Method for the Making Heteroepitaxial Thin Layers and Electronic Devices

1. Field of the Invention

The invention relates to a method for the making of heteroepitaxial thin layers and, more specifically, for the making of layers of different types of semiconductor materials. The invention also relates to the application of this method to the making of electronic and optoelectronic devices.

The invention can be used to obtain thin layers with a low density of defects, on a substrate that is also monocrystalline and that, in principle, has a distinct nature, by using seeds of a third material.

For example, the method according to the invention can be used to obtain thin layers of InP of high crystalline quality on a silicon substrate, using GaAs seeds. Furthermore, the method according to the invention can be used to obtain particular structures and associations of components (lasers, photodiodes, gun diodes etc.) based on III-V type material on a silicon substrate.

2. Description of the Prior Art

The heteroepitaxial growth of thin layers of III-V type compounds of high crystallographic quality on a silicon substrate enables the association of semiconductors having direct gaps and high mobility with a material that is technologically well mastered.

The result thereof is an extremely vast range of applications, in fields such as integrated optoelectronics (lasers and photodiodes with associated commands), optical telecommunications, power components made of III-V type materials, optical connection systems on silicon chips etc.

From a technical point of view, the growth of III-V type components on silicon is done by MBE (Molecular Beam Epitaxy) or MOVPE (Metalorganic Vapor Phase Epitaxy), which gives epitaxially grown thin layers having densities of dislocations greater than 10.sup.6 /cm.sup.2 (cf. S. F. Fang et al. in J. Appl. Phys., 68, R31-190), with which it is impossible to make optical components (such as lasers and photodiodes) having acceptable longevity.

Furthermore, while the epitaxy of GaAs on Si by MBE or MOVPE is relatively controlled, that of InP is very difficult to implement, for nucleation on Si takes place only at low temperature.

Consequently, it is difficult to decompose phosphine (PH.sub.3) when working with "source gas" MBE or else phosphine and metalorganics (trimethyl and triethylindium) when using MOVPE or MOMBE (Metalorganic Molecular Beam Epitaxy). Finally, the use of phosphorus at low temperature in standard MBE raises problems of pumping. In all cases, the InP layers show dislocation levels of over 10.sup.6 /cm.sup.2 as indicated here above.

The French patent applications Nos. 88 04437 and 88 04438 describe methods of growth that enable notable improvements in the quality of the heteroepitaxial layers of III-V materials on a silicon substrate.

Improvements to these methods have been described in the patent applications Nos. 90 12443 and 91 12352.

In the different documents referred to here above, the heteroepitaxy related to a material B (for example GaAs) on a substrate A (for example Si).

The present invention relates to a method for obtaining a material C that is monocrystalline and without defects (for example InP) from seeds of a material B (for example GaAs) on a substrate of a material A (for example silicon).

Taking the examples of materials indicated here above, the invention enables a "directed" growth of InP with high crystallographic quality on silicon, using very poor quality GaAs seeds. Furthermore, the step of direct growth (MBE or MOVPE) of InP on Si, which is very difficult to implement, is eliminated.

The invention therefore relates to a method for the growing of heteroepitaxial thin layers, said method comprising the following steps:

the making of a seed of monocrystalline material of a first type (B) in a cavity, the walls of which are made of at least one material on which there can be neither nucleation nor growth during the epitaxy step that will follow;

the growth, by epitaxy, of a material of a second type (C) in the cavity, from the seed so as to obtain a monocrystalline material of the second type (C) in the cavity.

