EP1514297A2

EP1514297A2 - Method for epitaxial growth of a gallium nitride film separated from its substrate

Info

Publication number: EP1514297A2
Application number: EP03755219A
Authority: EP
Inventors: Hacène Lahreche; Gilles La Closerie Marina - Bât. A Nataf; Bernard Beaumont
Original assignee: Lumilog SA
Current assignee: Luxium Solutions SAS
Priority date: 2002-05-28
Filing date: 2003-05-28
Publication date: 2005-03-16
Also published as: FR2840452A1; WO2003100839A2; JP2010251776A; JP2005527978A; AU2003255613A1; FR2840452B1; US20050217565A1; US7488385B2; AU2003255613A8; WO2003100839A3

Abstract

The invention concerns the preparation of gallium nitride films by epitaxy with reduced defect density levels. It concerns a method for producing a gallium nitride (GaN) film by epitaxial deposition of GaN. The invention is characterized in that it comprises at least a step of epitaxial lateral overgrowth and in that it comprises a step which consists in separating part of the GaN layer from its substrate by embrittlement through direct ion implantation in the GaN substrate. The invention also concerns the films obtainable by said method as well as the optoelectronic and electronic components provided with said gallium nitride films.

Description

PROCESS FOR THE PRODUCTION BY EPITAXY OF A GALLIUM NITRIDE FILM SEPARATE FROM ITS SUBSTRATE

The present invention relates to the production of gallium nitride (GaN) films by epitaxy with reduced defect densities.

It also relates to optoelectronic and electronic components provided with these gallium nitride films.

At the end of 1995, Nichia produced a laser diode from III-V nitrides. Such a result showed that one could obtain a laser emission in a heteroepitaxial structure where the density of dislocations obtain a laser emission during 10000 hours provided that it reached 10 ⁸ to 10 ¹⁰ cm ^"2. At the end of 1997, Nichia demonstrated that the structure could be provided with a layer of good quality GaN. These were GaN layers developed using ELO (Epitaxial Lateral Overgrowth) lateral overgrowth technology.

Although it has long been asserted that dislocations in GaN do not act as centers of non-radiative recombination, it is established that in fact certain dislocations with a screw component introduce non-radiative centers and that the performances are much higher on a structure of better crystallographic quality. Thus, the lifetime of the nitride-based laser diodes III-V depends critically on the density of dislocations in the GaN layers with which they are provided.

All current efforts are converging towards obtaining heteroepitaxial GaN with the best crystalline quality. This is why the lateral epitaxy technique (ELO) has been widely developed for GaN with many variants.

As there are no GaN films available with a satisfactory surface and in sufficient quantity, the nitride-based components III-V are produced by heteroepitaxy on substrates such as sapphire, SiC,

If or other. The sapphire commonly used as a substrate does not have a cleavage plane, which implies that in a laser diode structure with based on GaN epitaxied on sapphire, it is difficult to manufacture reflective facets.

Furthermore, the use of a substrate such as sapphire exhibiting both a lattice parameter disaggregation and a thermal expansion coefficient is responsible for the very high dislocation density in the heteroepitaxial layers of GaN / sapphire.

Different technologies of lateral epitaxial overgrowth have been developed for the implementation of ELO, in HVPE (Hydride Vapor Phase Epitaxy or vapor phase epitaxy from halides and hydrides), EPVOM (vapor phase epitaxy by pyrolysis organometallic) and even by sublimation (CSVT for Close Space Vapor Transport). All of them make it possible to obtain GaN layers with dislocation densities of less than 10 ⁷ cm ^"2 compared to 10 ⁸ to 10 ¹⁰ with standard technology. However, and this is inherent in the technology used, there are still areas where the dislocation density remains high, above openings and coalescing joints in a one-stage epitaxy technology, at the coalescing joints and in the middle of the openings in a two-stage technology, where during a first step one proceeds by epitaxy deposition of GaN in the openings after having masked and etched, in particular by photolithography, a dielectric layer to form these said openings and during a second step of lateral epitaxial overgrowth (ELO) one proceeds to the growth lateral of the GaN patterns first deposited which are continued until they coalesce.

A known variant of growth technology is based on vapor phase epitaxy by organometallic pyrolysis (EPVOM) according to a process which is now well established (on sapphire): treatment of the surface of the sapphire, nucleation at low temperature of a layer of GaN or AIN, annealed to the final growth temperature of this nucleation layer and high temperature growth of GaN (1000-1100 ° C). Several technologies have been developed to optimally optimize this heteroepitaxy and limit the density of dislocations to approximately 5x10 ⁸ cm ^"2 GaN in particular; P. Vennéguès et al, J. Cryst. Growth, 187, 167 (1998); S. Figge et al, J. Cryst. Growth, 221, 262 (2000)

On SiC, the nucleation layer at low temperature is no longer necessary, a layer of AIN at high temperature is first produced before the deposition of GaN. However, the density of dislocations remains appreciably of the order of approximately 5 × 10 ⁸ cm ^"2 .

