The present invention relates to semiconductor struc ^¬ tures and devices, in particular to structures and de- vices comprising an oxide-semiconductor or nitride- semiconductor interface, and to manufacturing of such structures and devices.

BACKGROUND

Various semiconductor devices, such as, for example, light emitting diodes LEDs, semiconductor lasers, pho- todetectors, photovoltaic cells, and transistors, com ^¬ prise one or more oxide-semiconductor or nitride- semiconductor interfaces. The purpose of an oxide or nitride layer often relates to passivation and/or electrical insulation of the underlying semiconductor surface .

In various types of devices having such oxide- semiconductor or nitride-semiconductor interface, the quality of the material interface between the semicon ^¬ ductor and the oxide or nitride is of crucial im ^¬ portance for the performance of the device. Generally, a chemically abrupt interface with low density of de- fects in the semiconductor surface is desired. High density of defect states may result in harmful ef ^¬ fects, such as increased leak current of the device. In the case of an oxide, defect states may be caused, for example, by oxidation of the semiconductor sur- face. Defect states induced by oxidation may relate e.g. to vacancies, dangling bonds and different oxide phases .

In the case of an oxide as the passivation or insula- tion material, conventionally, silicon dioxide S1O ₂ has been extensively used as a passivation layer mate- rial for silicon as the semiconductor material. Its use for other semiconductors such as germanium Ge and group III-V compound semiconductors has also been in ^¬ vestigated widely.

In one known approach exemplified e.g. by US 2005/0000566 Al, the semiconductor Ge surface on which an oxide layer is to be deposited is first cleaned from all oxide. Thereafter, an amorphous silicon layer is deposited on the cleaned semiconductor surface in order to avoid the substrate oxidation. A S1O ₂ pas ^¬ sivation layer is then formed on the amorphous Si lay ^¬ er. Another example of utilizing an initial Si layer on a Ge surface is disclosed in US 6352942 Bl . In the proposed approach, silicon can be deposited on the surface in a Ge growth chamber initially after growth of a Ge layer forming the surface layer of the sub ^¬ strate. Alternately, native oxides can be removed from the Ge surface before depositing the silicon layer. The silicon layer is exposed to dry oxygen gas at an elevated temperature for a time selected to oxidize only a portion of the initial Si layer.

In the above prior art examples, the silicon layer is in contact with the pure germanium surface. Such in ^¬ teraction may result in another undesired effect where silicon atoms diffuse into the Germanium crystal, leading to diffused (extended) interface. This may again result in harmful defect states negatively af- fecting the interface quality and consequently the performance of a device incorporating the passivated germanium surface.

The above phenomena are not limited to silicon dioxide and germanium as the oxide and the semiconductor mate ^¬ rials, respectively. Instead, it is believed that those are generic phenomena possible for a plurality of material combinations. For example, corresponding phenomena may exist for a nitride as the passivation or insulation material on a semiconductor substrate. To summarize, there is a need for a generic solution to produce high quality oxide-semiconductor and nitride-semiconductor interfaces which can be manufactured cost-efficiently at an industrial level. SUMMARY

In one aspect, a method may be implemented for forming a foreign oxide layer or a foreign nitride layer on a semiconductor substrate. By a semiconductor substrate is meant here a substrate having a surface, at least part of which is formed of a semiconductor material. Such semiconductor material may form a layer lying on a body or a support portion of the semiconductor sub ^¬ strate, such body or support portion being formed of some other material. Alternatively, the semiconductor material may cover the entire thickness of the semi ^¬ conductor substrate.

The semiconductor substrate may be a stand-alone body, structure, or element. Alternatively, it may form an integral part of a larger structure or device. For ex ^¬ ample, it may form a part of a structure of an optoe ^¬ lectronic device, a transistor, or any other device or component where a semiconductor-oxide interface or a semiconductor-nitride interface, preferably of high quality, is needed.

The semiconductor substrate may be formed and prepared by any appropriate method. Just as one example, a sem ^¬ iconductor substrate may be formed by depositing, for example by means of molecular beam epitaxy MBE, a lay ^¬ er of a III-V compound semiconductor on a sapphire deposition substrate. "Foreign" oxide or nitride refers here to oxidized or nitridized material, respectively, which is different from the oxide (s) or nitride (s) the semiconductor ma- terial of the semiconductor substrate itself may form.

