WO1996024709A1

WO1996024709A1 - PRODUCTION OF THIN SINGLE-CRYSTAL SiC LAYERS

Info

Publication number: WO1996024709A1
Application number: PCT/EP1996/000496
Authority: WO
Inventors: Joachim Bill; F. Frederick Lange; Thomas Wagner; Fritz Aldinger; Detlef Heimann
Original assignee: MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V.
Priority date: 1995-02-07
Filing date: 1996-02-06
Publication date: 1996-08-15
Also published as: DE19503976C1

Abstract

Thin single-crystal SiC layers can be obtained by means of a pyrolytic process in which a substrate is coated with a polysilane containing carbon, the adhesive layer is pyrolyzed using a protective gas and the amorphous SiC layer thus obtained is crystallized through being maintained at a temperature of over 700 °C. In a particular application, the process may be used for producing SiC layers doped in a simple way, the doping material being added in the form of a silane compound.

Description

Production of single-crystalline thin layers made of SiC

DESCRIPTION

This invention relates to a method for producing single-crystalline thin layers of silicon carbide, which may also contain a doping element.

Silicon carbide (SiC) is characterized by a combination of desirable properties, such as a high melting point, extreme hardness, good resistance to chemicals and mechanical damage, and moreover has favorable electronic properties, so that this material is a promising material, particularly in electronic applications , is seen. Several methods are known from the prior art with which thin SiC layers can be produced on a suitable substrate, mostly Si. However, the production of single-crystalline layers is relatively complex.

Liaw and Davis (J. Electrochem. Soc., 132 (1985) 642-648) describe a CVD process for producing single-crystalline SiC layers at at least about 1600 K and atmospheric pressure. A buffer layer of carbon is first applied to a Si substrate using C ₂ H. and then a single-crystalline SiC layer is produced using SiH ₄ and C ₂ H ₄ in H ₂ as the carrier gas.

In a modification of such a CVD strategy, according to Sumakeris et al. , (Thin Solid Films 225 (1993) 219-224) using a so-called ALE (Atomic Layer Epitaxy) method, produces single-crystal SiC layers on Si substrates. For this purpose, the substrate is exposed in a multistage process at a sample temperature of 820 to 980 ° C. to an amount of Si ₂ H ₆ which corresponds to a monomolecular silicon layer, then rinsed with H ₂ , then an excess of Exposed to C ₂ H ₄ and rinsed again with H ₂ . In the latter rinsing step, the sample is exposed to a heat source from 1450 to 1700 ° C, which is necessary to generate the single crystal. With this method, the above-mentioned application of a carbon buffer layer is not necessary.

Another approach to producing single-crystalline SiC layers is based on sublimation techniques, such as in Anikin et al. , (Materials Science and Engineering, Bll (1992) 113-115). A single-crystal SiC layer is produced on a Si substrate at temperatures between 1800 and 1900 ° C. using a starting material comprising polycrystalline silicon and SiC under vacuum.

Although single-crystalline SiC layers can thus be produced on suitable substrates by means of these CVD and sublimation processes known from the prior art, however, these techniques are only of limited suitability owing to their complicated process management and the associated high costs in industrial production, in particular of bulk goods. Another process route for the production of SiC is based on the pyrolysis of element-organic polymers (precursors), such as polysilanes and polycarbosilanes. Both SiC fibers (Yajima et al., Nature 261 (1976) 683 to 685) and SiC layers (Chu et al., Springer Proceedings in Physics, Vol. 43, A orphous and Crystalline Silicon Carbide and Related Materials II, Springer-Verlag, Berlin, Heidelberg 1989, pages 66 to 71). According to Chu, substrates to be coated are immersed in a solution or melt of preceramic polymers, ie of polycarbosilane polymers, and the polymer is then pyrolyzed at temperatures in the range from 500 to 1500 ° C., giving amorphous phases and microcrystalline phases at higher temperatures. However, the production of single-crystalline SiC layers by means of such a pyrolysis process is not known from the prior art. It was therefore an object of the present invention to provide a process with which single-crystalline SiC layers can be produced and which is distinguished from the processes known from the prior art by a simpler process control and thus greater economy.

Another object of the invention is to provide a method for producing a single-crystalline SiC layer which contains doping elements in a predetermined concentration range.

Yet another object of the present invention is a single-crystalline SiC layer obtainable by the process according to the invention, or a solid coated therewith.

The object of the invention is achieved by a process for producing single-crystalline epitaxial SiC layers, which is characterized in that the substrate to be coated is coated with at least one carbon-containing polysilane, the adhering layer is pyrolyzed under protective gas at about 200 to 1100 ° C and so generated ceramic layer of amorphous SiC crystallized by holding at a temperature of over 700 ° C.