Whatever the technology, the density of extended defects: dislocations, stacking faults, inversion domains, nanotubes reaches 5.10 ⁸ cm ^"2. Dislocations propagate in the direction of growth and emerge on the surface where they can be identified by atomic force microscopy (AFM) or cathodoluminescence (CL). These dislocations are harmful from several points of view. First, in high density (greater than 5x10 ⁸ cm ^"2 ), the defects degrade electronic mobility as well as optoelectronic properties (photoluminescence intensity, lifetime of the carriers). In addition, the emergence of surface dislocations results in a surface depression (Heying et al., J. Appl. Phys., 85, 6470, 1999). In a laser diode structure based on GalnN quantum multi-wells (MQWs), the dislocations disrupt the ordering of the MQWs and cause a non-homogeneous light emission. Finally, the metals used for ohmic contacts can also diffuse through these dislocations and nanotubes.

Thus, as explained above, the overgrowth by lateral epitaxy (ELO) with numerous variants, constitutes one of the most relevant methods for reducing the density of dislocations by several orders of magnitude, that is to say less than about 10 ⁷ cm ^"2 .

However, this method has other drawbacks, in particular for GaN / sapphire layers of ELO quality. Indeed, the substrate being sapphire, the cleavage of facets remains very difficult. In addition, there remain in the GaN layers obtained by ELO lines with high defect density corresponding to the coalescing joints, on which it is not recommended to make a component, which reduces the surface available for manufacturing. optoelectronic components. The ideal solution would therefore be to have high quality crystalline GaN of 50.8 mm (2 inches) in diameter as is the case for most semiconductors.

It is impossible to face these drawbacks and to obtain a single crystal by fusion meeting this diameter criterion, due to the physicochemical properties of GaN. Indeed, GaN single crystals can be obtained by growth in solution at high temperature (1800K) under hydrostatic pressure (1.5 GPa). But, although of very good crystalline quality (density of dislocations less than 10 ⁴ cm ^"2 ), the surface of these crystals does not exceed 1 cm ² and the mode of production does not meet global needs.

Furthermore, when using HVPE technology to produce a layer of GaN, the drawback remains that the density of dislocations is around 10 ⁷ cm ^"2. More precisely, a very thick layer (approximately 500 μm) is produced on sapphire by avoiding the formation of cracks. For thicknesses of this order, the density of dislocations decreases until around 10 ⁷ cm ^"3 . Then, the substrate is separated either by mechanical abrasion or by laser separation (LLO).

To obtain a GaN film separated from its substrate, of good quality, that is to say with a dislocation density of less than 10 ⁷ cm ² , on a diameter of at least 50.8 mm (2 inches), everything is necessary. first use ELO technology, then separate the GaN layer formed from its original substrate, and then re-thicken the GaN layer, or again reuse the GaN ELO layer thus separated for new growth. The substrate can be separated by a chemical route according to the technique disclosed in EP 1 041 610, in particular if the substrate is chosen from Si, NdGa0 ₃ or GaAs. This technique makes it possible to obtain a GaN film separated from its substrate, of good quality. This technique is not applicable for a sapphire substrate, which is chemically inert. Only laser ablation currently makes it possible to separate GaN from its sapphire substrate (LLO). This technique, as described in WO 98/14986, is based on the use of a pulsed UV laser emission which crosses the sapphire, but which is absorbed at the level of GaN causing local thermal decomposition of GaN at the interface.

However, there is a need to have alternative techniques for obtaining GaN films separated from their substrate, of 50.8 mm in diameter and of ELO quality, even if the existing techniques (LLO, sacrificial layer, abrasion) give good results. results.

It is also known from patent application EP 533 551 that thin films of semiconductor materials can be produced according to the following process:

In a first step, ions are implanted by bombardment creating in the semiconductor, at a depth close to the average depth of penetration of these ions, a layer of microcavities (or bubbles). In a second step, a heat treatment of the semiconductor thus implanted produces a rearrangement of the structure, and the pressure induced by the microbubbles allows a separation of a thin film from the rest of the semiconductor.

However, this so-called “smart-cut” technique has never been used for GaN.

The object of the invention is to propose a method for producing a GaN film separated from its substrate, which is simple, rapid and inexpensive, and which provides a GaN film of increased quality.

It is specified that in the context of the present invention, the GaN can be doped or not. Mention may in particular be made, as doping substances, of magnesium, zinc, beryllium, calcium, carbon, boron and silicon.

In the description which follows, the term “GaN film separated from its substrate” or “self-supported GaN film” is used.

