Method for producing III-N templates and the reprocessing thereof and III-N template

Abstract

There is provided a template comprising a substrate comprising sapphire and at least one III-N crystal layer, wherein III denotes at least one element of the main group III of the periodic table of the elements, selected from the group of Al, Ga and In, wherein in a region of the at least one III-N layer above the substrate comprises a mask material as an interlayer, wherein the III-N crystal layer of the template is defined by one or both of the following values (i)/(ii) of the deformation .sub.xx: (i) at room temperature the .sub.xx value lies in the range of <0; and (ii) at growth temperature the .sub.xx value lies in the range of .sub.xx0.

Claims

1. A template comprising a substrate comprising sapphire and at least one III-N crystal layer, wherein III denotes at least one element of the main group III of the periodic table of the elements, selected from the group of Al, Ga and In, wherein in a region of the at least one III-N layer above the substrate comprises a mask material as an interlayer, wherein the III-N crystal layer of the template is defined by one or both of the following values (i)/(ii) of the deformation .sub.xx: (i) at room temperature the .sub.xx value lies in the range of <0; and (ii) at growth temperature the .sub.xx value lies in the range of .sub.xx0, wherein the interlayer is a discontinuous layer that, as viewed along a face surface of the template, the interlayer comprises nanometer-scale islands of mask material and microscopic and/or nanometer scale gaps lacking mask material.

2. The template according to claim 1, wherein in the III-N crystal layer of the template the .sub.xx value: (i) lies in the range of 0>.sub.xx0.003 at room temperature; and (ii) lies in the range of 0>.sub.xx>0.0006 at growth temperature.

3. The template according to claim 1, characterized in that when, for the template, the substrate comprises sapphire with a thickness (d.sub.sapphire) of approximately 430 m (i.e. 20 m) and the III-N crystal layer comprises GaN with a thickness (d.sub.GaN) of approximately 7 m (i.e. 0.5 m) is used or set, a curvature of the template (K.sub.T) at the growth surface of the crystalline III-N material (i) is in the range from 0 to 150 km.sup.1 at the growth temperature, and (ii) is in the range from 200 to 400 km.sup.1 at room temperature, wherein the curvature value depends on respective layer thicknesses (d.sub.sapphire/d.sub.GaN) analogous to the Stoney equation in the following range:
K.sub.T(dGaN;dsapphire)=K.sub.T(7m;430m)(430 m/d.sub.sapphire).sup.2(d.sub.GaN/7 m).

4. The template according to claim 1, wherein the at least one III-N crystal layer comprises one or more crystals selected from the group consisting of GaN, AlN, AlGaN, InN, InGaN, AlInN and AlInGaN.

5. The template according to claim 1, wherein the distance between the interlayer and the substrate is at most 300 nm.

6. The template according to claim 1, wherein the distance between the interlayer and the substrate is at most 50 nm.

7. The template according to claim 1, wherein the mask material is selected from the group consisting of Si.sub.3N.sub.4, TiN, Al.sub.2O.sub.3, SiO.sub.2, WSi, and WSiN.

8. The template according to claim 1, wherein the mask material is deposited at a thickness of below 5 nm.

9. The template according to claim 1, wherein the mask material is deposited at a thickness of below 1 nm.

10. The template according to claim 1, wherein the mask material is deposited at a thickness 0.3 nm or less.

11. A template comprising a substrate comprising sapphire and at least one III-N crystal layer, wherein III denotes at least one element of the main group III of the periodic table of the elements, selected from the group of Al, Ga and In, wherein in a region of the at least one III-N layer above the substrate comprises a mask material as an interlayer, the interlayer being a discontinuous layer that, as viewed along a face surface of the template, comprises nanometer-scale islands of mask material and microscopic and/or nanometer scale gaps lacking mask material, wherein the distance between the interlayer and the substrate is at most 300 nm.

12. The template according to claim 11, wherein the distance between the interlayer and the substrate is at most 50 nm.

13. The template according to claim 11, wherein the mask material is selected from the group consisting of Si.sub.3N.sub.4, TiN, Al.sub.2O.sub.3, SiO.sub.2, WSi, and WSiN.

14. The template according to claim 11, wherein the at least one III-N crystal layer comprises one or more crystals selected from the group consisting of GaN, AlN, AlGaN, InN, InGaN, AlInN and AlInGaN.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIGS. 1A and 1B schematically show stages of the growth process for forming III-N templates with sapphire substrate, and in respectively different embodiments according to the present invention;

(2) FIG. 2 illustrates temporal temperature, reflection and curvature profiles during the exemplary growth of GaN on sapphire (distance 15 nm);

(3) FIG. 3 illustrates temporal temperature, reflection and curvature profiles during growth of GaN on sapphire (distance 300 nm);

(4) FIG. 4 shows the change of the curvature of the growth surface according to a different principle, in which optionally a deposition of an interlayer with mask material is combined with a change of the III-N growth temperature during the growth of the III-N layer of the template;

(5) FIGS. 5A and 5B show the change of the curvature of the growth surface mainly in dependence on the provision and location/positioning of an interlayer with mask material according to different possible embodiments of the present invention,

(6) FIG. 5C shows the results with respect to curvature of the growth surface when the templates defined according to FIGS. 5A and 5B are subjected to a further III-N (GaN) layer growth for producing thicker layers; and

(7) FIG. 6 illustrates temporal temperature, reflection and curvature profiles during conventional growth of GaN on sapphire.

(8) Without thus limiting the present invention, the following detailed description of the figures, aspects, embodiments and particular features will illustrate the invention and describe particular embodiments in detail.

DETAILED DESCRIPTION

(9) In the process for producing III-N starting substrates it was surprisingly found that by appropriately positioning a interlayer of mask material the templates can be significantly favourably influenced with respect to the relevant parameters of curvature of the growth surface at the template and/or suitable stress in the template such that the subsequent growth of III-N single crystals with excellent properties is enabled and in particular the subsequent tendency to crack formation in III-N single crystals growing on the template is significantly reduced.

(10) For producing the template initially a substrate is provided which is selected from starting substrates comprising sapphire or respectively consisting of sapphire as well as such starting substrates with structures formed thereon, for example with specific externally (ex situ) formed mask structures. A further possibility of providing a suitable starting substrate can comprise the formation of interlayers or intermediate structures for the purpose of supporting the later separation from the starting substrate, and/or the formation of a so-called GaN nano grass which is based on a substrate having formed thereon a GaN compliance layer with nano-column structure, as for example described in WO 2006035212 A1, WO 2008096168 A1, WO 2008087452 A1, EP 2136390 A2 and WO 2007107757 A2.

