WO2014026292A1

WO2014026292A1 - Arbitrarily thin ultra smooth film with built-in separation ability and method of forming the same

Info

Publication number: WO2014026292A1
Application number: PCT/CA2013/050633
Authority: WO
Inventors: Stephen Michael JOVANOVIC; Gabriel Allan DEVENYI; John Stewart Preston
Original assignee: McMaster University
Current assignee: McMaster University
Priority date: 2012-08-15
Filing date: 2013-08-15
Publication date: 2014-02-20
Anticipated expiration: 2015-02-15

Description

ARBITRARILY THIN ULTRA SMOOTH FILM WITH BUILT-IN SEPARATION ABILITY AND METHOD OF FORMING THE SAME

FIELD

The present disclosure relates to the field of thin film processing and methods for producing thin films and structures, having single crystal or multicrystalline quality. More particularly, that relate to the application of semiconducting thin films and structures as epitaxial platforms, and tandem devices respectively on carrier substrates.

BACKGROUND

Thin films are key components in microelectronics, optoelectronics, photovoltaics, and imaging applications. Thin films are used as an active component in many detector applications (infrared detectors), buffer layers (nuclear and visible detectors), and solar cells. These thin films are commonly deposited on substrates, either crystalline or amorphous, with thicknesses ranging from atomic layers to microns.

A benefit to microelectronic, optoelectronic, photovoltaic, and imaging sensor industries can be found in high quality films and structures integrated with or carried on low-cost readily available substrates, particularly if the cost to produce the films and process the film onto carrier substrates is low.

The prior art of producing free standing thin films involves the use of either ion implantation combined with heat treatments or chemical etching combined with stop-etch layers. Both these techniques result in rough and damaged film surfaces which require further processing before further use. In addition, such methods are not easily amenable to films with large dimensions such as several square meters. The other field of piror art involves strain induced delamination of GaN films through the growth of extremely strained epitaxial layers grown to a thickness on the order of millimeters. Such a process relies on spontaneous cracking of the grown film into smaller pieces and is not suitable for large area applications. An inevitable result of the large strain and thickess of such films is a high density of dislocations and a degraded quality of the film after liftoff. Moreover, this process has only been demonstrated over limited area of film removal.

It would be very beneficial to provide a method of producing highly crystalline thin films with large lateral dimensions, on the order of several square meters, limited only by the availability of substrates.

SUMMARY

The embodiment is comprised of a thin film grown on an growth oxide substrate, with a designed-in ability to detach from the substrate upon appropriate application of force or energy. This film can either be a single material, or a graded/stepped structure. The film is of arbitrary thickness. The film is flat on the order of a polished wafer. The thin film can be patterned via lithography prior to separation.

The film may be separated from the oxide substrate. After separation the film is free standing, independent of the growth substrate, or may be bonded to a new carrier substrate for support or electrical connection depending on the application. It may be separated from the growth oxide substrate by being bonded to the carrier substrate and then force or energy applied to remove it from the growth oxide substrate, with the film and growth oxide substrate remaining unchanged before and after separation. The film may be patterned via lithography pre or post separation. The original growth substrate is left free of the film and can be reused for subsequent film growths.

The freestanding film is separated through application of energy or force to trigger the built in separation of the film from a substrate upon which it was grown. These include, but are not limited to, mechanical, thermal, vibrational, piezoelectric, and optical energies or forces. The growth substrate is of any form which allows for the built in ability of the film to detach from the substrate. The crystalline nature of the film depends upon the presented template from the substrate and can range from single crystalline, to multicrystalline depending on the pairing of film material to substrate. Thus, an embodiment disclosed herein includes method of producing crystalline film, comprising:

a) growing a crystalline film on a surface of an oxide substrate with a bottom surface of the film being adhered to the surface of the oxide substrate, the film and the oxide substrate made from materials selected on the basis of being chemically dissimilar and being comparably lattice matched, the film being grown under conditions to produce a crystalline film adhered to the oxide substrate; and

b) bonding a carrier substrate to a top surface of the crystalline film to produce a substrate/crystalline film/oxide substrate composite and applying a separation agent to detach the oxide substrate from the bottom surface of the crystalline film to produce a detached substrate/crystalline film

composite, the crystalline film of the detached substrate/crystalline film composite characterized in that it is a single crystal or a multi-crystalline film, and wherein the oxide substrate and the crystalline film characterized by being, before and after the substrate/crystalline film composite is detached from the oxide substrate, chemically and physically unaltered.