Thus, the subject of the invention is a method for producing a self-supported film of gallium nitride (GaN) from a substrate, by deposition of GaN by epitaxy, characterized in that the deposition of GaN comprises at minus a step of lateral epitaxial overgrowth (ELO) and in that it comprises a step of separation of a part of the GaN layer from its substrate by embrittlement by implantation of ions in the GaN layer directly. More particularly, the invention relates to a process for producing a gallium nitride (GaN) film as described above, characterized in that it comprises the following successive steps:

(i) the deposition on a substrate of a GaN layer by vapor or liquid epitaxy. (ii) a step of implantation of embrittlement ions so as to create in the GaN layer deposited during the previous step a weakening zone,

(iii) a step of recovery by epitaxy by lateral epitaxial overgrowth

(ELO) to form a new layer of GaN and, (iv) a spontaneous separation step at the level of the embrittlement zone.

The step or steps of lateral epitaxial growth of GaN can be carried out in the vapor phase, for example using EPVOM, HVPE or even SVT or liquid (LPE) techniques. This method makes it possible in particular to place the embrittlement zone in a precisely desired zone because the GaN deposited during the resumption of epitaxy does not hinder the implantation of ions. This process also makes it possible to use the high temperatures of the resumption of epitaxy as being the useful heat treatment to ensure the rearrangement of the structure. This process finally makes it possible to obtain spontaneous separation after the various epitaxy phases, and heat treatments, namely from the resumption of growth until cooling at the end of the resumption phase of epitaxy.

This method has the particular advantage of not requiring large doses of implantation ions but also of providing GaN films of homogeneous and controlled thickness, namely in particular very thin, of the order of 0.1 μm. . The invention also relates to any GaN film capable of being obtained by this process. The GaN film thus obtained can have a thickness varying from 100 to 5000 μm. According to a particular embodiment of the invention, the GaN film obtained can have a thickness of at least 0.1 mm.

The range of thickness that can be targeted is therefore very wide.

For a large number of applications, it is sought to manufacture gallium nitride films having a thickness of more than 50 μm; these films are part of the invention.

An optoelectronic component is also proposed, and in particular a laser diode, a UV light-emitting diode, a photodetector or a transistor, characterized in that it is provided with a GaN film capable of being obtained by the process of the invention.

The initial substrates can have a thickness of a few hundred micrometers, generally of the order of 200 μm and can be chosen from sapphire, ZnO, SiC, LiAI02, LiAI02, LiGa0 ₂ , MgAI0, Si, GaAs, AIN or GaN. The substrates can be treated before any deposition of GaN by nitriding.

During the step of depositing on a substrate a layer of GaN by epitaxy in the vapor phase (i), a lateral epitaxial overgrowth in the vapor phase (ELO) is preferably carried out in order to minimize, from the start of the process of the invention, the defect density. We can notably use EPVOM, HVPE or CSVT technology. It is preferred during this step to use technology by EPVOM.

The two alternatives described below are both ELO technology, the second alternative using a technology called "spontaneous ELO", which has been described in MRS Internet J. Nitride Semicond. Res. 7, 8 (2002). Thus according to a first alternative of the step of depositing on the substrate a layer of GaN (i), first of all, after having deposited on the substrate a thin layer of GaN, a deposit on the substrate of a suitable dielectric, which is etched for example by photolithography, so as to define apertures, secondly an exposure of the zones of the thin layer of GaN which are opposite and finally a deposition by epitaxy of GaN so as to induce the deposition of GaN patterns on the facing zones and the anisotropic and lateral growth of the patterns, the lateral overgrowth being maintained until the different patterns coalesce. This first alternative in the ELO technique is known and notably described in patent application WO99 / 20816. In particular, the techniques for forming masks are known to those skilled in the art. The dielectric masks useful during step (i) can be made of silicon nitride (SiN), SiO ₂ or W. The dielectric is deposited according to techniques well known to those skilled in the art.

In the context of this first alternative of step (i), the prior deposition of the thin layer of GaN (before the deposition of the dielectric as a mask) can be preceded by the deposition of a nucleation layer by formation of a very thin film of silicon nitride so as to obtain spontaneous patterns or islands of GaN. This possibility is explained in great detail in Example 1 and also in patent application WO99 / 20816.

According to a second alternative of the step of depositing on the substrate a layer of GaN (i), the step of etching a dielectric mask is eliminated by virtue of the spontaneous formation of GaN patterns in the form of islets, playing the same role. More precisely, this second alternative can be described as follows: The substrate is covered with a thickness of silicon nitride of the order of a few atomic planes, in other words of the order of 10 nm to 20 nm in thickness. The deposition of SiN from silane and ammonia can last 360 seconds instead of 30, the SiN layer thus formed, is, as shown by the analysis in very high resolution electron microscopy presented in the aforementioned article, discontinuous; therefore, a SiN mask is formed spontaneously on a nanometric scale, which induces an ELO process. After completion of the formation of the silicon nitride layer, a layer of GaN, called a continuous buffer layer, is deposited. The thickness of this layer can be between 10 and 100 nm. The temperature during this operation can be between 500 and 700 ° C. An annealing is then carried out at high temperature between 900 and 1150 ° C. The buffer layer is converted from a continuous layer to a discontinuous layer formed of GaN patterns, or in other words of GaN patterns in the form of islands. The zones where the silicon nitride is exposed then function as a mask and the GaN patterns function as the GaN zones located in the openings made ex situ in the mask. Finally, we deposit by GaN epitaxy in the same way as in the previous alternative. This method, where the mask of silicon nitride is formed spontaneously, and which involves the same mechanisms of curvature of dislocations as in ELO is identified as "spontaneous ELO".