(11) A patterning optionally carried out ex situ, such as for example the opening of windows and other mask structures, thus belongs at the most to the step of providing the starting substrate, however not to the actual step of inserting the mask interlayer, as is described in the following in connection with the process according to the invention.

(12) For the provision of a starting substrate a foreign substrate with sapphire is used, preferably it consists of sapphire. More preferably a sapphire substrate with c orientation is used having a tilt towards (1-100) or (11-20) by 0.1 to 0.5 and a one-sided epi-ready polishing and a polished and/or preferably lapped backside. A further embodiment provides for the starting substrate to exhibit a surface structured by lithography or wet chemical or dry chemical etching processes (e.g. ICP etching).

(13) Based on the schematic FIGS. 1A and 1B, exemplary, non-limiting, however respectively varied embodiments are described. At this point it is noted that the thickness of a substrate (reference signs 100A and 100B, respectively) is substantially larger than the thicknesses of III-N material and III-N layers which are formed thereon, and further that a main part of the epitaxially grown III-N layer of the template (reference signs 105A and 105B, respectively) is substantially larger than a thickness of III-N material (reference signs 103A and 103B, respectively) below the interlayer with mask material (reference signs 102A and 102B, respectively), which is indicated by the respective breaks at the respective left-hand borders of the layers 100A/100B and 105A/105B.

(14) In the FIGS. 1A and 1B in the same step (1) initially the provision of the respective substrates 100A and 100B is shown. The respective substrates can optionally be pre-treated as described above, in particular said substrates can respectively be subjected to a desorption step and a nucleation step. In such an optional desorption step for example hydrocarbon residues but also other volatile contaminants can be removed from the starting substrate or a structured or otherwise pre-treated substrate. During the desorption step the starting substrate is heated in the process to an elevated temperature, preferably to a temperature from 1100 to 1300 C., more preferably to a temperature from 1150 to 1250 C., for example approximately around 1200 C. Due to the temperature gradient within the substrate the substrate is typically subjected to a bending (bowing, curvature), normally with a concave curvature with respect to the surface on which subsequently the III-N material is deposited. Optionally, the desorption step can further be followed by nitriding with ammonia. A further optional step consists of lowering the temperature after desorption occurred, for example to a temperature between 400 and 600 C., preferably to a temperature between 450 and 550 C. During this cooling thetypically concavecurvature decreases again, for example to the level like in the beginning of the heating to the desorption step.

(15) The provision and the pre-treatment of a substrate in the process for producing a template according to the present invention can preferably further comprise a nucleation step in which crystalline III-N material, especially minute III-N crystallites are grown onto the starting substrate. This step is schematically shown in the in this respect same step (2) of FIGS. 1A and 1B. The crystalline III-N material 101A and 101B, respectively, especially the III-N crystallites serve as seed crystals in the later further III-N crystal growth. III-N crystallites exhibit sizes from for example 1 to 40 nm with irregular forms, are generally present disorderedly on the starting substrate and suitably form initially a non-continuous nucleation layer. In the case of a low temperature GaN nucleation, this nucleation step typically takes place at a temperature from 400 to 600 C., preferably from 450 to 550 C. and more preferably from 500 to 540 C.

(16) An AlN nucleation typically takes place at a temperature from 850 to 1050 C., preferably from 900 to 1000 C. and more preferably from 950 to 960 C.

(17) During the low temperature nucleation step, optionally also during the subsequent heating to growth temperature, a re-crystallization can optionally occur.

(18) After the provision of a substrate, optionally with the optional measures described above, the further steps of the respective embodiments according to the invention can vary with respect to time point and position/location of the layer of the mask material and the resulting consequences thereof, as is illustrated respectively separately in FIGS. 1A and 1B.

(19) In the embodiment shown in FIG. 1A an interlayer made of mask material 102A is directly deposited already on the nucleation layer 101A, still before coalescence of the crystallites starts. In a further modification (not specifically shown here) this deposition of the interlayer is carried out not directly on the nucleation layer but only after a very short phase of a III-N growth, but still very close in the nanometer range to the nucleation layer, for example, at a distance in a range of up to 30 nm. In this distance range selected very close to the nucleation layer the subsequent steps occur practically analogous to the form shown in FIG. 1A.

(20) In the embodiment shown in FIG. 1B, initially a III-N growth is carried out on the nucleation layer 100B for a particular, generally still relatively short time, for example until a small thickness of the crystalline III-N layer 103B of 30 nm or beyond and suitably up to approximately 300 nm has formed, preferably up to approximately 100 nm, more preferably up to approximately 50 nm, and only then an interlayer made of mask material 102B is deposited in the corresponding distance from the nucleation layer of the substrate.

(21) Suitably and advantageously the deposition of the denoted interlayer 102A or respectively 102B is carried out in the same reactor with a process which is compatible with the technique for growing the III-N layer. For this purpose appropriate starting materials or reactive derived products or species of the mask material are reacted with each other in the reactor at appropriate temperature and further parameters which are suitable for the deposition of mask material. In its simplest form, a deposition of nitride mask material such as silicon nitride is carried out, because its deposition is well compatible with III-N deposition techniques. For this deposition, frequently same or similar, or at least compatible conditions with respect to reactor pressure and temperature can be chosen as for the III-N deposition and apart from that only suitable gas compositions and gas flow rates have to be adapted such that this process modification is easy to handle. For example, a silane gas and ammonia is flown into the reactor and reacted together at a suitable pressure and a suitable temperature of for example 800 C. to 1200 C., preferably at about 1050 to 1150 C. and is deposited in the form of Si.sub.3N.sub.4 and optionally further stoichiometric or over- or substoichiometric Si.sub.xN.sub.y compositions on the prepared substrate (100A; 101A). The step of depositing mask materials other than SiN, such as for example TiN, Al.sub.2O.sub.3, SiO.sub.2, WSi, and WSiN, can readily and accordingly be adjusted.

