A METHOD FOR PRODUCING A REGION DOPED WITH BORON IN A SiC-LAYER

Abstract: A method for producing a crystalline layer of SiC having at least a region thereof doped with boron atoms comprises a step a) of ion implantation of boron into a layer (1) of crystalline SiC and a step b) of heating the SiC-layer for annealing it for making the boron implanted therein electrically active. The method further comprises a step c) of implanting carbon atoms in said layer (1) for forming carbon interstitials in excess with respect to carbon vacancies present in the SiC-layer before carrying out step b).

Description: A method for producing a region doped with boron in a SiC- layer TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for producing a crystalline layer of SiC having at least a region thereof doped with boron atoms, comprising a step a) of ion implan- tation of boron into a layer of crystalline SiC and a step b) of heating the SiC-layer for annealing it for making the boron implanted therein electrically active, as well as a semiconductor device produced by carrying out such a method. SiC has, besides the properties of a high breakdown voltage, a high thermal conductivity and a high thermal stability and by that an ability to operate at high temperatures, namely upto 1000 K, the character of only allowing ex- tremely slow diffusion of dopants therein at convenient processing temperatures, so that the possibility to use the diffusion technique and the advantages of that technique is strongly reduced with respect to Si for obtaining doped regions in SiC. As a consequence of this, ion implantation of dopants is an attractive alternative for obtaining doped regions in SiC- layers . Furthermore, boron has turned out to be an element well suited as a dopant for p-type layers or regions of SiC. A method according to the introduction has therefore already been proposed, but that method has some drawbacks, which derive from the fact that SiC is a compound material, which means that doping atoms can potentially be incorpo- rated on Si as well as on C lattice sites, and the inherent property of boron to behave differently on Si and on C site in the SiC crystal. Boron forms on the Si site an acceptor with an ionisation energy of around 320 eV, whereas it on the C site forms a so called D-center - a deep center with an ionisation energy of around 650 meV. The character of the D-center has not been clarified yet, so that it is not sure whether it will act as an acceptor or a donor there. In any way, the active doping concentration, which is given by the doping atoms forming shallow levels (ionisation en- ergy of around 320 meV) is therefore reduced by the atoms forming said D-centers. This effect would be even enhanced, when the two levels would be of different character, i.e. acceptor and donor, through which a compensation phenomena would occur. Accordingly, it is not possible to obtain a region having at a given temperature a well defined concentration of charge carriers in the form of holes deriving from dopants in the form of boron atoms, neither is it possible to ob- tain such a high concentration of such charge carriers that would be desired for obtaining a low resistance of said region making it as suited as possible as a contact layer, by carrying out already known methods of the type defined in the introduction. By way of example it may be mentioned that a very precise control of the doping concentration is required for JTE . SUMMARY OF THE INVENTION The object of the present invention is to provide a method of the type defined in the introduction, which enables very precise control of the doping concentration obtained in said region of the SiC-layer. This object is according to the invention obtained by providing such a method with the additional step c) of implanting carbon atoms in said layer for forming carbon in- terstitials in excess with respect to carbon vacancies pre- sent in the SiC-layer before carrying out step b) , said step c) being carried out one of a) before, b) after and c) at the same time as step a) . This technique may be called co-implantation and it results in additional carbon interstitials besides those already created by the bombardment of the SiC-layer during the ion implantation of boron in step a) . When the SiC-layer after that is annealed there will be a site competition and as a consequence of the excess of C in the SiC-layer the incor- poration of the implanted boron atoms will be influenced in the direction of more such dopants on Si sites than on the C sites where they would form a D-center. In this way the incorporation of the implanted boron atoms can be influenced in a controlled way by adjusting the dose of carbon atoms implanted in the SiC-layer in step c) . It will in this way be possible to influence the boron atoms to sit exactly where they have to sit in the SiC-layer, so that the behaviour of this layer will be very well defined, which is also very important when a low doping concentra- tion of this layer is desired. Thus, the invention constitutes a further step towards precise control over doping concentrations