Abstract:
The invention relates to a post-activation method of dopants in a doped and activated GaN-base semiconductor layer, including the following successive steps: providing said doped and activated substrate, eliminating a part of the semiconductor material layer.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to production of a semiconductor layer made from n-doped or p-doped GaN. 
     STATE OF THE ART 
     Ion implantation is commonly used to dope semiconductor materials, and the nature of the implanted ions is chosen according to the type of doping that is desired to be obtained. 
     Si +  ions are often used to perform n-doping (excess electrons) in GaN-base semi-conductors, whereas Mg +  ions are for example used to perform p-doping (excess holes). Efficient p-doping by means of Mg +  ions is difficult to achieve, and it is therefore preferable to perform a Mg/P, Mg/N or Be/O co-implantation. 
     After implantation, the semiconductor then undergoes heat treatment in order to activate the dopant impurities more or less efficiently. 
     However, heat treatment may impair the surface of the semiconductor material if the latter is sensitive to temperature effects. For example, GaN is a semiconductor the surface of which may be impaired when the annealing temperature is higher than 850° C. Cracks or holes can appear at the surface of the semiconductor material, in particular when the heat treatment is performed at a temperature of more than 1000° C. for a period of more than a few minutes. 
     Heat treatment also generates diffusion of the dopants from the semiconductor matrix, but also causes contamination of the semiconductor by atoms such as H, O, and Si (when the semiconductor is deposited on a silicon substrate or in more general manner in contact with silicon). These atoms diffuse into the semiconductor matrix and generate a self-compensation of the n-doping or p-doping, which limits the activation rate of the dopant ions in the semiconductor. 
     To prevent impairment of the semiconductors and to increase the activation rate of the dopant ions after implantation, a protection layer, referred to as cap layer, can be deposited on top of the semiconductor. This deposition has to be performed before the heat treatment is performed. The cap layer enables the semiconductor to be annealed at a higher temperature than that which it would be able to withstand if it was bare, thereby enabling a doped semiconductor having less structural defects and a better activation rate to be obtained. 
     GaN-base semiconductor materials therefore appear to be particularly fragile materials, and in spite of the precautions taken, the electric properties of the semiconductor material are insufficient for industrial use, in particular if the semiconductor is p-doped. 
     OBJECT OF THE INVENTION 
     One object of the invention consists in implementing a method for producing a semiconductor material designed to improve its electric performances. 
     For this purpose, the method for fabricating a substrate comprising a layer made from doped and activated GaN-base semiconductor material comprises the following successive steps:
         providing said doped and activated substrate,   eliminating a part of the semiconductor material layer.       

     According to one implementation, the elimination step can be followed by a first heat treatment performed in an atmosphere containing nitrogen and devoid of hydrogen. 
     In preferential manner, the first heat treatment step can be followed by an implantation step of second dopant impurities. This step can be performed by ion implantation or by immersion in a nitrogen plasma. The implanted dose can advantageously be less than ⅔ of the dose initially implanted in the semiconductor material layer. 
     The method can also comprise a second heat treatment performed in an atmosphere containing nitrogen and devoid of hydrogen. 
     The method can also comprise a deposition step of an additional layer of GaN-base semiconductor material by epitaxial growth on the semiconductor material layer. 
     Deposition of the additional GaN-base semiconductor material layer can finally be followed by an additional heat treatment performed in an atmosphere containing nitrogen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention given for non-restrictive example purposes only and represented in the appended drawings, in which  FIGS. 1 to 8  illustrate in schematic manner an embodiment of the method for performing post-activation of dopants implanted in a GaN-base semiconductor layer. 
     
    
    
     DETAILED DESCRIPTION 
     The fabrication method is implemented on a GaN-base semiconductor material layer  1   b . In the illustrated example, the method is implemented on a substrate  1  which advantageously comprises a support  1   a  for example made from silicon, sapphire, Al2O3, or SiC, and a GaN semiconductor material layer  1   b . GaN-base semiconductor material layer  1   b  is doped with electrically dopant atoms and the latter have been activated, for example by means of high-temperature annealing. 
     Substrate  1  is advantageously doped by implantation of first dopant impurities  3  of Mg type or by a co-implantation of Mg +  and Si +  ions. The dose of first dopant impurities  3  is preferably comprised between 1.10 14  and 1.10 16  atoms/cm 2 . 
     Substrate  1  can have been covered by a cap layer  2  designed to protect semiconductor material layer  1   b  when the activation heat treatment of the first dopant impurities  3  is performed. Cap layer  2  then forms a barrier configured to prevent evaporation of nitrogen molecules from semiconductor material layer  1   b  during the heat treatment (cf.  FIG. 1 ). 
