Group 13 Element Nitride Wafer With Reduced Variation In Off-Cut Angle

Abstract

The invention relates to a two-dimensional crystal wafer of group 13 or III element nitride which is delimited by a face of orientation N, an opposing face of orientation E depending on the group 13 or III element, E being selected preferably from Ga, In, Al or a combination of these elements, charaterized in that the variation in crstalline off-cut angle in the largest dimension of said wafer is less than 5×10−3°/mm, and its curvature of geometric deformation of its faces exhibits a flexure in terms of absolute value of less than 10−.sup.3mm/mm of the largest dimension of said wafer.

Claims

1. Two-dimensional crystal wafer of group 13 or III element nitride, delimited by a face of orientation N, an opposite face of orientation E according to the group 13 or III element, E being chosen preferably from Ga, In, Al or a combination of these elements: wherein: the variation of the crystalline off-cut angle along the largest dimension of said wafer is less than 5×10.sup.−3°/mm, and its geometric curvature or deformation of its faces has a deflection less in absolute value than 10.sup.−3 mm/mm of the largest dimension of said wafer.

2. Wafer according to claim 1 whose crystalline curvature or crystalline deflection is less in absolute value than 10.sup.−3 mm/mm of the largest dimension of said wafer.

3. Wafer according to claim 1 whose E face is convex and the geometric deflection of said E face is negative and greater than −10.sup.−3 mm/mm of the largest dimension of said wafer, preferably greater than −0.7×10.sup.−3 mm/mm of the largest dimension of said wafer.

4. Wafer according claim 1 in which FWHME designates the width at mid-height of the x-ray diffraction peak of the line (002) around the angle w measured on the E face and where FWHME is less than 130 ArcSec and the quality gradient defined by the ratio (FWHM′N-FWHM′E)/thickness of said wafer, where FWHM′N designates the width at mid-height of the X-ray diffraction peak of the line (201) around the angle w measured on the N face (Nitrogen) and FWHM′E is that measured on E face, is comprised in absolute value between 0.005 and 5 ArcSec/micrometre.

5. Wafer according to claim 1, wherein at least one face, preferably the E face, has a surface with a crystalline quality and roughness allowing the growth of a subsequent layer along a single direction.

6. Wafer according to claim 1, whose crystalline off-cut angle along the largest dimension of said wafer is greater than 2×10.sup.−3o mm.

7. Wafer according to claim 1, whose crystalline off-cut angle along the largest dimension of said wafer and measured at the centre of said wafer is greater than 3×10.sup.−3° mm and/or less than 7×10.sup.−3° mm.

8. Wafer according to claim 1, wherein the group 13 or III element is chosen from Ga, In or Al, preferably the group 13 or III element comprises gallium.

9. Wafer according to claim 1, wherein the group 13 or III element is gallium.

10. Wafer according to claim 1 whose diameter is greater than 50 mm.

11. Wafer according to claim 1, whose ratio R of the largest length to the thickness of the wafer is comprised between 80 and 500, preferably between 100 and 500, preferably 100 and 450, even between 100 and 350.

12. Method for manufacturing the wafer according to claim 1, comprising the following steps: a: obtaining a self-supported raw crystal wafer of group 13 element nitride by epitaxial growth, b: determining the variation of the off-cut angle of said raw crystal at the end of growth, optionally after cutting said crystal, preferably by grinding or by cutting with a wire saw, to a relative thickness greater than 5x le and less than 10.sup.−2, said relative thickness corresponding to the ratio of the thickness of the wafer to the largest width of its faces, c: strain hardening of one face in order to reduce the variation of the off-cut angle, in particular by sandblasting, exposure to laser radiation, shot peening, ion bombardment or another technique making it possible to create a surface mechanical stress on said crystal, so as to obtain a non-zero geometric deflection, in particular a geometric deflection comprised in absolute value between 0.001 and 1, preferably between 0.01 and 0.1 mm/mm of the largest dimension of said wafer d: after strain hardening, grinding the N face by machining, e: after strain hardening, grinding the E face by machining, f: polishing the E face, g: chemical etching the N face after machining the N face or after polishing the E face, h: cleaning to decontaminate the surfaces of the wafer i: preparation of the E face, in particular by plasma engraving, for subsequent deposition of at least one layer necessary for the production of an optoelectronic or electronic component.

13. Manufacturing method according to claim 12, wherein the strain hardening step c) is carried out until the variation of the off-cut angle is less than 5×10.sup.−3o mm of the largest dimension of said wafer.

14. Manufacturing method according to claim 12, wherein strain hardening step c is carried out on the crystal face for which the curvature of the crystal lattice is concave.

15. Manufacturing method according to claim 12, wherein step c of strain hardening is carried out until a crystalline deflection comprised between 0.2 and 2 μm/mm of the largest dimension of the wafer is obtained, in the case where the N face of the wafer is concave.