It was surprisingly found that the amorphous SiC obtained after pyrolysis can be converted into single-crystalline SiC by holding for a sufficiently long period at a sufficiently high temperature.

In the process according to the invention, all materials which are compatible with SiC single-crystal layers are generally suitable as substrates to be coated. SiC single crystals, as well as Si wafers and single crystalline Al ₂ O ₃ , such as sapphires, are preferred, especially in technical applications, if appropriate also in doped form. Suitable precursor polymers are in general all carbon-containing polysilanes and polycarbosilanes and mixtures thereof which give SiC under suitable pyrolysis conditions. Suitable polymers as well as monomers and reaction pathways can be found in the review article by Laine and Babonneau (Chem. Mater. 5 (1993) 260 to 279), the disclosure of which is incorporated by reference herein. Currently preferred polymers include polyvinyl silanes, and particularly polymethyl vinyl silane.

Carbon-containing polysilanes are applied to the substrate using conventional methods known to those skilled in the art. When ordering a solid phase, e.g. In the case of a powder coating, it is preferred to subsequently keep the coated substrate at a temperature above the melting temperature of the carbon-containing polysilane for a sufficient period of time in order to bring about a melting of the solid phase and thus a uniform layer application. Preferably, however, the carbon-containing polysilane is applied from a liquid phase, i.e. depending on the properties of the material used from a solution or melt. Suitable solvents are all those in which the polymer is soluble and which do not affect the subsequent steps of the process. Typical examples of suitable solvents are toluene, hexane, xylene, etc.

It has proven particularly advantageous to carry out the layer application in such a way that the substrate to be coated is immersed in a solution or melt of the polymer. In this way, by suitably adjusting the viscosity of the solution or melt, the speed at which the substrate is pulled out of the solution or melt, and the temperature, the thickness of the layer adhering to the substrate and thus of the single-crystal SiC layer finally produced can be advantageous Ways to be regulated. After the application of the polymer layer, the coated substrate is exposed for a sufficient period of time in an inert environment, such as a He, Ar or N ₂ atmosphere, to a temperature sufficient to convert the adhering precursor layer thermally into a ceramic layer made of amorphous SiC. The temperature range used in the pyrolysis begins at around 200 ° C., since decomposition can already be observed during the crosslinking phase of the polymer. In general, however, higher temperatures are used in the pyrolysis, for example from about 500 ° C. to 1100 ° C., preferably from 900 ° C. to 1100 ° C. The time required depends on various parameters such as the precursor used, the layer thickness and the temperature and is generally from about 0.5 hours to 20 hours. The course of the pyrolysis can be monitored by means of known techniques such as ¹³ C and ²⁹ Si MAS NMR, as a result of which pyrolysis conditions suitable for individual applications can be determined by the experts.

In the subsequent process step, the amorphous SiC layer is converted into an SiC single crystal by holding it at a suitable temperature and for a sufficient period of time. The crystallization of the amorphous layer begins at around 700 ° C, depending on the precursor used. In other cases, however, it may be advantageous or even necessary to use higher temperatures, for example from about 1100 ° C. to 1600 ° C. or more than 1600 ° C. Suitable conditions for the respective application can be determined by the person skilled in the art using known techniques, e.g. by examining electron diffraction in the TEM.

Another object of the present invention is the production of SiC layers which contain a doping element. Suitable doping elements are known to the person skilled in the art and include, for example, boron for p-doping and, for example, nitrogen or phosphorus for n-doping. The doping elements can according to the inventive method be introduced advantageously by targeted synthesis of the precursors, so that homogeneous element distributions can be represented at the atomic level even in the preceramic state. For example, the introduction of boron via a hydroboration reaction of polyvinylsilanes, as described in Riedel et al. , (Journal of Material Science 28 (1993) 3931 to 3938). Analogously, polysilazanes can be used to incorporate nitrogen.

To produce doped SiC layers, a polymer is accordingly used as the precursor which contains the desired foreign atom in the concentration range provided in the molecule. Alternatively, mixtures of a precursor containing a foreign atom and an unmodified precursor can be used in a suitable ratio. It is an advantage of the method according to the invention for the production of doped SiC layers that by incorporating the foreign atoms into the precursors it is easy to maintain concentration ranges within narrow predetermined limits. Furthermore, a homogeneous element distribution is also achieved.

Coated substrates according to the present invention, i.e. Solids which are coated with a single-crystalline SiC layer according to the method according to the invention comprise such a layer on at least part of the surface thereof. It is understood that one or more substantially flat surfaces can be completely or partially coated with single-crystal SiC.