     When substrate  1  is a bulk GaN block, it is possible to cover the back surface with a cap layer  2  advantageously identical to that deposited on substrate  1  on the front surface. The front surface of substrate  1  is defined here as the surface impacted by the beam of dopant impurities, and the back surface as being the surface opposite the front surface. 
     According to the embodiments, cap layer  2  can be formed before or after the implantation step of the first dopant impurities. 
     Cap layer  2  can advantageously be silicon-base when the doping sought to be achieved is n-doping. What is meant by silicon-base is that the cap layer comprises at least 30% atomic of silicon. The material can then be SiO 2  or Si 3 N 4 . A silicon-base cap layer is particularly advantageous if an n-doping is sought for. When heat treatment is performed, atoms of Si type can diffuse in the direction of semiconductor material layer  1   b  to enhance the n-doping. 
     The material of cap layer  2  can also be devoid of silicon atoms. Advantageously, cap layer  2  is made from AlN. Cap layer  2  enables contamination of semiconductor layer  1   b  by silicon or oxygen molecules to be prevented. The cap layer for example enables a p-doping of layer  1   b  to be preserved. 
     If the doping is performed by means of ion implantation, the ion implantation will have created structural defects  4  in the semiconductor matrix. An activation heat treatment preferably implemented at a temperature of more than 1000° C. is performed after implantation (cf.  FIG. 2 ). This heat treatment enables the first dopant impurities  3  to be activated and the structural defects  4  to be partially repaired. The electric quality of semiconductor material layer  1   b  can be improved as there is a larger proportion of electrically active dopants and at least a part of the structural defects have been cured. 
     However, semiconductor material layer  1   b  can also have been impaired by the different technological steps to which it has been subjected up to this point, and in particular the implantation and high-temperature heat treatment steps. 
     The use of cap layer  2  as diffusion barrier results in an accumulation of the dopant species at the surface of layer  1   b  during the heat treatment. This accumulation of dopant species creates an inhomogeneous chemical composition of the surface of semiconductor material layer  1   b , in particular when cap layer  2  is made from AlN or from SiO 2 . It has been discovered that activation heat treatment also causes an increase of the roughness at the interface between substrate  1  and cap layer  2 . 
     To improve the electric performances of layer  1   b , it is particularly advantageous to improve the crystallographic quality of semiconductor material layer  1   b  and more particularly of the surface of semiconductor material layer  1   b . For this, several specific technological steps can be performed. 
     The top part of layer  1   b  being the most damaged and/or the most contaminated area, it is advantageous to eliminate not only cap layer  2  but also a part of layer  1   b  (see  FIG. 3 ). The value of the thickness removed from layer  1   b  depends on the process to which it was subjected beforehand. However, it appears advantageous to remove a portion of layer  1   b  having a thickness comprised between 1 and 40 nm, preferably 1 to 20 nm, to ensure elimination of a very large proportion of defects and/or contaminants, while at the same time preserving the electrically activated and doped part. 
     Removal of cap layer  2  and of the top part of semiconductor material layer  1   b  can for example be performed by Chemical Mechanical Polishing (CMP). 
     Chemical mechanical polishing consists in polishing the surface of a part by exerting a load pressure on a rotating part. Polishing is preferably performed in the presence of a slurry able to contain abrasive particles and particles designed to perform chemical etching. 
     To remove cap layer  2  and the top part of semiconductor material layer  1   b , substrate  1  is secured on a plate which is set in motion. A pressure advantageously comprised between 0.1 and 0.5 kg/cm is applied on substrate  1  so as to keep it in contact with the plate. 
     As far as the slurry is concerned, the latter can comprise 1 to 5% of Al 2 O 3  shot, or 0.1 to 30% of silica shot. The Al 2 O 3  and silica act as an abrasive and contribute to stripping of cap layer  2  when the plate is made to rotate with respect to substrate  1 . 
     To perform chemical etching of cap layer  2 , the pH of the slurry can be comprised between 2 and 11 and preferably be equal to 10. The slurry can for example be an ammonia-based solution. 
     When cap layer  2  and a sufficient thickness of semiconductor material layer  1   b  have been eliminated, substrate  1  is rinsed in order to remove the abrasive particles that may be present at its surface. The rinsing solution can be an ammonia solution. 
     As a variant, cap layer  2  can be eliminated by chemical mechanical polishing implementing a first set of operating conditions and layer  1   b  can be partially eliminated by chemical mechanical polishing implementing a second set of operating conditions different from the first set. 
     In alternative manner, cap layer  2  and the top part of semiconductor material layer  1   b  can be removed by chemical etching or by physical etching. It is possible to perform plasma etching and/or wet process etching. 