16. Manufacturing method according to claim 12, wherein step c of strain hardening is carried out until a crystalline deflection comprised between 0.1 and 1.5 μm/mm of the largest dimension of the wafer is obtained, in the case where the N face of the wafer is convex.

17. Manufacturing method according to claim 12, wherein the step of growing the wafer of group 13 or III element nitride comprises the following phases: a growth phase of a first layer of group 13 element nitride on a starting substrate having a crystallization plane whose off-cut angle is comprised between 0.1 and 5 degrees, preferably comprised between 0.2 and 0.8 degrees, and even more preferably comprised between 0.3 and 0.6 degrees; a formation phase of a separation zone; a regrowth phase to form a second layer of group 13 element nitride, a separation phase to obtain the crystal.

18. Use of a group 13 or III element nitride wafer according to claim 1 for the fabrication of optoelectronic or electronic components, such as light-emitting diodes, laser diodes, transistors with vertical or horizontal geometry, power electronics or electronic components for radio frequency transmission, reception or amplification, current rectifier diodes or sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Other advantages and characteristics of the method according to the invention and the associated product will appear more clearly from the following description of several variants of embodiment, given by way of non-limiting examples, from the attached drawings in which:

[0071] FIGS. 1A and 1B illustrate the determination of the geometric deflection of a raw crystal.

[0072] FIGS. 2A, 2B, 2C and 2D describe crystals having a crystalline curvature or deflection different from their geometric curvature or deflection.

[0073] FIG. 3 illustrates a manufacturing method for a raw crystal according to the invention.

[0074] FIG. 4 illustrates the obtaining of a raw crystal according to the invention.

[0075] FIG. 5 describes the raw crystal according to a first example of embodiment.

[0076] FIG. 6 describes the raw crystal according to a second example of embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0077] In reference to FIGS. 2 and 3, the major phases of the manufacturing method for GaN wafers is illustrated.

[0078] In the following, the method according to the invention will be described in reference to the manufacture of gallium nitride (GaN) wafers. However, it is quite obvious to the skilled person that the method described below can be used to grow a material including a layer of group 13 element nitride other than gallium nitride (GaN).

[0079] Preferred but non-limiting aspects of the product and method described above are described in the following parts: [0080] The crystal is a gallium nitride crystal. [0081] The crystal has a radius of curvature greater than 15 metres, preferably greater than 18 meters, more preferably greater than 20 meters, or even greater than or equal to 25 meters. [0082] The crystal has a dislocation density of 10.sup.−7 cm.sup.−2 or less.

[0083] Advantageously, the off-cut angle is non-zero in the vicinity of the center of the E face, for example of the gallium face, is comprised between 0.1 and 5 degrees, preferably between 0.2 and 0.8 degrees, still more preferably between 0.3 and 0.6 degrees, the cutting planes extending perpendicularly to the main axis A of the crystal, in particular along the plane a and m.

[0084] 1 . Manufacturing Method

[0085] The method according to the invention more particularly comprises the following successive steps: [0086] a growth phase (10) of a first layer of group 13 element nitride (5a) on a starting substrate (1) having a crystallization plane whose off-cut angle is comprised between 0.1 and 5 degrees. [0087] a formation phase (20) of a separation zone (4); [0088] a regrowth phase (30) to form a second layer of group 13 element nitride (5b), [0089] a separation phase (40) to obtain a self-supported crystal (5) [0090] a finishing phase (50) of the raw crystal obtained characterized in that it comprises: [0091] at least one strain hardening step (52) by damaging the surface corresponding to a concave crystalline curvature of said crystal, in order to straighten the crystalline planes, and [0092] at least one grinding step (53) making it possible to geometrically straighten the crystal, in order to obtain a wafer according to the invention. [0093] optionally, a prior thinning step (51) by removing material from the raw crystal according to the initial deflection of said crystal, in order to obtain a final wafer whose maximum deflection is less than 6×10.sup.4 mm/mm of the longest length of said wafer.

[0094] 1.1. Growth Phase 10

[0095] Optional growth phase 10 consists of forming an underlayer of GaN 5a by lateral overgrowth.

[0096] Lateral overgrowth makes it possible to minimize the density of defects contained in the GaN 5a underlayer.

[0097] The approach used to reduce the density of dislocations in the GaN 5a underlayer consists of: [0098] initiating a GaN island growth mode, then to [0099] promote the coalescence of the islands to obtain the GaN 5a underlayer. Advantageously, lateral overgrowth is implemented in a starting substrate 1 having a non-zero off-cut angle.

[0100] The fact of using a starting substrate 1 having a non-zero off-cut angle makes it possible to grow a first layer of GaN 5a having a non-zero off-cut angle.

[0101] The starting substrate 1 is chosen from among Si, AlN, GaN, GaAs, Al.sub.2O.sub.3 (sapphire), ZnO, SiC, LiAlO.sub.2, LiGaO.sub.2, MgAl.sub.2O.sub.4, 4H—SiC, or any other type of starting substrate known to the skilled person to implement growth of gallium nitride.

