Abstract:
A method is for treating a doped gallium nitride substrate of a first conductivity type, having dislocations emerging on the side of at least one of its surfaces. The method may include: a) forming, where each dislocation emerges, a recess extending into the substrate from the at least one surface; and b) filling the recesses with doped gallium nitride of the second conductivity type.

Description:
PRIORITY 
     This application claims the priority benefit of French Patent Application No. 13/58324, filed on Aug. 30, 2013, the contents of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
     FIELD OF THE INVENTION 
     The present disclosure relates to the forming of electronic components inside and on top of single-crystal gallium nitride substrates. It more specifically relates to a method for treating or preparing a doped single-crystal gallium nitride substrate for the forming of electronic components inside and on top of this substrate. 
     BACKGROUND 
     Electronic components, and especially power components such as Schottky diodes, have already been formed by using doped single-crystal gallium nitride as a semiconductor material. 
     Conventionally, gallium nitride substrates may be solid substrates, of a thickness ranging from a few tens to a few hundreds of micrometers, or appear in the form of a gallium nitride layer having a thickness of a few micrometers coating a support of another material, for example, a silicon support. 
     Whether they are solid (freestanding) or supported by a support material, gallium nitride substrates generally have dislocations, corresponding to discontinuities in the organization of their crystal structure. Such dislocations may raise issues in certain applications. 
     SUMMARY 
     Thus, an embodiment provides a method of treating a doped gallium nitride substrate of a first conductivity type, having dislocations emerging on the side of at least one of its surfaces, comprising: a) forming, where each dislocation emerges, a recess extending into the substrate from the at least one surface; and b) filling the recesses with doped gallium nitride of the second conductivity type. 
     According to an embodiment, step b) comprises a step of epitaxial deposition, on the at least one surface, of a doped single-crystal gallium nitride layer of the second conductivity type, followed by a step of planarization of this layer. 
     According to an embodiment, during the planarization step, the gallium nitride layer is removed everywhere except from the recesses. 
     According to an embodiment, the planarization step comprises a chem.-mech. polishing. 
     According to an embodiment, at step a), the recesses are formed by means of a wet etching solution applied on the at least one surface. 
     According to an embodiment, the etching solution comprises potassium hydroxide at a concentration in the range 10 to 90%. 
     According to an embodiment, the etching solution comprises phosphoric hydroxide at a concentration in the range 10 to 90%. 
     According to an embodiment, at step a), the recesses are formed by means of a chlorine plasma. 
     According to an embodiment, at step a), the forming of the recesses comprises a step of annealing the substrate at a temperature greater than or equal to 830° C. 
     According to an embodiment, the first and second conductivity types respectively are type N and type P. 
     Another embodiment provides a method for manufacturing a gallium nitride semiconductor component, comprising: treating, according to the above-mentioned method, a doped gallium nitride substrate of a first conductivity type, having dislocations emerging on the side of at least one of its surfaces; and depositing at least one conductive, semiconductor, or insulating layer on the at least one surface. 
     According to an embodiment, the component is a Schottky diode, and the conductive, semiconductor, or insulating layer is a metal layer. 
     The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  are cross-section views schematically illustrating an example of an embodiment of a method for treating a gallium nitride substrate for the forming of electronic components inside and on top of this substrate. 
       For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
     
    
    