The semiconductor material may be a single-element semiconductor material comprising just one semiconduc ^¬ tor element, such as germanium Ge . Alternatively, or in addition to, it may also comprise a compound semi ^¬ conductor material, such as gallium arsenide GaAs, in ^¬ dium arsenide InAs, indium gallium arsenide InGaAs, indium phosphide InP, aluminum gallium antimonide Al- GaSb, indium antimonide InSb, or any other compound semiconductor materials known in the art. In both cases, the semiconductor material may be doped or intrin ^¬ sic.

The method comprises providing a semiconductor sub- strate having an oxidized or nitridized surface layer; supplying at least one foreign element on the oxidized or nitridized surface layer; and keeping the oxidized or nitridized surface layer at an elevated temperature so as to oxidize or nitridize at least partially the supplied foreign element by the oxygen or nitrogen, respectively, initially present in the oxidized or ni ^¬ tridized surface layer.

The oxidized or nitridized surface layer means a layer formed by an oxide or a nitride of the semiconductor material of the semiconductor substrate. The oxidized or nitridized surface layer may cover only a part of the thickness of the semiconductor material. Alterna ^¬ tively, the entire thickness of the semiconductor ma- terial may be oxidized or nitridized, thereby forming the oxidized or nitridized surface layer. Said "providing" the semiconductor substrate with an oxidized or nitridized surface layer may comprise just receiving a complete semiconductor substrate having such oxidized or nitridized surface layer. Alterna- tively, forming such oxidized or nitridized surface layer may be a part of the method.

In itself, the semiconductor material may be crystal ^¬ line. However, oxides and nitrides of intrinsically crystalline semiconductor materials are typically amorphous. Therefore, the oxidized or nitridized sur ^¬ face layer may be amorphous also when the oxidized or nitridized surface layer lies on a crystalline semi ^¬ conductor layer.

The foreign element may be, in principle, any material capable of forming an oxide or a nitride layer on the semiconductor substrate. The primary physical require ^¬ ment is that the heat of formation per an oxygen or nitrogen atom of an oxide or a nitride of the foreign element is higher than the heat of formation per an oxygen or a nitrogen atom of the semiconductor oxide or nitride of the oxidized or nitrided surface layer. Such foreign element oxides and nitrides are typically dielectric materials which can be used to passivate and/or electrically insulate various semiconductor surfaces. Therefore, the foreign oxide or nitride lay ^¬ er can be considered generally as a dielectric layer comprising a foreign oxide or foreign nitride. The foreign element may be some semiconductor or metal, for example, silicon or aluminum, respectively. Other examples of metals for the foreign element are hafnium and barium. Said oxidizing or nitridizing the foreign element by the oxygen or nitrogen initially present in the oxi ^¬ dized or nitridized surface layer may take place sub- stantially simultaneously with supplying the foreign element on the oxidized or nitridized surface layer. In other words, the semiconductor substrate may be kept at an appropriately elevated temperature during supplying the foreign element on the oxidized or nitridized surface layer thereof, whereby the supplied foreign element and the oxygen or nitrogen may direct ^¬ ly form a layer of a foreign oxide or a foreign nitride. Said elevated temperature can be selected to be sufficiently high above the normal room temperature so that the oxygen or nitrogen initially bound in the ox ^¬ idized or nitridized surface layer is disentangled from the substrate semiconductor and diffuses towards the transient layer and reacts with the foreign ele- ment thereof, thereby forming an oxide or a nitride of the foreign element. The required temperature level depends on the material (s) of the semiconductor sub ^¬ strate and the foreign element. In an embodiment, the method comprises forming a tran ^¬ sient layer by depositing the foreign element on the oxidized or nitridized surface layer; and heat- treating the oxidized or nitridized surface layer and the transient layer thereon so as to oxidize or ni- tridize at least partially the transient layer by the oxygen or nitrogen, respectively, initially present in the oxidized or nitridized surface layer.