[0102] Starting substrate 1 can have a thickness of several hundred micrometres, generally 350 micrometres.

[0103] Advantageously, the starting substrate 1 can be treated by nitriding prior to any deposition step. This makes it possible to improve the crystal quality of the GaN obtained.

[0104] The off-cut angle can be comprised between 0.1 and 5 degrees, preferentially between 0.2 and 0.8 degrees, and even more preferentially between 0.3 and 0.6 degrees (especially to limit stacking faults).

[0105] The growth of the GaN 5a underlayer can be implemented according to different variants. Especially, lateral overgrowth can be based: [0106] on the use of a dielectric mask 3 including openings 3a in which islands form, such as described in document WO99/20816; [0107] on the use of a dielectric layer with no openings in which islands form spontaneously, such as described in document EP 1338683.

[0108] 1.1.1. First Variant of Lateral Overgrowth

[0109] In a first variant, the growth phase 10 consists of an epitaxial lateral overgrowth (ELO).

[0110] ELO includes a step of depositing a thicker flat layer 2 on the starting substrate 1.

[0111] This deposition is preferably done by metalorganic vapour phase epitaxy (MOVPE), for example at a temperature comprised between 500° C. and 700° C., especially 600° C.

[0112] The deposition of a layer 2 allows reducing the stresses between the starting substrate 1 and GaN underlayer 5a subsequently deposited by epitaxy there upon. Indeed, the deposition of layer 2 on substrate 1 ensures a “gentle” transition between substrate 1 and the GaN underlayer 5a whose crystalline structures are different.

[0113] The deposition of layer 2 also facilitates the subsequent separation of the GaN crystal 5, as will be seen from the description that follows. The layer 2 is, for example, a GaN layer, a AN layer, or a AlGaN layer.

[0114] In another step, a mask 3 including openings 3a is formed. The openings 3a can be point-like or in the form of bands, and make it possible to define positions for the subsequent selective growth of GaN islands.

[0115] The mask 3 can be a mask of dielectric material, such as, for example, SiN.sub.x (SiN, Si.sub.3N.sub.4, etc.) or SiO.sub.2 or TiN. This makes it possible to minimize the defects created at the edge of the mask and thereby improves the quality of the GaN layer deposited by epitaxy subsequently thereon.

[0116] Mask 3 can be formed by any technique known to the skilled person. For example, the formation of the mask can consist of: [0117] depositing a dielectric layer 3b from gaseous silane and ammonia precursors directly on layer 2, and [0118] engraving by photolithography of the dielectric layer 3b to form openings 3a.

[0119] Thus a starting substrate 1 coated with a layer 2 and a mask 3 is obtained. In addition to its function of improving the quality of the GaN underlayer 5a (by filtering through defects), the mask 3 makes it possible to weaken the interface between the starting substrate 1 and the first layer of GaN 5a.

[0120] Another step consists of forming GaN islands through the openings 3a of mask 3. The growth rate along an axis orthogonal to the primary plane of starting substrate 1 is kept higher than the lateral growth rate. In this way, islands or bands of triangular section are obtained (depending on the shape of openings 3a). Inside these triangular-section bands, through dislocations are bent at 90°. [0121] Then lateral overgrowth is performed to ultimately arrive at a flat ELO layer. At the end of this step of the process, a first GaN layer 5a is obtained having a dislocation density less than 10.sup.8 cm.sup.−2.

[0122] 1.1.2. Second Variant of Lateral Overgrowth

[0123] In a second variant, the growth phase 10 consists of an universal lateral overgrowth (ULO) such as described in document EP 1977028

[0124] ULO includes a step of depositing a nucleation layer 2 on the starting substrate 1.

[0125] The nucleation layer is, for example, a very thin film of silicon nitride SiN, of around a few atomic planes, in other words, of around 10 nm to 20 nm in thickness. The deposition of silane and ammonia based SiN can last 360 seconds.

[0126] A continuous buffer layer 2—for example GaN—is then deposited on the nucleation layer. The deposition of GaN buffer layer 2 makes it possible to filter the crystalline defects and thereby minimize from the start of the method the density of the defects that will be present in the GaN underlayer 5a subsequently deposited by epitaxy. The thickness of this GaN buffer layer 2 can be comprised between 10 and 100 nm. The temperature during this operation can be between 500 and 700° C.

[0127] Annealing is then carried out at a high temperature between 900 and 1150° C. Under the joint effect of the increase in temperature, the presence in the gaseous vehicle of a sufficient quantity of hydrogen and the presence of the very thin film of SiN, the morphology of the GaN buffer layer 2 undergoes a profound modification resulting from solid-phase recrystallization by mass transport. The initially continuous GaN buffer layer 2 is then converted into a discontinuous layer of GaN patterns. GaN patterns or islands of very good crystalline quality and retaining an epitaxial relationship with the starting substrate are thus obtained, thanks to the very small thickness of the nucleation layer.