     It can also be envisaged to remove cap layer  2  by chemical mechanical polishing and to remove the top part of semiconductor material layer  1   b  by means of another technique, or vice-versa. 
     Substrate  1  which is henceforth bare can advantageously be subjected to a first heat treatment designed to restructure the damaged surface of semiconductor material layer  1   b  and to eliminate the hydrogen present in the semiconductor matrix (cf.  FIG. 4 ). There is in fact no longer any cap layer preventing outgassing of the H 2  gas. 
     If Mg+ ions have been input to layer  1   b , for example by implantation, the first heat treatment can also be configured to cause dissociation of the Mg—H complexes created when the heat treatment for activation of the first dopant impurities  3  was performed. 
     The first heat treatment of the surface can be performed by a method referred to as “conventional” heat treatment (furnace annealing), or by Rapid Thermal Annealing (RTA) and Rapid Thermal Processing (RTP). 
     For example, when the first heat treatment is performed by conventional techniques, it may be advantageous to perform this treatment in an atmosphere advantageously devoid of hydrogen to increase the hydrogen outgassing. The annealing atmosphere advantageously contains nitrogen and can be a mixture of nitrogen and oxygen and can also contain Ar, He or NF 3 . 
     The temperature of the first heat treatment can advantageously be comprised between 600 and 850° C. and preferentially be equal to 750° C. The treatment time can be comprised between 5 and 120 min, and preferentially be equal to 20 min. The choice of the heat treatment technique used and of the associated parameters can advantageously be made according to the type of dopant impurities  3 , the quantity of structural defects  4  in semiconductor material layer  1   b , and the dimensions of the latter. 
     It is apparent that a part of the defects generally present in semiconductor layer  1   b  are vacancies. To fill the latter, it is advantageous to perform implantation of second dopant impurities  5  (cf.  FIG. 5 ). This can be achieved by any suitable technique and for example by an ion gun or by Plasma Immersion Ion Implantation (PIII). Second dopant impurities  5  are advantageously electrically active or non-electrically active elements designed to fill the vacancies, and can be N +  or P +  ions or a mixture of N +  and P +  ions. 
     Within the scope of an ion implantation by means of an ion gun, implantation can advantageously be performed at a temperature comprised between 15 and 700° C. and preferentially equal to 500° C. Remaining within this temperature range makes it possible to avoid entering temperature ranges where the surface of the GaN is impaired and where a nitrogen release is observed, these phenomena being all the more likely to occur as semiconductor material layer  1   b  is subjected to an ion bombardment. This temperature range therefore facilitates both insertion of second dopant impurities  5  into the matrix during implantation and reorganization of the matrix in the form of a crystal lattice. 
     The implantation energy of second dopant impurities  5  is advantageously lower than that of first dopant impurities  3  so as to implant dopants  5  mainly at the surface of semiconductor layer  1   b , i.e. in the area which is the most damaged. 
     The inventors further observed that the implanted dose of second dopant impurities  5  preferably has to be less than two thirds of the dose of first dopant impurities  3  implanted in semiconductor material layer  1   b . In this way, the vacancies created by the first implantation are filled by the second dopant impurities, without causing any further damage to the material. 
     The implanted dose of second dopant impurities  5  can advantageously be comprised between 0,1.10 15  and 1,5.10 15  atoms/cm 2 , and preferably be equal to 0,5.10 15  atoms/cm 2 . The energy of dopants  5  can further be comprised between 10 and 150 keV, and preferably be equal to 50 keV. 
     The implanted dose and the implantation energy of the second dopant impurities  5  enable N +  ions to be inserted where the semiconductor matrix  1   b  is the most damaged in sufficient quantity to repair it. In counter-intuitive manner, the GaN-base semiconductor matrix  1   b  is restored by high-temperature implantation of a controlled dose of dopant impurities  5  at low energy. The shortcomings of the doping and activation processing steps performed beforehand are thus overcome. 
     Unlike doping methods of the prior art wherein two types of materials are successively implanted before heat treatment is performed for activation of the dopants, the implantation step of second dopant impurities  5  is subsequent to the first activation heat treatment of the first dopant impurities  3 . This enables the dopant losses, in particular of p-type dopants, to be limited thereby improving the quality of the doping. The same area of semiconductor material layer  1   b  is doped and then annealed and then doped again. 
     Reconstruction of semiconductor matrix is moreover simpler to implement compared with those of the prior art, when the semiconductor comprises both p-doped areas and other n-doped areas. The second implantation of dopant impurities  5  can in fact be performed directly on the whole of semiconductor material layer  1   b  without having recourse to an etching mask. This embodiment is particularly advantageous when dopant impurities  5  are not electrically active or are possibly formed by a p-type dopant impurity. 
