential to increase the proximity between the optoelectronic component and the layer of retinal ganglion cells. This allows each optoelectronic component to activate a small portion of the retina to increase visual acuity. 
     Moreover, known retinal prostheses are flat two-dimensional (2D) devices that are not capable of making a three-dimensional (3D) simulation, which is a significant limitation of their performance. 
     SUMMARY 
     More effective solutions than the current ones, without the afore-mentioned drawbacks, are therefore needed. 
     The solution proposed is a retinal prosthesis featuring a matrix of optoelectronic components with semiconductor optical amplifiers SOAs, which contain an active layer of gallium nitride GaN with multiple quantum wells InGaN/GaAsN (gallium indium nitride/gallium arsenide nitride) or InGaN/AlGaN (gallium indium nitride/gallium aluminium nitride) on a substrate of gallium nitride GaN with p-type doping and covered with a layer of gallium nitride GaN with n-type doping. 
     The semiconducting material GaN has the advantage of having good chemical stability and bio-compatibility. For this reason, it is possible to encapsulate materials that are not well tolerated by the human organism in this material since it creates a protective barrier. 
     The semiconducting material GaN also has the characteristic of being transparent in the wavelength range of visible light. In this way, the retina cells that are still functional are not affected by the opacity of the retinal prosthesis. Furthermore, the retina cells not affected are still stimulated since the retinal prosthesis does not mask the visible light penetrating the eye. 
     Thus the optoelectronic devices with a matrix containing optoelectronic components based on a gallium nitride GaN structure with Multi Quantum Wells (MQW) InGaN/GaAsN or InGaN/AlGaN present the advantage of letting the light pass between two neighbouring optoelectronic components. 
     From one viewpoint, the substrate of gallium nitride (GaN) with p-type doping forms a column of pGaN. 
     From another viewpoint, the column of p-GaN is covered with an insulating layer of bio-compatible material chosen from carbon, diamond, titanium dioxide TiO 2 , silicon SiO 2 , silicon nitride Si 3 N 4  or gallium nitride GaN. 
     From yet another viewpoint, the ratio between the height and the cross dimension of the pGaN column is less than 20. 
     According to one method of construction, the optoelectronic device has a matrix of optoelectronic components that comprises semiconductor optical amplifiers SOAs of different heights. 
     According to another method of construction, the matrix of optoelectronic components has semiconductor optical amplifiers SOAs with vertical cavity or semiconductor optical amplifiers SOAs with horizontal cavity. When the matrix of optoelectronic components has at least one semiconductor optical amplifier SOA with vertical cavity, two distributed Bragg reflectors are placed on each side of the active GaN layer with multi quantum wells so that an optical cavity is defined. 
     According to yet another method of construction, the matrix of optoelectronic components is a three-dimensional (3D) matrix of semiconductor optical amplifiers SOAs with vertical cavity or semiconductor optical amplifiers SOAs with horizontal cavity. 
     The semiconductor optical amplifiers SOAs should be spaced at distance E such that E 2 =π(350/2) 2 ×1/n where n is the number of optoelectronic components in the matrix. 
     A transparent matrix of semiconductor optical amplifiers SOAs amplifies the blue, yellow or green light to improve the results obtained with the optogenetic technique, or with epiretinal or subretinal prostheses. 
     According to one method of construction, the optoelectronic component is a photodiode. The use of a transparent matrix of photodiodes with multi quantum wells InGaN/GaAsN or InGaN/AlGaN eliminates the need for the micro-camera used today with the epiretinal prosthesis. 
     According to another method of construction, the matrix of optoelectronic components also has vertical or horizontal photodiodes. The matrix of optoelectronic components should preferably contain at last one photodiode and at least one semiconductor optical amplifier SOA. 
     According to another method of construction, the optoelectronic device has a matrix of optoelectronic components that comprises vertical or horizontal photodiodes of different heights. The photodiodes and semiconductor optical amplifiers SOAs are of different heights in order to more accurately stimulate the layer of retinal ganglion cells and/or the optical nerve. 
     The vertical or horizontal photodiodes should be spaced at distance E such that E 2 =π(350/2) 2 ×1/n where n is the number of optoelectronic components in the matrix. 