The implantation of ions during step (ii) can be implemented during a single step or successive steps. The implantation energies can vary from 80 to 160 keV. The implantation ions can be chosen from H ⁺ , ions of rare gases such as helium, neon, krypton and xenon as well as boron, which can be used alone or in combination. H ⁺ ions are preferred as implantation ions in the context of the present invention.

According to the invention, the temperature during implantation can vary between 4K and 1000K. For example, this temperature can be maintained at room temperature during the implantation of H ⁺ ions in a GaN layer. The heat treatment temperature at which the crystal rearrangement takes place, which corresponds to the temperature of the epitaxy, can vary from 900 to 1150 ° C. In terms of dose of implantation ions, when this is the H ⁺ ion, the preferred dose is between 10 ¹⁶ and 10 ¹⁷ H ⁺ cm ^"2 ions. The implantation depth varies from 50 nm starting from from the free surface to the GaN / initial substrate interface. When step (i) of the method is implemented according to the second variant described above, the implantation can be carried out at different stages of growth either in the islets, either at an intermediate stage where the islets are not fully coalesced, or after complete coalescence.

The epitaxy recovery step (iii) can be implemented by epitaxy in EPVOM, HVPE, CSVT or even LPE (epitaxy in liquid phase - liquid epitaxy phase). We prefer during this step to implement HVPE technology.

The spontaneous separation in step (iv) takes place due to the thermal cycle (resumption of epitaxy at high temperature and cooling) which the layer of GaN ELO undergoes during step (iii) after implantation. The thickness ratio of the layer and the substrate can preferably be greater than 0.5 for spontaneous separation.

Other characteristics, objects and advantages of the invention will appear on reading the detailed description which follows of a particular embodiment of the invention, made with reference to Figures 1 to 5 in which: - Figure 1 is a representation of a first stage of two-stage lateral epitaxial overgrowth;

- Figure 2 is a representation of a second step of this lateral epitaxial overgrowth;

- Figure 3 is a representation of a step of implanting H + ions in an ELO layer;

- Figure 4 is a representation of a step of resumption of epitaxy in HVPE; - Figure 5 is a representation of a spontaneous ELO layer separation step.

Obtaining self-supported GaN is carried out, according to this particular embodiment of the invention, in three stages:

The first step consists of GaN growth by lateral epitaxial overgrowth on a sapphire substrate by EPVOM; The second step consists of implanting hydrogen; The third stage consists of a resumption of epitaxy in HVPE. The first step is shown diagrammatically in FIGS. 1 and 2: after having epitaxied a layer of GaN, referenced 2, on a sapphire substrate, referenced 1, a deposition of SiN is carried out in situ (masks 3) then, on this dielectric layer, 3bis openings are etched by photolithography in well defined crystallographic directions [1-100] or [11-20] GaN. Finally, we resume growth which first gives a selective epitaxy 4.

At the end of this first phase, where the growth speed along an axis C, orthogonal to the main plane of the substrate, is greater than the lateral growth speed, bands with triangular sections with facets {11-22} are obtained. Inside these triangular section bands, the through dislocations were bent at 90 °. In this first phase of ELO epitaxy, a lateral overgrowth is then carried out to finally result in a plane ELO 5 layer. At the end of this stage of the process, a GaN layer is obtained having a dislocation density less than 10 ⁷ cm ^"2 .

This assembly formed by a GaN ELO 5 layer and the sapphire substrate 1 is implanted by H ⁺ ions (FIG. 3) so that a weakening zone 6 is created in the ELO layer 5 at a depth between 50 nm and 5 μm. The ions are implanted at doses between 1 × 10 ¹⁵ and 1 × 10 ¹⁷ cm ^"2 .

During the resumption of epitaxy then carried out on this ELO layer, at high temperature, the implanted area, leads to embrittlement of the substrate consisting of the ELO layer in the cleavage zone. The ELO 5 layer therefore constitutes a privileged cleavage zone.

This layer weakened by the implantation of H + is then taken up by HVPE epitaxy. More precisely, after implantation, the weakened but entire ELO structure is placed on the substrate holder of an HVPE reactor. 10 to 500 μm of GaN are deposited to form a layer 7 (FIG. 4).

The HVPE technology is very widely documented and the resumption of epitaxy in HVPE is carried out here according to the state of the art.

During the resumption of GaN epitaxy by HVPE on this implanted ELO 5 structure H, three main interesting effects are obtained.

