A METHOD FOR PRODUCING A SILICON CARBIDE BIPOLAR DEVICE AND A SILICON CARBIDE BIPOLAR DEVICE

Abstract: A method for producing a bipolar semiconductor device having a first layer (3) doped according to a first doping type, said first layer being adapted to have minority charge carriers injected thereinto from a second layer (5) of the device of an opposite doping type to that of the first layer in a forward conducting state of the device, comprises a step of epitaxially growing said first layer during which at least one region (4) of said first layer is provided with a constitution implying a lower lifetime of said minority charge carriers therein than in other parts of said first layer by incorporating impurities in said region during the epitaxial growth thereof.

Description: A METHOD FOR PRODUCING A SILICON CARBIDE BIPOLAR DEVICE AND A SILICON CARBIDE BIPOLAR DEVICE TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for producing a bipolar semiconductor device having a first layer doped according to a first doping type being one of a) n and b) p, said first layer being adapted to have minority charge car- riers injected thereinto from a second layer of the device of an opposite doping type to that of the first layer in a forward conducting state of the device, said method comprising a step a) of epitaxially growing said first layer and a step b) of providing at least one region of said first layer with a constitution implying a lower lifetime of said minority charge carriers therein than in other parts of said first layer, as well as a bipolar semiconductor device according to the preamble of the appended independent device claim. The invention is occupied with all types of bipolar semiconductor devices, such as diodes, thyristors, BJTs and IGBTs, which have the common feature that minority charge carriers are injected into such a first layer, which normally is a low doped n-type drift layer, in the forward conducting state of the device resulting in an improved V( I)-characteristic of the device, i.e. the device requires a lower forward voltage for conducting a current of a certain value. However, it is also well known that the presence of these minority charge carriers in said first layer in the case of for instance a diode results in a reverse recovery of the device when it is turned off, i.e. switched into the reverse blocking state, which results in a reverse current in the reverse direction of the device during a certain period of time, the so called reverse recovery time, causing energy losses which may lead to cooling problems. In the case of a device of the type which is turned off like a GTO or an IGBT the presence of the minority charge carriers in said first layer results in a so called tail current in the forward direction upon turning off the device. This current also creates energy losses. Accordingly, there is a trade-off between conduction on- state losses and switching or turn-off losses in such bipolar semiconductor devices, and the growth of said first layer is normally carried out so as to obtain highest possible lifetime for the minority charge carriers therein and by that the possibility to obtain so low conduction on-state losses as possible. It is known to use different techniques to after that reduce the lifetime of said minority charge carriers in said first layer or at least a region thereof for obtaining a faster recombination of the minority charge carriers when the device is turned off or switched in the reverse blocking state and by that lower switching losses, but this will at the same time, as mentioned, affect the plasma formed by the minority charge carriers in the forward conducting state of the device and by that cause an increase of the conduction on-state losses. However, these methods of lowering said lifetime have the advantage that the uniformity of said lifetime is increased with respect to the cσndi- tions prevailing in said first layer after the growth thereof . It is known to introduce Au- or Pt-atoms into such a first layer of bipolar semicondutor devices of silicon by high temperature diffusion thereinto for creating recombination centres therein. However, this method is rather complicated to carry out and it is comparatively difficult to control the process and obtain a uniformity of the concentration of said atoms and by that a well defined minority charge carrier lifetime in said region of said first layer. This method has also a severe restriction, because it may only be used for materials in which the atoms to be introduced have a comparatively high diffusivity, so that it may for instance not be used for SiC-devices where the diffusitivity is very low. It is also known to use electron irradiation as a means of minority charge carrier lifetime control, and this technology provides a clean and simple process with greater control and uniformity than can be achieved by the impurity diffu- sion process. The main disadvantage of this irradiation is that the radiation-induced recombination centres tend to anneal out after a period of time, not only but particularly when the device is subjected to high temperatures. This electron irradiation has hitherto been carried out, so that it has changed said lifetime homogeneously throughout the device thickness. It is also known to use proton irradiation for causing damages of a restricted region of said first layer for obtain- ing a localised low lifetime region reducing the switching losses of bipolar devices but not lowering the conduction on-state losses that much. However, this proton irradiation has several