     A retinal prosthesis, which is an epiretinal prosthesis, is also proposed. 
     A retinal prosthesis, which is a subretinal prosthesis, is also proposed. 
     A retinal prosthesis featuring both a subretinal prosthesis and an epiretinal prosthesis, is also proposed. 
     According to one viewpoint, an epiretinal or subretinal prosthesis features a matrix with at least one vertical photodiode. 
     According to a second viewpoint, an epiretinal or subretinal prosthesis features a matrix with at least one horizontal photodiode. 
     According to a third viewpoint, an epiretinal or subretinal prosthesis features a matrix with at least one semiconductor optical amplifier with vertical cavity. 
     According to a fourth viewpoint, an epiretinal or subretinal prosthesis features a matrix with at least one semiconductor optical amplifier with horizontal cavity. 
     According to yet another viewpoint, at the same time at least one matrix of photodiodes and at least one matrix of semiconductor optical amplifiers SOAs can be incorporated into the same epiretinal or subretinal prosthesis in order to stimulate the neurones by both injecting an electric signal and amplifying the blue, green or yellow light. 
    
    
     
       BRIEF DESCRIPTION 
       Other characteristics and advantages of the present invention will become apparent upon reading the following description of embodiments, naturally given by way of illustrative and non-limiting examples, and in the attached drawing in which 
         FIG. 1  illustrates a schematic cross-sectional view of a human eye 
         FIG. 2  illustrates a schematic cross-sectional view of the retina 
         FIGS. 3 a , 3 b  and 3 c    illustrate schematically an embodiment of an optoelectronic component according to the invention 
         FIG. 4  illustrates schematically an embodiment of an optoelectronic device with a vertical photodiode applicable to a subretinal prosthesis 
         FIG. 5  illustrates schematically an embodiment of an optoelectronic device with a vertical photodiode applicable to an epiretinal prosthesis 
         FIG. 6  illustrates schematically an embodiment of an optoelectronic device with a semiconductor optical amplifier with vertical cavity applicable to a subretinal prosthesis 
         FIG. 7  illustrates schematically an embodiment of an optoelectronic device with a semiconductor optical amplifier with vertical cavity applicable to an epiretinal prosthesis 
         FIG. 8  illustrates schematically an embodiment of an optoelectronic device with a vertical photodiode and a semiconductor optical amplifier with vertical cavity applicable to a subretinal prosthesis 
         FIG. 9  illustrates schematically an embodiment of an optoelectronic device with a vertical photodiode and a semiconductor optical amplifier with vertical cavity applicable to an epiretinal prosthesis 
         FIG. 10  illustrates schematically an embodiment of a matrix of optoelectronic components 
         FIG. 11  illustrates the facet of the horizontal cavity GaN, for photodiode GaN and semiconductor optical amplifier SOA applications 
         FIGS. 12 a  and 12 b    illustrate schematically two perpendicular side views of one embodiment of an optoelectronic device consisting of a horizontal photodiode that can be applied to a sub-retinal prosthesis, 
         FIGS. 13 a  and 13 b    illustrate schematically two perpendicular side views of one embodiment of an optoelectronic device consisting of a horizontal photodiode that can be applied to an epiretinal prosthesis, 
         FIGS. 14 a  and 14 b    illustrate schematically two perpendicular side views of one embodiment of an optoelectronic device consisting of a semiconductor optical amplifier with horizontal cavity that can be applied to a sub-retinal prosthesis, 
         FIGS. 15 a  and 15 b    illustrate schematically two perpendicular side views of one embodiment of an optoelectronic device consisting of a semiconductor optical amplifier with horizontal cavity that can be applied to an epiretinal prosthesis, 
         FIGS. 16 a  and 16 b    illustrate schematically two perpendicular side views of another embodiment of an optoelectronic device consisting of a semiconductor optical amplifier with horizontal cavity that can be applied to an epiretinal prosthesis. 