A first effect is that the ELO 5 layer is thickened without losing its crystalline qualities (no new dislocation or crack is generated).

A second effect is that the dislocation density is further reduced when resuming epitaxy in HVPE, by a factor at least equal to 2. A third effect is that the overall layer 5,7 thus obtained spontaneously separates from its initial substrate of sapphire 1 during thermal cycle which, because of the difference in thermal expansion coefficients of sapphire 1 and of the GaN layers 5, 7 generates constraints, and a self-supported GaN film 8 of ELO quality is thus obtained. This self-supporting GaN film presents a surface where, as is common in HVPE, we observe growths in the form of hexagonal pyramids, and on a rear face, constituted by the fracture zone, we can indeed identify geometric patterns corresponding to the starting ELO structure. An ELO quality self-supported GaN film was thus obtained, that is to say with a dislocation density of less than 10 ⁷ cm ^"2 .

FIG. 6 shows the reflectivity curve measured in situ in real time during the growth of "spontaneous ELO" during step (i). Snapshots (a), (b) and (c) are scanning electron microscopy images of the GaN islets ranging from their spontaneous formation to their coalescence. This particular embodiment is illustrated by example 2 below. The GaN films obtained according to the process of the present invention can be polished and used as high quality films for the manufacture of GaN-based components (electronic or optoelectronic such as laser diodes, light emitting diodes, photodetectors, transistors ...). ).

Other advantages of the present technology compared to the state of the art are that it makes it possible to reuse the starting substrate several times after separation of the HVPE part and repolishing, and that it makes it possible to separate, for example by implanting H ⁺ ions in self-supporting GaN of very thin membranes, the thickness of which can be between 5 nm and 50 nm of GaN and then bringing them onto an inexpensive substrate (such as ceramic AIN).

Thus, the subject of the invention is also the substrate after separation of the gallium nitride layer by implantation of ions, comprising part of the GaN directly deposited on the substrate during step (i) of the method according to the invention, as a new substrate which can be used for recovery by epitaxy of GaN.

The use of the substrate after separation of the gallium nitride layer by implantation of ions, comprising a part of the GaN directly deposited on the substrate during step (i) of the method according to the present invention, as a new substrate for a recovery by epitaxy of GaN also forms part of the invention.

Having self-supporting GaN films is of considerable interest for the fabrication of laser diodes.

Indeed, the use of self-supported GaN rather than GaN ELO / sapphire (or SiC) makes it possible to produce laser components with contact on the front and rear face, and above all makes it possible to facilitate the cleavage of the facets to produce a Fabry-Perot cavity. Example 1: Step (i) according to the first alternative

A horizontal or vertical reactor is used for the EVPOM epitaxy. In the example described, a vertical reactor is used, with a cylindrical growth chamber 55 mm in diameter to receive a 2 "substrate.

Step (i)

A layer of GaN by EPVOM is deposited on a sapphire substrate with a thickness of 250 μm to 430 μm using a special procedure which makes it possible to spontaneously obtain islets by a treatment consisting in covering the substrate with a film of silicon nitride, of which the thickness is of the order of 0.1 nm. Explicitly, a substrate, in particular made of sapphire, is brought to a temperature of approximately 1050-1120 ° C. to be nitrided by exposure to a flow of NH ₃ for approximately 10 minutes.

Deposition of a nucleation layer

After this nitriding step, a very thin film of silicon nitride is formed on the surface, the film being obtained by reaction between NH ₃ and silane SiH ₄ for a time long enough to limit the thickness of the film to that of an atomic plane. The operating conditions are as follows:

The gaseous vehicle is a mixture of nitrogen and hydrogen in equal proportions (4 sl / min). The ammonia is introduced with a flow rate of 2 sl / min while the silane, in diluted form at 50 ppm in hydrogen, is introduced with a flow rate of 50 scc / min. Under these conditions the typical reaction time between NH ₃ and SiH ₄ is of the order of 30 seconds. The successive stages are followed by laser reflectometry.

After completion of the layer of silicon nitride, a layer of gallium nitride with a thickness of 20 to 30 nm is deposited on the dielectric film. The precursor of gallium is trimethylgallium (TMGa). The deposition layer is made at low temperature, of the order of 600 ° C.

