CA1333876C - Method for forming heteroepitaxial structures, and articles produced thereby - Google Patents

Abstract

A rapid thermal anneal (RTA) can substantially improve the quality of heteroepitaxial structures such as dielectrics on semiconductors (e.g., CaF2/Si), semiconductors on metal on semiconductor (e.g., Si/CoSi2/Si), semiconductor on semiconductor (e.g., SixGe1-x/Si), or semiconductor on insulator (e.g., Si/A12O3). The RTA involves heating a heterostructure, comprising a single crystal first material substrate with a layer of second material thereon, to an absolute temperature Ta2 for a time ta2, where typically 0.75 Tm2 < Ta2 < Tm2 and 1 < ta2 < 60 seconds, Tm2 being the absolute melting temperature of the second material.
The method can produce substantial improvement in such indicators of crystalline perfections as RBS ratio ?min or carrier mobility µ.

Description

~ETHOD FOR FOR~ING HETERO~PITAXI~L STRUCTURES, AND ~RTICLES PRO~UCED THEREBY

Field of the Invention This invention pertains to formation or improvement of single crystal heteroepitaxial structures by a method comprising solid-state rapid thermal annealing, and to articles, e.g., semiconductor devices, produced thereby.
Background of the Invention From the beginning of semiconductor device technology, scientists have discussed a host of heteroepitaxial structures, i.e., combinations of layers of different materials ln a single composite crystal.
The fascination in this kind of structure is traceable to the wide selection of electrical properties thus potentially available to the device designers. The early visions of a wide variety of such structures have not yet been realized, however, and it is now recognized that heteroepitaxial structures with perfection suitable for state of the art devices are in general extremely difficult to produce.
Some types of heteroepitaxial semiconductor structures, e.g., combinations of III-V compound semiconductors of varying compositions, are well known to the art and are widely used ir )ptical and electronic devices. Some heterostructures comprising a semiconductor substrate and a metal layer deposited thereon are also known to the art. For instance, U.5.
Patent 4,492,971 discloses silicon/metal silicide heterostructures. The patent teaches that such structures can, for instance, be formed by co-deposition of metal and silicon onto the single crystal silicon substrate, with the substrate maintained at a temperature that favors formation of the stoichiometric metal silicide. U.S. Patent 4,A77,308 discloses a method for forming an epitaxial layer of multiconstituent material on a substrate, the method comprising deposition of a very thin template layer onto the substrate material, followed by deposition of further material at a higher substrate temperature. And co-assigned U.S. patent application Serial No. 820,581 filed January 17, 1986 in the name of J. C. Hensel et al. teaches, inter alia, that a template method can also be used to form highly perfect epitaxial Si on metal silicide.
The above referred to references teach formation of heteroepitaxial structures by adjusting the conditions such that the deposition of overlayer material directly results in growth of the epitaxial and highly perfect overlayer. Although this approach is successful in a number of material combinations, e.g., silicon/metal silicide, there exist many material combinations for which this approach either does not work or is considered not likely to work.
The prior art also teaches that some heteroepitaxial structures can be formed by methods comprising transient melting of material. For instance, H. Ishiwara et al, Material Research Society Symposium, Proceedin~s, Volume 1, pp. 525-531 (1981), disclose that a heteroepitaxial Si/cosi2/si structure can be formed by depositing a layer of Co onto a Si substrate, heating the substrate to form the cobalt silicide, depositing a layer of amorphous Si atop the silicide, and melting the amorphous Si by means of a high powered laser.
Formation of a heteroepitaxial structure by means of transient melting of the top layer is also disclosed in co-assigned U.S. Patent 4,555,301 issued on November 26, 1985 to J. ~. Gibson et al.