A further understanding of the functional and advantageous aspects of the embodiments disclosed herein can be realized by reference to the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The following is a description of embodiments of portable stands for mobile electronic devices, constructed in accordance with the present embodiment, reference being had to the accompanying drawings, in which:

Figure 1 is a representative cross section of the film on the growth substrate;

Figure 2 is a representative cross section of the film bonded to a carrier substrate;

Figure 3A is a representative cross section of the film exposed to an optical pulse;

Figure 3B is a representative cross section of the film exposed to ultrasonic vibration; Figure 3C is a representative cross section of the film exposed to a thermal stress;

Figure 3D is a representative cross section of the film exposed to piezoelectric forces;

Figure 4A is a representative cross section of the film separated held on a carrier;

Figure 4B is a representative cross section of a separated film left free standing;

Figure 5A is a representative cross section of subsequent to separation deposition on a buffer layer film on a carrier substrate;

Figure 5B is a representative cross section of subsequent to separation deposition on a free standing film acting as a buffer material;

Figure 5C is a representative cross section of the top down integration of a thin film or structure on a buffer layer of separation material carried on a backing substrate;

Figure 5D is a representative cross section of the top down integration of a thin film or structure on a free standing buffer film;

Figure 6A is a representative cross section of the separation of a thin film or structure carried on a substrate from the growth substrate;

Figure 6B is a representative cross section of the separation of a free standing film or structure from the growth substrate;

Figure 6C is a representative cross section of subsequent to separation deposition on the original growth substrate to prepare a new film or structure;

Figure 7 is a flowchart of the possible production process for a thin film.

Figure 8a is a (1 1 1 ) Pole Figure of as-grown CdTe thin film on Al₂0₃. Figure 8b is a (1 1 1 ) Pole figure of post-liftoff CdTe thin film on epoxy carrier.

Figure 9 shows a generic liftoff process for CdTe thin film on sapphire substrate. A sapphire substrate (1 ) has a thin film of CdTe grown on it (2) it is coated with an optional protective layer of metal, adhesive, or polymer (3) the sample is then bonded to a carrier via the adhesive layer (4a) or through the addition of reflow solder and heat (4b) before being finally lifted off via mechanic forces (5a) or spontaneous temperature stress (5b).

Figure 10a is a Sapphire substrate post liftoff.

Figure 10b is a CdTe single crystal thin film on polysulfone carrier. Figure 11 is a low temperature PL at 10K of as-grown and lifted off thin film. Films post-liftoff are shifted 21 nm and the FWHM is reduced by half.

Figure 12a and Figure 12b show the thin film consists of a primary (1 1 1 )-up phase with some twinning on the 70.59 degree inclined (1 1 1 ) planes.

Figure 13a and Figure 13b show the thin film includes a primary

(1 1 1 )-up phase with substantial twinning along the (1 1 1 )-up directions and along the 70.59 degree inclined (1 1 1 ) planes.

Figure 14 shows the thin film consists of two equal (1 1 1 )-up phases, with some in-plane misalignment of the individual phases.

Figure 15 shows the thin film consists of four (1 1 1 )-up phases, with some in-plane misalignment of the individual phases.

DETAILED DESCRIPTION

Generally speaking, the embodiments described herein are directed to thin film processing and methods for producing thin films and structures, having single crystal or multicrystalline quality. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms. Thus, for purposes of teaching and not limitation, the illustrated embodiments are directed to thin film processing and methods for producing thin films and structures.

Various embodiments and aspects of the disclosure will be described with reference to details discussed below. The following description and drawings are illustrative of the disclosure and are not to be construed as limiting the disclosure. The drawings are not to scale. Numerous specific details are described to provide a thorough understanding of various embodiments of the present disclosure. However, in certain instances, well- known or conventional details are not described in order to provide a concise discussion of embodiments of the present disclosure.

As used herein, the terms, "comprises" and "comprising" are to be construed as being inclusive and open ended, and not exclusive.

Specifically, when used in this specification including claims, the terms, "comprises" and "comprising" and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.

As used herein, the terms "example", "exemplary" means "serving as an example, instance, or illustration," and should not be construed as preferred or advantageous over other configurations disclosed herein.

As used herein, the terms "about" and "approximately", when used in conjunction with ranges of dimensions of particles, compositions of mixtures or other physical properties or characteristics, are meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. It is not the intention to exclude embodiments such as these from the present disclosure.

As used herein, the phrases "lattice matched" and/or "exhibiting comparable lattice matching" and/or "comparably lattice matched" means the two materials have crystal structures which are geometrically compatible to within about 8%. In some cases this may be simply that there unit cells are comparable within about 8% or they may have a unit cell ratio (1 :2, 2:1 etc) comparable within about 8%. In some cases, the unit cell of the film may be rotated to attain lattice matching with the substrate.

As used herein, the phrase "single crystal film" means a thin film composed of a single phase and orientation of crystal epitaxial aligned to the single crystal substrate, containing less than 1 % of other phases or orientations

As used herein, the word "multicrystalline film" means a thin film composed of a single phase and a small countable number of crystal orientations which are epitaxially aligned to the crystal substrate As used herein, the word "chemically dissimilar" means that the materials are not drawn from the same family. Chemically dissimilar materials may have properties such as hardness, melting point and chemical reactivity that are very different. They may also differ dramatically in their optical, magnetic and electronic properties. It is the differences in their mechanical properties (hardness, Young's modulus etc) that enable the separation to take place without substantially damaging the film or substrate. The variation in other properties may be advantageous in allowing differing mechanisms to be deployed to initiate the release of the film.

As used herein, the phrase "physically unaltered" means the structure and character is not damaged or changed in a way that modifies the key properties for use. In the case of the substrate this means that the substrate could be readily used to initiate another growth without polishing or chemical processing. In the case of the film, it means that the electronic, optical and magnetic properties of the film are not substantively modified.

