Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B23/00—Single-crystal growth by condensing evaporated or sublimed materials
  • C30B23/02—Epitaxial-layer growth
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/36—Carbides
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/38—Nitrides

Definitions

• the present invention relates to a method for epitaxially growing objects of one of a) SiC, b) a Group Ill-nitride and c) alloys thereof on a substrate received in a suscep ⁇ tor having circumferential walls, in which these walls and by that the substrate and a source material for the growth are heated above a temperature level from which sublima ⁇ tion of the material grown starts to increase consider ⁇ ably, and a carrier gas flow is fed into the susceptor to ⁇ wards the substrate for carrying said source material to the substrate for said growth, as well as a device for epitaxially growing such objects according to the preamble of the appended independent device claim.
• the invention is applicable to the growth of SiC, Group Ill-nitrides and all types of alloys thereof, but the common problem of growing such objects of a high crystalline quality and at a reasonable growth rate from the commercial point of view will now by way of a non- limitative example be further explained for SiC.
• SiC single crystals are in particular grown for being used in different types of semiconductor devices, such as for example different types of diodes, transistors and thyris- tors, which are intended for applications in which it is possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions.
• the large band gap between the valence band and the conduction band of SiC makes devices fabricated from said material able to operate at high temperatures, namely up to 1000 K.
• the growth rate is determined by the de ⁇ gree of supersaturation of said vapours in the atmosphere around the seed crystal, which in its turn is determined by the temperature, the applied temperature gradient and the pressure in the system.
• the vapour transport is thus characterized by diffusion processes and convection.
• low pressures in the container are needed for making the transport of the sublimed SiC powder effective, while avoiding too many collisions of the vapour SiC on its way to the seed crystal.
• the obtained growth rates with such a system is in the order of a few mm/h.
• Typical tempera ⁇ tures, temperature gradients and pressures are in the or- der of 2400°C for the source material, 10-30°C/cm and 5-50 millibar, respectively.
• the ambient is normally Ar.
• the advantage with this method is its simplicity.
• the disad ⁇ vantage with the method is the limited control of the sys ⁇ tem, the unsatisfactory crystalline quality and the low purity which largely is governed by the purity of the source material and which indeed may be improved by the choice of a purer source material.
• Due to an inevitable escape of Si from the quasi-closed container the C/Si ra ⁇ tio of the vapourised source material cannot be kept con ⁇ stant during the entire growth. This will affect the growth in a negative way and cause crystalline defects.
• the growth In order to grow crystals of significant size for subsequent substrate production, the growth must also be interrupted from time to time in order to refill the container with new source material. These interrupts will also disturb the growing crystal.
• the presence of the temperature gradient at the growth interface causes forma ⁇ tion of crystalline defects such as icropipes, disloca ⁇ tion and point defect agglomerates.
• the gas needed for the growth is transported to the substrate by a carrier gas, which normally is hydrogen.
• the precur ⁇ sor gases used are in the SiC case normally silane and propane.
• the precursor gases decompose or are cracked and the silicon and carbon consituents migrate on the growing crystal surface to find a proper lattice site.
• the tem- perature of the system is normally kept below 1600°C. Es ⁇ sentially no temperature gradient is present in the growth front of the crystal.
• the advantage with the CVD process is the purity and the crystalline quality which mainly is limited by the substrate quality.
• the disadvantage with the CVD technique is the low growth rates which rules out any possibility of growing crystals for substrate produc ⁇ tion by this technique or even thick high quality layer ⁇ at a commercially interesting capacity.
• the typical growth rates of CVD grown SiC epitaxial layers are in the order of several ⁇ m/h at 1600°C.
• Recently another process, namely the High Temperature Chemical Vapour Deposition (HTCVD) process has been pre ⁇ sented (paper of High Temperature Chemical Vapour Deposi ⁇ tion released on Technical Digest of Int'l Conf. on SiC and Related Materials -ICSCRM-95-, Kyoto, Japan, 1995 and the US patent application No. 08/511 324).
• HTCVD High Temperature Chemical Vapour Deposition
• This process is technically a CVD process carried out at very high tem ⁇ perature where sublimation and etching of the seed crystal (substrate) and growing crystal or layer is significant.
• the etching of the growing surface has shown to improve the crystalline quality and also due to the purity of the precursor gases the purity of the grown crystals are very high.