in SiC necessary for producing tailor made doping profiles. The existence of a different behaviour of boron on Si and on C site in an SiC crystal has been studied and then disclosed by Ballandovich a.o. in Fizika i Tekhnika Polupro- vodnikov, 1995, volume 29, No. 2, page 370-377. It is there described that two different centers are formed by boron incorporated in an epitaxial film of SiC by diffusion. Furthermore, it is said that annealing at a temperature above 2300°C reduces the concentration of D-centers . However, an- nealing at such high temperatures will set very high requirements on the quality of the equipment used making it extremely expensive. Kimoto et. al describes in the journal Applied Physics Letters, 1995, volume 67, No. 16, pages 2385-2387, how the doping efficiency of aluminium and boron are increased under C-rich conditions during the epitaxy of SiC. However, the dopants are here incorporated during epitaxy under con- trol of the C/Si ratio. That this should work has also been suggested by Fukumoto in Physical Review B, volume 53, No. 8, 1996, in which simulations made concerning epitaxy of SiC under C-rich condition are presented. Furthermore, Mulpuri et.al. discloses in Journal of Electronic Materials, volume 25, No. 1, 1996 on pages 75-80 an attempt to use the site competition concept in the case of implanting aluminium into an SiC-layer for doping said layer. However, it has turned out that the site-competition did not function at all, and the conclusion was that C or Si co-implantations do not improve the acceptor activation problem in SiC, although Kimoto et.al. have assumed that this was possible to obtain for Al by incorporation thereof during epitaxy of SiC under C-rich condition. According to another preferred embodiment of the invention the dose of carbon implanted in step c) is selected so as to direct the majority of the boron implanted m step a) to sites intended for silicon atoms in said region of the SiC- layer during the annealing of step b) . By selecting said dose in that way the fact that boron may form D-centers in SiC may only have a neglectible influence on the behaviour of the dopants of said region in the SiC-layer so produced. Thus, it may with a high accuracy be said that the concen- tration of charge carriers deriving from the boron atoms in said region of the SiC-layer is only depending on the doping concentration of the boron atoms incorporated in the SiC and the ionisation energy of the shallow energy level on the Si sites as a function of other parameters, such as temperature, voltage etc. This means that the concentration of such charge carriers under a certain condition may be precisely controlled by selecting an appropriate dose of boron implanted into the SiC-layer during the ion implantation step a) . According to another preferred embodiment of the invention the dose of boron implanted is controlled during step a) for obtaining a determined concentration of boron sitting at said silicon sites in said region of the SiC-layer. The advantages of such a control of said dose of boron appear from the previous paragraph of this disclosure. According to another preferred embodiment of the invention being a further development of the preferred embodiment of the invention mentioned firstly, boron is implanted in step a) at a dose lying close to the solubility limit of boron in SiC for obtaining a low resistance of said region. It will in this way be possible to obtain highest possible concentration of boron atoms at Si sites and by that a very low resistance of said region at a given temperature making this region or SiC-layer suited as a contact layer. The present invention also comprises a semiconductor device according to the independent device claim, and the advan- tages of such a device clearly appear from the discussion above of the different embodiments of the method according to the invention. Further advantages and preferred features of the invention will appear from the following description. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below follows a specific description of a preferred embodiment of the invention cited as an example. In the drawing: Fig 1 illustrates schematically a semiconductor power device in the form of a rectifier diode of SiC, which has been produced while utilising a method according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig 1 illustrates by a way of a non-limitative example of a device to which the invention may be applied, a semiconductor diode with three semiconductor layers of SiC, namely a first highly doped layer 1 of p-type, a second lightly doped layer 2 of n-type and a third highly doped layer 3 of n-type. The three layers are superimposed in the order men- tioned. The device does also have an ohmic metal contact 4 arranged on the top of said first layer and forming the anode of the diode, and a second ohmic metal contact 5 arranged in contact with said third semiconductor layer and forming the cathode of the diode. Passivation layers have been left out, since they do not have anything to do with the invention. A device of this type may