(22) In this way a mask layer (102A and 102B, respectively) is formed on the nucleation layer (cf. FIG. 1A; 101A) or alternatively on the just growing III-N layer (cf. FIG. 1B; 101B), which is still very close to the nucleation layer. The mask layer can exhibit different forms. It is generally homogeneously distributed on the surface and may form a continuous layer, alternatively however it exhibits rather microscopic/nano-structured gaps; these possibilities are shown in the drawing schematically in the form of a dashed layer 102A or respectively 102B.

(23) The thickness of the interlayer 102A or respectively 102B comprising respectively mask material is very small which can be set by corresponding gas flow rates and short process times; said thickness suitably lies in the nanometer or sub-nanometer range, for example below 5 nm, more preferably below 1 nm, in particular down to below one monolayer (i.e. 0.2 to 0.3 nm or less).

(24) The distance of the interlayer 102A or respectively 102B from the substrate is small and suitably lies in the range of up to a maximum of 300 nm, for example at 1 to 200 nm and in particular at a few tens of nanometers, preferably 30 to 90 nm, more preferably 40 to 60 nm.

(25) After depositing the interlayer with mask material, the (continued) growth of a III-N layer 104A, 104B (stage (4) in FIG. 1A/1B) is carried out immediately thereafter until the template at the end of the growth (stage (5) in FIG. 1A/1B) exhibits a III-N layer 105A, 105B with desired thickness in the range from 0.1 to 10 m, preferably in the range from 3 to 10 m (total thickness of the III-N layer of the template including the interlayer of the mask material and optionally nucleation layer). According to the invention it is made sure that the characteristics curvature (measured at the growth surface) and/or stress of the III-N layer of the template are favourably influenced and advantageously used for subsequent processes. In contrast to conventional methods, in which typically a concave curvature of the growth surface forms, which then in the course of further crystal growth, i.e. with increasing thickness of the III-N layer, increases further, according to the invention it is effected that the curvature of the template decreases during the subsequent further growth of a growing III-N layer 104A or respectively 104B, as shown schematically in the respective steps (4) of FIGS. 1A/1B. In the particular case of FIG. 1A it is made sure that immediately subsequent to the deposition of the interlayer of mask material 101A the growing III-N layer untypically bows negatively/convexly with the coalescing of this III-N layer, thus building up a desired compressive stress in the template. Although in the specific case of FIG. 1B in step (3) initially a slightly concave curvature with tensile stress in the III-N crystal 103B is present, it is however made sure in this case that the curvature at least increases significantly less compared to a situation without deposition of the mask layer 102B at suitable location/position, and that optionally even a decrease of the curvature is achieved and thus a curvature difference K.sub.sK.sub.e0 is observed.

(26) In the case that the desired curvature behaviour is not or is not on its own achieved by the position/location of the interlayer with mask material in a suitable distance with respect to the surface of the foreign substrate or respectively the nucleation layer, this behaviour can be controlled by additional and purposive setting of other process parameters for the purpose that a supplemental contribution to the relation K.sub.sK.sub.e0 is provided, or that the template at growth temperature is essentially not bowed or is negatively bowed. An adjustment and optionally a variation of the III-N growth temperature is for this purpose a particularly suitable, new further process parameter. For the case of using sapphire as foreign substrate which has a higher thermal expansion coefficient than the III-N crystal to be grown the growth/the deposition occurs at a lowered growth temperature compared to that of the preceding growth. This change in temperature is carried out most effectively during a limited, preferably relatively early phase of the growth of the III-N layer of the template, and the growth is continued at this lowered temperature. A substantial curvature reduction with K.sub.sK.sub.e0 is achieved supplementarily for example by carrying out growth in at least one growth phase of the III-N crystal of the template at a growth temperature decreased by at least 10 C. compared to that of the preceding growth. The decrease of the growth temperature is preferably at least 20 C. and lies more preferably in the range from 20 to 50 C. and in particular in the range from 25 to 40 C.

(27) In the case of GaN a typical growth temperature lies for example in the range from 900 C. to 1200 C., preferably about 1020 to 1150 C., more preferably around approximately 1100 C.20 C. In the case of AlGaN with an Al fraction from 30% up to 90% a typical growth temperature lies for example in the range from 1070 to 1250 C., preferably at 1090 to 1270 C., and more preferably at 1170 C. Temperatures for the deposition of other III-N materials are adjusted accordingly on the basis of the general common knowledge.

(28) If optionally used and optionally desired, the system can, as described in the above denoted particular embodiment, initially be brought to a corresponding predetermined (first) temperature, wherein at this first temperature optionally only a re-crystallization occurs, and this temperature is then varied, however only to the extent of a changed (second) temperature at which still crystal growth and preferably epitaxial crystal growth can occur in order to eventually influence optionally the curvature behaviour additionally. When optionally used, this takes place preferably in the beginning or during the coalescence of the growing III-N crystallites or in the early phase of the growing III-N layer of the template. Because of the choice of sapphire as foreign substrate a decrease of the growth temperature is carried out. During continuing of the (preferably epitaxial) crystal growth in the range of the accordingly varied second growth temperaturei.e. below the respective first temperaturethe respective curvature of the growth surface further decreases continuously or intermittently. As soon as a sufficient supplemental curvature decrease is achieved via this optional step of temperature variation, the temperature for the further growth of the III-N layer can be again freely selected, for example in the range of the typical growth temperatures mentioned above, such as for GaN and AlGaN.

(29) The III-component, based on preceding steps such as during the formation of the initial III-N crystallites or respectively the nucleation layer on the substrate as described above can at the change to the step of the growth of the actual epitaxial III-N layer be kept the same, or alternatively it can be varied. The nucleation layer (cf. 102A or respectively 102B in FIGS. 1A/B) can for example be composed of GaN or AlN, and the epitaxial III-N layer of the template (cf. 104A-105A or respectively 104B-105B in FIGS. 1A/B) can independently therefrom be composed of GaN or AlGaN (preferably made of GaN). In a particular embodiment, the III-component is not changed.

(30) Referring again to the embodiments according to FIGS. 1A and 1B specifically described here, according to the invention it is made sure that in the course of the further growth of the entire III-N layer 105A or respectively 105B of the template in the micrometer range (typically up to 10 m) the curvature decreases further continuously by appropriate deposition of a single interlayer of the mask material 102A or respectively 102B, in the case of FIG. 1A with the tendency to further negative curvature values, in the case of FIG. 1B howeversince starting from a slightly positive/convex curvature in stage (3)towards a condition of an essentially absent curvature (cf. respectively stage (5) in FIGS. 1A/1B).