     
    
    
     Directional terminology like “left” and “right”, “top” and “bottom”, “front” and “rear”, “horizontal” and “vertical”, “above” and “below”, etc., is used here with reference to the orientation of the figures described. Since the components that make up the embodiments may be placed in different orientations, the directional terminology is used here only for illustrative purposes and is in no way limiting. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates schematically a cross-section of a human eye  1 . It is composed of three superposed membranes  2 ,  3 ,  4  surrounding a gelatinous substance called the vitreous humour  5 . 
     The anterior chamber of the eye, which receives the light, consisting of 
     the iris  6  with a round opening in its centre called the pupil  7 , which allows light to pass into the eye and the size of which adapts automatically to the brightness the eye is exposed to,
 
the cornea  8 , a round, transparent, domed membrane that allows light rays to pass through,
 
the lens  9 , which focuses the image on the retina depending on the distance.
 
     The retina  4  is the membrane that lines the inner surface of the eye&#39;s posterior chamber. The retina&#39;s nerve cells convert the light energy into electrical signals, which are transmitted to the brain by the optic nerve  10 . The blind spot  11  is the area of the eye where the fibres meet to form the optic nerve, and which contains no photosensitive cells. Nearby, the macula  12  (or yellow spot) is formed of numerous visual cells. 
     The most sensitive area of the retina, devoid of any blood capillaries, is called the fovea  13 . The fovea  13  is a small part of the retina found in the macula  12  (approximately 6 mm in diameter) that is sensitive to colours and is important for visual acuity. The foveola  14  (approximately 0.35 mm in diameter) is located in the middle of the fovea  13  (approximately 1.5 mm in diameter) and contains only cone cells. The fovea  13  is the part of the retina  4  with the highest visual acuity—this is where the rays of light have entered directly with the least interference, and is where the density of photoreceptor cells is at its highest. In the foveola  14 , the photoreceptor cones are longer, thinner, and more densely packed than elsewhere in the retina  4 . This ensures the foveola  14  has the highest visual acuity in the retina  4 . The photoreceptor cells convert the light energy into nervous impulses that are sent to the optic nerve. 
     As illustrated in the schematic cross-section view in  FIG. 2 , the retina  4  is composed of a stack of different layers arranged radially at the fovea  13 . The outer layer  20 , the layer of retinal ganglion cells (RGCs), stops the light from diffusing inside the eye. The inner layer  21 , the layer of photoreceptors (PRs), is formed of specialised nerve receptor cells  22 , with the rods and cones detecting light and the neurons processing and transmitting the visual information to the brain. The inner layer  22  is directly accessible by the foveola  14 . The middle layer  23 , or inner nuclear layer (INL), contains connecting cells such as bipolar cells. 
     There are several kinds of retinal prosthesis that use an optoelectronic device consisting of optoelectronic components based on a common concept as illustrated by  FIGS. 3 a  to 3 c   . This concept is based on carrying out one or more epitaxies on an intrinsic active GaN layer  30  with multiple quantum wells for InGaN/GaAsN (indium gallium nitride/arsenic gallium nitride) or InGaN/AlGaN (indium gallium nitride/aluminium gallium nitride) on a substrate  31  of p-type doped gallium nitride GaN. An intrinsic material is a semiconductive material that is not doped and/or has no impurities. Epitaxy is the crystalline growth of a material, generally carried out on the same material respecting the crystals&#39; meshing and orientation. At the top of each active GaN layer  30 , a layer of n-type doped GaN gallium nitride layer  32  is carried out to complete the epitaxy. 
     The process applied to the rear surface  33  of the p-GaN substrate  31 , wherein the p-GaN substrate  31  is thinned and polished to the desired height, results in p-GaN columns  34 . The p-GaN columns  33  are obtained by selective etching of the p-GaN substrate layer  31 , for example with a chloride inductively coupled plasma ICP. The p-GaN column  34  must be long enough to stimulate the cells of the retina. The ratio between the height and transverse measurement (width or diameter) of the p-GaN column  34  should preferably be less than  20 , to prevent the column from breaking. The p-GaN columns  34  may be different heights in order to stimulate different layers of the retina. The different heights are achieved through selective etching of the p-GaN substrate  31  for example, by starting from the rear surface. The p-GaN column  34  may take the shape of a rod with parallel edges ( FIG. 3 a   ), a truncated pyramid ( FIG. 3 b   ), or a thin rod on top of a wider base ( FIG. 3 c   ). In the remainder of this description, we shall consider a thin rod on top of a wider base, as shown in  FIG. 3   c.    