• the growth rate can be increased to the order of a few mm/h due to the increased surface mobility of the at- oms which thereby find their correct lattice sites faster.
• the temperatures are in the order of 1900°C-2500°C.
• the advantage with the HTCVD process is the high purity, the high crystalline quality and also the high growth rate.
• the disadvantage with the technique is the difficulty to establish favourable conditions for growth in an artificial way by adding silicon and carbon precursor gases in a correct amount at all times, i.e. during the temperature increase to the growth temperature and during growth. If a too small amount of precursor gases are added, a too high degree of etching or sublima ⁇ tion may occur, which thereby may cause a graphitisation of the crystal surface which will cause crystalline de ⁇ fects or even totally prevent growth. If a too high amount is added the supersaturation may be too high for the sur- face mobility and the growth may be polycrystalline. The conditions must thus artificially be kept close to a ther- modynamical equilibru , which may be very delicate to achieve.
• This technique has been considered to form the state of the art while drafting the preambles of the ap- pended independent claims, although the inventional tech ⁇ nique defined below is in fact no type of CVD technique.
• the object of the present invention is to advise a solu- tion to most of the problems discussed above by providing a method and a device making it possible to epitaxially grow objects, both layers and boules, of SiC, a group III- nitride or alloys thereof at a high growth rate while still obtaining a high crystalline quality of the object grown.
• This object is in accordance with the invention obtained by providing a method defined in the introduction with the steps of providing at least a part of said source material in a solid state as the material grown in a container com ⁇ prising the susceptor, by heating said source material part in said container bring it to a vapour state and carry it in a vapour state by the carrier gas flow to the substrate for said growth.
• the definition "as the material grown” is to be interpreted ⁇ o that when for instance SiC is grown said part of the source material is SiC in a solid state.
• the inventional method is an improve ⁇ ment of the High Temperature Chemical Vapour Deposition technique making it to a hybrid of that technique and the seeded sublimation technique utilising the advantages of each of these techniques. Thanks to the fact that this type of source material is present in a solid state in the container a very good control of a broad parameter range is obtained, so that a high quality crystal may be grown at a high growth rate.
• the Si and C containing vapours created either by etching or by sublimation of the SiC will be brought to the sub ⁇ strate by the carrier gas or by the carrier gas in combi- nation with a thermal gradient. Conditions close to ther- odynamical equilibrium can thus be obtained at all times and throughout the whole growth cycle provided that there is at all times a significant amount of SiC present in the growth chamber. No artificial means of creating a thermo- dynamical equilibrum are thus required during the tempera- ture ramp to the growth temperature. This improvement will make it simple to grow SiC boules without any temperature gradient.
• the transport of the source material will be much more ef- ficient which means that the growth rates can be raised substantially in comparison to the seeded sublimation technique.
• the vapour trans ⁇ port can be very easily controlled by increasing or de ⁇ creasing the flow of the carrier gas.
• the degree of super ⁇ saturation can be kept at a controllable level at all times. If the thermal gradient is made negative, i.e. the substrate is at a higher temperature than the source mate ⁇ rial, a controlled etching is also possible.
• the negative thermal gradient will not cause a problem once growth is desired since the transport of the source material by a carrier gas is so much more efficient than a thermal gra ⁇ hardware. Dopants may be easily added to the carrier gas in a manner similar to that of CVD. Since the gases pass through the container, much material will be lost and de ⁇ posited or transported to other parts of the container. These losses can, however, be assumed to be essentially equal for both the silicon and carbon containing vapours which will simplify the maintenance of a constant C/Si ra ⁇ tio. In this embodiment the growth rate may simply be con ⁇ trolled by controlling the temperature to which the sus ⁇ ceptor walls are heated and the flow rate of the carrier gas flow.
• the definition "container” is to be interpreted as an object defining a room in connection with the sus ⁇ ceptor room, in which a temperature above the temperature level from which sublimation of the material growth starts to increase considerably prevails.
• the container may be the same as the susceptor or in addition to the suscep- tor also comprise a room directly upstream the susceptor room. It is emphasised that the corresponding reasoning is valid also for the cases of growing objects of other mate ⁇ rials than SiC.
• said source material part is provided in said container by one of a) making the container of the material grown, b) in ⁇ ternally coating the container by the material grown and c) placing the material grown in the container.