[0128] The areas where silicon nitride SiN is stripped then function as a mask and the GaN patterns function as localized areas of GaN in the openings made ex situ in the mask. Then lateral overgrowth is performed to ultimately arrive at a flat ULO layer.

[0129] This method, where the silicon nitride mask is formed spontaneously and which involves the same mechanisms of curvature of the dislocations as in the ELO is identified as “ULO” (or “spontaneous ELO”).

[0130] 1.2. Phase (20) of Formation of a Separation Zone 4

[0131] The method also comprises a phase 20 of formation of a separation zone 4.

[0132] This phase 20 of formation of a separation zone can be implemented according to different variants. In particular, the phase 20 of formation of the separation zone can be implemented: [0133] prior to the phase 10 of growth of the GaN underlayer (first variant), or [0134] after the phase 10 of growth of the GaN underlayer (second variant), or [0135] during the phase 10 of growth of the GaN underlayer (third variant).

[0136] 1.2.1. First Variant of Formation of the Separation Zone 4

[0137] In a first variant, the phase 20 of formation of a separation zone 4 can consist of depositing a sacrificial intermediate layer prior to phase 10 of growth of the GaN underlayer 5a, such as described in document EP 1699951.

[0138] The intermediate layer, which can be SiO.sub.2; SiN; Si; SiON; AlON; Al; ZnO; Ti; TiO.sub.2; TiN; TiC; Ni; InN; MOSi.sub.2 or ZrN, serves as the sacrificial layer designed to be spontaneously vapourised during the subsequent growth by epitaxy of the GaN underlayer 5a.

[0139] 1.2.2. Second Variant of Formation of the Separation Zone

[0140] In a second variant, the phase 20 of formation of a separation zone 4 comprises an implantation step conducted after phase 10 of growth of the GaN underlayer 5a. This implantation makes it possible to create weakened zone in the GaN underlayer 5a.

[0141] The implantation consists of bombarding the GaN underlayer 5a with ions so as to create in the semiconductor at a depth close to the average depth of penetration of these ions, a layer of microcavities (or bubbles).

[0142] The implanted ions can be chosen from among tungsten, helium, neon, krypton, chromium, molybdenum, iron, hydrogen or boron. Preferably, the implanted ions are tungsten ions. These ions have the particular feature of breaking down GaN.

[0143] In terms of dose, when the ions implanted are H+, the dose of ions implanted can be comprised between 10.sup.16 and 10.sup.17 cm.sup.−2 and the depth of implantation can vary from 0 nm to 50 nm starting from the free surface—called growth face—of the GaN underlayer 5a.

[0144] The implantation of embrittlement ions can be implanted in a single step or successive steps. The temperature can be comprised between 4K and 1400K during the implantation step. The implantation can be followed by an annealing phase to treat the crystalline damage created during the ion implantation; this annealing can be carried out at a temperature comprised between 500° C. and 1500° C.

[0145] 1.2.3. Third Variant of Formation of the Separation Zone

[0146] In a third variant, the separation zone 4 can be formed during phase 10 of growth of the GaN underlayer 5a.

[0147] Especially when the growth phase is performed according to the first variant of embodiment, called ELO (i.e. deposition of the dielectric mask 3), the phase 20 of formation of the separation zone 4 can comprise the implantation of the buffer layer 2 prior to the deposition of mask 3.

[0148] This makes it possible to place the separation zone 4 at a precisely desired depth due to the fact that the first GaN layer 5a deposited during the ELO overgrowth step does not impede the implantation of ions.

[0149] Of course, the implantation can be done at different stages of the ELO (or ULO) lateral overgrowth phase, either in islands or at an intermediate stage where the islands have not entirely coalesced or after total coalescence of the islands.

[0150] 1.3. Epitaxial Regrowth Phase 30

[0151] At the end of the phases 20 of forming a separation zone 4 and growth 10 of the GaN underlayer 5a , the method comprises an epitaxial regrowth phase 30 for forming a first thick layer of non-doped GaN 5b, a second thick layer of doped GaN 5c and a third thick layer of non-doped GaN 5d.

[0152] The method could also be directly started at this phase 30 by the formation of a thick layer of GaN 5b, the growth phases 10 and the formation of the separation zone phase 20 being optional. In the following text, phases 10 and 20 are considered to be applied.

[0153] This epitaxial regrowth can be implemented by: [0154] Metalorganic vapour phase epitaxy (MOVPE), [0155] hydride vapour phase epitaxy (HVPE); [0156] Close-spaced vapor transport (CSVT); or even by [0157] Liquid phase epitaxy (LPE). [0158] Oxide vapour phase epitaxy (OVPE).