     In alternative manner, implantation of second dopant impurities  5  can be performed by immersion in a N +  ion plasma at a temperature comprised between 20 and 500° C., and preferentially equal to 500° C. The implanted dose can advantageously be comprised between 0,1.10 15  and 1,5.10 15  atoms/cm 2 , and preferentially be equal to 0,5.10 15  atoms/cm 2 . The implantation energy of second dopant impurities  5  can be comprised between 5 and 40 keV, and preferentially be equal to 30 keV. Implantation by plasma immersion is therefore preferred when defects  4  are mainly located at the surface of substrate  1 , as the implantation energies are lower. 
     The implantation step creates structural defects in semiconductor material layer  1   b , and the latter can therefore advantageously be subjected to a second heat treatment (cf.  FIG. 6 ). This heat treatment also enables H atoms to be eliminated from the semiconductor matrix by outgassing of H 2 . The combination of an implantation step followed by heat treatment enables the quality of semiconductor matrix  1   b  to be improved with application of a simple heat treatment. 
     The second heat treatment can be a treatment of RTA type performed in an atmosphere containing nitrogen and advantageously devoid of hydrogen and oxygen in order to prevent any contamination of semiconductor material layer  1   b  by H or O atoms, but also to enhance outgassing of hydrogen from the material. The atmosphere in which the heat treatment is performed may contain Ar, He or NF 3 . 
     The second heat treatment can be a single annealing or comprise a series of 5 to 40 annealing cycles, each cycle having a time period of 2 to 20 seconds. In a particular embodiment, the second heat treatment can comprise 10 cycles of 5 seconds each. 
     In alternative manner, the second heat treatment can be a standard annealing performed in an atmosphere devoid of oxygen for a period comprised between 15 and 25 min. 
     Whatever the method used to perform the second heat treatment, the temperature at which this annealing is performed can be comprised between 600 and 850° C., and preferentially be equal to 750° C. The high temperature of this heat treatment enables the defects of semiconductor layer  1   b  to be cured by atomic rearrangements, without however causing any evaporation of nitrogen. 
     On completion of the second heat treatment, the method can comprise a reconstruction step of the damaged surface of semiconductor material layer  1   b  (cf.  FIG. 7 ). This reconstruction step can comprise deposition of GaN-base semiconductor material  6  performed for example by Metalorganic Chemical Vapor Deposition (MOCVD) at a temperature comprised between 1000 and 1100° C. 
     Layer  6  can for example be a layer of AlGaN (i.e. Al(0-50%)Ga(0-50%)N), or of GaN (i.e. Al(0%)Ga(100%)N). Layer  6  can be doped by Mg atoms in the case of p-doping. 
     The deposited semiconductor material layer  6  can advantageously correspond to a layer having a thickness comprised between 1 and 40 nm. The thickness of layer  6  depends on the use for which substrate  1  is intended, and it is not necessarily identical to the thickness eliminated during the elimination step of the top part of semiconductor layer  1   b  represented in  FIG. 3 . This thickness can be comprised between 1 and 40 nm, preferentially comprised between 1 and 20 nm, and ideally be equal to 5 nm. 
     This thickness enables the electric quality of the semiconductor to be guaranteed. Deposition of the layer being performed at a temperature comprised between 1000 and 1100° C., the electric dopant impurities  3  and  5  can in fact migrate into layer  6  and ensure a similar electric conductivity to that of layer  1   b.    
     Subsequent to the epitaxy step, it is advantageous to perform a third heat treatment enabling elimination of a part of the hydrogen atoms present in layer  6  (cf.  FIG. 8 ). 
     The third heat treatment can advantageously be performed in an atmosphere advantageously devoid of hydrogen and oxygen, and containing nitrogen in order to prevent contamination of the GaN-base material by H, O or C atoms. The temperature of the third heat treatment can advantageously be comprised between 750 and 900° C., and preferentially be equal to 800° C. The duration of the heat treatment can be comprised between 15 and 120 min, and preferentially be less than or equal to 20 min when the temperature is 900° C., or be equal to 20 min otherwise. 
     In advantageous manner, the third heat treatment is performed subsequent to the epitaxy step without layer  6  being subjected to an oxidizing atmosphere. It can advantageously be performed in the susceptor that was used for epitaxy of semiconductor material layer  6 . 
     A semiconductor is thus obtained having a high dopant activation rate and an improved crystal quality, so as to have a material having not only a good electric quality but also an improved surface state, and that is less resistive than in the prior art.