(31) When the curvature value at the beginning of the crystal growth of the III-N layer or directly subsequently to the deposition of the interlayer of the mask material (such as for example in FIG. 1 approximately at stage (3)) is denoted K.sub.s or respectively K.sub.S (K.sub.start) and the curvature value at a later time point (such as for example in FIG. 1 at stage (4)) and in particular towards the end of the growth of the III-N layer of the template (for example according to FIG. 1 at stage (5)) is denoted as K.sub.e or respectively K.sub.E (K.sub.end), then the curvature difference (K.sub.sK.sub.e) of the template exhibits, respectively measured at growth temperature, a positive sign. Preferably, K.sub.sK.sub.e is at least 5 km.sup.1, more preferably at least 10 km.sup.1. On the other hand the curvature difference (K.sub.sK.sub.e) is preferably selected as not too large; it should thus preferably not be larger than 50 km.sup.1, more preferably not be larger than 20 km.sup.1.

(32) By recognizing and actively influencing this behavior and the relationships associated therewith, the process according to the present invention enables the production of a template comprising a first III-N layer, wherein said template at epitaxial growth temperature is not bowed, in any case is essentially not bowed (as is illustrated for example in FIG. 1B stage (5)), or is negatively bowed (as for example illustrated in FIG. 1A stage (5)). Separated by dashed lines, the stages (5) and (6) illustrate the respective final conditions of the completely produced template, respectively at growth temperature (stage (5)) or respectively after cooling at room temperature (stage (6)).

(33) In a preferred embodiment of the present invention all crystal growth steps described above in the first embodiment, including the optionally performed nucleation step, are carried out via Metal-Organic Vapor Phase Epitaxy (MOVPE). Alternatively or in combination the crystal growth steps described before can however also be performed via HVPE.

(34) When layer thicknesses of suitably at least 0.1 m, for example in the range from 0.1 to 10 m, preferably from 2 to 7.5 m of the III-N layer produced as described above are deposited on the substrate, a template is provided according to the invention, said template being excellently suited as starting template for further use or processing for the epitaxial growth of further layers and in particular of further III-N layers, and then the problem of the tendency to crack formation can be counter-acted, in particular when subsequently significantly thicker III-N layers such as III-N bulk crystals (ingots, boules) are grown or deposited. Suitable techniques for growing or depositing thicker III-N layers such as III-N bulk crystals can for example be selected from Vapor Phase Epitaxy (VPE) in particular Hydride Vapor Phase Epitaxy (HVPE), ammono-thermal processes, sublimation and the like.

(35) An exemplary course of the process of the invention according to a possible embodiment is illustrated in FIG. 2. Therein the following parameters are plotted as a temporal profile: The change of the growing surface (discernable by the profile of the decreasing amplitude of the reflectivity, given in arbitrary units a.u. in the lower part of the figure) as well as the temperature (left-hand ordinate, upper line corresponding to process temperature, lower line corresponding to wafer temperature) and the change of the curvature (right-hand ordinate) of the growth surface. The measurement of the curvature of the growth surface is carried out in situ, performable with an EpicurveTT curvature measurement device by the company LayTec (Berlin, Germany), said device allowing to simultaneously obtain data on temperature, reflection and curvature of the growth surface.

(36) Individual process stages or respectively phases are denoted in FIG. 2. These are correspondingly analogous to the depiction in FIG. 1. The phases which in FIG. 2 are denoted with desorption and GaN nucleation correspond to the stages denoted (1) and (2) in FIGS. 1A/1B and thus to the provision of a sapphire substrate. The interlayer with mask material is deposited without distance or respectively temporal delay and thus immediately on the nucleation (GaN or AlN nucleation; cf. FIG. 1A stage (3)), or close thereto or respectively after only short (optionally very short) growth duration (cf. FIG. 1B stage (3)), as shown in FIG. 2. The phase denoted with the term GaN layer corresponds to an epitaxial III-N crystal growth according to stages (4) to (5) of FIG. 1. In the course of the epitaxial III-N crystal growth which subsequently follows the deposition of the interlayer, the technical realization is recognizable that the curvature starting from K.sub.s decreases in the temporal course until at the end of the growth of the III-N (GaN) layer of the template the lower curvature K.sub.e is achieved, which in comparison to K.sub.s is smaller by approximately 20 to 30 km.sup.1. Finally, it can be cooled down to room temperature (cf. FIGS. 1A/B stage (6)). With such a procedure it becomes possible that the III-N (GaN) layer of the template at the end at growth temperature, i.e. at a thickness in the range of a few micrometers, is essentially not bowed; for example the curvature value (K.sub.e) at epitaxial growth temperature can lie in the range of maximum 30 km.sup.1, rather still around 20 km.sup.1 around zero. The procedure can, if desired, be varied such that at the end K.sub.e at growth temperature is negative and that thus the template exhibits a convex curvature. After cooling to room temperature, the template has a clearly increased convex curvature and thus a markedly compressive stress. When this template is heated again to growth temperature at a desired time pointfor example for growing a thick III-N bulk crystal, the condition with essentially absent or alternatively negative curvature is obtained again, which according to the surprising findings of the invention forms an excellent basis for further epitaxial III-N growth with decreased tendency to crack formation.

(37) FIG. 3 shows a corresponding course (illustration and labeling as given in FIG. 2) based on sapphire samples as starting substrate, when, as opposed to in FIG. 2, an interlayer made of mask material is deposited significantly later (namely 300 nm). Contrary to the case of FIG. 2 in which the mask material-interlayer was deposited in the very small distance of 15 nm, in the case of FIG. 3 a concave curvature indeed increases (i.e. K.sub.sK.sub.e is <0). However, also the embodiment according to FIG. 3 shows that with a predetermined specification of the distance the degree of curvature in the obtained template can be purposively set as desired. Accordingly, the template of FIG. 3 after cooling to room temperature has a markedly lower convex curvature than the template of FIG. 2. In view of avoiding crack formation during further epitaxial III-N growth, in particular with thicker III-N layers, the deposition of the mask material-interlayer with very small distance (approximately from 0 nm to approximately 50 nm, cf. representatively FIG. 2 with exemplary distance of 15 nm) is strongly advantageous here in comparison to the result according to FIG. 3 with a distance of about 300 nm.