After the deposition of the GaN layer has been completed, an annealing at high temperature of the order of 1080 ° C. is carried out. Under the joint effect of the rise in temperature, the presence in the gaseous vehicle of a sufficient quantity of hydrogen and the presence of the very thin film of silicon nitride under the layer of GaN, the morphology of said layer undergoes a profound modification resulting from '' Solid phase recrystallization by mass transport. When the temperature approaches 1060 ° C., it is noted that the reflectivity of the buffer layer suddenly decreases: the initially continuous buffer layer is then converted into a discontinuous layer formed by islands of gallium nitride. At the end of this spontaneous in-situ recrystallization process, patterns or islands of GaN of very good crystal quality are obtained and retaining an epitaxy relationship with the substrate thanks to the very thin thickness of the layer of silicon nitride. The patterns or islands of GaN are isolated from each other by zones where the layer of silicon nitride is exposed. The characteristic heights of the islands are of the order of 240 nm. During the subsequent resumption in epitaxy with gallium nitride on the surface of the sample, the zones where the silicon nitride layer is exposed function as a mask for ELO, and the patterns or islands of GaN as well spontaneously formed are analogous to point ELO patterns. Deposition of a thin layer of undoped gallium nitride prior to the ELO step

After the deposition of the nucleation layer, a thin layer of GaN 2 μm thick is deposited by vapor phase epitaxy by pyrolysis of organometallics. The source of gallium is Trimethylgallium (TMGa) and the source of nitrogen is ammonia. Such a method is described in numerous documents.

Deposition of a layer of GaN ELO (use of a dielectric mask)

After the growth of the thin layer of gallium nitride described above, a thin layer of silicon nitride is deposited as a dielectric mask using SiH ₄ and NH ₃ with flow rates of 50 sccm and 2 slm respectively. Although extremely thin, this layer of SiN has proven to be a perfectly selective mask. Engraving by photolithography and reactive ion attack is then carried out to produce linear openings of 3 μm spaced 7 μm apart. The linear openings are advantageously oriented in the direction [10-10] of GaN, although the variant of the method described in this example can be carried out for other linear orientations, in particular [11-20].

Resumption by epitaxy on the cleared areas is carried out by GaN unintentionally doped under operating conditions such that the growth speed in the direction of the patterns sufficiently exceeds the growth speed in the direction normal to the inclined flanks of said patterns . Under such conditions, the growth anisotropy leads to the disappearance of the facet (0001). The first phase of the implementation of the ELO process ends when the disappearance of the facet (0001) of the GaN pattern is ensured. At the end of the first step, the GaN patterns took the form of bands whose section is triangular. The second stage of ELO consists of resuming epitaxy by modifying the growth conditions to change the growth anisotropy so that it becomes favorable for the planerization of GaN patterns. As described in WO 99/20816, this can be achieved either by adding magnesium to the vapor phase or by increasing the temperature. During this second time, the GaN patterns develop with an expansion of the facet (0001) (which reappears at the top of each pattern) while the surface of the lateral facets decreases. The second step ends when the sides have disappeared, the upper surface of the deposit formed by the coalesced patterns is flat. This structure is implanted by H ⁺ ions in a step (ii) and taken up in HVPE as follows:

Step (iii): Resumption in HVPE of the implanted ELO layer.

After having undergone implantation of hydrogen ions in doses of between 1 × 10 ¹⁵ and 1 × 10 ¹⁷ cm ² , the ELO layer is reused as a substrate and placed in a HVPE reactor in order to obtain a thickness of GaN sufficient to produce 1 spontaneous separation effect. The temperature rise takes place in a mixed atmosphere of nitrogen (2.5 slm) and ammonia (0.5 slm). As soon as the temperature of

1030 ° C is reached, the vapor phase is modified, a mixture of 0.5 slm of nitrogen and 2 slm of hydrogen is maintained as a new carrier gas, while the flow of ammonia is reduced to 0.4 slm .

The first phase of the growth of a thick layer of GaN is then initiated by introducing into the vapor phase a flow rate of 15 sccm of gallium chloride obtained by reaction of 15 sccm of HCl with liquid gallium maintained at a temperature equal to that of the substrate (1030 ° C). As soon as the gallium chloride is brought into contact with the ammonia, there is instantaneous formation of GaN which is deposited on the substrate with a growth rate of approximately 40 μm per hour with these flow rates.

It is necessary to obtain a film of GaN sufficiently thick, therefore sufficiently resistant from a mechanical point of view so that the subsequent separation involves the entire treated surface and therefore to avoid fracturing of the GaN layer into pieces of small surface. The growth thus continues for several hours under these experimental conditions in order to reach a thickness of at least 200 μm of the GaN layer. The growth is then interrupted and while remaining under a flow of ammonia, the experimental parameters are modified in order to reduce the roughness of the surface which is very important under the operating conditions described above. The growth temperature is brought to 1050 ° C., the flow of ammonia is increased to 1 slm and the composition of the carrier gas is modified to have a mixture of 1 slm of hydrogen and 1.5 slm of nitrogen. .

As soon as the temperature of the substrate reaches 1050 ° C., growth is then resumed by introducing a flow rate of 5 sccm of HCl on the liquid gallium leading to a flow rate of 5 sccm of gallium chloride in the vapor phase. The growth is continued under these new conditions for approximately 2 hours.

The growth is then definitively completed by diverting the flow of HCI towards the outside and the cooling takes place in an atmosphere consisting only of nitrogen and ammonia with a temperature ramp of 2.5 ° C per minute. When the temperature of the substrate is below 800 ° C, the ammonia flow can be completely interrupted.