Thus, in this embodiment, after formation of the tran- sient layer by supplying the foreign element on the oxidized or nitridized surface layer, there is a tem ^¬ porary structure effectively comprising a layered stack of the semiconductor substrate, the oxidized or nitridized surface layer thereof, and the transient layer on the oxidized or nitridized surface layer. This stack may be exposed to a heat treatment where the temperature of the oxidized surface layer and the transition layer of the foreign element is heated to a sufficiently high temperature so that the oxygen ini ^¬ tially bound in the oxidized surface layer is disen ^¬ tangled from the substrate semiconductor and diffuses towards the transient layer and reacts with the for ^¬ eign element thereof, thereby forming an oxide of the foreign element. The heat treatment is preferably car ^¬ ried out in vacuum conditions to prevent any external impurities from affecting the oxidation or nitridation of the foreign element. During the formation of the transient layer, the substrate may be kept at any ap ^¬ propriate temperature which may be, for example, sub ^¬ stantially lower than the temperature of the oxidized or nitridized surface layer during the heat treatment.

In the above embodiment, the transient layer formation and the oxidation or nitridation of the foreign element may thus take place separately in time. The transient layer may also be formed so as to com ^¬ prise more than one foreign element. Therefore, "de ^¬ positing a foreign element" shall be understood as meaning deposition of at least one foreign element. In the case of more than one foreign element, in said ox- idation or nitridation of the transient layer, oxides or nitrides of different foreign elements, or an alloy oxide or nitride of several foreign elements, may form. Correspondingly, the semiconductor material may generally comprise any semiconductor material on which a foreign oxide layer may be formed. Examples are germa ^¬ nium and III-V compound semiconductors, i.e. compound semiconductors formed of the group III and V elements. One example of the latter, commonly used, for example, in high speed electronics and high-end photodiodes, is gallium arsenide GaAs . In supplying the foreign element on the oxidized or nitridized surface layer, any deposition method known in the art, such as sputtering, evaporation, or atomic layer deposition, may be used. In the case of forming a transient layer on an oxidized surface layer, the deposition is preferably carried out in non-oxidizing conditions, i.e. in vacuum or with a low amount of ex ^¬ ternal oxygen present, to prevent oxidation of the foreign element at this stage.

Alternatively, the substrate with the oxidized surface layer thereon may be kept at such high temperature during the formation of the transient layer, whereby said disentangling and diffusing of the oxygen as well as the reaction thereof with the foreign element may start already during the transient layer formation. Thus, in this approach, the heat-treatment may be car ^¬ ried at least partially simultaneously with the tran- sient layer formation.

Thus, in all embodiments above, an appropriate heat treatment at an elevated temperature can cause the diffusion of the oxygen or nitrogen in the oxidized or nitridized surface layer of the semiconductor sub ^¬ strate towards the foreign element supplied on the surface of the oxidized or nitridized surface layer, possibly forming a transient layer. Thereby, all or part of the supplied foreign element may be converted into the foreign oxide or foreign nitride layer. Thus, a single heat treatment may serve for both removing the oxygen or nitrogen, i.e. the harmful oxidized or nitridized surface layer, from the substrate, and transforming at least part of supplied foreign ele- ment, possibly forming a transient layer, into the foreign oxide or foreign nitride layer. Consequently, a high quality, chemically abrupt interface may result between the possibly crystalline semiconductor surface and the foreign oxide or foreign nitride layer. Advantageously, such high-quality interface may be achieved in a well-controlled, cost-effective process.

The "elevated" or "high" temperature required to ini ^¬ tiate said decomposing of the initial oxide or nitride of the semiconductor, and to form the oxide or nitride of the foreign element, depends on the materials used. This temperature shall be sufficiently high to provide the oxygen or nitrogen atoms bound in the semiconduc ^¬ tor oxygen with an energy exceeding the potential barrier binding the semiconductor and the oxygen or nitrogen together.

In an embodiment, the elevated temperature is equal to or higher than 300 °C. Such high temperatures may pro ^¬ vide efficient formation of the oxide or nitride for various combinations of the substrate and foreign ele- ment materials, such as for germanium and silicon, re ^¬ spectively.

The "elevated temperature" may be also clearly higher than 300 °C. For example, in the case in which an amorphous silicon transient layer is deposited on an oxidized germanium surface, temperature of about 600 °C may be appropriate. The temperature should not be too high, although, to avoid decomposition of the semiconductor and the foreign oxide or foreign nitride materials. For example, silicon dioxide Si02 decompos ^¬ es in temperatures exceeding 900 °C. On the other hand, in the case in which GaAs is used as the semi ^¬ conductor material of the semiconductor substrate, the maximum temperature should be below about 700 °C to avoid decomposition of GaAs. The duration of the heat treatment or, in general, keeping the oxidized or nitridized surface layer at an elevated temperature, may vary according to the mate ^¬ rials and layer thicknesses used. It may be in the range of minutes, for example, 5 to 20 minutes.