     Several optoelectronic components with a similar structure consisting of multiple quantum wells can be created using selective area growth (SAG) technology, in order to amplify or detect several wavelengths (blue, green, yellow). The various optoelectronic components found on the same matrix are electrically separated by an area of implanted GaN or semi-isolating GaN, so that the optoelectronic components are isolated from each other and the matrix remains transparent. 
       FIG. 4  illustrates a schematic representation of an optoelectronic device embodiment, composed of at least one GaN-based vertical photodiode and intended for use in a sub-retinal prosthesis. 
     An absorbent active GaN layer  40  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a substrate  41  of p-type doped gallium nitride GaN. A layer  42  of n-type doped gallium nitride GaN is laid on top of the absorbent GaN layer  40 . By thinning and polishing the p-GaN substrate  41 , a p-GaN column  43  is obtained at the desired height. The p-GaN column  43  is coated with an isolating layer  44  of dielectric or semi-isolating material. Furthermore, the material composing the isolating layer  44  must offer a good level of biocompatibility, such as carbon, diamond, titanium dioxide, common dielectric materials (silica, silicon nitride, etc.) or semi-isolating GaN material. The isolation is completed by implanting semi-isolating GaN  45  to separate the optoelectronic components from each other, in order to polarise the optoelectronic components in a matrix independently. 
     A metal contact  46  is placed on the front surface of the n-type doped gallium nitride GaN layer  42 . The metal contact  46  on the front surface of the n-GaN layer  42  polarises the retinal prosthesis. Another metal contact  47  is placed at the end of the p-GaN column  43  corresponding with the area of the retina  48  that is stimulated. The metal contacts  46  and  47  are connected by an electrochemical generator  49  (battery or accumulator), which establishes a voltage between them. Because the light L must pass through the p-GaN column  43  to reach the absorbent active GaN layer  40 , the metal contact  47  must not cover the entire upper surface of the p-GaN column  43 . A photocurrent appears, which will stimulate the retina&#39;s various layers. 
     The embodiment schematically illustrated in  FIG. 5  shows an optoelectronic device consisting of at least one GaN-based vertical photodiode that is designed for use with an epiretinal prosthesis. 
     An absorbent active GaN layer  50  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a substrate  51  of p-type doped gallium nitride GaN. A layer  52  of n-type doped gallium nitride GaN is laid on top of the absorbent GaN layer  50 . By thinning and polishing the p-GaN substrate  51 , a p-GaN column  53  is obtained at the desired height. The p-GaN column  53  is coated with an isolating layer  54  of dielectric or semi-isolating material. Isolation is completed by implanting semi-isolating GaN  55 . Each photodiode in a matrix may be independently polarised from its neighbour in this way, depending on the medical requirement. A metal contact  56  is placed on the front surface of the n-GaN layer  52 , and another metal contact  57  is placed at the end of the p-GaN column  53  that corresponds to the area of the retina  58  to be stimulated. Because the light L must pass through the nGaN column  52  to reach the absorbent active GaN layer  50 , the metal contact  56  must not cover the entire front surface of the n-GaN column  52 . A photocurrent appears, which will stimulate the retina&#39;s various layers. However, the metal contact  57  may cover the entire surface at the end of the pGaN column  53  because the induced photocurrent is enough to stimulate the different layers of the retina. Indeed, there is no need to transmit the light outside of the epiretinal area where there are no photoreceptor cells. 
     Replacing the retina with matrices containing thousands, if not millions, of optoelectronic components based on semiconductors, like these photodiodes, will make it possible to convert the light into an electrical signal, which will then be transmitted to the visual fibres that are still functioning. 
     We will now consider  FIG. 6 , which illustrates a schematic view of an optoelectronic device embodiment, composed of at least one GaN-based semiconductor optical amplifier with vertical cavity and intended for use in a sub-retinal prosthesis. 
     Remember that an optical amplifier is a device that amplifies an optical signal directly, without the need to convert it into an electrical signal beforehand. An optical amplifier is different from a laser in that it has no optical cavity, or there is no retroaction produced from the cavity. The semiconductor optical amplifiers SOAs are optical amplifiers that use semiconductive material to provide the gain medium. These semiconductor optical amplifiers SOAs contain anti-reflective parts at its end surfaces, which results in energy loss from the cavity that is above the gain, thus preventing the optical amplifier from working like a laser. 
     An amplifying active GaN layer  60  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a substrate  61  of p-type doped gallium nitride GaN. A layer  62  of n-type doped gallium nitride GaN is laid on top of the amplifying GaN layer  60 . Two distributed Bragg reflectors DBRs  63  are placed on either side of the amplifying GaN layer  60 . 
     By thinning and polishing the p-GaN substrate  61 , a p-GaN column  64  is obtained at the desired height. The p-GaN column  64  is coated with an isolating layer  65  of dielectric or semi-isolating material. 
     A metal contact  66  is placed on the front surface of the n-GaN layer  62 , and another metal contact  67  is placed at the end of the p-GaN column  64  that corresponds to the area of the retina  68  to be stimulated. Since on the one hand the incident light L must be able to penetrate the p-GaN substrate  61  to reach the amplifying GaN layer  60 , and on the other hand the amplified light AL must be able to reach the area that requires stimulation  68 , the metal contact  67  must not cover the entire surface at the end of the p-GaN column  64 . 
     The distributed Bragg reflectors DBRs  63  define an optical cavity in which blue light is amplified. All of the blue light is reflected on the mirror created by the metal contact  66  covering the front surface of the n-GaN layer  62 . 
     After carrying out an optogenetic operation, the retinal cells will be selectively stimulated by the amplified blue light AL. An anti-reflective coating  69  is necessary on the top end of the pGaN column  64  in order to prevent parasite reflections and improve the quality of optical transmission. 
       FIG. 7  illustrates a schematic view of an optoelectronic device embodiment, composed of at least one GaN-based semiconductor optical amplifier with vertical cavity and intended for use in an epiretinal prosthesis. 
     An amplifying active GaN layer  70  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a substrate  71  of p-type doped gallium nitride GaN. A layer  72  of n-type doped gallium nitride GaN is laid on top of the amplifying GaN layer  70 . Two distributed Bragg reflectors DBRs  73  are placed on either side of the amplifying GaN layer  70 . 
     By thinning and polishing the p-GaN substrate  71 , a column of p-GaN of the desired height  74  is produced. The p-GaN layer  74  is covered with an insulating layer  75  of dielectric or semi-insulating material. 
     A metal contact  76  is placed on the front face of the n-GaN layer  72  and another metal contact  77  is placed at the end of the p-GaN column  74  corresponding to the area of retina  78  to be stimulated. Because the incident blue light L has to cross the n-GaN layer  72  to reach the GaN amplifying layer  70 , the metal contact  76  must not cover the entire surface of the front face of the n-GaN layer  72 . Once the blue light LA has been amplified it has to leave the column of p-GaN  74  to stimulate the neighbouring layers  78  of the retina, where the optogenetic therapy has been active, and the metal contact  77  must therefore not cover the entire surface of the end of the column of p-GaN  74 . 
     The blue light is amplified in the optical cavity defined by the two distributed Bragg reflectors DBRs  73 , positioned either side of the GaN amplification layer  70 . After an optogenetic operation, the retina cells will be selectively stimulated by this amplified blue light LA. An anti-reflection coating  79  is required at the upper end of the p-GaN column  74  to prevent parasitic reflections, and to improve the optical transmission quality. 
     It may be advantageous to combine an optoelectronic device intended as a subretinal prosthesis with an optoelectronic device intended as an epiretinal prosthesis, whether or not they have the same operational mode. For example, a subretinal prosthesis containing optical amplifiers can be combined with an epiretinal prosthesis containing photodiodes, in particular in cases where optogenetic therapy proves more effective for cells close to the layer of ganglion cells than for photoreceptive cells such as cones. Or inversely, an epiretinal prosthesis containing optical amplifiers can be combined with a subretinal prosthesis containing photodiodes. It is also possible to combine photodiodes and optical amplifiers in a single epiretinal or subretinal prosthesis. This can be achieved by the use of vias (metallised holes) to produce direct and indirect polarisation of the optoelectronic components. 