Various means for heating wafer-like samples during processing are known to the art. For instance, D. J. Lischner et al, Materials Research Society Symposium, Proceedings, Volume ~, pp. 759-764 (1982), disclose annealing of ion-implanted Si by means of a battery of tungsten halogen lamps, J. C. C. Fan et al, (ibid, pp. 751-758), disclose transient heating of wafer-shaped materials (e.g., Si) by means of a graphite strip heater, and C. Hill, Materials Research Society Symposium, Proceedings, Volume 13 (1983), pp. 381-392, discusses beam processing in semiconductor technology.
As demonstrated by the above discussed references, the prior art knows techniques that can be used to produce some heteroepitaxial structures. Chief among the possible combinations are metal silicides on Si. These prior art techniques typically involve either co-deposition of the metal and silicon under closely specified conditions, reaction of the deposited metal with the substrate, or melting of the deposited material. Some of these techniques can also be used with material combinations other than Si/metal silicide.
For instance, Ishiwara et al, Applied Physics Letters, Volume ~0, pp. 66-68 (1982), found a relatively narrow substrate temperature range in which good quality epitaxial CaF2 could be grown on Si(100). Deviations of as little as 25C from the optimum temperature during growth seriously degraded the quality of the CaF2 crystal as measured by Rutherford backscattering spectroscopy (RBS).
In view of the fact that many material combinations, including combinations comprising one or more semiconductors, metals and/or insulators, are of potential interest in technology, especially in semiconductor device technology, a broadly applicable processing technique that can result in the transformation of lower quality deposited material into higher quality material that is epitaxial with its - 4 ~ 1 ~ 3 3 8 7 6 substrate would be of much interest. This application discloses such a technique.
Glossary A crystalline layer atop a single crystal substrate herein is considered to be "epitaxial" with the substrate if the overlayer is a single crystal and the crystalline orientation of the overlayer is determined by the crystalline orientation of the substrate.
A layer of crystalline material herein is considered to be "polycrystalline" if Xmin (i.e., the room temperature ratio of RBS in an appropriate low-index lattice direction (typically chosen normal to the layer) to that in a random lattice direction) is greater than at least about 20-~-, for a 1-2 ~eV He probe beam of about 1 mm2 cross section. In amorphous material xmin is of course 100%.
A crystalline layer is considered herein to be ~single crystalline" if its Xmin, determined with a similar probe beam, is less than about 20%, preferably less than 10 or even 5%, along two crystalline directions.
The Xmin of crystals known to be essentially perfect is found to be about 3%, the exact value depending on the material. ~or Xmin between about 3 and about 20%, xmin is a generally accepted measure of crystal perfection. However, in epitaxial films that are found to have Xmin ~ 3% and that are thus considered to be essentially perfect according to the Xmin criterion, possible differences of crystalline perfection may be established by other means. One such other measure of crystalline perfection is the observed room temperature carrier mobility (~) in the material.
This measure can be used, for instance, for heteroepitaxial semiconductors, e.g., SixGel_x/Si.

13338~S

Herein the relevant pre-RTA (Rapid _hermal _nneal) measure of second material crystal perfection is identified by the subscript i (e.g., xmjn ;, ,u;), and the post-RTA
measure by the subscript f (e.g., xmjnp ~ld- The "normalized change" in the appropriate measure of second material crystal perfection herein is designated ~xmjn, ~,u, etc., where ~Xmin = (Xmin,i - xm;n,r)/ xmjn ;~ and ~,u = (~r - ~ Lt An epitaxial layer of material astop a single crystal substrate is herein considered to form a "heteroepitaxial" structure if the substrate (the "first material") substantially differs from the overlayer (the "second material") in chemical composition.
In particular, at least one chemical element that is a major constituent of the first 10 (second) material is not a major constituent of the second (first) material. For purposes of this definition a constituent is a "major" constituent if it makes up at least 5% by weight of the material.
Summary of the Invention In accordance with one aspect of the invention there is provided method 15 for fabricating an article comprising a composite structure of a single crystal first material substrate and a layer of a crystalline second material overlying the substrate, the lattice constants of the first and second materials differing by less than ten per cent, the second material layer being epitaxial with the substrate CHARACTERIZED BY a) providing a composite structure of a first material single crystal substrate and a crystalline layer of 20 second material on the substrate, the second material having a RBS ratio xmin i and a predetermined chemical composition, the chemical composition of the second material differing from that of the first material, the first material chosen from the group consisting of the semiconductors, metal silicides and Al2O3, and the second material chosen from the group consisting of the semiconductors and the metal silicides; and b) heating at least the second material to an annealing temperature Ta2 for a time ta2, with 0.75 Tm2 <Ta2<Tm2, and 1<ta2<60 seconds, where Tm2 is the second material absolute melting temperature, and with Ta2 and ta2 chosen such that, after completion of the heating, the second material still has essentially the predetermined chemical composition and (i) the second material has Xminr such that ~Xmjn>O.1, where l~xmin = (xmin~i~xmin~d/xmin~i or (ii) if prior to the heating step a room temperature carrier mobility ~; is associated with the composite structure, then the heating is conducted such that a room temperature carrier mobility 1l, is associated with the composite structure, such that ~,u>0.1, where /~ li)/,Ub or (iii) both.