(38) FIG. 4 schematically shows in a possible variant the change to be expected of the curvature of the growth surface (right-hand ordinate) and respectively applied temperature (left-hand ordinate, upper line corresponding to process temperature, lower line corresponding to wafer temperature) for the case of a possible further embodiment of the present invention, in which optionally a deposition of an interlayer with mask material is combined with a reduction of the III-N growth temperature during the growth of the III-N layer of the template. In this possible and optional embodiment, after providing a sapphire foreign substrate, which also comprises the deposition of a very thin GaN or AlN nucleation layer (discernable in the diagram in the phases of an initial high temperature phase with a subsequent low temperature phase) initially heating to growth temperature is again performed, then optionally followed by ahere not shownre-crystallization phase, to then deposit an interlayer with mask material as described above, which can as described for example occur at a time point when already a III-N layer of above 50 nm or above 100 nm and or for example 300 nm, has grown. Additionally and in contrast to the basic embodiments illustrated in FIGS. 1 and 2, now however, either simultaneously with this interlayer deposition or in a particular, preferably short time period before or thereafter, a temperature decrease (cf. the temperature ramp of approximately 30 C. after increasing to growth temperature shown in FIG. 4) is applied, and then the growth of the III-N layer of the template is continued at this lowered temperature to obtain an additional contribution to the curvature reduction. In the overall result of this combination it is to be expected that the curvature of the template at the growth surface decreases in the course of the further growth, i.e. K.sub.sK.sub.e lies markedly above zero, as shown in principle in FIG. 4.

(39) Alternatively to the temperature decrease for continued growth of the III-N layer of the template, further process parameters can be applied to provide for the observation of the relation K.sub.sK.sub.e>0.

(40) From FIGS. 5A, 5B and 5C further possibilities and different embodiments for controlling curvature and/or stress in the III-N layer of the template, in order to grow on this basis at least one further III-N layer and optionally a thick bulk crystal with reduced tendency to crack formation on the template become apparent, wherein FIGS. 5B and 5C also contain results for comparisons without interlayer (respectively lines (B) and (E)). In FIGS. 5A-C the curvature behaviour of the III-N layer of the template at growth temperature is plotted against its thickness. In this connection, FIGS. 5A and 5B show the phase of the producing of corresponding templates, wherein here exemplarily as growth technique MOVPE was used, whereas FIG. 5C shows a later phase of the producing of thicker III-N (here GaN) layers using these corresponding templates, wherein here the HVPE growth technique was used, which is particularly well suited for producing thicker layers up to bulk crystals.

(41) The results show a consistent trend that the curvature behaviour is significantly influenced by how the distance of a single interlayer of mask material is set with respect to sapphire starting substrate or respectively an optional nucleation layer thereon. It is apparent that for the used experimental constellation in the system sapphire/GaN used here, the condition of K.sub.sK.sub.e>0 is safely observed for the cases of no distance (i.e. line 0 nm) and for the cases of distances 15 nm and 30 nm and still obvious for distances up to about 50 nm. For larger distances of above 50 nm, i.e. as shown at 60 nm, 90 nm and 300 nm, this condition is primarily not fulfilled, but surprisingly the increase of the curvature is comparatively strongly attenuated with the consequence that such an increase can be kept smaller than in cases in which no interlayer of mask material was deposited (cf. FIGS. 5B and C, comparative lines (B) and (E)), and in particular when growth was on a nucleation layer which was deposited at low temperature (LT-GaN-Nucl) (cf. FIGS. 5B and C, comparative line (E)). The procedure according to the present invention shows the significantly better controllability of favorable template properties as well as the possibility to precisely set template curvature as desired, which can be used for the further III-N epitaxial growth.

(42) FIG. 5A thus confirms that the provision of only a single interlayer of mask material in a distance which is set in a controlled way relative to the sapphire starting substrate, optionally with an optionally formed nucleation layer thereon, allows a very precise setting and control of a desired curvature value. It can be further seen that when observing relevant factors such as distance of the interlayer of mask material and thickness of the grown III-N layer and thus the growth duration such a condition in the respective finished, substrate-bound III-N template can be set at which the template according to the alternative solution principle for avoiding crack formation at growth temperature is not or essentially not bowed or is negatively bowed, i.e. the curvature can be limited to at most 30 km.sup.1, better to at most 20 km.sup.1 or better still to at most 10 km.sup.1 or preferably can even be set to negative values. Furthermore, as described, it is possible to influence the difference in a supplementary fashion with further process parameters such as temperature variations during growth of the III-N layer of the template, and if desired to make sure that the particular relation of K.sub.sK.sub.e>0 is safely observed.

(43) The obtained template according to the invention exhibits advantageous properties and features, which will be further described in the following. As such it is an interesting commercial object, it can however also be further processed as template within further steps described below, directly subsequently or alternatively indirectly after providing, storing or shipping.

(44) A template for producing of further III-N single crystal according to the present invention is in the temperature range of an epitaxial crystal growth not bowed or essentially not bowed, or it is negatively bowed. When for example for the template as substrate sapphire with a thickness (d.sub.sapphire) of 430 m (approximately, i.e. 20 m) and as III-N crystal layer GaN with a thickness (d.sub.GaN) of 7 m (approximately, i.e. 0.5 m) is used or set, then the term essentially not bowed is preferably defined such that the curvature value (K.sub.e) at epitaxial growth temperature lies in the range of maximum 30 km.sup.1, preferably in the range of maximum 10 km.sup.1 around zero; the term not bowed then denotes a K.sub.e value of approximately zero, for example 05 km.sup.1 and in particular 02 km.sup.1; and the term negatively bowed is then defined by a curvature at growth temperature in the range of below 0 km.sup.1, for example in the range up to 150 km.sup.1, more preferably in the range from 25 to 75 km.sup.1.

(45) It is noted that when using other materials for III-N than GaN, the exact curvature value can vary; however, according to the invention the intended setting of a (essentially) non-curvature or a negative curvature is maintained. Furthermore, when setting different layer thicknesses, the curvature value can vary depending on the respective layer thicknesses analogous to the following simplified Stoney equation, according to whichas long as the film (d.sub.III-N) is significantly thinner than the substrate (d.sub.substrate)the following relationship applies [wherein R=curvature radius and .sub.xx=strain]:
1/R=6*(d.sub.III-N/d.sup.2.sub.substrate)*.sub.xx.