As used herein, the phrase "initiating layer" means-of indeterminate thickness adjacent to the substrate which due to its proximity to the substrate is of a different structure or character than the rest of the thin film, a character which enables the liftoff process. This layer can be an intrinsic property of the thin film material, or it can be an extrinsic buffer layer grown to facilitate liftoff.

The present method is based on the surprising discovery that single crystal CdTe films grown on c-plane sapphire can be readily detached.

CdTe is a soft, ionic 11 -VI semiconductor with a modest melting point of 1092 °C. Sapphire is a very hard oxide, stable to high temperatures approaching 2000 °C. CdTe is lattice matched to c-plane sapphire to within 3.6%. Similar results have been obtained for several semiconductors grown on chemically dissimilar substrates. In all cases, the film and substrate are mechanically dissimilar and come from distinct families of compounds.

Referring to the Figures, Figure 1 is a schematic of the initial system shown generally at 10, comprised of a film 12 of arbitrary thickness (1 nm- 500 μητι), deposited on a substrate 16 by a deposition method for depositing thin films, such method including, but not limited to metal-organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), PLD (Pulsed Laser Deposition), thermal/electron beam evaporation, or sputtering, to mention just a few known methods. The thickness of the growth substrate 12 is arbitrary, and is typically selected to be the standard thickness available commercially. The lateral dimensions are also arbitrary, limited only by the available materials and maximum deposition area. A virtual "initiating layer" 14 is also shown to indicate the area of the thin film key in production of the built in ability to separate from the growth substrate 12. The virtual "initiating layer" 14 is that layer which initiates the growth of the film or structure on the substrate template as well has the built in separation from substrate characteristic. In some embodiments, the initiating layer may be a

compositionally unique layer introduced to facilitate the separation, before continuing growth with another material. In other embodiments, the initiating layer will be compositionally identical to the film but will have differing properties due to the adjacent substrate. The substrate 16 is an oxide, and the following oxides have been used which give good quality single crystal or multicrystalline thin films. This has been explicitly shown to work with c- plane sapphire, magnesium oxide, and yittrium aluminum garnite, based on the broad variation in these oxides we anticipate it will also be effective for AI₂O₃, BaTiOg, Bi₄Ge₃O₁₂, Bi₁₂GeO₂₀, GGG, KTN, KTaO₃, LSAT, LiAIO₂, LiNbO₃, LiTaO₃, MgAI₂O₄, MgO, NdCaAIO₄, SrTiO₃, SrLaAIO₄, PbWO₄, PMNT, SrLaGaO₄, Quartz, TiO₂, YAG, YVO₄, YSZ, ZnO, CaO, LaAIO₃, CoO, FeO, MnO, NiO, Cr₂O₃, Fe₃O₄, L₂-_xSr_xCO, Muscovite and KaTaO3. The thin film may be a 11 -VI binary alloy, including CdS, CdTe, CdSe, ZnS, ZnSe, ZnTe, HgSe, CuSe, CuTe, Bi₂Se₃, Bi₂Te₃ and HgTe. The thin film may be a ll-VI ternary alloy, including CdZnTe, HgCdTe, HgZnTe, CuCdTe, CuZnTe, CdBiTe,ZnBiTe, BiCuTe, HgBiTe, CdZnSe, HgCdSe, HgZnSe, CuCdSe, CuZnSe, CdBiSe,ZnBiSe, BiCuSe, and HgBiSe. The thin film may be a lll-V binary alloy, including GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AIN, AIP, AIAs, AlSb, ΒΝ,ΒΡ, BAs, and BSb. The thin film may be a lll-V ternary alloy including AIBN, AIGaN, AllnN,BGaN, BlnN, GalnN, AIBP, AIGaP, AllnP,BGaP, BlnP, GalnP, AIBAs, AIGaAs, AllnAs,BGaAs, BlnAs, GalnAs, AIBSb, AIGaSb, AllnSb,BGaSb, BlnSb, GalnSb, BNP, BNAs, BNSb, BPAs, BPSb, BAsSb, AINP, AINAs, AINSb, AIPAs, AlPSb, AIAsSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, GaAsSb, InNP, InNAs, InNSb, InPAs, InPSb, InAsSb. The thin film being grown on the growth substrate may be silicon, germanium or carbon.

The present method makes use of close, but not necessarily exact, matching of lattice parameters of the substrate and the thin film being grown. The matching in lattice parameters needs to be sufficiently close to induce an epitaxial alignment between the film and substrate. The substrate material and the thin film being grown thereon are chemically dissimilar. This contributes to producing an initiating layer that is abrupt and has a large variation in mechanical properties on the two sides of that layer. The separation of the film from substrate is ultimately a mechanical process whether it is directly induces mechanically or via thermal expansion mismatch or other indirect process. The sharp change in mechanical properties at the interface (i.e. the initiating layer) is essential but not necessarily sufficient criteria to ensure that neither the film nor substrate are substantially damaged or altered.