disadvantages. It is difficult to obtain a homogenous influence of the whole region, in which said lifetime is to be lowered. Furthermore, it is associated with high costs to provide a source of proton irradiation. It is also difficult to reach regions located deeper into a device, especially in hard irradiation materials, such as SiC. SUMMARY OF THE INVENTION The object of the present invention is to provide a method for producing a bipolar semiconductor device as defined in the introduction, which solves the problems adhered to the methods discussed above. This object is according the invention obtained by forming said lower lifetime region of said first layer directly dur- ing the epitaxial growth of this region during step a) by changing the composition of the substances fed to said first layer for the growth thereof when said region is grown. This means that a localised low lifetime region may be obtained without the need of any source of proton irradiation or the like simplifying the process considerably. Furthermore, it will be possible to obtain an excellent uniformity of the lifetime in said region if so desired. It will also be easy to chose any desired location for said region to give the best device performance, and the thickness (depth) of the lower lifetime region can easily be controlled during the epitaxial growth process. Another advantage of this method is that no high diffusivity of any element in the material of said first layer is necessary for obtaining recombination centres in said region. According to a preferred embodiment of the invention said region is formed by incorporating impurities therein during the growth thereof. An advantage of this method is that no high diffusivity of said impurities in the material of said first layer is necessary for obtaining the incorporation of said impurities forming recombination centres in said region. According to another preferred embodiment of the invention the ratio of those of said substances that contain the basic elements for the growth of said first layer is changed dur- ing this growth when said region is grown. This is an easy and efficient way to influence the lifetime of said minority charge carriers in the region, and this will result in a lower quality of the region and by that a lower lifetime. "Basic elements" is defined as the elements participating in forming the basic structure of the material grown, so that they will in for instance the case of growing SiC be Si and C. According to another preferred embodiment of the invention said region is formed by incorporating impurities during the growth of a certain thickness of said first layer, so that said region will be a sub-layer of the first layer having a lower lifetime of said minority charge carriers. A well de- fined such sub-layer having a lower minority charge carrier lifetime may so be simply realised by carrying out the method according to the invention. According to another preferred embodiment of the invention concerning a method carried out for obtaining a device in which said second layer is located next to said first layer, said lower lifetime region is grown in a part of said first layer located close to said second layer. It has turned out that this location of said region is very advantageous. Such a localised lower lifetime region will only increase the conduction on-state losses of a device so produced to a comparatively small extent, but the switching losses will be reduced considerably. This is due to a strongly enhanced recombination rate of minority charge carriers present in said first layer in said region when the device is turned off or switched to be reversed biased, which reduces the charge, i.e. the tail current, to flow until the steady state is reached in a device turned off and the charge which has to flow in the reverse direction after reverse biasing a device until the steady reverse blocking state is reached. Furthermore, in the latter case, the device will start to block at a comparatively lower reverse current than without this lower lifetime region reducing the switching losses. It has also turned out that said reverse current will after said blocking state has been reached decrease slowly, so that a very "soft" reverse recovery is obtained, which will be of great importance when this current has to be led through an inductance. According to another preferred embodiment of the invention said first layer is provided with a lifetime of said minority charge carriers varying throughout said first layer. It will in this way be possible to give different regions of said first layer exactly the minority charge carrier lifetime that is desired for each particular device and adapted for the later task of the device in question. According to another very preferred embodiment of the invention at least said first layer is produced by epitaxially growing SiC. The method according to the invention is par- ticularly well suited for producing a bipolar semiconductor device with at least one region of said first layer provided with a constitution implying a lower lifetime of the minority charge carriers therein when said first layer is made of SiC, since the low diffusivity of most elements in SiC, also at comparatively high temperatures, which is a disadvantage when trying to obtain a lower lifetime region by the impurity diffusion process discussed above and which makes it impossible to use that process for SiC, here is turned into an advantage, since the recombination centres formed through the incorporation of said impurities into the SiC- layer will still be