(46) Assuming a very thin layer .sub.xx is considered to be constant, i.e. when the layer thicknesses change the system reacts with a change of R (the change of .sub.xx resulting from a change of the curvature is neglected). Thus, when using the exemplary materials sapphire and GaN, but for setting layer thicknesses (d.sub.sapphire/d.sub.GaN) other than the afore-mentioned, the curvature value lies depending on the respective layer thicknesses analogous to the Stoney equation in the following range:
K.sub.T(dGaN;dsapphire)=K.sub.T(7m)(430 m)(430 m/d.sub.sapphire).sup.2(d.sub.GaN/7 m),
wherein in the case of choosing other materials this equation is calculated with respective values of d.sub.substrate/d.sub.III-N.

(47) For a template according to the present invention this for example means further that when for 430 m of sapphire and for a 3.5 to 4 m thick GaN layer a curvature of 250 km.sup.1 is present, for the same process a curvature of 425 km.sup.1 results for a sapphire wafer with a thickness of 330 m.

(48) It is further noted that the curvature at room temperature is changed compared to the curvature at growth temperature, and can possibly be markedly different. For example, when using sapphire as foreign substrate the templateas a result of the plastic deformation during the cooling from the growth temperature to room temperature, mainly due to the different thermal expansion coefficients of the different crystalline materialsis additionally imprinted with a stress (which is only produced through extrinsic compression). This is schematically illustrated in FIGS. 1A and 1B in a status of the final stage (6) of the produced template according to the present invention at room temperature, wherein in comparison to the final stage (5) at growth temperature respectively a markedly stronger negative curvature is present. Accordingly it is observed, supplementarily or alternatively, that for example for the materials sapphire and GaN the curvature at room temperature is specified in the range from K.sub.T(7m; 430m)<200 km.sup.1, preferably from 200 to 400 km.sup.1, preferably in the range from 300 to 350 km.sup.1; wherein again for cases of other layer thicknesses it is referred to the simplified Stoney equation,
K.sub.T(dGaN;dsapphire)=K.sub.T(7m;430m)(430 m/d.sub.sapphire).sup.2(d.sub.GaN/7 m).

(49) In a further preferred embodiment, the template at room temperature exhibits a curvature radius in the range from 4 to 6 m for the case of d.sub.sapphire=430 m and d.sub.GaN=3.5 m.

(50) Another possibility to characteristically describe the product properties or structure properties of the template obtained according to the present invention is to specify the strain of the lattice constants or the stress.

(51) The strain .sub.xx is defined as follows:

(52) ${\mathrm{.Math.}}_{\mathrm{XX}}=\frac{\mathrm{lattice}\mathrm{constant}a-\mathrm{lattice}\mathrm{constant}{a}_0}{\mathrm{lattice}\mathrm{constant}{a}_0}$
wherein a denotes the actual lattice constant in the crystal and a.sub.0 denotes the theoretical ideal lattice constant.

(53) X-ray methods for determining absolute lattice constants are discussed in detail in M. A. Moram and M. E. Vickers, Rep. Prog. Phys. 72, 036502 (2009).

(54) Thereby the determination is carried out using Bragg's Law
n=2d.sub.hkl sin
initially for the lattice constant c from a 2theta-scan with three-axes-geometry in symmetrical reflexes such as for example 004. The ideal lattice constant according to V. Darakchieva, B. Monemar, A. Usui, M. Saenger, M. Schubert, Journal of Crystal Growth 310 (2008) 959-965) is c.sub.0=5.185230.00002 . The determination of the lattice constant a is then carried out using the equation,

(55) $\frac{1}{d_{\mathrm{hkl}}^2}=\frac{4}{3}\frac{h^2+k^2+\mathrm{hk}}{a^2}+\frac{l^2}{c^2}$
also given for example in M. A. Moram and M. E. Vickers, Rep. Prog. Phys. 72 (2009) 036502, from asymmetrical reflexes hkl such as for example 105 in the 2theta-scan. According to V. Darakchieva, B. Monemar, A. Usui, M. Saenger, M. Schubert, Journal of Crystal Growth 310 (2008) 959-965), the ideal lattice constant a.sub.0 for unstressed GaN can be assumed to be a.sub.0=3.189260.00004 .

(56) As to the background of the phenomena of intrinsic and extrinsic stress, among others considering lattice constants, cf. Hearne et al., Appl. Physics Letters 74, 356-358 (2007).

(57) Furthermore, the properties can also be given by the stress .sub.xx, wherein
.sub.xx=M.sub.f.Math..sub.XX(Hooke's formula)
wherein M.sub.f denotes the biaxial elastic modulus. The determination of the stress .sub.xx is readily possible via Raman spectroscopy, for example as described in I. Ahmad, M. Holtz, N. N. Faleev, and H. Temkin, J. Appl. Phys. 95, 1692 (2004); therein the biaxial elastic modulus of 362 GPa is derived from the literature as a value, wherein a very similar value of 359 GPa can be taken from J. Shen, S. Johnston, S. Shang, T. Anderson, J. Cryst. Growth 6 (2002) 240; thus a suitable and consistent value for the biaxial elastic modulus M.sub.f is about 360 GPa.

(58) A template according to the present invention exhibits in the temperature range of an epitaxial crystal growth a value of .sub.Xx0 (i.e. including .sub.XX=0), but in particular of .sub.XX<0. This value can be directly determined from an in situ measurement of the curvature.

(59) Besides the presence of an interlayer with mask material, a template according to the present invention can further exhibit at room temperature a compressive stress of .sub.xx<0.70 GPa, and/or the strain .sub.xx of the template at room temperate can be set to a value in the range of .sub.xx<0, preferably in the range 0>.sub.xx0.003, more preferably in the range 0.0015.sub.xx0.0025 (or 0.0015>.sub.xx0.0025) and in particular in the range 0.0020.sub.xx0.0025.

(60) A suitable curvature measurement device, which is applicable in combination with an apparatus for vapor phase epitaxy, is for example the curvature measurement device of Laytec AG, Seesener Strasse, Berlin, Germany (cf. for example DE102005023302 A1 and EP000002299236 A1). These curvature measurement devices are well adapted to be combined with available equipments for vapor phase epitaxy, such as MOVPE, HVPE or MBE (Molecular Beam Epitaxy) and furthermore enable a measurement of the temperature at the wafer surface.