be produced by known growing and etching techniques . However, in this case the first layer of p-type has been produced by carrying out a method according to a preferred embodiment of the invention. First of all, the different layers have been epitaxially grown in a known way, preferably by Chemical Vapour Deposition (CVD) . After that, boron atoms have been ionised and accelerated towards a surface of the SiC crystal created by correspondingly patterning a mask covering the top surface thereof while using acceleration energies exceeding 30 keV, Boron atoms are .in this way implanted into the n-type layer 2 while forming the p-type layer 1 illustrated in the figure. The dose of the boron implanted is in the region of 1 • 10^^ cm ~ 2. Carbon atoms are then also implanted into the layer 1, so that carbon interstitials are formed in excess with respect to carbon vacancies present in the layer 1. After that, the SiC crys- tal is heated for annealing the layer 1 for making the boron implanted therein electrically active, and the boron atoms will then compete with Si and C interstitials about the different Si and C vacancies. Thanks to said excess of carbon, a majority of the boron atoms activated will after the annealing step be found on Si sites, i.e. they have formed the shallow energy level, so that the doping concentration so obtained in the layer 1 will predominantly be active above a certain temperature. In this way a very well defined active dopant concentration is formed in said first layer 1, and in this case this doping concentration will be high lying close to the solubility limit of boron in SiC resulting in a low resistance of said layer 1 making it well suited as contact layer. The invention is of course not in any way restricted to the preferred embodiment described above, but many possibilities to modifications thereof will be apparent to a man with ordinary skill in the art. Although it has been shown how the method according to the invention is used as a part of the process for producing a diode, the method according to the invention may be used in production of all types of semiconductor devices, such as IGBT's, MOSFETs, thyristors, and so on. It is pointed out that the method according to the invention is only occupied with producing a crystalline layer of SiC having at least a region thereof doped with boron atoms starting from a layer of crystalline SiC already produced. This layer of crystalline SiC will of course have some inevitable dopants and impurities although this is not said in the claims. It will be possible to dope the entire SiC-layer with boron atoms or only a region thereof. It is not necessary that all the semiconductor layers of the device, for the production of which the method according to the invention is utilised, are made of SiC, but the device may for instance be a device having a so called het- erojunction. The definition layer is to be interpreted broadly and comprises all types of volume extensions and shapes. The word crystalline means a good periodicity of the lattice in the three dimensions over greater regions, i.e. typically polycrystalline structures are excluded. The steps of implantation of the boron atoms and the carbon atoms may be carried out in an arbitrary order and even at the same time should any equipment enabling this be available.

Claims: Claims 1. A method for producing a crystalline layer of SiC having at least a region thereof doped with boron atoms, compris- ing a step a) of ion implantation of boron into a layer (1) of crystalline SiC and a step b) of heating the SiC-layer for annealing it for making the boron implanted therein electrically active, characterized in that it comprises a step c) of implanting carbon atoms in said layer (1) for forming carbon interstitials in excess with respect to carbon vacancies present in the SiC-layer before carrying out step b) , said step c) being carried out one of a) before, b) after and c) at the same time as step a) . 2. A method according to claim 1, characterized in that in step c) the dose of carbon implanted is selected so as to direct the majority of the boron implanted in step a) to sites intended for silicon at- oms in said region of the SiC-layer during the annealing of step b) . 3. A method according to claim 2, characterized in that the dose of boron implanted is con- trolled during step a) for obtaining a determined concentration of boron sitting at said silicon sites in said region of the SiC-layer. 4. A method according to claim 2 or 3, characterized in that boron is implanted in step a) at a dose lying close to the solubility limit of boron in SiC for obtaining a low resistance of said region. 5. A semiconductor device comprising at least one layer of SiC, characterized in that said layer is produced while carrying out a method according to any of claims 1-4.

Inventors: ROTTNER KURT, SCHOENER ADOLF, ROTTNER, KURT, SCHOENER, ADOLF
Applicants: ABB RESEARCH LTD, ROTTNER KURT, SCHOENER ADOLF, ABB RESEARCH LTD., ROTTNER, KURT, SCHOENER, ADOLF
Application year: 1997
Main classification: C30B29
IPC codes: C30B29, C30B31, H01L21, H01L29