there also if the device is excerted to very high temperatures thanks to the low diffusivity of such impurities in SiC. It is also an object of the present invention to provide a bipolar semiconductor device according to the preamble of the appended independent device claim having a good tradeoff between conduction on-state losses and switching losses with respect to such devices already known. This object is according to the invention obtained by providing such a device, in which said lower lifetime region of said first layer has another composition than said other parts of said first layer, said composition being obtained during an epitaxial growth of this region. The advantages of such a semiconductor device will be apparent from the discussion above of the method according to the invention. According to a preferred embodiment of the invention at least said first layer of the device is of SiC, and such a device will have particular advantages with respect to prior art devices of this type. Further advantages and preferred features of the invention will appear from the following description and the other de- pendent claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of a preferred embodiment of the invention cited as an example. In the drawings: Fig. 1 is a schematic cross-section view through a rectifying diode produced through a method according to the invention, Fig. 2 is a graph showing how the lifetime of minority charge carriers changes in the thickness direction of the diode in Fig. 1, Fig. 3 is a view of an electrical circuit in which the diode shown in Fig. 1 may be incorporated and for which simulations have been done for comparison with a cir- cuit having a similar diode without any lower lifetime region in said first layer, Fig. 4 shows a graph of the current versus voltage in the forward conducting state of two diodes having differ- ent minority charge carrier lifetimes in the drift layer thereof, Fig. 5 shows a graph of the reverse recovery charge which has to flow in the reverse direction of a diode upon turning-off thereof versus the on-state forward voltage of a diode according to Fig. 1 and a corresponding diode having no lower lifetime region, and Fig. 6 shows a graph of the current through a diode versus time when it is turned-off for the diodes according to Fig. 5. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 illustrates a rectifying diode, to which the invention is applied. All semiconductor layers of this diode is made of SiC, and this diode and the features thereof will hereinafter be explained for illuminating the invention and the advantages thereof, since the invention is particularly advantageous in the SiC-case, but it is emphasised that the invention is not to be considered to be restricted to this semiconductor material. Fig. 1 is a very simplified view of the device, in which passivation layers and so on having nothing to do with the invention have been left out. The diode has superimposed a cathode 1, a comparatively thick highly doped n-type substrate layer 2, which here has a thickness of 50 μm and a doping concentration of 3x10-*-^cm~3, a low doped n-type drift layer 3 having a total thickness of 30μm and a doping concentration of 2xlO- L 5 ιrT , the uppermost region of which forming a sub-layer 4 having a lower lifetime of minority charge carriers than the rest of the drift layer and a thickness of 5μm, a highly doped p-type layer 5 having a thickness of 5μm and a doping concentration of lθ cm~3 and which forms a pn-junction with the drift layer, and finally an anode 6. In the forward conducting state of the diode the layer 5 will function as a hole emitter and inject minority charge carriers in the form of positive holes into the drift layer 3 forming a plasma therein and improving the conductivity of the device. However, this plasma does also give rise to energy losses when the diode is switched to be biased in the reverse direction. The different layers 2-5 of this device are epitaxially grown by using the Chemical Vapour Deposition (CVD) tech- nique or developments thereof, such as the High Temperature Chemical Vapour Deposition (HTCVD) technique and the first major part 7 of the drift layer 3 is grown with an aim to keep the minority charge carrier lifetime therein as high as possible, and the uppermost part 4 of the drift layer is epitaxially grown while introducing additives reducing said lifetime, so that these are incorporated in the layer 4 during the epitaxial growth thereof. Such impurities are in this case introduced to such a concentration that the minority charge carrier lifetime in the sub-layer 4 will be 10 ns, whereas the minority charge carrier lifetime will be 100 ns in said major part 7 of the drift layer. This is illustrated in Fig. 2, which illustrates said lifetime τ vs. the device thickness in said drift layer. Accordingly, the minority charge carrier lifetime is in this way varied in the axial direction of the device, so that the method according to the invention is here used to axially control the life- time profile in the drift layer. The dashed line in Fig. 2 corresponds to the lower limitation of the sub-layer 4. The impurities incorporated in said sub-layer 4 during the epitaxial growth thereof may be any type of element suitable to lower the lifetime of the minority charge carriers when present in the SiC-layer. Compensation may also be used for lowering the lifetime at a given doping concentration. For instance may N be incorporated at a concentration of 2xl0 1 ^cm ~ 3 and vanadium as compensating acceptor at a concentration