• the preferred conditions may be contained by placing the mate ⁇ rial grown, for instance as a powder or as lumps in the container, but it may also be obtained by making the con ⁇ tainer of the material grown or coating it internally by the material grown, so that material from the walls of the container may be released and used for the growth.
• At least a part of said source material for said growth is added to the carrier gas flow upstream the container and carried by the carrier gas flow to the container in one of a) a solid state or b) a liquid state for being brought to a vapour state in the container by heating there.
• a) a solid state or b) a liquid state for being brought to a vapour state in the container by heating there may be made so that crystals of unlim ⁇ ited length may be grown without interruption.
• Fig. 1 is a longitudinal cross-section view of a device according to a first embodiment of the invention
• Fig. 2 is a perspective view of the susceptor used in the device of Fig. 1 according to a first preferred embodiment adapted for epitaxially growing layers, parts of the susceptor walls being broken away so as to illustrate the interior of the susceptor,
• Fig. 3 is a view similar to Fig. 2 of a susceptor accord ⁇ ing to a second preferred embodiment adapted for growing boules by the same inventional method as used for the growth in the embodiment according to
• Fig. 4 is a view similar to Fig. 2 and 3 of a susceptor according to a third preferred embodiment adapted to enable the growth of boules of nearly unlimited lengths.
• Fig 1 shows schematically a device according to a pre ⁇ ferred embodiment of the invention for epitaxially growing SiC by a method according to a preferred embodiment of the invention being a hybrid of seeded sublimation and High Temperature Chemical Vapour Deposition on a SiC-substrate in a simplified manner, and it is obvious that the device in question also comprises other means, such as pumps, but conventional equipment having nothing to do with the in ⁇ vention has been omitted for the sake of clearness and concentration to the inventional characteristics.
• the de ⁇ vice comprises a casing 1 constituted by a tube 2 of quartz extending substantially vertically and two opposite end flanges 3 and 4.
• the end flange 4 is preferably remov ⁇ able so as to get access to the interior of the tube 2.
• a conduit 5 for supplying a stream of at least a carrier gas intended for the transport of source material for said growth to the substrate inside a susceptor is introduced through the lower end flange 3.
• the composition and task of this gas mixture containing at least a carrier gas will be discussed more in detail further on.
• the conduit 5 is connected to separate conduits 20-23 leading to sources for different additions of source material for said growth to said carrier gas and these conduits are provided with flow regulating means not shown for regulating the content of each component in said gas mixture a ⁇ desired. Such flow regulating means will also be there for the carrier gas flow.
• the conduits 20-23 are in Fig. 1 for the sake of clearness illustrated as emerging into the conduit 5 close to the casing 1, but in practi ⁇ e they will probably be at a greater distance therefrom.
• the device comprises a funnel 6 for concen ⁇ trating the gas flow from the conduit 5 into a ⁇ usceptor 7 (see also Fig 2).
• the susceptor 7 shown in Figs 1 and 2 is adapted for epitaxially growing layers of SiC.
• the suscep ⁇ tor is substantially cylindrical with circumferential walls 8 of a substantially uniform thickness.
• the walls are made of graphite, but they are internally coated by a layer of SiC 9 or alternatively covered by a cylindrical plate made of SiC.
• the space surrounding the susceptor is enclosed and filled by graphite foam 10 for thermal insu ⁇ lation for protecting the ⁇ urrounding quartz tube 2.
• Rf- field radiating means 11 in the form of a Rf-coil sur ⁇ rounds the tube 2 along the longitudinal extension of the susceptor 7.
• This heating means 11 is arranged to radiate a Rf-field uniformly heating the walls 8 of the susceptor and thereby the gas mixture introduced into the susceptor.
• the susceptor 7 comprises a lid 12 of the same material as the rest of the susceptor, on the lower side of which a
• SiC substrate 13 is arranged and which may be removed from the rest of the susceptor so as to remove the substrate after a layer has been grown thereon.
• the lid 12 is provided with peripheral gas outlet holes 14, so that a preferable laminar gas flow will enter the susceptor room 18 through the lower inlet 15 and flow close to the substrate and leave the sus ⁇ ceptor through the upper outlets 14 and then the device through a conduit 16 connected to a pump not shown.
• the temperature inside the susceptor 7 may be checked py- rometrically through looking into the susceptor 7 through a window indicated at 17.
• the source material for the growth is located in the form of a SiC pow ⁇ der with high purity.