     In the embodiment illustrated schematically in  FIG. 8 , an optoelectronic device containing at least one GaN-based photodiode and at least one vertical cavity GaN-based semiconductor optical amplifier combined, is intended for use in a subretinal prosthesis. 
     Photodiode  80 , analogous to that in  FIG. 4 , contains an active absorbent GaN layer  81  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells deposited on a p-GaN substrate  82  and surmounted by an n-GaN layer  83  which is cut to form a column of p-GaN  84 . A metal contact  85  is deposited on the n-GaN layer  83  and another metal contact  86  partially covers the upper end of the p-GaN column  84  corresponding to the area of retina  87  to be stimulated. 
     The vertical cavity semiconductor optical amplifier  100  contains a GaN amplifying layer  101  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells, deposited on a p-GaN substrate  102  and surmounted by a n-GaN layer  103  which is cut to form a column of p-GaN  104 . Two distributed Bragg reflectors DBRs  105  are placed either side of the GaN amplifying layer  101 . A metal contact  106  is deposited on the n-GaN layer  103  and another metal contact  107  partially covers the upper end of the p-GaN column  102  corresponding to the area of retina  108  to be stimulated. 
     The multiple quantum well structure of the photodiode and the multiple quantum well structure of the vertical cavity semiconductor amplifier can be adapted with distributed Bragg reflectors, by using butt-joint epitaxy. 
     We now consider  FIG. 9 , schematically illustrating an embodiment for an optoelectronic device containing at least one GaN-based photodiode and at least one vertical cavity GaN-based semiconductor optical amplifier combined, intended for use in an epiretinal prosthesis. 
     Photodiode  90 , analogous to that in  FIG. 5 , contains an active absorbent GaN layer  91  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells deposited on a p-GaN substrate  92  and surmounted by an n-GaN layer  93  which is cut to form a column of p-GaN  94 . A metal contact  95  is deposited on the n-GaN layer  93  and another metal contact  96  partially covers the upper end of the p-GaN column  94  corresponding to the area of retina  97  to be stimulated. 
     The vertical cavity semiconductor optical amplifier  110  contains a GaN amplifying layer  111  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells, deposited on a p-GaN substrate  112  and surmounted by an n-GaN layer  113  which is cut to form a column of p-GaN  114 . Two distributed Bragg reflectors DBRs  115  are placed either side of the GaN amplifying layer  111 . A metal contact  116  is deposited on the n-GaN layer  113  and another metal contact  117  partially covers the upper end of the p-GaN column  112  corresponding to the area of retina  118  to be stimulated. 
     It thus becomes possible to replace the retina by a prosthesis consisting of an optoelectronic device containing thousands or even millions of optoelectronic components in a matrix, as illustrated in  FIG. 10 . 
     An important parameter is the distance between two optoelectronic components in a matrix. There must be enough free space between the optoelectronic components for the active cells in the internal nuclear layer INL or the ganglion cell layer GCL to function normally. It may in particular be interesting to enable organic tissues to be introduced between the individual optoelectronic components. But there must also be a sufficient number of optoelectronic components (photodiodes or optical amplifiers) to allow the patient good image definition. 
     The foveola has a diameter of approximately 0.35 mm. The spacing E between two adjacent devices is given by the following relation, where n is the number of optoelectronic components in the matrix: 
         E   2 (μm)=π(350/2) 2 ×1/ n  
 
     In the case of a matrix with 2000 optoelectronic components, the spacing D is about 48 μm. The height H of the p-GaN column must be less than 480 μm, given that the thickness of the retina is generally less than 0.5 mm. In an optoelectronic device containing optoelectronic components in which the p-GaN column has a transverse dimension D (width or diameter) of about 24 μm, there remains 24 μm available to allow, for example, for metal contacts and electrical connections. 