of lO^cm-^ f or obtaining a resulting doping concentration of n-type of lθ l ^cm ~ 3. Titanium is another good candidate for such a compensating acceptor in SiC. Fig. 3 illustrates an electrical circuit, in which the SiC- diode 8 is built in, and the function of this switching circuit has been simulated by calculations and compared with the function thereof when the diode 8 is replaced by an identical diode except for the absence of said lower life- time layer 4. This circuit is used to make a simple test of the diode turn-off in a circuit which gives a fairly good model of the situation as a free-wheeling diode in e.g. an IGBT converter. In the simulation the following circuit data was used: forward current 100A/cm 2 , reverse DC voltage at 9 2,8 kV, clamped by the capacitance 10, di/dt for the commutation at turn-off 500A/cm^μs (controlled by the inductance 11) dV/dt during reverse blocking approximately 5kV/μs (adjusted by introducing the capacitance 12 into the circuit ) . Before making comments upon the results of these simulations the influence of the minority charge carrier lifetime in the drift layer of a rectifier diode on the forward voltage at a given current through the diode in the forward conducting state thereof will be briefly discussed while referring to Fig. 4. The dashed line corresponds to a diode having a lower minority charge carrier lifetime in the drift layer than a corresponding diode, which then gets a Vp( I)-characteristic according to the line 13 in Fig. 4. Accordingly, a higher voltage has to be applied over a diode having a lower minority charge carrier lifetime in the drift layer thereof for obtaining a given current through the diode, which will then increase the on-state energy losses of the diode. Fig. 5 illustrates how the charge Q to be led through the diode in the reverse direction upon turning-off thereof depends upon the forward voltage over the diode for a given current. The line 14 corresponds to a diode according to Fig. 1, but with an homogenous lifetime throughout the entire drift layer. Expressed in another way, it is illus- trated how said charge is changed with the minority charge carrier lifetime in said drift layer, and travelling along the curve 14 from the top thereof and downwardly will mean going towards lower and lower minority charge carrier lifetimes and at the same time lower switching losses which have to be paid by increasing conduction on-state losses. The simulations referred to above result in the dashed line 15 in Fig. 5 for the diode shown in Fig. 1. Accordingly, it will be possible to obtain a smaller charge to be led in said reverse direction upon turning-off the diode for a given forward voltage when a localised lower lifetime region is placed next to the p-emitter than when the lifetime has been lowered for the entire drift layer. Accordingly, a very good trade off between conduction on-state losses and switching losses may be obtained in this way. The current I through a diode without the lower lifetime layer 4 (curve 16) and of the diode shown in Fig. 1 (curve 17) are illustrated in Fig. 6 as a function of the time elapsed after turning the diode off. It may be seen that the diode will reach the point 18 at which the pn-junction thereof will start to block at a much lower reverse current than in a diode having no such lower lifetime layer. It may also be seen that the total charge led through the diode in the reverse direction thereof after turning the diode off, and by that the turn-off energy loss, will be much lower for the diode according to the invention. A further important characteristic of the curve 17 is that a very "soft" reverse recovery is obtained, i.e. the reverse current is decreasing comparatively slowly, so that di/dt will be low and by that no high voltages will be created over a possible inductance of the switching circuit through which said current has to flow. It is true that the energy losses will be somewhat larger for such an extension of the curve 17 than if it would go faster towards zero from the point 18, but this will mostly be gratefully accepted, since the problems with a very large di/dt over an inductance may be avoided. The invention is of course not in any way restricted to the preferred embodiment described above, but many possiblities to modifications thereof will be apparent to a man with ordinary skill in the art. Although it has been shown in the figures how the minority charge carrier lifetime is varied in the axial direction of the device, it is within the scope of the invention to vary said lifetime in other directions by incorporating impurities during the epitaxial growth of said first layer, for instance by using the so called regrowth technique. It would also be possible to incorporate different impurities in different parts of said first layer for obtaining different lifetimes in these different parts. The concentration of said impurities lowering said lifetime may also be varied so as to vary said lifetime throughout said first layer. Said first layer may be doped according to the p-type as well as according to the n-type. In the first case electrons will form said minority charge carriers. All definitions concerning the materials of the different device layers do of course also include inevitable impurities as well as intentional doping when SiC is concerned. The definition layer is to be interpreted broadly and com- prises all types of volume extensions and shapes. Another technique of epitaxial growth than CVD, such as LPE, is also conceivable and within the scope of the present invention.