(61) Accordingly after the epitaxial crystal growth a template is obtained which, based on the above-described properties, is suited to produce crystals of outstanding quality and with outstanding features in further epitaxial growth steps. The template according to the invention is thus excellently suited for the further use, it can also as such be provided, stored or shipped for further use, or it can be directly further used in an entire process.

(62) A further aspect of the present invention is a process for producing III-N single crystals, wherein III denotes at least one element of the main group III of the periodic table of the elements, selected from Al, Ga and In, wherein the process comprises the following steps:

(63) aa) providing a template which comprises a starting substrate comprising sapphire and at least one III-N crystal layer, wherein the starting substrate and the at least one III-N crystal layer are formed such that the template in the temperature range of an epitaxial crystal growth exhibits no or almost no curvature or exhibits a negative curvature, and

(64) bb) carrying out an epitaxial crystal growth for forming further III-N crystal on the template according to aa), optionally for producing a III-N bulk crystal,

(65) cc) optionally separating III-N single crystal or III-N bulk crystal and foreign substrate.

(66) This aspect of the invention starts from the alternative solution principle of minimizing or eliminating altogether the risk of crack formation by the preconditions specified in the steps aa) and bb).

(67) In a preferred embodiment, the template provided in the step aa) comprises the interlayer with mask material described above, wherein in this respect it is referred to the above description concerning the formation of the template exhibiting such an interlayer. In this aspect of the invention according to the alternative solution principle, the presence of such an interlayer is however not necessarily required, because the curvature condition defined in the step aa) can alternatively also be adjusted by other conditions, specifically by suitable temperature control and temperature variation during the III-N growth of the template, as is also described elsewhere in other passages.

(68) As a result of imprinting lattice deformation and compressive stress according to the present invention, the condition of the template provided in the step aa) can also be defined in that the III-N crystal of the template at growth temperature exhibits a value of .sub.xx0 (i.e. including .sub.xx=0), but in particular a value of .sub.xx<0, wherein the value lies preferably in the range from 0>.sub.xx>0.0006, and more preferably in the range from 0.0003>.sub.xx>0.0006. At room temperature, a compressive stress of .sub.xx<0.70 GPa can be present. The strain .sub.xx at room temperature of the template according to the present invention exhibits preferably a value in the range from 0>.sub.xx0.003, more preferably in the range from 0.0015.sub.xx0.0025 (or 0.0015>.sub.xx0.0025), and in particular in the range from 0.0020.sub.xx0.0025.

(69) In a further embodiment of the present invention, III-N single crystals are produced which are obtained bywithout or with interruption between the steps aa) and bb)carrying out an additional epitaxial crystal growth on the template obtained according to the invention for forming further III-N crystal. Further epitaxial III-N crystal growth can be performed at a growth temperature which can be selected independently from the above-mentioned crystal growth temperatures.

(70) Also other conditions of the further crystal growth on the template can now be chosen freely. Thus, III-N materials can be grown in which the III-component can be selected and varied as desired. Accordingly, at least one (optionally further) GaN, AlN, AlGaN, InN, InGaN, AlInN or AlInGaN layer(s) can be deposited for producing accordingly thicker III-N layers or III-N single crystals. Preferably, the III-N crystal layer of the template as well as the III-N crystal epitaxially grown thereon form a purely binary system, e.g. GaN, AlN or InN, or the III-N crystal layer of the template is a binary system, in particular GaN (at least mainly, since the nucleation layer can optionally be composed of a different material, such as for example AlN), and the III-N crystal epitaxially grown thereon is a binary or ternary III-N material which can be freely chosen, in particular again binary GaN.

(71) Step bb) can follow step aa) immediately, alternatively the process can be interrupted in between. It is possible to change the reactor between the steps, which in turn enables the growth of III-N crystals in the step bb) via a different growth method than the one used for producing the template provided according to step aa), in order to choose optimum conditions for the respective steps. Thus, the additional epitaxial crystal growth on the template produced according to the invention is preferably carried out via HVPE. The advantageous selection of the step bb) under HVPE conditions enables high growth rates and accordingly the obtaining of thicker layers. However, also all steps of the process, relating to the entire growth including the template formation and the subsequent deposition of the further epitaxial III-N layer, can be carried out in a single equipment using a particular growth technique, for example only via HVPE, such that the steps aa) and bb) are performed in the same reactor.

(72) According to the invention in the process for producing III-N single crystals according to the embodiments described above an epitaxial crystal growth on the provided template can be carried out such that after finishing the epitaxial growth with markedly reduced risk of crack formation thick III-N single crystals of very good crystal quality with layer thicknesses of at least 1 mm, preferably of at least 5 mm, more preferably of at least 7 mm and most preferably of at least 1 cm are obtained. Owing to the absence of cracks, the entire thickness of the bulk crystal can advantageously be used.

(73) After finishing the epitaxial crystal growth for producing a III-N single crystal, the crack-free III-N single crystal can optionally be separated from the substrate (optional step cc)). In a preferred embodiment, this takes place via self-separation, such as during the cooling from a crystal growth temperature. In a further embodiment, the separation of III-N single crystal and the substrate can be performed by grinding-off, sawing-off or a lift-off process.

(74) When the epitaxially grown III-N single crystal exhibits a sufficiently large thickness, wherein a so-called III-N boule or ingot is obtained, it is possible to separate this single crystal for forming a multitude of individual thin disks (wafers) using suitable methods. The separation of the single crystals comprises common methods for cutting or sawing of III-N single crystals. The wafers thus obtained are excellently suited as a basis for producing semiconductor devices and components, for example opto-electronic and electronic components. The wafers produced according to the present invention are well suited for use as power components, high-frequency components, light-emitting diodes and lasers.

(75) In all of the process stages, in particular for the actual, epitaxially grown III-N layers of a III-N boule or ingot and correspondingly in the III-N single crystal for the obtained wafers the inclusion of dopants is possible. Suitable dopants comprise n-dopants as well as p-dopants and can comprise elements selected from the group consisting of Be, Mg, Si, Ge, Sn, Pb, Se and Te. For semi-isolating material suitable dopants can comprise elements selected from the group consisting of C, Fe, Mn and Cr.