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Brief Description of the Drawings FIGS. 1 and 2 schematically depict exemplary heterostructures;
FIG. 3 schematically depicts apparatus for rapid thermal annealing;
FIG. 4 shows xmjn of a CaF2 layer on Si, prepared according to a prior art method and according to the invention, respectively;
FIG. 5 shows xmjn of CaF2/Si(100) structures as a function of anneal time; and FIGS. 6 and 7 show xmjn of CaF2Si(100) structures as a function of annealing temperature.
The Invention We have discovered that the crystalline perfection (as expressed by xmjn, ~, or in other appropriate ways) of a layer of material (the "second" material) deposited onto an appropriate single crystal substrate (the "first" material) can, under appropriate - 6 ~ 133~87~

conditions, be substantially improved by means of a rapid thermal anneal (RTA). Typically, the inventive method produces a normalized change in the appropriate measure of crystal quality of at least about 0.1. Thus, ~ Xmin ~ 0.1, or ~ ~ > 0.1. For instance, the Xmin i f as deposited second material frequently is greater than 10%, even 20 or 50%, and the inventive method can produce therefrom high quality epitaxial material having Xmin f of less than 10%, even 5%.
The method is currently believed to have wide applicability and may, for instance, permit (when applied sequentially) the fabrication of heteroepitaxial structures comprising any desired number of layers of appropriately chosen single crystal material. In such a case, the last layer to be transformed into single crystal material forms the substrate for the next layer to be formed, and the term "substrate" herein is to be understood in this broader sense. Exemplary material combinations to which the invention method can advantageously be applied are CaF2 on Si (CaF2/Si), Si/CaF2/Si, ~aF2/metal silicide/Si, SixGe(l x)/Si, with 0 < x < 1, BaF2/Ge~ and Si/A12O3. Typically, the first and second material lattice constants ~ust differ by less than 10% for epitaxy to be possible.
The inventive method typically comprises preparation of a major surface of a single crystal substrate, typically a semiconductor wafer that is cut substantially parallel to a low-index (e.g., within about + 5 of a (100), (111), or (1102)) lattice plane, by known methods, such as to provide a substantially atomically clean and undamaged substrate surface. See, for instance, A. Ishizaka et al, Proceedings, Second International Symposium on Molecular Beam Epitaxy, Tokyo, 1982, pp. 183-186. Onto the thus prepared first material surface all the material needed to yield the desired layer thickness of second material is deposited.
In case the second material is a multiconstituent material, deposition is often advantageously co-deposition. During deposition the substrate temperature can be raised to any desired level, so long as the materials neither melt nor chemically react. However, it is currently considered to be a significant feature of the instant invention that the substrate can be maintained at a relatively low temperature during deposition, typically below 0.5 Tm2, where Tm2 is the absolute melting temperature of the second material, and even below 300 or 200C.
The thus deposited second material, which has the same overall chemical composition as the desired single crystal epitaxial second material but which is typically polycrystalline or even amorphous, is then subjected to a RTA by any appropriate means. The RTA
involves heating at least the deposited second material to an annealing temperature Ta2, and maintaining the deposited second material at the annealing temperature for an annealing time ta2.
By the "annealing temperature" we mean herein a temperature that permits atomic rearrangement of the deposited second material, typically a temperature greater than about 0.75 Tm2 but less than Tm2, preferably between 0.8 and 0.95 Tm2. By an "annealing time" we mean herein a time sufficient to result in transformation of the deposited second material into single crystal second material at the annealing temperature. For instance, for CaF2 on Si, ta2 typically is in the range 1-60, or frequently 5-40 seconds, and these ranges we consider to be broadly applicable.
