LIQUID PHASE EPITAXY

Abstract: Liquid phase epitaxially grown crystals are prepared in a tilt tube apparatus pivotable about a pivoting arrangement (21). The apparatus comprises a silica crucible (10) within a silica reaction tube (11) having a palladium diffused hydrogen atmosphere which is heated by electric furnace (12). The crucible (10) is in contact with a silica rod (13) which is connected through a flexible coupling (15) to a transducer (14) which draws power from a power oscillator (17). Vibrations are thereby induced in the crucible (10) during preparation of a crystal.

Description: LIQUID PHASE EPITAXY Technical Field The present invention relates to a method of producing and an apparatus for the production of liquid phase epitaxially grown crystals. The requirement for such materia has increased in line with growth in the electronic industry. Background of the Invention Crystalline material may be produced by a number of well established techniques including Czochralski pulling, a technique in which a crystal is grown at high pressure and rotated slowly to encourage uniform growth as it is pulled from a melt, and to a lesser extent the horizontal Bridgman techniques. However, epitaxial growth methods have emerged a more pure material has been required. Molecular beam epita (MBE) is one such technique, but is restricted in that only epitaxial thicknesses in the order of 10 x 10 ~ m may be produced. Liquid phase epitaxy (LPE) was developed to produce layers of greater thickness. The LPE technique is based on the tilt-tube furnace first described by Nelson (H. Nelson, RCA Review 24, 603 (1963)), and later adapted by others, particularly Hicks et al (H.G.B. Hicks and D.F. Manley, Soli State Communications, Vol.7, pp.1463-1465 (1969)), and is successful in producing epitaxial layers that are substantially thicker than those produced by MBE. LPE involves a liquidous-solidus reaction in which a single crystal epitaxial layer is grown on a single crystal substrate. The resultant epitaxial layer mimics the crystallographic arrangement of the substrate lattice. Two characteristics must be considered when assessing t quality of a crystal, namely the integrity of the crystallographic morphology and the level of impurities in t crystal. Generally, LPE utilizes substrates of high morphologic integrity as the base for the production of crystals' of enhanced purity. Alternatively however, the technique may b used to produce material that has the same crystallographic structure, but differing chemical and/or physical properties to that of the substrate material. Such would be the case when LPE is used in the production of "doped" semiconductor materials, where an impurity is deliberately introduced into a crystal to change its electrical properties from that of an ideal intrinsic material to that with either an excess of conduction electrons or electron vacancies (holes) in the valence band. The established LPE technique utilizes an apparatus, a 0 component of which is a crucible, in which is secured the substrate upon which the epitaxial layer is to be grown. The crucible also contains a mixture of two components (hereinafter referred to as the melt), namely the material to be grown on the substrate (hereinafter referred to as the 5 source material) and a solvent. The apparatus also includes a means of heating the mixture and isolating it from the substrate until interaction is required. Upon continued application of heat, the source material will dissolve in the solvent at a temperature which is 0 dependent upon the materials involved. When the solvent is nearly saturated with source material, the melt is introduced to the substrate such that it covers the exposed surface of the substrate. The substrate surface partially dissolves until a solution equilibrium is established. Then upon 25 cooling, the source material will begin to precipitate from the melt and epitaxial growth upon the substrate commences. The partial substrate dissolution (melt back) which occurs prior to the commencement of epitaxial growth is an important feature of the LPE technique as it is the solution 5.0 fjront that is formed at that time which coincides with the crystallographic arrangement of the substrate and allows for j. the subsequent mimicry of the substrate morphology. This established LPE technique, however, is inherently limited by the quality of the material that it is capable of 35 producing. Poor quality results from a combination of imperfect melt back of the substrate, resulting in inexact mimicry of the substrate morphology and the presence of imperfections in the crystalline structure. Such imperfections include non-uniform doping profiles and the incorporation of solvent inclusions in the growing epitaxial layer. The likely cause of the poor quality crystals has bee identified as a combination of thermal convection currents an constitutional supercooling. Summary of the Invention As an advance over the established LPE technique, the present invention provides a method of producing liquid phase epitaxially grown crystals comprising the steps of dissolving a source material into a solvent to form a melt, introducing the melt to a substrate upon which the epitaxial layer is to be grown, agitating the melt during partial substrate dissolution which results from introduction of the melt to th substrate, and further agitating the melt during deposition o an epitaxial layer on the substrate which results from allowing the melt to cool. It is a preferred feature of the above method that the melt is also agitated during dissolution of the source material into the solvent prior to the introduction of the melt to the substrate. The present method may be particularly useful in preparing semiconductor materials, particularly compound semiconductor materials, though it is equally applicable to the preparation of single element material such as silicon or germanium. Furthermore, the present invention is also equall applicable to the preparation of both doped and undoped semiconductor materials. Compound semiconductors are semiconductors made of a compound of two or more elements. Such semiconductors are commonly III-V semiconductors made from a compound of elemen from group III of the periodic table (such as aluminium, gallium, and indium) and group V of the periodic table (such as nitrogen, phosphorus, arsenic and antimony). Examples of compound III-V semiconductors include binary compounds such gallium arsenide (GaAs), ternaries such as aluminium gallium arsenide (AlGaAs), and quaternaries such as indium gallium arsenide phosphide (InGaAsP). According to a second aspect of the present invention, there is provided an apparatus for the preparation of liquid phase epitaxially grown crystals, the apparatus comprising a controlled atmosphere reaction vessel, separation means for isolating- a melt from a substrate in the