• the funnel 6 has circumferential open ⁇ ings 25 for the passage of the flow of said gas mixture into the susceptor room 18.
• the function of the device is as follows:
• the heating means 11 will heat the susceptor walls 8 and by that the susceptor room 18 and the substrate 13 and the SiC powder 24 contained therein to a temperature above the tem ⁇ perature level from which sublimation of the SiC starts to increase considerably, in the present case to a temperature of about 2300°C.
• a gas flow containing a carrier gas which may be H2, Ar or He, is fed to the suscep ⁇ tor room 18 through the conduit 5 and the funnel 6.
• the defi ⁇ nition of carrier gas is a gas not actively participating in the growth, i.e. not having components which are grown into said object of the substrate.
• He and Ar have the advantage with respect to H2 that they do not react with reactive ele ⁇ ments of cracked precursor gases, such as propane, and He is particularly well suited to be used for the growth owing to the comparatively high thermal conductivity thereof compared with Ar.
• the carrier gas flow will carry SiC powder sublimed and being present in the vapour state in the susceptor room 18 towards the substrate 13, so that no temperature gradient is necessary for the transport of the SiC vapour to the sub ⁇ strate.
• the Si and C escape with this system will be large, however, it may be kept at a reasonable level since atmos ⁇ pheric pressure operation is possible.
• SiC vapour carried in this way to the substrate will form an atmosphere around the substrate having a certain "SiC gas pressure", which will counteract sublimation of SiC from the substrate and etching thereof. In this way it is ensured at all times that a super ⁇ saturation is maintained and that no etching of the substrate occurs, so that an SiC crystal of high quality may be grown on the substrate at high growth rates. It has turned out that it will be no problem to grow an object of 0,5 m/hrs at 2300°C in this way and probably much higher growth rates may be envisaged.
• SiC atmosphere not be provided around the substrate, the following may happen: all SiC begins to sublime at a temperature being high enough, which would mean that Si components from the substrate will leave the substrate at a higher rate than the carbon, so that a film of graphite would be formed on the substrate.
• Si- and C-containing precur ⁇ sor gases must be supplied at a correct ratio being very delicate to control in the beginning of said growth. If this happens the result of the growth may be very bad or the growth will in the worst case be impossible.
• the parameters needed for the control of the growth rate may in the method according to the invention be easily controlled within a vast parameter range, so that an adequate control of the relevant parameters is always ensured.
• the tem ⁇ perature inside the susceptor may be controlled by control ⁇ ling the heating means 11 and the growth rate and the quality of the crystal grown may be raised with the temperature.
• higher temperatures also involve a risk of impuri ⁇ ties coming out of the walls of the susceptor.
• the carrier gas flow rate will control the flow of vapourized SiC to the atmosphere around the substrate.
• the growth rate may by con ⁇ trolling the carrier gas flow be very effectively varied, and the device comprises for this sake means 26 schematically in ⁇ dicated for allowing regulation of the flow rate of the car ⁇ rier gas flow.
• a higher carrier gas flow rate will mean a higher growth rate, but too high growth rates may involve a risk of a lowered quality of the material grown.
• the sub ⁇ strate should not have a higher temperature than the SiC powder, since this would mean a temperature gradient normally resulting in a transport of SiC to the colder region, i.e. towards the SiC powder.
• the carrier gas flow is totally dominating and forces "the SiC-vapour" in the right direction, so that a positive growth always takes place regardless of a possible negative temperature gradient in the room, which easily may be the case due to the vertical ar ⁇ rangement of the susceptor.
• the ratio of Si- and C-containing molecules can be kept constant in the susceptor room 18 by additions of silane and propane into the carrier gas through the conduits 22 and 23.
• the silane may partially be decomposed cr cracked on its way to the casing 1, which will lead to an at least partial plugging of the conduits in question and a degraded and less reliable function of the entire device.
• the risk of such cracking is higher at low gas flow rates.
• This problem is however solved by adding Si or SiC as a powder to the carrier gas flow through the conduit 20. In this way also carbon or graphite could be added as a powder if this should prove to be necessary.
• the initial stage of the growth being of particular importance for the quality of the layers grown closely thereafter may be adequately controlled, and very high growth rates are after that achievable - well in the order of mm/h.
• the efficient transport ob ⁇ tained a high growth rate may be obtained at these high tem ⁇ peratures.