       FIG. 11  schematically illustrates the facet of the horizontal GaN cavity, for GaN photodiode and semiconductor optical amplifier SOA applications. The facet is bevelled at an angle α. To obtain total reflection on the guide layers of the MQW-based optical guide OG with an overall optical index n1 and the confinement layers with an overall optical index n2, the angle θ must be greater than the Brewster angle θ Brewster  and defined by the following inequalities: 
     n1&gt;n2 
     θ&gt;θ Brewster    
     α&gt;θ Brewster    
     β&lt;π/2−θBrewster 
     θ Brewster =arcsin (n2/n1) 
       FIGS. 12 a  and 12 b    schematically illustrate an embodiment of an optoelectronic device, containing at least one GaN-based horizontal photodiode, intended for use in a subretinal prosthesis.  FIG. 12 a    is a side view of the device in which light is propagated in the plane of the figure, and  FIG. 12 b    is another side view of the device perpendicular to  FIG. 12 a   . 
     An active absorbent GaN amplifying layer  120  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a p-doped gallium nitride GaN substrate  121 . A layer  122  of n-doped gallium nitride GaN is deposited above the GaN absorbent layer  120 . By selective etching of the p-GaN substrate  121  using an inductively coupled plasma ICP, a column of p-GaN  123  is formed up to the desired height, sufficient to allow stimulation of the retinal cells. The p-GaN layer  123  is covered with an insulating layer  124  of dielectric or semi-insulating material. Furthermore, the material composing the insulating layer  124  must have good biocompatibility, such as carbon, diamond, titanium dioxide, common dielectric materials (silica, silicon nitride, etc.) or the semi-insulating material GaN. The insulation is completed by implanting semi-insulating GaN  125  to separate the optoelectronic components from each other, to allow each of the optoelectronic components in the matrix to be polarised independently. 
     On the front face of the n-doped gallium nitride GaN layer  122 , a metal layer  126  is deposited. The metal contact  126  on the front face of the n-GaN layer  122  allows the retinal prosthesis to be polarised. Another metal contact  127  is placed at the upper end of the p-GaN column  123  corresponding to the area of the retina  128  that is stimulated. The metal contacts  126  and  127  are connected by an electrochemical generator  129  (primary or rechargeable battery) which applies a voltage between them. Because the light L has to cross the p-GaN column  123  to reach the absorbent GaN amplifying layer  120 , the metal contact  127  must not cover the entire surface of the front face of the p-GaN column  123 . There appears a photocurrent which will stimulate the various layers of the retina. 
     In the embodiment illustrated in  FIGS. 13 a  and 13 b   , an optoelectronic device containing at least one GaN-based horizontal photodiode intended for use in an epiretinal prosthesis is illustrated.  FIG. 13 a    is a side view of the device in which light is propagated in the plane of the figure, and  FIG. 13 b    is another side view of the device perpendicular to  FIG. 13 a   . 
     An active absorbent GaN amplifying layer  130  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a p-doped gallium nitride GaN substrate  131 . A layer  132  of n-doped gallium nitride GaN is deposited above the GaN absorbent layer  130 . From the p-GaN substrate  131 , a column of p-GaN of the desired height  133  is produced. The p-GaN layer  133  is covered with an insulating layer  134  of dielectric or semi-insulating material. The insulation is completed by implanting semi-insulating GaN  135 . Each photodiode in a matrix can thus be polarised independently from its neighbour according to medical requirements. A metal contact  136  is placed on the front face of the n-GaN layer  132  and another metal contact  137  is placed at the end of the p-GaN column  133  corresponding to the area of retina  138  to be stimulated. 
     The light must enter through the bevelled edge of the optical guide OG. The bevelled edge inclined at an angle a has a TiO 2  and SiO 2 -based anti-reflection coating that has been deposited to ensure good optical transmission between the exterior of the device and the guide layers. A photoelectric current appears, stimulating the various layers of the retina. 
       FIGS. 14 a  and 14 b    schematically illustrate an embodiment of an optoelectronic device, containing at least one horizontal cavity GaN-based semiconductor optical amplifier, intended for use in a subretinal prosthesis.  FIG. 14 a    is a side view of the device in which light is propagated in the plane of the figure, and  FIG. 14 b    is another side view of the device perpendicular to  FIG. 14 a   . 