It is obvious to the person skilled in the art that the process according to the invention can also be used to apply a plurality of layers in succession to a substrate by suitable process control, these layers optionally being able to contain different doping elements. Likewise, the concentration of a doping element within a single layer can be varied by suitable layer application. The method according to the invention is thus a simple Stable new method for producing single-crystalline epitaxial layers from pure and doped silicon carbide and is due to its simple process procedure superior to the methods known from the prior art in terms of economy. Solids with an epitaxial monocrystalline SiC layer according to the invention are suitable for a large number of fields of application, in particular for electronic applications.

The following examples are intended to explain the process according to the invention in conjunction with FIG. 1. Figure 1 shows a schematic representation of the dip application process.

Example 1 Representation of the polyvinylsilane

154.03 g (6.7 mol) of sodium and a mixture of 1200 ml of toluene and 50 ml of tetrahydrofuran are placed in a 1000 ml three-necked flask with reflux condenser, KPG stirrer and dropping funnel. A mixture of 104.1 ml (115.04 g, 1.0 mol) of dichloromethylsilane, 121.3 ml of dichlorodimethylsilane (129.06 g; 1.0 mol) and 130.62 ml (141 , 07 g; 1.0 mol) dichloromethylvinylsilane and 88.8 ml (76.05 g, 0.7 mol) chlorotrimethylsilane. The reaction mixture is heated under reflux for 8 hours. The soluble PVS are separated from the insoluble ones via a protective gas frit. The higher molecular insoluble PVS content can be obtained by washing out the salt and then drying in vacuo. This reaction gives 120 g (52% yield) of soluble fractions and 90 g (38%) of insoluble PVS polymer.

Example 2 Production of Single-Crystalline Epitaxial SiC Layers on SiC Single Crystals To apply the synthesized polymethylvinylsilane to the substrate, it was first dissolved in toluene. The polysilane content in the solution was 15.9% by mass. The viscosity was determined to be 1.3 mPas and the density was 0.87 g / cm ³ . A substrate made of single-crystal SiC was then immersed in this solution, as shown in FIG. 1, and pulled out again at a constant speed.

The polymer layer adhering to the substrate was then pyrolyzed under an argon atmosphere at 1000 ° C. The layer obtained consisted of amorphous SiC, as could be shown with the aid of electron diffraction in a transmission electron microscope (TEM). In a subsequent step, this coated substrate was then aged in a graphite furnace at 1600 ° C. for a period of 10 h and then examined with electron diffraction in the TEM. From the diffraction pattern it was found that the layer is present in single crystal on the exposed substrate. The thickness of the layer obtained is approximately 300 nm.

Claims

claims

1. A process for producing single-crystalline epitaxial SiC layers, characterized in that the substrate to be coated is coated with at least one carbon-containing polysilane, the adhering layer is pyrolyzed under protective gas at about 200 to 1100 ° C and the ceramic layer made of amorphous SiC by holding crystallized at a temperature of over 700 ° C.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that one immerses the substrate to be coated in a solution or melt of the carbon-containing polysilane.

3. The method of claim 2, d a d u r c h g e k e n n z e i c h n e t that the thickness of the adhering layer by adjusting the viscosity of the solution or melt, the speed at which the substrate is pulled out of the solution or melt and the temperature is controlled.

4. The method according to any one of the preceding claims, that the use of a polysilane or polycarbosilane as the carbon-containing polysilane.

5. The method according to claim 4, characterized in that to achieve a doped SiC layer of the solution or melt, the doping element in the form of a compound of the doping element together with one Silane compound or as polysilane containing the doping element in the molecule.

6. The method according to claim 5, d a d u r c h g e k e n n z e i c h n e t that hydroborated polyvinylsilane is added to obtain a boron-doped SiC layer.

7. The method of claim 5, d a d u r c h g e k e n n z e i c h n e t that a polysilazane is added to produce an N-doped SiC layer.

8. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that one carries out the pyrolysis at a temperature of 900 ° C to 1100 ° C.

9. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that one carries out the crystallization at a temperature of about 1100 ° C to 1600 ° C.

10. Single-crystalline epitaxial SiC layer, characterized in that it is obtainable by coating a substrate to be coated with at least one carbon-containing polysilane, pyrolyzing the adhering layer at about 200 ° C. to 1100 ° C. under protective gas and crystallizing the ceramic layer of amorphous SiC thus obtained by holding at a temperature of over 700 ° C.

11. Solid body which is coated with at least one layer of monocrystalline SiC, characterized in that that it is obtainable by applying at least one carbon-containing polysilane to at least part of the surface of the solid, pyrolyzing the adhering layer at about 200 ° C. to 1100 ° C. under protective gas and crystallizing the ceramic layer thus obtained from amorphous SiC by holding at a temperature of over 700 ° C.