• the purity of the Si powder or silane and graphite or propane fed to the susceptor room by the carrier gas the introduction of unwanted compensating accep ⁇ tors into the layers grown is dramatically reduced with re ⁇ spect to the epitaxial growth of such layers by CVD at norrr.al temperatures resulting in a significant prolongation of the minority carrier lifetime in the crystals grown. This is a vital improvement for the production of high-power bipolar devices.
• Fig 3 shows a susceptor 7' according to a second preferred embodiment of the invention, which is adapted for growing boules 19 of SiC on a substrate in the form of a seed crystal indicated at 13'.
• This susceptor is intended to be incorporated in a device according to Fig 1 in the same way as the susceptor according to Fig 2.
• the susceptor ac ⁇ cording to Fig 3 only differs from that according to Fig 2 by the arrangement of gas outlet holes 14' at the bottom of the susceptor.
• the gas flow will as indi- eated reach the region of the SiC boules grown where the source material will be deposited and the re ⁇ ulting compo ⁇ nents thereof will be diverted back and leave the suscep ⁇ tor through the holes 14'.
• SiC boules with a high crystalline quality may in this way be epitaxially grown at a sufficiently high growth rate thanks to the high temperature used.
• a thermodynamic equilibrium in the susceptor room 18' and the C/Si ratio therein may easily be held constant by any appropriate addition as discussed above. This means that the degree of supersaturation which influ ⁇ ences both the growth rate and the formation of micropipes can be varied without any thermal gradient.
• Fig. 4 shows a susceptor according to a third preferred embodiment of the invention.
• This su ⁇ ceptor has only one different principle feature than the other two embodi ⁇ ments, namely the lid 12" and by that the crystal 19" grown may be constantly rai ⁇ ed, preferably by rotation thereof a ⁇ in the Czochral ⁇ ki growth. Without this possi ⁇ bility it would be neces ⁇ ary to heat a much longer suscep ⁇ tor room for growing long objects. And the great distance between the solid source material and the crystal would also be harmful with respect to depo ⁇ itions on the suscep- tor wall before reaching the crystal.
• source material for said growth is added to the carrier gas flow upstream the susceptor 7 and car- ried by the carrier gas flow to the susceptor in a solid or liquid state for ensuring a continuous feeding of source material for said growth.
• Such material may be pow ⁇ der of Si, C and/or SiC.
• a disadvantage of the method according to the invention may be the larger material losses that may be encountered when compared to seeded sublimation growth, however, thanks to the carrier gas transport the pre ⁇ ure may be kept at atmospheric pressure which thereby will limit these losses to some extent.
• the losses will es ⁇ entially be governed by the gas velocity. As long as the ⁇ e losse ⁇ do not cau ⁇ e a degraded function of the sy ⁇ tem by for in ⁇ stance blocking filters or tubes on the downstream side this is a minor problem since a fresh supply of new ⁇ ource material can at all time ⁇ be maintained.
• the invention is also applicable to the growth of a group Ill-nitride, an alloy of group III- nitrides or an alloy of SiC and one or more group Ill-ni- trides, for which the corresponding positive result may be expected.
• object in the claims is made for includ- ing the epitaxial growth of all types of crystals, such as layers of different thicknesse ⁇ as well as thick boules. All definitions concerning the material of course also in ⁇ clude inevitable impurities as well as intentional doping.
• a source material as the material grown in the susceptor in another state than as a powder, such as lumps, or as a combination of powder and lumps.
• a source material as the material grown in the susceptor in another state than as a powder, such as lumps, or as a combination of powder and lumps.
• this may also be obtained by providing a susceptor or container made of the material grown or internally coated thereby.
• the cry ⁇ tal may in an alternative embodiment of the inven ⁇ tion be moved in the direction out of the ⁇ usceptor room in another way than by screwing, for instance by a dis ⁇ placement caused by pulling.

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• Inorganic Chemistry (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)

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• 1995
  • 1995-10-04 SE SE9503427A patent/SE9503427D0/xx unknown
• 1996
  • 1996-10-02 WO PCT/SE1996/001231 patent/WO1997013012A1/en not_active Application Discontinuation
  • 1996-10-02 EP EP96933698A patent/EP0853689A1/de not_active Withdrawn
  • 1996-10-02 JP JP9514192A patent/JPH11513352A/ja active Pending

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