     An active GaN amplifying layer  140  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a p-doped gallium nitride GaN substrate  141 . A layer  142  of n-doped gallium nitride GaN is deposited above the GaN amplifying layer  140 . The p-GaN layer  141  is covered with an insulating layer  143  of dielectric or semi-insulating material. 
     A metal contact  144  is placed on the front face of the n-GaN layer  142  and another metal contact  145  is placed at the end of the p-GaN layer  141  corresponding to the area of retina  146  to be stimulated. Because part of the incident light L has to be able to cross the p-GaN substrate  141  to reach the GaN amplifying layer  140 , and another part of the amplified light LA has to be able to reach the area to be stimulated  146 , the metal contact  145  must not cover the entire surface of the end of the p-GaN layer  141 . After an optogenetic operation, the retina cells will be selectively stimulated by the amplified blue light LA. 
       FIGS. 15 a  and 15 b    schematically illustrate an embodiment of an optoelectronic device, containing at least one horizontal cavity GaN-based semiconductor optical amplifier, intended for use in an epiretinal prosthesis.  FIG. 15 a    is a side view of the device in which light is propagated in the plane of the figure, and  FIG. 15 b    is another side view of the device perpendicular to  FIG. 15 a   . 
     An active GaN amplifying layer  150  with multiple InGaN/GaAsN or InGaN/AlGaN quantum wells is made by epitaxy on a p-doped gallium nitride GaN substrate  151 . A layer  152  of n-doped gallium nitride GaN is deposited above the GaN amplifying layer  150 . The p-GaN layer  151  is covered with an insulating layer  153  of dielectric or semi-insulating material. 
     A metal contact  154  is placed on the front face of the n-GaN layer  152  and another metal contact  155  is placed at the end of the p-GaN layer  151  corresponding to the area of retina  156  to be stimulated. The incident blue light L must reach the GaN amplifying layer  150 , and once the blue light LA is amplified it will stimulate the layers close to the retina. After an optogenetic operation, the retina cells will be selectively stimulated by this amplified blue light LA. 
       FIGS. 16 a  and 16 b    schematically illustrate another embodiment of an optoelectronic device, containing at least one semiconductor optical amplifier based on horizontal cavity GaN, intended for use in an epiretinal prosthesis.  FIG. 16 a    is a side view of the device in which light is propagated in the plane of the figure, and  FIG. 16 b    is another side view of the device perpendicular to the plane of  FIG. 16 a   . 
     In this other version, the p-doped gallium nitride GaN substrate  160  is etched to allow light L to pass. It is also useful to first of all etch the substrate and then the edge of the semiconductor optical amplifier SOA to create a bevelled edge. The blue light LA is amplified in the optical cavity defined by the two bevelled edges. After the optogenetic treatment, the retina cells are selectively stimulated by this amplified blue light LA. One of the bevelled edges  161  is the input of the signal which is to be amplified. The second  162  is the output of the amplified blue light LA. The bevelled edges are inclined at an angle α that is below the limit of the Brewster angle. An anti-reflection coating based on layers of TiO 2  and SiO 2  has been deposited to ensure good optical transmission between the exterior of the device and the guiding layers. 
     It may be interesting to mix a subretinal prosthesis with an epiretinal prosthesis having the same or a different operating mode. It is also possible to mix the two operating modes, subretinal and epiretinal, in a single retinal prosthesis by the use of metallised holes or vias, to cause the direct and indirect polarisation of the optoelectronic components. It is possible to adapt the structure of multi-quantum wells of the photodiode and the multi-quantum wells of the horizontal cavity of the semiconductor optical amplifier SOA, by the use of butt-joint epitaxy. 
     Naturally, this invention is not limited to the described embodiments, and is open to many variants accessible to the person skilled in the art in the field without departing from the spirit of the invention. In particular, the composition of the active layer could be modified for any III-V semiconductor tuned to the GaN and active in the visible domain, i.e. with a photoluminescence peak in the blue-green-yellow zone.