The invention also relates to a method for the growth of heteroepitaxial thin layers, comprising the following steps:

the growth, on a face of a crystalline substrate, of a layer of a monocrystalline material of the first type (B);

the deposition of an insulator layer;

the making of at least one first aperture in the insulator layer;

the etching, through the first aperture, of a part of the layer of the first type (B) of monocrystalline material along a sufficient length from the edges of the aperture for the making of a seed;

the growth of a seed, made of a material of the second type (C), through the aperture from the material of the first type (B);

the making of at least one second aperture in the insulator layer, above the material of the first type (B);

the etching, through the second aperture, of the material of the first type (B);

the growth, by epitaxy, of a material of the second type (C) by the second aperture, starting from the seed made of material of the second type (C)

The different objects and features of the invention shall appear more clearly from the following description, and from the appended drawings, of which:

FIG. 1 shows a general view of the manufacturing method of the invention;

FIGS. 2a and 2b show explanatory diagrams of the method of the invention;

FIGS. 3a to 3d shows manufacturing steps of the method according to the invention;

FIGS. 4a to 4f show an example, in detail, of a method of manufacture according to the invention;

FIG. 5 show a variant of the method of manufacture according to the invention;

FIGS. 6a to 6f show another example, in detail, of the method of manufacture according to the invention;

FIGS. 7a to 7d show a variant of the method described with reference to FIGS. 6a to 6f;

FIGS. 8a to 8c show a method for the manufacture of an optoelectronic device;

FIG. 9 shows a variant of the method represented by FIGS. 8a to 8b;

FIGS. 10 to 10c show a method for the manufacture of several associated devices.

Referring to FIG. 1, we shall first of all give a general description of the method of the invention.

On a substrate 1, made of a monocrystalline material A, there is made a seed 4, of a material B that is also monocrystalline, sandwiched between a face 10 of the substrate 1 and a face 20 of a confinement element (layer) 2 made of a material D.

Through an aperture 5, a monocrystalline material C is made to grow in the cavity 3 from the seed 4. The growth front of the monocrystalline material C develops as indicated by an arrow in FIG. 1. As shall be seen here below, a preferred method of growth will be a chloride or hydride vapor phase epitaxy (VPE).

According to the invention, the material C to be made to grow is of a type different from the material B of the seed 4.

As for the materials A and D, they may have chemically different or similar natures, but they are chosen so that, during the growth of the material C, there can be neither nucleation nor growth of the material C on the faces 10 and 20 under the chosen conditions of growth.

According to the exemplary embodiments that shall be described here below, the materials A and B are different. However, it is also possible to envisage a case where they are identical.

For example, the material A of the substrate 1 is silicon, the material B of the seed 4 is GaAs and the material C to be grown is InP.

The method of the invention consists in achieving the growth, by directed epitaxy on silicon, of the material such as, for example, InP from the GaAs seed. The blocking of the GaAs dislocations generated at the GaAs/InP interface during the epitaxial operation is done either at the face 20 of the element 2, positioned beforehand above the GaAs seed. FIGS. 2a and 2b give a schematic view of the principle of this blocking of the dislocations (the dislocations being shown in the form of inclined lines).

In order to prevent parasitic nucleation and easily obtain conditions of selective epitaxy, the element 2 is constituted by a typically dielectric material, with a high nucleation barrier. Furthermore, the nucleation on the surface 10 of the silicon is also prevented, here again because of the intrinsically high value of the barrier of nucleation of InP on Si but also because this surface 10 is partially passivated by native oxide. The method preferably used for this type of growth will be one that enables operation close to conditions of thermal equilibrium for the total reaction of deposition, namely a method of chloride VPE (vapor phase epitaxy) or else hydride VPE (HVPE). In the former case, the precursors are phosphorus trichloride (PCl.sub.3) and indium, while in the latter case the precursors are phosphine (PH.sub.3) and indium.

FIGS. 3a to 3d give a succinct description of a succession of technological steps of the method according to the invention, making it possible to obtain thin layers of InP on Si using GaAs seeds on silicon.

The procedure starts with a GaAs layer 4 on a silicon substrate 1 obtained by MBE or by MOCVD and having a density of dislocations typically greater than 10.sup.6 /cm.sup.2 (FIG. 3a). A deposition is made, on this structure, of a thin layer of Si.sub.3 N.sub.4 or any other appropriate material in which there are made apertures 5, 5' such as parallel bands with a width of 1 to 20 .mu.m and with a pitch or spacing of 10 to 200 .mu.m (FIG. 3b). The major part of the GaAs is removed from the layer 4 (either chemically or in vapor phase at 600.degree.-800.degree. C., using hydrochloric acid HCl) through the previously etched apertures 5, 5' so as to keep only the bands 4, having a width of 1 to 10 .mu.m for example (FIG. 3c): these bands 4 will serve as seeds. The InP is then epitaxiated through the apertures 5, 5'. Starting from the lateral seeds of GaAs such as 4, the growth is achieved in a confined way in the cavities 3, 3' between the lower surface 20 of the cap 2 and the upper surface 10 of the silicon wafer (FIG. 3d). Owing to this confinement, the dislocations are blocked according to the mechanism of FIGS. 2a and 2b. Thus, defect-free InP on silicon is obtained.

Although the dislocations are blocked, it may happen that, should the lattice parameter mismatching and/or the difference in thermal expansion coefficients be too great, cracks will appear in the material obtained by directed growth (InP in the above example) beyond a certain width of growth.

In order to overcome this drawback, the invention provides for a two-step procedure for directed growth, as is shown schematically in FIGS. 4a to 4f.

Just as in the foregoing procedure, a layer 2 of nitride, Si.sub.3 N.sub.4 for example, is deposited on a layer 4 of GaAs heteroepitaxiated (by MBE or MOCVD) on a silicon substrate 1. Apertures 5, 5', in the form of bands for example, are opened in this nitride layer (FIG. 4a). For the different geometrical characteristics, reference will be made to the commentary on FIG. 3b. Through the apertures 5, 5' thus made, the GaAs layer 4 is very slightly etched so as to clear a width L, starting from the edges of the bands 5, 5', that is smaller than the critical width beyond which cracks are liable to appear during the re-epitaxy of the third material (InP in the chosen example). Typically, this clearing is of the order of 0.5 to some micrometers (see FIG. 4b). Then a first growth (of InP according to the chosen example) is made, starting from the lateral flanks of GaAs left bare after the etching operation. The width of this growth is kept below the critical width corresponding to the appearance of the cracks (FIG. 4c). New seeds 14, 14', etc., made of InP, are obtained.

A second layer of Si.sub.3 N.sub.4 is re-deposited on the entire piece, so as to block the previously etched apertures (FIG. 4d).

Using a second mask (or else the same mask as the one used to make the first apertures, but translated by a half-pitch), a second series of bands 15, 15' is opened in the nitride layers (see FIG. 4e). This second series of bands is preferably positioned midway between the previous bands 5, 5' etc. Through these second apertures, the remaining layer 4 of GaAs is eliminated by chemical etching so as to keep only the new seeds 14, 14' of InP (again according to the chosen example). For this etching, preferably a selective chemical mixture is used, in such a way that there is no need to control the etching of the InP. This latter operation could indeed prove to be extremely difficult, should the InP bands have a small width. The structure thus obtained is shown in FIG. 4e. Finally, the seeds 14, 14' etc. are used for the re-growing, by directed epitaxy, of the layers 7, 7' etc. of InP without defects on a silicon substrate (see FIG. 4f).

For different reasons, and depending on the nature of the components that are to be made in the layers of III-V materials on silicon, it may be desired to insulate the thin layer from the substrate.

In this case, the invention provides for a chemical oxidation of the surface of the silicon substrate 1, after etching of the layer 4 of material B (GaAs in this case) and before growth by directed epitaxy of the material C (InP). To make this chemical oxide on the surface 10 of the silicon substrate, it is possible to use a solution of H.sub.2 O.sub.2 /H.sub.2 SO.sub.4 or else, again, a solution of HNO.sub.3. Depending on the period of exposure and on the concentration of the solutions in question, the thicknesses obtained for the "chemical" SiO.sub.2 layer 8 will range from some tens to some hundreds of angstroms.

FIG. 5 shows a method such as this for growth after chemical oxidation of the face 10 of the substrate 1.

Should a layer 8 of higher quality oxide, or even a thermal oxide, have to be obtained on the surface of the silicon substrate, then the initial substrate used will be a wafer of SOI (silicon on insulator) type silicon. Different techniques are commonly used to make SOI wafers, and the IEEE CIRCUITS AND DEVICES MAGAZINE, Vol 3, Nos. 4 and 6, July and November 1987, for example, may be referred to for many details of these techniques.

In the following example, a SIMOX (Separation by IMplantation of Oxygen) type SOI wafer is used, but it would have been possible also to use SOI wafers of the (Silicon Direct Bonding) or ZMR (Zone Melting Recrystallization) type.

FIGS. 6a to 6f describe a sequence of technological steps by which it is possible to obtain thin layers insulated from the substrate by a high quality oxide.

The procedure therefore starts from a SIMOX type wafer comprising a layer 8 of SiO.sub.2 and a layer 9 of Si (100) (FIG. 6a) on which there is epitaxially grown, by MBE or MOVPE, a thin layer 4 of GaAs (FIG. 6b), which will therefore have more than 10.sup.6 dislocations/cm.sup.2. A layer 2 of Si.sub.3 N.sub.4 is deposited. In this layer 2, just as in the foregoing example, a series of apertures 5, 5' is made in the form of parallel bands for example (FIG. 6c). In succession, the GaAs layer 4 is etched to form seeds 4 (see FIG. 6d) and then the silicon layer 9 is etched (see FIG. 6e). Then the directed growth of InP is achieved through the apertures 5, 5' from GaAs seeds. Just as in the case of FIG. 1, there is no nucleation of InP on the flanks of the underlying Si (layer 9) in the form of a GaAs seed if the directed growth of InP is done by VPE for example.

FIGS. 7a to 7d show a variant of a method for making a seed of material B at an end of a cavity surrounded by insulator material, including a thermal oxide.

According to FIG. 7a, on a substrate 1 made of silicon for example, there is made a layer 8 of thermal oxide (thermal SiO.sub.2), then a layer 10 of amorphous semiconductor (amorphous Si) and finally a layer 11 made of a material different from the two previous materials (8 and 10), for example a metal such as aluminum.

Apertures are etched in all these layers: an epitaxy is made of a material (GaAs) in order to make seeds 4. The GaAs material deposited in the apertures on the substrate Si is monocrystalline at 4 and it is polycrystalline on the aluminum layer 11.

The epitaxy is such that the thickness of the seeds 4 enables the upper face of the layer 10 to be reached (FIG. 7b).

Then, the aluminum layer 11 is etched so as to achieve the "lift off" of the layer 13 of polycrystalline GaAs (FIG. 7c).

The structure is covered by a layer 2 of insulator (Si.sub.3 N.sub.4) and apertures 5, 5' are made in the layer 2. Through the apertures 5, 5', the amorphous silicon 10 is etched and cavities 3, 3' (FIG. 7d) are obtained.

It is now possible to achieve a directed epitaxy of InP through the apertures 5, 5' out of seeds such as 4, and then a layer of InP on thermal silica 8 is obtained.

The different methods of insulation described here above are necessary when components are made by directed epitaxy on silicon. Thus, FIGS. 8a to 8c give a description, by way of an example, of the making of an InP/GaInAsP/InP laser obtained by directed epitaxy, but by modulating the doping and the composition of the materials epitaxially grown in the cavity 3 of FIG. 8a.

The method starts, for example, with a structure of the type shown in FIG. 6e and carried over to FIG. 8a. It is possible to start, for example, by causing the growth, by VPE, of a layer of GaAs, using the seed 4, so as to improve the crystalline quality of the seed; in most cases, this is not necessary. The bands 5 and 5' in the nitride 2 will preferably have been oriented beforehand in a direction [110] of the plane of the substrate (with an orientation of [100]). The bands of seed 4 keep this orientation [110]. This makes it possible, during the directed growth, to obtain a single lateral facet. Thus, a homogeneous incorporation of the dopants is achieved, during the directed growth. FIG. 8b shows a case where a [110] type lateral growth facet is obtained. After the growth of GaAs, an n+ doped InP growth is achieved, followed by an n doped InP growth. An epitaxy of the quaternary material Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y is then done, the lattice parameter of this quaternary material being matched with InP. Depending on the laser wavelength to be obtained, values will be chosen such that, for example, x=0.27 and y=0.60 or else x=0.40 and y=0.85 or else again x=0.47 and y=1.

The width 1 of the quaternary material will typically range from 0.1 to 0.8 .mu.m.

Finally, a growth of InP (p) is carried out, followed by a growth of InGaAs (p+) on which it will be easy to make an ohmic contact. The succession of the layers thus made is shown in FIG. 8b. Finally, a layer 20 of Si.sub.3 N.sub.4 or of SiO.sub.2 is deposited on the structure to passivate it. Windows are opened for the contacts 21, 22. The contacts 21, 22 are deposited and they are etched. FIG. 8c shows the structure of a laser with coplanar electrodes thus obtained.

In order to improve the upper interface of the active layer of GaInAsP, it is possible, as shown in FIG. 9, to etch the layer 20 of Si.sub.3 N.sub.4 locally and to carry out a growth, by operations of selective epitaxy, of an InP layer 23 that is either semi-insulating or doped. Thus, an excessive recombination is avoided at the upper interface of GaInAsP: such an excessive recombination would have had the effect of reducing the longevity of the laser.

On a single substrate, it is possible to fabricate components of different types. Indeed, on the substrate 1 of FIG. 10a, there are shown, for example, two cavities 3 and 3' with seeds 4, 4', 4". The cavity 3 can be used to make one or two components of a first type by the performing an epitaxial growth from the seeds 4, 4'. The cavity 3' can be used to make one or two components of a second type. To this end, it is sufficient to achieve the growth of materials having different compositions and/or dopings in the two cavities.

For example if a structure such as the one described here above with reference to FIGS. 8a to 8c is made to grow in the cavity 3, it is clear that, on the same substrate of silicon, in the cavity 3', it is possible to manufacture also by an identical process GaAs/GaAlAs type lasers. FIGS. 10b and 10c describe a quantum well structure such as this.

As shown in FIG. 10b, the following materials are made to grow successively from a GaAs seed:

GaAs (n+)

GaAs (n)

GaAlAs (n)

1/2 optical cavity of GaAlAs with Al varying continually from 20 to 40%

1 quantum well of GaAlAs with Al chosen between 0 and 5

1/2 optical cavity of GaAlAs with Al varying continually from 20 to 40%

GaAlAs (p)

GaAs (p+)

After the making of an insulator layer on the structure and the making of contacts, there is obtained the structure shown in FIG. 10c wherein dimensions of the different zones of the materials referred to earlier are indicated on an exemplary basis.

If a GaAs/GaAlAs laser such as the one shown in FIG. 10c and an InP/GaInAsP/InP laser such as the one shown in FIG. 8c have to be made on a single substrate, the method will start with the making of the GaAs/GaAlAs structure of FIG. 10b, this structure being made at higher temperature. Then the structure of FIG. 8b, corresponding to the second type of laser, will be made. The metallizations and the etchings corresponding to FIGS. 8c and 10c will be made at the last stage.

It is clear that the above structures and devices represent only particular examples. According to transistor technology, it is also possible to make InP gun diodes, GaInAs type photodiodes or any other types of photodiodes.