Once the thin film 12 has been produced on the lattice matched substrate 16, the film is then removed from the lattice matched substrate by bonding a carrier substrate to the top surface of the thin film. Figure 2 shows the layered system of Figure 1 now bonded to a carrier substrate 20 via a bonding layer 22 bonding substrate 20 to thin film 12. The bonding of the carrier substrate 22 to film 12 by bonding layer 22 can be done via variety of methods, such as, but not limited to, two-part resin adhesives, polymers, metal diffusion bonding or reflow soldering, wafer bonding, and oxide bonding. The carrier substrate 20 may include any other rigid or flexible material which provides a continuous surface. The fact that the film 12 can be easily physically detached from the lattice matched substrate is very surprising. All of the films described here require elevated temperatures in order for the atoms to have the mobility necessary to form crystalline or multi-crystalline material. These temperatures range from somewhat less than 300 °C to over 800^ depending on the material. This process also requires time for the atoms to organize themselves epitaxially which means that high quality materials often require slower growth rates. Typically, this results in maximum growth rates ranging from one unit cell per minute up to one unit cell per microsecond. Thermal physics teaches that atoms adsorbed onto a substrate can readily desorb via a thermal process and will do so on the scale of nanoseconds to microseconds if not bound to the surface. The conclusion is that the initial atoms that eventually form the film are bound to the substrate with substantive bonding energy. For the range of

temperatures given above, this bonding must be on the order of ~1 eV per atom. If this bonding persisted once the film was formed the detachment process would not be possible. The conclusion is that the bonding at the interface changes in the vicinity of the interface either during the growth, subsequent to the growth during cool-down, or upon application of direct or indirect mechanical force. This change is radical in nature, with the average bonding becoming many orders of magnitude weaker than during the initial stages of growth. The region in which the bonding is changed defines the initiating layer shown in the figures.

Figure 3A shows one method of separating the thin film from the growth substrate after bonding. The application of high energy photons 30 via a laser or other high intensity light source supplies energy to the initiating layer 14 causing the release of the thin film from the substrate. This approach would be applicable where the substrate is substantively transparent at photon wavelengths and the film and/or initiating layer are absorbing. This allows the localized electronic excitation to take place at the interface and via thermal expansion or alternate effects such as piezoelectric activation leads to mechanical action releasing the film. In the case of CdTe grown on sapphire, high energy ultraviolet photons from an excimer laser can readily pass through the sapphire and then be strongly absorbed in the CdTe. The subsequent thermal expansion of the CdTe is sufficient to initiate the release of the film. Figure 3B shows another method of separating the thin film from the growth substrate after bonding. A thin film or device adhered to the carrier substrate is placed in a sonication medium 32, typically liquid, and ultrasonic agitation is applied to the medium. The resulting ultrasonic forces cause the release of the thin film from the substrate. The sonication process is a reliable method to introduce relatively gentle, localized stresses to the system. In one embodiment, we have adhered a CdTe film to a glass slide using a commercial two-part epoxy. During the sonication process, the sapphire substrate is released from the film, resulting in a clean, detached sapphire substrate and a CdTe film bonded to the glass slide. Figure 3C shows another method of separating the thin film from the growth substrate after bonding. A thin film or device adhered to a carrier substrate is placed into a cryogenic medium 34 which causes the combined structure to go through a temperature change. The resulting thermal forces cause a release of the thin film from the growth substrate. The chemical dissimilarity between the film and substrate typically implies that their coefficient for thermal expansion will be distinct. Any change in temperature is then translated into a mechanical stress. One embodiment of this would be the metallization of the CdTe using Cu/Au or In/AI layered structures. Once this system is metalized it can be connected electrically and mechanically to other metallic pads via standard soldering techniques. The mechanical strain that results from the soldering is sufficient to result in the release of the film, resulting in an unaltered sapphire substrate and a CdTe film, bonded via the Cu/Au or In/AI layers and the solder to the new metallic pad. In a second embodiment the CdTe film grown on sapphire has a heated polymer film attached to it. The cooling to room temperatures is sufficient to detach the CdTe polymer film combination from the sapphire substrate.

Figure 3D shows another method of separating the thin film from the growth substrate after bonding. A thin film or device adhered to a carrier substrate has a large voltage 36 applied across the combined structure. Through piezoelectric properties, planar strain is developed in the structure causing the separation of the thin film from the growth substrate. Many of the film examples provided here have piezoelectric properties. A rapid application of an electric field to such materials will result in a mechanical stress sufficient to induce removal of the film. For example, a voltage of 10 V applied to a 100 nm thick GaN film on sapphire substrate would result in a mechanical stress sufficient to initiate detachment. After following any of the separation procedures in Figure 3, the result is the embodiment shown in Figure 4A, a thin film attached to a carrier substrate. Depending upon the carrier substrate type, the film can then be released, resulting in another form of the embodiment, Figure 4B, where the carrier substrate has been detached resulting in a freestanding film 12.

The resulting free standing thin film, bonded or unbonded to a carrier substrate, can be used to grow subsequent thin film layers by a variety of methods. Such a process is shown in Figure 5A for a free standing thin film and Figure 5B for a thin film bonded to a carrier substrate. These films can be of the same or different composition to the original thin film depending upon the desired application. The resulting composite structure is shown in Figure 5C for a thin film still bonded to a carrier substrate and Figure 5D for a free standing thin film.