A further condition on Ta2 and ta2 is the requirement that the chemical composition of the resulting second material layer be substantially identical to that of the as deposited second material.
This obviously requires that substantially no mass transfer across, or chemical reaction at, - 8 ~ 1333876 substrate/deposit interface occurs, and sets upper limits on the allowable combinations of Ta2 and ta2. A
further requirement is that Ta2 be less than the melting temperature of the first material, as well as of any other material in a multi-material heterostructure.
FIG. 1 schematically depicts the simplest possible epitaxial heterostructure, namely, a first material single crystal substrate 10 and a second material single crystal overlayer 11 that is epitaxial with 10. For instance, 10 could be a Si wafer, and 11 a semiconductor such as Ge, an insulator such as CaF2, or a metal such as CoSi2. FIG. 2 schematically shows a slightly more complicated epitaxial heterostructure comprising single crystal material 10 (e.g., Si), epitaxial single crystal material 11 (e.g., CaF2) thereon, and epitaxial single crystal material 20 te.g., Si) atop 11. It will be recalled that herein 10 is considered to be the substrate for purposes of formation of 11, whereas 11 is considered to be the substrate for purposes of formation of 20. The scheme is to be extended in the case of more complicated epitaxial heterostructures. In electronic devices according to the invention means for making electrical contact to appropriate layers of the heterostructure would of course be provided. Such means are well known and are not shown.
The inventive method comprises a RTA, carried out by any appropriate means. Such means are well known in the art. See, for instance, D. J. Lischner et al (op. cit.), and J. C. C. Fan et al (op. cit.).
Exem~lary apparatus for carrying out a RTA according to the invention is schematically shown in FIG. 3, wherein sample 30 (e.g., a Si wafer with a layer of composition Si 8Ge 2 thereon) is supported by transparent (e.g., fused silica) sample holder 31 having sample supporting posts 32. The sample holder is attached to cover plate 33 which can form an airtight seal with transparent chamber 35 by means of O-ring 34. Means 37 are provided for attaching the apparatus to a vacuum system. Not shown are other well known means that may advantageously be present in a RTA apparatus, e.g., resistive heater means, temperature measuring means (e.g., thermocouples), ~eans for admitting inert gas to the chamber, and means for depositing second material onto the substrate (e.g., e-beam, Knudsen, or other evaporators, or sputtering means). Radiant energy sources 36 are shown placed outside of 35, and such placement is currently preferred for high intensity lamps, e.g., tungsten halogen lamps. However, other heat sources (e.g., a graphite wire heater) could be advantageously placed inside the chamber. In order to achieve substantially uniform heating of 30 during RTA
it is desirable that the radiant flux from the sources 36 be substantially homogeneous. This can be achieved, for instance, by interposing a diffusing surface between the lamps and the sample, e.g., by sandblasting or otherwise roughening the outside of 35, or by providing a large area heat source.
Particularly advantageous apparatus for practicing the invention comprises means for final in situ preparation of the initial substrate (e.g., by means of oxidation, followed by heating in UHV to remove the oxide, or by sputtering), means for in situ forming the second material layer atop the initial substrate, and, if desired, means for forming in situ one or more further layers of material. With such apparatus it is in principle possible to form heterostructures comprising any desired number of layers without exposing the incomplete heterostructure to a contaminating atmosphere, and this is currently considered by us to be a preferred approach. In particular, we believe that it is frequently advantageous to carry out the deposition(s) and the RTA(s) under conditions such that the 2 partial pressure is at most about 10-2 Pa, lO 133387G