Claims: Claims 1. A method for producing a bipolar semiconductor device having a first layer ( 3 ) doped according to a first doping type being one of a) n and b) p, said first layer being adapted to have minority charge carriers injected thereinto from a second layer ( 5 ) of the device of an opposite doping type to that of the first layer in a forward conducting state of the device, said method comprising a step a) of epitaxially growing said first layer and a step b) of providing at least one region of said first layer with a constitution implying a lower lifetime of said minority charge carriers therein than in other parts of said first layer, characterized in that said lower lifetime region (4) of said first layer is formed directly during the epitaxial growth of this region during step a) by changing the composition of the substances fed to said first layer for the growth thereof when said region is grown. 2. A method according to claim 1, characterized in that said region (4) is formed by incorporating impurities therein during the growth thereof. 3. A method according to claim 1, characterized in that the ratio of those of said substances that contain the basic elements for the growth of said first layer ( 3 ) is changed during this growth when said region (4) is grown. 4. A method according to claim 2, characterized in that said region is formed by incorporating impurities during the growth of a certain thickness of said first layer ( 3 ) , so that said region will be a sub-layer (4) of said first layer having a lower lifetime of said minority charge carriers. 5. A method according to any of claims 1-4, said second layer ( 5 ) being located next to said first layer ( 3 ) , characterized in that said lower lifetime region (4) is grown in a part of said first layer located close to said second layer. 6. A method according to any of claims 1-3, characterized in that said first layer is provided with a lifetime of said minority charge carriers varying throughout said first layer. 7. A method according to claim 6, characterized in that said lifetime is varied by incorporating impurities therein with one of a) a concentration varying throughout said first layer, b) a composition varying throughout said first layer and c) both a) and b) . 8. A method according to any of claims 1-7, characterized in that at least said first layer ( 3 ) is produced by epitaxially growing SiC. 9. A method according to claims 3 and 8, characterized in that the C/Si-ratio of said substances is changed during the growth of said first layer (3) when said region (4) is grown. 10. A method according to any of claims 1-9, characterized in that at least said first layer is grown by Chemical Vapour Deposition ( CVD ) . 11. A method according to any of claims 1-10, characterized in that said lower lifetime region (4) is created in a first layer (3) which is in the form of a low doped drift layer of said device. 12. A method according to any of claims 1-11, characterized in that said first layer (3) is grown as a n-type layer. 13. A method according to claims 8 and 11, characterized in that one of a) titanium and b) vanadium is incorporated as a compensating acceptor in said region (4) during the growth thereof for lowering said lifetime therein. 14. A method according to claim 11, characterized in that said first layer (3) is epitaxially grown with a very low doping concentration and with an aim to keep the minority charge carrier lifetime thereof as high as possible except from during the growth of a specific thickness interval of said first layer for which additives reducing said lifetime are introduced. 15. A method according to any of claims 1-14, characterized in that said lower lifetime region (4) is provided with a lifetime of said minority charge carriers being at least five times lower than in the major part (7) of said first layer ( 3 ) . 16. A method according to any of claims 1-15, characterized in that said region ( 4 ) is made thin with respect to the thickness of the first layer ( 3 ) . 17. A method according to claim 16, characterized in that said region (4) is made thinner than 1/3 of the thickness of said first layer (3). 18. A method according to claim 2 or claim 2 and any of claims 3-17, characterized in that impurities strongly enhancing the recombination rate of said minority charge carriers are incorporated in said region (4) during the epitaxial growth thereof. 19. A bipolar semiconductor device having a first layer (3) doped according to a first doping type being one of a) n and b) p, said first layer being adapted to have minority charge carriers injected thereinto from a second layer (5) of the device of an opposite doping type to that of the first layer in a forward conducting state of the device, at least one region (4) of said first layer having a constitution implying a lower lifetime of said minority charge carriers therein than in other parts of said first layer, character- ized, in that said lower lifetime region of said first layer has another composition than said other parts of said first layer, said composition being obtained during an epitaxial growth of this region. 20. A device according to claim 19, characterized in that said region has impurities reducing said lifetime incorpo- rated therein during an epitaxial growth of this region. 21. A device according to claim 19 or 20, characterized in that at least said first layer is of SiC. 22. A device according to any of claims 19-21, in which said second layer ( 5 ) being located next to said first layer ( 3 ) , characterized in that said lower lifetime region (4) is located close to said second layer.

Inventors: HERMANSSON WILLY, HERMANSSON, WILLY
Applicants: ABB RESEARCH LTD, HERMANSSON WILLY, ABB RESEARCH LTD., HERMANSSON, WILLY
Application year: 1997
Main classification: H01L21
IPC codes: H01L21, H01L29