Figure 6A and Figure 6B illustrate the physical separation of the lattice matched substrate 16 from the grown thin film 12 using any separation method such as, but not limited to, those illustrated in Figures 3A to 3D. After the separation of the film 12 from substrate 16, the growth substrate 16 may be used to grow further thin films 12 and since the growth substrate is physically unaltered has a surface ready for subsequent film growth as seen in Figure 6C.

Figure 7 is a process flow chart of the broad preparations and procedures to produce a single free standing or carried film. It can be repeated any number of times with the original growth substrate 16 once the film 12 has been separated from its surface. Growth substrate preparations for epitaxy or deposition may include minimal surface cleaning to remove atmospheric particulates, but may not be necessary depending on the growth mode for films.

The present method will now be illustrated with the following non- limiting examples. EXAMPLE 1

CdTe/Sapphire

CdTe thin films were deposited on single crystal c-plane α-ΑΙ₂0₃ +/- 0.5^Q wafers, obtained from MTI Crystals Inc and wafer saw diced into 1 2 mm x 12 mm squares. Prior to deposition, substrates were solvent cleaned in an ultrasonic bath. Samples were loaded into a custom pulsed laser deposition chamber at a base pressure of 1 x10^"7 Torr and in-situ annealed at 450^QC for 30 minutes. CdTe thin films were deposited by pulsed laser deposition using a GSI Lumonics IPEX-848 KrF excimer laser with a wavelength of 248 nm. Pulses from the laser were focused and rastered radially onto a rotating CdTe 2.54 cm diameter target with a spot size of 4.25 mm² and average energy density of 1 .8 J/cm². The CdTe 5N (99.999%) pressed powder target obtained from Princeton Scientific was stoichiometric and undoped. During growth samples were kept at a nominal temperature of 300 °C via a Pt-Rh thermocouple on the growth furnace surface. Films were grown to a thickness of 100 nm, as determined by optical and stylus profilometry.

Structural information was obtained using two dimensional X-ray diffraction (2DXRD) techniques. A Bruker SMART6000 CCD detector on a Bruker 3-circle D8 goniometer with Rigaku RU-200 rotating anode X-ray generator and parallel-focusing mirror optics were used for the data collection. In 2DXRD a two dimensional X-ray detector is used to map the x- ray reflections in the upper-half sphere of reciprocal space, resulting in a complete representation of all crystal spacings and their directions. After collection 2DXRD data can be sliced by a variety of methods, pole figures are extracted by selecting a given 2theta range (d-spacing) and mapping the resulting intensity in a stereographic projection. The resulting intensity maps indicate the out-of-plane directions of all crystal d-spacings in the selected integration range. 2DXRD data was processed into pole figures using Bruker GADDS. 2DXRD measurements of as-grown samples, generating a (1 1 1 ) pole figure, indicate a dominant (1 1 1 )-up phase of CdTe, with 0.1 -0.5% of first and second order twins, as shown in Figure 8. High resolution X-ray diffraction (HRXRD) performed on the as-grown films indicate a partially strain relaxed (0.2% residual strain) film with in-plane tension. Samples were prepared for liftoff according to the generalized process detailed in Figure 9. Key factors ensuring a complete and defect free liftoff process were the cleanliness of the film surface after deposition and the use of an adhesion layer which is strong and uniformly bonded to the thin film.

Samples which have undergone the generalized liftoff procedure are found to have maintained their single crystal nature after liftoff as

characterized by 2DXRD. Samples prepared using epoxy resin as a carrier were measured using the same procedure as as-grown samples and data was processed into the pole figure shown as Figure 8b. A notable difference between Figure 8a and Figure 8b is the absence of the central peak in the scaled region, which is the bleed-through of a strong sapphire substrate reflection, no longer present with an epoxy carrier. Peaks in the lifted off 2DXRD pole figures are also significantly sharper both in the radial and azimuthal direction, due to the now complete relaxation of residual strain.

Low temperature photoluminesence of as-grown CdTe thin films have previously been demonstrated to be of exceptionally high quality, due to the absence of twins, contaminants and other defects. As-grown thin films were measured at 10K, and were found to have a single emission peak at 808 nm and a FWHM of 23 nm as shown in Figure 10. The film was then underwent lift off processing via epoxy carrier and was again low temperature PL at 1 0K was collected also shown in Figure 10. The single emission peak shifts by 21 nm to 787 nm and the FWHM of the emission narrows to 1 1 nm. These changes to the PL emission indicate a relaxing of the residual strain in the thin film, indicating an improvement in the quality of the film post liftoff.

The generalized liftoff procedure described can be achieved by a number of specific methods. The simplest method is the application of a strong adhesive tape to the top surface of the single crystal, followed by peeling of the tape, causing a release of the thin film from the substrate. The adhesive tape acted as the carrier for the thin film. The high curvature accompanying adhesive tape lift off generally causes crystallographic cleaving of the thin film. Careful control of tape curvature can reduce or eliminate cleaving. Liquid and semi-liquid adhesives, such as cyanoacrylate, two part epoxy resin and UV-cured adhesives, have also been used to successfully achieve liftoff of thin film samples. Bonding to a variety of rigid carriers including glass, silicon, metal sheet and printed circuit boards has been successful at transferring films while maintaining the thin film crystallinity. After adhesion, thin films are lifted off by the application of a peeling force. Liftoff can also be achieved through the force applied by differential thermal expansion triggered by rapid cooling of the sample in LN₂ or rapid heating from solder reflow.

Polymer films (polysulphone, polycarbonate and

polymethylmethacrylate) have also been used as combined adhesive/carrier materials. A sample-sized square of the polymer carrier, approximately 500 microns thick, was layered over the sample. The polymer carrier is in the glassy state at room temperature. The layered sample was then placed on a hot plate and heated to approximately 50 °C - 80 °C above the glass transition temperature of the polymer carrier (Tg (PSU) = 186°C, Tg (PC) = 150°C, Tg(PMMA) = 105^). A sufficient annealing time was chosen to ensure the polymer film was in the melt state. At this point, a metal cylinder was rolled over the molten polymer film, ensuring good contact between the polymer and sample. The sample was allowed to slowly cool ambiently to avoid crack formation in the CdTe film as a result of the mismatch in thermal expansion coefficients between the layers. After cooling, peeling force was applied to release the film. Liftoff with these polymer films was a combination of cooling stresses and mechanical forces. A representative carrier/substrate pair is shown in Figure 10.

Electrical contact during liftoff processing is achievable via conductive epoxy as detailed above and metal-metal bonding. As-grown thin films were sputter coated with 100 nm of Platinum and 500 nm of Copper to provide a thermally stable capping layer. Samples were then coated with a thin layer of reflow solder (Sn63Pb37) and placed metal side down onto Cu metal films. Samples were then placed in an oven and cycled through the solder manufacturer's recommended heating cycle. Upon removal thin films were found to have bonded to the metal foil and separated from the substrate. Liftoff was achieved in this process via the thermal stresses imposed the cooling solder.

Post liftoff, sapphire substrates have been utilized for additional growths of thin film CdTe through a process of thermal annealing. Sapphire substrates, post liftoff were placed in a tube furnace and heated to 600°C for 30 minutes to desorb any residual CdTe remaining after liftoff processing. Substrates were then processed as detailed above to grow new CdTe thin films. Thin films grown on re-used substrates were indistinguishable within growth parameter variation from first-grown samples, as characterized by 2DXRD.

The mechanism by which liftoff of CdTe on sapphire substrates occurs has not been definitively determined. A number of theoretical models have been examined by this group to explain the exceptional interface which yields epitaxial single crystal thin film growth and then releases the resulting film with relatively little effort. The models proposed to explain such a process are 1 ) finite thickness polarization induced interface reorganization (a.k.a. the polarization catastrophe) 2) double layer Te nucleation, 3) van- der-waals induced epitaxy and 4) generalized weak covalent bonding.

Polar oxides such as sapphire are known to build up an unstable polarization due to unbalanced charge distribution (dipoles) in their unit cells, resulting in a small by finite voltage. This unstable polarization must be resolved as it builds to millions of volts over a crystal. This unstable polarization can be resolved by reorganization of the surfaces to counteract the dipole, or via charge transfer from an overlayer. Either of these mechanisms could have a significant impact on the interface between the

CdTe and the sapphire. Both atomic reorganization and charge transfer have the possibility of drastically altering the bonding environment leading to the liftoff phenomenon.

Investigations by the inventors into the liftoff phenomenon have included the use of density functional theory (DFT) modeling of the energy landscape of Cd and Te atoms on a sapphire surface, and their preferred position. Modeling of this system has indicated a preference for two layers of Te to bond to the sapphire substrate before the atoms form the typical CdTe structure. This two-layer structure immediately at the interface may be beneficial during nucleation however after some critical thickness the CdTe grown above may influence this layer such that the bonding is significantly weakened.

Finally, it is possible that the bonding energies between the initial thin film nucleation layer and substrate to be very small, with bonding being more akin to Van der Waals forces than to the typical covalent or semi-ionic bonding seen in solids. The bonding is sufficiently strong enough to provide a good template for epitaxial growth, but when strong perturbative forces (thermal strains, mechanical peeling) are applied, the bonding can be overwhelmed. This weak bonding regime may be due to the strongly ionic nature of the CdTe semiconductor (-70% ionicity) combined with the highly stable oxide surface of sapphire.

The present disclosure discloses a mechanism by which single crystal thin films can be mechanically separated from their single crystal substrates. While this mechanism has been initially discovered to apply to CdTe on sapphire substrates, it is expected that other material combinations will demonstrate this phenomenon if 1 ) they still offer an epitaxial template for growth and 2) the bonding strength between the thin film and the substrate is sufficiently small to be overcome by mechanical forces. Such freestanding thin films produced by this mechanism offer a route to producing new high efficiency low cost devices for a variety of applications.

EXAMPLE 2

Zinc Telluride (ZnTe)/Sapphire

Grown from pressed powder target via pulsed laser deposition, 315°C for 2 hr at 1 Hz repetition rate, energy density 1 .88 J/cm². Grown on (0001 ) sapphire substrate, diced into 12 mm x 12 mm squares, cleaned with solvents and annealed at 450 °C for 1 hour before deposition.

Liftoff processing is achieved by the application of a polysulfone (plastic) thick film on hot plate and peel force. As-grown pole Figures 12a and 12b show the thin film consists of a primary (1 1 1 )-up phase with some twinning on the 70.59 degree inclined (1 1 1 ) planes.

2DXRD measurements after liftoff indicate a film consisting of a single phase of (1 1 1 )-up ZnTe, with a reduction in the width of the diffraction peaks and a suppression of the twinned peaks.

EXAMPLE 3

Indium Antimonide (lnSb)/Sapphire

Grown in an MBE system from two solid sources at -300 °C for 0.5 hr at 500 um/hr for total thickness -250 nm. Grown on (0001 ) sapphire substrates 9.45 mm x 5.45 mm, cleaned with solvents and annealed in UHV at 450°C for 15 min.

Liftoff processing was achieved by the application of a commercial high strength construction adhesive tape followed by LN₂ submersion to trigger tape thermal contraction and shear force.

As-grown pole Figures 13a and 13b show the thin film consists of a primary (1 1 1 )-up phase with substantial twinning along the (1 1 1 )-up directions and along the 70.59 degree inclined (1 1 1 ) planes.

2DXRD measurements after liftoff indicate a film consisting of a single phase of (1 1 1 )-up InSb hosting twinning but with a reduction in both twining and the width of diffraction peaks.

EXAMPLE 4

Cadmium Telluride (CdTe)/MgO

CdTe films were grown from a pressed powder target via pulsed laser deposition, 300 °C for 30 minutes at 1 Hz repetition rate, energy density 1 .75 J/cm². These films were grown on (1 1 1 ) MgO, cleaned with solvents and annealed at 450°C in oxygen for 10 minutes prior to growth. Liftoff was achieved by bonding of thin film to glass slide using commercial 5 minute two part resin epoxy, followed by a simple peeling force. As grown pole Figure 14 shows the thin film consists of two equal (1 1 1 )-up phases, with some in- plane misalignment of the individual phases. EXAMPLE 5

Indium Arsenide (lnAs)/YAG

Grown in an MBE system from two solid sources at -300 °C for 0.5 hr at 500 um/hr for total thickness -250 nm. Grown on (001 ) YAG substrates 9.45 mm x 5.45 mm, cleaned with solvents and annealed in UHV at 450°C for 15 min. Liftoff processing was achieved by the application of a

commercial high strength construction adhesive tape followed by LN₂ (liquid nitrogen) submersion to trigger tape thermal contraction and shear force. As grown pole Figure 15 shows the thin film consists of four (1 1 1 )-up phases, with some in-plane misalignment of the individual phases.

Various electrical and or optical devices may be constructed from the produced according to the method disclosed herein. Examples include, but are not limited to, diodes, a single junction photovoltaic, a graded junction photovoltaic, a tandem multi junction photovoltaic, a field effect photovoltaic, a photodiode, a radiation detector, and a photoconductor to mention just a few. The resulting devices may be subsequently aligned with existing integrated circuits, CMOS or otherwise, and bonded to form heterojunction devices. They may be processed by conventional techniques to create horizontal devices such as transistors. The films may be patterned to give thin film of a piezoelectric nature and subsequently processed by

conventional techniques to produce microelectromechanical or

nanoelectromechanical devices. The film may be further patterned by lithographic methods.

The foregoing description of the preferred embodiments has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiments illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1 . A method of producing crystalline film, comprising:

2. The method according to claim 1 wherein in step b) the bonding of the carrier substrate to the top surface of the crystalline film includes releasable bonding between the carrier substrate to the top surface of the crystalline film, and the method further including detaching the crystalline film from the carrier substrate to produce a free standing crystalline film, and wherein the free standing crystalline film is, before and after detachment from the carrier substrate, chemically and physically unaltered.

3. The method according to claim 1 or 2 wherein the oxide substrate is any one of sapphire (Al₂0₃), magnesium oxide (MgO), Yttria Aluminum Garnet (YAG, Y₃AI₅0i₂).

4. The method according to claim 1 or 2 wherein the oxide substrate is any one of MgAI₂0₄, PbW0₄, Ti0₂, SrTi0₃, GGG, YSZ, CoO, LiAI0₂, LaAI0₂.

5. The method according to claim 1 , 2, 3 or 4 wherein the selected material is a ll-VI binary alloy.

6. The method according to claim 1 , 2, 3 or 4 wherein the selected material is a ll-VI quaternary alloy.

7. The method according to claim 1 , 2, 3 or 4 wherein the selected material is a ll-VI ternary alloy.

8. The method according to claim 1 , 2, 3 or 4 wherein the selected material is a lll-V binary alloy.

9. The method according to claim 1 , 2, 3 or 4 wherein the selected material is a lll-V ternary alloy.

10. The method according to claim 1 , 2, 3 or 4 wherein the selected material is a lll-V quaternary alloy.

1 1 . The method according to claim 1 , 2, 3 or 4 wherein the selected material is group IV semiconductor (carbon, silicon, germanium, tin).

12. The method according to any one of claims 1 to 1 1 wherein the growing of the crystalline film on the oxide film is by any one of metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), plasma enhanced chemical vapor deposition (PECVD), sputtering, thermal evaporation, electron beam evaporation, atomic layer deposition (ALD), liquid phase epitaxy (LPE), electrodeposition, pulsed electron deposition (PED), hydride vapor phase epitaxy (HVPE).

13. The method according to any one of claims 1 to 12 wherein the crystalline film is grown to a thickness in a range from about 1 nm to about 500 urn.

14. The method according to any one of claims 1 to 12 wherein the separation agent includes applying energy and/or a force to any one or combination of the substrate/crystalline film composite and the oxide substrate to affect detachment of the substrate/crystalline film composite from the oxide substrate.

15. The method according to claim 14 wherein the separation agent is addition of heat to the substrate/crystalline film composite and the oxide substrate.

16. The method according to claim 14 wherein the separation agent is removal of heat from the substrate/crystalline film composite and the oxide substrate.

17. The method according to claim 14 wherein the separation agent is application of a mechanical peel force to substrate/crystalline film composite and the oxide substrate.

18. The method according to claim 14 wherein the separation agent is application of a mechanical shear force to substrate/crystalline film composite and the oxide substrate.

19. The method according to claim 14 wherein the separation agent is application of a mechanical vibrational force to substrate/crystalline film composite and the oxide substrate.

20. The method according to claim 14 wherein the separation agent is application of a piezoelectric peel force to the substrate/crystalline film composite and the oxide substrate.

21 . The method according to claim 14 wherein the separation agent is application of a piezoelectric shear force to the substrate/crystalline film composite and the oxide substrate.

22. The method according to claim 14 wherein the separation agent is application of photonic energy to the substrate/crystalline film composite and the oxide substrate.

23. The method according to any one of claims 1 to 22, including repeating steps a) and b) a plurality of times to sequentially produce a plurality of substrate/crystalline film composites on the same oxide substrate without a need for chemical or mechanical surface preparation of the oxide substrate between production of each of the sequentially produced plurality of substrate/crystalline film composites.

24. The method according to claim 5, wherein said ll-VI binary alloy includes CdTe, CdSe, ZnSe, ZnTe, HgSe, HgTe, CuSe, CuTe, Bi₂Se₃, and Bi₂Te₃.

25. The method according to claim 7, wherein said ll-VI ternary alloy includes CdZnTe, HgCdTe, HgZnTe, CuCdTe, CuZnTe, CdBiTe,ZnBiTe, BiCuTe, HgBiTe, CdZnSe, HgCdSe, HgZnSe, CuCdSe, CuZnSe,

CdBiSe,ZnBiSe, BiCuSe, and HgBiSe.

26. The method according to claim 8, wherein said lll-V binary alloy includes GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AIN, AlP, AIAs, AlSb, ΒΝ,ΒΡ, BAs, and BSb.

27. The method according to claim 9, wherein said lll-V ternary alloy includes AIBN, AIGaN, AllnN,BGaN, BlnN, GalnN, AIBP, AIGaP,

AllnP,BGaP, BlnP, GalnP, AIBAs, AIGaAs, AllnAs,BGaAs, BlnAs, GalnAs, AIBSb, AIGaSb, AllnSb,BGaSb, BlnSb, GalnSb, BNP, BNAs, BNSb, BPAs, BPSb, BAsSb, AINP, AINAs, AINSb, AIPAs, AlPSb, AIAsSb, GaNP, GaNAs, GaNSb, GaP As, GaPSb, GaAsSb, InNP, InNAs, InNSb, InPAs, InPSb, and InAsSb.

28. A structure having a crystalline film on its surface, comprising:

a) a crystalline film having a top and a bottom surface; and b) an oxide substrate adhered to the bottom surface of the crystalline film ,

wherein said crystalline film and oxide substrate are selected on the basis of being chemically dissimilar and being comparably lattice matched, such that, upon applying a separation agent, both the film and the substrate are chemically and physically unaltered after separation.

29. The structure of claim 28, wherein the crystalline film is made from a single crystal or multi crystalline material.

30. The structure of any one of claims 28 or 29, wherein the crystalline film is made from any one of a l l-VI binary alloy, a ll-VI ternary alloy, a l l-VI quaternary alloy, a l ll-V binary alloy, a lll-V ternary alloy, a lll-V quaternary alloy, and a IV semiconductor.

31 . The structure of any one of claim 28 or 29, wherein the crystalline film is made from any one of CdTe, CdSe, ZnSe, ZnTe, HgSe, HgTe, CuSe, CuTe, Bi₂Se₃, Bi₂Te₃, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AIN, AIP, AIAs, AlSb, ΒΝ,ΒΡ, BAs, and BSb.

32. The structure of any one of claims 28 to 31 , wherein the oxide substrate isany one of sapphire (Al₂0₃), magnesium oxide (MgO) and Yttria Aluminum Garnet (YAG, Y₃AI₅0i₂).

33. The structure of any one of claim 32, wherein sapphire is c-plane sapphire.

34. A device comprising:

the structure as defined in any one of claims 28 to 33; and a carrier substrate bound to the top surface of the crystalline film.