[0159] Preferably, this step implements the HVPE technology, which makes it possible to obtain three main interesting effects: [0160] A first effect is that the GaN underlayer 5a is thickened without loss of its crystalline qualities (neither new dislocations nor cracks are generated), [0161] A second effects is that the dislocation density is further reduced during repeat epitaxy by HVPE, by a factor at least equal to 2, beyond 100 μm of GaN growth (0001) (ref https://doi.org/10.1143/APEX.5.095503) [0162] A third effect is that the thick GaN layer 5 thus obtained can, in certain cases, allow the spontaneous separation of its starting substrate 1 at the separation zone 4 in case of sublimation or mechanical fracture of said zone during HVPE growth. More precisely, the regrowth is performed according to the following method: the temperature is increased under a mixed atmosphere of nitrogen, ammonia and hydrogen. As soon as a stable temperature of approximately 1000° C. is reached, the growth phase of a thick layer of GaN is then initiated by introducing gallium chloride (GaCl), for example, into the vapour phase. The GaCl and ammonia are partially pyrolyzed in the growth chamber, whose temperature is maintained at approximately 1000° C. Thus a monocrystalline deposit of GaN is progressively formed at the nucleation substrate (formed during the first growth phase).

[0163] It is necessary to obtain a sufficiently thick layer of GaN, therefore sufficiently strong from the mechanical viewpoint to prevent the fracture of the GaN layer into pieces of small area during the separation as well as to make it easier to handle it without breaking. Growth continues for several hours under these experimental conditions in order to reach a thickness of at least 200 microns of the GaN layer and preferentially a thickness greater than 1 mm.

[0164] Growth is then definitively completed by diverting the flow of HC1 to the outside and cooling takes place in an atmosphere made up of nitrogen and ammonia.

[0165] The growth conditions for these first, second and third monocrystalline layers 5b, 5c, 5d are typically at a growth temperature comprised between 900 to 1200° C., with a growth rate that can be comprised between 50 and 500 micrometres/h, preferably between 70 and 200 micrometres/h.

[0166] The raw self-supported GaN crystal thus obtained has a thickness greater than 200 μm and preferentially greater than 1 mm. Its maximum thickness is less than 10 mm or even less than 5 mm.

[0167] The raw self-supported GaN crystal thus obtained has a diameter greater than 50 mm and preferentially a diameter greater than 100 mm. Its maximum diameter is less than 250 mm or even less than 200 mm.

[0168] In reference to FIG. 4, the layer 5b has areas of permanent defects corresponding to recesses or wells of decreasing section in the direction opposite the direction of growth.

[0169] In a second step, under the same growth conditions, doping is performed by introducing the doping element n, to obtain a second monocrystalline GaN layer 5c on the layer 5b, according to the following process: [0170] The introduction and concentration of oxygen are controlled by the purity of precursor element III and by carrying out very thorough purges of the reactor before growth under vacuum at a residual pressure of less than 500 torr. [0171] For germanium: from a solid source, GeCl.sub.4, germane, tetramethylgermanium, isobutylgermane and derivatives thereof. These doping gases are then vapourised in the reaction chamber. Preferably, these doping gases can be mixed beforehand in vapour phase with the flow of GaCl to improve the homogeneous distribution of the doping flow in the growth chamber. In the case of a gaseous precursor, the gas reservoir is kept at a pressure comprised between 1 and 3 bar and a carrier gas flow (N2 and/or H2) of a flow rate comprised between 0.25 sccm and 20 sccm is applied. [0172] For silicon, from silane, dichlorosilane, silicon tetrachloride and derivatives are vapourised in the reaction chamber. In the case of dichlorosilane (1% diluted in 99% N.sub.2 (or H.sub.2) a flow comprised between 1 and 20 sccm is applied. Preferably, these doping gases can be mixed beforehand in vapour phase with the flow of GaCl to improve the homogeneous distribution of the doping flow in the growth chamber. [0173] The silicon and germanium can be introduced together, which leads to a system with 3 dopants.

[0174] Typically, the thickness of this first monocrystalline GaN layer 5b is from 100 to 1000 micrometres.

[0175] The permanent defect zones 6 of layer 5b cause the defects to also propagate in the layer 5b. Thus, during the growth of the second doped layer 5c, depressions are obtained in the material of the layer 5c in the depression zones coinciding with the defect zones of layer 5b.

[0176] Other forms of material depression can appear linked to inclusions or nucleations that locally modify the growth rate without, however, emerging on the upper surface of the layer 5b or even 5c.

[0177] In a third step, the introduction of doping gas n is discontinued and the growth conditions are maintained, making it possible to obtain on the preceding doped layer 5c, typically of 100 to 2000 micrometres thick, a third monocrystalline GaN layer 5d, typically 200 to 5000 micrometres thick.

[0178] The thickness obtained for the monocrystalline layer and the growth method for GaN 5d allow the depression zones of layer 5c to be filled by deposition of this third layer.

