MULTILAYER STRUCTURE

Abstract

The invention provides a multilayer structure comprising at least two monocrystalline layers A and B, wherein layer A comprises ?-Ga.sub.2O.sub.3 and layer B comprises ?-Ga.sub.2O.sub.3 and wherein layers A and B are adjacent

Claims

1. A multilayer structure comprising at least two monocrystalline layers A and B, wherein layer A comprises ?-Ga.sub.2O.sub.3 and layer B comprises ?-Ga.sub.2O.sub.3 and wherein layers A and B are adjacent.

2. The multilayer structure as claimed in claim 1, wherein the thickness of the layers is in the range of 10 nm to 10 ?m.

3. The multilayer structure as claimed in claim 1, wherein said structure comprises three layers in the order BAB.

4. The multilayer structure as claimed in claim 1, wherein each layer is homogenous.

5. The multilayer structure as claimed in claim 1, wherein the interface between the layers is continuous.

6. A method for producing ?-Ga.sub.2O.sub.3, said method comprising the step of irradiating ?-Ga.sub.2O.sub.3 with an ion beam.

7. The method as claimed in claim 6, wherein said ion beam is a medium to heavy ion beam.

8. The method as claimed in claim 6, wherein said ?-Ga.sub.2O.sub.3 is monocrystalline.

9. The method as claimed in claim 6, wherein said ?-Ga.sub.2O.sub.3 forms a layer in a multilayer structure comprising at least two monocrystalline layers A and B, wherein layer A comprises ?-Ga.sub.2O.sub.3 and layer B comprises ?-Ga.sub.2O.sub.3 and wherein layers A and B are adjacent.

10. The method as claimed in claim 6, wherein the method does not require applying external pressures.

11. The method as claimed in claim 6, wherein the irradiation takes place at room temperature.

12. The method as claimed in claim 6, wherein said ion beam has a dosage of 1?10.sup.13 to 1?10.sup.17 ions cm.sup.?2.

13. A semiconductor device comprising a multilayer structure as claimed in claim 1.

14. A multilayer structure prepared by a method which comprises the step of irradiating a ?-Ga.sub.2O.sub.3 substrate with an ion beam capable of inducing a phase transition.

15. The multilayer structure prepared by a method as claimed in claim 14, wherein said ion beam is a medium to heavy ion beam.

16. The multilayer structure prepared by a method as claimed in claim 14, wherein said ion beam has a dosage of 1?10.sup.13 to 1?10.sup.17 ions cm.sup.?2.

17. The multilayer structure prepared by a method as claimed in claim 14, wherein said ion beam comprises nickel, gallium, or gold ions.

18. The multilayer structure prepared by a method as claimed in claim 17, wherein the nickel, gallium or gold ions are selected from .sup.58Ni.sup.+, .sup.69Ga.sup.+ and .sup.197Au.sup.+.

19. The multilayer structure prepared by a method as claimed in claim 14, wherein the ?-Ga.sub.2O.sub.3 substrate is provided in the form of a (010) or (?201) oriented ?-Ga.sub.2O.sub.3 single crystal wafer.

Description

[0030] FIG. 1. (a) ABF-STEM image of ?-Ga.sub.2O.sub.3 sample irradiated with 1?10.sup.16 Ni/cm.sup.2. Panels (b) and (c) illustrate SAED patterns from the unimplanted and implanted regions respectively; (d-f) FFTs from high-resolution images taken from different regions of the sample as indicated in the panel (a).

[0031] FIG. 2. (a) ABF-STEM and (b) HAADF-STEM high-resolution images acquired simultaneously from the maximum ion-concentration region of (010) ?-Ga.sub.2O.sub.3 irradiated with 1?10.sup.16 Ni/cm.sup.2. FFTs extracted from two adjacent grains, indicated as 1 and 2 in panel (a), are shown in the corresponding bottom panels.

[0032] FIG. 3. EELS spectra of the oxygen-K edge, acquired from different areas of the high-dose and low-dose implanted ?-Ga.sub.2O.sub.3, illustrating characteristic intensity changes correlated with ?-to-? transition.

[0033] FIG. 4. (a) RBS/C spectra and (b) XRD 2 theta scans of the (010) oriented ?-Ga.sub.2O.sub.3 crystals implanted with 400 keV .sup.58Ni.sup.+(1?10.sup.15 cm.sup.?2), 500 keV .sup.69Ga.sup.+(1?10.sup.15 cm.sup.?2) and 1.2 MeV .sup.197Au.sup.+ ions (3?10.sup.14 cm.sup.?2).

EXAMPLES

[0034] (010) and (?201) oriented ?-Ga.sub.2O.sub.3 single crystal wafers (Tamura Corp.) were implanted with different ions, specifically .sup.58Ni.sup.+, .sup.69Ga.sup.+, and .sup.197Au.sup.+. The ballistic defect production rates (without accounting for non-linear cascade density effects) for .sup.69Ga.sup.+ and .sup.197Au.sup.+ implants were normalized to that of .sup.58Ni.sup.+ ion implanted with 400 keV in a wide dose range of 6?10.sup.13 to 1?10.sup.16 cm.sup.?2. The implantations were performed at room temperature (if not indicated otherwise) and 7? off the normal direction.