In supplying the foreign element, e.g. by forming the transient layer, the amount of the supplied foreign element or the thickness of the transient layer formed may be adjusted according to the thickness of, and the amount of oxygen or nitrogen contained in, the oxi ^¬ dized or nitridized surface layer of the semiconductor substrate. In other words, amount of the supplied for ^¬ eign element or the transient layer thickness may be selected so that, taking into account the stoichiome- try of the foreign element oxide or nitride to be formed, there are sufficient foreign element atoms to ensure that in the oxidation or nitridation process, all or at least most of the oxygen or nitrogen in the oxidized or nitridized surface layer of the substrate can be consumed in the oxidation or nitridation process.

On the other hand, the amount of the supplied foreign element may exceed the amount required by the stoichi- ometry conditions. It has been found, for example, that formation of the foreign oxide may start at the side of the transient layer adjacent to the oxidized surface layer of the semiconductor substrate. Thereby, a protective foreign oxide layer may form at an early stage between the transient layer and the oxidized surface layer of the semiconductor substrate, prevent ^¬ ing the diffusion of the foreign element atoms into the semiconductor even if there is an excess amount of the foreign element. The rest of the foreign element, not oxidized by the oxygen initially contained in the oxidized surface layer, may be then oxidized by means of external oxygen.

In an embodiment for forming a foreign oxide layer on a semiconductor substrate, the oxidized surface layer may comprise native oxide. In this embodiment, there is no need for any specific oxidation of the semiconductor surface; the semiconductor substrate may be simply exposed to ambient air for a period sufficient for the native oxide layer to form. Typically, native oxide formation on a semiconductor surfaces saturates so that a material-specific native oxide thickness is formed . The oxidized or nitridized surface layer may be also formed by a specific oxidation or nitridation process, in which case providing a semiconductor substrate having an oxidized or nitridized surface layer may com ^¬ prise, for example, cleaning a surface of a semicon- ductor substrate, and oxidizing or nitridizing the thereby cleaned surface. By forming the oxidized or nitridized surface layer by a controlled oxidation or nitridation process, the thickness and the properties thereof may be adjusted more easily than in the case of a native oxide. Such cleaning and oxidation or nitridation processes may be carried out according to principles and equipment as such known in the art.

In one aspect, a method may be implemented for forming a semiconductor device, wherein the method comprises forming a foreign oxide layer or a foreign nitride layer on a semiconductor substrate according to any of the embodiments discussed above. Thus, in this aspect, a method may comprise formation of a complete, opera- ble semiconductor device. Such device may be, for ex ^¬ ample, a light emitting diode LED, a photodiode, a semiconductor laser, a photovoltaic (solar) cell, or any other optoelectronic device. It may be also any type of transistor, such as a metal-oxide- semiconductor field-effect transistor MOSFET or any other type of field-effect transistor, or a high- electron-mobility transistor HEM . In addition to MOSFET, the device may also be any other building block of complementary metal oxide semiconductor CMOS circuits. As some specific examples, the method can be used for manufacturing a germanium-based or gallium arsenide-based infrared photodiode or a metal-oxide- semiconductor capacitor MOSCAP. Yet further semiconductor device examples are capacitors, diodes, fuses, flash memory cells and circuits, and semiconductor photosynthetic cells. In general, the possible imple- mentations of the method are not limited to the above examples only. Instead, the semiconductor device may be any device incorporating a semiconductor-oxide or semiconductor-nitride interface. The semiconductor device may be formed as a discrete component or module. It may also be formed as an inte ^¬ gral, inseparable part of a larger device, module, circuit, or assembly. In a device aspect, a semiconductor structure may com ^¬ prise a foreign oxide or foreign nitride layer formed on a semiconductor substrate by a method as defined above. The semiconductor structure may form a part of a semiconductor device comprising such semiconductor structure. Similarly to the method aspect discussed above, the semiconductor device may be, for example, one of a light emitting diode, a photodetector , a semiconductor laser, a photovoltaic cell, a transistor such as a metal-oxide-semiconductor field-effect tran- sistor, a metal-oxide-semiconductor capacitor, a high- electron-mobility transistor, a flash memory cell or circuit, and a semiconductor photosynthetic cell. In a semiconductor structure formed using the above method, an abrupt, high-quality interface with low de ^¬ fect density may be present between the semiconductor and the foreign oxide or foreign nitride layer. In particular, there may be clearly less foreign element atoms diffused into the semiconductor of the semicon ^¬ ductor substrate than in the case of a similar struc ^¬ ture manufactured using any prior art process.