[0179] 1.4. Separation Phase 40

[0180] A separation phase 40 is also implemented. This phase is dependent on the variant implemented for phase 20 of formation of separation zone 4.

[0181] In the case of an ion implantation, the spontaneous separation phase 40 takes place due to the thermal cycle (epitaxial regrowth at high temperature and cooling) that the thick GaN layer 5 undergoes which, because of the difference of thermal expansion coefficients between the starting substrate 1 and the thick GaN layer 5, generates stresses causing its separation.

[0182] In the case where an intermediate sacrificial layer is deposited, this separation occurs during the epitaxial regrowth (repeat epitaxy) by spontaneous vaporisation of the intermediate layer or even by mechanical fracture at the said sacrificial layer. In the case of post-growth separation, a laser can be used to vaporise the sacrificial layer.

[0183] Thus a self-supported GaN crystal 5 is obtained as illustrated in FIGS. 1A or 1B.

[0184] Such a crystal may be curved and has a radius of curvature typically greater than 5 metres and less than 25 metres and preferably less than 20 metres. Moreover, crystal 5a also has a dislocation density of 10.sup.7 cm.sup.−2 or less, preferably 5×10.sup.6 cm .sup.2.

[0185] Since the GaN 5 crystal was formed on a starting substrate having a non-zero off-cut angle (or miscut angle), it also has a non-zero off-cut angle, the orientation of crystalline planes being propagated from one layer to the other. For example, in the case of a sapphire substrate 1 of off-cut angle equal to 4 degrees, the growth face of crystal 5 has a off-cut angle equal to 4 degrees, preferably comprised between 0.1 and 1 degree over its entire surface.

[0186] 1.5. Finishing Phase 50

[0187] Once the GaN crystal 5 is separated from the starting substrate 1, a finishing phase 50 of the obtained raw crystal is performed in order to obtain a final wafer with a maximum deflection less than 6×10.sup.−4 mm/mm of crystal width. Finishing phase 50 comprises: [0188] Optionally, a prior thinning step 51 by removing material from the raw crystal depending on the initial deflection of said crystal. Preferentially, thinning is done by machining to a relative thickness greater than 5×10.sup.−3 and less than 10.sup.−2, said thickness corresponding to the ratio of the thickness of the wafer to the longest length of its faces. [0189] At least one strain hardening step 52 by damaging the surface corresponding to a concave crystalline curvature of said crystal, in order to straighten the crystalline planes to reduce the variation of the off-cut angle. Strain hardening can be performed by surface abrasion, for example by sandblasting, shot blasting, grinding, lapping or exposure to laser radiation to generate a strain hardening layer. Strain hardening can also be carried out by another technique suitable for cutting group 13 element nitride monocrystals under mechanical stress. According to a particular embodiment, strain hardening is carried out until the variation of the off-cut angle is less than 5×10.sup.−3°/mm of the largest dimension of said wafer.

[0190] Preferably, strain hardening is carried out on the face of the wafer on which the crystal deformation is concave or for which the curvature of the crystal lattice is concave.

[0191] Preferably, strain hardening is carried out on a wafer not attached to a support. Advantageously, strain hardening is done until a crystalline deflection is obtained comprised in absolute value between 0.2 and 2, preferably between 0.3 and 1.5, even between 0.4 and 1.0 μm/mm of the largest dimension of the wafer, in the case where the N face of the wafer is concave. In the case where the N face of the wafer is convex, the strain hardening is preferably carried out until a crystalline deflection is obtained comprised in absolute value between 0.1 and 1.5, preferably between 0.2 and 1.2, even between 0.2 and 1.0 μm/mm of the largest dimension of the wafer,

[0192] At least one grinding step making it possible to geometrically straighten the crystal. Then N face and/or E face and the sides or edges of the wafer are ground with polishing, in order to obtain an acceptable surface state for the application of an optoelectronic or electronic component.

[0193] The finishing step can also comprise other steps, such as chemical etching of the N face after machining the N face or after polishing the E face, a cleaning to decontaminate the wafer surfaces and/or a preparation of the E face, in particular by plasma engraving, for subsequent deposition of at least one layer necessary to create an optoelectronic or electronic components.

[0194] Thus, the proposed method is particularly suited to the manufacture of slices or wafers of semiconductor material, in particular slices or wafers of element nitride of group 13 or III of the periodic table, more particularly slices or wafers composed of group 13 nitride, preferably of GaN, of large diameter, greater than 50 mm, than 100 mm or even 150 to 200 mm.

[0195] The final slice or wafer of semiconductor material according to the invention has a thickness of 200 to 2000 micrometres, preferentially of approximately 400 micrometres, and an excellent crystalline quality such that the width at mid-height of the x-ray diffraction peak (XRD) of line (002) around the angle w in symmetrical condition of the GaN films (0001) is less than 130 ArcSec in the case of a gallium nitride wafer.