reaction vessel, heating means for causing the dissolution of a source material in a solvent to form the melt, transfer means for introducing the melt to the substrate so that partial substrate dissolution occurs and during cooling deposition of epitaxially grown crystals takes place and agitation means for agitating the melt during partial substrate dissolution and the deposition of the epitaxial layer onto the substrate. The reaction vessel preferably includes a crucible within which heat induced dissolution of the source material within the solvent occurs. The crucible is preferably made from materials selected from high purity silica, graphite, boron nitride and alumina as all are resistant to breakdown at elevated temperatures and do not contribute impurities to the melt contained within the crucible. The substrate also is preferably located within the crucible, in which case, both the separation means for isolating the melt from the substrate and the transfer means for intrσducing the melt to the substrate may be provided by means to mount the reaction vessel on a pivotal arrangement which facilitates movement of the reaction vessel from a first position to a second position. In the first position, the crucible is inclined such that the substrate is elevated relative to the melt and conversely in the second position the crucible is inclined such that the melt is initially elevated relative to the substrate and subsequently covers the substrate. Separation means and transfer means are thus facilitated when the reaction vessel is configured in the first and second position respectively. Alternatively however, the substrate may be located elsewhere in the reaction vessel. The heating means to facilitate dissolution of the source material within the solvent is preferably provided by a furnace and the controlled atmosphere within the reaction vessel is preferably provided by a flow of Pd diffused hydrogen. The agitation means for agitating a melt during substrat dissolution and the deposition of the epitaxial layer onto th substrate is preferably provided by means to cause vibration of the crucible containing the melt and substrate. The means to cause vibration preferably comprises a rod or similar vibration conductor having a tip in contact with the crucible and connected to a transducer which in turn is connected to a power oscillator. Preferably, the rod is composed of silica and is connected to the transducer by way of a flexible coupling which seals the silica rod into the reaction vessel (which thus remains gas tight) and minimises dampening of the vibrations induced by the transducer. The transducer is preferably operated with a frequency o 50 to 70 Hz and with a power input such that the crucible displacement is 0.045 to 0.055 mm. Alternatively, the agitation means may not involve physical contact between the crucible and some vibration inducing mechanism, but rather agitation may be achieved by way of electro-magnetically induced eddy currents or similar non-contact agitation means. Brief Description of the Drawings One preferred form of the invention will now be described, by way of example only, with reference to the accompanying drawings. Figure 1 is a schematic representation of an apparatus for the preparation of liquid phase epitaxially grown galliu arsenide (GaAs) wherein the apparatus is configured in a fir position. Figure 2 is a schematic representation of the apparatus of figure 1 when configured in a second position. Figure 3 is an enlarged view of a portion of the apparatus illustrated in figure 2 but at time subsequent to that illustrated in figure 2. Best Mode of Carrying out the Invention The GaAs crystal is grown in a silica crucible 10 contained within a controlled atmosphere silica reaction tube 11. The atmosphere is controlled by a flow of palladium diffused hydrogen (not illustrated) . Heat is supplied to the crucible 10 by way of the electric furnace 12 which partially surrounds the reaction tube 11. The crucible 10 is in physical contact with a silica rod 13 which is connected to a transducer 14 by way of a flexible coupling 15 which seals the silica rod 13 into the reaction tube 11 at its base 16. When utilized, the transducer 14 is operated with a frequency of 60Hz, drawing a power input from the power oscillator 17 such that a crucible displacement of 0.05mm results. The crucible 10 contains a melt 18 consisting of GaAs source material and a gallium solvent together with a GaAs substrate 19 of the <100> orientation which is anchored in the crucible 10 by way of a silica clamp 20. The reaction tube portion of the apparatus is mounted on a pivoting arrangement 21 which facilitates movement of the reaction tube 11 from a first position as illustrated in figure 1 to a second position as illustrated in figure 2. Prior to the growth of a GaAs crystal, the apparatus is arranged with the reaction tube 11 in the first position (figure 1) and the crucible 10 is loaded with both the melt 18 and the substrate 19. Heat is applied to the crucible 10 fro the furnace 12 and vibration of the crucible 10 is commenced by applying power to the transducer 14. As the temperature rises, GaAs particles begin to dissolve in the gallium solven and the resultant melt's homogeneity is maintained by the "stirring" effect of the vibrations transmitted to the crucible. When the temperature of the melt has reached approximately 830°C at which time the gallium solvent will be practically saturated with GaAs, the reaction tube 11 is transferred to the second position (figure 2) by movement of the pivoting arrangement 21. The source of heat is then reduced to provide a controlled rate of temperature decrease. The melt 18 is thus introduced to the substrate 19 such 19 (figure 3) with resulting partial dissolution (melt back) of the substrate surface until a solution equilibrium is established, followed by growth of the GaAs from the gallium onto the substrate surface. Vibration of the crucible 10 is continued throughout this process with the result that the problems of imperfect initial dissolution of the substrate a gallium inclusions in the growing epitaxial layer due to a combination of thermal convection currents and constitutional supercooling are mitigated with resulting production of GaAs crystals of enhanced quality. When the required crystalline growth is completed, vibration of the crucible 10 is discontinued and the reactio tube 11 is returned to the first position as illustrated in figure 1 by movement of the pivoting arrangement 21. The remaining melt 18 is decanted from the surface of the freshl grown GaAs crystal. The resultant crystal may subsequently removed and further processed as desired. The remaining mel can be successfully reused in the growth of further crystals and additional GaAs may be periodically added to the melt to replenish that consumed in epitaxial growth.