The separation step (iv) takes place spontaneously by cooling.

Example 2: Step (i) according to the second alternative

In Example 2, a 3 ^χ 2 "vertical reactor is used where the active gases are distributed by a shower system which makes it possible to separate the gaseous flows of ammonia and trimethylgallium.

Explicitly, a substrate, as described in MRS Internet J.

Nitride Semicond. Res. 7, 8 (2002), in particular in sapphire, is brought to a temperature of approximately 1050-1120 ° C. to be nitrided by exposure to a flow of NH ₃ for approximately 10 minutes.

Step (i): thin layer of GaN by spontaneous ELO

After this nitriding step, a thin film of silicon nitride is formed on the surface, the film being obtained by reaction between NH ₃ and silane SiH for a time long enough to limit the thickness of the film to a few atomic planes. The operating conditions are as follows: The gaseous vehicle is a mixture of nitrogen and hydrogen in equal proportions (10 sl / min). The ammonia is introduced with a flow rate of 8 sl / min while the silane, in the form diluted to 1000 ppm in hydrogen, is introduced with a flow rate of 50 scc / min. Under these conditions the typical reaction time between NH ₃ and SiH ₄ is of the order of 300 seconds. The growth technology is then identical to Example 1:

-Deposition of a GaN layer at 600 ° C - Annealing at 1080 ° C (formation of islets). The successive stages are followed by laser reflectometry. (Fig. 6). Figure 6 also shows the reflectivity curve corresponding to Example 1. The longer duration of the deposition of silicon nitride (360 seconds compared to 30 seconds when depositing a nucleation layer as in Example 1) induced a much longer total coalescence of all the islets (that is to say which lasts longer? details?), but produces a better quality GaN substrate, typically with dislocation densities of less than 10 ⁸ cm ^"2 .

It is this substrate, developed according to this technology identified as "spontaneous ELO", which will then be implanted during a step (ii) (we no longer produce an ELO structure by depositing a mask, opening in this mask and resuming growth).

Step (iii): Resumption in HVPE of the "spontaneous ELO" layer implanted

After having undergone an implantation of hydrogen ions in doses of between 1 × 10 ¹⁵ and 1 × 10 ¹⁷ cm ^"2 , the" spontaneous ELO "layer is reused as a substrate and placed in an HVPE reactor in order to obtain a sufficient thickness of GaN to produce the spontaneous separation effect. Unlike example 1, implantation can be carried out at different stages of growth either in the islets, or at an intermediate stage where the islets are not fully coalesced, or after coalescence These three possibilities are indicated by an arrow in Figure 6.

The resumption of epitaxy takes place in a horizontal HVPE reactor 1 ^χ 2 "The temperature rise takes place in a mixed atmosphere of nitrogen (2.5 slm) and ammonia (0.5 slm). the temperature of 1030 ° C is reached, the vapor phase is modified, a mixture of 0.5 slm of nitrogen and 2 slm of hydrogen is maintained as a new carrier gas, while the flow of ammonia is reduced to 0 , 4 slm.

The first phase of the growth of a thick layer of GaN is then initiated by introducing into the vapor phase a flow of 15 sccm of gallium chloride obtained by reaction of 15 sccm of HCl with liquid gallium maintained at a temperature equal to that of the substrate (1030 ° C). As soon as the gallium chloride is brought into contact with the ammonia, there is instantaneous formation of GaN which is deposited on the substrate with a growth rate of approximately 40 μm per hour with these flow rates.

It is necessary to obtain a film of GaN sufficiently thick, therefore sufficiently resistant from a mechanical point of view so that the subsequent separation involves the entire treated surface and therefore to avoid fracturing of the GaN layer into pieces of small surface. The growth thus continues for several hours under these experimental conditions in order to reach a thickness of at least 200 μm of the GaN layer.

The growth is then interrupted and while remaining under a flow of ammonia, the experimental parameters are modified in order to reduce the roughness of the surface which is very important under the operating conditions described above. The growth temperature is brought to 1050 ° C., the flow of ammonia is increased to 1 μm and the composition of the carrier gas is modified to have a mixture of 1 μl of hydrogen and 1.5 μm of nitrogen. As soon as the temperature of the substrate reaches 1050 ° C., growth is then resumed by introducing a flow rate of 5 sccm of HCl on the liquid gallium leading to a flow rate of 5 sccm of gallium chloride in the vapor phase. The growth is continued under these new conditions for approximately 2 hours. Growth is then definitively completed by deriving the flow of

HCI to the outside and cooling takes place in an atmosphere consisting only of nitrogen and ammonia with a temperature ramp of 2.5 ° C per minute. When the temperature of the substrate is below 800 ° C, the ammonia flow can be completely interrupted.

The self-supporting substrate which separated spontaneously during a step (iv) after the thermal cycle, resumption of HVPE growth and cooling has a dislocation density of less than 10 ⁷ cm ^"2 .