Further, in the case with silicon dioxide as the foreign oxide, as one unique feature produced by the above methods, the silicon +1 oxidation state may be substantially completely lacking in the foreign oxide layer. Thus, in the case of silicon as the foreign el ^¬ ement and the substrate having initially an oxidized surface layer, a semiconductor structure may be manu ^¬ factured comprising a foreign oxide which comprises silicon, the silicon dioxide being substantially free of +1 oxidation states. The lack of +1 oxidation states indicates a chemically abrupt, well defined in ^¬ terface between the silicon dioxide and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, some embodiments are described with reference to the accompanying drawings, wherein:

Figures 1 and 2 are schematic illustrations of the steps of methods in accordance with the invention; and Figures 3 to 5 show measurements of test structures manufactured in accordance with the invention.

DETAILED DESCRIPTION

In the method illustrated in Figure 1, a semiconductor substrate 1 is first provided in step a) . In this ex ^¬ ample, the semiconductor substrate 1 is illustrated as a stand-alone substrate element being entirely formed of a semiconductor material. Alternatively, a semicon ^¬ ductor substrate could have a semiconductor material layer lying on a support layer of some other material. A semiconductor substrate as illustrated in Figure 1 could also form an integral part of a larger device or structure .

Possible semiconductor materials of the semiconductor substrate are, for example, doped and un-doped germa ^¬ nium Ge and III-V compound semiconductors, such as gallium arsenide GaAs, indium gallium arsenide InGaAs, indium arsenide InAs, indium antimonide InSb and alu ^¬ minum indium phosphide AllnP.

Said "providing" may comprise just receiving or having the semiconductor substrate which has been formed pre ^¬ viously. Alternatively, forming the semiconductor sub ^¬ strate may be an integral part of the method. For ex- ample, providing the semiconductor substrate may comprise depositing it by molecular beam epitaxy MBE in vacuum conditions.

The substrate has a top surface 2 to be coat- ed/insulated/passivated by a dielectric oxide or ni ^¬ tride of a foreign element, i.e. an element different from the element (s) of the substrate. The foreign ele ^¬ ment may be, for example, silicon or aluminum, the ox ^¬ ide or nitride then being, for example, silicon diox- ide S1O ₂ or silicon nitride SiN or S1 ₃ N ₄ for silicon, or aluminum oxide AI ₂ O ₃ or aluminum nitride A1N for aluminum as the foreign element.

"Top" is used here just to refer to the location of the surface at issue at the top of the semiconductor substrate in the drawing of Figure 1, without binding the position of the substrate in any specific coordi- nates. Thus, it does not limit the positioning of such surface in reality: naturally, the substrate may be kept in positions where the surface thereof to be coated by the foreign oxide may face to any direction and may be even, for example, a bottom surface of the substrate .

In step b) , the top surface 2 of the semiconductor substrate formed in step a) is oxidized or nitridized so that an oxidized or nitridized surface layer 3 is formed on top of the semiconductor substrate. The oxi ^¬ dized or nitridized surface layer thus comprises ox ^¬ ide (s) or nitride (s) of the semiconductor material. Oxidation may be carried out simply by exposing the top surface to an ambient atmosphere where oxygen is present, thereby letting a native oxide layer form on the top surface. Alternatively, the top surface may be first cleaned from possible oxides and impurities, for example, chemically or, in the case of germanium as the semiconductor material, by argon ion sputtering or hydrogen exposure which may be followed by post heat ^¬ ing in ultrahigh vacuum conditions. Details of those cleaning processes are well known in the art. After cleaning, the top surface may be oxidized or ni- tridized in a controlled manner according to processes and principles as such known in the art. Naturally, oxidation or nitridation may also be carried out di ^¬ rectly after formation of the semiconductor substrate e.g. by means of MBE, in which case separate cleaning may be not necessary.

