Patent ID: 12217964

DESCRIPTION OF THE EMBODIMENTS

Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties.

For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the following description mainly concerns the obtaining of a doped semiconductor layer. The different structures where such a layer may be used have not been detailed. Further, the steps that may be implemented, before or after the forming of the doped layer, to obtain such structures, have not been detailed.

Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements.

In the following disclosure, unless otherwise specified, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures.

Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%.

FIGS.1A to1Care cross-section views illustrating successive steps of an example of a method of obtaining a doped semiconductor layer according to an embodiment.

The forming of a light-emitting cell stack comprising a first semiconductor layer101of a first conductivity type forming an anode or cathode layer of the cell, of an active layer103, and of a second doped semiconductor layer105of the second conductivity type forming a cathode or anode layer of the cell is considered in the present example. Layers101and105are for example layers of a semiconductor material III-V, for example, gallium nitride layers. Active layer103for example comprises confinement means corresponding to multiple quantum wells. As an example, active layer103is formed of an alternation of semiconductor layers of a first material and of semiconductor layers of a second material, each layer of the first material being sandwiched between two layers of the second material, the first material having a narrower bandgap than that of the second material, to define multiple quantum wells. Layers101,103, and105are for example formed by epitaxy. The stack of layers101,103, and105is arranged on a support substrate107, for example, made of sapphire or of silicon. A stack109of one or a plurality of buffer layers may form an interface between substrate107and the stack of layers101,103, and105. In the shown example, stack109is arranged on top of and in contact with the upper surface of substrate107, layer101is arranged on top of and in contact with the upper surface of stack109, layer103is arranged on top of and in contact with the upper surface of layer101, and layer105is arranged on top of and in contact with the upper surface of layer103.

The doping of the upper semiconductor layer105of the stack is here more particularly considered.

FIG.1Aillustrates the structure obtained at the end of a step of deposition of an insulating protection layer111, for example, made of silicon nitride (Si3N4), on the upper surface side of layer105, for example, in contact with the upper surface of layer105. Layer111for example extends over the entire surface of layer105. The thickness of layer111is for example in the range from 5 to 500 nm, and preferably from 10 to 50 nm, for example, in the order of 20 nm. At this stage, layer105may be a non-doped layer. As a variant, layer105may have been already previously doped in situ during its epitaxial growth. It is desired in this case to increase the doping level of layer105.

FIG.1Billustrates a step of ion implantation, in semiconductor layer105, through protection layer111, of an element which is a dopant for the material of layer105. During this step, as will be explained in further detail hereafter, an element which is not a dopant for the material of layer105is further implanted, preferably with no intermediate anneal step to limit, at the cell scale, the stress introduced by the dopant element.

The energies and doses of implantation of the dopant element and of the non-dopant element are selected according to the desired doping profile. The implantations energies and doses are further selected to obtain a full amorphization of an upper portion105aof layer105, and to keep the original crystal reference in a lower portion105bof layer105. Preferably, the thickness of the lower reference single-crystal layer105bis relatively small to enable to carry off possible dislocations or other crystal defects during a subsequent step of recrystallization anneal of layer105a. As an example, the thickness of the lower reference single-crystal layer105bis smaller than half the thickness of original layer105, for example smaller than one fifth of the thickness of original layer105. As an example, the thickness of lower reference single-crystal layer105bis in the range from 2 to 100 nm, preferably from 2 to 10 nm. Layer105for example has a thickness in the range from 10 to 500 nm, for example from 100 to 400 nm.

Protection layer111particularly enables to protect layer105against the sputtering during the step of ion implantation of the dopant element and of the non-dopant element.

FIG.1Cillustrates a step of anneal of the structure obtained at the end of the steps ofFIGS.1A and1B, to obtain a solid phase recrystallization of the upper portion105aof layer105. The anneal is for example performed at a temperature in the range from 300 to 1200° C. The duration of the recrystallization anneal is for example in the range from 1 minute to 10 hours. Preferably, the anneal is performed at low temperature, for example, at approximately 400° C., for example for approximately 1 hour. During this step, a recrystallization of layer105ais obtained. The crystal reference is provided by the underlying single-crystal layer105b. At the end of this step, a doped crystal semiconductor layer105ais obtained. The doping level of layer105adepends on the dose of the dopant implanted at the step ofFIG.1B.

Protection layer111may be removed after the anneal. As a variant, layer111may be removed before the anneal. Subsequent steps, not detailed, may then be implemented to form one or a plurality of light-emitting cells from the obtained structure. In particular, a step of deposition of an electrode on top of and in contact with layer105amay be provided.

The doping method described in relation withFIGS.1A to1Cis particularly advantageous for the doping of a layer of a III-V-type semiconductor material. The method may however be adapted to the doping of other semiconductor alloys, and in particular semiconductor alloys having a large bandgap, for example, greater than 1.5 eV and preferably greater than 3 eV.

Generally, layer105may be a single-crystal layer of an alloy of at least one first element which will be called element A1hereafter, for example, a group-III element, and one second element, which will be called element A2hereafter, for example, a group-V element. The dopant element implanted at the step ofFIG.1B, which will be called element B hereafter, may be a P-type or N-type dopant element. As an example, dopant element B is intended to substitute to atoms of element A1of the initial alloy to obtain a P-type or N-type doping. In the case where element A1of the alloy is a group-III element, dopant element B may be a group-II element to obtain a P-type doping, or a group-IV element to obtain an N-type doping. The covalent radius of dopant element B may be different from that of substituted element A1. The non-dopant element implanted at the step ofFIG.1B, which will be called element C hereafter, is selected to compensate for the stress introduced, at the cell level, by dopant level B. As an example, if dopant element B has a covalent radius smaller than that of substituted element A1, an element of the same group as element A1having a covalent radius greater than or equal to and preferably greater than that of element A1may be selected as non-dopant element C. Conversely, if dopant element B has a covalent radius greater than that of substituted element A1, an element of the same group as element A1having a covalent radius smaller than or equal to and preferably smaller than that of element A1may be selected as non-dopant element C.

During the implantation step ofFIG.1B, in addition to elements B and C, atoms of element A2of the initial alloy may be implanted, preferably with no intermediate anneal step, to compensate for the addition of elements B and C and to keep the general stoichiometry of the material.

The implantation dose of dopant element B during the step ofFIG.1Bis preferably relatively high, for example, greater than 1020atoms/cm3, to favor the amorphization of the upper portion105aof layer105.

Examples of application of the method ofFIGS.1A to1Cto the doping of a gallium nitride layer (GaN) will now be described. In this case, initial semiconductor layer105is a single-crystal GaN layer. Elements A1and A2of the semiconductor alloy forming initial layer105respectively are gallium (Ga) and nitrogen (N). The layer105awhich is desired to be obtained at the end of the process is a CyBxGa1-x-yN layer, with x and y respectively defining the concentration of dopant element B and the concentration of non-dopant element C in the final layer. Dopant element B must substitute to gallium (Ga). The concentration x of dopant element B is selected to obtain the desired doping level. Non-dopant element C and concentration y of non-dopant C are selected according to the covalent radius of dopant element B and to the concentration x of dopant B to obtain in fine a generally non-stressed layer105a.

As an example, concentrations x and y are selected to respect the following rule of mixtures:

a*x+b*y=0⁢⁢with⁢:[Math⁢⁢1]a=13*(1-RBRh)3*1S⁢⁢and⁢:[Math⁢⁢2]b=13*(1-RCRh)3*1S[Math⁢⁢3]
where RB, RC, and Rh respectively designate the covalent radiuses of elements B, C, and A1(Ga in the present example), and S designates the site concentration in the host matrix, that is, the number of gallium atoms in the initial cell of layer105.

More generally, to define the concentration y of non-dopant element C, other rule of mixtures may be defined, based on a modelization of the stress in a crystal semiconductor alloy.

During the implantation step ofFIG.1C, in addition to elements B and C, it is provided to implant nitrogen (element A2), preferably with no intermediate anneal step, to compensate for the addition of elements B and C and to keep the general stoichiometry of the material. In the absence of such a co-implantation of nitrogen, the stoichiometry of the final layer105awould be CyBxGa1-x-yN1-x-y. To compensate for the implantation of elements B and C, a co-implantation of nitrogen at a concentration z=x+y is here provided.

Case of the P Doping:

To obtain a P-type doped layer105a, the dopant element B implanted at the step ofFIG.1Bmay be a group-II element, for example magnesium (Mg), beryllium (Be), zinc (Zn), or calcium (Ca). Non-dopant element C may be an element of the same group as gallium, that is, a group-III element, for example aluminum or indium. Preferably, dopant element B is magnesium. In the case where dopant element B is magnesium, non-dopant element C is preferably aluminum. Indeed, magnesium has a greater covalent radius than that of gallium, while aluminum has a smaller covalent radius than that of gallium, which enables to balance the stress in the cell.

As an example, the implanted magnesium dose is in the order of 3*1015atoms/cm2with an implantation energy in the order of 23 keV, the implanted aluminum dose is in the order of 4.6*1015atoms/cm2with an implantation energy in the order of 120 keV, and the implanted nitrogen dose is in the order of 9.6*1015atoms/cm2with an implantation energy in the order of 15 keV.

Case of the N Doping:

To obtain an N-type doped layer105a, the dopant element B implanted at the step ofFIG.1Bmay be a group-IV element, for example, silicon (Si), germanium (Ge), or carbon (C). Non-dopant element C may be an element of the same group as gallium, that is, a group-III element, for example aluminum or indium. Preferably, dopant element B is silicon. In the case where dopant element B is silicon, non-dopant element C preferably is indium. Indeed, silicon has a smaller covalent radius than that of gallium, while indium has a greater covalent radius than that of gallium, which enables to balance the stress in the cell.

It will be within the abilities of those skilled in the art to adapt the above-described method to the doping of other semiconductor alloys. For example, in the case where layer105is made of silicon carbide (SiC), elements A1and A2are respectively silicon (Si) and carbon (C). To obtain a P-type doping, dopant element B may be a group-II element, for example, boron (B) and non-dopant element C may be a group-III element, for example, germanium. To obtain an N-type doping, dopant element B may be a group-IV element, for example, arsenic, and non-dopant element C may be a group-III element, for example, carbon.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. In particular, the described embodiments are not limited to the examples of materials or to the examples of numerical values mentioned in the description.

Further, although an example of application of the doping method to the forming of light-emitting cells has been described hereabove, the described embodiments are not limited to this specific application. As a variant, the method of obtaining a doped semiconductor layer described hereabove may be used for other applications, for example, for the forming of semiconductor power components (transistors, diodes, etc.).

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.