Claims

1. Method for producing a gallium nitride (GaN) film from a substrate, by deposition of GaN by epitaxy, characterized in that the deposition of GaN comprises at least one step of lateral epitaxial overgrowth (ELO) and in that it comprises a step of separating part of the GaN layer from its substrate by embrittlement by implanting ions in the GaN layer directly.

2. Method for producing a gallium nitride (GaN) film according to claim 1, characterized in that it comprises the following successive steps:

(i) the deposition on a substrate of a GaN layer by vapor or liquid epitaxy,

(ii) a step of implantation of embrittlement ions so as to create in the GaN layer deposited during the previous step a weakening zone,

(iii) a step of recovery by epitaxy by lateral epitaxial overgrowth

(ELO) to form a GaN layer and,

(iv) a spontaneous separation step at the level of the embrittlement zone.

3. Method according to claim 2, characterized in that the deposition of GaN during step (i) is carried out by epitaxy by lateral epitaxial overgrowth ELO in vapor or liquid phase.

4. Method according to claim 3, characterized in that step (i) is implemented by technology by vapor phase epitaxy from halides and hydrides (HVPE), by technology by epitaxy in vapor phase by organometallic pyrolysis (EPVOM) or by sublimation (CSVT).

5. Method according to claim 3 or 4, characterized in that step (i) comprises the following steps:

- deposition of a GaN layer,

- deposition of a dielectric layer which is etched to obtain openings. - deposition of GaN in the GaN zones located in the openings then,

- GaN deposition giving rise to lateral overgrowth until the GaN patterns coalesce.

6. Method according to claim 3 or 4, characterized in that step (i) is a spontaneous ELO step which comprises the following steps:

- deposition of silicon nitride with a thickness of the order of 10 to 20 nm, - deposition of a continuous buffer layer of GaN,

annealing at high temperature between 1050 and 1120 ° C. so that the buffer layer is converted from a continuous layer to a discontinuous layer formed of GaN patterns in the form of islands, then

- deposition by GaN epitaxy.

7. Method according to claimδ, characterized in that the implantation is carried out either in the islets, or at an intermediate stage where the islets are not entirely coalesces, or after total coalescence of these islets.

8. Method according to any one of claims 1 to 7, characterized in that the implantation ions can be chosen from H ⁺ , ions of rare gases such as helium, neon, krypton as well as boron.

9. Method according to any one of claims 1 to 8, characterized in that the implantation energies can vary from 80 to 160 keV.

10. Method according to any one of claims 1 to 9, characterized in that the ions implanted in the GaN layer are H ⁺ ions.

11. Method according to claim 10, characterized in that the implantation ions are H ⁺ ions and the implantation dose in H ⁺ ions varies from 10 ¹⁶ to 10 ⁷ cm ^"2 .

12. Method according to any one of claims 2 to 11, characterized in that the spontaneous separation at the level of the zone of weakness of the layer formed during step (i) defined in claim 2, is implemented by returning to ambient temperature after resuming epitaxy.

13. Method according to any one of claims 1 to 12, characterized in that the implantation depth varies from 50 nm to the GaN / initial substrate interface.

14. Method according to any one of claims 1 to 13, characterized in that the substrate is chosen from sapphire, ZnO, 6H-SiC, LiAI0 ₂ , UAIO2, LiGa0 ₂ , MgAI0 ₄ , Si, GaAs, AIN or GaN .

15. The method of claim 14, characterized in that the substrate is a sapphire substrate.

16. Method according to any one of claims 2 to 15, characterized in that the lateral epitaxial overgrowth according to step (iii) as defined in claim 2 is carried out by epitaxy in EPVOM, in HVPE, in CSVT or in liquid phase epitaxy (LPE).

17. Method according to any one of claims 1 to 16, characterized in that the gallium nitride is doped during at least one of the steps of lateral epitaxial overgrowth with a doping substance which can be chosen from magnesium, zinc, beryllium, calcium, carbon, boron or silicon.

18. Gallium nitride film, characterized in that it is capable of being obtained by a process according to any one of claims 1 to 17.

19. Gallium nitride film according to claim 18, characterized in that it has a thickness greater than 50 μm.

20. Substrate after separation of the gallium nitride layer by implantation of ions according to the method as defined in any one of claims 1 to 17, comprising a part of the GaN directly deposited on the substrate during the step ( i) as defined in claim 2, as a new substrate which can be used for recovery by subsequent GaN epitaxy.

21. Use of the substrate after separation of the gallium nitride layer by implantation of ions according to the method as defined in any one of claims 1 to 17, comprising part of the GaN directly deposited on the substrate during step (i) as defined in claim 2, as a new substrate for recovery by epitaxy of GaN.

22. Optoelectronic component, characterized in that it is provided with a GaN film according to claim 18 or 19.

23. Laser diode, UV light-emitting diode, photodetector or transistor, characterized in that it is provided with a GaN film according to claim 18 or 19.