(76) In a further preferred embodiment, the crack-free III-N single crystal is composed of gallium nitride, and this crystal exhibits in the growth direction a lattice constant a in the range of <a.sub.0, in particular in the range from 0.31829 nm<a0.318926 nm. As reference value of the lattice constant a.sub.0 of GaN here the value of a.sub.0=0.318926 nm can be assumed (cf. V. Darakchieva, B. Monemar, A. Usui, M. Saenger, M. Schubert, Journal of Crystal Growth 310 (2008) 959-965). This corresponds approximately to a lattice constant c in the range from 0.sub.zz<+0.0001.

EXAMPLES

Example 1

(77) As growth technique a MOVPE on pre-treated sapphire (which is subjected to a desorption and a nucleation) is used with the details given in the following. The temperatures given here relate to the nominally set temperature of the heaters; the temperature at the template or respectively the crystal is lower, in some cases up to about approximately 70 K lower (cf. FIGS. 2 and 3: here the heater temperature is denoted by the upper line and the measured temperature of the wafer support is given by the lower line).

(78) Reactor:

(79) MOVPE reactor Aixtron 200/4 RF-S, single wafer, horizontal

(80) Foreign Substrate:

(81) c-plane sapphire substrate, off-cut 0.2 in m-direction 430 m thickness unstructured

(82) Desorption Step (FIG. 1 (1); 100)

(83) Reactor pressure: 100 mbar

(84) Heating: from 400 C. to 1200 C. in 7 min

(85) Reactor temperature: 1200 C.

(86) Process temperature duration: 10 min in H.sub.2 atmosphere

(87) Cooling to 960 C.

(88) Nucleation Step (FIG. 1 (2); 101)

(89) Gas flows: 25 sccm trimethyl aluminium (TMAl), bubbler: 5 C.,

(90) 250 sccm NH.sub.3

(91) Cooling to 960 C.

(92) Opening of the valves

(93) Nucleation: 10 min

(94) Increase of the ammonia flow to 1.6 slm

(95) T-Ramp; Optionally Crystal Growth (FIG. 1 (2) to Before (3); 103)

(96) Heating from 960 C. to 1100 C. in 40 sec

(97) Reactor pressure: 150 mbar, H.sub.2 atmosphere

(98) Gas flows: optionally 16-26 sccm trimethyl gallium (TMGa), 2475 sccm NH.sub.3

(99) Crystal growth time: 0-10 min (corresponding to 0-300 nm)

(100) SiN Deposition (FIG. 1 (3); 102)

(101) Gas flows: 0.113 mol/min silane, 1475 sccm NH.sub.3

(102) No TMGa

(103) Pressure: 150 mbar

(104) Temperature: 1100 C.

(105) Duration: 3 min

(106) Further Crystal Growth: (FIG. 1 (4); 104)

(107) 1100 C.

(108) reactor pressure: 150 mbar, H.sub.2 atmosphere

(109) gas flows: 26 sccm TMGa, 2000 sccm NH.sub.3

(110) crystal growth time 90-240 min, corresponding to 3-10 m GaN thickness

(111) Growth end and cooling: (FIG. 1 (5)-(6))

(112) Switching-off of heating and TMGa flow

(113) Lowering of NH.sub.3: 2000 sccm to 500 sccm in 40 sec

(114) Switching-off: NH.sub.3 flow under 700 C.

(115) Switching-over: NH.sub.3 flow to N.sub.2 flow

(116) FIG. 5A shows the course of the curvature at growth temperature (1350 K), plotted against the thickness of the grown GaN layer and thus in the temporal course, distinguished respectively according to distance of the SiN (Si.sub.xN.sub.y) with respect to the AlN nucleation layer. In this respect the zero point relates to the beginning of the continued growth of the III-N layer 104A, 104B (i.e. after stage (3) and before or respectively during stage (4) in FIGS. 1A/1B). The curvature behaviour can be purposively and precisely controlled. The following Table 1 gives the in situ, i.e. measured at growth temperature, .sub.xx values and the curvature values C (km.sup.1) measured at room temperature and the .sub.xx values at room temperature determined from C towards the end of the template production with respective thicknesses of about approximately 7 m.

(117) TABLE-US-00001 TABLE 1 distance AlN Thickness and SiN (m) in-situ C @ RT (km.sup.1) @ RT 0 nm 7.21 6.00E04 396 2.27E03 15 nm 7.09 4.50E04 365 2.13E03 30 nm 6.76 4.00E04 367 2.24E03 60 nm 6.73 1.10E04 298 1.83E03 90 nm 6.81 1.00E04 299 1.82E03 300 nm 7.29 2.50E04 293 1.66E03

Example 2 and Comparative Examples

(118) On selected templates produced according to Example 1 for which GaN layers with SiN interlayers directly on the nucleation layer (sample A) or after a very small (15-30 nm; sample D) or larger (300 nm; sample C) distances were deposited or according to Comparative Examples for which GaN was grown without SiN (sample B) or on low temperature GaN nucleation layer (sample E), the curvature was followed analogous to Example 1, namely in the range of a MOVPE growth to approximately 7 m as shown in FIG. 5B, or during performing further HVPE growth to approximately 25 m as shown in FIG. 5C. The results of the FIGS. 5B and 5C show once more the significantly better results regarding setting and behaviour of the curvature of the templates according to the invention (A), (C) and (D) compared to the comparative templates (B) and (E) without SiN interlayer.

Further Comparative Examples

(119) In further Comparative Examples again similar experimental conditions can be used, except that no interlayer of mask material is deposited.

(120) FIG. 6 shows typical in situ data of the MOVPE growth of GaN on sapphire in a further Comparative Example, wherein in the bottom diagram the development of the curvature during the process for three different sapphire substrates is shown (cf. Brunner et al. in J. Crystal Growth 298, 202-206 (2007)). The arrows point to the curvature values K.sub.s and K.sub.e to be read off. With a curvature K.sub.s of 50 km.sup.1 and a curvature K.sub.e of 70 km.sup.1 here K.sub.sK.sub.e<0 applies, i.e. the GaN layer is intrinsically stressed in a tensile manner at growth temperature. Through cooling this stress of the GaN layer is superimposed by an extrinsic compressive stress.