Thus, after step b) , a semiconductor substrate with an oxidized or nitridized surface layer is provided. In step c) , a foreign element 5 is deposited, prefera ^¬ bly in non-oxidizing conditions, on the oxidized or nitridized surface layer 3, thereby forming a transi- ent layer 4 on top of the structure. The deposition may be carried out by, for example, sputtering, evapo ^¬ ration, or atomic layer deposition. The deposition process details may be selected according to the prin- ciples, and by means of equipment, which are as such known in the art. Preferably, a layer with substantially uniform thickness is formed. During the deposi ^¬ tion, the semiconductor substrate may be kept at room temperature. In some other cases, the substrate may be heated or cooled above or below the room temperature, respectively. The non-oxidizing conditions mean that the deposition may be carried out in vacuum or with a low oxygen partial pressure present. The transient layer thickness and the foreign element amount contained therein may be adjusted according to the amount of oxygen or nitrogen present in the oxi ^¬ dized or nitridized surface layer 3. In other words, the amount of the foreign element in the transient layer may be set to equal to or exceed the amount, as calculated on the basis of the stoichiometry of the foreign element oxide or nitride to be formed, corre ^¬ sponding to the oxygen or nitrogen amount in the oxidized or nitridized surface layer.

The stack of the substrate with the oxidized or ni ^¬ tridized surface layer thereof and the transient layer of the foreign element is annealed, i.e. exposed to a heat treatment at an elevated temperature which is sufficiently high to initiate decomposition of the ox ^¬ idized or nitridized surface layer and to initiate diffusion of the oxygen or nitrogen atoms. It has been observed by the inventors that the oxygen or nitrogen atoms from the semiconductor oxide or nitride may dif- fuse towards the foreign element atoms of the transi ^¬ ent layer and react with them, thereby forming oxide or nitride of the foreign element. The heat treatment is preferably continued as long as all the semiconduc ^¬ tor oxide or nitride of the oxidized or nitridized surface layer is decomposed. Typical duration of the heat treatment may be, for example, 5 to 30 minutes.

Thus, as a consequence of the annealing, as illustrat ^¬ ed by step d) in Figure 1, the oxidized or nitridized surface layer of the semiconductor substrate and the transient layer thereon may be converted into a pure semiconductor substrate 1 and a foreign oxide or nitride layer 6 thereon. Advantageously, it has been ob ^¬ served by the inventors that it is thereby possible to achieve a high quality, chemically abrupt interface between the semiconductor and the oxide or nitride of the foreign element.

In the above, the method has been discussed at a gen ^¬ eral level with no connection to any concrete device. However, it is to be noted that the above process may be used to form a foreign oxide layer on a semiconduc ^¬ tor substrate, wherein the substrate and the oxide or nitride layer may form a part of any semiconductor device where a high quality foreign oxide-semiconductor or nitride-semiconductor interface is needed. In this sense, the above method may form a part of a method for forming a complete, operable semiconductor device. Correspondingly, the structure illustrated in Figure 1 may be considered illustrating a semiconductor device or a part thereof.

Such semiconductor device may be, for example, a light emitting diode, a photodiode, a semiconductor laser, a photovoltaic cell, a metal-oxide-semiconductor field- effect transistor or some other building block of com- plementary metal oxide semiconductor CMOS circuits, a high-electron-mobility transistor, or any other elec ^¬ tronic component with a foreign oxide-semiconductor interface. As one specific example of photovoltaics , not limiting the scope of possible applications, the method can be used to form a passivating foreign oxide layer on a germanium bottom layer of a four-terminal mechanical photovoltaic stack having a III-V top lay ^¬ er. An example of the general structure of a four- terminal mechanical photovoltaic stack is disclosed in Flamand et al, "Towards highly efficient 4-terminal mechanical photovoltaic stacks", Science Direct, III- Vs Review, Vol. 19, Issue 7, Sept-Oct 2006, pp 24 - 27. The top layer of such stack could be passivated, for example, by a method disclosed in WO 2012/062966 Al . The process of Figure 2 differs from that of Figure 1 in that no separate transient layer is formed. In ^¬ stead, in step c) , foreign element 5 is supplied on the surface of the oxidized or nitridized surface lay ^¬ er, as illustrated by the arrows marked in the drawing of Figure 4, while keeping the semiconductor substrate and the surface layer thereon at a sufficient high temperature to initiate odixation or nitridation of the foreign element adsorbed on the surface layer. Thereby, formation of a foreign oxide or foreign ni- tride layer 6 at least starts already during the sup ^¬ ply of the foreign element. Also in this case, the amount of the supplied foreign element atoms and the duration of keeping the semiconductor substrate at an elevated temperature may be selected so that all the semiconductor oxide or nitride of the oxidized or ni ^¬ tridized surface layer is decomposed. Finally, in step d) , a foreign oxide or nitride layer 6 is formed on the semiconductor substrate 1 with a high quality, chemically abrupt interface therebetween.