Claims: Claims : 1. An apparatus for the preparation of liquid phase epitaxially grown crystals, the apparatus comprising a controlled atmosphere reaction vessel, separation means for isolating a melt from a substrate in the reaction vessel, heating means for causing dissolution of a source material in a solvent to form the melt, transfer means for introducing the melt to the substrate so that partial substrate dissolution occurs and during cooling deposition of epitaxially grown crystals takes place, and agitation means for agitating the melt during partial substrate dissolution and the deposition of the epitaxial layer onto the substrate. 2. An apparatus as claimed in claim 1 further comprising a crucible located within the reaction vessel, within which heat - induced dissolution of the source material within the solvent occurs. 3. An apparatus as claimed in claim 2 wherein the crucible is manufactured from a material selected from the group consisting of high purity silica, graphite, boron nitride and alumina. 4. An apparatus as claimed in claim 2 or claim 3 wherein both the separation means for isolating the melt from the substrate and the transfer means for introducing the melt to the substrate are provided by means to mount the reaction vessel on a pivotal arrangement which facilitates movement of the reaction vessel from a first position in which the crucible is inclined such that the substrate is elevated relative to the melt, to a second position in which the melt is initially elevated relative to the substrate and can subsequently cover the substrate, whereby in addition to the source material and the solvent, the substrate is located within the crucible. 5. An apparatus as claimed in any one of the preceding claims wherein the heating means is an electric furnace. 6. An apparatus as claimed in any one of claims 2-5 wherein the agitation means comprises means to induce vibration of the crucible. 7. An apparatus as claimed in claim 6 wherein the means to induce vibration of the crucible comprises an oscillator connected through a transducer to a vibration conductor havin a tip in contact with the crucible. 8. An apparatus as claimed in claim 7 wherein the vibration conductor is a silica rod which is connected to the transduce by a flexible coupling which seals the silica rod into the reaction vessel. 9. An apparatus as claimed in claim 7 or claim 8 wherein th transducer is adapted to be operated with a frequency of 50-7 Hz and with a power input such that the crucible displacement is from 0.045 - 0.055 mm. 10. An apparatus as claimed in any one of claims 1-5 wherei the agitation means comprises means for inducing eddy current electro-magnetically. 11. An apparatus as claimed in any one of the preceding claims wherein the controlled atmosphere within the reaction vessel comprises palladium diffused hydrogen. 12. A method of producing liquid phase epitaxially grown crystals comprising the steps of dissolving a source materia into a solvent to form a melt, introducing the melt to a substrate upon which the epitaxial layer is to be grown, agitating the melt during partial substrate dissolution whic results from introduction of the melt to the substrate, and further agitating the melt during deposition of an epitaxial layer on the substrate which results from allowing the melt cool. 13. A method as claimed in claim 12 wherein the melt is als agitated during dissolution of the source material into the solvent prior to the introduction of the melt to the substra 14. A method as claimed in claim 12 or claim 13 which utilises an apparatus as claimed in any one of claims 1-11. 15. A method as claimed in any one of claims 12-14 wherein the source material is gallium arsenide, the solvent is gallium, and the substrate is gallium arsenide. 16. A crystal prepared by a method as claimed in any one of claims 12-15.

Inventors: ALEXIEV DIMITRI, TAVENDALE ALISTER JAMES, ALEXIEV, DIMITRI, TAVENDALE, ALISTER, JAMES +DI
Applicants: AUSTRALIAN NUCLEAR SCIENCE TEC, TAVENDALE OLWYN & HF, AUSTRALIAN NUCLEAR SCIENCE & TECHNOLOGY ORGANISATI, TAVENDALE, OLWYN +HF
Application year: 1990
Main classification: C30B19
IPC codes: C30B19