     DETAILED DESCRIPTION 
     In the rest of the present description, unless otherwise indicated, terms “on the order of”, “approximately”, “substantially” and “around” mean to within ten percent. 
       FIG. 1A  schematically shows an N-type doped single-crystal gallium nitride substrate  101  (GaN). In this example, substrate  101  is formed on a silicon support  103 . The described embodiments are however compatible with solid gallium nitride substrates (freestanding). 
     As appears in  FIG. 1A , substrate  101  comprises dislocations. Some of these dislocations, referred to as  105  in the drawing, emerge on the side of the surface of substrate  101  opposite to silicon support  103 , which will be called hereafter, by convention, the upper surface of substrate  101 . 
     Emerging dislocations  105  are capable of causing malfunctions in electronic components where one or several conductive, semiconductor, or insulating layers (not shown) coat the upper surface of substrate  101 . They are particularly problematic when a Schottky diode comprising a Schottky contact between substrate  101  and a conductive layer (not shown), for example, made of metal, coating the upper surface of substrate  101 , is desired to be formed. Indeed, the contact areas between dislocations  105  and the conductive layer form areas of lower potential barrier in the Schottky junction, which locally decreases the reverse withstand voltage of the diode and increases reverse leakage currents with respect to a diode comprising no dislocation emerging on the Schottky contact. 
     It is here provided to treat substrate  101  to overcome all or part of the disadvantages linked to the presence of dislocations  105  emerging on its upper surface side. 
     To achieve this, it is provided to open an upper portion of dislocations  105  emerging on the upper surface side of substrate  101 , that is, to form in substrate  101 , on its upper surface side, recesses in front of dislocations  105 , and then to fill the openings with gallium nitride of a conductivity type opposite to that of the substrate. 
       FIG. 1B  illustrates a step of forming recesses  107  in substrate  101 , on the upper surface side thereof, opposite to dislocations  105  emerging on the upper surface side of the substrate. 
     In a preferred embodiment, to form recesses  107 , the upper surface of substrate  101  is placed in contact with a chemical etching solution capable of preferentially etching the areas of substrate  101  surrounding dislocations  105  over the areas of substrate  101  comprising no dislocations emerging on the upper surface of the substrate. A solution based on potassium hydroxide (KOH) may for example be used. As a variation, a solution based on orthophosphoric acid (H 3 PO 4 ) may be used. To obtain a marked etching of the substrate areas surrounding dislocations  105 , the concentration of the etching agent in the solution is preferably relatively high, for example, in the range 10% to 90% in the case of potassium hydroxide or of orthophosphoric acid. 
     As a variation, to form recesses  107 , the upper surface of substrate  101  may be submitted to an etching plasma, for example, a chlorinated or argon plasma or any other appropriate etching gas. 
     As a variation, to form recesses  107 , substrate  101  may be annealed at a relatively high temperature, preferably higher than 830° C., which causes an opening of the upper portion of dislocations  105  emerging on the upper surface of the substrate. 
     More generally, any method capable of forming recesses  107  extending in substrate  101  from its upper surface, opposite to dislocations  105 , may be used. 
     As an example, recesses  107  extend in substrate  101  from its upper surface down to a depth approximately in the range from 0.2 to 10 μm, and have a diameter or a width approximately in the range from 0.001 to 0.5 μm. 
     In the shown example ( FIGS. 1C and 1D ), to fill recesses  107 , it is provided to form, by epitaxy, a doped gallium nitride layer of a conductivity type opposite to that of the substrate (that is, a P-type layer in this example) coating the upper surface of substrate  101 , and then to planarize this layer to remove it everywhere except inside of recesses  107 . 
       FIG. 1C  illustrates a step, subsequent to the step of forming of recesses  107 , of epitaxial deposition of a P-type doped single-crystal gallium nitride layer  109  over the entire surface of substrate  101 . Layer  109  is for example formed by chemical vapor deposition, according to a method currently called metal organic chemical vapor deposition, MOCVD, in the art. As a variation, layer  109  is formed according to a method currently called MBE (“Molecular Beam Epitaxy”) in the art or by a method currently called RPCVD (“Remote Plasma Chemical Vapor Deposition”) in the art. Any other method capable of depositing a doped gallium nitride layer of a conductivity type opposite to that of substrate  101  may be used. To obtain a P-type doping, magnesium may for example be provided in the deposition source. The doping level of layer  109  is preferably greater than that of substrate  101 . As an example, substrate  101  has a doping level in the range 10 15  to 2*10 16  atoms/cm 3 , and layer  109  has a doping level greater than 10 17  atoms/cm 3 . 
     The thickness of layer  109  is preferably selected to totally fill recesses  107 , for example all the way to the upper surface level of substrate  101 . As an example, layer  109  has a thickness in the range 0.2 to 15 μm. 
       FIG. 1D  illustrates a planarization step, following the deposition of layer  109 , during which layer  109  is thinned from its upper surface, until it is totally removed opposite to the regions substrate  101  which have not been recessed at the step described in relation with  FIG. 18 , to clear the access to the upper surface of substrate  101  in these regions. During the planarization step, the P-type doped gallium nitride of layer  109  is removed everywhere except from recesses  107 . A small thickness of substrate  101  may possibly be removed during the planarization, to guarantee the removal of any P-type doped gallium nitride residue from the upper surface of the regions of substrate  101  which have not been recessed at the step described in relation with  FIG. 1B . The planarization is unterrupted before reaching the bottom of recesses  107 , to keep P-type doped gallium nitride areas forming an interface between dislocations  105  and the upper surface of the semiconductor structure. In a preferred embodiment, the planarization step of  FIG. 1D  is performed at least partly and preferably totally by chem.-mech. polishing (CMP). 
     An advantage of the embodiment described in relation with  FIGS. 1A to 1D  is that it enables to avoid for dislocations of the substrate to directly emerge on a surface of the substrate intended to receive conductive, semiconductor, or insulating layers of a semiconductor gallium nitride component. 
     This embodiment is particularly advantageous for the forming of a Schottky diode comprising a Schottky contact between substrate  101  and a conductive layer  110 , for example, made of metal, coating the upper surface of substrate  101 . Indeed, the presence of the P-type doped local interface regions enables to avoid a drop of the potential barrier at dislocations  105  when the diode is reverse-biased. This enables to improve the reverse withstand voltage of the diode. This further enables to decrease reverse current leakages in the diode via dislocations  105 . It should be noted that in the case of a Schottky diode, the doping level of the P-type regions should be sufficiently high for the reverse withstand voltage of the diode to take place at the level of the Schottky interface, and not at the level of the PN diodes formed between the P-type gallium nitride filling recesses  107  and substrate  101 . 
     Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
     In particular, the above-described embodiments are not limited to the forming of a diode comprising a Schottky contact between substrate  101  and a conductive layer  110  coating the upper surface of the substrate. The method for treating substrate  101  described in relation with  FIGS. 1A to 1D  may be used for the forming of other gallium nitride semiconductor components, for example, PN power diodes, bipolar power transistors, light-emitting diodes, heterojunction transistors, heterojunction diodes, or any other gallium nitride component capable of taking advantage of the provided treatment of the dislocations emerging on the upper surface side of the substrate. 
     Further, the described embodiments are not limited to the treatment of only the dislocations emerging on the upper surface side of the substrate. Thus, in the case of a solid (freestanding) substrate, it will be within the abilities of those skilled in the art to adapt the method described in relation with  FIGS. 1A and 1B  to treat not only dislocations emerging on the upper surface side of the substrate, but also dislocations emerging on the lower surface side of the substrate if this is advantageous for the envisaged use of the substrate. 
     Further, the described embodiments are not limited to the treating of an N-type doped gallium nitride substrate, but may be applied to the treating of a P-type doped substrate. In this case, it will be provided to fill with N-type doped gallium nitride recesses  107  formed at the step of  FIG. 1B . 
     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.