[0035] The samples were analyzed by a combination of Rutherford backscattering spectrometry in channeling mode (RBS/C), x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). The RBS/C was performed using 1.6 MeV He.sup.+ ions incident along [010] direction and 1650 backscattering geometry. The XRD 2 theta measurements were performed using Bruker AXS D8 Discover diffractometer using Cu K.sub.?1 radiation in locked-coupled mode.

[0036] The STEM and EELS investigations were conducted on an FEI Titan G2 60-300 kV at 300 kV with a probe convergence angle of 24 mrad. The simultaneous STEM imaging was conducted with 3 detectors: high-angle annular dark field (HAADF) (collection angles 101.7-200 mrad), annular dark field (ADF) (collection angles 22.4-101.7 mrad) and annular bright field (ABF) (collection angles 8.5-22.4 mrad). The resulting spatial resolution achieved was approximately 0.08 nm. EELS was performed using a Gatan Quantum 965 imaging filter. The energy dispersion was 0.1 eV/channel and the energy resolution measured using the full width at half maximum (FWHM) of the zero-loss peak was 1.1 eV. Electron transparent TEM samples with a cross-sectional wedge geometry were prepared by mechanical polishing with the final thinning performed by Ar ion milling and plasma cleaning.

[0037] Formation of the new phase in ?-Ga.sub.2O.sub.3 due to high dose implantation is supported by STEM investigations and FIG. 1(a-f) summarizes the STEM data for the sample implanted with 1?10.sup.16 Ni/cm.sup.2. Specifically, FIG. 1(a) shows an ABF-STEM image and strain contrast reveals the formation of two distinct regionsthe film and the substrateof the initially homogeneous ?-Ga.sub.2O.sub.3 wafer. Selected area electron diffraction (SAED) patterns taken from the unimplanted and implanted regions, i.e. FIGS. 1(b) and 1(c), illustrate a prominent transformation from monoclinic ?- to ordered orthorhombic ?-phase. This phase transformation extends to ?300 nm from the surface and stops abruptly, forming a sharp interface with the J-phase wafer/substrate, see FIG. 1(a). The contrast associated with defects/strain inside the ?-Ga.sub.2O.sub.3 film gradually increases towards the ?/? interface, see FIG. 1(a). However, fast Fourier transforms (FFTs) from high-resolution images taken at the interfacial area (e) and the upper part (f) of the implanted region show that the ordered orthorhombic phase is retained through the depth of the film as compared with the FFT at the ?-Ga.sub.2O.sub.3 substrate, see FIG. 1(f). Thus, SAED and FFTs show the formation of a single-phase ordered orthorhombic ?-phase region both at meso and nano-scale. The sharp spots, in addition to the lack of extra reflections and/or striking of the main reflections, indicate a highly-oriented crystalline film with no signs of high-angle mis-orientations, high-density of mis-oriented grains or amorphization as also supported by FIG. 2 showing an analysis of the adjacent ?-phase grains exhibiting different strain contrast. Indeed, even though the film in FIG. 1(a) was clearly interpreted as single ?-phase film, there are still remaining questions, in particular related to potential mis-orientation between the grains as well as regarding potential chemical variations. For that reason, we investigated two adjacent grains as seen in FIG. 2. FFTs extracted from two adjacent grains reveal that both grains are stabilized in ?-phase and have the same orientation. The stable contrast in HAADF (pure Z-contrast image) indicates no chemical variations and the contrast in ABF is attributed only to the strain. Thus, the grains are nicely co-oriented and there are no chemical inhomogeneities observed.

[0038] Moreover, the comparison between EELS spectra in FIG. 3 provides additional arguments. Indeed, because of different atomic coordination in ?- and ?-phases we detected characteristic changes in the fine structure of the EELS spectra acquired in STEM-mode, by comparing low and high-dose implanted samples. In particular, the oxygen K-edge is characterized by two main peaks, labelled A and B in FIG. 3, related to the O 2p-Ga 4s and O 2p-Ga 4p bonding, respectively. As seen from FIG. 3, prominent changes in the A/B intensity ratio (I.sub.A/I.sub.B) occur when the phase transition takes place. Specifically, I.sub.A/I.sub.B decreases in the x-phase. This can be attributed either to the increase in O 2p-Ga 4p hybridization or to the transfer of electrons from O 2p-Ga 4p band into another band.

[0039] The next figure demonstrates that the phenomenon of the ?-to-? phase transition is generically related to the accumulation of the lattice disorder and not to the chemical nature of the implanted ions. This was proved by performing control implants with Ga and Au ions. Note that, the ballistic defect production rates for Ga, and Au implants were normalized to that of Ni ion implanted with 400 keV in a wide dose range of 6?10.sup.13-1?10.sup.16 cm.sup.?2. FIG. 4 provides a representative example of the corresponding RBS/C and XRD data. As seen from FIG. 4, both the RBS/C profiles and XRD 2 theta scans exhibit very similar trends for the all ion species used. Specifically, FIG. 4(a) shows the formation of the box-shape disorder layer reaching ?90% of the random signal for the all ions used. In its turn, FIG. 4(b) demonstrates that the virgin ?-Ga.sub.2O.sub.3 wafer is characterized by a strong reflection around 60.9? attributed to the (020) planes of ?-Ga.sub.2O.sub.3, while the diffraction peaks in the implanted samples centered at ?63.4? was interpreted as signatures of the ?-Ga.sub.2O.sub.3. Thus, the observed polymorph transitions are attributed to the disorder-induced effects with negligible impact of the chemical nature of the ions.