preferably less than 10 3 Pa.
The remainder of the discussion will largely be in terms of Si/CaF2 heterostructures. This is done for the sake of concreteness only.
As is known to the prior art, single crystal epitaxial CaF2 can be formed on Si by vapor deposition of CaF2 onto a (100) oriented Si substrate, provided the substrate is maintained at about 600C. FIG. 4 provides a comparison between results obtained by means of the prior art technique and the inventive method, respectively. Presented is the dependence of Xmin on the substrate temperature during deposition of CaF2, with curve 40 being Xmin,i and 41 being Xmin,f~ with the deposition having been followed by a RTA at about 1080C
for about 20 seconds. Since the melting temperature of CaF2 is about 1360C, T~2 in this case is 1633K, and Ta2 therefore was about 0.83 Tm2.
As is evident from FIG. 4, the inventive method can produce high quality single crystal CaF2 over a wide range of deposition temperatures, including very low ones, whereas deposition without RTA (i.e., essentially the prior art technique) produces single crystal CaF2 only over a relatively narrow temperature range (e.g., 600 + 25, -50C). We currently believe that the inventive method can produce single crystal material even from amorphous deposited material, and consequently we believe that there is generally substantially no lower limit on the deposition temperature. This is considered to be a significant aspect of the instant invention. For instance, deposition onto a cold substrate can avoid the island formation frequently observed when it is attempted to form very thin layers of second material on a hot substrate.
FIG. 5 shows exemplary data on the dependence of Xmin on ta2 in CaF2/Si (100) heterostructures, for a deposition temperature of 300C, and Ta2 of 1080C- As ll 1333876 can be seen, under these conditions Xmin < 20% results for 10 < ta2 < 47 seconds, with lowest Xmin resulting for 20 < ta2 < 30 seconds. As will be appreciated, the appropriate annealing time depends not only on the second material and possibly also on the first material, but is also a strong function of the annealing temperature. No universally valid limits can thus be determined, but we believe that typically ta2 will be in the range 1-60 seconds, with ta2 decreasing as Ta2 increases.
FIG. 6 shows exemplary data on the dependence of Xmin on Ta2 in the CaF2 Si(100) system. The anneal time was 20 seconds, all samples measured came from the same wafer, the wafer was maintained at about 580C
during CaF2 deposition. As FIG. 6 shows, significant improvement in crystal quality of the overlayer occurred when the RTA carried out at temperatures above about 970C. It will, however, be apparent that the use of a shorter (longer) annealing time would have shifted the curve to slightly higher (lower) temperatures.
FIG. 7 gives further exemplary data on the dependence f Xmin on the anneal temperature, and shows that for conditions substantially as used to obtain the data of FIG. 6 (except that ta2 = 7 seconds), the crystalline perfection of CaF2 in CaF2/Si(100) precipitously decreases for annealing temperatures above about 1150C. FIG. 7 thus shows the existence of an upper limit in Ta2. It will undoubtedly be appreciated by those skilled in the art that the exact value of the upper limit depends on the second material, ta2, and possibly also on the first material. Typically, a2 < 0.95 Tm2, and frequently T 2 < 0.9 T
For some material combinations, typically when the differential thermal expansion of substrate and overlayer exceeds about 1~ between room temperature and Ta2, cracking or buckling of the overlayer may occur during cool-down after the RTA. This can often be prevented by a thermal soak after RTA, i.e., by maintaining the sample at a soak temperature Ts2 ( Tm2 < Ts2 < 0-75 Tm2) for a time tS2 ~1 < tS2 < 60 seconds).
Example I
A layer of CaF2 on Si(100) was grown as follows: a 7.6 cm (3 inch) wafer of Si was prepared by a standard method (see Ishizaka et al, op. cit.) to be atomically clean. The final preparatory step involved placing the wafer, with a thin oxide layer thereon, into a UHV chamber with a base pressure < l~lO~lOTorr (< 1.3-10-8 Pa), and heating the wafer to about 850C to desorb the oxide and leave an atomically clean surface.
The substrate temperature was then lowered to about 580C and a 500 n~ thick fil~ of CaF2 deposited thereon.