[0196] The crystal undergoes a trimming operation then is cut into several slices or wafers typically of 100 to 600 micrometres thick using loose wire saw (abrasive particles in a slip which impregnates the wire before cutting), or fixed wire saw (abrasive particles previously fixed onto the wire).

[0197] The crystal is cut along transverse cutting planes P orthogonal to the main axis A of the crystal. This makes it possible to obtain a much better material yield than that described in document US 2012/0184090. Current techniques make it possible to control the elimination of a layer thickness to within 10 micrometres.

[0198] The present invention and its advantages are illustrated by means of the following examples. Of course, the examples according to the invention should not be considered as limiting its implementation.

Examples of Embodiment

[0199] Raw GaN crystals of 100 mm diameter were obtained by growth via HVPE on the base of a substrate such as described, for example, by WO/03100839A2 according to step 3 incorporated by reference. The flow rate (volume) ratio of N.sub.2(N.sub.2+H.sub.2) is 0.2. The crystals typically have a thickness between 500 and 2000 micrometres.

[0200] In the following examples, crystals according to FIG. 5 have a concave N face and curved crystalline planes following the geometric curvature of the crystal were obtained. A first series was treated according to the finishing method of the invention and the other series according to a known method for comparison purposes.

[0201] The final crystals of the examples below have a ratio (FWHMN FWHMGa)/thickness of the wafer, where FWHMN designates the width at mid-height of the X-ray diffraction peak (XRD) of the line (201) around the angle ω in asymmetric condition of the GaN planes (0001) on the nitrogen face and FWHME the one on the group 13 face, which is −0.4 ArcSec/micron of wafer thickness.

[0202] The geometric deflection or curvature of the wafer for a diameter of 100 mm was measured on the N face using a micrometre comparator of the profilometer type as described by EP1528591 A2 [0089] and the crystalline deflection for a diameter of 100 mm of the wafer was measured by x-ray diffraction as described in the publication “Curvature and bow of bulk GaN substrates” by Humberto M. Foronda et al. published in the Journal of Applied Physics 120, 035104 (2016).

[0203] The off-cut angle, expressed in degrees per 100 mm of width of the wafer, was measured on the wafer over the entire width of the wafer except the ends, i.e. over at least approximately 95 mm of the width of the wafer, by X-ray diffraction according to the standard SEMI-MF26. At each point of the wafer, the off-cut angle corresponds to the difference between the plane m of the crystal and the plane normal to the surface at this point. The variation of the off-cut angle is defined as being the deviation between the highest angle and the lowest angle, for 100 mm diameter of the wafer. Steps f), h) and i) have not been detailed in the examples below because the angle variation deviations are small but an overall angle variation has been specified for all of these steps.

[0204] The results are shown in Table 1 below:

TABLE-US-00001 TABLE 1 Steps of the finishing process Resulting deflection at each successive treatment step (the geometric and crystalline Comparative Comparative deflections are measured in Invention Example Example reference to the N face) Example 1 1a 1b Initial step- raw crystal +115 +121 +115 Geometric deflection (μm) Crystalline deflection (μm) +115 +121 +115 Variation of the off-cut angle 0.20 0.20 0.20 b)-Thickness adjustment (machining Na (not applied) (machining of the Ga face) of the Ga face) Geometric deflection (μm) +155 +155 Crystalline deflection (μm) +155 +155 Variation of the off-cut angle 0.71 0.71 c)-Strain hardening the N face (Sandbiasting the N face) Na Sandblasting the N face until a zero geometric deflection is obtained Geometric deflection (μm) −50 Crystalline deflection (μm) −50 Variation of the off-cut angle 0.23 d)-grinding the N face −40 +55 +48 Geometric deflection (μm) Crystalline deflection (μm) −40 +140 +135 Variation of the off-cut angle 0.18 0.64 +62 g) chemical etching the N face −20 −21 13 Geometric deflection (μm) Crystalline deflection (μm) −20 +110 +105 Variation of the off-cut angle 0.10 0.50 0.48 d) -grinding Ga face +70 +61 +52 Geometric deflection (μm) Crystalline deflection (μm) +70 +150 +140 Variation of the off-cut angle 0.32 0.69 0.64 f) h) i) Finished product after +40 +41 +27 cleaning and plasma engraving Geometric deflection (μm) Crystalline deflection (μm) +40 +115 +107 Crystalline quality gradient −0.8 −0.8 −0.8 (arcsec/micron) 0.15 0.51 0.49 Variation of the off-cut angle Na = not applied

[0205] As can be seen in this Table 1, the proposed method, in comparison to a method without a strain-hardening step (1a) makes it possible to obtain a substantial reduction of the crystalline deflection and the off-cut angle variation. In particular the prior strain-hardening step makes it possible to straighten or even reverse the crystalline curvature in order to anticipate the deformations generated by the subsequent finishing operations, in particular the operation of grinding the wafer. This prior strain-hardening step thus makes it possible to obtain a wafer whose crystalline and geometric deflection or deformation are both very low.

