Abstract:
A method of controlling the partial pressure of at least one substance or substance mixture comprising arranging the substance or mixture of substances in a chamber, arranging at least one element containing the substance or mixture of substances in the chamber and selecting the structure or crystal structure of the element to provide the desired partial pressure in the chamber.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method of controlling the partial pressure of at least one substance or mixture of substances in simultaneously liquid and solid phase. 
     The invention relates in particular to so called liquid phase epitaxy techniques which serve to produce semiconductor components. 
     In one such method, a so called epitaxy boat has a chamber wherein for example, there is a depression, which contains a subtance or mixture of substances in the form of a so called growth substrate. The growth substrate is covered by a displaceable lid during a heating phase necessary during manufacture, the lid having a so called epitaxial growth melt in a recess. After the heating phase the lid 4 is displaced such that the epitaxial growth melt is located above the growth substrate so that a doped semiconductor layer can grow for example. 
     This method has the disadvantage that a proportion of the substance or mixture of substances is vapourised during the heating phase such that the growth substrate is provided with a flawed surface. This thermal damage may produce faulty semiconductor components. 
     In order to avoid this thermal damage it was proposed that a recess should be arranged in the lid this recess accomodating a body for example a so called covering substrate which has a similar chemical composition to the epitaxial growth substrate. Thermal damage to the surface always recurs in a method of this type and makes it impossible to produce reliable semiconductor components for example. 
     SUMMARY OF THE INVENTION / 
     It is an object of the invention to provide a method of controlling the partial pressure of a substance or mixture of substances which facilitates simple reliable and very accurate control such that more particularly a substance or mixture of substances in solid phase, is not subject to damage at its surface due to heat when there is thermal loading during manufacture. 
     According to a first aspect of the invention there is provided a method of controlling the partial pressure of at least one substance or mixture of substances comprising arranging said substance or mixture of substances in a chamber, arranging at least one element containing said substance or mixture of substances in said chamber and selecting its structure or crystal structure to provide the desired partial pressure in said chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail by way of example, with reference to the drawing, the single FIGURE of which shows, schematically, one form of apparatus for use with the method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawing shows an apparatus used in the previously described known method. Thus the drawing shows a so called epitaxy boat 1 with a chamber 2, for example a depression, which contains a substance or mixture of substances in the form of a so called growth substrate 3. The growth substrate 3 is covered by a displaceable lid 4 during a heating phase necessary during manufacture, the lid having a so called epitaxial growth melt 5 in a recess. After the heating phase the lid 4 is displaced such that the epitaxial growth melt 5 is located above the growth substrate 3 so that a doped semiconductor layer can grow for example. 
     This method has the disadvantage that a proportion of the substance or mixture of substances is vaporized during the heating phase such that the growth substrate 3 is provided with a flawed surface 6. This thermal damage produces faulty semiconductor components. 
     As mentioned previously, in order to avoid this thermal damage it was proposed that a recess should be arranged in the lid 4 said recess accomodating a body 7, for example a so called covering substrate which has a similar chemical composition to the epitaxial growth substrate. Thermal damage to the surface 6, which surface defines a phase boundary between substrate 3 and a gaseous medium in space 9, always recurs in a method of this type and makes it impossible to produce reliable semiconductor components for example. 
     The present invention is based on the knowledge that the partial pressure of the substance or mixture of substances does not only depend on the chemical composition of the body or its surroundings but to a very large extent on the structure and/or crystal structure of the body. Surprisingly it has been shown that with the same physical conditions a crystal formed from a substance or mixture of substances has a different partial pressure of the substance or mixture of substances at differently oriented crystal surfaces, for example (110)-surface or (111)-surface respectively. This effect is utilized for the method in accordance with the invention of which one embodiment is described hereafter. 
     An InP crystal, which forms the growth substrate 3, is used as the mixture of substances and the surface 6 is formed for example by an (100)-crystal surface. An InP crystal is also used for the body 7, the covering substrate and, is oriented such that its surface 8 which faces the surface 6 is formed by a (111)-B crystal surface. The two crystal surfaces are separated from each other spacially by means of a spacing 9. During the heating phase mentioned at the outset, which causes thermal loading, the volatile P(phosphorus) component of the InP only vaporized from the surface 8 since because of its crystal orientation it brings about a higher partial pressure of phosphorus at the temperatures used than is the case for the surface 6 which is to be protected. Taking further easily set conditions into account, for example size of the spacing 9, and the material and shape of the epitaxy boat 1, the partial pressure of the phosphorus is controlled during the heating phase such that thermal damage to the surface 6 is avoided. 
     It is an advantage of the above described embodiment that such a good quality of the surface of individual semiconductor layers is provided when manufacturing so called Group III-V semiconductor components such as InP, GaAs and GaP that subsequent processing steps designed to smooth away unevenness, for example by etching and/or initially dissolving the semiconductor layer, are superfluous. 
     The invention is not restricted to the example of embodiment described but can be used for any crystallizing substances or mixtures of substances in which differently oriented crystal surfaces cause a different partial pressure respectively of at least one chemical component of the substance or mixture of substances. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations.