The present invention relates to a method of fabricating a device comprising an un-twinned epitaxial layer of one or more semiconducting materials deposited on a surface of a single-crystal sapphire substrate in a deposition apparatus. Further, the present invention relates to a device comprising an un-twinned epitaxial layer of one or more semiconducting materials deposited on a surface of a single-crystal sapphire substrate. In addition, the present invention relates to a deposition apparatus for fabricating said device, the deposition apparatus comprising a reaction chamber sealable with respect to the ambient atmosphere for arranging the single crystal sapphire substrate, a gas system for providing an adjustable atmosphere in the reaction chamber, a heating means for heating the substrate, and a deposition means for depositing the epitaxial layer of one or more semiconducting materials on the surface of the substrate.

Devices with epitaxial layers of one or more semiconducting materials deposited on substrates are the backbone of modern electronics and computer technology. However, the quality of said epitaxial layers strongly depends on the surface finish of the substrate used for said epitaxial growth of the semiconducting layer. For instance, on the proceeding miniaturization of electronic devices towards quantum components, such as qubits, an extremely low density of structural defects both within the deposited epitaxial layer as well as at its interfaces to the layers above and below, in particular on its interface to the underlying substrate, is required.

Sapphire (AI2O3) is a material very well suited for the usage as universal substrate in microelectronics and nanofabrication. However, due to the chemical structure of sapphire as a single-crystal based on two elemental components, most of the usually used semiconducting materials, such as for instance silicon (Si), grow on a given surface of a sapphire substrate in two energetically equivalent orientations, which are chosen arbitrarily. Therefore, domains of these orientations with uncontrollable sizes form during epitaxial growth, and the boundaries between these domains in turn act as defects and limit the usability of the fabricated device.

In summary, for improving the usability of a device comprising a semiconducting layer deposited onto a single-crystal sapphire substrate, growing said semiconducting epitaxial layer in only one of these energetically equivalent orientations, so called “un-twinned”, would be of advantage.

In view of the above, it is an object of the present invention to provide an improved method of fabricating a device comprising an epitaxial layer of one or more semiconducting materials deposited on a surface of a single-crystal sapphire substrate in a deposition apparatus, an accordingly provided device, and an accordingly constructed deposition apparatus, in which the epitaxial layer can be provided as un-twinned epitaxial layer.

This object is satisfied by the respective independent patent claims. In particular, this object is satisfied by a method of fabricating a device according to claim 1 , by a device according to claim 26, and by a deposition apparatus for fabricating a device according to claim 27. The dependent claims describe preferred embodiments of the invention. Details and advantages described with respect to the method according to the first aspect of the invention also refer to the device according to the second aspect of the invention, and to a deposition apparatus for fabricating said device according to the third aspect of the invention and vice versa, if of technical sense.

According to the first aspect of the invention, the object is satisfied by a method of fabricating a device comprising an un-twinned epitaxial layer of one or more semi- conducting materials deposited on a surface of a single-crystal sapphire substrate in a deposition apparatus. The method according to the present invention comprises the following steps: a) Providing the sapphire substrate with a miscut angle selected in the range of

0.001 ° to 1 ° with respect to the (0001 ) plane surface of the single crystal sapphire; b) Arranging the sapphire substrate provided in step a) in a reaction chamber of the deposition apparatus, subsequently sealing the reaction chamber with respect to the ambient atmosphere and providing a preparation atmosphere in the reaction chamber; c) Heating the sapphire substrate for providing the sapphire substrate with a preparation temperature selected in the range of 1400 °C to 2000 °C; d) Heating the sapphire substrate for providing the sapphire substrate with a deposition temperature different to the preparation temperature selected in the range of 300 °C to 1400 °C; and e) Depositing the epitaxial layer on the surface of the substrate in the reaction chamber, whereby the substrate is further heated for continuously providing the deposition temperature.

The method according to the first aspect of the present invention allows the fabrication of a device comprising an un-twinned epitaxial layer of one or more semiconducting materials. Said epitaxial layer is deposited in a deposition apparatus on a surface of a single-crystal sapphire. A deposition apparatus according to the present invention is any apparatus capable of providing evaporated and/or sublimated material, which subsequently is deposited onto a surface of the sapphire substrate. The evaporated and/or sublimated material is suitably selected for coating said surface with the respective epitaxial layer. In that sense, epitaxial layers consisting of only the evaporated and/or sublimated material, and also epitaxial layers comprising next to the evaporated and/or sublimated material other material components, for instance provided by a likewise suitably selected reaction atmos- phere, respectively, can be provided by implementing the method according to the present invention.

As mentioned above, simply providing the single-crystal sapphire substrate is not sufficient for ensuring a growth of an un-twinned epitaxial layer of one or more semiconducting materials. That is why the method according to the present invention comprises the following steps for preparing the surface of the sapphire substrate and subsequently controlling the coating of said surface with the epitaxial layer of one or more semiconducting materials.

In a first step a) of the method according to the present invention, the sapphire substrate is provided with an actively selected miscut. Said miscut is selected with a miscut angle selected in the range of 0.001 ° to 1 ° with respect to the (0001 ) plane surface of the single crystal sapphire.

When cutting the substrate from the bulk single crystal, the cutting plane may be directed slightly away from the crystal plane, wherein according to the present invention the (0001 ) plane of the sapphire crystal is used as reference plane. Depending on this so called miscut angle, the prepared surface of the substrate will be terraces with terrace widths and terrace orientations that depend on the direction of the cut and can therefore be controlled at will. In other words, the miscut angle is the angle at which a single crystal is cut from a bulk substrate, wherein depending on this miscut angle, the pre-prepared surface will have terrace widths and terrace orientations that depend on the direction of the cut.

Apart from the absolute value of the miscut, its orientation is important, since the direction in which the steps on the surface are oriented with respect to the periodic arrangement of the crystal they form on defines the symmetry breaking which allows selecting between the different, energetically equivalent, in-plane surface reconstruction orientations. According to the method according to the present inven- tion, the crystallographic directions in the (0001 ) plane of the single crystal sapphire are used for defining the orientation of the miscut angle, as said (0001 ) plane maps best the hexagonal symmetry of said single crystal sapphire.

After preparing the sapphire substrate with said defined miscut angle, it is arranged in a reaction chamber of the deposition apparatus in the following step b) of the method according to the present invention. Said reaction chamber is seal- able with respect to the ambient atmosphere and hence allows continuing the remaining steps of the method according to the present invention in a controlled environment, in particular with respect to the atmosphere present at the surface of the sapphire substrate to be coated. In particular, step b) also comprises providing a preparation atmosphere suitably selected for the following measures of preparing the surface of the sapphire substrate, in particular with respect to the pressure and/or the constituents of said preparation atmosphere.

Preferably, the reaction chamber encloses all means necessary for both the preparation of the surface of the sapphire substrate and for the subsequent coating of the sapphire substrate. By that the reaction chamber can be kept sealed with respect to the ambient atmosphere and contaminations of the fabricated device by interactions with the environment can be minimized.

As already described above, after providing the sapphire substrate with the defined selected miscut in step a), the surface of the sapphire substrate is provided with terraces. As sapphire is a material comprising two different elemental components, namely AhOs and hence aluminum and oxygen as AI2O3, these terraces will comprise an alternating arrangement of surfaces composed of the two elemental components. As the selection of the respective orientation, with which the one or more semiconducting material will grow on said terraces, is strongly influenced by the actual elemental composition of said surface of the substrate, depositing the epitaxial layer directly on a further untreated terraced surface of the sapphire substrate will result in domains of these orientations of the grown epitaxial layer.

Hence, for further preparing the surface of the sapphire substrate, in the following step c) of the method according to the present invention, the substrate is heated for providing the substrate, in particular its surface, with a preparation temperature selected in the range of 1400 °C to 2000 °C. Any suitable method for heating the substrate can be used, for instance direct contact heating or radiation heating.

Said heating provides several advantages. First of all, impurities on the substrate surface can be evaporated. In addition, if a preparation atmosphere containing oxygen is used, also oxidizing said surface is possible.

In addition, the heating of the substrate can also lead to annealing processes. In other words, the number of missing or additional atoms on the substrate surface can be reduced and also discontinuities of a symmetry present on the substrate surface or even within its bulk can be healed.

Finally, the temperature of the substrate for the heating of the substrate in step c) of the method according to the present invention selected in the range of 1400 °C to 2000 °C is high enough that the most volatile constituent of the crystal, namely oxygen, sublimates from the surface. This on the one hand exposes on the surface a much more reactive aluminum reconstruction layer of sapphire and thereby enables an easier deposition of the epitaxial layer. On the other hand, all of the above-described terraces caused by the suitably selected miscut when preparing the surface of the sapphire substrate are provided with the same termination. Hence, the selection of the respective orientation, with which the one or more semiconducting material will grow on said terraces, will be the same for all terraces. In other words, after step c), the surface of the single-crystal sapphire substrate is suitably prepared for a deposition of an un-twinned epitaxial layer of one or more semiconducting materials. In summary, said preparation is based on a miscut with respect to the (0001 )-plane surface of the single crystal sapphire when cutting the actual sapphire substrate, and a subsequent heating of the substrate for annealing the surface and providing the surface with a single surface termination throughout all formed terraces on the surface of the substrate.

As mentioned above, the substrate is heated in step c) for providing a preparation temperature most suitable for the preparation of the surface of the sapphire substrate. However, said preparation temperature is adapted for said preparation, but inappropriate for the subsequent deposition of the epitaxial layer of the one or more semiconducting materials.

Hence, in the next step d) of the method according to the present invention, the substrate is again heated, but to a deposition temperature suitably selected for the subsequent coating. Said deposition temperature is different, in particular lower, than the preparation temperature. Essentially, the deposition temperature is selected in the range of 300 °C to 1400 °C. Preferably, the deposition temperature is selected such that on the one hand an influence on the substrate due to the heating, for instance annealing processes or sublimation of volatile constituents of the substrate, is minimized, and on the other hand the mobility of the materials deposited onto the surface of the substrate for forming the epitaxial layer is high enough that said epitaxial layer can grow essentially defect free.

After reaching the deposition temperature of the substrate, either by relative cooling from the preparation temperature and subsequent heating for maintaining the deposition temperature, or after cooling to a temperature lower than the deposition temperature, e. g. for surface analysis, and subsequent re-heating the substrate to the deposition temperature, the actual deposition of the epitaxial layer of one or more semiconducting materials takes place in the final step e) of the method according to the present invention. Basically, any deposition method can be used for said deposition, in fact the deposition method most suitable for the respective one or more semiconducting materials can be used. During the deposition, the substrate is further heated for continuously maintaining the substrate on the suitably selected deposition temperature. By that, a highest quality of the deposited epitaxial layer can be ensured. Thereby, the epitaxial layer can be provided un-twinned and essentially defect free.

In summary, by implementing the method according to the present invention, untwinned epitaxial layers of one or more semiconducting materials can be provided essentially defect free on single crystal sapphire substrates. Such layers potentially allow the integration of microelectronics based on semiconducting layers, for instance silicon-based microelectronic circuits, on sapphire substrates. This allows the use of sapphire as a universal substrate for the heterogenous monolithic integration of diverse materials.

The method can also be characterized in that in step a) the miscut angle is selected in the range of 0.01 ° to 0.1 °, in particular in the range of 0.03° to 0.07°, preferably wherein the miscut angle is 0.05°. By selecting the miscut angle, the sizes of the height and width of the terraces formed on the surface of the substrate can be adjusted. A miscut angle selected in the range of 0.01 ° to 0.1 °, in particular in the range of 0.03° to 0.07°, preferably wherein the miscut angle is 0.05°, has been found as especially suitable with respect to both, providing enough terrace steps on the surface for actively breaking the hexagonal symmetry of the (0001 ) plane surface of the single crystal sapphire, and simultaneously maximizing the sizes of said terraces, respectively.

In addition, the method according to the present invention can comprise that a vacuum atmosphere with a pressure selected in the range of 10’ ⁸ hPa to 10’ ¹² hPa is provided as the preparation atmosphere, or the preparation atmosphere comprises, preferably consists of, oxygen, in particular O2 and/or O3, with a pressure selected in the range of 10 ⁴ hPa to 10’ ⁶ hPa. Providing the preparation atmosphere as a vacuum atmosphere provides the advantage of a very pure environment during the preparation of the surface of the sapphire substrate. However, as sapphire essentially is an oxide of aluminum, also a preparation atmosphere containing oxygen in particular O2 and/or O3 can be useful, as this allows additionally oxidizing of said surface. In both ways, a surface of the substrate can be provided which comprises a uniform surface termination across all terraces.

According to another embodiment of the method according to the present invention, in step c) and/or step d) and/or step e) the sapphire substrate is heated by impinging laser light, in particular laser light with a wavelength selected in the range of 1 pm to 20 pm, preferably provided by a CO2 laser source, onto the sapphire substrate. Lasers can advantageously be used to heat substrates to a desired and defined temperature and are comparatively simple to use. The substrate can preferably be irradiated on this back side with a laser to be heated to the desired preparation and/or deposition temperatures. Sapphire, which is transparent at visible wavelengths, absorbs well at long infrared wavelengths, therefore a CO2 laser at around 10 pm may be used. The temperature can preferably be controlled, for instance with a pyrometer aimed at the back side of the wafer. Preferably, the single crystal sapphire substrate may be prepared to have a rough surface on its back side to help absorb the laser radiation. Therefore, the back side of the substrate is roughened either by not performing any further grinding or polishing steps after cutting it from the bulk crystal, by coarse grinding or other procedures that produce surface roughness with locally large deviations from the average surface, on a length scale at or above the wavelength of the heating laser.

Further, the method according to the present invention can comprise that in step c) the sapphire substrate is provided with the preparation temperature for a duration of 200 s or more. The above-described annealing and sublimation processes during the preparation of the surface of the sapphire substrate are not instantaneous, but take some time. It has been found that 200 s is a time span that is basically sufficient for the necessary preparatory processes. The time span is chosen to on the one hand be as short as possible to provide an efficient process. On the other hand, it needs to be long enough such that uncertainties in the definition of the starting and end points of the time span, such as timing errors, settling processes, control transients and transients due to the heat capacity of the substrate, do not essentially affect the reproducibility of the heating procedure. In general, there is a dependency of the necessary duration of said processes on the actual provided preparation temperature, wherein a higher preparation temperature allows to shorten the duration.

The method according to the present invention can also be characterized in that in step c) the preparation temperature is selected in the range of 1600 °C to 1800 °C, preferably wherein the preparation temperature is 1700 °C. As mentioned above, the preparation temperature has to be selected high enough to ensure the occurrence of the necessary processes during the preparation of the surface of the substrate. It has been found that a preparation temperature selected in the range of 1600 °C to 1800 °C, preferably a preparation temperature of 1700 °C, fulfils this requirement, while simultaneously limiting the demands on the heating means used for heating the substrate in step c) of the method according to the present invention.

In another embodiment of the method according to the present invention, in steps d) and e) the deposition temperature is selected in the range of 700 °C to 1200 °C, in particular in the range of 900 °C to 1100 °C, preferably wherein the deposition temperature is 1000 °C, and wherein in step e) the epitaxial layer comprises silicon (Si), preferably consists of silicon (Si) as semiconducting material. Silicon is one of the most commonly used semiconducting materials used in modern microe- lectronics. Selecting the deposition temperature in the range of 700 °C to 1200 °C, in particular in the range of 900 °C to 1 100 °C, preferably wherein the deposition temperature is 1000 °C, a deposition of un-twinned epitaxial layers of silicon on sapphire substrates can be ensured with the lowest defect density, in fact, the epitaxial layer comprising silicon, preferably consisting of silicon, can be provided essentially defect free.

Further, the method according to the present invention can be characterized in that the reaction chamber stays continuously sealed with respect to the ambient atmosphere after providing the preparation atmosphere in step b) until the deposition is completely finished in step e). In other words, all steps c) to e) are successively carried out within the same reaction chamber, and the sealing against the ambient atmosphere stays intact during the whole process. Hence, all steps c) to e) of the method according to present invention are carried out in-situ. External influences on the substrate and the subsequent formed device, in particular a contact to the ambient atmosphere, can thereby avoided.

In addition, the method according to the present invention can comprise that after step e), the epitaxial layer comprises a thickness of 50 nm or more, preferably of 3 pm or more. A wide variety of thicknesses can be provided, in particular suitably selected for the respective purpose of the device to be fabricated using the method according to the present invention.

According to another embodiment of the method according to the present invention, during the deposition of the epitaxial layer in step e) additional material is deposited into the epitaxial layer for doping said epitaxial layer. By doping a semiconducting material with additional material, its intrinsic properties with respect to a conductivity or other physical properties can be specifically changed. The industrial application possibilities of the device fabricated in this way can thus be significantly expanded. In addition, the method according to the present invention can comprise that during the deposition of the epitaxial layer in step e) two or more different material groups of semiconducting materials are used, wherein each of said material groups comprises one or more semiconducting materials, and wherein the two or more material groups are used to continuously and/or stepwise and/or repeatedly stepwise modulate a composition of the epitaxial layer. Microelectronic elements, such as for instance transistors, often are based on an interaction of layers of different semiconducting materials. Hence, by using two or more different material groups for the deposition of the epitaxial layer during step e), usage of the device fabricated by the method according to the present invention for a wide variety of microelectronic elements can be provided.

Further, the method according to the present invention can be characterized in that the deposition apparatus uses one or more of the following deposition methods:

Thermal laser epitaxy (TLE) Pulsed laser deposition (PLD) Physical vapor deposition (PVD) Electron-beam physical vapor deposition (EBPVD) Sputter deposition Molecular beam epitaxy (MBE) Chemical vapor deposition (CVD) Metal-organic chemical vapor deposition (MOCVD)

This list is not closed, and also other suitable deposition methods can be used in the respective deposition apparatus. In particular, for the respective one or more semiconducting material to be used for depositing the epitaxial layer in step e) of the method according to the present invention, the most suitable deposition method can be selected. Also, the method can comprise that after step c) and before step e) a deposition atmosphere suitable for depositing the epitaxial layer in step e) is provided in the reaction chamber. Some of the one or more semiconducting materials to be used for depositing the epitaxial layer may need a special deposition atmosphere. This can be the case, if the respective semiconducting material is a compound material comprising a compound element not providable as a solid target but only in its gaseous form. For instance, for a nitride as semiconducting material, such as GaN or InN, a deposition atmosphere comprising nitrogen can be used. In summary, by providing a deposition atmosphere, the variety of semiconducting materials usable in the method according to the present invention can be enlarged.

The method according to the present invention can be enhanced further by that the deposition atmosphere comprises a pressure selected in the range of 10’ ⁶ to 10 ¹ hPa, in particular in the range of 10’ ⁴ to 10 ¹ hPa, especially in the range of 10’ ⁴ to 10’ ² hPa. In particular, for the respective device to be fabricated and in particular for the respective epitaxial layer of one or more semiconducting materials to be deposited in step e), the most suitable pressure of the deposition atmosphere can be selected. The resulting quality of the fabricated device can thereby be enhanced.

According to another enhanced embodiment of the method according to the present invention, the deposition atmosphere comprises a process gas selected from the group of members consisting of oxygen (O), ozone (O3), plasma-activated oxygen (O), nitrogen (N), plasma-activated nitrogen (N), phosphorus (P), sulfur (S), selenium (Se), mercury (Hg), NH3, N2O, CH4 and combinations of the foregoing. This list is not closed, and also other suitable process gases can be used in the respective deposition apparatus. A wide range of semiconducting materials demanding a suitably selected deposition atmosphere can thereby be provided for the epitaxial layer of the fabricated device. Further, the method according to the present invention can be characterized in that one of the one or more semiconducting materials is an elemental semiconductor. Elemental semiconductors are the simplest semiconducting materials, as they consist of a single chemical element and hence require only a single material source during the deposition of the epitaxial layer in step e) of the method according to the present invention. In addition, elemental semiconductors such as for instance silicon are an especially widespread semiconducting material used in modern microelectronics.

The method according to the present invention can be enhanced further by that as elemental semiconductor one of the following metalloids is selected from the group of members comprising silicon (Si), germanium (Ge), diamond-like carbon (C), arsenic (As), boron (B), sulfur (S), selenium (Se), and tellurium (Te). This list is not closed, and also other suitable elemental semiconductors can be used. A wide range of semiconducting materials suitably selected for the purpose of the fabricated device can thereby be provided.

Alternatively, or additionally, the method according to the present invention can comprise that one of the one or more semiconducting materials is silicon carbide (SiC). SiC is used for instance in semiconductor electronics devices that operate at high temperatures or high voltages, or both. Hence, by using silicon carbide as one of the one or more semiconducting materials a device can be fabricated by the method according to the present invention suitable for high temperatures or high voltages.

Also, the method according to the present invention can be characterized in that one of the one or more semiconducting materials is a lll-V compound semiconductor comprising one or more Ill-element and one or more V-element, wherein the one or more Ill-element is selected from the group of members comprising boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl), and wherein the one or more V-element is selected from the group comprising nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). Ill-V compound semiconductors have the advantage over elemental semiconductors that their band gap can be varied, often continuously and/or over a wide range, with the material composition. Thereby, the electrical properties of a device with an epitaxial layer comprising a lll-V compound semiconductor as semiconducting material can be changed in a targeted manner. Such devices comprising a Ill-V compound semiconductor are mainly used in optical devices such as detectors, light-emitting diodes or lasers, but increasingly also for high-power electronic applications.

Further, the method according to the present invention can be enhanced by that the Ill-V compound semiconductor is a binary compound consisting of one Ill- element and one V-element, in particular wherein the Ill-V compound semiconductor is selected from the group of members comprising GaN, AIN, InN, BN, GaP, AIP, InP, BP, GaAs, AlAs, InAs, BAs, GaSb, AlSb, and InSb. This list is not closed, and also other suitable binary Ill-V compound semiconductors can be used. A wide range of semiconducting materials suitably selected for the purpose of the fabricated device can thereby be provided. Binary compound semiconductors only require two material sources during the deposition of the epitaxial layer in step e) of the method according to the present invention, wherein, for instance for nitrides and/or phosphides, one of the material sources can be provided by a suitably selected deposition atmosphere.

Alternatively, the method according to the present invention can be enhanced by that the Ill-V compound semiconductor is a multernary compound consisting of a mixture of three or more Ill-elements and V-elements, in particular wherein the III- V compound semiconductor is selected from the group of members comprising InGaN, InGaP, AIGaAs InGaAs, AIGaSb, InGaSb, and Gai-xln _x Asi- _y Py. This list is not closed, and also other suitable multernary Ill-V compound semiconductors can be used. Multernary compounds are one option to vary the band gap of semicon- ductors almost continuously over a wide energy range. A wide range of semiconducting materials suitably selected for the purpose of the fabricated device can thereby be provided.

According to a further embodiment, the method according to the present invention can be characterized in that one of the one or more semiconducting materials is a ll-VI compound semiconductor comprising one or more ll-element and one or more Vl-element, wherein the one or more ll-element is selected from the group of members comprising beryllium (Be), zinc (Zn), cadmium (Cd), and mercury (Hg), and wherein the one or more Vl-element is selected from the group comprising oxygen (O), sulfur (S), selenium (Se), and tellurium (Te). Similar to lll-V compound semiconductors, also ll-VI compound semiconductors have the advantage over elemental semiconductors that their band gap can be varied with the material composition. Thereby, the electrical properties of a device with an epitaxial layer comprising a ll-VI compound semiconductor as semiconducting material can be changed in a targeted manner. Such devices comprising a ll-VI compound semiconductor generally exhibit large direct band gaps, making them popular for short wavelength applications in optoelectronics.

Further, the method according to the present invention can be enhanced by that the ll-VI compound semiconductor is a binary compound consisting of one ll- element and one Vl-element, in particular wherein the ll-VI compound semiconductor is selected from the group of members comprising ZnO, ZnS, CdS, ZnSe, CdSe, BeTe, ZnTe, and CdTe. This list is not closed, and also other suitable binary ll-VI compound semiconductors can be used. A wide range of semiconducting materials suitably selected for the purpose of the fabricated device can thereby be provided. Binary compound semiconductors only require two material sources during the deposition of the epitaxial layer in step e) of the method according to the present invention, wherein, for instance for oxides and/or sulfides, one of the material sources can be provided by a suitably selected deposition atmosphere. Alternatively, the method according to the present invention can be enhanced by that the ll-VI compound semiconductor is a multernary compound consisting of a mixture of three or more ll-elements and Vl-elements, in particular wherein the ll- VI compound semiconductor is selected from the group of members comprising (Zn Cd)Se, (Be, Zn)Se, (Be, Cd)Se, and Zn(S, Se). This list is not closed, and also other suitable multernary ll-VI compound semiconductors can be used. Multernary compounds are one option to vary the band gap of semiconductors almost continuously over a wide energy range. A wide range of semiconducting materials suitably selected for the purpose of the fabricated device can thereby be provided.

According to a further enhanced embodiment of the method according to the present invention, one of the one or more semiconducting materials is an alloy of two or more semiconducting materials, in particular wherein the two or more semiconducting materials are selected from members of a group comprising elemental semiconductors, lll-V compound semiconductors, and ll-VI com-pound semiconductors. In other words, also combinations of two or more different semiconducting materials forming an alloy can be used as part of the epitaxial layer. The wide range of possible compositions of the un-twinned epitaxial layer deposited onto a sapphire substrate can thereby be extended further.

According to a second aspect of the invention, the object is satisfied by a device comprising an un-twinned epitaxial layer of one or more semiconducting materials deposited on a surface of a single-crystal sapphire substrate. The device according to the second aspect of the present invention is characterized in that the device is obtainable by the method according to the first aspect of the present invention. In other words, by implementing the method according to the first aspect of the present invention, a device according to the second aspect of the present invention is fabricated. Hence, the device according to the second aspect of the pre- sent invention provides all features and advantages described above with respect to the method according to the first aspect of the present invention.

According to a third aspect of the present invention, the object is satisfied by a deposition apparatus for fabricating a device according to the second aspect of the present invention, comprising a reaction chamber sealable with respect to the ambient atmosphere for arranging the single crystal sapphire substrate, a gas system for providing an adjustable atmosphere in the reaction chamber, a heating means for heating the substrate, and a deposition means for depositing the epitaxial layer of one or more semiconducting materials on the surface of the substrate. The deposition apparatus according to the third aspect of the present invention is characterized in that the deposition apparatus is constructed for carrying out the method according to the present invention for fabricating the device according to the second aspect of the present invention.

The deposition apparatus according to the present invention can be constructed but is not limited to carry out a deposition method selected from TLE, PLD, PVD, EBPVD, Sputter deposition, MBE, CVD, and/or MOCVD. The respective single crystal sapphire substrate to be coated can be arranged in a reaction chamber of the deposition apparatus which is sealable with respect to the ambient atmosphere. Subsequently, the substrate can be heated by accordingly provided heating means of the deposition apparatus. A gas system of the deposition apparatus ensures the presence of a suitable adjustable atmosphere within the reaction chamber, both of a preparation atmosphere and, when needed, also of a deposition atmosphere.

In summary, as the deposition apparatus according to the third aspect of the present invention is constructed to carry out the method according to the first aspect of the present invention, in particular for fabricating the device according to the second aspect of the present invention, it also provides all features and ad- vantages described above with respect to the method according to the first aspect of the present invention and with respect to the device according to the second aspect of the present invention, respectively.

The invention will be explained in detail in the following by means of embodiments and with reference to the drawings. In particular, in the figures are shown:

Fig. 1 A method according to the present invention for fabrication of a device according to the present invention,

Fig. 2 A deposition apparatus according to the present invention during execution of a method according to the present invention,

Fig. 3 Two measurements of a device according to the present invention, and

Fig. 4 Two top views of a device according to the present invention.

Fig. 2 depicts in subfigures A, B a deposition apparatus 40 during different stages of the method according to the present invention as depicted in Fig. 1 . Hence, Figs. 1 , 2 are described together in the following.

The deposition apparatus 40 according to the present invention can be constructed but is not limited to carry out deposition means 60 for a deposition method selected from TLE, PLD, PVD, EBPVD, Sputter deposition, MBE, CVD, and/or MOCVD. Preferably, and as depicted in Fig. 2, the deposition apparatus 40 can be constructed as TLE system 46.

The deposition apparatus 40 comprises a reaction chamber 42. The reaction chamber 42 is sealable with respect to the ambient atmosphere 70. A gas system 44 of the deposition apparatus 40 allows providing an adjustable atmosphere 72, 74 within the reaction chamber 42. Arrangement means (not depicted) are used for positioning a substrate 20 within the reaction chamber 42 for a subsequent coating of a surface 22 of said substrate 20.

According to the method according to the present invention, said substrate 20 is provided as a single-crystal sapphire substrate 20, which is according to step a) A of the method according to the present invention provided with a miscut angle selected in the range of 0.001 ° to 1 ° with respect to the (0001 ) plane surface of the single crystal sapphire. More preferably, the miscut angle is selected in the range of 0.01 ° to 0.1 °, in particular in the range of 0.03° to 0.07°, preferably wherein the miscut angle is 0.05°. Said substrate 20 is arranged in the reaction chamber 42.

In the following step b) B of the method according to the present invention, the reaction chamber 42 is sealed with respect to the ambient atmosphere 70 and filled with a suitably selected preparation atmosphere 72, such as for instance a vacuum atmosphere with a pressure selected in the range of 10’ ⁸ hPa to 10’ ¹² hPa or an atmosphere comprising, preferably consisting of, oxygen, in particular O2 and/or O3, with a pressure selected in the range of 10 ⁴ hPa to 10’ ⁶ hPa. Said preparation atmosphere 72 is provided by the gas system 44 of the deposition apparatus 40.

As depicted in subfigure A of Fig. 2, for the preparation of the surface 22 of the substrate 20, in the subsequent step c) C of the method according to the present invention, the substrate 20 is heated to a preparation temperature selected in the range of 1400 °C to 2000 °C. For ensuring an effective preparation of the surface 22 of the substrate, a duration of said heating of the substrate 20 of 200 s or more is provided. In particular, the preparation temperature is selected in the range of 1600 °C to 1800 °C, preferably wherein the preparation temperature is 1700 °C. As depicted, the heating means 50 for heating the substrate can be a substrate heating laser 52 providing a laser beam impinging onto substrate 20, in particular on the back side of the substrate 20. As the substrate 20 is a single-crystal sapphire, a laser beam with a wavelength selected in the range of 1 pm to 20 pm, can be used. Preferably, the substrate heating laser 52 comprises a CO2 laser source.

Said heating provides several advantages. First of all, impurities on the surface 22 of the substrate 20 can be evaporated. In addition, if a preparation atmosphere 72 containing oxygen is used, also oxidizing said surface is possible. In addition, the heating of the substrate 20 also leads to annealing processes. Finally, the most volatile constituent of the crystal, namely oxygen, sublimates from the surface 22 of the substrate 20. This exposes on the surface 22 a much more reactive aluminum reconstruction layer of sapphire and thereby enables an easier deposition of the epitaxial layer. Further, the entire surface 22 of the sapphire substrate 20 is provided with the same reconstruction and hence termination, respectively. Hence, an un-twinned growth of the one or more semiconducting material forming the epitaxial layer 30 (see subfigure B of Fig. 2) can be provided.

In summary, after step c), the surface 22 of the single-crystal sapphire substrate 20 is suitably prepared for a deposition of an un-twinned epitaxial layer 30 of one or more semiconducting materials. Said preparation is based on a miscut with respect to the (0001 )-plane surface of the single crystal sapphire substrate 20 when cutting the sapphire, and a subsequent heating of the substrate 20 for annealing the surface 22, providing the surface with a single surface termination throughout the surface 22 of the substrate 20.

After the preparation of the substrate 20 in steps a) A to c) C of the method according to the present invention, the substrate 20 in most of the cases is still at the preparation temperature, which normally is not perfect or even unsuitable for the following coating of the surface 22 with an epitaxial layer 30. Hence, in the next step d) D of the method according to the present invention, the temperature of the substrate 20 is changed to a more suitable selected deposition temperature differ- ent to the preparation temperature. Depending on the intended one or more semiconducting materials used for the epitaxial layer 30, this deposition temperature can be selected in the range of 300 °C to 1400 °C, for instance for silicon as semiconducting material in the range of 700 °C to 1200 °C, in particular in the range of 900 °C to 1 100 °C, preferably to 1000 °C. Again, preferably a source heating laser 52, in particular a CO2 laser can be used for heating the substrate 20. The deposition temperature may also be subsequently changed during the deposition process, for example if a certain gradient in defects such as dislocations, or a heterostructure of different layers 30 of semiconducting materials, is prepared.

Said heating of step d) D is kept activated in the subsequent step e) E of the method according to the present invention for continuously providing the surface 22 of the substrate 20 during the deposition of the epitaxial layer 30, as depicted in subfigure B of Fig. 2. As the shown deposition apparatus 40 is a TLE system 46, the respective deposition means 60 comprise a source heating laser 66, depicted as an arrow representing the laser beam provided by the source heating laser 66, for evaporating and/or sublimating source material 64 provided as source 62 within the reaction chamber 42. The evaporated and/or sublimated source material 64 impinges onto the surface 22 prepared in the first steps a) to c) of the method according to the present invention and thereby forms the un-twinned epitaxial layer 30. By continuously providing the substrate 20 at its deposition temperature, a high quality of the deposited epitaxial layer 30 can be ensured. Thereby, the epitaxial layer 30 can be provided un-twinned and essentially defect free.

Preferably, the reaction chamber 42 stays sealed with respect to the ambient atmosphere throughout the complete method according to the present invention, namely from the sealing in step b) B up to the end of the coating of the substrate 20 in step e) E. Any harmful influence of a contact of the epitaxial layer 30 with the ambient atmosphere can thereby be avoided.

Said sealing of the reaction chamber 42 during the execution of the method according to the present invention also allows after step c) C to fill the reaction chamber 42 with a deposition atmosphere 74 suitable for the intended epitaxial layer 30 to be deposited onto the surface 22 of the substrate 20. Said deposition atmosphere 74 can for instance comprises a process gas selected from the group of members consisting of oxygen (O), ozone (O3), plasma-activated oxygen (O), nitrogen (N), plasma-activated nitrogen (N), phosphorus (P), sulfur (S), selenium (Se), mercury (Hg), NH3, N2O, CH4 and combinations of the foregoing. Additionally, or alternatively, the deposition atmosphere 74 can comprise a pressure selected in the range of 10’ ⁶ to 10 ¹ hPa, in particular in the range of 10’ ⁴ to 10 ¹ hPa, especially in the range of 10’ ⁴ to 10’ ² hPa.

In subfigure B of Fig. 2 the epitaxial layer 30 is depicted only schematically. Said epitaxial layer 30 can comprise for instance a thickness of 50 nm or more, preferably of 3 pm or more. Also, additional material can be deposited into the epitaxial layer 30 for doping said epitaxial layer 30. Alternatively, or additionally, sublayers comprising different semiconducting materials can be used to continuously and/or stepwise and/or repeatedly stepwise modulate a composition of the epitaxial layer 30.

Also, a wide range of different semiconducting materials can be uses for forming the epitaxial layer 30. The range of possible materials encompass elemental semiconductors such as for instance silicon or germanium, lll-V compound semiconductors such as for instance GaN, GaAs or InGaN, and/or ll-VI compound semiconductors such as for instance ZnO, CdSe or (Zn Cd)Se. Also, combinations and/or alloys of the foregoing can be used. In summary, by implementing the method according to the present invention and/or by using the deposition apparatus 40 according to the present invention, un-twinned epitaxial layers 30 of one or more semiconducting materials can be provided essentially defect free on single crystal sapphire substrates 20. Such epitaxial layers 30 potentially allow the integration of microelectronics based on semiconducting layers, for instance silicon-based microelectronic circuits, on sapphire substrates 20. This allows the use of sapphire as a universal substrate 20 for the heterogenous monolithic integration of diverse materials.

In Figs. 3 and 4, actual measurements of an un-twinned epitaxial layer 30 of silicon on a single-crystal sapphire substrate 20 fabricated in a deposition apparatus 40 according to the present invention (see Fig. 2, in particular subfigure B) by implementing a method according to the present invention (see Fig. 1 ) are shown.

Due to the uniform selection of only one of the possible two rotational domains of the sapphire surface 22 of the sapphire substrate 20, during deposition of the epitaxial layer 30 silicon nucleates and grows in only one orientation, without twinning and the associated defects. This is verified by the x-ray analysis of such a silicon layer on sapphire (0001 ) shown in subfigure A of Fig. 3. Two X-ray diffraction scans are shown, in the upper panel of the epitaxial layer 30 of silicon, in the lower panel of the underlying sapphire substrate 20. As is clearly visible, the silicon peaks repeat only every 120°, which is the case only if a single in-plane orientation of the (1 11 )-oriented silicon is present. For a twinned layer, six instead of three peaks per rotation would be measured.

Subfigure B of Fig. 3 depicts an X-ray 9-29 scan of the same sample encompassing an epitaxial layer 30 of silicon on a sapphire substrate 20. The uniform (11 1 ) orientation of the epitaxial layer 30 comprising silicon is confirmed by the X-ray 9- 29 scan. Only the (1 1 1 ) and (333) reflections associated to silicon are found paral- lei to the (Z-A2O3 (0006) and (000.12) planes normal (apart from the miscut) to the surface 22 in the growth direction of the epitaxial layer 30.

Together, these two measurements depicted in Fig. 3 confirm that the silicon epitaxial layer 30 is un-twinned and has only one single orientation with respect to the substrate 20.

In Fig. 4, top views of two samples of an epitaxial layer 30 on a sapphire substrate 20 are shown. The difference between the two samples is the respective thickness of the epitaxial layer 30, namely 300 nm in subfigure A and 1 ,3 pm in subfigure B.

The respective silicon epitaxial layer 30 still has some defects due to its nonperfect lattice mismatch of 32.8% with the substrate 20. This means a close match of three AI2O3 lattice constants with two silicon lattice constants (ideally 33.333... %). However, these defects seem to annihilate to a significant degree with increasing thickness of the epitaxial layer 30.

As already mentioned above, subfigure A of Fig. 4 shows epitaxial layers 30 of approximately 300 nm thickness, about 1/3 the distance of the image size tickmarks at the top and left of the image. At this stage, the surface of the epitaxial layer 30 is still quite rough, with the lowest regions probably reaching down to or very close to the surface 22 of the substrate 20.

Thicker epitaxial layers 30, such as depicted in subfigure B of Fig. 4, however, show a closed surface of the epitaxial layer 30. The depicted epitaxial layer 30 is approximately as thick as the distance of the lateral tickmarks. The highest and lowest points in the scan, as indicated by the numbers on the scale, now span only 1/20 of the overall thickness of the epitaxial layer 30. Flat terraces extend several micrometers in the lateral direction. For even thicker layers, further smoothing and improvement in surface crystal quality occurs, allowing in particular the usage of the fabricated layers and devices according to the present invention in silicon- based devices and nanostructures.

Reference list

10 Device

20 Substrate

22 Surface

30 Epitaxial layer

40 Deposition apparatus

42 Reaction chamber

44 Gas system

46 TLE system

50 Heating means

52 Substrate heating laser

60 Deposition means

62 Source

64 Source material

66 Source heating laser

70 Ambient atmosphere

72 Preparation atmosphere

74 Deposition atmosphere

A Step a)

B Step b)

C Step c)

D Step d) E Step e)