Deposition was carried out using a Knudsen effusion cell, with the CaF2 maintained in a graphite crucible within the cell. The cell was operated at about 1150C, the pressure in the vacuum chamber during deposition was < 1-10 9Torr (< 1.3-10 7Pa), and the growth rate was about 1 nm/min. Following deposition the wafer was allowed to cool and then transferred to an RT~ furnace of the type shown in FIG. 3. An identically prepared CaF2 film had Xmin The furnace was sealed and flushed with Ar for about 10 minutes, thereafter the furnace power was ramped up at about 15~/second, until the annealing temperature of about 1080C was reached. This temperature was held for about 20 seconds, then the furnace power was decreased at 15%/second. The wafer was maintained in the furnace until the wafer temperature had dropped to about 100C. ~xamination of the wafer by RBS (using a 1.8 ~eV He probe beam of about 1 mm2 cross section) showed Xmin of about 4%, indicative of single crystal material of high perfection.
Transmission electron microscopy revealed that the CaF2/Si interface was smooth and substantially defect free. The chemical composition of the overlayer material remained CaF2, with no evidence of mass transport across the interface, or of reaction with the substrate.
Example II
A Si(100) substrate was prepared substantially as described in Example I, and about 6 nm of Co deposited thereon by electron beam evaporation with the substrate maintained at about room temperature. After completion of the Co deposition, the substrate temperature was raised to about 650C and maintained there for about 10 minutes, resulting in formation of a 20 nm thick layer of epitaxial single crystal CoSi2.
After lowering the substrate temperature to 550C, a 700 nm thick layer of CaF2 was deposited, substantially as described in Example I. The layer was subjected to a RTA at about 1080C for about 20 seconds, also substantially as described. The resulting heterostructure was single crystal throughout, the CoSi2 layer epitaxial with the Si substrate, and the CaF2 (Xmin = 4%) epitaxial with the silicide.
Example III
A 500 nm thick layer of CaF2 ( min about 14%) is deposited on Si(100) substantially as described in Example I, but without Ar flushing. After cooling of the sample, the sample is moved, without breaking of the vacuum, to a RTA furnace, and subjected to a RTA, also substantially as described in Example I, with an annealing temperature of about 1080C, and annealing time of about 20 seconds. Identically prepared CaF2 samples are known to have Xmin ~ 4%. Following the RTA
the sample is returned, without breaking of the vacuum, to the deposition chamber, the sample temperature is raised to about 600C, and a 500 nm thick layer of Si deposited (0.1 nm/second) on the CaF2 substrate by electron beam evaporation. After cooling of the sample, the sample is moved again, without breaking of the 14 1 33 3~ 76 vacuum, to the RTA furnace, and the Si deposit subjected to a RTA (annealing temperature about 1200C, annealing time about 20 seconds) substantially as described alone. The resulting heterostructure is epitaxial single crystal throughout, with the Si layer having xmjn about 4%.
Example IV
The 0.5 nm thick Si layer of a commercially available (Union Carbide, San Diego, California) Si/Al2O3 heterostructure, grown at a substrate temperature of about 500C, has a xmjn of about 20%. Subjecting the sample to a RTA
(Ta2 = 1200C, ta2 = 20 seconds) substantially as described in Example I, results in a heteroepitaxial structure, with the Si having a xmjn of about 5%.
Example V
On a n Si(100) substrate were sequentially deposited by MBE (using multiple guns) a) 470 nm Si (p+, 1 I018 cm~3);
b) 25 nm of undoped Si;
c) 50 nm of undoped Ge 2S i 8 alloy;
d) 25 nm of undoped Si; and e) 75 nm of Si(p+, 1 1018 cm~3).
All layers were deposited at a substrate temperature of 500C, in a vacuum of at least about 10-7 Pa. Further details of the procedure can be found in U.S. Patent No. 4,529,455 which issued on July 16, 1985 to J.C. Bean et al.
The thus produced heterostructure had essentially ideal xmjn (3%) throughout, the hole mobility in the alloy at 60K was about 140 cm2/volt sec. After a RTA (Ta2 = 1080C, ta2 = 11 seconds) substantially as described in Example 1, the hole mobility was 210 cm2/volt sec, with xmjn remaining about 3%.