[0206] In the following examples, crystals according to FIG. 6 have a convex N face and curved crystalline planes following the geometric curvature of the crystal were obtained. A first series was treated according to the finishing method according to the invention and the other series according to a known method for comparison purposes.

[0207] Also in comparison with a process where, in step c), the wafer is straightened in a sandblasting step until a zero geometric deflection (1b) is obtained, a significant crystalline deflection is obtained for the finished product. The results are shown in Table 2 below:

TABLE-US-00002 TABLE 2 Steps of the finishing method resulting deflection at each successive treatment step (the geometric and crystalline deflections are measured in reference to the N Invention Comparative Example face) Example 2 2 Initial step - raw crystal −141 −132 Geometric deflection (μm) Crystalline deflection (μm) −141 −132 Variation of the off-cut angle 0.6 0.5 b) thickness adjustment (machining of the Ga face) Geometric deflection (μm) −60 Crystalline deflection (μm) −60 Na Variation of the off-cut angle 0.28 c) Strain hardening face Ga (Sandblasting face Ga) Geometric deflection (μm) −30 Na Crystalline deflection (μm) −30 Variation of the off-cut angle 0.14 d) grinding the N face Geometric deflection (μm) −90 −50 Crystalline deflection (μm) −91 −178 Variation of the off-cut angle 0.41 0.82 g) chemical etching the N face −60 −20 Geometric deflection (μm) Crystalline deflection (μm) −61 −145 Variation of the off-cut angle 0.28 0.66 e) -grinding face Ga +74 +102 Geometric deflection (μm) Crystalline deflection (μm) +12 −60 Variation of the off-cut angle 0.06 0.28 f) h) i) Finished product after +23 +50 cleaning and plasma engraving Geometric deflection (μm) Crystalline deflection (μm) −42 −115 Crystalline quality gradient −0.8 −0.8 (arcsec/micron) 0.19 0.53 Variation of the off-cut angle

[0208] As can be seen in this Table 2, the proposed method, in comparison to a traditional method makes it possible to obtain a substantial reduction of the crystalline deflection and the off-cut angle variation.

[0209] The proposed method therefore makes it possible to obtain a group 13 element nitride crystal wafer having a high crystalline quality and a crystalline gradient greater than 5% whose variation of the off-cut angle and geometric deformation are permanently very low despite the multiple deformations generated by the finishing steps between obtaining the raw crystal and the final wafer ready to create an optoelectronic or electronic component.

[0210] In particular, it is possible to obtain a very low variation of the off-cut angle by combining strain hardening, not for purposes of geometrically straightening the crystal but rather for purposes of reducing its crystal off-cut angle variation. The geometric deformation of the crystal due to this strain hardening step c is then corrected by subsequent grinding steps which will allow a reduction in the geometric deformation of the crystal.

[0211] Advantageously, the off-cut angle at the centre of the wafer is around 0.5 degrees.

[0212] Advantageously, the curvature or geometric deformation of its faces has a deflection less than 10.sup.−3 mm/mm of the largest dimension of said wafer.

[0213] Also, advantageously, the quality gradient is comprised between −0.005 ArcSec/micron and −5 ArcSec/micron.

[0214] Also, advantageously, the crystal has a dislocation density of 10.sup.7 cm.sup.−2 or less.

[0215] Also, advantageously, the proposed method makes it possible to obtain a wafer in which: [0216] at least one face, preferably the E face, has a mean square surface roughness (defined according to the standard NF EN ISO 4287 December 1998) less than 0.5 nm over a surface area of 4μm.sup.2, and/or in which the crystal has a dislocation density of 10.sup.7 cm.sup.−2 or less; and/or [0217] the electrical resistivity measured is less than 25 mOhms.cm; and/or [0218] the absorption coefficient measured at a wavelength of 400 nm is less than 4 cm.sup.−1.
Also, the proposed method is particularly advantageous for obtaining a crystal wafer of group 13 element nitride, the curvature or geometric deformation of which and the off-cut angle variation are very low, having a ratio R of the longest length (in micrometres) to the thickness of the wafer (in micrometres) comprised between 80 and 500, preferably between 100 and 500, preferably between 100 and 450, or even between 100 and 350.

[0219] Table 3 below presents the ratio R obtained for wafers of different dimensions obtained according to the proposed method:

TABLE-US-00003 TABLE 3 Longest length Thickness Ratio R:length Wafer (inches) (mm) (μm) (μm)/thickness (μm) 2″ 50.8 100-600  80-500 4″ 100 200-1000 100 to 500, preferably 100 to 450 6″ 150 300, preferably 400 100 to 500, preferably 100 to to 1500 450, preferably 100 to 400 8″ 200 400, preferably 500, 100 to 500, preferably 100 to preferably 600 to 450, preferably 100 to 400; 2000 preferably 100 to 350