The principles of the method as discussed above were tested in practice. In one example, an amorphous sili- con film as a transient layer was deposited on a natu ^¬ rally oxidized germanium (100) substrate, after which the structure was heated step by step up to 600 °C in vacuum. In the graph of Figure 3, x-ray photoelectron spectra (XPS) of germanium 3d peak, measured between the heating/annealing steps, are shown.

The lowermost curve a) of the graph of Figure 3 repre ^¬ sents the spectrum measured from the sample with a na- tive oxide layer thereon before heating. The next curve b) was measured from the sample after deposition of a Si transient layer with a thickness of 2.5 nm on the native oxide. Curve c) represents the spectrum af ^¬ ter annealing the sample at 300 °C for 10 minutes. Curve d) was measured from the sample after annealing at 630 °C for 5 minutes. The uppermost curve repre ^¬ sents measurements from the sample after deposition of a further 2.5 nm Si layer on the surface thereof. The measurements were performed in situ, i.e. without re- moving the sample from the vacuum chamber in which it was annealed.

In the lowermost curve a) , the highest peak at the binding energy of about 33 eV results from germanium oxide. When the treatment proceeds, the peak position and shape shift clearly so that at step d) and finally at step e) , the signal from Ge oxides has effectively disappeared. Instead, there is a clear peak in inten ^¬ sity from pure Ge crystal at about 29 eV. The spectrum after annealing at 630 °C corresponds well with the bulk spectrum of germanium. Thus, the change of the spectrum in result of heating the sample implies com ^¬ plete decomposition of the germanium oxide (s). Other measurements were performed to confirm the for ^¬ mation of the silicon dioxide from the oxygen of the decomposed germanium oxide and the silicon of the amorphous silicon layer deposited initially on the ox ^¬ idized germanium surface. Further, tests were carried out where similar Ge/Si0 ₂ samples were additionally annealed in oxygen atmosphere (dose 3000 Langmuir) at 300 °C. The annealing did not change the germanium 3d spectrum, implying that the Ge substrate was not any ^¬ more oxidized during the additional annealing. Thus, the silicon dioxide effectively protects the underly ^¬ ing germanium surface, thereby preserving the chemical abruptness of the silicon dioxide-germanium interface.

In another test example, a metal-oxide-semiconductor capacitor containing a Si0 ₂ /Ge interface was manufac ^¬ tured. The Si0 ₂ /Ge interface was prepared similarly to the previous examples, i.e. by forming a transient layer of silicon on an oxidized surface layer of the Ge substrate, and annealing the thereby formed layer stack. Figure 4 represents capacitance-voltage curves measured from the sample for various frequencies. Ca- pacitance modulation with the voltage supports the as ^¬ sumption of formation of a high quality Si0 ₂ /Ge interface .

Figure 5 shows the leakage current measured from the same sample as the capacitance-voltage curves of Fig ^¬ ure 4. The very low leakage current is a further evi ^¬ dence of the assumed formation of a high-quality Si0 ₂ /Ge interface during the annealing of the germani ^¬ um/germanium oxide/amorphous silicon stack.

In the above test examples, germanium and silicon were used as the semiconductor material and the foreign el ^¬ ement of the foreign oxide layer, respectively. Other tests made with, for example, gallium arsenide GaAs as the semiconductor, support the applicability of the method for great variety of semiconductors, not only to Ge . In those tests, aluminum was used as one alter- native to silicon as the foreign element. Therefore, the embodiments of the invention are not limited to any specific semiconductor and foreign element material or their combinations. In particular, although the above test examples concerned formation of a foreign oxide layer on a semiconductor substrate, similar or corresponding results are achievable also in the case of forming a foreign nitride layer on a semiconductor substrate .

In general, it is important to note that the above ex ^¬ amples and embodiments do not limit the scope of pos ^¬ sible embodiments of the inventions, but the embodi ^¬ ments may freely vary within the scope of the claims.