Claims (9)

1. Method for fabricating an article comprising a composite structure of a single crystal first material substrate and a layer of a crystalline second material overlying the substrate, the lattice constants of the first and second materialsdiffering by less than ten per cent, the second material layer being epitaxial with the substrate CHARACTERIZED BY
a) providing a composite structure of a first material single crystal substrate and a crystalline layer of second material on the substrate, the second material having a RBS ratio ?min,i and a predetermined chemical composition, thechemical composition of the second material differing from that of the first material, the first material chosen from the group consisting of the semiconductors, metalsilicides and A12O3, and the second material chosen from the group consisting ofthe semiconductors and the metal silicides; and b) heating at least the second material to an annealing temperature Ta2 for a time ta2, with 0.75 Tm2 <Ta2 <Tm2, and 1?ta2?60 seconds, where Tm2 is the second material absolute melting temperature, and with Ta2 and ta2 chosen such that, after completion of the heating, the second material still has essentially the predetermined chemical composition and (i) the second material has ?min,f such that .DELTA.?min?0.1, where .DELTA.?min = (?min,i-?min,f)/?min,i or (ii) if prior to the heating step a room temperature carrier mobility µi is associated with the composite structure, then the heating is conducted such that a room temperature carrier mobility µf is associated with the composite structure, such that .DELTA.µ>0.1, where .DELTA.µ = (µf-µi)/µf, or (iii) both.

2. Method according to claim 1, CHARACTERIZED IN THAT
at least step b) is carried out in an atmosphere having an O2 partial pressure less than about 1x10-2 Pa.

3. Method according to claim 1, CHARACTERIZED IN THAT
?min,i > 10% and ?min,f?5%.

4. Method according to claim 1, CHARACTERIZED IN THAT
the single crystal first material substrate consists of a layer (11) of said first material formed on a single crystal substrate (10) of a third material, the third material composition differing from at least the first material composition, said first material having ?min < 10%, the crystalline layer (20) of second material being formed on said layer of first material.

5. Method according to claim 4, CHARACTERIZED IN THAT
the formation of the layer of first material comprises depositing first material on the third material substrate and heating the deposited first material to an annealing temperature Tal for a time tal, with Tal and tal chosen such that the first material with ?min<10% is produced.

6. Method according to claim 1, CHARACTERIZED IN THAT
the first material is A12O3 and the second material is Si.

7. Method according to claim 6, CHARACTERIZED IN THAT
Ta2 is in the range from about 1250°C to about 1410°C.

8. Method according to claim 1, CHARACTERIZED IN THAT
the first material consists substantially of Si, and the second material consists substantially of material of composition SixGe1-x, 0<x<1.

9. Method according to claim 1, CHARACTERIZED IN THAT
after completion of step b), the temperature of the composite structure is reduced to a soak temperature Ts2, where 0.5 Tm2 < Ts2 < 0.75 Tm2, and the composite structure maintained at the soak temperature for a time ts2, where 1 < ts2 < 60 seconds.